Sinter-HIPing Process & Mechanism Of Inhibiting Crystal Growth In Nanocomposite WC-Co Materials

WC-Co hardmetal is a kind of tool materials with good properties. With the development of nanocomposite technology and materials, it is imported in more and more fields which can resolve effectively the contradiction of strength and hardness in conventional cemented carbides. Up till now, none of companys can produce a cermet with grain size of less than 100 nm in the world. This is because that the current technology can't effectively control WC crystal growth during sintering. Scientific researchers over the world are having been applied themselves to synthesizing nanocomposite powder, sintering hardmetals with new technology and inducting crystal growth inhibitors in new way.The ultrafine grained WC-Co heardmetals having both high hardness and high strength was prepared in this work by die pressing, powder extrusion molding Sinter-HIPing from nanocomposite WC-Co powder mixed with proper inhibitors. The powder was fabricated by spray pyrogenation & continuous reduction and carbonization process. The influences of grain size to hardness and transverse rupture strength, the mechanism of inhibitors to WC crystal growth were also studied.Results showed from reactive thermal performance analysis and XRD testing that it is feasible that inhibitor oxide (V₂O₅ and Cr₂O₃) can be inducted by spray pyrogenation process in stock powder. The tungsten carbide-cobalt powder was then prepared by continuous reduction and carbonization process in vacuum circumstance. According to XRD, SEM, TEM determination and mechanical properties tests, the proper inhibitors can effectively inhibit WC crystal growth and the as-fabicated hardmetal has better properties than the un-inducted samples. When composite inhibitors (VC and Cr₃C₂) are doped with 0.4wt% individually, they can effectively inhibit WC crystal growth and hardmetal with excellent properties can be obtained. The hardmetal has better properties with more even and smaller grain size.By making a comparison of structures and properties between vacuum sintered hardmetal and sinter-HIPed ones, the advantage of sinter-HIPing process was proved. Results showed that the mechanisms of inhibitors to WC crystal growth were that poly-steps between WC polycrystals are formed when VC inhibitor is inducted. They increase boundary energy in WC polycrystals and slow down nuclear growing velocity. The Cr₃C₂ inhibitor is distributed evenly between WC polycrystals and solid-dissolved in cobalt having no partiality.At last, ultrafine hardmetal was fabricated by sinter-HIPing process from WC-0.4VC-0.4Cr3C2-10Co nanocomposite powder. The hardmetal has the average grain size of 350 nm, with the HRA of 93.4 and TRS of 3810 MPa.