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See detailEstimation of rubber sliding friction from asperity interaction modeling
Bui, Q. V.; Ponthot, Jean-Philippe ULg

in Wear (2002), 252(1-2), 150-160

Interaction between a soft rubber asperity and its hard counterpart is traced with the help of a finite element computation. The analysis is aimed to estimate the influence of adhesion between rubber and ... [more ▼]

Interaction between a soft rubber asperity and its hard counterpart is traced with the help of a finite element computation. The analysis is aimed to estimate the influence of adhesion between rubber and rigid surfaces and the energy losses arising from the deformation of rubber bulk to the sliding resistance. At the contact zone, interfacial bonds are formed due to adhesion and their resistance to sliding is represented by the shear strength of the contact interface. In the rubber bulk, the hysteresis loss is calculated using an appropriate model of the viscoelastic mechanical behavior of rubber for large strains. Dependence of friction on sliding speeds and temperature is hence detected. Influence of surface roughness and contact pressure on friction is also examined. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailNanoindentation and scanning force microscopy as a novel method for the characterization of tribological transfer films
Randall, N. X.; Bozet, Jean-Luc ULg

in Wear (1997), 212(1), 18-24

In conventional pin-on-disk testing of the tribological characteristics of two different materials in sliding contact, the main parameters of interest are notably the friction and wear properties of the ... [more ▼]

In conventional pin-on-disk testing of the tribological characteristics of two different materials in sliding contact, the main parameters of interest are notably the friction and wear properties of the material pair. However, when two bodies consisting of hard and soft materials respectively are subjected to such testing, the appearance of a transfer film, or third body, which can be a composite mixture of the two, is often observed. Until now the characterization of transfer films in terms of their mechanical properties has been hampered by their nonhomogeneous distribution across a tested surface, their small size, low thickness and the difficulty in accurately positioning a test probe such that the film properties can be measured independently from those of the substrate. In this paper a new method is introduced, consisting of nanoindentation and scanning force microscopy (SFM), which is capable of highly localized indentation testing of a specified sample site with high resolution imaging of the area prior to and after indentation. In this way the hardness and modulus of a transfer film can be obtained, as well as valuable surface topographical information concerning the material response to the indentation. Measurements are presented for the material pair A286/polyimide after testing on a pin-on-disk tribometer in ambient air and liquid nitrogen. Distinct variations in hardness between the transfer films and their contacting bodies have een observed and correlated to the wear behaviour and testing environment. (C) 1997 Elsevier Science S.A. [less ▲]

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See detailTYPE OF WEAR FOR THE PAIR TI6A14V/PCTFE IN AMBIENT AIR AND IN LIQUID-NITROGEN
Bozet, Jean-Luc ULg

in Wear (1993), 162(Part B), 1025-1028

In the development of large, high pressure valves for cryotechnic rocket engines, the Tribology Department of the University of Liege had the opportunity to evaluate the pair Ti6Al4V ... [more ▼]

In the development of large, high pressure valves for cryotechnic rocket engines, the Tribology Department of the University of Liege had the opportunity to evaluate the pair Ti6Al4V/polychlorotrifluoroethylene (PCTFE) on a pin-on-disk tribometer in ambient air and liquid nitrogen with the contact pressure and sliding speed ranging, respectively, from 3 to 9 MPa and 0.03 to 0.05 m s-1. Because of the affinity of titanium for halogen elements, such as chlorine and fluorine, which are both present in PCTFE, we noticed the formation of an abrasive compound made of chlorine, fluorine, titanium and aluminium. These elements were detected by electron microscopy. In both environments the heat produced during sliding is responsible for the decomposition of the PCTFE. The surface of the polymer is softened by the heat and embedded abrasive particles then scratch the sliding track of the Ti6Al4V disk. This abrasiveness is confirmed with a three-dimensional stylus profilometer used on the Ti6Al4V disks. The low temperature and neutrality of liquid nitrogen hardly inhibit the decomposition of the polymer. This is related to the poor thermal conductivity of both titanium and PCTFE: the heat produced at the contact is fully available for the chemical reaction between the chlorine, fluorine and titanium, all the more so since this reaction requires little energy to take place. A better understanding of the wear behaviour of the pair PCTFE/Ti6Al4V in both air and liquid nitrogen should lead to the development of a surface treatment which prevents the formation of abrasive particles. We could then use titanium alloys as bulk materials for sliding parts in cryogenic valves and, because of the low specific mass of titanium, could benefit from the subsequent weight reduction. [less ▲]

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