| TiCN-based cermets are promising materials in cutting tools,due to their high hardness at elevated temperature,excellent wear resistance and very low friction coefficient to stainless steel,superior oxidation resistance etc.However,their potential applications in the field of cutting tools is restricted by their low strength and toughiness.In this paper,TiCN powders were synthesized with TiCl4 and carbon black by hydrolysis precipitation-carbothemal reduction and nitridation method.The powder products were characterized by differential scanning calorimetry(DSC),X-ray diffraction(XRD)and scanning electron microscopy(SEM).Two kinds of cermet materials were prepared by ultrafine titanium carbonitride powder as the raw mgaterial.The matrix of one kind of cermet is ultrafine titanium carbonitride powder,and the matrix of the other kind of cermet is the mixed powder of ultrafine titanium carbonitride and micron titanium carbonitride.In addition,chromium,boron,cobalt and nickel powders were mixed by mechan ical alloying to prepare CoNiCrx and CoNiBx alloy powders.The influence.of CoNiCrx and CoNiBx on the mechanical properties of titanium carbonitride based cermets was also discussed.In the synthesis process of titanium carbonitride powder,Titanium atoms could:only exist in a compound.of TiO2.TiO2 and carbon black formed agglomerate when baking at 350℃,for 2h.Titanium oxide transformed into a complex compound of TiCxNyOz during carbothermal reduction and nitridation process at 1200～1400℃.Nitrogen accelerate the transformation proeesses from TiO2 to TiCxNyO2.Oxygen atoms were gradually replaced by.carbon and nitrogen atoms to form TiCxNy solid soliution.TiC0.547N0.453 powders contenting 0.3%of oxygen could be synthesized at 1530℃ for 4h.The ultrafine titanium carbonitride powder was used as the raw material of hard phase matrix to prepare cermet,and its microscopic structure and mechanical properties were studied.The results shows that the mechanical properties of cermet are better at 1450℃ wih sintering for 1h.Its relative density is 99.4%,bending strength is 153 QMPa andickcrs hardness is 17893.Hv30.By SEM-BSE and SEM-EDS,t wsa found that the structure of cermet presents obvious black-core,gray-ring structure and white phase.Black-core and grey-ring are the same structure,which are recarded as(Ti,Me)(C,N)(Me=W,Mo,Nb)phase,while the white phase is(W,Mo,Ti,Nb)(Co,Ni)C.The fracture mode of materials is brittle fracture,including transgranlar fracture and intergranu lar fracture.Transgranular fracture mainly occurs in white phase.The mixed powders were used to produce cermet The microstructure and mechanieal properties of cermet were investigated.The results shows that when the content of u ltrafine titanium carbonitride powder is 20wt%,the mtaterial has the best performance,its density,bending strength and Vickers hardness are 99.6%,1672MPa and 1702Hv30 respectively.There were two kinds of "blak-core" phases with different compositions in cermet,one is titanium carbonitride particles which were not completely dissolved in the shape of large bars,the other was solid solution formed by carbide such as TiCN,WC,Mo2Ci,etc.The fracture mode.of cermet is tra sgranular fracture and intergranular fracture.transgr anular fracture mainly occurs in the solid solution of titanium carbonitride which is not completely dissolved.Because of the addition of ultrafine titanium carbonitride particles to refine the grain,crack deflection and microcrack were easy to occur at the fine grain during the fracture prrocess.Boron and chrom ium elements were added into the b;onder phase by mechanical alloying,and the process of mechanical alloying and the effect of boron and chromium elements on the mechanical properties of titanium carbonitride based cermets were studied.The results shows that with the increase of chromium and boron content,the physical properties of the materials are significantly reduced.Th erefore,boron and chromium are not suitable to be added to cermet in large quantities. |
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