Effect of spray coated SiO2 layers on the low temperature oxidation of Si3N4

Si3N4 using Y2O3 as a sintering additive provides good mechanical properties and high temperature oxidation resistance, but can suffer from crack formation when oxidized at 1000 °C. In order to minimize this problem, a SiO2 layer was deposited onto Si3N4 by the spray coating method. The dispersion of the SiO2 slurry for the spray coating process was optimized, and dense silicate layers were formed by the spray coating and heat treatment in nitrogen atmosphere. Penetration of the coating layer into the sample was observed when the heat treatment temperature was higher than 1400 °C. The crystallization behavior of silicate coatings is similar to that of native oxide layers, but the formation of cristobalite is suppressed. The SiO2 coating inhibits the rapid oxidation of Si3N4 at 1000 °C effectively.