Tribological Properties Of Poly (Aryl Ether Sulfone)s

Special engineering plastic such as poly(aryl ether sulfone)s(PAESs) are widely used as anti-friction and wear-resistance materials in transmission and rotation system of some high technical fields such as precision machinery, aerospace,automotive industry, medical equipment and weapon equipment because of their considerable excellent properties like high strength, light weight, self-lubricating,wear resistance and no corrosion. Compared with traditional metal frictional materials, special engineering plastic have obvious advantages in attempting to lower weight, reduce noise, enhance the service stability of equipments and prolong service life. As the increasing application of special engineering plastic and their composites used on wear-resistance parts of engineering equipment, the study of triboligical properties and mechanism of these materials has gradually become a hot research topic in the field of tribology. After the rapid development during the last decades, the tribology has formed a series of classical theory and mechanism that based on metal and ceramic frictional materials. Because of its unique viscoelasticity property, the frictional properties of polymers are more complex and sensitive when compared with metal and ceramic, which are not only influenced by the application conditions but also their inherent properties. As a consequence, a considerable of factors should be taken into consideration when study the wear and friction properties of polymers. The development of nano-tribology provides researchers an access to do experiment and theoretical study of the adhesion, friction and wear of the sliding surfaces from the micro perception in terms of polymer tribology.Researchers have found that apart from macro testing conditions such as load,sliding speed and temperature, many micro properties such as cohesive energy density, solid surface energy, microscopic real contact area can also impact the tribological behaviors of polymer frictional materials.At present, efforts in attempting to improve wear resistance and lower friction coefficient of special engineering plastic are mainly focusing on the blending modification such as using inorganic fillers and fluorine containing resins to modifyspecial engineering plastic, and this part of work has achieved certain success and some industrial applications. But blending modification method should also take thermal, mechanical and processing properties of materials into consideration, which limits the improvement of the composites’ tribological properties. In order to make a further progress in improving the tribological behaviors of polymer composites, wear and friction of matrix resins are important factors that cannot be ignored. Currently,the study concerning the relationship between molecular structure and tribological properties of special engineering plastic is less systematic. Moreover, the micro factors that have influence on the tribological behavior of the polymer should be explored more deeply. Aiming at these potential problems, this dissertation selected amorphous PAESs as research targets and carried out a series of tests to study their friction and wear performance, and systematicly discussed the impact and mechanism of macro testing conditions and micro molecular structure on the tribological properties of PAESs. And this work provides basic foundation and guidance for the design of polymer materials with high tribological properties.First of all, we prepared commercialized PPSU, PES and PSF by nucleophilic condensation polymerization according to the previous work in our research group,and also prepared PEES which has a similar molecular structure with the three PAESs mentioned above. The Mn of the four PAESs distribute between 50~80 kDa, and the inherent viscosity of NMP solution distribute between 0.49~0.57; The Tg of PPSU, PES and PSF are 222 ℃, 225 ℃ and 184 ℃ respectively, and that of PEES is 201℃,which means the basic performance of PEES completely reach the application level of special engineering plastic. A series of friction tests under different sliding speed were carried out for the four normal PAESs, and the mechanism and controlling factors of the friction and wear behaviors of them were studied carefully by a combination of their surface energy, Tg and worn surface. Results show that with the increase of sliding speed, the friction coefficient(cof) of the four PAESs firstly goes up because of the plastic deformation behavior of micro convex bodies on the tribo-surface of PAESs, and then the cof shows a sharp decline because of the self-lubrication properties of the transfer film; Under low sliding speed conditions,the change of the surface energy that caused by the difference in molecular structure is the key factor that determines the cof of the four PAESs. A higher surface energy leads to a stronger bonding force between the surface of the steel plate and the PAESs. As a consequence, there will be a higher cof for PAESs. The sequence of surface energy of the four PAESs is PPSU<PES<PEES<PSF and the corresponding cof order is PPSU>PES>PEES>PSF. The specific wear rate of the four PAESs is determined by the transfer ability of the molecular chain under this testing condition, and the wear mechanism is cutting abrasive wear according to the morphologies of worn surfaces; Under high sliding speed conditions, because of the higher friction-heating effect, the Tg of the four PAESs has become the main parameter that impacts their tribological properties. The cof of PAESs with a higher Tg such as PPSU and PES is significantly higher than that of PAESs with a lower Tg such as PEES and PSF,but the wear rate shows an opposite trend. The wear mechanism of the four PAESs is adhesive wear according to the morphologies of worn surfaces; Because of the existing of biphenyl group, PPSU owns high Tg and low surface energy compared with other three PAESs, which enables PPSU to exhibit the most excellent tribological properties among the four normal PAESs.Based on the above results, we then prepared another two PAESs with similar molecular structures both containing biphenyl groups, PEDESDS and PEESDS. The Tg of PEDESDS and PEESDS are 265 ℃and 259 ℃ respectively, and the inherent viscosity of m-chlorophenol solution are 0.70 and 0.64. The basic performance of them reach the application level of special engineering plastic. Then we carried out a same tribological test.Results show that under low sliding speed conditions, the cof of PEDESDS with a smaller surface energy is lower than that of PEESDS with a higher surface energy, and there is a same trend for the wear rate of these two PAESs because the molecular chain segment of PEDESDS has a stronger rigidity and weaker movement ability compared with PEESDS. These results certify the previous conclusions under low sliding speed conditions;Under high sliding speed conditions,the introduction of biphenyl group into PAESs molecular structure significantly improves the Tg of PAESs, leading that the surface energy of the two PAESs still hasan impact on the cof of them under high friction-heating effect conditions, but the difference of Tg is gradually presenting influence on the cof; According to the comparison of the tribological behaviors and molecular structures of PEDESDS and PEESDS, we found that biphenyl group have the function of reducing the cof and wear rate of PAESs.The results of this dissertation provide with us an opportunity to get access to a deep understanding about tribological properties of PAESs, expand polymer tribology related theoretical research and have important theoretical significance and application value. At the same time, this work provides us with criterions in terms of designing and perfecting polymer frictional materials with excellent behaviors.