Graded coatings by gradient temperature densification
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Graded coatings by gradient temperature densification Sung Kang Hur Department Materials Science and Technology, Chang-won University, Korea
Sang H. Yoo and Joanna R. Groza Department of Chemical Engineering/Materials Science and Mechanical Engineering, University of California at Davis, Davis, California 95616-5294
Jung Man Doh Division of Metals, Korea Institute of Science and Technology, Seoul, Korea
Kazuo Yamazaki Department of Chemical Engineering/Materials Science and Mechanical Engineering, University of California at Davis, Davis, California 95616-5294
Kazuo Shoda Sodick Co., Ltd., 3-12-1 Nakamuchidai, Midoriky, Yokohama, Japan (Received 21 October 1996; accepted 3 July 1997)
Functionally gradient materials (FGM) were prepared using layers of ZrO2 –3 mol % Y2 O3 ceramic and NiCrAlY powders. A fine-grained zirconia powder was chosen to lower the ceramic sintering temperature and achieve simultaneous metal and ceramic densification. Consolidation of FGM’s was achieved by a short time field-assisted sintering technique. Sintering was performed either at a constant temperature or in a temperature gradient by using punches made of different materials (i.e., one graphite and one tungsten). A temperature gradient of at least 100 ±C was required with a low value of 1200 ±C at the metal end and exceeding 1300 ±C at the ceramic end. Increasing the number of intermediate layers alleviates some of the cracks formed during sintering due to different coefficients of thermal expansion.
I. INTRODUCTION
Thermal barrier coatings made of ceramic materials are largely used to impart corrosion and oxidation resistance to metal parts at high temperatures. Particularly, engine efficiency may be increased by combining the good mechanical behavior of superalloys with the superior high temperature resistance of the ceramic coatings. The heat resistant coating allows an increase in turbine inlet temperatures and reduces coolant requirements. The conventional ceramic coatings suffer from cracking and delaminations at the interface due to large ceramic-metal difference in the coefficients of thermal expansion (CTE). To minimize the thermal stresses at an abrupt metal-ceramic interface, a gradual change from a ceramic coating to metal base support, such as in functionally gradient materials1 (FGM), may be adopted. FGM’s offer the promise to minimize thermal stresses at an otherwise abrupt metal-ceramic interface, but, at the same time, provide challenges for processing simultaneously the two types of materials. The usual processing methods of FGM’s are powder metallurgy (PM),2,3 chemical4 or physical vapor deposition,5 plasma spraying,6,7 and self-propagating high temperature combustion synthesis.8 Among them, PM processing of FGM’s provides a wide range of compositional and microstructural flexibility, in addition to shape-forming J. Mater. Res., Vol. 13, No. 5, May 1998
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capability. However, in simultaneous sintering of ceramic
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