Oxidation resistance and strength retention of silicon nitride coated with lutetium disilicate

Silicon nitride (Si3N4) coated with lutetium disilicate (Lu2Si2O7) layer through the intermediate silica (SiO2)rich phase was fabricated by reaction sintering after dip-coating. The Lu2Si2O7 layer with a thickness of 3-29 mum was formed on Si3N4 substrate. It was relatively dense with several pores, and strongly jointed to the substrate through the intermediate phase. Oxidation resistance and strength retention of the coated Si3N4 were investigated and compared with those of the uncoated Si3N4. Apparent activation energy for oxidation at 1300-1500degreesC in air increased up to 523.1 kJ/mol with increasing thickness of the coating layer, which was about 1.7 times higher than that of the uncoated Si3N4 (310.3 kJ/mol). After long-term cycling oxidation at 1500degreesC for up to 1000 h in air, the total weight gain decreased by up to 51%, indicating that oxidation resistance improved by about 100%. The improvement was attributed to the protection of the substrate from an oxidative environment by the Lu2Si2O7 layer and underlying SiO2-rich phase. Flexural strength of the coated Si3N4 was not deteriorated before and after the cycling oxidation at 1500degreesC for up to 1000 It in comparison with that of the uncoated Si3N4. However, the high-temperature strength at 1400-1500degreesC in air was a little less than that of the uncoated Si3N4. The deterioration was attributed to the softening of amorphous SiO2 phase, which existed in the intermediate phase (between the Lu2Si2O7 layer and substrate) and inside the substrate.