Study On The Hot Pressed Sintering Technique, Microstructures And Properties Of WC-MgO Composites

Cemented carbides with superior strength and hardness, high elastic modulus and low thermal expansion coefficient, have been taking an important role in the industrial areas of cutting tools, milling and drilling, molds and wear resistant parts. Among hard alloys, WC-Co materials embrace the widest applications, in which Co guarantees the good toughness and WC ensures the high hardness. However, Co is rare and expensive strategic resource. Moreover, due to its attracting physical and chemical properties, the demand of Co in catalyst, batteries, electronic components and other aspects is increasing rapidly. In addition, the hardness and corrosion resistance of WC-Co composites are deteriorated because of the low melting point, easily soften phenomenon and high chemical activity of Co. Therefore, it is of great urgency and significance to fabricate new composites with higher hardness and strength using economical raw materials, which could be considered as the alternative materials for WC-Co composites.In this paper, MgO was chosen as the substitute of Co. The nanocomposite WC-MgO powders were produced using mechanical alloying method and the subsequent consolidation of WC-MgO bulk materials was conducted using hot pressing sintering technique. This study was begun with the investigation on the densification process of composite powders. Methods were conducted to improve the microstructures and mechanical properties of sintered materials. Then, the corrosion and wear behavior of sinterined samples were studied. The main aims of this study are: Studying the densification process of composite powders during sintering; Observing the microstructural evolution of bulk materials under different designed sintering regimes; Evaluating the method to determine the hardness and fracture toughness of as-sintered materials; Studying the corrosion and corrosive wear behaviors of compacts in the acidic and alkaline solutions, respectively; Investigating the solid particle erosion behavior of compacts; Comparing the effects of adding grain growth inhibitors on the microstructures properties of as-sintered samples. Significant original results are listed as follows:Firstly, sintering temperature and time largely affect the densification process of the nanocomposites WC-MgO powders. The densification process of composites powders was investigated. Master sintering curve was also established based on the results of densification rate under different heating rate. The experimental results demonstrated that the optimum sintering temperature regime for WC-4.3wt%MgO nanocomposites powder is1500℃-1800℃. The master sintering curve was established based on the combined sintering theory. The apparent activation energy for the powders is361.8kJ/mol. The ability of MSC models to predict and control the sintering process for identical WC-MgO compacts was verified by a series of sintering experiments. The established sintering curve could be used to predict the final density and desification process of this materials under different heating rate and sintering time. In addition, suitable sintering parameters could be decided based on a required final density using this master sintering curve.Secondly, the same nanocomposites powders were used as raw materials powders. The influence of sintering parameters on the density and grain size of as-sintered samples was studied. The effects of adding VC and Cr3C2on the densification process, grain growth and mechanical properties were also studied. The results demonstrated that optimum sintering parameters for this composites were found after carefully investigating the densification process and gran growth during sintering. Hot pressing sintering at the temperature of1650℃with applied pressure of39.6MPa for90min could achieve compacts with a relative density of97.58TD%and desired mechanical properties, i.e. Vickers hardness of15.43GPa and flexural strength of1065.3MPa. The addition of0.25wt.%VC and0.25wt.%Cr3C2decreased the densification rate of sintered samples and retarded the grain growth during sintering. The apparent activation energy for grain growth during isothermal sintering was increased from492.53kJ/mol to426.85kJ/mol. Correspondingly, this addition of VC/Cr3C2decreased the grain size, improved the dispersion statues of second phase, increased the interfacial bonding strength between the matrix and second phase and thus enhanced the mechanical properties of sintered samples.Thirdly, WC-MgO compacts sintered at the optimum sintering parameters were used to investigate their fracture toughness. The influences of applied load on the fracture toughness of sintered compacts were also studied. Then the techniques to accurately determine the fracture toughness and mechanisms responsible for increasing tougheness were discussed. The experimental results revealed that values of fracture toughness determined using the Niihara’s equation under the load of49N was chose to represent the indentation fracture toughness of WC-MgO composites. The improved toughness of this composite can be attributed to the second phase toughening effects. Cracks deflections and extentions that enhance the toughenss were induced by smaller particle size and better degree on dispersion of-second phase.Fourthly, the corrosion behavior of WC-MgO compacts in the acidic (pH=1) and alkaline (pH=13) solutions was investigated using immersion corrosion method and electrochemical tests. The influence of adding VC/Cr3C2on the corrosion resistance was also studied. Then the corrosive wear behavior of samples in the same corrosive environments was observed. The experimental results demonstrated that the MgO phases were preferentially dissolved and WC matrix had a chemical stability in the acidic solution. Samples could become passivate in this solution and the passive layers were made of WO3·H2O and WO3·2H2O. The addition of VC/Cr3C2improved the corrosion resistance in the acidic solution. However, in the alkaline solution, WC matrix was severely attacked and the interfacial bonding between the matrix and second phase was deteriorated. Samples had no the ability to become passivate in this situation. Interestingly, the addition of VC/Cr3C2accelerated the corrosion rate in the alkaline solution. Similar to the corrosion behaviors of the materials, samples revealed a higher wear resistance in the acidic solution. VC/Cr3C2addition also increased the wear resistance in the acidic solution. On the contrary, the wear loss of materials in the alkaline was more pronounced and could be reduced slightly by improving the mechanical properties.Fifthly, SiO2was taken as erodent particles. The solid particle erosion behaviors of sintered samples under different impingement angles were investigated. A micro scale dynamic model (MSDM) was established to study the effect of experimental parameters and microstructure on the erosion resistance. The results revealed that the erosion behavior of WC-MgO composites was similar to that of ceramics based materials. The wear loss caused by solid particle erosion increased with increased impingement angle. The dominant fracture model for this composites is combined by intergranular and transgranular fracture. With the addition of VC/Cr3C2, the erosion resistance at each impingement angle was improved. The grain growth inhibitors particularly benefited the high-angle erosion resistance of the material due to the elevated toughness. A micro scale dynamic model was established based on the Newton’s law and mechanical properties (Young’s modulus, yield modulus, tensile strength and fracture strength) to study erosive behaviors and mechanisms of WC-MgO composites. Modeling results demonstrated that grain size and grain boundary strength play considerably roles in the erosion resistance. Reduced grain size could benefit the erosion resistance only when the grain boundary is reasonably strong. The wear loss caused by erosion became much more pronounced with a weak grain boundary.The experiments and results in this dissertation were based on the synthesis of nanocomposites WC-MgO powders and its hot pressed sintering technique. The microstructures, mechanical properties, corrosion, corrosive wear and erosion wear of the as-sintered samples were investigated in detail. The effects of adding a small amount of VC/Cr3C2on the microstructures and properties of WC-MgO composites were also discussed. This study laid a solid and profound foundation for the understanding and applications of WC-MgO composites.