Chemical, mechanical and tribological characterization of ultra-thin and hard amorphous carbon coatings as thin as 3.5 nm: recent developments

Diamond material and its smooth coatings are used for very low wear and relatively low friction. Major limitations of the true diamond coatings are that they need to be deposited at high temperatures, can only be deposited on selected substrates, and require surface finishing. Hard amorphous carbon (a-C), commonly known as diamondlike carbon (DLC), coatings exhibit mechanical, thermal and optical properties close to that of diamond. These can be deposited with a large range of thicknesses by using a variety of deposition processes, on variety of substrates at or near room temperature. The coatings reproduce substrate topography avoiding the need of post finishing. Friction and wear properties of some DLC coatings can be very attractive for tribological applications. The largest industrial application of these coatings is in magnetic storage devices. Recent developments in the chemical, mechanical and tribological characterization of the ultra-thin coatings are reviewed in this paper. The prevailing atomic arrangement in the DLC coatings is amorphous or quasi-amorphous with small diamond (sp3), graphite (sp2) and other unidentifiable micro- or nanocrystallites. The mechanical and tribological properties of the DLC coatings are dependent upon the deposition technique. Thin coatings deposited by filtered cathodic arc, ion beam and ECR-CVD hold a promise for tribological applications. Coatings as thin as 5 nm in thickness provide wear protection.