Study on loading capacity of the coating-substrate system of vapour-deposited Tin and TiC on bearing balls II. Rubbing method in experiments and analysis

The critical load of failure L_c is an important index for the loading capacity of a coating-substrate system. In consideration of the fact that a coating-substrate system in practice may take any one form of the four failure modes, coating failure, substrate yielding, interfacial failure or the mixed failure, this paper defines the critical load of failure of a coating-substrate system as the maximum normal load under a certain friction coefficient before any failure occurs. It also puts forward a new method, the rubbing method, to measure L_c. A multifunctional tester, Model CM-I, is developed on the basis of this method. The critical loads of bearing balls with physical vapor deposited (PVD) TiN or chemical vapor deposited (CVD) TiC or CVD TiC + Ti(C_xN_y) + TiN coatings under different friction coefficients are tested on this machine and their failing processes are analysed. The results show that the rubbing method is a sound new method for the measurement of L_c, and the results agree well with the those of field effect microscopy. The L_c of the CVD coating is greater than that of the PVD coating, and the L_c under smaller friction coefficient f is greater than that under larger f.