Principles of protection from high-temperature oxidation of niobium and carbon materials

摘要

This work determines, basing on analysis of experimental and literature data, some fundamental problems for working out of heat-proof coatings on niobium alloys and carbon materials (CM), and proposes detinite ways to sotoe them. Speccay, the necessary ot two-leveled protection as minimum - the barrier layer on the refractory materials/coating border plus the outer protective layer - is shown. The choice of high-enthalpy compounds of the transition metals of group IV-a as components for barrier layer, preventing the diffusion of elements from the base to the oxidation surface, is being proved. The results of our investigations together with analysis of literature shows that the direct use of the traditional approaches on heat-protection, elaborated for refractory metals, does not always work for CM. This is related, as we suggest, to the fact that any coating on CM is subjected at high temperature oxidation conditions to effect of the two controversial diffusion streams: oxidation - the stream of oxygen atoms from the oxidation surface, and reducing - the stream of carbon atoms from the carbon base. The presence of the carbon stream under these conditions may introduce significant differences to the behavior of heat proof coatings of the same kind on CM (graphite, C-C composites and others) compare to those on refractory metals.