Diamond-coated hard material, throwaway insert and a process for the production thereof

A diamond-coated hard material having a diamond- and/or diamond-like carbon-coated layer formed on the surface of a hard material consisting of Si.sub.3 N.sub.4 as a predominant component, in which:

• (1) microscopic roughness is present on the surface of the substrate and
• (2) the protrusive parts are defined by a surface roughness represented by an Rmax of 1.5 to 30 .mu.m in the standard length of 50 .mu.m in the diamond-coated layer and substrate interface.

A diamond-coated throwaway insert having a diamond- or diamond-like carbon-coated layer with a thickness of 0.1 to 200 .mu.m, deposited from gaseous phase, on the surface of a substrate consisting of Si.sub.3 N.sub.4 as a predominant component, in which the surface state of the substrate is maintained as sintered and a part or whole of the sintered surface is coated with the diamond- or diamond-like carbon-coated layer.

A process for the production of a diamond-coated throwaway insert, which comprises sintering a mixed powder to be a substrate, comprising Si.sub.3 N.sub.4 powder containing at least 50 % of a Si.sub.3 N.sub.4 , as a predominant component, and 1 to 50 wt % of at least one sintering assistant selected from the group consisting of Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 MgO AlN and SiO.sub.2 at a temperature of 1,600.degree. to 2,000.degree. C. in a gaseous N.sub.2 atmosphere for 30 minutes to 5 hours, converting at least the rake face thereof into a sintered surface or heat-treated surface and then coating at least the rake face with diamond.