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Thermal expansion of Tribomet MCrAlY coatings
Surface & Coatings Technology 201 (2006) 3819 – 3823 www.elsevier.com/locate/surfcoat
Thermal expansion of Tribomet MCrAlY coatings Thomas Taylor ⁎, John Foster Praxair Surface Technologies, United States Available online 20 September 2006
Abstract The thermal expansion behavior of seven Tribomet™ MCrAlY coatings was determined, using the PST Sapphire Dilatometer. Expansion equations were found vs. temperature and also vs. chemical composition. TM 301, NiCrAlY, has the lowest expansion of the group. © 2006 Elsevier B.V. All rights reserved. Keywords: Abradable coating; NiCrAl-bentonite
1. Introduction In the thermal barrier coating (TBC), one usually considers the metallic bondcoat and the zirconia-based ceramic as a system. Knowledge of the thermal expansion behavior of the bondcoat and ceramic topcoat is important because the relatively large expansion mismatch between bondcoat and ceramic imposes a tension stress in the ceramic on heating. Assume that the system is held at temperature for awhile, and the bondcoat relaxes by creep. Upon cooling, this same expansion mismatch creates a compression stress in the ceramic. These stresses in the ceramic contribute to interface crack propagation and potentially to separation. If the mismatch expansions were less the situation should lead to longer TBC life. The zirconia ceramic, typically yttria-stabilized, already has one of the largest expansion coefficients for an oxide. The reduction in mismatch then focuses on the bondcoat. In a previous program [1,2] common plasma sprayed MCrAlY alloys were studied, where M is Co, Ni, or a Ni–Co mixture. There it was found that indeed the actual composition of the MCrAlY coating mattered relative to its thermal expansion: typically all Ni–Co matrix MCrAlY coatings had relatively high thermal expansion, Co-base had somewhat less, but NiCrAlY coatings had the lowest thermal expansion. Using Multiple Correlation techniques, equations were found to predict the thermal expansion values for any MCrAlY coating from its composition. Further, a new range of Cr and Al compositions for the NiCrAlY was found from this predictive equation, coating samples were thus made and found to have the low expansion predicted [1,2]. ⁎ Corresponding author. E-mail address: [email protected] (T. Taylor). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.261
However, PST also makes MCrAlY coatings by the advanced electroplating process, Tribomet™. In principal the composition of a given Tribomet could be made to match that of the powder and coating of the earlier studied thermally sprayed coatings. The process to make Tribomet is quite different, described below, including the heat treatment cycle used to homogenize the components to produce the final MCrAlY coating. Thermally sprayed coatings start from fully pre-alloyed powder of uniform composition, and they also typically have a heat treatment cycle after coating to densify the coating and improve bonding to the substrate. In both coating approaches the final state will have multiple phases, such as aluminide and face-centered cubic matrix phases, with others possible. However, the metallurgical process to reach the final, stabilized coating state is different between the two processes, and the final microstructures are different in detail. The question to be answered in the present study is does the coating process, in addition to the coating composition, affect the resulting thermal expansion? Will it be possible to correlate expansion to composition in Tribomet coatings and will that relation be similar to the one for thermal spray MCrAlY coatings? In this thermal expansion study, like the previous work, we only include expansion data of thermally stabilized coatings, that is, after the initial heat treatment has completed the reaction and densification processes. 2. Experimental 2.1. Tribomet samples Seven different MCrAlY-type coatings were prepared for this study. All were made by PST's advanced Tribomet electroplate
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T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823
process at the Weston Super-Mare facility. In the Tribomet process, a wide range of different NiCoCrAlY compositions can be deposited on complex substrates. Fine particles of CrAlYtype compositions are designed in conjunction with a Ni, Co or variable Ni–Co electroplated matrix to produce the final desired composition. The particles are in suspension in the electroplating solution and are entrapped as the electrodeposited matrix is built up. This “green” coating is then vacuum heat treated to form the MCrAlY alloy coating. The present program coatings only include Ni, Co, Cr and Al variations, except for TM 304 which contained 4.5 wt.% Ta in addition. TM 286 does have 0.25 Hf and 0.4 wt.% Si, and these were considered negligible in the multiple correlation fit to composition. Likewise, all coatings had about 0.5 wt.% Y, so it was not included in the fit since it was not a variable. All coatings were deposited to about 10–15 mils thickness on 0.50-in. OD aluminum tubes. The coated tube was parted to 1.00 in. long cylinders, and most of the aluminum was bored out. The final step was to leach residual aluminum in 25% NaOH at a controlled temperature (b60 °C) for about 90 min. The NaOH solution does not attack the MCrAlY coating. After leaching the coating sample was rinsed in de-ionized (DI) water, ultrasonically rinsed in DI water, rinsed in methanol and warm air dried. 2.2. Dilatometer methodology All samples were heat-treated 4 h at 1080 °C in argon to reach a stable sintered state by running the “green” or as-coated samples in the dilatometer. The same dilatometer cycle was used for the green and all subsequent thermal expansion runs: heating at 5 °C/min to 1080 °C, holding 4 h and cooling at 5 °C/min. If any residual sintering occurred, more than a few 0.1 mils/in., the data was not included in this study, but in the sample re-run until it was stable. The dilatometer is calibrated by running a 1.00-in. long sample of pure Ni, traceable to NIST. The sample is run multiple
Table 1 Average thermal expansion dependence on temperature TM coating
301 302 304 308 309 312 286
½LðT Þ−Lð25-CÞ=Lð25-CÞ ¼ A þ B4T þ C4T 2 ðmm=mÞ A
B
C × 106
Max. T, °C
No. runs
−0.1454 −0.2990 −0.1833 −0.3134 −0.2775 −0.3877 −0.2866
0.0098 0.0134 0.0108 0.0130 0.0129 0.0136 0.0131
7.248 3.898 6.604 4.029 5.624 4.416 5.360
1050 1050 1050 1050 1050 1050 1050
3 4 3 3 3 3 4
times and the average heating and cooling curves are compared to the accepted Ni expansion data published by TPRL [4]. Any deviation is formed into a correction list which the computer applies to all subsequent samples. 2.3. Dilatometer testing and expansion data analysis All samples reported here were run at least three to four times, some even eight. The practice has always been to use just the cooling curve to generate the thermal expansion data reported, although the heating curve for a well-stabilized coating is very close. The cooling curve is shifted if necessary to give the zero expansion value at 25 °C, so all expansions are relative to 25 °C. The fully corrected data is entered as temperature and time pairs, from 1075 °C down to 25 °C, and given a leastsquares parabolic fit of expansion to temperature. This second order, upward curving line fits the data very well, returning an R-squared value close to 1.00 in each case. To make a judgment on the goodness of the data, the fit equation is used to calculate the expansion at 525 and 825 °C, and these two values are compared to all other similar values for the numerous runs on that exact same sample. There were a few cases of run data that was dropped from the final average. In some of these cases there was an experimental issue; in others the expansion values just were unexplainably different, high or low, from the main trend. Using this screening, the final standard deviation of the data that was accepted was about 2% or less of the expansion value average. After the screening, for each coating sample the individual run fit equations were used to calculate the expansion at 25, 100 and each increasing 100 degree increment to 1100 °C. Then the average at each temperature increment was determined. These average expansion values were then least-squares fit as a second order equation for temperature. This final result is what is reported here for the average expansion equation for the seven Tribomet coatings. The final equation was very close to the individual run fit equations, as to be expected. 2.4. Coating chemical analysis
Fig. 1. Average thermal expansion as a function of temperature for Tribomet 301.
Mating coatings were made in each case to the tube samples sent to PST's Speedway Laboratory for thermal expansion testing. These were weighed and the electroplated matrix was dissolved in the green state in nitric acid, and the “CrAlY”
T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823 Table 2 Average thermal expansion of coatings from 25 °C (mm/m)
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The three independent variables sets for trial multiple correlation fitting were as follows:
TM coating
525 °C
SD
SD as % aver
825 °C
SD
SD as % aver
No. of runs
301 302 304 308 309 312 286
7.009 7.821 7.333 7.862 8.032 7.991 8.079
0.073 0.080 0.043 0.181 0.046 0.108 0.105
1.04 1.02 0.59 2.30 0.57 1.35 1.29
12.891 13.426 13.263 13.442 14.172 13.872 14.186
0.133 0.175 0.064 0.233 0.075 0.149 0.238
1.04 1.30 0.48 1.66 0.53 1.08 1.68
3 4 3 3 3 3 4
particle residue collected, dried and weighed. The Co of the dissolved solution was chemically analyzed by atomic absorption, nickel taken as balance. The matrix and CrAlY weight fractions thus obtained gravimetrically, and with the Ni–Co analysis and the earlier wet chemical lot analysis of the CrAlY particles, allowed calculation of the final coating composition.
A B C
Ni; Cr; Al and Ni*Co Co; Cr; Al and Ni*Co Ni; Co; Cr; Al and Ni*Co
Sets A and B gave good and very similar fits with the ability to predict back the expansion values for each coating close to the original data. Set C gave an exact fit to the original data (R-squared = 1.000), but it gave very unlikely results when predicting expansion beyond the original range of the coating compositions. Set C was in essence, over-fitting the data. Set A is also the same set of independent variables used before in the study of thermally sprayed MCrAlY's [1], and it will be seen to have produced an equation that is similar in form to before, but it is not the same in detail which will be discussed below. 3. Results
2.5. Expansion–composition multiple correlation 3.1. Stabilized thermal expansion The last data analysis was to fit the average expansion of the coatings to the chemical analysis of the same coating. For this, the multiple correlation software SPC KISS 97 [5] was used. The wet chemical analysis of the coatings was entered for Ni, Co, Cr and Al. The minor elements Y, Si, Hf were neglected. All coatings had about 0.5 wt.% Y so it was not a variable, but was not expected to have an effect on expansion. Two coatings were of some concern; TM 308 had 2.3 wt.% Hf and TM 304 had 4.1 wt.% Ta. With only seven coatings, four major variables and one interaction gave only one extra degree of freedom. Further, the range of the compositions between coatings was not large. The multiple correlation program was struggling to find a good fit with all coatings included. TM 304 appeared to be lower in expansion than the closest similar composition, TM 309 which had no Ta. When TM 304 was removed from the correlation, the fit came together, since the Ta effect apparently was a wild card that was not being addressed by the major elements in the fit. TM 304 will be discussed separately. With six coatings, using four major elements as variables and one interaction, the fit was reasonable, able to explain about 90% of the variation by the resulting equations. It remains to be noted that the range of variation of Cr and Al were not large, which make the power of the correlation difficult. Three sets of independent variables were tried for the fit of the dependent variable thermal expansion. The thermal expansion terms were taken from the original parabolic fit equations to the data, averaged over several runs to give the final coating expansion equation, as described above. Expansion values were calculated at 525 and 825 °C and these were used in two separate correlation exercises. The reasons for selecting these two temperatures was given earlier [1], suggesting that at 525° the coatings still had high yield strength and could still impart thermal expansion mismatch stress to the YSZ layer in a TBC. At 825°, while low in yield strength the thermal expansion values were larger and maybe more divergent between coatings and would allow a better correlation to composition.
The final average expansion behavior for a typical Tribomet coating is shown in Fig. 1, including the best parabolic fit equation to that data. These equations are listed in Table 1 for all seven coatings. The expansion values to be used with the multiple correlation fit to coating composition are given in Table 2, including the standard deviation of the values and the number of dilatometer runs used to make the average result. 3.2. Wet chemical and SEM/EDS analyses The wet chemical analysis method was described above, and the results are given in Table 3. 3.3. Correlation of expansion to composition 3.3.1. 525 °C correlation The resulting multiple correlation equation for thermal expansion at 525 °C was expð525Þ ¼ 25:765 0:03644Ni 0:40654Cr 0:89984Al þ 0:00077994Ni*Co where the indicated elemental compositions are entered as weight percent (by wet chemical analysis). The units of expansion are Table 3 Wet chemical analyses of actual coatings used in dilatometry Coating
TM 301 TM 302 TM 304 TM 308 TM 309 TM 312 TM 286
wt.% Al
Cr
8.6 9.60 8.20 9.30 8.90 9.24 11.10
20.7 23.10 19.80 23.30 21.50 22.30 15.32
Co 66.60 21.50 65.10 18.20 35.40 14.36
Ni
Y
70.2
0.5 0.50 0.80
45.50
Hf
Ta
Si
Sum
0.41
100.00 99.80 99.90 100.00 99.80 99.79 99.73
4.10 2.30
50.70 32.30 58.14
0.50 0.55 0.20
0.20
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T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823
tion from the measured expansion, with an average deviation of 0.13 mm/m or about 0.9% of the average expansion value at 825 °C. The surface plot trend of Tribomet expansion at 825 °C as a function of Ni and Co, with Cr and Al fixed at the mean values of the range evaluated, has a twist or warp, as did the 525 °C plot, due to the Ni*Co interaction term in the Exp (825) equation. In Fig. 3, the 825 °C expansion data as predicted by the fit equation is seen as a two-dimensional plot for Ni and Co. The trend takes a different shape but it again indicates that expansion is reduced as the composition moves toward high Ni and low Co. Similarly for the 825 °C expansion data as a function of Cr and Al, with Ni and Co held constant at their mean values the plot is linear in each variable because that is what we selected for the form of the fit equation. As found at 525 °C this fit equation predicts lower expansion as either or both Cr and Al are increased. Fig. 2. Plot of thermal expansion (mm/m) variation with Ni and Co (wt.%) at 525 °C, with Cr and Al concentrations held constant at the mean values of the range studied.
mm/m, or equivalently, mils/in. This equation accounts for about 94% of the variation of the data. Each coating's expansion as predicted by the equation was found to be very close to the data, usually within 0.1 mm/m or less. The overall average deviation was 0.075 mm/m, or about 1% of the expansion value. Graphical plots of the above equation help visualize the trend with composition. An expansion surface for 525 °C as a function of Ni and Co, with Cr and Al held constant at their mean values, is seen to have a slight warp, which is the effect of the interaction term Ni*Co in the equation. The two-dimensional plot more clearly shows expansion is reduced toward the corner of low Co and high Ni (Fig. 2). A two-dimensional plot with variation in Cr and Al at the mean values of Ni and Co as constant shows another prediction for expansion. Expansion is reduced as the composition moves toward high Cr and high Al.
3.4. Ta effect in TM 304 The Tribomet coating TM 304 was not included in the above multiple correlation analyses since it had about 4.5 wt.% Ta, which appeared to have a significant effect on expansion. For example, TM 309 is similar to TM 304, except with no Ta, and it has higher expansion than TM 304. Yet, 309 also has slightly more Ni, less Co, and more Cr and Al than 304, and these chemical trends in themselves should decrease expansion. To properly measure a Ta effect, we should have at least two similar coatings with different Ta additions. The following is another way to estimate the Ta effect using the data we have. If the composition of TM 304 is normalized to 100% after
3.3.2. 825 °C correlation The resulting multiple correlation equation for thermal expansion at 825 °C was expð825Þ ¼ 38:030 0:041134Ni 0:551694Cr 1:20184Al þ 0:001064Ni*Co where the expansion value is the change from 25 °C to 825 °C in mm/m (or mils/in.) as before. This is the 825 A version of the three sets of variables tried, outlined above. The B versions, using Co instead of Ni as one linear term was very slightly better fit than the B version, but the above equation is directly comparable with the fit equations for thermal spray coating expansion [1], so will be adopted here as well. The above equation accounts for about 90% of the variation in the experimental data. The predicted expansions were about 0.1–0.2 mm/m in devia-
Fig. 3. Plot of thermal expansion (mm/m) variation with Ni and Co (wt.%) at 825 °C, with Cr and Al concentrations held constant at the mean values of the range studied.
T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823
conceptually removing the Ta (about 5 wt.%), we would have the following composition: TM 3044 ðno TaÞ ¼ 47:89Ni 22:63Co 20:84Cr 8:63Al If we now use the fit equations derived here to estimate the thermal expansion of such a composition we can compare that to the measured value of TM 304 which does contain Ta. TM 304* TM 304
525 °C 8.63 7.33
825 °C 14.92 13.26
(calculated) (measured)
It thus appears that the Ta effect is significant in reducing the expansion of a Tribomet NiCoCrAlY coating. 4. Discussion 4.1. Composition effect in Tribomet coatings on thermal expansion Without even use of the multiple correlation analysis of the original data, it can be seen from Table 3 that there is a significant difference in expansion between the low (TM 301) and high (TM 286) expansion coatings. The difference is considerably greater than even a 3-sigma allowance for the standard deviation of the data. It is about 1 mm/m difference at 525 °C, which is about 12% of the mean expansion of all coatings. One can also see that the low expansion coating has high Ni and no Co, which is the same finding obtained from multiple correlation. The effect of Cr and Al are difficult to pick out without the multiple correlation exercise. There we found that higher Cr and Al are predicted to reduce thermal expansion. We also found that Ta appears to reduce thermal expansion in itself, not from multiple correlation, but from separate comparison and calculation. This finding agrees with a previous PSTRR study which developed the advanced thermal spray coating of CoNiCrAlY + Pt + Ta, LCR-2106 [3]. If we are looking for a new composition in Tribomet coatings with lower thermal expansion, one strategy would be to target a NiCrAlY (no cobalt), with as high Cr and Al levels that can be achieved with the process, and to add Ta as well. 4.2. TM 301 expansion behavior Six of the seven coatings had thermal expansion curves vs. temperature that were very closely the same in form and curvature. As shown in Table 1, TM 301 had a lower coefficient for the T term and a higher coefficient for the T2 term. This describes a line with higher curvature, giving lower expansion at low temperatures and coming back to the higher expansion values of the group at high temperature. The behavior is similar to the plasma sprayed LN-33 NiCrAlY, but not nearly as abrupt in change from low to high temperature behavior. It still could be caused by the same mechanism proven earlier [1,2], where
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the low-expansion alpha-Cr phase is present in only NiCrAlY at low temperature and it dissolves upon heating above about 950 °C. The microstructures of TM 301 were not extensively studied at high and low temperature in this program, but may be examined later to confirm this point. 4.3. Comparison of Tribomet and thermal spray coatings It is of interest to compare the expansion of Tribomet MCrAlY coatings to thermal spray counterparts. While there are some coatings that are similar in composition, none are exactly the same, and it was shown that small changes in composition can change expansion. What can be done is to take the multiple correlation vs. composition equations from the earlier thermal spray study [1,2] and use it to predict the expansion of Tribomet compositions. Using the following: Thermal spray coatings: 525 : exp ¼ 8:6892 0:012424Ni 0:052554Cr 0:001044Al þ 0:00026934Ni*Co ðmm=mÞ 825 : exp ¼ 15:079 0:023764Ni 0:079724Cr 0:011634Al þ 0:001044Ni*Co ðmm=mÞ For the six Tribomet coating compositions of Table 3 (excluding Ta-bearing TM 304) the expansion prediction average is 0.53 mm/m lower than the measured values at 525 °C and 0.71 mm/m lower at 825 °C, corresponding to 6.8 and 5.2% of the average expansion values, respectively. This exercise shows that the thermal spray equations, which fits thermal spray MCrAlY coatings very well, predicts a lower expansion for the Tribomet compositions than what is measured. This suggests that, on average, Tribomet coatings would expand about 7% more than the same composition coating by thermal spray. How is this possible for the same compositions? The answer must reside in the morphology of the two coating types, and the size and distribution of the phases. This could be a useful comparison to further understand our MCrAlY coatings. Acknowledgements The help of the technical staff at Weston made all the coatings samples. Bob Schotts, R&D Machinist cut and bored all sample tubes. Brian Thompson did the thermocouple calibration tests. References [1] [2] [3] [4]
T.A. Taylor, P.N. Walsh, Surf. Coat. Technol. 177–178 (2004) 24. T.A. Taylor, P.N. Walsh, Surf. Coat. Technol. 188–189 (2004) 41. Thomas Taylor, David Bettridge, Surf. Coat. Technol. 86–87 (1996) 9. Touloukian, et al., Thermal Expansion, Metallic Elements and Alloys, TPRL, vol. 12, Plenum, New York, 1976, p. 225. [5] Air Academy Associates, Colorado Springs, CO, 80920.
Thermal expansion of Tribomet MCrAlY coatings Thomas Taylor ⁎, John Foster Praxair Surface Technologies, United States Available online 20 September 2006
Abstract The thermal expansion behavior of seven Tribomet™ MCrAlY coatings was determined, using the PST Sapphire Dilatometer. Expansion equations were found vs. temperature and also vs. chemical composition. TM 301, NiCrAlY, has the lowest expansion of the group. © 2006 Elsevier B.V. All rights reserved. Keywords: Abradable coating; NiCrAl-bentonite
1. Introduction In the thermal barrier coating (TBC), one usually considers the metallic bondcoat and the zirconia-based ceramic as a system. Knowledge of the thermal expansion behavior of the bondcoat and ceramic topcoat is important because the relatively large expansion mismatch between bondcoat and ceramic imposes a tension stress in the ceramic on heating. Assume that the system is held at temperature for awhile, and the bondcoat relaxes by creep. Upon cooling, this same expansion mismatch creates a compression stress in the ceramic. These stresses in the ceramic contribute to interface crack propagation and potentially to separation. If the mismatch expansions were less the situation should lead to longer TBC life. The zirconia ceramic, typically yttria-stabilized, already has one of the largest expansion coefficients for an oxide. The reduction in mismatch then focuses on the bondcoat. In a previous program [1,2] common plasma sprayed MCrAlY alloys were studied, where M is Co, Ni, or a Ni–Co mixture. There it was found that indeed the actual composition of the MCrAlY coating mattered relative to its thermal expansion: typically all Ni–Co matrix MCrAlY coatings had relatively high thermal expansion, Co-base had somewhat less, but NiCrAlY coatings had the lowest thermal expansion. Using Multiple Correlation techniques, equations were found to predict the thermal expansion values for any MCrAlY coating from its composition. Further, a new range of Cr and Al compositions for the NiCrAlY was found from this predictive equation, coating samples were thus made and found to have the low expansion predicted [1,2]. ⁎ Corresponding author. E-mail address: [email protected] (T. Taylor). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.261
However, PST also makes MCrAlY coatings by the advanced electroplating process, Tribomet™. In principal the composition of a given Tribomet could be made to match that of the powder and coating of the earlier studied thermally sprayed coatings. The process to make Tribomet is quite different, described below, including the heat treatment cycle used to homogenize the components to produce the final MCrAlY coating. Thermally sprayed coatings start from fully pre-alloyed powder of uniform composition, and they also typically have a heat treatment cycle after coating to densify the coating and improve bonding to the substrate. In both coating approaches the final state will have multiple phases, such as aluminide and face-centered cubic matrix phases, with others possible. However, the metallurgical process to reach the final, stabilized coating state is different between the two processes, and the final microstructures are different in detail. The question to be answered in the present study is does the coating process, in addition to the coating composition, affect the resulting thermal expansion? Will it be possible to correlate expansion to composition in Tribomet coatings and will that relation be similar to the one for thermal spray MCrAlY coatings? In this thermal expansion study, like the previous work, we only include expansion data of thermally stabilized coatings, that is, after the initial heat treatment has completed the reaction and densification processes. 2. Experimental 2.1. Tribomet samples Seven different MCrAlY-type coatings were prepared for this study. All were made by PST's advanced Tribomet electroplate
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T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823
process at the Weston Super-Mare facility. In the Tribomet process, a wide range of different NiCoCrAlY compositions can be deposited on complex substrates. Fine particles of CrAlYtype compositions are designed in conjunction with a Ni, Co or variable Ni–Co electroplated matrix to produce the final desired composition. The particles are in suspension in the electroplating solution and are entrapped as the electrodeposited matrix is built up. This “green” coating is then vacuum heat treated to form the MCrAlY alloy coating. The present program coatings only include Ni, Co, Cr and Al variations, except for TM 304 which contained 4.5 wt.% Ta in addition. TM 286 does have 0.25 Hf and 0.4 wt.% Si, and these were considered negligible in the multiple correlation fit to composition. Likewise, all coatings had about 0.5 wt.% Y, so it was not included in the fit since it was not a variable. All coatings were deposited to about 10–15 mils thickness on 0.50-in. OD aluminum tubes. The coated tube was parted to 1.00 in. long cylinders, and most of the aluminum was bored out. The final step was to leach residual aluminum in 25% NaOH at a controlled temperature (b60 °C) for about 90 min. The NaOH solution does not attack the MCrAlY coating. After leaching the coating sample was rinsed in de-ionized (DI) water, ultrasonically rinsed in DI water, rinsed in methanol and warm air dried. 2.2. Dilatometer methodology All samples were heat-treated 4 h at 1080 °C in argon to reach a stable sintered state by running the “green” or as-coated samples in the dilatometer. The same dilatometer cycle was used for the green and all subsequent thermal expansion runs: heating at 5 °C/min to 1080 °C, holding 4 h and cooling at 5 °C/min. If any residual sintering occurred, more than a few 0.1 mils/in., the data was not included in this study, but in the sample re-run until it was stable. The dilatometer is calibrated by running a 1.00-in. long sample of pure Ni, traceable to NIST. The sample is run multiple
Table 1 Average thermal expansion dependence on temperature TM coating
301 302 304 308 309 312 286
½LðT Þ−Lð25-CÞ=Lð25-CÞ ¼ A þ B4T þ C4T 2 ðmm=mÞ A
B
C × 106
Max. T, °C
No. runs
−0.1454 −0.2990 −0.1833 −0.3134 −0.2775 −0.3877 −0.2866
0.0098 0.0134 0.0108 0.0130 0.0129 0.0136 0.0131
7.248 3.898 6.604 4.029 5.624 4.416 5.360
1050 1050 1050 1050 1050 1050 1050
3 4 3 3 3 3 4
times and the average heating and cooling curves are compared to the accepted Ni expansion data published by TPRL [4]. Any deviation is formed into a correction list which the computer applies to all subsequent samples. 2.3. Dilatometer testing and expansion data analysis All samples reported here were run at least three to four times, some even eight. The practice has always been to use just the cooling curve to generate the thermal expansion data reported, although the heating curve for a well-stabilized coating is very close. The cooling curve is shifted if necessary to give the zero expansion value at 25 °C, so all expansions are relative to 25 °C. The fully corrected data is entered as temperature and time pairs, from 1075 °C down to 25 °C, and given a leastsquares parabolic fit of expansion to temperature. This second order, upward curving line fits the data very well, returning an R-squared value close to 1.00 in each case. To make a judgment on the goodness of the data, the fit equation is used to calculate the expansion at 525 and 825 °C, and these two values are compared to all other similar values for the numerous runs on that exact same sample. There were a few cases of run data that was dropped from the final average. In some of these cases there was an experimental issue; in others the expansion values just were unexplainably different, high or low, from the main trend. Using this screening, the final standard deviation of the data that was accepted was about 2% or less of the expansion value average. After the screening, for each coating sample the individual run fit equations were used to calculate the expansion at 25, 100 and each increasing 100 degree increment to 1100 °C. Then the average at each temperature increment was determined. These average expansion values were then least-squares fit as a second order equation for temperature. This final result is what is reported here for the average expansion equation for the seven Tribomet coatings. The final equation was very close to the individual run fit equations, as to be expected. 2.4. Coating chemical analysis
Fig. 1. Average thermal expansion as a function of temperature for Tribomet 301.
Mating coatings were made in each case to the tube samples sent to PST's Speedway Laboratory for thermal expansion testing. These were weighed and the electroplated matrix was dissolved in the green state in nitric acid, and the “CrAlY”
T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823 Table 2 Average thermal expansion of coatings from 25 °C (mm/m)
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The three independent variables sets for trial multiple correlation fitting were as follows:
TM coating
525 °C
SD
SD as % aver
825 °C
SD
SD as % aver
No. of runs
301 302 304 308 309 312 286
7.009 7.821 7.333 7.862 8.032 7.991 8.079
0.073 0.080 0.043 0.181 0.046 0.108 0.105
1.04 1.02 0.59 2.30 0.57 1.35 1.29
12.891 13.426 13.263 13.442 14.172 13.872 14.186
0.133 0.175 0.064 0.233 0.075 0.149 0.238
1.04 1.30 0.48 1.66 0.53 1.08 1.68
3 4 3 3 3 3 4
particle residue collected, dried and weighed. The Co of the dissolved solution was chemically analyzed by atomic absorption, nickel taken as balance. The matrix and CrAlY weight fractions thus obtained gravimetrically, and with the Ni–Co analysis and the earlier wet chemical lot analysis of the CrAlY particles, allowed calculation of the final coating composition.
A B C
Ni; Cr; Al and Ni*Co Co; Cr; Al and Ni*Co Ni; Co; Cr; Al and Ni*Co
Sets A and B gave good and very similar fits with the ability to predict back the expansion values for each coating close to the original data. Set C gave an exact fit to the original data (R-squared = 1.000), but it gave very unlikely results when predicting expansion beyond the original range of the coating compositions. Set C was in essence, over-fitting the data. Set A is also the same set of independent variables used before in the study of thermally sprayed MCrAlY's [1], and it will be seen to have produced an equation that is similar in form to before, but it is not the same in detail which will be discussed below. 3. Results
2.5. Expansion–composition multiple correlation 3.1. Stabilized thermal expansion The last data analysis was to fit the average expansion of the coatings to the chemical analysis of the same coating. For this, the multiple correlation software SPC KISS 97 [5] was used. The wet chemical analysis of the coatings was entered for Ni, Co, Cr and Al. The minor elements Y, Si, Hf were neglected. All coatings had about 0.5 wt.% Y so it was not a variable, but was not expected to have an effect on expansion. Two coatings were of some concern; TM 308 had 2.3 wt.% Hf and TM 304 had 4.1 wt.% Ta. With only seven coatings, four major variables and one interaction gave only one extra degree of freedom. Further, the range of the compositions between coatings was not large. The multiple correlation program was struggling to find a good fit with all coatings included. TM 304 appeared to be lower in expansion than the closest similar composition, TM 309 which had no Ta. When TM 304 was removed from the correlation, the fit came together, since the Ta effect apparently was a wild card that was not being addressed by the major elements in the fit. TM 304 will be discussed separately. With six coatings, using four major elements as variables and one interaction, the fit was reasonable, able to explain about 90% of the variation by the resulting equations. It remains to be noted that the range of variation of Cr and Al were not large, which make the power of the correlation difficult. Three sets of independent variables were tried for the fit of the dependent variable thermal expansion. The thermal expansion terms were taken from the original parabolic fit equations to the data, averaged over several runs to give the final coating expansion equation, as described above. Expansion values were calculated at 525 and 825 °C and these were used in two separate correlation exercises. The reasons for selecting these two temperatures was given earlier [1], suggesting that at 525° the coatings still had high yield strength and could still impart thermal expansion mismatch stress to the YSZ layer in a TBC. At 825°, while low in yield strength the thermal expansion values were larger and maybe more divergent between coatings and would allow a better correlation to composition.
The final average expansion behavior for a typical Tribomet coating is shown in Fig. 1, including the best parabolic fit equation to that data. These equations are listed in Table 1 for all seven coatings. The expansion values to be used with the multiple correlation fit to coating composition are given in Table 2, including the standard deviation of the values and the number of dilatometer runs used to make the average result. 3.2. Wet chemical and SEM/EDS analyses The wet chemical analysis method was described above, and the results are given in Table 3. 3.3. Correlation of expansion to composition 3.3.1. 525 °C correlation The resulting multiple correlation equation for thermal expansion at 525 °C was expð525Þ ¼ 25:765 0:03644Ni 0:40654Cr 0:89984Al þ 0:00077994Ni*Co where the indicated elemental compositions are entered as weight percent (by wet chemical analysis). The units of expansion are Table 3 Wet chemical analyses of actual coatings used in dilatometry Coating
TM 301 TM 302 TM 304 TM 308 TM 309 TM 312 TM 286
wt.% Al
Cr
8.6 9.60 8.20 9.30 8.90 9.24 11.10
20.7 23.10 19.80 23.30 21.50 22.30 15.32
Co 66.60 21.50 65.10 18.20 35.40 14.36
Ni
Y
70.2
0.5 0.50 0.80
45.50
Hf
Ta
Si
Sum
0.41
100.00 99.80 99.90 100.00 99.80 99.79 99.73
4.10 2.30
50.70 32.30 58.14
0.50 0.55 0.20
0.20
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T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823
tion from the measured expansion, with an average deviation of 0.13 mm/m or about 0.9% of the average expansion value at 825 °C. The surface plot trend of Tribomet expansion at 825 °C as a function of Ni and Co, with Cr and Al fixed at the mean values of the range evaluated, has a twist or warp, as did the 525 °C plot, due to the Ni*Co interaction term in the Exp (825) equation. In Fig. 3, the 825 °C expansion data as predicted by the fit equation is seen as a two-dimensional plot for Ni and Co. The trend takes a different shape but it again indicates that expansion is reduced as the composition moves toward high Ni and low Co. Similarly for the 825 °C expansion data as a function of Cr and Al, with Ni and Co held constant at their mean values the plot is linear in each variable because that is what we selected for the form of the fit equation. As found at 525 °C this fit equation predicts lower expansion as either or both Cr and Al are increased. Fig. 2. Plot of thermal expansion (mm/m) variation with Ni and Co (wt.%) at 525 °C, with Cr and Al concentrations held constant at the mean values of the range studied.
mm/m, or equivalently, mils/in. This equation accounts for about 94% of the variation of the data. Each coating's expansion as predicted by the equation was found to be very close to the data, usually within 0.1 mm/m or less. The overall average deviation was 0.075 mm/m, or about 1% of the expansion value. Graphical plots of the above equation help visualize the trend with composition. An expansion surface for 525 °C as a function of Ni and Co, with Cr and Al held constant at their mean values, is seen to have a slight warp, which is the effect of the interaction term Ni*Co in the equation. The two-dimensional plot more clearly shows expansion is reduced toward the corner of low Co and high Ni (Fig. 2). A two-dimensional plot with variation in Cr and Al at the mean values of Ni and Co as constant shows another prediction for expansion. Expansion is reduced as the composition moves toward high Cr and high Al.
3.4. Ta effect in TM 304 The Tribomet coating TM 304 was not included in the above multiple correlation analyses since it had about 4.5 wt.% Ta, which appeared to have a significant effect on expansion. For example, TM 309 is similar to TM 304, except with no Ta, and it has higher expansion than TM 304. Yet, 309 also has slightly more Ni, less Co, and more Cr and Al than 304, and these chemical trends in themselves should decrease expansion. To properly measure a Ta effect, we should have at least two similar coatings with different Ta additions. The following is another way to estimate the Ta effect using the data we have. If the composition of TM 304 is normalized to 100% after
3.3.2. 825 °C correlation The resulting multiple correlation equation for thermal expansion at 825 °C was expð825Þ ¼ 38:030 0:041134Ni 0:551694Cr 1:20184Al þ 0:001064Ni*Co where the expansion value is the change from 25 °C to 825 °C in mm/m (or mils/in.) as before. This is the 825 A version of the three sets of variables tried, outlined above. The B versions, using Co instead of Ni as one linear term was very slightly better fit than the B version, but the above equation is directly comparable with the fit equations for thermal spray coating expansion [1], so will be adopted here as well. The above equation accounts for about 90% of the variation in the experimental data. The predicted expansions were about 0.1–0.2 mm/m in devia-
Fig. 3. Plot of thermal expansion (mm/m) variation with Ni and Co (wt.%) at 825 °C, with Cr and Al concentrations held constant at the mean values of the range studied.
T. Taylor, J. Foster / Surface & Coatings Technology 201 (2006) 3819–3823
conceptually removing the Ta (about 5 wt.%), we would have the following composition: TM 3044 ðno TaÞ ¼ 47:89Ni 22:63Co 20:84Cr 8:63Al If we now use the fit equations derived here to estimate the thermal expansion of such a composition we can compare that to the measured value of TM 304 which does contain Ta. TM 304* TM 304
525 °C 8.63 7.33
825 °C 14.92 13.26
(calculated) (measured)
It thus appears that the Ta effect is significant in reducing the expansion of a Tribomet NiCoCrAlY coating. 4. Discussion 4.1. Composition effect in Tribomet coatings on thermal expansion Without even use of the multiple correlation analysis of the original data, it can be seen from Table 3 that there is a significant difference in expansion between the low (TM 301) and high (TM 286) expansion coatings. The difference is considerably greater than even a 3-sigma allowance for the standard deviation of the data. It is about 1 mm/m difference at 525 °C, which is about 12% of the mean expansion of all coatings. One can also see that the low expansion coating has high Ni and no Co, which is the same finding obtained from multiple correlation. The effect of Cr and Al are difficult to pick out without the multiple correlation exercise. There we found that higher Cr and Al are predicted to reduce thermal expansion. We also found that Ta appears to reduce thermal expansion in itself, not from multiple correlation, but from separate comparison and calculation. This finding agrees with a previous PSTRR study which developed the advanced thermal spray coating of CoNiCrAlY + Pt + Ta, LCR-2106 [3]. If we are looking for a new composition in Tribomet coatings with lower thermal expansion, one strategy would be to target a NiCrAlY (no cobalt), with as high Cr and Al levels that can be achieved with the process, and to add Ta as well. 4.2. TM 301 expansion behavior Six of the seven coatings had thermal expansion curves vs. temperature that were very closely the same in form and curvature. As shown in Table 1, TM 301 had a lower coefficient for the T term and a higher coefficient for the T2 term. This describes a line with higher curvature, giving lower expansion at low temperatures and coming back to the higher expansion values of the group at high temperature. The behavior is similar to the plasma sprayed LN-33 NiCrAlY, but not nearly as abrupt in change from low to high temperature behavior. It still could be caused by the same mechanism proven earlier [1,2], where
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the low-expansion alpha-Cr phase is present in only NiCrAlY at low temperature and it dissolves upon heating above about 950 °C. The microstructures of TM 301 were not extensively studied at high and low temperature in this program, but may be examined later to confirm this point. 4.3. Comparison of Tribomet and thermal spray coatings It is of interest to compare the expansion of Tribomet MCrAlY coatings to thermal spray counterparts. While there are some coatings that are similar in composition, none are exactly the same, and it was shown that small changes in composition can change expansion. What can be done is to take the multiple correlation vs. composition equations from the earlier thermal spray study [1,2] and use it to predict the expansion of Tribomet compositions. Using the following: Thermal spray coatings: 525 : exp ¼ 8:6892 0:012424Ni 0:052554Cr 0:001044Al þ 0:00026934Ni*Co ðmm=mÞ 825 : exp ¼ 15:079 0:023764Ni 0:079724Cr 0:011634Al þ 0:001044Ni*Co ðmm=mÞ For the six Tribomet coating compositions of Table 3 (excluding Ta-bearing TM 304) the expansion prediction average is 0.53 mm/m lower than the measured values at 525 °C and 0.71 mm/m lower at 825 °C, corresponding to 6.8 and 5.2% of the average expansion values, respectively. This exercise shows that the thermal spray equations, which fits thermal spray MCrAlY coatings very well, predicts a lower expansion for the Tribomet compositions than what is measured. This suggests that, on average, Tribomet coatings would expand about 7% more than the same composition coating by thermal spray. How is this possible for the same compositions? The answer must reside in the morphology of the two coating types, and the size and distribution of the phases. This could be a useful comparison to further understand our MCrAlY coatings. Acknowledgements The help of the technical staff at Weston made all the coatings samples. Bob Schotts, R&D Machinist cut and bored all sample tubes. Brian Thompson did the thermocouple calibration tests. References [1] [2] [3] [4]
T.A. Taylor, P.N. Walsh, Surf. Coat. Technol. 177–178 (2004) 24. T.A. Taylor, P.N. Walsh, Surf. Coat. Technol. 188–189 (2004) 41. Thomas Taylor, David Bettridge, Surf. Coat. Technol. 86–87 (1996) 9. Touloukian, et al., Thermal Expansion, Metallic Elements and Alloys, TPRL, vol. 12, Plenum, New York, 1976, p. 225. [5] Air Academy Associates, Colorado Springs, CO, 80920.