Spatially Resolved Triboemission Measurements

Abstract

Electron emission that occurs when surfaces undergo wear has been studied since the 1950s and is the subject of continued speculation due to its possibly crucial role in the formation of boundary lubricating films and the degradation of hard drive lubricants. Despite this, the mechanisms by which this type of emission occurs are poorly understood. In order to shed light on this phenomenon, we report for the first time how friction-stimulated electron emission can be visualized, by scratching an aluminium oxide surface, positioned 10 mm from a microchannel plate (MCP) in a high vacuum. The MCP is coupled with a phosphor screen displaying the spatial distribution of electrons that are emitted in individual bursts as wear is accruing. The probability density of this emission was calculated and the Shannon spectral entropy for a 10-ms time window was found to be 0.11, suggesting that partially deterministic mechanisms are responsible. These emission maps help to elucidate the obscure mechanisms that cause emission, in a way that has not been possible with previous spatially averaged measurements. For example, linear distributions support the theory that electron emission arises due to the formation of surface cracks. In addition to this, as the contact moves over the specimen surface, certain locations show an increased propensity to emit, which further suggests the presence of localized defects. Furthermore, in certain cases, sustained after-emission is observed from localized defects exiting the contact. These results highlight the possibility of using the technique to monitor very low levels of wear and even image crack formation in real time.