| As wear-resistant machining tool materials,Ti（C,N）-based cermets have gradually replaced expensive WC-Co cemented carbides,showing great application prospects in the fields of cutting tools,thermo-forming molds,seals,and shaft sleeves.The preparation of cermets involves complex thermal and kinetic processes.Different formulations of components lead to diversity of the gradient structure and organizational structures of the materials,which affect their performance.However,limited by traditional research methods,there is still a lack of comprehensive research on the relationship between cermet composition-process-structure-property.It has been shown that using（Ti,Me）（C,N）（Me is W,Mo,Ta,Nb,etc.）solid-solution powder could be a feasible way to jointly improve the strength and toughness of cermets,which is also suitable for batch production without adjusting the traditional process route and equipment.However,there is a scarcity of studies on the preparation of high strength and toughness Ti（C,N）-based cermets from（Ti,Me）（C,N）solid-solution powder.At present,research methods based on thermodynamic calculations combined with key experiments have greatly promoted the research and application of cermets.In this work,based on three types of multi-component solid-solution powders,namely（Ti,W）（C,N）,（Ti,Mo）（C,N）,and（Ti,Nb）（C,N）,the effect of C content,N partial pressure and multicomponent（Ti,Me）（C,N）solid-solution powders on the structure and properties of cermets were systematically studied by performing key experiments and thermodynamic calculations.The innovative research results of this work are as follows:（1）The effect of different C addition on the gradient structure of（Ti,Mo）（C,N）-based cermets was studied,and the formation mechanism of gradient structure of cermets was discussed.The content of C in the binder phase of（Ti,Mo）（C,N）-Ni and Ti（C,N）-Mo2C-Ni cermets prepared in the denitrification atmosphere shows the trend of high surface concentration and low core concentration,which is due to the thermodynamic coupling of Ti-C and the rapid migration of C atoms in the binder phase Ni,resulting in the gradient distribution of C in the binder.At the same time,the Ni-rich gradient layer becomes thinner with the increase of C content,which is because the surface enrichment of free C reduces the local activity of Ti atoms on the surface and inhibits their migration to the core so that the hard phase grain can be retained on the cermet surface.The increase of C content suppresses the solubility of Ti and Mo in the binder phase,inhibits the grain growth of the hard phase,and reduces the spherical phase.（2）The effects of different partial pressure of N on the structure and mechanical properties of（Ti,Nb）（C,N）-based cermets were studied.The existence of Nb can effectively improve the wettability of hard phase and binder phase,and avoid the phenomenon of merging and growing between particles.The Ni-rich gradient layer can be formed on the surface of（Ti,Nb）（C,N）-based cermets when sintered in a vacuum,which is due to the higher N equilibrium partial pressure and the enhancement of N concentration gradient of（Ti,Nb）（C,N）-based cermets.The hard phase particles in the surface layer grow abnormally when the cermets are sintered in the N2 atmosphere of 100 mbar and 400 mbar,and this phenomenon is more obvious with the increase of N partial pressure.The fracture toughness of（Ti,Nb）（C,N）-based cermets increases at first and then decreases slightly with the increase of N partial pressure.（3）The effect of（Ti,Nb）（C,N）solid solution powder on the microstructure,gradient structure,and properties of cermets was studied.Compared with Ti（C,N）,（Ti,Nb）（C,N）has poor thermodynamic stability,the addition of Nb increases the N equilibrium partial pressure of（Ti,Nb）（C,N）-based cermets,and denitrification is more likely to occur during vacuum sintering.Compared with Ti（C,N）+Nb C cermets,（Ti,Nb）（C,N）-based cermets tend to form a free/weak core-rim structure.The cermet prepared by（Ti,Nb）（C,N）solid solution powder can effectively inhibit the formation of a Ni-rich gradient layer on the surface.Compared with the cermets prepared by Ti（C,N）,the hardness of the cermets prepared by（Ti,Nb）（C,N）is increased by 11%,and its high temperature oxidation resistance（at 800°C）has been improved by 67%.（4）The effects of the conventional mixed powder and new（Ti,W）（C,N）solid solution powder on the structure and properties of cermets were studied.The cermets prepared by conventional powder mixing and new（Ti,W）（C,N）solid solution powders are composed of fcc-（Ti,Me）（C,N）and fcc-（Co-Ni）phases.Due to the differences in the stability of Ti C,Ti N,Ti（C,N）,and（Ti,W）（C,N）phases and their solubility in（Co-Ni）,the lattice constants of each cermet phase are slightly different.Powder composition segregation leads to defects such as dislocations and internal stress in the hard phase particles of（Ti,W）（C,N）cermet.Under the action of external force,cleavage steps are easily formed in the grains,which makes the crack propagation more difficult.At the same time,the bonding strength between hard phase and binder phase can be greatly improved by using solid solution powder.The direct contact between Ti（C,N）and（Co-Ni）binder phase with undesirable wettability is avoided,and the crack initiation tendency is reduced and the fracture toughness is improved.Compared with the cermets prepared by conventional powder mixing,the wear rate in a wide temperature range and corrosion rate of cermets prepared by（Ti,W）（C,N）solid solution powder are greatly reduced.In summary,This work focuses on the role of C addition,N pressure,（Ti,Me）（C,N）solid solution powder in cermets.The relationship between thermodynamic properties of（Ti,Me）（C,N）and core-rim structure,gradient structure,and mechanical properties of cermets was revealed by using the scientific method of thermodynamic calculation and key experiments,so as to provide some important theoretical guidance and technical reference for the fabrication of hard yet tough cermets using multicomponent solid-solution powders. |
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