**4. Composite manufacturing**

Preparation of dense particulate composite bodies with randomly distributed inclusions meets potential difficulties during composite powder preparation and during sintering. The most popular method of second phase dispersion in the matrix is simple mixing. This method is widely used for zirconia/WC system [26-28]. The mixing process utilizing intensive mills (atrittors, rotation-vibrational mills) in short time assures the proper tungsten carbide particles distribution within the matrix in the wide range of WC content 10 – 50 vol. %. More sophisticated methods as for instance decomposition of organic WC precursors are nowadays too expensive for wider application [23, 29, 30].

Tungsten Carbide as an Reinforcement in Structural Oxide-Matrix Composites 89

Al2O3 ZrO2 WC

Commercial powders were used as a starting materials (alumina – TM-DAR Taimicron, zirconia – Tosoh 3Y-TZ, tungsten carbide – Baildonit). Powders homogeneity was assured

Materials for test were fabricated by hot-pressing technique (HP) due to guarantee the maximum of the densification of investigated samples. The sintering conditions were as follow: the maximum temperature - 1500°C (for zirconia and zirconia/WC composite) and 1650°C (for alumina and alumina/WC composite) with 1 hour soaking time and maximum

A typical SEM microstructures of hot-pressed composites were showed in the Figs. 7 and 8.

by 30 min. of attrition mixing of constituent powders in ethyl alcohol.

Table 2 collects data about the grain size of individual phases.

Material Mean grain size, μ<sup>m</sup>

**Table 2.** The mean grain size of phases existing in sinters containing 10 vol. % of WC.

**Figure 7.** The typical SEM image of thermally etched ZrO2/WC composite microstructure.

Alumina 5.20 ±2.90 - - Alumina/10vol.% WC 1.25 ±0.80 - 0.45 ±0.30 Zirconia solid solution - 0.32 ±0.18 - Zirconia/10vol.% WC - 0.27 ±0.15 0.47 ±0.35

applied pressure - 25 MPa.

In alumina/WC composite system the mechanical mixing is also the main preparation method of the composite powder. Anyway, there were some experiments [23] utilizing selfpropagating high temperature synthesis (SHS) process for *in-situ* synthesis of alumina/tungsten carbide composite powder. In this process both tungsten carbides (WC, W2C) were present in the product.

Sintering of composites with oxide matrices and dispersed WC particles is a typical example of sintering with "rigid inclusions" widely described in literature [31-33]. In fact, during this process, diffusional mechanisms of densifications appear only in the oxide matrix. The presence of carbide particles makes the sintering driving forces much weaker. This effect is as stronger as higher is tungsten carbide particles volume content. The high relative density demand for structural applications (> 97 % of theo.) can be assured using pressureless sintering method when WC content not exceed 20 vol. %. Additionally, sintering temperature in this case must be relatively high (1550C for zirconia and 1600C for alumina). It is not profitable for sinters microstructure because the grain growth phenomenon. The inert sintering atmosphere demanded for preservation of WC from oxidation at high temperatures causes some additional factor of stabilization in zirconia [22, 34].

Practically, the most often sintering method for both type of composites is hot-pressing technique. Application of this method allows to assure high densities (> 98 % of theo.) in relatively short time (30 – 60 min.). Such conditions limits the grain growth in the matrix (see Table 3).

There were some investigations utilizing pulsed electric current sintering (PECS) for zirconia/WC composite densification [27]. These methods were profitable when WC content in the composite was relatively high (~30-40 vol. %).
