The role of nickel in the oxidation resistance of tungsten-based alloys

In this work, W-N-Ni coatings were produced with different contents of nick el and nitrogen. The films were oxidised in pure air at increasing temperat ures up to 800 degrees C. An increase in the oxidation resistance upon the addition of an increasing amount of nickel was observed. The oxidation mech anism involves the formation of two nickel-rich, external oxide layers of N iO and NiWO4. The limiting step seems to be the outward diffusion of either Ni2+ or W6+ ions through the oxide layers. To retain the oxidation resista nce of the amorphous W-N-Ni coatings at 800 degrees C, it is necessary that crystallisation does not induce a dimensional stress capable of destroying the protective oxide layers. (C) 1999 Elsevier Science S.A. All rights res erved.