The lubrication and automotive industries are seeking on-line sensors capable of determining the chemical condition and degree of degradation of industrial lubricants. Further development in understanding fundamental electrochemical characteristics of lubricants and their contaminants will allow for improvements in monitoring the performance of existent and the formulation of new advanced lubricants. Contamination by a few hundred ppm of water can cause detrimental changes in the lubricating properties of engine oil. Levels of acidity and basicity are parameters common to a wide variety of industrial fluids and are closely related to rate of lubricant oxidative breakdown. Related to the rate of lubricant oxidative breakdown is the formation and build-up of soot, which also diminishes lubricant quality.
Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronopotentiometry (CP) were employed as a basis for on-line electrochemical sensors for composition analysis of fully formulated industrial lubricants. These electrochemical techniques appeared as powerful yet inexpensive tools specifically capable of on-line detection and monitoring of major contaminants of typical industrial lubricants as well as oxidative degradation. Microelectromechanical systems (MEMS) technology was employed for the design and fabrication of these electrochemical sensors.
The application of these techniques for the analysis of the electrochemical properties of industrial lubricants focused on establishing relationships between lubricant chemical composition and experimental data. Presented data demonstrated a need to rely on several output parameters collected over wide frequency, electrochemical potential, temperature, and electrode geometry ranges in order to assess complex processes and decomposition pathways occurring both at the electrode interface and in the bulk solution. An equivalent circuit model was developed to describe the electrochemical structure of a typical industrial lubricant. The model was used for the analysis of electrochemical results as a reflection of chemical composition and changes in the system.
Following this, a series of MEMS-based interdigitated sensors were developed and tested in typical industrial lubricants. Results showed that performance of the interdigitated sensors was consistent with the design equations; however, the comparison of EIS results using the interdigitated prototypes and the solid parallel plate electrodes showed that the prototype design needs further optimization.