The Effect Of Morphology And Dual-scale Structure Of WC On Mechanical Properties Of WC-Co Hardmetals

WC-Co hardmetals have wide industrial applications due to their high hardness,outstanding wear resistance and good fracture toughness.However,the conventional hardmetals cannot well meet the requirements of the modern industry,because of the trade-off between the hardness and strength,toughness.Therefore,it is improtant to develop the WC-Co hardmetals with combination of high hardness,toughness and strength simultaneously.In this dissertation,we have improved the mechanical properties of WC-8Co hardmetals through controlling the size and morphology of WC with the method of plasma-milling technique as well as the synchronous method of carbonizing and sintering.In addition,the relationship between microstructure and mechanical properties has been revealed.The W,C and Co powders were used as raw materials.By controlling plasma-milling process,granular and flake-like W-C-Co nanometer composite powders were respectively obtained.With increasing the sintering temperature,the density of sintered WC-Co hardmetals increased,in addition,the hardness and transverse rupture strength firstly increased and then decreased.The optimal comprehensive mechanical properties were obtained at the sintering temperature of 1390^oC.Moreover,better mechanical properties can be achieved by using the low-pressure sintering process.In process of sintering,granular W-C-Co powder tended to form prismatic WC grains,while flake-like W-C-Co powder was more likely to generate plate-like WC grains.The prismatic WC grained WC-8Co hardmetals have homogeneous mechanical properties due to its WC randomly distributing.For plate-like WC grained hardmetals,its mechanical properties are closely related to the degree of preferred orientation of plate-like WC.WC-Co hardmetals with arranged randomly plate-like WC exhibit homogeneous mechanical properties.However,highly oriented plate-like WC hardmetals present anisotropy of mechanical properties.Compared with prismatic WC grained hardmetal,the WC-Co hardmetals with plate-like WC has better comprehensive properties.In order to overcome the anisotropy of mechanical properties resulting from highly oriented plate-like WC and further improve the mechanical properties,we designed the dual-shaped WC hardmetals.The results showed that mixing the granular W-C-Co composite powder and flake-like W-C-Co composite powder at different mass ratio,the proportion of prismatic WC and plate-like WC as well as the degree of preferred orientation of WC can be well controlled.Not only the degree of anisotropy of mechanical properties can be reduced,but also TRS and fracture toughness can be improved.In addition,it was found that coarser prismatic WC had better effect on improving TRS and fracture toughness than the smaller prismatic WC.When using the 12μm raw W,and the granular W-C-Co composite powder and flake-like W-C-Co composite powder mixing at mass ratio of 1:1,the dual-shaped WC hardmetals achieves excellent comprehensive mechanical properties.The hardness on its section V and P are 1733 kgf/mm² and 1721 kgf/mm²,TRS are 3795MPa and 3724MPa respectively,fracture toughness are 21.56 MPa·m^1/2 and 21.13 MPa·m^1/2.To further improve the mechanical properties of WC-8Co hardmetals,the hardmetals with dual scale structured plate-like WC were prepared.By mixing two different sized flake-like W-C-Co composite powders,fine and coarser plate-like WC can be retained after sintering.Moreover,the amount ratio of the fine plate-like WC to coarser plate-like WC and the degree of preferred orientation of plate-like WC can be well controlled by changing mass ratio of two different sized flake-like W-C-Co powders.Compared with individual dual scale effect or plate-like WC morphological effect,synergetic effects of dual scale structure and the plate-like morphology could better increasing fracture toughness and TRS.Moderate sized plate-like WC grouped together with coarse plate-like WC,both the fracture toughness and TRS increased first and then decreased.The best mechanical properties were listed as follows:hardness,TRS and fracture toughness on section V were 1768 kgf/mm²,4084MPa、23.11MPa·m^1/2 respectively,meanwhile,that on the section P were 1733kgf/mm²,3924MPa and21.56 MPa·m^1/2,respectively.We have revealed the toughing mechanism through observing the crack propagation and the fracture morphologies.For prismatic WC hardmetals,the bigger prismatic WC triggers more zigzagged crack which is benefit for toughness.Similarly,the plate-like WC triggers lots of crack deflections and increases the proportion of transgranular fracture of plate-like WC from basal plane.For the dual-shaped WC hardmetals,both the size effect of prismatic WC and morphological effect of plate-like WC play combined role to increase the degree of crack deflections and amount of transgranular fracture.For dual scale structured plate-like WC hardmetals,the toughening mechanism mainly reflects in the dual scale structure and the plate-like morphological.The area reinforcement effect of fine plate-like WC can leads the crack deflection to occur within coarser plate-like WC and increases the crack deflection,as well as reduces the proportion of WC/WC interface fracture.In addition,plate-like WC also has morphological effect which triggers lots of crack deflections and amount of transgranular fracture of plate-like WC from basal plane.The Lee-Gurland hardness model has been modified,and made it suitable for prismatic WC and plate-like WC grained hardmetals.In addition,by taking the effect of crack deflection into consideration,the Ravichandran facture toughness model for WC-Co hardmetals is modified,and we setup the modified fracture toughness model for WC-Co hardmetals with distributing disorderly plate-like WC grains.