Study Of The Effect Of Graphite And Copper On Tribological Properties And Third Body Action Mechanism For Phenolic Resin

Phenolic resin（PF）-based friction composites have many advantages,such as high friction coefficient,light weight,low cost and chemical stability.They are widely used in train and automotive as friction braking materials.Graphite（Gr）,as a lubricant in friction composites,can improve the friction stability and wear resistance of friction composites.However,the content and morphology of graphite are different,which leads to reducing friction coefficient and weakening the strength of composites.Copper can improve the tribological and wear properties of PF-based composites.The interaction between copper and Gr is also an important factor affecting the tribological properties.Therefore,the comparative analysis of the mechanism of PF,Gr and copper in the friction process is of guiding significance to optimize the properties of PF-based composites.In this thesis,PF,Gr-PF composites,copper/Gr-PF composites and copper coated graphite（CCG）-PF composites were prepared.The relationship between friction coefficient,wear rate and the shape,particle size and content of Gr,as well as the copper and CCG were measured on a constant speed friction tester in the friction speed range of less than 3000 r/min.The effects of Gr,copper and PF on the friction and wear properties of PF-based composites and the relationship between the tribological behavior and friction conditions were revealed by means of optical microscope and electron microscope.The results as following:1.The fluctuation of friction coefficient of PF is related to the third body structure and chemical reaction during friction.In the rotational speed range of 3000～200 r/min,the friction coefficient fluctuates in the range of 0.22～0.67,and the friction coefficient fluctuates to 0.1when the rotational speed is 1500 r/min.The fluctuation of friction coefficient is due to the speed sensitivity of PF.At low speed,the brittle spalling of PF asperity forms granular third body,which intensifies the surface roughness and leads to high friction coefficient;At high rotational speed,the main chain of ether bond of PF is broken by friction heat,which leads to the decrease of strength of PF and the formation of refined third body,which increases the smoothness of friction surface and intensifies the attenuation of friction coefficient.2.The influence of Gr on the tribological properties of PF is related to the particle size.When the rotational speed is less than 1000 r/min,the influence of Gr particle size on friction coefficient and wear of PF is not obvious.When the rotational speed is higher than 1000 r/min,the friction coefficient increases with the increase of Gr particle size.The reason is that when the rotational speed is higher than 1000 r/min,the dispersion of large particle size Gr is large,the graphite particles cover the surface insufficiently,and the PF directly adheres to the friction disc,which is easy to form large damage,resulting in the increase of friction coefficient and wear rate.3.Compared with flake Gr,the friction coefficient and wear rate of granular Gr are lower than that of flake Gr at low friction speed;When the friction speed exceeds 1000 r/min,the friction coefficient and wear rate of granular Gr(friction coefficient:0.39,wear rate:2.9×10^-6mm³N^-1m^-1)are higher than that of flake Gr(friction coefficient:0.19,wear rate:1.36×10^-6mm³N^-1m^-1).The reason is that flake Gr weakens the strength of the composites.At low rotational speed,the ploughing effect of low strength composites is obvious,and the ploughing area accounts for 70%of the total area,resulting in high friction coefficient and wear rate.When the rotational speed exceeds 1000 r/min,the flake Gr which is easy to disperse reduces the friction surface roughness,resulting in the decrease of friction coefficient and wear rate.However,granular Gr is not conducive to crack propagation,so the weakening degree of the composites is low,and the high-speed impact resistance is strong,so the friction coefficient and wear rate have no obvious attenuation.4.There are different wear forms between granular Gr and flake Gr.Granular Gr is a kind of micro convex body with meshing shear and adhesive wear at the contact point.The generalized friction coefficient equation is as follows:（?）The flake Gr is micro convex interface adhesive wear.The generalized friction coefficient equation is as follows:（?）5.The effect of copper on the friction and wear of PF is related to the morphology of copper.The friction coefficient of copper/Gr-PF composite is above 0.47 when the rotational speed is lower than 2000 r/min;When the rotational speed is higher than 2000 r/min,the friction coefficient decreases to 0.37.The friction coefficient of CCG-PF composite increases with the increase of rotational speed,and reaches 0.84 at 3000 r/min.The reason lies in the dispersion of copper in the composites.When the copper particles in the composite are agglomerated to a large extent,the agglomerated copper is easy to adhere to the friction disc under low-speed friction conditions,resulting in high friction coefficient.In high-speed friction,the enrichment of copper is not conducive to the strengthening of the composite,and the larger wear leads to the increase of the coverage of Gr particles on the surface,which plays a role in reducing the friction coefficient.CCG makes copper highly dispersed in the composite,improves the strength of the composite and increases the copper content in the third body,and makes the friction coefficient show a higher value at high speed.6.The influence of friction mode on friction coefficient is related to the composition of friction composites.For PF,Gr-PF composite and copper/Gr-PF composite,when the composites first carry out at high-speed friction,the surface structure formed by high-speed friction is beneficial to the increase of friction coefficient due to the increase of the hardness of the third body.For CCG-PF composite,the dispersion of copper can strengthen the composite and the third body on the surface,so that,the friction coefficient increases with the increase of rotational speed.