Processing and characterization of cerium-doped O' + beta' mixed-phase silicon aluminum oxygen nitrogen ceramic composite materials

The goal of this study was to develop an O{dollar}spprime{dollar} + {dollar}betaspprime{dollar} SiAlON ceramic composite material combining the mechanical strength of {dollar}betaspprime{dollar} SiAlON and oxidation resistance of O{dollar}spprime{dollar} SiAlON for structural applications. A range of O{dollar}spprime{dollar} + {dollar}betaspprime{dollar} two-phase ceramic composites were fabricated by pressureless liquid-phase sintering with CeO{dollar}sb2{dollar} as a sintering aid. Phase dependence of densification, strength, elastic properties, and oxidation resistance were studied.; Densification kinetics were modelled by liquid-phase sintering. The densification of O{dollar}spprime{dollar} SiAlON was more rapid than {dollar}betaspprime{dollar} SiAlON in the initial rearrangement stage due to the formation of relatively large amounts and low viscosity initial liquid phase. Densification of {dollar}betaspprime{dollar} and O{dollar}spprime{dollar} SiAlON was controlled by diffusion and solution-precipitation, respectively, while densification of O{dollar}spprime{dollar} + {dollar}betaspprime{dollar} SiAlON composites was controlled by either diffusion or solution-precipitation depending on the sintering temperature. Densification due to solid-state reactions was negligible. Final density achieved for all compositions was approximately 99% of theoretical density.; The reaction sequence of Ce-doped O{dollar}spprime{dollar} + {dollar}betaspprime{dollar} SiAlON composites was similar to Y-doped O{dollar}spprime{dollar} + {dollar}betaspprime{dollar} SiAlON composites. O{dollar}spprime{dollar} started to form at 1450{dollar}spcirc{dollar}C and increased in amount until 1575{dollar}spcirc{dollar}C. At 1575{dollar}spcirc{dollar}C, the formation of {dollar}betaspprime{dollar} began, and the mixture equilibrated to its expected O{dollar}spprime{dollar}: {dollar}betaspprime{dollar} ratio.; The oxidation resistance of these materials increased with increasing O{dollar}spprime{dollar} content. Oxidation resistance was improved by limiting crystallization of the oxide scale. A mixture law of oxidation kinetics was developed to model oxidation behavior of the two-phase composite.; A trade-off existed between the mechanical strength of the {dollar}betaspprime{dollar} SiAlON and the oxidation resistance of the O{dollar}spprime{dollar} SiAlON. Three-point bending strength increased with increasing {dollar}betaspprime{dollar} content. The strength was improved by the high fracture toughness of the {dollar}betaspprime{dollar} phase. Strain to failure was independent of phase content. Preliminary investigation showed that an optimum combination of oxidation resistance and mechanical strength of O{dollar}spprime{dollar} + {dollar}betaspprime{dollar} SiAlON was possible by carefully tailoring the composition and microstructure.