Effects of ceramic material in laser-directed energy deposition of titanium/ceramic functionally graded materials

The titanium/ceramic functionally graded materials (FGMs) have much higher wear resistance than traditional titanium alloys, while cracks are easily formed during the manufacturing process. The zirconia-toughened alumina (ZTA) was adopted to suppress the crack and improve the performances in the laser-directed energy deposition (L-DED) of TC4/ZTA FGMs. The mechanisms influencing the microstructure, hardness, and friction wear properties of the ZTA were investigated. The interfacial reactions and solid-state phase transformation during the L-DED process formed TiAl2O5 compounds and nanoscale yttria-stabilized tetragonal zirconia (YSZ) particles in the ZTA-deposited layer, where the β and lamellar martensitic secondary α' phases were increased by Zr. The nano-YSZ particles refined the grains in the ZTA-deposited layer by promoting heterogeneous nucleation and inhibiting grain growth, thereby causing a columnar-to-equiaxed transition. The hardness and friction wear properties increased with an increasing ZTA concentration, which is caused by the fine grain and Orowan strengthening effects of the nano-YSZ particles. Additionally, the better ductility and lower mismatch in the coefficient of thermal expansion of ZTA compared with Al2O3 ceramic were beneficial for crack suppression.