Lubrication Mechanism of IF-MS2 A Drag Delivery Model

Formation of M0S2 single sheets inside the contact area has already explained friction-reducing properties of several additives the molybdenum dilhiocarbamate (MoDTC) and the molybdenum dithiophosphate (MoDTP) [58]. Grossioid et al. showed the presence of M0S2 sheets in a carbon matrix or a phosphate matrix, these sheets coming from the chemical decomposition of the molecule of additive. Only a few sheets are necessary to obtain low friction. With these two additives, a low coefficient of friction is obtained after only several minutes of test, which corresponds to the activation time of the additive [98]. Moreover, these additives are efficient only if they are thermally activated (70 °C), whereas IFs are efficient at ambient temperature. In the case of IF, low friction is obtained from the first cycles. 

The action mode of IF-M0S2 is based on an exfoliation of their external sheets to release single sheets inside the contact area. These sheets lead to a reduction of the friction coefficient. Some of these sheets are in incommensurate conditions. This explains the very low friction coefficients observed. The results show that the action mechanism of IF-WS2 is identical to 

Schwarz et al. studied the effect of adhesion and pressure on the tribological properties of fullerenes [94,99]. Particles adhere to surfaces by van der Waals forces. These forces are proportional to the radius of the particles and independent of the number of layers. Deformations due to adhesion of the particles on surfaces are very weak. Adhesion supports the exfoliation of the particles, but does not trigger it. The exfoliation of fullerenes is due to the pressure exerted on the particles, which destabilizes the structure, a critical pressure being necessary. Exfoliation occurs only under adhesion conditions and concerns only the first layers (1 or 2 first layers). Layers then adhere on surfaces, which minimizes the edge effects and leads to the formation of films on surfaces [99], 

Drummond et al. studied the properties of films formed on surfaces with IF-WS2 dispersed in tetradecane [45]. After a friction test, they carried out rinsing of surfaces using solvent to eliminate all IFs present. A new friction test was then performed with pure tetradecane and a friction coefficient similar to that obtained with IF in tetradecane was measured. This experiment illustrates the tribological properties of WS2 films formed on surfaces. Prasad and Zaenski studied transfer films formed during friction between a steel ball and WS2 coatings [100], A very low coefficient of friction (0.04) was obtained in an N2 atmosphere, with a smooth transfer film made up of very thin WS2 layers formed on the pin. These thin layers are very flexible and can be curved and thus adhere to surfaces. 

TEM observations of the wear particles collected on the pin after the friction test (P = 1.12 GPa, V = 2.5 mm/s, T = 20°C) show the presence of many WS2 single sheets. The diffrac-togram of Eigure 2.99 clearly shows 18 diffraction points, which can be divided into three 

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