Ball Milling is an indispensable process in the production of cemented carbide materials. This paper introduces the application of plasma milling technology.

Plasma is a quasi-neutral gas with collective behavior, formed by gas ionization and composed of a large number of positive and negative charged particles, electrons, neutral particles, and free radicals. Benefiting from the abundant and diverse active particles, plasma readily reacts with the surface of contacted materials, thereby altering the surface structure, composition, functional groups, and wettability of materials. Dielectric barrier discharge (DBD) is a common cold plasma technology that eliminates the stringent vacuum requirements of gas ionization and is widely used in material surface treatment.

According to the plasma milling schematic, pulse voltage is applied to the electrodes at both ends of a discharge milling jar with a dielectric barrier structure, and the discharge parameters of the plasma power supply are adjusted based on the milling load to excite gases (argon, nitrogen, oxygen, ammonia, etc.) in the jar and generate low-temperature discharge plasma. Notably, the cold plasma generated by DBD has an extremely high electron temperature but a low overall macroscopic temperature (controllable below the metal phase transition point or even at room temperature), and its dielectric barrier layer can suppress spark or arc discharge, avoiding damage to the milling system by thermal plasma.

Mixed powders form a large lamellar morphology through layer stacking under the extrusion deformation of high-energy milling and the thermal effect of plasma particle flow. During heating carbonization, carbon diffuses between lamellae, easily generating WC via in-situ reaction and forming a plate-like structure. Co coated on the surface of W particles bonds WC layers in both transverse and longitudinal directions.

In other words, plasma milling facilitates the formation of alternating lamellar structures of W and C, shortening the W/C reaction interface and significantly reducing the activation energy and temperature for WC synthesis. On the other hand, highly active plasma particles (ions, electrons, excited atoms/molecules, free radicals, etc.) easily adsorb to other substances, causing changes in material microstructure and properties, enhancing defects and surface activity of powder materials during milling, and promoting diffusion, phase transformation, and chemical reactions.

Plasma milling for 1~3 hours achieves ultra-rapid refinement and activation of W-C-Co composite powders. After compression molding, the green compacts can realize synchronous WC synthesis and alloy densification during a single solid-phase sintering process (around 1340~1390℃), successfully developing the “one-step carbonization-sintering method” for cemented carbide preparation.

Figure 3 One-step preparation of WC-Co cemented carbide based on plasma milling technology

Compared with traditional preparation processes for WC-Co alloys, this method features simplicity, high efficiency, and short production cycle. More importantly, W-C-Co powders prepared by plasma milling have a fine lamellar structure with nanoscale thickness, which induces the formation of plate-like WC during subsequent sintering.

Nanoscale WC obtained by carbonization at 1000℃ is generally truncated triangular, with an average size of 100~300 nm and thickness less than 100 nm. The “one-step carbonization-sintering method” can synthesize WC grains with multiple morphologies (plate-like, prismatic) and scales, potentially greatly improving the mechanical properties of WC-Co cemented carbides.

Figure 4 After 3 hours of plasma milling: Morphology of W-C powder

Compared with conventional milling, plasma milling can obtain plasma with different discharge intensities by adjusting discharge parameters, which significantly affects the microstructure and properties of plate-like WC-Co cemented carbides. Powders from conventional milling form spherical aggregates, while plasma-milled powders exhibit more distinct lamellar morphology with increasing discharge intensity. This is mainly attributed to two aspects:

(1) Local high temperature from electron temperature causes recrystallization annealing of powders extruded and deformed by milling balls, facilitating the formation of W lamellar structures;

(2) Possible association with electroplasticity. Transmission electron microscopy (TEM) observation of milled W-C-Co composite powders shows that plasma-milled powders are distinctly lamellar, whereas conventional milled powders tend to agglomerate with larger particle sizes. Graphite in conventional milling presents flocculent amorphous state, while plasma-milled graphite forms finer nanoscale lamellar structures, greatly increasing the contact area between W and graphite, which further explains the lower carbonization temperature. The activation energy of W-C-Co powders prepared by plasma milling (P-milling-D) is 276.13 kJ/mol, significantly lower than 330.71 kJ/mol of conventional milling (C-milling-A).

Figure 5 TEM images of W-C-Co composite powders after 3 hours of milling: a. Conventional milling; b. Plasma milling

After compression and low-pressure sintering, the effect of different plasma intensities on the microstructure and properties of milled W-C-Co powders translates to the morphology and properties of WC grains. WC prepared by non-discharged conventional milling is prismatic, while plasma-milled WC is plate-like with highly oriented arrangement of plate-like WC grains in the alloy. Additionally, with increasing plasma intensity, the prepared plate-like WC has smaller size and thinner thickness.

Figure 6 Thickness distribution of plate-like WC

Differences in WC morphology and size lead to significant variations in the mechanical properties of WC-8%Co alloys: the bending strength and hardness of C-milling-A samples (conventional milling) on the V-plane are only 1581 MPa and HRA 91.5, while those of P-milling-B and P-milling-D samples (plasma milling) reach 3371 MPa, 3567 MPa and HRA 92.3, HRA 92.7, respectively.

This indicates that directionally arranged plate-like cemented carbides prepared by plasma milling have outstanding performance advantages over conventional milling. The mechanical properties of cemented carbides improve significantly with increasing plasma discharge intensity.

Three types of W powders with different initial particle sizes (0.5 μm, 1~5 μm, 12 μm) were mixed with graphite and Co powders for plasma milling at different durations. The particle size of W aggregates increases with the initial W particle size; W aggregates transform from nearly spherical to lamellar as milling time increases from 1 h to 3 h.

Figure 7 W-C-Co composite powders prepared by plasma milling for (a) 1 h and (b) 3 h (initial W powder: 12 μm)

After powder compression and low-pressure sintering at 1390℃ for 1 h, it is found that nearly spherical W aggregates in samples milled for 1 h tend to form prismatic WC during sintering, while lamellar W aggregates in samples milled for 3 h mainly form plate-like WC.

Figure 8 Microstructure of WC-8%Co cemented carbides sintered from powders milled by plasma for 1 h and 3 h at different cross-sections: (a) Sample B1, V-section of alloy;(b) Sample B3, V-section of alloy

Grain growth inhibitors were added during the one-step carbonization-sintering preparation of cemented carbides. SEM observations show that with plasma milling, particles in W-C-Co-VC mixed powders are uniformly distributed, continuously refined, and stacked in lamellae, with graphite tightly coating W particles (consistent with the case without grain growth inhibitors). XRD results of sintered WC10%Co-1.2%VC alloy indicate that the alloy is mainly composed of WC and Co, with no VC or harmful subcarbides detected.

Figure 9 SEM images (after corrosion) of WC-10%Co cemented carbides sintered at 1340℃ with different inhibitor contents: (a) 0%VC; (b) 0.6%VC; (c) 0.9%VC; (d) 1.2%VC

Although plate-like WC-Co cemented carbides prepared by plasma milling have excellent mechanical properties, the highly oriented arrangement of WC causes anisotropy in mechanical properties, especially fracture toughness and transverse rupture strength (TRS). A simple and feasible method for preparing dual-scale plate-like WC cemented carbides is proposed: any two types of W-C-Co mixed powders with initial W particle sizes of S (0.5 μm), M (1~5 μm), and L (12 μm) are separately plasma-milled for 3 h to obtain two types of lamellar W-C-Co powders with different sizes; the powders are mixed, compressed, and subjected to one-step carbonization-sintering to form dual-scale plate-like WC.

The MxLy series dual-scale plate-like WC-Co cemented carbides prepared by this simple mixing method significantly improve the highly oriented arrangement of plate-like WC in the alloy bulk. By calculating the peak intensity ratio of WC (0001) crystal plane to (10$\overline{1}\(0) crystal plane on V and P planes, mixing M and L powders at different mass ratios can control the quantity ratio of medium-grained M-plate-like WC and coarse-grained L-plate-like WC, thereby regulating the overall preferred orientation of plate-like WC in the alloy. The difference in I(0001)/I(10\)\overline{1}$0) of plate-like WC on V and P planes is adjustable in the range of 1.92~3.02.

Figure 10 Difference in I(0001)/I(10$\overline{1}$0) of MxLy cemented carbides prepared by low-pressure sintering on V and P planes

Plasma milling realizes the coupling of plasma and mechanical milling, effectively improving milling efficiency and introducing a new microstructure evolution mechanism, which endows unique advantages for WC-Co cemented carbide preparation.

Using W, C, and Co powders as raw materials, plasma milling for only 1~3 hours achieves rapid refinement and activation to obtain highly reactive W-C-Co nanocomposite powders, reducing the carbonization temperature by about 400℃ compared with conventional carbonization processes. After compression molding, the highly reactive powders realize synchronous WC synthesis and alloy densification in a single solid-phase sintering process. The “one-step carbonization-sintering method” has the advantages of simple process, high efficiency, and short production cycle, suitable for large-scale production via conventional sintering.

By controlling plasma milling parameters, the morphology of WC can be regulated to obtain prismatic and plate-like WC; directionally arranged plate-like WC cemented carbides can be prepared by controlling WC size and compression process. Adding VC or V₂O₅ grain growth inhibitors directly to the initial plasma milling powders exerts an effective inhibitory effect: with increasing VC content, WC morphology transforms from long strip to thin sheet (about 100 nm) with obvious grain refinement; WC-10%Co cemented carbide with 0.9% VC addition exhibits optimal mechanical properties.

Mixing two types of plasma-milled powders with different initial scales and morphologies enables the preparation of cemented carbides with dual-scale and dual-morphology WC. Adjusting the ratio of two-scale lamellar W-C-Co powders controls the proportion and preferred orientation of dual-scale plate-like WC. Both the plate-like effect and dual-scale structure effect improve the TRS and fracture toughness of cemented carbides, and their synergistic effect enhances these properties more significantly.

When medium-grained plate-like WC is combined with coarse-grained plate-like WC, the TRS and fracture toughness of the alloy first increase and then decrease. Under appropriate microstructural conditions, the comprehensive mechanical properties reach a high level. For example, the hardness, TRS, and fracture toughness of WC8%Co alloy on the plane perpendicular to the compression direction are as high as 1768 kgf/mm², 4084 MPa, and 23.11 MPa·m^(1/2), respectively; on the plane parallel to the compression direction, they reach 1733 kgf/mm², 3924 MPa, and 21.56 MPa·m^(1/2), respectively.