**3. Results and discussion**

#### **3.1 Structural characterization**

XRD patterns of the WC10wt.%Co samples are presented in Fig. 2. As expected, the results showed that at room temperature (Fig. 2a) the mixed powder is composed of the crystalline phases WC and Co. The low intensity of the peaks of cobalt may be related to Cu-Kα radiation used. When the mixed powder is sintered under high pressure and high temperature (5.5 GPa and 1400 ºC), as shown in Fig. 2b, characteristic peaks of WC, Co and Co3W3C are identified. In addition, a small amount of the y phase was also identified.

Fig. 2. XRD patterns of the WC10wt.%Co: a) before sintering; and b) after sintering.

The XRD analysis showed the formation of intermediate phases such as Co3W3C and y. These phases formed are undesired because they are harmful to the mechanical properties of cemented carbide. The appearance of these phases during sintering is consistent with the binary diagram of WC-Co (Silva, 1996). It is possible that these phases can be related to the

XRD patterns of the WC10wt.%Co samples are presented in Fig. 2. As expected, the results showed that at room temperature (Fig. 2a) the mixed powder is composed of the crystalline phases WC and Co. The low intensity of the peaks of cobalt may be related to Cu-Kα radiation used. When the mixed powder is sintered under high pressure and high temperature (5.5 GPa and 1400 ºC), as shown in Fig. 2b, characteristic peaks of WC, Co and Co3W3C are identified. In addition, a small amount of the y phase was also identified.

**20 30 40 50 60 70 80 90**

**2**θ **(Degree)**

**20 30 40 50 60 70 80 90**

**2**θ **(Degree)**

The XRD analysis showed the formation of intermediate phases such as Co3W3C and y. These phases formed are undesired because they are harmful to the mechanical properties of cemented carbide. The appearance of these phases during sintering is consistent with the binary diagram of WC-Co (Silva, 1996). It is possible that these phases can be related to the

**Co3W3C**

γ

**Co3W3C Co3W3C**

γ

Fig. 2. XRD patterns of the WC10wt.%Co: a) before sintering; and b) after sintering.

**WC**

**WC**

**Co**

**WC**

**WC**

**WC**

**WC**

**WC**

**WC**

**Co3W3C**

**Co3W3C**

**Co3W3C**

**b)**

**a)**

**Intensity (a.u.)**

**Intensity (a.u.)**

**WC WC**

**WC WC WC WC**

**WC**

**WC**

**WC**

**WC**

**3. Results and discussion 3.1 Structural characterization**  following factors: i) sensitivity of the WC to the loss of carbon as consequence of their low formation energy; ii) sintering atmosphere with characteristic oxidant; and iii) the presence in the starting powders of compounds that react with the carbide, consumining the carbon. Another important aspect that must be considered is that the HPHT sintering process is very fast. This can take the carbon to react with the adsorbed oxygen also quickly, resulting in decrease or loss of carbon of the WC. The consequence is not having a good dissolution of free carbon in the liquid phase, so new phases can occur by diffusion.

Figures 3 and 4 show the X-ray diffraction patterns of the cemented carbide samples doped com rare-earth elements.

Fig. 3. XRD pattern of the WC10wt.%Co doped with 2 % of La2O3 sintered under HPHT.

Fig. 4. XRD pattern of the WC10wt.%Co doped with 2 % of CeO2 sintered under HPHT.

As can be observed, the samples doped with rare-earth elements (La2O3 (Fig. 3) and CeO2 (Fig. 4)) had a phase composition very similar to that of the rare-earth free samples. It is noticed that only small differences in the peak intensities occurred. This means that the

High Pressure Sintering of WC-10Co Doped with Rare-Earth Elements 387

According to the literature (Arbilla et al., 1996), the manufacturing of cemented carbide (WC10wt.%Co) is basically the liquid phase sintering of powder mixture of tungsten carbide and cobalt at a temperature of approximately 1400 ºC. At this temperature, the cobalt becomes liquid and diffuses into the structure and, after cooling, the desired properties are obtained. In this work, however, an unusual process was used for the production of

The results in Fig. 5 indicated that the WC10wt.%Co-free rare-earth elements showed a low value of apparent density (10.56 g/cm3), reaching a densification of about 73 % (Fig. 6). Whereas the samples studied were subjected to the HPHT process, it was expected a higher value. Furthermore, this value is lower than in the cemented carbide densification obtained

The sintering when performed via the HPHT process tends to promote greater adhesion between the particles and higher densification in the material. However, the actions of certain factors must be taken into account because they can lead to some variations such as (Rodrigues et al., 2006; Bobrovnitchii, 2001): i) the presence of gradients of temperature and pressure in the compression chamber of the DAP may result in residual stresses and cracks in the samples; and ii) the rapid reduction of pressure and temperature can cause defects in the samples.

It can also be seen in Figure 6 that the samples containing rare-earth element showed a high densification. In particular, the samples with higher degree of densification were those containing 2 % of La2O3 (96.63 %) and 0.5 % of CeO2 (93.10 %) in relation the cobalt phase. Importantly, all cemented carbide samples obtained had the same procedure of manufacture. This fact confirms that the rare-earth elements incorporated into the WC10wt.%Co contribute effectively to the high densification of the pellets sintered under HPHT conditions. This result is consistent with the literature (Xu et al., 2001; Gomes, 2004). According to the literature (Ji et al., 1996), the addition of rare-earth element leads to decreased porosity of the cemented carbide. As the cemented carbide is a fragile PM material, porosity always causes great tension, which is a source of fracture. There is a direct relationship between mechanical strength and porosity of the cemented carbide, in which fewer pores and smaller pore size results in higher mechanical strength. During the sintering cycle, gaseous impurities, for example, oxygen and sulfur in cemented carbide is partially released in gaseous form. If they are not partially removed such defects as pores will take shape. Since the form of sulfide and oxide compounds of rare earths, which are stable at high temperatures, gaseous impurities are reduced and thus reduce the possible formation of pores. However, reducing the temperature required for the liquid phase by the addition of rare-earth element implies that under the same conditions of sintering, the sintering time by liquid phase has been extended, which leads to significant displacement of material, filling of pores with the liquid phase and decrease in number and size of pores. This fact is combinated with the effect of high pressure and high temperature that promote

Figures 7 and 8 present the values of coercive force (Hc) of the cemented carbide produced via HPHT as a function of the rare-earth elements added. The results show that the addition of rare-earth elements in the structure of the cemented carbide influences the coercive field.

cemented carbide doped with rare-earth elements.

in industrial routines (Gomes et al., 2004).

the filling of empty spaces by the binder metal.

**3.3 Coercive force** 

formation of these intermediate phases is not associated with the rare-earth elements incorporated in the cemented carbide studied.

#### **3.2 Densification**

The apparent density of the pellets of WC10wt.%Co doped with rare-earth elements sintered under HPHT is shown in Fig. 5. It can be observed that the samples containing rareearth elements showed higher apparent density. The highest values were obtained for samples containing 2 % La2O3 (14.00 g/cm3) and 0.5 % CeO2 (13.53 g/cm3).

Fig. 5. Apparent density of the cemented carbides sintered under HPHT.

Fig. 6. Relative density of the cemented carbides sintered under HPHT.

The densification behavior of the cemented carbide as a function of the content of rare-earth elements is shown in Fig. 6. The value of theoretical density for the cemented carbide used (WC10wt.%Co) was 14.54 g/cm3.

formation of these intermediate phases is not associated with the rare-earth elements

The apparent density of the pellets of WC10wt.%Co doped with rare-earth elements sintered under HPHT is shown in Fig. 5. It can be observed that the samples containing rareearth elements showed higher apparent density. The highest values were obtained for

**-0.5 0.0 0.5 1.0 1.5 2.0 2.5**

**Rare-earth content (%)**

**-0.5 0.0 0.5 1.0 1.5 2.0 2.5**

**Rare-earth content (%)**

The densification behavior of the cemented carbide as a function of the content of rare-earth elements is shown in Fig. 6. The value of theoretical density for the cemented carbide used

samples containing 2 % La2O3 (14.00 g/cm3) and 0.5 % CeO2 (13.53 g/cm3).

**La2O3 CeO2**

Fig. 5. Apparent density of the cemented carbides sintered under HPHT.

**La2O3 CeO2**

Fig. 6. Relative density of the cemented carbides sintered under HPHT.

incorporated in the cemented carbide studied.

**10**

**70**

(WC10wt.%Co) was 14.54 g/cm3.

**75**

**80**

**85**

**Relative density (%)**

**90**

**95**

**11**

**12**

**Apparent density (g/cm3)**

**13**

**14**

**15**

**3.2 Densification** 

According to the literature (Arbilla et al., 1996), the manufacturing of cemented carbide (WC10wt.%Co) is basically the liquid phase sintering of powder mixture of tungsten carbide and cobalt at a temperature of approximately 1400 ºC. At this temperature, the cobalt becomes liquid and diffuses into the structure and, after cooling, the desired properties are obtained. In this work, however, an unusual process was used for the production of cemented carbide doped with rare-earth elements.

The results in Fig. 5 indicated that the WC10wt.%Co-free rare-earth elements showed a low value of apparent density (10.56 g/cm3), reaching a densification of about 73 % (Fig. 6). Whereas the samples studied were subjected to the HPHT process, it was expected a higher value. Furthermore, this value is lower than in the cemented carbide densification obtained in industrial routines (Gomes et al., 2004).

The sintering when performed via the HPHT process tends to promote greater adhesion between the particles and higher densification in the material. However, the actions of certain factors must be taken into account because they can lead to some variations such as (Rodrigues et al., 2006; Bobrovnitchii, 2001): i) the presence of gradients of temperature and pressure in the compression chamber of the DAP may result in residual stresses and cracks in the samples; and ii) the rapid reduction of pressure and temperature can cause defects in the samples.

It can also be seen in Figure 6 that the samples containing rare-earth element showed a high densification. In particular, the samples with higher degree of densification were those containing 2 % of La2O3 (96.63 %) and 0.5 % of CeO2 (93.10 %) in relation the cobalt phase. Importantly, all cemented carbide samples obtained had the same procedure of manufacture. This fact confirms that the rare-earth elements incorporated into the WC10wt.%Co contribute effectively to the high densification of the pellets sintered under HPHT conditions. This result is consistent with the literature (Xu et al., 2001; Gomes, 2004).

According to the literature (Ji et al., 1996), the addition of rare-earth element leads to decreased porosity of the cemented carbide. As the cemented carbide is a fragile PM material, porosity always causes great tension, which is a source of fracture. There is a direct relationship between mechanical strength and porosity of the cemented carbide, in which fewer pores and smaller pore size results in higher mechanical strength. During the sintering cycle, gaseous impurities, for example, oxygen and sulfur in cemented carbide is partially released in gaseous form. If they are not partially removed such defects as pores will take shape. Since the form of sulfide and oxide compounds of rare earths, which are stable at high temperatures, gaseous impurities are reduced and thus reduce the possible formation of pores. However, reducing the temperature required for the liquid phase by the addition of rare-earth element implies that under the same conditions of sintering, the sintering time by liquid phase has been extended, which leads to significant displacement of material, filling of pores with the liquid phase and decrease in number and size of pores. This fact is combinated with the effect of high pressure and high temperature that promote the filling of empty spaces by the binder metal.
