Impact of beads and drops on a repellent solid surface: a unified description
Srishti Arora, J-M Fromental, Serge Mora, Ty Phou, Laurence Ramos, Christian Ligoure
We investigate freely expanding sheets formed by ultrasoft gel beads, and liquid and viscoelastic drops, produced by the impact of the bead or drop on a silicon wafer covered with a thin layer of liquid nitrogen that suppresses viscous dissipation thanks to an inverse Leidenfrost effect. Our experiments show a unified behavior for the impact dynamics that holds for solids, liquids, and viscoelastic fluids and that we rationalize by properly taking into account elastocapillary effects. In this framework, the classical impact dynamics of solids and liquids, as far as viscous dissipation is negligible, appears as the asymptotic limits of a universal theoretical description. A novel material-dependent characteristic velocity that includes both capillary and bulk elasticity emerges from this unified description of the physics of impact.
Work of Adhesion Measurements of MoS2 Dry Lubricated 440C Stainless Steel Tribological Contacts
Simo Pajovic, Guillaume Colas, Aurélien Saulot, Mathieu Renouf, Tobin Filleter
The tribological behavior of dry lubricants depends on their mechanical and physicochemical environment, making it difficult to predict in practice. Discrete Element Method‐based modeling has been one successful approach to provide valuable insight into the tribology of dry lubricated contacts. However, it requires well‐defined interactions between discrete elements, in particular between those simulating different materials. Measuring the properties governing those interactions, such as the work of adhesion (W), is therefore critical. The present work describes a method for measuring the W between AISI440C steel and MoS2‐based coatings used in spacecraft. Using Atomic Force Microscopy local asperity and adhesion measurements, the W between steel microbeads and MoS2 coatings is determined at different stages in its wear life. The distributions of W values in the worn coatings and pristine coatings agree well with earlier Time‐of‐Flight Secondary Ion Mass Spectroscopy studies on the physicochemistry of the samples, as well as contact angle measurements. Additional measurements between the same materials on a ball bearing from a real life‐test unit of a spacecraft instrument also show a similar W distribution, suggesting that the approach used here provides relevant data for use in numerical simulations.
Space Grease Tribological Behavior for Refourmulation: Numerical and Experimental Investigations
Magali Busquet, Mathieu Renouf, David Leveque, Yves Berthier, Nathalie Bouscharain, Jacques Sicre
An unexpected behavior of space bearings lubricated with new substitute grease at low speeds was investigated. The complex discontinuous tribological behavior of such a biphasic grease was investigated by combining numerical and experimental approaches. From a numerical point of view, using the Discrete Element Method, the grease tribological behavior seems to be controlled by both the thickener particle size and physico-chemical parameters associated to the interactions between the different grease components (PTFE, oil / surface, volume). From an experimental point of view, the influence of the controlling parameters was investigated on a bearing test bench by using different greases composed of two sizes of PTFE particles and two base oils. The considered grease behaviours were discriminated for a given protocol in terms of bearing torque evolution (presence or not of torque peaks) and of surface morphologies (grease cluster formation, grease structural changes,…). Correlations between modelling and experiments are then proposed.
Self-lubricating polymer composites: using numerical trbology to hightlight their design criterion
Maria Villavicencio, Mathieu Renouf, Aurélien Saulot, Y Michel, Yves Mahéo, Guillaume Colas, Tobin Filleter, Yves Berthier
After the cessation of RT/Duroid 5813, manufacturing tests were performed by CNES and ESA/ESTL in order to find an alternative material. Although PGM-HT was selected as the best candidate, limitations about its tribological capabilities to replace RT/Duroid 5813 were later pointed out. Today, the predictability of the tribological behaviour of those materials is not fully overcome. The motivation to this work is to complement studies of self-lubricating materials by coupling experimental analyses with numerical modelling, in order to predict their tribological behaviour. A Discrete Element Method is chosen to construct the numerical material, because it allows to represent wear and the third body generation at the scale of the ball/retainer contact. An underlying role of the adhesion between components in controlling the tribological properties of the transfer film has been observed.
Adhesion measurements in MOS2 dry lubricated contacts to inform predictive tribological numerical models: comparison between laboratory-tested samples and ball bearings from the niriss mechanism
Guillaume Colas, Simo Pajovic, Aurélien Saulot, Mathieu Renouf, Peter Cameron, Alexander Beaton, Andrew Gibson, Tobin Filleter
Predicting the tribological behaviour of dry lubricants remains difficult because it greatly depends on their mechanical and physicochemical environment. While it is difficult to analytically model dry lubrication, Discrete Element Method (DEM)-based modelling has been able to provide valuable insight into the tribological behaviour of dry lubricated contacts. The present study aims to experimentally define interactions between the discrete elements used for simulating different materials in contact, in order to accurately model and predict the tribological behaviour of dry lubricants. Those interactions are here defined by using the work of adhesion (W) between engineering materials: AISI440C, pristine MoS2 coating, as well as the related transfer film. A method was developed and applied on regular laboratory tribological test samples and ball bearings from the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument of the James Webb Space Telescope. Measured W values were consistent between all worn surfaces. The first DEM modelling results exhibit behaviours similar to those observed experimentally including surface plasticization and transfer.
Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling
MD Villavicencio, Mathieu Renouf, Aurélien Saulot, Yann Michel, Yves Mahéo, Guillaume Colas, Tobin Filleter, Yves Berthier
Self-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F).
Precision ball bearings with these composites are manufactured since the 60’s in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation.