FGM-Oxidation Protection System for Nonoxide Ceramics

摘要

Chemically graded composites, consisting of layers withdifferent amounts of Si_3N_4, A1_2O_3 and SiO_2 ranging fromSi_3N_4-rich plus Y_2O_3 to Si_3N_4-free were hot-pressed at 1600deg C under 12.5MPa uniaxial pressure in argon atmosphere. The aimof the study was the development of a functionally graded laminatesystem capable for oxidation protection of non-oxide substrates athigh temperatures.Starting materials for the composite layers are #beta#-Si_3N_4,#alpha#-Si_3N_4, #alpha#-A1_2O_3 and Y_2O_3 submicron powderscoated with amorphous SiO_2 by means of a sol-gel process. Theindividual layers of the FGM display favourable densificationbehaviour due to viscous flow sintering of the amorphous silica,and is almost complete at 14000C. Up to 14000C no reaction betweenconstituents is observed. Increasing the sintering temperatures to1600 deg C causes extensive formation of mullite (2.SiO_2.3A1_2O_3),and X-phase sialon (Si_(12)A1_(18)O_(39)N_8) in the surface andintermediate layers, respectively. In the substrate layer Si_3N_4and a glassy Y-Si-Al-O-N-phase forms which crystallises at furtherheat-treatment at 1200 deg C to Y_2Si_2O_7.The FGM composites display few cracks in the surface layer but whichare stopped or deflected in the adjacent intermediate layer. Thisis explained with low stresses in the substrate and intermediatelayers due to the similar low thermal expansion coefficients ofSi_3N_4 and X-phase sialon, while the expansion coefficient ofmullite being the only phase in the surface layer is considerablyhigher. Since in oxidation protection layers crack formation shouldbe prevented, it is reasonable to omit the mullite surface layerand to use the intermediate layer for oxidation protection only:Experiments show clearly that up to 1300 deg C the oxidationbehaviour of the intermediate layer material is excellent, andafter isothermal heat-treatment at 1200 deg C no interdiffusionbetween substrate and intermediate layer does occer.