Microstructure and tribological properties of Ti2AlC-B particle-enhanced self-lubricating coatings on Ti6Al4V by ultrasonic impact treatment and laser cladding

To improve the wear resistance of Ti6Al4V (TC4) alloy, the TC4/Ti2AlC/B composite coatings were cladded on Ti6Al4V substrate by laser, and then the TC4/Ti2AlC/B composite coatings were treated with ultrasonic impact treatment (UIT). The microstructure, phase composition, microhardness and residual stress of composite coatings with and without UIT were analyzed. The dry-sliding tribological properties and corresponding wear mechanisms of these composite coatings at different temperatures (RT, 500 degrees C) were also systematically investigated. The results indicate that the variation of B content and (UIT) did not change the phase composition of the composite coatings. Furthermore, the composite coatings consist of an a-Ti and Fe-Ti-V solid solutions, hard phases of TiB, TiC and Ti3Al, as well as a Ti2AlC ceramic phase. During the laser cladding process, TiB is formed in situ from B and Ti, and Ti2AlC is partially dissolved and converted to TiC and Ti3Al, resulting in a significant increase in the microhardness of the coating compared to the substrate. Specifically, the compressive stress of all composite coatings increased significantly, where the surface stress of the coating (composition: 9 wt%B/Ti2AlC/TC4) varied from 22 MPa to 42 MPa (negative values represent compressive stress). Due to the synergetic effect of the Ti2AlC lubricant, Al2O3, TiO2 oxide films and hard phase, as well as the increased compressive stress on the surface of the composite coating after UIT, the coefficient of friction (COF) is the lowest (0.242 at 500 degrees C) when the composition is 6 wt% B/Ti2AlC/TC4 with UIT, and has the most excellent wear resistance (0.69 x10(-5) mm(3)/N center dot m at 500 degrees C). The composite coatings are dominated by hard phase exfoliation, three-body abrasive wear, adhesive wear and slight oxidation wear at room temperature (RT), while the main wear mechanisms at 500 degrees C are adhesive wear, slight abrasive wear and oxidation wear.