Study Of Copper, Iron And Graphite Third Body On Tribological Property And The Action Mechanism

A large quantity of third body is formed during the braking process of copper-based powder metallurgy materials used in the manufacture of high-speed train brake pads.The composition of the third body is complicated and it changes with the friction condition varying.So the third body is an important factor affecting the performance of friction.Thus it is a key issue to clarify the relationship between the composition and structural state of third body and the friction coefficient with the aim of designing excellent performance brake pad material.Since the third body is a mixture of multiple components,the friction coefficient is a comprehensive reflection of a variety of components.Therefore,it is impossible to judge the influence degree of each component in the third body on the friction coefficient by testing the relationship between the composition of the material and the coefficient of friction.As a solution,in this paper,distinctive experiment methods of artificially adjusting the third body composition and interfering the friction surface have been proposed,which provide a new way to explore the influence degree of the third body composition on the friction coefficient.Aiming at copper,iron and graphite components,various components third body is prepared by the prefabricated third body powder.Through comparing the variation magnitude of friction coefficient with and without adding prefabricated third body to the friction surface,combining with the surface interference treatment case,to explore the relationship between the third body content,particle size,quantity and friction coefficient under different friction conditions.The main contents and main conclusions are as follows:1.The method of counting parameters of surface spalling area ratio and the surface roughness were introduced to evaluate the evolution characteristics of third body.The results showed that the surface spalling area ratio and surface roughness of the copper-based friction material decreased gradually with the friction speed increasing,constituting the densification process of the third body.On the contrary,as the speed decreased,the surface spalling area ratio and surface roughness gradually increased,manifesting the fragmentation process of dense third body.2.The influence of the amount and particle size of third body as well as the state under dry and wet conditions on the friction coefficient was studied by surface interference treatments during friction experiment.The results showed that the friction coefficient of the material was average reduced by 0.08,0.15 and 0.35 respectively under the condition of dry wiping,dripping water and wet wiping,which proved that the amount of third body played the role of changing the friction coefficient.The little amount of third body reduced the mechanical engagement force of the friction pair,resulting in a decrease in friction coefficient.Under wet friction condition,water played the role of lubrication,the less amount of third body resulted in a greater decline in the friction coefficient.The test under abrasive paper interference showed that the Si C particles in the third body increased the friction coefficient by 0.07~0.09.It was proved that the hard particles with large particle size increased the surface roughness by plowing effect,leading the friction coefficient increasing.The hard particles with small particle size increase the number of hard components in the third body,resulting in an increase in the coefficient of friction.3.The role of copper in third body was invesitgated by the method of adding copper third body.It showed that copper third body played the role of increasing the friction coefficient,and the average increment was 0.03 generally.The effect of copper third body on the friction coefficient was related to the friction speed.At low speed,the adhesion and shear of copper on the surface resulted in a higher coefficient of friction.At high speed,because of the softening of copper,the smooth and dense third body layer formed on the friction surface,which was unfavorable to the increase of the friction coefficient.As the friction pressure increased,the variation of copper third body caused the increment of the friction coefficient reduced.4.The quantitative relation formula between the graphite content in copper-graphite third body and the friction coefficient was obtained by adding Cu90%-Gr10%,Cu50%-Gr50%,Cu10%-Gr90%and pure graphite third body:y(28)(1.39E-07)x~3(10)(1.46E-05)x~2-0.002x(10)0.487The threshold graphite content in copper-garaphite third body stabilized coefficient of friction was 90 wt%.Friction coefficient decreased owing to the good flowability and lubricity of the graphite particles on the surface,reducing the adhesion between the metallic materials.5.The role of iron in third body was invesitgated by adding iron third body.It showed that the effect of iron third body on the improvement of the friction coefficient of the material mainly occurs at low pressure and low speed,i.e.,when the P·V value is less than 255,the average friction coefficient increases 0.04.When the P·V value is greater than 275,the average friction coefficient reduces 0.04.Due to low speed and low pressure,the iron third body played the role of abrasive,increasing the engaging force between the frictioncouple,causing the friction coefficient increased.When the speed and pressure became higher,the surface temperature increased,and the plastic deformation of the iron was prone to oxidation,which promoted the formation of smooth and dense oxide film on the surface,so friction coefficient decreases.6.It found that the friction coefficient of the friction material reduced with the increasing concentration of iron component in copper-iron mixed third body by adding Cu90%-Fe10%,Cu50%-Fe50%,Cu10%-Fe90%and pure iron third body.When frction speed is low ranging within 100-500 rpm,the average friction coefficient for Cu90%-Fe10%,Cu50%-Fe50%,Cu10%-Fe90%and pure iron third body increased 0.07,0.05,0.03 and 0.02,respectively.When frction speed is higher ranging within 500-1500 rpm,the average friction coefficient for Cu90%-Fe10%,Cu50%-Fe50%,Cu10%-Fe90%and pure iron third body decreased 0.01,0.04,0.06 and 0.06,respectively.Reduction of the friction coefficient can be attributed to the iron oxide reducing the adhesion of copper third body.