Jérôme Cavoret

An Investigation on the Reduced Ability of IF-MoS2 Nanoparticles to Reduce Friction and Wear in the Presence of Dispersants

Abstract Inorganic fullerene-like molybdenum disulfide (IF-MoS2) nanoparticles are known to exhibit great friction and wear-reducing abilities in severe boundary lubrication regimes, when added to a base oil alone. Their use in fully formulated lubricants was investigated in this study, and the tribological benefits attributed to the IF-MoS2 nanoparticles were found to be lost in the presence of dispersants. Various experimental techniques were used on three reference oils (base oil containing only IF-MoS2, only dispersants and both IF-MoS2 and dispersants) in order to understand the effect of succinimide-based dispersants on the three phases needed for effective nanoparticle-based lubrication, namely (1) the passing of the nanoparticles through the contact (2) the exfoliation of the IF-MoS2 inside the contact and (3) the adhesion of the released MoS2 platelets on the friction surfaces. The dispersants were shown to improve the dispersion of the nanoparticles in the oil by reducing their agglomeration, but prevented the adhesion of a low-friction MoS2 tribofilm on the steel surfaces. In-situ contact visualization revealed that the well-dispersed nanoparticles passed through the contact and exfoliated nanoparticles were observed after tribological testing. These results imply that nanoparticle dispersion itself does not seem to be an issue concerning nanoparticle effectiveness, even though the reduced agglomerate size and inertia may have affected nanoparticle flow near the contact, as well as entrapment and exfoliation conditions inside the contact. The use of succinimide-based dispersants may, however, have affected the tribochemistry of the contact, by an excessive adsorption on the steel surfaces and/or by encapsulating the released MoS2 platelets, preventing tribofilm adhesion. A balance was finally found between nanoparticle dispersion and friction reduction, but for very low dispersant concentrations and after a running-in period. The role of succinimide-based dispersants and their effect on nanoparticle lubrication were discussed in the light of these results.

Gear tooth pitting modelling and detection based on transmission error measurements

In this study, an experimental validation of a 3D gear dynamic model in the presence of localised faults such as pitting on tooth flanks is proposed. The corresponding numerical model accounts for spur and helical gear systems including gear errors and deviations along with the supporting shafts and bearings. Simulation results are compared with the evidence from a back-to-back test rig and the model validation relies on loaded transmission error (TE) measurements. Many numerical and experimental results on dynamic behaviours due to the presence of tooth pitting in geared systems are presented. Based on TE measurements, it is demonstrated that the actual vibrations generated by gear tooth pitting validate the gear model and its extension to consider such tooth surface failures.

Particle Entrapment in Hybrid Lubricated Point Contacts

ABSTRACT This study proposes an innovative approach for the study of particle entrapment in rolling element bearings (REBs). Two couples of contacting materials were considered, the classical steel–steel and silicon nitride–steel used in hybrid bearings. Numerical simulations, as well as experiments, combine theoretical trajectories for incoming contaminant particles and effective entrapment ratios observed within a twin-disc machine. Linking both approaches allows the highlighting of some key parameters leading to particle entrapment under pure rolling conditions in elastohydrodynamic point contacts.

An Experimental Study and Model Determination of the Mechanical Stiffness of Paper Folds

Over the past few decades, folding paper has extended beyond the origami deployable applications to reach the engineering field. Nevertheless, mechanical information about paper behavior is still lacking, especially during folding/unfolding. This article proposes an approach to characterize the paper fold behavior in order to extract the material data that will be needed for the simulation of folding and to go a step further the single kinematics of origami mechanisms. The model developed herein from simple experiments for the fold behavior relies on a macroscopic local hinge with a nonlinear torsional spring. Though validated with only straight folds, the model is still applicable in the case of curved folds thanks to the locality principle of the mechanical behavior. The influence of both the folding angle and the fold length is extracted automatically from a set of experimental values exhibiting a deterministic behavior and a variability due to the folding process. The goal is also to propose a methodology that may extend the simple case of the paper crease, or even the case of thin material sheets, and may be adapted to other identification problems.

Particle Entrapment in Rolling Element Bearings: the Effect of Ellipticity, Nature of Materials and Sliding

ABSTRACT This study on particle entrapment within elastohydrodynamic contacts reports experimental tests mimicking rolling element bearing contacts using a twin-disc machine with a contaminated lubricant as well as numerical simulations. The ball bearing contacts might be nonideally circular, composed of a hybrid couple of contact materials, and operated under rolling–sliding conditions. This study focused on the relative importance of each of these configurations to the mechanisms responsible for particle entrapment. It was found experimentally and confirmed numerically that the contact ellipticity is of primary importance to entrapment probability. Tests mixing two-by-two pure rolling and non-zero slide-to-roll ratios (SRRs) and different materials and contact configurations revealed combined mechanisms leading to entrapment and surface damage.

Thermal analysis of twin-disc machine for traction tests and scuffing experiments

Literature agrees that thermal effects have a great influence on friction. However, the heat flows and the thermal behaviour of test rigs used to characterize friction coefficient are largely dependent on their architecture. As a consequence, some discrepancies can be noted between the frictional responses measured by different test benches. Therefore, a thermal model of a twin-disc machine has been developed using the thermal network methodology. This model allows thermal analyses of the traction tests performed with this twin-disc machine. The results reveal that the disc bulk temperature has a significant impact on friction coefficient. Some scuffing experiments are also studied thanks to this numerical model, and a thermal interpretation of classical scuffing criteria is proposed.

Rolling contact fatigue crack propagation in nitrided alloyed steels

Experimental investigations were carried out to better understand the rolling contact fatigue mechanisms in nitrided layers of the 33CrMoV12-9 steel grade. Surface-initiated pitting failure mode was reproduced on a twin-disc machine to analyse crack growth and compressive residual stress behaviour within the nitrided layers. Metallographic examinations, 3D observations by means of high-resolution X-ray computed tomography and residual stress analysis were realised on nitrided 33CrMoV12-9 specimens before and after rolling contact fatigue tests. The study revealed that if the initial compressive residual stresses associated with the surface treatment are released during the process of rolling contact fatigue, pre-existing superficial cracks propagate in the nitrided layers along the intergranular carbides. These precipitates induced by the nitriding process therefore act as preferential crack propagation sites.