The Tribology Mechanism Of Exogenous Copper Third Body On Copper-based Powder Metallurgy Materials

The friction materials used on high-speed trains are mostly copper-based powder metallurgy friction materials.During the braking process,the product between the friction pairs is the third body.Its composition and organization are not only related to the brake disc and the friction material,but also change with changes in friction conditions.Therefore,in order to control the friction braking performance,it is very important to study the relationship between the composition and organization state of the third body and the friction performance.In order to clarify the influence of copper component on copper-based friction materials with different properties,this paper uses powder metallurgy technology to prepare three copper-based friction materials with different properties,and uses the exogenous powder method to add copper third body powder to the friction interface.By comparing the friction coefficient,surface morphology,and element distribution under the two conditions,the mechanism of the copper third body is analyzed.The main conclusions obtained are as follows:（1）The addition of exogenous copper third body shows that the copper third body can increase the friction coefficient at the friction interface.The exogenous copper powder tends to increase the friction coefficient.The average friction coefficient increases by 0.21,0.09 and0.004 for pure copper,copper-graphite and Cu-Si O₂ materials than those without Cu addition,respectively.In comparison,the copper third body has a significant increase in the friction coefficient of pure copper material and copper-graphite material,and has a small effect on Cu-Si O₂ material.（2）The adhesion of exogenous copper third body on the surface of pure copper material is helpful to increase the friction coefficient.This causes the copper third body to accumulate on the friction surface to form a pure copper layer with a thickness of about 50μm.As the friction speed further increases,severe adhesive wear occurs and the friction coefficient increases sharply.When the maximum increment occurs at 600 rpm,the value is 0.34.（3）For copper-graphite material,the plasticity of the exogenous copper third body will aggravate the deformation of the surface third body.The copper powder increases the friction force after extrusion deformation and work hardening.At the same time,the layered accumulation distribution of the exogenous copper third body at high speed can effectively hinder the lubrication of graphite,increase the friction force,and increase the friction coefficient.（4）For Cu-Si O₂ material,the presence of Si O₂ leads to the formation of many spalling pits on the surface.These spalling pits effectively weaken the ability to form a continuous copper third body and make the distribution of the third copper body more dispersed.At high speed,the exogenous copper third body can be mixed into the original third body,causing uneven distribution of the composition of the third body and causing large-area peeling of the friction surface.This spalling phenomenon is not conducive to the full contact between the brake disc and the material,so that the friction coefficient decreases.（5）On the cross-section of the third body,the exogenous copper powder has little impact on the copper-graphite sample,and has a significant impact on the pure copper sample and the Cu-Si O₂ sample.The exogenous copper third body will increase the thickness of the third body significantly.At the same time,the composition and density of the third body will be uneven in the depth direction,which will cause the third body to peel off,thereby affecting the friction performance.