. 

Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2436 March 2001 

. 

Evaluation of Monolithic Ceramics and 

Ceramic Thermal Barrier Coatings for 

Diesel Engine Applications 

Jeffrey J. Swab and Paul J. Huang 

Weapons and Materials Research Directorate, ARL 

Aooroved for public release; distribution is unlimited. 
Abstract 

The Metals and Ceramics Research Branch (MCRB) of the Weapons and Materials 

Research Directorate is providing ceramic material characterization and evaluation to the 

Tank Automotive Research, Development, and Engineering Center (TARDEC) as a part 

of a project entitled “Fighting Vehicle Propulsion Technology Using Ceramic Materials.” 

Through research and exploratory development of advanced ceramics and other 

technologies, the objective of this project is to improve the mobility of future Army 

fighting vehicles. The purpose is to demonstrate the capabilities of ceramics, other 

advanced materials, tribology, and combustion technologies to contribute to a higher 

power density and lower fuel consumption diesel engine. 

Four commercial silicon nitrides were purchased and characterized for potential 

application as piston crowns, valves, and valve seats. A sodium-zirconium-phosphate 

(NZP) family of materials was examined for exhaust port applications, and several 

thermal barrier coating (TBC) materials were examined as possible additions to metallic 

piston crowns, rings, and cylinder liners. This report contains the results of this 

preliminary characterization and evaluation. 

ii 
Acknowledgments 

The authors would like to acknowledge the financial support of the Propulsion Systems 

Division of the U.S. Army Tank-Automotive Research, Development, and Engineering Center 

(TARDEC) provided by Dr. Walter Bryzik, Ernest Schwarz, and Eugene Danielson. The authors 

would also like to acknowledge the following people who assisted in this effort: Matthew 

Motyka for his efforts in the mechanical property testing of these materials, Dr. Parimal Pate1 for 

performing the thermal expansion testing, and Anand Rulnari at State University of New York 

(SUNY) for helping with the thermal conductivity tests. 

, 

. . . 

111 
iv 
Table of Contents 

Acknowledgments .+ .. . . . __ f.........._...f........._.. _. . . . . . ..-. _ ._..............~.f........f...................... 111 

List of Figures ..,.................................._.... e .. . . . . _ -............................. _ .. . . . . . . . . . . . . . . . . . . . . .._. vii 

List of Tables . . . . . ..f____...........__.............-..........................---........... _ .. . . . . . . . . . . ..f............. ix 

1. Introduction,. __.. _......._____........f.__ _ _......... _ ____.........__. _.........._. ._.........._...~......,..._f.... 1 

2. Silicon Nitride . . . . . . . . . . ..__...........__............__.. e .. . . ..- .._..*......-_..__................**.................... 3 

2.1 Background .......................................................................................................... 3 

2.2 Test Results.. ........................................................................................................ 4 

2.2.1 Density ................................................................................................................. 6 

2.2‘2 Elastic Modulus ................................................................................................... 6 

2.2.3 Composition ......................................................................................................... 6 

2.2.4 Thermal Conductivi(y ........................................................................................... 7 

2.2.5 Thermal Expansion .............................................................................................. 7 

2.2.6 Mechanical Strength ............................................................................................ 8 

2.2.7 Fracture Analysis.. ............................................................................................... 9 

2.2.8 Hardness .............................................................................................................. 9 

2.2.9 Fracture Toughness ............................................................................................. 12 

2.3 s-w .............................................................................................................. 12 

3. Low Thermal Expansion/Low Thermal Conductivity Ceramics .._._.......,.._........ 12 

3.1 Background .......................................................................................................... 12 

3.2 Test Results .......................................................................................................... 13 

3.2.1 Density ................................................................................................................. 13 

3.2.2 Elastic Modulus ................................................................................................... 13 

3.2.3 Composition ......................................................................................................... 13 

3.2.4 Thermal Conductivity. .......................................................................................... 14 

3.2.5 Thermal Expansion .............................................................................................. 14 

3.2.6 Mechanical Strength ............................................................................................ 14 

3.2.7 Hardness .............................................................................................................. 14 

3.2.8 Fracture Toughness ............................................................................................. 1s 

3.3 s-w .............................................................................................................. 1s 

4. Ceramic Thermal Barrier Coatings (TBCs)...... . . . . . ..*....__.*......*..........................-.. 1s 

4.1 Background . . ..f....*......_.__*.~*f.~................_................fff.*..f.~.......*........~~f................. 1s 

V 
4.2 Test Results .......................................................................................................... 16 

4.2.1 Density ................................................................................................................. 16 

4.2.2 Hardness .............................................................................................................. 17 

4.2.3 Thermal Conductiviq ........................................................................................... 17 

4.2.4 Thermal Shock ..................................................................................................... 17 

4.2.5 Thermal Expansion .............................................................................................. 17 

4.2.6 Adhesion Strength ................................................................................................ 17 

4.3 SUCK .............................................................................................................. 18 

5. References . . ..__. . _._ _....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Distribution List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..~*.........*...*..~.*..........-..-.....- 23 

Report Documentation Page .*..*......*.~f.**..*..~**..*......~....~~.....*................................... 25 

19 

, 

. 

vi 
List of Figures 

. Figure 

1. 

2(a). 

2(b). 

w. 

2(d). 

Two-Parameter Weibull Plot .._ .__...... . . . . . . . _ _........... ..*.+ . . . . . . .._... . . . . . . . . . _ .................._...f.. 

Strength-Limiting Flaw in GS-44CL Is a Pore (P”, Surface, =30 pm) .................f_.. 

Strength-Limiting Flaw in C&V15 Is an Agglomerate of Sic Whiskers 

(A”, Surface, =80 pm) . . . . . . . . . . . . . . . . . . . . . . . ..f......f.._.___......................_................_f....._...... 

Strength-Limiting Flaw in NT55 1 Is Machining Damage 

(MDS, Surface, =75 pm) . . . ..f._.___.............................__.._...._.__f........................._._._._.__ 

Strength-Limiting Flaw in Ceralloy 147-3 IN Is Machining Damage 

(MDS, Surface , ==60 pm x 200 pm).. . . . . . . . +. . . . . . , . . . . . . . . . . . . . . . . . . . . . . . .__. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

u 

8 

10 

10 

11 

11 

vii 
. 

. 

. . . 

Vlll 
List of Tables 

Table 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

Silicon Nitrides Evaluated ....................................................................................... 

Properties of the Silicon Nitrides Evaluated ............................................................ 

Elemental Analysis of Si3N4 .................................................................................... 

Properties of the NZP Low Thermal Expansion Materials. ...................................... 

Elemental Composition of NZP Materials ............................................................... 

Ceramic TBC Materials Evaluated .......................................................................... 

Screening Data for Ceramic TBC Materials ............................................................ 

Adhesion Strength of Ceramic TBCs.. ..................................................................... 

& 

3 

5 

7 

13 

14 

15 

16 

16 
hTENTIONALLY LEFT BLANK. 

X 
1. Introduction 

The oil embargo of the 1970’s spurred the need to improve the performance of engines to 

reduce the dependence on fossil fuel. Modern high performance ceramics with unique properties 

(high temperature- strength, strength retention after time at high temperatures, and 

oxidation/corrosion resistance) were identified as materials that have the potential to increase 

engine performance and reduce fuel consumption. Since that time, significant efforts have been 

made by the materials and engine development communities to take advantage of advanced 

ceramic materials to increase engine performance and achieve this goal. Recently, 

environmental concerns have also contributed to the push to increase the applications of ceramic 

materials in engines. Although the benefits envisioned almost thirty years ago have yet to be 

fully realized, tremendous potential for ceramic materials to enhance engine performance still 

exists. 

A variety of ceramic materials are being considered for applications in advanced engines. 

Monolithic ceramic materials (silicon nitride is the leading candidate), which are under 

consideration as structural components to replace metallic piston crowns, valves, and valve seats, 

have better corrosion resistance (ability to tolerate low-grade fuels), higher strength at elevated 

temperature, and higher hardness (better wear performance) than most metals. Other monolithic 

ceramics, specifically tailored with low thermal conductivity and thermal expansion, are being 

considered as liners for exhaust ports. Using a material with a low thermal conductivity in the 

exhaust port (the transfer of thermal energy to the cylinder head) can reduce the thermal energy 

loss at the cylinder head and lead to a smaller cooling system. Additional savings can also be 

gained in a turbo charging system by using the retained thermal energy to increase engine output 

power. Another class of ceramic materials with low thermal conductivity are ceramic thermal 

barrier coatings (TBCs). Ceramic TBCs are not designed as structural components, but instead 

are used to reduce the operating temperature a metallic component experiences, which extends 

the component life. It is anticipated that incorporating ceramic materials into diesel engines will 

reduce the in-cylinder heat rejection by almost 40%, improve fuel economy by approximately 

6%, and cut particulate emission by up to 50%. Other benefits may include multifuel capability, 
improved power output and combustion efficiency, decreases in engine size and weight, reduced 

noise levels, and reduced maintenance costs [ 1, 21. 

The Metals and Ceramics Research Branch (MCRB) of the Weapons and Materials Research 

Directorate is providing ceramic material characterization and evaluation to the Tank and 

Automotive Research Development and Engineering Center (TARDEC), as part of an 

international program to examine advanced materials for diesel engines. Through research and 

exploratory development of advanced ceramics and other technologies, the objective is to 

improve the mobility of future Army fighting vehicles. The purpose is to demonstrate the 

capabilities of ceramics, other advanced materials, tribology, and combustion technologies to 

contribute to a diesel engine with a higher power density and lower fuel consumption. 

The objective of this project will be accomplished through a series of five cooperative tasks: 

(1) a preliminary analysis of combustion phenomena, (2) an analysis of engine parameters, (3) 

ceramic material testing and evaluation, (4) the design, fabrication, and optimization of a test 

bed integrated single cylinder engine (TI-SCE), and (5) final testing and evaluation of the 

TI-SCE. The United States and its international partner* will be collaborating with major engine 

manufacturers in their respective countries to develop and test the TI-SCE. The main engine 

contractor for the DOD will be the Detroit Diesel Corporation, while the main engine contractors 

of the international partner are also leading diesel engine manufacturers. 

As part of Task 3, MCRB is evaluating the thermal and mechanical properties of monolithic 

ceramics and thermal barrier-coating materials. The information generated will be incorporated 

into the development of the TI-SCE. A reciprocal agreemento exchange specimens and data 

with our international partner is part of the project. Ceramic materials will be examined to use in 

the following engine components: piston crowns, rings, cylinder heads and liners, valves, valve 

seats, and intake exhaust ports. 

* The identity of the international partner is restricted to a need-to-know basis. 

2 
. 

This report covers the initial screening and characterization of monolithic ceramic materials 

and ceramic thermal barrier-coating materials that have potential applications in the combustion 

area of advanced diesel engines. Additional testing is presently underway on the “‘best” 

materials and will be reported in the future. All materials evaluated were domestically produced 

and most are commercially available. The report is divided into three sections: Silicon Nitrides, 

Low Thermal Expansion/Low Thermal Conductivity Ceramics, and Ceramic TBCs. In each 

section, the properties measured and analyzed will be discussed and (if possible) compared to the 

values provided by the manufacturers. 

2. Silicon Nitride 

2.1 Background. Four silicon nitride (SijNd) materials were examined for potential diesel 

engine applications such as piston crowns, valves, and valve seats (see Table 1). Recent studies 

in Japan [3] and Germany [4] using silicon nitride valves in automotive applications have 

reported great success. Over 500,000 km of road testing has been completed without any 

problems in the Japanese study, while the German effort produced 30,000 high-quality valves 

over a three-month period for field testing by Mercedes-Benz. 

Table 1. Silicon Nitrides Evaluated 

Manufacturer Material Code 

Allied Signal, Inc. GS-44CL 

Ceradyne, Inc. Ceralloy 147-3 1N 

Cercom, Inc. clw15 

Billet Sizea Processing Technique 

504x50t CIP/Sinter 

100x 100x9 Reaction Bonded 

150x 150x8 Hot Pressed w/15 v/o 

Sic whiskers 

St. Gobain Industrial 

Ceramics NT55 1 

‘All dimensions are nominal and in millimeters. 

57 x 57 x 13 Sinter/HIP 

3 
Typically, the material manufacturers produce a “family” of S&N4 materials with each 

member of the family tailored to a specific application. After a discussion with each of the 

manufacturers, the GS-44CL, NT551, and Ceralloy 147-3 1N silicon nitrides listed in Table 1 

were selected as the test materials. Each S&N4 may be considered in-situ toughened and was 

developed with engine applications in mind. During processing, steps were taken to ensure the 

elongation of some grains to provide a duplex microstructure which could enhance some of the 

mechanical properties. (The CIW15 silicon nitride is not an in-situ toughened material. It is 

however a toughened silicon nitride, as it contains approximately 15 volume-percent SIC 

whiskers which have been added to enhance toughness. Although not designed for engine 

applications, this material was included to compare with the in-situ toughened silicon nitrides.) 

All materials were produced in 1997 and purchased in large billet sizes that were subsequently 

machined to the appropriate test specimen geometries. Only one billet was purchased of the 

CIWlS, while multiple billets were needed from the other three silicon nitrides to obtain 

sufficient specimens for the entire test matrix. 

Rectangular specimens with nominal dimensions of 3 mm x 4 mm x 50 mm were machined 

for all tests, except for determining thermal expansion and conductivity tests. All of the 

rectangular specimens were machined according to guidelines in ASTM Cl 161 [S]. For thermal 

expansion measurements, the specimens were 3 mm x 3 mm x 4 mm, and for thermal 

conductivity, a disk measuring 12.5 mm in diameter by 1 mm thick was used, 

2.2 Test Results. The physical and mechanical properties determined in this study are listed 

in Table 2. Many of the properties listed in Table 2 have been determined elsewhere over the 

past 6-10 years for these same silicon nitrides [6-l 1). However, it is virtually impossible to 

compare these sets of data without knowing the vintage of the material. Manufacturers are 

continually optimizing the composition and processing methods in order to make a better product 

and/or tailor the product to a specific application. Although most of these modifications are 

subtle, they can have a significant impact on a property or properties, which makes comparing 

different vintages of the same material a daunting (if not impossible) task. In this report, product 

data supplied by the maunfacturers will be available for comparison. The maunfacturers’ data is 

4 
Note: NA - not available. 

a Short beam chevron notch. 

b IO-kg load. 

’ Indentation strength method. 

d Indentation load unknown. 

e Indentation - crack length with 5-kg vickers indent. 

f j-kg load. 

g Single-edge notched beam. 

h Knoop hardness - l-kg load. 

i Characteristic flexure strength of the bend bar. 

1 Ceralloy 147-3 1N 

1 

Company ARL 

3.28 ) 3.3 ] 3.20 

347 I 320 I 305 

Table 2. Properties of the Silicon Nitrides Evaluated 

Product Data GS-44CL NT 551 

14.3 1 15.2f 1 15.0 

22 1 35 j 23 

4.3 I 3.5 I 3.3 

CIW15 II 

3.27 3.25 

335 380 

800 770’ 

17 16 

10.28 6.03 f 0.28 

15.gh 18.4 

NA 18 

3.3 4.4 
typical for the material they produced, but it may not be representative of the vintages tested in 

this study. 

2.2.1 Density. Density was measured at room temperature using the Archimedes water 

immersion method with distilled water. A minimum of 10 specimens were used to determine the 

average density of each Si3N4. When specimens were obtained from multiple billets, the density 

was determined on specimens randomly selected from all billets. This was done to check for 

possible billet-to-billet variability. The densities measured in this study are in agreement with 

the respective company data (3.20 g/cm3 to 3.28 g/cm3). No billet-to-billet variability was 

detected in the density data for any of these materials. 

2.2.2 Elastic Modulus. Elastic modulus was obtained on a minimum of five specimens of 

each Si3N4 using the procedures in ASTM C 1259 [ 121. In this test method, the fundament 

flexural resonant frequency is measured when a rectangular specimen is mechanically exited by 

a single elastic strike. A transducer senses the resultant mechanical vibration and transforms it 

into an electrical signal. The fundamental frequency is isolated, and this value, along with the 

specimen dimensions and mass, are used to calculate the dynamic elastic modulus. For all but 

the Ceralloy 147-31N, the elastic modulus obtained is considerably higher than the 

manufacturer’s values. The reason for this discrepancy is unknown and additional analysis is 

needed. 

2.2.3 Composition.* Semiquantitative energy dispersive x-ray fluorescence (XRF) analysis 

was used to determined the elemental composition of each Si3N4. The analysis was completed 

on a fine powder under a vacuum of approximately 300 mTorr. Spectra were collected with a 

thin window solid state Si(Li) detector system using tungsten white radiation for excitation. 

Elements present in each material were determined from the spectra. This method is not capable 

of detecting Al (n = 13) or elements with lower atomic numbers; thus, the results in Table 3 may 

not reflect the complete elemental composition of each material. 

‘XRF analysis completed by Lamda Research Inc., Cincinnati, OH, under contract DAAD17-99-P-0338. 

6 
Table 3. Elemental Analysis of SijN4 

. 

Si3N4 Material+ 

Element 

(weight-percent) 4 

Si 

Y 

Yb 

Ca 

GS-44CL NT 551 Ceralloy 147-3 1 N CIW15 

92 82 82 88 

7 6 7 11 

<l <l <l 4 

Cl - - 4 

Fe <l - - Cl 

co cl <l Cl Cl 

Nd - 11 11 - 

La - Cl Cl - 

rote: The dash (-) denotes an element not detected. 

The presence of yttrium in each of these silicon nitrides is expected since it was found that 

Y203 is an excellent densification [ 131 aid for silicon nitride, and that the resultant SijN4 

exhibited excellent properties at room and elevated temperatures. In recent years, research has 

focused on other rare earth oxides (especially Nd203, La203 and CeOz) and rare earth oxide in 

combination with YZOJ to reduce the cost of the densification aid without compromising the 

properties [l&16]. 

2.2.4 Thermal Conductivi@. The average room temperature thermal conductivity was 

determined using a laser flash method in accordance with JIS R1611 [ 173 on five specimens 

from each silicon nitride. Uniformly thick specimens were coated with a thin layer of graphite to 

minimize reflection of the pulse. The rise in temperature of the specimen as it is subjected to the 

laser pulse is obtained to determine the thermal diffusivity of the specimen. The thermal 

diffusivity, specimen density, and thickness are used to calculate the thermal conductivity. 

Values obtained in this study are consistently lower when compared to the manufacturer’s 

values. This may be due to the use of different measurement techniques. 

2.2.5 Thermal Expansion. The coefficient of thermal expansion from room temperature to 

1000 “C was determined using a single-rod dilatometer. Specimens were heated at 100” C/hr, 

7 
and the CTE value was determined from the linear regression of the data. The resulting values 

listed in Table 2 are in good agreement with the manufacturer’s values. 

2.2.6 Mechanical Strength. The flexure strength was measured in accordance with ASTM 

C 1161 using a standard 20 mm x 40 mm fully articulating four-point flexure fixture and a 

cross-head speed of 0.5 mm/min. A minimum of 30 longitudinally-machined specimens of each 

Si3N4 were fractured to obtain the average flexural strength. A two-parameter Weibull plot 

(using uncensored data) was then generated following the procedure in ASTM Cl239 [ 1 S] to 

determine the characteristic flexure strength and unbiased Weibull modulus using maximum 

likelihood estimations. The characteristic flexural strength and Weibull modulus of each silicon 

nitride were in excellent agreement with the values reported by the manufacturers. Figure 1 

contains the Weibull plots for each silicon nitride. These Weibull plots indicate that there is a 

single flaw population limiting the strength of each silicon nitride. 

2 

1 

- 0 

h 

Ii -1 

I 

F 

=* -2 

T 

V 

c -3 

- 

c 

- 

-4 

-5 

-6 

1 

- Ceralloy 147-31N MLE 

. Ceralloy 147-31 N Data 

- CIWI5 MLE 

z CIW15 Data 

- GS44 MLE 

0 GS44 Data 

- NT551 MLE 

. NT551 Data 

-9 

-9 

-9 

-5 

-2 

-1 

400 600 800 

Failure Stress (MPa) 

1000 1200 

9.9 

9.0 

‘0.0 D 

z 

K 

IO.0 u 

-. =1 

G 

10.0 0, 

0.0 2 -’ 

c 

5.0 $ 

. 

w 

2.0 - * 

h 

1.0 g 

0.5 

Figure 1. Two-Parameter Weibull Plot. 
In order to check for possible billet-to-billet variability in cases where multiple billets were 

needed to obtain the requisite number of specimens, the 30 specimens were randomly and 

equally selected from each billet. A simple student t-test was used to determine if any 

differences existed in strength between billets. Such a difference was only observed in the 

Ceralloy 147-3 1N silicon nitride. 

2.2.7 Fracture Analysis. The fracture surfaces from all 30 specimens of each silicon nitride 

were optically examined using a low magnification microscope. Selected fracture surfaces were 

examined in more detail using an SEM (according to ASTM Cl322 [ 193) to determine the 

strength-limiting flaw(s) in each material. Figures 2 (a-d) are SEM photographs that represent 

the dominate strength-limiting flaw type in each silicon nitride. 

The dominant flaw in GS-44CL was a volume-distributed pore (see Figure 2[a]), while the 

strength of CIW15 was limited by a volume-distributed agglomerate of SIC whiskers, as in 

Figure 2(b). The fracture surfaces of the NT55 1 and Ceralloy 147-3 1N (Figures 2[c] and 2[d]) 

revealed that the strength of both was limited by surface flaws created during machining. 

Previous work reported [lo] that under very similar testing conditions, machining-induced 

damage limited the strength of NT551. In the same study, NT551 exhibited a color 

inhomogeniety resembling a “snowflake” on the specimen surface when examined optically. 

The inhomogeniety was linked to the secondary phase in the material. Elemental mapping 

showed that the snowflake regions were comprised of SiO2, while the nonsnowflake regions 

contained A1203, Y203, and Ndz03. It was concluded that the inhomogeneity was the result of a 

separation of this secondary phase or that two different secondary phases are present as a result 

of the processing. The vintage of NT551 examined in this research also exhibited the same 

inhomogeniety. 

2.2.8 Hardness. Vickers hardness (HVl) was determined through the indentation of 

polished sections using a l-kg indentation load and the procedures outlined in ASTM Cl327 

[20]. Five valid indentations were made on each specimen, and the average hardness was 

reported. In cases where multiple billets were used, the hardness from each billet was 

9 
Figure 2(a). Strength-Limiting Flaw in GS-44CL Is a Pore (P”, Surface, ~30 pm). 

Figure 2(b), Strength-Limiting Flaw in CIW15 Is an Agglomerate of Sic Whiskers 

(A”, Surface, ~80 pm). “T” Denotes the Specimen Tensile Surface. 

10 
Figure 2(c). Strength-Limiting Flaw in NT551 Is Machining Damage (MD’, Surface, 

=75 pm). “T” Denotes the Specimen Tensile Surface, and the Arrows 

Indicate the Subsurface Semielliptical Cracks Created During Machining. 

Figure 2(d). Strength-Limiting Flaw in Ceralloy 147-31N Is Machining Damage (MD’, 

Surface, ~60 urn x 200 pm). “T” Denotes the Specimen Tensile Surface, 

and the Arrows Indicate the Subsurface Semielliptical Crack Created 

During Machining. 

13 
determined. The hardness was consistent throughout the various billets in all materials. The 

HVI values of the GS-44CL, NT551, and Ceralloy 147-3 1N are in fairly good agreement with 

the manufacturer”s values, even though these values were obtained with higher indentation loads. 

A direct comparison is impossible for CIWl5 since the company reports Knoop hardness. 

2.2.9 Fracture Toughness. The room temperature Mode I fracture toughness (I&) was 

determined with a minimum of five specimens from each material using the single-edge 

precracked beam (SEPB) technique outlined in ASTM Cl421 [al]. With its long diagonal 

perpendicular to the long axis of the specimen, a single Knoop indentation was placed in the 

middle of one 3-mm-wide face of each specimen using a lo-kg indentation load. A precrack was 

subsequently popped in using a compression anvil with a 6-mm bridge span. Specimens were 

then fractured in four-point flexure using inner and outer spans of 20 and 40 mm, respectively, 

and the length of the popped-in crack measured. The KIC measured in this study was 

consistently below that reported by the manufacturers. This is not surprising since the 

manufacturers use test methods which typically yield higher values of I&. There were no 

observed billet-to-billet variations in toughness for any of these silicon nitrides. 

2.3 Summary. The GS-44CL silicon nitride was selected to exchange with our international 

partner because it had a combination of highest fracture toughness, excellent strength, a high 

Weibull modulus, and because there was no billet-to-billet variability in the properties. 

3. Low Thermal Expansion/Low Thermal Conductivity 

Ceramics 

3.1 Background. A sodium-zirconium-phosphate (NZP) family of materials with low 

thermal expansion, low thermal conductivity, and high thermal shock resistance is produced by 

Low Thermal Expansion Ceramics, Co. (LoTEC). A 70% dense (low density) and an 80% dense 

(medium density) version of this material was purchased and evaluated for exhaust port 

applications. 

12 
3.2 Test Results. Specimens (the same dimensions as those used to determine the properties 

of the silicon nitrides) were machined from large billets nominally 200 mm x 200 mm x 13 mm 

in size. Unless noted, the testing procedures for property determination were the same as those 

previously used for the silicon nitrides. Table 4 is a summary of the material properties. 

Table 4, Propertics of the NZP Low Thermal Expansion Materials 

Product Data Low Density (70%) Medium Density (80%) 

Company 1 ARL Company 1 ARL 

‘he strength, fracture toughness, and elastic modulus data provided by the company are for a 90% dense mater 

3.2.1 Density. Attempts to obtain a valid density for these porous ceramics using the 

Archimedes water immersion method were unsuccessful, as the resultant values were typically 

higher than the manufacturer’s values. Simply immersing the specimen in distilled water was 

insufficient to fill the pores and provide an accurate saturated weight. A true saturated weight 

was achieved by placing the specimens in a desiccator, pulling a vacuum, and back filling with 

water. The densities determined by this modified method are in excellent agreement with the 

respective manufacturer’s data for both versions of the material. 

3.2.2 Elastic Modulus. Elastic modulus values are in excellent agreement with the 

manufacturer’s values. 

3.2.3 Composition. Table 5 summarizes the elemental content of both NZP materials. As 

expected, there is no difference in the elemental composition of the two versions of this material, 

13 
Table 5. Elemental Composition of NZP Materials 

Element 

(weight-percent) 

P 

Sr 

Zr 

11 Ba I 15 I 14 II 

Material 

Medium Density Low Density 

(80%) (70%) 

28 27 

+=l il 

56 57 

II Fe I Cl I Cl II 

I Hf 4 1 Ni - <l I 

Note: The dash (-) denotes an element not detected. 

3.2.4 Therm02 Conductivity. Both of the thermal conductivity values reported are below the 

company value. The low-density NZP has a thermal conductivity of about one half of the 

medium-density NZP, due to the higher content of porosity in the former material. 

3.2.5 Thermal Expansion. The thermal expansion values obtained in this study are higher 

than the manufacturer’s values. This may be caused by differences in the test technique, the 

heating rates, or both. 

3.2.6 Mechanical Strength. As expected, the flexural strength was very low for both 

versions because of the porous nature of the material. The average strength of each material is in 

excellent agreement with the values reported by the manufacturer. No fracture analysis was 

conducted due to the high porosity content and the difficulties associated with fiactography of 

such materials. 

3.2.7 Hardness. Hardness measurements were complicated by the porous nature of these 

NZPs-it took many indents to obtain five valid measurements. Since these are porous 

materials, it is not surprising that the hardness @IV,) of both is relatively low. 

14 
3.2.8 Fracture Toughness. Due to the coarse-grained microstructure and the significant 

amount of porosity in both versions, valid fracture toughness values could not be obtained using 

the SEPB method. No other method was tried. 

3.3 Summary. Both versions of the NZP material appear to have the necessary properties 

for consideration as exhaust port materials. The medium-density NZP was selected to exchange 

with our international partner because the material was readily available at the time of the 

exchange. 

4. Ceramic Thermal Barrier Coatings (TBCs) 

4.1 Background. Ceramic TBCs were examined for potential diesel applications such as 

coatings for piston crowns, rings, cylinder head and liner 

ports. The TBC materials evaluated are listed in Table 6. 

valves, valve seats, and intake exhaust 

Table 6. Ceramic TBC Materials Evaluated 

Manufacturer Material Bond Coat Comments 

Heany Industries Ce02-PSZa NiCrAlY - 

Heany Industries Ce02-PSZ (sealed) NiCrAlY Sol-gel sealed top layer 

Heany Industries Mullite NiCrAlY - 

SUNYb at Stony Brook 8% YzOx-PSZ Ni-4.5Al - 

Praxair Surface Technologies Y2O3-PSZ (LZ16) CoNiCrAlY - 

%Z - Partially stabilized zirconia. 

SUNY - State University of New York. 

The ceramic TBC materials evaluated were selected based on the attempt to push the current 

technology of TBC, and selection from a current commercial-grade coating as the baseline 

material. The three coatings selected from Heany Industries were selected to push the 

technology. The coating from the Thermal Spray Laboratory, SUNY at Stony Brook, was 

selected to see the traditional TBC with a novel bond coat on 4140 steel substrates. The coating 

from Praxair Surface Industries was selected as the baseline TBC. All materials were produced 

15 
using a standard atmospheric plasma spray technique, some were sprayed as coating only on 

removable substrate, and some were sprayed with a bond coat on 4140 steel substrates (as 

indicated in the test method section). 

4.2 Test Results. The physical and mechanical properties of these coatings, as determined 

in this study, are listed in Tables 7 and 8. These property values vary from what has been 

previously reported in literature for similar types of TBCs. This is most likely due to differences 

in coating deposition parameters and different bond coats. 

Table 7. Screening Data for Ceramic TBC Materials 

Thermal Average 

Materials Density Hardness (HK,) Conductivity Thermal Shock CTE 

(g/cc) @‘a) (W/m K) (x 10-6/“c) 

Ce-PSZ 5.22 2.1 + 0.4 0.49 fail 13.4 

Ce-PSZ (sealed) 5.54 3.2 + 0.3 0.65 pass 11.7 

Mullite 2.76 3.4 f 0.2 0.78 fail 6.6 

8% Y-PSZ 5.58 2.9 zk 0.1 0.83 fail 11.7 

,LZ16 5.46 5.3 Ik 0.4 1.52 pass N/A 

Table 8. Adhesion Strength of Ceramic TBCs 

II Materials 1 No. of Specimens Tested 1 Average Adhesion Strength 1 

Ce-PSZa 

Ce-PSZ” 

5 

5 

(MPa) - 

3.2 

4.1 

Ce-PSZ 4 7.5 

LZ-16 4 2.7 

I 

‘TBC coating only, no bond coat used. 

1 

4.2.1 Density. The density was measured by the Archimedes method using an inert gas 

pycnometer at room temperature with lo-mm diameter TBC-only specimens. The average 

density was calculated from a minimum of 10 runs from three specimens. The measured density 

appears to agree with published results found in literature. 

16 
4.2.2 Hurdness. The hardness test was measured in accordance to ASTM Cl326 [22] using 

a Knoop microhardness indenter at room temperature with a l-kg load and minimum of five 

valid indents to generate the standard deviation. The specimen was a polished cross-section of a 

lo-mm-diameter TBC with a bond coat on 4140 steel substrate. 

4.2.3 Thermal Conductivity. The thermal conductivity test was in accordance to JIS R1611, 

as described previously, at room temperature on five, lo-mm-diameter TBC-only specimens. 

4.2.4 Thermal Shock. The thermal shock testing was conducted by placing the lo-mm 

diameter TBC with a bond coat on 4140 steel substrate in a furnace at 800 “C for 30 min in air, 

and then rapidly quenching it in water at room temperature. This sequence constituted one cycle, 

and each specimen was subjected to only one cycle. Three specimens of each material were 

examined to see if delamination or cracking occurred at the interfaces. Each specimen was then 

classified as “pass” or “fail” based on whether delamination occurred. Visual examination 

indicated that delamination occurred at the bond coat/substrate interface, but an additional 

examination with the scanning electron microscope should be conducted to verify whether 

delamination occurred at the bond coat/substrate interface, or at the bond coat/TBC interface. 

4.25 Thermal Expansion. The coefficient of thermal expansion (CTE) was measured from 

room temperature to 800 “C using a dilatometer. The test procedure was in accordance to 

ASTM E228-95 [23]. The CTE measurement was conducted through the coating thickness 

(parallel to the spray direction) of the coating. Future work will be conducted to measure the 

CTE in the direction normal to the spray direction. 

4.2.6 Adhesion Strength. The adhesion strength was measured according to ASTM C633 

[24]. This method is used to determine the adhesive strength of a coating to a substrate when 

subjected to a tensile stress fiel&normal to the surface. In this study, testing consisted of coating 

one flat face of l-inch-diameter cylindrical steel subtrate with the TBUbond coat system, and 

then bonding this coating to the face of an uncoated steel substrate with a two-part, room 

temperature curing epoxy. The entire assembly was then subjected to a tensile load normal to 

17 
the plane of the TBC, at room temperature, until failure. Testing was conducted on the Ce-PSZ 

and LZ-16 coatings, and the results are listed in Table 8. The Ce-PSZ materials were tested with 

and without bond coat (BC), and the LZ-16 material was tested with bond coat. As expected, the 

adhesion strength was significantly better with BC, but still lower than typically reported 

strengths for TBCs. This discrepancy is probably due to the much slower cross-head speed, 

which was set to 0.127 mm/min. A literature review shows that the cross-head speed is typically 

1.27 mm/min. Future work will investigate the effect of cross-head speed on the adhesion 

strength. 

4.3 Summary. The Ce-PSZ and Ce-PSZ sealed materials were selected as the “best” 

materials to exchange with our international partner since they had the lowest thermal 

conductivity and met the need to push ahead the current thermal barrier coating technology. 

Upon further evaluation, TBCs may not be viable candidates for certain engine components due 

to poor wear resistance and processing-related issues. 

18 
5. References 

1. 

. 2. 

. 3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

c 

11. 

Parker, D. W. “Thermal Barrier Coatings for Gas Turbines, Automotive Engines and Diesel 

Equipment.” Materials and&sign, vol. 13, no. 6, pp. 345-351, 1992. 

Winkler, M. F., D. W. Parker, and J. A. Bonar. “Thermal Barrier Coatings for Diesel 

Engines: Ten Years of Experience.” SAE Paper 922438,1992. 

Kinsohita, T., Y, Miyairi, K. Kawasaki, S. Miwa, and M. Masuda. “Development of Silicon 

Nitride Valves for Automotive Engines.” 6th International Symposium on Ceramic 

Materials and Components for Engines, Technoplaza, Co., Ltd., Japan, pp. 193-196, edited 

by K. Niihara, S. Hirano, S. Kanzaki, and K. Morinaga, 1998. 

Woetting, G., H. Lindner, and E. Gugel. “Experience With High-Volume Production of 

Silicon Nitride Automotive Engine Valves.” 6th International Symposium on Ceramic 

Materials and Components for Engines, Technoplaza, Co., Ltd., Japan, pp, 197-202, edited 

by K. Niihara, S. Hirano, S. Kanzaki, and K. Morinaga, 1998. 

American Society for Testing of Materials. “Standard Test Method for Dynamic Young’s 

Modulus, Shear Modulus, and Poisson’s Ratio for Advanced Ceramics by Impulse 

Excitation.” ASTM C1161,1998AnnuaZBookofASTMStundards, vol. 15.01. 

Breder, K., and V. J. Tennery. “Comparison of Flexure Strength, Weibull Parameters and 

Failure Modes for Five High Strength Silicon Nitrides.” Interagency Exchange Agreement 

II, Subtask 7 Report, February 1994. 

Hecht, N. I;, “Environmental Effects in Toughened Ceramics,” Propulsion System 

Materials Program, Bimonthly Technical Progress Report, ORNL/CF-95/15, February 1996. 

Ceramic Machining Consortium, National Institute of Standards and Technology. 

8th Program Review Meeting, March 1996. 

Ott, R. D., K. Breder, T. R. Watkins, M. K. Ferber, and J. M. Rigsbee. “Characterization of 

Machining-Induced Sub-Surface Damage in a High Strength Silicon Nitride.” Ceramic 

Science and Engineering Proceedings, vol. 18, no. 4, pp. 93-103, 1997. 

Andrews, M. J., A. A. Wereszczak, T. P. Kirkland, and K. Breder. “Strength and Fatigue of 

NT55 1 Silicon Nitride and NT55 1 Diesel Exhaust Valves.” ORNL/TM-19991332, February 

2000. 

Ives, L. K., S. Jahamir, L. M. Gill, and J. J. Filliben. “Effect of Grinding on the Strength of 

a Sintered Reaction Bonded Silicon Nitride.” Interceram, vol. 47, no. 3, pp* 168-173, 1998. 

19 
12. American Society for Testing of Materials. “Standard Test Method for Dynamic Young’s 

Modulus, Shear Modulus, and Poisson’s Ratio for Advanced Ceramics by Impulse 

Excitation.” ASTM C 1259, I998 Annual Book of ASTM Standards, vol. 15.01. 

13. Gazza, G. E. “Hot Pressed SiJNJ.” J. American Ceramic Society, vol. 56, no. 12, p. 662, 

1973, 

14. Ekstrtim, T. “Sialon Ceramics Sintered With Yttria and Rare Earth Oxides.” Material 

Resource Socieq Symposium Procedure, vol. 287, pp* 121-l 32, 1993. 

15. Tiegs, T. N., S. D. Nurm, K. L. Ploetz, P. A. Menchoffer, and C. A. Walls. “Fabrication and 

Properties of S&N4 With Rare Earth Apatite Grain Boundary Phases.” Material Resource 

Society Symposium Procedure, vol. 287, pp. 353-358, 1993. 

16. Nunn, S, D., T. N. Tiegs, K. L. Ploetz, C. A. Walls, and N. Bell. “Silicon Nitrides 

Containing Rare Earth Silicate Intergranular Phases.” Material Resource Society Symposium 

Procedure, vol. 287, pp. 359-364, 1993. 

17. Testing and Methods of Thermal D#usivity, Specific Heat Capacity, and l%ermai 

Conductivity for High Pevormance Ceramics by Laser Flash Method. JIS R16 11, Japanese 

Industrial Standard, 1991 e 

18. American Society for Testing of Materials. “Standard Practice for Reporting Uniaxial 

Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics.” 

ASTM C1239,1998 Annual BookofASTMStandards, vol. 15.01. 

19. American Society for Testing of Materials. “Standard Practice for Fractography and 

Characterization of Fracture Origins in Advanced Ceramics,” ASTM C1322, 1998 Annual 

Book of ASTM Standards, vol. 15.01. 

20. American Society for Testing of Materials. “Standard Test Method for Vickers Indentation 

Hardness of Advanced Ceramics.” ASTM C1327-96a, 1998 Annual Book of ASTM 

Standards, vol. 15.01 e 

21. American Society for Testing of Materials. “Standard Test Method for Determination of 

Toughness of Advanced Ceramics at Ambient Temperature.” Annual Book of ASTM 

Standards, vol. 15.01. 

22. American Society for Testing of Materials. “Standard Test Method for Kiroop Indentation 

Hardness of Advanced Ceramics.” ASTM C1326-96a, 1998 Annual Book of ASTM 

Standards, vol. 15.01. 

23. American Society for Testing of Materials. “Standard Test Method for Linear Thermal 

Expansion of Solid Materials With a Vitreous Silica Dilatometer.” ASTM E228, 1998 

Annual Book of ASTM Standards, vol. 14.02. 

20 
24. American Society for Testing of Materials. “Standard Test Method for Adhesion or 

Cohesive Strength of Flame-Sprayed Coatings.” ASTM C633-79, 1998 Annual Book of 

ASTM Standards, vol. 15 .O 1. 

21 
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REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 07040188 

Ott 96-Sep 97-785K3 1 

olithic Ceramics and Ceramic Thermal Barrier Coatings for 

Ott 97-&p 98--88M439 

Jeffrey J. Swab and Paul J. Huang 

Ott 9X-Sep 99-9XM43 1 

U.S. Army Research Laboratory 

ATTN: AMSRL-WM-MC 

ARL-TR-2436 

TARDEC 

Propulsion System Division, Engine Research Function, AMSTA-TR-R, MS 12 1 

Warren, MI 48397-5000 

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Approved for public release; distribution is unlimited. 

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13. ABSTRACT(Maximum 200 words) 

The Metals and Ceramics Research Branch (MCRB) of the Weapons and Materials Research Directorate is providing 

ceramic material characterization and evaluation to the Tank Automotive Research, Development, and Engineering 

Center (TARDEC) as a part of a project entitled “Fighting Vehicle Propulsion Technology Using Ceramic Materials.” 

Through research and exploratory development of advanced ceramics and other technologies, the objective of thk 

project is to improve the mobility of future Army fighting vehicles. The purpose is to demonstrate the capabilities ot 

ceramics, other advanced materials, tribology, and combustion technologies to contribute to a higher power density and 

lower fuel consumption diesel engine. 

Four commercial silicon nitrides were purchased and characterized for potential application as piston crowfl~, valves! 

and valve seats. A sodium-zirconium-phosphate (NZP) family of materials was examined for exhaust port applications! 

and several thermal barrier coating (TBC) materials were examined as possible additions to metallic piston crowns: 

rings, and cylinder liners. This report contains the results of this preliminary characterization and evaluation. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

ceramics, silicon nitride, thermal barrier coatings, diesel engines, partially stabilized 31 

zirconia 16. PRICE CODE 

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Figure Z(a). Strength-Limiting Flaw in GS-44CL Is a Pore (P”, Surface, =30 pm). 

Figure 2(b), Strength-Limiting Flaw in CIW15 Is an Agglomerate of SIC Whiskers 

(A”, Surface, ~80 pm). $‘T” Denotes the Specimen Tensile Surface. 

10 
Figure 2(c). Strength-Limiting Flaw in NT551 Is Machining Damage (MD”, Surface, 

=75 pm). “T” Denotes the Specimen Tensile Surface, and the Arrows 

Indicate the Subsurface Semielliptical Cracks Created During Machining. 

Figure 2(d). Strength-Limiting Flaw in Ceralloy 147-31N Is Machining Damage (MDs, 

Surface, ~60 pm x 200 pm). “T” Denotes the Specimen Tensile Surface, 

and the Arrows Indicate the Subsurface Semielliptical Crack Created 

During Machining.