| Ceramic tools were considered as a competitive tool in the field of high-speed cutting hardened steels because of their high hardness,wear resistance and good chemical stability.However,their intrinsic brittleness limited their population and application.Graphene possessed large specific area,high aspect ratio,good mechanical,thermal and electrical properties,which made it a promising candidate to strengthen and toughen ceramic tools.In this work,graphene was introduced into the Al2O3 matrix composite ceramic tool material,and the graphene/Al2O3 matrix composite ceramic tool with the staggered distribution of strong and weak interfaces was successfully fabricated through the interfacial physical regulation based on functional modification of the graphene surfaces and the interfacial stress regulation regulation based on the staggered distribution of strong and weak interfaces.And with the aid of this interfacial distribution characteristics,multiple strengthening and toughening mechanisms were introduced into the ceramic tool,which significantly improved its mechanical properties.In the end,the graphene/Al2O3 matrix composite ceramic tool was used in high-speed cutting hardened steels.Based on the electrostatic repulsion dispersion mechanism,steric hindrance dispersion mechanism and cooperative dispersion mechanism,respectively,different graphene dispersion surfactants were introduced and used to modify graphene.PVP with the steric hindrance dispersion effect was determined as the optimal surfactant.Ultraviolet/visible spectrophotometry was used to determine the content of surfactant and the pH of the dispersion.And the optimal surfactant content was determined as 80%of the mass fraction of graphene and the optimal PH of the dispersion was 6.The effect of graphene content on microstructure and mechanical properties of ceramic tool materials was investigated.It has been found that the microstructure of the composite tool material was optimized,and the mechanical properties were also greatly improved,when the graphene content was 0.5 vol%.However,when the graphene content exceeded 1 vol%,graphene agglomeration led to the significant decrease in mechanical properties of composite tool materials.Therefore,it could be determined that 0.5 vol%of graphene was the critical volume fraction to achieve interfacial physical regulation in the graphene/Al2O3 matrix composite ceramic tool material.The effect of graphene geometry sizes on interfacial characteristics,microstructures and mechanical properties of the graphene-Al2O3 matrix composite ceramic tool material was investigated.The results indicated that the interfacial phases induced by the interfacial chemistry reaction between graphene and ceramic matrix can change the interfacial stress state,and the interfacial phases and sizes of the phases were subject to graphene sizes.With the increase of graphene thickness,the interfacial phases between graphene and Al2O3 changed from Al4O4C to Al4C3 and the sizes of the phases were increased.Al4O4C and Al4C3 interfacial phases contributed to the formation of the strong-weak staggered interfaces in ceramic tool materials.It was determined that graphene with a thinner thickness and larger lateral size was more beneficial to realize the interfacial stress regulation based on the staggered distribution of strong and weak interfaces in the graphene/Al2O3 matrix composite ceramic tool material.The effects of sintering parameters on microstructure,graphene structure stability and mechanical properties of the graphene/Al2O3 matrix composite ceramic tool material were investigated.It has been found that the graphene structure stability was most closely related to the dwell time,and that too long or too short dwell time might trigger adestruction effect on graphene structure.The sintering parameters of 1600℃ sintering temperature,15 min dwell time and 30 MPa applied pressure were demonstrated to be the optimal sintering parameters to obtain optimal mechanical properties.The main reasons for obtaining the optimal mechanical properties were grain refinement,the formation of a moderate amount of weak bonding interfaces and good structure stability of graphene.Based on the results of interfacial physical regulation and interfacial stress regulation,the graphene/Al2O3 matrix composite ceramic tool was prepared by two-step hot pressing（HP）.The interfacial adhesion energy and interfacial shear strength were calculated by first-principles which verified the composite ceramic tool with the strong and weak staggered interfaces was successfully obtained.The staggered strong-weak interfaces could induce strengthening mechanisms such as load transfer,grain refinement,etc.and toughening mechanisms such as crack bridging,graphene pull-out,etc.,which effectively improved mechanical properties.The grain growth kinetics and grain refinement mechanism of Al2O3 grains and WC grains in Al2O3/WC/TiC/graphene ceramic composite were investigated in this work.The grain growth exponent of Al2O3 was determined as 2,which conformed to the growth mechanism of grain boundary diffusion.Meanwhile,the grain growth exponent of WC was determined as 3,which conformed to the grain growth mechanism of volume diffusion or liquid-phase diffusion.The different wettability of graphene to Al2O3 and WC was responsible for their different grain growth mechanisms.The grain refinement mechanism of Al2O3 grains was graphene pinning effect and intragranular structure,and the grain refinement of WC grains could be attributed to the strong bonding interface between WC and graphene.The self-made graphene/Al2O3 matrix composite ceramic tool（AWTG0.5）was used to continuously turn the hardened 40Cr steel at different cutting speeds,and compared with the self-made graphene-free AWTG0 ceramic tool and the commercial ceramic tools SG4 and LT55.It has been found that the cutting performance of AWTG0.5 tool was significantly better than that of AWTG0,SG4 and LT55 tools.And the main failure mechanism of AWTG0.5 tool was wear,and the main wear mechanisms were adhesive wear and abrasive wear.The effect of graphene orientation distribution on the antifriction and anti-wear performance and cutting performance of ceramic tools was investigated.The results indicated that it was more beneficial to improve the antifriction and anti-wear performance and cutting performance of ceramic tools when the graphene orientation distribution was perpendicular to the direction of the loading pressure. |
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