Impact toughness and strength synergy enabled by a stress-adaptive high-entropy alloy interlayer in laser-nitrided coatings

In complex working conditions, the nitriding layer in traditional nitriding steel is prone to crack failure due to the residual stress caused by the incompatibility between ceramic nitride layer and metallic substrate. In the present study, a multi-layer coating system of “38CrMoAl substrate/FeCoCrNi–Al 0.5 HEAs/dual phase nitrides” was prepared by “laser cladding + ion nitriding” technology. By introducing HEA intermediate layer, the nitriding product was the dual-phase AlN and CrN, compared to the single iron nitrides of 38CrMoAl. The intermediate HEA layer played the important role in residual stress releasing. It not only significantly reduced the residual stress, but also changed the stress property from tensile to compressive, which was quantitatively and qualitatively proved by nanoindentation, EBSD and theoretical calculations. During dynamic impact process, the intermediate HEA layer timely accommodated strain, making stress uniform rather than concentrated. The “hard-soft phase” coordinated deformation leads to the fracture mechanism of the triple-layer structure changing from the brittle fracture of the double-layer structure to the "brittle-ductile" mixed fracture, thus protecting the substrate and improving the impact resistance of the triple-layer coating. As a result, the charpy impact energy of triple-layer structure ( A k = 52.3 J) is higher than that of two-layer structure ( A k = 40.3 J). By split Hopkinson pressure bar test, the impact yield strength and ultimate strength of the triple-layer coating were 259 MPa and 164 MPa higher than those of the double-layer coating.