A perspective on the optimisation of hard carbon and related coatings for engineering applications

Hard carbon coatings hold the key to improved performance for many types of products. However the achievement of these improvements requires the selection of the appropriate type of carbon coating and therefore the correct process and appropriate deposition parameters. The huge range of properties achievable in carbon coatings is mainly due to the ability of carbon to form different types of interatomic bonds, to take up different sites, and to adopt different structures. In addition to intrinsic material properties, other factors must also be considered for each application, such as the adhesion level achievable and coating cost. This complex situation explains why the number of applications for hard carbon films is still more limited than originally expected. Despite the considerable progress achieved during the last decade in hard coating technologies, practical results often appear conflicting, with differences in properties occurring even within the same types of coatings. Furthermore, the many different deposition systems and processes which have been developed introduce further complications in regard to (for example) achievable coating uniformity and deposition rates. Thus, there is often confusion in the use of certain fundamental principles, especially regarding the growth mechanisms and the effects which produce more dense homogeneous and stable coating materials. This is especially true for the improved properties of tetrahedral amorphous carbon films, which are different from previously reported diamond-like carbon materials, and can be created by adapting and improving existing industrial processes, to offer advantages compared to earlier coatings, and hence possibilities for important new applications. This paper discusses issues relating to intrinsic material properties, and practical aspects such as adhesion, to provide a framework for the development, selection and use of hard carbon coatings in practical situations.