Research On The Microstructure And Properties Of Double Ti(C,N)-based Cermets

Ti(C,N)-based cermets are mainly used as cutting tools,however,the low toughness of the cermets limits applications.The structure design of Ti(C,N)-based cermets can expand application including semi finishing machining,finishing machining and oil well drill bit inserts and improve effective of finishing machining.Based on the needs of Ti(C,N)-based cermets in practical engineering applications,it is very necessary and urgent to carry out a systematic theoretical analysis and experimental research on the material design.On the basis of fracture mechanics of materials,Ti(C,N)-based cermets with the different composite structures was designed.The specimens of Ti(C,N)-based cermets with the different composite structures were prepared by sintering of gas pressure,vacuum and spark plasma.Effects of WC-Co granules,Ti(C,N)-based cermets granules,double structure and NiTi fiber on the microstructure and mechanical properties of Ti(C,N)based cermets were studied.The main research works are summarized as follows:(1)Ti(C,N)-based cermets with various WC-Co granules were prepared by vacuum sintering.Effect of WC-Co granules addition on the microstructure and mechanical properties of Ti(C,N)-based cermets was studied.The results show that the WC-Co granules are surrounded by the Ti(C,N)based cermets homogeneously.A new transitional layer,which,is the mixture of intermetallic compound and(Ti,W,Mo)C was found between the WC-Co granules and the matrix of Ti(C,N)based cermets.With the increase of rough WC-Co granules content,transverse rupture strength TRS and fracture toughness and relative density increase firstly,and decrease with the further increase of rough WC-Co granules content.The hardness decreases obviously with increasing rough WC-Co granules.With the increase of fine WC-Co granules content,TRS,fracture toughness and relative density increase.The hardness decreases obviously with increasing fine WC-Co granules.The toughening mechanisms were characterized by crack deflection,branching.(2)Ti(C,N)-based cermets with various Ti(C,N)-based cermets granules were prepared by gas pressure sintering.Effect of Ti(C,N)-based cermets granule content on the microstructure and mechanical properties of Ti(C,N)-based cermets were studied.Results show that the Ti(C,N)-based cermets granules are surrounded by the matrix homogeneously.With the increase of Ti(C,N)based cermets granules content,TRS decreases obviously,while hardness has the opposite trend.Fracture toughness firstly increases,then decreases with the further increase of granules.The toughening mechanisms was characterized by crack branching and high fracture energy of coarse granule.(3)Double Ti(C,N)-based cermets with various Ti(C,N)based cermets granules size and WC content were prepared by vacuum sintering.The effect of Ti(C,N)based cermets granules size on the microstructure,mechanical properties and fracture behavior of double Ti(C,N)based cermets was investigated.Results show that the double Ti(C,N)based cermets consists of Ti(C,N)based cermets granules and the matrix.Ti(C,N)based cermets granules distribute homogeneously in the matrix.The matrix is composite of binder Co(primary),a black core/grey rim,a black core/white inner rim/grey outer rim,a white core/grey rim as well as a coreless structure.A new phase with the four layer composite structures was also found.Addition of WC into double Ti(C,N)based cermets should not lead to the change of size of granule.While lager amount of the white core/grey rim as well as the coreless structures of the matrix increase by increasing WC content.With the increase of granule size,TRS and hardness decrease obviously,while fracture toughness has the opposite trend.Addition of WC into double Ti(C,N)based cermets can obtain higher TRS,which is related to the structure of the matrix.With the increase of WC content,TRS and fracture toughness increase,while the hardness decreases.Fracture modes of the cermets are mainly pull-out effect of granule and tear ridges.Double Ti(C,N)based cermets has enhanced fracture toughness owing to extrinsic energy dissipation mechanisms such as crack branch,crack bridge,crack deflection and microcracks and intrinsic energy absorbing mechanisms such as tear ridges.(4)Double Ti(C,N)-based cermets with various Ti(C,N)based cermets granules size were prepared by vacuum sintering.The effect of Ti(C,N)based cermets granules size on the microstructure and mechanical properties of double Ti(C,N)based cermets was investigated.Results show that the Ti(C,N)based cermets granules are surrounded by matrix homogeneously.The matrix is composite of binder Co(primary),a black core/grey rim,a black core/white inner rim/grey outer rim,a white core/grey rim as well as a coreless structure.Double Ti(C,N)based cermets can obtain higher fracture toughness,but it is disadvantageous to transverse rupture strength(TRS)and hardness(HRA).With the increase of granule size,TRS and hardness decrease obviously,while fracture toughness has the opposite trend.Fracture modes of the cermets are mainly pull-out effect of granule and tear ridges.The toughening mechanisms was characterized by crack branch,crack bridge,crack deflection and microcracks.(5)Coarse double Ti(C,N)-based cermets with different Ti(C,N)based cermets granules size were prepared by vacuum sintering.Microstructure,mechanical properties and fracture behavior of the coarse double Ti(C,N)based cermets were studied,which is to examine the relationship between microstructure and mechanical properties.Double Ti(C,N)based cermets consist of "the large granules" and the matrix.The large granules distribute homogeneously within the matrix.Coreless structure can be found in the matrix.In order to avoid the transverse rupture strength(TRS)to decrease obviously,the WC addition into double Ti(C,N)based cermet was investigated.It can be found that double Ti(C,N)based cermets with WC addition has no effect on the large granules size.While the white core/grey rim and coreless can be observed in the matrix.With the increase of granule size,TRS and hardness decrease obviously,while fracture toughness increase obviously.Addition of WC into double Ti(C,N)based cermets can obtain higher TRS and fracture toughness.Double Ti(C,N)based cermets have higher fracture toughness than conventional Ti(C,N)based cermets at equivalent mass fraction.Fracture images show that failure mode of the cermets are a mixture of brittle fracture and ductile tearing.Dominant toughening mechanisms including crack deflect,tearing,crack branch,crack bridge and microcracks,can be observed.(6)Ti(C,N)-based cermets with various NiTi fiber were prepared by spark plasma sintering.Effect of NiTi fiber content on the microstructure and mechanical properties of Ti(C,N)-based cermets was studied.Results show that NiTi fibers are surrounded by the matrix homogeneously.Temperature-induced phase transition can be found by Differential Scanning Calorimetry(DSC).With the increase of NiTi fiber content,grain size of hard phase decreases.With the increase of NiTi fiber content,relative density increase firstly,and then decreases obviously,TRS increase obvious,while hardness decreases gradually.The strengthening mechanisms were characterized by superelastic,pull-out and debonding of NiTi fiber.