Submitted Abstract
Ultra-high molecular weight polyethylene (UHMWPE) is a material widely used as bearing material in joint prostheses whose durability depends on its chemical and mechanical properties. One of the main steps in the production of UHMWPE parts for prostheses is an irradiation-induced cross-linking that considerably improves wear resistance, but at the same time lowers drastically the chemical stability, tensile strength, and toughness of UHMWPE.Texturing like e.g. the formation of a stretched molecular network, could be an alternative treatment to cross-linking, since it can increase strength and toughness of a polymer without altering its chemical properties. However, little information is yet available about the effect of chain orientation on the sliding and wear behaviour of textured UHMWPE. In this PhD thesis, the effect of the chain orientation state on the sliding and wear behaviour of UHMWPE is investigated as well as the underlying mechanisms.Different texture states of UHMWPE were obtained by deformation under tensile stresses performed under an accurately control of the strain level. SAXS/WAXS analyses, SEM observations, WLI tests, and DMA investigations showed a transformation of the initial lamellar structure into a micro-fibrillar structure containing lamellae forming chevrons and thinner lamellae, and characterized by a disappearance of the b-relaxation, and an increased roughness. To evaluate the tribological behaviour of such strained UHMWPE, test samples were subjected to reciprocating sliding with a ball-on-plate configuration, using corundum balls as counter-body. Such tests were performed parallel and perpendicular to the chain direction. From an axial strain level of 0.9, the sliding dissipated energy of textured UHMWPE decreases whatever the sliding direction compared to non-textured UHMWPE. In particular, the cumulative dissipated energy at 50,000 sliding cycles decreases from 110–104 mJ for untreated UHMWPE down to 65 and 43 mJ for the most textured UHMWPE (axial strain of 1.3), parallel and perpendicular to the texture direction respectively. At higher number of cycles (500,000 sliding cycles), a decrease of loss volume by a factor of 3 and 6 was noticed when comparing non-textured UHMWPE with the most textured UHMWPE at 0° and 90° respectively. The results were discussed on the basis of a competition between microstructural features causing a high wear resistance, and microstructural features causing a low wear resistance. Based on our results, the first features are prominent and include the presence of lamellae chevrons (spring effect), the disappearance of b-relaxation (loss of mobility), a possible increase of thermal conductivity (limited temperature elevation), the solicitation of crystalline lamella residues perpendicular to chain axis (unfavourable conditions for a deformation of the lamellae), and the solicitation of amorphous chains network parallel to chain axis (unfavourable conditions for a deformation of amorphous phase). Despite the anisotropic microstructure of textured UHWMPE, this material has anti-wear properties both in parallel and perpendicular directions to the texture direction, and appears suitable for medical device applications.Keywords:UHMWPE, microstructure, sliding, wear, texturing