Preparation For Carbide-Metal Composite Powders And Cermets By Direct Reduction And Carburization Process

Transition-metal carbides(NbC,Cr₃C₂,Mo₂C,VC,and TiC)possess characteristics of high melting points,high hardness,and hard wear.And some of them can be widely used as cutting-tool,mould and catalysts.In the industry of WC-Co hardmetals,these carbides are usually used as grain-growth-inhibitors.Cermets,consisting of transition-metal carbides cemented by the metal phase (Co,Ni,et al),exhibit high hardness and wear resistance.A few studies were related to the grain growth behavior of reforactory metal carbides and mechanism of liuid phase sintering in NbC-Co,Cr₃C₂-Co,Mo₂C-Co,VC-Co cermets.As to TiC-Ni cermets,they have been studyed widely,but the manufacture method is only restricted in sintering from the mixture of TiC powder and Ni powder.However,mechanical properties of such cermets haven't been fully investigated now.In this study,by using transition-metal oxides(Yb₂O₅,Cr₂O₃,MoO₃,V₂O₅, TiO₂),Co₃O₄,NiO,and carbon black as the raw materials,composite powders (NbC- 10wt.%Co,Cr₃C₂-10wt.%Co,Mo₂C- 10wt.%Co,VC- 10wt.%Co,TiC-40wt.%Ni) were synthesized via direct reduction & carbonization technology under vacuum circumstance.After mould shaping and cold isostatic pressing forming,green samples from composite powders were used to prepare cermets by vacuum sintering.Phases of the composite powders synthesized at different temperatures and in different dwell time were detected by XRD.SEM was performed to observe the morphology of the composite powders.Results show that cermets composite powders could be synthesized at low temperatures,which were effectively reduced by the participation of Co₃O₄ and NiO.The mechanism of low-temperature carbonization was studied by comparative experiments and thermodynamic datas.The relative density,TRS,SEM,and XRD of sintered samples were tested and the results show:by vacuum sintering,the following cermets were required:NbC-Co cermets whose TRS is 608.00 MPa and relative density is 99.42%under 1420℃for 2 hours,Cr₃C₂-Co cermets whose TRS is 560.52 MPa and relative density is 97.01% under 1420℃for 2 hours,Mo₂C-Co cermets whose TRS is 409.48 MPa and relative density is 97.01%under 1370℃for 2 hours,VCoCo cermets whose TRS is 348.01 MPa and relative density is 87.87%under 1420℃for 2 hours,and TiC-Ni cermets whose TRS is 890.42 MPa and relative density is 97.01%under 1370℃for 2 hours.The sinstering mechanism of green samples from composite powders is typical liquid phase sintering.The flow of the liquid phase makes the loose powders into bulk materials.The mechanical properties of cermets are related to their microstructures. The content of free carbon,the contact angles between carbide grains,the grain growth mold are the key factors to the mechanical properties of cermets.The direct reduction & carbonization technology under vacuum circumstance has a good promising in industry,which production cost is low and the process is easy to control.