Effect Of Diamond/La₂O₃ Addition On The Sintering Behavior,Microstructure And Mechanical Properties Of Ti（C,N）-based Cermets

Ti（C,N）-based cermets are regard as a promising substitute for traditional WC-Co cement to make high-speed and high-precision cutting tools,due to self-lubricating,wear resistance,and high chemical stability,etc.Insufficient strength and toughness is still the key factor restricting the development of Ti（C,N）-based cermets.In this paper,the role of diamond or La₂O₃additive on the sintering behavior,microstructure and mechanical properties were studied.The aim of this work was to better understand the physical and chemical reactions of cermets during sintering,discuss the effects of diamond and La₂O₃ on the sintering behavior,microstructure and mechanical properties of cermets,elucidate the mechanism of the two additives,and thus provide a guide to the design of high-performance cermets.（1）Combining thermal analysis and dilatometry experiments,the sintering process of cermet was investigated systematically.The key factors affecting its final structure and mechanical properties were revealed.During 1246～1320℃,the cermet undergoes solid phase diffusion reaction,accompanied by endothermic and denitrification behaviour.During 1375～1392℃,with the full melting of the metallic binder phase,the cermet enters liquid phase sintering stage.Based on the analysis of densification kinetics,it is revealed that the dissolution-precipitation stage dominated by the interface-reaction mechanism,which can explain the relationship between the number of reaction sites and the densification of cermets.The hardness,bending strength and plane fracture toughness of cermets prepared by vacuum sintering at 1450℃for 1h are 1520 HV₃₀,1592 MPa and 11.30 MPa·m^1/2,respectively.Their microstructure can be characterised by three typical structures,including“white core-grey rim”,“black core-grey rim”and“rimless-Ti（C,N）”.The white core grains exhibit better chemical activity than Ti（C,N）grains,which is conducive to the construction of low internal stress“core-rim”interface.This provides an important theoretical basis for the subsequent research.（2）Diamond additive can promote the solid diffusion reaction and dissolution-precipitation process of cermets.The solid phase diffusion reaction between W,Mo and Ti（C,N）is enhanced,thus more multicomponent solid solution are generated.During the subsequent dissolution-precipitation stage,these solid solution particles become more effective precipitation sites than Ti（C,N）.Owing to their appreciable dispersion uniformity and highly activated C atoms formed via diamond graphitization,diamond shows a more significant effect on the densification and the uniformity of final sintered structure.With the increase of diamond content,the number of“white core-grey rim”structure increases,while the volume fraction of black core decreases gradually.The cermet with a 0.6wt.%diamond content exhibited optimal mechanical properties with a hardness of 1835 HV₃₀,transverse rupture strength of 2026 MPa,and plane fracture toughness of 12.95 MPa·m^1/2.This is due to grain refinement and the uniformly distributed“white core/grey rim”grains.When diamond was excessive,the integrity of“black core-grey rim”structures decrease significantly.however,the“white core-grey rim”structures remained intact and the white cores coarsened obviously.（3）Due to the interface-purification and directional migration of La element,La₂O₃ additive has an important influence on the solid phase diffusion reaction and dissolution-precipitation process of cermets.La₂O₃was believed to convert into La under the reduction of carbon donating component（Mo₂C）.Metallic La tended to distribute at the surface of Ti（C,N）particles with more oxide impurities and plays a role in removing impurities and purifying the interface.This not only promotes the formation of coherent interface between binder phase and Ti（C,N）,but also further reduces the stress of core/rim interface.With the increase of La₂O₃ addition（0～0.6wt.%）,the denitrogenation event of cermets moved towards higher temperature,and its adverse effect on sintering densification was gradually intensified.The volume fraction of black Ti（C,N）in microstructure increased first and then decreased,while the volume fraction of white core phase increased and the rim phase decreased monotonously.When La₂O₃content reached 0.2wt.%,the optimal mechanical properties were obtained with a hardness value of1670HV₃₀,a bending strength value of 1884 MPa and a fracture toughness value of 16.8 MPa·m^1/2.With excessive ultrafine La₂O₃additive（>0.2wt.%）,the agglomeration of binder phase were observed,causing a sharp decline of mechanical properties and obstruction to densification.