Sliding friction-induced subsurface structures and severe surface oxidation can be the major causes influencing the wear resistance of ductile metallic materials. Here, we demonstrated the role of subsurface and surface structures in enhancing the wear resistance of an equiatomic metastable CoCrNiCu high-entropy alloy (HEA). The CoCrNiCu HEA is composed of a CoCrNi-rich face-centered cubic (FCC) dendrite phase and a Cu-rich FCC inter-dendrite phase. Copious Cu-rich nano-precipitates are formed and distributed uniformly inside the dendrites after tuning the distribution and composition of the two phases by thermal annealing. Although the formation of nano-precipitates decreases the hardness of the alloy due to the loss of solid solution strengthening, these nano-precipitates can be deformed to form continuous Cu-rich nanolayers during dry sliding, leading to a self-organized nano-laminated microstructure and extensive hardening in the subsurface. In addition, the nano-precipitates can facilitate the formation of continuous and compacted glaze layers on the worn surface, which are also beneficial for the reduction of the wear rate of CoCrNiCu. The current work can be extended to other alloy systems and might provide guidelines for designing and fabricating wear-resistant alloys in general.

The excellent properties of metallic glass (MG) films make them perfect candidates for the use in miniature systems and tools. However, their high coefficients of friction (COFs) and poor wear resistance considerably limit their long-term performance in nanoscale contact. We report the fabrication of a MG/graphene multilayer by the repeated deposition of Cu₅₀Zr₅₀ MG with alternating layers of graphene. The microstructure of the multilayer was characterized by the transmission electron microscopy (TEM). Its mechanical and nanotribological properties were studied by nanoindentation and nanoscratch tests, respectively. A molecular dynamics (MD) simulation revealed that the addition of graphene endowed the MG with superelastic recovery, which reduced friction during nanoscratching. In comparison with the monolithic MG film, the multilayer exhibited improved wear resistance and a low COF in repeated nanowear tests owing to the enhanced mechanical properties and lubricating effect caused by the graphene layer. This work is expected to motivate the design of other novel MG films with excellent nanowear properties for engineering applications.