Investigation Of Tribological Performance And Simulation Of Temperature Field During Sliding Contact For Polyimide Composites

The friction and wear behavior are universal phenomena for the moving parts in contact which including friction, wear, lubrication and damage etc. In tribology field, the materials need to have a good mechanical performance, heat and corrosion resistance besides wear. Nowadays, the polymers especially the high performance polymers have be applicated in tribological field more and more for their excellent properties. As one of the most excellent engineering plastics, polyimide (PI) is well known for its marked thermal-stability, excellent mechanical properties and resistance to most of solvents, and is becoming one of the best candidates for high temperature tribological applications. Many studies are focused on the improvements in mechanical and tribological properties of PI filled with solid lubricants and fiber-reinforces. MOS2, PTFE and short-cut carbon fiber (CF) were filled to a novel aromatic of thermoplastic PI(TPI)and the tribological and mechanical properties for those composites were investigated in this thesis. Due to structure characters which depend highly on temperature, the materials failure was mainly caused by high temperature on the contact surface. Most polymers can easily be destructed and deformed for their poor heat conductivity, which caused large temperature gradient. So to understand temperature distribution of PI composites during sliding contact is very helpful for failure analysis. By using the finite element (FE) method, physical and numerical model were established to simulate the temperature distribution of the composite within sliding time according to the tribological behaviors of 30wt% PTFE-TPI here. And then, the temperature field distribution of CF reinforced TPI composites were predicted by this model to inspect its accuracy and applicability. The tribological and mechanical properties for pure TPI and its composite modified by MoS₂ were evaluated under the dry sliding and water lubricated conditions. The composites worn morphologies and elements distribution were also analyzed by SEM. The results showed that the materials main mechanical performances decreased partly compared with pure TPI and became more stable with MoS₂ content increasing. Under dry sliding condition, the materials friction coefficient decreased obviously and MoS₂ enriched the worn surface. The worn surface of 10wt%MoS₂-TPI composite showed local melt phenomenal, meanwhile having the lowest wear rate, 1.14×10^-13 m³Â·N^-1·m^-1. The wear rate increased and melt degree became deeper when the MOS2 content reached 20wt%. Accordingly, the composites tribological performances were improved evidently and MOS2 still had an effective lubricating work, while fatigue wear was the main factor during the tribology process under water lubrication condition. It reflected that the tribological performances were affected by temperature evidently. The effect of PTFE content on composites tribological performances was also investigated. The friction coefficient and wear rate decreased in low PTFE contents, and then increased with further increase of PTFE content, both of them seemed to be minimums at 30wt% PTFE content, which was 0.075 and 2.21?á10-15 m 3 ? N?1 ?m?1 respectively. At the same time we built up a simple physical model and carried out a numerical simulation for the temperature distribution of the composite by using MATLAB PDE TOOL. It was analyzed that: at the primary stage, the real load Pr should be greater than the nominal value Pa for the micro peaks on the contact surface, which was modified as Pr = Pa???? 12t.5+1??? through optimizing method. When the friction coefficient became stable gradually, the Pr was near to Pa while the real contact area was close to the nominal area. It was simulated again for the unstable state by the modified model and the mean deviation decreased from 8.9 ?? to 2.1 ??. The performances of polymer material can be improved by filling CF due to its high strength and modulus, friction reduction and heat conductivity. The tribological properties of composites reinforced by short-cut carbon fiber were further investigated in this thesis. The minimum friction coefficient was 0.17 at 5wt% CF content, and wear coefficient decreased an order at 15wt% CF content respectively. And then, the temperature fields for this system were simulated. According to the CF content increase, the rigidity was enhanced, and thus the value of |áincreased. Taking 20wt%CF-TPI as an example, it showed that the temperature descended at first and then ascended along the radial direction under the effects of heat production and exchange, and the temperature on the friction surface was the highest, descending on axle successively, furthermore the temperature gradient was large for the polymers poor heat conduction. The material will be damaged and wear become serious at melt point for the heat accumulation. So it is help to understand the intrinsic of the friction and wear byresearch on the temperature field during sliding process. Due to the relative moving and closed contact between the couples, it is difficult to measure the temperature changes online. The model proposed here can predict the temperature distribution for polymer in tribology process. The work above has actual meanings.