Present progress in the development of low friction coatings

Abstract

Low friction coatings are being developed for a variety of reasons. Practical conditions in which machines and machine elements have to operate are ever expanding and more often involve features which preclude liquid or grease lubrication. An example of such conditions very common in everyday life is the food-processing industry, where any kind of lubrication is impossible owing to the risk of contamination of a product. Some other conditions include hostile environments such as high temperature or vacuum. High temperatures are common, for example, in the chemical industry and power generation, whereas vacuum is often associated with space technology. A new motivation for development of low friction coatings is a general trend to reduce lubrication for reasons of environmental compliance. All these together challenge materials technology and surface engineering especially. Accordingly, activity in the area of the development of coating materials, sophisticated deposition methods as well as post treatments of coatings has been vigorous in recent years. In this paper we first review typical conditions in which low friction coatings are sought as well as the demands for properties which coatings should fulfil in order to function in an appropriate way. This is followed by a more detailed presentation of three different coatings for three different conditions. Amorphous hard carbon films deposited by arc discharge deposition are hydrogen free, have high hardness and are best characterized as an amorphous diamond. The films have been shown to possess a friction coefficient below 0.2 or even below 0.1 in dry-sliding conditions associated with good wear resistance. Potential conditions in which these films could be used are dry sliding at room temperature and in a normal atmosphere with adequate humidity. The second coating family which will be presented is based on molybdenum disulphide (MoS 2). These are typical coatings for use in a vacuum. Many attempts have been made to overcome the many limitations of these coatings with post-treatments. In this presentation, ion beam and laser processing of MoS 2 coatings with or without alloying are dealt with. The third set of conditions includes high temperatures. This is one of the most difficult areas of low friction coatings and no final solutions for practical purposes have been developed.