Tribologie

vignette|upright=1.5|Les premières études empiriques sur les forces de friction sont attribuées à Léonard de Vinci. La tribologie (du grec ancien , « frottement » et , « science, étude ») est la science qui étudie les phénomènes susceptibles de se produire entre deux systèmes matériels en contact, immobiles ou animés de mouvements relatifs. Ce terme recouvre, entre autres, tous les domaines du frottement, de l’usure, de l'étude des interfaces et de la lubrification. C'est une branche du génie mécanique et de la science des matériaux qui a des applications concrètes en archéologie, dermatologie, cosmétique, dans l'industrie, les transports et la biomécanique. Ce terme désigne aussi la discipline psycho-physiologique qui étudie comment les êtres vivants perçoivent les frottements dont ils sont le siège et les vibrations qui en résultent. Histoire vignette|Tribomètre (1742), construit par Jean Antoine Nollet, . Malgré la désignation relativement récente du domaine de la trib

Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Fatemeh Saeidi

Reducing friction and wear is essential for building efficient systems with low energy consumption and a long lifetime. Surface texturing is one of the methods to reduce friction and wear, especially in oil-lubricated systems. However, there is still a lack of knowledge about the fundamental aspects involved in the improvement of tribological performance, especially for the mechanical parts sliding under starved lubrication conditions. Moreover, failure in such lubricated contacts is poorly understood and there are little agreements regarding the mechanisms leading to scuffing. The present research focuses on three main objectives. The first is to characterize a cast iron-steel tribo-system under starved lubrication conditions, and to improve its tribological performance by laser surface texturing. All tribo-tests are performed using a flat-on-flat set-up with reciprocal movements. This set-up simulates specific industrial operating conditions of a semi-journal bearing used in a cutting machine. A Design of Experiments (DoE) approach is selected to investigate the effect of the different geometrical micro-texture parameters on the coefficient of friction (COF) and the lifetime of the cast iron samples. An optimum micro-texture geometry leading to a low COF and a long lifetime is determined based on a fractional factorial design. The second goal is to investigate the failure mechanisms of the selected tribo-pair. Failure of the tribo-system is characterized for both textured and un-textured surfaces. Scuffing is found to be the failure mechanism for the un-textured cast iron samples. For the textured cast iron samples, in contrast, two different failure mechanisms are observed depending on the distance between the micro-textures in direction of sliding (DMS). The textured surfaces with a DMS > 3.5 mm fail via the scuffing mechanism, whereas the surfaces with a DMS < 3 mm fail by a different mechanism named the oxidation mechanism. In contrary to the scuffing mechanism, the oxidation failure is found to be more a gradual failure. Based on the experimental observations, two hypotheses are suggested for the scuffing mechanism, and one for the oxidation mechanism. Finally, predicting the lifetime of a tribo-system is of the utmost importance to save costs. The DoE model developed for the lifetime was found to be a weak predictive model due to the sudden nature of scuffing. The accuracy of this model could only be improved by performing large statistics, which is not time and cost effective. Consequently, the last objective of this work is to employ an acoustic emission (AE) technique to detect precisely the surface state of our tribo-system. The application of wavelet packet decomposition, as an advanced signal processing method used for AE signals, is found to be promising for early detection of scuffing. The extension of this work using random forest regression shows the possibility of predicting scuffing 5 min before it occurs.

EPFL2016

Tribological Approach to the Watch Movements

Valentine Agnès Madeleine Magnin

Mechanical watches are complex and sensitive systems that have to maintain a faultless precision over their lifetime. However, this precision can be altered by disturbances occurring during the functioning time, such as friction, lubricant drying, or wear, leading to surface modification. Due to the significant number of contacts, and diversity of materials, geometries, motions, and lubricants, these disturbances and their consequences over time are difficult to control. To deeply understand these phenomena and limit their effect on watches precision over time, a tribological study of watch movements is necessary. The goal of this Ph.D. research is to contribute to the development of a scientific methodology to appraise friction and wear evolution over time in watch movements. To achieve this, a system approach combined with a modelling of the phenomena was used to anticipate the evolution of two critical watch components: the barrel and the escapement. Based on existing theories of lubrication, the performance of the Swiss lever escapement was appraised as a function of well defined geometrical, kinematic, and dynamic parameters. The developed formalism allowed to highlight the relevance of the radii of curvature of the escapement tooth and the anchor pallet on the effectiveness of the fluid lubrication of the Swiss lever escapement. The tribology of the barrel, a brass piece coated with a nickel sublayer and a flash of gold, was then investigated. Wear characterization of real pieces allowed to identify the relevant wear mechanisms at the barrel drum/spring contact. Friction tests were performed to reproduce the observed phenomena in controlled conditions. A predictive law was proposed to estimate the loss of power at the escapement wheel and showed that the wear of the barrel drum can significantly affect the energy efficiency of the watch. Model surfaces composed of a nickel substrate of a specific waviness profile coated with different gold thicknesses were used for further investigation on the wear mechanisms of the gold coating. A simple mechanistic model considering contact pressure, geometrical, and frictional properties was developed to predict the minimum gold thickness necessary to lubricate the system. Two different relations were proposed according to the sliding direction. These relations were used to evaluate possible improvement strategies in terms of materials properties and surface finishing. With the intention to perform further tribological studies of any contacts of a mechanical watch, a methodology was proposed. This methodology stands on three phases: analysis, modelling and application to the watch. This thesis emphasizes the need of a tribological study to better understand the tribological response of a mechanical watch. By combining reviews of literature, experimental observations, and theoretical laws, analytical models were developed to identify critical parameters, materials and design strategies for improving the tribological response of watch components.

EPFL2020

Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Fatemeh Saeidi

EPFL2016

Tribological Approach to the Watch Movements

Valentine Agnès Madeleine Magnin

EPFL2020

Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Tribological Approach to the Watch Movements

Molecular tribology of highly ordered monolayers

Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Tribological Approach to the Watch Movements

Molecular tribology of highly ordered monolayers