Effect of V Content on Microstructure and Properties of TiNbZrVx Medium-Entropy Alloy Coatings on TC4 Substrate by Laser Cladding

In order to improve the wear resistance of titanium alloy and apply it to the high-speed train brake disc, TiNbZrVx (x = 0, 0.2, 0.4, 0.6, 0.8) refractory medium-entropy alloy coatings were prepared on Ti-6Al-4V (TC4) substrate. The effect of V content on the microstructure, mechanical properties, and friction and wear properties of the coatings was studied. TiNbZrVx coatings achieved good metallurgical bonding with the substrate, forming BCC and B2 phases and AlZr3 intermetallic compound (IMC). From TiNbZr coating to TiNbZrV0.8 coating, V promotes element segregation and new phase formation, which decreased the average grain size from 85.055 μm to 56.515 μm, increased the average hardness from 265.5 HV to 343.4 HV, and reduced the room temperature (RT) wear rate by 97.8%. However, the ductility of the coatings decreased from 15.7% to 5.8% because the grain boundary precipitates changed the dislocation arrangement, and the tensile fracture mode changed from ductile fracture to brittle fracture. Abrasive wear was the main wear mode at RT, and adhesive wear and oxidation wear were the main wear modes at elevated temperature. The COF at elevated temperature was lower than that at RT, because a large number of friction pair components were transferred to the coating surface at high temperature and were repeatedly rolled to form a dense film, which played a certain lubricating role.