Binders in cemented carbides can be systematically categorized into several functional types based on their introduction purposes and core effects. These additives interact with the basic components (WC and Co) of cemented carbides through physical or chemical means, exerting a profound impact on the alloy’s microstructure, phase composition, interface properties, and macro-mechanical performance during both sintering and service. In general, the main roles of additives can be summarized as follows:
According to the well-known Hall-Petch relationship, the strength and hardness of materials typically increase with decreasing grain size. During the liquid-phase sintering of cemented carbides, WC grains tend to grow significantly via the “dissolution-reprecipitation” mechanism. Particularly under high-temperature and long-term holding conditions, this easily leads to grain coarsening or even abnormally large grains, which severely impair the alloy’s strength and toughness.
To address this issue, grain growth inhibitors are essential. Their core function is to retard or prevent WC grain coarsening during sintering, thereby achieving a fine and uniformly distributed microstructure. Common types include:
High-melting-point cubic carbides (e.g., VC, Cr₃C₂, TaC, NbC, TiC);
Specific metal elements (e.g., Mo, Cr, V);
Rare earth elements (e.g., La, Ce) and their oxides (e.g., La₂O₃, CeO₂, Y₂O₃).
These inhibitors regulate grain growth by increasing the kinetic barrier for WC grain coarsening (detailed in subsequent chapters). Ultimately, they simultaneously enhance the alloy’s hardness, wear resistance, bending strength, and fracture toughness, achieving a balance between strength and toughness.
Fig.1 HRTEM images of the WC(0001)Co interface in WC-10Co-0.5VC submicron alloy under normal cooling (0.67K/s) and rapid cooling (50K/s) conditions
Figure 2 High-resolution transmission electron microscopy (HRTEM) images of the WC(0001)/Co and W0C(10T0)/Co interfaces in WC-10Co-0.9Cr3C2 submicron alloy under rapid cooling conditions
Dispersion strengthening is a classic mechanism in metal matrix composites. By introducing fine, uniformly distributed, and insoluble hard second-phase particles into the matrix, the movement of dislocations is effectively pinned, thereby improving the matrix’s yield strength and hardness. In cemented carbides, some additives act directly as dispersion-strengthening phases, while others in-situ form reinforcement phases during sintering.
For example:
Nano-sized hard particles (e.g., TiC, NbC, TiN) disperse in the cobalt binder phase or at WC grain boundaries, hindering the plastic deformation of the binder phase under stress and enhancing the alloy’s overall strength and high-temperature creep resistance.
In recent years, two-dimensional materials such as graphene have been explored as reinforcement phases. Studies show that trace additions of graphene significantly improve the hardness and fracture toughness of WC-Co alloys, attributed to graphene’s unique high strength, high modulus, and toughening mechanisms (e.g., crack deflection and bridging) during crack propagation.
The introduction of these reinforcement phases constructs a secondary microstrengthening system alongside the traditional WC skeleton, enriching the performance design dimensions of cemented carbides.
Cobalt (Co) is the most classic and effective
binder phase in WC-Co cemented carbides, offering excellent wettability, dissolution-precipitation capacity, and a good balance of strength and toughness. However, the scarcity of cobalt resources, price volatility, and potential biological toxicity have driven researchers to explore modification or replacement strategies. The main purposes of binder phase modifiers are to improve the performance of the cobalt phase itself, or partially/totally replace Co with other elements to reduce costs and enhance specific properties (e.g., corrosion resistance, high-temperature performance).
Common modification approaches include:
Replacement with Ni, Fe, or Ni-Fe alloys: Significantly improves corrosion resistance and toughness but usually sacrifices some hardness and high-temperature strength.
Solid solution strengthening: Elements such as Cr, Mo, and W dissolve in Co, enhancing the strength, hardness, high-temperature oxidation resistance, and softening resistance of the binder phase.
Non-metallic modification: Adding stable oxide particles (e.g., Al₂O₃) achieves dispersion strengthening of the binder phase; rare earth oxides (e.g., CeO₂) not only refine WC grains but also purify the binder phase and improve WC/Co interface bonding, comprehensively enhancing alloy performance.
Cobalt, the binder phase, exists in two allotropes at room temperature: face-centered cubic (fcc, γ-Co) and hexagonal close-packed (hcp, ε-Co). The fcc phase, with better plasticity and toughness, is generally the desired stable phase in cemented carbides. However, under cooling or stress induction, fcc-Co tends to undergo martensitic transformation to the more brittle hcp-Co. This transformation causes volume changes and internal stress, potentially initiating microcracks and reducing the alloy’s toughness and transverse rupture strength (TRS).
Phase transition regulators inhibit or control this adverse transformation. Rare earth elements (e.g., La, Ce, Pr, Nd) have shown significant effectiveness in stabilizing the fcc structure of Co and suppressing its martensitic transformation to hcp-Co. The mechanism may involve altering the stacking fault energy between fcc-Co and hcp-Co, increasing the phase transition barrier. By stabilizing fcc-Co, internal stress concentration from phase transformation is avoided, improving the alloy’s TRS and overall reliability.
Figure 3 High-resolution transmission electron microscopy (HRTEM) images of the two WC/Co interfaces in WC-10Co-0.5VC-0.9Cr3C2 submicron alloy under rapid cooling conditions (9) Copy
To maximize the performance potential of cemented carbides, additives with a single function often struggle to meet complex requirements. Thus, composite addition of multiple additives with different functions, leveraging their synergistic effects, has become a key direction in modern cemented carbide R&D.
Examples include:
Composite grain growth inhibitors (e.g., VC + Cr₃C₂): Achieve more significant grain refinement and a better balance of strength and toughness than single additions.
Carbides + rare earth oxides (e.g., VC + Y₂O₃): VC primarily inhibits WC grain growth, while Y₂O₃ refines grains, purifies grain boundaries, improves interface bonding, and stabilizes the binder phase.
This “multi-pronged” strategy comprehensively optimizes the alloy’s microstructure, achieving a “1+1>2” performance enhancement. The table below summarizes the types, main functions, and typical representatives of common additives in cemented carbides.
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