The Regulation Of Core-Rim Interface And Mechanical Properties Of Ti(C,N)-Based Cermets

Owing to the advantages of high hardness,low friction coefficient with steel and good bending strength,Ti（C,N）-based cermets is an ideal candidate material for stainless steel,hardened steel,copper and other non-ferrous metal machining tool and hot working die.However,compared with traditional tungsten-cobalt carbide,the wettability of Ti（C,N）and Ni/Co is not ideal,leading to the lack of strength and toughness of Ti（C,N）-based cermets.To improve the bonding strength between Ti（C,N）hard phase and metal phase,secondary carbides such as molybdenum carbide and tungsten carbide with good wettability of metal phase are mainly added.Through the liquid phase sintering dissolution-precipitation process,equilibrium phase（Ti,W,Mo...）（C,N）solid solution is precipitated on the surface of undissolved Ti（C,N）particles,forming a core-rim structure.The core-rim structure is characterized by multiple internal interfaces and composition differences,and its microstructure and gradient composition determine the final mechanical properties of Ti（C,N）-based cermets.Aiming at the problems such as incomplete inner rim phase structure,large difference of core-rim phase and poor interface matching caused by nitrogen removal reaction in sintering process,the effects of sintering atmosphere（vacuum,nitrogen partial pressure）and sintering system,secondary carbide addition,metal phase composition and nano-titanium carbide addition on microstructure and mechanical properties of cermet are systematically studied in this paper.According to the metallurgical reaction law of core-rim structure,controlled solid phase nitrogen partial pressure sintering technology and titanium-rich bonding phase were used to construct solid phase diffusion mass transfer and nitrogen equilibrium sintering conditions conducive to inner rim phase precipitation,which is expected to provid technical ideas and references for the preparation of high strength and toughness Ti（C,N）-based cermets based on core-rim phase interface regulation.The main content and results of this paper are as follows:（1）Ti（C,N）-based cermets was prepared by vacuum sintering,sintering under different atmospheres and controlled solid phase nitrogen partial pressure sintering technology,respectively.The effects of different sintering atmospheres and systems on the microstructure and mechanical properties of Ti（C,N）-based cermets were studied.It was found that the cermet prepared by solid nitrogen partial pressure sintering had the best comprehensive properties.At 1200℃,cermets undergo rapid nitrogen removal under vacuum environment,resulting in the generation of carbon-deficient M₆C phase and incomplete rim phase after sintering,which reduces the fracture toughness of materials.Controlled solid phase nitrogen partial pressure sintering technology could inhibit nitrogen removal at opening stage,increasing the grain size of core-rim phase and black Ti（C,N）core phase,and significantly improve the mechanical properties of cermets.（2）The effects of Ni/Co ratio,Cu powder addition and Ti-rich bonding on the microstructure and mechanical properties of cermet were studied.With the decrease of Ni/Co ratio,the particle size of core-rim phase decreases and the thickness of the rim phase becomes thickens.When the Ni/Co ratio is 13/7,the mechanical properties of Ti（C,N）-based cermets are the best,the bending strength of the cermet is 1898 MPa and the Rockwell hardness is 89.2 HRA,the fracture toughness K_IC is 11.5MPa·m^1/2.High activity titanium rich binder（Ni-Ti-C）powder was prepared by high energy ball milling.The titanium rich binder could inhibit the growth of Ti（C,N）by accelerating the dissolution and precipitation process,reduce the core-rim interface lattice mismatch.As the content of Ti-C in the bonding phase increases from 0 wt.%to 20 wt.%,the average grain size of cermet decreases from 0.79μm to 0.63μm,and the bending strength of the material reaches 2073 MPa.（3）The effects of secondary carbide addition on microstructure,core-rim interface structure and mechanical properties of Ti（C,N）-based cermets were studied.The addition of VC and Zr C effectively inhibit the growth of grain,and Ta C and Nb C make the rim phase transition thicker.Compared with Ta C,Zr C and Nb C,the addition of VC can shrink the lattice of Ti（C,N）,and reduce the lattice mismatch of inner interface from 4.0%to0.8%.The reduction of lattice mismatch at the rim-binder interface causes the fracture mode of cermet to change from intergranular fracture to transgranular fracture.Due to strengthening the interface between the rim phase and the binder phase,the cermet containing VC has obtained excellent mechanical properties.Its bending strength is 2099 Mpa,and its fracture toughness is 10.3 MPa·m^1/2.（4）Nano-Ti C can accelerate the solution-precipitation reaction process,promote the precipitation of white core-rim phase,reduce the size of the core phase and increase the thickness of the ring phase.By changing the equilibrium crystal form of（Ti,M）（C,N）in Ni-Co,nano-Ti C transforms from cube to octahedron,which greatly affects the interface between the core phase and the bond phase.The orientation relationship of nano-Tic changes from（200）core phase//（111）bonded phase to（111）core phase//（111）binder phase.Submicron Ti（C,N）-based cermets with an average grain size of 0.62μm were prepared using ultra-fine Ti（C,N）powders as precursor,which were synthesized by high-energy activation-heat treatment process.The mechanical properties of submicron Ti（C,N）-based cermets are 1960±135 MPa（TRS）,1750±40 N/mm²(HV₁₀)and 9.2MPa·m^1/2,respectively.