Study On Material System Development,Processing,and Microstructure Of Functionally Graded Cemented Carbides With Cubic Phase Rich Surface

Functionally graded carbides?FGCs?with cubic phase rich surface have significant advantages over conventional carbides.Due to the facts that the surface layer of FGCs is cubic phase of nitrides or carbon nitrides with higher hardness and hence higher wear resistance compared with core section,and the intermediate layer of FGCs has better fracture toughness than the surface layer,FGCs enjoy the synergy of high wear resistance and fracture toughness,which is not easily obtained for conventional carbides.Therefore,FGCs can be directly used as cutting tools for machining,mining,and energy exploration.Furthermore,cubic phase rich surface layer in FGCs,produced through nitriding process,can effectively reduce the difference in coefficient of thermal expansion between carbide substrate and coatings,making FGCs an ideal substrate for coating.Currently,the research on material system for FGCs with cubic phase rich surface is focused on WC-Ti C-Co system and little other material system has been reported yet.The research on the processing and gradient structure mechanisms is quite incomplete so that the precision control of gradient structure cannot be achieved,which significantly limits the application of FGCs with cubic phase rich surface.In this study,high pressure nitriding was used as the main processing.The new carbide materials with Zr and Zr-Ti were creatively employed for the fabrication of FGCs with cubic phase rich surface.The effect of Zr content on microstructure and mechanical behavior of FGCs was systematically investigated.And microstructure evolution modeling of Zr and Zr-Ti containing FGCs with cubic phase rich surface was successfully constructed.This study supports the research on material design and microstructure mechanisms for precision control of gradient structure of FGCs.Moreover,it also provides material and processing development for significant expansion of the applications of FGCs.The study found:1.For WC-Co/Ni-Zr carbides,cubic phase Zr N rich surface layer forms during high pressure nitriding process and the growth mode of surface layer is epitaxial growth.2.For WC-Co/Ni-Zr carbides,when Zr content is 5 wt.%,a uniform and dense Zr N surface layer forms,and when Zr content is 10 wt.%or 15 wt.%,the surface layer is composed of the mixture of Zr N and WC.3.The content of Zr N for WC-Ni-Zr carbides is apparently higher than that for WC-Co-Zr carbides,manifesting that Zr has more significant thermal coupling reaction with nitrogen in WC-Ni-Zr carbides than in WC-Co-Zr carbides,and thus making cubic phase Zr N rich surface layer grow faster.4.For WC-Co/Ni-Zr carbides,the microhardness of surface layer is more than 2400 HV,significantly higher than that of core section?2000-2100 HV?,showing the microstructure-property advantage of WC-Co/Ni-Zr carbides.5.For WC-Ti C-Zr-Co/Ni carbides,cubic phase Zr N rich surface layer forms during high pressure nitriding while the contribution of Ti C to the formation of surface layer is suppressed,showing that Zr is a more effective component for gradient structure formation as compared with Ti C.As compared with the gradient surface layer of WC-Ti C-Co/Ni,the gradient surface layer of WC-Ti C-Zr-Co/Ni is thinner,more uniform,and almost WC free.Along with the increase of Zr content,the gradient surface layer of WC-Ti C-Zr-Co/Ni becomes thicker but has more WC and higher porosity.6.For WC-Ti-Zr-Co/Ni carbides,cubic phase Ti N rich surface layer forms during high pressure nitriding and no Zr N is observed.The gradient surface layer is thin and has high porosity and much cracks.As compared with WC-Ti-Co/Ni carbide,WC-Ti-Zr-Co/Ni carbide has slightly lower relative density and its core section has similar hardness but 52.4%higher fracture toughness,but still as low as 7.40 MPa·m^1/2,demonstrating that WC-Ti-Zr-Co/Ni is not an ideal material candidate for FGCs.7.The high pressure nitriding process for WC-Co/Ni-Zr carbides can be divided into four stages,which include?1?the alloying of Co/Ni and Zr,formation of liquid phase,and the densification process,?2?the iteraction of high pressure nitrogen with liquid metal and the dissolution of nitrogen into liquid metal,?3?the formation of gradient surface layer,and?4?the Zr diffusion in sub-surface region and the continuous growing of gradient surface layer.8.The dynamics modeling for gradient layer growth of WC-Zr-Co/Ni carbides is:???where X is gradient layer thickness,D_[Zr],sis the diffusion coefficient of Zr in gradient surface layer,C_[Zr],i is the concentration of Zr in liquid metal of inner surface of gradient surface layer,C_{Zr N} is the concentration of Zr in gradient surface layer.This modeling shows that the gradient layer growth is mainly controlled by the diffusion coefficient of Zr in gradient surface layer and the concentration of Zr in liquid metal of inner surface of gradient surface layer.