Application of Photoluminescent Nanoparticles for in situ Temperature and Pressure measurements in Elastohydrodynamic regime
Résumé
Temperature and pressure are two relevant parameters for the optimization of lubrication performance in the elastohydrodynamic lubrication (EHL) regime. To date, various experimental methods have been developed to measure these two parameters with more or less success. In the current work, a new in situ technique is under development. This techniques exploits the photoluminescence (PL) sensitivity of some nanoparticles (NPs) to changes in temperature and pressure, and it aims at achieving simultaneous and local measurements of these two parameters throughout the contact. In this respect, temperature/pressure calibrations have been carried out in order to evaluate the sensitivity of these NPs to the two parameters. Moreover, some simple dynamic tests have been performed in view of elucidating the influence of shear stress on the PL response of the NPs, and inversely the influence of such NPs on the rheological properties of the lubricants. The different findings in these preliminary tests have been finally used to apply this technique for mapping temperature and pressure in an actual EHL contact.The past 60 years have witnessed a growing research effort in developing in situ techniques for the study of elastohydrodynamic lubrication (EHL), which is a frequently encountered regime in highly stressed machine elements such as rolling bearings, gears, cams and followers. These techniques have been of crucial importance for achieving our current understanding of the multi-scale and multi-physics nature of EHL. However, the main challenge in developing such techniques resides in the severity of the physical conditions experienced by the extremely small volume of lubricant, together with accessibility restrictions due to the confinement of the lubricant film in contacts under elastohydrodynamic (EHD) conditions.Different parameters have been measured via the above-mentioned techniques. Among those, temperature and pressure are of particular importance in the optimization of lubrication performance. Indeed, accurate and highly resolved temperature and pressure mappings are needed for achieving faithful prediction of film thickness under the different working conditions. In this context, the current work aims at developing a new in situ technique for probing simultaneously the local temperature and pressure throughout the contact. This technique is based on the sensitivity of some photoluminescent nanoparticles (NPs) to temperature and pressure variations. Measurements are performed by (i) dispersing these NPs in the lubricant within the contact and (ii) monitoring temperature and pressure-induced variations in photoluminescence (PL) wavelength.Two different natures of NPs were studied for this purpose. Their respective temperature and pressure sensitivities were calibrated in the ranges found in actual EHL contacts (i,e. 0 – 1.3 GPa and 20 – 100 °C respectively) using a thermally regulated, high pressure diamond cell. Moreover, the versatility of NPs for sensing applications have been examined by testing two different lubricants, namely squalane and a mixture of squalane and cyclopentane. These different tests revealed the existence of other parameters that affect, in addition to temperature and pressure, the response of the NPs, such as the viscosity of the lubricant and the concentration of the NPs. Thus, a comprehensive study was necessary in order to elucidate the mechanisms behind these findings and, more importantly, to define a procedure minimizing the undesired influence of these additional parameters.Unlike in calibration measurements, the lubricant is not static in an operating machine element. For this reason, simple dynamic conditions have been simulated using an optical rheometer with parallel plate geometry. Tests have been performed in order to (i) study the influence of shear stress on the PL of NPs, and inversely (ii) verify if the presence of NPs modifies the rheological properties of the lubricant.These tests, together with the previous calibration measurements, different findings in the previous calibration tests were finally used to perform temperature and pressure mapping in a realistic EHL regime. The latter was obtained in a ball-on-plate tribometer in which the plate is made of glass and the experimental conditions such as load, velocity, sliding and lubricant bulk temperature can be varied simultaneously.