Laser Processing of High-Entropy VNb2TaCrMoWTi0.3B0.6 Alloy Coatings for Wear Reduction in Dry Friction with Different Counterfaces

Abstract

The microstructure, phase composition, and microhardness of the cast high-entropy VNb₂TaCrMoW alloy with the addition of titanium diboride were studied. The initial VNb₂TaCrMoW alloy consisted of two bcc solid solutions, slightly differing in lattice parameters (a = 0.3139 nm and 0.3200 nm). The addition of boron as titanium diboride and repeated remelting led to a bcc solid solution with a larger lattice parameter (a = 0.3217 nm) and a boride with W_3.5Fe_2.5B₄ structure (a = 0.6054 nm and c = 0.3256 nm). The bcc solid solution was the first to crystallize, and the boride was part of the eutectic grains and precipitated from the last melt portions, forming a closed network. The resulting alloy was applied to a carbon steel substrate as a coating using electrospark deposition employing an Elitron-24A installation with varying electrical pulse energy. Higher pulse energy during coating deposition increased the layer thickness and surface roughness but did not influence the phase composition. The microstructure of the coatings was more uniform compared to the cast alloys, and X-ray diffraction showed that the coatings contained bcc solid solutions, Fe₇W₆ intermetallic compound, and a small amount of TaO₂ oxide. The coatings had a hardness of about 10 GPa and were 11–15 μm and 16–20 μm thick at discharge energies of 0.52 and 1.1 J, respectively. A comparative analysis of the phase composition, hardness, and microstructure of the cast alloy and associated coatings was carried out. The coatings deposited at a discharge energy of 0.52 J were subjected to laser processing. Laser processing of the coatings resulted in a thermally affected zone, while the surface layer hardness hardly changed. The wear resistance of the coatings deposited at a discharge energy of 0.52 J was analyzed. Wear resistance testing was conducted for three counterface materials (VK6, Al₂O₃, and Si₃N₄) in quasistatic and dynamic loading modes. Laser processing of the electrospark coatings changed the wear mechanism and significantly increased the wear resistance regardless of the counterface material and loading mode.