FGM-Oxidation Protection System for Nonoxide Ceramics

Abstract

Chemically graded composites, consisting of layers with different amounts of Si 3 N 4 , Al 2 O 3 and SiO 2 ranging from Si 3 N 4 -rich plus Y 2 O 3 to Si 3 N 4 -free were hot-pressed at 1600°C under 12.5MPa uniaxial pressure in argon atmosphere. The aim of the study was the development of functionally graded laminate system capable for oxidation protection of non-oxide substrates at high temperatures. Starting materials for the composite layers are β-Si 3 N 4 , α-Si 3 N 4 , α-Al 2 O 3 and Y 2 O 3 submicron powders coated with amorphous SiO 2 by means of a sol-gel process. The individual layers of the FGM display favourable densification behaviour due to viscous flow sintering of the amorphous silica, and is almost complete at 1400°C. Up to 1400°C no reaction between constituents is observed. Increasing the sintering temperatures to 1600°C causes extensive formation of mullite (2.SiO 2 .3Al 2 O 3 ), and X-phase sialon (Si 12 Al 18 O 39 N 8 ) in the surface and intermediate layers, respectively. In the substrate layer Si 3 N 4 and a glassy Y-Si-Al-O-N-phase forms which crystallises at further heat-treatment at 1200°C to Y 2 Si 2 O 7 . The FGM composites display few cracks in the surface layer but which are stopped or deflected in the adjacent intermediate layer. This is explained with low stresses in the substrate and intermediate layers due to the similar low thermal expansion coefficients of Si 3 N 4 and X-phase sialon, while the expansion coefficient of mullite being the only phase in the surface layer is considerably higher. Since in oxidation protection layers crack formation should be prevented, it is reasonable to omit the mullite surface layer and to use the intcnnediate layer for oxidation protection only: Experiments show clearly that up to 1300°C the oxidation behaviour of the intermediate layer material is excellent, and after isothermal heat-treatment at 1200°C no interdiffusion between substrate and intermediate layer does occur.