Strengthening mechanism of a CoCrFeNi-based high-entropy alloy composite coating with in-situ formation multi-phase ceramic reinforcement

Although the strengthening behavior of single-phase ceramics has been widely studied, research on the synergistic reinforcement of multiphase ceramics remains relatively limited. In this study, in situ TiB _x, TiC, and Ti(C,N) multiphase ceramic-reinforced CoCrFeNi-based high-entropy alloy composite coatings were fabricated on Q235 steel using a CoCrFeNi(Ti) _x(B ₄C) _y powder system via laser cladding under an Ar-N ₂ mixed atmosphere. The effects of Ti and B ₄C contents on the phase composition, microstructure, microhardness, and wear properties of the coatings were systematically investigated. The results indicate that the coatings are mainly composed of in situ formed TiB, TiB ₂, TiC, and Ti(C,N) ceramic phases embedded within a BCC solid-solution matrix. When the Ti and B ₄C contents are 7.7% and 1.8%, respectively, TiB ₂ becomes the dominant reinforcing phase. With further increases in Ti and B ₄C, the coatings develop a microstructure characterized by network-like TiB encasing TiB ₂ particles, while the contents of TiB, TiC, and Ti(C,N) increase synergistically. The maximum microhardness of the coatings reaches 442.1 HV, approximately 2.4 times that of the unreinforced matrix, which is mainly attributed to the synergistic strengthening of multiphase ceramics and Ti-induced solid-solution strengthening. Wear analysis indicates that, at room temperature, the CoCrFeNi(Ti) _1.0(B ₄C) _0.2 coating exhibits the best wear resistance among all compositions, achieving the lowest wear rate of 4.98 × 10−6 mm3 N−1m−1, which is attributed to its multiphase synergistic strengthening structure. In addition, the CoCrFeNi matrix coating (C0) predominantly undergoes adhesive wear, whereas the introduction of Ti and B ₄C leads to a transition toward abrasive wear due to the presence of hard ceramic phases.