A new capacitive sensor for displacement measurement in a surface force apparatus

We present a new capacitive sensor for displacement measurement in a Surface Forces Apparatus (SFA) which allows dynamical measurements in the range of 0-100 Hz. This sensor measures the relative displacement between two macroscopic opaque surfaces over periods of time ranging from milliseconds to in principle an indefinite period, at a very low price and down to atomic resolution. It consists of a plane capacitor, a high frequency oscillator, and a high sensitivity frequency to voltage conversion. We use this sensor to study the nanorheological properties of dodecane confined between glass surfaces.Comment: 7 pages, 8 figure

Adhesion forces due to nano-triboelectrification between similar materials

Contact electrification and triboelectrification are well-known in the case of dissimilar materials, however the case of charge exchange during friction between nominally identical insulating materials is less documented. We experimentally investigated the triboelectrification between two smooth monocrystalline α-Al 2O 3 (sapphire) antagonists by surface force measurements with a Surface Force Apparatus (SFA). The force between a sphere and a plane, both in sapphire, was measured as a function of the sphere-plane distance D, before and after nano-friction tests, under dry argon atmosphere. Respective contributions of van der Waals, water meniscus and electrostatic forces were determined. The estimated Hamaker constant was in good agreement with the Lifshitz theory, and the dominant meniscus attraction at low separation could be overcome with small radius sphere. We demonstrated that electrostatic forces were generated by the nano-friction test and we quantified the adhesion that results from this contact-electrification. In the first stage of the unloading process, the short range electrostatic force was found to vary both with time and distance D. Experimental results were correlated with surface densities of mobile charges on the two surfaces, and the time-dependence was related to classical surface transport phenomena on alumina surfaces

Development of a new mechano-chemical model in boundary lubrication

A newly developed tribochemical model based on thermodynamics of interfaces and kinetics of tribochemical reactions is implemented in a contact mechanics simulation and the results are validated against experimental results. The model considers both mechanical and thermal activation of tribochemical reactions instead of former thermal activation theories. The model considers tribofilm removal and is able to capture the tribofilm behaviour during the experiment. The aim of this work is to implement tribochemistry into deterministic modelling of boundary lubrication and study the effect of tribofilms in reducing friction or wear. A new contact mechanics model considering normal and tangential forces in boundary lubrication is developed for two real rough steel surfaces. The model is developed for real tribological systems and is flexible to different laboratory experiments. Tribochemistry (e.g. tribofilm formation and removal) and also mechanical properties are considered in this model. The amount of wear is calculated using a modified Archard’s wear equation accounting for local tribofilm thickness and its mechanical properties. This model can be used for monitoring the tribofilm growth on rough surfaces and also the real time surface roughness as well as changes in the λ ratio. This model enables the observation of in-situ tribofilm thickness and surface coverage and helps in better understanding the real mechanisms of wear

Nanoindentation, nano-scratch and nano-friction tests with a surface force apparatus

7 - 9 novembre 200

Nano-machining with a Surface Force Apparatus

April 13-17 199

Nanoindentation and nanofriction on DLC films

International audienceDiamond-like carbon (DLC) coatings have been studied for many years as wear-resistant and low friction materials. Their tribological behaviour depends on the nature of the coating, which is determined by the deposition process. It is also strongly affected by environmental conditions. In this study, the nanomechanical properties and the nanofriction behaviour of two a-C:H coatings deposited on silicon substrate and exhibiting different macroscopic friction behaviours were investigated using a three-axial surface force apparatus equipped with a diamond tip. The hardness, Young's modulus and Poisson's ratio of the two coatings were determined. Viscous recovery of the indents was observed a few days after the indentation tests. Low friction coefficients (around 0.06) were measured for the coating which exhibited macroscopic low friction. For the coating which exhibits a higher friction coefficient, the dissipative behaviour measured during nanofriction test was enhanced by sliding, compared to the dissipative behaviour measured during normal indentation, suggesting a link between normal viscous dissipation at nanoscale and high friction

Nanomechanical properties of boundary lubricating films

First the nanomechanical properties of ZnDTP antiwear tribofilms were measured by nanoindentation at ambient temperature. As the tribofilms are very thin, the elastic properties of the substrate influence the measurements. A simple model was used to extract the elastic modulus of the ZnDTP tribofilm from the composite measured modulus. During the loading stage of the nanoindentation test, it was observed that the mechanical properties of the tribofilm significantly increase. Additional measurements performed after local nano-machining of the tribofilm have shown that this phenomenon does not correspond to a gradient of properties throughout the film but to an accommodation process in response to the applied pressure, which permits to describe the ZnDTP tribofilms as "smart" materials. Because of the patchy structure of the tribofilms, it was tried to go further by having an estimation of the properties of the tribofilms from a very high number of tests. A 10 µm x 10 µm cartography of the stiffness from 1600 points of measurement was realized. It was clearly visible that the lowest stiffness values were obtained in the valleys and the highest were obtained on the pads. This work was extended to the measurement of the nanomechanical properties of the tribofilm at temperatures up to 80°C (near service conditions) using a Nanoindenter XP® entirely set into a climatic chamber. The effect of applied pressure on the elastic properties was also observed at 80°C. The results show that the elastic modulus of the tribofilm remains constant in the studied temperature range. The use of the F/S2 parameter, independent of contact geometry, permitted to point out a significant decrease in hardness while the temperature increases. Furthermore, AFM observations of the residual pile-up pointed out an evolution of the film deformation process with temperature. Pile-up seems to have been formed by successive pronounced steps at ambient temperature whereas at 80°C, it seems to result from a more continuous flow of matter. This point was investigated by extending the temperature range to lower values and the viscous-plastic properties of the tribofilm were studied by varying the deformation rate during the nanoindentation tests