"The origins of triboemission -- Correlating wear damage with electron emission"

Alessandra Ciniero,Julian Le Rouzic,Iain Baikie,Tom Reddyhoff

doi:10.1016/j.wear.2017.01.013

Abstract

Triboemission – i.e. the emission of photons, electrons and other charged particles that arise from a sliding contact – may play a key role in tribochemical processes, such as lubricant degradation. However, the mechanisms that give rise to this type of emission are not well understood. For the first time, we present spatially resolved measurements of electron emission, obtained as a range materials are worn. These are obtained from scratch tests, carried out under vacuum conditions (10−5Torr), in which microchannel plates coupled to a phosphor screen are used to image electron emission.The results show that electron emission occurs at specific locations on the worn surface and, depending on the conductivity of the material, these sites remain active and decay with a time constant of up to several seconds. SEM images of the worn surface at these sites reveal that either surface fractures or grain defects are present. This suggests that fractoemission mechanisms are at least partially responsible for triboemission (however, the possible contribution of tribocharging mechanisms are also discussed). Specifically, this provides evidence to support the theory that triboemission results from the imbalance of charge on opposing faces of wear cracks and that this generates an electric field sufficient to accelerate molecular fracture products, which then bombard the surface leading to secondary emission.The strong geometric correlation between damage topography and electron emission distributions shows the potential of using this technique to monitoring wear and crack formation in real time and under high (30x) magnification.

Highlights

The emission of particles, that are stimulated by frictional contact between sliding bodies, is often referred to as “triboemission” [1,2,3]
The intensity rises rapidly and decreases gradually. This is in contrast to triboemission measurements by previous researchers who observed bursts that fall rapidly after their initial rise, even for tests carried out in alumina specimens [1,2,3,5,8]
Triboemission may play a role in tribochemical processes, there is uncertainty regarding the mechanisms responsible

Summary

Introduction

The emission of particles, (photons electron, protons, positive and negative ions), that are stimulated by frictional contact between sliding bodies, is often referred to as “triboemission” [1,2,3]. Numerous studies have demonstrated that this type of emission can occur under vacuum conditions, the majority of which measured spatially averaged signals from optical and electrical probes, located in the vicinity of a sliding contact [1,2,3,4,5,6,7]. Ciniero et al / Wear 374-375 (2017) 113–119 rubbing can lead to particle emission These include: i) thermal emission due to frictional heating [24,20,25]; ii) field emission due to an accumulation of surface charge caused by triboelectrification [26]; iii) fractoemission [6]. SEM images of the wear track show how the distribution of emitted particles is strongly correlated to damage geometry on the sample surface

Test methods

Results and discussion

Conclusions