**1. Introduction**

378 Sintering of Ceramics – New Emerging Techniques

Zdujić, M., Jovalekić, Č., Poleti, D., Veljković, I. & Karanović, L., Journal of Non-Crystalline

Zhi-hui, C., Jun-fu, Q., Cheng, L., Jian-ning, D. & Yuan-yuan, Z., Ceramics International, 36

Solids, 352 (2006) 3058.

(2010) 241.

The cemented carbides are usually known as the hard metals. They are typically two phase composite materials, in which a hard refractory carbide phase is bonded by an iron-group metal, usually metallic cobalt (Thümmler e Oberacker, 1993). This is due to its ability to wetting the carbide particles, which allows the agglomeration of particles, giving the material toughness. The tungsten carbide (WC) due to its metallic nature presents an excellent thermal conductivity. Thus, the cemented carbide is a typical example of composite material.

The properties of cemented carbides are related to a combination of the properties of individual constituents. Thus, the attractive properties of such materials are related to the carbide hardness combined with toughness of the binder metal. When compared to other systems, WC-Co exhibits the best combination of mechanical strength, wear resistance and good toughness (Schwarzkopf and Kieffer, 1986). Considering this fact, WC-Co is an unique system in this field of materials.

The demands of the industrial and economical development worldwide at the beginning of the twentieth century resulted in the development of the cemented carbide (Williams, 1998). Since that time the cemented carbide is a class of materials of great technological importance, whose main applications are in machining operations and as tooling for metal forming or cutting. In addition, the cemented carbides can also be applied in drawing dies, drawing, stamping and metal forming, mining and wear-resistant pieces. Thus, these materials are applied in those activities that require high abrasion resistance, hardness and impact resistance, such as heavy machining and drilling.

The properties of the cemented carbides can be modified from changes in the proportions of the materials used in its manufacture. There are several cemented carbide compositions in which the cobalt phase can vary from 3 to 30 wt.%. However, the most usual composition and of high interest in this chapter is that based on 90 wt.% tungsten carbide (WC) and 10 wt.% cobalt (Co) (German, 1994).

The cemented carbide is conventionally produced by powder metallurgy processing via liquid phase sintering. The elemental powders are comminuted, mixed, compacted, and

High Pressure Sintering of WC-10Co Doped with Rare-Earth Elements 381

The mixed powders initially were submitted to a single action die-compaction step using a hydraulic press (Danpresse, model DC 20) in a cylindrical 7 mm diameter steel die without lubrificants at 800 MPa. The compact dimensions were 7 mm in diameter and 7 mm in height. After uniaxial compaction, the consolidated samples were pressed under high

The high pressure compaction needs an apparatus that can stably generate a pressure of more than 1 GPa. A toroidal type high-pressure device was used in the experiments. Fig. 1 shows a typical drawing of the high-pressure device (Ramalho, 1998). This device allows the compaction tests in cylindrical powder samples until 7 mm of diameter and 9 mm of height. The high-pressure tests were carried out in a special hydraulic press (Ryazantyashpressmash, DO 138B model). The tests were carried out as follows: the precompacted mixed powder sample was placed without slackness into central aperture of the deformable capsule (Fig. 1). The deformable capsule is made of a calcite based powder compact (95 % CaCO3 + 5 % (SiO2, Al2O3, Fe2O3)). The calcite based material is a solid pressure medium well known in the field of ultrahigh pressure technology (Vianna et al., 2001). For effective heat conduction in the sample, the capsule was covered with two caps composed of 50 % of graphite and 50 % of calcite. A high pressure anvil made of hard metal was used as mould, where the capsule was fixed in its concavity, and then sealed with the outer side of the mould. Finally, the complete device was axially placed in the mobile table of the press. When the force is applied, the capsule is deformed with concomitant formation of a gasket and the generation of a high pressure. A high pressure of 5.5 GPa was applied to the samples. The high pressure into the compression chamber was transmitted to the cemented carbide powder samples by the capsule. The high pressure level reached was maintained for 30 s, and then the temperature was raised to 1400 ºC. These conditions of

La2O3 CeO2

Samples WC10wt.%Co Rare-earth

AL1 100.0 - - AL2 99.5 0.5 - AL3 99.0 1.0 - AL4 98.5 1.5 - AL5 98.0 2.0 - AL6 99.5 - 0.5 AL7 99.0 - 1.0 AL8 98.5 - 1.5 AL9 98.0 - 2.0

Table 2. Composition of the cemented carbides doped with rare-earths.

high pressure and high temperature were maintained for a time of 40 s.

**2.2 High pressure sintering of the cemented carbide** 

pressure and high temperature (HPHT).

sintered under various conditions. The processes commonly used are pressing/sintering, hot pressing, and sinter/hot isostatic pressing (HIP) (North et al., 1991; White, 1998). Recently, an unusual process for the production of cemented carbide using a high pressure technique has been investigated (Rodrigues et al., 2006). On the other hand, the effect of the rare-earth elements addition on the sintering behavior of cemented carbide has also been investigated. In particular, the beneficial effects of the rare-earth elements on the mechanical properties, microstructure and cutting performance have been extensively investigated (Li et al., 1986; Yao et al., 1987; Liang et al., 1989; Shan, 1990; Luo, 1991; Liu et al., 1992; Chenguang, 1992; Pan, 1993; Yang et al., 1993; Deng, 1993; Li et al, 1993; He et al., 1994; Li et al, 1994; Yan et al., 1995; Cheng and Yu, 1995; Yuan et al., 1995; Li, 1996; He, 1996; Ji et al., 1996; Xu et al., 2001; Gomes, 2004). However, the production of cemented carbide doped with rare-earth elements using a high pressure technique is still to be investigated.

This chapter focuses on the possibility of production of WC-10wt.%Co doped with rareearth elements under high pressure and high temperature (HPHT) conditions. Emphasis is given on the effects of the rare-earth additions on the densification behavior and physical and mechanical properties of the end product.
