Online lubricating oil condition monitoring by laser-induced breakdown spectroscopy with a micrometer-diameter liquid filament

The metallic elements presented in lubricants are not only indications of oil life, but also reveal the healthiness of mechanical system; thus, the online monitoring of lubricant composition can be of great value. In this work, we demonstrated the feasibility of online lubricant condition monitoring using laser-induced breakdown spectroscopy (LIBS). The lubricant was sampled by a piezoelectric jet valve, generating micrometer-diameter liquid filaments. Using magnesium as a representative element, a set of reference lubricating oils were prepared with Mg concentration ranged from 30 to 500 μg/g. By adjusting the operating conditions of piezoelectric jet valve, the diameter of lubricating oil filaments was regulated between 21.3 μm and 60.4 μm. It was shown that the absolute intensity and signal-to-noise ratio (SNR) of LIBS signals decreased monotonically with the increasing filament diameters. Under the optimized filament diameter of 21.3 μm, the intensity of C I line at 245.86 nm was used as an internal reference to normalize the Mg II line at 279.55 nm. A linear calibration curve was established between the concentration of Mg and the intensity of Mg emission line, which yielded a coefficient of determination 0.969. With the actual Mg concentrations of 100 μg/g and 300 μg/g, the relative measurement errors were 16.5% and 11.8%, respectively, while the relative standard deviations were 9.9% and 11.1%. The limit of detection for Mg element was found to be 10.9 μg/g, which meets the requirement of lubricating oil condition monitoring.