Microstructural evolution and high-temperature tribological properties of in situ-synthesized TiC/TiBx-reinforced Ti-6Al-4V functionally graded coatings by laser directed energy deposition

Abstract

This study addresses the limited high-temperature wear resistance of Ti-6Al-4V alloy and the cracking susceptibility associated with conventional homogeneous ceramic-reinforced coatings. A functionally graded material (FGM) strategy was employed to achieve the synergistic enhancement of strength, toughness, and crack resistance, while elucidating its influence on microstructural evolution and high-temperature tribological behavior. A functionally graded composite coating with progressively increasing B₄C content (0 wt% → 2 wt% → 4 wt%) was successfully fabricated on a Ti-6Al-4V substrate via laser directed energy deposition (L-DED). The coating exhibits a dense and defect-free microstructure with strong metallurgical bonding, as confirmed by the absence of obvious cracks and porosity. With increasing B₄C content, in situ synthesized TiC, TiB, and TiB₂ ceramic phases are generated within the coating. These reinforcing phases are uniformly distributed throughout the matrix, promoting grain refinement, enhancing interfacial bonding strength, and resulting in a gradual increase in microhardness along the coating thickness. Microstructural analysis reveals that the morphology and distribution of the reinforcing phases are governed by the coupling effect of in situ reactions and solidification kinetics. Specifically, TiC exhibits an equiaxed morphology, TiB presents whisker-like or prismatic structures, and TiB₂ mainly displays a plate-like morphology. These phases influence grain orientation and texture evolution through heterogeneous nucleation and interfacial pinning effects. Energy dispersive spectroscopy (EDS) analyses confirm a well-defined and continuous compositional gradient across the coating thickness, thereby validating the effectiveness of the designed functionally graded architecture. Electron backscatter diffraction (EBSD) results reveal an increased fraction of low-angle grain boundaries and a transition toward more randomized grain orientations. Meanwhile, the kernel average misorientation (KAM) and geometrically necessary dislocation (GND) densities decrease, indicating the effective relaxation of lattice distortion. The reduced Schmid factor further suggests enhanced resistance to dislocation slip. High-temperature tribological tests conducted at 700 ℃ demonstrate a transition from the running-in stage to steady-state wear, while the dominant wear mechanism evolves from adhesive and abrasive wear to oxidation-controlled wear. Under moderate loads (40–60 N), a stable oxide film forms on the worn surface, effectively reducing friction and wear. However, under a higher load of 80 N, fracture and spallation of the oxide film induce severe third-body abrasion, resulting in deteriorated wear resistance. Overall, the FGM design effectively mitigates thermal mismatch and stress concentration, enabling a smooth transition in microstructure and mechanical properties. This effectively suppresses crack formation and enhances high-temperature wear resistance through the synergistic regulation of ceramic phase distribution and oxidation behavior, thereby providing theoretical guidance for the design of advanced titanium alloy composite coatings.