The Research On The Preparation,Microstructure And Mechanical Properties Of(Ti,M)C-based Cermets

Ti(C,N)-based cermets are promising materials in producing high performance tools and dies, due to their high hardness at elevated temperature, excellent wear resistance and very low friction coefficient to stainless steel, superior oxidation resistance etc. In this paper,(Ti,M)C solid solution powders (M=W, Nb) were synthesized via carbothermal reactions after high-energy ball milling.(Ti,M)C based cermets were produced, in order to obtain better mechanical properties and expand the range of applications. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), oxygen/nitrogen analyzer, carbon/sulfur analyzer, Material Studio7.0, bending strength tester, rockwell hardness and vickers hardness tester were used in this study. Phase evolution, reaction mechanism and the powder characteristics during the synthesis process of (Ti1-x,Wx)C were studied;(Ti,W,Nb)C solid solution was obtained via microwave-assisted carbothermal reduction (MWCR)/carbothermal reduction (CR), respectively, and the effect of microwave irradiation on powder synthesis and characteristics were investigated; effect of changes in chemical composition and methods of adding WC on microstructure and mechnical properties of (Ti1-x,Wx)C-20Ni cermet were studied; effect of Mo and TiC addition on microstructure and mechanical properties of (Ti,W)C based cermet were also analyzed.The milled oxides were reduced and carburized gradually during carbothermal reduction, the reaction sequences of oxides were determined. The main mechanism of weight transfer during carbothermal reduction was relied on the reactions between CO2and CO. The lattice parameter and crystallite size of (Ti,W)C solid solution powders decreased with increasing W content. The calculation results obtained by CASTEP suggested that the total cell energy of (Ti1-x,Wx)C decreased firstly with increasing W content; the total cell energy reached the lowest value when12.5at.%WC dissolved in TiC, then the total cell energy increased with increasing W content; the bulk modulus increased with increasing W content while the shear modulus appeared reverse variation tendency.Using a cost-effective and timesaving process, ultrafine (Ti,W,Nb)C solid solution powders with size in the range of100～350nm were produced via MWCR at only1250℃for30min. Microwave irradiation accelerates the reactions during deoxidization and carburization process; the complete (Ti,W,Nb)C solid solution can be formed at relative lower temperature (100～200℃) for a shorter holding time than conventional CR process.(Ti1-x,Wx)C solid solutions were used to produce cermet. The microstructure and mechanical properties of (Ti1-x,Wx)C-20Ni cermets were investigated. The result showed that (Ti1-x,Wx)C based cermets do not show obvious core/rim structure, the ceramic grains were refined while the number of pores decreased with the increase of W content. Compared with the cermet produced from TiC and WC, the (Ti,W)C solid solution based cermet possesses higher toughness but lower hardness.The (Tio.68,Wo.32)C-Mo-Ni cermets contain a weak dark core-grey outer rim structure. When Mo was added, the dark core was transformed into (Ti,W,Mo)C solid solutions. The concentration of Mo atom in (Ti,W,Mo)C solid solution increased with increasing Mo addition. The morphology of ceramic grains in (Ti,W)C-30Ni cermets is relatively refined and becomes spherical with increasing Mo content. However, more pores and inhomogeneous of ceramic grains were exhibited when excessive Mo was added (20wt.%).Ceramic grains with weak core/rim structure and black core/grey outer rim coexisted in (Tio.68,Wo.32)C-15Mo-30Ni cermet with TiC addition. The ceramic grain size appeared "double peak" distribution. The bigger ceramic grains (-3.5μm) distributed uniformly in the microstructure which would cause crack deflection and improve the toughness of the material.