Influence of the ball material on friction and wear in microabrasion tests

This work aims to contribute to the understanding of the complex relationship between ball material, friction, particle dynamics and wear during microabrasion tests. For that, five different balls (empty set 25.4 mm) were used: one ceramic (Si3N4), one metal (AISI 52100 steel) and three thermoplastic polymers (polypropylene, polyacetal, and polyamide 6.6). The tests used a fixed ball micro-abrasion test rig, AISI 304 austenitic stainless steel specimens, silica abrasive particles (10 %wt. slurry concentration), rotational speed of 150 rpm and total test time of 90 min. The harder balls induced higher friction but resulted in lower wear rates than the softer polymeric balls. The wear craters showed that particle entrainment was favoured for the low friction, softer balls (polypropylene and polyamide 6.6), leading to sliding of the abrasives and thus grooving. The craters produced with the harder balls (AISI 52100 steel and Si3N4) presented a few, sparsely spaced deep grooves, but with significant amount of microindentations between the grooves, characterizing the mechanism of microrolling. The polyacetal ball resulted in intermediate results between the other two groups. It was hypothesised that for the lower friction materials the larger difference in friction between the abrasive and the ball (mu(ab)) and between the abrasive and the specimen (mu(as)) favoured particle entrainment and increased wear rates. For the harder balls this difference should be smaller, leading to fewer particles entraining into the contact, but fragmentation of the abrasives could induce microrolling between the grooves.