Nanotechnology is an umbrella term for a wide range of technologies concerned with structures and processes of materials that have nanometer scale. Nanocomposites are one of those advanced materials that received much attention due to their unique and unusual properties that proposing them as promising candidates for several structural and wear resistance applications [1]. Nanocomposite materials are formed by dispersing nanocrystalline reinforcement ceramics into metallic matrix, leading to significant improvement in the mechanical and physical properties. It has been reported that both strength and fracture toughness are increased by the order of two to four times than conventional composite materials [2]. Among the transitionmetal carbides, WC has excellent high temperature strength and good corrosion resistance. It shows extremely high hardness value and possesses high values of Young’s modulus [3]. Due to its poor fracture toughness and the difficulties in powders consolidation to obtain fully dense compacts, WC is usually mixed with metallic binders, such as Co, Fe, and Ni to form so-called cemented carbides. WC-Co cements with different Co volume fractions ranging from 4% to 14% have been widely used for cutting tools and wear resistant materials. Mechanical mixing method, using ball milling of the reinforcement materials (WC) with several concentrations of Co (metallic matrix) shows significant advantage to obtain nanocomposite WC-Co powders [4,5]. The powders were then consolidated into bulk objects, using hot pressing technique. The hot-pressed WCCo powders show remarkable increase in the fracture toughness; however the existence of the metallic binding material leads to a decrease in the hardness and elastic module values. WC-Co cements have some industrial limitations because of the presence of metallic Co matrix (binder) leads to failure at high temperature due to softening. Many efforts have been carried out to achieve superior hardness and toughness combinations through replacing the metallic Co by different types of ceramic nanocrystalline materials to form ceramic matrices (WC nanocomposites) [6-8].

The present work has been addressed in order to study the influence of nanocrystalline ZrO₂ additives on improving the fracture toughness and Poisson’s ratio of mechanically mixed WC-ZrO₂ nanocomposites. The selection of ZrO₂ comes from the fact that it has a high thermal stability and excellent mechanical properties such as high bending strength and excellent fracture toughness. We are also proposing a powerful tool for obtaining fully dense nano-ceramic composites, using spark plasma sintering (SPS) technique for the mechanically mixed ceramic powders of WC-ZrO₂.

In the present study, elemental powders of WC (99.5%, 30 µm) were mixed with different selected volume fractions of ZrO₂ (2% Y₂O₃) powders (99.5%, 10 µm) of 0.5, 5, 10, 15 and 20 vol.%. The mixed powders of each ZrO₂ concentration were sealed in a cylindrical WC vial (250 ml in volume) together with fifty WC balls (10 mm in diameter) in a glove box under argon gas atmosphere. The ballto-powder weight ratio was maintained at the level of 10:1. The ball-milling experiments were carried out at room temperature, using Fritsch P5 high-energy ball mill at a rotation speed of 250 rpm. The milling experiments were interrupted at regular intervals and small amounts of the milled powders were taken out from the vial in the glove box. The powders were characterized by means of X-ray diffraction (XRD) with CuKα radiation, scanning electron microscope (SEM), transmission electron microscope (TEM) using 200 kV and/or high-resolution transmission electron microscopes (HRTEM).

The end product of the ball-milled nanocomposite powders (after 360 ks) at different ZrO₂ concentrations were individually consolidated into bulk samples, using spark plasma sintering (SPS) method. The consolidation procedure took place in vacuum at 1673 K with a pressure of 19.6 to 38.2 MPa. In order to avoid any undesired grain growth, the sintering process was applied for only 0.18 ks without adding any binding materials. The densities of consolidated WC/ZrO₂ materials were determined by Archimedes’ principle, using water immersion method. Vickers indenter with a load of 50 kg was employed to determine the hardness of the compacted samples. The size of the indentation cracks has been used to determine the fracture toughness (K_c) of the sample [9]. The hardness and K_c values reported below are averaged from at least ten indentations. The elastic properties of the bulk samples were determined by nondestructive test using pulse-echo overlap ultrasonic technique using ultrasonic detector.

XRD technique was employed to follow the structural changes that may occur during ball milling of hcp-WC with different volume fractions t-ZrO₂ powders and after the consolidation process that was achieved at 1673 K, using SPS technique. Figure 1(a) displays the XRD pattern of ball milled WC-10% ZrO₂ powders after 43 ks of the milling time. The powders at this early stage of milling still consist of coarse grains, indicated by the existence of sharp Bragg-peaks which are corresponding to the matrix and reinforcement materials of t-ZrO₂ and hcp-WC, respectively. Contrary, the XRD pattern of the final-product Figure 1(b), which was obtained after longer milling time (360 ks), shows a significant broadening in the Bragg lines for both ZrO₂ and WC materials, suggesting the formation of nanocomposite WC-ZrO₂. Figure 1(c) depicts the

XRD pattern of the final-product (360 ks) that was consolidated at 1673 K indicates the absence of any intermediate phase (s) other than WC and ZrO₂. The absence of any reacted phases during this sintering step implies the thermodynamic compatibility of WC and ZrO₂ at the applied consolidation temperature. Furthermore, there is no obvious dramatic change in the grain size of both the matrix and reinforcement materials can be detected after sintering, indicating that the consolidated sample maintains its nanocrystalline properties (Figure 1(c)).

Figure 2 shows the bright field image (BFI) of WC-10% ZrO₂ powders after ball milling for 43 ks (Figure 2(a)) and 360 ks (Figure 2(b)). The light gray region in Figure 2(a) shows the ZrO₂ matrix, whereas the dark coarse grains embedded into the matrix, present the WC grains. The WC grains that are heterogeneously distributed in the matrix have irregular shapes with a wide grain size distribution, ranging from 23 to 280 nm in diameter (Figure 2(a)). Obviously, the matrix material at this early stage of milling (43 ks) is either rich or poor with WC. Increasing the milling time (360 ks) leads to successive increase in the impact and shear forces that are generated by the grinding tools (balls) so that the brittle WC grains disintegrated into finer cells with an average diameter of 18 nm in diameter as shown in Figure 2(b). This dramatic disintegration causes an increase in the WC surface area, leading to the formation of nanocrystalline spherical lenses of WC, which are fairly distributed into the whole matrix material to form a homogeneous WC/ZrO₂ nanocomposite. The formation of these nanomaterials is attributed to the plastic deformation that is produced in the WC crystal lattice during the high-energy ball milling process and this occurs by slip and twinning in the lattice of the milled powders. Due to the successive accumulations of the dislocations density, the crystals are disintegrated into sub-grains that are initially separated by low angle grain boundaries. The formation of these sub grains is attributed to the decrease in the atomic level strain. Increasing the ball milling time from 43 ks to 360 ks leads to further lattice distortion and consequently to grain size reduction. Reduction in grain size is very important factor for the consolidation procedure because it increases the sinterability of the powders.