Impact tests were conducted on the silicon nitride substrates coated with Si3N4 and SiC by chemical vapour deposition (CVD) For both 100- and 200-mum-thick Si3N4-coated silicon nitride, Hertzian crack extension was reduced by debonding at the interface. Although Hertzian crack extension was not reduced for 100-mum-thick SiC-coated silicon nitride, it was reduced for 200-mum-thick SiC-coated silicon nitride. Theoretical calculations suggest that debonding at the interface consumed the fracture energy of Hertzian crack extension in the case of Si3N, coatings, but it was observed that Hertzian cracks were not arrested at the interface.