Adhesion, Friction and Leakage in Contacts with Elastomers
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Adhesion, friction and leakage in contacts with elastomers is central to many technologically demanding applications such as tires, seals, wiper blades and syringes to just name a few. These tribological systems are very complex having different material properties (e.g elasticity, viscoelasticity), surface characteristics (multiscale roughness, surface energies) which are consequential in deciding properties such as area of real contact, interfacial separation, stresses acting at the interface. In this thesis, seven different industrially important elastomers which differed in their chemical nature and viscoelastic behavior were chosen for experimentation pertaining to adhesion, friction and leakage. The viscoelastic properties of materials and surface characteristics of the counterface were characterized in detail and the results were analyzed using JKR (for adhesive contacts) and Persson’s theory of contact mechanics (for rubber friction and leakage studies). We have distinguished the three major contributions to rubber adhesion acting at different length scales: bulk viscoelasticity, roughness and molecular mobility, which contribute differently to determine work of adhesion. We showed through experimentation and further analysis that if only partial contact occur at the interface (which is almost always the case in practical applications due to surface roughness), the pull-off force will increase with increasing loading force. This fact open up the possibility to gain information about the area of real contact from the load dependency of the work of adhesion deduced from the JKR theory. Despite the similar surface energies of the elastomers used in study the leakage behavior of elastomers showed different leak rates. The varied leak rates had strong correlation to the adhesion of the studied elastomers to glass in water (the media being sealed), which we think can lead to different way of fluid squeeze out from interface leading to dewetting transition and can be cause of nucleation and growth of gas bubbles. We discuss on the origin of adhesive contribution to rubber friction using model tribological system and attributed this to polymer bonding-stretching-debonding processes as compared to propagation of opening cracks. We found that the contribution from area of real contact to rubber friction has different contribution depending on the temperature and sliding speed. At temperature T > Tg, the temperature dependency of the shear stress τf is weaker than that of the bulk viscoelastic modulus. We proposed that physical origin of τf for T > Tg is determined by the rubber molecule segment mobility at the sliding interface, which is higher than in the bulk due to increased free-volume effect due to short-wavelength surface roughness. For temperatures T < Tg the shear stress τf is nearly velocity independent, and of similar magnitude as observed for glassy polymers such as PMMA or polyethylene. In this case the rubber undergoes plastic deformations in the asperity contact regions, and the frictional shear stress is due to slip at the interface between the rubber and a hydrocarbon film adsorbed on the concrete surface. The hydrocarbon film (of nanometer thickness or more) is assumed to result from the transfer of molecules from the rubber to the substrate surface. In rubber friction studies it is usually assumed that the friction force does not depend on the sliding direction, unless the substrate has anisotropic properties, like a steel surface grinded in one direction. We presented experimental results for rubber friction, where we observed a strong asymmetry between forward and backward sliding, where forward and backward refer to the run-in direction of the rubber block. The observed asymmetry was attributed either to the formation of short-length-scale geometrical structures on the rubber surface which did not show inversion symmetry when rubber was slided forwards and backwards and Baushinger effect (usually seen in metals due to plastic deformation). Elastomer composites based on HNBR containing 50 weight percent microencapsulated phase change materials (MEPCM) were compounded, characterized and numerically simulated to predict their response to transient blowdown situations. Experiments revealed that processing of these composite needs to be taken care of for utilizing MEPCM’s full heat release capability during transient cooling. Numerical simulations showed the temperature decrease retarding capability of these elastomer composites during a blowdown scenario. Sensitivity analysis of the thermal properties of phase change materials for making such rubber seals showed various avenues of optimization for better sealing solution at low temperatures. The latent heat of the MEPCMs and the thermal conductivity of the solid phase of the MEPCM were shown to be most significant to improve transient heating of the materials.