Theoretical Design And Experimental Study Of Graded Cemented Carbides Based On Carbonitride

As a composite material with high hardness,high strength,good wear resistance and corrosion resistance,cemented carbide is widely used in industrial fields such as high-speed cutting,exploration drilling,mine-excavation.In order to improve its mechanical properties,a hard coating is usually covered on its surface by physical vapor deposition or chemical vapor deposition.Due to the difference of thermal expansion coefficient between substrate and coating material,it is easy to generate congenital crack source at the coating surface,and then cause crack propagation in various heavyduty processing and extremely harsh service environment,which significantly shortens the service life of cemented carbide tools.In order to restrain crack propagation and prolong tool life,a new type of WC-Ti(C,N)-Co based graded cemented carbides came into being.Through microstructure design,it gives the organic combination of strength and toughness of the alloy surface and core,greatly improves the service performance of the alloy and prolongs its service life.The preparation process of graded cemented carbides involves the theory of thermodynamics and kinetics.Both process parameters and alloy composition will affect the microstructure and properties of graded layer.At present,through the combination of theoretical calculation and experiment,reasonable parameters can be selected in complex environment,which greatly improves the efficiency of preparing new graded cemented carbides.In this paper,multi-layer graded cemented carbides were prepared through certain thermodynamic calculations and key experiments.The effects of carbides and nitrides systems of Ti,Zr and Hf with miscibility gap on the graded structure were systematically studied.The main research contents and innovations of this paper are as follows:(1)Several kinds of multi-layer graded cemented carbides of TiC-ZrC-HfC system were prepared by means of theoretical calculation and key experiments.The introduction of carbides miscibility gap provides a variety of possibilities for the evolution of graded structure.Unlike the traditional single-layer gradient of fcc-freesurface-layer and fcc-rich-surface-layer,the new graded cemented carbides have graded structure and element distribution in several cases.(2)The TiN-ZrN-HfN graded cemented carbides with fcc-free-surface-layer were prepared by denitrification process.The effects of different nitrogen sources and various nitrogen source combinations on the graded surface layer were systematically studied.The thickness of graded layer of nitrides system and the formation laws of residual cubic phase in surface graded layer were analyzed and summarized,combined with their element distributions,the effects of different nitrogen sources on cobalt distribution in graded cemented carbides and microhardness in graded direction were discussed.(3)Based on thermodynamics and kinetics,the diffusion behavior of Ti,Zr and Hf in their carbides and nitrides graded cemented carbides system were systematically studied.It was proposed for the first time that high-temperature spinodal decomposition can affect the diffusion behavior of transition metal atoms as a bidirectional driving force in the process of liquid phase sintering,The element distribution process of graded cemented carbides under N-coupling mechanism and spinodal decomposition mechanism is analyzed,which provides a new idea for the design and preparation of novel graded cemented carbides.