Stability of Silicon Nitride Coated with Lutetium Disilicate in an Oxidative Environment

Abstract

Silicon nitride (Si 3 N 4 ) coated with lutetium disilicate (Lu 2 Si 2 O 1 ) was fabricated by reaction sintering after dip coating. The Lu 2 Si 2 O 7 layer of about 14 μm in thickness with a small amount of silica (SiO 2 ) phasewas homogeneously formed on the Si 3 N 4 substrate. It was also strongly jointed to the substrate and relatively dense with a few pores. The oxidation resistance of the coated Si 3 N 4 was almost half that of the uncoated one in static air atmosphere at 1500°C for 500 hours. A corrosion test was also conducted in 30 vol%-H 2 O/air balance at 1.8 atm and 1500°C for 100 hours. The coating layer effectively protected the substrate from the corrosion. The flexural strength of the coated Si 3 N 4 was not deteriorated after the oxidation and corrosion tests. These chemical and physical stabilities were attributed to the protection of Si 3 N 4 by the coating layer of Lu 2 Si 2 O 7 in an oxidative environment.