DOE/NASA/21749-1 

NASA TM-103130 

Assessment of Fundamental Materials 

Needs for Thick Thermal Barrier 

Coatings (TTBC's) for Truck 

Diesel Engines 

Robert A. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

May 1990 

Prepared for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

Office of Transportation Systems 

DOE/NASA/21749-1 

NASA TM-103130 

Assessment of Fundamental Materials 

Needs for Thick Thermal Barrier 

Coatings (TTBC's) for Truck 

Diesel Engines 

Robert A. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

May 1990 

Prepared for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

Office of Transportation Systems 
DISCLAIMER 

This report was prepared as an account of work sponsored by an agency 

of the United States Government. Neither the United States Government 

nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, apparatus, 

product, or process disclosed, or represents that Its use would not 

infringe privately owned rights. Reference herein to any specific 

commercial product, process, or service by trade name, trademark, 

manufacturer, or otherwise, does not necessarily constitute or imply its 

endorsement, recommendation, or favoring by the United States 

Government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United 

States Government or any agency thereof. 

Printed in the United States of America 

Available from 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

NTIS price codes 1 

Printed copy: A03 

Microfiche copy: A01 

1Codes are used for pricing all publications. The code is determined by 

the number of pages in the publication. Information pertaining to the 

pricing codes can be found in the current issues of the following 

publications, which are generally available in most libraries: Energy 

Research Abstracts (ERA); Government Reports Announcements and Index 

(GRA and I); Scientific and Technical Abstract Reports (STAR); and 

publication, NTIS-PR-360 available from NTIS at the above address. 

DISCLAIMER 

This report was prepared as an account of work sponsored by an agency 

of the United States Government. Neither the United States Government 

nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, apparatus, 

product, or process disclosed, or represents that Its use would not 

infringe privately owned rights. Reference herein to any specific 

commercial product, process, or service by trade name, trademark, 

manufacturer, or otherwise, does not necessarily constitute or imply its 

endorsement, recommendation, or favoring by the United States 

Government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United 

States Government or any agency thereof. 

Printed in the United States of America 

Available from 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

NTIS price codes 1 

Printed copy: A03 

Microfiche copy: A01 

1Codes are used for pricing all publications. The code is determined by 

the number of pages in the publication. Information pertaining to the 

pricing codes can be found in the current issues of the following 

publications, which are generally available in most libraries: Energy 

Research Abstracts (ERA); Government Reports Announcements and Index 

(GRA and I); Scientific and Technical Abstract Reports (STAR); and 

publication, NTIS-PR-360 available from NTIS at the above address. 
r 

DOE/NASA/21749-1 

NASA TM-103130 

Assessment of Fundamental Materials 

Needs for Thick Thermal Barrier 

Coatings (TTBC's) for Truck 

Diesel Engines 

Robert A. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

May 1990 

Work performed for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

Office of Transportation Systems 

Washington, D.C. 20545 

Under Interagency Agreement DE-AI05-870R21749 

r 

DOE/NASA/21749-1 

NASA TM-103130 

Assessment of Fundamental Materials 

Needs for Thick Thermal Barrier 

Coatings (TTBC's) for Truck 

Diesel Engines 

Robert A. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

May 1990 

Work performed for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

Office of Transportation Systems 

Washington, D.C. 20545 

Under Interagency Agreement DE-AI05-870R21749 
-- 

- - -----

Assessment of Fundamental Materials Needs for Thick Thermal 

Barrier Coatings (TTBC's) for Truck Diesel Engines 

Summary 

Robert A. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

The present state of development of thick thermal barrier 

coatings for truck diesel engines is assessed and areas where 

improved fundamental understanding is needed to properly pursue 

development are identified. Emphasis is given to the coating 

systems and design approaches that are being developed for the 

next generation of truck diesel engines under DOE/NASA support. 

It is noted that, while considerable progress has been made, the 

current level of understanding of coating system behavior is 

inadequate and this lack of fundamental understanding may impede 

current and future development. Several areas where improved 

understanding would be especially valuable are identified and 

recommendations for research into those areas are offered. 

Introduction 

The first objective of this study is to assess the present state 

of development of thick thermal barrier coatings (TTBCs) for 

truck diesel engines. Thick shall be defined here as a total 

coating system thickness of at least 0.060 inch (0.15 cm). This 

is in contrast to thin (less than about 0.015 inch or 0.04 cm) 

thermal barrier coatings which were initially developed for high 

temperature service in aircraft gas turbine engines (ref. 1). 

However, a brief overview of relevant materials aspects of thin 

thermal barrier coatings will be given in Part IV of this paper. 

The second objective is to identify those areas where fundamental 

understanding is needed to properly pursue further development. 

This paper is based on discussions with representatives of diesel 

and gas turbine engine manufacturers, research institutions, 

plasma spray equipment manufacturers, spray powder manufacturers, 

and coating vendors. Input was taken from site visits, phone 

interviews, and the publicly available literature. site visits 

where made to Caterpillar Inc., Peoria, Illinois; Cummins Engine 

Company, Columbus, Indiana; Southwest Research Institute, San 

Antonio, Texas; United Technologies Research Center, East 

-- - - -----

Assessment of Fundamental Materials Needs for Thick Thermal 

Barrier Coatings (TTBC's) for Truck Diesel Engines 

Summary 

Robert A. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

The present state of development of thick thermal barrier 

coatings for truck diesel engines is assessed and areas where 

improved fundamental understanding is needed to properly pursue 

development are identified. Emphasis is given to the coating 

systems and design approaches that are being developed for the 

next generation of truck diesel engines under DOE/NASA support. 

It is noted that, while considerable progress has been made, the 

current level of understanding of coating system behavior is 

inadequate and this lack of fundamental understanding may impede 

current and future development. Several areas where improved 

understanding would be especially valuable are identified and 

recommendations for research into those areas are offered. 

Introduction 

The first objective of this study is to assess the present state 

of development of thick thermal barrier coatings (TTBCs) for 

truck diesel engines. Thick shall be defined here as a total 

coating system thickness of at least 0.060 inch (0.15 cm). This 

is in contrast to thin (less than about 0.015 inch or 0.04 cm) 

thermal barrier coatings which were initially developed for high 

temperature service in aircraft gas turbine engines (ref. 1). 

However, a brief overview of relevant materials aspects of thin 

thermal barrier coatings will be given in Part IV of this paper. 

The second objective is to identify those areas where fundamental 

understanding is needed to properly pursue further development. 

This paper is based on discussions with representatives of diesel 

and gas turbine engine manufacturers, research institutions, 

plasma spray equipment manufacturers, spray powder manufacturers, 

and coating vendors. Input was taken from site visits, phone 

interviews, and the publicly available literature. site visits 

where made to Caterpillar Inc., Peoria, Illinois; Cummins Engine 

Company, Columbus, Indiana; Southwest Research Institute, San 

Antonio, Texas; United Technologies Research Center, East 
Hartford, Connecticut; Purdue Thermophysical Laboratory, West 

Lafayette, Indiana; and Union Carbide coatings, Indianapolis, 

Indiana. Phone and mail discussions were held with Alloy Metals, 

Inc., Troy, Michigan; Allison Gas Turbines, Indianapolis Indiana; 

Detroit Diesel Corporation, Detroit, Michigan; Pratt & Whitney 

Aircraft, East Hartford, Connecticut; Solar Turbines, San Diego, 

California; and State University of New York, Stony Brook, New 

York. 

This study is supported by DOE Oak Ridge Operations, Ceramic 

Technology for Heat Engines Program Office, D. Ray Johnson, 

Program Manager under DOE/NASA Interagency Agreement No. 

DE-AI05-870R21749, Task 2. 

Part I. Needs for Future Truck Diesel Engine 

Truck diesel engines by approximately the year 2000 will be 

characterized by: higher fuel economy; improved reliability and 

durability: higher power density; lower NOx and hydrocarbon 

emissions, much lower particulate emissions; and lower noise 

(ref. 2). 

In order to meet these goals the Low Heat Rejection (LHR) Diesel 

Engine Concept has evolved (ref. 3). This engine will have 

insulated pistons, cylinder heads, and/or exhaust ports. The 

insulation may be TTBCs, monolithic ceramics, or air gaps -- most 

likely, combinations of the three approaches will be used. The 

water cooling system will be eliminated. Advanced lubricants 

capable of higher temperature operation will be used, and wear 

coatings will be applied to the piston rings and the cylinder 

liners. Waste energy in the exhaust will be recovered through 

turbocharging and turbocompounding (ref. 4). A Rankine bottoming 

cycle may also be included but only if fuel prices rise enough 

for it to be cost effective (ref. 5). A schematic of a LHR engine 

is shown in figure 1. A TTBC coated piston cap and cylinder head 

from a Caterpillar single cycle research engine are shown in 

figures 2a and 2b. 

Anticipated benefits associated with the LHR engine include: 

improved fuel economy (especially when combined with exhaust gas 

recovery); increased power density for greater horsepower from 

smaller displacement; and decreased maintenance costs along with 

increased reliability from elimination of the water cooling 

system (refs. 2, 6, and 7). 

Several questions, however, still remain. For example, the 

thermodynamic benefits must still be quantified (ref. 3). Many 

other of the remaining questions are related to materials 

technology. That is, questions about the durability, reliability, 

2 

------- -------

Hartford, Connecticut; Purdue Thermophysical Laboratory, West 

Lafayette, Indiana; and Union Carbide coatings, Indianapolis, 

Indiana. Phone and mail discussions were held with Alloy Metals, 

Inc., Troy, Michigan; Allison Gas Turbines, Indianapolis Indiana; 

Detroit Diesel Corporation, Detroit, Michigan; Pratt & Whitney 

Aircraft, East Hartford, Connecticut; Solar Turbines, San Diego, 

California; and State University of New York, Stony Brook, New 

York. 

This study is supported by DOE Oak Ridge Operations, Ceramic 

Technology for Heat Engines Program Office, D. Ray Johnson, 

Program Manager under DOE/NASA Interagency Agreement No. 

DE-AI05-870R21749, Task 2. 

Part I. Needs for Future Truck Diesel Engine 

Truck diesel engines by approximately the year 2000 will be 

characterized by: higher fuel economy; improved reliability and 

durability: higher power density; lower NOx and hydrocarbon 

emissions, much lower particulate emissions; and lower noise 

(ref. 2). 

In order to meet these goals the Low Heat Rejection (LHR) Diesel 

Engine Concept has evolved (ref. 3). This engine will have 

insulated pistons, cylinder heads, and/or exhaust ports. The 

insulation may be TTBCs, monolithic ceramics, or air gaps -- most 

likely, combinations of the three approaches will be used. The 

water cooling system will be eliminated. Advanced lubricants 

capable of higher temperature operation will be used, and wear 

coatings will be applied to the piston rings and the cylinder 

liners. Waste energy in the exhaust will be recovered through 

turbocharging and turbocompounding (ref. 4). A Rankine bottoming 

cycle may also be included but only if fuel prices rise enough 

for it to be cost effective (ref. 5). A schematic of a LHR engine 

is shown in figure 1. A TTBC coated piston cap and cylinder head 

from a Caterpillar single cycle research engine are shown in 

figures 2a and 2b. 

Anticipated benefits associated with the LHR engine include: 

improved fuel economy (especially when combined with exhaust gas 

recovery); increased power density for greater horsepower from 

smaller displacement; and decreased maintenance costs along with 

increased reliability from elimination of the water cooling 

system (refs. 2, 6, and 7). 

Several questions, however, still remain. For example, the 

thermodynamic benefits must still be quantified (ref. 3). Many 

other of the remaining questions are related to materials 

technology. That is, questions about the durability, reliability, 

2 

------- -------
and cost effectiveness of materials in the LHR engine must be 

demonstrated. 

Part II. Current status of TTBCs for Diesel Engines 

II A. The Heavy Duty Transport Technology Program (HDTT) 

II.A.l Background to the HDTT program 

Most of the discussion in this part will focus on the DOE/NASA 

Heavy Duty Transport Technology Program (HDTT). This is because 

most of the ongoing, publicly available research and development 

in TTBCs is being conducted under this program. HDTT is also the 

most "scientific" of current ongoing programs. The source of 

funding for the HDTT program is the DOE Office of Transportation 

Systems in Washington D.C. The program manager is John Fairbanks. 

The program is managed by the NASA-Lewis Research Center, 

Terrestrial Propulsion Office. James Wood is the NASA HDTT 

program manager and M. Murray Bailey is the TTBC project 

engineer. 

The overall objective of the HDTT program is to develop a 

technology base that can be applied to Low Heat Rejection diesel 

engines. The technology of thick thermal barrier coatings is one 

of the options being explored under HDTT (ref. 8). This is 

because TTBCs are believed to be very promising, especially for 

insulating the piston. The goals of the TTBC program are to 

develop the analytical tools needed to allow the TTBCs and the 

components to be designed as a system, to improve control or 

processing to the point where near-net shapes coatings can be 

applied, to improve NDE inspection for analysis of the component 

after processing, and to verify the reliability and durability of 

coated components in an engine environment (Private 

communication, M. M. Bailey, NASA-Lewis Research Center, 1989). 

Additional goals are evolving in this program as reference engine 

designs for meeting overall fuel economy goals evolve. 

TTBC research under the HDTT program is component and performance 

oriented and has been conducted primarily via research at diesel 

engine companies. An initial research and development contract 

was awarded to Allison Gas Turbines (ref. 9). This initial 

contract demonstrated 24 hour durability, which is over one 

million firing cycles, in an uncooled research engine having 

0.100 inch (0.25 cm) coating system on the head and a 0.060 inch 

(0.15 cm) coating system on the piston and valves. The TTBC 

system on the head included a sintered metal fiber strain 

isolator pad, a thin metallic bond coat layer and two layers of 

zirconia-yttria with the outer layer being more porous than the 

inner layer. This is shown schematically in figure 3a, and the 

3 

and cost effectiveness of materials in the LHR engine must be 

demonstrated. 

Part II. Current status of TTBCs for Diesel Engines 

II A. The Heavy Duty Transport Technology Program (HDTT) 

II.A.l Background to the HDTT program 

Most of the discussion in this part will focus on the DOE/NASA 

Heavy Duty Transport Technology Program (HDTT). This is because 

most of the ongoing, publicly available research and development 

in TTBCs is being conducted under this program. HDTT is also the 

most "scientific" of current ongoing programs. The source of 

funding for the HDTT program is the DOE Office of Transportation 

Systems in Washington D.C. The program manager is John Fairbanks. 

The program is managed by the NASA-Lewis Research Center, 

Terrestrial Propulsion Office. James Wood is the NASA HDTT 

program manager and M. Murray Bailey is the TTBC project 

engineer. 

The overall objective of the HDTT program is to develop a 

technology base that can be applied to Low Heat Rejection diesel 

engines. The technology of thick thermal barrier coatings is one 

of the options being explored under HDTT (ref. 8). This is 

because TTBCs are believed to be very promising, especially for 

insulating the piston. The goals of the TTBC program are to 

develop the analytical tools needed to allow the TTBCs and the 

components to be designed as a system, to improve control or 

processing to the point where near-net shapes coatings can be 

applied, to improve NDE inspection for analysis of the component 

after processing, and to verify the reliability and durability of 

coated components in an engine environment (Private 

communication, M. M. Bailey, NASA-Lewis Research Center, 1989). 

Additional goals are evolving in this program as reference engine 

designs for meeting overall fuel economy goals evolve. 

TTBC research under the HDTT program is component and performance 

oriented and has been conducted primarily via research at diesel 

engine companies. An initial research and development contract 

was awarded to Allison Gas Turbines (ref. 9). This initial 

contract demonstrated 24 hour durability, which is over one 

million firing cycles, in an uncooled research engine having 

0.100 inch (0.25 cm) coating system on the head and a 0.060 inch 

(0.15 cm) coating system on the piston and valves. The TTBC 

system on the head included a sintered metal fiber strain 

isolator pad, a thin metallic bond coat layer and two layers of 

zirconia-yttria with the outer layer being more porous than the 

inner layer. This is shown schematically in figure 3a, and the 

3 
coated components are shown in figure 3b. Allison is no longer 

involved in diesel engine research (that portion of the business 

is now a separate company named Detroit Diesel corporation), 

however, they are continuing to pursue the strain isolator 

approach for certain gas turbine applications. 

II.A.2 Ongoing HDTT contracts 

There are two ongoing TTBC contracts supported by the HDTT 

program. One is with Caterpillar Inc. under program manager H. J. 

Larson, and the other is with Cummins Engine Company under 

program manager T. M. Yonushonis. Coatings under these contracts 

are being designed and evaluated for application to pistons and 

cylinder heads. Performance benefits and durability are being 

confirmed in a single cylinder engine test modified to LHR 

conditions. The specific coating system conductance goal for 

these contracts is 0.5 BTU/hr-oF (408 W/M2-K), and the durability 

goal is 100 hours under accelerated conditions. Selection of TTBC 

systems drew heavily from prior aircraft engine experience on 

plasma sprayed abradable seals. Cummins selected a system based 

on the aircraft ceramic seals experience of subcontractor UTRC 

(United Technologies Research Center). Caterpillar developed a 

similar coating system. On both contracts, coating system 

selection was based on analytical design, mechanical properties 

determination, and a limited amount of rig testing. Since the 

programs were component and performance oriented and not 

materials oriented, there was no attempt to test a wide variety 

of coating system concepts. 

In each case four layer coating system designs were developed. 

These designs consisted of a metallic bond coat, two layers of 

mixed ceramic and metal, and a layer of a zirconia-yttria 

ceramic. A schematic of each companies design is given in figure 

4. Some of the key properties measured for each coating system 

are given in table 1. 

The extensive analysis conducted under the HDTT program at 

Caterpillar and Cummins has quantified our knowledge of the 

cyclic stresses and strains encountered by thick coatings on 

diesel engine components. This analysis began with a recognition 

of low cycle and high cycles stress and temperature excursions 

encountered in the diesel engine. As shown schematically in 

figure 5, the low cycle excursions involve stop/start transients 

and no load/full load transients (e.g., down hill/up hill) and 

the high cycle transients are due to combustion in the cylinder. 

These combustion transients are, of course, superimposed on the 

lower cycle transients. The cyclic environment in the diesel 

engine differs greatly from the typical commercial aircraft 

engine cycle which involves take off, climb, and steady state 

cruise. The transfer ot the aircraft ceramic seals technology to 

4 I 

I 

coated components are shown in figure 3b. Allison is no longer 

involved in diesel engine research (that portion of the business 

is now a separate company named Detroit Diesel corporation), 

however, they are continuing to pursue the strain isolator 

approach for certain gas turbine applications. 

II.A.2 Ongoing HDTT contracts 

There are two ongoing TTBC contracts supported by the HDTT 

program. One is with Caterpillar Inc. under program manager H. J. 

Larson, and the other is with Cummins Engine Company under 

program manager T. M. Yonushonis. Coatings under these contracts 

are being designed and evaluated for application to pistons and 

cylinder heads. Performance benefits and durability are being 

confirmed in a single cylinder engine test modified to LHR 

conditions. The specific coating system conductance goal for 

these contracts is 0.5 BTU/hr-oF (408 W/M2-K), and the durability 

goal is 100 hours under accelerated conditions. Selection of TTBC 

systems drew heavily from prior aircraft engine experience on 

plasma sprayed abradable seals. Cummins selected a system based 

on the aircraft ceramic seals experience of subcontractor UTRC 

(United Technologies Research Center). Caterpillar developed a 

similar coating system. On both contracts, coating system 

selection was based on analytical design, mechanical properties 

determination, and a limited amount of rig testing. Since the 

programs were component and performance oriented and not 

materials oriented, there was no attempt to test a wide variety 

of coating system concepts. 

In each case four layer coating system designs were developed. 

These designs consisted of a metallic bond coat, two layers of 

mixed ceramic and metal, and a layer of a zirconia-yttria 

ceramic. A schematic of each companies design is given in figure 

4. Some of the key properties measured for each coating system 

are given in table 1. 

The extensive analysis conducted under the HDTT program at 

Caterpillar and Cummins has quantified our knowledge of the 

cyclic stresses and strains encountered by thick coatings on 

diesel engine components. This analysis began with a recognition 

of low cycle and high cycles stress and temperature excursions 

encountered in the diesel engine. As shown schematically in 

figure 5, the low cycle excursions involve stop/start transients 

and no load/full load transients (e.g., down hill/up hill) and 

the high cycle transients are due to combustion in the cylinder. 

These combustion transients are, of course, superimposed on the 

lower cycle transients. The cyclic environment in the diesel 

engine differs greatly from the typical commercial aircraft 

engine cycle which involves take off, climb, and steady state 

cruise. The transfer ot the aircraft ceramic seals technology to 

4 I 

I 
the diesel engine began with the assumption that the designs 

which perform well in commercial aircraft engine cycle could be 

adapted to perform well in the very different diesel engine 

cycle. While that assumption was a good starting point, the task 

of developing thick thermal barrier coatings for diesel engines 

is proving to be more challenging than a simple adaptation of gas 

turbine seals technology. 

Figure 6 is an example of the temperature and stress excursions 

predicted to occur in a TTBC system on a piston based on a 10 

infinite plate finite element analysis (refs. 7 and 10). These 

calculations predict a very low gradient across the coating 

system thickness for no-load operation. At full load a gradient 

of about 500°C is predicted for the conditions of this 

calculation. The figure 6a also shows that, as the load is 

decreased the surface of the coating cools significantly faster 

than the interior. Also note that the metal temperatures remain 

below 300°C which is well below the 800°C required to oxidize the 

MCrAIY in the graded layers. As shown in figure 6b, the coating 

surface is in a high state of compression during full-load 

conditions. At no load the residual stress state dominates. This 

residual stress state still involves compression at the surface 

and tension in the graded layer just above the bond coat. Both 

UTRC and Caterpillar have chosen to preheat the components before 

spraying which reduces the tensile residual stresses at the 

interface region while placing the surface further into 

compression. This differs from the practice with thin aircraft 

thermal barrier coatings where substrates are kept comparatively 

cool to minimize compressive residual stress. The cyclic stress 

state described above may be designated as a low cycle thermal 

fatigue (a LCF thermal fatigue). Figure 7 from a Caterpillar 

study (Private communication, H. J. Larson, Caterpillar, Inc, 

1989) illustrates how temperatures vary across the complicated 

geometry of the diesel piston with the highest temperatures 

occurring at the edge of the bowl region. 

The HOTT program has also revealed that severe temperature 

fluctuations occur on the surface of the coating. This is due to 

the combustion of the mixture of diesel fuel and air every two 

revolutions of the crank shaft in a four cycle engine. This means 

that the piston is subjected to, for example, 22 explosions per 

second when the engine is operating at 2600 RPM. Gas 

temperatures, cylinder pressures, and heat flux rise to very high 

instantaneous values with every explosion. In fact, peak cylinder 

pressures over 3000 psi, gas temperatures up to 1400 0C (2600 0F), 

and heat fluxes up to 5 MW/M2 are predicted for some advanced 

designs. These peak, instantaneous conditions are more severe 

than those encountered in a gas turbine engine. On the other 

hand, the mean conditions are much less severe than those 

encountered in the gas turbine. Figure 8a illustrates 

5 

the diesel engine began with the assumption that the designs 

which perform well in commercial aircraft engine cycle could be 

adapted to perform well in the very different diesel engine 

cycle. While that assumption was a good starting point, the task 

of developing thick thermal barrier coatings for diesel engines 

is proving to be more challenging than a simple adaptation of gas 

turbine seals technology. 

Figure 6 is an example of the temperature and stress excursions 

predicted to occur in a TTBC system on a piston based on a 10 

infinite plate finite element analysis (refs. 7 and 10). These 

calculations predict a very low gradient across the coating 

system thickness for no-load operation. At full load a gradient 

of about 500°C is predicted for the conditions of this 

calculation. The figure 6a also shows that, as the load is 

decreased the surface of the coating cools significantly faster 

than the interior. Also note that the metal temperatures remain 

below 300°C which is well below the 800°C required to oxidize the 

MCrAIY in the graded layers. As shown in figure 6b, the coating 

surface is in a high state of compression during full-load 

conditions. At no load the residual stress state dominates. This 

residual stress state still involves compression at the surface 

and tension in the graded layer just above the bond coat. Both 

UTRC and Caterpillar have chosen to preheat the components before 

spraying which reduces the tensile residual stresses at the 

interface region while placing the surface further into 

compression. This differs from the practice with thin aircraft 

thermal barrier coatings where substrates are kept comparatively 

cool to minimize compressive residual stress. The cyclic stress 

state described above may be designated as a low cycle thermal 

fatigue (a LCF thermal fatigue). Figure 7 from a Caterpillar 

study (Private communication, H. J. Larson, Caterpillar, Inc, 

1989) illustrates how temperatures vary across the complicated 

geometry of the diesel piston with the highest temperatures 

occurring at the edge of the bowl region. 

The HOTT program has also revealed that severe temperature 

fluctuations occur on the surface of the coating. This is due to 

the combustion of the mixture of diesel fuel and air every two 

revolutions of the crank shaft in a four cycle engine. This means 

that the piston is subjected to, for example, 22 explosions per 

second when the engine is operating at 2600 RPM. Gas 

temperatures, cylinder pressures, and heat flux rise to very high 

instantaneous values with every explosion. In fact, peak cylinder 

pressures over 3000 psi, gas temperatures up to 1400 0C (2600 0F), 

and heat fluxes up to 5 MW/M2 are predicted for some advanced 

designs. These peak, instantaneous conditions are more severe 

than those encountered in a gas turbine engine. On the other 

hand, the mean conditions are much less severe than those 

encountered in the gas turbine. Figure 8a illustrates 

5 
i 

I 

_. _

schematically how the gas temperature, cylinder pressure, and 

heat flux of an engine operating at full load rise to very high 

instantaneous values once every 2 revolutions (720°) of the crank 

shaft. The temperature gradient in the coating system is 

illustrated in figure 8b. This is similar to the upper curve in 

figure 6a except that figure 8b shows that the outer 0.5 mil 

(0.13mm) of the coating experiences severe temperature gradients. 

Temperature swings at the surface have been calculated to be on 

the order of 225°C for a LHR engine operating at 2600 RPM (refs. 

7 and 10). This effect has also been demonstrated experimentally 

in an HDTT sponsored program (ref. 11) where thin film 

thermocouples were used to experimentally measure this 

temperature swing. Temperature swings of 140°C were measured for 

an engine operating at 1300 RPM at 75% of full load. These 

temperature swings lead to high cycle cyclic thermal stress at 

the surface superimposed on the compressive mean stress as shown 

in figure 8c. The instantaneous nature of the temperature maximum 

leads to asymmetry of the cyclic stress about the mean stress 

with a greater amplitude of the compressive component. In 

reference 7 the maximum compressive stress was calculated to be 

approximately equal to the compressive strength of the coating. 

The preheat applied during processing increases the magnitude of 

the compressive mean stress at the surface and therefore may 

increase the possibility of fatigue damage. Additionally, a high 

cycle mechanical stress is superimposed on this thermal stress as 

the high peak pressures cause the piston to bend about the pin. 

The magnitude of this bending stress is sensitive to piston 

design and is therefore and important factor to consider when 

designing for TTBCs. This cyclic stress state may be designated 

as a high cycle fatigue (HCF). 

Surface spalling of specimens prepared by caterpillar was induced 

in a purely mechanical four point bend fatigue test conducted at 

the Southwest Research Institute under HDTT funding (refs. 12 and 

13). Tensile and compressive S-N curves taken from that study are 

presented in figures 9a and b. The tensile fatigue specimens used 

for these tests were free standing ceramic coatings while the 

compressive fatigue specimens remained attached to a substrate. 

It should be noted that although these curves are drawn with 

upward curvature suggesting the presence of an endurance limit, 

it is still not know whether an endurance limit actually exists. 

This is an important question because if an endurance limit 

exists a coating which survived to that limit in a test should 

not suffer fatigue failure in a much longer test. 

In the diesel engine, the LCF type of delamination spalling and 

the HCF type of surface spalling are two conditions that could 

lead to coating fail ure. The relative importance of these two 

potential failure modes and the conditions which would favor one 

mode over the other are still unknown. When spalling has 

6 L_ 

i 

I 

_. _

schematically how the gas temperature, cylinder pressure, and 

heat flux of an engine operating at full load rise to very high 

instantaneous values once every 2 revolutions (720°) of the crank 

shaft. The temperature gradient in the coating system is 

illustrated in figure 8b. This is similar to the upper curve in 

figure 6a except that figure 8b shows that the outer 0.5 mil 

(0.13mm) of the coating experiences severe temperature gradients. 

Temperature swings at the surface have been calculated to be on 

the order of 225°C for a LHR engine operating at 2600 RPM (refs. 

7 and 10). This effect has also been demonstrated experimentally 

in an HDTT sponsored program (ref. 11) where thin film 

thermocouples were used to experimentally measure this 

temperature swing. Temperature swings of 140°C were measured for 

an engine operating at 1300 RPM at 75% of full load. These 

temperature swings lead to high cycle cyclic thermal stress at 

the surface superimposed on the compressive mean stress as shown 

in figure 8c. The instantaneous nature of the temperature maximum 

leads to asymmetry of the cyclic stress about the mean stress 

with a greater amplitude of the compressive component. In 

reference 7 the maximum compressive stress was calculated to be 

approximately equal to the compressive strength of the coating. 

The preheat applied during processing increases the magnitude of 

the compressive mean stress at the surface and therefore may 

increase the possibility of fatigue damage. Additionally, a high 

cycle mechanical stress is superimposed on this thermal stress as 

the high peak pressures cause the piston to bend about the pin. 

The magnitude of this bending stress is sensitive to piston 

design and is therefore and important factor to consider when 

designing for TTBCs. This cyclic stress state may be designated 

as a high cycle fatigue (HCF). 

Surface spalling of specimens prepared by caterpillar was induced 

in a purely mechanical four point bend fatigue test conducted at 

the Southwest Research Institute under HDTT funding (refs. 12 and 

13). Tensile and compressive S-N curves taken from that study are 

presented in figures 9a and b. The tensile fatigue specimens used 

for these tests were free standing ceramic coatings while the 

compressive fatigue specimens remained attached to a substrate. 

It should be noted that although these curves are drawn with 

upward curvature suggesting the presence of an endurance limit, 

it is still not know whether an endurance limit actually exists. 

This is an important question because if an endurance limit 

exists a coating which survived to that limit in a test should 

not suffer fatigue failure in a much longer test. 

In the diesel engine, the LCF type of delamination spalling and 

the HCF type of surface spalling are two conditions that could 

lead to coating fail ure. The relative importance of these two 

potential failure modes and the conditions which would favor one 

mode over the other are still unknown. When spalling has 

6 L_ 
'~--------'- --~ "" --, _._._-- -----

occurred, as evident in the publicly referenceable engine tests 

at Cummins (refs. 14 and 15), it appears from post test 

inspection that a delamination type of failure has occurred. An 

example is shown in figure 10. Although, in certain cases it may 

be possible for repeated surface (HCF) spalling to mimic the 

appearance of delamination CLCF) spalling. caterpillar, in their 

pUblications under the HDTT program, indicates a concern with at 

least the possibility of the HCF failure mode. 

The laboratory test rigs which have been used at UTRC for Cummins 

and at Solar Turbines for caterpillar have been successful in 

duplicating the LCF type of stress state that is found in the 

engine but much less able to reproduce a severe HCF type of 

stress state. A schematic of the test rig used at Caterpillar/ 

Solar Turbines is shown in figure 11. A similar rig was used to a 

limited extent by Cummins/UTRC. These rigs subject flat specimens 

to front side heating and back side cooling to duplicate the 

temperature gradients encountered in the engine. stop/start and 

no load/full load transients are achieved by moving the specimen 

in and out of the flame. The rotating carousel produces a HCF 

cycle which in these rigs is much milder than the HCF cycles 

encountered in the engine. Coating failure in the laboratory rigs 

is generally associated with edge initiated delamination or 

surface initiated vertical cracking. Edge initiated failure is a 

concern for coatings tested in an engine especially for 

coatings on the cylinder head. 

In summary, the HDTT TTBC program is component, design, .and 

performance oriented and not strongly materials oriented. The 

program has been successful in taking the ceramic abradable seals 

developed by the aircraft industry and adapting them to create 

the first generation diesel engine TTBCs. Finite element modeling 

has been conducted under the program, first generation coating 

system designs have been established, coating system properties 

have been measu red, and engine testing which will be published in 

the open literature is being conducted. A risk of the HDTT TTBC 

program is that experience on TTBCs is being gained on very few 

coating concepts. Thus, the success or failure of coatings based 

on these few concepts will influence decisions as to the 

viability of TTBCs that may effect their use in diesel engines 

well into the next century. 

II.A.3 Future HDTT efforts 

The next phase of the HDTT TTBC program will continue to develop 

the technology for applying coatings to diesel engines. Efforts 

will focus on establishing an NDE technology for inspecting 

coatings on components, on improving processing so that 

near-net-shape coatings can be sprayed, on expanding 

characterization of materials properties, and possibly on 

7 

'~--------'- --~ "" --, _._._-- -----

occurred, as evident in the publicly referenceable engine tests 

at Cummins (refs. 14 and 15), it appears from post test 

inspection that a delamination type of failure has occurred. An 

example is shown in figure 10. Although, in certain cases it may 

be possible for repeated surface (HCF) spalling to mimic the 

appearance of delamination CLCF) spalling. caterpillar, in their 

pUblications under the HDTT program, indicates a concern with at 

least the possibility of the HCF failure mode. 

The laboratory test rigs which have been used at UTRC for Cummins 

and at Solar Turbines for caterpillar have been successful in 

duplicating the LCF type of stress state that is found in the 

engine but much less able to reproduce a severe HCF type of 

stress state. A schematic of the test rig used at Caterpillar/ 

Solar Turbines is shown in figure 11. A similar rig was used to a 

limited extent by Cummins/UTRC. These rigs subject flat specimens 

to front side heating and back side cooling to duplicate the 

temperature gradients encountered in the engine. stop/start and 

no load/full load transients are achieved by moving the specimen 

in and out of the flame. The rotating carousel produces a HCF 

cycle which in these rigs is much milder than the HCF cycles 

encountered in the engine. Coating failure in the laboratory rigs 

is generally associated with edge initiated delamination or 

surface initiated vertical cracking. Edge initiated failure is a 

concern for coatings tested in an engine especially for 

coatings on the cylinder head. 

In summary, the HDTT TTBC program is component, design, .and 

performance oriented and not strongly materials oriented. The 

program has been successful in taking the ceramic abradable seals 

developed by the aircraft industry and adapting them to create 

the first generation diesel engine TTBCs. Finite element modeling 

has been conducted under the program, first generation coating 

system designs have been established, coating system properties 

have been measu red, and engine testing which will be published in 

the open literature is being conducted. A risk of the HDTT TTBC 

program is that experience on TTBCs is being gained on very few 

coating concepts. Thus, the success or failure of coatings based 

on these few concepts will influence decisions as to the 

viability of TTBCs that may effect their use in diesel engines 

well into the next century. 

II.A.3 Future HDTT efforts 

The next phase of the HDTT TTBC program will continue to develop 

the technology for applying coatings to diesel engines. Efforts 

will focus on establishing an NDE technology for inspecting 

coatings on components, on improving processing so that 

near-net-shape coatings can be sprayed, on expanding 

characterization of materials properties, and possibly on 

7 
developing seal coatings. Coatings in the next phase will have a 

10000 hour design life, and will be verified in a 1000 hour 

engine test run at accelerated conditions. Failure mechanisms 

and life prediction will also be emphasized. 

The HDTT program has also supported the in-house NASA-Lewis 

thermal barrier coatings program. Initially this support 

consisted of upgrading the plasma spray process automation, 

monitoring, and control equipment to bring capabilities up to the 

state-of-the-art. A study, supported by NASA aeronautical base 

research and technology funding, was than conducted to learn how 

to use the equipment to produce a wide variety of coating 

microstructures from a variety of starting powders each of which 

had nominally the same chemistry • Each of the coatings sprayed 

under this effort has been characterized with respect to 

porosity, microstructure, deposition efficiency, and in the near 

future thermal conductivity. Current plans are to extend 

characterization to mechanical properties and to construct a room 

temperature flash diffusivity apparatus and to study the effects 

of high temperature exposure on properties. Also a test rig is 

being constructed to determine whether a plasma torch impinging 

on a rotating, coated disc can produce high temperature swings on 

the surface of the coating . 

II.B. other Research into Thick Thermal Barrier Coatings for 

Truck Diesel Engines 

II.B.l Army research into TTBCs 

The U.S. Army - TACOM supported the first research into low heat 

rejection engines and has continued to support work in this area. 

Army requirements for their on-road trucks are rather similar to 

needs of the commercial industry. One important difference is 

that they wish to have the capabil1ty to burn lower quality 

fuels. Army supported research has concentrated on evaluating 

commercially available coatings in diesel engines. Most of the 

reports published under Army contracts have limited distribution 

(e.g., references 16 and 17) and therefore will not be discussed 

here. A discussion of the Army's efforts as of 1987 may be found 

in reference 6. 

II.B.2 LHR diesel engine research in Europe and Japan 

One of the most prolific researchers into TTBCs for diesel 

engines are I. Kvernes and his associates in Oslo, Norway. This 

research into diesel engine coatings has been going on for many 

years, and in fact some of it received U.S. DOE support (ref. 

18). Recently, Kvernes reported test results for TTBCs on a 

single cylinder version of a marine diesel engine. The tests 

8 

_________________________________ ________________ J 

developing seal coatings. Coatings in the next phase will have a 

10000 hour design life, and will be verified in a 1000 hour 

engine test run at accelerated conditions. Failure mechanisms 

and life prediction will also be emphasized. 

The HDTT program has also supported the in-house NASA-Lewis 

thermal barrier coatings program. Initially this support 

consisted of upgrading the plasma spray process automation, 

monitoring, and control equipment to bring capabilities up to the 

state-of-the-art. A study, supported by NASA aeronautical base 

research and technology funding, was than conducted to learn how 

to use the equipment to produce a wide variety of coating 

microstructures from a variety of starting powders each of which 

had nominally the same chemistry • Each of the coatings sprayed 

under this effort has been characterized with respect to 

porosity, microstructure, deposition efficiency, and in the near 

future thermal conductivity. Current plans are to extend 

characterization to mechanical properties and to construct a room 

temperature flash diffusivity apparatus and to study the effects 

of high temperature exposure on properties. Also a test rig is 

being constructed to determine whether a plasma torch impinging 

on a rotating, coated disc can produce high temperature swings on 

the surface of the coating . 

II.B. other Research into Thick Thermal Barrier Coatings for 

Truck Diesel Engines 

II.B.l Army research into TTBCs 

The U.S. Army - TACOM supported the first research into low heat 

rejection engines and has continued to support work in this area. 

Army requirements for their on-road trucks are rather similar to 

needs of the commercial industry. One important difference is 

that they wish to have the capabil1ty to burn lower quality 

fuels. Army supported research has concentrated on evaluating 

commercially available coatings in diesel engines. Most of the 

reports published under Army contracts have limited distribution 

(e.g., references 16 and 17) and therefore will not be discussed 

here. A discussion of the Army's efforts as of 1987 may be found 

in reference 6. 

II.B.2 LHR diesel engine research in Europe and Japan 

One of the most prolific researchers into TTBCs for diesel 

engines are I. Kvernes and his associates in Oslo, Norway. This 

research into diesel engine coatings has been going on for many 

years, and in fact some of it received U.S. DOE support (ref. 

18). Recently, Kvernes reported test results for TTBCs on a 

single cylinder version of a marine diesel engine. The tests 

8 

_________________________________ ________________ J 
_ .

-- ~ _. 

showed that a 1 mm thick coating on the cylinder head and a 2 mm 

coating on the piston reduced heat losses into the water coolant 

by 9% while increasing the heat losses into the oil by 3% for an 

overall 6% reduction. 

Other European research into TTBCs for diesel engines and in LHR 

engines are 

reported to be at a standstill (ref. 6). This is based on the 

discouraging experimental results on insulated European engines 

(ref. 3) and their interpretation by Waschni et ale (ref. 19). 

The "Waschni analysis" concluded that, at least for engines of 

European design, heat transfer would actually increase with 

increasing component temperature causing a net loss in 

efficiency. This analysis has been disputed in another study 

(ref. 20). 

Japanese researchers are developing LHR engines but are focussing 

on monolithic ceramics (ref. 6). 

Part III. Areas Where Fundamental Materials Understanding is 

Incomplete 

The purpose of Part III is to identify and discuss briefly the 

many areas where understanding of the fundamentals aspects of 

thick thermal barrier coatings for diesel engines is incomplete. 

This discussion draws heavily from discussions with 

representatives of the companies and research institutions noted 

in the introduction. Ultimately, the opinions are those of the 

author. Prior to the interviews a list of those areas where 

fundamental knowledge is lacking was drawn up and mailed to 

potential interviewees. This list, which is reproduced in a 

revised form in the appendix, served as a starting point for 

discussion. Interviewees were encouraged to comment as to the 

relevance of each topic and to offer additional topics. six 

general areas selected for discussion were A) Fundamental 

Knowledge Needed to Optimize Zirconia TTBCs, B) Effect of 

Environment and stress state on Properties and Performance, C) 

Thermophysical and Thermomecanical Properties Measurement, D) 

Lower Cost TTBCs, E) Alternate Approaches, and F) Development of 

Meaningful Bench Tests 

Before discussing the above areas, it is necessary to adopt a 

working definition of "fundamental research" and of "materials 

research." Thus for the purposes of this discussion "fundamental 

research" shall be research enabling but not including the design 

and testing of thick thermal barrier coatings on diesel engine 

components or the commercialization of those designs. That is, 

the actual design, engine testing, and commercialization will be 

considered to be beyond the scope of this study, but all of the 

9 

_ .-- ~ _. 

showed that a 1 mm thick coating on the cylinder head and a 2 mm 

coating on the piston reduced heat losses into the water coolant 

by 9% while increasing the heat losses into the oil by 3% for an 

overall 6% reduction. 

Other European research into TTBCs for diesel engines and in LHR 

engines are 

reported to be at a standstill (ref. 6). This is based on the 

discouraging experimental results on insulated European engines 

(ref. 3) and their interpretation by Waschni et ale (ref. 19). 

The "Waschni analysis" concluded that, at least for engines of 

European design, heat transfer would actually increase with 

increasing component temperature causing a net loss in 

efficiency. This analysis has been disputed in another study 

(ref. 20). 

Japanese researchers are developing LHR engines but are focussing 

on monolithic ceramics (ref. 6). 

Part III. Areas Where Fundamental Materials Understanding is 

Incomplete 

The purpose of Part III is to identify and discuss briefly the 

many areas where understanding of the fundamentals aspects of 

thick thermal barrier coatings for diesel engines is incomplete. 

This discussion draws heavily from discussions with 

representatives of the companies and research institutions noted 

in the introduction. Ultimately, the opinions are those of the 

author. Prior to the interviews a list of those areas where 

fundamental knowledge is lacking was drawn up and mailed to 

potential interviewees. This list, which is reproduced in a 

revised form in the appendix, served as a starting point for 

discussion. Interviewees were encouraged to comment as to the 

relevance of each topic and to offer additional topics. six 

general areas selected for discussion were A) Fundamental 

Knowledge Needed to Optimize Zirconia TTBCs, B) Effect of 

Environment and stress state on Properties and Performance, C) 

Thermophysical and Thermomecanical Properties Measurement, D) 

Lower Cost TTBCs, E) Alternate Approaches, and F) Development of 

Meaningful Bench Tests 

Before discussing the above areas, it is necessary to adopt a 

working definition of "fundamental research" and of "materials 

research." Thus for the purposes of this discussion "fundamental 

research" shall be research enabling but not including the design 

and testing of thick thermal barrier coatings on diesel engine 

components or the commercialization of those designs. That is, 

the actual design, engine testing, and commercialization will be 

considered to be beyond the scope of this study, but all of the 

9 
background knowledge that goes into those efforts shall be 

considered to be fundamental. "Materials research" is research 

related to processing and evaluation of current and future TTBCs 

and does not include thermodynamic effects such as performance 

improvements and fuel savings. However, the diesel engine 

companies stress that all fundamental materials work must be 

calibrated to engine experience, and that a considerable amount 

of fundamental engine-related thermodynamic and mechanical 

engineering research is also needed. 

III.A. Fundamental knowledge needed to optimize zirconia TTBCs 

In this section the factors affecting optimization and 

reproducibility of thick thermal barrier coatings are considered. 

The factors considered include those associated with the starting 

powder, with the processing, with selection of optimum materials, 

and with scaling up from the laboratory to the 

component. 

III.A.l. Effect of powder preparation method, chemistry, 

manufacturer, and lot on properties and performance 

A set of spray parameters developed for one lot of zirconia 

powder will not necessarily yield the same coating when another 

lot of zirconia is sprayed. Sources of variability include 

manufacturer, preparation method, and even lot-to-Iot variations. 

There is no universal standard for plasma spray powders, and our 

understanding of which factors are most important is incomplete. 

Furthermore, these factors will vary depending on the 

application. In the aircraft industry each turbine engine 

company has developed specifications for spray powders. Powder 

vendors must then meet the specification in order to compete for 

orders, with competition being encouraged for cost reasons. 

Current specifications are tighter than they were a few years ago 

out of necessity for maintaining high quality. This is one area 

where a fundamental study designed to determine the nature of 

this variability would help to provide the background needed for 

future TTBC specifications. 

III.A.2 Effect of processing, microstructure, and phase 

distributions on properties and performance 

The variations in coating properties and performance arising from 

processing variability may be as significant as the powder lot 

variabilities unless great care is taken to monitor the quality 

control during processing. This fact is being accepted more and 

more by the plasma spraying industry. However, there is no 

generally accepted approach for dealing with processing 

variability. In addition to problems of variability, there are 

10 

background knowledge that goes into those efforts shall be 

considered to be fundamental. "Materials research" is research 

related to processing and evaluation of current and future TTBCs 

and does not include thermodynamic effects such as performance 

improvements and fuel savings. However, the diesel engine 

companies stress that all fundamental materials work must be 

calibrated to engine experience, and that a considerable amount 

of fundamental engine-related thermodynamic and mechanical 

engineering research is also needed. 

III.A. Fundamental knowledge needed to optimize zirconia TTBCs 

In this section the factors affecting optimization and 

reproducibility of thick thermal barrier coatings are considered. 

The factors considered include those associated with the starting 

powder, with the processing, with selection of optimum materials, 

and with scaling up from the laboratory to the 

component. 

III.A.l. Effect of powder preparation method, chemistry, 

manufacturer, and lot on properties and performance 

A set of spray parameters developed for one lot of zirconia 

powder will not necessarily yield the same coating when another 

lot of zirconia is sprayed. Sources of variability include 

manufacturer, preparation method, and even lot-to-Iot variations. 

There is no universal standard for plasma spray powders, and our 

understanding of which factors are most important is incomplete. 

Furthermore, these factors will vary depending on the 

application. In the aircraft industry each turbine engine 

company has developed specifications for spray powders. Powder 

vendors must then meet the specification in order to compete for 

orders, with competition being encouraged for cost reasons. 

Current specifications are tighter than they were a few years ago 

out of necessity for maintaining high quality. This is one area 

where a fundamental study designed to determine the nature of 

this variability would help to provide the background needed for 

future TTBC specifications. 

III.A.2 Effect of processing, microstructure, and phase 

distributions on properties and performance 

The variations in coating properties and performance arising from 

processing variability may be as significant as the powder lot 

variabilities unless great care is taken to monitor the quality 

control during processing. This fact is being accepted more and 

more by the plasma spraying industry. However, there is no 

generally accepted approach for dealing with processing 

variability. In addition to problems of variability, there are 

10 
optimization related problems. In fact it may be argued that no 

coating can be fully optimized without resorting to an infinite 

amount of testing and evaluation. Thus optimization becomes a 

trade off between the need to develop a reliable coating system 

and the need to minimize development costs. Furthermore, 

variability problems compound optimization problems. This is 

because excessive variability makes it difficult to optimize a 

given coating system or to evaluate a new system against an old 

one. As described in Part II, the HDTT program was component 

oriented and not materials oriented. Therefore only a few coating 

systems could be considered and the amount of time that could be 

devoted to optimization was limited. Therefore fundamental 

research designed to identify and solve variability problems, 

improve quality control, and find viable back-up coating systems 

would be very useful. 

III.A.3 optimum zirconia composition for TTBCs 

Even if spray powder preparation and the plasma spray process 

were "optimized" it would still be necessary to ensure that the 

ceramic material selected for the TTBC was theoretically the best 

available material for use in the diesel engine. Zirconia based 

materials, especially zirconia-yttria, has received the most 

serious consideration based on the successful use of these 

materials in the gas turbine industry and the lack of any strong 

contenders. One major difference between the two coating systems 

developed under HDTT is that CumminsjUTRC employs zr02-20%Y203 

and Caterpillar employs ZrOz-8%Y203. These two compositions are 

very different from each other with the 20% yttria composition 

forming an equilibrium cubic phase when sprayed and with the 8% 

yttria composition forming a non-equilibrium tetragonal phase 

plus lesser amounts of the monoclinic and cubic phases (refs. 21 

and 22). Additionally, the starting powders for these two 

coatings come from two different manufacturers and are prepared 

by two very different techniques. Thus, a fundamental question 

is how do these two materials compare with respect to properties 

and performance when compared head to head? A second question is 

how would the properties and performance of other zirconias 

compare? 

III.A.4 Effect of scaling up from test coupons to components on 

properties and performance 

This is a very important factor -- especially when the processing 

is transferred from spraying flat coupons in a research shop to 

spraying components in a production shop as was the case with the 

UTRC coatings (refs. 23 and 24). While this topic may be more 

applied than fundamental, a fundamental study of such factors as 

the effect of spray angle on coating properties and the effect of 

substrate mass could help to identify potential problem areas. 

11 

optimization related problems. In fact it may be argued that no 

coating can be fully optimized without resorting to an infinite 

amount of testing and evaluation. Thus optimization becomes a 

trade off between the need to develop a reliable coating system 

and the need to minimize development costs. Furthermore, 

variability problems compound optimization problems. This is 

because excessive variability makes it difficult to optimize a 

given coating system or to evaluate a new system against an old 

one. As described in Part II, the HDTT program was component 

oriented and not materials oriented. Therefore only a few coating 

systems could be considered and the amount of time that could be 

devoted to optimization was limited. Therefore fundamental 

research designed to identify and solve variability problems, 

improve quality control, and find viable back-up coating systems 

would be very useful. 

III.A.3 optimum zirconia composition for TTBCs 

Even if spray powder preparation and the plasma spray process 

were "optimized" it would still be necessary to ensure that the 

ceramic material selected for the TTBC was theoretically the best 

available material for use in the diesel engine. Zirconia based 

materials, especially zirconia-yttria, has received the most 

serious consideration based on the successful use of these 

materials in the gas turbine industry and the lack of any strong 

contenders. One major difference between the two coating systems 

developed under HDTT is that CumminsjUTRC employs zr02-20%Y203 

and Caterpillar employs ZrOz-8%Y203. These two compositions are 

very different from each other with the 20% yttria composition 

forming an equilibrium cubic phase when sprayed and with the 8% 

yttria composition forming a non-equilibrium tetragonal phase 

plus lesser amounts of the monoclinic and cubic phases (refs. 21 

and 22). Additionally, the starting powders for these two 

coatings come from two different manufacturers and are prepared 

by two very different techniques. Thus, a fundamental question 

is how do these two materials compare with respect to properties 

and performance when compared head to head? A second question is 

how would the properties and performance of other zirconias 

compare? 

III.A.4 Effect of scaling up from test coupons to components on 

properties and performance 

This is a very important factor -- especially when the processing 

is transferred from spraying flat coupons in a research shop to 

spraying components in a production shop as was the case with the 

UTRC coatings (refs. 23 and 24). While this topic may be more 

applied than fundamental, a fundamental study of such factors as 

the effect of spray angle on coating properties and the effect of 

substrate mass could help to identify potential problem areas. 

11 
III.B The effect of environment and cyclic stress on properties 

and performance 

In this section the environmental and stress related factors 

which contribute to coating degradation will be considered. A 

variety of cyclic, time at temperature, chemical and mechanical 

conditions that may be encountered in the engine will be 

considered 

III.B.1 Effects of cycling 

The cyclic conditions in the diesel engine are much more severe 

than was generally realized before the HDTT program. Under HDTT 

the magnitude of the start/stop transients, no load/full load 

transients, and combustion transients have been characterized. 

However, the thermal and mechanical LCF and HCF response of TTBCs 

is still not well characterized. A concern over the detrimental 

effect of edges and corners has been raised, as well as the 

possibility that the hot spots which result from the fuel 

injector pattern may contribute to the fatigue of the coatings. 

Mudflat cracking has been observed on the surface in some tests 

but it is not even known whether this is a beneficial or 

detrimental effect. Subcritical crack growth and microcrack link 

up mechanisms have been proposed for thin TBCs, and similar 

effects may occur in diesel TTBCs. Thus, numerous opportunities 

exist for important, fundamental research into the fatigue 

response of TTBCs. 

III.B.2 Deposit related effects 

The effects of the deposits which plate out on the coating are 

not well understood. These deposits form from the additives in 

the oil and the sulfur contaminant in the fuel. Calcium sulfate 

is a commonly observed deposit. It is interesting to note that 

although there are many papers in the literature which discuss 

the destabilization of zirconia from reactions between a deposit 

and yttria, the calcium ion should instead further stabilize the 

zirconia if a reaction occurred. It is not known whether the 

presence of a deposit on the surface effects the HCF response of 

the coating. Also, Cummins has noted that a deposit can build up 

at the edge of the piston to the point where it may slide against 

the liner. This sliding could then damage the edge of the TTBC. 

If diesel engines were run on lower grade fuel containing higher 

levels of sulfur plus other contaminants then the deposit could 

be much more detrimental. Fortunately, the trend is towards even 

cleaner fuels for environmental reasons. However, dirty fuel 

could be used in certain situations by Army trucks. The effect of 

deposits originating from the oil could be one factor used in the 

12 

III.B The effect of environment and cyclic stress on properties 

and performance 

In this section the environmental and stress related factors 

which contribute to coating degradation will be considered. A 

variety of cyclic, time at temperature, chemical and mechanical 

conditions that may be encountered in the engine will be 

considered 

III.B.1 Effects of cycling 

The cyclic conditions in the diesel engine are much more severe 

than was generally realized before the HDTT program. Under HDTT 

the magnitude of the start/stop transients, no load/full load 

transients, and combustion transients have been characterized. 

However, the thermal and mechanical LCF and HCF response of TTBCs 

is still not well characterized. A concern over the detrimental 

effect of edges and corners has been raised, as well as the 

possibility that the hot spots which result from the fuel 

injector pattern may contribute to the fatigue of the coatings. 

Mudflat cracking has been observed on the surface in some tests 

but it is not even known whether this is a beneficial or 

detrimental effect. Subcritical crack growth and microcrack link 

up mechanisms have been proposed for thin TBCs, and similar 

effects may occur in diesel TTBCs. Thus, numerous opportunities 

exist for important, fundamental research into the fatigue 

response of TTBCs. 

III.B.2 Deposit related effects 

The effects of the deposits which plate out on the coating are 

not well understood. These deposits form from the additives in 

the oil and the sulfur contaminant in the fuel. Calcium sulfate 

is a commonly observed deposit. It is interesting to note that 

although there are many papers in the literature which discuss 

the destabilization of zirconia from reactions between a deposit 

and yttria, the calcium ion should instead further stabilize the 

zirconia if a reaction occurred. It is not known whether the 

presence of a deposit on the surface effects the HCF response of 

the coating. Also, Cummins has noted that a deposit can build up 

at the edge of the piston to the point where it may slide against 

the liner. This sliding could then damage the edge of the TTBC. 

If diesel engines were run on lower grade fuel containing higher 

levels of sulfur plus other contaminants then the deposit could 

be much more detrimental. Fortunately, the trend is towards even 

cleaner fuels for environmental reasons. However, dirty fuel 

could be used in certain situations by Army trucks. The effect of 

deposits originating from the oil could be one factor used in the 

12 
· ~ .. --- -------

selection of the particular additive package for the oil. A 

fundamental experimental and thermodynamic analysis of the 

interaction between the observed deposits and the coating 

material would help to clarify whether chemical reactions may be 

expected at the temperatures encountered in the diesel engine. 

A conceivable beneficial effect of the deposits would be to block 

the radiation of infrared energy through the partially 

translucent TTBC. It has been suggested that translucence would 

reduce the effective thermal conductivity of ceramic materials 

(ref. 25). This would be especially important if the coatings 

were not going to be sealed. Emisivity measurements on 

deposit-plated TTBCs could be informative. 

III.B.3 Time at temperature related effects 

Time at temperature factors such as oxidation, phase 

transformations, sintering, and creep are very important for 

ceramic coatings and seals operating in gas turbine engines. In 

the diesel engine, materials temperatures are comparatively low 

so that very long times would be required for these factors to 

become important. While extremely long time testing is generally 

impractical, in many cases the effect of time may be 

"accelerated" by conducting tests at elevated temperatures. That 

is the behavior of a material at very long times often can be 

estimated by shorter time testing at higher temperatures. 

Furthermore, the interest of the DOE in gas turbines for 

transportation and power generation also warrants investigation 

at higher temperatures. 

1II.B.4 Erosion and foreign object damage 

Erosion and to a lesser extent foreign object damage is another 

topic that is important under certain conditions for gas turbine 

engines but is not believed to be very important for diesel 

engines. This is primarily because diesel engines employ filtered 

air. The possibility of damage of other engine components from 

spalled pieces of the coating is not considered to be a problem. 

I1I.C Thermophysical and thermomechanical properties measurement 

Characterization of thermomechanical and thermophysical 

properties of TTBCs will be considered in this section. 

Mechanical properties such as various strengths and moduli, 

physical properties such as thermal conductivity and 

permeability, and NDE techniques will be addressed. This topic 

has assumed increasing importance as thermal barrier coatings 

begin to be designed into engines. This is having a positive 

effect on thermal barrier coating development which in the past 

13 

· ~ .. --- -------

selection of the particular additive package for the oil. A 

fundamental experimental and thermodynamic analysis of the 

interaction between the observed deposits and the coating 

material would help to clarify whether chemical reactions may be 

expected at the temperatures encountered in the diesel engine. 

A conceivable beneficial effect of the deposits would be to block 

the radiation of infrared energy through the partially 

translucent TTBC. It has been suggested that translucence would 

reduce the effective thermal conductivity of ceramic materials 

(ref. 25). This would be especially important if the coatings 

were not going to be sealed. Emisivity measurements on 

deposit-plated TTBCs could be informative. 

III.B.3 Time at temperature related effects 

Time at temperature factors such as oxidation, phase 

transformations, sintering, and creep are very important for 

ceramic coatings and seals operating in gas turbine engines. In 

the diesel engine, materials temperatures are comparatively low 

so that very long times would be required for these factors to 

become important. While extremely long time testing is generally 

impractical, in many cases the effect of time may be 

"accelerated" by conducting tests at elevated temperatures. That 

is the behavior of a material at very long times often can be 

estimated by shorter time testing at higher temperatures. 

Furthermore, the interest of the DOE in gas turbines for 

transportation and power generation also warrants investigation 

at higher temperatures. 

1II.B.4 Erosion and foreign object damage 

Erosion and to a lesser extent foreign object damage is another 

topic that is important under certain conditions for gas turbine 

engines but is not believed to be very important for diesel 

engines. This is primarily because diesel engines employ filtered 

air. The possibility of damage of other engine components from 

spalled pieces of the coating is not considered to be a problem. 

I1I.C Thermophysical and thermomechanical properties measurement 

Characterization of thermomechanical and thermophysical 

properties of TTBCs will be considered in this section. 

Mechanical properties such as various strengths and moduli, 

physical properties such as thermal conductivity and 

permeability, and NDE techniques will be addressed. This topic 

has assumed increasing importance as thermal barrier coatings 

begin to be designed into engines. This is having a positive 

effect on thermal barrier coating development which in the past 

13 
had tended to be rather "Edisonian". Properties of thin 

bill-of-material aircraft TBCs have only recently been well 

characterized under the NASA HOST program (ref. 26). UTRC has 

historically done the best job in characterizing the properties 

of TTBCs, and UTRC and Caterpillar are now both characterizing 

the properties of the TTBC systems being developed under HDTT. 

III.C.l Thermomechanical properties 

The coatings and the components for the next generation of diesel 

engines are being designed together as a system. Thus, measuring 

the properties has become effectively the first step in design, 

and close cooperation between designers and the experimentalist 

is now essential. Among the properties to be considered are 

ultimate strength, yield strength, strain to failure, and modulus 

measured in tension and compression over a range of temperatures 

through the use of uniaxial or bend tests. The high and low cycle 

fatigue behavior of the coating system should also be considered 

as well as tensile adhesion tests for bond strengths and fracture 

toughness tests. Anisotropy in properties and nonlinear effects 

should also be addressed. For each of the above tests care should 

be taken to ensure the use of proper specimen geometries. The 

properties in this section are fundamentally important and 

necessary for any coating systems under consideration. 

III.C.2 Thermal conductivity measurement 

Thermal conductivity is a property with obvious relevance to 

thermal barrier coatings. Despite the great relevance, this is 

another property which in the past has historically been given 

secondary importance by coating developers who have been more 

concerned with durability than the conductivity. Included among 

the approaches which may be used to measure thermal conductivity 

are flash diffusivity, thermal wave (modulated), cut bar, and 

radial techniques (ref. 27). 

The flash diffusivity approach appears to have gained the most 

favor for measuring the conductivity of thermal barrier coatings. 

That is because, with proper care, this approach is able to 

quickly yield accurate results on simple specimens. However other 

techniques cannot yet be ruled out, and with any technique 

erroneous results may be obtained by the naive. Thermal 

conductivity measurements should included as part of any 

fundamental study of TTBCs. 

III.C.3 NDE and metrology 

There is currently a strong need for NDE techniques which would 

be sensitive to coating properties, porosity, and thickness. No 

techniques have yet been found which will yield an accurate 

14 

had tended to be rather "Edisonian". Properties of thin 

bill-of-material aircraft TBCs have only recently been well 

characterized under the NASA HOST program (ref. 26). UTRC has 

historically done the best job in characterizing the properties 

of TTBCs, and UTRC and Caterpillar are now both characterizing 

the properties of the TTBC systems being developed under HDTT. 

III.C.l Thermomechanical properties 

The coatings and the components for the next generation of diesel 

engines are being designed together as a system. Thus, measuring 

the properties has become effectively the first step in design, 

and close cooperation between designers and the experimentalist 

is now essential. Among the properties to be considered are 

ultimate strength, yield strength, strain to failure, and modulus 

measured in tension and compression over a range of temperatures 

through the use of uniaxial or bend tests. The high and low cycle 

fatigue behavior of the coating system should also be considered 

as well as tensile adhesion tests for bond strengths and fracture 

toughness tests. Anisotropy in properties and nonlinear effects 

should also be addressed. For each of the above tests care should 

be taken to ensure the use of proper specimen geometries. The 

properties in this section are fundamentally important and 

necessary for any coating systems under consideration. 

III.C.2 Thermal conductivity measurement 

Thermal conductivity is a property with obvious relevance to 

thermal barrier coatings. Despite the great relevance, this is 

another property which in the past has historically been given 

secondary importance by coating developers who have been more 

concerned with durability than the conductivity. Included among 

the approaches which may be used to measure thermal conductivity 

are flash diffusivity, thermal wave (modulated), cut bar, and 

radial techniques (ref. 27). 

The flash diffusivity approach appears to have gained the most 

favor for measuring the conductivity of thermal barrier coatings. 

That is because, with proper care, this approach is able to 

quickly yield accurate results on simple specimens. However other 

techniques cannot yet be ruled out, and with any technique 

erroneous results may be obtained by the naive. Thermal 

conductivity measurements should included as part of any 

fundamental study of TTBCs. 

III.C.3 NDE and metrology 

There is currently a strong need for NDE techniques which would 

be sensitive to coating properties, porosity, and thickness. No 

techniques have yet been found which will yield an accurate 

14 
measure of properties and porosity (with porosity the accuracy 

needed is probably about +2%). An optical technique has recently 

been developed for measuring coating thickness on-line during 

processing (ref. 28 and 29), and this and other techniques will 

be considered for diesel engines under HDTT. There are many other 

opportunities for fundamental research into NDE/property 

relationships which could have a place in a fundamental program. 

III.C.4 Residual stress measurement 

The residual stress in a TTBC system (defined as the 

"macrostress" imposed on the ceramic layer by the constraints of 

the substrate) may be manipulated during processing by substrate 

heating or cooling, ceramic-side cooling, varying the thickness 

per pass, and varying the traverse speed. UTRC has measured 

residual stress by applying strain gages to thin coated 

substrates and grinding off successive layers of the coating. 

Caterpillar has experimented with using x-ray technique to 

measure residual stresses at the surface. The Brookhaven National 

Laboratory synchrotron radiation facility has been used to 

measure residual stress at the ceramic-metal interface in thin 

plasma sprayed alumina coatings (ref. 30). Others have deduced 

residual stress from the deflection observed when coatings are 

placed on thin substrates (Private communication, R.C. Novak, 

united Technologies Research Center, 1989). Since residual stress 

is intimately connected to coating durability, fundamental 

research in this area would useful. 

Another type of residual stress present in plasma sprayed 

ceramics is the "microstress" which develops as the plasma 

sprayed ceramic splats rapidly solidify. This type of stress may 

be deduced from the broadening of x-ray diffraction peaks (ref. 

31) • 

III.C.S Permeability 

As discussed in Part III pressure in the cylinder will rapidly 

vary from atmospheric pressure to possibly over 3000 psi many 

times per second. The response of the porous TTBC to these 

fluctuations has not been well characterized and therefore 

represents another opportunity for fundamental research. The 

intrinsic oxygen conductivity that zirconia displays at high 

temperatures is not considered to be a issue for concern for the 

temperatures encountered in diesel engines. Oxygen conductivity 

may become important for zirconia in higher temperature gas 

turbine engines. 

15 

----------- ----

I 

! 

I 

I 

J 

measure of properties and porosity (with porosity the accuracy 

needed is probably about +2%). An optical technique has recently 

been developed for measuring coating thickness on-line during 

processing (ref. 28 and 29), and this and other techniques will 

be considered for diesel engines under HDTT. There are many other 

opportunities for fundamental research into NDE/property 

relationships which could have a place in a fundamental program. 

III.C.4 Residual stress measurement 

The residual stress in a TTBC system (defined as the 

"macrostress" imposed on the ceramic layer by the constraints of 

the substrate) may be manipulated during processing by substrate 

heating or cooling, ceramic-side cooling, varying the thickness 

per pass, and varying the traverse speed. UTRC has measured 

residual stress by applying strain gages to thin coated 

substrates and grinding off successive layers of the coating. 

Caterpillar has experimented with using x-ray technique to 

measure residual stresses at the surface. The Brookhaven National 

Laboratory synchrotron radiation facility has been used to 

measure residual stress at the ceramic-metal interface in thin 

plasma sprayed alumina coatings (ref. 30). Others have deduced 

residual stress from the deflection observed when coatings are 

placed on thin substrates (Private communication, R.C. Novak, 

united Technologies Research Center, 1989). Since residual stress 

is intimately connected to coating durability, fundamental 

research in this area would useful. 

Another type of residual stress present in plasma sprayed 

ceramics is the "microstress" which develops as the plasma 

sprayed ceramic splats rapidly solidify. This type of stress may 

be deduced from the broadening of x-ray diffraction peaks (ref. 

31) • 

III.C.S Permeability 

As discussed in Part III pressure in the cylinder will rapidly 

vary from atmospheric pressure to possibly over 3000 psi many 

times per second. The response of the porous TTBC to these 

fluctuations has not been well characterized and therefore 

represents another opportunity for fundamental research. The 

intrinsic oxygen conductivity that zirconia displays at high 

temperatures is not considered to be a issue for concern for the 

temperatures encountered in diesel engines. Oxygen conductivity 

may become important for zirconia in higher temperature gas 

turbine engines. 

15 

----------- ----

I 

! 

I 

I 

J 
111.0. Lower cost TTBCs 

Cost is obviously an important factor since the overall costs of 

the LHR engine must not exceed the value of the fuel economy, 

power density, and durability gains. The cost of the ceramic 

powders alone is so significant that high deposition efficiency 

spraying techniques must be developed and the possibility of 

overspray reclamation must be considered. There appears to be low 

interest at the present time in searching for lower cost ceramics 

since much work in the past has yielded few alternatives. The 

high costs of metallic bond coats, especially the large amount 

used in the graded regions of layered coatings, is a concern. Low 

cost alternatives to NiCrAIY and CoCrAlY should be considered. 

III.E Alternative approaches 

In this section alternatives to the current four layer approach 

(figure 4) and alternatives to plasma spray processing are 

considered. In general, there appears to be satisfaction with the 

four-layer approach ,and there is relatively low interest in 

investigating how bonding might be improved to the point where 

the intermediate layers would not be necessary. Approaches 

discussed included efforts to improve bond strength, to tailor 

surface roughness, and to manage residual stress. strain isolator 

pads appear to be ruled out for automotive applications although 

they still may have a place in certain aircraft gas turbine 

designs. Garrett (ref. 32) has developed a thick ceramic seal 

whose design avoids the need for graded layers by inducing 

vertical segmentation cracking through most of the thickness of 

the ceramic. This is accomplished by plasma spraying the ceramic 

at a low angle onto a substrate with finely spaced ridges 

machined onto its surface. 

The possibility of using molecular orbital theory to provide 

insights into the detailed mechanisms of bonding came up in 

discussions at southwest Research (Private communication, J. F. 

Maguire, southwest Research Institute, 1989) Similar 

calculations, performed under a NASA grant, have investigated the 

bonding of alumina to nickel aluminum alloys (refs. 33 and 34). 

The insights provided by a quantum mechanical study could 

potentially influence the design of the next generation of 

thermal barrier coatings. 

There was low interest in investigating alternatives to plasma 

spray processing, especially for the ceramic layer. One area that 

could be looked into would be to see if any of the new flame and 

plasma spray approaches that were developed for wear coatings 

have the potential to spray TTBCs. Shrouded plasma spraying 

(which is a low cost alternative to low pressure plasma spraying) 

16 

---------' 

111.0. Lower cost TTBCs 

Cost is obviously an important factor since the overall costs of 

the LHR engine must not exceed the value of the fuel economy, 

power density, and durability gains. The cost of the ceramic 

powders alone is so significant that high deposition efficiency 

spraying techniques must be developed and the possibility of 

overspray reclamation must be considered. There appears to be low 

interest at the present time in searching for lower cost ceramics 

since much work in the past has yielded few alternatives. The 

high costs of metallic bond coats, especially the large amount 

used in the graded regions of layered coatings, is a concern. Low 

cost alternatives to NiCrAIY and CoCrAlY should be considered. 

III.E Alternative approaches 

In this section alternatives to the current four layer approach 

(figure 4) and alternatives to plasma spray processing are 

considered. In general, there appears to be satisfaction with the 

four-layer approach ,and there is relatively low interest in 

investigating how bonding might be improved to the point where 

the intermediate layers would not be necessary. Approaches 

discussed included efforts to improve bond strength, to tailor 

surface roughness, and to manage residual stress. strain isolator 

pads appear to be ruled out for automotive applications although 

they still may have a place in certain aircraft gas turbine 

designs. Garrett (ref. 32) has developed a thick ceramic seal 

whose design avoids the need for graded layers by inducing 

vertical segmentation cracking through most of the thickness of 

the ceramic. This is accomplished by plasma spraying the ceramic 

at a low angle onto a substrate with finely spaced ridges 

machined onto its surface. 

The possibility of using molecular orbital theory to provide 

insights into the detailed mechanisms of bonding came up in 

discussions at southwest Research (Private communication, J. F. 

Maguire, southwest Research Institute, 1989) Similar 

calculations, performed under a NASA grant, have investigated the 

bonding of alumina to nickel aluminum alloys (refs. 33 and 34). 

The insights provided by a quantum mechanical study could 

potentially influence the design of the next generation of 

thermal barrier coatings. 

There was low interest in investigating alternatives to plasma 

spray processing, especially for the ceramic layer. One area that 

could be looked into would be to see if any of the new flame and 

plasma spray approaches that were developed for wear coatings 

have the potential to spray TTBCs. Shrouded plasma spraying 

(which is a low cost alternative to low pressure plasma spraying) 

16 

---------' 
-~ . 

_.__... _

is a technique to limit the amount of bond coat oxidation that 

occurs upon spraying. Possible benefits of reduced oxide content 

of diesel TTBC bond coats are not immediately apparent, however. 

III.F. Development of Meaningful screening Tests 

There appears to be a need to evaluate many more coatings in an 

engine environment or an engine simulative environment. These 

tests are needed both to thoroughly evaluate current systems, to 

study failure mechanisms, and to investigate new concepts. 

Caterpillar through subcontractor Solar Turbines and CUmmins 

through subcontractor UTRC have used bench rig tests to a limited 

extent but otherwise have relied on past aircraft experience and 

design to optimize coatings. As described in Part II these rigs 

were able to simUlate the no load/full load type of LCF cycle but 

were not designed to reproduce a severe HCF cycle. If a simple 

bench test that reproduced the thermal gradients encountered in a 

diesel engine could be developed, then it could be used to 

evaluate a much greater number of coating systems than can be 

handled with current test engines. 

Part IV. Materials Aspects of Current Thin Thermal Barrier 

Coatings (TBCs) for the Turbine section of Aircraft Gas 

Turbine Engines 

In this section, thin TBCs for the turbine section of aircraft 

gas turbine engines will be examined with emphasis on the history 

of their development and on certain aspects of their performance 

with emphasis on the materials science view point. Factors which 

have led to the selection of the current state-of-the-art 

zirconia-yttria/MCrAlY TBCs will be emphasized as will 

observations fundamentally related to the their behavior. 

oxidation related TBC failure mechanisms and life prediction 

approaches for aircraft gas turbine engines will not be discussed 

since those topics are not directly relevant to diesel engines 

and they have been covered in detail elsewhere (e.g. references 

1 and 35) . The plasma spray process will not be discussed in 

detail either. Again several reviews are available on this topic 

(e.g. , references 36, 37, and 38). 

IV.A. The current state-of-the-art thin TBC 

Current TBCs for the turbine section of gas turbine engines 

consist of a layer of ZrOz-(6-8%)YZ0 3 (by weight) ceramic over a 

layer of MCrAlY bond coat. Typical thicknesses for the ceramic 

and bond coat layers are 0.010 and 0.005 inch (0.025 and 0.012 

cm.), respectively. The ceramic layer is applied by air plasma 

17 

-~ . 

_.__... _

is a technique to limit the amount of bond coat oxidation that 

occurs upon spraying. Possible benefits of reduced oxide content 

of diesel TTBC bond coats are not immediately apparent, however. 

III.F. Development of Meaningful screening Tests 

There appears to be a need to evaluate many more coatings in an 

engine environment or an engine simulative environment. These 

tests are needed both to thoroughly evaluate current systems, to 

study failure mechanisms, and to investigate new concepts. 

Caterpillar through subcontractor Solar Turbines and CUmmins 

through subcontractor UTRC have used bench rig tests to a limited 

extent but otherwise have relied on past aircraft experience and 

design to optimize coatings. As described in Part II these rigs 

were able to simUlate the no load/full load type of LCF cycle but 

were not designed to reproduce a severe HCF cycle. If a simple 

bench test that reproduced the thermal gradients encountered in a 

diesel engine could be developed, then it could be used to 

evaluate a much greater number of coating systems than can be 

handled with current test engines. 

Part IV. Materials Aspects of Current Thin Thermal Barrier 

Coatings (TBCs) for the Turbine section of Aircraft Gas 

Turbine Engines 

In this section, thin TBCs for the turbine section of aircraft 

gas turbine engines will be examined with emphasis on the history 

of their development and on certain aspects of their performance 

with emphasis on the materials science view point. Factors which 

have led to the selection of the current state-of-the-art 

zirconia-yttria/MCrAlY TBCs will be emphasized as will 

observations fundamentally related to the their behavior. 

oxidation related TBC failure mechanisms and life prediction 

approaches for aircraft gas turbine engines will not be discussed 

since those topics are not directly relevant to diesel engines 

and they have been covered in detail elsewhere (e.g. references 

1 and 35) . The plasma spray process will not be discussed in 

detail either. Again several reviews are available on this topic 

(e.g. , references 36, 37, and 38). 

IV.A. The current state-of-the-art thin TBC 

Current TBCs for the turbine section of gas turbine engines 

consist of a layer of ZrOz-(6-8%)YZ0 3 (by weight) ceramic over a 

layer of MCrAlY bond coat. Typical thicknesses for the ceramic 

and bond coat layers are 0.010 and 0.005 inch (0.025 and 0.012 

cm.), respectively. The ceramic layer is applied by air plasma 

17 
spraying which produces a porous and microcracked structure with 

a density of about 90% of the theoretical value and a phase 

composition that is predominantly a non-equilibrium, high-yttria 

tetragonal phase (T'). The 10% "porosity" in the ceramic lowers 

the thermal conductivity and the elastic modulus by factors that 

are much greater than predicted by a simple rule of mixtures. As 

a result the plasma sprayed ceramic layer is both strain tolerant 

and highly insulating. The effect that the non-equilibrium phase 

distribution has on coating system behavior is not well 

understood. 

The MCrAIY bond coat is typically applied by low pressure plasma 

spraying which produces a dense structure that is free of 

internal oxidation. coating systems that have low pressure plasma 

sprayed bond coats generally have greater cyclic lives at higher 

temperatures than systems with air plasma sprayed bond coats. 

Also, low pressure plasma sprayed bond coats are inherently more 

protective of the underlying substrate. There is no generally 

agreed upon composition of the MCrAIY bond coat. Typically 

chromium is in the range of 16 to 36% (by weight), aluminum is in 

the range of 5 to 12%, and yttrium is less than 1%. The balance 

"M" usually consists of nickel or nickel plus cobalt. 

The evolution from early TBCs to the current standard thin TBC 

and fundamental observations regarding the performance of thin 

TBCs will be examined in the remainder of this part of the paper. 

IV.B. Early ceramic coatings for aerospace applications 

The first ceramic coatings for aerospace applications were frit 

enamels. The first of these frit coatings were developed by NACA 

and the NBS (refs. 39 and 40. Frit enamels found considerable 

use in aircraft engines throughout the 1950's (ref. 41). Later, 

flame sprayed ceramic coatings were developed (refs. 42-47). Of 

the various early ceramic materials that were evaluated for 

thermal barrier applications, alumina and zirconia-calcia evolved 

as the leading materials. The bond coat material for these early 

applications, if one was used at all, was typically nichrome or 

molybdenum for non-oxidizing environments. Early applications 

included the protection of sheet metal in jet engines and in 

rocket engine thrust chambers. The most visible coatings during 

this period were the flame sprayed zirconia-calcia coatings on 

the regeneratively cooled XLR99 thrust chambers for the X-15 

experimental rocket planes (refs. 45 and 46). with the 

subsequent development of plasma spray processing -- which 

evolved from research into low thrust plasma arc engines for 

space craft and from plasma arc test facilities developed for 

reentry simulation (ref. 48) -- the utility of using the very 

high temperature plasmas for spraying ceramics was soon 

18 

spraying which produces a porous and microcracked structure with 

a density of about 90% of the theoretical value and a phase 

composition that is predominantly a non-equilibrium, high-yttria 

tetragonal phase (T'). The 10% "porosity" in the ceramic lowers 

the thermal conductivity and the elastic modulus by factors that 

are much greater than predicted by a simple rule of mixtures. As 

a result the plasma sprayed ceramic layer is both strain tolerant 

and highly insulating. The effect that the non-equilibrium phase 

distribution has on coating system behavior is not well 

understood. 

The MCrAIY bond coat is typically applied by low pressure plasma 

spraying which produces a dense structure that is free of 

internal oxidation. coating systems that have low pressure plasma 

sprayed bond coats generally have greater cyclic lives at higher 

temperatures than systems with air plasma sprayed bond coats. 

Also, low pressure plasma sprayed bond coats are inherently more 

protective of the underlying substrate. There is no generally 

agreed upon composition of the MCrAIY bond coat. Typically 

chromium is in the range of 16 to 36% (by weight), aluminum is in 

the range of 5 to 12%, and yttrium is less than 1%. The balance 

"M" usually consists of nickel or nickel plus cobalt. 

The evolution from early TBCs to the current standard thin TBC 

and fundamental observations regarding the performance of thin 

TBCs will be examined in the remainder of this part of the paper. 

IV.B. Early ceramic coatings for aerospace applications 

The first ceramic coatings for aerospace applications were frit 

enamels. The first of these frit coatings were developed by NACA 

and the NBS (refs. 39 and 40. Frit enamels found considerable 

use in aircraft engines throughout the 1950's (ref. 41). Later, 

flame sprayed ceramic coatings were developed (refs. 42-47). Of 

the various early ceramic materials that were evaluated for 

thermal barrier applications, alumina and zirconia-calcia evolved 

as the leading materials. The bond coat material for these early 

applications, if one was used at all, was typically nichrome or 

molybdenum for non-oxidizing environments. Early applications 

included the protection of sheet metal in jet engines and in 

rocket engine thrust chambers. The most visible coatings during 

this period were the flame sprayed zirconia-calcia coatings on 

the regeneratively cooled XLR99 thrust chambers for the X-15 

experimental rocket planes (refs. 45 and 46). with the 

subsequent development of plasma spray processing -- which 

evolved from research into low thrust plasma arc engines for 

space craft and from plasma arc test facilities developed for 

reentry simulation (ref. 48) -- the utility of using the very 

high temperature plasmas for spraying ceramics was soon 

18 
recognized (refs. 44, 47, 49, and 50). By 1964, plasma sprayed 

thermal barrier coatings were in use on the combustors in 

commercial gas turbine engines (ref. 51). 

Alumina and zirconia-calcia did not prove to be a viable material 

for the more advanced thermal barrier applications. In the case 

of alumina this is primarily because its thermal conductivity is 

relatively high (ref. 52). Also, alumina forms non-equilibrium 

phases which are variously described as gamma, eta, or delta and 

these non-equilibrium phases shrink when they convert to the 

equilibrium alpha phase upon high temperature exposure. This 

shrinkage is expected to have a detrimental effect on coating 

life. The phase transformation to alpha alumina appears to become 

a factor above about 1100 0C (refs. 36, 42, 50, 53, 54, and 55). 

The problem with zirconia-calcia is related to "destabilization" 

from the cubic fluorite (F-ZrOz) phase that is observed in the 

as-sprayed material to the monoclinic (M-ZrOz) phase. 

Zirconia-based ceramics containing excessive amounts of this 

monoclinic phase are not usable as structural materials -- due to 

the shrinkage observed upon a rapid martensitic transformation 

from the low temperature monoclinic phase to the higher 

temperature tetragonal phase. Although the calcia is added to 

zirconia with the intent of extending the high temperature cubic 

phase field down to room temperature, it is now known that the 

cubic phase in zirconia-calcia is actually not stable below about 

1140 0C (ref. 56). Therefore, even though the cubic phase is 

observed in the plasma sprayed material, diffusional 

rearrangement below 1140 0C can destabilize the material. 

Magnesia stabilized zirconia (often called magnesium zirconate) 

has been widely used as a sUbstitute for calcia stabilized 

zirconia. However, magnesia, like calcia, does not stabilize the 

F-ZrOz phase at low temperatures. According to the equilibrium 

phase diagram, this stabilization only occurs above about 1400 0C 

(ref. 56), and therefore diffusional rearrangement at 

temperatures below 1400~ destabilize this material. The upper 

use temperature for zirconia-calcia or zirconia-magnesia will be 

related to the temperature above which cation diffusion becomes 

important in these materials. An apparently small but detectable 

decomposition to the monocolinic phase was observed after 

ZrOz-MgO was heated for 40 days at 800CC (ref. 57). Toriz et ale 

(ref. 58) gave 9500C as the practical upper use temperature for 

zirconia magnesia in gas turbine applications, and a similar 

upper use temperature may apply for zirconia-calcia. 

There are two other potential difficulties with zirconia-calcia 

and zirconia-magnesia. One problem with zirconia-magnesia and, 

certainly also with zirconia-calcia, is a sulfation reaction that 

occurs between the alkaline earth oxide and sulfur dioxide (refs. 

59 and 60). This reaction -- which is analogous to the 

19 

recognized (refs. 44, 47, 49, and 50). By 1964, plasma sprayed 

thermal barrier coatings were in use on the combustors in 

commercial gas turbine engines (ref. 51). 

Alumina and zirconia-calcia did not prove to be a viable material 

for the more advanced thermal barrier applications. In the case 

of alumina this is primarily because its thermal conductivity is 

relatively high (ref. 52). Also, alumina forms non-equilibrium 

phases which are variously described as gamma, eta, or delta and 

these non-equilibrium phases shrink when they convert to the 

equilibrium alpha phase upon high temperature exposure. This 

shrinkage is expected to have a detrimental effect on coating 

life. The phase transformation to alpha alumina appears to become 

a factor above about 1100 0C (refs. 36, 42, 50, 53, 54, and 55). 

The problem with zirconia-calcia is related to "destabilization" 

from the cubic fluorite (F-ZrOz) phase that is observed in the 

as-sprayed material to the monoclinic (M-ZrOz) phase. 

Zirconia-based ceramics containing excessive amounts of this 

monoclinic phase are not usable as structural materials -- due to 

the shrinkage observed upon a rapid martensitic transformation 

from the low temperature monoclinic phase to the higher 

temperature tetragonal phase. Although the calcia is added to 

zirconia with the intent of extending the high temperature cubic 

phase field down to room temperature, it is now known that the 

cubic phase in zirconia-calcia is actually not stable below about 

1140 0C (ref. 56). Therefore, even though the cubic phase is 

observed in the plasma sprayed material, diffusional 

rearrangement below 1140 0C can destabilize the material. 

Magnesia stabilized zirconia (often called magnesium zirconate) 

has been widely used as a sUbstitute for calcia stabilized 

zirconia. However, magnesia, like calcia, does not stabilize the 

F-ZrOz phase at low temperatures. According to the equilibrium 

phase diagram, this stabilization only occurs above about 1400 0C 

(ref. 56), and therefore diffusional rearrangement at 

temperatures below 1400~ destabilize this material. The upper 

use temperature for zirconia-calcia or zirconia-magnesia will be 

related to the temperature above which cation diffusion becomes 

important in these materials. An apparently small but detectable 

decomposition to the monocolinic phase was observed after 

ZrOz-MgO was heated for 40 days at 800CC (ref. 57). Toriz et ale 

(ref. 58) gave 9500C as the practical upper use temperature for 

zirconia magnesia in gas turbine applications, and a similar 

upper use temperature may apply for zirconia-calcia. 

There are two other potential difficulties with zirconia-calcia 

and zirconia-magnesia. One problem with zirconia-magnesia and, 

certainly also with zirconia-calcia, is a sulfation reaction that 

occurs between the alkaline earth oxide and sulfur dioxide (refs. 

59 and 60). This reaction -- which is analogous to the 

19 
industrial use of limestone or dolemite scrubbers for the removal 

of sulfur dioxide from power plant exhausts -- converts the 

magnesia portion of the solid solution to magnesium sulfate. The 

effect of this sulfation reaction on coating behavior is 

uncertain. Another difficulty is the volatility of the alkali 

earth oxides -- especially magnesia -- at very high temperatures. 

This volativity could cause loss of the stabilizer upon plasma 

spraying (ref. 61) and could affect the performance of coatings 

subjected to surface temperatures of perhaps 1600 0C (ref. 62). 

In summary, application processes for aerospace ceramic coatings 

evolved from frit enamels to flame spray and finally to plasma 

spray. During this time alumina and initially zirconia-calcia but 

later zirconia-magnesia were the ceramic materials most often 

used. However, alumina proved to be inadequate because of high 

thermal conductivity and questions about high temperature phase 

stability. Zirconia-caleia and zirconia-magnesia were found to 

have upper use temperatures of about 950°C and were susceptible 

to sulfation reactions. 

IV.C. Current zirconia-yttria/MCrAlY TBCs 

IV.C.1 Zirconia-yttria thermal barrier coatings 

The current era in thermal barrier coatings began in the mid 

1970's with the development at NASA-Lewis of a two layer TBC 

consisting of a plasma sprayed zirconia-yttria ceramic over a 

plasma sprayed NiCrAlY bond coat (refs. 63, 64, 65) and the 

successful testing of this coating on the turbine blades in a 

research gas turbine engine (ref. 66). 

There were three keys to the success of this new coating system. 

First, yttria was used to stabilize the zirconia. Second, the 

bond coat was an oxidation resistant NiCrAlY alloy of the type 

that was being developed for metallic overlay coatings. Finally, 

their approach employed only two layers -- the ceramic and the 

bond coat. The approach being taken at that time by Pratt & 

Whitney also involved zirconia-yttria and an MCrAlY. However 

their approach favored an intermediate region of mixed metal and 

ceramic to mitigate thermal expansion mismatch stresses (ref. 

67). It was soon discovered that this graded expansion approach 

was inappropriate for temperatures high enough for oxidation to 

occur in the graded region (ref. 35). Later work places an upper 

limit of about 800~ for the maximum temperature to which the 

graded region may be exposed (refs. 59 and 60). Union Carbide was 

another early developer of zirconia-yttria/MCrAlY TBCs (ref. 68). 

The initial zirconia-yttria TBCs contained from 12 to 20% of 

yttria which was added to fully stabilize the cubic phase. Later, 

20 

industrial use of limestone or dolemite scrubbers for the removal 

of sulfur dioxide from power plant exhausts -- converts the 

magnesia portion of the solid solution to magnesium sulfate. The 

effect of this sulfation reaction on coating behavior is 

uncertain. Another difficulty is the volatility of the alkali 

earth oxides -- especially magnesia -- at very high temperatures. 

This volativity could cause loss of the stabilizer upon plasma 

spraying (ref. 61) and could affect the performance of coatings 

subjected to surface temperatures of perhaps 1600 0C (ref. 62). 

In summary, application processes for aerospace ceramic coatings 

evolved from frit enamels to flame spray and finally to plasma 

spray. During this time alumina and initially zirconia-calcia but 

later zirconia-magnesia were the ceramic materials most often 

used. However, alumina proved to be inadequate because of high 

thermal conductivity and questions about high temperature phase 

stability. Zirconia-caleia and zirconia-magnesia were found to 

have upper use temperatures of about 950°C and were susceptible 

to sulfation reactions. 

IV.C. Current zirconia-yttria/MCrAlY TBCs 

IV.C.1 Zirconia-yttria thermal barrier coatings 

The current era in thermal barrier coatings began in the mid 

1970's with the development at NASA-Lewis of a two layer TBC 

consisting of a plasma sprayed zirconia-yttria ceramic over a 

plasma sprayed NiCrAlY bond coat (refs. 63, 64, 65) and the 

successful testing of this coating on the turbine blades in a 

research gas turbine engine (ref. 66). 

There were three keys to the success of this new coating system. 

First, yttria was used to stabilize the zirconia. Second, the 

bond coat was an oxidation resistant NiCrAlY alloy of the type 

that was being developed for metallic overlay coatings. Finally, 

their approach employed only two layers -- the ceramic and the 

bond coat. The approach being taken at that time by Pratt & 

Whitney also involved zirconia-yttria and an MCrAlY. However 

their approach favored an intermediate region of mixed metal and 

ceramic to mitigate thermal expansion mismatch stresses (ref. 

67). It was soon discovered that this graded expansion approach 

was inappropriate for temperatures high enough for oxidation to 

occur in the graded region (ref. 35). Later work places an upper 

limit of about 800~ for the maximum temperature to which the 

graded region may be exposed (refs. 59 and 60). Union Carbide was 

another early developer of zirconia-yttria/MCrAlY TBCs (ref. 68). 

The initial zirconia-yttria TBCs contained from 12 to 20% of 

yttria which was added to fully stabilize the cubic phase. Later, 

20 
stecura showed that better performance could be achieved by using 

only 6-8% yttria (ref. 69). The TBC literature, in general, now 

supports the view that zirconia-(6-8%)yttria is superior to 

zirconia-(12-20%)yttria for advanced gas turbine applications and 

that zirconia-yttria TBCs are in turn superior to 

zirconia-magnesia and zirconia-calcia (refs. 58-60, 70-72). 

IV.C.2 Phase distributions in plasma sprayed zirconia-yttria 

The predominant phase observed in plasma sprayed partially 

stabilized zirconia-yttria is a high yttria, non-equilibrium 

tetragonal phase (refs. 21, 22, 73-75). This phase was first 

observed by Scott (ref. 76) who showed that this non-equilibrium 

phase forms when partially stabilized zirconia-yttria is quenched 

from high temperature. This phase may be called 

"nontransformable" tetragonal, T'-ZrOz• This distinguishes it 

from the low yttria tetragonal phase, T-Zr04, which is an 

equilibrium phase in partially stabilized zlrconia between about 

600 and 2000~. The term nontransformable refers to the fact 

that, in contrast to the low yttria T-Zr02 tetragonal phase, the 

T'-ZrOz phase does not undergo a martensitic transformation to 

the M-Zr02 monoclinic phase upon cooling. A version of the 

zirconia-yttria phase diagram from reference 76 -- taken from 

reference 21 -- is presented in figure 12. The bars near the 

bottom of this figure represent the phases having the lowest free 

energy for a diffusionless transformation, i.e. these are the 

phases encountered upon quenching. The arrows in the figure 

represent, schematically, the processes of plasma spraying and 

subsequent high temperature ageing. The "X" in the figure 

represents Zr02-8%Y203 (in weight percent) or Zr02-8.6%Y01 5 (in 

mole per cent) that has been melted by the plasma. Quenching this 

composition, as indicated by the virtical line extending to room 

temperature, yields the nontransformable T' phase. It is believed 

that this phase forms because the process of quenching does not 

allow sufficient time for diffusional rearrangement to the lower 

free energy (i.e. equilibrium) combination of phases. These 

equilibrium phases are the M-ZrOz monoclinic and F-ZrOz cubic 

fluorite phases that are predicted by the phase diagram. 

The T'-ZrOz phase that is predominant in the state-of-the-art 

zirconia-yttria thermal barrier coatings is, of course, 

inherently unstable. However, the high temperature 

transformations to the equilibrium phases are found to be 

sluggish. For example, for the particular batch of plasma 

sprayed ZrOz-8%Y.z03 reported in reference 21, 10 hours at 1200 't 

was an insufficlent amount of time to produce a detectable 

decrease in the T'-zroz phase. However 100 hours at 1200 0C and 

only 1 hour at 1400 0C did produce a detectable change. (Tie 

lines drawn on figure 12 indicate the path of the 1400 0C 

decomposition.) In current aircraft gas turbine engines the 

21 

stecura showed that better performance could be achieved by using 

only 6-8% yttria (ref. 69). The TBC literature, in general, now 

supports the view that zirconia-(6-8%)yttria is superior to 

zirconia-(12-20%)yttria for advanced gas turbine applications and 

that zirconia-yttria TBCs are in turn superior to 

zirconia-magnesia and zirconia-calcia (refs. 58-60, 70-72). 

IV.C.2 Phase distributions in plasma sprayed zirconia-yttria 

The predominant phase observed in plasma sprayed partially 

stabilized zirconia-yttria is a high yttria, non-equilibrium 

tetragonal phase (refs. 21, 22, 73-75). This phase was first 

observed by Scott (ref. 76) who showed that this non-equilibrium 

phase forms when partially stabilized zirconia-yttria is quenched 

from high temperature. This phase may be called 

"nontransformable" tetragonal, T'-ZrOz• This distinguishes it 

from the low yttria tetragonal phase, T-Zr04, which is an 

equilibrium phase in partially stabilized zlrconia between about 

600 and 2000~. The term nontransformable refers to the fact 

that, in contrast to the low yttria T-Zr02 tetragonal phase, the 

T'-ZrOz phase does not undergo a martensitic transformation to 

the M-Zr02 monoclinic phase upon cooling. A version of the 

zirconia-yttria phase diagram from reference 76 -- taken from 

reference 21 -- is presented in figure 12. The bars near the 

bottom of this figure represent the phases having the lowest free 

energy for a diffusionless transformation, i.e. these are the 

phases encountered upon quenching. The arrows in the figure 

represent, schematically, the processes of plasma spraying and 

subsequent high temperature ageing. The "X" in the figure 

represents Zr02-8%Y203 (in weight percent) or Zr02-8.6%Y01 5 (in 

mole per cent) that has been melted by the plasma. Quenching this 

composition, as indicated by the virtical line extending to room 

temperature, yields the nontransformable T' phase. It is believed 

that this phase forms because the process of quenching does not 

allow sufficient time for diffusional rearrangement to the lower 

free energy (i.e. equilibrium) combination of phases. These 

equilibrium phases are the M-ZrOz monoclinic and F-ZrOz cubic 

fluorite phases that are predicted by the phase diagram. 

The T'-ZrOz phase that is predominant in the state-of-the-art 

zirconia-yttria thermal barrier coatings is, of course, 

inherently unstable. However, the high temperature 

transformations to the equilibrium phases are found to be 

sluggish. For example, for the particular batch of plasma 

sprayed ZrOz-8%Y.z03 reported in reference 21, 10 hours at 1200 't 

was an insufficlent amount of time to produce a detectable 

decrease in the T'-zroz phase. However 100 hours at 1200 0C and 

only 1 hour at 1400 0C did produce a detectable change. (Tie 

lines drawn on figure 12 indicate the path of the 1400 0C 

decomposition.) In current aircraft gas turbine engines the 

21 
temperatures at the interface, where stresses a r e max i mum, remain 

well below 1200 0C even in hot streaks during the t akeoff portion 

of the cycle. Also, the total accumulated time that a component 

spe nds at the maximum temperature is relat ively short. 

The refore, decomposition of the T'-ZrOz phase to the equilibrium 

M-Zroz and F-ZrOz phases is not expected t o be i mportant near the 

interface region of current TBCs in current engines. 

Trans formations near the higher temperatu re s urface of these 

coa tings does appear to be possible. Howeve r, at the present 

time there are no known adverse effects of transformations near 

the surface. 

Powder preparation method and plasma spray processing parameters 

are two factors which strongly affect the microstructures, phase 

distributions, thermophysical properties, and thermomechanical 

properties of thermal barrier coatings. As a r esult the 

performance of two nominally identical coatings ma y v ary widely. 

In theory this variability could be benefici al if coating 

engineers fully understood how to vary start ing powders and 

processing parameters to produce coatings tailo r ed for a specific 

need. Unfortunately, the technology is still in a rather chaotic 

state where no two coatings from any two sources c an be counted 

on to give comparable performance. 

It is interesting to compare the nontransformable T'-ZrOz phase 

that forms in plasma sprayed, partially stabilized 

zirconia-yttria to the metastable T-ZrO pha se tha t f orms in 

yttria-stabilized tetragonal pOlycrystals -- Y- TZ P (ref . 77). 

Y-TZP is a fine grained material containing greater than about 2% 

and less than 4 or 5% yttria that has bee n p r ocessed to form a 

metastable T-ZrOz phase. This metastable phase can undergo a 

stress-induced martensi tic transformation to the M-Zr Oz phase 

which imparts a high fracture toughness on Y-TZP v i a the 

transformation toughening mechanism. The frac ture toughness is 

generally observed to falloff rapidly with increasing yttria as 

the T'-ZrOz phase begins to replace the T-ZrOz ph ase (refs. 78 

and 79). An exception is the zirconia-yttria ma t erial described 

by Lange (ref. 80) which retained low yttria, metastable T-Zroz 

precipitates in a cubic matrix throughout the e ntire partially 

stabilized range. The gradual falloff of the measured fracture 

toughness values with increasing yttria c ontent can be taken as 

evidence that metastable T-ZrOz precipitates h ad formed rather 

than the higher yttria, nontransformable T' - zrOz. In Lange's 

material, the metastable tetragonal zirconia phase was retained 

at higher yttria compositions by processing s o as to produce fine 

grain size and high density. 

The transformation toughening mechanism does not appear to play a 

role plasma sprayed ZrOz-(6-8%)YZ03 -- i.e., the pre dominant 

T'-Zr02 phase cannot transform martensitically at temperature too 

22 

temperatures at the interface, where stresses a r e max i mum, remain 

well below 1200 0C even in hot streaks during the t akeoff portion 

of the cycle. Also, the total accumulated time that a component 

spe nds at the maximum temperature is relat ively short. 

The refore, decomposition of the T'-ZrOz phase to the equilibrium 

M-Zroz and F-ZrOz phases is not expected t o be i mportant near the 

interface region of current TBCs in current engines. 

Trans formations near the higher temperatu re s urface of these 

coa tings does appear to be possible. Howeve r, at the present 

time there are no known adverse effects of transformations near 

the surface. 

Powder preparation method and plasma spray processing parameters 

are two factors which strongly affect the microstructures, phase 

distributions, thermophysical properties, and thermomechanical 

properties of thermal barrier coatings. As a r esult the 

performance of two nominally identical coatings ma y v ary widely. 

In theory this variability could be benefici al if coating 

engineers fully understood how to vary start ing powders and 

processing parameters to produce coatings tailo r ed for a specific 

need. Unfortunately, the technology is still in a rather chaotic 

state where no two coatings from any two sources c an be counted 

on to give comparable performance. 

It is interesting to compare the nontransformable T'-ZrOz phase 

that forms in plasma sprayed, partially stabilized 

zirconia-yttria to the metastable T-ZrO pha se tha t f orms in 

yttria-stabilized tetragonal pOlycrystals -- Y- TZ P (ref . 77). 

Y-TZP is a fine grained material containing greater than about 2% 

and less than 4 or 5% yttria that has bee n p r ocessed to form a 

metastable T-ZrOz phase. This metastable phase can undergo a 

stress-induced martensi tic transformation to the M-Zr Oz phase 

which imparts a high fracture toughness on Y-TZP v i a the 

transformation toughening mechanism. The frac ture toughness is 

generally observed to falloff rapidly with increasing yttria as 

the T'-ZrOz phase begins to replace the T-ZrOz ph ase (refs. 78 

and 79). An exception is the zirconia-yttria ma t erial described 

by Lange (ref. 80) which retained low yttria, metastable T-Zroz 

precipitates in a cubic matrix throughout the e ntire partially 

stabilized range. The gradual falloff of the measured fracture 

toughness values with increasing yttria c ontent can be taken as 

evidence that metastable T-ZrOz precipitates h ad formed rather 

than the higher yttria, nontransformable T' - zrOz. In Lange's 

material, the metastable tetragonal zirconia phase was retained 

at higher yttria compositions by processing s o as to produce fine 

grain size and high density. 

The transformation toughening mechanism does not appear to play a 

role plasma sprayed ZrOz-(6-8%)YZ03 -- i.e., the pre dominant 

T'-Zr02 phase cannot transform martensitically at temperature too 

22 
low for diffusional rearrangement and there is no evidence that 

metastable T-Zr02 is present. The fracture toughness values for 

this material are low (refs. 81 and 82) and the force of grinding 

does not cause an increase in the amount of the M-Zr02 phase 

(ref. 21). Also, plasma sprayed materials do not have the high 

density needed to retain the metastable T-Zr02 phase in the 

partially stabilized range -- even though the grain size is fine 

(ref. 83). However, there is evidence that the metastable T-Zr02 is present in plasma sprayed Zr02_4\Y2

03 • This is because the 

amount of M-ZrOz increases by a factor of two when that material 

is ground. A similar increase in the amount of the monoclinic 

phase also occurs upon ageing this material for two years at room 

temperature (ref. 22). (Decomposition of metastable T-ZrOz to 

M-Zr02 is now know to occur when the Y-TZP material is aged at 

temperatures as low as 100°C in the presence of water or water 

vapor (ref. 84». When ZrOz-4%Y203 is sprayed as a thermal 

barrier coating and subject to thermal cycling it performs rather 

well but is apparently mechanically unstable since it gradually 

crumbles to powder (ref. 69). It is not clear what role possible 

compositional inhomogenaities may have played in this mechanical 

instability. 

IV.C.3 Effects of impurities in plasma sprayed zirconia-yttria 

Most impurities in typical lots of zirconia-yttria plasma spray 

powder are low level with few apparent effects -- the most 

obvious of which is a yellowing of the color of the ceramic. On 

the other hand, hafnia and silica may be present in 

zirconia-yttria plasma spray powders in amounts up to about 2% 

(by weight). The effects of a few percent of hafnia are 

generally considered minor because hafnia and zirconia are 

chemically very similar (although in certain nuclear applications 

it is desirable to remove the hafnia component due to its higher 

neutron cross section; also, pure hafnia has a higher Ms 

temperature than pure zirconia). The silica impurity, on the 

other hand, may strongly affect coating behavior. The typical 

level of silica in the zirconia-yttria compositions reported by 

Stecura, e.g., in reference 69, was below 0.2%. When a sample of 

zirconia-yttria powder that had been prepared to stecura's 

specifications was analyzed using x-ray photoelectron 

spectroscopy (XPS) , a considerable amount of silicon was detected 

on the surface. In fact, the estimated percentage of silicon 

atoms in at least the outer 20 A of the powder particles was 

about two-thirds as large as the sum of the percentages of 

zirconia plus yttria (Informal report from M.V. Zeller of 

NASA-Lewis Research Center to R.A. Miller, 1988). This implies 

that a low melting silica-rich compound such as zircon may be 

present on the surface of these plasma spray powders. In another 

study, XPS was used to show that a sodium-yttria-silicon oxide 

was present on the surface of a zirconia-yttria powder (ref. 85). 

23 

low for diffusional rearrangement and there is no evidence that 

metastable T-Zr02 is present. The fracture toughness values for 

this material are low (refs. 81 and 82) and the force of grinding 

does not cause an increase in the amount of the M-Zr02 phase 

(ref. 21). Also, plasma sprayed materials do not have the high 

density needed to retain the metastable T-Zr02 phase in the 

partially stabilized range -- even though the grain size is fine 

(ref. 83). However, there is evidence that the metastable T-Zr02 is present in plasma sprayed Zr02_4\Y2

03 • This is because the 

amount of M-ZrOz increases by a factor of two when that material 

is ground. A similar increase in the amount of the monoclinic 

phase also occurs upon ageing this material for two years at room 

temperature (ref. 22). (Decomposition of metastable T-ZrOz to 

M-Zr02 is now know to occur when the Y-TZP material is aged at 

temperatures as low as 100°C in the presence of water or water 

vapor (ref. 84». When ZrOz-4%Y203 is sprayed as a thermal 

barrier coating and subject to thermal cycling it performs rather 

well but is apparently mechanically unstable since it gradually 

crumbles to powder (ref. 69). It is not clear what role possible 

compositional inhomogenaities may have played in this mechanical 

instability. 

IV.C.3 Effects of impurities in plasma sprayed zirconia-yttria 

Most impurities in typical lots of zirconia-yttria plasma spray 

powder are low level with few apparent effects -- the most 

obvious of which is a yellowing of the color of the ceramic. On 

the other hand, hafnia and silica may be present in 

zirconia-yttria plasma spray powders in amounts up to about 2% 

(by weight). The effects of a few percent of hafnia are 

generally considered minor because hafnia and zirconia are 

chemically very similar (although in certain nuclear applications 

it is desirable to remove the hafnia component due to its higher 

neutron cross section; also, pure hafnia has a higher Ms 

temperature than pure zirconia). The silica impurity, on the 

other hand, may strongly affect coating behavior. The typical 

level of silica in the zirconia-yttria compositions reported by 

Stecura, e.g., in reference 69, was below 0.2%. When a sample of 

zirconia-yttria powder that had been prepared to stecura's 

specifications was analyzed using x-ray photoelectron 

spectroscopy (XPS) , a considerable amount of silicon was detected 

on the surface. In fact, the estimated percentage of silicon 

atoms in at least the outer 20 A of the powder particles was 

about two-thirds as large as the sum of the percentages of 

zirconia plus yttria (Informal report from M.V. Zeller of 

NASA-Lewis Research Center to R.A. Miller, 1988). This implies 

that a low melting silica-rich compound such as zircon may be 

present on the surface of these plasma spray powders. In another 

study, XPS was used to show that a sodium-yttria-silicon oxide 

was present on the surface of a zirconia-yttria powder (ref. 85). 

23 
When that powder was fabricated and eva lua t ed as an electrical 

ceramic, anomalous electrical and grain growt h behavior was 

observe d. The silica rich phase was o ffe r ed as an explanation for 

this a nomalous behavior. Hoel and Kvernes (ref. 83) have used 

TEM with EDX-spectroscopy to study thin foi ls of plasma sprayed 

zirconia-yttria. They observed, by employing techni ques involving 

"ca reful tilting and diffuse dark-field imaging," a silica-rich 

glassy phase concentrated at the grain boundaries. TEM 

observations of silica-rich intergra nular phases have also been 

observed in certain monolithic zirconia -yttria ceramics (e.g., 

references 86 and 87). It is belie v e d tha t these silica-rich 

layers may affect high temperature creep p roperties, sintering 

rates, grain growth, and possibly the T'to M plus F 

transformation. Additionally, unite d Tech nologies (Private 

Communication, R.C. Novak, United Techno l ogies Research Center, 

1989) has observed a direct relations hip between sintering rate 

and silica level in plasma sprayed zirconia-yt tria. 

IV.C . 4 Bond coat development 

The composition of bond coats for aircraft TBC applications has 

gener ally followed the development o f oxida tion resistant overlay 

coati ngs. The early compositions were materials such as NiCr and 

NiAI . stecura (ref. 63) was the first to report t he use of a 

MCrAl Y coating. Examples of bond coat composition s in use today 

for higher temperature applications include 

Ni-22%Co-18%Cr-12%AI-0.4%Y which is the b ill-of-material bond 

coat at Pratt & Whitney (ref. 88) and Ni-22%Cr-10%AI-0.3%Y which 

is b i ll-of-material at General Elect ric (ref. 89). stecura (ref. 

90) achieved the best TBC performance in f urnace tests using 

Ni-36%Cr-6%AI-1%Y. 

The bond coats initial ly reported by stecura and othe r s were 

prepared by air plasma spraying. However, l ow pressure plasma 

spraying is now considered the state- of- the-art for the more 

advanced applications (ref. 35) where bond coat temperatures may 

exceed about 1000CC. Other approaches which have not yet been 

fully evaluated include shrouded spraying (r ef. 81) and various 

types of flame spraying incl~ding the new h igh velocity torches. 

There are also many materials besides NiCrAlY and NiCoCrAIY that 

are available for use as bond coats. For example s t ecura (ref. 

91) has reported that FeCrAlY bond coats a re the best performers 

on iron-based superalloys. For relati vely l ow temperature 

operation (even in oxidizing conditions such as encountered in 

diesel engine operation) a wide number of bond coa ts may be 

considered. These may include for example s tainless steel or any 

of various mechanically alloyed bond c oat materials such as NiAI 

24 

When that powder was fabricated and eva lua t ed as an electrical 

ceramic, anomalous electrical and grain growt h behavior was 

observe d. The silica rich phase was o ffe r ed as an explanation for 

this a nomalous behavior. Hoel and Kvernes (ref. 83) have used 

TEM with EDX-spectroscopy to study thin foi ls of plasma sprayed 

zirconia-yttria. They observed, by employing techni ques involving 

"ca reful tilting and diffuse dark-field imaging," a silica-rich 

glassy phase concentrated at the grain boundaries. TEM 

observations of silica-rich intergra nular phases have also been 

observed in certain monolithic zirconia -yttria ceramics (e.g., 

references 86 and 87). It is belie v e d tha t these silica-rich 

layers may affect high temperature creep p roperties, sintering 

rates, grain growth, and possibly the T'to M plus F 

transformation. Additionally, unite d Tech nologies (Private 

Communication, R.C. Novak, United Techno l ogies Research Center, 

1989) has observed a direct relations hip between sintering rate 

and silica level in plasma sprayed zirconia-yt tria. 

IV.C . 4 Bond coat development 

The composition of bond coats for aircraft TBC applications has 

gener ally followed the development o f oxida tion resistant overlay 

coati ngs. The early compositions were materials such as NiCr and 

NiAI . stecura (ref. 63) was the first to report t he use of a 

MCrAl Y coating. Examples of bond coat composition s in use today 

for higher temperature applications include 

Ni-22%Co-18%Cr-12%AI-0.4%Y which is the b ill-of-material bond 

coat at Pratt & Whitney (ref. 88) and Ni-22%Cr-10%AI-0.3%Y which 

is b i ll-of-material at General Elect ric (ref. 89). stecura (ref. 

90) achieved the best TBC performance in f urnace tests using 

Ni-36%Cr-6%AI-1%Y. 

The bond coats initial ly reported by stecura and othe r s were 

prepared by air plasma spraying. However, l ow pressure plasma 

spraying is now considered the state- of- the-art for the more 

advanced applications (ref. 35) where bond coat temperatures may 

exceed about 1000CC. Other approaches which have not yet been 

fully evaluated include shrouded spraying (r ef. 81) and various 

types of flame spraying incl~ding the new h igh velocity torches. 

There are also many materials besides NiCrAlY and NiCoCrAIY that 

are available for use as bond coats. For example s t ecura (ref. 

91) has reported that FeCrAlY bond coats a re the best performers 

on iron-based superalloys. For relati vely l ow temperature 

operation (even in oxidizing conditions such as encountered in 

diesel engine operation) a wide number of bond coa ts may be 

considered. These may include for example s tainless steel or any 

of various mechanically alloyed bond c oat materials such as NiAI 

24 
which is so adherent even to a relatively smooth substrates that 

it is called "self-bonding". 

IV.D. other zirconia-based TBCs 

other zirconia-based thermal barrier coatings that have been 

reported in the recent literature are zirconia-ceria (refs. 82, 

92-94), zirconia-yttria-ceria (ref. 95), and zirconia-ytterbia 

(ref. 96). The ceria-based ceramics are believed to be more 

resistant to hot corrosion than zirconia-yttria (refs. 92 and 

93). The levels of ceria in those ceramics tends to be greater 

than 15% (by weight), i.e., 20 molt -- which is the amount of 

ceria needed to place the material in the tetragonal plus cubic 

region of the equilibrium zirconia-ceria phase diagram (ref. 97). 

For somewhat lower levels of yttria there is a discrepancy 

between the phase diagram and recent experimental observations. 

That is, according to the phase diagram only the tetragonal phase 

should be stable between about 10 and 14% (by weight), i.e. 14 

to 18 molt. However, plasma sprayed ZrOz_l0%CeOz contains 8% of 

the M-ZrOz phase while ZrOz-15%CeOz contains 23% M-Zroz Furthermore both of these compositions exhibit transformation 

toughening (ref. 82). Those observations and other fracture 

toughness measurements on monolithic zirconia-ceria (ref. 98) 

indicate that M-ZrOz is the room temperature equilibrium phase 

even in the 14 to 18% ceria range. Also, it should be noted that 

the behavior of plasma sprayed ZrOz-(10-15%)CeOz appears to be 

similar to the behavior discussed earlier for plasma sprayed 

ZrOz-4%YZ03 -- a material which is in or near the Y-TZP range. 

Zirconia-ytterbia thermal barrier coatings have performed well in 

furnace tests. The optimum composition of those tested was 12% 

(by weight), i.e. 4 molt (ref. 96). This composition appears to 

be in the same position relative to the equilibrium phase diagram 

(ref. 56) as zirconia-yttria in the 6 to 8% range. 

IV.F. Bonding of zirconia to MCrAIY or alumina 

Plasma sprayed zirconia adheres to the underlying bond coat with 

a strength that is comparable to the tensile strength of the 

plasma sprayed zirconia. This value has been variously measured 

to be in the 6-36 MPa (1000-5000 psi) range (refs. 70, 99-101). 

The detailed mechanism of this adherence and the detailed effect 

of MCrAIY oxidation on adherence are not well understood. 

Inspection by transmission electron microscopy of the interface 

between intragranular alumina and zirconia particles in zirconia 

toughened alumina has revealed certain aspects of the zirconia to 

alumina bond (refs. 102 and 103) That is, the interface appeared 

25 

- ~ --.----. 

which is so adherent even to a relatively smooth substrates that 

it is called "self-bonding". 

IV.D. other zirconia-based TBCs 

other zirconia-based thermal barrier coatings that have been 

reported in the recent literature are zirconia-ceria (refs. 82, 

92-94), zirconia-yttria-ceria (ref. 95), and zirconia-ytterbia 

(ref. 96). The ceria-based ceramics are believed to be more 

resistant to hot corrosion than zirconia-yttria (refs. 92 and 

93). The levels of ceria in those ceramics tends to be greater 

than 15% (by weight), i.e., 20 molt -- which is the amount of 

ceria needed to place the material in the tetragonal plus cubic 

region of the equilibrium zirconia-ceria phase diagram (ref. 97). 

For somewhat lower levels of yttria there is a discrepancy 

between the phase diagram and recent experimental observations. 

That is, according to the phase diagram only the tetragonal phase 

should be stable between about 10 and 14% (by weight), i.e. 14 

to 18 molt. However, plasma sprayed ZrOz_l0%CeOz contains 8% of 

the M-ZrOz phase while ZrOz-15%CeOz contains 23% M-Zroz Furthermore both of these compositions exhibit transformation 

toughening (ref. 82). Those observations and other fracture 

toughness measurements on monolithic zirconia-ceria (ref. 98) 

indicate that M-ZrOz is the room temperature equilibrium phase 

even in the 14 to 18% ceria range. Also, it should be noted that 

the behavior of plasma sprayed ZrOz-(10-15%)CeOz appears to be 

similar to the behavior discussed earlier for plasma sprayed 

ZrOz-4%YZ03 -- a material which is in or near the Y-TZP range. 

Zirconia-ytterbia thermal barrier coatings have performed well in 

furnace tests. The optimum composition of those tested was 12% 

(by weight), i.e. 4 molt (ref. 96). This composition appears to 

be in the same position relative to the equilibrium phase diagram 

(ref. 56) as zirconia-yttria in the 6 to 8% range. 

IV.F. Bonding of zirconia to MCrAIY or alumina 

Plasma sprayed zirconia adheres to the underlying bond coat with 

a strength that is comparable to the tensile strength of the 

plasma sprayed zirconia. This value has been variously measured 

to be in the 6-36 MPa (1000-5000 psi) range (refs. 70, 99-101). 

The detailed mechanism of this adherence and the detailed effect 

of MCrAIY oxidation on adherence are not well understood. 

Inspection by transmission electron microscopy of the interface 

between intragranular alumina and zirconia particles in zirconia 

toughened alumina has revealed certain aspects of the zirconia to 

alumina bond (refs. 102 and 103) That is, the interface appeared 

25 

- ~ --.----. 
to be sharp with no discernible interaction zone, althou gh in 

some locations facet e d particles were noted at the inte r face 

indicating a low energy interface. In most locations t h e 

particles were spherical with ledge-like or misfit 

dislocation-like structures. Molecular orbital calculations such 

as those reported for alumina-metal interfaces (refs . 33 and 34) 

have not yet been report e d for zirconia-alumina or 

zirconia-MCrAIY interfaces. 

IV.G. Brief overview o f the mechanical behavior of 

zirconia-yttria 

Plasma sprayed z i rconia-yttria is a material with very 

int eresting mechanical properties (refs. 81, 88, and 89). The 

str ess-strain curve a s me asured in uniaxial tension i s nonlinear 

wit h the modulus d ecre asing with increasing load. The compressive 

str ength is more t han a n order of magnitude greater than the 

tensile strength . s train to failure in compression as large as 

-4 % have been measured at 1200 oC. Creep like behavior is observed 

at temperatures below half of the melting point (refs. 88 and 

104 ) . It is possible that the very high strain tolerance and and 

c r e ep observed at higher temperature are a function of splat 

s liding wh i c h may in turn be controlled by low melting phases at 

the splat boundaries. This low temperature phase may involve the 

sil ica impurity in zirconia (ref. 83). As noted earlier, silica 

impurities may a lso strongly affect sintering rates. 

Pa r t V. Pri oritized List of Fundamental Materials Resea r ch Needs 

The l i s t o f a rea s where our understanding of the fundamentals of 

the behavior of TTBCs in diesel engines is formidable. (For gas 

turbine engines t h e list is even greater since temperatures are 

higher.) Due to competitive, time, and financial pressures the 

i mmediate interests of diesel engine companies tends towards the 

more applied problems. Similar pressures are encountered at gas 

turbine compani e s, coating companies, as well as researc h 

i ns titutions. As a result, most fundamental needs rema i n u nmet. 

While the HDTT a nd Army funded programs help to meet the more 

pressing applied n e e ds of the diesel engine companies, t here are 

numerous areas where fundamental research would complime nt and 

potentially greatly benefit the more applied research programs. 

I t is instructive to review the immediate technical barr i ers 

i dentif ied by the HDTT program. These include: 

1) improvi ng des ign tools, 

2) improving understanding of failure mechanisms, 

3) improving ma terials characterization approaches , 

26 

to be sharp with no discernible interaction zone, althou gh in 

some locations facet e d particles were noted at the inte r face 

indicating a low energy interface. In most locations t h e 

particles were spherical with ledge-like or misfit 

dislocation-like structures. Molecular orbital calculations such 

as those reported for alumina-metal interfaces (refs . 33 and 34) 

have not yet been report e d for zirconia-alumina or 

zirconia-MCrAIY interfaces. 

IV.G. Brief overview o f the mechanical behavior of 

zirconia-yttria 

Plasma sprayed z i rconia-yttria is a material with very 

int eresting mechanical properties (refs. 81, 88, and 89). The 

str ess-strain curve a s me asured in uniaxial tension i s nonlinear 

wit h the modulus d ecre asing with increasing load. The compressive 

str ength is more t han a n order of magnitude greater than the 

tensile strength . s train to failure in compression as large as 

-4 % have been measured at 1200 oC. Creep like behavior is observed 

at temperatures below half of the melting point (refs. 88 and 

104 ) . It is possible that the very high strain tolerance and and 

c r e ep observed at higher temperature are a function of splat 

s liding wh i c h may in turn be controlled by low melting phases at 

the splat boundaries. This low temperature phase may involve the 

sil ica impurity in zirconia (ref. 83). As noted earlier, silica 

impurities may a lso strongly affect sintering rates. 

Pa r t V. Pri oritized List of Fundamental Materials Resea r ch Needs 

The l i s t o f a rea s where our understanding of the fundamentals of 

the behavior of TTBCs in diesel engines is formidable. (For gas 

turbine engines t h e list is even greater since temperatures are 

higher.) Due to competitive, time, and financial pressures the 

i mmediate interests of diesel engine companies tends towards the 

more applied problems. Similar pressures are encountered at gas 

turbine compani e s, coating companies, as well as researc h 

i ns titutions. As a result, most fundamental needs rema i n u nmet. 

While the HDTT a nd Army funded programs help to meet the more 

pressing applied n e e ds of the diesel engine companies, t here are 

numerous areas where fundamental research would complime nt and 

potentially greatly benefit the more applied research programs. 

I t is instructive to review the immediate technical barr i ers 

i dentif ied by the HDTT program. These include: 

1) improvi ng des ign tools, 

2) improving understanding of failure mechanisms, 

3) improving ma terials characterization approaches , 

26 
4) investigating variations in properties across the piston 

head, 

5) developing the capability for near-net-shape plasma 

spraying, 

6) developing seal coatings, and 

7) developing one-sided NDE techniques for inspection of 

coated components. 

Most of these items will be addressed in future HDTT programs. 

Several of the items in the above list have fundamental aspects 

-- especially those items involving failure mechanisms and 

properties characterization. Thus, the remainder of Part IV will 

be devoted to identifying those areas where fundamental research 

would complement the ongoing applied research and which would 

provide the insights needed to help properly pursue future 

developments in this area. Recommendations will be divided into 

two broad classes. Those with near-term relevance to diesel 

engine TTBC development will be referred to as 

"Fundamental/Problem-Solving Recommendations." Those with perhaps 

more far-reaching but less near-term relevance will be referred 

to as "Fundamental/Basic Recommendations." 

V.A Fundamental/Problem-Solving Recommendations 

Recommendation AI. Development of Bench Screening Tests which 

Duplicate the Thermal Gradients and Cyclic Stresses 

Encountered in the Engine 

Only a few types of coating systems have been evaluated in the 

diesel engine under the current design-oriented programs. There 

is a need to develop the capability to inexpensively evaluate a 

wider range of options, to develop statistically significant life 

data, and to conduct critical experiments designed to elucidate 

failure mechanisms (to be discussed in Recommendation II). The 

aircraft industry relies heavily on relatively simple laboratory 

burner rigs for screening, life data collection, and mechanistic 

studies. These rigs duplicate the high temperature exposure and 

the cyclic thermal expansion mismatch stresses experienced in the 

engine. They do not fully duplicate the severity of the heating 

transients but they are believed to be secondary in importance. 

As a result these rigs are known to rank coating performance in 

the same order as in the engine. In the author's opinion. the 

diesel industry needs a comparable rig to allow low cost 

simUlation of the diesel engine cyclic environment. Current 

Caterpillar/ Solar and UTRC rigs have been used to a limited 

extent to screen coatings. These rigs are useful for low cycle 

thermal fatigue testing and they would probably work very well 

for screening out the worst of any new coating concepts. However, 

they do not duplicate the high cycle fatigue component of the 

27 

4) investigating variations in properties across the piston 

head, 

5) developing the capability for near-net-shape plasma 

spraying, 

6) developing seal coatings, and 

7) developing one-sided NDE techniques for inspection of 

coated components. 

Most of these items will be addressed in future HDTT programs. 

Several of the items in the above list have fundamental aspects 

-- especially those items involving failure mechanisms and 

properties characterization. Thus, the remainder of Part IV will 

be devoted to identifying those areas where fundamental research 

would complement the ongoing applied research and which would 

provide the insights needed to help properly pursue future 

developments in this area. Recommendations will be divided into 

two broad classes. Those with near-term relevance to diesel 

engine TTBC development will be referred to as 

"Fundamental/Problem-Solving Recommendations." Those with perhaps 

more far-reaching but less near-term relevance will be referred 

to as "Fundamental/Basic Recommendations." 

V.A Fundamental/Problem-Solving Recommendations 

Recommendation AI. Development of Bench Screening Tests which 

Duplicate the Thermal Gradients and Cyclic Stresses 

Encountered in the Engine 

Only a few types of coating systems have been evaluated in the 

diesel engine under the current design-oriented programs. There 

is a need to develop the capability to inexpensively evaluate a 

wider range of options, to develop statistically significant life 

data, and to conduct critical experiments designed to elucidate 

failure mechanisms (to be discussed in Recommendation II). The 

aircraft industry relies heavily on relatively simple laboratory 

burner rigs for screening, life data collection, and mechanistic 

studies. These rigs duplicate the high temperature exposure and 

the cyclic thermal expansion mismatch stresses experienced in the 

engine. They do not fully duplicate the severity of the heating 

transients but they are believed to be secondary in importance. 

As a result these rigs are known to rank coating performance in 

the same order as in the engine. In the author's opinion. the 

diesel industry needs a comparable rig to allow low cost 

simUlation of the diesel engine cyclic environment. Current 

Caterpillar/ Solar and UTRC rigs have been used to a limited 

extent to screen coatings. These rigs are useful for low cycle 

thermal fatigue testing and they would probably work very well 

for screening out the worst of any new coating concepts. However, 

they do not duplicate the high cycle fatigue component of the 

27 
diesel engine cycle. If the high cycle fatigue component proves 

to be important these existing rigs may not be adequate to 

accomplish the goals outlined above. 

Therefore the first recommendation is that an effort be initiated 

to design and construct a laboratory test rig capable of 

reproducing the cyclic temperature swings and thermal stresses 

encountered in the diesel engine (as depicted in figures 5 to 7). 

This rig may involve rotating carousel (or rotating disk) similar 

to the rigs mentioned above but with a higher heat flux input 

combined with front side cooling to create the HCF type of cycle. 

Alternatively, the rig could involve intermittent combustion such 

as occurs in a D-gun (ref. 105) or a pulsejet combustor (ref. 

106), or it could involve a mechanical HCF load superimposed on 

thermal LCF loads. Once completed, this rig could be used to 

evaluate a wide variety of coatings. Items to investigate 

include variations in processing parameters and properties, 

variations in starting powders, low cost bond coats, the effect 

of yttria level, lot-to-Iot variability, and the effect of 

sealing. Specimens which perform well in this rig could then be 

engine tested under the more applied programs. The rig could also 

be used for basic mechanism studies, as will be discussed in the 

next recommendation. 

It should be mentioned that rig tests are being recommended 

primarily so that coating designers would have the opportunity to 

evaluate many more coating systems then can be accommodated by 

engine testing. However, rig testing would only supplement and 

not replace engine testing. The performance of coatings in the 

engine still must be the final arbitrator for evaluating new 

materials and new designs. Also, the current trend of attempting 

to minimize the need for the trial and error "Edisonian" approach 

by relying more heavily on analytical design should continue. 

still, the experience of the gas turbine industry has 

demonstrated the value of screening a wide variety of coating 

systems. This experience has shown that stress analysis cannot be 

used reliably to predict the behavior of new coating systems and 

that rigs may be used to elucidate failure mechanisms and to 

collect life data that can be fed into life prediction models. 

Recommendation A2. Investigations of the Conditions Leading to 

LCF and HCF coating System Failures 

There is no general agreement as to the relative importance of 

failure modes due to low cycle fatigue (LCF) and high cycle 

fatigue (HCF). with the LCF mode, the relative importance of 

frequent no load/full load transients versus fewer but more 

severe stop/start transients needs to be quantified. with the HCF 

mode, the relative importance of thermal stress (due to surface 

28 

diesel engine cycle. If the high cycle fatigue component proves 

to be important these existing rigs may not be adequate to 

accomplish the goals outlined above. 

Therefore the first recommendation is that an effort be initiated 

to design and construct a laboratory test rig capable of 

reproducing the cyclic temperature swings and thermal stresses 

encountered in the diesel engine (as depicted in figures 5 to 7). 

This rig may involve rotating carousel (or rotating disk) similar 

to the rigs mentioned above but with a higher heat flux input 

combined with front side cooling to create the HCF type of cycle. 

Alternatively, the rig could involve intermittent combustion such 

as occurs in a D-gun (ref. 105) or a pulsejet combustor (ref. 

106), or it could involve a mechanical HCF load superimposed on 

thermal LCF loads. Once completed, this rig could be used to 

evaluate a wide variety of coatings. Items to investigate 

include variations in processing parameters and properties, 

variations in starting powders, low cost bond coats, the effect 

of yttria level, lot-to-Iot variability, and the effect of 

sealing. Specimens which perform well in this rig could then be 

engine tested under the more applied programs. The rig could also 

be used for basic mechanism studies, as will be discussed in the 

next recommendation. 

It should be mentioned that rig tests are being recommended 

primarily so that coating designers would have the opportunity to 

evaluate many more coating systems then can be accommodated by 

engine testing. However, rig testing would only supplement and 

not replace engine testing. The performance of coatings in the 

engine still must be the final arbitrator for evaluating new 

materials and new designs. Also, the current trend of attempting 

to minimize the need for the trial and error "Edisonian" approach 

by relying more heavily on analytical design should continue. 

still, the experience of the gas turbine industry has 

demonstrated the value of screening a wide variety of coating 

systems. This experience has shown that stress analysis cannot be 

used reliably to predict the behavior of new coating systems and 

that rigs may be used to elucidate failure mechanisms and to 

collect life data that can be fed into life prediction models. 

Recommendation A2. Investigations of the Conditions Leading to 

LCF and HCF coating System Failures 

There is no general agreement as to the relative importance of 

failure modes due to low cycle fatigue (LCF) and high cycle 

fatigue (HCF). with the LCF mode, the relative importance of 

frequent no load/full load transients versus fewer but more 

severe stop/start transients needs to be quantified. with the HCF 

mode, the relative importance of thermal stress (due to surface 

28 
temperature swings) versus bending stress (due to deflections of 

the pistons) needs to be quantified. An understanding of the 

conditions favoring failure by one mode versus failure by the 

other is needed to guide future design and to enable the 

development of a life prediction methodology. 

Therefore the second recommendation is that an effort be 

initiated to determine which factors can shift failure from the 

LCF mode to the HCF mode. This study would require a rig -- such 

as the one suggested in Recommendation Al -- that could produce 

severe LCF and HCF transients, and the use of this rig to 

independently vary the relative amplitude of the LCF and HCF 

cycles. The study should also include mechanical fatigue tests 

capable of producing HCF and possibly LCF by purely mechanical 

means. The mechanical HCF tests would help to determine the 

relative importance of the thermal and mechanical components to 

HCF failure. Mechanical tests would also be available for 

generating S-N plots, Goodman diagrams, and to search for the 

existence of an endurance limit. Materials factors to vary 

include coating layer thicknesses, substrate temperature, coating 

chemistry (especially 8% vs. 20% yttria), coating strength, 

coating pre-stress, specimen geometry (flat vs. concave vs. 

convex), sealing of the surface, and surface contamination. 

Recommendation A3. Properties Tailoring, Characterization, and 

Degradation 

The plasma spray process can be manipulated to produce a wide 

variety of microstructures. It should be possible to use this 

processing flexibility to tailor microstructures and properties 

to meet the needs of engine designers. While a detailed study to 

learn how to tailor coatings would be extremely useful, support 

for this type of work is not generally available. 

Therefore, the third recommendation is that a thorough study be 

conducted to learn how to manipulate properties by means of 

plasma spray processing variations. This study would include an 

investigation of the best ways to measure each property, a 

statistically significant number of measurements including 

several lots of powder to yield lot-to-lot variability, finite 

element sensitivity analysis to determine which properties are 

most important, and a correlation between NDE measurements and 

properties. The effect of the engine environment, cyclic stress, 

contaminants and surface treatments on properties should also be 

investigated. The value of such a study to the general thermal 

barrier coatings community would be enhanced if properties were 

also collected at temperatures above those expected in the diesel 

engine. Such information would then be of value to the gas 

turbine community. 

29 

temperature swings) versus bending stress (due to deflections of 

the pistons) needs to be quantified. An understanding of the 

conditions favoring failure by one mode versus failure by the 

other is needed to guide future design and to enable the 

development of a life prediction methodology. 

Therefore the second recommendation is that an effort be 

initiated to determine which factors can shift failure from the 

LCF mode to the HCF mode. This study would require a rig -- such 

as the one suggested in Recommendation Al -- that could produce 

severe LCF and HCF transients, and the use of this rig to 

independently vary the relative amplitude of the LCF and HCF 

cycles. The study should also include mechanical fatigue tests 

capable of producing HCF and possibly LCF by purely mechanical 

means. The mechanical HCF tests would help to determine the 

relative importance of the thermal and mechanical components to 

HCF failure. Mechanical tests would also be available for 

generating S-N plots, Goodman diagrams, and to search for the 

existence of an endurance limit. Materials factors to vary 

include coating layer thicknesses, substrate temperature, coating 

chemistry (especially 8% vs. 20% yttria), coating strength, 

coating pre-stress, specimen geometry (flat vs. concave vs. 

convex), sealing of the surface, and surface contamination. 

Recommendation A3. Properties Tailoring, Characterization, and 

Degradation 

The plasma spray process can be manipulated to produce a wide 

variety of microstructures. It should be possible to use this 

processing flexibility to tailor microstructures and properties 

to meet the needs of engine designers. While a detailed study to 

learn how to tailor coatings would be extremely useful, support 

for this type of work is not generally available. 

Therefore, the third recommendation is that a thorough study be 

conducted to learn how to manipulate properties by means of 

plasma spray processing variations. This study would include an 

investigation of the best ways to measure each property, a 

statistically significant number of measurements including 

several lots of powder to yield lot-to-lot variability, finite 

element sensitivity analysis to determine which properties are 

most important, and a correlation between NDE measurements and 

properties. The effect of the engine environment, cyclic stress, 

contaminants and surface treatments on properties should also be 

investigated. The value of such a study to the general thermal 

barrier coatings community would be enhanced if properties were 

also collected at temperatures above those expected in the diesel 

engine. Such information would then be of value to the gas 

turbine community. 

29 
Properties to consider would include deposition efficiency, 

por osity, phase distributions, thermal conductivity, and failure 

str ains, strengths, and moduli. The study should also consider 

non -linear stress strain response and property anisotropy. 

V. B Fundamental/Basic Recommendations 

Re c ommendation B1. Mapping the effects of time and temperature on 

phase transformations, microstructure, sintering, and 

s elected thermomechanical and thermophysical properties 

Therma l barrier coati ngs in diesel and other heat engines are 

sub ject t o high temperatures and high cyclic strains in oxidizing 

or c orrosi ve environments. These conditions may combine to 

d e g rade the microstructure, phase distributions, and properties 

of z irconia-yttria thermal barrier coatings. A basic 

u nderstand i ng of the kinetics (and in certain instances the 

thermody nami cs) of these time/temperature/cycle dependent 

p r ocesse s i s needed to help guide the design of future coating 

systems . Wh i le certain aspects of this topic have been reported 

previou s ly , no studies to date have thoroughly characterized all 

of the impo r tant degradative processes that can occur. 

Unfortunately , t his is a very difficult task. One major 

difficulty i s t hat there is no standard thermal barrier coating. 

Th at i s , both i ntentional and happenstance variability in 

s t arting p owders, process parameters, etc. causes coatings which 

may be nomina l ly identical to vary widely in properties and 

performance. Therefore the results of even the most thorough 

study on any one coati ng system cannot be assumed to apply to any 

other coating sys tem. Since, the cost in time and money of a 

thorough study on a l arge number of coatings would be 

prohibitive, it wou l d be necessary to compromise and limit the 

s cope of suc h a study. 

Therefore, the recommendation is to select a limited number of 

plasma sprayed ceramic coating materials and a limited number of 

spray parame t er sets (perhaps three widely differing starting 

p owders and t wo sets of parameters for each) and to conduct a 

d e tailed investigation of the effect of time and temperature on a 

wide variety o f properties. These properties may include 

qualitative f e atures of the microstructure, phase distributions 

(e specially t h e percentage of the monoclinic phase), porosity 

(sinte ring rate), thermal conductivity, strength, strain 

tolerance, mod u lus (actually secant modulus if nonlinear effects 

are t o be igno r ed . Temperatures should be extended to above 

t hose involved in diesel engine operation under the assumption 

t h at mat hemat ical relationships could be developed to allow an 

e xchange b e t ween t i me and temperature (e.g., a Larsen-Miller 

30 

____ J 

Properties to consider would include deposition efficiency, 

por osity, phase distributions, thermal conductivity, and failure 

str ains, strengths, and moduli. The study should also consider 

non -linear stress strain response and property anisotropy. 

V. B Fundamental/Basic Recommendations 

Re c ommendation B1. Mapping the effects of time and temperature on 

phase transformations, microstructure, sintering, and 

s elected thermomechanical and thermophysical properties 

Therma l barrier coati ngs in diesel and other heat engines are 

sub ject t o high temperatures and high cyclic strains in oxidizing 

or c orrosi ve environments. These conditions may combine to 

d e g rade the microstructure, phase distributions, and properties 

of z irconia-yttria thermal barrier coatings. A basic 

u nderstand i ng of the kinetics (and in certain instances the 

thermody nami cs) of these time/temperature/cycle dependent 

p r ocesse s i s needed to help guide the design of future coating 

systems . Wh i le certain aspects of this topic have been reported 

previou s ly , no studies to date have thoroughly characterized all 

of the impo r tant degradative processes that can occur. 

Unfortunately , t his is a very difficult task. One major 

difficulty i s t hat there is no standard thermal barrier coating. 

Th at i s , both i ntentional and happenstance variability in 

s t arting p owders, process parameters, etc. causes coatings which 

may be nomina l ly identical to vary widely in properties and 

performance. Therefore the results of even the most thorough 

study on any one coati ng system cannot be assumed to apply to any 

other coating sys tem. Since, the cost in time and money of a 

thorough study on a l arge number of coatings would be 

prohibitive, it wou l d be necessary to compromise and limit the 

s cope of suc h a study. 

Therefore, the recommendation is to select a limited number of 

plasma sprayed ceramic coating materials and a limited number of 

spray parame t er sets (perhaps three widely differing starting 

p owders and t wo sets of parameters for each) and to conduct a 

d e tailed investigation of the effect of time and temperature on a 

wide variety o f properties. These properties may include 

qualitative f e atures of the microstructure, phase distributions 

(e specially t h e percentage of the monoclinic phase), porosity 

(sinte ring rate), thermal conductivity, strength, strain 

tolerance, mod u lus (actually secant modulus if nonlinear effects 

are t o be igno r ed . Temperatures should be extended to above 

t hose involved in diesel engine operation under the assumption 

t h at mat hemat ical relationships could be developed to allow an 

e xchange b e t ween t i me and temperature (e.g., a Larsen-Miller 

30 

____ J 
approach). Since it would be desirable to multiply replicate all 

measurements, the amount of effort required would be great for 

even a modest study. Also, while this recommendation overlaps 

those made in Recommendation A3, that recommendation was aimed at 

providing information for the design engineer while 

Recommendation B1 is aimed at providing basic materials science 

knowledge. 

Recommendation B2. Investigation of the role of silica impurities 

on microstructure, high temperature strain tolerance, creep 

rate, sintering, and phase transformations 

As discussed in Part IV, silica impurities may play an important 

role in the microstructural development of plasma sprayed 

zirconia-yttria which in turn may affect the T'-Zr02 to M-Zr02 

plus F-ZrOzphase transformation. Also, the presence of silica at 

grain or splat boundaries could strongly affect high temperature 

properties such as sintering rates, creep rates, and strain 

tolerances (where values up to 4% have been measured at 1200 0C in 

uniaxial compression). Silica on the surface of plasma spray 

particles could also affect plasma spray deposition rates. 

Thus, it is recommended that a detailed study be conducted on the 

effect of silica impurities on the properties mentioned above and 

that emphasis be given to the measurement of thermomechanical 

properties at high temperatures, sintering rates, and any effect 

on phase transformations. Such a study should involve plasma 

sprayed ceramic materials having at least three levels of silica 

-- from very low to high -- in otherwise identical 

zirconia-yttria spray powder, and at least two different sets of 

plasma spray parameters. Also, analytical techniques such as TEM 

or XPS should be incorporated in such a study and used to 

determine the "micro-distribution" of the silica-rich phases in 

the starting powder and in the plasma sprayed ceramic material. 

Recommendation B3. Residual stress measurement 

Residual stress plays a very important role in the TBC behavior 

and techniques are needed for measurement of these stresses both 

in the as-sprayed coating systems and in coatings that have been 

subject to high temperature cyclic exposure. 

It is recommended that a variety of approaches be evaluated for 

measuring the residual stresses present in the ceramic at room 

temperature due to the constraints imposed by the substrate and 

that the results from the various techniques compared against 

each other. The study should also include a detailed 

investigation on the effect of processing conditions and high 

31 

approach). Since it would be desirable to multiply replicate all 

measurements, the amount of effort required would be great for 

even a modest study. Also, while this recommendation overlaps 

those made in Recommendation A3, that recommendation was aimed at 

providing information for the design engineer while 

Recommendation B1 is aimed at providing basic materials science 

knowledge. 

Recommendation B2. Investigation of the role of silica impurities 

on microstructure, high temperature strain tolerance, creep 

rate, sintering, and phase transformations 

As discussed in Part IV, silica impurities may play an important 

role in the microstructural development of plasma sprayed 

zirconia-yttria which in turn may affect the T'-Zr02 to M-Zr02 

plus F-ZrOzphase transformation. Also, the presence of silica at 

grain or splat boundaries could strongly affect high temperature 

properties such as sintering rates, creep rates, and strain 

tolerances (where values up to 4% have been measured at 1200 0C in 

uniaxial compression). Silica on the surface of plasma spray 

particles could also affect plasma spray deposition rates. 

Thus, it is recommended that a detailed study be conducted on the 

effect of silica impurities on the properties mentioned above and 

that emphasis be given to the measurement of thermomechanical 

properties at high temperatures, sintering rates, and any effect 

on phase transformations. Such a study should involve plasma 

sprayed ceramic materials having at least three levels of silica 

-- from very low to high -- in otherwise identical 

zirconia-yttria spray powder, and at least two different sets of 

plasma spray parameters. Also, analytical techniques such as TEM 

or XPS should be incorporated in such a study and used to 

determine the "micro-distribution" of the silica-rich phases in 

the starting powder and in the plasma sprayed ceramic material. 

Recommendation B3. Residual stress measurement 

Residual stress plays a very important role in the TBC behavior 

and techniques are needed for measurement of these stresses both 

in the as-sprayed coating systems and in coatings that have been 

subject to high temperature cyclic exposure. 

It is recommended that a variety of approaches be evaluated for 

measuring the residual stresses present in the ceramic at room 

temperature due to the constraints imposed by the substrate and 

that the results from the various techniques compared against 

each other. The study should also include a detailed 

investigation on the effect of processing conditions and high 

31 
temperature exposure on residual stress state, and it should 

include finite element modeling of residual stress. Processing 

properties to vary could include substrate temperature, ceramic 

temperature, thic~ness per pass, and traverse speed. Measurement 

of the microstressses present in the ceramic splats as a result 

of the plasma spraying should also be considered. 

Recommendation B4. Basic molecular orbital theory analysis of 

zirconia to NiCrAlY and alumina bonding 

Very little is known about the detailed mechanism of bonding of 

plasma sprayed zirconia to the underlying metallic bond coat and 

of the bonding of zirconia to the layer of alumina or other 

oxides which may form on the surface of the NiCrAIY. Such 

knowledge would have low short term value and has therefore never 

received support even though the cost of such a study should be 

modest. However, there are potential long term benefits since 

understanding in this area would help to influence future coating 

system concepts. 

Therefore the fourth recommendation is that molecular orbital 

theory be employed to investigate the detailed mechanisms of the 

bonding of zirconia to both the oxidized and unoxidized bond coat 

be supported. 

Concluding Remarks 

Fundamental research into thermal barrier coatings has 

historically received only modest support. Now that thermal 

barrier coatings are becoming accepted by designers, the gaps in 

our knowledge are threatening to impede further development. 

Many opportunities for fundamental research in thick thermal 

barrier coatings for diesel engines have been identified and 

cataloged in Part III of this paper. Most of these areas have 

direct and near term relevance to the ongoing TTBC development 

programs which are discussed in Part II. A brief discussion of 

the factors which have lead to current state-of-the-art thin 

thermal barrier coatings for aircraft applications and certain 

aspects of the materials science of these coatings are discussed 

in Part IV. Recommendations for future fundamental research are 

given in Part V. The three recommendations in Part V.A were 

selected for their relatively near term relevance to problems 

encountered while developing coatings for diesel engine 

applications. The recommendations in Part V.B are more basic and 

are believed to have significant long term value. Both sections 

include topics that have in general been neglected in the past. 

Additional materials based research into these topics, into other 

32 

temperature exposure on residual stress state, and it should 

include finite element modeling of residual stress. Processing 

properties to vary could include substrate temperature, ceramic 

temperature, thic~ness per pass, and traverse speed. Measurement 

of the microstressses present in the ceramic splats as a result 

of the plasma spraying should also be considered. 

Recommendation B4. Basic molecular orbital theory analysis of 

zirconia to NiCrAlY and alumina bonding 

Very little is known about the detailed mechanism of bonding of 

plasma sprayed zirconia to the underlying metallic bond coat and 

of the bonding of zirconia to the layer of alumina or other 

oxides which may form on the surface of the NiCrAIY. Such 

knowledge would have low short term value and has therefore never 

received support even though the cost of such a study should be 

modest. However, there are potential long term benefits since 

understanding in this area would help to influence future coating 

system concepts. 

Therefore the fourth recommendation is that molecular orbital 

theory be employed to investigate the detailed mechanisms of the 

bonding of zirconia to both the oxidized and unoxidized bond coat 

be supported. 

Concluding Remarks 

Fundamental research into thermal barrier coatings has 

historically received only modest support. Now that thermal 

barrier coatings are becoming accepted by designers, the gaps in 

our knowledge are threatening to impede further development. 

Many opportunities for fundamental research in thick thermal 

barrier coatings for diesel engines have been identified and 

cataloged in Part III of this paper. Most of these areas have 

direct and near term relevance to the ongoing TTBC development 

programs which are discussed in Part II. A brief discussion of 

the factors which have lead to current state-of-the-art thin 

thermal barrier coatings for aircraft applications and certain 

aspects of the materials science of these coatings are discussed 

in Part IV. Recommendations for future fundamental research are 

given in Part V. The three recommendations in Part V.A were 

selected for their relatively near term relevance to problems 

encountered while developing coatings for diesel engine 

applications. The recommendations in Part V.B are more basic and 

are believed to have significant long term value. Both sections 

include topics that have in general been neglected in the past. 

Additional materials based research into these topics, into other 

32 
,

--~-- 

--_._--- -----.

topics from Part III, and into still others that the reader may 

wish to include will begin to fill many of the gaps in our 

knowledge. This research will also ensure that future decisions 

regarding the viability of thick thermal barrier coatings for 

diesel engines are made from an adequate experience base. 

33 

,--~-- 

--_._--- -----.

topics from Part III, and into still others that the reader may 

wish to include will begin to fill many of the gaps in our 

knowledge. This research will also ensure that future decisions 

regarding the viability of thick thermal barrier coatings for 

diesel engines are made from an adequate experience base. 

33 
APPENDIX I. AREAS WHERE 'l'HE TECHNOLOGY OF THICK TBCs FOR DIESEL 

ENGINES MAY BENEFIT FROM FUNDAMENTAL RESEARCH 

A. FUNDAMENTAL KNOWLEDGE NEEDED TO OPTIMIZED ZIRCONIA TTBCs 

1. EFFECT OF POWDER PREPARATION METHOD, CHEMISTRY, 

MANUFACTURER, AND LOT ON PROPERTIES AND ON PERFORMANCE 

a. TYPES OF POWDERS INCLUDE FUSED AND CRUSHED, SINTERED, 

SOL GEL, SPRAY DRIED, AND HOSP 

2. EFFECT OF PLASMA SPRAY PROCESSING, MICROSTRUCTURE, AND 

PHASE DISTRIBUTION ON THERMOPHYSICAL AND THERMOMECHANICAL 

PROPERTIES AND ON PERFORMANCE 

3. OPTIMUM ZIRCONIA COMPOSITION FOR THICK TBCs 

a. CANDIDATES INCLUDE Zr02 - 8%Y203, Zr02 - 20%Y203, 

Zr02-CeO-Y203 AND, zr02-MgO 

b . OTHERS? 

4. EFFECT OF SCALING UP FROM COUPONS TO COMPONENTS ON 

PROPERTIES AND PERFORMANCE 

B. EFFECT OF ENVIRONMENT AND STRESS STATE ON PROPERTIES AND 

PERFORMANCE 

1 . EFFECTS OF CYCLING 

a. TRANSIENTS ENCOUNTERED IN THE ENGINE 

i. STOP/START TRANSIENTS 

ii. NO LOAD/FULL LOAD TRANSIENTS 

iii. IGNITION TRANSIENTS 

b. FATIGUE 

i. LOW CYCLE VS . HIGH CYCLE 

ii. THERMAL VS. MECHANICAL 

iii. POSSIBLE ENDURANCE LIMIT 

c. FACTORS WHICH MAY EFFECT THE RESPONSE TO TRANSIENTS 

i. EDGES AND CORNERS 

ii. HOT STREAKS 

d. CRACK GROWTH 

i. MUDFLAT CRACKING 

ii. MICROCRACK LINK UP 

2. DEPOSIT RELATED EFFECTS 

a. POSSIBLE STABILIZATION OR DESTABILIZATION FROM DEPOSITS 

b. MECHANICAL DAMAGE FROM DEPOSITS AT LOAD POINTS 

c. EFFECT ON HCF 

d. EFFECT OF EMISIVITY CHANGES ON HEAT TRANSFER 

3 . TIME AT TEMPERATURE RELATED EFFECTS 

a . OXIDATION 

b . PHASE TRANSFORMATIONS 

c . SINTERING 

d. CERAMIC CREEP 

34 

APPENDIX I. AREAS WHERE 'l'HE TECHNOLOGY OF THICK TBCs FOR DIESEL 

ENGINES MAY BENEFIT FROM FUNDAMENTAL RESEARCH 

A. FUNDAMENTAL KNOWLEDGE NEEDED TO OPTIMIZED ZIRCONIA TTBCs 

1. EFFECT OF POWDER PREPARATION METHOD, CHEMISTRY, 

MANUFACTURER, AND LOT ON PROPERTIES AND ON PERFORMANCE 

a. TYPES OF POWDERS INCLUDE FUSED AND CRUSHED, SINTERED, 

SOL GEL, SPRAY DRIED, AND HOSP 

2. EFFECT OF PLASMA SPRAY PROCESSING, MICROSTRUCTURE, AND 

PHASE DISTRIBUTION ON THERMOPHYSICAL AND THERMOMECHANICAL 

PROPERTIES AND ON PERFORMANCE 

3. OPTIMUM ZIRCONIA COMPOSITION FOR THICK TBCs 

a. CANDIDATES INCLUDE Zr02 - 8%Y203, Zr02 - 20%Y203, 

Zr02-CeO-Y203 AND, zr02-MgO 

b . OTHERS? 

4. EFFECT OF SCALING UP FROM COUPONS TO COMPONENTS ON 

PROPERTIES AND PERFORMANCE 

B. EFFECT OF ENVIRONMENT AND STRESS STATE ON PROPERTIES AND 

PERFORMANCE 

1 . EFFECTS OF CYCLING 

a. TRANSIENTS ENCOUNTERED IN THE ENGINE 

i. STOP/START TRANSIENTS 

ii. NO LOAD/FULL LOAD TRANSIENTS 

iii. IGNITION TRANSIENTS 

b. FATIGUE 

i. LOW CYCLE VS . HIGH CYCLE 

ii. THERMAL VS. MECHANICAL 

iii. POSSIBLE ENDURANCE LIMIT 

c. FACTORS WHICH MAY EFFECT THE RESPONSE TO TRANSIENTS 

i. EDGES AND CORNERS 

ii. HOT STREAKS 

d. CRACK GROWTH 

i. MUDFLAT CRACKING 

ii. MICROCRACK LINK UP 

2. DEPOSIT RELATED EFFECTS 

a. POSSIBLE STABILIZATION OR DESTABILIZATION FROM DEPOSITS 

b. MECHANICAL DAMAGE FROM DEPOSITS AT LOAD POINTS 

c. EFFECT ON HCF 

d. EFFECT OF EMISIVITY CHANGES ON HEAT TRANSFER 

3 . TIME AT TEMPERATURE RELATED EFFECTS 

a . OXIDATION 

b . PHASE TRANSFORMATIONS 

c . SINTERING 

d. CERAMIC CREEP 

34 
r4. EROSION AND FOF~IGN OBJECT DAMAGE 

a. DAMAGE TO THE CERAMIC 

b. DAMAGE CAUSED BY THE CERAMIC 

C. THERMOPHYSICAL AND THERMOMECHANICAL PROPERTIES MEASUREMENT 

1. THERMOMECHANICAL PROPERTIES 

a. DEFINITION OF APPROPRIATE PROPERTIES 

i. COMPRESSIVE UTS, YIELD STRENGTH, CREEP, MODULUS 

ii. TENSILE 

iii. FATIGUE 

iv. FOUR POINT BEND STRENGTHS AND FATIGUE VS. UNIAXIAL 

v. BOND STRENGTH 

vi. FRACTURE TOUGHNESS 

vii. PROPERTY ANISOTROPY 

viii. OTHERS? 

b. BEST SPECIMEN CONFIGURATION FOR EACH MEASUREMENT 

2. THERMAL CONDUC'rIVITY MEASUREMENT 

a. BEST APPROACH 

i. FLASH DIFFUSIVITY, THERMAL WAVE, CUT BAR, RADIAL 

ii. OTHERS? 

3. NDE AND METROLOGY 

a. DENSITY, PROPERTIES AND/OR FLAW SENSITIVE TECHNIQUES 

i. ULTRASOUND, X-RAY, GAMMA RAY 

ii. OTHERS? 

b. THICKNESS MEASUREMENT 

i. SUITABLE FOR POST TEST INSPECTION 

ii. SUITABLE FOR ON-LINE NON-CONTACT 

4. RESIDUAL STRESS MEASUREMENT 

a. X-RAY 

b. STRAIN GAGE 

5. PERMEABILITY 

a. SEALING 

b. OXYGEN CONDUCTION 

D. LOWER COST TTBCs 

1. HIGH DEPOSITION EFFICIENCY SPRAYING 

2. LOWER COST ALTERNATE CERAMICS 

a. eg CaTi03, CaSi04, ZrSi04 

b. PERFORMANCE/PROPERTIES CHARACTERIZATION WOULD BE NEEDED 

3. LOWER COST AND LOWER CTE ALTERNATIVE BOND COATS 

a. STAINLESS STEEL? 

35 

r4. EROSION AND FOF~IGN OBJECT DAMAGE 

a. DAMAGE TO THE CERAMIC 

b. DAMAGE CAUSED BY THE CERAMIC 

C. THERMOPHYSICAL AND THERMOMECHANICAL PROPERTIES MEASUREMENT 

1. THERMOMECHANICAL PROPERTIES 

a. DEFINITION OF APPROPRIATE PROPERTIES 

i. COMPRESSIVE UTS, YIELD STRENGTH, CREEP, MODULUS 

ii. TENSILE 

iii. FATIGUE 

iv. FOUR POINT BEND STRENGTHS AND FATIGUE VS. UNIAXIAL 

v. BOND STRENGTH 

vi. FRACTURE TOUGHNESS 

vii. PROPERTY ANISOTROPY 

viii. OTHERS? 

b. BEST SPECIMEN CONFIGURATION FOR EACH MEASUREMENT 

2. THERMAL CONDUC'rIVITY MEASUREMENT 

a. BEST APPROACH 

i. FLASH DIFFUSIVITY, THERMAL WAVE, CUT BAR, RADIAL 

ii. OTHERS? 

3. NDE AND METROLOGY 

a. DENSITY, PROPERTIES AND/OR FLAW SENSITIVE TECHNIQUES 

i. ULTRASOUND, X-RAY, GAMMA RAY 

ii. OTHERS? 

b. THICKNESS MEASUREMENT 

i. SUITABLE FOR POST TEST INSPECTION 

ii. SUITABLE FOR ON-LINE NON-CONTACT 

4. RESIDUAL STRESS MEASUREMENT 

a. X-RAY 

b. STRAIN GAGE 

5. PERMEABILITY 

a. SEALING 

b. OXYGEN CONDUCTION 

D. LOWER COST TTBCs 

1. HIGH DEPOSITION EFFICIENCY SPRAYING 

2. LOWER COST ALTERNATE CERAMICS 

a. eg CaTi03, CaSi04, ZrSi04 

b. PERFORMANCE/PROPERTIES CHARACTERIZATION WOULD BE NEEDED 

3. LOWER COST AND LOWER CTE ALTERNATIVE BOND COATS 

a. STAINLESS STEEL? 

35 
E . ALTERNATIVE APPROACHES 

1 . ENHANCED BONDING/I ~TERFACIAL STRAIN ACCOMMODAT I ON 

a. ALTE~~ATIVES o MIXED CERAMIC/METAL INTERMEDIATE LAYERS 

i. IMPROVED CHEMICAL BONDING 

i i . INCREASED ROUGHNESS 

iii . RESIDUAL STRESS MANAGEMENT 

b. STRAIN ISOLATOR PADS 

2 . ALTERNATIVE PROCESSING 

a. ROCKIDE OR OTHER FLAME SPRAYING, PVD, SPUTTERING, 

SLURRY, SOL GEL 

b . F I BER TOUGHENING 

c . LPPS BOND COATS, SHROUDED BOND COATS 

F . DEVELOPMENT OF MEANINGFUL BENCH TESTS 

1 . S IMPLIF ICATION OF SINGLE CYLINDER ENGINE TESTS 

2 . DEVELOPMENT OF RIGS ABLE TO SIMULATE COMPONENT THERMAL 

STRESS 

36 

E . ALTERNATIVE APPROACHES 

1 . ENHANCED BONDING/I ~TERFACIAL STRAIN ACCOMMODAT I ON 

a. ALTE~~ATIVES o MIXED CERAMIC/METAL INTERMEDIATE LAYERS 

i. IMPROVED CHEMICAL BONDING 

i i . INCREASED ROUGHNESS 

iii . RESIDUAL STRESS MANAGEMENT 

b. STRAIN ISOLATOR PADS 

2 . ALTERNATIVE PROCESSING 

a. ROCKIDE OR OTHER FLAME SPRAYING, PVD, SPUTTERING, 

SLURRY, SOL GEL 

b . F I BER TOUGHENING 

c . LPPS BOND COATS, SHROUDED BOND COATS 

F . DEVELOPMENT OF MEANINGFUL BENCH TESTS 

1 . S IMPLIF ICATION OF SINGLE CYLINDER ENGINE TESTS 

2 . DEVELOPMENT OF RIGS ABLE TO SIMULATE COMPONENT THERMAL 

STRESS 

36 
REFERENCES 

1. Miller, R.A.: Current Status of Thermal Barrier Coatings 

An OVerview. Surf. Coatings Technol., vol. 30, no. 1, 

Jan. 1987, pp. 1-11. 

2. Reichenbach O.H . : Development of Advanced High Temperature 

In-Cylinder Components and Tribological Systems for Low Heat 

Rejection Diesel Engines. Automotive Technology Development 

Contractors' Coordination Meeting, 26th Proceedings, Society 

of Automotive Engineers, Warrendale, PA, 1989, pp. 77-81. 

3. Fairbanks J.W.: The Heavy Duty Transport Technology Program. 

Automotive Technology Development Contractors' Coordination 

Meeting, 26th Proceedings, Society of Automotive Engineers, 

Warrendale, PA, 1989, pp. 17-20. 

4. Wilson, D.E.: The Design of a Low Specific Fuel Consumption 

Turbocompound Engine. SAE Paper 860072, 1986. 

5. Bailey, M.M.: Comparative Evaluation of Three Alternative 

Power Cycles for Waste Heat Recovery from the Exhaust of 

Adiabatic Diesel Engines. NASA TM-86953, 1985. 

6. Bryzik, W.: U.S. Army Tank-Automotive Command Engine 

Program. Coatings for Advanced Heat Engines Workshop: Book 

of Abstracts. Department of Energy, Washington, DC, 1987, 

pp. 1-9 to 1-20. 

7. Yonushonis, T.M., et al.: Thick Thermal Barrier Coatings 

for Diesel Engines. Automotive Technology Development 

Contractors' Coordination Meeting, 25th Proceedings, Society 

of Automotive Engineers, Warrendale, PA, 1988, pp. 61-66. 

8. Wood, J.C.: Heavy Duty Transport Technology Program. 

Coatings for Advanced Heat Engines Workshop: Book of 

Abstracts . Department of Energy, Washington, DC, 1987, 

pp . 11-29 to 11-32. 

9. Holtman, R.L.; Layne, J.L.; and Schechter, B.: An Investigation of Enhanced Capability Thermal Barrier Coating Systems 

for Diesel Engine Components. NASA CR-174820, 1984. 

10. Hoag, K.L., et al.: Modeling of Thick Thermal Barrier 

Coatings, ASME Paper 88-ICE-20, 1988. 

11. Morel, T.; Wahiduzzaman, S.; and Fort, E.F.: LHR Engine 

Design Analysis Methodology -- Heat Transfer Measurement in 

an Insulated Diesel. Automotive Technology Development 

Contractors Coordination Meeting, 26th Proceedings, Society 

of Automotive Engineers, Warrendale, PA, 1989, pp. 21-26. 

37 

J 

REFERENCES 

1. Miller, R.A.: Current Status of Thermal Barrier Coatings 

An OVerview. Surf. Coatings Technol., vol. 30, no. 1, 

Jan. 1987, pp. 1-11. 

2. Reichenbach O.H . : Development of Advanced High Temperature 

In-Cylinder Components and Tribological Systems for Low Heat 

Rejection Diesel Engines. Automotive Technology Development 

Contractors' Coordination Meeting, 26th Proceedings, Society 

of Automotive Engineers, Warrendale, PA, 1989, pp. 77-81. 

3. Fairbanks J.W.: The Heavy Duty Transport Technology Program. 

Automotive Technology Development Contractors' Coordination 

Meeting, 26th Proceedings, Society of Automotive Engineers, 

Warrendale, PA, 1989, pp. 17-20. 

4. Wilson, D.E.: The Design of a Low Specific Fuel Consumption 

Turbocompound Engine. SAE Paper 860072, 1986. 

5. Bailey, M.M.: Comparative Evaluation of Three Alternative 

Power Cycles for Waste Heat Recovery from the Exhaust of 

Adiabatic Diesel Engines. NASA TM-86953, 1985. 

6. Bryzik, W.: U.S. Army Tank-Automotive Command Engine 

Program. Coatings for Advanced Heat Engines Workshop: Book 

of Abstracts. Department of Energy, Washington, DC, 1987, 

pp. 1-9 to 1-20. 

7. Yonushonis, T.M., et al.: Thick Thermal Barrier Coatings 

for Diesel Engines. Automotive Technology Development 

Contractors' Coordination Meeting, 25th Proceedings, Society 

of Automotive Engineers, Warrendale, PA, 1988, pp. 61-66. 

8. Wood, J.C.: Heavy Duty Transport Technology Program. 

Coatings for Advanced Heat Engines Workshop: Book of 

Abstracts . Department of Energy, Washington, DC, 1987, 

pp . 11-29 to 11-32. 

9. Holtman, R.L.; Layne, J.L.; and Schechter, B.: An Investigation of Enhanced Capability Thermal Barrier Coating Systems 

for Diesel Engine Components. NASA CR-174820, 1984. 

10. Hoag, K.L., et al.: Modeling of Thick Thermal Barrier 

Coatings, ASME Paper 88-ICE-20, 1988. 

11. Morel, T.; Wahiduzzaman, S.; and Fort, E.F.: LHR Engine 

Design Analysis Methodology -- Heat Transfer Measurement in 

an Insulated Diesel. Automotive Technology Development 

Contractors Coordination Meeting, 26th Proceedings, Society 

of Automotive Engineers, Warrendale, PA, 1989, pp. 21-26. 

37 

J 
12. Larson, H.J.: Thick Thermal Barrier Coatings. Automotive 

Technology Development Contractors' Coordination Meeting, 

26th Proceedings, Society of Automotive Engineers, 

Warrendale, PA, 1989, pp. 27-33. 

13. Brink, R.C.: Material Property Evaluation of Thick Thermal 

Barrier Coating Systems. J. Eng. Gas Turbines Power, 

vol. III, no. 3, July 1989, pp. 570-577. 

14. Yonushonis, T.M.: Diesel Engine Evaluation of Thermal 

Barrier Coatings. Thermal Spray Technology: New Ideas and 

Processes; Proceedings of the National Thermal Spray 

Conference, Oct. 24-27, 1988, Cincinnati, OH, D.L. Houck, 

ed., ASM International, Metals Park, OH, 1989, pp. 239-243. 

15 . Yonushonis, T.M.: Thick Thermal Barrier Coatings. 

Automotive Technology Development Contractors' Coordination 

Meeting, 26th Proceedings, Society of Automotive Engineers, 

Warrendale, PA, 1989, pp. 35-38. 

16. Coers, R., et al.: Development of an Adiabatic Engine. 

TACOM TR-13182, 1986. 

17. Woods, M.E.; and Kamo, R.: Development of Tribological and 

Advanced High-Temperature In-Cylinder Components for Advanced 

High Temperature Diesel Engines. TACOM TR-13374, 1988. 

18. Kvernes, I.; and Noerholm, 0.: Coatings for Diesel Engines 

and Associated Problems. Coatings for Advanced Heat Engines 

Workshop: Book of Abstracts, Department of Energy, 

Washington, DC, 1987, pp. 1II-73 to 111-81 . 

19. Woschni, G. ; Spindler, W.; and Kolesa, K.: Heat Insulation 

of combustion Chamber Walls -- A Measure to Decrease the 

Fuel Consumption of 1. C. Engines? SAE Paper 870339, 1987. 

20. Cheng, W.K.; and Wong, V.W.: Does Heat Transfer Increase in 

Low Heat Rejection Engines? Automotive Technology Development Contractors' Coordination Meeting, 25th Proceedings, 

Society of Automotive Engineers, Warrendale, PA, 1988, 

pp. 57-60. 

21. Miller, R A.; Smialek, J.L.; and Garlick, R.G .: Phase 

Stability in Plasma-Sprayed Partially Stabilized ZirconiaYttria. Science and Technology of Zirconia (Advances in 

Ceramics, Vol. 3), A.H. Heuer and L.W. Hobbs, eds., American 

Ceramic Society, Inc., Columbus, OH, 1981, pp. 241-253. 

22. Miller, R.A.; Garlick, R.G.; and Smialek, J.L.: Phase 

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Soc. Bull., vol. 62, no. 12, Dec. 1983, pp. 1355-1358. 

38 

12. Larson, H.J.: Thick Thermal Barrier Coatings. Automotive 

Technology Development Contractors' Coordination Meeting, 

26th Proceedings, Society of Automotive Engineers, 

Warrendale, PA, 1989, pp. 27-33. 

13. Brink, R.C.: Material Property Evaluation of Thick Thermal 

Barrier Coating Systems. J. Eng. Gas Turbines Power, 

vol. III, no. 3, July 1989, pp. 570-577. 

14. Yonushonis, T.M.: Diesel Engine Evaluation of Thermal 

Barrier Coatings. Thermal Spray Technology: New Ideas and 

Processes; Proceedings of the National Thermal Spray 

Conference, Oct. 24-27, 1988, Cincinnati, OH, D.L. Houck, 

ed., ASM International, Metals Park, OH, 1989, pp. 239-243. 

15 . Yonushonis, T.M.: Thick Thermal Barrier Coatings. 

Automotive Technology Development Contractors' Coordination 

Meeting, 26th Proceedings, Society of Automotive Engineers, 

Warrendale, PA, 1989, pp. 35-38. 

16. Coers, R., et al.: Development of an Adiabatic Engine. 

TACOM TR-13182, 1986. 

17. Woods, M.E.; and Kamo, R.: Development of Tribological and 

Advanced High-Temperature In-Cylinder Components for Advanced 

High Temperature Diesel Engines. TACOM TR-13374, 1988. 

18. Kvernes, I.; and Noerholm, 0.: Coatings for Diesel Engines 

and Associated Problems. Coatings for Advanced Heat Engines 

Workshop: Book of Abstracts, Department of Energy, 

Washington, DC, 1987, pp. 1II-73 to 111-81 . 

19. Woschni, G. ; Spindler, W.; and Kolesa, K.: Heat Insulation 

of combustion Chamber Walls -- A Measure to Decrease the 

Fuel Consumption of 1. C. Engines? SAE Paper 870339, 1987. 

20. Cheng, W.K.; and Wong, V.W.: Does Heat Transfer Increase in 

Low Heat Rejection Engines? Automotive Technology Development Contractors' Coordination Meeting, 25th Proceedings, 

Society of Automotive Engineers, Warrendale, PA, 1988, 

pp. 57-60. 

21. Miller, R A.; Smialek, J.L.; and Garlick, R.G .: Phase 

Stability in Plasma-Sprayed Partially Stabilized ZirconiaYttria. Science and Technology of Zirconia (Advances in 

Ceramics, Vol. 3), A.H. Heuer and L.W. Hobbs, eds., American 

Ceramic Society, Inc., Columbus, OH, 1981, pp. 241-253. 

22. Miller, R.A.; Garlick, R.G.; and Smialek, J.L.: Phase 

Distributions in Plasma Sprayed Zirconia-Yttria. Am. Ceram. 

Soc. Bull., vol. 62, no. 12, Dec. 1983, pp. 1355-1358. 

38 
23 . Novak, R.C.; Materese, A.P.; and Huston, R. P.: Development 

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24. Novak, R.C.; Materese, A.P.; and Huston, R.P.: Automotive 

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25. Wahiduzzaman, S.; and Morel, T.: Effect of Transluscence of 

Engineering Ceramics on Heat Transfer in Diesel Engines. 

Automotive Technology Development Contractors Coordination 

Meeting, 25th Proceedings, Society of Automotive Engineers , 

Warrendale, PA, 1988, pp. 239-245. 

26. Miller, R.A.: Life Modeling of Thermal Barrier Coatings for 

Aircraft Gas Tur bine Engines. J. Eng. Gas Turbines Power, 

vol. III, no. 2, Apr. 1989, pp. 301-305. 

27. Touloukian, Y.S., et al.: Thermophysical Properties of 

Matter., vols. I, 2, and 10. IFI/Plenum, New York, 1973 . 

28. Fetheroff, C.W.; Derkacs, T.; and Matay, I.M.: Automated 

Plasma Spray (APS) Process Feasibility Study. 

(TRW-ER-8019-F, TRW, Inc.; NASA Contract: NAS3-20112) NASA 

CR-165418, 1981 . 

29. Zatorski, R.A.; and Lee, S.G.: Manufactoring Technology for 

an In-Process Thermal Spray Coating Thickness Gauge. 

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30. Georgopoulos, P.; Cohen, J.B.; and Herman, H.: Residual 

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31 . Watkins, T.R.; Ryba, E.; and Green, D.J.: Microstrains in 

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32. Strangeman, T.S . : Abradable Strain Tolerant Ceramic Turbine 

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33 . Anderson , A.B . ; Mehandru, S.P.; and Smialek, J.L.: Dopant 

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no. 7, July 1985, pp. 1695-1701. 

39 

23 . Novak, R.C.; Materese, A.P.; and Huston, R. P.: Development 

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the National Thermal Spray Conference, Metals Park, OH, 

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24. Novak, R.C.; Materese, A.P.; and Huston, R.P.: Automotive 

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25. Wahiduzzaman, S.; and Morel, T.: Effect of Transluscence of 

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29. Zatorski, R.A.; and Lee, S.G.: Manufactoring Technology for 

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30. Georgopoulos, P.; Cohen, J.B.; and Herman, H.: Residual 

Stress Measurements of a Plasma-Sprayed Coating System Using 

Synchrotron X-Ray Radiation. Mater. Sci. Eng., vol. 80, 1986, 

pp. L4I-L43. 

31 . Watkins, T.R.; Ryba, E.; and Green, D.J.: Microstrains in 

Thermal Barrier Coatings. Presented at the 89th Annual 

Meeting of the American Ceramics Society, Cincinnati, OH, 

May 2, 1988, Paper No. 43-C-88. Submitted to the J. Am. 

Ceram. Soc . , 1990. 

32. Strangeman, T.S . : Abradable Strain Tolerant Ceramic Turbine 

Shroud. U.S. Patent 4,764,089, Aug. 16, 1988. 

33 . Anderson , A.B . ; Mehandru, S.P.; and Smialek, J.L.: Dopant 

Effect of Yttrium and the Growth and Adherence of Alumina on 

Nickel-Aluminum Alloys. J. Electrochem. Soc.; vol. 132, 

no. 7, July 1985, pp. 1695-1701. 

39 
34. Anderson, A.B.; Hong, S.H.; and Smialek, J.L.: Comparison 

of Bonding in First Transition-Metal Series: Diatomic and 

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1987, pp. 4250-4254. 

35. DeMasi-Marcin, J.T.; Sheffler, K.D.; and Bose, S.: 

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36. Chagnon, P.; and Fauchais, P.: Thermal Spraying of Ceramics. 

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37. Nicoll, A.R., et al.: Future Developments in Plasma Spray 

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38. Herman, H.: Plasma-Sprayed Coatings. Sci. Am., vol. 259, 

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39. Harrison, W.N.; Moore, D.G.; and Richmond, J.C.: Review of 

an Investigation of Ceramic Coatings for Metallic Turbine 

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40. Garrett, F.G.; and Gyorgak, C.A.: Adhesive and Protective 

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43 . Wheildon, W.M., Jr.: Oxide Coated Metal Articles with Metal 

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44 . Levy, A.V.: Ceramic Coating for Insulation. Met. Prog., 

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45 . Hjelm, L.N.; and Bornhorst, B.R.: Research-AirplaneCommittee Report on Conference on the Progress of the X-15 

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46 . Davies, H.: The Design and Development of the Thiokol XLR99 

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47 . Grisaffe, S.G.: Simplified Guide to Thermal-Spray Coatings. 

Mach. Des., vol. 39, July 20, 1967, pp. 174-181. 

40 

34. Anderson, A.B.; Hong, S.H.; and Smialek, J.L.: Comparison 

of Bonding in First Transition-Metal Series: Diatomic and 

Bulk Sulfides and Oxides, J. Phys. Chern., vol. 91, no. 16, 

1987, pp. 4250-4254. 

35. DeMasi-Marcin, J.T.; Sheffler, K.D.; and Bose, S.: 

Mechanisms of Degradation and Failure in a Plasma Deposited 

Thermal Barrier Coating. ASME Paper 89-GT-I32, June 1989. 

36. Chagnon, P.; and Fauchais, P.: Thermal Spraying of Ceramics. 

Ceram. Int., vol. 10, no. 4, Oct.-Dec. 1984, pp. 119-131. 

37. Nicoll, A.R., et al.: Future Developments in Plasma Spray 

Coating. Mater. Sci. Technol., vol. 2, Mar. 1986, 

pp. 214-219. 

38. Herman, H.: Plasma-Sprayed Coatings. Sci. Am., vol. 259, 

Sept. 1988, pp. 112-117. 

39. Harrison, W.N.; Moore, D.G.; and Richmond, J.C.: Review of 

an Investigation of Ceramic Coatings for Metallic Turbine 

Parts and Other High-Temperature Applications. NACA 

TN-1186, 1947. 

40. Garrett, F.G.; and Gyorgak, C.A.: Adhesive and Protective 

Characteristics of Ceramic Coating A-417 and Its Effect on 

Engine Life of Forged Refractaloy-26 (AMS 5760) and Cast 

Stellite (AMS 5385) Turbine Blades. NACA RM-E52130, 1953. 

41. Cannistraro, N.I.: Where Do We Stand in Ceramic Coatings? 

Met . Prog., vol. 74, no. 5, Nov. 1958, pp. 111-113. 

42 . Ault, N.N.: Characteristics of Refractory Oxide Coatings 

Produced by Flame Spraying. J. Amer. Ceram. Soc., vol. 40, 

no. 3, Mar. 1957, pp. 69-74. 

43 . Wheildon, W.M., Jr.: Oxide Coated Metal Articles with Metal 

Undercoating. U.S. Patent 3,006,782, Oct. 1961. 

44 . Levy, A.V.: Ceramic Coating for Insulation. Met. Prog., 

vol. 75, no. 3, Mar. 1959, pp. 86-89. 

45 . Hjelm, L.N.; and Bornhorst, B.R.: Research-AirplaneCommittee Report on Conference on the Progress of the X-15 

Project. NASA TM X-57072, 1961, pp. 227-253. 

46 . Davies, H.: The Design and Development of the Thiokol XLR99 

Rocket Engine for the X-I5 Aircraft. J. Royal Aeronaut. 

Soc., vol. 67, no. 626, Feb. 1963, pp. 79-91. 

47 . Grisaffe, S.G.: Simplified Guide to Thermal-Spray Coatings. 

Mach. Des., vol. 39, July 20, 1967, pp. 174-181. 

40 
48. Dennis, P.R.; Gates, O.W.; Smith, C.R.; and Bond, J.B.: 

Technology Survey -- Plasma Jet Technology. NASA SP-5033, 

1965, pp. 117-120. 

49. Ingham, H.S.; and Shepard, A.P.: Metco Flame Spray Handbook, 

vol. II, Powder Processes. Second ed., Metco Inc., Westbury, 

NY, 1964. 

50. Curren, A.N.; Grisaffe, S.G.; and Wycoff, K.C.: Hydrogen 

Plasma Tests of Some Insulating Coating Systems for the 

Nuclear Rocket Thrust Chamber. NASA TM X-2461, 1972. 

51. Grot, A.S.; and Martyn, J.K.: Behavior of Plasma-Sprayed 

Ceramic Thermal-Barrier Coating for Gas Turbine Applications. 

Amer. Ceram. Soc. Bull., vol. 60, no. 8., Aug. 1981, 

pp. 807-811. 

52. Wilkes, K.E.; and Lagedrost J.F.: Thermophysical Properties 

of Plasma Sprayed Coatings. NASA CR-121144, 1973. 

53. Humink, J., Jr.: High-Temperature Inorganic Coatings. 

Reinhold Publishing Co., New York, 1963, pp. 14-15. 

54. McPherson, R.: The Relationship Between the Mechanism of 

Formation, Microstructure and Properties of Plasma-Sprayed 

Coatings. Thin Solid Films, vol. 83, no. 3, Mar. 1981, 

pp. 297-310. 

55. Chuanian, D.; Zatorski, R.A.; and Herman, H.: Oxide Powders 

for Plasma Spraying -- The Relationship Between Powder 

Characteristics and Coating Properties. Thin Solid Films, 

vol. 118, 1984, pp. 467-475. 

56. Stubican, V.S.: Phase Equilibria and Metastabilities in the 

Systems Zr02-MgO, Zr02-CaO, and Zr02-Y203' Adv. Ceram., 

vol. 24A, 1984, pp. 71-85. 

57. Sim, M.S.; and Stubican, V.S.: Phase Relations and Ordering 

in the System Zr02-MgO. J. Am. Ceram. Soc., vol. 70, no. 7, 

July 1987, pp. 521-526. 

58. Toriz, F.C.; Thakker, A.B.; and Gupta S.K.: Thermal Barrier 

Coating for Jet Engines. ASME 88-GT-279, June 1988. 

59. Bratton, R.J.; Lau, S.K.; and Lee, S.Y.: Evaluation of 

Present Thermal Barrier Coatings for Potential Service in 

Electric Utility Gas Turbines. NASA CR-165545, 1982. 

60 . Levine, S.R; and Miller, R.A.: Thermal Barrier Coatings for 

Utility Gas Turbines. NASA TM-85349, 1982. 

61. Boch, P., et al.: Plasma-Sprayed Zirconia Coatings. Adv. 

Ceram., vol. 12, 1984, pp. 488-502. 

41 

-----------

48. Dennis, P.R.; Gates, O.W.; Smith, C.R.; and Bond, J.B.: 

Technology Survey -- Plasma Jet Technology. NASA SP-5033, 

1965, pp. 117-120. 

49. Ingham, H.S.; and Shepard, A.P.: Metco Flame Spray Handbook, 

vol. II, Powder Processes. Second ed., Metco Inc., Westbury, 

NY, 1964. 

50. Curren, A.N.; Grisaffe, S.G.; and Wycoff, K.C.: Hydrogen 

Plasma Tests of Some Insulating Coating Systems for the 

Nuclear Rocket Thrust Chamber. NASA TM X-2461, 1972. 

51. Grot, A.S.; and Martyn, J.K.: Behavior of Plasma-Sprayed 

Ceramic Thermal-Barrier Coating for Gas Turbine Applications. 

Amer. Ceram. Soc. Bull., vol. 60, no. 8., Aug. 1981, 

pp. 807-811. 

52. Wilkes, K.E.; and Lagedrost J.F.: Thermophysical Properties 

of Plasma Sprayed Coatings. NASA CR-121144, 1973. 

53. Humink, J., Jr.: High-Temperature Inorganic Coatings. 

Reinhold Publishing Co., New York, 1963, pp. 14-15. 

54. McPherson, R.: The Relationship Between the Mechanism of 

Formation, Microstructure and Properties of Plasma-Sprayed 

Coatings. Thin Solid Films, vol. 83, no. 3, Mar. 1981, 

pp. 297-310. 

55. Chuanian, D.; Zatorski, R.A.; and Herman, H.: Oxide Powders 

for Plasma Spraying -- The Relationship Between Powder 

Characteristics and Coating Properties. Thin Solid Films, 

vol. 118, 1984, pp. 467-475. 

56. Stubican, V.S.: Phase Equilibria and Metastabilities in the 

Systems Zr02-MgO, Zr02-CaO, and Zr02-Y203' Adv. Ceram., 

vol. 24A, 1984, pp. 71-85. 

57. Sim, M.S.; and Stubican, V.S.: Phase Relations and Ordering 

in the System Zr02-MgO. J. Am. Ceram. Soc., vol. 70, no. 7, 

July 1987, pp. 521-526. 

58. Toriz, F.C.; Thakker, A.B.; and Gupta S.K.: Thermal Barrier 

Coating for Jet Engines. ASME 88-GT-279, June 1988. 

59. Bratton, R.J.; Lau, S.K.; and Lee, S.Y.: Evaluation of 

Present Thermal Barrier Coatings for Potential Service in 

Electric Utility Gas Turbines. NASA CR-165545, 1982. 

60 . Levine, S.R; and Miller, R.A.: Thermal Barrier Coatings for 

Utility Gas Turbines. NASA TM-85349, 1982. 

61. Boch, P., et al.: Plasma-Sprayed Zirconia Coatings. Adv. 

Ceram., vol. 12, 1984, pp. 488-502. 

41 

-----------
62. Jacobson, N.S.: Thermodynamic Properties of Some Metal 

Oxide-Zirconia .:>ystems. NASA TM-102351, 1989. 

63. Stecura, S.: Two-Layer Thermal Barrier Coating for Turbine 

Airfoils -- Furnace and Burner Rig Test Results. NASA 

TM-X-3425, 1976. 

64 . Stecura, S; and Liebert, G.H.: Thermal Barrier Coating 

System. U.S. Patent 4,055,705, Oct. 1977. 

65 . Stecura, S.: Two-Layer Thermal Barrier Coating for High 

Temperature Components. Am. Ceram. Soc. Bull., vol. 56, 

no. 12, Dec. 1977, pp. 1082-1085. 

66. Liebert, C. H. , et al.: Durability 6f Zirconia ThermalBarrier Ceramic Coatings on Air-Cooled Turbine Blades in 

Cyclic Jet Engine Operation. NASA TM-X-3410, 1976. 

67. Goward, G.W.; Grey, D.A.; and Rrutenat, R.C.: Thermal 

Barrier Coating for Nickel and Cobalt Base Super Alloys. 

U.S. Patent 4,248,940, Feb. 1981. 

68. Tucker, R.C.; Taylor, T.A.; and Weatherly, M.H.: Plasma 

Deposited NiCrAlY Airfoil and Zirconia/NiCrAlY Thermal 

Barrier Coatings. Presented at the Third Conference on Gas 

Turbine Materials in a Marine Environment, Bath University, 

Bath, England, Sept. 20-23, 1976, Session VII, Paper 2. 

69. Stecura, S. : Effects of Compositional Changes on the 

Performance of a Thermal Barrier Coating System. NASA 

TM-78976, 1978. 

70. Siemers, P.A.; and Hillig, W.B.: Thermal-Barrier-Coated 

Turbine Blade Study. NASA CR-165351, 1981. 

71. Anderson, N.P.; and Sheffler, R.D.: Development of Strain 

Tolerant Thermal Barrier Coating Systems, Tasks I-III . 

(PWA-5777-29 Pratt & Whitney Aircraft; NASA Contract: 

NAS3-22548) NASA CR-168251, 1983. 

72. Duderstadt, E.C.; and Agarwal P.: Energy Efficient Engine, 

High Pressure Turbine Thermal Barrier Coating Support 

Technology Report. (R82AEB293 General Electric Co., Aircraft 

Engine Business Group; NASA Contract: NAS3-20643) NASA 

CR-168037, 1983. 

73 . Shankar, N.R.; Berndt, C.C.; and Herman, H: 

Acoustic Emission Response of .Plasma-Sprayed 

Y202 Coatings. Ceram. Eng. Sci. Proc., vol. 

1982, pp. 772-792. 

Failure and 

Zr02-8 wt % 

3, no. 9-10, 

74 . Lanteri, V.; Heuer, A .H.; and Mitchell, T.E.: Tetragonal 

Phase in the System Zr02-Y203' Adv. Ceram., vol. 12, 1984, 

pp. 118 -130 . 

42 

_________ 1 

62. Jacobson, N.S.: Thermodynamic Properties of Some Metal 

Oxide-Zirconia .:>ystems. NASA TM-102351, 1989. 

63. Stecura, S.: Two-Layer Thermal Barrier Coating for Turbine 

Airfoils -- Furnace and Burner Rig Test Results. NASA 

TM-X-3425, 1976. 

64 . Stecura, S; and Liebert, G.H.: Thermal Barrier Coating 

System. U.S. Patent 4,055,705, Oct. 1977. 

65 . Stecura, S.: Two-Layer Thermal Barrier Coating for High 

Temperature Components. Am. Ceram. Soc. Bull., vol. 56, 

no. 12, Dec. 1977, pp. 1082-1085. 

66. Liebert, C. H. , et al.: Durability 6f Zirconia ThermalBarrier Ceramic Coatings on Air-Cooled Turbine Blades in 

Cyclic Jet Engine Operation. NASA TM-X-3410, 1976. 

67. Goward, G.W.; Grey, D.A.; and Rrutenat, R.C.: Thermal 

Barrier Coating for Nickel and Cobalt Base Super Alloys. 

U.S. Patent 4,248,940, Feb. 1981. 

68. Tucker, R.C.; Taylor, T.A.; and Weatherly, M.H.: Plasma 

Deposited NiCrAlY Airfoil and Zirconia/NiCrAlY Thermal 

Barrier Coatings. Presented at the Third Conference on Gas 

Turbine Materials in a Marine Environment, Bath University, 

Bath, England, Sept. 20-23, 1976, Session VII, Paper 2. 

69. Stecura, S. : Effects of Compositional Changes on the 

Performance of a Thermal Barrier Coating System. NASA 

TM-78976, 1978. 

70. Siemers, P.A.; and Hillig, W.B.: Thermal-Barrier-Coated 

Turbine Blade Study. NASA CR-165351, 1981. 

71. Anderson, N.P.; and Sheffler, R.D.: Development of Strain 

Tolerant Thermal Barrier Coating Systems, Tasks I-III . 

(PWA-5777-29 Pratt & Whitney Aircraft; NASA Contract: 

NAS3-22548) NASA CR-168251, 1983. 

72. Duderstadt, E.C.; and Agarwal P.: Energy Efficient Engine, 

High Pressure Turbine Thermal Barrier Coating Support 

Technology Report. (R82AEB293 General Electric Co., Aircraft 

Engine Business Group; NASA Contract: NAS3-20643) NASA 

CR-168037, 1983. 

73 . Shankar, N.R.; Berndt, C.C.; and Herman, H: 

Acoustic Emission Response of .Plasma-Sprayed 

Y202 Coatings. Ceram. Eng. Sci. Proc., vol. 

1982, pp. 772-792. 

Failure and 

Zr02-8 wt % 

3, no. 9-10, 

74 . Lanteri, V.; Heuer, A .H.; and Mitchell, T.E.: Tetragonal 

Phase in the System Zr02-Y203' Adv. Ceram., vol. 12, 1984, 

pp. 118 -130 . 

42 

_________ 1 
75. Keller, R.J.: Research and Development for Improved Thermal 

Barrier Coating . AFWAL-TR-84-4008, 1984. 

76. Scott H.G.: Phase Relationships in the Zirconia-Yttria 

System. J. Mater. Sci., vol. 10, no. 9, Sept. 1975, 

pp. 1527-1535. 

77. Heuer, A.H.; Chaim, R.; and Lanteri, V.: Review Article: 

Phase Transformations and Microstructural Characterization 

of Alloys in the System Y203-Zr02' Adv. Ceram., vol. 24a, 

1988, pp. 3-20. 

78. Claussen, N: Microstructural Design of Zirconia-Toughened 

Ceramics (ZTC). Adv. Ceram., vol. 12, 1984, pp. 325-351. 

79. Ingel, R.P., et al.: Physical, Microstructural, and Thermomechanical Properties of Zr02 Single Crystals. Adv. Ceram., 

vol. 12, 1984, pp. 408-414. 

80. Lange, F.F.: Transformation Toughening, Part 3, Experimental 

Observations in the Zr02-Y203 System. J. Mater. Sci., 

vol. 17, no. 1, Jan. 1982, pp. 240-246. 

81. Strangman, T.E.; Neumann, J.; and Liu, A.: Thermal Barrier 

Coating Life-Prediction Model Development. Final Report. 

NASA CR-179507, 1987. 

82. Heintze, G.N.; and McPherson, R.: Fracture Toughness of 

Plasma-Sprayed Zirconia Coatings. Thermal Spray: Advances 

in Coatings Technology; Proceedings of the National Thermal 

Spray Conference, Sept. 14-17, 1987, D.L. Houck, ed., ASM 

International, Metals Park, OH, 1988, pp. 271-275. 

83. Hoel R.H.; and Kvernes, I: The Microstructure of Thermal 

Barrier Coatings. Thermal Spray: Advances in Coatings 

Technology; Proceedings of the National Thermal Spray 

Conference, Sept. 14-17, 1987, D.L. Houck, ed., ASM 

International, Metals Park, OH, 1988, pp. 291-296. 

84. Nakajima, K; Kobayashi, K.; and Murata, Y: Phase Stability 

of Y-PSZ in Aqueous Solutions. Adv. Ceram., vol. 12, 1984, 

pp. 399-407. 

85. Hughes, A.E.; and Sexton B.A.: J. Mater. Sci., vol. 24, 

no. 3, Mar. 1989, pp. 1057-1061. 

86. Ruhle, M.; Clausen, N.; and Heuer, A.H.: Microstructural 

Studies of Y203-Containing Tetragonal Zr02 Polycrystals 

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87 . Mecartney, 

Properties 

crystals. 

pp. 54-58. 

M.L.: Influence of an Amorphous 2nd Phase on the 

of Yttria-Stabilized Tetragonal Zirconia PolyJ. Amer. Ceram. Soc., vol. 70, no. 1, Jan. 1987, 

43 

75. Keller, R.J.: Research and Development for Improved Thermal 

Barrier Coating . AFWAL-TR-84-4008, 1984. 

76. Scott H.G.: Phase Relationships in the Zirconia-Yttria 

System. J. Mater. Sci., vol. 10, no. 9, Sept. 1975, 

pp. 1527-1535. 

77. Heuer, A.H.; Chaim, R.; and Lanteri, V.: Review Article: 

Phase Transformations and Microstructural Characterization 

of Alloys in the System Y203-Zr02' Adv. Ceram., vol. 24a, 

1988, pp. 3-20. 

78. Claussen, N: Microstructural Design of Zirconia-Toughened 

Ceramics (ZTC). Adv. Ceram., vol. 12, 1984, pp. 325-351. 

79. Ingel, R.P., et al.: Physical, Microstructural, and Thermomechanical Properties of Zr02 Single Crystals. Adv. Ceram., 

vol. 12, 1984, pp. 408-414. 

80. Lange, F.F.: Transformation Toughening, Part 3, Experimental 

Observations in the Zr02-Y203 System. J. Mater. Sci., 

vol. 17, no. 1, Jan. 1982, pp. 240-246. 

81. Strangman, T.E.; Neumann, J.; and Liu, A.: Thermal Barrier 

Coating Life-Prediction Model Development. Final Report. 

NASA CR-179507, 1987. 

82. Heintze, G.N.; and McPherson, R.: Fracture Toughness of 

Plasma-Sprayed Zirconia Coatings. Thermal Spray: Advances 

in Coatings Technology; Proceedings of the National Thermal 

Spray Conference, Sept. 14-17, 1987, D.L. Houck, ed., ASM 

International, Metals Park, OH, 1988, pp. 271-275. 

83. Hoel R.H.; and Kvernes, I: The Microstructure of Thermal 

Barrier Coatings. Thermal Spray: Advances in Coatings 

Technology; Proceedings of the National Thermal Spray 

Conference, Sept. 14-17, 1987, D.L. Houck, ed., ASM 

International, Metals Park, OH, 1988, pp. 291-296. 

84. Nakajima, K; Kobayashi, K.; and Murata, Y: Phase Stability 

of Y-PSZ in Aqueous Solutions. Adv. Ceram., vol. 12, 1984, 

pp. 399-407. 

85. Hughes, A.E.; and Sexton B.A.: J. Mater. Sci., vol. 24, 

no. 3, Mar. 1989, pp. 1057-1061. 

86. Ruhle, M.; Clausen, N.; and Heuer, A.H.: Microstructural 

Studies of Y203-Containing Tetragonal Zr02 Polycrystals 

(Y-TZP). Adv. Ceram., vol. 12, 1984, pp. 352-370. 

87 . Mecartney, 

Properties 

crystals. 

pp. 54-58. 

M.L.: Influence of an Amorphous 2nd Phase on the 

of Yttria-Stabilized Tetragonal Zirconia PolyJ. Amer. Ceram. Soc., vol. 70, no. 1, Jan. 1987, 

43 
L 

88 . DeMasi, J.T.; and Ortiz, M.: Thermal, Barrier Coating Life 

Prediction Model Development Phase I -- Final Report. 

(PWA-5970-40 Pratt & Whitney Aircraft; NASA Contract: 

NAS3-23944) NASA CR-182230, 1989. 

89 . Hillery, R.V., et al.: Thermal Barrier Coating Life 

Prediction Model, Final Report. (R87-AEB586 General 

Electric Co., Aircraft Engine Business Group; NASA 

Contract: NAS3-23493) NASA CR-180807, 1987. 

90 . Stecura, S.: Optimization of the NiCrAl-Y/Zr02-Y203 Thermal 

Barrier System . NASA TM-86905, 1985. 

91. Stecura, S.: Advanced Thermal Barrier System Bond Coatings 

for Use on Ni-, Co-, and Fe-Base Alloy Substrates. NASA 

TM-87062, 1985. 

92 . Siemers, P .A.; and McKee, D.: U.S. Patent 4,328,285. 

93 . Jones, R.L.; Jones, S.R.; and Williams, C.E.: Sulfation of 

Ce02 and Zr02 Relating to Hot Corrosion. J. Electrochem. 

Soc . , vol. 132, June 1985, pp. 1498-1501. (Also, Naval 

Research Laboratory Memorandum Report 5489, 1984.) 

94 . Holmes, J .W.; and Pilsner, B.H.: Cerium Oxide Stabilized 

Thermal Barrier Coatings. Thermal Spray: Advances in 

Coatings Technology; Proceedings of the National Thermal 

Spray Conference, Sept. 14-17, 1987, D.L. Houck, ed., ASM 

International, Metals Park, OH, 1988, pp. 259-270. 

95 . Dorfman, M.R.; and Reardon, J.D.: Advanced Thermal Barrier 

Coating Systems. Presented at NASA Lewis Research Center 

Thermal Barrier Coatings Workshop, May 21-22, 1985. 

Abstracts and Figures, pp. 171-178. 

96 . Stecura, S.: New Zr02-Yb203 Plasma-Sprayed Coatings for 

Thermal Barrier Applications. Thin Solid Films, vol. 150, 

1987, pp. 15-40. 

97 . Duwez, P.; and Odell, F.: Phase Relationships in the System 

Zirconia-Ceramic. J. Am. Ceram. Soc., vol. 33, no. 9, 

Sept . 1950, pp. 274-283. 

98 . Lange, F.F. : Transformation Toughening, Part 5, Effect of 

Temperature and Alloy on Fracture Toughness. J. Mater. Sci., 

vol . 17, no. 1, Jan. 1982, pp. 225-234. 

99 . Levine, S.R. : Adhesive/Cohesive Strength of a Zr02-12 w/o 

Y203/NiCrAlY Thermal Barrier Coating. NASA TM-73792, 1978. 

44 

L 

88 . DeMasi, J.T.; and Ortiz, M.: Thermal, Barrier Coating Life 

Prediction Model Development Phase I -- Final Report. 

(PWA-5970-40 Pratt & Whitney Aircraft; NASA Contract: 

NAS3-23944) NASA CR-182230, 1989. 

89 . Hillery, R.V., et al.: Thermal Barrier Coating Life 

Prediction Model, Final Report. (R87-AEB586 General 

Electric Co., Aircraft Engine Business Group; NASA 

Contract: NAS3-23493) NASA CR-180807, 1987. 

90 . Stecura, S.: Optimization of the NiCrAl-Y/Zr02-Y203 Thermal 

Barrier System . NASA TM-86905, 1985. 

91. Stecura, S.: Advanced Thermal Barrier System Bond Coatings 

for Use on Ni-, Co-, and Fe-Base Alloy Substrates. NASA 

TM-87062, 1985. 

92 . Siemers, P .A.; and McKee, D.: U.S. Patent 4,328,285. 

93 . Jones, R.L.; Jones, S.R.; and Williams, C.E.: Sulfation of 

Ce02 and Zr02 Relating to Hot Corrosion. J. Electrochem. 

Soc . , vol. 132, June 1985, pp. 1498-1501. (Also, Naval 

Research Laboratory Memorandum Report 5489, 1984.) 

94 . Holmes, J .W.; and Pilsner, B.H.: Cerium Oxide Stabilized 

Thermal Barrier Coatings. Thermal Spray: Advances in 

Coatings Technology; Proceedings of the National Thermal 

Spray Conference, Sept. 14-17, 1987, D.L. Houck, ed., ASM 

International, Metals Park, OH, 1988, pp. 259-270. 

95 . Dorfman, M.R.; and Reardon, J.D.: Advanced Thermal Barrier 

Coating Systems. Presented at NASA Lewis Research Center 

Thermal Barrier Coatings Workshop, May 21-22, 1985. 

Abstracts and Figures, pp. 171-178. 

96 . Stecura, S.: New Zr02-Yb203 Plasma-Sprayed Coatings for 

Thermal Barrier Applications. Thin Solid Films, vol. 150, 

1987, pp. 15-40. 

97 . Duwez, P.; and Odell, F.: Phase Relationships in the System 

Zirconia-Ceramic. J. Am. Ceram. Soc., vol. 33, no. 9, 

Sept . 1950, pp. 274-283. 

98 . Lange, F.F. : Transformation Toughening, Part 5, Effect of 

Temperature and Alloy on Fracture Toughness. J. Mater. Sci., 

vol . 17, no. 1, Jan. 1982, pp. 225-234. 

99 . Levine, S.R. : Adhesive/Cohesive Strength of a Zr02-12 w/o 

Y203/NiCrAlY Thermal Barrier Coating. NASA TM-73792, 1978. 

44 
100. Berndt, C.C.; and Miller, R.A.: Mechanical Property 

Measurements of Plasma-Sprayed Thermal-Barrier Coatings . Ceram. Eng. Sci. Proc., vol. 5, no. 7-8, July-Aug. 1984, 

pp. 479-490. 

101. Berndt, C.C.: Instrumented Tensile Adhesion Tests on Plasma 

Sprayed Thermal Barrier Coatings . J. Mater. Eng., vol. 11, 

no. 4, Apr. 1989, pp. 275-282. 

102 . Heuer, A.H., et al.: HREM Studies of Coherent and Incoherent 

Interfaces in Zr02-Containing Ceramics: A Preliminary 

Account . Ultramicroscopy, vol. 18, 1985, pp. 335-348. 

103. Kraus-Lanteri, S.P.; Mitchell, T.E.; and Heuer, A.H.: 

Structure of Incoherent Zr02/A1203 Interfaces. J. Am. Ceram . Soc., vol. 69, no. 3, Mar. 1986, pp. 256-258. 

104. Firestone, R.F., et al: Creep of Plasma-Sprayed-Zr02 

Thermal-Barrier Coatings. Ceram. Eng. Sci. Proc., vol. 3, 

Mar. 1982, pp. 158-171. 

105. Tucker, R.C., Jr.: Plasma and Detonation Gun Deposition 

Techniques and Coating Properties. Deposition Technologies 

for Films and Coatings. R.F. Bunshah, ed., Noyes, 1982, 

pp. 454-489. 

106. Casamassa, J.V. ; and Bent, R.D.: Jet Aircraft Power Systems. 

Third ed., McGraw-Hill, 1965, pp. 64-65. 

45 

100. Berndt, C.C.; and Miller, R.A.: Mechanical Property 

Measurements of Plasma-Sprayed Thermal-Barrier Coatings . Ceram. Eng. Sci. Proc., vol. 5, no. 7-8, July-Aug. 1984, 

pp. 479-490. 

101. Berndt, C.C.: Instrumented Tensile Adhesion Tests on Plasma 

Sprayed Thermal Barrier Coatings . J. Mater. Eng., vol. 11, 

no. 4, Apr. 1989, pp. 275-282. 

102 . Heuer, A.H., et al.: HREM Studies of Coherent and Incoherent 

Interfaces in Zr02-Containing Ceramics: A Preliminary 

Account . Ultramicroscopy, vol. 18, 1985, pp. 335-348. 

103. Kraus-Lanteri, S.P.; Mitchell, T.E.; and Heuer, A.H.: 

Structure of Incoherent Zr02/A1203 Interfaces. J. Am. Ceram . Soc., vol. 69, no. 3, Mar. 1986, pp. 256-258. 

104. Firestone, R.F., et al: Creep of Plasma-Sprayed-Zr02 

Thermal-Barrier Coatings. Ceram. Eng. Sci. Proc., vol. 3, 

Mar. 1982, pp. 158-171. 

105. Tucker, R.C., Jr.: Plasma and Detonation Gun Deposition 

Techniques and Coating Properties. Deposition Technologies 

for Films and Coatings. R.F. Bunshah, ed., Noyes, 1982, 

pp. 454-489. 

106. Casamassa, J.V. ; and Bent, R.D.: Jet Aircraft Power Systems. 

Third ed., McGraw-Hill, 1965, pp. 64-65. 

45 
0'1 

TABLE 1. - SELECTED PROPERTIES FOR CATERPILLAR AND CUMMINS/UTRC TTBCs 

K(W/M-OC) a.(}JID/M-OC) E(GPa) Tensile Compressive Tensi

bend strength, failur

strength, MPaa strain

MPaa 

Caterpillar: 

NiCrAIY ---- 12.0 61 380 ---- 0.75 

50% Zr02-Y2031 3.25 9.3 49 150 ---- .45 

50% NiCrAIY 

75% Zr02-Y2031 1. 95 8.8 43 80 ---- .35 

25% NiCrAIY 

Zr02-8%Y203 .94 7.7 44 50 600 .15 

CUImlins IUTRC: 

NiCraAl 3.7 13.8 105 252 1048 ----

40% Zr02-Y2031 2.0 12.6 66 140 748 ----

60% CoCrAIY 

85% Zr02-Y2031 1.0 10.3 51 47 437 ----

15% CoCrAIY 

Zr02-20%Y203 .5 9.0 37 40 390 ----

.. 

-- ----- -- ---

L-

____ 

aMeasured in bending .at 400°C. 

TABLE 1. - SELECTED PROPERTIES FOR CATERPILLAR AND CUMMINS/UTRC TTBCs 

K(W/M-OC) a.(}JID/M-OC) E(GPa) Tensile Compressive Tensi

bend strength, failur

strength, MPaa strain

MPaa 

Caterpillar: 

NiCrAIY ---- 12.0 61 380 ---- 0.75 

50% Zr02-Y2031 3.25 9.3 49 150 ---- .45 

50% NiCrAIY 

75% Zr02-Y2031 1. 95 8.8 43 80 ---- .35 

25% NiCrAIY 

Zr02-8%Y203 .94 7.7 44 50 600 .15 

CUImlins IUTRC: 

NiCraAl 3.7 13.8 105 252 1048 ---- 

40% Zr02-Y2031 2.0 12.6 66 140 748 ---- 

60% CoCrAIY 

85% Zr02-Y2031 1.0 10.3 51 47 437 ---- 

15% CoCrAIY 

Zr02-20%Y203 .5 9.0 37 40 390 ---- 

aMeasured in bending .at 400°C. 

-------------
Figure 1. - Schematic of the low heat rejection diesel engine showing key problem 

areas (ref. 8). 

C-89-01481 ______________ ~9~ 

(a) Piston cap. (b) Cylinder head (before final grinding of valve seats). 

Figure 2. - nBC coated Caterpillar Inc. r_ch engine. 

47 

i _1 

Figure 1. - Schematic of the low heat rejection diesel engine showing key problem 

areas (ref. 8). 

C-89-01481 ______________ ~9~ 

(a) Piston cap. (b) Cylinder head (before final grinding of valve seats). 

Figure 2. - nBC coated Caterpillar Inc. r_ch engine. 

47 

i _1 
Tg 

---q 

NOTE: 

T g = TEMPERATURE OF GAS 

q = HEAT FLOW 

T c = TEMPERATURE OF COOLANT 

,- BOND 

/ COAT / 

/ 

0 

0 

0 0 

0 0 0 

0 

0 0 0 

/ / , L- YSZ L-SINTEREO '- METALLIC 

THERMAL FIBER SUBSTRATE 

BARRIER STRAIN 

LAYER ISOLATOR 

(al Schemalic of Ihe coating syslem. (bl Coated piston, cylinder head, and valves before engine testing. 

Figure 3. - Strain isolator based nBC developed by Allison under HOn. 

CATERPILLAR CUMMINSIUTRC 

75% CERAMIC! 

25%NICrAIY 

,- LOW THERMAL 

/ CONDUCTIVITY 

,/ CERAMIC ~ , 

, , , 

- ... .-....... 

, 

" 

85% CERAMICI 

MIXED METAL --., 15% CoCrAIY , AND ;.. ___ ------ - __ ....... ___ •,-CERAMIC ~ _ 40% CERAMIC! 

/ / --- 60% CoCrAIY 

,/ r"------":':NICrAI 

50% CERAMIC 

5O%NICrAIY 

NICrAIy'-' ------- __ ... ~ .... v __ ... /. 

STAINLESS 

STEEL PISTON 

CAP 

," 

~METALLIC / / / 1/ I BONDCOAT-' 

T 

1.0mm 

..L 

DUCTILE IRON, 

FIBER REINFORCED 

ALUMINUM OR Bl900 

SUPER ALLOY PISTON 

CAP 

Figure 4. - Schematlc of caterpillar and CUmmlnslUTRC nBC designs developed under HOn. 

48 

Tg 

---q 

NOTE: 

T g = TEMPERATURE OF GAS 

q = HEAT FLOW 

T c = TEMPERATURE OF COOLANT 

,- BOND 

/ COAT / 

/ 

0 

0 

0 0 

0 0 0 

0 

0 0 0 

/ / , L- YSZ L-SINTEREO '- METALLIC 

THERMAL FIBER SUBSTRATE 

BARRIER STRAIN 

LAYER ISOLATOR 

(al Schemalic of Ihe coating syslem. (bl Coated piston, cylinder head, and valves before engine testing. 

Figure 3. - Strain isolator based nBC developed by Allison under HOn. 

CATERPILLAR CUMMINSIUTRC 

75% CERAMIC! 

25%NICrAIY 

,- LOW THERMAL 

/ CONDUCTIVITY 

,/ CERAMIC ~ , 

, , , 

- ... .-....... 

, 

" 

85% CERAMICI 

MIXED METAL --., 15% CoCrAIY , AND ;.. ___ ------ - __ ....... ___ •,-CERAMIC ~ _ 40% CERAMIC! 

/ / --- 60% CoCrAIY 

,/ r"------":':NICrAI 

50% CERAMIC 

5O%NICrAIY 

NICrAIy'-' ------- __ ... ~ .... v __ ... /. 

STAINLESS 

STEEL PISTON 

CAP 

," 

~METALLIC / / / 1/ I BONDCOAT-' 

T 

1.0mm 

..L 

DUCTILE IRON, 

FIBER REINFORCED 

ALUMINUM OR Bl900 

SUPER ALLOY PISTON 

CAP 

Figure 4. - Schematlc of caterpillar and CUmmlnslUTRC nBC designs developed under HOn. 

48 
800 

iii" 

a: 

:;) 

~ 800 

a: 

~o 

~,. 400 

I-

~ 200 

~ 0 

TIME 

(a) Stoplstart. 

w 

a: 

a: 

~ w 

Cl. 

::I 

w 

I-

~ o 

o 

TIME 

(b) No Ioad/fullload. 

100m. 

TIME 

(c) Combustion transients. 

Figure 5. - Schematic of cydlc environment In trucK diesel engines . 

.2 

~ 0 

ffi -.2 a: 

~ -.4 

~ -.6 

COMPRESSION 

-.8 SURFACE 

TTBC THICKNESS 

(a) Coating system tem~ (b) Stress to strength ratio 

erature versus thickness. versus thickness. 

Figure 6. - Schematic of temperatures and stress states across the 

coating system for full load, two seconds Into no load, and no load 

conditions (refs. 7 and 10). 

49 

800 

iii" 

a: 

:;) 

~ 800 

a: 

~o 

~,. 400 

I-

~ 200 

~ 0 

TIME 

(a) Stoplstart. 

w 

a: 

a: 

~ w 

Cl. 

::I 

w 

I-

~ o 

o 

TIME 

(b) No Ioad/fullload. 

100m. 

TIME 

(c) Combustion transients. 

Figure 5. - Schematic of cydlc environment In trucK diesel engines . 

.2 

~ 0 

ffi -.2 a: 

~ -.4 

~ -.6 

COMPRESSION 

-.8 SURFACE 

TTBC THICKNESS 

(a) Coating system tem~ (b) Stress to strength ratio 

erature versus thickness. versus thickness. 

Figure 6. - Schematic of temperatures and stress states across the 

coating system for full load, two seconds Into no load, and no load 

conditions (refs. 7 and 10). 

49 
Figure 7. - Mean temperature contours calculated for a LHR piston assuming gas temperature 

of 756 ·C and heat nux of 2080 watts. 

,MAX TENSION 0 -w ID wa: :I: 

a:::>>< ~ ... 

0 ~I/)::> Ii. z -cl/)-J 0 W a:WIi. W a: wa: ... a: Q.Q.~ ::> Ii! ~~:I: ~ I/) I/) ... za: a: W 

1/)-0 W a: 

~¢ Q. 

~ Ii; COMPRESSION 0 I!! 

0 360 720 SURFACE SURFACE 

CRANK ANGLE, deg nec THICKNESS nec THICKNESS 

(a) Gas temperature, (b) Coating system temp- (c) Stress versus 

cylinder pressure, erature versus thickness. thickness. 

or heat nux. 

Figure 8. - Schematic of the eflec:t of CX)I11bustion transients. 

50 

Figure 7. - Mean temperature contours calculated for a LHR piston assuming gas temperature 

of 756 ·C and heat nux of 2080 watts. 

,MAX TENSION 0 -w ID wa: :I: 

a:::>>< ~ ... 

0 ~I/)::> Ii. z -cl/)-J 0 W a:WIi. W a: wa: ... a: Q.Q.~ ::> Ii! ~~:I: ~ I/) I/) ... za: a: W 

1/)-0 W a: 

~¢ Q. 

~ Ii; COMPRESSION 0 I!! 

0 360 720 SURFACE SURFACE 

CRANK ANGLE, deg nec THICKNESS nec THICKNESS 

(a) Gas temperature, (b) Coating system temp- (c) Stress versus 

cylinder pressure, erature versus thickness. thickness. 

or heat nux. 

Figure 8. - Schematic of the eflec:t of CX)I11bustion transients. 

50 
10 

I:. 

Wo ~40 ::!z 

I/)C 

fficr: 

... 0 

THERMAL 

CYCLE 

:. 800 

~~ _UI 

1/)0 

m~ 400 

ft:cr: 

~~ 8 II! 

... 

I/) 

2 4 6 0 2 4 

LOG CYCLES TO LOG CYCLES TO 

FAILURE FAILURE 

(a) Tensile fatigue. (b) Compressive fatlgue. 

Figure 9. - Example of high cycle fatigue results for the caterpillar 

cenIIT1lc (ref •. 12 and 13). 

Figure 10. - Example of a coating system failure in an engine test 

(refs. 14 and 15). 

SLIP 

RING 

ASSEMBLY~ ,-

/ 

CAROUSEL 

INTERNAL 

COOLING 

AIR ROTATING 

CAROUSEL 

<=> --.Y""'rVr~~~ 

CAROUSEL 

EXTERNAL 

COOLING AIR 

PREHEATED 

INLET AIR 

6 

Figure 11. - Bench rig used by caterpillar/Solar to simulate thermal shock exposure 

(refs. 12 and 13). 

51 

10 

I:. 

Wo ~40 ::!z 

I/)C 

fficr: 

... 0 

THERMAL 

CYCLE 

:. 800 

~~ _UI 

1/)0 

m~ 400 

ft:cr: 

~~ 8 II! 

... 

I/) 

2 4 6 0 2 4 

LOG CYCLES TO LOG CYCLES TO 

FAILURE FAILURE 

(a) Tensile fatigue. (b) Compressive fatlgue. 

Figure 9. - Example of high cycle fatigue results for the caterpillar 

cenIIT1lc (ref •. 12 and 13). 

Figure 10. - Example of a coating system failure in an engine test 

(refs. 14 and 15). 

SLIP 

RING 

ASSEMBLY~ ,-

/ 

CAROUSEL 

INTERNAL 

COOLING 

AIR ROTATING 

CAROUSEL 

<=> --.Y""'rVr~~~ 

CAROUSEL 

EXTERNAL 

COOLING AIR 

PREHEATED 

INLET AIR 

6 

Figure 11. - Bench rig used by caterpillar/Solar to simulate thermal shock exposure 

(refs. 12 and 13). 

51 
~ 

ui 

a: 

~ a: 

1&1 

IL 

:. 

I!! 

3000 

2500 

2000 

1500 

1000 

500 

LIQUID 

(L) 

x 

1 

1 CUBIC 

1 (F) 1 

1 

1 r-TRANSFORMABLE 

F TETRAGONAL (T) 

1 

MONO- M+F 1 1 

CLINIC 1 1 1 

(M) 1 , 1 

I--.,....--..!..' = == ---:-1 --- , 

o L/MONOCLINIC : ',- CUBIC 

L NONTRANSFORMABLE 

TETRAGONAL (T) 

~L-____ ~ ______ ~ ______ ~ ______ ~ 

o 5 10 

Y01.5• mol % 

15 

Figure 12. - Zirconla-ynrta phase diagram showing phases 

encountered on quenching and upon high temperature 

ageing (ref. 21). 

52 

20 

~ 

ui 

a: 

~ a: 

1&1 

IL 

:. 

I!! 

3000 

2500 

2000 

1500 

1000 

500 

LIQUID 

(L) 

x 

1 

1 CUBIC 

1 (F) 1 

1 

1 r-TRANSFORMABLE 

F TETRAGONAL (T) 

1 

MONO- M+F 1 1 

CLINIC 1 1 1 

(M) 1 , 1 

I--.,....--..!..' = == ---:-1 --- , 

o L/MONOCLINIC : ',- CUBIC 

L NONTRANSFORMABLE 

TETRAGONAL (T) 

~L-____ ~ ______ ~ ______ ~ ______ ~ 

o 5 10 

Y01.5• mol % 

15 

Figure 12. - Zirconla-ynrta phase diagram showing phases 

encountered on quenching and upon high temperature 

ageing (ref. 21). 

52 

20 
f\U\SI\ Report Documentation Page NatiOll8i Aeronautics and 

Space Administration 

1. Report No. NASA TM-I03130 2. Government Accession No. 3. Recipient's Catalog No. 

DOE/NASA/21749-1 

4. Title and Subtitle 5. Report Date 

Assessment of Fundamental Materials Needs for Thick Thermal May 1990 

Barrier Coatings (TTBC's) for Truck Diesel Engines 6. Performing Organization Code 

7. Author(s) B. Performing Organization Report No. 

Robert A. Miller E-5460 

10. Work Unit No. 

505-63-lA 

9. Performing Organization Name and Address 

11 . Contract or Grant No. 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Address Technical Memorandum 

U.S. Department of Energy 14. Sponsoring Agency Code 

Office of Transportation Systems 

Washington, D.C. 20545 

15. Supplementary Notes 

Final Report. Prepared under Interagency Agreement DE-AI05-870R2l749. 

16. Abstract 

The present state of development of thick thermal barrier coatings for truck diesel engines is assessed and areas 

where improved fundamental understanding is needed to properly pursue development are identified. Emphasis is 

given to the coating systems and design approaches that are being developed for the next generation of truck 

diesel engines under DOE/NASA support. It is noted that, while considerable progress has been made, the 

current level of understanding of coating system behavior is inadequate and this lack of fundamental understanding 

may impede current and future development. Several areas where improved understanding would be especially 

valuable are identified and recommendations for research into those areas are offered. 

17. Key Words (Suggested by Author(s)) lB. Distribution Statement 

Thermal barrier coatings Unclassified - Unlimited 

Diesel engines Subject Category 26 

DOE Category UC-332 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages 22. Price" 

Unclassified Unclassified 54 A04 

NASA FORM 1626 OCT 86 'For sale by the National Technical Information Service, Springfield, Virginia 22161 

f\U\SI\ Report Documentation Page NatiOll8i Aeronautics and 

Space Administration 

1. Report No. NASA TM-I03130 2. Government Accession No. 3. Recipient's Catalog No. 

DOE/NASA/21749-1 

4. Title and Subtitle 5. Report Date 

Assessment of Fundamental Materials Needs for Thick Thermal May 1990 

Barrier Coatings (TTBC's) for Truck Diesel Engines 6. Performing Organization Code 

7. Author(s) B. Performing Organization Report No. 

Robert A. Miller E-5460 

10. Work Unit No. 

505-63-lA 

9. Performing Organization Name and Address 

11 . Contract or Grant No. 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Address Technical Memorandum 

U.S. Department of Energy 14. Sponsoring Agency Code 

Office of Transportation Systems 

Washington, D.C. 20545 

15. Supplementary Notes 

Final Report. Prepared under Interagency Agreement DE-AI05-870R2l749. 

16. Abstract 

The present state of development of thick thermal barrier coatings for truck diesel engines is assessed and areas 

where improved fundamental understanding is needed to properly pursue development are identified. Emphasis is 

given to the coating systems and design approaches that are being developed for the next generation of truck 

diesel engines under DOE/NASA support. It is noted that, while considerable progress has been made, the 

current level of understanding of coating system behavior is inadequate and this lack of fundamental understanding 

may impede current and future development. Several areas where improved understanding would be especially 

valuable are identified and recommendations for research into those areas are offered. 

17. Key Words (Suggested by Author(s)) lB. Distribution Statement 

Thermal barrier coatings Unclassified - Unlimited 

Diesel engines Subject Category 26 

DOE Category UC-332 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages 22. Price" 

Unclassified Unclassified 54 A04 

NASA FORM 1626 OCT 86 'For sale by the National Technical Information Service, Springfield, Virginia 22161