3.1. Evaluation of primary powders
Figure 1 shows the x-ray diffraction patterns for tungsten carbide, cobalt, and titanium carbide powders. As can be seen, the tungsten carbide powder used in this study has only mono-tungsten carbide (WC) peaks and peaks related to the W2C phase are not seen in this powder which indicates the high purity of tungsten carbide powder (in terms of phase analysis). Also, in this powder, the peaks related to the mono-tungsten carbide phase have the maximum intensities at the angles of 2θ = 41.635 and 2θ = 56.728, which are related to plates (100) and (101), respectively. It can be seen that the peaks position and the intensity of peaks in titanium carbide powder are very similar to the standard sample (JCPDS/ICDD: 00-032-1383). The peak at 2θ = 48.839 has a maximum intensity, which is related to the (200) plates. The x-ray diffraction patterns of cobalt powder are shown in Fig. 1. As can be seen, the used cobalt powder has two types of allotropic. These two allotropic types of cobalt are close-packed hexagonal (ε) and face-centered cubic (α) which are shown in Fig. 1. It has been reported that the close-packed hexagonal type of cobalt (ε cobalt) is approximately stable at temperatures below 400°C and the face-centered cubic type of cobalt (α cobalt) is stable at higher temperatures[21]. It has also been reported that cobalt powders used in various industries contain approximately the same amounts of both types of allotropy. However, during the grinding or crushing process, the amount of hexagonal cobalt powder increases[22].
Scanning electron microscopy images of tungsten carbide, cobalt, and titanium carbide powders are shown in Fig. 2. As can be seen, tungsten carbide powder has a spherical shape with rounded and almost irregular corners. At higher magnifications, these quasi-spherical particles with rounded corners are more clearly seen (Fig. 2-A). Also, due to the presence of gravitational forces between the particles, agglomerates are observed in the primary powder. For cobalt powder, the particles also accumulate in the form of agglomerates in certain areas (Fig. 2-B). Electron microscopy images of titanium carbide powder are also shown in Fig. 2. As can be seen, the morphology of this powder is in the form of edged particles with sharp corners.
The results of particle size distribution analysis for tungsten carbide powder are shown in Fig. 3. It is observed that the particle size distribution for tungsten carbide powder is approximately 3 to 5 µm and most particles have a size of approximately 4 µm. It is also observed that 3.98% of the particles are smaller than 10 µm and 6.76% of the particles are smaller than 5 µm (d50 = 3.427µm).
3.2. Densification process and sintering phenomena
The variation of temperature and displacement (shrinkage) with time during the SPS process is shown in Fig. 4 (for samples sintered at 1200 ℃ and 1400 ℃ for 10 minutes). As can be seen, for the sintered sample at 1200 ℃ (Fig. 4-A), very little displacement (shrinkage) occurred approximately before 1500 seconds. This is due to the change in the grain arrangement and the filling of large cavities as well as the growth of tungsten carbide and titanium carbide particles (grain growth densification)[23]. Also, with a further increase in temperature, the displacement (contraction) increases (Approximately after 1500 seconds).
For the sintered sample at 1400 ℃ (Fig. 4-B), a slight displacement occurred approximately before 1700 seconds. But after this time, with increasing temperature, the displacement increases rapidly. Viscous flow densification and Liquid phase sintering, as well as particle rearrangement, are the most important factors in increasing the displacement at higher temperatures for these hard materials.
Furthermore, as shown in Fig. 4, the displacement for the sintered sample at 1400 ℃ is higher than for the sintered sample at 1200 ℃. This is due to the increased fluidity of the cobalt and the filling of porosity[24, 25].
It has been reported that during the heating process of powders, the volume decreases with increasing densification [23]. According to the SPS sintering mechanism, when the spark current flows through the powders, the temperature rapidly increases at the point of contact between the particles. As a result, according to the Joule Heating Effect, a temperature gradient is created from the center to the surfaces of the powder particles, and in a very small area, the temperature rises sharply. Cobalt melting also occurs at much lower temperatures, which leads to densification and filling of porosity [27].
3.4. Microstructure, density, and mechanical properties
Figure 6 andFigure 7 show electron microscopy images of samples sintered at temperatures of 1200℃, 1300℃, and 1400℃. As can be seen, at a temperature of 1200℃ some tungsten carbide particles remain circular. As the sintering temperature increases to 1300℃, some circular particles are completely removed and angular particles with sharp edges of tungsten carbide are seen. It is also observed that by performing the sintering process at 1400℃, the particles also tend to grow abnormally. It has been reported that tungsten carbide particles grow and become larger in the sintering process by dissolution and deposition mechanism. Smaller particles dissolve in the binder phase and then begin to precipitate, and this phenomenon causes the smaller particles to disappear and the larger particles to grow[29]. At a temperature of 1200℃, very fine particles of tungsten carbide can be seen, but with increasing sintering temperature to 1400℃, the amount of these fine particles decreases, which is due to the dissolution of these particles in the cobalt matrix and finally the growth of tungsten carbide grains.
The particle size distribution analysis of tungsten carbide for sintered samples at different temperatures is also shown in Fig. 7.
As can be seen, the average particle size of tungsten carbide was obtained for sintered samples is 0.2, 0.3, and 0.7 µm for samples sintered at 1200, 1300, and 1400℃, respectively. Therefore, sintering temperature can affect the average particle size of tungsten carbide. In order to control the grain size of tungsten carbide, the addition of cubic carbides in this type of material has been proposed to improve some properties during the sintering process[30]. It is also very important to control the sintering temperature and determine the optimal sintering temperature.
It has been reported that there are many parameters to increase porosity in 94wt% WC − 3wt% TiC − 6wt% Co hard materials. Among the most important of these factors are the presence of impurities, lack of homogeneity during mixing, trapped gases and sintering temperature. which prevents filling between the carbide particles by the binder phase[31].
In addition, as can be seen in these hard materials, tungsten carbide particles are dissolved in cobalt and the outer layers of the surfaces related to titanium carbide particles. The microstructure of these materials shows that parts of the tungsten carbide phase are replaced by the core-shell phase of TiC/(Ti, W)C. These observations have been reported in other studies[26, 28]. Figure 8 shows a schematic of the particles in these hard materials.
The density of sintered samples at different temperatures of 1200, 1300, and 1400°C is shown in Fig. 9. It can be seen that the apparent density of the samples increases with increasing sintering temperatures from 1200 to1300°C. This is due to increased cobalt phase fluidity and capillary pressure. Therefore, the porous is filled, and the amount of empty spaces between particles in the sample is reduced and the density is improved[11]. It has been reported that the wettability between tungsten carbide, titanium carbide and cobalt is highly dependent on the sintering temperature and wettability increases with increasing sintering temperature and sintering time. Increasing the atomic and liquid phase diffusion at the sintering temperature and decreasing the viscosity at high temperatures facilitate the movement of the liquid phase and leads to the filling of the pores and the improvement of the density[32]. As shown in Fig. 9, the apparent density decreases with increasing sintering temperature from 1300 to 1400°C. It has been reported that the density decreases with the excessive increase of sintering temperature by evaporation of Co[33]. Therefore, the use of the SPS method due to the short sintering time increases the density of samples compared to other sintering methods[34].
Figure 10 shows the increase of microhardness with the increase in sintering temperature from 1200 to 1400°C. For the sintered samples at 1200℃, 1300℃, and 1400℃, the microhardness values were 1746.41HV, 2094.34HV, and 2280.97HV, respectively. Therefore, the mechanical properties of 94wt% WC − 3wt% TiC − 6wt% Co hard materials depend on the size of tungsten carbide grains after the sintering process. Inhibiting the growth of tungsten carbide grains and reducing the grain size of tungsten carbide, increases the hardness of these materials[26]. Due to the higher amount of WC and higher its modulus of elasticity rather than cobalt and titanium carbide, when an external force is applied to these materials, a very large amount of this force is applied to the tungsten carbide grains. Therefore, the hardness of these materials depends on the grain size of tungsten carbide. It has been reported that when the cobalt amount in these materials is constant, there is a linear relationship between hardness and the grain size of tungsten carbide[35].
Although, titanium carbide has been reported to improve the hardness and abrasion resistance of these materials, but reduces the toughness[36]. Furthermore, creating a solid solution between titanium carbide particles and tungsten carbide and creating a core-shell structure including TiC-core/(W, Ti)C-shell can increase the hardness[26].
Two factors affect the growth mechanism of tungsten carbide grains with increasing sintering temperature. The first factor is the increase in cobalt phase fluidity with increasing sintering temperature. Atomic mobility in the cobalt phase increases with increasing sintering temperature, which causes the pore in samples to fill faster. This factor increases the strength of the sample by increasing the density. The second factor is the increase in the diffusion coefficient of carbon and tungsten atoms in the cobalt phase with increasing sintering temperature. This factor also increases the growth rate of tungsten carbide grains. These two factors simultaneously affect the growth of tungsten carbide grains, and increasing the effect of one is associated with decreasing the effect of the other[11].
Electron microscopy images of the fracture surface of sintered samples at 1200, 1300, and 1400°C are shown in Fig. 11. As can be seen, the particle size increases with increasing sintering temperature from 1200 to 1400°C. Particles with abnormal size are also shown in Fig. 11-C. It is observed that with increasing sintering temperature to 1300°C, the porosity decreases. These results are consistent with the results for density(Fig. 9).
Investigating the fracture surface of the sintered samples at different temperatures indicates that the fracture mode in 94wt% WC − 3wt% TiC − 6wt% Co hard material is the brittle intergranular mode, and coarse WC fracture can be observed from the fracture surface. Since coarse WC grain can easily lead to stress concentration under loading conditions, so the micro crack generally originates from the rupture of coarse WC and the phase interface between coarse WC and binder. It is seen that the percentage of intergranular fracture is much higher than the percentage of transgranular fracture.
It is reported that the high intergranular fracture percentage in these materials is due to the high Young's modulus and the specific crystalline structure of tungsten carbide[37–39]. According to the Griffith-Orowan strength theory, the grain boundary strength between two tungsten carbide grains is 0.35 times the cleavage fracture strength of the tungsten carbide grain on the plate (100). And when a force is applied, the stress at the boundary between two tungsten carbide grains is greater than the stress at the tungsten carbide grain. This is the main reason for the higher percentage of intergranular fracture in this type of material[40].
It has been reported that very little plastic deformation occurs in these hard metals due to the fracture originating from micro-pores and coarse grains of tungsten carbide. The fracture of the sample occurs at the interface between the WC and Co phases and the crack grows through the Co phase. Therefore, the crack propagation is reduced by hard phases, and the fracture mode is observed as a brittle and intergranular fracture[40]. As can be seen in Fig. 11, the fracture morphology in these materials is relatively flat and the direction of cracks growth is relatively constant. Smooth plates caused by cleavage fracture create a lot of reflection. In all the specimens that were sintered at different temperatures, the fracture surfaces had a large reflection, and these surfaces were visible. Therefore, it can be concluded that fracture in cemented carbide with 6 wt% cobalt and 3 wt% of titanium carbide is a cleavage type.
Cleavage fracture is a brittle fracture that occurs along with certain crystallographic plates and is usually associated with small fracture energy. Cleavage sheets are the characteristics of any crystal lattice. Cleavage fracture has been observed in the structure of hcp, that tungsten carbide is one of the materials that has hcp structure[39].