Tribological properties and structure of ultra-high molecular weight polyethylene after gamma irradiation

The aim of the present study was to investigate the effects of irradiation and post treatment on the molecular structure, mechanical, and tribological. properties of ultra-high molecular weight polyethylene (UHMWPE). UHMWPE was irradiated by gamma ray at doses of 120,250, and 500 kGy in air and encapsulation by plastic film, and then posttreated in vacuum. The tribological properties were investigated by sliding UHMWPE against Si3N4 ceramic ball on a ball-on-disc tribometer in the presence of distilled water lubricant. The result showed that, after gamma irradiation, the crystallinity and cross-linking degree of UHMWPE increased with dose increasing, whereas its toughness and the elongation decreased sharply. All irradiated samples exhibited lower wear rates with dose in the range from 120 to 250 kGy, but, higher friction coefficients, compared with those un-irradiated. However, over 250 kGy, the wear factor began to increase, which may be associated with the ductile-brittle transition in its mechanical properties. Posttreatment in vacuum promoted residual radicals to cross-link and improved wear resistance of UHMWPE further. The 250 kGy irradiated and posttreated sample showed the best wear resistance. The wear mechanism of un-irradiated UHMWPE and those posttreated ones was mainly plowing, plastic deformation, and adhesion, whereas the irradiated samples were characterized mainly by abrasive wear.