40 research outputs found
Formalisation of the tribo-oxidation mechanisms of a cobalt-based alloy subjected to fretting wear : influence of the temperature
No full textCe mémoire de thèse présente une analyse multi-physique des endommagements par usure provoqués sur un alliage base cobalt soumis à des sollicitations de fretting pour des températures allant de 25˚C à 600˚C. On montre alors que l’usure est sévère à basse température puis douce à haute température. À basse température, l’usure est contrôlée par une synergie entre l’oxydation de l’interface et l’abrasion de la couche d’oxyde nouvellement formée. Une formulation analytique est développée, prenant en compte ces deux aspects. Cependant, cette loi n’est valable que lorsque le produit de l’endommagement, à savoir les débris d’usure oxydés, sont immédiatement évacués de l’interface. Or, si la température dépasse une température seuil T1, les débris commencent à stagner dans l’interface, ils s’agglomèrent et modifient complètement la réponse tribologique. À haute température, une tribocouche protectrice est formée par compaction et frittage des débris, limitant drastiquement l’usure. La loi d’usure par oxydo-abrasion, décrivant le comportement tribologique à basse température, est alors modifiée pour prendre en compte l’apparition de cette tribocouche. La tribocouche, également appelée glaze layer, possède une structure multi-couches, où les propriétés d’oxydation et de diffusion des éléments d’alliage jouent un rôle prépondérant dans les modalités de sa formation. À la lumière de ces résultats, sont finalement discutés les mécanismes mis en jeu dans l’absence totale d’usure lorsque la glaze layer est efficace à l’interface.This manuscript presents a multi-physical analysis of the fretting wear damages observed on a cobalt-based alloy. It has been showed that the wear process is severe at low temperature whereas a mild wear process takes place at high temperature. At low temperature, the wear process is driven by a synergetic action of the oxidation of the surface and the abrasion of the newly formed oxide layer. An analytical formulation is then proposed by taking into account these two phenomena. However, the proposed wear law is only valid when the wear debris is immediately ejected out of the interface. Yet, when the temperature is high enough, the debris starts to agglomerate into the interface changing completely the tribological response. At high temperature, a protective tribolayer is formed by a compaction and sintering processes of the wear debris which drastically limits wear. The oxido-abrasive wear law, describing the wear behaviour at low temperature, is then modified in order to take into account the tribolayer formation. The tribolayer, also called "glaze layer", has a multi-layerered structure whose formation is strongly linked to the oxidation and diffusive properties of the alloying elements. In the light of these results, the tribological mecanisms providing complete protection of the interface from further wear are discussed
Formalisation des mécanismes de tribo-oxydation d'un alliage de cobalt soumis à des sollicitations de fretting-usure : effet de la température
No full textThis manuscript presents a multi-physical analysis of the fretting wear damages observed on a cobalt-based alloy. It has been showed that the wear process is severe at low temperature whereas a mild wear process takes place at high temperature. At low temperature, the wear process is driven by a synergetic action of the oxidation of the surface and the abrasion of the newly formed oxide layer. An analytical formulation is then proposed by taking into account these two phenomena. However, the proposed wear law is only valid when the wear debris is immediately ejected out of the interface. Yet, when the temperature is high enough, the debris starts to agglomerate into the interface changing completely the tribological response. At high temperature, a protective tribolayer is formed by a compaction and sintering processes of the wear debris which drastically limits wear. The oxido-abrasive wear law, describing the wear behaviour at low temperature, is then modified in order to take into account the tribolayer formation. The tribolayer, also called "glaze layer", has a multi-layerered structure whose formation is strongly linked to the oxidation and diffusive properties of the alloying elements. In the light of these results, the tribological mecanisms providing complete protection of the interface from further wear are discussed.Ce mémoire de thèse présente une analyse multi-physique des endommagements par usure provoqués sur un alliage base cobalt soumis à des sollicitations de fretting pour des températures allant de 25˚C à 600˚C. On montre alors que l’usure est sévère à basse température puis douce à haute température. À basse température, l’usure est contrôlée par une synergie entre l’oxydation de l’interface et l’abrasion de la couche d’oxyde nouvellement formée. Une formulation analytique est développée, prenant en compte ces deux aspects. Cependant, cette loi n’est valable que lorsque le produit de l’endommagement, à savoir les débris d’usure oxydés, sont immédiatement évacués de l’interface. Or, si la température dépasse une température seuil T1, les débris commencent à stagner dans l’interface, ils s’agglomèrent et modifient complètement la réponse tribologique. À haute température, une tribocouche protectrice est formée par compaction et frittage des débris, limitant drastiquement l’usure. La loi d’usure par oxydo-abrasion, décrivant le comportement tribologique à basse température, est alors modifiée pour prendre en compte l’apparition de cette tribocouche. La tribocouche, également appelée glaze layer, possède une structure multi-couches, où les propriétés d’oxydation et de diffusion des éléments d’alliage jouent un rôle prépondérant dans les modalités de sa formation. À la lumière de ces résultats, sont finalement discutés les mécanismes mis en jeu dans l’absence totale d’usure lorsque la glaze layer est efficace à l’interface
The Two Pictures Cycles in Early Manuscripts of St Anselm’s Prayers. Illuminating the Middle Ages: Tributes to Prof. John Lowden from his Students, Friends and Colleagues, eds Laura Cleaver, Alixe Bovey and Lucy Donkin (Leiden 2020),
No full textTwo distinct series of miniatures are identified accompanying early manuscripts of St Anselm's Prayers. Both originate in the circle of the author himself but register his changing attitudes in a turbulent period of his life. The first c.1100 is political and strongly pro-papal, the second c.1104 focuses on the individual's experience of God
Understanding and formalization of the fretting-wear behavior of a cobalt-based alloy at high temperature
No full textInternational audienceThe purpose of this study is to investigate the mechanisms involved in the wear of cobalt-based interfaces at high temperature. The studied contact is a cobalt-based alloy subjected to fretting against an alumina sample. At high temperature, a protective third body is spontaneously created at the interface and presents excellent tribological properties. The formation of the so-called "glaze layer" leads to an absence of wear. The investigation presents on complete description of the hightemperature tribolayer with microstructural, chemical and mechanical characterizations. The glaze layer regime is mainly related to a threshold temperature above which a thin cobalt-rich layer is formed by a tribo-sintering process. A formalization of the tribo-sintering process is proposed to predict the necessary number of fretting cycles N GL to form the glaze layer. The tribo-sintering process prevents wear debris ejection by continuously re-incorporating the wear debris particles in the glaze layer. The re-incorporation of the wear debris may be the reason for the absence of wear of the fretted interface from a macroscopic point of view. Finally, the paper presents an extended friction energy wear approach taking into account tribo-oxidation and tribo-sintering considerations. The formulation is able to predict wear for a large range of tribological parameters (temperature, frequency, sliding amplitude, number of cycles), when applied to the Co-based/alumina contact
A combined friction energy and tribo-oxidation formulation to describe the high temperature fretting wear response of a cobalt-based alloy
No full textInternational audienc
Investigating the origins of the 'double rise' shape in hydrogen permeation transients on pure iron,
No full textInternational audienceThis study delves into the underlying causes of the atypical 'double rise' shape observed in hydrogen permeation rising transients on pure iron and low alloy steels. Electrochemical permeation experiments on pure iron reveal a fast initial rise, a short pseudo-plateau, and a slow second rise. Similar patterns emerge in the decaying transients. The micro-porosity present in material appears to act as reversible traps, affecting hydrogen diffusion. Surface damage, confirmed by SEM analysis, exacerbates the issue. Utilizing numerical simulations, an FEM model effectively replicates the 'double rise' behavior, attributed to limited recombination/dissociation kinetics at bulk-cavity interfaces. Overall, micro-porosity is identified as the primary factor behind this unique permeation curve shape
Etude expérimentale de la diffusion et du piégeage de l’hydrogène dans un acier contenant de la porosité
No full textNational audienc
Design and experimental validation of hydrogen trapping features in model Nickel alloys
No full textNational audienc
Design and experimental validation of hydrogen trapping features in model nickel alloys
No full textInternational audienc
