METALS AND MATERIALS International, Vol. 14, No. 4 (2008), pp. 411~417 doi: 10.3365/met.mat.2008.08.411 Published 26 August 2008 Functionally Gradient (YSZ-20 %Al 2 O 3 )-SUS422 Composites D. H. Kuo 1 , C. L. Chang 2 , K. S. Chen 2 , T. Y. Yeh 3 , R. K. Shiue 3,* , and M. H. Wei 4 1 Department of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, R.O.C. 2 Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan, R.O.C. 3 Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, R.O.C. 4 Chemical Systems Research Division, Chung-Shan Institute of Science and Technology, Lung-Tan, Tao-Yuan, Taiwan, R.O.C. Functionally gradient materials (FGMs) were prepared by mixing 5 layers comprised of different ratios of (YSZ-20 %Al O ) and 422 stainless (SUS422) powders, followed by hot pressing for densification. Two design concepts were proposed: One was a FGM with a monotonic change of the CTE (coefficient of thermal expan- sion) for each layer, and is designated as the monotonic mode, and the other was a FGM with a change of CTE that is not monotonic for each layer, and is termed the non-monotonic mode. The FGM with a monotonic CTE mode cracked at the ceramic surface after it was removed from the hot pressing furnace. In contrast, the FGM with a non-monotonic CTE mode survived after hot pressing. Based on ABAQUS simulation results, a non-monotonic change in CTE resulted in a decrease of residual stress on the ceramic side but an increase inside the metal-rich layers of the FGMs. The induced change in the stress distribution inside the FGMs was compromised by the deformation of the metal-rich ingredient (SUS422) in the FGM. Thermal shock tests of FGMs were performed between 25 C and 600 C. The non-monotonic FGM endured up to 100 thermal cycles with only slight bending, and was free of delamination and cracking. The use of composition-adjusted layers to manipulate thermal expansion coefficients of each layer greatly changed the stress contour of the FGM. It is noted that a modified functional-gradient FGM can be fabricated with a hard ceramic surface on one side to resist high temperature, and a ductile metallic surface on the other side to provide toughness. Keywords: hot pressing, FGMs, composites, Alumina, structural applications 1. INTRODUCTION Functionally gradient materials (FGMs), defined as com- posite materials with a continuously changed composition, were proposed by M. Niino in 1984 at the National Aero- space Laboratory of Japan [1]. The primary aim of his project was to develop advanced heat-shielding structural materials for the future space program. Functionally graded heat resistant materials, which have a ceramic material on one side and a metallic alloy on the other side, are representative FGMs. The amount of the ceramic is gradu- ally modified towards the metallic side. The ceramic mate- rial provides an excellent heat resistant property, while thermal conductivity and toughness are derived from the metallic alloy. FGMs, therefore, combine not only the advantages of ceramics and metals but also benefits derived from the graded composition, e.g., the release of residual stresses upon thermal cycling of the FGMs. The highest temperature on the leading edge of space vehi- cles has been estimated to reach 1800 o C [2]. Hence, materi- als at the surface must withstand temperatures up to 1800 o C and huge temperature fluctuations between 25 o C and 1800 o C in air. Non-oxide ceramics have high strength at elevated temperatures, but oxidation and room temperature brittleness are major problems. Although most oxides have excellent oxidation resistance, the loss of creep strength at elevated temperatures and room-temperature brittleness are primary disadvantages. In contrast, metallic alloys possess excellent room-temperature toughness, but they lack high-temperature strength. Accordingly, a thermal barrier coating (TBC) of stabilized zirconia has been applied to turbine blades for the purpose of heat shielding. FGMs combining the advantages of both ceramics and metals are potential candidates for var- ious structural applications. *Corresponding author: rkshiue@ntu.edu.tw