TECHNIQUES FOR IMPROVING THIN-FILM ADHESION

In most engineering applications the minimum criterion for adequate performance of a coated component is that the coating survive in the operating environment for the component lifetime. In practice this means that good adhesion of the coating to the substrate is of prime importance in dictating the lifetime of the component and a number of techniques have been developed to improve coating/substrate adhesion based on this requirement. Cleaning of the components prior to coating in order to remove protective layers, organic contaminants, adsorbed layers, dust and fingerprints is essential and several multi-stage cleaning processes have been developed to achieve this. Once loaded in the coating system some pre-heating of the components in vacuum or in a reducing plasma can be used to remove friable adsorbates and weak surface oxides. At this stage, sputter cleaning is also an important technique; this is used to break up more tenacious oxides and sputter contaminants from the component surface. However, this process can change the surface chemistry and properties of the components and does tend to lead to redistribution of sputtered material elsewhere in the system which can be a disadvantage. For deposition technologies which operate at high temperature a certain amount of diffusional intermixing at the interface can be achieved once coating starts which can be beneficial for adhesion and similar mixing is possible by the use of ion bombardment during the early stages of film deposition. Finally, for ceramic coatings, the use of metallic interlayers or bond coats can be used to improve adhesion and there is an optimum interlayer thickness to maximise coating/substrate adhesion. In this paper the effects of cleaning pre-treatments, sputter cleaning and interlayers on adhesion (assessed using the scratch adhesion test) will be illustrated with results for titanium nitride coatings on a stainless steel substrate.