Wear and friction properties of hard PVD coatings

Tribological properties of physically vapour deposited coatings of TiN, TiAIN, TiCN, and CrN deposited on discs made of two different tool steels have been studied using different pin-on-disc test procedures. Seizing and adhesive wear were studied by a step-load test using a steel ball, friction properties by using a flat steel pin under low load, and the wear resistance was studied by using an Al2O3 ball, simulating a more abrasive wear situation. In all tests, coating improved the performance of the samples, reducing seizing and wear considerably. TiAIN exhibited the most stable friction and wear mode, which is attributed to a stable aluminium oxide surface layer. The wear tests against Al2O3 showed that the Ti-based coatings reduced the severe wear of the uncoated hardened steel by approximately 4 times, while the softer, but more ductile, CrN reduced the wear by a factor of 2. The results have been related to nanoindentation hardness measurements and compared with results from held tests of coated tools. In particular, the test results for TiAIN seem to be in very good accordance with practical results from field tests. Thus, pin-on-disc tests can make up a relevant and convenient test method for evaluation of fundamental tribological properties of physically vapour deposited coatings. However, proper test conditions must be carefully chosen in order to evaluate the relevant applications.