In this work, B4C ceramic particles were introduced into CoCrFeNiMn high-entropy alloy coatings to construct in situ carbide-reinforced heterogeneous microstructures during laser cladding. CoCrFeNiMn-based composite coatings containing 0wt.%, 1.0wt.%, 1.5wt.% and 2.5wt.% B4C were fabricated on Q345 steel substrates by coaxial powder-feeding laser cladding under identical processing parameters, including a laser power of 1 800 W, scanning speed of 8 mm/s, powder feeding rate of 12 g/min, carrier gas flow rate of 8 L/min, shielding gas flow rate of 15 L/min and track overlap of 40%. The actual coating thicknesses were 4.91-5.05 mm and the dilution rates were 8.3%-8.9%, indicating that the differences in microstructure and properties mainly originated from B4C addition rather than coating geometry or dilution variation. X-ray diffraction, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, tensile testing, Vickers microhardness testing, dry sliding wear tests at room temperature and 600 ℃, and laser confocal profiling were used to reveal the phase evolution, heterogeneous microstructure formation, strengthening behavior and wear mechanism. The results showed that B4C did not remain as an independent ceramic phase after laser cladding. Under the transient high-temperature condition of the molten pool, B4C decomposed, and the released C atoms preferentially reacted with Cr atoms in the matrix to form M7C3-type Cr-rich carbides, while B atoms were mainly dissolved into the FCC matrix. This reaction behavior was confirmed by the disappearance of B4C diffraction peaks, the appearance of M7C3 diffraction peaks, the low-angle shift and broadening of FCC peaks, and the Cr-B-C enrichment in the second-phase regions. With the increasing B4C content, the coating evolved from a relatively homogeneous FCC solid solution into a multiphase and multiscale heterogeneous structure consisting of a ductile FCC matrix, in situ M7C3 carbides, refined near-equiaxed grains and network-like second phases. The carbide morphology changed from discontinuous precipitates to semi-continuous or continuous network-like distributions, and the average grain size decreased from (25.6±1.5) μm in the B4C-free coating to (9.6±0.7) μm in the 2.5wt.% B4C coating. This refinement was attributed to heterogeneous nucleation induced by B4C-derived particles, enhanced constitutional undercooling caused by B and C redistribution, and Zener pinning by in situ carbides. The tensile strength increased from about 580 MPa for the B4C-free coating to over 1 050 MPa for the 2.5wt.% B4C coating, while the average microhardness increased from 196HV to 460HV. The enhanced strength and hardness resulted from the synergistic effects of interstitial solid-solution strengthening, grain refinement strengthening, and precipitation strengthening/load-bearing by M7C3 carbides. Fracture analysis further indicated that B4C addition reduced Mn-rich oxide inclusions, probably because B promoted oxygen scavenging and improved melt cleanliness. The tribological results showed that the 2.5wt.% B4C coating had the best wear resistance. Its steady-state friction coefficient at room temperature decreased to approximately 0.32, and its wear scar depth at 600 ℃ was reduced by about 47% compared with that of the B4C-free coating. The improved wear resistance was mainly attributed to the hard carbide skeleton, refined grains, reduced plastic deformation and the formation of a relatively continuous Mn-rich oxide film during high-temperature sliding. Overall, 2.5wt.% B4C is identified as the optimal addition under the present laser-cladding conditions, providing an effective route for designing high-strength and high-temperature wear-resistant CoCrFeNiMn-based composite coatings.
Key words
laser cladding /
CoCrFeNiMn alloys /
B4C reinforcing phase /
microstructures /
mechanical properties /
high-temperature wear resistance
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Funding
Shandong Provincial Natural Science Foundation Project (ZR2025MS894); Supported by Shandong Laboratory of Aluminum Advanced Manufacturing in Binzhou Open Project (SLAAMB OP2025YB04)
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