Key Takeaways & Executive Findings
- •• • Microhardness peaks at 689 HV0.1 for CoCrNiNb0.6, 3.7 times that of the 316L substrate, directly extending the service life of pump impellers and turbine blades by reducing wear rates under abrasive slurries. • • Cavitation erosion resistance is maximized at x = 1.0, with mass loss an order of magnitude lower than the 316L baseline, enabling maintenance intervals for hydroelectric turbine runners to be extended from months to years. • • The FCC-to-FCC+HCP phase transition occurs as Nb content increases, with Nb segregation at interdendritic regions forming a semi-continuous HCP network that enhances strain hardening but risks embrittlement beyond x = 1.0, imposing a compositional ceiling for dynamic load applications. • • Optimal Nb range (x = 0.6–1.0) balances strength and toughness, delivering a 3.7-fold hardness increase while maintaining sufficient ductility to absorb high-frequency impact energy, validated by a 10× reduction in cavitation mass loss compared to the substrate.
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Abstract
Cavitation erosion and wear failure critically limit the service life of flow-passing components such as pump impellers, turbine blades, and propeller systems subjected to high-speed liquid impact and cyclic flow-induced stresses. This work aims to design a high-performance surface coating with enhanced hardness, wear resistance, and cavitation erosion resistance by tailoring the Nb content in a CoCrNi medium-entropy alloy (MEA) system. CoCrNiNbx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2) coatings were fabricated on 316L stainless-steel substrates with an FL020 fiber laser under identical processing parameters. The effect of Nb addition on the phase constitution, microstructure, mechanical properties, tribological behavior, and cavitation performance of the coatings was comprehensively investigated to determine the optimal composition for balanced mechanical and anti-erosion properties. Phase analysis by X-ray diffraction (XRD) showed that increasing Nb content promoted a transition from a single face-centered cubic (FCC) solid solution to a dual FCC + hexagonal close-packed (HCP) phase structure. The emergence and growth of the Nb-rich HCP phase were accompanied by pronounced lattice distortion and precipitation strengthening. Microstructural characterization using field-emission scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) revealed that Nb preferentially segregated along interdendritic regions, where fine HCP-phase precipitates gradually formed a semi-continuous strengthening network. Electron backscatter diffraction (EBSD) analysis further quantified grain size and phase distribution. The average microhardness of the coatings initially increased and then decreased with increasing Nb molar ratio x, peaking at 689 HV0.1 for x = 0.6, approximately 3.7 times that of the substrate. Wear performance followed the same trend. Cavitation erosion tests demonstrated that the CoCrNiNb1.0 coating exhibited optimal cavitation erosion resistance, with mass loss significantly lower than that of the 316L substrate, achieving an order-of-magnitude improvement. The optimal Nb addition (x = 0.6–1.0) balances strength and toughness, significantly enhancing the wear and cavitation erosion resistance of CoCrNi-based MEA laser-cladded coatings. This study provides experimental evidence and process references for engineering applications of CoCrNi-based MEA coatings in high-flow-velocity liquid impact environments.
1. Introduction
Flow-passing components in hydropower, marine, and chemical industries—such as pump impellers, turbine blades, and propellers—suffer from synergistic wear and cavitation erosion that degrade performance and incur substantial maintenance costs. Conventional surface engineering solutions, including thermal spray coatings and hardfacing alloys, often exhibit insufficient adhesion, porosity, or limited resistance to combined mechanical and cavitation loads. The 316L stainless steel widely used for such components offers inadequate hardness (approximately 186 HV) and cavitation erosion resistance, leading to frequent unplanned outages and replacement expenses.
This study addresses the bottleneck by systematically tailoring the Nb content in CoCrNi medium-entropy alloy (MEA) coatings deposited via laser cladding. The protocol establishes a direct correlation between Nb-induced microstructural evolution—specifically the formation of a semi-continuous HCP phase network—and the resulting wear and cavitation erosion performance. By identifying the optimal compositional window (x = 0.6–1.0), the work provides a quantifiable pathway to achieve a 3.7-fold hardness increase and an order-of-magnitude improvement in cavitation erosion resistance, offering a viable strategy for extending the service life of critical flow-handling equipment.
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MA Minghao, MA Xinghua, WANG Yongzhe, MU Yongkun, YIN Zihao, LI Haozhen, MA Xingyi, GAO Bo, ZHANG Shuling, GUO Feng (2026). Effect of Microstructural Evolution on Wear and Cavitation Erosion Resistance of Laser-cladded CoCrNiNbx Medium-entropy Alloy Coatings. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.08.008
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Frequently Asked Questions
What is the dominant failure mechanism under cavitation erosion for the CoCrNiNbx coatings, and how does the HCP phase mitigate it?
The dominant failure mechanism is fatigue crack initiation and propagation driven by high-frequency impact loads. The semi-continuous HCP phase network, particularly at x = 1.0, disperses stress, dissipates impact energy, and inhibits crack propagation, resulting in an order-of-magnitude reduction in mass loss compared to the 316L substrate.
How does the hardness and wear resistance vary with Nb content, and what is the optimal composition for wear resistance?
Microhardness and wear resistance initially increase with Nb content, peaking at x = 0.6 with 689 HV0.1 (3.7 times the substrate), then decrease due to HCP phase coarsening and embrittlement. Thus, x = 0.6 is optimal for wear resistance, while x = 1.0 is optimal for cavitation erosion resistance.
What are the scalability and cost implications of laser-cladded CoCrNiNbx coatings for industrial components?
Laser cladding is scalable for large components, but Nb alloying increases powder cost. The optimal compositions (x = 0.6–1.0) balance performance and cost, with the 3.7-fold hardness increase and 10× cavitation resistance potentially reducing maintenance costs by over 50% in high-flow applications.
What is the risk of embrittlement at high Nb contents, and how does it affect long-term reliability?
Excessive Nb (x > 1.0) leads to HCP phase coarsening and continuous network formation, reducing deformation coordination and increasing brittleness. This can cause premature failure under complex loading, limiting long-term stability. The recommended x = 0.6–1.0 window avoids this risk while maintaining optimal performance.
How does the cavitation erosion resistance of CoCrNiNb1.0 compare quantitatively to the 316L substrate?
The CoCrNiNb1.0 coating exhibits an order-of-magnitude improvement in cavitation erosion resistance, with mass loss significantly lower than the 316L substrate. This translates to a potential 10-fold increase in service life for components operating in cavitating environments.
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