This thesis utilized the concept of an optically stimulated Contact Potential Difference (osCPD) sensor to monitor oil properties. The osCPD technique is a variant of the contact potential difference (CPD) method used to obtain surface properties of materials. The technique uses modulated light to stimulate electron charge carriers in silicon coated with a layer of oil. Demonstration of this oil monitoring design was done by placing different oil samples on the silicon surface and monitoring the corresponding electrical signal with the osCPD sensor. Experiments showed that the osCPD sensor produced an electrical signal that was related to the amount of time an oil sample was aged in an engine (or mileage). Further, a linear relationship was found between the relative conductivity of these oils and the osCPD signal. It is theorized that this osCPD signal is dependant on the charge transfer at the silicon and oil interface. Investigation of this interaction was carried out. Experiments showed that adding a silicon nitride passivation layer on the silicon surface eliminated the change in osCPD signal with oil properties. A model of this charge interaction was developed.

This thesis incorporates an array of Contact Potential Difference (CPD) sensors to measure and monitor the degree of skew in the rolling elements of a spherical roller bearing. Skewing is the motion of a roller as it turns about an axis normal to the roller race interface. Roller skew is generated as part of the kinematic effects of roller bearings. Skew monitoring is important for bearing design as it is an indirect measure of bearing life. For the purpose of this thesis, roller skew was measured utilizing multiple pairs of CPD probes located around the bearings outer raceway at varying points of the loading zone. These CPD probes are not in direct contact with the rollers, but in close proximity to their surface (through the bearing outer ring). The skew angle measured is related to different operating conditions such as applied load, shaft speed, and lubrication. The pair of CPD probes detected a signal as the roller surface passed by and the phase difference between the two distinct signals measured the skew angles in the range of 0.016 to 1.10. The shaft is rotated both clockwise and counterclockwise to capture any probe misalignment which was in the range of 0.5 up to 2.0 . This thesis also provides a model for the probe signal as a spherical roller surface passes the probe surface.

Oil analysis is a broad field comprised of hundreds of individual tests that provide meaningful benefit by assessing one or more properties of lubricants or machines. Many tests are performed on new types of oil during research and development. The lubricants chemical, physical, or lubricating properties are validated for quality control purposes and product performance classification. Much of the research in this area is devoted to the online oil degradation systems which allow getting a prompt response about the condition of lubricant. This thesis investigates the concept for monitoring oil degradation with a vibrating Kelvin probe technique. The Vibrating Kelvin probe method for measuring the work function of metals has been used since 1932. Among the applications of this technique are adsorption, corrosion, friction and other studies. A novel application of this method is proposed in this thesis. The vibrating Kelvin system was created with one static surface acting as a sampling surface and the other one electrically isolated. The interaction of the oil with one of the surfaces of a capacitor results in a signal which is synchronously measured. The oil molecules adsorb on the surface of one of the plates and form a space charge layer which changes the work function of that surface. Oil prepared by intentional oxidation was used to evaluate and to monitor the ability to see changes in oil.

This thesis addresses the fabrication, evaluation, and analysis of the probe tip of a non-vibrating contact potential difference sensor. The non-vibrating contact potential difference (nvCPD) probe measures the work function variation on a conducting surface and recent experiments performed to measure the size of surface features have shown poor correlation between actual and calculated probe tip dimensions. In order to address this deficiency, experiments were done and an analytical model was developed, including fringe electrical fields, that predicts the shape of the nvCPD probe signal as a function of probe tip geometry, work function variation, and experimental parameters. Probe tips were constructed with varying geometric properties and experiments using these probe tips were compared to a model. There was good correlation of the nvCPD probe output for a known work function change and probe tip geometry. The effective area of the probe tip resulting from electrical field fringing is expected to increase with dielectric thickness to a finite value, based on pre-existing electrostatic models for a shielded parallel plate capacitor. The minimum fringe field obtained in these experiments was for a 3.18mm diameter probe tip with a dielectric thickness of 0.20mm. The fringe field diameter was 3.38mm at a fly height of 0.60mm, representing an effective probe tip area increase of 13%.