目的 明确B4C添加对CoCrFeNiMn高熵合金激光熔覆涂层物相演化、异质组织形成及力学与高温耐磨性能的影响规律,阐明B4C分解、原位碳化物析出和组织强化之间的关系,确定较优B4C添加量。方法 采用同轴送粉激光熔覆技术制备B4C质量分数分别为0、1.0%、1.5%和2.5%的CoCrFeNiMn基复合涂层,并通过X射线衍射、光学显微镜、扫描电子显微镜及能谱分析对其物相组成与微观组织进行表征,结合拉伸试验、显微硬度测试及室温和600 ℃摩擦磨损试验系统评价其力学及摩擦学性能。结果 激光熔池的瞬时高温促使B4C发生分解,C原子与Cr原位反应生成M7C3型富Cr碳化物,B原子主要固溶于FCC基体中;随着B4C含量增加,涂层逐渐形成由软韧FCC基体、M7C3型富Cr碳化物及细化近等轴晶共同构成的多相、多尺度异质组织,M7C3碳化物由断续析出逐渐演变为半连续或连续网络状分布。质量分数为2.5%的B4C复合涂层表现出较优综合性能,其抗拉强度超过1 050 MPa,显微硬度约为460HV,较纯合金涂层分别提高约80%和134%,室温稳定摩擦系数降低至0.32,600 ℃高温磨痕深度降低约47%。结论 B4C的引入可通过M7C3碳化物析出强化、间隙固溶强化、晶粒细化以及氧化物夹杂减少等机制,提高涂层的强度、硬度和高温耐磨性能。其中,质量分数为2.5%的B4C为本研究条件下较优添加量,可为高熵合金复合涂层的组织调控及高温耐磨应用提供实验依据。
Abstract
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.
关键词
激光熔覆 /
CoCrFeNiMn合金 /
B4C增强相 /
微观组织 /
力学性能 /
高温耐磨性能
Key words
laser cladding /
CoCrFeNiMn alloys /
B4C reinforcing phase /
microstructures /
mechanical properties /
high-temperature wear resistance
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基金
山东省自然科学基金资助项目（ZR2025MS894); 滨州铝产业先进制造山东省实验室开放课题项目资助（SLAAMB OP2025YB04）
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