Analysis of the formation of plasma sprayed thermal barrier coatings

摘要

Increase in heat-resistance of a ZrO_2 coating consists of change in its surface properties caused by short-term concentrated thermal effect. It is known that in the process of heat treatment the external atmosphere effects the surface of a Ni-Cr-Al-Y understratum due to porous structure of the top ceramic coating and causes oxidation of this understratum. As a result an Al_2O_3 layer is formed on metal-ceramic border. This layer changes the stress condition of the coating that results in infringement of metal-ceramic contact and exfoliation of ceramics [1]. To lower velocity of oxidation of understratum the surface of a ZrO_2 thermal barrier ceramic coating is fused by laser radiation. The fracturing occurs on the surface of samples because of a thermal stress. Usually these fractures are of 1 μm max. in width and about 10μm in depth. In most cases the depth fractures are less than the thickness of modified layer, i.e., the partial hermetic sealing of protective surface is observed. As a rule, the diameter of formed fragments do not exceed 50 μm. The complex dependence of coating heat-resistance on porosity has formed the basis of structural or fragmentary theory. According to this theory a material with separate fragments having some possibility of moving because of thermal loading are characterized by high heat-resistance [2]. The fragmentary structure has microfractures which interfere with development of destroying fractures. Heat-resistance of such a structure raises with increase in concentration of microfractures. The fragmentary structure can be produced by different technological ways, in particular, by the use of several components having various shrinkage during burning, various coefficients of thermal expansion, large and fine granularity, etc. Fusion of the surface by laser radiation with formation of fragmentary structure is of special interest as fracturing and development thereof in modified layer because of laser processing of the surface of a ZrO_2coating can result in increased heat resistance. Besides the partial hermetic sealing of understratum occurs because of ZrO_2 fusion.