Hydrogen release in UHMWPE upon He-ion bombardment

Ultrahigh molecular weight polyethylene (UHMWPE) due to its good wear resistance is the material of choice for the load-bearing surfaces of total joint implants. Unfortunately, failure is often observed caused by the UHMWPE fragility. As a consequence, the formation of microcracks leads to production of microparticles, which reduce the lifetime of the joint. The aim of this work was to improve micromechanical properties of UHMWPE by He-ion bombardment. Energetic ions are slowed down in material by interaction with the electronic system of target atoms. Such interaction producing excitation of atomic electrons or ionization leads to the cleavage of C-H bonds leading to the release of hydrogen. In this work, hydrogen release due to He-ion bombardment at energies ranging from 0.8 to 2.0 MeV was studied in situ by RBS. The data were analyzed using the statistical model of molecular recombination. The H-release cross-section and the saturation concentration were found to depend strongly on the electronic stopping power of the He-beam and exhibit quadratic dependence on the incident ion energy. Our data make it possible to extrapolate the H-release parameters to much lower energies where no RBS measurements are possible. The consistency of such a procedure was independently checked by the NRA measurements. To study the role of temperature on H-release some experiments were performed at 50 K.