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It is well known that elastomers usually possess poor dry sliding friction properties due to their highly adhesive character. In order to overcome this problematic behavior in industrial applications, interfacial materials such as oils, greases, coatings, or lacks are normally used in order to separate or to functionalize the contact surfaces of elastomers. Alternatively, the high adhesion tendency of elastomers may be explicitly reduced by modifying the elastomer composition itself or by enabling a reduction of its effective contact area through, for example, surface laser texturing. This second approach, i.e., the reduction of the adhesive character of elastomers through laser structuring, will be the main topic of the present study. For this purpose, different micro-sized grooved structures were produced on flat injection molds using an ultra-short pulsed laser. The micro-structured molds were then used to produce injection molded micro-ridged Liquid Silicone Rubber (LSR) sample pads. The investigations consisted firstly of determining the degree of replication of the mold micro-structures onto the surface of the LSR pads and secondly, to ascertain the degree of reduction of the friction force (or coefficient of friction) of these micro-ridged LSR pads in comparison to the benchmark (unstructured LSR pads) when tested under dry conditions against Aluminum alloy (Al-6082) or PA6.6-GF30 plates. For this second part of the investigation, the normal force (or contact pressure) dependency of the coefficient of friction was determined through stepwise load increasing friction tests. The results of these investigations have shown that the production of micro-ridged surfaces on LSR pads through laser structuring of the injection molds could be successfully achieved and that it enables a significant reduction of the friction force for low normal forces (or contact pressures), where the component of adhesion friction is playing an important and determining role in the overall friction behavior of the LSR elastomer. [ABSTRACT FROM AUTHOR]

This paper presents a systematic procedure for the development of a constitutive model of friction with focus on the application in bulk metal forming simulations. The empirically based friction model describes friction as a function of sliding distance and the most relevant friction influencing parameters. The latter were determined by means of designed experiments. An optimal friction model is obtained as a trade-off between model accuracy and complexity by using stepwise nonlinear regression and a modified version of the Akaike information criterion. Within this study, the procedure is applied to determine a friction model for tube drawing. However, the same approach can also be used for modeling friction of any other bulk metal forming process. [ABSTRACT FROM AUTHOR]

Abstract Elastomers are currently used intensively in various industrial applications. However, their deployment in dry tribological contact is normally inhibited by their poor dry sliding friction behaviour due to their intrinsically high adhesive characteristics. This problem is normally addressed in the industry by using various fluid (greases or oils) or solid (coatings or lacks) lubricants. Alternatives to the use of lubricants for tribosystems involving elastomers exist in the form of modifications of the elastomer composition or by reducing their effective nominal contact area by using surface texturing. The latter approach of reducing the adhesive nature of elastomers by surface structuring is the main focus of the present study. Two different micro-sized structures (consisting of micro-pits) were produced on thin Al inlays using a microscopically scaled punching process. These inlays were thereafter glued onto laboratory-scale injection moulds. These micro-pitted moulds were subsequently used to produce injection moulded micro-pillared L iquid S ilicone R ubber (LSR) pads. The present study was focused on three different aspects: 1. Determination of the degree of replication of microstructures from injection moulds to the surface of LSR pads; 2. Evaluation of any possible friction reduction induced by a decrease of the nominal contact area through micro-texturing of LSR pads in dry tribological tests; and 3. Evaluation of the wear resistance of micro-pillared LSR pads under dry conditions. The results have shown that successful production and relatively accurate replication of micro-pitted structures from injection moulds onto LSR pad surfaces in the form of micro-pillars may be achieved. Furthermore, it was shown that a decrease of the effective nominal contact area through micro-pillars may enable a reduction of friction in comparison to the benchmark (unstructured LSR pads), but only for low normal loads where the adhesive component of friction is playing a determining role. Finally, wear tests have shown that the wear resistance of both micro-pillared structures produced on LSR pads under dry conditions against Al or PA6.6-GF30 was relatively poor. Both structures exhibited extensively worn regions after a testing distance of 11 m: structure 1 showed a larger worn area ratio than structure 2 (ratio worn area/total area ~45%–60% for structure 1 and ~30% for structure 2). Highlights • Production of micro-pits in thin Al inlays using a microscopically scaled punching process is highly feasible. • High accuracy reproduction of micro-pits from Al inlays as moulded micro-pillared LSR pads has been successfully achieved. • At normal loads >150 N(adhesive friction as secondary role), micro-pillared pads have friction coefficients similar to benchmark. • At normal loads <40 N(adhesive friction is predominant), reduction of friction coefficients by a factor of 2 may be realized. • Both micro-pillared structures possess relatively poor wear resistance when tribologically tested in dry conditions. [ABSTRACT FROM AUTHOR]

It is generally known that gaseous atmospheres can significantly affect the friction and wear behaviour of non-lubricated contacts; however there still exists a lack of knowledge on the subject, since the tribological behaviour and the tribochemistry of the gas-lubricated systems are highly sensitive to the selection of the operating parameters, and the results from available literature are often scattered and sometimes even contradictable. This study was focused on the identification of the friction and wear mechanisms of technical gases used at atmospheric gas-pressure in non-lubricated DIN 100Cr6 (AISI 52100) bearing steel contacts at severe operating conditions (high-frequency oscillation at high contact pressure). Argon (Ar), nitrogen (N 2 ) and carbon dioxide (CO 2 ) were used as gas atmospheres and air atmosphere was used as a reference. The gases are analysed in terms of their chemical reactivity with the steel surfaces and a correlation between the properties of different tribochemical products and the observed friction and wear mechanisms is made. In N 2 and CO 2 atmospheres, wear was significantly lower than in air atmosphere, with a wear reduction comparable to the effect of using a liquid lubricant. In N 2 atmosphere, a slightly higher friction was measured than in air atmosphere, while in CO 2 atmosphere, friction was 60% lower than in air atmosphere. In Ar atmosphere, both friction and wear were slightly higher than in air atmosphere. With X-ray photoelectron spectroscopy (XPS) of the wear particles, it was observed that the concentrations of different tribochemical products and their mutual ratios critically determine the friction and wear behaviour of the non-lubricated steel contact. [ABSTRACT FROM AUTHOR]