PRESENT PROGRESS IN THE DEVELOPMENT OF LOW-FRICTION COATINGS

Low friction coatings are being developed for a variety of reasons. Pr actical conditions in which machines and machine elements have to oper ate are ever expanding and more often involve features which preclude liquid or grease lubrication. An example of such conditions very commo n in everyday life is the food-processing industry, where any kind of lubrication is impossible owing to the risk of contamination of a prod uct. Some other conditions include hostile environments such as high t emperature or vacuum. High temperatures are common, for example, in th e chemical industry and power generation, whereas vacuum is often asso ciated 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 t echnology and surface engineering especially. Accordingly, activity in the area of the development of coating materials, sophisticated depos ition methods as well as posttreatments of coatings has been vigorous in recent years. In this paper we first review typical conditions in w hich low friction coatings are sought as well as the demands for prope rties which coatings should fulfil in order to function in an appropri ate way. This is followed by a more detailed presentation of three dif ferent coatings for three different conditions. Amorphous hard carbon films deposited by are discharge deposition are hydrogen free, have hi gh hardness and are best characterized as an amorphous diamond. The fi lms have been shown to possess a friction coefficient below 0.2 or eve n below 0.1 in dry-sliding conditions associated with good wear resist ance. Potential conditions in which these films could be used are dry sliding at room temperature and in a normal atmosphere with adequate h umidity. The second coating family which will be presented is based on molybdenum disulphide (MoS2). 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 bea m and laser processing of MoS2 coatings with or without alloying are d ealt with. The third set of conditions includes high temperatures. Thi s is one of the most difficult areas of low friction coatings and no f inal solutions for practical purposes have been developed.