Refractory ceramic coatings: processes, systems and wettability/adhesion

Refractory materials such as ceramics and refractory metals/alloys are widely used for structural applications in severe environments where more efficient use of energy is a prime need. Owing to properties such as high strength and refractoriness, low thermal expansion and electric conductivity these materials may be used as turbocharger rotors, valves, valve seats and turbocharger turbine wheels in gas turbine engines, tubes in industrial heat exchangers, fuel elements in nuclear reactors, fusion reactor wall material, tool materials in machining and casting and as electronic packaging. In these applications, however, these materials often experience extreme mechanical, thermal and chemical loading conditions and are expected to perform effectively beyond existing limits. The existing limits of performance for these materials can be extended only via coating technology, which presently appears to be the most suitable technical and economic approach. Thus there remains a critical need to develop a suitable and reliable refractory coating system and coating technique. In view of the above requirements, the present paper embarks on a brief review of refractory coatings. The review begins with a set of criteria required for an effective refractory coating. The criteria include general thermophysical and mechanical properties for the refractory coating only and properties of the coating-substrate as a system. The review continues with a section on methods of coating. The classification of methods of coating is based on categories such as gaseous, solution, molten/semi-molten and solid state processes. Each category is explained with conventional and novel techniques and coatings that can be synthesized using these techniques. These categories are supplemented also with examples of applications. A major emphasis has been put on refractory materials in the following section on coating material systems. The most important materials systems have been grouped as oxides and nitrides, carbides, borides, silicides and aluminides. The general properties that make them useful as refractory coatings, along with specific materials and applications, have been reviewed. Wettability and adhesion between coating and substrate are very important for the coating- substrate system to perform effectively and reliably. The last section discusses the fundamentals of wettability and work of adhesion, based on which wetting in metal-metal, metal oxide, metal-carbon/carbide systems has been discussed. Finally, the review sets the motion of thoughts with a description on future developments. Copyright (C) 2001 John Wiley & Sons, Ltd.