Hard Problems with Soft Materials: Wear of Semicrystalline Polymeric Solids

SEBASTIAN LAINO; RICARDO DOMMARCO; JEREMIAS DE BONA; VALERIA PETTARIN; PATRICIA FRONTINI

Mar del Plata

Encuentro; III MaB – III Encuentro Nacional de Materia Blanda; 2010

Facultad de Ingeniería - Universidad Nacional de Mar del Plata

The successful applications of bulk polymers in engineering led to the work of using polymers as composites where modifications to properties could be achieved by adding fillers such as strong fibers, lubricating solid particles and even metallic particle and fibers. Polymer composites have been very successful in finding many engineering applications as tribological materials for gears, bearing cage, shoe soles, automotive brakes and so on. Presently, polymers are used as bulk or as films for various applications where surface friction and wear are important design requirements. Wear is defined as the removal of materials from solid surfaces as a result of mechanical action. Much of the knowledge on the tribological behavior of polymeric materials is empirical and very limited predictive capability currently exists. Since wear is a result of the accumulation of plastic deformation and hardness is the measure of a material’s ability to combat plastic deformation, the relationship between wear and hardness has been extensively studied. For traditional materials or “hard materials” like metals and ceramics, both theory and experiments show that the wear resistance increases with changes in the hardness and friction. A strong positive correlation between friction and wear and a strong negative correlation between hardness and wear have been found. Conversely, the experimental results shown here carried out on “polymeric soft composites” demonstrate that the above mention strong correlation between hardness, coefficient of friction and wear behavior does not state any more. This work first tempted to diminish the coefficient of friction of a semicrystalline polymer by adding a solid lubricant. HMW-HDPE compounded with different amounts of solid lubricant (MoS2 powder) exhibited enhanced wear resistance under both sliding and abrasive wear conditions despite of the lack of significant changes in friction and hardness. The improvements were then interpreted in terms of dissipation of the generated heat and changes in wear mechanisms. Amounts of filler larger than 10% resulted in a detriment of wear resistance due to weak microstructures which lead to the occurrence of micro-cracking wear mechanism. Work was further extended to the case of “hard and strong” fillers in a “softer” matrix. In this case the filler was expected to increase the overall hardness of the composite while the coefficient of friction remains low or increases. Results presented here are on nano-clay modified PP. Despite the largest hardness of nanoclay no increase in hardness of the composites was achieved. Also, no improvements in wear behavior were found. These results indicate that the tribological properties of polymers and composites seem to be largely determined not by their surface properties, but rather by their volume properties. In addition another abnormal behavior was found. Samples appeared grooved and damaged after rubbing against metal counterface but no weight lost was detected, i.e. wear was generated by plastic deformation and subsequent hipping up of material without producing particle debris.