Cemented carbides, known as the “teeth of industry”, especially ultrafine-grained tungsten carbide (WC) ones with high hardness and wear resistance—are widely used in mechanical processing, mining, and other fields. WC-Co is the most produced and widely applied type: expensive cobalt (Co) acts as a binder to boost the alloy’s flexural strength, but it also reduces WC’s corrosion resistance, hardness, and wear resistance. Binder-free WC cemented carbides have thus become a key direction for high-wear-resistance materials. However, pure WC has a high melting point (2800°C), making traditional sintering ineffective. While special high-temperature methods (e.g., plasma discharge sintering, microwave sintering) work, their high production costs limit industrial use.

Refining powder particles lowers their melting point and significantly improves WC cemented carbides’ strength, hardness, and toughness. This study first refined WC powder via ball milling, then prepared ultrafine-grained binder-free WC cemented carbides with high hardness through hot-pressing sintering. The sintered samples reached a hardness of 2294 HV.

Experimental Process and Methods

In this experiment, pure WC powder with a particle size of 0.5–1 μm was selected. The original WC powder was first subjected to ball milling treatment on an S6-2 high-energy ball mill, and argon protection was applied after vacuuming. The rotational speed of the ball mill was 600 r/min, the grinding balls were made of stainless steel, the ball-to-powder ratio was 40:1, and the ball milling time was 6 hours.

After ball milling, the WC powder and paraffin were ground uniformly in an agate mortar at a mass ratio of 98:2. Then, under a pressure of 500 MPa, the mixture of WC powder and paraffin was pressed into 20 mm × 20 mm thin sheets. To densify the pre-pressed samples and prevent the formation of pores, the samples were subjected to pre-sintering for dewaxing before the final sintering. The pre-sintering was carried out in an argon-protected tube furnace, with the samples placed in a quartz tube. The temperature was slowly increased from room temperature to 750°C at a heating rate of 3°C/min, held at this temperature for 30 minutes, and finally cooled to room temperature along with the furnace.

Hot pressing sintering was performed on a YT27-200T hydraulic press with a maximum pressure of 200 tons. First, the pre-sintered samples were placed in a mold shell, filled with glass powder around, and then placed in a resistance furnace for heating, where the temperature was raised to 1200–1300°C. To prevent the samples from being reduced at high temperatures, a mold cover should be added on top of the mold shell and argon should be introduced for protection during heating. The samples were placed into the preheated mold for hot pressing, with a pressure of 140 MPa maintained for 10 seconds.

X-ray analysis of the samples was conducted on a D/max2500Te X-ray diffractometer. A JSM-6360LV scanning electron microscope (SEM) was used to observe the microstructure morphology of the samples, and energy dispersive spectroscopy (EDS) analysis was performed. A F-700 numerical control microhardness tester was used to carry out Vickers microhardness tests, with a load of 500 g maintained for 10 seconds. For each sample, 5 points were randomly measured, and the average value was taken.

 

Experimental Results and Analysis

Scanning Electron Microscope Morphology Observation

Figure 4 shows the SEM images of WC powder before and after ball milling. Among them, Figure 1(a) is the original WC powder without ball milling, and Figure 1(b) is the WC powder after ball milling. It can be seen from the figures that the particles of WC powder become significantly smaller after ball milling. The particle size of the original WC powder is approximately 0.5–1 μm, while that of the powder after ball milling decreases to 0.2 μm. Due to the refinement of the powder, powder agglomeration occurs in local areas.

 

X-ray Crystal Structure Analysis

Figure 5 shows the X-ray diffraction pattern of the original WC powder. From the X-ray pattern of the original WC powder, it can be clearly observed that the peaks of WC are distinct, indicating that the sample is basically pure WC powder with almost no other impurities.

To investigate whether oxidation and decarburization occurred during the experimental process, X-ray diffraction analysis was performed on WC samples under different treatment states in this study, as shown in Figure 6. Among them, (a), (b), and (c) represent the X-ray diffraction patterns of the WC powder after ball milling, the pre-sintered WC sample, and the hot-pressed sintered WC sample, respectively.

 

From Figure 6(a), it can be seen that the WC powder remained unchanged (still WC) after ball milling. However, compared with the original WC powder, the diffraction peaks of WC were significantly weakened. This indicates that the WC powder particles had been refined after ball milling, which is consistent with the aforementioned scanning electron microscope (SEM) observation results. After pre-sintering, the sample was completely composed of WC, and no oxidation or decarburization occurred (Figure 6(b)).

Figure 6(c) shows the X-ray diffraction pattern of the ball-milled WC powder after hot-pressing sintering under the conditions of 1200–1300°C, 140 MPa, and 10 seconds. As can be seen from the figure, the sample was mostly WC, with a small amount of Fe₃W₃C formed simultaneously. As mentioned earlier, the ball-milled WC powder contained trace amounts of Fe and Cr elements (Figure 2). During the high-temperature sintering process, Fe may react chemically with WC to form Fe₃W₃C; however, the small amount of impurities did not affect the microstructural properties of the cemented carbide after sintering.

 

Microhardness and Density Tests

The microhardness values of the samples obtained by pressure sintering of the original WC powder and the ball-milled WC powder are shown in the attached table. Among them, the average hardness of the WC cemented carbide prepared from the original WC powder was 939 HV, while the average hardness of the WC cemented carbide prepared from the ball-milled WC powder reached 2294 HV. Therefore, the finer the powder particles, the higher the hardness of the sample after hot-pressing sintering.

In addition, the relative density of the two types of samples was tested using the Archimedes drainage method. As shown in the attached table, the relative density of the sample sintered from the original WC powder was relatively low, only 73.1%; in contrast, the relative density of the sample sintered from the ball-milled WC powder was relatively high, reaching 96.7%.

A comparison between Figure 3 and Figure 4 reveals that the sample sintered from the original WC powder was relatively loose with incomplete bonding, whereas the sample sintered from the ball-milled WC powder exhibited relatively dense bonding. Therefore, the difference in the metallurgical bonding status of the two samples led to the variation in their relative densities— the finer the powder particles, the higher the density of the sample after hot-pressing sintering.

The test results of microhardness and relative density indicate that the finer the WC powder particles, the better the performance of the cemented carbide after sintering. Refining powder particles is an effective method to improve the performance of WC cemented carbides.

Attached Table: Hardness and Density of Hot-Pressed Sintered Samples from Different Powders

 

Powder Type	Preforming Pressure (MPa)	Hot-Pressing Pressure (MPa)	Microhardness (HV)	Relative Density (%)
As-Received WC Powder	500	140	939	73.1
Ball-Milled WC Powder	500	140	2294	96.7

 

Sintering is a heat treatment process that forms metallurgical bonding between powder particles at a temperature lower than the melting point of the main component. According to whether a liquid phase appears during the sintering process, sintering is generally divided into two major categories: solid-phase sintering and liquid-phase sintering.

The sintering of binder-free WC cemented carbides belongs to solid-phase sintering, which is usually carried out at a temperature ranging from 2/3 to 4/5 of its melting point. However, the melting point of WC is as high as 2800°C, making it difficult to achieve binder-free sintering below 1300°C. The relatively high surface energy and distortion energy of powder particles are the driving forces for powder sintering . The surface of powder particles usually has high surface energy, which increases rapidly as the powder particles are refined; during ball milling and pressing deformation, the powder particles also generate distortion energy due to deformation.

Foreign scholars have also found similar research results in the ball milling treatment of WC . Therefore, after ball milling treatment, the refinement of the original WC powder particles in this study also enables them to accumulate high surface energy; the deformation of powder particles during the ball milling process makes them accumulate relatively high distortion energy, and a decrease in melting point may also occur. All these factors are conducive to the formation of metallurgical bonding between WC powder particles during the sintering process.

In addition, carbon content has a significant impact on the microstructural properties of WC cemented carbides. The X-ray diffraction patterns of the ball-milled WC powder, pre-sintered WC sample, and hot-pressed sintered WC sample do not contain W or W₂C, indicating that no oxidation or decarburization occurred. This suggests that under the argon atmosphere protection conditions adopted in this study, the oxidation and decarburization phenomena (which lead to the transformation of WC into W or W₂C) have been effectively suppressed. Therefore, the process exploration in this study regarding the sintering of binder-free WC cemented carbides will provide valuable technical references for the industrial application of WC cemented carbides.

 

Conclusions

• The WC powder particles were refined through the ball milling process. After hot-pressing sintering, a binder-free WC cemented carbide with a dense structure was formed. The microhardness of the sample reached 2294 HV. X-ray analysis results showed that the dense structure formed after hot-pressing sintering was WC cemented carbide.
• The ball milling treatment caused the original WC powder particles to accumulate high surface energy and large distortion energy, which was beneficial to the formation of metallurgical bonding between WC powder particles during the sintering process.