Reactive Friction and Numerical Experimentation : Mechano-chemical Behavior of C/C composites used in high energy brakes – FReIN

The objective of a brake is to convert kinetic energy in thermal energy, and then to diffuse this thermal energy. The increase of temperature induced by a braking is related to the calorific value and to the mass of the material. For airplanes, energies are high and mass is a keypoint, then carbon/carbon composites (C/C) are the best choices. On large aircrafts, they have almost replaced the conventional steel brakes since they exhibit good mechanical properties whatever the temperature, a calorific value which is twice the one of steel, and their lifetime is significantly higher than the one of steel brakes.

It has been shown that molecules presents in atmosphère have a great influence on wear and friction factor of C/C composites. In the frame of four PhD thesis done in collaboration between Messier-Bugatti and the Institut de Science des Matériaux de Mulhouse (IS2M), and also between Messier-Bugatti, the Centre de Recherche sur les Matériaux Divisés (CRMD) and the Laboratoire de Mécanique des Contacts et des Structures (LaMCoS), it was shown that carbon materials which are submitted to mechanical sollicitations typical of tribology (quasi-hydrostatic pressure and shear gradient) develop a significant increase of their reactivity. This chemical reactivity leads to interactions between the friction material and the gas (chemisorption reactions). This leads to a coupling between mechanical and chemical phenomena, and therefore to modifications of the friction factor and of the wear.

Other studies done on a model material (graphite) have lead to development of a new methodology which allows to analyze the coupling between chemical and mechanical phenomena. This method is based on the fact that the processes occurring in a miller are similar to the ones observed in high energy brakes. Then, the aim of the project is to perform milling in well controlled conditions (gas composition, temperature). These experiments will be a model case in order to separate the chemical and the mechanical phenomena. to this end, a discrete element model will be built to simulate the behavior of the composite material inside the miller. This simulation will include the mechanical, thermal and chemical processes which are occurring in the miller. This model will allow to study the evolution of surface reactivity of particles inside the miller, and its consequence on adhesion and agglomeration process between particles. Then a finite element model of the composite will be built, which includes the relations obtained from the discrete element milling simulation. The results of the finite element model will be compared with braking tests on two different devices: a tribometer equipped with a mass spectrometer which is able to analyze the gas evolving from the interface, and an industrial scale test device which use experimental conditions close to the ones of the application.

The scientific results which are expected from this project are important: it is the first time that the interactions between the chemical and mechanical processes which occur during the degradation of the C/C composite will be completely quantified (evolution of surface reactivity, kinetics of chemical and mechanical phenomena, influence of temperature). This is a key point in order to understand the evolution of the friction factor and of the wear during the use of carbonaceous materials in brakes. A second important result will be the quantitative description of the adhesion component in friction of carbon materials, which will be included in a finite element model of the behavior of the composite during the braking. From an industrial application point of view, this project will help to understand some unexpected results which are observed on the braking test devices. The knowledge of the phenomena which occur during the friction will also help to optimize the composite materials.