Numerical Simulation Of The Transfer Process On The Friction Interface Of Polymer Composites

The interface transfer occurs when a polymer material slides against a metalcounterpart, which leads to the formation of the transfer film on the counterface. Theexistence of the transfer film can effectively reduce the wear of the polymer material. Ithas been proved that the formation of transfer film is due to the effect of the mechanicalembedding, physical and chemical bonding which makes the polymer adhere andgradually expand on the contact surface of the metal, finally a layer of uniform,complete transfer film will generate. The formation and growth of the transfer film isalways accompanied by a dynamic process of partially peeling off and repairing process,which reaches to a dynamic balance after the overall transfer film forms. But in fact, theexperimental study is difficult to observe and reproduce the dynamic process of frictiontransfer and wear, such as the dynamic change of the interface characteristics, stress andstrain, etc. And this dynamic process is very important for the effective control of thetransfer film and the optimization of the tribological performance. Also, it is the keyproblem to realize the friction performance control. On this account, the Particle FlowCode in2Dimensions (PFC2D), based on the discrete element method, was employedto simulate and model the process of friction and wear of PTFE/45steel in this thesis.The effects of the surface roughness (Rz) of45steel on the friction transfer and wearrule of PTFE, the influence of the filler content on the tribological properties of theCu/PTFE composites and the motion characteristics of the PTFE matrix particles duringthe process of the friction and wear were systematically investigated in our work. Thedynamic process of the friction transfer and wear of the PTFE/45steel friction pair wasreproduced at the micro-level. And the transfer and wear were quantitatively exhibited.It will be very significant for the optimization and effective control of friction and wearof polymer/metal sliding system. The main research conclusions are as follows:(1) The numerical simulation of the process of the friction transfer and wear ofPTFE/45steel showed that due to the action of shear and extrusion, some of theparticles near the friction interface falling off the PTFE can be act as transferredparticles to adsorb on the counterface. And the others were discharged from the frictioninterface as the debris. At the beginning of the formation of the transferred particle layer,the number of the transferred particles increased gradually. But after a period of time,the transferred particle number tended to a constant value and maintained a dynamic balance with the formation and completion of the transferred particle layer. The wearrate decreased significantly and maintained at a stable value as a layer of uniform andcontinuous transfer particle layer (transfer film) was formed.(2) The simulation showed that the counterpart surface roughness (Rz=0.3~1.5μm)of45steel have a certain effect on the number of the transferred particles and wornparticles of the PTFE. In a certain range, the number of the transferred particlesincreases significantly along with the increase of the Rz value (Rz=0.3~1.2μm). Butafter the Rz increases to a certain value (Rz=1.2μm), the increase trend of the numberof the transferred particles with the increase in the Rz is less than the Rz=0.3~1.2μm.The number of the worn particles generally increases with the increase of the Rz value.But there was a maximum number of the worn particles when Rz=0.9μm.(3) The influence of filler content on the tribological properties of the Cu/PTFEcomposites was simulated. The results showed that on the one hand, the addition of Cuparticles in PTFE can promote the formation of the transfer layer, and the bondingstrength between the transfered particles and the counterface has been improved. On theother hand, the addition of Cu particles improve the overall strength of compositematerial, which has certain inhibitory effect on the formation of the transfered particles.When the Cu content was20%, the number of the transfered particles was the most, andthe transfered particle number was the least as the Cu content was50%. Meanwhile, thewear of the PTFE-based composites was effectively reduced due to the addition of theCu particles. And the more the content of the Cu, the better the antifriction effect was.When the Cu content was50%, the number of the wear particles of the PTFE-basedcomposites was decreased by nearly half of the pure PTFE.(4) The motion characteristics of the PTFE matrix particles during the process ofthe friction and wear were studied. The simulation results showed that PTFE matrixparticles will move in the horizontal and vertical direction during friction. And the rangeof the motion mainly occurred in the quasi-fluid layer of PTFE. The moving direction ofthe horizontal position of the particles mainly keep consistent with the movement of45steel, which means that the movement direction of the horizontal position of theparticles have the following features. In the vertical direction, the particles in the PTFEmatrix will move mainly upwards. The analysis about the horizontal moving range ofthe three particles distributed in a sample level in the PTFE showed that the movingrange of the middle particle was larger than the other two particles.