Influence of Nanocrystalline ZrO 2 Additives on the Fracture Toughness and Hardness of Spark Plasma Activated Sintered WC / ZrO 2 Nanocomposites Obtained by Mechanical Mixing Method

The present study reports the formation of ultrafine hard particles of nanocomposite WC with different additions of ZrO2 powders (0.5 20 vol.%). The initial mixed powders of WC with the desired ZrO2 concentrations were mechanically mixed for 360 ks (end-product) under argon gas atmosphere at room temperature, using high energy ball mill. The end-product consists of average grain size of about 17 nm in diameter. The obtained nanocomposite powders were consolidated into fully dense compact, using spark plasma sintering (SPS) technique in vacuum. The experimental results revealed that the consolidation step, which was conducted at 1673 K with uniaxial pressure ranging from 19.6 to 38.2 MPa for short time (0.18 ks), does not lead to dramatic grain growth in the powders so that the consolidated nanocomposite bulk objects maintain their nanocrystalline behavior, being fine grains with an average size of 63 nm in diameter. The relative densities of consolidated nanocomposite WC/ZrO2 materials increase from 99.1% for WC-0.5% ZrO2 to 99.93% for WC-20% ZrO2. The indentation fracture toughness of the composites can be tailored between 7.31 and 19.46 MPa/m by controlling the volume fraction of ZrO2 matrix from 0.5% to 20%. The results show that the Poisson’s ratio increased monotonically with increasing the ZrO2 concentrations to get a maximum value of 0.268 for WC-20% ZrO2. In the whole range of ZrO2 concentrations (0.5 20 vol.%), high hardness values (20.73 to 22.83 GPa) were achieved. The Young’s modulus tends to decrease with increasing the volume fraction of the ZrO2 matrix to reach a minimum value of 583.2 GPa for WC-20% ZrO2. These hard and tough WC/ZrO2 nanocomposites are proposed to be employed as higher abrasive-wear resistant materials.


Introduction
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 transition-metal 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 WC-Co 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][7][8].
The present work has been addressed in order to study the influence of nanocrystalline ZrO 2 additives on improving the fracture toughness and Poisson's ratio of mechanically mixed WC-ZrO 2 nanocomposites.The selection of ZrO 2 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-ZrO 2 .

Experimental
In the present study, elemental powders of WC (99.5%, 30 µm) were mixed with different selected volume fractions of ZrO 2 (2% Y 2 O 3 ) powders (99.5%, 10 µm) of 0.5, 5, 10, 15 and 20 vol.%.The mixed powders of each ZrO 2 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 elec-tron microscopes (HRTEM).
The end product of the ball-milled nanocomposite powders (after 360 ks) at different ZrO 2 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/ZrO 2 materials were determined by Archimedes' principle, using water immersion method.Vickers indenter with a load of 50 kg was employed to deter-mine 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.

Results and Discussions
XRD technique was employed to follow the structural changes that may occur during ball milling of hcp-WC with different volume fractions t-ZrO 2 powders and after the consolidation process that was achieved at 1673 K, using SPS technique.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 ZrO 2 .The absence of any reacted phases during this sintering step implies the thermodynamic compatibility of WC and ZrO 2 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% ZrO 2 powders after ball milling for 43 ks (Figure 2 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/ZrO 2 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.Figure 3 presents the outer macroscopic view of as consolidated samples of WC-10% ZrO 2 (Figure 3(a-i)) and WC-20 % ZrO 2 (Figure 3(a-ii)) that were obtained after consolidation the powders of the end-product at 1673 K, using SPS technique.The consolidated mirror-like buttons have excellent macroscopic characteristics with the absence of any crack or macro pores (Figure 3(a)).To ensure the homogeneity and the vertical density variation of the compacted samples along their longitudinal section, the consolidated button of WC-10% ZrO 2 (Figure 3(b-i)) was sectioned into 4 slices with about 20 mm in height for each (Figure 3(b-ii)) and the density of each slice was individually measured.However, the highest bulk and relative densities are brained at the top of this compact; the results do not imply any serious values fluctuations through the longitudinal of the sample, as depicted in Figure 4(a).The variation in the measured relative density of this sample does not exceed above 0.55% (Figure 4(b)) suggests the absence of large numbers of pores and also may elucidate the excellent distributions of the WC throughout the whole compact.
The BFI of as-consolidated WC-10% ZrO 2 sample is shown in Figure 5.The ZrO 2 matrix (light gray region) consists of equi-axed grains with less than 40 nm in diameter.Likewise the matrix materials, the WC grains (dark grains) were affected by this consolidation step, which leads to a slight grain growth and the formation of edgedlike WC grains with a grain size distribution ranging from 20 nm up to 60 nm in diameter (Figure 5).It is worth notifying that the WC grains are distributed at the grain boundaries of ZrO 2 , however some WC grains tend to agglomerate together in some regions.Thus, we can conclude that WC grains are perfectly interfacial bonded with the ZrO 2 ceramics matrix to form full-dense nanocomposite WC/ZrO 2 material.We should emphasis that in the SPS process, several factors contribute to enhance the densification; 1) the use of rapid heating rate of heating and cooling (~600 K/s), 2) rapid transfer of heat because the WC die acts by it-self as a heating element and 3) the application of pulsed DC current to heat the sample, implying that samples are also exposed to a pulsed electric field during the consolidation process.It is believed that application of mechanical pressure promotes the removal of pores and enhances diffusion [10].However, it is frequently argued the improved densification rates stem mostly from the use of pulsed DC current of high energy.When pulse voltage is applied to the powders, micro-discharge takes place among the particles, which generates plasma.Thus the atoms on the surface of each particle are activated, leading to high sintered densities in very short time.In addition to the application of the pulse electric current that leads to thermal and electric field diffusions within a short time, one important factor is the grain size of the prepared composite powders.The small grain size of the ball-milled powders is considered as important reason for enhancing the driving force during SPS consolidation in that ultrafine particles with their large surface area would be greater inter-particle contact (number of necks) and, hence, more paths for volume diffusion can be achieved [11].
The bulk densities of as-consolidated nanocomposites WC/ZrO 2 materials were measured and plotted in Figure 7(a) as a function of ZrO 2 concentrations.Two mathematical expressions were used to predict the effect of ZrO 2 on the density of the composites and to compare the measured values with the theoretical ones.The theoretical density values for different volume fractions were calculated from the rules of mixture, using the upper bound and lower bound equations, that is respectively presented by where, ρ and V refer to the density and the volume fraction, respectively.It can be seen that the measured density values of WC/ZrO 2 composites fall between the upper and lower curves, indicating a good matching between the calculated and measured density.The values of the measured density for WC mixed with different ZrO 2 contents are listed in Table 1.The correlation between the relative densities of the WC/ZrO 2 composites were calculated and presented in Figure 7(b) as a function of ZrO 2 content.All the consolidated samples have relative densities higher than 99.1%, indicating that WC/ZrO 2 nanocomposites can be fully densified at 1673 K, using SPS technique.It can be inferred from the figure that increasing the volume fraction of the matrix material leads to a monotonical increase in the relative density of the composites.This may suggest a monotonical decreasing in the pores and a good bonding between the matrix and reinforcement materials. (a) A SEM micrograph of the Vickers hardness indentation for consolidated WC-15% ZrO 2 button is shown in Figure 8.The hardness of this compact sample was measured and found to be 21.13GPa, taking the average of at least ten indentations.It is worth noticing that this value is higher than that of bonded nanocomposite WC-14% Co (13.2 GPa) [4].The measured hardness values of the other WC/-ZrO 2 nanocomposites, which are listed in Table 1 were plotted in Figure 9 against the ZrO 2 contents (closed symbols).WC/ZrO 2 nanocomposites posses high hardness values (higher than 20.50 GPa) however, the hardness tends to decrease slightly with increasing the soft phase of ZrO 2 content.Going back to Figure 8, where the demonstrated cracks in the micrograph were developed in the product during the indentation and extended to 235 µm away from it.These values for different ZrO 2 contents that were used as indicator for estimating the fracture toughness (K c ) via the model that was proposed by Anstis et al. [10] are listed in Table 1 and     and a maximum K c of 19.46 MPa/m 1/2 was achieved for the composite with 20% ZrO 2 characteristics of the obtained WC/ZrO 2 nanocomposites, indicated by the monotonical increasing in the K c with increase the ZrO 2 contents.The addition of ZrO 2 increases the fracture toughness due to so-called transformation toughening [13,14].It is believed that the polymorphic tetragonal-monoclinic phase transformation that takes place in ZrO 2 leads to finite amount of volume change (4% -5%) and a large shear strain (14% -15%) [14].It has been suggested by Miyazaki et al. [12] that the improvement in fracture toughness of the powder-mixture composites is attributed mainly to the "stress-induced" transformation of the ZrO 2 phase.
Figure 10 shows the dependence of the Poisson's ratio (closed symbols) and Young's modulus (open symbols) on the ZrO 2 content of the WC/ZrO 2 nanocomposites.These values, which are summarized in Table 1, were estimated from the measured densities and the constant parameters of the nondestructive testing apparatus.Increasing the soft ZrO 2 against the hard brittle phase of WC phase in the composites is greatly improve the ductility of the composites, indicated by a monotonical increasing in the values of Poisson's ratio to achieve a maximum value of 0.268 for the composite with 20% ZrO 2 .Compare this value with that one for pure WC (0.004) [14]; one can say that the addition of ZrO 2 improves the ductile characteristics of the fabricated nanocomposite materials.The Young's modulus of the consolidated WC/ZrO 2 nanocomposites is strongly influenced by increasing the ZrO 2 that lead to a linear decreasing in the values of Young's modulus to reach a minimum of 583.2 GPa for WC-20% ZrO 2 .This monotonical decrease is attributed to the decreasing of the volume fraction of the hard WC phase.

Conclusions
We have employed high-energy ball milling technique to fabricate nanocomposite WC/ZrO 2 powders up to 20 vol.% of ZrO 2 .The powders that were milled for 360 ks were consolidated at 1673 K, using SPS technique.The as-consolidated objects have full dense and maintain their nanocrystalline characteristics.The indentation fracture toughness of the composites has been tailored between 7.31 and 19.46 MPa/m 1/2 by controlling the volume fraction of ZrO 2 matrix between 0.5% to 20%.The results show that the Poisson's ratio increased monotonically with increasing the ZrO 2 concentrations to get a maximum value of 0.268 for WC-20 vol.%ZrO 2 .High hardness values (20.73 to 22.83 GPa) and Young's modulus (583.2 to 688.2 GPa) were achieved.

Figure 1 (Figure 1 .
Figure 1.XRD patterns of nanocomposite WC-10 vol.%ZrO 2 after ball milling for (a) 43 ks; (b) 360 ks; (c) for the consolidated sample after ball milling for 360 ks of the final product.
(a)) and 360 ks (Figure 2(b)).The light gray region in Figure 2(a) shows the ZrO 2 matrix, whereas the dark coarse grains embedded into the matrix, present the WC grains.

Figure 5 .
Figure 5. BFI planner view of the end-product of nanocomposite WC-10 vol.%ZrO 2 after consolidation.Further microscopic information was obtained by SEM examinations.The micrograph of the back scattering electron microscope and the elemental dot mapping for W and Zr of the consolidated WC-10% ZrO 2 compact are shown together in Figure 6.Obviously, the WC particles that have sharp edges (Figure 6(a)) are embedded and homogeneously distributed along the ZrO 2 matrix.Apparently, neither voids nor cracks can be detected, indicating an excellent interfacial bonding.Moreover, no undesirable byproduct could be identified, as suggested by the segregation of the

Figure 6 .
Figure 6.Back scattering view (a) and the dot mapping of elemental W (b) and Zr (c) of nanocomposite WC-10 vol.%ZrO 2 after consolidation.

Figure 7 .
Figure 7. (a) The calculated/theoretical (open symbols) and measured densities (closed symbols) of WC mixed with different ZrO 2 contents; (b) The dependence of the relative density of WC/ZrO 2 on the ZrO 2 contents.

Figure 8 .
Figure 8. SEM micrograph of Vickers hardness indentation developed by applying a load of 50 kg on the consolidated sample of nanocomposite WC-15 vol.%ZrO 2 .