Abstract

An improved method is proposed for providing a coating film of diamond on the surface of, for example, a cutting tool as the substrate by the plasma jet deposition method, in which the deposited diamond film has, different from the cubic crystalline structure formed under conventional conditions, a predominantly hexagonal crystalline structure so as to greatly enhance the advantages obtained by the diamond coating of the tool in respect of the hardness and smoothness of the coated surface. The improvement comprises: using hydrogen alone as the plasma-generating gas; controlling the pressure of the plasma atmosphere not to exceed 300 Torr; keeping the substrate surface at a temperature of 800°-1200° C.; and making a temperature gradient of at least 13,000° C./cm within the boundary layer on the substrate surface.