Preparation Of Carbide-cobalt Composite Powders And Hardmetals There Of And Research On Its Mechanism

Refractory metal carbides possess of high melting point, high hardness, chemical stability, good electrical conductivity and thermal conductivity, thus are widely used in cutting, wear resistant parts, coating, high temperature structural materials, and grain growth inhibitors for ultrafine WC-Co hardmetals.Hardmetals, consisting of refractory metal carbides cemented by the cobalt phase, exhibit high hardness, wear resistance, and excellent hot hardness. Ultrafine WC-Co hardmetals have been applied in areas such as miniature drills for highly integrated printed circuit boards (PCBs), pins for dot-printers, and so on, owing to its both high hardness and high strength.WC-Co composite powder has attracted substantial interests due to its uniform distribution in composition, and proper routes suited for industrialization have been exploring by scientists and engineers in recent decades. However, disadvantages such as high cost, inaccurate control industrially, and environment contamination are difficult to conquer with such routes.In this work, ultrafine tungsten carbide-cobalt (WC-10wt. %Co) composite powder was synthesized via spray-drying and direct reduction and carburization process in vacuum, which includes precursor preparation by spray-drying of a suspension of ammonium metatungstate (AMT) and cobalt carbonate (CoCO₃), calcination to evaporate volatile components, formation of tungsten-cobalt mixed oxide powder (CoWO₄/WO₃), ball-milling with carbon black, and subsequent direct reduction and carburization reaction in vacuum. The synthesis temperature of WC-10wt.%Co composite powder withoutηor graphite phases is lower than 1000℃. The calculated particle size by BET test is 0.29μm. The carburization process is described as follow: CoWO₄/WO₃→CoWO₄ + WO₂ + W₁₈O₄₉→CoWO₄ + WO₂ + W₁₈O₄₉ + W→WE + Co₆W₆C + Co₃W₃C + Co→WC + Co. Coarse WC powder (FSSS: 0.9μm) and Co powder (FSSS: 1.0μm) (WC: Co = 9: 1 in mass) were added into the obtained WC-10wt.%Co composite powder with addition of 30wt.%, 50wt.% and 70wt.%, respectively. Results show that the hardmetal fabricated from 70wt.% (WC-10wt.%Co composite powder) + 30wt.% (90wt.%WC + 10wt.%Co coarse powder) mixed powders exhibits a fine microstructure as well as optimum mechanical properties.Cobalt-inhibitor composite powders of Co-30wt.%VC, Co-30wt.%Mo₂C, Co-30wt.%TaC, Co-30wt.%TiC and Co-30wt.%NbC were synthesized by direct reduction and carburization process in vacuum, using grain-growth-inhibitor oxides, Co₃O₄, and carbon black as the raw materials. Results show that the carburization temperature can be effectively lowered by the participation of Co₃O₄, and the obtained powders exhibit obvious core/rim structure.The process of direct reduction and carburization is investigated. The low temperature carburization mechanism of refractory metal oxides with the participation of Co₃O₄ is analyzed, and the relationship of hardness and strength is also discussed.