The Effect of Crystal Structure on the Properties of Cemented Carbide Balls

Cemented carbide balls are spherical products made through powder metallurgy, using tungsten carbide (WC) as the primary hard phase and a metal such as cobalt (Co) as a binder phase. Their crystal structure (including the crystal type, lattice parameters, defects, and interfacial structure of the hard and binder phases) plays a decisive role in properties such as hardness, toughness, wear resistance, and corrosion resistance.

I. The Effect of the Crystal Structure of the Hard Phase WC

As the core hard phase of tungsten-based cemented carbide balls, the crystal structure of WC significantly influences material properties.

1. Crystal Type and Bonding Characteristics

WC has a hexagonal (α-WC) structure, and the W-C bond is a mixture of strong covalent and metallic bonds. This unique bonding pattern imparts WC with high hardness (HV approximately 2000-2500) and a high melting point (2870°C), making it an ideal hard, load-bearing phase in cemented carbide balls. However, its hexagonal structure also results in anisotropy in the cutting direction.

2. Lattice Parameters and Defects

Lattice Distortion: When WC contains lattice defects (such as vacancies and dislocations) or is doped with other elements (such as Cr or V), the lattice parameters change, affecting the electron cloud distribution. For example, the Cr-doped WC lattice contracts, resulting in a slight increase in hardness.

Grain Size: According to the Hall-Petch relationship, grain refinement increases hardness, but excessive grain refinement decreases toughness. In cemented carbide balls, the WC grain size is typically controlled within the range of 0.2-5μm to achieve a balance between hardness and toughness.

3. Phase Composition and Stability

Duplex Structure: In WC-Co cemented carbide balls, WC can form two phases: α-WC (hexagonal) and β-WC (cubic). β phase is unstable at high temperatures and easily decomposes into α-WC and C, leading to performance degradation. Therefore, controlling the β phase content is key to improving the high-temperature stability of cemented carbide balls.

Carbon Content: Insufficient carbon content will cause the formation of carbon-deficient phases (such as η phase, e.g., Co₃W₃C) in WC, reducing hardness and toughness; excessive carbon may form free graphite, weakening material properties.

II. Influence of the Crystal Structure of the Binder Phase

Cobalt (Co) is a common binder phase in cemented carbide balls, and its crystal structure is crucial to the material's toughness and other properties.

1. Crystal Type and Slip System

Co has a hexagonal close-packed (HCP) structure at room temperature, which transforms to a face-centered cubic (FCC) structure at high temperatures. The HCP structure has fewer slip systems and limited plastic deformation capacity, while the FCC structure has more slip systems and better high-temperature toughness. This allows Co-based cemented carbide balls to maintain good toughness even at high temperatures.

2. Grain Size and Distribution

Fine binder phase grains improve bond strength, but excessively fine grains may increase brittleness. Controlling the Co grain size optimizes the toughness and hardness balance of cemented carbide balls. Furthermore, the uniform distribution of the binder phase is crucial. If Co aggregates or forms coarse grains, the flexural strength and wear resistance of the carbide balls will be reduced.

3. Solid Solution Strengthening and Phase Transformation

Solid Solution Strengthening: Dissolving small amounts of elements such as W and C in Co can form a solid solution, hindering dislocation motion and increasing strength. For example, dissolving W in the Co phase in WC-Co significantly increases hardness.

Phase Transformation Hardening: Co undergoes a HCP to FCC phase transformation at high temperatures, accompanied by volume changes that may generate residual stresses and further strengthen the material.

III. Influence of the Interface Structure between the Hard and Binder Phases

The interface is a key region for stress transfer and crack propagation, and its structure significantly influences the fracture toughness of carbide balls.

1. Interfacial Bond Strength

Strong interfacial bonds (such as chemical bonds) effectively transfer stress and improve flexural strength. Weak interfacial bonds can easily lead to crack propagation along the interface, reducing toughness. For example, the interface between WC and Co forms a solid solution layer through the diffusion of W atoms, enhancing the bonding strength.

2. Interface Phase Formation

During high-temperature sintering, WC and Co may react to form an interface phase (such as Co₃W₃C). Its crystal structure and properties are intermediate between those of WC and Co, which can regulate the interfacial stress distribution and improve toughness.

IV. The Comprehensive Influence of Crystal Structure on Properties

1. Balance between Hardness and Toughness

High hardness requires fine hard phase grains, few defects, and strong bonding; high toughness requires fine, evenly distributed binder phase grains with abundant slip systems. By optimizing WC grain size and Co content, cemented carbide balls can achieve a balance between hardness (HRA 89-93) and flexural strength.

2. Wear Resistance and Corrosion Resistance

Wear resistance primarily depends on the hardness of the hard phase and the strength of the binder phase; corrosion resistance is related to the crystal structure of the binder phase (for example, materials with an FCC structure are generally more corrosion-resistant) and interface sealing. In cemented carbide balls, rationally designing the crystal structure can enhance both performance aspects. 3. High-Temperature Performance

At high temperatures, the crystal structure stability of the binder phase (such as the FCC phase of Co) and the oxidation resistance of the hard phase jointly determine the high-temperature hardness, strength, and creep resistance of cemented carbide balls.

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