**3.3 Coercive force**

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.

High Pressure Sintering of WC-10Co Doped with Rare-Earth Elements 389

The measures of coercive force of the sintered pellets indicated an increase in the coercive field of the WC10wt.%Co doped with rare-earth elements. This increase in the coercive field may have been influenced by the WC grain size resulting from the refinement of the microstructure caused by the rare-earth elements. It is reported in the literature (Xu et al., 2001) that these elements inhibit the grain growth of WC thereby making thinner. The values of Hc increase with the refinement of microstructure, because this makes it difficult to magnetization of the material in the direction of the applied field. Thus, it is necessary to generate higher coercive force to magnetize the WC10wt.%Co. The high value of Hc indicates that the cemented carbide studied corresponded to the class of magnetically hard materials, the application it is intended for light machining. The results also showed that the lanthanum oxide (La2O3) was the most effective in increasing the coercive field of the WC10wt.%Co. The sample with 0.5 %of lanthanum oxide reached the highest value of Hc (14.0 – 20.8 kA/m). In the case of addition of cerium oxide (CeO2), the sample with 0.5 % of

The behavior of the axial compressive strength as a function of the rare-earth content of the

**0.0 0.5 1.0 1.5 2.0**

**Rare-earth content (%)**

The effect of the addition of rare-earth elements was to increase the compressive strength of the pieces of WC10wt.%Co. This is mainly associated with increased densification of the cemented carbide containing rare-earth during the sintering process. In addition, the rareearth additive can inhibit the martensitic phase transformation of Co phase (ε-Co), which weakens the material resulting in the increase of α-Co (CFC) that contributes directly to

The results of Figure 9 also showed different behavior for the axial compressive strength, depending on the rare-earth added. The highest values of mechanical strength of the

Fig. 9. Axial compressive strength of the cemented carbides sintered under HPHT.

CeO2 reached the highest value of Hc (9.2 – 18.5 kA/m).

pellets of cemented carbides sintered under HPHT is shown in Fig. 9.

**La2O**3 **CeO2**

**3.4 Axial compressive strength** 

**0**

increased mechanical strength (Xu et al., 2001).

**200**

**400**

**600**

**Axial compressive strength (MPa)**

**800**

**1000**

This magnetic property is very sensitive to microstructure and chemical composition of the cemented carbide.

Fig. 7. Coercive force of the cemented carbides doped with lanthanum oxide sintered under HPHT.

Fig. 8. Coercive force of the cemented carbides doped with cerium oxide sintered under HPHT.

As observed in Figs 7 and 8 there is variability in the values obtained for the coercive force. However, this variability is also observed in the literature (Brookes, 1998) for the cemented carbide. The coercive force values obtained in this work are also within the range of Hc for the cemented carbide (WC10wt.%Co).

The measures of coercive force of the sintered pellets indicated an increase in the coercive field of the WC10wt.%Co doped with rare-earth elements. This increase in the coercive field may have been influenced by the WC grain size resulting from the refinement of the microstructure caused by the rare-earth elements. It is reported in the literature (Xu et al., 2001) that these elements inhibit the grain growth of WC thereby making thinner. The values of Hc increase with the refinement of microstructure, because this makes it difficult to magnetization of the material in the direction of the applied field. Thus, it is necessary to generate higher coercive force to magnetize the WC10wt.%Co. The high value of Hc indicates that the cemented carbide studied corresponded to the class of magnetically hard materials, the application it is intended for light machining. The results also showed that the lanthanum oxide (La2O3) was the most effective in increasing the coercive field of the WC10wt.%Co. The sample with 0.5 %of lanthanum oxide reached the highest value of Hc (14.0 – 20.8 kA/m). In the case of addition of cerium oxide (CeO2), the sample with 0.5 % of CeO2 reached the highest value of Hc (9.2 – 18.5 kA/m).

#### **3.4 Axial compressive strength**

388 Sintering of Ceramics – New Emerging Techniques

This magnetic property is very sensitive to microstructure and chemical composition of the

**0.0 0.5 1.0 1.5 2.0**

Fig. 7. Coercive force of the cemented carbides doped with lanthanum oxide sintered under

**0.0 0.5 1.0 1.5 2.0**

As observed in Figs 7 and 8 there is variability in the values obtained for the coercive force. However, this variability is also observed in the literature (Brookes, 1998) for the cemented carbide. The coercive force values obtained in this work are also within the range of Hc for

Fig. 8. Coercive force of the cemented carbides doped with cerium oxide sintered under

**Cerium oxide content (%)**

**Lanthanium oxide content (%)**

**Coercive force (kA/m)**

the cemented carbide (WC10wt.%Co).

**Coercive force (kA/m)**

cemented carbide.

HPHT.

HPHT.

The behavior of the axial compressive strength as a function of the rare-earth content of the pellets of cemented carbides sintered under HPHT is shown in Fig. 9.

Fig. 9. Axial compressive strength of the cemented carbides sintered under HPHT.

The effect of the addition of rare-earth elements was to increase the compressive strength of the pieces of WC10wt.%Co. This is mainly associated with increased densification of the cemented carbide containing rare-earth during the sintering process. In addition, the rareearth additive can inhibit the martensitic phase transformation of Co phase (ε-Co), which weakens the material resulting in the increase of α-Co (CFC) that contributes directly to increased mechanical strength (Xu et al., 2001).

The results of Figure 9 also showed different behavior for the axial compressive strength, depending on the rare-earth added. The highest values of mechanical strength of the

High Pressure Sintering of WC-10Co Doped with Rare-Earth Elements 391

The results of Figure 11 also showed that the highest values of axial compressive elasticity modulus of the cemented carbide doped with rare-earth elements were obtained for the

Figure 12 shows the values of microhardness for the pellets of WC10wt.%Co doped with rare-earth elements. The results show that the microhardness increases with the addition of rare-earth elements. This increase may be due to increased densification with the rare earth added. On the other hand, the complex variation in microhardness values may have been influenced by the geometric irregularity at the top and bottom of the samples caused by the

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

**Rare-earth content (%)**

It can be seen in Fig. 12 an increase in microhardness when 0.5 % of La2O3 is added. A decrease in microhardness occurs with the addition of 1 % of La2O3. With the addition of 1.5 % of La2O3, the microhardness increases again and back to decrease with 2 % of La2O3. For cemented carbide containing cerium oxide, there is an increase with the addition of 0.5 % and a subsequent decrease between 1 to 1.5 %. An increase occurs again with the addition of 2 % of CeO2. It can also be observed that the highest values of microhardness of the cemented carbide doped with rare-earth elements were obtained for the samples containing

Table 3 presents the values of wear resistance for the pellets of cemented carbide doped with rare-earth elements obtained under HPHT conditions. The results indicated that all samples of cemented carbide containing rare-earth elements showed less mass loss compared to the reference sample (WC10wt.%Co). This was expected, since the decrease in grain size caused by the rare-earth elements and also the positive influence of high pressure leads to increased

samples containing 1 % of La2O3 (9095 MPa) and 1 % of CeO2 (8573 MPa).

**La2O3 CeO2**

Fig. 12. Microhardness of the cemented carbides sintered under HPHT.

**3.6 Microhardenss** 

sintering process as previously described.

0.5 % of La2O3 (2609 HV) and 0.5 % of CeO2 (1979 HV).

microhardness and, consequently, to reduce the wear.

**3.7 Wear resistance** 

**Microhardness (kgf/cm2)**

cemented carbide doped with rare-earth elements were obtained for the samples containing 1 % of La2O3 (1055 MPa) and 1 % of CeO2 (943 MPa). In addition, the complex variation observed for the mechanical strength may be associated with the fact that some samples did not show regular cylinder geometry (Fig. 10).

Fig. 10. Appearance of the pellets of WC10wt.%Co obtained via HPHT.
