On the improvement in Nanomechanical Behavior Induced by Nitrogen Ion Irradiation on UHMWPE

FASCE, LAURA; CURA, JOSEFINA; PETTARÍN, VALERIA; FRONTINI, PATRICIA; DEL GROSSO, MARIELA; CHAPPA, VERÓNICA; GRACÍA BERMÚDEZ, GRERARDO

Salerno, Italia

Simposio; Symposium S20 - Polymers for Biomedical and Pharmaceutical Applications of the Annual Meeting of the Polymer Processing Society, PPS-24; 2008

Polymer Processing Society

Inadequate mechanical surface properties limit the service life of ultra-high molecular weight polyethylene (UHMWPE) hip and knee joint components. It is well known that in vivo wear resistance of UHMWPE can be improved by crosslinking, but concomitantly its fatigue strength is reduced. M. del Grosso et al. have shown that irradiation with 33MeV N ion beam at fluences ranging from 5x1010 to 1x1014 ions/cm2 induces strong chemical modifications in the outer layers of the polymer [1]. Thus it appears that, by means of this irradiation technique, it may be possible to generate suitable surface modifications without affecting the desirable bulk properties of UHMWPE. The motivation of this work was to examine nanoscale mechanical behavior of the outer layer region of N beam irradiated UHMWPE using depth-sensing indentation techniques. Reduced elastic modulus and universal hardness were determined from nanoindentation experiments and by means of the Oliver-Pharr approach. Apparent friction coefficient and scratch depth profiles were evaluated from nanoscratch tests. Nanowear experiments were also carried out to image and study the materials deformation characteristics in worn regions. The irradiated samples exhibited better performance than unirradiated ones, i.e. higher elastic modulus and hardness, and lower friction coefficient. The magnitudes of these properties were shown to be dependent on fluence under the irradiation conditions used. UHMWPE irradiated with 1x1012 ions/cm2 exhibited a four-fold increase of hardness and elastic modulus and at one point five-fold decrease of friction coefficient. This sample also showed a remarkable improvement in the wear behavior at the nanoscale.