Добірка наукової літератури з теми "Tribologically Transformed Structure (TTS)"

The evolution of fretting wear behavior and damage mechanism in Alloy 690TT with cycle number was investigated via laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), focus ion beam (FIB), and transmission electron microscopy (TEM). The results showed that the fretting running status underwent a transition from partial slip and mixed stick-slip to final gross slip with the transformation of Ft–D curves from the ellipse to the parallelogram. The coefficient of friction (COF) experienced three drops throughout the fretting process, which indicated the transformation from high-friction wear to low-friction wear. The first drop was due to the transition from two-body to three-body contact. The second and third drops were mainly related to the evolution of the glaze layer from a localized distribution to completely covering the whole contact surface. The competition between fretting induced fatigue cracking (FIF) and fretting induced wear (FIW) ran through the entire fretting wear process. Before the 1.2 × 104th cycle, the fatigue crack growth was faster than wear, and FIF won the competition. As the fretting cycle continued to increase, the wear velocity was obviously faster than that of FIF, which indicated that FIW defeated FIF. The tribologically transformed structure (TTS) participated in the competition between FIF and FIW. The gain boundaries and dislocations in the TTS were a suitable pathway for crack initiation and propagation and oxygen permeation.

Fretting wear is a common phenomenon that happens between contact parts when there is an oscillatory relative movement. To investigate wear characteristics history in the fretting process, the finite element method (FEM) is commonly applied to simulate the fretting by considering the wear in the model. In most literature publications, the wear coefficient is considered as a constant, which is not a real case based on the experimental results. To consider the variation of wear coefficient, a double-linear model is applied in this paper, and the tribologically transformed structure (TTS) phase is considered in the study of the wear coefficient variation model. By using these models for variable wear coefficient for both flat and cylinder, the difference of wear characteristics, plastic strain, and stress between variable wear coefficient model (VWCM) and constant wear coefficient model (CWCM) are analyzed. The results show that the variable wear coefficient has no significant effect on the wear characteristic at the end of the process in the gross sliding regime. However, in the partial slip regime, the effect of variable wear coefficient on wear characteristics is significant. Due to the difference in contact geometry in the fretting process between VWCM and CWCM, the tangential and shear stress and equivalent plastic strain also show differences during the fretting process.

Cold spray is now well recognized as one of the most powerful and efficient coating process because it is cost-attractive and “green”. However, this process still shows limitations to achieve coatings for highly-demanding service conditions such as those required in certain automotive and/or aircraft applications. Beyond these limitations, cold spray is expected to compete with conventional P/M routes.The present work therefore focussed on the study of damage mechanisms in cold-sprayed AISI 316L and 316L-matrix–Cu composites coatings due to high-loading conditions. Different damage mechanisms could occur depending on the content of Cu particle addition, due to changes in the response of the microstructure to the loading. These mechanisms were studied using the newly-developed “impact-sliding” test. In this test, a steel ball impacts the coating surface at a given frequency, with a fixed angle. The influence of major testing parameters was investigated.Microstructures before and after testing were studied using optical microscopy, scanning electron microscopy (SEM), and microprobe analysis in addition to 3D optical profilometry of impacted areas. Damage mechanisms were seen to be of two types, i.e. plastic deformation and wear. These resulted in decohesion of splats, formation of wear debris and formation of a layer with a tribologically-transformed structure (TTS) at the contact surface.Results showed that cold spray could be claimed to be suitable for the achievement of high-performance coatings for industrial applications provided that the coating microstructure can be controlled. This could be done using a composite approach to the coating composition.

Le fretting est un phénomène de dégradation des surfaces sous faibles amplitudes de glissements alternés. Durant les sollicitations de fretting, des couches superficielles subissent une transformation de leur microstructure, apparaissant blanche après attaque chimique, et deviennent fragiles. Ces couches sont appelées Transformations Tribologiques Superficielles (TTS) et elles participent à la formation de débris. Ces travaux de recherche visent à comprendre la genèse des TTS dans un contact plan-plan de TA6V soumis à des sollicitations de fretting, ainsi qu'à caractériser leur comportement mécanique.Les conditions d'apparition des TTS sont tout d'abord évaluées expérimentalement en faisant varier les paramètres de la sollicitation de fretting, notamment la pression de contact. La cinétique de formation est étudiée en analysant la présence et la morphologie de la TTS pour différents nombres de cycles de fretting. Des analyses chimiques EDX, observation optiques et MEB des coupes transverses des traces de fretting après attaque chimique sont réalisées. Les résultats montrent que les TTS apparaissent d'abord localement, par îlots, avant de former une zone unique et élargie au centre du contact avec une épaisseur limitée à moins de 100 microns. Cependant, des pressions inférieures ou égales à 200 MPa ne permettent pas l'observation des TTS, suggérant l'établissement d'une valeur seuil de pression pour leur formation dans le contact.La deuxième partie de l'étude se consacre à la caractérisation de la microstructure et de la texture cristallographique des TTS. Les TTS étant des matériaux à nanograins, des analyses MET ont permis de décrire précisément leur structure. Les phases en présences sont identifiées grâce aux clichés de diffraction électronique. Des analyses chimiques EDS et EELS sont réalisées. La méthode Astar est utilisée afin d'établir la texture locale des TTS à une résolution nanoscopique. Il en ressort que les TTS sont constituées de deux couches de nanograins alternées. L'une est constituée de grains de phase alpha de 20 à 50 nm ayant une texture cristallographique marquée, l'autre de grains (aussi de phase alpha) faisant quelques nanomètres de diamètre et sans texture évidente. La présence d'azote est également détectée dans cette dernière couche. Un mécanisme de formation des TTS par recristallisation dynamique continue, couplée à une localisation de la déformation plastique en bandes, est avancé afin d'expliquer la microstructure hétérogène observée.La destruction des TTS est un enjeu lui aussi capital. Cependant, l'épaisseur très faible des TTS (< 100 microns) rend la détermination de leurs caractéristiques mécaniques difficile. Des essais de flexion de micro-poutre entaillées, usinées et sollicitées sous MEB-FIB, ont permis d'identifier la ténacité des TTS et de révéler leur fragilité importante. Leur microstructure hétérogène a également un impact vis-à-vis du chemin de fissuration.Dans une dernière partie, une nouvelle approche numérique a été mise en place afin d'estimer les niveaux de déformations plastiques cumulées dans nos contacts. Cette simulation est faite en deux étapes. D'abord l'usure est simulée par un modèle multiphysique prenant en compte des phénomènes d'oxydation des surfaces. Ensuite, un calcul élasto-plastique est réalisé sur la surface usée numériquement afin d'estimer la déformation plastique cumulée lors du fretting. Ces simulations ont permis de confirmer l'apparition des TTS sous forme d'îlots par un processus plastique et démontrent l'intérêt des simulations pour expliquer la formation des TTS
Fretting is a surface degradation phenomenon occurring under low-amplitude alternating sliding. During fretting loading, superficial layers undergo microstructural transformations, appearing white after chemical etching, and become brittle. These layers are named Tribologically Transformed Structures (TTS) and contribute to debris formation. This research aims to understand the genesis of TTS in a plane-plane contact of Ti-Al-4V alloy subjected to fretting loading and to characterize their mechanical behavior.First, TTS formation was evaluated under various contact conditions, particularly by modifying the contact pressure. The kinetics of TTS formation are studied by analyzing the localization and morphology of TTS for different fretting cycle numbers. EDX chemical analyses, optical observations, and SEM of cross-sectional cuts of fretting scars after chemical etching are conducted. The results show that TTS initially appear locally as islands before forming a single, enlarged zone at the center of the contact with a thickness smaller than 100 microns. However, pressures inferior or equal to 200 MPa do not allow TTS to form, suggesting the establishment of a pressure threshold for their appearance in the contact.The second part of the study focuses on the characterization of the microstructure and crystallographic texture of TTS. Since TTS are nanograin materials, TEM analyses are needed to describe their structure. The phases are identified through electron diffraction patterns, while EDS and EELS chemical analyses are executed. The Astar method is employed to establish the local texture of TTS zones at a nanoscopic resolution. It emerges that TTS consist of two alternating layers of nanograins. One layer comprises larger alpha phase grains (20 to 50 nm) with a distinct crystallographic texture, while the other consists of smaller grains (also of the alpha phase) with a few nanometers in diameter and lacking an untextured. The presence of nitrogen is also detected in this layer. A mechanism of TTS formation by continuous dynamic recrystallization coupled with localization of plastic deformation into bands is introduced to explain the observed heterogeneous microstructure.The destruction of TTS is also a critical issue. However, the very low thickness of TTS (<100 microns) renders the determination of their mechanical characteristics challenging. Flexural tests of notched micro-cantilevers, machined and loaded in a SEM-FIB, were used to identify the fracture toughness of TTS and revealed their significant brittleness. Their heterogeneous microstructure also impacts the crack propagation path.Finally, a novel numerical approach has been implemented to estimate cumulative levels of plastic deformation within the contacts. This simulation is conducted in two stages. First, wear is simulated using a multiphysical model that takes into account surface oxidation phenomena. Then, an elasto-plastic calculation is performed on the worn surface to estimate the cumulative plastic deformation during fretting. These simulations confirm the appearance of TTS in the form of islands via a plastic process, highlighting the utility of simulations in explaining TTS formation

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