Grain size and scratching depth dependent tribological characteristics of CrCoNi medium-entropy alloy coatings: A molecular dynamics simulation study

In this work, we have investigated the impact of grain size and scratching depth on the tribological behavior of nanocrystalline CrCoNi medium-entropy alloy (MEA) coatings during scratching by molecular dynamics (MD) simulations. Regardless of the grain size, a monotonically increase in both friction coefficient and wear rate with increasing the scratching depth was observed in the nanocrystalline CrCoNi MEA coating. When its microstructure becomes coarser, the friction coefficient becomes lower. As a function of grain size, the evolutions of friction coefficient and wear rate can be divided into three regions, namely high friction coefficient (or wear rate) region in a grain size range of below 6 nm, relatively stable friction coefficient (or wear rate) region in a grain size range of 6–12 nm, and a relatively low friction coefficient (or wear rate) region in a grain size range of larger than 12 nm. According to the MD simulation results, the higher wear rate for a finer CrCoNi MEA coating can be explained by its lower hardness. In contrast, the reduction of friction force was mainly attributed to the scratching induced grain boundary (GB) sliding and dislocation emissions from GBs. Moreover, the scratching induced phase transformation from crystalline to amorphization and plastic deformation mode transitions also affect its tribological properties.