Research On The Preparation Of Graded Ti(C,N) Based Cermets And Formation Kinetics Of The Fine Grained Surface Layer

Due to the irreconcilable conflict between hardness and strength of homogeneous Ti?C,N?-based cermets and the weak bonding between coating and substrate material,a new method for the fabrication of functional gradient cermets was proposed according to the mass transfer principle in this dissertation.The graded cermets were prepared in one sintering cycle,which were characterized by wear resistant surface enrich in hard phase and tough bulk with higher binder phase.The relationship among manufacturing process,microstructure and mechanical properties of the graded Ti?C,N?-based cermets were systematically investigated by thermal analysis?DTA?,X-ray diffraction?XRD?,scanning electron microscope?SEM?,energy dispersive X-ray detector?EDS?,electron-probe microanalysis?EPMA?,and transmission electron microscope?TEM?.In the first part of the dissertation,the development and present research situation of Ti?C,N?based cermets were critically overviewed,which included the influence of composition and sintering process on the microstructures and properties,the present research situation of graded structure cermets was summarized,and then the preparation principle and formation kinetics of gradient layers of the classical graded structure hardmetals are described.Based on above,the purpose and significance of the dissertation were pointed out.The effects of carbon addition on the densification behavior,microstructure evolution and mechanical properties of Ti?C,N?-based cermets were studied to prepare the graded cermet by the low-pressure carburizing in one sintering cycle.The full densification temperature was almost identical for cermets with carbon addition in the range of 0 to 1 wt.%,but it obviously decreased from 1340°C to 1280°C for cermets with excess carbon addition?up to1.5wt.%?.The carbon addition had no obvious effect on the phase transformation.However,the carbon addition had a great influence on the final microstructure of cermets sintered at 1400°C,inner rim gradually became thicker while outer rim became thinner with the increasing of carbon addition,which was attributed to the fact that the carbon addition influenced the dissolution-precipitation process of carbides in Ni phase.Typically,Ti?C,N?-based cermets with 0 wt.%carbon addition showed good mechanical properties with TRS of 2070MPa and hardness of 88.3HRA.On the basis of the above-mentioned research,the effects of main process parameters of low-pressure carburizing treatment on the microstructure and mechanical properties of Ti?C,N?-based cermets were studied.Some conclusions were drawn as follows:the surface zone deficient in binder was introduced after carburization due to the difference of liquid pressure and the affinity of carbon and the other alloying elements.Surface hardness of the cermets was improved by carburization,while transverse rapture strength was hardly changed.It was found that a considerable thick nickel-deficient layer was introduced by carburization during liquid phase sintering,while the higher amplitude of deficient nickel of the original surface was formed after carburization within the temperature of the triple phase coexistent region.Furthermore,combined with the advantages of the two kind of carbonization processes mentioned above,the thickness of binder-deficient layer and the amplitude of deficient nickel of the surface were800?m and 38.2%,respectively,after the two step carburizing treatment.Accordingly,Ti?C,N?-based cermets carburized by two step treatment showed excellent mechanical properties with surface hardness of 1913HV and TRS of 2084MPa.The effects of nitriding treatment during different sintering stages on the microstructure and mechanical properties of Ti?C,N?based cermets were studied,in order to provide the basis for the preparation of graded cermet by the nitriding treatment in a complete sintering cycle.The results showed that:compared with the cermets sintered in vacuum,the surface microstructures of these cermets changed obviously when nitrogen gas was introduced in entire sintering stage,heating-up stage and final sintering stage.Due to the existence of high nitrogen potential,the rim phase was difficult to form,and the agglomeration and growth of ceramic particles became more serious,resulting in higher porosity,and the mechanical properties decreased dramatically.However,the cermet which nitrogen gas was introduced at cooling stage had formed an about10?m graded surface layer enriched in hard phase,which was attributed to the fact that the high nitrogen activity in the surface region drove the titanium to diffuse outwards and combined with nitrogen.When the nitriding temperature was over 1240?,WC and Mo₂C were precipitated on the surface of the sample,and the morphology of those particles showed a growing trend with the increase of nitriding time.The effects of main process parameters of nitriding treatment under low temperature environment at cooling stage on the microstructure and mechanical properties of Ti?C,N?-based cermets were studied.When the cermets were treated at 1200?for 30h in the nitrogen atmosphere during cooling,a 6.6?m deep hard surface layer enriched in fine grained carbonitride was introduced,which improved significantly the surface hardness of the graded cermets.During nitriding treatment,the high nitrogen activity on the surface caused the rim phase to dissolve gradually into the binder phase and precipitate in different forms,which significantly changed the hard phase appearance of this graded surface layer,mainly composed of fine TiN particles and near spherical hard phase particles with core-shell structure.It was found that at the same nitriding time the thickness of the hard-layer became thicker when the nitriding temperature was incereased;and at the same nitriding temperature the longer nitriding time was,the thicker hard-layer was formed.The development of the graded layer enriched in hard phase was controlled by the diffusion of the alloy elements and the growth rate did not follow the parabolic law.On this basis,the kinetic equation of the formation of the harden layer enrich in fine grained carbonitride was established.According to the equation,the thickness of the hardened layer under different nitriding treatment processes was calculated,which were in good agreement with the experimental results.