Research On The Microstructure Controlt And Cutting Performance Of Graphene/WC Based Graded Nanocomposite Tools

Precision and ultra-precision machining,including cutting,is one of the twelve key breakthroughs of advanced basic technology in consolidation of industrial foundation(one of the five major programs of "Made in China 2025").High-speed cutting tool technology,the core element of high-speed cutting,raised higher requirements for the performance of tool materials.The hardness of cemented carbide tool is slightly lower than that of ceramics,cubic boron nitride and diamond,but much higher than that of high speed steel.The toughness of cemented carbide tool is slightly lower than that of high speed steel,but much higher than that of ceramics,cubic boron nitride and diamond.Thus,cemented carbide enjoys an irreplaceable status in the tool industry.Recently there have been concerns about the challenges associated with the contradiction of hardness vs toughness as well as the use of Co,i.e.price instability(the major incentive for alternative binder development),toxicity and properties degeneration.Thus,three kinds of novel WC based tool materials,MLG(multilayer graphene)/WC-Co graded cemented carbide,binderless cemented carbide and MLG/WC-Al2O3-TiC,are developed in this desertation,further expanding the application of cemented carbide cutting tools.With a two-dimensional layered structure and large specific surface area,graphne is easy to agglomerate.The distribution uniformity of graphene in the materials matrix has an important impact on its reinforcing effect.The suitable dispersing media and dispersants were chosen.The results showed that MLG can be best dispersed in N-methyl-2-pyrrolidone(NMP)with polyvinylpyrrolidone(PVP)being the dispersing agent.The dispersing mechanism was the synergistic effect of electrostatic dispersion mechanism and steric hindrance dispersion mechanism.Nano-laminated WC-Co composites are prepared by introducing MLG as the superior cobalt gradient stability enhancement phase.It was found that MLG with a predesigned gradient opposite the predesigned cobalt gradient could maximally enhance the stability of predesigned cobalt gradient during liquid phase sintering.The three crucial cobalt gradient stability enhancing mechanisms are MLG preferential orientation,MLG induced generation of the liquid cobalt phase and MLG wrapping matrix grains.Functionally graded WC-Al2O3-TiC-Cr3C2-VC tool materials are synthesized by employing two-step hot-pressing sintering(TSS).Results show that the highest densification,the most homogeneous dispersion of nano-particulates in WC matrix as well as the excellent mechanical properties are achieved for Al2O3-TiC additions with Al2O3/(Al2O3+TiC)weight ratio 0.6 together with Cr3C2/(Cr3C2+VC)weight ration 0.6.Densification mechanisms are studied.Nano-grained Al2O3 and Cr3C2 facilitate the sintering by producing a small amount of liquid phase,whereas nano-grained TiC promotes the densification process by forming(Ti0.8,W0.2)C solid solution phase and introducing defects(case displacement,dislocation,etc.).Toughening mechanisms are studied in this paper,including dispersion toughening,microcrack toughening,intracrystalline toughening,two-step sintering induced a small amount of plate-like WC toughening,multi-dimensional multi-scale toughening and gradient structure introduced surface residual compressive stress toughening.In addition,a large number of dislocations found by TEM possess an indispensable contribution to the toughening of the materials.MLG-reinforced graded WC-based nono-composites were prepared through TSS.Multi-stage toughening mechanism is proposed by combining structural toughening and component toughening.The theoretical system of multi-stage toughening cutting tools is constructed by combining multi-stage toughening with friction reduction and lubrication effect.Primary toughening mechanism:gradient structure introduced surface residual compressive stress toughening;secondary toughening mechanism:MLG toughening,including MLG bending,wrapping,MLG wall and network,grains bridging by MLG,MLG induced weak interface,MLG pullout,crack deflection,crack bridging and crack stopping;tertiary toughening mechanism:multi-component nanoparticles toughening,including grain refinement,substructure as well as dislocation toughening.The dramatic improvement in anti-friction and anti-wear performance is attributed to the self-lubrication effect of MLG and easily formed friction layer in the contact interface.Furthermore,the unrivalled thermal conductivity of MLG and its rather significant effect in inhibiting the grain growth are the important contribution to the improved anti-friction and anti-wear performance.The cutting performances of two kinds of developed tools named MLG/WC-Co and WCAT0.6CrV are studied in dry turning of pure iron.Compared with YG6,the two developed cutting tools induce lower cutting forces,cutting temperatures and tool-chip average friction coefficients with a bigger shear angles and tool lives.The cutting performance improving mechanisms of two kinds of tools are revealed.Firstly,the wear resistances of two kinds of cutting tools are improved because of the remarkable improvement of mechanical properties.Secondly,for MLG/WC-Co,graphene lubrication laminated friction layer at the tool-chip interface in the cutting process,reducing the friction and adhesions.The higher surface hardness of WCAT0.6CrV reduces the friction at the tool-chip interface.Thirdly,the thermal conductivity of MLG can reach 6000W/(m·K),which is 1.5 times higher than that of the diamond possessing the highest thermal conductivity in natural materials.The uniformly dispersed MLG can make the cutting heat transfer out,reducing the cutting temperature at the tool tip and improving the tool life of MLG/WC-Co.Employing nano-Al2O3 and nano-TiC as ceramic binder phase,WCAT0.6CrV tool has better anti-oxidation wear and anti-bonding wear ability in comparison with YG6 tool with the low melting point metal Co binder phase.