Microstructure evolution and fretting wear performance of laser melt deposited titanium matrix composite coatings strengthened by two-scale reinforcements

Titanium alloys exhibit poor wear resistance under severe contact conditions. Although in situ reinforced titanium matrix composite (TMC) coatings provide an effective solution, how reinforcement content influences multiphase solidification behavior and tribological performance, especially under fretting wear conditions, remains unclear. In this study, wear-resistant TMC coatings were fabricated on TC4 alloy via laser cladding using TC4, TiB₂, and graphite powders. TiB and TiC reinforcement phases were in situ synthesized during processing, and the nominal volume fractions of the in-situ formed TiB and TiC reinforcements were designed to range from 5 to 20 vol%, with a theoretical TiB/TiC ratio of 1:1. The evolution of microstructure and fretting wear behavior was investigated to clarify the role of reinforcement content in governing phase formation and tribological performance. The results demonstrate that the reinforcement precipitation follows a content-dependent solidification pathway. When the reinforcement fraction reached 15 vol%, a two-scale heterogeneous reinforcement architecture consisting of dendritic primary ceramic phases and network-like eutectic reinforcements was formed. This structural transition significantly refined the primary β-Ti grains and enhanced fine-grain strengthening. Consequently, the coating exhibited optimal wear resistance, with the friction coefficient and wear volume reduced by 47% and 52%, respectively, compared with the TC4 substrate. The superior wear performance arises from a synergistic reinforcement architecture achieved at an optimal reinforcement content, where coarse TiC dendrites provide load-bearing capability and the eutectic network promotes stress redistribution and restricts wear debris propagation, demonstrating that excessive reinforcement addition does not necessarily lead to improved wear resistance. This study reveals the existence of an optimal reinforcement threshold and establishes a clear correlation between solidification behavior, microstructural architecture, and wear resistance, providing new insights for composition design and microstructure optimization of laser manufactured TMC coatings.