Fabrication And Performance Of Core-shell Coated Self-Lubricating Cemented Carbide Tools For Cutting Compacted Graphite Cast Iron

Compacted graphite cast iron is considered to be an ideal material for manufacturing thinwalled lightweight new engines.Compared to gray cast iron,it has excellent mechanical properties.However,due to the lower thermal conductivity,ferrite is easily bonded to the cutting edge and MnS lubricating layer cannot be formed during the cutting process,making the tool more wear-prone and resulting in worse processing performance than gray cast iron.In this paper,a novel self-lubricating cemented carbide tool material was designed for friction reduction while keeping better comprehensive mechanical properties.By using a non-uniform nucleation method,MoS2@Al2O3 core-shell coated solid lubricant was created,and a new selflubricating cemented carbide tool with this solid lubricant was prepared by hot pressed sintering process.Through the study of friction and wear characteristics and cutting performance,it is shown that the self-lubricating tool has good friction reduction and wear resistance,which can improve the problem of serious tool wear in the process of cutting compacted graphite cast iron and prolong the service life of the tool.Based on the design principles of self-lubricating tool materials and the processing characteristics of compacted graphite cast iron materials,the material system of WC/Co/Cr3C2/Y2O3/MoS2@Al2O3(abbreviated as HPYG6@M)self-lubricating tool was determined.The physical and chemical compatibility of tool materials was analyzed,and the WC/Co/Cr3C2/Y2O3 tool matrix material components other than solid lubricants were optimized by experiments.The results demonstrate that the WC/Co/Cr3C2/Y2O3 tool matrix material produced by vacuum hot pressed sintering has the better mechanical properties when the mass fractions of Co and Cr3C2 are 6.0%and 0.4%,respectively.Flexural strength is 1584 MPa,the Vickers hardness is 20.97 GPa,and the fracture toughness is 10.92 MPa·m1/2.MoS2@Al2O3 core-shell coated solid lubricant was prepared by non-uniform nucleation method.The effect of MoS2@Al2O3 content on the mechanical properties of self-lubricating cemented carbide tool materials was studied,and the toughening and strengthening mechanism of self-lubricating tool materials was analyzed.It is found that HPYG6@M tool material can achieve improved better mechanical properties when the content of MoS2@Al2O3 is 3 vol%.The fracture toughness is 11.32 MPa·m1/2,the Vickers hardness is 19.33 GPa,and the flexural strength is 1485 MPa.Compared with the tool material directly adding the corresponding content of solid lubricant MoS2,its mechanical properties are greatly improved.Combined with the analysis of the fracture morphology of the tool material,it can be seen that the Al2O3 shell coats MoS2 in the core,which improves the bonding strength with the matrix material,prevents the melting and diffusion of MoS2 during the sintering process,and is beneficial for increasing the density of the tool material.As a second-phase toughening particle,MoS2@Al2O3 causes a transgranular fracture mode,consumes more fracture energy,and enhances the mechanical properties comparing to directly adding solid lubricant.The friction and wear characteristics of tool materials with MoS2 and MoS2@Al2O3 coated solid lubricants were compared and studied.Although the friction coefficient of the tool material added with MoS2@Al2O3 is slightly higher than that of the tool material directly added with MoS2,the wear rate of the tool material added with MoS2@Al2O3 is significantly lower than that of the tool material directly added with MoS2 due to its hardness,fracture toughness and lubricant release rate.Effects of load and rotation speed on the friction characteristics were investigated.The friction coefficient of the tool material with MoS2@Al2O3 decreased with increasing load,but the wear rate increased.The friction coefficient and wear rate decrease as rotating speed increases.The wear-resistant and self-lubricating mechanism of MoS2@Al2O3 coated solid lubricant was investigated.The friction process was found to liberate layered MoS2 from the Al2O3 shell,forming a lubricating film on the surface of the friction pair,which decreased the friction coefficient.And meanwhile,Al2O3 improved the bonding strength of the lubricant and the tool matrix material,so that the formed lubricating film had strong adsorption and was not easy to fall off,thereby slowing the release rate of MoS2 and improving the wear resistance of the tool material.The cutting performance of a new self-lubricating cemented carbide tool with MoS2@Al2O3 coated solid lubricant in milling compacted graphite cast iron was studied.The results show that the tool with coated solid lubricant can significantly enhance the machining quality of the workpiece surface,decrease tool wear and greatly improve the tool life compared with the tool without lubricant and the commercial coated tool.Compared with the cemented carbide tool without lubricant,the cutting temperature is reduced by about 19%.According to the observation and analysis of the wear morphology of the rake face and flank face,adhesive wear,oxidation wear,and diffusion wear are the main wear mechanisms of the rake face.Boundary groove wear is the failure mode of the flank face,and diffusion and adhesive wear are the main causes of wear.In comparison to the HG tool,the wear degree of the HG@M selflubricating tool obviously reduced,and the good effect of friction reduction and wear resistance is achieved.