Microstructure and Tribological Properties of CoCrFeNiAl x Ti y Gradient High-Entropy Alloy Coatings Fabricated by Direct Energy Deposition

In this study, direct energy deposition (DED) was used to fabricate a four-layer gradient high-entropy alloy coating, with the compositions from bottom to top of CoCrFeNi, CoCrFeNiAl 0.3, CoCrFeNiAl 0.3 Ti 0.3, and CoCrFeNiAl 0.5 Ti 0.5 . The microstructure, microhardness, and tribological properties of the gradient coating along the deposition direction were investigated. The gradient coating exhibited a progressive increase in Al and Ti contents from bottom to top, resulting in typical dendritic structures in the CoCrFeNiAl 0.3 Ti 0.3 layer and grain refinement in the CoCrFeNiAl 0.5 Ti 0.5 layer. The phase structure evolved from a single face-centered cubic (FCC) phase to a mixed structure consisting of FCC and body-centered cubic (BCC) phases, accompanied by the precipitation of Al–Ti-rich phases. The average microhardness of the gradient coating increased from 214 HV at the bottom layer to 586 HV at the top layer. The significant enhancement in microhardness is mainly attributed to the increased fraction of BCC phases and the impediment to dislocation motion by Al–Ti-rich precipitates. Under ambient air conditions, the gradient coating demonstrated superior tribological performance compared with the four constituent homogeneous alloys. Wear morphology analysis revealed fewer spalling pits on the worn surface of the gradient coating, which can be attributed to its strength-toughness composite structure. This work provides a feasible strategy for the development of wear-resistant gradient high-entropy alloy coatings.