Molecular Dynamics Simulation of Al_xCoCrFeNi High Entropy Alloy Coating at High Temperature

High-entropy alloys (HEAs) possess excellent mechanical and tribological properties, making them a promising material for high-temperature coatings. Al_xCoCrFeNi (x=0, 3, 6, 10) high entropy alloy coatings were formulated on Ni substrates to augment their wear and oxidation resistance, thus broadening the application scope of Ni and its alloys. In this study, the mechanical properties and friction and wear behavior of Al_xCoCrFeNi (x=0, 3, 6, 10) high entropy alloy coatings on Ni substrates were analyzed at temperatures ranging from 300~1 500 K by molecular dynamics simulations, where the molecular dynamics model incorporated both the Ni-based coatings and spherical diamond abrasives. Throughout the scratching process, the abrasive particle moved along the positive x-direction at a predetermined speed of 100 m/s. The results demonstrated that the normal and tangential friction forces of Al_xCoCrFeNi HEA coatings progressively decreased with increasing Al content, exhibiting a negative correlation with temperature. Furthermore, with the increase of Al content, the average normal and tangential friction forces also decreased accordingly, and the maximum reduction was 46.74% when x=10. During the scratching process of Al_xCoCrFeNi HEA coatings, the overall fluctuation of the average friction coefficient was small, initially decreasing and then increasing with the increase of Al content. At 1 500 K, Al₁₀CoCrFeNi coating showed the maximum friction coefficient, at 0.529. The displacement of damaged atoms and the shear strain of the coating were positively correlated with Al content and temperature. The increase of Al content or temperature reduced the shear strength of the friction surface, resulting in an increasing number of damaged atoms and a deterioration in the uniformity of material deformation. The primary damaged atoms were identified as abrasive dust atoms, which accumulated unevenly on either side of the scratch, exhibited various shapes, and increased proportionally with the distance of the scratch. As the temperature rose, the shear strain value increased, which resulted in the increase of atomic displacement, and the shear strain with high Al content was greater than that with low Al content. The phase structure and dislocation defects had correlations with both Al content and temperature. As the Al content increased and the temperature rose, the number of softer FCC phase atoms diminished, leading to a reduction in the ductility of the material. More BCC phase atoms and other phase structure atoms emerged in the coating. The atomic structure under went a smooth transition between 300 K and 900 K, with the rate of change accelerating after 900 K. At high temperatures, the depth of defect generation increased, and the dislocation density decreased. This indicated that the migration of dislocations became more difficult at high temperatures, leading to a reduced strengthening effect of dislocations and increased severity of damage in the material's subsurface layer. The bonding strength between the Al_xCoCrFeNi coatings and the Ni substrate decreased with increasing Al content and incrementally rose with temperature. The stress at the interface between the HEA coating and the Ni substrate was one of the factors affecting the bonding strength. The interfacial stresses in the coating were significantly higher for x=0 than in the other three cases.