Study On Nanocomposite WC/MgO Prepared By High-energy Balling Milling And Sintering

Tungsten-cobalt cemented carbide（WC/Co） became one of the focal points in the research of modern tool and structural materials for its exceptional high hardness,good wear/corrosion resistance and excellent thermal hardness.However,the binder Co is a rare and noble metal,and conventional cemented carbides with coarse grains are high in hardness but low in fracture toughness.Thus,it is of great significance to find a new material with both superior hardness and fracture toughness,whose raw material is easy of access,as the substitute material for the commonly used WC/Co.The high energy planetary ball milling,followed by hot-press sintering and spark plasma sintering method is adopted to study the synthesis of nanocomposite WC/MgO with both superior high hardness and fracture toughness.The effects of elemental composition,ball milling process and sintering process on the structures and properties of the nanocomposite WC/MgO powders and bulk materials are systematically studied by XRD,SEM,TEM,HRTEM,DSC,etc.Furthermore,the thermal dynamics of the formation reaction of nanocomposite WC/MgO powders and the toughening mechanism of the bulk materials are further discussed in the present work.The study is divided into two parts:Part 1:high energy planetary ball milling is applied to two different systems,i.e.WO₃-C-3Mg and WC-MgO,in the hope of investigating the feasibility of fabricating nanocomposite WC/MgO powders.The results show that under argon gas atmosphere,both of these two powder mixtures can be mechanically alloyed to synthesize nanocomposite WC/MgO powders with 50h ball milling.The average crystallite size of the obtained powders are 25～35 nm and the sizes of many particles are under 100 nm.During the research on the high energy ball milling process of WO₃-C-3Mg system,it can be found that the nanocomposite WC/MgO powders can be synthesized through two reaction modes under different conditions,i.e.diffusion and self-propagating explosive reaction,which can be transformed into each other with the ball milling parameters.In addition,optimization of high energy ball milling process for WC-MgO system is carried out,and process parameters in favor of fining grain and particle size are obtained.Part 2:The obtained nanocomposite WC/MgO powders are consolidated into bulk materials by spark plasma sintering and hot-press sintering,and the effect of the sintering method and process conditions of sintering on the properties of bulk materials is studied.（1） Through the analysis of phases,microstructures and mechanical properties,it can be found that the nanocomposite WC/MgO consolidated by spark plasma sintering exhibits purer phase,finer crystallite size and better mechanical performance,when compared to the nanocomposite WC/MgO obtained by hot-press sintering.（2） The effects of MgO content on the properties of bulk materials consolidated by spark plasma sintering are studied.The results demonstrate that the addition of second phase MgO may hinder the movement of boundary,thus inhibit the grain growth,and the particle size of the matrix decreases with the increase of the second phase volume fraction,the best combination of hardness and fracture toughness of the WC/MgO bulk materials can be achieved when the content of MgO is 8 wt.%.However,a further increase in the content of MgO may result in the reduction of hardness and fracture toughness for the reason that the toughen effect may be weakened due to the increasing aggregation extents of particles.（3） The effects of sintering pressure and temperature on the properties of bulk materials consolidated by spark plasma sintering are systematically studied,and found that when a proper sintering temperature is chosen,the higher the sintering pressure,the higher the relative density,hardness and fracture toughness of composite WC/MgO will be.Sintering temperature can also greatly affect the properties of the composite.At low sintering temperature,the density decreases,while at high sintering temperature,grains grow and the value of hardness and fracture decreases too,although the density increases.（4） According to the optimization result of sintering process,the best combination of hardness and fracture toughness(HV=17.78 GPa, KIC=12.21 MPa·m^1/2) of nanocomposite WC/MgO can be achieved when the MgO content,the sintering pressure and the sintering temperature are designed at 8 wt.%,70Mpa and 1650℃, respectively.Compared to conventional non-nanometer WC / MgO powder sintering,properties of the bulk Nano WC / MgO material have more apparent advantages,which show that nano-grained WC-MgO bulk materials exhibit evident nano effect.And compared to the WC-8 wt.%Co (HV=16.65GPa,KIC=12.26MPa·m^1/2) prepared by the identical method,nanocomposite WC/MgO has a significant advantage in both superior hardness and great strength.The above results have laid a solid foundation for the further research on the milling process of WC/MgO composite powder and its bulk batch-preparation process.