Guillaume Colas

Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling

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 MoS 2). 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 60s 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. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM).

Surface and Mechanical Characterization of Dental Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP) After Different Aging Processes.

Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) is a ceramic material used in indirect dental restorations. However, phase transformation at body temperature may compromise the materials mechanical properties, affecting the clinical performance of the restoration. The effect of mastication on 3Y-TZP aging has not been investigated. 3Y-TZP specimens (IPS E-max ZirCAD and Z5) were aged in three different modes (n=13): no aging (control), hydrothermal aging (HA), or chewing simulation (CS). Mechanical properties and surface topography were analyzed. Analysis of variance showed that neither aging protocol (p=0.692) nor material (p=0.283) or the interaction between them (p=0.216) had a significant effect on flexural strength, values ranged from 928.8 MPa (IPSHA) to 1,080.6 MPa (Z5HA). Nanoindentation analysis showed that material, aging protocol, and the interaction between them had a significant effect (p<0.001) on surface hardness and reduced Youngs modulus. The compositional analysis revealed similar yttrium content for all the experimental conditions (aging: p=0.997; material: p=0.248; interaction material×aging: p=0.720). Atomic force microscopy showed an effect of aging protocols on phase transformation, with samples submitted to CS exhibiting features compatible with maximized phase transformation, such as increased volume of the material microstructure at the surface leading to an increase in surface roughness.

Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide

In this work, the effect of humidity and water intercalation on the friction and wear behavior of few-layers of graphene and graphene oxide (GO) was studied using friction force microscopy. Thickness measurements demonstrated significant water intercalation within GO affecting its surface topography (roughness and protrusions), whereas negligible water intercalation of graphene was observed. It was found that water intercalation in GO contributed to wearing of layers at a relative humidity as low as ∼30%. The influence of surface wettability and water adsorption was also studied by comparing the sliding behavior of SiO2/GO, SiO2/Graphene, and SiO2/SiO2 interfaces. Friction for the SiO2/GO interface increased with relative humidity due to water intercalation and condensation of water. In contrast, it was observed that adsorption of water molecules lubricated the SiO2/SiO2 interface due to easy shearing of water on the hydrophobic surface, particularly once the adsorbed water layers had transitioned from ice-like water to liquid-like water structures. Lastly, an opposite friction trend was observed for the graphene/SiO2 interface with water molecules failing to lubricate the interface as compared to the dry graphene/SiO2 contact.

An Insight into the Phase Transformation of WS2 upon Fluorination

The transformation from semiconducting to metallic phase, accompanied by a structural transition in 2D transition metal dichalcogenides has attracted the attention of the researchers worldwide. The unconventional structural transformation of fluorinated WS2 (FWS2 ) into the 1T phase is described. The energy difference between the two phases debugs this transition, as fluorination enhances the stability of 1T FWS2 and makes it energetically favorable at higher F concentration. Investigation of the electronic and optical nature of FWS2 is supplemented by possible band structures and bandgap calculations. Magnetic centers in the 1T phase appear in FWS2 possibly due to the introduction of defect sites. A direct consequence of the phase transition and associated increase in interlayer spacing is a change in friction behavior. Friction force microscopy is used to determine this effect of functionalization accompanied phase transformation.

Tribochemical Competition within a MoS2/Ti Dry Lubricated Macroscale Contact in Ultrahigh Vacuum: A Time-of-Flight Secondary Ion Mass Spectrometry Investigation

Controlling and predicting the tribological behavior of dry lubricants is a necessity to ensure low friction, long life, and low particle generation. Understanding the tribochemistry of the materials as a function of the environment is of primary interest as synergistic effects exist between the mechanics, the physicochemistry, and the thermodynamics within a contact. However, in most studies the role of the coating internal contaminants in the process is often discarded to the benefit of a more common approach in which the performances of the materials are compared as a function of different atmospheric pressure environments. The study focuses on the understanding of the tribochemical processes occurring between the materials and their internal contaminants inside an AISI440C contact lubricated by a MoS2/Ti coating. Time-of-flight secondary ion mass spectrometry is used to study at the molecular level, the material before and after friction. Friction tests with different durations are performed in ultrahigh vacuum at the macroscale to stay relevant to the real application (space). The adsorption/desorption of gaseous species during friction is monitored by mass spectrometry to ensure reliable study of the tribochemical processes inside the contact. The study shows that a competition exists between the Ti- and MoS2-based materials to create the appropriate lubricating materials via (i) recrystallization of MoS2 materials with creation of a MoS xO y material via reactions with internal contaminants (presumably H2O), (ii) reaction of Ti-based materials with internal contaminants (mostly H2O and N2). The biphasic material created is highly similar to the one created in both humid air and dry N2 environments and providing low friction and low particle generation. However, the process is incomplete. The study thus brings insight into the possibility of controlling friction via a rational inclusion of reactants in a form of contaminants to control the tribochemical processes governing the low friction and long life.

Nanoscale Mechanical Characterization of 1D and 2D Materials with Application to Nanocomposites

In this chapter the critical recent advances in the area of nanoscale mechanical characterization of 1D and 2D nanostructures with application to nanocomposites are presented. CNTs and graphene have been the most widely studied within this class of materials; however, a number of additional 1D and 2D nanostructures such as molybdenum disulfide (MoS2) ultrathin films have also recently emerged and will be discussed. The chapter will cover a variety of nanomechanical characterization techniques that have been developed recently, with a particular focus on methods which characterize the nature of interactions (shear strength, interfacial friction, etc.) between the nanocomposite constituents which can play a significant role in governing macroscopic nanocomposite behavior.