NASA / CR-2003-212210 

Flame Spray Strain Gages With Improved 

Durability and Lifetimes 

Otto Gregory 

University of Rhode Island, Narragansett, Rhode Island 

Prepared under Grant NAGS2652 

National Aeronautics and 

Space Administration 

. Glenn Research Center 

April 2003 
Flame Spray Strain Gages with Improved Durability and Lifetimes 

Otto Gregory 

University of Rhode Island 

Narrangansett, Rhode Island 

As the temperature capability of engine components is increased and new materials are 

developed to meet these new challenges, there is a growing need to assess material 

behavior in these environments so that structural models can be validated and engine 

materials can be further characterized. Towards this end, improved flame sprayed strain 

gages are being developed to meet these instrumentation needs and will be required to 

operate under more severe thermo-mechanical loadings and at higher temperatures than 

are possible with existing materials and designs. Specifically, new thermal spray 

instrumentation will help NASA and the propulsion engine companies meet the future 

demands of new materials and designs developed under NASA’s Aerospace Propulsion 

and Power (APP) program. The overall program goal is to improve thermal sprayed 

instrumentation by focusing on ways to stabilize the high temperature mechanical and 

electrical properties of the two thermal sprayed layers in which the instrumentation is 

embedded: the metallic bond coat and the ceramic overcoat. Initially, the objective of the 

program was to extend the thermal fatigue life of commercially available thermal sprayed 

coatings. Since oxidation of the NiCoCrAlY and NiCrAlY based bond coats is a leading 

cause of failure in thermal spray instrumentation, a number of different approaches to 

improve the high temperature oxidation resistance of the thermally sprayed bond coats 

were investigated. 

The first approach utilized different heat treatments in reduced oxygen partial pressures, 

to selectively oxidize the aluminum and chromium in the bond coat, i.e. as-sprayed bond 

coats were thermally oxidized in reduced oxygen partial pressures in the temperature 

range 1600 to 1900 OF. As a result of these heat treatments, the interfacial stresses that 

develop in the ceramic overcoats during oxidation of the metallic bond coats were 

dramatically reduced. This was due in part to the presence of both internal and external 

oxides formed within the surface layers of the bond coat, which resulted in a graded 

coating, of sorts, whereby the thermal expansion coefficient (TCE) of the bond coat was 

graded to more closely match that of the ceramic overcoat. Another important benefit of 

these heat treatments was an overall reduction in the oxidation of the metallic bond coat 

onto which the dielectric layer was applied. Several other approaches to improve 

oxidation resistance of the NiCoCrAlY bond coats were investigated, including the 

development of combinatorial libraries to determine the optimal composition of the 

starting bond coat powders, platinum diffusion barriers and/or heat treatments and the 

development of combinatorial libraries to optimize an intermediate coating that would 

effectively match the TCE of the metal bond coat to the ceramic overcoat at the “rough” 

or “wavy” interface formed between the bond coat and overcoat. In addition to the 

improvements made to the thermal sprayed bond coats, the thermal sprayed coatings and 

surface modifications were evaluated in our recently developed computer-controlled 

burner rig facility, which is ideally suited for studying the thermo-mechanical properties 

of the ceramic top coats and bond coats in which instrumentation will be embedded. 

NASNCR-2003-2 122 10 1 
Preliminary data collected during the first nine months of the program indicate that the 

results of this research will substantially increase the lifetime and durability of thermal 

spray strain gage instrumentation, the industry standard for more than 25 years, at 

temperatures well above 1000°C. A detailed summary of the key accomplishments 

completed to date under the APP program is presented below and the major tasks 

(milestoneshenchmarks) that have been accomplished within the proposed time frame 

are highlighted in italics. 

Improved thermal sprayed bond coats via heat treatment in reduced oxygen ambients 

The thermal fatigue properties of commercially available thermal sprayed bond coats 

including Praxair N171 (NiCoCrAIY) and Praxair N343 (NiCrAlY) were evaluated using 

a retractable Inconel 718 test fixture placed inside a conventional tube furnace as shown 

in Figure 1. Thermal fatigue lifetime was based on the of number of cycles to failure 

where each cycle consisted of a 1 hour hold at 1100 "C, rapid cooling to 150 "C (6 to 

8 min) and reheating to 11 00 "C. The fatigue lifetimes of the as-sprayed Praxair N171 

(NiCoCrAlY) and Praxair N343 (NiCrAlY) bond coats were used as a baseline for 

comparison purposes. The lifetimes of the Praxair N17 1 (NiCoCrAlY) bond coats that 

were thermally oxidized in reduced oxygen partial pressures in the temperature range 

1600 to 1900 OF, were substantially improved relative to the as-sprayed Praxair N171 

bond coats. Of these different heat treatments investigated, those done at 1750 "F in 

reduced oxygen partial pressures yielded the largest increase in coating lifetime, relative 

to as-sprayed Praxair N171 coatings (1 10 cycles to failure vs. 42 cycles to failure). Heat 

treatment of the Praxair N343 (NiCrAlY) coatings in reduced oxygen partial pressures 

had little or no effect on coating lifetime at 1100 "C, with the Praxair N343 bond coats 

only surviving 2 to 3 cycles at 1100 "C, independent of heat treatment and surface 

treatment. In all cases, a top coat of magnesium aluminate spinel was thermally sprayed 

onto the Praxair N 17 1 and Praxair N343 bond coats such that two '/z in wide stripes were 

formed on the surface of the coupon. In this way, more than one thermally sprayed 

coating could be evaluated per coupon to reduce testing cycle time. A typical heat 

treatment schedule is shown in Figure 2. It consisted of a series of 20 min ramps and 1 

hour holds until the desired temperature was reached. After this temperature was 

reached, the furnace was ramped down to room temperature at a rate of 3/min until the 

sample was cool enough to remove from the furnace. 

Inconel 718 coupons prepared with thermally sprayed Praxair N171 bond coats survived 

considerably more cycles to failure than Praxair N343 bond coats, when tested at 

1100 "C. For comparison purposes, the as-sprayed Praxair N171 bond coats survived 

42 cycles to failure and the heat treated Praxair N171 bond coats survived 79 to 

144 cycles to failure at 1100 "C, whereas the Praxair N343 bond coats only survived 2 to 

3 cycles to failure at 1 100 "C, independent of heat treatment and surface treatment. 

A summary of the thermal fatigue test results obtained from our furnace rig is presented 

in Table 1. Specifically, the lifetimes of the various thermal sprayed bond coats, heat 

treatments and coatings applied to the bond coats that were thermally cycled from 150 to 

1100 "C are summarized in Table 1 below. 

NASNCR-2003-2 122 10 2 
Figure 1. Photograph of an Inconel 7 18 test coupon being removed from the hot zone of a 

conventional tube furnace maintained at 1100 "C. Note the retractable lnconel 718 test 

fixture on which the test coupon was placed and the test coupon itself that was 

overcoated with 2 stripes of thermally sprayed magnesium aluminate spinel and attached 

to the test bed by platinum wire. A series of three heat shields was required to maintain 

the furnace temperature as the coupon was withdrawn from the hot zone and a 

thermocouple was placed underneath the coupon to monitor temperature. 

2000 

1750 

--4 500 

21 250 

k 

E1 000 

3 

+d 

Q, e 750 

500 

250 

0 

Ramp down 

from 1750 F at 

~- ~- ~ ~~~~ 

ti 

0 5 10 15 20 25 30 

Time (hours) 

Figure 2. Typical heat treatment schedule for a Praxair N171 bond coat heated to 

1750 OF in a reduced oxygen partial pressure ambient. The schedule consisted of a series 

of 20 min ramps and 1 hour holds until the desired temperature was reached. After 

reaching the terminal temperature the furnace was ramped down at a rate of 3/min until 

the Inconel 71 8 coupon was cool enough to be removed from the furnace. 

NASNCR-2003-2 122 IO 3 
-N171-0501-B 

-N171-0501-pB 

-N171-0610-C 

NASNCR-2003-2 122 10 

+0.5%Ce N171 after cooling to 60 C 

0.001 

0.5% Ce 

NiCoCrAlY 0.003 none 1750 Rokide 1100 1 124 TC debonded from BC 

NiCoCrAlY 0.003 Pt 1750 Rokide 1100 1 74* *TC failure after cooling to R1 

when furnace shutdown after 

power outage 

NiCoCrAIY 0.003 - none 1750 Rokide 1100 1 144 TC debonded from BC 

,004 

4 
. 

2-N171Si-0610 NiCoCrAlY 

+ 1%Si 

2-Nl71Ce- NiCoCrAlY 

0610-B + 0.5% Ce 

2-N171Ce-C NiCoCrAlY 

+ 0.5% Ce 

2-N171-0-C NiCoCrAlY 

2-N171Ce-0715 NiCoCrAlY 

+ 0.5% Ce 

2-Nl71Cel- NiCoCrAlY 

0715 + 1.0% Ce 

2-N171-0-D NiCoCrAlY 

0.002 - none 1750 Rokide 1100 1 49* * Sample failed over the 49th 

cycle is last time when sample 

was know to be intact 

.004 

0.0025 - none 1750 Rokide 1100 1 71 * *Sample prematurely failed 

0.003 after cooling to 68 C 

0.002- none none Rokide 1100 1 49 TC debonded from BC 

0.003 

0.003 none none Rokide 1100 1 55 TC debonded from BC 

0.0025 none 1750 Rolude 1100 1 47 TC debonded from BC 

0.003 none 1750 Rokide 1100 1 69 TC debonded from BC; TC 

started to buckle in the center 

of the strips 

0.003 none none Rokide 1100 1 71 014 HA; TC debonded 

from BC 

Evaluation of platinum (and rhodium) diffusion barrier coatings for NiCoCrAlY bond 

coats 

Platinum difision barriers substantially increased the lifetimes of thermal sprayed 

coatings when used in conjunction with heat treatments in reduced partial pressures of 

oxygen. Platinum coatings applied to the surfaces of N171 bond coats by rf sputtering 

and heat treated at 1800 OF in reduced oxygen partial pressures, yielded the highest 

number of cycles to failure (192 cycles to failure) at 1100 "C. A thermal sprayed N171 

bond coat with a platinum diffusion barrier applied to the surface was the best performer 

of all the coatings and heat treatments investigated to date and represented an increase in 

the fatigue life of Praxair N171 coated samples of over 200%. By comparison, the 

as-sprayed Nl71 bond coats only survived 42 cycles to failure at 1100 "C and the N343 

bond coated specimens only survived 2 to 3 cycles to failure at 1100 OC, regardless of 

heat treatment and surface modification. The sputtered platinum films were on the order 

of 2 pm in thickness and proved to be sufficiently thick to act as an effective oxygen 

diffusion barrier, slowing the growth of the internal oxides and promoting an alumina 

rich outer scale on the surface of the N171 bond coat. 

In a similar series of experiments, rhodium difision barriers were investigated to try and 

extend the lifetimes of the thermal sprayed N171 bond coats. However, these 

experiments were done with electroplated rhodium coatings and not sputtered coatings as 

was the case for the platinum difision barriers. The reason being, that this plating 

technique could be readily transferred to bench level technicians without the need for 

specialized training and deposition equipment. In essence, this pen plating process was 

used in conjunction with a special rhodium plating solution that was developed for the 

kind of control needed for this precision plating process. The results were not as 

NASA/CR-2003-2 122 10 5 
dramatic as those obtained form the platinum diffusion barriers prepared by sputtering 

but they resulted in a substantial increase in bond coat lifetime relative to the as-sprayed 

N171 bond coats. Ultimately, a pen plating process is envisioned for the platinum 

diffusion barriers but is still under development at this time. 

Evaluation of alternative coatinns and additives to NiCoCrAl Y bond coats 

Several other approaches to improve the oxidation resistance of the NiCoCrAlY bond 

coats were investigated, including the use of PVD alumina coatings to slow the growth of 

the internal oxides formed within the surface layers of the N 1 7 1 bond coats. When A1203 

coatings on the order of 1.5 pm in thickness were deposited onto Praxair N171 bond 

coats and heat treated in reduced oxygen partial pressures at 1800 OF, a substantial 

increase in fatigue life was observed relative to the as-sprayed Praxair N171 substrates 

(94 cycles to failure vs. 42 cycles to failure), as shown in Table 1. However, when PVD 

AI203 coatings on the order of 1.5 pm in thickness were deposited onto the N343 bond 

coated coupons, they only survived 3 cycles to failure at 1100 OC, as shown in Table 1 of 

this report. 

In the literature, partially stabilized zirconia (PSZ) thermal barrier coatings were applied 

to superalloy substrates by a combination of PVD and thermal spraying to achieve 

improved adherence and oxidation resistance. The thermo-mechanical behavior of these 

coated superalloys at high temperature was dramatically improved using this approach. 

This study provided the motivation to use a combination of PVD and thermally sprayed 

alumina to improve the fatigue life of the Inconel 7 18 coupons bond coated with Praxair 

N171 by improving the adhesion and oxidation resistance. Recent experiments 

employing 5 to 20 pm thick AI203 coatings have indicated that thermal fatigue life of the 

thermal sprayed coatings can be further improved by using thicker pre-coats of alumina. 

The thermal fatigue results of thermal sprayed coatings employing these thicker alumina 

diffusion barrier coatings are presented in a latter section of this report where a computer 

controlled burner rig was used to evaluate the lifetime of the coatings. 

In addition to the approaches described above to improve oxidation resistance of the 

NiCoCrAlY bond coats, metallic powders were added to the bond coat powders prior to 

spraying to improve the fatigue life of the Praxair N171 bond coats. These additives 

showed only marginal improvement in fatigue life. Specifically, silicon additions (1 wt%) 

to the Praxair N171 powders in conjunction with heat treatments to 1750 OF in reduced 

02 partial pressures showed little or no improvement in the fatigue life of the Inconel 7 18 

coupons tested at 1100 "C. Similarly, ceria additions (0.5 to lwt%) in conjunction with 

heat treatments to 1750 OF in reduced 02 partial pressures yielded a marginal increase in 

the fatigue life of Praxair N171 coated samples (69 cycles to failure vs. 63 cycles to 

failure) as shown in Table 1 of this report. In all cases, a ceramic top coat of magnesium 

aluminate spinel was thermally sprayed onto the Praxair N I 7 1 bond coats. 

NASNCR-2003-2 122 10 6 
A comuuter controlled burner rig for evaluating new thermal suraved coatings 

Two different test beds were used to evaluate the lifetimes of the thermal sprayed Inconel 

718 coupons. Initially, testing of the thermal sprayed coupons was carried out in 

retractable Inconel 7 1 8 test fixture placed inside a conventional tube furnace. Thermal 

fatigue tests were based on the number of cycles to failure and consisted of a 1 hour hold 

at 1 100 "C, followed by rapid cooling to 150 "C and then reheating to 1100 "C. More 

recently, a computer-controlled, burner rig (oxygen fueled torch) was designed and 

constructed to evaluate the various bond coats (and heat treatments) under conditions 

more similar to jet engine behavior. These tests consisted of rapid heating of the thermal 

sprayed ceramic top coat (front of Inconel 718 coupon) to 1150 to 1200 "C in an open 

flame, holding in the flame for 60 min followed by rapid cooling to 150 "C (3 min) and 

then reheating to 1150 to 1200 "C. A 150 "C temperature difference was maintained 

between the front of the Inconel 7 18 coupon and the back of the Inconel 71 8 coupon. 

The surface temperature of the ceramic was continuously monitored using an optical 

pyrometer. The burner rig provided a more realistic testing environment to evaluate the 

combination of the thermal sprayed bond coats and top coats while the furnace tests were 

employed to study sintering and oxidation effects of the various thermal spray coatings 

but did not have the severity of the burner rig tests. A photograph of an Inconel 7 18 test 

coupon overcoated with 2 stripes of thermally sprayed magnesium aluminate spinel and 

placed in front of an oxygen fueled torch is shown in Figure 3. The coupon shown in 

Figure 3 was periodically moved in and out of the flame via our computer-controlled 

burner rig. A block diagram of the computer controlled burner rig with the components 

comprising the apparatus is shown in Figure 4 and a photograph of the actual computercontrolled burner rig used to evaluate the thermal fatigue properties of the various 

coatings is shown in Figure 5. Here, the various components comprising the rig are 

visible, including the oxygen fueled torch and a small fan designed to cool the coupon 

from the test temperature within a few minutes. Located underneath the table are a 

variety of motors, controllers and interface boxes that control the movement of the 

Inconel 7 1 8 test coupon on the rail. 

A summary of the thermal cycling test results obtained on our computer controlled burner 

rig showing the lifetimes of the various thermal sprayed coatings and treatments at 

1150 C is presented in Table 2. Only Praxair N171 (NiCoCrAlY) bond coats were 

evaluated using this much more severe testing protocol. Those heat treatments and 

coatings applied to the Praxair N171 (NiCoCrAIY) bond coats that showed the most 

promise in terms of improving the fatigue life were repeated on the computer controlled 

burner rig; i.e. not all of the Inconel 71 8 test coupons bond coated with NiCoCrAlY were 

evaluated using this burner rig. 

. 

NASNCR-2003-2 1221 0 7 
Figire 3. Photograph of an Inconel 7 18 test coupon overcoated with 2 stripes of thermally 

sprayed magnesium aluminate spinel and placed in front of an oxygen fueled torch. The 

coupon is moved into and out of the flame via the computer-controlled burner rig. 

Stepping 

Motor ~ I 

Sample 

- Thermo couple Vice (BLhl"d SamLl, 

~ XL 

~ 

Computer1 

Con troller 

Interface 

Micro 

Computer 

~ 

~ ~ 

~ 

Side View Stage 

Top View 

Oxy-Propane 

Torch 

Figure 4. Schematic of our recently developed computer-controlled burner rig showing 

the various components comprising the apparatus. The rail mechanism, including the 

motors, controllers and interface boxes that control the movement of the Inconel 71 8 test 

coupon are also shown. 

NASNCR-2003-2 1221 0 8 
Figure 5. Photograph of our recently developed computer-controlled burner rig showing 

the various components comprising the apparatus, including the oxygen fueled torch. 

Located underneath the table are a variety of motors, controllers and interface boxes that 

control the movement of the Inconel 718 test coupon on a rail. Also pictured is a small 

fan that is designed to cool the coupon from the test temperature within a few minutes. 

Presentation of kev accomplishments at P WIG meetinas (Mav 2002 and October (2002) 

An initial review of key accomplishments within the first five months of NASA’s APP 

program to improve thermal sprayed instrumentation was presented to members of the 

Ohio Aerospace Institute’s Propulsion Instrumentation Working Group (PW IC) in 

San Diego, CA, at the May 2002 meeting. A second review of key accomplishments 

within the first 10 months of the program will be made at the October 2002 meeting of 

the Ohio Aerospace Institute’s Propulsion Instrumentation Working Group (PWIG) in 

Morgantown, WV. The improvements in thermal spray instrumentation made over the 

first five months of the program were well received at the first PWIG meeting and all 

indications to date are that the temperature range and reliability of the new thermal spray 

instrumentation will be extended well beyond the current state of the art with the 

improvements made thus far in this program. 

NASNCR-2003-2 1221 0 9 
Table 2. Summary of thermal fatigue test results obtained from our computer controlled 

burner rig. Only Praxair N 17 1 (NiCoCrAlY) bond coats, associated surface modifications and 

heat treatments were evaluated using this testing protocol. 

Bond Coat Thermal cycling 

Heat 

treat 

T (F) 

Surface 

Treat 

none 

none 

Comments 

Sample 

previously 

fatigued using 

1 min heating 

and 3.5 min 

cooling cycles 

Failure during 

cooling from 

1 st cycle 

Failure during 

heating to 2nd 

cycle 

Failure during 

heating to 2nd 

cycle 

Failure during 

cooling from 

1st cycle 

Failed during 

heating to 3rd 

cycle 

Spalling of TC 

during cooling 

from 4th cycle 

Decomposition 

of TC during 

cycle 

Gradual 

spalling of TC 

during cycling; 

Extensive 

blistering 

during 4th & 

5th cycle 

Extensive 

blistering 

apparent during 

4th cycle, 

limited spalling 

on heating and 

cooling; 

Extensive 

spalling of the 

TC 

Specimen 

ID 

2R-NI 7 1 - 0 

2R-N 17 I - 

0-B 

2R-N I 7 I - 

0805 

2R-NI71- 

0805-B 

2R-N I7 1 - 0811-2p 

2R-N 171 - 081 I-2pB 

2R-N 17 I - 0-c 

2R-N 17 1 - 0-D 

2R-NI71- 

0909-Rh 

2R-N I71 - 

0909-2Rh 

2R-N 17 1 - 0909- 

20AI-B 

2R-N 17 1 - 0909- 

20AI 

cycles 

to 

failure 

2 

1 

thickness 

(inches) 

0.016 - 

0.017 

0.018 

thickness 

(inches) 

0.0035 

0.003 - 0.004 

type 

Rokide 

Rokide 

type 

NiCoCrAlY 

NiCoCrAlY 

none 

none 

1200 

- 

1 I50 

- 

I I50 

0.0035 - 

0.004 

0.005 - 

0.006 

0.003 - 

0.0035 

Rokide 

Rokide 

Rokide 

NiCoCrAlY 

NiCoCrAlY 

NiCoCrAlY 

1750 

1750 

1800 

none 

none 

2h Pt 

NiCoCrAlY 0.003 2h R 1150 

- 

1 I50 

Rokide 

Rokide 

1800 

NiCoCrAlY 0.007 none none 0.014 

NiCoCrAlY 0.006 none none Rokide 0.014 1 I50 1 4 

1 layer Rh 

plating 

Pen 

0.013- 

0.014 NiCoCrAlY 0.007 1800 Rokide 1 I50 1 5 

Rokide 

2 layers 

Rh pen 

plating 

0.007- 

0.009 

0.013- 

0.014 NiCoCrAlY 1800 1150 1 5 

NiCoCrAlY 0.003 20h 

A1203 Rokide 0.014- 

0.017 1800 1150 1 IO 

NiCoCrAlY 0.003 1800 20h 

A1203 

0.0 14- 

0.017 1 I50 

- 

Extensive 

spalling of the 

TC 

Rokide 1 4 

NASNCR-2003-212210 10 
2R-Nl71- 

2R-N171- 

NiCoCrAlY 

1 

1 

1 

1 I NiCoCrAIY 

Extensive 

spalling and 

blistering in 

contact area 

Strong 

8 degradation of 

TC 

TC debonded 

from BC during 

cooling from 

6th cycle 

*No failure or 

degradation of 

three 

0.014HT 24* the TC after 

I 

1150 

I 

0.015 

0.004- 

0.005 

0.004 

1150 

0.004 

Y 

0.003 

NiCoCrAlY 0.003 

0.003 0.015 

2h Pt 

2h Pt 

1150 

none 

none 

none 

none 

1800 

1800 

none 

none 

none 

none 

Rokide 

Rokide 

100% 

Alumin 

a 

100% 

Alumin 

a 

Rokide 

+ EZ 

TBC 

Rokide 

+ EZ 

TBC 

0.016 1 ~~~~ 

0.017 

0.014HT 1150 

I 

1 

6 

distance 

Failure on 

heating to 6th 

cycle; inside 

comers of TC 

started peeling 

Off 

Ovtimization of bond coat chemistry and intermediate TCE coating chemistw using 

combinatorial libraries. 

Two types of combinatorial chemistry experiments were conducted to improve the 

fatigue lifetimes and electrical properties of the thermal sprayed bond coats. The first 

combinatorial approach to optimize bond coat chemistry was to reformulate the 

composition of the starting NiCoCrAlY powders, such that the electrical properties of the 

thermally sprayed bond coat could be optimized for dielectric strength; i.e. a 

combinatorial method for synthesizing thin film libraries that could be rapidly screened 

by electrical measurements (probing the surface of each library) to identi@ optimal 

compositions that can lead to improved dielectric strength. To accomplish this polished 

Inconel 718 substrates were placed in between an aluminum target and a NiCoCrAlY 

target such that different compositions could be achieved by co-sputtering into welldefined areas via a shadow mask to form the combinatorial library. 

A schematic of the combinatorial library, complete with the location of the substrates and 

spatial arrangement of the shadow masks is shown in Figure 6. In this way, a wide range 

of alloys were produced on the Inconel 718 coupons depending on the distance (spatial 

arrangement of the shadow mask) from each sputtering target as shown in Figure 7. The 

specimens were oxidized under reduced oxygen partial pressures at 1750 OF after which 

each combinatorial library was tested for dielectric strength using a Kiethley 

picometerholtage source. The breakdown voltage was determined by measuring the 

NASA/CR-2003-212210 11 
current as the voltage was ramped from zero to the breakdown voltage in 10 volt 

increments. At the breakdown voltage, the current rapidly increased by orders of 

magnitude. The areas showing the greatest dielectric strength were analyzed for 

chemical composition by energy dispersive analysis of x-rays (EDS). Those compositions 

will be used as starting point in preparing new thermal spray powders. The results of 

combinatorial approach to optimize dielectric strength of the combinatorial library 

produced by co-sputtering aluminumlNiCoCrA1Y films (formed on Inconel 7 18 

substrates) are shown in Figure 6. Based on this analysis, intermediate combinations of 

aluminum/NiCoCrAlY provided the largest dielectric strengths of all combinations tested 

in this manner. The chemical analysis using energy dispersive analysis of x-rays (EDS) 

to determine the optimal chemical composition for dielectric strength has recently been 

completed. It was determined that alloys containing - 20% AI as compared to 13% AI in 

the NiCoCrAlY provided the greatest dielectric strength. 

A second combinatorial approach to optimize the chemistry of an intermediate TCE 

coating was also completed. Here the basic idea was to match the TCE of the metallic 

and ceramic layers at the wavy interface formed by the thermal sprayed coatings. This 

requires mixing the two materials in a ratio that will minimize the thermal stresses in the 

wavy interface. It is generally agreed upon in the literature that these thermal sprayed 

coatings fail as a result of thermal expansion mismatch between the ceramic and metallic 

layers in these coating systems. To form the combinatorial library, Inconel 71 8 coupons 

were thermally sprayed with Praxair N 17 1 bond coats and were used as substrates. These 

substrates were placed between an aluminum oxide target and a NiCoCrAlY target and 

the material was co-sputtered into well-defined areas via a shadow mask to form a 

combinatorial library, as shown in Figure 8. Thus, a wide range of alloy compositions 

was produced, depending on the distance (spatial arrangement of the shadow masks) 

fiom each sputtering target as shown in Figure 9. These segmented regions or 

combinatorial libraries were thermally sprayed with a magnesium aluminate top coat and 

subjected to repeated thermal cycling (from 150 to 1100 “C) in our furnace rig to 

determine which compositions minimize cracking of the ceramic top coat, i.e. optimize 

the composition of the intermediate coating that leads to minimal thermal expansion 

differences between the peaks and valleys of the wavy interface formed by the thermal 

sprayed bond coat. The combinatorial library showing the least cracking was analyzed for 

chemical composition by energy dispersive analysis of x-rays (EDS). The optimized 

composition will be made into a new powder mix that will be thermally sprayed as a thin 

layer on the N171 bond coat to affect the TCE mismatch at the very “irregular” bond 

coat/ top coat interface. The chemical analysis using energy dispersive analysis of x-rays 

(EDS) is still underway to determine the optimal chemical composition of the 

intermediate TCE coating. 

NASNCR-2003-2 122 10 12 
003 HA 005 HA 

002 HA 004 HA 

Figure 6. Results of combinatorial chemistry experiments to evaluate dielectric strengths 

(breakdown voltages) of the combinatorial libraries formed by co-sputtering aluminum 

and NiCoCrAlY on Inconel 7 18 substrates. The breakdown voltages associated with each 

combinatorial library are included in the schematic. The boxed numbers correspond to 

the breakdown voltages within each combinatorial library. 

1 

Figure 7. Photograph of combinatorial libraries formed on Inconel 

co-sputtering aluminum and NiCoCrAlY onto Inconel 7 18 substrates. 

718 substrates by 

NASA/CR-2003-212210 13 
Figure 8. Results of combinatorial chemistry experiments to optimize intermediate 

coating chemistry that would match the TCE of the metallic and ceramic layers at the 

wavy interface formed by the thermal sprayed coatings. The combinatorial libraries were 

formed by co-sputtering aluminum oxide and NiCoCrAlY onto Inconel 71 8 substrates 

that were thermally sprayed with Praxair N171 bond coats. The lifetimes of each 

combinatorial library (in hours) are included in the schematic. These times (boxed 

numbers) correspond to the onset of microcracking and delamination within each 

combinatorial library. 

Figure 9. Photograph of combinatorial libraries formed on the surface of Praxair N171 

bond coats by co-sputtering aluminum oxide and NiCoCrAlY. 

NASNCR-2003-2 1221 0 14 
Characterization of thermal sway ceramic coatings 

All failed test coupons were cross sectioned using standard metallographic procedures 

and examined by scanning electron microscopy (SEM) using our state of the art JEOL 

5900 Environmental Scanning Electron Microscope, equipped with light element EDS 

and 3 backscattered electron detectors. Cross sections of the failed coatings were 

examined in backscattered mode, since this provides the best contrast from an imaging 

viewpoint; i.e. this mode provides the greatest sensitivity to atomic number differences of 

the various constituents comprising the thermal sprayed coatings. The interface regions 

of failed Inconel 718 test coupons thermally sprayed with Praxair N343 indicated 

adhesive failure within the bond coat whereas those sprayed with Praxair N171 failed by 

decohesiod delamination at top coatbond coat interface. Representative examples of the 

two different types of failure in the thermally sprayed bond coats are shown in the SEM 

micrographs of Figures l.O(a) and (b), respectively. The white phase present in each cross 

section is the platinum diffusion barrier applied to each bond coat. In general, those test 

coupons thermally sprayed with thicker bond coats and top coats resulted in a greater 

number of cycles to failure at 1100 "C. 

8 

,-- -- 

(a> 

Figure 10. (a) SEM micrograph of a failed Praxair N171 thermal sprayed bond coat 

showing failure by decohesion / delamination at top coatbond coat interface and 

(b) SEM micrograph of a failed Praxair N343 thermally sprayed bond coat showing 

adhesive failure within the bond coat. 

The failure mechanisms observed for the thermal sprayed coatings fatigue tested in our 

computer-controlled burner rig were very different than those observed for the thermal 

sprayed coatings tested in our furnace rig. The Inconel 71 8 coupons with Praxair N 17 1 

bond coats that were overcoated with high purity alumina, failed by adhesive failure and 

delamination at the top coatbond coat interface similar to the magnesium aluminate 

spinel overcoated Inconel 718 coupons that were tested in our furnace rig. However, 

Inconel 71 8 coupons with Praxair N 1 7 1 bond coats that were overcoated with magnesium 

aluminate spinel and fatigue tested in our computer-controlled burner rig failed by 

cohesive failure due to disintegration (localized spalling andor flaking of the ceramic top 

coat). It was also observed that the surfaces of the magnesium aluminate spinel top coats 

NASNCR-2003-2 12210 15 
became very rough after thermal cycling and the surface morphology changed 

considerably, similar to the changes observed in thermal sprayed zirconia TBC's. In 

these systems, sintering effects and phase changes in the thermal sprayed zirconia TBC 

coatings have been linked to failure. 

SEM micrographs of as-sprayed magnesium aluminate spinel and as-sprayed high purity 

alumina are shown in Figures ll(a) and (b). The surface morphology of the two top 

coats shown in Figure I1 appears to be somewhat similar. However, after thermal 

cycling to 1400 "C, fine structure that is barely visible in the microstructure of the 

magnesium aluminate spinel shown in Figure 1 I(a), becomes much more pronounced as 

shown in the SEM micrograph in Figure 12. Here there is evidence that crystallization 

and/or a phase change has occurred in the bulk ceramic upon heating to these 

temperatures. This surface morphology is very similar to that of amorphous silicon 

dioxide that has been thermally cycled to 1200 "C. When Si3N4 ceramics are oxidized at 

1200 "C, an amorphous Si02 is formed on the surface. However, after repeated thermal 

cycling, devitrification of the silica occurs and small crystallites of cristobalite 

(crystalline Si02) grow within the amorphous Si02 matrix, similar to the surface 

morphology shown in Figure 12. 

These SEM micrographs may hold several clues as to why Inconel 7 18 coupons prepared 

with Praxair N171 bond coats and overcoated with high purity alumina survived a 

significantly greater number of cycles to failure when tested in our computer-controlled 

burner rig than did the N171 bond coats overcoated with magnesium aluminate spinel 

(24 + cycles to failure vs. 4 cycles to failure). A differential scanning calorimetry (DSC) 

spectra of the two thermaIIy sprayed top coats (magnesium aluminate spinel and high 

purity alumina) cycled from room temperature to 1400 "C is shown in Figures 13 and 14. 

These DSC spectra confirm the SEM findings in that a large peak is present in the DSC 

spectra of the magnesium aluminate spinel at 850 "C (Figure 13) but no such peaks are 

present in the DSC spectra of the high purity alumina (Figure 14). 

The fatigue lifetimes of the Praxair N171 (NiCoCrAIY) bond coats that were thermally 

oxidized in reduced oxygen partial pressure, were not improved relative to the as-sprayed 

Praxair N 171 bond coats, when tested in our computer-controlled burner rig. Platinum 

diffusion barriers (2 pm thick) applied to Praxair N 171 (NiCoCrAlY) bond coats resulted 

in considerable improvement in fatigue life relative to the as-sprayed Praxair N 171 bond 

coats, essentially doubling the lifetime (8 cycles to failure vs. 4 cycles to failure) as 

shown in Table 2. When relatively thick AI203 pre-coats (5 pm thick) have been applied 

to Praxair N171 (NiCoCrAlY) bond coats as diffusion barrier coatings, the fatigue life of 

the thermal sprayed coatings was substantially improved relative to the as-sprayed 

Praxair N 171 (NiCoCrAlY) bond coats (1 0 cycles to failure vs. 4 cycles to failure) when 

tested in our computer-controlled burner rig, as shown in Table 2. 

NASNCR-2003-2 12210 16 
(a) (b) 

Figure 1 1. (a) SEM micrograph of as-sprayed magnesium aluminate spinel coating and 

(b) SEM micrograph of as-sprayed high purity alumina coating. 

Figure 12. SEM micrograph of an as-sprayed magnesium aluminate spinel coating. 

There is evidence that crystallization and/or a phase change has occurred in the bulk 

ceramic upon thermal cycling to 1400 "C. 

, 

NASNCR-2003-2 1221 0 17 
DSC bsulti For RameQpnpd Magnesium Aluminate 

2- 

15 

1- 

05 - 

g 

2 0- 

H 

Y 

-05 

- 

- 

- 

. 

100 

~~ 13 40 

.,-: I 

-2 - 

remperature IT,] 

Figure 13. DSC scan of an as-sprayed magnesium aluminate spinel coating. Note the 

peak at 850 "C, suggesting that crystallization and/or a phase change has occurred in the 

bulk ceramic upon thermal cycling to 1400 "C. 

DSC Data of SDT Norton 1702-12 Powder 

20 

10 

0 

- 

m 

s 

- g -10 Y 

.d 

P 

-20 

-30 

Temperature ("C) 

110 

1M) 

90 

80 

605 ~~~ 

-Heat Flow '- m -Weight% 

50 ~ 

-40 

Figure 14. DSC scan of an as-sprayed high purity alumina coating. Note that there are 

no sharp peaks present in the spectra, suggesting that no crystallization/phase changes 

should occur in the bulk ceramic when thermally cycled to 1400 "C. 

NASNCR-2003-2 1221 0 18 
Conclusions 

The focus of this APP research program was to improve the bond coats used in the 

fabrication of flame sprayed instrumentation. Typically, a bond coat is applied to a 

superalloy surface prior to the application of a thin dielectric coating onto which 

instrumentation is placed. After affixing the instrumentation, a much thicker ceramic 

topcoat is typically applied to protect the instrumentation from harsh environments. The 

fatigue life of NiCoCrAlY coated superalloys was extended beyond current state-of-theart by relatively simple and cost effective means. Heat treatment in reduced oxygen 

partial pressures at 1750 to 1800 OF effectively doubled the fatigue life of NiCoCrAlY 

coated substrates relative to as-sprayed substrates and when used in conjunction with 

platinum diffusion barriers yielded a four fold increase in the fatigue life of NiCoCrAlY 

coated substrates. Further improvements in the fatigue life of thermally sprayed coatings 

were made by employing intermediate coatings, which minimized thermal expansion 

differences between the bond coat and top coat. Combinatorial chemistry experiments 

yielded an optimum composition for an intermediate TCE matching coating that showed 

considerable promise in extending the fatigue life of thermal spray instrumentation. The 

intermediate coating had two functions: to reduce the surface roughness of the peaks and 

valleys associated with the as-sprayed NiCoCrAlY bond coat, and to produce a thin layer 

of a mixture of A1203 and NiCoCrAlY that exhibited an intermediate TCE. The optimal 

composition of the intermediate coating consisted of 60 wt% A1203 and 40 wt% 

NiCoCrAIY, as determined by energy dispersive analysis of x-rays (EDS). Intermediate 

coatings having this composition were prepared by physical vapor deposition and the 

resulting coating systems are being evaluated in our test facility. 

The Ohio Aerospace Institute’s PWIG team members from Pratt and Whitney and 

Honeywell engines have supplied all of the necessary superalloy substrates to date. 

These included both Inconel 71 8 coupons and Inconel 71 8 parts for the test rig as well as 

single crystal superalloys (Pratt and Whitney). NASA and The Ohio Aerospace Institute’s 

Propulsion Instrumentation Working Group will verify the modifications to the existing 

flame spray technology by applying them to propulsion hardware such that structural 

models can be validated and the lifetime of components predicted. 

. 

NASA/CR-2003-212210 19 
Presentations-NASA APP program: “Flame Spray Strain Gages with Improved 

Durability and Lifetimes 

Ohio Aerospace Institute -Propulsion Instrumentation Working Group; “Flame Spray 

Strain Gages with Improved Lifetimes and Durability”, O.J. Gregory, Markus Downey, 

Steve Wnuk and Vince Wnuk, San Diego, CA, May 2002 

Ohio Aerospace Institute -Propulsion Instrumentation Working Group; “Flame Spray 

Strain Gages with Improved Lifetimes and Durability”, O.J. Gregory, Markus Downey, 

Tim Starr, Steve Wnuk and Vince Wnuk, San Diego, CA, Morgantown, WV, Oct. 2002 

Students funded on NASA APP program: 

Markus Downey-MS student in Chemical Engineering 

Nick Pascucci-Undergraduate student in Chemical Engineering 

Tim Starr-Undergraduate student in Chemical Engineering 

NASNCR-2003-2 122 10 20 
REPORT DOCUMENTATION PAGE 

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 

April 2003 

Form Approved 

OMB NO. 0704-0188 

3. REPORT TYPE AND DATES COVERED 

Annual Contractor Report-December 2001 -October 200 

4. TITLE AND SUBTITLE 

1 Flame Spray Strain Gages With Improved Durability and Lifetimes 

WU-708-87-23-00 

NAG3-2652 6. AUTHOR(S) 

Otto Gregory 

5. FUNDING NUMBERS 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

University of Rhode Island 

15 S. Ferry Road 

Narragansett, Rhode Island 02882 

9. SPONSORlNG/MONlTORlNG AGENCY NAME(S) AND ADDRESS(ES) 

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

14. SUBJECT TERMS 

Strain gauge; Flame spray; High temperature 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 

OF REPORT OF THIS PAGE OF ABSTRACT 

Unclassified Unclassified Unclassified 

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26 

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AGENCY REPORT NUMBER 

NASA CR-2003-212210 

I 

11. SUPPLEMENTARY NOTES 

Project Manager, Gustave Fralick, Instrumentation and Controls Division, NASA Glenn Research Center, organization 

code 5510,216-433-3645. 

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Subject Category: 35 Distribution: Nonstandard 

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Available electronically at httu://pltrs.erc.nnsa.pov 

This publication is available from the NASA Center for Aerospace Information, 3014214390. 

13. ABSTRACT (Maximum 200 words) 

The focus of this AF'P research program was to improve the bond coats used in the fabrication of flame sprayed instrumentation. Typically. a bond coat 

is applied to a superalloy surface prior to the application of a thin dielectric coating onto which instrumentation is placed. After affixing the instrumentation, a much thicker ceramic topcoat is typically applied to protect the instrumentation from harsh environments. The fatigue life of NiCoCrAlY 

coated superalloys was extended beyond current state-of-the-art by relatively simple and cost effective means. Heat treatment in reduced oxygen partial 

pressures at 1750 to 1800 OF effectively doubled the fatigue life of NiCoCrAlY coated substrates relative to as-sprayed substrates and when used in 

conjunction with platinum diffusion barriers yielded a four fold increase in the fatigue life of NiCoCrAlY coated substrates. Further improvements in 

the fatigue life of thermally sprayed coatings were made by employing intermediate coatings, which minimized thermal expansion differences between 

the bond coat and top coat. Combinatorial chemistry experiments yielded an optimum composition for an intermediate TCE matching coating that 

showed considerable promise in extending the fatigue life of thermal spray instrumentation. The intermediate coating had two functions: to reduce the 

surface roughness of the peaks and valleys associated with the as-sprayed NiCoCrAlY bond coat, and to produce a thin layer of a mixture of A$0, and 

NiCoCrAlY that exhibited an intermediate TCE. The optimal composition of the intermediate coating consisted of 60 wt% A1,0, and 40 wt% 

NiCoCrAIY, as determined by energy dispersive analysis of x-rays (EDS). Intermediate coatings having this composition were prepared by physical 

vapor deposition and the resulting coating systems are being evaluated in our test facility. 

NSN 7540-01 -280-5500 Standard Form 298 (Rev. 2-89) 

Prescribed by ANSI Std. 239-18 

298-1 02