Tribological aspects in modern aircraft industry

Preface : The problem of contact materials subject to relative movement is becoming increasingly important for lifetime of the equipment, pollution and cost in energy. One of the aims of tribology is to understand how friction occurs and wear in dynamical contacts. This can vary considerably depending on the choice of the pair of materials and the experimental device. Few milligrams of material lost in a mechanism is sufficient to make it unusable and cause replacement with significant costs. There is still no specific and general formalism to solve the problem of tribology, that is to say, friction, tearing, wear and lubrication of solid and liquid. Friction behavior or wear depends on complex conditions of stress, environment and physicochemical properties of surfaces, themselves dependent on tribological mechanisms. The intrinsic mechanical properties of materials are complementary and play a direct role in the tribological process. The work presented at the ACMA’14 conferences are multidisciplinary, by interacting mechanics, physical chemistry and materials science on a different scale ranging, the goal is to explain, even if the reactions of the contact surfaces are very complex, the basics of tribology, the process of friction and wear, to apply the test methods on the materials in contact and get material coatings on other surfaces by different methods and processes mechanical, physical and chemical. This special issue of the journal "KEM: Key Engineering Materials" consists of a collection of studies both in the fields of experimental simulation of tribology than the science of surfaces in contact by relative movements in relation to the mechanical properties materials

Speed and Atmosphere Influences on Nanotribological Properties of NbSe2

Nanotribological properties of NbSe2 are studied using an atomic friction force microscope. The friction force is measured as a function of normal load and scan speeds ranging from 10 nm s−1 to 40 μm s−1 under two atmospheres (air and argon). At low speed, no effect of atmosphere is noticed and a linear relationship between the friction and normal forces is observed leading to a friction coefficient close to 0.02 for both atmospheres. At high speed, the tip/surface contact obeys the JKR theory and the tribological properties are atmosphere dependent: the shear stress measured in air environment is three times lower than the one measured under argon atmosphere. A special attention is paid to interpret these results through numerical data obtained from a simple athermal model based on Tomlinson approach

Experimental and theoretical investigations of friction properties of graphite intercalated compounds.

It is classically admitted that the goodfriction properties of lamellar compounds are strongly related to their anisotropic structure and especially to the existence of weak interlayer interactions through the van der Waals gap separating the basal layers. As it is also known, the presence of the van der Waals gap in the structure of lamellar compounds will allow lot of chemical species to be intercalated in the structure leading both to the expansion of structure parameters and inter layer interactions modifications. The present work is concerned with the experimental and theoretical study of friction propertiesof Graphite Intercalated Compounds (GICs) in order to better understand thetribologiclamellar compounds. In order to modulate the interlayer interactions, two types of intercalated species were used, electrophylic species (AlCl3, FeCl3, SbCl5) and nucleophilic species (Li, K, Rb)

Evolution des propriétés mécaniques de surface suivie par spectrométrie Raman sur des couples en carbure de silicium et en carbones imprégnés

Les garnitures mécaniques en carbure de silicium (SiC) sont généralement utilisées en milieu lubrifié. Toutefois afin d'accéder à de nouvelles applications technologiques, il peut être nécessaire de les employer en contact sec. Cependant, l'emploi du couple SiC/SiC dans les conditions de frottement sec conduit à une usure sévère des garnitures, voire à leur rupture. Afin de développer cette application une des bagues est remplacée par un matériau en carbone-graphite (CG) dont les propriétés mécaniques et tribologiques sont renforcées par le procédé d'imprégnation qui consiste à introduire dans les porosités du CG un polymère ou un métal ductil à basse température. Dans cette étude, nous avons choisi l'imprégnation soit par une résine phénolique, du PTFE, ou de l'antimoine. Ces ajouts ont pour effet respectif de renforcer la structure de la bague en CG, de diminuer son énergie de surface et de permettre l'accomodation par déformation du film interfacial. Nous proposons d'étudier les mécanismes de dégradations tribologiques qui surviennent à l'interface entre le couple SiC/CG au moyen d'un tribomètre rotatif réalisant un contact conforme anneau/anneau, à température ambiante, avec une vitesse de glissement de 0,5 m/s et une pression de contact de 0,1 MPa. Les faciès d'usure et les tribofilms sont examinés par microscopie électronique à balayage et par spectroscopie de dispersion des rayons X. La spectrométrie Raman vient compléter l'identification chimique des films tribologiques. Cette technique permet la mesure et la cartographie de la taille des cristallites de graphite et la répartition des contraintes résiduelles à la surface des bagues en SiC ou en CG.La description des mécanismes de dégradation des matériaux s'appuient sur le concept du circuit tribologique qui exprime un bilan des débits de matière dans le contact. Dans le cas des couples SiC/CG, les débits sources proviennent principalement des bagues en CG. Les bagues en SIC sont préservées de l'usure. Pour tous les couples de frottement, le troisième corps contient du carbone amorphe et des cristallites de grandes dimensions dont le maintien dans le contact est favorisé par la dynamique du tribomètre. L'atmosphère continue à jouer un rôle dans l'interface car, bien que le contact soit conforme, l'oxydation de l'antimoine est détectée à la surface des bagues qui en sont imprégnées. Les tribofilms les plus stables réduisent le débit d'usure et cette stabilité est fonction de l'imprégnant employé

Assessment of parameters variations for treatment of CO2 laser carburizing over AISI 4340 steel

This study evaluated the best parameters for the carburizing treatment using a CO2 laser of low power over an AISI 4340 steel previously subjected to heat treatments of quenching and tempering. The coating process used a liquid dispersion of carbon black in ethanol and carboxyl methylcellulose. The laser power was kept constant at 125 W and the resolution is 600 dots per inch. The variable parameter of CO2 laser was the speed. Two speeds were selected for the coating preparation: 1200 and 2000 m/s. After treatment, metallographic preparation and optical microscopy observation were performed in addition to the evaluation of parameter variations through hardness measurements and microscratch tests

Thermal behavior of silicon carbide/carbon tribological tests

During friction, the materials in contact undergo a thermal field that can accelerate their deterioration or promote the creation of protective layers. Silicon carbide/carbon dry contact often experiences these phenomena through an oxidation process and a material transfer. In this study, three carbon samples with different impregnation conditions have been considered. Dry friction experiments using a ring-on-ring tribometer were carried out and thermal fields induced were followed by means of Infrared Thermography. Wear volumes and rates were analyzed as well as the third body generated during the tests. Energetic flows were identified and a wear mechanism for the silicon carbide/carbon dry contact is proposed, with respect to the concept of the third body approach

High temperature tribological behaviour of metal matrix composites produced by SPS

Materials used as friction components in transportation field are subjected to extreme working conditions: they rapidly reach their structural limits and critical parts require to be regularly replaced. Alternative solutions withstanding higher operating conditions imply to find innovative materials. Steel matrix composites including various solid lubricants, WS2 and h-BN, able to support high temperatures were developed using a Spark Plasma Sintering technique, which makes possible the formation of new microstructures out of reach by conventional means. Sliding tests were conducted using a pin-on-disc tribometer in air at 450°C, with a normal load of 15 N and various velocities ranged from 0.1 to 1.5 m/s. Influence of solid lubricant content and sensitivity to test parameters were studied in terms of friction and wear responses of the contacting materials. Test results reveal an improvement of friction properties for composites containing highest WS2contents. A reduction of wear is quantified for all composites, and the best behaviour is observed for those that contain WS2. In agreement with the third body approach, interpretations are proposed to describe the interphase dynamics within the contact

Friction properties of fluorinated carbons

In boundary lubrication regime, friction reduction and antiwear processes are associated to the presence of additives in the lubricating oils or greases. These processes are due to the formation of protective tribofilms resulting from chemical reactions between the additives and the sliding surfaces, in the physico-chemical conditions of the sliding contact. Conventional antiwear additives mainly consist of transition metal organo phosphate or thiophosphates which present a remarkable efficiency in the case of contacts between ferrous alloys. In the case of non reacting surfaces, these additives become inactive. Recently developped lubrication strategies consist in the use of dispersion in oils of nano additives able to build the protective tribofilm in the sliding contact without reaction with the surfaces. Carbon fluorinated phases, due to their lamellar structure and their high chemical stability even at relatively high temperature (400°C) represent interesting candidates as lubricant nano-additives subjected to present friction reduction, anti wear and anti corrosion actions. This work presents the tribologic behaviour of some carbon fluorinated derivatives such as graphite fluorides, fluorinated carbon nanofibers, fluorinated carbon nanodiscs and fluorinated carbon blacks. The influence, on the tribologic performances, of the structure of the initial carbon phases, of the fluorination rate (0<F/C<1) and the structure of the fluorinated compounds is discussed

Tribological behavior of a silicon carbide/carbone dry contact

The development of new high-performance mechanical seals working in severe conditions requires higher material performances. Sintered silicon carbide (SSC), widely used as a hard mating material, is a potential candidate but its friction and wear properties need to be investigated in the scope of these new applications. Silicon carbide offers good mechanical properties (high hardness, high Young modulus), good corrosion resistance and good thermal conductivity, that make it suitable for tribological applications in different atmosphere (in air, argon or vacuum) and in dry or lubricated sliding. Combined with a counter-face ring made of a softer carbon-graphite, the dry sliding of SSC can be sustained even under severe conditions of pressure and speed. Graphite has been intensively studied in tribology since Bragg first described its lamellar structure. It has been thought during many years that graphite could act as a solid lubricant thanks to this structure. In fact, the environmental conditions strongly influence its tribological behavior. The hardness of the ceramic facing the carbon seal has also an impact on its friction properties. A transfer layer of carbon is generally found on the ceramic surface. In this study, a first experiment assesses the tribological behavior of SSC sliding against itself and three different carbon-graphite materials. Dry friction and ring-on-ring configuration are considered. A second test uses an infrared camera to estimate the temperature variations of a SiC/C couple during sliding, which determines relationship between displacement resistance and the heat generation

Établissement d’un critère de qualité des taillants diamant/WC-Co pour l’excavation de formations rocheuses

Sous des conditions géologiques extrêmes, les outils de forage composés d’inserts en diamant polycristallins (PDC) sont particulièrement intéressants au vu de leur performance de coupe [1]. La qualité des matériaux innovants utilisés pour la fabrication de ces outils PDC doit être déterminée avec précision par la mesure de leur efficacité de coupe et de leur résistance à l’usure. Un banc d’usure (tour) composé de roches formées par un mortier spécifique a été utilisé pour réaliser les expériences de cette étude. Durant les essais, des inserts PDC ont été usés sur une distance d’abrasion supérieure à 15 km avec un effort normal moyen compris entre 3000 et 5000 N, une vitesse de coupe autour de 1,8 m.s-1 et une profondeur de coupe de 2 mm. Les analyses sont basées sur des modèles établissant des relations couplées entre les efforts de coupe et de frottement reliées aux mécanismes d’excavation réalisés par les outils de coupe [2]. Dans cette étude, les modèles sont implémentés dans le but d’évaluer l’efficacité de coupe et pour estimer la durée de vie de ces inserts diamantés. Les inserts testés ont révélés des taux d’usure [3] compris entre 1·10-8 et 16·10−8 mm3·N-1·m-1 et des efficacités de coupe à 10 km évaluées entre 31 et 55 %. Une approche originale est ensuite développée pour établir analytiquement un critère de qualité globale d’un taillant, tenant compte à la fois de son comportement tribologique (résistance à l’usure) et de ses capacités d’excavation (efficacité de coupe). Une hiérarchie clairement marquée a été révélée sur les différents taillants étudiés. Les variations de composition des matériaux et des méthodes de production distinctes utilisées pour la mise en œuvre des échantillons en sont à l’origine