Study On Fabrication And Properties Of WC-Al₂O₃ Composite Toughened By Graphene

WC-Co cemented carbide has excellent hardness,toughness,wear resistance and bending strength,and is widely used in cutting tools,drilling tools,drawing dies,wear-resistant coatings and other fields.However,Co is a scarce strategic resource,and the demand of Co in many fields is increasing.In China,Co resources are poor and mainly rely on imports.In order to cope with the increasing demand for Co and the scarcity of resources,many scholars have carried out research on Co-free cemented carbides.Among them,WC-Al₂O₃ composite material has high hardness and excellent corrosion resistance,and it is a kind of Co-free hard composite material with application prospects.However,brittleness is still an important common problem faced by Co-free cemented carbide,which restricts its further engineering application.In recent years,graphene has received extensive attention from academia because of its high specific surface area and excellent mechanical properties.Existing studies have shown that the introduction of graphene can effectively improve the mechanical properties of ceramic materials,and its improvement in the fracture toughness of brittle materials has also been verified by certain experiments.At the same time,graphene also has certain self-lubricating properties,which provides the possibility to further improve the friction and wear properties of the material.Therefore,from the perspective of"microstructure optimization"and"simultaneous improvement of mechanical,friction and wear properties",the author of this article introduces graphene into WC-Al₂O₃ composites in order to further improve the mechanical properties and friction and wear properties of the materials.The specific research work and results are as follows:1.Research and solve the dispersion problem of graphene in WC-Al₂O₃ composite materials,and give full play to the toughening effect of graphene.The difficulty of graphene dispersion is not only to overcome the sedimentation problem caused by the density difference of WC,Al₂O₃ and graphene,but also to retain the original radial size and complete crystal structure of graphene.In view of the above problems,the composite dispersion process was selected and the best solution to obtain WC,Al₂O₃ and graphene composite powders was:ultrasonic dispersion of graphene for 4 hours,planetary ball mill dispersion of WC-Al₂O₃composite powder for 50 hours,and then the graphene and WC,Al₂O₃ powders were mixed and planetary ball milled for 4h.Studies have shown that adding 10vol%polyethylene glycol to absolute ethanol as a dispersant can form a coating effect on the surface of ceramic particles,reduce the attractive force between particles,significantly delay the sedimentation rate of the material in the dispersion,and reduce the layering phenomenon.The dispersion results show that when the graphene content is less than 0.5wt%,the graphene can be uniformly dispersed in the WC,Al₂O₃ composite powder,and its structural integrity is better,the number of layers is small,and the radial original size is basically retained;When the graphene content is higher than 0.7wt%,the chance of contact between graphene increases,and under the action of van der Waals force,adsorption and agglomeration occur again.1.To solve the dispersion problem of graphene in WC-Al₂O₃ composite is the first challenge to give full play of the toughening effect of graphene.The difficulty of dispersing this system is that the grain size of WC-Al₂O₃ composite powder is required to be as small as possible,while the number of layers of graphene is required to be as few as possible,but the original radial size and complete crystal structure of graphene are expected to be retained.In addition,the density difference among WC,Al₂O₃ and graphene is large,so the problem of settlement and stratification will occur during the process of dispersion.In order to solve the dispersion problem,the composite dispersion process was adopted.The dispersion process mainly includes dispersing graphene by ultrasonic wave for 4 hour and ball-milling the WC,Al₂O₃ composite powder for 50 hours,respectively.After that,the dispersed graphene and WC/Al₂O₃composite powder were ball-milled for another 4 hours.It is found that adding 10 vol%polyethylene glycol as dispersant in anhydrous ethanol significantly delay the settling speed of materials in the dispersion and improve the stratification phenomenon,because polyethylene glycol can form coating effect on the surface of ceramic particles and reduce the attraction between particles.It is also found that when the content of graphene is less than 0.5wt%,the dispersion level of graphene in WC and Al₂O₃composite powder is better;where the structure integrity of graphene is better,the number of layers is less,and the original radial size is basically retained.When the content of graphene is higher than 0.7wt%,the contact chance between graphene sheets increases,and the adsorption and agglomeration occur again under the effect of van der Waals force.2.The WC-Al₂O₃-graphene composite material was prepared by hot pressing sintering to obtain a composite material with high density,fine grains,and a basically complete graphene structure.The influence of the introduction of graphene on the microstructure and mechanical properties of the material was studied.The results show that the introduction of graphene refines the grain size of the WC-Al₂O₃ composite material,and simultaneously improves the hardness and fracture toughness of the material.With the addition of graphene,the WC grain size first decreased and then increased,and the hardness and toughness showed a trend of first increasing and then decreasing;when the amount of graphene added was 0.3wt%,the hardness and toughness reached the maximum value of 18.78 GPa and 11.09MPa·m^1/2 increased by 18.7%and 40.8%,respectively.The main toughening mechanism is that the uniformly distributed graphene effectively hinders the crack propagation and enhances the original crack deflection and crack bridging effects in the WC-Al₂O₃ composite.At the same time,the pull-out process of graphene further consumes fracture energy;the graphene distributed between the particles makes the fracture mode change from intercrystalline fracture to transcrystalline fracture,consumes more fracture energy,and improves the fracture toughness of the material.3.The effect of graphene addition on the friction and wear properties of WC-Al₂O₃composites was studied.It is found that with the addition of graphene,the friction coefficient and wear rate first decrease and then increase.Compared with the WC-Al₂O₃ composite without graphene,when 0.5 wt%graphene is added,the friction coefficient reaches the minimum value,which reduces by 41.2%;when 0.3 wt%graphene is added,the wear rate reaches the minimum value,which decreases by64.5%.When 0.3wt%graphene is added,the lowest wear rate of the material is mainly attributed to the fine grain size and optimal mechanical properties.At this time,the wear mechanism is mainly abrasive wear.When 0.5wt%graphene is added,the graphene will undergo shear slip,forming a large number of graphene friction films with fewer layers.The lubricating effect of the friction film significantly reduces the friction coefficient.At the same time,the protective effect of the friction film on the wear surface reduces the wear rate of the material to a certain extent.The wear mechanism is broken and peeling.4.The effect of temperature on the friction and wear properties of WC-Al₂O₃-graphene composites was studied,and the wear mechanism under high temperature conditions was analyzed.The research results show that the friction coefficient decreases with the increase of temperature,while the wear rate increases with the increase of temperature;graphene can significantly improve the high-temperature wear resistance of WC-Al₂O₃ composites.Under high temperature conditions,the decrease in the mechanical properties of WC-Al₂O₃ composites caused serious fragmentation on the surface of the material;WO₃ formed by high temperature oxidation reduced the friction coefficient.However,due to the lower mechanical properties of WO₃ and the mismatch with the thermal expansion coefficient of the WC matrix,the oxidative wear of the material is accelerated.The wear mechanism at this time is mainly crushing and peeling,abrasive wear and oxidative wear.The introduction of graphene improves the high temperature wear resistance of the material.The reason is that the uniformly distributed graphene prevents microcrack propagation and the extraction of matrix particles,reducing the peeling of the compacted layer;at the same time,the graphene directly interacts with part of the WC matrix,hinder the reaction of WC and O₂,slow down oxidative wear.As mentioned above,the author of this article studied WC-Al₂O₃-graphene composite and found that the introduction of graphene has achieved the simultaneous improvement of the mechanical properties and friction and wear properties of the WC-Al₂O₃ composite.On this basis,the toughening mechanism of graphene on WC-Al₂O₃composites was analyzed and discussed,and the influence of graphene on the wear mechanism of WC-Al₂O₃ composites was revealed.This work will enrich research content of the properties of cobalt-free cemented carbide materials toughened by graphene.