Caractérisation du comportement mécanique de surfaces hyper-déformées par des phénomènes de contact

The mechanical surface treatments confer better local mechanical properties against wear or fatigue service conditions. In the case of impact-based treatments, the material is exposed to repeated mechanical loadings, producing a severe plastic deformation in the near-surface. It leads to a local and progressive refinement of the microstructure into the affected zone, commonly known as Tribologically Transformed Surface (TTS). For this project, two mechanical surface treatments are used in a model material (pure α-iron): (i) shot-peening and (ii) micro-percussion.The resulting surfaces are characterized by a mechanical property gradient in-depth as a consequence of the microstructural transformation over a few tens of microns. Nowadays, it is well-known that this rise of local mechanical properties could improve the service lifetime of materials. However, a simple micro-hardness test is not quite enough to quantify precisely the engendered variation of mechanical properties and understand the influence of several microstructural effects. For this purpose, two micro-mechanical tests are considered: (i) nano-indentation and (ii) in situ micro-pillar compression.The main issue of this work is to characterize the mechanically-induced transformed surfaces and correlate the mechanical properties gradients with the local microstructural evolutions. Indeed, three main goals are considered: (i) quantify the mechanical and microstructural gradients induced by the surface treatments (shot-peening and micro-percussion), (ii) correlate the results obtained by the means of both mechanical tests (nano-indentation and micro-pillar compression) and finally (iii) investigate the influence of several microstructural effects related with the graded strengthening of hyper-deformed surfaces.Dans l’industrie, les traitements mécaniques de surface métalliques permettent d’améliorer les conditions de service des pièces mécaniques. Les effets de contact de ces types de procédés engendrent une forte déformation plastique du matériau et par conséquent une transformation microstructurale en sous-surface. Cette transformation se manifeste dans le raffinement progressif de la microstructure dans une couche de quelques dizaines de micromètres. Celle-ci est souvent dénommé "surface tribologiquement transformée" (en anglais : Tribologically Transformed Surface - TTS). Une telle transformation microstructurale conduit à une augmentation des propriétés mécaniques en extrême surface et rend le matériau plus résistant aux conditions de frottement, usure et fatigue.Dans le cadre de cette étude, deux procédures de transformation microstructurale ont été employées sur un matériau modèle : le fer-α. Pour la première technique (grenaillage), la surface est impactée de façon répétitive avec des billes métalliques projetées à grande vitesse. Concernant la deuxième méthode (micro-percussion), la surface est impactée répétitivement à un endroit précis avec un indenteur conique rigide.L’objet de ce projet se centre sur trois aspects principaux : (i) déterminer les gradients mécaniques et microstructuraux induits sur les deux types de surfaces transformées (grenaillage et micro-percussion), (ii) établir un lien quantitatif entre les mesures faites par deux types d’essais micromécaniques (nano-indentation et micro-compression de piliers) et (iii) mettre en évidence les effets microstructuraux impliqués (taille de grain, densité de dislocations, etc...) dans l’augmentation des propriétés mécaniques par hyper-déformation de surfaces

Characterization of the mechanical behavior of hyper-deformed surfaces induced by contact effects

Dans l’industrie, les traitements mécaniques de surface métalliques permettent d’améliorer les conditions de service des pièces mécaniques. Les effets de contact de ces types de procédés engendrent une forte déformation plastique du matériau et par conséquent une transformation microstructurale en sous-surface. Cette transformation se manifeste dans le raffinement progressif de la microstructure dans une couche de quelques dizaines de micromètres. Celle-ci est souvent dénommé "surface tribologiquement transformée" (en anglais : Tribologically Transformed Surface - TTS). Une telle transformation microstructurale conduit à une augmentation des propriétés mécaniques en extrême surface et rend le matériau plus résistant aux conditions de frottement, usure et fatigue.Dans le cadre de cette étude, deux procédures de transformation microstructurale ont été employées sur un matériau modèle : le fer-α. Pour la première technique (grenaillage), la surface est impactée de façon répétitive avec des billes métalliques projetées à grande vitesse. Concernant la deuxième méthode (micro-percussion), la surface est impactée répétitivement à un endroit précis avec un indenteur conique rigide.L’objet de ce projet se centre sur trois aspects principaux : (i) déterminer les gradients mécaniques et microstructuraux induits sur les deux types de surfaces transformées (grenaillage et micro-percussion), (ii) établir un lien quantitatif entre les mesures faites par deux types d’essais micromécaniques (nano-indentation et micro-compression de piliers) et (iii) mettre en évidence les effets microstructuraux impliqués (taille de grain, densité de dislocations, etc...) dans l’augmentation des propriétés mécaniques par hyper-déformation de surfaces.The mechanical surface treatments confer better local mechanical properties against wear or fatigue service conditions. In the case of impact-based treatments, the material is exposed to repeated mechanical loadings, producing a severe plastic deformation in the near-surface. It leads to a local and progressive refinement of the microstructure into the affected zone, commonly known as Tribologically Transformed Surface (TTS). For this project, two mechanical surface treatments are used in a model material (pure α-iron): (i) shot-peening and (ii) micro-percussion.The resulting surfaces are characterized by a mechanical property gradient in-depth as a consequence of the microstructural transformation over a few tens of microns. Nowadays, it is well-known that this rise of local mechanical properties could improve the service lifetime of materials. However, a simple micro-hardness test is not quite enough to quantify precisely the engendered variation of mechanical properties and understand the influence of several microstructural effects. For this purpose, two micro-mechanical tests are considered: (i) nano-indentation and (ii) in situ micro-pillar compression.The main issue of this work is to characterize the mechanically-induced transformed surfaces and correlate the mechanical properties gradients with the local microstructural evolutions. Indeed, three main goals are considered: (i) quantify the mechanical and microstructural gradients induced by the surface treatments (shot-peening and micro-percussion), (ii) correlate the results obtained by the means of both mechanical tests (nano-indentation and micro-pillar compression) and finally (iii) investigate the influence of several microstructural effects related with the graded strengthening of hyper-deformed surfaces

Effect of sliding velocity on friction-induced microstructural evolution in Copper

International audienceDurability of engineering workpieces surfaces is well-known to be strongly related to the microstructural evolutions induced by machining processes. One current challenge is to choose the right process parameters, such as the sliding speed, in order to optimize both the subsurface microstructure and the surface properties. In this paper, a special tribometer, able to simulate contact pressures and cutting speeds occurring during machining, has been used to characterize the effect of sliding velocity on microstructural evolution induced in copper. Significant recrystallization and grain refinement phenomena have been observed for the highest sliding speed tested (250m/min). Finite element analysis have been performed to extract local variables near the pin/copper bar interface. A good agreement is noticed between the equivalent plastic strain level, the temperature rise, the resulting grain size and the hardness gradient

Investigation of surface mechanical property evolution using nano-indentation and micro-compression testing

International audienc

Investigation of graded strengthened hyper-deformed surfaces by impact treatment: micro-percussion testing

International audienc

Microstructural and mechanical properties of hyperdeformed surfaces: in situ micro-pillar compression and EBSD investigations in α-iron

International audienc

Measurement of mechanical properties gradient on impact based transformed surfaces: Nano-mechanical testing in graded micro-structured α-iron

International audienc

Assessment of mechanical property gradients after impact-based surface treatment: application to pure alpha-iron

International audienceMechanical surface treatments are known for their ability to improve material resistance to abrasive wear and local fatigue crack microstructure of the home-made crack propagation. These treatments are based on repeated contact loadings which create large plastic strains in the near-surface that can induce a local grain refinement. In this case, a significant increase in the near-surface local mechanical properties is thus usually observed. In this paper, nano-mechanical tests are used to quantify the mechanical property gradient in the near-surface of a purity-controlled alpha-iron after an impact-based treatment. A methodology based on the combination of two different techniques is proposed: nano-indentation and in-situ micro-pillar compression. The resulting in-depth mechanical properties gradient is compared to the average grain size measured by EBSD. A positive relationship with the well-known Hall-Fetch effect is observed