DIRECT-CURRENT BIAS APPLIED TO HOT FLAME DIAMOND DEPOSITION PRODUCES SMOOTH LOW-FRICTION COATINGS

As-deposited diamond coatings generally have a high surface roughness which results in a high friction coefficient and extensive wear of the counter material in sliding contact. Therefore several methods for sm oothening diamond coatings have been proposed, such as laser polishing , molten metal etching, thermochemical polishing and mechanical polish ing. All these methods have some disadvantage e.g. long processing tim e or high processing temperature. Furthermore, they are all post-depos ition treatments i.e. the manufacture of these coatings requires at le ast two processing steps, deposition and smoothening. With the present method which combines d.c. bias with hot dame diamond deposition, a s mooth diamond surface is produced during the actual growth of the film . No post-deposition treatment is necessary. The surface roughness is not dependent on the coating thickness which means that thick coatings with smooth surface can be produced. In fact, the method has a smooth ening effect, i.e. rough surfaces can be made smooth. The method is co mparable to conventional hot flame deposition of diamond as to growth rate and cost of producing the coatings. The coatings have a nano-crys talline structure and a surface roughness of Ra = 25 nm, and result in a friction coefficient of 0.1 or less in dry sliding and about 0.05 i n water-lubricated sliding against cemented carbide. Their wear resist ance is virtually the same as that of conventional diamond films.