Study On Synthesis Of Nanocomposite WC-MgO Powders And Their Subsequent Hot-Pressing Consolidation

Among hard alloys, the ultra-fine WC-Co cemented carbides with superior hardness and toughness find wide industrial applications as tips for cutting tools and wear-resistant parts. The intrinsic resistance to oxidation and corrosion at high temperature also makes them desirable as protective coating for devices at elevated temperatures. Metallic binder (typically Co) is introduced to improve WC interparticle binding and to increase compact toughness. However, Co is expansive and rare and its reserves all over the world are very limited. Moreover, metallic binders result in reduced hardness and corrosion/oxidation resistance, and enhance grain growth, particularly in conventional liquid phase sintering due to rapid diffusion in the liquid phase. Therefore, efforts to obtain harder materials have attempted the preparation of WC with low amounts of Co and WC with no metal binder.A new composite material, WC-MgO is considered as an ideal material for use in industrial applications. Compared with the commercial micron- and submicron-grained structure WC-Co composites, the WC-MgO can achieve superior high value of hardness and toughness combination. In the current work, high energy planetary ball milling and its subsequent hot-pressing sintering were adopted to the synthesis of nanocomposite WC-MgO powders and the bulk material. The formation of nano-sized WC and WC-MgO composite powders were investigated at first. The solid-state reaction mechanism, the influence of its previous mechanical activation and the process of the composite powder synthesis were discussed. These were followed by an understanding on the hot-pressing sintering behavior and its improvements. Rare earth oxide addition and two-step-sintering method were selected for the further developing on the mechanical properties of the consolidated bulks. Some significant results have been achieved.Firstly, solid-state carbothermic reduction of tungsten oxide (WO3) to nano-sized tungsten carbide (WC) particles was obtained by calcining mechanically activated mixtures of WO3 and graphite at 1215℃under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, Magneli phase (WO2.72 and WO2) and metallic tungsten (W), were observed at 741℃, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.Secondly, a series of artificial-neural-network (ANN) models was developed for the analysis and prediction of correlations between processing (high-energy planetary ball milling) parameters and the morphological characteristics of nanocomposite WC-4wt%MgO powders by applying the back-propagation (BP) neural network technique. The input parameters of the BP network were milling speed, milling ball diameter and ball-to-powder weight ratio. The properties of the as-milled powders (specifically crystallite size, specific surface area and median particle size) were the output for three individual BP network models. These models were based on the mathematic statistical approach and seemed suitable for the complicated ball milling process which is difficult to be accurately described by physical models. Well acceptable performances of the neural networks were achieved. The model can be used for the prediction of properties of composite WC-MgO powders at various milling parameters. It can also be used for the optimization of processing and ball milling parameters.Thirdly, the obtained nanocomposite WC-4wt%MgO powders were consolidated into bulk materials via hot-pressing sintering. The influence of sintering regimes on the microstructures and properties of bulk materials was studied. It can be found that sintering temperature and holding time can greatly affect the properties of the as-sintered bulks. At lower sintering temperature and shorter holding time, the dense bulk structure cannot be obtained, while at higher sintering temperature or longer isothermal treatment, grains might coarsen. As a result, the mechanical properties of the as-sintered WC-MgO bulks might become unsatisfactory as well. The optimized sintering temperature can be determined regarding the bulk density and the best combination of hardness and fracture toughness. Hot-pressing sintering at the temperature of 1650℃with applied pressure of 39.6 MPa for 90 min can obtain a relative density of 94.56 %TD and the sintered compacts maintain their unique properties, i.e. superior hardness (HV= 17.78 GPa), toughness (Kc= 12.21 MPa·m1/2), and flexural strength (σ= 1065.3 MPa) combination. The improved toughness of WC-MgO composite can be attributed to the second phase toughening effects. The observations on the indentation cracks on the surface of the WC-MgO indicates that once the crack has reached particulate-matrix interface, the difference in the crack-tip opening displacement between the ductile particle and the brittle matrix would cause crack to be locally blunted, thus produce closure stress bridging the crack along its length. These effects require more external load to force the crack propagate further, thus induce improvement of toughness. In addition, crack deflections that enhance the energy for crack growth were also observed. It can be concluded achieving a high density and a small grain size are very important for the structural ceramic materials because it brings about an improvement of mechanical properties.Fourthly, a detailed investigation was carried out into the influences of the lanthanum oxide (La2O3) addition upon the microstructural characteristics and the mechanical properties of the WC-MgO composite bulk prepared by hot-pressing sintering. The results indicate that due to the unique properties of rare earth element such as high surface activity and large ionic radius, the addition of trace La2O3 can suppress the decarburization, promote the microstructural refinement and improve the particulate dispersion homogeneity and the particulate/matrix interfacial coherence. Consequently, the relative density of the sintered sample with 0.1 wt% La2O3 addition can be increased by 4.2% as compared with the sample without La2O3 addition. This indicates the possibility of preparing high-hardness (18.02 GPa) and flexural fracture strength (1265.9 MPa) WC-MgO composite material with adding the RE oxide (La2O3) using conventional hot-pressing sintering method.Fifthly, two-step hot-pressing sintering (TSS) was applied to consolidate nanocomposite WC-4wt%MgO powders. The first step sintering was employed at a higher temperature to obtain an initial high density, and the second step was held at a lower temperature by isothermal sintering for several hours to increase bulk density without significant grain growth. The experimental results showed the sintering temperature plays an important role in densification and grain growth of WC-MgO compacts. The optimum TSS regime consisted of heating at 1750℃(1st step) and 1550℃(2nd step), resulting in the formation of near full dense microstructure (99%TD) with suppressed grain growth (2.59μm). Accordingly, the improvement on the mechanical properties, including increase in the hardness (from 16.7 to 18.4 GPa), fracture toughness (from 10.2 to 12.95 MPa-m1/2) and flexural strength (from 976.6 to 1283.7 MPa), was also observed due to the grain refining and full dense bulk.In the current work, nanocomposite WC-MgO powders and the composite bulks, which achieve competitive values of hardness and fracture toughness, can be an ideal engineering material as the alternative of WC-Co. This study laid a solid foundation for the understanding of WC-MgO synthesis process and its batch-preparation application.