Effects of Y2O3 Content on the Microstructure and Tribological Properties of WC-Reinforced Ti-Based Coatings on TC4 Surfaces

To extend the safety service life of aviation TC4 alloy, the composite coatings of TC4 + Ni-MoS2 + WC + xY2O3 (x = 0, 1, 2, 3, 4 wt.%) were prepared on TC4 by coaxial powder feeding laser cladding technology. The results showed that all the coatings had the same generated phases which mainly consisted of TiC, Ti2Ni, Ti2S, matrix β-Ti, and unfused residual WC. Y2O3 formed co-dependent growth relationships with TiC, Ti2S, and Ti2Ni. Meanwhile, TiC-Ti2S, TiC-Ti2Ni, and Ti2S-Ti2Ni coherent composite structure phases were effectively synthesized in all the coatings. With the increase in the Y2O3 content, the exposed area of the matrix increased and other phases refined progressively. When the Y2O3 content in the coatings were 3 and 4 wt.%, the degree of phase refinement in the coatings was consistent and the phases grew along grain boundaries, but microstructure segregated in the 4 wt.% Y2O3 coating. The microhardness of all the coatings was higher than that of TC4 and decreased with the increase in the Y2O3 content. Higher friction coefficients and lower wear rates both appeared in all the coatings than in the substrate, and they presented a trend of decreased first and then increased with the addition of Y2O3, in which the 3 wt.% Y2O3 coating had the lowest friction coefficient and optimal wear resistance. The research found that the Y2O3 could not change the types of phases in the coatings and could serve as a heterogeneous nucleation center for the refinement of the TiC-Ti2S-Ti2Ni coherent structure phase. Meanwhile, except for the matrix phase, Y2O3 could attract other phases to pinning on the grain boundaries of the coatings. The content of Y2O3 was negatively correlated with the hardness and wear resistance of the coating and it had the optimal tribological properties with the moderate amount of Y2O3. The wear mechanism of all coatings was abrasive wear.