Microstructural effects on surface mechanical properties of ion-implanted polymers
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Tefzel, a copolymer of tetrafluoroethylene and ethylene, was implanted simultaneously with 400 keV boron, 700 keV nitrogen, and 600 keV carbon to a dose of 3 X 1015 ions/cm 2 for each ion. The implanted layer was examined using transmission electron microscope and compared with the pristine Tefzel for microstructural changes. The microhardness of the implanted and pristine Tefzel was determined using a nanoindentation technique. TEM bright-field images of the implanted layer show a patch-type contrast with distinguishable bright and dark regions. Electron energy loss spectroscopy (EELS) was used to show that the bright regions had a higher carbon concentration, as compared with the dark regions. The carbon-rich regions had an average size of approximately 40 ran. The pristine material showed a fairly featureless contrast with occasional local patchy regions. These were determined to be due to local thickness variations. The triple implantation improved the hardness of pristine Tefzel by over 66 times. The structure of the carbon-rich regions appears to be clusters of sp2 bonded C atoms with sp3 sites present and hydrogen preferentially bonded in the sp3 C configuration. It was speculated that the carbon-rich regions could be harder than the surrounding regions, but this could not be resolved due to the small size of the regions.
I. INTRODUCTION Recent studies at Oak Ridge National Laboratory have shown that ion-beam treatment of polymers can dramatically improve surface properties such as hardness and wear resistance. In particular, it has been shown that multiple ion implantation is more effective than single ion implantation.1 Currently, there is significant interest in understanding these changes caused by ion irradiation at the microstructural level, to comprehend better the macroscopic effects in polymers. Ion implantation induces various changes in the structure and chemistry of polymers. Residual gas analysis has shown that there is a liberation of various gaseous species such as hydrogen, carbon monoxide, carbon dioxide, and an assortment of hydrocarbons during irradiation, particularly in the initial stages.1 This leads to a reorganization of the near-surface structure and chemistry. It has been shown that the irradiation results in the formation of a carbonized layer on the surface.2'3 The structure of this carbon-rich layer is of interest in understanding the effects of the irradiation since it is directly responsible for the surface property changes. Our current knowledge regarding surface microstructure of ion-implanted polymers comes mainly from work done on conductivity enhancement in polymers by ion implantation. The current model for conductivity enhancement is based on the presence of carbon-rich islands separated by insulating channels. Conductivity occurs by J. Mater. Res., Vol. 8, No. 4, Apr 1993 http://journals.cambridge.org
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an "electronic hopping mechanism" wherein electrons "tunnel" through the insulating channels while moving from island to island.4 Only indirect
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