Tribological properties of thin hard coatings: demands and evaluation

Thin hard coatings are currently being used in a large number of tribological applications. Some important examples are tools for metal cutting and forming, and machine elements like sliding bearings, seals, valves, etc. The present paper deals with the problems of identifying the limiting factors in given tribological applications of coating composites, and how to experimentally determine their tribological responsse, with special attention given to wear.

As a means for coating development, the intricate relations between coating deposition parameters and the triboological response of the coated composite are indicated by a flow chart of relevant coating and substrate properties. It is demonstrated how the general characteristics (topography, composition, microstructure, etc.) and fundamental properties (chemical, thermal, mechanical, etc.) of coating and substrate combine to generate the tribological properties (resistance against abrasion, erosion, sliding wear, etc.), which given the tribological loading, determines the tribological response in terms of friction and wear.

A number of different test methods for tribological evaluation of coating composites are presented together with illustrative examples. The test range from four-point bending for evaluating coating fracture strength, to designated tribological tests to determine the resistance against abrsion, particle erosion, and sliding wear.