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简体中文 Plate-like WC grain cemented carbides are widely used in machining, mining, aerospace and other fields for their excellent hardness and wear resistance, but traditional types often face a hardness-toughness trade-off that limits their use in complex service environments. In 1969, MEADOWS et al. first achieved oriented WC grain arrangement via hot pressing and extrusion, successfully preparing plate-like WC grain cemented carbides with Group VIII binders (Co, Ni, Fe, etc.). This new material exhibited high transverse rupture strength, hardness, good toughness and corrosion resistance. Subsequently, Sandvik launched the TD900 series of plate-like grain toughened cemented carbides with controllable composition and simple preparation, establishing “tungsten + graphite + Group VIII binder” mixed powder as the main technical route.
Owing to their outstanding performance and broad industrial prospects, plate-like WC grain cemented carbides have gained increasing attention globally, with significant progress in preparation method optimization and performance enhancement. This review summarizes the research progress by focusing on WC grain structure and properties, plate-like formation mechanism, raw material composition, ball milling and sintering processes, as well as heterogeneous structure research.
Structure and Properties of Plate-like WC Grain Cemented Carbides
Crystal Structure of WC
WC crystal is a hexagonal close-packed (HCP) interstitial compound with lattice parameters a=0.2906nm and c=0.2837nm, giving a c/a ratio of 0.9763. Tungsten atoms are located at the (0,0,0) positions, while carbon atoms occupy the (2/3,1/3,1/2) positions. The atomic arrangement varies significantly among different crystal plane families: the (0001) plane has a dense atomic packing, while the side planes are relatively sparse. This anisotropic atomic distribution leads to obvious anisotropy in the mechanical properties of the alloy, especially in hardness. The Vickers hardness of the (0001) plane can reach 2100HV, which is twice that of the (1100) plane (1100HV).
Key Properties of Plate-like WC Grain Cemented Carbides
By optimizing the preparation process to reduce the c/a ratio of WC grains and promote their oriented growth into plate-like structures with well-developed (0001) planes, the macrohardness of cemented carbides can be significantly improved due to the increased proportion of high-hardness (0001) planes. Compared with traditional cemented carbides, plate-like WC grain cemented carbides have two prominent advantages:
Higher toughness
When cracks propagate along the grain boundaries of plate-like WC, they undergo more deflections and tortuous paths. Additionally, cracks tend to experience crazing and bridging, which effectively hinders their further expansion and consumes more energy during the fracture process.
Excellent high-temperature mechanical properties
The plate-like structure of WC grains reduces the stacking fault energy of the alloy, facilitating dislocation propagation while inhibiting dislocation jogging, cross-slip and climb.
Improved high-temperature stability
As a result of the optimized dislocation behavior induced by the plate-like structure, the high-temperature creep rate of the cemented carbide is reduced, and its overall high-temperature stability is enhanced.
Heterogeneous Structure of Plate-like WC Grain Cemented Carbides
Recent research on plate-like WC grain cemented carbides has expanded to the field of heterogeneous structures, yielding several forward-looking achievements.
ZHENG Yong et al. prepared gradient-structured plate-like WC grain cemented carbides by adding TiC and VC to flattened tungsten-graphite-cobalt raw materials. This gradient alloy is rich in hard phases and poor in cobalt on the surface, with a gradient distribution of cobalt content within a certain thickness of the surface layer. Compared with homogeneous structured plate-like cemented carbides, it exhibits higher surface hardness and core toughness, making it suitable for mining and drilling applications.
In recent years, dual-scale/multi-scale grain structures have emerged as a research hotspot for toughening. The strengthening and toughening mechanism lies in the fact that fine grains can effectively hinder dislocation movement and maintain high alloy strength, while coarse grains can absorb strains around fine grains and delay crack propagation, thereby improving the toughness of the alloy. Studies have shown that the fracture toughness of dual-scale WC-10Co cemented carbide is increased by approximately 15% compared with that of homogeneous structured alloys with the same composition, without significant loss of hardness.
Formation Mechanism of Plate-like WC Grains
The formation of plate-like WC grains is closely related to the preparation process, and there are three main mainstream formation methods:
The first method involves heating fine-grained WC above a critical temperature (determined by the particle size of the original WC powder and the composition and content of the metal binder), leading to the formation of plate-like WC grains through a dissolution-precipitation mechanism.
The second method utilizes chemical media for nucleation and preferential growth along the (0001) crystal plane. Currently, the media used during sintering mainly include plate-like WC seeds, TiC and Y₂O₃. During the liquid-phase sintering stage, WC grains undergo recrystallization. The media uniformly dispersed in the liquid phase promote the uniform precipitation and growth of the WC phase from the binder phase, while inhibiting the growth of WC grains along the c-axis.
The third method uses flattened tungsten powder as the base material, mixed with graphite and cobalt for sintering. In this preparation process, near-spherical tungsten particles are first flattened by ball milling and then oriented arranged under pressing force, with the (0001) planes of most grains perpendicular to the pressing direction. Subsequently, chemical reactions occur among tungsten, graphite and cobalt during liquid-phase sintering, successively forming plate-like CoₓWᵧC_z and W₂C phases. Since it is easier for cobalt-tungsten-carbon carbides (Co₉W₆C, etc.) and tungsten to obtain carbon atoms on the (0001) plane than on other prismatic planes, and the interface energy between the WC (0001) plane and cobalt-tungsten-carbon carbides is lower than that of other crystal planes, the WC (0001) plane grows preferentially, resulting in the formation of a large number of oriented plate-like WC grains perpendicular to the pressing direction. Currently, this preparation method has been widely recognized by researchers.
Preparation Technology of Plate-like WC Grain Cemented Carbides
Raw Material Composition and Basic System
The raw materials for plate-like WC grain cemented carbides mainly include tungsten-containing materials, graphite and Group VIII metal binders. Tungsten-containing materials are classified into three categories: elemental tungsten powder, Co-W-C compound powder and WC powder. Current research mostly involves doping modification on this basis to optimize the morphology of plate-like grains and the performance of the alloy.
Preparation Process Using Elemental Tungsten Powder as Raw Material
Using tungsten powder + graphite + cobalt as raw materials, plate-like WC grains can be obtained after flattening treatment of tungsten powder followed by sintering. The grains exhibit a high degree of oriented arrangement, leading to obvious anisotropy of the material. The advantage of this process is the regular orientation of plate-like grains, resulting in higher fracture toughness and transverse rupture strength in specific directions. However, the flattening process is complex, time-consuming and prone to contamination, making it difficult to batch-produce high-quality tungsten powder stably.
Preparation Process Using Co-W-C Compounds as Raw Materials
Adopting CoₓWᵧC_z + graphite as raw materials, non-oriented plate-like WC grains can be obtained through synthesis, mixed milling and hot-pressing sintering, which effectively improves the hardness anisotropy caused by oriented arrangement. However, this method has obvious limitations: Co-W-C compounds are unstable, high-purity powders are difficult to produce in batches, and plate-like twins are difficult to control, making it unsuitable for large-scale production.
Preparation Process Using WC Powder as Raw Material
When using ordinary WC powder as the tungsten-containing raw material, it is necessary to add certain inducing media to promote the growth of plate-like grains, mainly including the following methods: adding plate-like WC seeds, which act as nucleation centers during the liquid-phase sintering stage to guide the precipitation and growth of WC along the seed surface, significantly improving the flexural strength and fracture toughness; adding compounds such as TiC, where trace TiC can change the interfacial energy between phases, prompting WC to grow preferentially along the (0001) plane to achieve grain flattening; adding rare earth elements and carbides (such as Cr₃C₂, VC, La₂O₃), which can improve the crystalline integrity of plate-like grains and the density and hardness of the alloy.
Current Status and Development Trends of the Process
Currently, various preparation methods can produce a certain number of plate-like grains, but they generally face problems such as difficulty in controlling the morphology of plate-like grains, unstable production quality and high cost. Therefore, the further development of stable and reliable preparation processes for plate-like WC grain cemented carbides remains an important research direction in the future. The raw material system will develop towards multi-component doping and grain refinement.
Ball Milling Process
Ball milling is a core process for tungsten powder flattening, directly affecting plate-like grain morphology and alloy performance by refining powder and introducing defects, which lays the foundation for the oriented growth of plate-like WC grains during liquid-phase sintering.
KINOSHITA et al. found that prolonged ball milling enhances the oriented arrangement of plate-like WC grains (independent of initial particle size); larger tungsten/cobalt particles, smaller graphite particles, and extended milling result in larger and more numerous plate-like grains.
ZHU Min et al. developed the PBMS-type plasma ball mill, and WANG Wei et al. confirmed it can refine 0.5μm tungsten grains to 81-86nm in 1-3h, achieving more uniform dispersion and prominent plate-like grain characteristics with higher efficiency than conventional high-energy ball milling.
High-energy ball milling is currently the primary method but has drawbacks (complex procedures, low efficiency, powder contamination leading to sintering pores); developing efficient, low-pollution processes is a key future direction, with plasma ball milling being a promising option if equipment costs are not a constraint.
Sintering Process
Sintering process is a key link to improve the density and mechanical properties of plate-like WC grain cemented carbides. Currently, the mainstream processes include vacuum sintering, low-pressure sintering, hot-pressing sintering and spark plasma sintering (SPS). No research reports on the application of microwave sintering in plate-like grain preparation have been found yet.
Vacuum Sintering (Conventional Basic Process)
As a conventional method for cemented carbide preparation, the core problem of vacuum sintering is that gas is difficult to be completely removed, which easily forms small pores, limiting the comprehensive mechanical properties of the alloy. Studies have shown that increasing the sintering temperature and extending the holding time can increase the number of plate-like WC grains and improve the density, but it is accompanied by the obvious disadvantage of significant increase in WC grain size.
Low-Pressure Sintering (Preferred for High Density)
Compared with vacuum sintering (density 98.5%), low-pressure sintering (pressure can increase the alloy density to 99.5%, achieving near-complete densification. Its advantage stems from the faster cooling rate, which can promote the formation of more face-centered cubic (fcc) structured Co₃W₃C phase in the alloy. The content of this phase directly determines the deformation ability of the binder phase, which is conducive to significantly improving the toughness of the alloy.
Hot-Pressing Sintering (Oriented Arrangement and Performance Regulation)
Under nitrogen protection, hot-pressing sintering (pressure 10~40MPa) at 25MPa is conducive to the oriented arrangement of plate-like WC grains. Taking WC-10Ni₉Al alloy as an example, within the temperature range of 1250~1350°C, the density, hardness, transverse rupture strength and fracture toughness of the alloy increase with the increase of temperature. When the sintering temperature exceeds 1400°C, WC grains aggregate and grow significantly, and the small pores inside the alloy aggregate into round holes, leading to performance degradation.
Spark Plasma Sintering (Efficient and High-Performance Process)
The core advantages of spark plasma sintering are high efficiency and superior mechanical properties. At temperatures ≤1450°C, the higher the temperature, the more sufficient the plate-like formation of WC grains. The increase in heating rate is beneficial to grain refinement and improvement of hardness and fracture toughness. Compared with hot-pressing sintering, its samples have slightly lower density (due to differences in binder phase volatilization), but more uniform microstructure, finer grains and higher plate-like grain formation rate. Ultimately, the hardness, transverse rupture strength and fracture toughness are all better. This is attributed to the fact that during the SPS process, each particle generates Joule heat by itself, the heat source obtained by the particles is uniform, the sintering densification is carried out simultaneously in the entire powder body, resulting in a more uniform microstructure of the sintered body.
Process Performance Comparison
Overall, low-pressure sintering and hot-pressing sintering focus on improving the density of the alloy; spark plasma sintering performs more prominently in terms of efficiency, toughness and comprehensive mechanical properties, and is a preferred direction that balances high performance and production efficiency.
結論
Plate-like WC grain cemented carbides feature excellent high-temperature performance, solving the traditional hardness-toughness trade-off and boasting broad application prospects. However, most preparation methods are still in the experimental stage, while new heterogeneous systems (e.g., gradient, bimodal structures) offer promising optimization directions.
Key challenges remain: complex raw material nano-ization and doping processes lead to contamination and unstable quality; low-pressure/hot-pressing sintering causes grain coarsening; advanced processes (plasma ball milling, SPS) increase costs; and practical engineering application research is limited.
In summary, developing efficient, reliable, and low-cost preparation technologies will be the long-term research focus and development trend for plate-like WC grain cemented carbides.
