Effects of sequential He + and Ar + implantation on surface properties of polymers
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Effects of sequential He+ and Ar+ implantation on surface properties of polymers Gopal R. Rao and Eal H. Lee Oak Ridge National Laboratory, Bldg. 5500, MS 6376, Oak Ridge, Tennessee 37831-6376 (Received 28 February 1996; accepted 22 May 1996)
Three important polymers: polystyrene (PS), poly ether ether ketone (PEEK), and polyimide Kapton, were irradiated separately with 1 MeV He+ , 1 MeV Ar+ , and 1 MeV He+ followed by 1 MeV Ar+ sequentially, to a fluence of 3 3 1019 ionsym2 for each ion. The specimens were characterized for changes in surface hardness using a nanoindentation technique, and wear resistance using a reciprocating sliding wear apparatus with a steel ball counterface. Results indicated that while all polymers showed higher hardness values after ion irradiation, the dual irradiation resulted in the largest hardness increase, greater than for the single ion-irradiated specimens. Wear test results also indicated that the dual He+ 1 Ar+ irradiation resulted in the best improvement in wear resistance of the polymers. These improvements in properties are a consequence of cross-linking of the polymer material caused by the ion irradiation. Linear energy transfer considerations showed that the dual He+ 1 Ar+ implantation was better because it combined a deeper implant, in the form of He, along with Ar irradiation which resulted in a shallower but more highly cross-linked layer at the near surface. Thus a deeper and graded cross-linked surface region was formed. The study shows that there is greater flexibility for tailoring surface properties of polymers by using a judicious combination of ion species, ion energies, and fluences.
I. INTRODUCTION
Ion irradiation of polymers using high energy ions has been shown to significantly alter surface-sensitive properties.1–3 The effectiveness of ion implantation for improving surface-related mechanical properties of polymers, such as hardness and tribological properties, has been convincingly demonstrated in several recent studies at Oak Ridge National Laboratory (ORNL).3–7 Ionirradiated polymers retain all the advantages and useful properties of polymers while possessing, in addition, a hard, wear-resistant surface. The technique is, however, limited by the shallow depth of the surface-modified layer. For ion energies up to a few MeV, typical of commercial implanters, the modification depth varies from a few micrometers for low mass ions to less than a micrometer for higher mass ions. Typically, lower mass ions penetrate deeper, but are less effective in altering near-surface properties than higher mass ions which, however, penetrate only to shallow depths. The process of bombarding a polymer with energetic ions causes significant microstructural changes in the material. A large number of bonds are broken, smaller molecular species diffuse away, some of which escape from the surface, and several of the remaining unattached atoms form bonds, ultimately resulting in a well-interconnected, cross-linked network. This scenario
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