Depth-Dependent Hardness Improvements in Ion Irradiated Polystyrene
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thicker modified layer than for a higher mass ion. Also, fluences higher than 1x10W9 ions/m2 have yielded significant improvements in hardness of polystyrene. The irradiation was performed at the triple ion irradiation facility in the Metals and Ceramics Division at Oak Ridge National Laboratory [6], using a 2.5 MV Van de Graaff accelerator. The specimen was cut and mounted edge-on in epoxy resin. The surface was then polished using fine diamond paste (successively with 6 pm, 3 pm and 0.25 pm diamond particle sizes) thus yielding a cross-section of the irradiated layer. Hardness measurements were performed using the Nanoindenter" (Nanoinstruments, Knoxville, Tennessee) [5]. Indents were made at 10 pm intervals along a path that was inclined approximately 6' to the epoxy-polystyrene interface intersectionn with the plane of the polished surface, starting in the epoxy, into the irradiated layer and terminating in the unirradiated PS substrate. For each indent, a 20 mN load was used and the total indentation depth was 500 nm with hardness values reported for a 100 nm depth.
The distance along the path was converted into equivalent normal depth from the epoxy/polystyrene, and the results are reported in terms of hardness as a function of irradiation depth. The indents were subsequently imaged using optical microscopy and scanning force microscopy (SFM). SFM was conducted using a Topometrix* Explorer Scanning Force Microscope. RESULTS AND DISCUSSION Hardness values as a function of irradiation depth are shown in figure 1. Hardness increased from a value of 0.4 GPa for the epoxy to a high of 12 GPa for a depth of around 6.5 Am, before dropping to a value of 0.45 pm for the unirradiated PS beyond the 9 pm ion range. The variation in hardness was not smooth as shown in the figure. For comparison, the hardness at 100 nm depth, measured while indenting normal to the
15+ 2 MeV He+ Irradiated Polystyrene
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o Figure 1.
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Nanoindentation hardness values as a function of irradiation depth taken across the cross-section showing a peak hardness of 12 GPa at a depth of approximately 6.5 pm.
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irradiated surface, was 3.32 GPa. The series of indents is shown in the composite optical micrograph in figure 2, which distinctly shows the irradiated layer between the epoxy and the underlying unirradiated PS. The epoxy-polystyrene interface can be seen as a dark line in figure 2. The residual images of the indents, however, were not visible within the irradiated polymer layer. Scanning force microscopy also did not show indents within the modified region. It is to be noted from figure 2 that the irradiated layer does not appear to be flat but seems to have the appearance of a "hill" with the peak, corresponding to the white band at a depth within the irradiated layer cross-section corresponding approximately to the maximum hardness. This is probably due to the fact that since this region of higher hardness is cross-linked to a greater extent, it was more resistant to the polishing t
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