Effect of ATP-TiO₂ Hybrid with Two Morphologies on Fretting Wear Properties of UHMWPE

Ultra high molecular weight polyethylene (UHMWPE) is a kind of thermoplastic engineering plastic with good biocompatibility, high impact resistance and wear resistance, which is widely used in machinery, transportation, construction, medical and other fields. However, UHMWPE also has some disadvantages, such as low strength, poor heat resistance and creep resistance. Those properties result in UHMWPE matrix being susceptible to friction heat, then softening and deformation, which seriously affects the service life of workpieces. Using rigid filler with good heat resistance is one of the effective methods to improve the strength of UHMWPE. Attapulgite (ATP) is a kind of natural rigid silicate of nanoscale size, which can improve the heat resistance and decrease the friction coefficient of the composite by filling in UHMWPE matrix. In order to facilitate more even dispersion of fillers in matrix, titanium dioxide (TiO₂) was introduced to modify ATP nanofibers. As a kind of frequently-used reinforcement elements, TiO₂ had excellent mechanical properties, low expansion, high thermal conductivity and heat resistance. Taking ATP nanofibers as a carrier, TiO₂ grew in-situ on the surface to prepare ATP-TiO₂ hybrid. The product was usually applied as the adsorption material in catalysis field. However, the tribological properties of polymer composites reinforced with ATP-TiO₂ hybrid and the effect of the filler’s microstructure on the tribological mechanism were rarely studied. In this study, two kinds of ATP-TiO₂ hybrids with different morphologies using tetrabutyl titanate as precursor solution and ATP as carrier were prepared by a sol-gel method and a steam method, respectively. With the addition of 5% ATP-TiO₂ hybrids, UHMWPE based composites were obtained, and their fretting wear properties were tested for comparison with those of neat UHMWPE and the composite reinforced by ATP nanofibers. The effect of ATP-TiO₂ hybrids and their microstructures on the fretting wear performance of UHMWPE was investigated. The results showed that TiO₂ particles were successfully loaded on the surface of ATP nanofibers under two preparation methods, and the heat resistance of ATP-TiO₂ hybrids was significantly improved compared with ATP nanofibers. Among them, the size of TiO₂ in the ATP-TiO₂ hybrid prepared by the steam method was about 10 nm, which was far less than that of 50~200 nm in the hybrid prepared by the sol-gel method. Because of the small size and uniform cladding of TiO₂, the ATP-TiO₂ hybrid prepared by the steam method had higher specific surface area even than ATP nanofibers, leading to more even dispersion in the matrix than ATP and the hybrid prepared by the sol-gel method. The results of shore hardness and elastic modulus both of UHMWPE and of the composites showed that the addition of ATP nanofibers and the ATP-TiO₂ hybrids increased the strength of UHMWPE matrix. Meanwhile, the composite filled by ATP-TiO₂ hybrid prepared by the steam method possessed the lowest friction coefficient and wear. It was attributed to the regular morphology of ATP-TiO₂ hybrid prepared by the steam method, so that the hybrid could effectively bear the friction load, and promoted the formation of transfer film, thus reducing the wear of the composites, and displaying flat wear surface that rarely occur obvious plastic deformation and abrasive wear.