Hertzian Crack Suppression and Damage Tolerance of Silicon Nitride Bilayer

Abstract

Hertzian crack suppression phenomena and relatively high damage tolerance were investigated in hard/soft silicon nitride (Si₃N₄) bilayers. Coarse α-Si₃N₄ powder was used for the hard coating layer and fine α-Si₃N₄ powder was used for the soft substrate layer. The two layers were designed with a strong interface. Hertzian indentation was used to investigate contact fracture and damage tolerance property. Hertzian crack suppression has occurred with increasing applied load and decreasing coating thickness. The crack suppression contributed strength improvement, especially in the bilayers with thinner coatings. Ultimately, the combination of hard coating with soft but tough underlayer improved the damage tolerance of brittle Si₃N₄ ceramics.