Transfer of 319 Al alloy to titanium diboride and titanium nitride based (TiAlN, TiCN, TiN) coatings: Effects of sliding speed, temperature and environment

In order to develop coatings to be used in dry machining aluminum alloys, it is necessary to have a better understanding of the factors that influence transfer and adhesion of aluminum to the surfaces of coatings that are potential candidates for this function. In this work, the material transfer and adhesion phenomena between 319 Al alloy samples and different coatings deposited on M2 type steel substrates were examined using a pin-on-disc type wear machine. The coatings included titanium diboride (TiB₂), CrN, titanium nitride based compounds, (TiN, TiCN, TiAlN) and uncoated M2 steel discs. The effects of sliding speed, sliding distance, test temperature and working atmosphere were studied. Sliding distances were kept short because the focus of the work was on the initial transfer behavior. The effect of surface roughness was examined using two TiB₂ coatings; one with an R_a value of 16nm and the other with a large R_a of 83 nm. The morphology of sliding tracks was examined using a scanning electron microscope (SEM). With the aid of image analysis software, the SEM images were used to rank the coatings according to the amount of aluminum transferred and the loose debris generated during the tests. In general, the TiB₂ and TiCN coatings exhibited the least amount of aluminum transfer on their surfaces compared to the other coatings. TiN and CrN exhibited the largest amount of aluminum transfer, but the amount of loose debris generated on the surfaces of these two coatings was small. Increasing the sliding speed from 0.12 to 0.6 m/s led to a decrease in the amount of aluminum transferred to the surfaces of all the coating tested. Increasing the test temperature to 160 °C did not significantly change the amount of aluminum transferred to TiB₂ and TiCN coatings. However TiN, TiAlN, CrN and M2 steel picked up significantly less aluminum compared to that at room temperature. When tested under argon atmosphere, all coatings (except TiB₂) exhibited a sharp decrease in the amount of aluminum adhered to their surfaces suggesting that the presence of oxygen and water vapor promoted aluminum adhesion to these coatings. The amount of aluminum transfer to the TiB₂ coating was a weak function of the inertness of the environment, which was attributed to the high chemical stability of TiB₂. Surface roughness played a critical role in aluminum transfer, especially in the early stages of sliding of TiB₂.