Achieving low friction and superior wear resistance in WC cemented carbides via CoxFeNiCrCu HEA/VC co-doping

Technical Summary: Achieving Low Friction and Superior Wear Resistance in WC Cemented Carbides via CoxFeNiCrCu HEA/VC Co-doping

Problem Statement
High-precision cutting tools face severe operational challenges, including extreme temperatures, high pressure, and intense impact, leading to rapid tool wear. While tungsten carbide (WC) cemented carbides are the industry standard for wear resistance, conventional WC-Co alloys face critical limitations. These include the scarcity and high cost of cobalt (Co), its classification as a Group 1 carcinogen, and environmental concerns regarding Co contamination. Furthermore, the significant mismatch in thermal expansion coefficients between the WC hard phase and the Co binder weakens interfacial bonding at high temperatures, accelerating wear and compromising tool life. Although High-Entropy Alloy (HEA) binders have emerged as promising alternatives due to their superior strength and grain-refining capabilities, existing studies often utilize fixed-composition alloys and lack specific functional elements (such as low-shear-strength Cu) to optimize self-lubricating tribological performance. Additionally, the dynamic evolution of solid-solution strengthening in response to varying base element content (specifically Co) remains underexplored.

Methodology
To address these gaps, the authors designed a novel multi-principal element binder system, CoxFeNiCrCu, with varying Cobalt content (x = 1.0, 1.5, 2.0 molar ratios). This system was combined with Vanadium Carbide (VC) micro-alloying to act as a grain growth inhibitor.

• Material Synthesis: HEA binder powders were prepared via mechanical alloying of Co, Fe, Ni, Cr, and Cu powders. These were mixed with WC powder (90:10 mass ratio) and varying amounts of VC (0–15% relative to the binder mass). A traditional WC-Co alloy served as the control.
• Processing: The mixed powders underwent vacuum hot-press sintering at 1450°C under 30 MPa pressure.
• Characterization: The study employed X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) to analyze phase composition and microstructure. Mechanical properties (hardness, fracture toughness, bending strength) were evaluated using Vickers indentation and three-point bending tests. Tribological performance was assessed via dry sliding friction tests against Si₃N₄ ceramic balls, measuring the coefficient of friction (COF) and wear volume.

Key Contributions and Results
The study demonstrates that the synergistic combination of a tunable HEA binder and VC micro-alloying effectively decouples the traditional hardness-toughness trade-off.

1. Microstructural Evolution:

   • The HEA binder inherently refines WC grains, reducing the average size from 4.01 μm (in WC-Co) to a range of 2.55–2.96 μm.
   • Increasing Co content in the binder (x = 1.0 to 2.0) induces a phase transformation from a single FCC structure to a dual-phase FCC/BCC solid solution.
   • While VC effectively refines grains in the conventional WC-Co system (reducing size to 3.12 μm), its effect is marginal in the HEA system due to competitive adsorption between the HEA binder and VC on the WC surface.

2. Mechanical Properties:

   • Hardness and Strength: The WC-Co1.5FeNiCrCu binder yielded the optimal balance, achieving 1635 HV and 1454 MPa bending strength.
   • Synergistic Optimization: The addition of 10% VC to the optimal binder (WC-Co1.5FeNiCrCu) further enhanced performance, resulting in a peak hardness of 1842 HV, a bending strength of 1523.66 MPa, and a fracture toughness of 11.38 MPa·m¹/². This represents a significant improvement over the conventional WC-Co baseline.

3. Tribological Performance:

   • The HEA-bound composites exhibited lower friction coefficients (0.298–0.308) compared to WC-Co (0.323).
   • The WC-Co1.5FeNiCrCu10%VC composite achieved the lowest friction coefficient of 0.256 and a 49.2% reduction in wear volume compared to the WC-Co baseline.
   • Wear Mechanisms: The superior wear resistance is attributed to the formation of an in-situ Cr₂O₃ glaze layer via tribo-chemical reactions, which acts as a protective film. Additionally, the refined microstructure suppressed delamination and severe abrasive wear (plowing).

Significance
The paper claims that this approach provides a promising pathway for developing next-generation wear-resistant materials. By leveraging the intrinsic grain-refining capability of the CoxFeNiCrCu HEA binder and the microstructural tailoring of VC, the study successfully achieves an exceptional strength-toughness balance and superior tribological performance without relying on traditional cobalt binders. The work highlights the potential of multi-principal element binders containing specific functional elements (like Cu for low shear strength and Cr for oxidation resistance) to overcome the limitations of conventional cemented carbides in high-performance machining applications.