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 nonmetallic layer .[.of an iron group metal aluminate.].. The coated product combines a wear resistance substantially as high as aluminum oxide cutting materials and a transverse rupture strength of at least 150,000 p.s.i. The coated product is prepared by passing water vapor, hydrogen gas and an aluminum halide over the substrate at a temperature of from 900°-1250° C., the ratio of water vapor to hydrogen gas being between about 0.025 and 2.0.
and other materials..]. .[.5. The coated cemented carbide insert of claim 4 having a centrally disposed hole therein, the insert adapted to be mounted in a pin-type toolholder..]. .[.6. The coated cemented carbide product of claim 1 in which the coating is less than 15 microns in thickness..]. .[.7. The coated cemented carbide product of claim 1 in which the cemented carbide substrate comprises titanium carbide and a matrix selected from the group consisting of iron, nickel and cobalt..]. .[.8. The coated cemented carbide product of claim 7 containing tantalum carbide..]. .[.9. The coated cemented carbide product of claim 1 in which the cemented carbide substrate comprises tungsten carbide, titanium carbide and tantalum carbide and a cobalt matrix..]. .Iadd.10. A cemented carbide tool provided with a surface coating produced by coating at least a portion of the surface of the cemented carbide with a layer 1.0 to 20 microns thick of refractory aluminum oxide. .Iaddend. .Iadd.11. A cemented carbide tool as claimed in claim 10 wherein said surface coating has a thickness of between 1.0 and 10 microns. .Iaddend. .Iadd.12. A cemented carbide tool as claimed in claim 10 wherein said cemented carbide is composed of a carbide of a metal selected from the group consisting of tungsten, titanium, tantalum and niobium, or a mixed carbide of tantalum and niobium, and a binder metal selected from the group consisting of cobalt, iron, and nickel. .Iaddend.