| Aiming at the problem that the traditional self-lubricating ceramic cutting tool materials cannot give consideration to both antifriction and mechanical properties,a new type of self-lubricating ceramic cutting tool material added with metal coated solid lubricant composite powders was developed by microstructural modification with toughening and reinforcing effect of metal.The developed cutting tool material applied alumina(Al2O3)as matrix,tungsten carbide titanium solid solution((W,Ti)C)as reinforcing phase,magnesium oxide(MgO),yttrium oxide(Y2O3)as sintering aid,hexagonal boron nitride(h-BN)as solid lubricant,and nickel(Ni)as coating metal.The solid lubricant was three scaled h-BN:micron h-BN(h-BNμ),nano h-BN(h-BNn),and h-BN nanosheets(BNNS),respectively.The scale effect of hexagonal boron nitride on the microstructure,mechanical properties,friction and wear properties,and cutting performance of the self-lubricating ceramic cutting tool materials was revealed.Ni-coated three scaled h-BN composite powders,namely h-BNμ@Ni,h-BNn@Ni and BNNS@Ni,were prepared by electroless plating with hydrazine hydrate as reducing agent.Because the traditional acid-based sensitizing solution has poor wettability to h-BN powders,an alcohol-based sensitizing solution was proposed and applied to improve the sensitizing effect.In order to solve the problem that h-BNn and BNNS are easy to cause decomposion of the plating solution due to their large specific surface area,a staged constant temperature electroless plating process was proposed to make the plating process performed smoothly.The coating layer of the prepared h-BN@Ni composite powders was relatively complete and uniform,consisted of crystalline nano Ni particles,and was tightly bonded with the h-BN.In this study,self-made BNNS was used,accoundingly a device and method for producing boron nitride nanosheets via synergistic exfoliation with pure shear ball milling and ultrasonication was proposed.Self-lubricating ceramic cutting tool materials respectively added with h-BNμ@Ni,h-BNn@Ni,and BNNS@Ni composite powders were prepared by vacuum hot pressing sintering process.Their mechanical properties were higher than those of the corresponding self-lubricating ceramic cutting tool materials added with h-BN powders.Among them,the flexural strength,Vickers hardness and fracture toughness of cutting tool material added with 5 vol.%coated micron h-BN were 599 MPa,16.1 GPa and 5.2 MPa·m1/2,respectively,which were 8.7%,3.9%and 20.9%higher than those of cutting tool material added with 5 vol.%micron h-BN,respectively.The flexural strength,Vickers hardness and fracture toughness of cutting tool material added with 5 vol.%coated nano h-BN were 632 MPa,17.2 GPa and 5.4 MPa·m1/2,respectively,which were 8.4%,5.5%and 14.9%higher than cutting tool material added with 5 vol.%nano h-BN,respectively.The flexural strength,Vickers hardness and fracture toughness of cutting tool material added with 1 vol.%coated BNNS were 760 MPa,18.7 GPa and 6.7 MPa·m 1/2,respectively,which were 3.4%,3.3%and 11.7%higher than cutting tool material added with 1 vol.%BNNS,respectively.The increase in strength and hardness of the cutting tool materials was attributed to the addition of h-BN@Ni composite powders instead of h-BN powders,which made the h-BN grains uniformly distributed in the cutting tool materials and tightly bonded with the ceramic matrix.The improvement of fracture toughness of the cutting tool materials was because that the Ni coating layer of h-BN@Ni composite powders increased the interfacial bonding strength between h-BN grains and ceramic matrix,which enhanced the crack propagation resistance and increased the debonding and pulling-out work of h-BN,and the metal Ni itself played the toughening effect.The mechanical properties of the self-lubricating ceramic cutting tool materials increased with the decrease of h-BN scale.The effect of h-BN@Ni content on the friction and wear characteristics of the self-lubricating ceramic cutting tool materials was studied.Friction and wear tests showed that with the increase of h-BNμ@Ni content,the friction coefficient of cutting tool material added with coated micron h-BN decreased,and the wear firstly decreased and then increased.With the increase of h-BNn@Ni content,the friction coefficient of cutting tool material added with coated nano h-BN decreased and the wear rate increased.The influence of test conditions on the friction and wear characteristics of the self-lubricating ceramic cutting tool materials was studied.The test results showed that the friction coefficients of cutting tool materials respectively added with 10 vol.%coated micron h-BN,10 vol.%coated nano h-BN and 1 vol.%coated BNNS firstly increased and then decreased with the increase of load,while the wear rate increased with the increase of load.Both friction coefficient and wear rate decreased with the increase of sliding speed.Under the same test conditions,the antifriction and wear resistance of the above-mentioned three cuting tool materials were superior to the corresponding cuting tool materials respectively added with 10 vol.%micron h-BN,10 vol.%nano h-BN and 1 vol.%BNNS.The friction and wear properties of the self-lubricating ceramic cutting tool materials increased with the decrease of h-BN scale.The cutting performance of the self-lubricating ceramic cutting tools on 40Cr hardened steel was studied.The results showed that the cutting performance of cutting tools added with h-BN@Ni composite powders was better than that of cutting tools added with h-BN powders.Under the conditions of cutting speed of 200 m/min,back engagement of 0.2 mm,and feed rate of 0.102 mm/r,the tool-chip friction coefficients of cutting tools respectively added with 5 vol.%coated micron h-BN,5 vol.%coated nano h-BN and 1 vol.%coated BNNS were 2%,6.8%and 5%lower than those of corresponding cutting tools respectively added with 5 vol.%micron h-BN,5 vol.%nano h-BN and 1 vol.%BNNS.The cutting temperature decreased by 6.7%,5.9%and 4.2%,respectively.The effective cutting distance increased by 22.5%,19.7%and 21%,respectively.The rake face wear of the self-lubricating ceramic cutting tools was mainly crater wear and micro-chipping of cutting edge caused by fatigue wear.The wear in the high stress affected region of the rake face was more severe than that in the low stress affected region.h-BN in the cutting tool materials was precipitated on the rake face to form a friction film,which reduced the adhesive wear of chips to the cutting tool.No friction film was formed on the flank face of the self-lubricating ceramic cutting tools,and its wear was mainly abrasive wear.The cutting performance of the self-lubricating ceramic cutting tools increases with the decrease of h-BN scale. |
PDF Full Text Request