Abstract: A high-strength, coated cemented carbide product comprising a cemented carbide substrate and a fully dense alpha aluminum oxide coating on the substrate. The coating has a thickness of from 1-20 microns and is firmly and adherently bonded to the cemented carbide substrate through a thin intermediate nonometallic layer of an iron group metal aluminate. The coated product combines a wear resistance substantially as high as aluminum oxide coating materials and a transverse rupture strength of at least 150,000 psi. The coated product is prepared by passing water vapor, hydrogen gas and an aluminum halide over the substrate at a temperatue of from 900.degree.-1250.degree.C. the ratio of water vapor to hydrogen gas being between about 0.025 and 2.0.