Tribochemistry in sliding wear of TiCN–Ni-based cermets

The tailoring of cermet composition to improve tribological properties requires careful choice of the type of secondary carbide. To investigate this aspect, a number of sliding tests were carried out on baseline TiCN–20Ni cermet and TiCN–20wt%Ni–10 wt% XC cermets (X = W/Nb/Ta/Hf) at varying loads of 5N, 20N, and 50N against bearing steel. With these experiments, we attempted to answer some of the pertinent issues: (i) how does the type of secondary carbide (WC/NbC/TaC/HfC) influence friction and wear behavior, and is such influence dependent on load?; and (ii) how does the secondary carbide addition affect the stability and composition of the tribochemical layer under the selected sliding conditions? Our experimental results reveal that the added secondary carbides influence chemical interactions between different oxides and such interactions dominate the friction and wear behavior. A higher coefficient of friction (COF) range, varying from 0.75 to 0.64 was recorded at 5N; whereas the reduced COF of 0.46–0.52 was observed at 20N or 50N. The volumetric wear rate decreased with load and varied on the order of 10−6 to 10−7 mm3/Nm for the cermets investigated. The cermet containing HfC exhibited high friction and poor wear resistance. At low load (5N), the abrasion and adhesion of hard debris containing various oxides dominated the wear, and resulted in high friction and wear loss. In contrast, the more pronounced increase in friction-induced contact temperature (below 500 °C) and compaction of hard debris resulted in the formation of a distinct tribochemical layer at higher loads (20N and 50N). The formation of a dense tribolayer containing oxides of iron and/or titanium is responsible for the reduced friction and wear, irrespective of secondary carbides.