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The ratio of two forces acting respectively perpendicular and normal to a contact surface of two bodies, the coefficient of friction, is widely used in engineering and science depicting the friction resistance of materials sliding over one another. Ruled by the so-called Amontons-Coulomb friction laws (independence from the load, the contact area and the sliding speed), this dimensionless quantity appears to be convenient for engineering and relatively easy to determine. Nevertheless, the use of tabulated friction coefficients becomes somewhat an issue to predict friction behavior of mechanical systems. The system dependence of friction is sometimes ignored, leading to misapplication. Moreover, the fundamental origins of sliding resistance are not as clear and care should be taken when attributing a fundamental significance to the friction coefficient. This paper aims to clarify findings on friction Charles Augustin Coulomb did and that have been used for hundred of years.

During braking, third-body flows and layers govern friction mechanisms, which are fully responsible of the friction coefficient and wear. In the context of development of brake friction pairs, the involved tribological circuit has to be well understood and mastered. This paper concerns a sintered metal matrix composite used for TGV very high speed train. A series of low-energy stop brakings allows a detailed study of the third-body formation at the pad-disc contact. The pin surface is observed after each test. The evolution of the rubbing-area expansion all along the series is explained, and the friction behaviour, typical of the studied friction material, is related to the formation of a well-established third body at the pad-disc interface.

Friction materials for braking applications are made of a great number of components. Mineral Roxul®1000 fibres have been widely used as a reinforcement product for such materials. Especially in NAO/non-steel disc pad applications these fibres are an important component of the new generation formulations. In this paper, specific material formulations have been tested on an inertia braking tribometer. It is shown that the NVH performances may be strongly affected by the fibre type. Especially, a rubber coating on the surface of Roxul®1000 fibres reduces the squeal of NAO/non-steel disc pads significantly. Numerical simulations of the braking system have been also developed to better understand the effect of the material. Results show that the squealing modes are selected among instable modes corresponding to coupling between the pad-disc modal frequencies in frictional conditions.

During railway braking, dissipation of high energy due to friction leads to transient and localised thermal phenomena such as hot bands and hot spots. These localisation phenomena interact with third body flows and friction mechanisms activated in the contact. To study these couplings, an experimental approach has been developed, based on an inertial tribometer able to reproduce high thermo-mechanical brake-disc loadings. This paper focuses on the coupling between flows of third-body and hot-band migration. Transient localised thermal phenomena are described for high-energy stop-braking. The monitoring of the disc track during friction in the visible and infrared wavelengths evidences the interactions between third-body flows and hot-band migration.