FORMATION OF CHROMIUM COATINGS ALLOYED WITH NICKEL ON DIAMOND

The methods of X-ray phase analysis are used to study the composition of chromium-nickel coatings and metallizing mixtures (metal spraying c ompositions) as a function of the annealing conditions. The thickness of the coatings is calculated from the weight gain by diamond. It is s hown that coatings are formed mainly by the transfer of chromium and n ickel through the gas phase to the reaction surface (diamond or the ou ter surface of the coatings) and diffusion Of carbon from the diamond (to the coating) which determines the phase composition of the coating : Cr3C2, Cr, Cr7C3, and Ni. Nickel appears in the metal spraying compo sitions because of the reduction of NiO by the carbon of chromium and carbon monooxide. The formation of the phases in the metal spraying co mpositions, which block the intensity of evaporation of chromium (due to NiO restoration with chromium) and of Ni(Cr) (due to the dissolutio n of chromium in nickel), is the factor that limits the growth of the coating. This is the twin reason for the decrease in the thickness of the coating when nickel monoxide is added to the chromium metallizing mixture.