Oxidation Resistance and Strength Retention of Silicon Nitride Coated with Lutetium Disilicate

Abstract

Silicon nitride (Si 3 N 4 ) coated with lutetium disilicate (Lu 2 Si 2 O 7 ) layer through the intermediate silica (SiO 2 )-rich phase was fabricated by reaction sintering after dip-coating. The Lu 2 Si 2 O 7 layer with a thickness of 3-29 μm was formed on Si 3 N 4 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 Si 3 N 4 were investigated and compared with those of the uncoated Si 3 N 4 . Apparent activation energy for oxidation at 1300-1500°C 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 Si 3 N 4 (310.3 kJ/mol). After long-term cycling oxidation at 1500°C 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 Lu 2 Si 2 O 7 layer and underlying SiO 2 -rich phase. Flexural strength of the coated Si 3 N 4 was not deteriorated before and after the cycling oxidation at 1500°C for up to 1000 h in comparison with that of the uncoated Si 3 N 4 . However, the high-temperature strength at 1400-1500°C in air was a little less than that of the uncoated Si 3 N 4 . The deterioration was attributed to the softening of amorphous SiO 2 phase, which existed in the intermediate phase (between the Lu 2 Si 2 O 7 layer and substrate) and inside the substrate.