Effect of the Type of Rare-Erath Oxides on the Oxidation and Thermal Shock Behavior of In-Situ α-Sialon

Effect of the Type of Rare-Erath Oxides on the Oxidation and Thermal Shock Behavior of In-Situ α-Sialon
　　α-Sialon ceramics composites doped with Nd, Sm, Dy, and Yb rare-earth additives were synt-hesised by hot-pressing. The effects of adding excess 2 wt.% rare earth oxide as sintering aids on the densification, phase components, and microstructure were studied by XRD, SEM and TEM. Room temperature flexure strength and fracture toughness were measured through three-point bending, and single-dege-notch beam bending method. The influence of the rare earth, phase Ball Neodymium Magnets components, and microstructures on the themophysical properties, thermal shock and oxidation resistance of the composites had been investigated.Results showed that all theα-Sialon ceramics achieved densities higher than 98% of their theoretical values after hot pressing at 1800℃for 1h under pressure of 30MPa in a nitrogen atomosphere of 0.1 MPa.
　　The Dy and Yb-dopedα-Sialons were successfully densified by adding 2 wt.% corresponding rare earth oxide as additives, and by using utrafine Si3N4、AlN、Al2O3 and RE2O3(RE= Dy and Yb)as the starting powders.XRD results revealed that the Nd-Sialons hadα-Sialon, trace ofβ-Sialon and M′(R2Si3-xAlxO3+xN4-x) in their phase components regardless of the 2 wt.% excess rare earth oxide. On the contrary, the other Sm-, Dy-, and Yb-sialon were pureα-Sialon, and had high hardness (Hv10=21GPa), accordingly.The microstructure of the materials varied with rare earths. Yb and Dy-sialon obtained fine equiaxed grains in their microstructures, because that the smaller rare earths promoted the formation ofα-Sialon. After the transient liquid was depleted, anisotropical growth of theα-Sialon was stunted.
　　This was contrarory to the case of Nd and Sm-Sialon, Ball Neodymium Magnets where elongated grains were presented. The flexure strength and fracture toughness of the materials falled into the range from 300MPa～600MPa and 5.1MPa·m1/2, depending on their microstructures. Elongated grains of composites impart high toughness and strength to the material throngh energy-absorbing mechanisms of crack bridging,grain pullout,and crack path deflection.The research results show that the composites have excellent thermal shock resistance propertier. The minimum residual strength remained 75% of its original flexural strength at a thermal shock resistance temperature dirrerence (△T) up to1200℃. The residual strength of the composites improved at a thermal shock resistance temperature dirrerence (△T) up to1100℃after water quenching for 14 times.Combination of the excellent mechanical properties together with the good thermophysical properties result in its excellent thermal shock Ball Neodymium Magnets resistance properties.The high temperature oxidation resistance of -Sialon ceramics composites doped with different rare-earth additives was farely good. Weigh gains by area at 1300℃for 32h were 3.5 mg/cm2 for NdE2 and NdE0, but 1.5 mg/cm2 for SmE2, DyE2, YbE2 materials. Oxidation activation energies of NdE2, NdE0, SmE2, DyE2,YbE2,were determined to be 475kJ/mol,488kJ/mol,505kJ/mol,512kJ/mol, 515kJ/mol respectively.