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Hot isostatic pressing of plasma sprayed yttria-stabilized zirconia
March 1998
Materials Letters 34 Ž1998. 263–268
Hot isostatic pressing of plasma sprayed yttria-stabilized zirconia K.A. Khor ) , Y.W. Gu School of Mechanical and Production Engineering, Nanyang Technological UniÕersity, Nanyang AÕenue, 63 9798 Singapore, Singapore Received 29 May 1997; revised 24 June 1997; accepted 27 June 1997
Abstract The porosity in plasma sprayed ceramics is caused by the irregular splat formation of the lamellae structure as the molten droplets impact the substrate. This paper reports the influence of hot isostatic pressing ŽHIP. as a post-spray treatment of plasma sprayed coatings. Investigations are made on the physical properties, pore size distribution and thermo-physical properties, namely thermal diffusivity and thermal conductivity, of plasma sprayed yttria-stabilized zirconia ŽZrO 2 –7.8 wt% Y2 O 3 . coatings after HIP at 1000, 1100 and 12008C for 1 h. Results show that HIP processing the plasma sprayed coatings reduced the porosity by ; 2.5% and increased the microhardness by ; 40%. Correspondingly, the thermal diffusivity and thermal conductivity of the coatings were found to be significantly changed after HIP treatment at 12008C. q 1998 Elsevier Science B.V. PACS: 81.05.-t; 81.05.Je; 81.15.Rs Keywords: Plasma spray; Hot isostatic pressing; Zirconia; Porosity; Microstructure; Thermal diffusivity; Thermal conductivity
1. Introduction Porosity in plasma sprayed coatings can be viewed by a two level model where pores in the size-range of 1–10 m m arise from entrapped gas, unmelted particles, cracking or premature solidification by some particles while pores - 0.1 m m are formed as a result of poor interlamellar contacts w1x. The latter type of pore is rather predominant in ceramic coatings. Plasma sprayed coatings are formed by melting the powder feedstock and impinging the molten droplets onto a prepared substrate surface. The molten
)
Corresponding author.
droplets solidify rapidly on impact. There are also varying degrees of spreading of the liquid before final solidification. Thus, giving rise to the characteristic lamellae microstructure of plasma sprayed coatings. The inter-lamella adhesion is predominantly mechanical interlocking of the atoms. Bad contacts amongst the lamellae can be found and in many cases the space between two overlapping lamellae can be up to 100 nm w1x. The outcome is a coating that is mechanically weak. A recent study attempts to further categorize the micro-pores according to the nature of their formation w2x. Basically, the micropores are formed from resulting dropletrdroplet and dropletrliquidsurface-layer interactions. In addition, they can be formed through the ejection or rebounding of liquid
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 1 8 4 - 5
264
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
Žthe pores are formed when the detached liquid eventually falls back onto the coating and some pores result from entrapped gases. and dispersoids as in the case of carbides, oxides and nitrides reinforced alloy systems. Porosity in plasma sprayed coatings generally depend on the process settings such as average particle size and size distribution of the feedstock, the power level of the spray, the spray distance and the type of cooling received by the coating. The influence of the porosity on the thermal conductivity has been analyzed by several researchers w3–5x. These models help to predict the effective thermal conductivity, k eff , which is the conductivity of the porous material given the value of the volume fraction of the pores, P, the thermal conductivity of the matrix, k m , and the thermal conductivity of the dispersed phase, k f . Hot isostatic pressing ŽHIP. is a technology that has encouraging success in consolidation of advanced materials such as ceramics and intermetallics w6–11x. The simultaneous application of high temperature and pressure often results in a material with high density. This paper reports the effect of HIP on the microstructure, and, physical and thermal properties of plasma sprayed yttria stabilized zirconia ŽYSZ. coatings. The YSZ coating is a popular material for thermal barrier coatings ŽTBC. in turbine blades of jet engines and gas turbines w12,13x. It has good chemical and thermal stability and is an excellent thermal insulator. A bond coat of composition MCrAlY Žwhere M s Ni, Co or Fe. is usually applied as an intermediate layer to enhance the adhesion of the ceramic. However, oxidation of the samples in air results in the formation of a reaction layer at the bond coatrceramic coat interface w14,15x. Failure of the TBC has been attributed, in many instances, to the presence of this layer of oxide that eventually leads to spalling of the TBC. Thus, in order for the TBC to have long term effectiveness the pore structure and pore distribution has to be altered to reduce oxidation of the bond coat.
2. Experimental procedure The zirconia-based coatings are prepared by a 40 kW dc plasma spray system ŽMiller Thermal, WI.. A
Table 1 Plasma spraying parameters Primary gas Žpressure.; flowrate Auxiliary gas Žpressure.; flowrate Powder feed rate Arc current Arc voltage Spray distance
argon Ž50 psi.; 82 scfh helium Ž50 psi.; 26 scfh 30 grmin 800 A 50 V 120 mm
commercial yttria stabilized zirconia thermal spray powder ŽAI 1075, Alloy International, TX. was used as the feedstock. The HIP system used was the System 5X from Kobelco, Japan. This system is capable of attaining a maximum temperature of 20008C and a maximum pressure of 200 MPa when a graphite furnace heating element is used. A molybdenum heating element was used in the present study. Table 1 lists the plasma spray conditions while Table 2 shows the HIP parameters for the post-spray treatment of plasma sprayed coatings. Scanning electron microscopy ŽSEM. was performed with the Cambridge Stereo scan S360 equipped with an energy dispersive X-ray analyzer ŽLink AN 10r85S.. Phase analysis was done on the Philips MPD 1880 X-ray diffractometer system. Density measurement was accomplished with a helium gas pycnometer ŽUltrapycnometer, Quantachrome, USA.. Porosity measurement was executed using the mercury intrusion porosimeter, Autopore III from Micromeritics, USA. In this technique, mercury is intruded into the coating to measure the pore size. The technique is based on the non-wetting characteristics of mercury. Pore sizes are determined by Washburn’s equation Pr s y2g cos u ,
Ž 1.
where g is the surface tension of mercury and u is the contact angle between mercury and the pore wall. Pr is the applied external pressure that ranged from Table 2 Parameters for hot isostatic pressing of Y2 O 3 stabilized ZrO 2 ŽHYZ. Specimen
Temperature Ž8C.
Pressure ŽMPa.
Time Žh.
HYZ 1 HYZ 2 HYZ 3
1000 1100 1200
180 180 180
1 1 1
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
several bars to several tens of thousand bars. The variation of the external applied pressure caused changes in the intruded volume. This in turn was related to pore-size distribution. This technique requires pores that are inter-connected, a common feature in plasma sprayed ceramic coatings. The thermal diffusivity and thermal conductivity of the coatings were measured by the laser flash method ŽThermaflash 2200, Holometrix, USA.. The samples tested were disks ; 2 m m thick and 12.7 m m in diameter. The samples were coated with approximately 0.1 m m of gold and 5 m m of graphite for testing. This method conformed to ASTM E1461-92 for the measurement of thermal diffusivity.
3. Results and discussion 3.1. Microstructure and physical properties of plasma sprayed and HIPed yttria stabilized zirconia Fig. 1 shows the fractured surface of the assprayed YSZ coating. The typical features described earlier viz. pores, inter-lamellae separation, internal lamellae cracking can be observed. The fractured surface of HIPed coatings showed, generally, a lower incidence of pores and relatively better inter-lamellae adhesion ŽFig. 2.. There is no evidence of any grain growth in all the HIPed samples ŽHYZ 1–3..
Fig. 1. SEM view of as-sprayed ZrO 2 –7.8 wt% Y2 O 3 ŽYSZ. coating.
265
Fig. 2. SEM view of ZrO 2 –7.8 wt% Y2 O 3 coating after HIP.
The MIP results showed some changes to the pore size distribution of the YSZ coatings after HIP ŽFig. 3.. HIP has effected a reduction in pores of the size-range 30–100 m m, 1.5–4 m m and a corresponding increase in the pores of size 0.2–1.0 m m. It can also be observed that at a HIP temperature of 12008C, the fine pores that are - 0.2 m m are significantly reduced. These pores are likely to be the inter-lamellae pores and the reduction or elimination of these pores would serve to strengthen the inter-lamellae bonds. There is also a decrease in the overall porosity, of ; 2.5%, in the HIPed coatings, as shown in Table 3. The density measurements showed a corresponding increase in the density of the HIPed coatings. Microhardness measurements taken on polished cross-sections of the coatings showed a progressive increase in hardness values ŽFig. 4.. Previous investigation has shown that the porosity in the plasma sprayed coatings can be reduced to - 1% when the coatings are HIPed with encapsulation w16x. Complete elimination of the pores in the YSZ coatings is, of course, not practical considering the application of the coatings as thermal barrier coatings. A certain level of porosity in the TBC is necessary for efficient thermal insulation. Hence, the coatings are HIPed without encapsulation. Nevertheless, a reduction of the various pore networks and inter-lamellae pores will help to slow down the oxidation process of the bond coats, that ultimately lead to spallation during usage. The reduction of these pores would contribute significantly to the increase in the lifespan of the TBC. The pores, in
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
266
Fig. 3. Pore size distribution of the YSZ coatings after HIP.
particular the large pores Ž50–100 m m., appear to collapse during HIP, as evident from the MIP results. This lead to particle fragmentation and formation of many ‘new’ inter-particle contacts. The MIP results seems to indicate that the small pores in the sizerange 0.3–1.0 m m increased correspondingly, quite likely as a result of the collapse of the large pores. The reduction in the amount of pores that are - 0.2 m m appears to be a consequence of the elimination of the inter-lamellae pores through sintering effects of the HIP process. This will lead to strengthening of the inter-lamellae bonds and may account for the
increase in microhardness values of the HIPed coatings despite the rather modest reduction in the overall porosity.
Table 3 Porosity of HIPed Y2 O 3 -stabilized ZrO 2 coatings measured by mercury intrusion porosimetry Specimen
Porosity Ž%.
Average density Žgrml.
Average pore diameter Ž m m.
As-sprayed HYZ 1 HYZ 2 HYZ 3
12.48 12.42 11.72 9.97
5.6278 5.7588 5.7720 5.9427
0.2145 0.1910 0.1836 0.1871 Fig. 4. Microhardness of as-sprayed and HIPed coatings.
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
267
the thermo–physical properties of the coatings. Whilst the reduction in the fine pores Ž- 0.2 m m. apparently improved the physical properties of the YSZ, the thermo–physical properties were correspondingly diminished. This suggests that there is a limit to the improvement of the physical properties of the plasma sprayed YSZ at the expense of the thermo–physical properties. Bearing in mind the application of this material as a thermal barrier coating.
4. Conclusions Fig. 5. Thermal diffusivity versus temperature plot for as-sprayed and HIPed YSZ coatings.
3.2. Thermal diffusiÕity and thermal conductiÕity of HIPed yttria stabilized zirconia Figs. 5 and 6 shows the thermal diffusivity and thermal conductivity values of the as-sprayed and HIPed coatings respectively. It can be observed that the thermal diffusivity and thermal conductivity values of the sample that was HIPed increased significantly over the values of as-sprayed coatings. These values can be related to the pore size distribution of the coatings, and highlight the impact on the reduction of pores in the size-range of 50–100 m m as well as the 1.5–4 m m pores on the thermo–physical properties of the plasma sprayed ceramic. The increase in pores of size 0.3–1.0 m m in the HIPed samples apparently did not contribute positively to
Hot isostatic pressing of plasma sprayed YSZ has revealed changes to the pores size distribution, physical and thermal properties. The reduction of porosity was modest Ž; 2.5%., however, improvement of their physical properties such as microhardness Ž; 40%. was more significant. The MIP data of the pore size distributions showed that HIP evoked changes to the microstructures of the plasma sprayed and HIPed YSZ coatings that in turn influenced the physical and thermal properties of the coatings. The large pores Ž50–100 m m. in the YSZ coatings apparently collapsed and fragmentation occurred, under the HIP pressure, to yield many additional particle contacts that helped to improve the physical properties such as hardness and density. The improvement in physical properties was also believed to have been contributed by the reduction of fine pores that are - 0.2 m m. The average pore diameter was reduced from 0.2145 to 0.1836 m m for the HIPed samples ŽHYZ 1–3.. However, pores in the size-range 0.3–1.0 m m was found to increase following the HIP treatment. These pores are believed to have no significant impact on the thermal properties of the YSZ coatings. Instead, the thermal diffusivity and thermal conductivity results indicated that the reduction in the 30–100 m m and 1.5–4 m m pores caused the HIPed samples to exhibit higher values for thermal diffusivity and thermal conductivity.
Acknowledgements Fig. 6. Thermal conductivity versus temperature plot for as-sprayed and HIPed YSZ coatings.
The Hot Isostatic Press ŽHIP. unit was financed by a Research Grant from the Ministry of Finance
268
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
ŽSingapore.. Financial assistance through Applied Research Grant No. RP 56r92 and RG 25r96 is gratefully acknowledged. The technical assistance provided by Daniel Gan, Alex Ng, Sunny Low, Jimmy Yip and Yong Mei Yoke is gratefully acknowledged.
References w1x R. McPherson, Thin Solid Films 83 Ž1981. 297. w2x H. Liu, E.J. Larvenia, R.H. Rangel, Acta Metall. Mater. 43 Ž1995. 2053. w3x R. McPherson, Thin Solid Films 112 Ž1984. 89. w4x K.E. Wilkes,J.F. Lagedrost, Thermophysical properties of plasma sprayed coatings, NASA Rep. CR-121144 Ž1973.. w5x R.C. Progelhof, J.L. Throne, R.R. Ruetsch, Polymer Eng. Sci. 16 Ž9. Ž1976. 615. w6x L.M. Sheppard, Am. Ceram. Soc. Bull. 71 Ž1992. 313.
w7x J.H. Moll, C.F. Yolton, B.J. McTiernan, Inter. J. Powder Metall. 26 Ž2. Ž1990. 149. w8x P.G. Bailey, W.H. Schweikert, Superalloys Source Book, ASM, OH, 1984, pp. 342–346. w9x H. Kuribayashi, K. Suganuma, Y. Miyamoto, M. Koizumi, Am. Ceram. Soc. Bull. 65 Ž9. Ž1986. 1306. w10x K.A. Khor, N.L. Loh, M.J. Bannister ŽEd.., Ceramics — Adding the Value, vol. 2, CSIRO, Melbourne, 1992, p. 804. w11x K.A. Khor, Y. Murakoshi, T. Horihata, M. Takahashi, T. Sano, in: Y. Bando, K. Kosuge ŽEds.., Proceeding 1993, Powder Metallurgy World Congress, Japan Society of Powder and Powder Metallurgy, Tokyo, 1993 p. 800. w12x W.J. Brindley, R.A. Miller, Adv. Mater. Proc. 140 Ž8. Ž1989. 29. w13x T.N. Rhys-Jones, F.C. Toriz, High Temp. Technol. 7 Ž2. Ž1989. 73. w14x B. Wu, E. Chang, S. Chang, D. Tu, J. Am. Ceram. Soc. 72 Ž2. Ž1989. 212. w15x A.H. Bartlett, R.D. Maschio, J. Am. Ceram. Soc. 78 Ž1995. 1018. w16x K.A. Khor, N.L. Loh, in: T.S. Sudarshan, M. Jeandin ŽEds.., Surface Modification Technologies VIII, The Institute of Materials, London, 1995, p. 688.
Materials Letters 34 Ž1998. 263–268
Hot isostatic pressing of plasma sprayed yttria-stabilized zirconia K.A. Khor ) , Y.W. Gu School of Mechanical and Production Engineering, Nanyang Technological UniÕersity, Nanyang AÕenue, 63 9798 Singapore, Singapore Received 29 May 1997; revised 24 June 1997; accepted 27 June 1997
Abstract The porosity in plasma sprayed ceramics is caused by the irregular splat formation of the lamellae structure as the molten droplets impact the substrate. This paper reports the influence of hot isostatic pressing ŽHIP. as a post-spray treatment of plasma sprayed coatings. Investigations are made on the physical properties, pore size distribution and thermo-physical properties, namely thermal diffusivity and thermal conductivity, of plasma sprayed yttria-stabilized zirconia ŽZrO 2 –7.8 wt% Y2 O 3 . coatings after HIP at 1000, 1100 and 12008C for 1 h. Results show that HIP processing the plasma sprayed coatings reduced the porosity by ; 2.5% and increased the microhardness by ; 40%. Correspondingly, the thermal diffusivity and thermal conductivity of the coatings were found to be significantly changed after HIP treatment at 12008C. q 1998 Elsevier Science B.V. PACS: 81.05.-t; 81.05.Je; 81.15.Rs Keywords: Plasma spray; Hot isostatic pressing; Zirconia; Porosity; Microstructure; Thermal diffusivity; Thermal conductivity
1. Introduction Porosity in plasma sprayed coatings can be viewed by a two level model where pores in the size-range of 1–10 m m arise from entrapped gas, unmelted particles, cracking or premature solidification by some particles while pores - 0.1 m m are formed as a result of poor interlamellar contacts w1x. The latter type of pore is rather predominant in ceramic coatings. Plasma sprayed coatings are formed by melting the powder feedstock and impinging the molten droplets onto a prepared substrate surface. The molten
)
Corresponding author.
droplets solidify rapidly on impact. There are also varying degrees of spreading of the liquid before final solidification. Thus, giving rise to the characteristic lamellae microstructure of plasma sprayed coatings. The inter-lamella adhesion is predominantly mechanical interlocking of the atoms. Bad contacts amongst the lamellae can be found and in many cases the space between two overlapping lamellae can be up to 100 nm w1x. The outcome is a coating that is mechanically weak. A recent study attempts to further categorize the micro-pores according to the nature of their formation w2x. Basically, the micropores are formed from resulting dropletrdroplet and dropletrliquidsurface-layer interactions. In addition, they can be formed through the ejection or rebounding of liquid
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 1 8 4 - 5
264
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
Žthe pores are formed when the detached liquid eventually falls back onto the coating and some pores result from entrapped gases. and dispersoids as in the case of carbides, oxides and nitrides reinforced alloy systems. Porosity in plasma sprayed coatings generally depend on the process settings such as average particle size and size distribution of the feedstock, the power level of the spray, the spray distance and the type of cooling received by the coating. The influence of the porosity on the thermal conductivity has been analyzed by several researchers w3–5x. These models help to predict the effective thermal conductivity, k eff , which is the conductivity of the porous material given the value of the volume fraction of the pores, P, the thermal conductivity of the matrix, k m , and the thermal conductivity of the dispersed phase, k f . Hot isostatic pressing ŽHIP. is a technology that has encouraging success in consolidation of advanced materials such as ceramics and intermetallics w6–11x. The simultaneous application of high temperature and pressure often results in a material with high density. This paper reports the effect of HIP on the microstructure, and, physical and thermal properties of plasma sprayed yttria stabilized zirconia ŽYSZ. coatings. The YSZ coating is a popular material for thermal barrier coatings ŽTBC. in turbine blades of jet engines and gas turbines w12,13x. It has good chemical and thermal stability and is an excellent thermal insulator. A bond coat of composition MCrAlY Žwhere M s Ni, Co or Fe. is usually applied as an intermediate layer to enhance the adhesion of the ceramic. However, oxidation of the samples in air results in the formation of a reaction layer at the bond coatrceramic coat interface w14,15x. Failure of the TBC has been attributed, in many instances, to the presence of this layer of oxide that eventually leads to spalling of the TBC. Thus, in order for the TBC to have long term effectiveness the pore structure and pore distribution has to be altered to reduce oxidation of the bond coat.
2. Experimental procedure The zirconia-based coatings are prepared by a 40 kW dc plasma spray system ŽMiller Thermal, WI.. A
Table 1 Plasma spraying parameters Primary gas Žpressure.; flowrate Auxiliary gas Žpressure.; flowrate Powder feed rate Arc current Arc voltage Spray distance
argon Ž50 psi.; 82 scfh helium Ž50 psi.; 26 scfh 30 grmin 800 A 50 V 120 mm
commercial yttria stabilized zirconia thermal spray powder ŽAI 1075, Alloy International, TX. was used as the feedstock. The HIP system used was the System 5X from Kobelco, Japan. This system is capable of attaining a maximum temperature of 20008C and a maximum pressure of 200 MPa when a graphite furnace heating element is used. A molybdenum heating element was used in the present study. Table 1 lists the plasma spray conditions while Table 2 shows the HIP parameters for the post-spray treatment of plasma sprayed coatings. Scanning electron microscopy ŽSEM. was performed with the Cambridge Stereo scan S360 equipped with an energy dispersive X-ray analyzer ŽLink AN 10r85S.. Phase analysis was done on the Philips MPD 1880 X-ray diffractometer system. Density measurement was accomplished with a helium gas pycnometer ŽUltrapycnometer, Quantachrome, USA.. Porosity measurement was executed using the mercury intrusion porosimeter, Autopore III from Micromeritics, USA. In this technique, mercury is intruded into the coating to measure the pore size. The technique is based on the non-wetting characteristics of mercury. Pore sizes are determined by Washburn’s equation Pr s y2g cos u ,
Ž 1.
where g is the surface tension of mercury and u is the contact angle between mercury and the pore wall. Pr is the applied external pressure that ranged from Table 2 Parameters for hot isostatic pressing of Y2 O 3 stabilized ZrO 2 ŽHYZ. Specimen
Temperature Ž8C.
Pressure ŽMPa.
Time Žh.
HYZ 1 HYZ 2 HYZ 3
1000 1100 1200
180 180 180
1 1 1
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
several bars to several tens of thousand bars. The variation of the external applied pressure caused changes in the intruded volume. This in turn was related to pore-size distribution. This technique requires pores that are inter-connected, a common feature in plasma sprayed ceramic coatings. The thermal diffusivity and thermal conductivity of the coatings were measured by the laser flash method ŽThermaflash 2200, Holometrix, USA.. The samples tested were disks ; 2 m m thick and 12.7 m m in diameter. The samples were coated with approximately 0.1 m m of gold and 5 m m of graphite for testing. This method conformed to ASTM E1461-92 for the measurement of thermal diffusivity.
3. Results and discussion 3.1. Microstructure and physical properties of plasma sprayed and HIPed yttria stabilized zirconia Fig. 1 shows the fractured surface of the assprayed YSZ coating. The typical features described earlier viz. pores, inter-lamellae separation, internal lamellae cracking can be observed. The fractured surface of HIPed coatings showed, generally, a lower incidence of pores and relatively better inter-lamellae adhesion ŽFig. 2.. There is no evidence of any grain growth in all the HIPed samples ŽHYZ 1–3..
Fig. 1. SEM view of as-sprayed ZrO 2 –7.8 wt% Y2 O 3 ŽYSZ. coating.
265
Fig. 2. SEM view of ZrO 2 –7.8 wt% Y2 O 3 coating after HIP.
The MIP results showed some changes to the pore size distribution of the YSZ coatings after HIP ŽFig. 3.. HIP has effected a reduction in pores of the size-range 30–100 m m, 1.5–4 m m and a corresponding increase in the pores of size 0.2–1.0 m m. It can also be observed that at a HIP temperature of 12008C, the fine pores that are - 0.2 m m are significantly reduced. These pores are likely to be the inter-lamellae pores and the reduction or elimination of these pores would serve to strengthen the inter-lamellae bonds. There is also a decrease in the overall porosity, of ; 2.5%, in the HIPed coatings, as shown in Table 3. The density measurements showed a corresponding increase in the density of the HIPed coatings. Microhardness measurements taken on polished cross-sections of the coatings showed a progressive increase in hardness values ŽFig. 4.. Previous investigation has shown that the porosity in the plasma sprayed coatings can be reduced to - 1% when the coatings are HIPed with encapsulation w16x. Complete elimination of the pores in the YSZ coatings is, of course, not practical considering the application of the coatings as thermal barrier coatings. A certain level of porosity in the TBC is necessary for efficient thermal insulation. Hence, the coatings are HIPed without encapsulation. Nevertheless, a reduction of the various pore networks and inter-lamellae pores will help to slow down the oxidation process of the bond coats, that ultimately lead to spallation during usage. The reduction of these pores would contribute significantly to the increase in the lifespan of the TBC. The pores, in
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
266
Fig. 3. Pore size distribution of the YSZ coatings after HIP.
particular the large pores Ž50–100 m m., appear to collapse during HIP, as evident from the MIP results. This lead to particle fragmentation and formation of many ‘new’ inter-particle contacts. The MIP results seems to indicate that the small pores in the sizerange 0.3–1.0 m m increased correspondingly, quite likely as a result of the collapse of the large pores. The reduction in the amount of pores that are - 0.2 m m appears to be a consequence of the elimination of the inter-lamellae pores through sintering effects of the HIP process. This will lead to strengthening of the inter-lamellae bonds and may account for the
increase in microhardness values of the HIPed coatings despite the rather modest reduction in the overall porosity.
Table 3 Porosity of HIPed Y2 O 3 -stabilized ZrO 2 coatings measured by mercury intrusion porosimetry Specimen
Porosity Ž%.
Average density Žgrml.
Average pore diameter Ž m m.
As-sprayed HYZ 1 HYZ 2 HYZ 3
12.48 12.42 11.72 9.97
5.6278 5.7588 5.7720 5.9427
0.2145 0.1910 0.1836 0.1871 Fig. 4. Microhardness of as-sprayed and HIPed coatings.
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
267
the thermo–physical properties of the coatings. Whilst the reduction in the fine pores Ž- 0.2 m m. apparently improved the physical properties of the YSZ, the thermo–physical properties were correspondingly diminished. This suggests that there is a limit to the improvement of the physical properties of the plasma sprayed YSZ at the expense of the thermo–physical properties. Bearing in mind the application of this material as a thermal barrier coating.
4. Conclusions Fig. 5. Thermal diffusivity versus temperature plot for as-sprayed and HIPed YSZ coatings.
3.2. Thermal diffusiÕity and thermal conductiÕity of HIPed yttria stabilized zirconia Figs. 5 and 6 shows the thermal diffusivity and thermal conductivity values of the as-sprayed and HIPed coatings respectively. It can be observed that the thermal diffusivity and thermal conductivity values of the sample that was HIPed increased significantly over the values of as-sprayed coatings. These values can be related to the pore size distribution of the coatings, and highlight the impact on the reduction of pores in the size-range of 50–100 m m as well as the 1.5–4 m m pores on the thermo–physical properties of the plasma sprayed ceramic. The increase in pores of size 0.3–1.0 m m in the HIPed samples apparently did not contribute positively to
Hot isostatic pressing of plasma sprayed YSZ has revealed changes to the pores size distribution, physical and thermal properties. The reduction of porosity was modest Ž; 2.5%., however, improvement of their physical properties such as microhardness Ž; 40%. was more significant. The MIP data of the pore size distributions showed that HIP evoked changes to the microstructures of the plasma sprayed and HIPed YSZ coatings that in turn influenced the physical and thermal properties of the coatings. The large pores Ž50–100 m m. in the YSZ coatings apparently collapsed and fragmentation occurred, under the HIP pressure, to yield many additional particle contacts that helped to improve the physical properties such as hardness and density. The improvement in physical properties was also believed to have been contributed by the reduction of fine pores that are - 0.2 m m. The average pore diameter was reduced from 0.2145 to 0.1836 m m for the HIPed samples ŽHYZ 1–3.. However, pores in the size-range 0.3–1.0 m m was found to increase following the HIP treatment. These pores are believed to have no significant impact on the thermal properties of the YSZ coatings. Instead, the thermal diffusivity and thermal conductivity results indicated that the reduction in the 30–100 m m and 1.5–4 m m pores caused the HIPed samples to exhibit higher values for thermal diffusivity and thermal conductivity.
Acknowledgements Fig. 6. Thermal conductivity versus temperature plot for as-sprayed and HIPed YSZ coatings.
The Hot Isostatic Press ŽHIP. unit was financed by a Research Grant from the Ministry of Finance
268
K.A. Khor, Y.W. Gu r Materials Letters 34 (1998) 263–268
ŽSingapore.. Financial assistance through Applied Research Grant No. RP 56r92 and RG 25r96 is gratefully acknowledged. The technical assistance provided by Daniel Gan, Alex Ng, Sunny Low, Jimmy Yip and Yong Mei Yoke is gratefully acknowledged.
References w1x R. McPherson, Thin Solid Films 83 Ž1981. 297. w2x H. Liu, E.J. Larvenia, R.H. Rangel, Acta Metall. Mater. 43 Ž1995. 2053. w3x R. McPherson, Thin Solid Films 112 Ž1984. 89. w4x K.E. Wilkes,J.F. Lagedrost, Thermophysical properties of plasma sprayed coatings, NASA Rep. CR-121144 Ž1973.. w5x R.C. Progelhof, J.L. Throne, R.R. Ruetsch, Polymer Eng. Sci. 16 Ž9. Ž1976. 615. w6x L.M. Sheppard, Am. Ceram. Soc. Bull. 71 Ž1992. 313.
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