INTRODUCTION: The complexity of understanding the behavior of concentrated lubricated contact arises mainly from the coupling between the local stresses induced by the contacting solids and the rheological behavior of the lubricant at the interface. Nevertheless, most of the physical phenomena that occur are highly dependent on the distributions of pressure and temperature. However, up to now, experimental techniques providing pressure and/or temperature measurements in lubricated contact, such as electrical resistance, infrared emissivity and Raman spectroscopy are limited by intrusiveness, lower spatial resolution and the intrinsic weakness of the signal respectively [1].
Our study describes an innovative in situ technique which allows a local measurement of temperature and pressure throughout lubricated contacts with a spatial resolution of about 10 µm. This technique exploits the photoluminescence sensitivity of semiconductor-based nanoparticles (NPs) to changes in pressure and temperature [2, 3]. The selection of these NPs was done according to several criteria:
- Their nanometric size, typically in the range 4-8 nm, therefore one or two orders of magnitude smaller than the film thickness of a lubricated point contact.
- Their dispersibility in lubricants.
- Their resistivity to the contact severe conditions.
- Their ultrafast response timescale, faster than the lubricant transit time (nanosecond vs. millisecond).
METHODS: In this work, the NPs were dispersed in small concentration in squalane. Among lubricated contacts, the elastohydrodynamic (EHD) contact appeared to be an ideal candidate for our study due to its stationary character in time and in space. However, prior to in situ experiments, it was necessary to calibrate the NPs response in the squalane suspension as a function pressure and temperature by using a diamond anvil cell (DAC). Rheological tests were performed as well, this time under atmospheric pressure, in order to check out whether the presence of NPs modifies the lubricant rheological properties or not. These experiments can provide confirmation for using NPs as reliable nanosensors in realistic/representative EHD conditions.
Then, measurements were carried out in an EHD contact produced by loading a steel ball against a transparent disc, so that the film is viewed using a photoluminescence microscope focusing through the disc. Only the first results under pure rolling condition will be shown in this presentation. These first in situ experimental measurements were compared with values predicted using an in–house finite element EHD model inspired by the works of Habchi et al. [4].
RESULTS: Preliminary results, displayed, in Figure 1A, show that the NPs don’t modify the rheological behavior of the carrier fluid at ambient pressure and different temperatures and, by extension, in a thin film as those found in lubricated interfaces. Figure 1B illustrates the calibration curves obtained from experiments carried out with the DAC by fitting the experimental