Two-Step Hot-Pressing Sintering Of Nanocomposite WC-AL₂O₃Compacts

Tungsten carbide with superior characteristics including excellent hardness and strength and oxidation resistance encourages their widespread use for industrial field. Pure WC was embedded in a ductile metal Co matrix by liquid sintering. The hardness and wear resistance of the cemented carbides are inversely proportional to the metallic binder content, and also, the metallic binders are inferior to WC in corrosion and elevated temperature applications. All of these have limited the application and the service life of the cemented carbides with metallic binder phase. To overcome these disadvantages, the replacement of Co with other material as great as possible a reduction in the amount of Co phase have been attempted. Because of the high melting point of WC, however, it is almost impossible to sinter pure it has been found difficult to achieve a better WC-Co bulk product with an ultrafine microstructure by a conventional sintering process due to particle coarsening during sintering and so on. The WC-Al2O3composite powder synthesized by high-energy ball milling was used as raw material followed by Two-Step hot-pressing Sintering (TSS).The first step sintering at a constant heating rate with a higher temperature to obtain an initial high density and guarantees the disappearance of supercritical pores while the other pores become unstable. The second step was held at lower temperature by isothermal for a period of hours to increase bulk density with a limited grain size.The present work deals with the influence of deliberate temperature and holding time at second step has been well documented by orthogonal analysis, on microstructure and mechanical properties of WC-Al2O3nano-composite. The mechanisms of solid sintering were discussed. The results show that when the sample sintered at1600℃(1st step) and holding6hours at1450℃obtain a relative density of99.03%, a fracture toughness of10.43MPa-ml/2and a bending strength of1162.14Mpa. These mechanical properties have been greatly improved compared with convention sintering. At a relatively lower second step temperature (T2=1400℃), no notable densification but a remarkable grain growth was observed even after a prolonged isothermal treatment. Two-step sintering conducted at T1=1600℃,T2=1450℃leads to nearly fully dense structure, accompanied a magnificent suppression of grain growth. The grain boundary diffusion remains active while the grain boundary migration was retarded.1500℃was chosen as second step sintering for failing to eliminated the velocity of grain boundary migration and promotes the coarsening of grain size.These as-milled composite powders added in different contents of La2O3and Cr3C2were prepared by the optimized two-step hot pressing in a graphite die. The addition of La2O3can accelerate the sintering WC-Al2O3composite and increase the density, enhance the particulate dispersion homogeneity and the matrix/particulate interfacial coherence, inhibit the coarsening of particle and grain. But an excessive addition of La2O3results in the obvious pores throughout the sintered bulk which leads to decrease in the strength. The addition of Cr3C2suppresses the decarburization during the hot pressing sintering, promotes the micro-structure refinement and enhances the particulate dispersion homogeneity.