Microstructure evolution of the W-C hard coatings using directed energy deposition on tungsten alloy surface

Herein, we propose a method for producing W–C composite ceramic coatings on tungsten alloy surfaces using directed energy deposition (DED). Different microstructures of DED single tracks were obtained with different line energy densities. Two parts of DED tracks were discovered, which contained the W–C fusion zone and the WC-Ni-Co melting injection zone. The W–C fusion zone mainly comprised four typical phases, WC, W₂C, W, and C(diamond, graphite (D, G)), which were present in the microstructures of the tracks in different combinations. The key factor for determining the microstructure evolution was C content. With higher line energy density, more tungsten alloy melted, which decreased C content in the melt pool. As C content decreased, the composition of the microstructure transformed to C(D, G) + WC + W₂C → WC + W₂C → W₂C + W + a small amount of γ-(Ni, Co). C content gradually decreased from the top to the bottom of the melt pool, resulting in a gradient microstructure transition. The microstructure and property could be tuned by controlling the C content. The hardness was the highest, 2300 HV, at 40.7 at.% C with WC-W₂C eutectoid. This study provides practical insights for producing coatings on tungsten alloy surfaces with optimized process parameters and for tailoring mechanical properties.