Physical vapour deposition processes

Abstract

Physical vapour deposition (PVD) is the process whereby metals and alloys may be transferred in the vapour state from one source to another. It was known as early as 1857, and since 1912 has been operated in vacuum form, both experimentally and commercially, for applying coatings and fabricating bulk shapes. Over the years the basic PVD process has been modified and used in a number of different ways, and these have been given different names. The development of this sometimes conflicting and confusing nomenclature is reviewed as an introduction to a discussion of the background and present status of the PVD process for depositing overlay protective coatings on gas turbine components, a process which has been in use commercially for over 15 years. The requirements for this application are very different from those for most other uses, and these are discussed in terms of their effects on the adherence, structure, and post-coating processing of the applied coatings. The limitations of the process, such as required compositional ranges, growth defect tolerance, line-of-sight deposition, low-angle coatings, and the ability to deposit elements having widely different vapour pressures, are considered. The latest forms of the PVD process are discussed in terms of how these limitations affect them. The present status of PVD processes is examined in the light of the current economic climate and future technological developments. MSTj283 IntroductionIn the early 1960s the development of gas turbine design and materials resulted in a requirement for aerofoil protective coating systems beyond the capabilities of the diffusion applied aluminide coatings then in use. 1 In these systems approximately 70% of the coating was substrate, and increasing the strength of the substrate often reduced the lifetimes of coating systems made from materials that would have lasted longer at higher temperatures. 2 From this new requirement the concept of an overlay coating system was born, and with it a search for a suitable method of deposition.One. process then receiving considerable attention for other applications, physical vapour deposition (PVD), came to be considered for the application of a coating composition developed separately and formed independently of the substrate.Although neither PVD nor the gas turbine were new technologies, the performance requirements that arose in the early 1960s brought them together, and a m'odified coating processing technology emerged. The use of the PVD process for coating gas turbine components presented a set of requirements and problems not normally found with other PVD applications.In this paper, discussion is limited to a metal vapour produced by electron beam evaporation (EB-PVD). However, many of the considerations are also relevant to sputtering or other vapour processes. Within the limits of this paper it is neither possible nor desirable to cover the number of detailed reviews 3 -5 and the immense literature in this area. Instead, following a short historical summary, an attempt is made to e...

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