WC carbide, as a key material in modern industry, has always been a research hotspot in the field of materials science for performance optimization. In this study, WC-TiC, WC-TaC, and WC-TiC-TaC were prepared using hot-pressing sintering (HP). The effects of TaC addition, TiC addition, and their co-addition on the microstructural evolution and properties of WC were systematically evaluated.

Research Background

In recent years, numerous researchers have attempted to prepare high-density binderless tungsten carbide (WC) using various processing methods, such as hot-pressing sintering (HP), spark plasma sintering (SPS), and pressureless sintering (PS).

The results indicate that without high-pressure conditions, a sintering temperature above 1,800 °C is required to prepare binderless WC via solid-phase sintering. However, in practical sintering processes, the high temperatures needed for high densification can easily lead to decarburization of WC, ultimately forming the W₂C phase.

Nanoscale or submicron WC powders are typically used to achieve fine grains and enhance the mechanical properties of WC while lowering the sintering temperature. However, nanopowders are prone to oxidation, forming a surface oxide layer that consumes carbon during sintering, leading to the formation of the decarburized W₂C phase. The presence of W₂C increases brittleness and reduces the Young’s modulus, thereby degrading the mechanical properties of the material.

In practice, since surface oxides or adsorbed oxygen on nanopowders consume carbon, a certain amount of additional carbon must be introduced during sintering to obtain pure, high-performance WC. Carbon addition helps achieve pure WC without reducing the Young’s modulus and improves sinterability, but it also promotes grain growth.

High-density binderless WC follows the Hall-Petch principle: the smaller the grain size, the higher the hardness (strength). Therefore, inhibiting WC grain growth is essential to obtain WC products with higher hardness (strength).

An effective method to suppress WC grain growth is the addition of transition metal carbides as grain growth inhibitors, such as Cr₃C₂, VC, TaC, TiC, and ZrC. Among these, TiC and TaC typically contain carbon vacancies in their crystal structures and exist as non-stoichiometric compounds, meaning their crystal structures remain unchanged despite fluctuations in carbon content.

Both TiC and TaC can dissolve WC and form solid solutions, promoting the densification process and effectively inhibiting WC grain growth, thereby improving mechanical properties. Studies report that adding TiC and TaC can enhance the hardness and corrosion resistance of WC, expanding its applications in mechanical seals and sliding components. Additionally, TiC addition can reduce tool wear caused by chemical interactions near the cutting edge.

However, the effects of TaC addition, TiC addition, and their synergistic addition on the microstructure and properties of WC still require comprehensive investigation.

Significance of the Research

Liquid-phase sintered WC carbide (WC-Co) consist of a hard tungsten carbide phase and a cobalt (Co) binder phase. They possess high hardness and wear resistance, making them widely used in metal cutting and rock drilling tools. The presence of the Co binder phase enhances the sintering ability of tungsten carbide and improves its fracture toughness, but it reduces the hardness, Young’s modulus, and corrosion resistance of the tungsten carbide WC carbide, limiting its application under extreme conditions. Therefore, the development of binder-free tungsten carbide WC carbides serves as a supplement to traditional WC carbides. However, due to the high melting point, rigid covalent bonds, and low diffusion mobility of binder-free tungsten carbide, it is difficult to achieve high densification during sintering.

Traditional WC-Co carbides are widely used in cutting processing, mining tools, and other fields due to their excellent comprehensive properties. Nevertheless, although the presence of the Co binder phase improves the toughness of the material, it inevitably reduces its hardness, heat resistance, and corrosion resistance. As modern industry continues to raise requirements for material performance, the development of binder-free WC carbides has become an important direction to break through the performance bottlenecks of existing materials.

The main challenges faced by binder-free WC carbides include:

Difficulty in sintering densification: due to the high melting point (2870°C) and low diffusion coefficient of WC.

Grain growth tendency: WC grains tend to grow abnormally during high-temperature sintering.

Formation of decarburized phases: brittle W₂C phases are easily formed during sintering.

To address these issues, this study selects TiC and TaC as additives based on the following considerations:

Both are transition metal carbides and have good compatibility with WC.

They can effectively inhibit the growth of WC grains.

They can improve sintering performance without significantly impairing the material’s hardness.

Experimental Methods

In this experiment, TiC, TaC powders, and pure nano-WC powder were used as raw materials to prepare WC-TiC, WC-TaC, and WC-TiC-TaC binder-free carbides via hot-press sintering technology. The effects of TiC, TaC, and TiC-TaC additions on the microstructure and properties of binder-free WC carbides were systematically studied.

High-purity nano-WC powder (with an average particle size of 50 nm) was used as the matrix material, with the addition of 10wt% TiC, 10wt% TaC, and 5wt% TiC + 5wt% TaC, respectively. Samples were prepared through the following processes:

1.Raw material ball milling and mixing (using ethanol as the medium, for 24 hours).

2.Drying and sieving.

3.Hot-press sintering (1800°C, 30 MPa, holding for 1 hour).

4.Furnace cooling.

Multiple characterization methods were used to systematically analyze the material properties:

1.XRD analysis for phase composition.

2.SEM observation for microstructure.

3.Vickers hardness tester for mechanical properties.

4.Friction and wear tests for wear resistance evaluation.

Research Results

The results show that:

1.Under the sintering temperature of 800°C, the relative density of all samples reaches over 99%, and the average grain size is less than 600 nm. Among them, TaC has the most significant inhibitory effect on WC grain growth (with an average grain size of 370.1 nm), but there is a W₂C decarburized phase.

2.WC-TiC, which has no decarburized phase and good microstructure uniformity, exhibits the highest fracture toughness and bending strength, with values of 5.68 MPa·m¹/² and 1027.48 MPa, respectively. WC-TaC has fine grain size and contains brittle W₂C phases, showing the best hardness and wear resistance, with a hardness of 2662.8 HV0.3 and a wear rate of only 1.3×10⁻⁷ mm³/(N·m).

3.Due to the high hardness, high density, and small grain size of all samples, abrasive wear is the main wear mechanism in friction and wear tests. Among them, the wear surface of WC-TaC has shallow plowing grooves and only a small number of fine pits, demonstrating excellent wear resistance.

Fig.4  Friction and Wear Properties of WC-TiC, WC-TaC and WC-TiC-TaC

Выводы

(1) The addition of TiC, TaC, and TiC-TaC all contributes to improving the sintering performance of binder-free WC and limiting the growth of WC grains. The relative density of all samples reaches over 99%. TaC has a more significant inhibitory effect on WC grain growth. WC-TaC exhibits the smallest grain size with the narrowest distribution range, with an average grain size of 370.1 nm, but forms a brittle W₂C decarburized phase. The grain sizes of WC-TiC and WC-TiC-TaC samples are larger than that of WC-TaC, with relatively narrow distribution ranges; their average grain sizes are 450.7 nm and 510.3 nm, respectively, and no decarburized phase is generated.

(2) WC-TiC shows the highest fracture toughness, bending strength, and the best polishing performance, with fracture toughness and bending strength reaching 5.68 MPa·m¹/² and 1027.48 MPa, respectively. Due to the formation of the brittle W₂C phase, WC-TaC achieves the highest hardness, up to 2662.8 HV0.3.

(3) The main wear mechanism of WC-TiC, WC-TaC, and WC-TiC-TaC is abrasive wear. WC-TaC exhibits the most excellent wear resistance, with a wear rate of only 1.3×10⁻⁷ mm³/(N·m), which is 92.9% and 94.3% lower than that of WC-TiC (1.84×10⁻⁶ mm³/(N·m)) and WC-TiC-TaC (2.27×10⁻⁶ mm³/(N·m)), respectively.  

Fig.5 Wear surface morphology of WC-TiC, WC-TaC and WC-TiC-TaC

(4) This study not only reveals the influence mechanism of different additives on the properties of binder-free WC carbides but also provides important insights for future material design. It is particularly noteworthy that the WC-TaC carbide, with an ultra-high hardness of 2662.8 HV0.3 and an extremely low wear rate of only 1.3×10⁻⁷ mm³/(N·m), demonstrates application potential under extreme wear conditions. Moreover, all samples exhibit abrasive wear as the main characteristic in friction and wear tests; in particular, the excellent performance of the WC-TaC sample, with only shallow plowing grooves and a small number of fine pits on its surface, indicates broad prospects for such materials in precision machining and harsh environment applications. These findings not only enrich the theoretical system of WC carbides but also provide important technical support for the development of next-generation high-performance cutting tools and wear-resistant components. With the deepening of further research, we have reason to expect that these materials will play a revolutionary role in aerospace, precision manufacturing, and other fields.