Combiningin situtribometry and triboscopy to understand third body behavior of a Cd coating

Abstract

The dynamic nature of third bodies and the buried nature of a sliding contact makes it difficult to interpret and correlate changes in the coefficient of friction to instantaneous materials changes. That is, full utilization of the 'tribological circuit' as proposed by Berthier, to describe tribological flows is often limited in practice unless specialized techniques are used. In this paper, an in situ tribometer is used to understand the evolution and flow of third bodies and their effect on the coefficient of friction. The in situ tribometer consisting of a transparent sapphire countersphere coupled with optical microscope is used to study the interface during linearly reciprocating sliding with simultaneous measurement of the coefficient of friction by measuring the tangential force with a time-resolved piezo sensor at a sampling rate of 800 Hz along the track length. This provides instantaneous coefficient of friction measurements along the localized track length, allowing one to plot 'spatial friction' maps, also known as triboscopic images. These graphs are cycle number on the x-axis and track position on the y-axis with a color coding for coefficient of friction. The in situ images of third bodies are correlated with the triboscopic images to identify third body flows and how they relate to local frictional changes. A soft coating (Cd) on hard substrate (low carbon steel) is used to study these third body phenomenon.

Friction transitions and connections to third bodies for a Cd coating on steel substrate

“…In situ studies done by Sriraman et al [3] on Cd coated steel observed transfer film formation with frequent recirculation of Cd at the contact region. Studies done by Behera et al [4] integrating in situ and triboscopy method for Cd coating observed similar temporal features occurring due to transfer film formation and subsequent change in CoF with change in transfer film morphology. Spectral analysis performed on similar features identified from the triboscopic imagery indicated change in third body morphology with progress of the test with some similarities between sapphire and steel countersphere.…”

“…These lead to discrete events of increased friction lasting a few cycles with no significant spatial features. That is, the friction is increased all along the wear track as the cause of the frictional rise is associated with third body flows occurring with the transfer film attached to the ball, which traverses the entire track [4]. As described above, the low average CoF (Regime II) is primarily due to favorable contact conditions of steelon-steel with thin layers of Cd smear at the interface.…”

“…7(a)). This is due to the resolution of the spatial friction technique and the fact that not all friction changes are expected to be directly correlated with wear track features [4]. Third body flows such as detachment of transfer film or adhesion of coating material to the counterface may also modify the CoF, complicating direct correlations, except for some special cases, like the one noted above.…”

“…In Regime I, the transfer film is formed by adhesive transfer of Cd to the countersphere. However, for metallic wear, these early stages of wear are dynamic and periodical, transfer film material is lost or replenished [3,4,9,13]. These lead to discrete events of increased friction lasting a few cycles with no significant spatial features.…”

“…Cadmium (Cd) has been used extensively for its sacrificial corrosion protection of steel [1,2] and for its solid lubricating properties. In terms of the tribology of Cd coatings, most researchers use standard tribology testings [3,4], such as pin-on-disc, and report average friction versus cycle and overall wear or wear rate. Testing of this sort provides an overall description of the tribological behavior but makes it difficult to examine the mechanisms behind time and spatial dependent variability of the friction or wear.…”