Objective Copper (Cu) and its alloys have excellent ductility and good electrical/thermal conductivity. However, their low strength, low hardness, and poor wear resistance restrict their application in industrial and military fields. Adding lubricating phase particles to Cu and its alloys can give the material excellent self-lubricating properties. The Cu-based graphite composite material not only has excellent thermal/electrical conductivity of Cu, but also have low thermal expansion coefficient and excellent solid lubricating property of graphite. Supersonic laser deposition (SLD) is a material deposition technology that combines laser irradiation and cold spray (CS). By introducing laser irradiation, instantaneous heating and softening of the sprayed particles and substrates improve their plastic deformation abilities, which can facilitate the deposition of low-plasticity materials, greatly broadening the range of sprayable materials for CS. Therefore, based on the characteristics and advantages of SLD, we use SLD technology to prepare Cu-based graphite composite coating on the surface of Cu substrate and systematically investigate the effects of different graphite contents on the microstructure, microhardness, and friction of the composite coating in this study. The study can provide a reference for surface modification and additive manufacturing of Cu-based materials.

Methods Firstly, graphite/Cu composite coatings with different graphite contents are deposited on Cu substrates by SLD technology. Then, the microstructures and morphologies of the worn surfaces of the composite coatings with different graphite contents are analyzed by scanning electron microscope. The phases and compositions of composite coatings with different graphite contents are analyzed by X-ray diffractometer, energy dispersive spectrometer, and Raman spectrometer. Furthermore, the microhardnesses of composite coatings with different graphite contents are tested by automatic Vickers hardness tester. The effect of graphite content on the microhardness of composite coating is investigated. Finally, wear resistances of the composite coatings with different graphite contents are tested by friction and wear testing machine. The effect of graphite content on wear resistance of the composite coating is elucidated.

Results and Discussions The thickness of the composite coating decreases as the proportion of graphite in the original powder increases. When the mass fraction of graphite in the original powder is 5%, 10%, and 15%, the thickness of the composite coating is 908.2, 741.2, and 688.9 μm, respectively (Fig. 3). Owing to the work hardening effect caused by particle impact during the deposition process, the average hardness of the pure Cu coating is as high as 133.60 HV_0.2. When the mass fraction of graphite in the original powder increases from 5% to 15%, the hardness of the composite coating decreases from 122.48 HV_0.2 to 95.02 HV_0.2 (Fig. 7). The wear resistance study of the as-prepared composite coating shows that the mass loss of CuGr0 coating is 4.398 mg. However, when the mass fraction of graphite in the original powder increases from 5% to 15%, the mass loss of the composite coating decreases from 2.058 mg to 0.746 mg (Fig. 8). During the wear process, the graphite in the composite coating can generate continuous solid lubricating films, preventing direct contact between the grinding ball and composite coating and reducing further wear of the grinding ball to the composite coating. Additionally, the morphologies of the worn surfaces of the composite coatings with different graphite contents reveal that the wear mechanism of the coating without graphite is adhesive wear and the wear mechanism of the coating with graphite is abrasive wear (Fig. 11).

Conclusions The deposition efficiency of the composite coating decreases as the content of graphite in the original powder increases. Graphite is a soft solid lubricant, increasing its content in the composite coating will reduce the ability of coatings to resist plastic deformation, microhardness, friction coefficient and wear rate.The wear mechanism of the graphite-free coating is adhesive wear. The wear mechanism of the coating changes from adhesive wear to abrasive wear when graphite is added to the coating.