Wear 263 (2007) 1230–1242 Coupling between friction physical mechanisms and transient thermal phenomena involved in pad–disc contact during railway braking A.-L. Cristol-Bulth´ e ∗ , Y. Desplanques, G. Degallaix Laboratoire de M´ ecanique de Lille (CNRS UMR 8107), Ecole Centrale de Lille, BP 48, F-59651 Villeneuve d’Ascq Cedex, France Received 14 August 2006; received in revised form 22 December 2006; accepted 23 December 2006 Available online 23 May 2007 Abstract During railway braking, friction dissipates high energy, leading to transient and localized thermal phenomena such as hot bands and hot spots. These thermal phenomena interact with third-body flows and friction mechanisms activated in the contact. To provide information on the transient character of these physical mechanisms, severe stop-braking conditions are reproduced on a specially designed braking tribometer. Thermal phenomena observed by infrared thermography on the disc-track are characterised and correlated to the evolution of the friction coefficient. In view to studying the coupling between thermal phenomena and friction mechanisms, rubbing surfaces are observed and analysed at typical stages of braking in correlation with the corresponding disc-track thermograms. The results show how third-body flows, resulting from the waviness distortion of the disc, feed the contact and lead to the formation of the third-body in the form of flat plates that stabilize the friction coefficient. © 2007 Elsevier B.V. All rights reserved. Keywords: Railway braking; Hot spots; Hot band; Infrared thermography; Thermal stress; Third-body flow 1. Introduction In railway braking, high energy is dissipated by friction dur- ing short periods. Under this condition, friction causes transient and localized thermal phenomena with high thermal gradients such as hot bands and hot spots [1,2]. The study of these phe- nomena is usually done on a full-scale test bench and does not provide information on the local and transient friction mecha- nisms involved in the contact. On the other hand, investigations on friction materials are often performed on continuous friction devices that do not account for the global influence of the tribo- logical triplet. These studies show that the friction of pad and disc materials generates a third-body mainly composed of powder and flat plates [3–10]. However, even when couplings between thermal and physical aspects are investigated using continuous friction tribotesters [11], interactions between thermal localiza- tion phenomena and friction physical mechanisms involved in braking cannot be studied on these devices. Consequently, a braking tribometer has been specifically developed to reproduce stop-braking conditions at reduced-scale [12]. The present study ∗ Corresponding author. Tel.: +33 3 20 33 53 79; fax: +33 3 20 33 53 93. E-mail address: Anne-Lise.Bulthe@ec-lille.fr (A.-L. Cristol-Bulth´ e). deals with the coupling between thermal phenomena and fric- tion mechanisms. Infrared thermography of the disc-track during braking tests coupled with observations of the rubbing surfaces provide information on the transient character of the third-body during the braking process [13,14]. The first part of this paper is concerned by the experimental aspects of the work and presents the laboratory device and the test conditions. The second part presents the results, i.e. transient thermal phenomena observed by infrared thermography corre- lated to the evolution of the friction coefficient. In the third part, the physical mechanisms involved during braking are investi- gated, by observations and analyses of rubbing surfaces at typical braking stages. Finally, a discussion summarises the results and proposes a tribological circuit involved in braking, highlighting the importance of coupling between friction mechanisms and thermal transient phenomena. 2. Experiments 2.1. Laboratory device: the braking tribometer All the tests were performed on a pin-on-disc braking tri- bometer developed to reproduce at reduced-scale railway-type stop-braking loading conditions as close as possible to those 0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.12.052