OXIDATION OF CR-NI COATINGS ON DIAMOND

The kinetics of the oxidation of chromium and chromium - nickel coated diamond powders in air at 750 and 850-degrees-C was studied by means of interrupted weighing. The coatings and metallized powders were subj ected to x-ray phase analysis. Decrease of the powder mass confirms th at permeation of the mass with volatile oxides predominates over the i ncrement due to the weight of the absorbed oxygen in the oxides NiO an d Cr2O3. The weight loss is caused by the burning away of both the coa ting (oxidation of chromium and its carbides to CrO with formation of CO and CO2) and the diamond. Carbon from the diamond is also removed w ith CO and CO2 as a result of successive reduction of chromium oxide b y carbon, which diffuses to the oxidized layers, and further reduction of the carbides by the same mechanism. The higher the oxidation tempe rature and the thinner the coating, the greater the contribution of th ese reactions is to the processes responsible for the decrease in weig ht of the metallized diamonds. It is also encouraged by diffusion of c arbon to the scale formed and by simultaneous diffusion of oxygen to t he coating. The reactions of primary oxidation of chromium contained i n the nickel solid solution, reduction of nickel by carbon and chromiu m, and dissolution of chromium in the reduced nickel contribute signif icantly to the oxidation of chromium - nickel coatings.