Nanomechanical properties of boundary lubricating films
Résumé
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.