Current research on self-expanding super-elastic nitinol stents is mainly focused on designing geometries suited to the human venous and arterial systems. This study specifically considers a patented one-piece double cross-sectional stent designed to address venous stenosis affecting the vena cava, iliac veins, and their bifurcations. Before its placement within the vein, numerous tribological challenges arise as the stent slides along the guide-wire (so-called catheter). These are mainly connected to the lack of knowledge related to (i) its frictional behaviour and (ii) the level of contact pressure linked to the unknown real contact area between the compressed stent and the polytetrafluoroethylene (PTFE) catheter. This paper describes an original multi-scale approach, based on the Persson's contact theory, that allows determination of (i) the contact pressure, (ii) the evolution of the real contact area, and (iii) the shear stresses at the interface during the stent sliding. A topographical optimization criterion will finally be provided, enabling control of both friction and stick-slip phenomena at the stent-catheter interface.

For a decade, Triboelectric Nanogenerators (TENG) have been massively developed and optimized either as (i) triboelectric energy harvesters or, (ii) triboelectric self-powered sensors. Unfortunately, it is obvious that the classical tribologists' concerns - like the optimization of both real contact area, friction laws and wear processes - have never really been integrated so far neither in their development processes nor in their triboelectrical responses. Thus, a question as basic as "What kind of tribological information can we expect from multi-asperity triboelectric sensor rubbing on a plane in dry friction?'' does currently not lead to a trivial answer. This paper tries to get out this paradoxical situation by simulating triboelectrical behavior of a multi-asperity real contact area in dry friction. Results reveal that a sliding triboelectric sensor assesses the instantaneous and averaged velocities of every interacting asperities within the contact area, finally probing, in real time, the tribological actual contact area behavior during sliding.

One aspect of current research on self-expanding super-elastic nitinol stents is mainly focused on the development of a new design, which itself depends on the human venous and arterial systems to be treated.In this context, the present study focuses on a one-piece double cross-sectional stent used in the treatment of venous stenosis, affecting the vena cava, iliac veins and their bifurcations.During stent placement, many tribological problems occur when the stent slides in the guide-wire (catheter), mainly due to the lack of knowledge about their frictional behaviour.Among this lack of information, the level of contact pressure -related to the unknown real contact area between the compressed stent and the polytetrafluoroethylene (PTFE) catheter -is largely misunderstood.The aim of this work is to describe a method allowing to determine both the contact pressure and the evolution of the real contact area during sliding movement using a multi-scale approach involving (i) X-ray tomography at the macroscopic scale for determining the static contact pressure, and (ii) a 3D optical measurement instrument at the microscopic scale coupled with the Persson's theory to calculate the evolution of the real contact areas.The latter takes into account both (i) the elastic properties of each material, and (ii) the variations of the topographic parameters with contact pressures, and (iii) the sliding parameters (ie, velocities and sliding distance).These results lead to a better prediction of the true frictional behaviour between the nitinol stent and the PTFE catheter.In addition, the methodology allows to simplify the complex numerical simulation of a real stent sliding within a catheter by an equivalent model using a much more simplified geometry including the frictional behaviour of the complex stent.

Friction is an important limitation of energy efficiency performances of MEMS/NEMS but is, in the same time, a great opportunity for harvesting energy by designing optimized tribo-electric nano-Generators (TENG). Thus, frictional behaviour can be accurately controlled in real time by using thermally sensitive periodic patterned self-assembled monolayers of n-octadecyltrichlorosilane (OTS) grafted on MEMS surfaces. Nanopatterns are currently used in order to limit the wear rate without modifying the frictional behaviour. In this work, patterns have been created by micro-contact printing ($$\upmu \hbox {CP}$$) using a polydimethylsiloxane (PDMS) stamp displaying a trapezoidal profile. Hence, pattern periodicity can be continuously changed—and then optimized from discontinuous to pseudo-continuous—by applying a controlled normal load on the soft PDMS stamp. A multiscale tribological study has been carried out on these nanopatterns by using both single-asperity and multi-asperity nanotribometers. Lateral force microscopy (LFM) provides the individual frictional behaviour of each pattern’s component, whereas the multi-asperity nanotribometer rather gives the emerging frictional behaviour induced by the patterning according to temperature. As a macroscopic crucial parameter while designing TENG’s devices, this macroscopic behaviour has to be carefully optimized for each practical applications at the molecular scale. Thus, the microscale frictional behaviour can be precisely optimized by the pattern’s periodicity, whereas the macroscopic one can be accurately controlled with values of friction coefficient ranging from 0.12 to 0.04 by varying the contact temperature. In addition, any inertial effects observed in the thermal-controlled frictional behaviour of nanopatterns can be drastically reduced using infra-red emission as thermal source.

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO3 (97% wt% - 40 nm in size) and two organic matrices: (i) the interlamellar mainly composed of beta-chitin and proteins, and (ii) the intracrystalline composed by silk-fibroin-like proteins. This material is currently contemplated for the manufacture of small prostheses (e.g., rachis and dorsal vertebra prostheses) which are subjected to micro-slip or fretting motion. In this work, the tribological behavior of nacre is studied by varying the frictional dissipated power from few nW to several hundred mW, in order to assess the various responses of the different nacre's components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet's elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale and anisotropy of its structure - and especially by its both matrices and respective orientation vs. the sliding direction.

The design and control of materials at the nanoscale are the foundation of many new strategies for energy generation, storage and efficiency. Besides, friction is an important limitation of energy efficiency performances in MEMS/NEMS [1]. Multi-asperity nanotribology studies are needed to develop a fundamental understanding of interfacial phenomena where frictional behaviour is controlled by interactions between nano-asperities [2]. Controlling these interactions is clearly the first step for designing triboactive surfaces – ie, surfaces whose frictional behavior can be controlled in real time by means of external stimuli [2]. A promising way consists to apply a suitable stimulus – eg. IR [2] or UV beam

Friction is known to be a multi-scale phenomenon which starts at the atomic or molecular scale, and emerges at the macro-scale by means of the so-called real contact area. This multi-scale process generally involves some self-organization processes within this real contact area, which generally occurs with the reduction of the number of degrees of freedom. Hence, macro-scale properties of physical systems – such as the coefficient of friction, the Young modulus, the yield strength, or the fracture toughness – cannot be deduced directly from the molecular scale properties because these properties are defined by meso-scale objects such as defects, grains, and asperities – especially, roughness for tribological purpose. Thus, (i) tribological properties – including friction and wear processes – need to be studied by considering a multi-scale approach, in order to understand how these phenomena emerge from atomic to macroscopic scale; (ii) specific emerging tribological behaviors can be tailored or adjusted by means of self-assembled monolayers or hierarchical surfaces or materials. In this paper, this approach will be illustrated by considering various tribological studies, which have been investigated at various scales in our respective teams.

Nanoforce sensors using passive magnetic springs associated to a macroscopic seismic mass are known to be a possible alternative to force sensors based on elastic microstructures like Atomic Force Microscopes if the nanoforces that have to be measured are characterized by a bandwidth limited to a few Hertz. The estimation of the unknown force applied to the seismic mass is based on the deconvolution of the noisy measurement of the mass displacement which has an under-damped dynamic. Despite their high performances in terms of linearity, resolution and measurement range, such force sensors are extremely sensitive to low frequency environmental mechanical disturbances, like the angular variations of the anti-vibration table supporting the device or the residual seismic vibrations that are not filtered by the table. They are also sensitive to the temperature evolution of the ambient air. The evaluation, modeling and compensation of such environmental disturbances have to be specifically studied in the context of magnetic springs associated to a macroscopic seismic mass because of their important negative effects in terms of low frequency drifts and oscillatory disturbances. This article presents an estimation and a passive compensation strategy of the low frequency and non-stationary mechanical disturbances that is based on a differential principle. This approach is applied to a nanoforce sensor based on diamagnetic levitation developed in the last decade. It does not necessitate to add new types of sensors in the measurement chain such as very high resolution and low frequency inclinometers or accelerometers in order to estimate the mechanical disturbances. In term of performances, the force estimation error remains in the nanonewton level over periods of time of several minutes when external temperature remains constant.

Nanoforce sensors using passive magnetic springs associated to a macroscopic seismic mass are known to be a possible alternative to force sensors based on elastic microstructures like Atomic Force Microscopes if the nanoforces that have to be measured are characterized by a bandwidth limited to a few Hertz. The estimation of the unknown force applied to the seismic mass is based on the deconvolution of the noisy measurement of the mass displacement which has an under- damped dynamic. Despite their high performances in terms of linearity, resolution and measurement range, such force sensors are extremely sensitive to low frequency environmental mechanical disturbances. They are also sensitive to the temperature evolution of the ambient air. The evaluation, modeling and compensation of such environmental disturbances have to be specifically studied in the context of magnetic springs associated to a macroscopic seismic mass. This article presents an estimation and a passive compensation strategy of the low frequency and non- stationary mechanical disturbances that is based on a differential principle. This approach is applied to a nanoforce sensor based on diamagnetic levitation developed in the last decade and gives an uncertainty below the nanonewton level.

Wear analysis at the micro/nanoscale appears as a great challenge for MEMS/NEMS devices. At this scale classical post mortem analysis – like profilometry or AFM assessment – often failed because (i) the error in wear assessment owing to the elastic recovery is no longer negligible at this scale and (ii) the presence of nanometric tribolayer within the contact cannot be take into account when the contact is opened afterward. So, this paper deals with an in situ wear assessment based on a triboscopic approach where the final position zf of the ball is known without opening the contact because its vertical position is assessed at every instant of the process. This triboscopic assessment considers the initial approach of the surfaces z0 and gives the wear rate by taking into account the presence of any tribolayer within the contact. It requires some corrections as (i) the tilt of the sample and (ii) the initial displacement of the surfaces, which is a function of the mechanical properties of the samples. The latter are determined by using an inverse method combining spherical nanoindentation and boundary element numerical simulations, which are both described too. Validation and application of this in situ approach to the running-in wear assessment of thin soft and hard coatings currently used in MEMS manufacturing are finally presented.

This study presents a new multi-axis friction sensor. This chip is designed to measure normal forces up to 1 mN and friction forces up to 100 μN and will be used to study multi-asperity nanotribology. First prototypes have been manufactured on a p-type Silicon on Insulator wafer (SOI) and characterized.

At the nanoscale and for particular applications such as dexterous micro-manipulation, two degrees of freedom nanotribometers are no longer adequate for studying and characterising the contacts. This paper deals with the specifications and working principle of a new multi-axis friction sensor designed for nanotribological testing applied to this purpose in order to extract each contribution independently (i.e., sliding, rolling and spin motion). It is composed of a central platform with a fixed ball and surrounded by a compliant table. Its sensing ability is based on piezoresistivity: four sets of piezoresistors are symmetrically distributed at the root of four central beams. Finite elements method simulations are performed to find the optimal dimensions of the sensor. As results, this sensor could measure independently normal and friction forces in the range of 1 mN and 100 µN, respectively and the three rotation components. Estimated crosstalk is lower than 1% with a good sensitivity.

Running-in wear of MEMS can be described as a process which involves mechanical, chemical and physico-chemical phenomena at various scale levels. Extracting each sort of components would enable to better understand wear mechanisms in order to prevent it. Unfortunately, these components are generally hard to extract experimentally because their own time responses are generally not in the same order. A suitable approach is to combine multi-asperity nanotribological tests, using an in situ wear assessment, with numerical simulations using Movable Cellular Automata (MCA), which are able to interact together within the contact. Experimental tests enable to control the actual physico-chemical environment with an environmental enclosure, and MCA simulate the multi-asperity contact, where interactions between automata pairs can be controlled by various fracture and bonds criteria. There is generally an optimal set of interaction criteria which provides numerical results that match correctly with the experimental ones. By studying the influence of experimental environment on this optimal set, assumptions can be made about mechanical, chemical and physico-chemical phenomena that are likely to occur within the actual tribocontact. In this work, these assumptions have been studied for various samples like silicon wafers displaying various crystallographic orientations and nanostructures, and self-assembled monolayers grafted on silicon wafers, and carbon nitride coatings rubbing under different environmental conditions.

Micro/nanotribological study of self-assembled monolayers (SAMs) derived from n-alkyltrichlorosilanes deposited on silicon wafers displaying various crystallographic orientations—Si (100), Si (111) and Si (110)—has been conducted using a ball-on-disc nanotribometer. The parameters that have been varied are (i) the alkyl chain length, (ii) the tribological parameters (i.e. the normal load, the sliding velocity, the sliding distance and the relative humidity level) and (iii) some surface characteristics of the silicon substrates (i.e. crystallographic orientation, roughness, and thickness of the amorphous native oxide). Experimental results show that the key parameter controlling the tribological behaviour is not the alkyl chain length as generally reported for adsorbed and grafted monolayers, but rather the film’s homogeneity—i.e. the degree of packing and the surface coverage of the monolayer—in connection with the crystallographic orientation of the substrate.

Chromium thin films were sputter deposited implementing the GLancing Angle Deposition (GLAD) method, which is a thin film deposition technique where the incident vapor flux – composed of atoms and molecules from gas phase – strikes onto the substrate at tilted angles α. Oriented chromium columns were produced with various column angles β (from 0 to 60°) closely linked to the sputtering pressure and incidence angle α. Three sputtering pressures of 0.11, 0.40 and 0.53 Pa were used. Incidence angle α of the sputtered particles was systematically changed from 0 to 80°. Tribological properties were investigated as a function of these operating parameters. Results reveal that the tribological behaviour is strongly correlated with the structure and especially the growth mechanism of the films, which are both linked with the operating sputtering parameters. Thus, at the lowest sputtering pressure (0.11 Pa), gradual variations of the tribological properties and wettability are observed as a function of the incidence angle α, which are interesting for tailoring surfaces displaying a gradient of wettability. In contrast, at higher sputtering pressures (>0.2Pa), local variations of static friction coefficient, wettability and lateral contact stiffness are systematically observed as a function of the column angle β—and then the incidence angle α. Basically, these results enable to tailor tribological properties by tuning the incidence angle α in order to control the transition from sticking to sliding in micro-gripping.

Wear can be described as a process which involves mechanical and chemical phenomena at various scale levels [1]. Extracting each sort of components should enable to better understand wear mechanisms from the macroscale down to the nanoscale and the atomic level. However, it is well known that these components are generally hard to extract independently because their own time responses are generally not in the same order [1, 2]. Mixing chemical reactions and mechanical deformations in a same model is quite difficult to carry out, whereas they work together rather well in the real world. A suitable approach is to combine experimental multi-asperity nanotribological tests and numerical simulations using discrete elements, which are able to interact together within the contact. Experimental tests enable to control the actual chemical environment [1], and numerical simulations give the opportunity to access to the hidden parameters controlling agglomeration and fracture mechanisms of particles which are trapped within the contact [2, 3].

Sheet nacre (so-called mother–of–pearl) is a hybrid biocomposite with a multiscale structure including nanograins of calcium carbonate (97wt%–40nm in size) and two organic matrices: (i) the “intracrystalline” (mainly composed by silk-fibroin-like proteins), and (ii) the “interlaminar” one (mainly composed of β-chitin and proteins). Micro/nanotribological behaviour was investigated on sheet nacre displaying various configurations (so-called face-on and edge-on), by varying the orientation of the matrices vs. the sliding direction. Different levels of frictional dissipated energy were observed as a function of both: (i) the type of matrix which is involved in the dissipation mechanism (intracrystalline or interlaminar), and (ii) the orientation of the matrices themselves vs. the sliding direction. These various dissipative ways can involve either, multiscale wear processes entailing the both matrices, or irreversible deformation only, without any wear process. They have been identified and explained by considering the double composite structure of sheet nacre.

Pearls are produced from a natural biomineralisation process. The structural unit is a biocrystal whose formation is controlled by organic molecules. In the present paper we studied a major growth defect to highlight the role of the organics in the growth mechanism. Some freshwater pearls exhibit a lack of lustre, also known as ‘milky pearl’ defect. This defect is related to the formation of vaterite instead of aragonite during the biomineralisation growth process. We used Rock Eval pyrolysis, a new technique in this field, to quantify a noticeable increase of the organic matrix in milky pearls. At least 20% more organics were found when vaterite forms than aragonite. To further study its role during the growth process, the organic matrix was extracted using three different protocols and characterised by infrared spectroscopy (FTIR) and liquid chromatography (HPLC). The fraction of the organic matrix which is soluble in water (WSM fraction) was significantly different when analysed by FTIR and by HPLC. This fraction is very likely occluded within the mineral (intracrystalline matrix). To examine its role, WSM extracted from milky pearls was used as additive in controlled calcium carbonate growth experiments. In this case, vaterite crystals were obtained for the greater part. When WSM extracted from healthy pearls was used, aragonite and calcite crystals were grown. The polymorph (vaterite vs aragonite) appeared to be clearly connected to the WSM organic fraction. Several hypotheses may explain this result: e.g., a minor disorder of the organic composition of the extrapallial fluid, an excess of Mn or Mg or a blocking of the growth at a transient stage. Finally, we propose to use cathodoluminescence as a non‐destructive technique to sort these defective pearls.

The micro/nanotribological behaviors of monocrystalline silicon wafers – displaying various orientations and roughnesses – rubbing on metals and ceramics balls are investigated under multi-asperity low contact pressures. Whatever the ball's materials, results reveal a crystallography-induced anisotropy in friction due to crystallographic orientation of the wafer. This anisotropy leads to a time-dependent seizure mechanism – connected to the surface energy of the wafer itself linked to its crystal orientation. The wafer roughness mainly influences the third body generation rate and finally controls the seizure mechanism. When carbon nitride coatings are deposited on wafers, anisotropy does completely vanish: friction and wear mechanisms are instead controlled by the surrounding environment.