A. L. Zharin

Application of the contact potential difference technique for on-line rubbing surface monitoring (review)

The Kelvin technique for measuring contact potential differences has been adapted for continuous non-destructive monitoring of changes in the electron work function of a rubbing surface. The method can be used to investigate tribological materials for a wide range of conditions, including changes in load, sliding speed, and environment, with or without lubrication. It relies on the sensitivity of the work function to the various events which accompany friction, e.g., plastic deformation, creation of new surface material, adsorption, oxidation, phase changes and redistribution of alloy components. At present, this is the only method which is sensitive to both surface and near-surface defects and permits study of one of the two interacting surfaces during sliding. Current work emphasizes two aspects of sliding behavior: (1) critical points with respect to changes in normal load, with relevance to materials selection and optimization, and (2) the kinetics of friction processes, including periodic changes which may be related to those in fatigue.

Applications of a non-contacting Kelvin probe during sliding

Sliding contact causes dramatic changes near the interface in metals. These include the development of large plastic strains and associated changes in defect concentration, microstructure, crystallographic texture and phase content. In addition, interactions with the counterface material and the environment and mechanical mixing processes can modify the material on both sliding surfaces. Structural and chemical characterization techniques have provided some insight into friction and wear processes, but many questions remain concerning mechanisms. A non-contacting Kelvin probe which detects changes in electron work function offers a useful tool for in situ monitoring of changes associated with sliding. Use of such a probe allows improved design of friction and wear experiments and provides new information on wear mechanisms. Earlier results reported by Zharin were interpreted in terms of damage accumulation and fatigue-like processes. The present results for the unlubricated sliding of various metal combinations confirm the experimental observations of Zharin but suggest that chemical reactions and changes in composition influence the changes detected by the Kelvin probe. For copper-based alloys, the Kelvin probe signal (KPS) can vary with a well-defined period. Images from a video system and from post-test observations with SEM and EDS indicate that the KPS correlates well with the amount of oxygen incorporated in the surface material. Results from materials which do not exhibit nearly-periodic behavior are also described. Differences in the development of the friction signal and the KPS with time can be correlated with differences in the development, accumulation and removal of material affected by tribochemical processes.

The Nonvibrating Kelvin Probe and Its Application for Monitoring Surface Wear

This paper reports on the design and development of the nonvibrating Kelvin probe that could be used as a noncontact sensor for tribological damage. This device detects surface charge through temporal variation in the electron work function of a material. Experiments are performed to demonstrate the operation of the probe on a rotating aluminum shaft. The probe, made of lead, is placed adjacent (<0.5-mm distance) to the shaft. The two surfaces, which are electrically connected, form a capacitor. Benchmark experiments on the variation of the work function with changes in surface chemistry were performed by coating a segment along the shaft circumference with a colloidal silver paint. As the shaft rotated, the probe senses changes in the contact potential difference with the shaft surface, due to the compositional variation. The temporal variation in the contact potential difference induces a current in an external electrical circuit. This current is amplified and converted to a voltage signal. The magnitude of the signal decreases asymptotically with the electrode-shaft distance and increases linearly with the rotational frequency, as expected. These results are consistent with the theoretical. Preliminary tests to detect mechanical surface damage, using the probe, were also done. These results show that the Kelvin probe can distinguish geometrical features on the surface.

CONTACT POTENTIAL MEASUREMENTS OF HARD DISK DRIVE SURFACES IN HUMID ENVIRONMENTS

Contact potential difference (CPD) experiments were conducted by the vibrating probe (Kelvin) method on humidity‐exposed hard disk drive surfaces. The measurements were made at various locations along the circumference of a wear track caused by the sliding contact with a spherical silicon pin, operated in a pin‐on‐disk configuration. The load on the pin, rotational speed of the disk, and the humidity were controlled. The CPD signals varied along the wear track and the magnitude of the CPD change increased with the load from 29 to 98 mN at a fixed relative humidity. The CPD signals also increased linearly with relative humidity ranging from 30% to 70% as the normal load was fixed at 29 and 49 mN. The CPD signal appears to saturate at a load of 98 mN where the relative humidity is 50% and higher.

Macroscopic effects of delamination wear

Abstract Measurement using a pin-on-disc machine of periodic changes in the energy parameters of sliding surfaces under conditions of unlubricated contact has provided experimental evidence for the delamination theory of wear. The experimental procedure is suitable for the investigation of lubricated frictional contact.

The effect of high-dispersion fillers on adhesive and frictional properties of ethylene-vinyl acetate copolymer

The adhesive strength of adhesive joints between metal surfaces has been improved considerably by modifying an adhesive based on a copolymer of ethylene with montmorillonite vinyl acetate. It as been proved that the static (adhesive joint) and dynamic (frictional interaction) adhesion modes are correlated. It has been shown that the surface potential found by a modified Kelvin-Zisman method increases with increasing adhesion of the metal-polymer joint.

Contact Potential Difference Techniques as Probing Tools in Tribology and Surface Mapping

Contact potential differences techniques have been adapted for continuous nondestructive monitoring of changes in the electron work function of a rubbing surface. The method can be used to investigate tribological materials for a wide range of conditions, including changes in load, sliding speed, and environment, with or without lubrication. It relies on the sensitivity of the work function to the various events, which accompany friction, for example, plastic deformation, creation of new surface of material, adsorption, oxidation, phase changes, and redistribution of alloy components. At present, this is the only method sensitive to both surface and near-surface defects and permits study of one of the two interacting surfaces during sliding. For metals and alloys, the thickness of a layer contributing to the electron work function measurement is equal to several atomic distances, that is, even traditional contact potential differences measurements is really related to nanoscale. Kelvin probe force microscopy allows to determine not only the surface topography as does atomic force microscopy, but in addition also delivers images of the surface work function on a nanometer scale. Modern contact potential differences techniques cover the range from macro/micro to nanoscales. The current paper focused on an in situ contact potential difference measurement during the sliding of materials.

Surface Characterization with an Ionization Probe

This paper discusses the application of an ionizing source coupled with galvanic differences between metals in a measure of the work function difference between the metal surfaces. The electrical field generated from the contact potential difference (CPD) between two electrodes will cause the gaseous ions to discharge at both surfaces, creating a measurable current. The current depends on the surface size, spacing, and ionizing source power. One of the surfaces (probe) can vary in shape and size, and if inert, can be used to obtain the work function or surface potential of the second surface. The ionic current is proportional to ion mobility, ion generation rate, CPD, and the probe size, but inversely proportional to the spacing between the probe and the sample. It is found, as expected, that there is an approximate linear relationship between the ionization probe signal and the work function of the surfaces of metals.

Macro- and Micro Kelvin Probe in Tribological Studies

The Kelvin method of electronic work function (EWF) measurement of contact potential difference (CPD) technique is an excellent non-destructive monitoring technique. Lord Kelvin offered the CPD in 1898. The CPD technique was developed considerably parallel with quantum theory of solids. As researchers were trying to correlate experimental data with theory, the EWF was explained according to the fundamental quantum mechanical parameters of solids. However, a strong influence of surface conditions on the experimental results was found and the technique was practically forgotten. Later, problems in the measurement of surface conditions have gained a special importance with the development of solid-state electronics. However, systems of surface analysis began to appear during the same years. Such systems were complicated devices attached to ultrahigh vacuum systems. These systems have overshadowed the CPD technique. An analysis of published papers has shown that surface analysis systems yield interesting results when conducting fundamental experiments with pure model surfaces. Results are not reliable for most of engineering surfaces. It is explained that surface analysis systems, in most cases, do not analyse the surface, but instead analyse artefacts on the surface. According to our experience, CPD does give reliable information about the surface.