Effect of crystalline and amorphous phases on the transfer of polytetrafluoroethylene (PTFE) onto metallic substrates
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The transfer of polytetrafluoroethylene (PTFE) onto stainless steel (AISI 316) and silicon wafers in a static contact was characterized at temperatures ranging from 21 °C to 300 °C. The amount of transferred PTFE on AISI 316 substrate was found to be strongly time- and temperature-dependent, and reached a steady state after a certain period of contact time. On testing the effect of temperature on this steady transfer state, an almost temperature independent amount of PTFE was observed to be transferred onto AISI 316 at temperatures between 130 and 180 °C. This steady state started at the glass transition at about 127 °C. This was significantly consistent with an almost constant amorphous-to-crystalline phase ratio at about the same temperature range as determined using infrared spectrophotometry (IR). In general, the amount of the transferred PTFE on AISI 316 depended on the relative amount of amorphous phase in the PTFE contact area. Thus, the amorphous phase in semicrystalline PTFE evidently plays an active role in the transfer process.
I. INTRODUCTION In solid contacts between polytetrafluoroethylene (PTFE) and metallic materials, the tribological models and explanations for the mechanisms of transfer and adhesion are, generally, based only on qualitative, and not quantitative, analyses of the structure of PTFE. For instance, the mechanism of the characteristic transfer film formation of PTFE in the sliding contact with other solid materials has been considered to be due to the adhesive junctions formed at the interface, being broken under the load by the tangent shear stress.1 These junctions are thought to be caused by the cohesive bonding between PTFE end caps (-CF 2 -) or other free radicals and metallic surface.2 Moreover, slippage of the crystalline lamellae, due to the amorphous region existing between the crystalline slices, results in adhesive transfer.3 The demonstration of slippage within the amorphous regions was also shown by other workers.4 Kar and Bahadur pointed out that the interlamellar shear can be attributed to this slippage based on their electron microscopy and differential thermal analysis.5 They have not observed any thermal softening or melting at both low and high sliding speeds. Taneka et al, on the other hand, suggested that the transfer film is formed because of the destruction of the banded structure due to the low activation energy (7 kcal/mol) of the slippage between crystalline slices.6 With respect to the effect of crystallinity, Lontz and Kumnick7 showed that the wear of PTFE in sliding contact against steel in a pin-on-ring configuration decreases as the crystallinity decreases, since, obviously, the mechanical and physical properties of PTFE such J. Mater. Res., Vol. 7, No. 11, Nov 1992 http://journals.cambridge.org
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as the strength, modulus, density, and hardness vary with the crystallinity. Generally, the yield strain and hardness decrease, but modulus in flexure increases, as the crystallinity increases.8 Similarly, the ultimate tensile strength,
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