Coating evaluation methods: a round robin study

This paper reports work carried out at three laboratories, aimed at assessing the repeatability of various test methods used in the study of tribological coatings. The test methods were the ball crater technique for coating thickness, the Vickers microhardness test, the scratch test for adhesion and the pin on disc test for friction and wear. Three different coatings were studied, based on various compounds of titanium, boron, nitrogen and aluminium. Also different novel ionisation assisted PVD methods were used to produce the coatings The paper demonstrates the spread of results obtained by each of the tests on each of the coatings, both within each laboratory and also between laboratories. The results show that assessment of properties such as thickness and hardness can be made with reasonable repeatablity; greater care must be taken however in ascribing quantitive measures to adhesion levels. In particular it is necessary to define the criteria to be used for failure. In the case of friction and wear assessment the laboratories involved demonstrated that considerable variability can be seen, both in friction coefficients and wear rates. Nevertheless we demonstrate that the tests described provide a good basis for the assessment of coating properties and for the investigation of tribological behaviour, permitting further coating development.