The Transient Friction Response of a Laser-Textured, Reciprocating Contact to the Entrainment of Individual Pockets

Abstract

To shed light on the mechanisms with which surface texture improves the tribological performance of piston-liner contacts, we have measured the transient friction response as individual pockets pass through a reciprocating sliding contact. Tests were performed at different sliding speeds and results compared to those from a non-textured, reference specimen under different lubrication regimes. At low speed when the contact is in the boundary regime, friction force falls abruptly as each pocket leaves the contact zone, before gradually returning to an approximately steady-state value. This suggests that each pocket acts to temporarily increase the film thickness, which then decays to its non-textured value as oil is squeezed out. At higher speeds, friction is seen to reduce in a stepwise fashion, since the period between pockets being entrained is less than the time taken for the film to decay. In addition, friction results obtained when the contact is operating in the middle of the mixed regime point to a temporary film thickness collapse as the pocket enters the contact, and this agrees with recent modelling predictions. At higher speeds, the compound effect of successive pockets is to shift the contact to the right on Stribeck curve. These results imply that each pocket gives rise to an increase in film thickness that is both short-lived and small in magnitude (we estimate a few tens of nm). However, the resulting effect on friction can be significant (up to 82 % in this study) for two reasons: (1) provided the pocket frequency is sufficiently high, each successive pocket entrainment builds the film up without there being time for it to reduce back to its steady-state value; (2) when the contact is in the mixed regime, the Stribeck curve is at its steepest and friction is therefore most sensitive to film thickness changes. This has important practical implications in that pocket spacing on piston liners should be varied as a function of reciprocating sliding speed.

Looking into a laser textured piston ring-liner contact

“…The "U" shaped variation of the frictional response in the highlighted raw data plots is explained by the contact operating under mixed lubrication conditions (along the stroke, friction decreases with speed due to film formation). At the beginning of the test, when the composite roughness of the two surfaces is very low (i.e., 15 nm), the fully-flooded non-textured sample gives the lowest friction; this behaviour is in agreement with previous studies [13,[31][32][33] and is due to the contact operating around the transition between hydrodynamic and mixed lubrication regime. However, as shown in [33], the roughening of the surfaces due to wear causes a transition into the mixed regime where the textured surface begins to outperform the non-textured one (in this case after approximately 40 minutes of rubbing).…”

“…During experiments to study the impact of viscosity on cavitation pattern, the oil temperature was controlled using the setup described in [13]. Friction force stability over time and test repeatability were analysed for both textured and nontextured specimens and confirmed previously in [13,31,32]. Under both mixed/boundary and hydrodynamic lubrication conditions, measurements were shown to be repeatable to within 0.1 N, over a period of months.…”

“…In order to apply texture effectively in practice, it is vitally important to understand which combinations of these mechanisms occur and under which conditions. Recent work at Imperial College [13,[31][32][33][34][35][36] has investigated a variety of surface texture geometries in a contact closely replicating an automotive piston ring-cylinder liner pair. The relationship between the friction reduction capability of surface texture and lubrication regime was characterised in [13] for a variety of textured shapes, while in [35,37] rules for the optimum pocket width, depth and spacing was established for the best performing shape determined previously (i.e.…”

“…This showed that pockets act to increase the oil film thickness by 10s of nanometres, causing a reduction in asperity contact and hence significant reductions in friction due to the steepness of the Stribeck curve in the mixed regime. The transient behaviour of individual pockets passing through the contact was studied in [32], where it was shown that pocket entrainment frequency is more important than physical spacing between pockets. In the most recent study [33], a close correlation was found between the amount of wear in the vicinity of the reversal point and the volume of oil within the pockets.…”

“…In addition to high resolution friction force recordings, the apparatus described in detail in [13,31,32], allows for film thickness and cavitation pattern to be captured using two high sensitivity optical imaging techniques: i) a modified version of the ultrathin film interferometry technique that is able to measure nanometer size film thicknesses in textured reciprocating contacts operating in mixed and boundary regimes, and ii) a laser-induced fluorescence (LIF) technique to qualitatively image the distribution of oil in and around the reciprocating ring-liner pairing. The latter technique will be the focus of the present study.…”