The effect of under-layers and pre-test temperature on the tribological characteristics of TiN films rubbing against steel

Four kinds of coating film applied to the steel substrate of lower disc specimens were tested using the ring-on-disc adapter to simulate dry surface contact. The thermal effect on the tribological behavior was investigated by comparing two pre-test temperatures (28 and 300 °C). An analytical approach to the temperature solutions of the lower specimen at the contact area was developed by assuming a parabolic temperature polynomial. The maximum contact temperature Tc is then calculated using temperatures measured at several depth points as the boundary conditions. Experimental results for friction coefficient, independent of the specimen type, can be expressed as a nonlinear function of either the applied power or the contact temperatures parameter. The entire range of the friction power parameter (fLV; f: average friction coefficient, L: applied load, V: sliding velocity) can be divided into three subregions according to different tribological behaviors exhibited in each subregion. As to the wear rate of the ring-type upper specimen made of the SKD-61 steel, two jumps are formed in the entire range of friction powers. The first jump is determined by the change in the predominance of Fe2O3 and Fe3O4; whereas the second jump is primarily related to the severity of surface adhesion. The wear rate behavior of the lower specimen is strongly related to the wear mechanisms arising at the three subregions. The mild-oxidational wear model can also account for the wear rates and the tribological behavior of the upper specimens (in ring-type) operating at two pre-test temperatures.