Lutetium disilicate (Lu2Si2O7) layer of 4-12 μm in thickness with a small amount of silica (SiO2) phase was coated on silicon nitride (Si3N4) by reaction sintering after dip coating. The oxidation resistance and flexural strength of the coated sample were investigated and compared with those of the uncoated one. After cycling oxidation at 1500°C for up to 100 h in air, the total weight gain of the coated sample was almost half that of the uncoated one, indicating that oxidation resistance of Si3N4 was improved by about 100%. The flexural strength of the coated sample was not deteriorated in comparison with that of the uncoated one, and it was also maintained after the cycling oxidation at 1500°C for 100 h in air. Those physical and chemical stabilities can be attributed to the presence of Lu2Si2O7 layer, which effectively prevents oxidation of Si3N4 at high temperature.
