Research On The Coated Cutting Tools Of Functionally Graded Cemented Carbide Substrates With FCC-rich Surface

Cemented carbides have been used widely as cutting and mining tools due to their high wear resistance and strength.The applications of conventional cemented carbides are significantly limited due to the problem that wear resistance and toughness cannot be improved simultaneously for conventional cemented carbides with homogeneous microstructures.Functionally graded cemented carbides(FGCs)with fcc phase-riched surface can solve this problem with high wear resisting surface and high toughness core.Moreover,the fcc phase-riched surface can improve the wear resistance of surface and the thermal expansion coefficient difference can be reduced so the binding strength can be improved between FGC substrates and coatings.Currently,the research on FGCs with fcc phase-riched surface has been solely focused on Ti C containing carbides and the carbide substrates for cutter coatings are mainly conventional high toughness carbides with homogeneous microstructures.In this study,two types of Zr containing FGCs,WC-Co-Zr and WC-Ni-Zr,have been synthesized through high pressure nitrogen infiltration process.The effects of binder type,binder contents,and carbon contents have been investigated on the formation process and microstructure evolution mechanisms of gradient microstructures.Furthermore,these Zr containing FGCs with fcc phase-riched surface have been creatively used as the carbide substrates for cutting coatings.The effect of these FGC substrates has been investigated on both the wearing and cutting performance of coated cutters.It is found that,1.Zr N(fcc phase)-riched surface layer formed for both WC-Co-Zr and WC-Ni-Zr carbides during nitrogen infiltration process in an outward growth mode.2.For both WC-Co-Zr and WC-Ni-Zr carbides,the surface layer thickness increases with binder content.Compared with WC-Co-Zr carbide,WC-Ni-Zr carbide has thicker but looser structured surface layer.3.For both WC-Co-Zr and WC-Ni-Zr carbides,the surface layer thickness increases with the decrease of carbon content.Compared with that for WC-Co-Zr,the surface layer thickness for WC-Ni-Zr increases more significantly with the decrease of WC equivalent carbon content,while the surface layer for WC-Ni-Zr has more defects.4.The binder and WC equivalent carbon contents have different effects on mechanical properties of WC-Co-Zr and WC-Ni-Zr carbides.Both the increase of binder content and the decrease of carbon content lead to the decrease of sintered densities.The increase of binder content results in the decrease of hardness and the increase of toughness.The WC equivalent carbon content has little effect on both hardness and toughness.Compared with WC-Co-Zr,WC-Ni-Zr has lower hardness but higher toughness.5.Compared with the carbides with homogeneous microstructure,the FGCs shows lower friction coefficient and wear rate during friction and wear testing.Compared with the coating on the carbide substrate with homogeneous microstructure,the coating on FGC substrates shows lower friction coefficient and wear rate during friction and wear testing.6.The scratch testing shows that for WC-Co-Zr carbide,the bonding strength between coating and FGC substrate is higher than that between coating and carbide substrate with homogeneous microstructure while for WC-Ni-Zr carbide,the binding strength trend is opposite.7.For both WC-Co-Zr and WC-Ni-Zr cutting tools,the cutting test for HT250 shows that the cutter wear rate sequence from the highest to the lowest is the cutter with homogeneous microstructure,the FGC cutter,the coated cutter on the carbide substrate with homogeneous microstructure,and the coated cutter on the FGC substrate.The cutter wear modes include adhesive wear,oxidation wear,and abrasive wear.8.For both WC-Co-Zr and WC-Ni-Zr cutters,the cutting test for P20 shows that the cutter wear rate sequence from the highest to the lowest is the cutter with homogeneous microstructure,the FGC cutter,the coated cutter on the carbide substrate with homogeneous microstructure,and the coated cutter on the FGC substrate.The cutter wear modes include mainly groove wear and other modes such as adhesive wear,oxidation wear,and abrasive wear.