Thermal properties and microstructure of two thermal barrier coatings

Surface and Coatings Technology, 54/55 (1992) 53-57

53

Thermal properties and microstructure of two thermal barrier coatings Thomas A. Taylor Union Carbide Coatings Service, 1500 Polco Street, Indianapolis, IN 46222 (USA)

Abstract Two plasma sprayed yttria stabilized zirconia coatings are described in terms of their microstructure. The structures differ widely as

a result of controlling the coating density, and in one producing a vertical macrocrack pattern that segments the structure. The thermal properties, including thermal expansion, thermal diffusivity and specific heat, were measured from room temperature to 1200°C. The thermal conductivity was calculated from these properties. It was found that a small increase in thermal conductivity occurs as a result of the first heating cycle, and this effect was examined in relation to microstructure.

1. Introduction

Plasma sprayed zirconia thermal barrier coatings (TBCs) have been applied to a wide range of engineering structures to help control heat flow, to lower substrate temperature or to mitigate local hot spots. In order to help designers work with TBCs the thermal properties must be provided, which is the purpose of this paper. One key variable defining thermal conductivity, once the composition is established, is the coating density. Typically, 6-8 wt.% yttria stabilized zirconia (YSZ)coatings are made at 85-88% of theoretical density. We have recently developed a new high density YSZ coating which has a controlled population of vertical macrocracks, giving the coating much enhanced thermal shock resistance and at the same time excellent erosion resistance and finishability [1, 2]. In this paper, vertical cracks are defined as having their crack plane essentially perpendicular to the plane of the coating, while horizontal cracks are parallel to the plane of the coating. A vertical macrocrack will be considered to be at least 100 11m long and traverse many splat layers, while a microcrack is considered to be 2-10 11m long. The thermal properties of a lower density, non-macrocracked coating and the new dense macrocracked coating were thus measured and compared.

2. Experimental details The starting powders had the analysis given in Table 1. Free standing coupons were made by spraying 1.27em diameter substrates with a 0.19 em thick coating of YSZ, then carefully mechanically removing the coating. This was made possible by using only a light grit blast to roughen the substrates. These specimens were used for

0257 -8972/92/$5.00

TABLE I. Chemical analysis of starting powders (wt.%)

Zircoat-16 LZ-17

y 203

Si0 2

Ti0 2

Al20 3

Fe 20 3

Hf0 2

Total of all others

7.03 8,30

0,33 0.18

0.150 0.017

0.093 0.008

0.090 0,011

1.5"

0,31 0.10

1.7"

"Not analyzed, estimated from other similar lots.

specific heat and thermal diffusivity measurements. Thermal expansion samples were sprayed on 1.27 em diameter aluminum tubes; they were then parted to 2.54 em in length and the aluminum was leached with warm sodium hydroxide. The coatings were characterized in terms of microstructure (Fig. 1),density and phase analysis. The macrocracked coating was made using spraying parameters to, give an average vertical macrocrack spacing of 0.032 em with a 0.015 cm standard deviation. The crack spacing frequency followed a statistical normal distribution. In addition, the vertical macrocracks in the dense coating have horizontal branch cracks, and both high and low density coatings have very short horizontal and vertical microcracks. The coating density was measured (ASTM B-328) and then related to the theoretical density (TD) calculated for the specificY203 analysis [3]. The macrocracked coating was 90.4% TD (standard deviation TABLE 2. X-ray diffraction phase analysis of as-sprayed coatings (vol.%) Monoclinic Zircoat-16 (macrocracked) LZ-17

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16.8

Tetragonal

Cubic

97.5

2.5

74.6

8.6

1992-Union Carbide Coatings Service Technology Corporation

54

T . A . Ta ylor I Thermal properties ami microstructure of tiro thermal harrier coutings

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0.20), while the lower density coating was 86.8% TO (standard deviation 0.06). The phase analysis was performed by X-ray diffraction using the algorithms of Miller [4]. Table 2 shows that the dense macrocracked coating (Zircoat-16™) was essentially tetragonal with a small amount of cubic phase. The lower density coating (UCAR™ LZ-17) was principally tetragonal but had a significant monoclinic phase content.

nrZircoat and DCAR are trademarks of Union Carbide Coatings Service Technology Corporation.

3. Results The thermal expansion of Zircoat-16 was linear from room temperature to 1075 DC, with an expansion coefficient of 11.4 x 10- 6 ern cm - I DC - I, for the first or subsequent heating cycles. LZ-17 showed a 0.1 % length contraction upon first cycle cooling, and a smaller contraction on the second cycle. There were small departures from a linear expansion curve for LZ-17 , slight contractions at 900 and 1000 DC on both first and second cycle heating runs. These effects may indicate local inhomogeneities in the Y203 concentration, while the

T. A. Taylor

I Thermal properties and microstructure of tll'O thermal harrier coatings

net contraction may indicate that the coating is slightly less monoclinic after thermal cycling. Overall, LZ-17 had the same expansion coefficient as Zircoat-16. Linear expansivity at about 8 wt. % y 203 is in agreement with the results of Brandon and Taylor [5]. The specific heat was measured with a differential scanning calorimeter with sapphire as the reference material. Figure 2 shows the measured data points for both coatings, which were indistinguishable by virtue of their similar Y203 content. The solid line is the specific heat calculated from the pure oxide values [6] and a simple rule of mixtures based on weight percent. The thermal diffusivity was measured by the laser flash method [6] in vacuum (10- 5 Torr) with readings taken at 23 and 100"C and then in steps of 100°C up to 1200 "C. The material was assumed stable with regard to sintering densification so measurements upon cooling were generally not taken. However, in one case, the diffusivity was measured at 500 DC upon cooling, and it was unexpectedly found to be higher than the heating value by 10%. This observation gave a new direction to the study and we reran the same coating specimens through the apparatus for a second cycle, but only up to 500 "C, The results for Zircoat-16 and LZ-17 are shown in Figs. 3 and 4. The initial portions of the curves for the first and second runs for each coating are parallel up to about 400 DC. Then the first run data are seen to flatten and show little temperature dependence up to 1200 DC. The indication was that some process was occurring, starting at about 500°C, which was increasing the thermal diffusivity on the first heating run. The effect is greater in Zircoat-16 than in LZ-17. The assumption was made that the high temperature excursion of run I had stabilized the material and that no further changes would occur during the second run. The high

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temperature data from run I were combined with the run 2 data and fitted with a curve as indicated. The cause of thermal instability in these coatings will be discussed later. Thermal conductivity as a function of temperature from room temperature to 1200 DC was calculated using measured specific heat and thermal diffusivity, and measured coating density corrected for thermal expansion as follows:

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56

T. A. Taylor / Thermal properties and microstructure of t\\'o thermal barrier coatings

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calculated for both coatings using thermally stabilized diffusivity,

4. Discussion The thermal diffusivity curve shape for the present zirconia coatings is quite similar to that for sintered powders of 3.3 wt.% magnesia stabilized zirconia ceramics over a range of densities (76-96% of theoretical) [7J, and to relatively high density sintered zirconia ceramics stabilized with 2.6 and 3.4 wt.% MgO and with 2.4-9.0 wt.% Y203 [8]. The absolute value of the thermal diffusivity is, of course, dependent upon the composition and density of the material. The specific heat measurements are in excellent agreement with calculated values using pure oxide data, and with other measured values for dense sintered YSZ ceramics [8]. The thermal conductivity curve has a downturn at low temperature, but this can be seen to be a consequence of decreasing specific heat and the product having a thermal diffusivity value that is density dependent. An interesting effect found in this study is the increase in thermal diffusivity which occurred during the first heating cycle to 1200DC. The increase at room temperature was 12% for Zircoat-16 and about 6% for LZ-17. Several causes were considered to explain this effect. We made new measurements to see if densification could have occurred at temperatures as low as 1200 DC, for times even greater than in the thermal diffusivity run exposure. In the vacuum thermal diffusivity apparatus, the automatic cycle remains at each measurement temperature for 8 min, then heats to the next 100DC increment in 8 min. Thus the maximum time above, say, 1000 DC was 40 min. New free standing YSZ samples of

both coating types using similar powder lots were prepared as described earlier. One set of free standing YSZ samples was vacuum heat treated at 1152 DC for 16 h, and another set was heat treated in air at 1164 "C for 4 h. The density ratios for the vacuum and air heat treatments to the as-coated density were found to be 1.0025 and 1.0018 (Zircoat-16), and 0.9984 and 0.9975 (LZ-17). Thus, at most, a 0.25% increase in density was observed for Zircoat-16, while LZ-17 decreased slightly in density. Clearly, densification is not an adequate explanation for the increased conductivity. A phase change was considered next. Williams [9J has shown that monoclinic and tetragonal zirconia have about the same thermal conductivity but the cubic phase has considerably less. X-ray diffraction phase analysis was carried out on the LZ-17 specimen that had been subjected to both the first and second diffusivity runs. An as-coated specimen of Zircoat-16 was also run using the heating stage of the diffractometer. The results (Table 3) showed that in Zircoat-16 the cubic phase increased slightly with increasing temperature and was partially retained on cooling. This phase change should decrease thermal conductivity, which is opposite to what was observed. In LZ-17 the cubic phase was slightly less after the diffusivity runs, and using Williams's values a 1.4% increase in diffusivity can be calculated, which is considerably less than the 6% increase measured. The phase change results are thus not consistent or adequate to account for the thermal instability in diffusivity. Another mechanism considered was that the impurities present might have redistributed upon heating to locate at splat edges and boundaries, possibly increasing the boundary conductivity. In fact, the Zircoat-16 coating had a higher impurity analysis and greater diffusivity increase than the LZ-17 coating. However, SEM analyses could find no systematic variation in Al and Si impurity elements from grain centers to splat boundaries. Even etching in HF proved inconclusive for showing impurity distributions in as-coated compared with the 1152 DC (vacuum) or 1164 DC (air) heat treated specimens. TABLE 3. X-ray diffraction phase analysis of zirconia coatings (vol."!o)

Zircoat-Iti 25°C 250°C 500°C 800°C 25 DC, cooled IZ-I7 As coated Cycled in diffusivity rig"

Monoclinic

Tetragonal

Cubic


97.5 97 96 94.5 95.5

2.5 3 4 5.5 4.5

74.5 74

8.5 7

17

19

"RT to 1200°C to RT; RT to 500°C to RT.

T. A. Taylor / Thermal properties ami microstrllcture of two thermal barrier coatings

Optical microscopy did reveal that some of the fine horizontal microcracks had healed or closed, particularly after the air heat treatment. The large vertical macrocracks in Zircoat-Io were not affected. Healing horizontal cracks which are perpendicular to the laser flash heat flow direction should increase diffusivity and conductivity [10, 11].

5. Conclusions

The thermal diffusivity and conductivity of plasma sprayed 7-8 wt. % YSZ coatings increase with increasing coating density. They further increase somewhat with the first high temperature exposure in the present coatings, most probably as a result of the healing of some of the horizontal microcracks, even when the exposure is below the densification range.

Acknowledgments

The help preparing samples by D. L. Appleby and the X-ray phase analyses made by Dr G. C. Whichard are greatly appreciated. The careful thermal diffusivity

57

measurements were made at TPRL, Purdue University. W. A. Warren and Dr R. C. Tucker made constructive suggestions for the manuscript.

References I T. A. Taylor, D. L. Appleby, A. E. Weatherill and 1. Griffiths, Surf Coat. Technol .. 43/44 (1990) 470. 2 US Patent 5,073,433, December 17, 1991. 3 R. P. lnge! and D. Lewis, J. Am. Ceram. Soc., 69 (4) (1986) 325. 4 R. A. Miller, J. L. Smialek and R. G. Garlick, in A. H. Heuer and L. W. Hobbs (eds.), Science and Technology of Zirconia. Advances ill Ceramics, VoL 3, American Ceramic Society, Columbus, OH, 1981, p. 241. 5 J. R. Brandon and R. Taylor, Surf Coat. Technol., 39/40 (l989) 143. 6 Y. S. Touloukian, R. W. Powell, L. C. Ho and M. C. Nicolaou, Thermal Diffusioit», Thermophysical Properties of Matter, VoL 10, Plenum, New York, 1973, p. 22a. 7 M. V. Swain, L. F. Johnson, R. Syed and D. P. H. Hasselman, J. Mater. Sci. Lett., 5 (1986) 799. 8 D. P. H. Hasselman, L. F. Johnson, L. D. Bentsen, R. Syed, H. L. Lee and M. V. Swain, Am. Ceram. Soc. Butl., 66 (5) (1987) 799. 9 R. K. Williams, J. B. Bates, R. S. Graves, D. L. McElroy and F. J. Weaver, Int. J. Thermophys .. 9 (4) (1988) 587. 10 D. P. H. Hasselman, J. Composite Mater., 12 (1978)403. 11 R. McPherson, Thin Solid Films. 112 (1984) 89.