Re‐melting of carbide coatings by TIG welding

The combination of mechanical and thermal properties, with which one is confronted when dealing with ceramics, makes them potentially very attractive constructional materials. This is particularly so since at both low and high temperatures they posses high hardness, resistance to both abrasive wear and erosion, as well as to the action of corrosive substances. In a variety of combinations and compositions these materials are also used as protective coatings. The range of applications of protective surface coatings not only in ceramic or cermet form is determined by the type of material used, the structure and the degree of adhesion of the layer, the generated residual stress system, etc. It is well known that, for instance, a coating characterised by a considerable surface porosity will not provide effective anti-corrosive protection, whereas the layer with a low adhesive property will be useless under dynamic loading conditions. It is therefore essential that the process of coating deposition be conducted with care and with regard to the future use of the component. One of the most common methods of the deposition of ceramic coatings is that of plasma spraying. This is partly due to the high melting temperatures of ceramics and therefore to the necessity of selecting a source of heat which would either produce melting of the material or, at last, its plastisization. Plasma spraying makes it possible to deposit a wide range of coating materials on metallic, ceramic and even polymeric bases." However, the very short time interval in which the powder remains in the stream of plasma measured in milliseconds and, as a rule, the low thermal conductivity of ceramics are responsible for the fact that in spite of the very high temperature of the plasma itself, some of the particles are only partially surface-melted. Full adhesion is not achieved and, therefore, porosity-free surfaces are not produced. Also, particle velocity remains insufficient to produce pore and other defect-free surfaces.