Effect of Nb content on the tribological properties of laser-cladded Ti-Al-C MAX phase composite coatings

Ti-Al-C-Nb_x (x = 0, 2, 4, 6, and 8 at%) composite coatings are prepared on the surface of TC4 titanium alloy via laser cladding. The effect of Nb content on the microstructure, hardness and tribological properties of the Ti-Al-C-based coatings are investigated. The results show that the coatings are composed of Ti₃Al, TiC, Ti₂AlC, Ti₂AlNb, and Ti₃AlC₂ phases. In the coating with an Nb content of 2 at%, a large amount of flocculent Ti₃AlC₂ phase precipitated. This is attributed to the solid solution of a small amount of Nb into the Ti₃AlC₂ phase, promoting the heterogeneous nucleation of Ti₃AlC₂. With the increase of the Nb content, more Nb atoms are incorporated into the Ti₃AlC₂ phase through solid solution, causing the decomposition of Ti₃AlC₂ into Ti₂AlC and TiC. When the Nb content is 8 at%, the flocculent Ti₃AlC₂ phase almost completely disappeared. The Ti-Al-Nb matrix, hard TiC particles and Ti₃AlC₂ MAX phase form a composite structure with synergistic effects. This enhanced the friction and wear reduction properties of the coatings and reduced the hardness difference between the matrix and the reinforcing phases.