Improvement of surface mechanical properties of polymers by helium ion implantation

The use of ion implantation as a surface treatment technology has been dominated by its applications to prevent wear and oxidation in metal alloys, although some early works already pointed out that ion implantation could also be effective as a surface treatment for other materials, including polymers. Further research has shown that low-dose implantation of energetic light ions could be very effective for improving properties such as hardness and wear resistance in many different polymers. Cross linking of polymeric chains due to ionization energy provided by the stopping process is the main mechanism to explain the changes in mechanical properties. According to this model the lighter the ion is, the stronger is the effect: hydrogen and helium ions would be the ideal options to be employed in the treatment of polymers.This paper presents the results obtained by helium implantation in ultra high molecular weight polyethylene (UHMWPE). He+ ions were at 90 and doses between 5 × 1015 and 2 × 1016 ions/cm2. Nitrogen was also implanted in separate samples, under the same conditions, as a reference. Universal Hardness was measured using micro indenter at a maximum load of 2 mN. The results clearly show a higher increase in hardness and surface elastic modulus for samples implanted with helium, in comparison to those implanted with nitrogen or unimplanted. The effect also increases with the dose, although it saturates at 1 × 1016 ions/cm2.