THERMAL-OXIDATION OF TUNGSTEN-BASED SPUTTERED COATINGS

The effect of the addition of nickel, titanium, and nitrogen on the ai r oxidation behavior of W-based sputtered coatings in the temperature range 600 to 800 degrees C was studied. In some cases these additions significantly Improved the oxidation resistance of the tungsten coatin gs. As reported for bulk tungsten, all the coatings studied were oxidi zed by layers following a parabolic law. Besides WO3 and WOx phases de tected in all the oxidized coatings, TiO2 and NiWO4 were also detected for W-Ti and W-Ni films, respectively. WOx was present as an inner pr otective compact layer covered by the porous WO3 oxide. The best oxida tion resistance was found for W-Ti and W-N-Ni coatings which also pres ented the highest activation energies (E(a) = 234 and 218 kJ mol(-1), respectively, as opposed to E(a) approximate to 188 kJ mol(-1) for the other coatings). These lower oxidation weight gains were attributed t o the greater difficulty of the inward diffusion of oxygen ions for W- Ti films, owing to the formation of fine particles of TiO2, and the fo rmation of the external, more protective layer of NiWO4 for W-N-Ni coa tings.