Nanostructured Coatings

Abstract

In many branches of technology where surfaces are playing a growing role, the use of coatings is often the only way to provide surfaces with specific functional properties. For example, the austenitic stainless steels or titanium alloys exhibit poor resistance to wear and low hardness values, which limits the field of applications. The idea then is to develop new solutions which would improve the mechanical performance and durability of objects used in contact and subjected to mechanical forces in hostile gaseous or liquid environments. Hard coatings are generally much sought after to enhance the resistance to wear and corrosion. They are of particular importance because they constitute a class of protective coatings which is already widely used on an industrial scale to improve the hardness and lifetime of cutting tools.With new vapour deposition processes, treatments and coatings can be combined to produce films with composition gradients or periodic multilayers, for example, associating a hard layer with a more elastic layer, whereupon the structure and properties of coatings can be controlled on a nanometric scale. Films with very high hardness (40-80 GPa) and resistance to wear and corrosion can thereby be fabricated.There are several different architectures for nanostructured coatings, but the two most important are multilayers made from periodic stacks of bilayers A/B with nanometric thicknesses and nanocomposites comprising two phases, one nanocrystalline and the other usually amorphous. This is shown schematically in Fig. 24.1.The very high hardness values of this new class of materials have led to a proposed classification: hard for hardnesses in the range 20-40 GPa, superhard for values in the range 40-80 GPa, and ultrahard for values in excess of 80 GPa. These new mechanical properties (see Chap. 8) are mainly due to a size reduction effect in the distribution of phases, i.e., the size of the crystallites or the thickness of the films.Before describing the vapour deposition techniques used to make these coatings, let us first spell out the main ideas and methodology, giving a few