Synergistic strengthening of CoCrFeNi high-entropy alloy composite coating by in-situ formed multiphase ceramic reinforcements and La2O3 rare-earth oxides

Q235 steel is widely used in friction- and wear-related service conditions; however, its relatively low hardness leads to limited wear resistance. In this study, CoCrFeNi composite coating were fabricated on a Q235 substrate by laser cladding in an 80%Ar–20%N ₂ mixed-gas atmosphere, with the addition of Ti, B ₄C, and LaB ₆ to improve the performance of the coatings. The effects on morphology, microstructure, microhardness, and tribological properties were examined. Ti and B ₄C promoted in situ formation of ceramic phases (TiB _x, TiC, Ti(C,N)), leading to significant grain refinement, with the average grain size reducing from 46.3 μm to 1.4 μm. LaB ₆ did not alter the phase composition but introduced La ₂O ₃ rare-earth oxide, improving coating density and reducing cracks and pores. The microhardness of the coatings containing Ti and B ₄C increased to 330.4 HV, 1.8 times that of the matrix. With the addition of LaB ₆, the microhardness further increased to 432.1 HV, significantly improving over the coatings without LaB ₆. At room temperature, the wear volume of coating C1 was 26.4 × 10 ⁶ μm ³, only 14% of that of the matrix. With the addition of LaB ₆, the wear volume decreased to 11.8 × 10 ⁶ μm ³, a 45% reduction compared to coating C1. These improvements are attributed to the synergistic strengthening of ceramic phases, grain refinement, and solid-solution strengthening from B, C, N, and Ti. LaB ₆ improved the densification of the coatings and reduced internal defects. At 200 °C, wear resistance decreased, with wear volume higher than at room temperature, but still better than that of the matrix.