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Textured surfaces offer the potential to promote friction and wear reduction by increasing the hydrodynamic pressure, fluid uptake, or acting as oil or debris reservoirs. However, texturing techniques often require additional manufacturing steps and costs, thus frequently being not economically feasible for real engineering applications. This experimental study aims at applying a fast laser texturing technique on curved surfaces for obtaining superior tribological performances. A femtosecond pulsed laser (Ti:Sapphire) and direct laser interference patterning (with a solid-state Nd:YAG laser) were used for manufacturing dimple and groove patterns on curved steel surfaces (ball samples). Tribological tests were carried out under elasto-hydrodynamic lubricated contact conditions varying slide-roll ratio using a ball-on-disk configuration. Furthermore, a specific interferometry technique for rough surfaces was used to measure the film thickness of smooth and textured surfaces. Smooth steel samples were used to obtain data for the reference surface. The results showed that dimples promoted friction reduction (up to 20%) compared to the reference smooth specimens, whereas grooves generally caused less beneficial or detrimental effects. In addition, dimples promoted the formation of full film lubrication conditions at lower speeds. This study demonstrates how fast texturing techniques could potentially be used for improving the tribological performance of bearings as well as other mechanical components utilised in several engineering applications.

This paper presents an experimental study into the flow behaviour of lubricant in a reciprocating contact simulating a piston ring–cylinder liner pair. The aim was to understand the effects of cavitation, starvation and surface texture, as well as the interaction between these, in order to improve automotive engine performance. A custom-built test rig was used, in which a section of piston ring is loaded against a reciprocating, laser-textured, fused silica pad representing the liner. A fluorescence microscope focusses through the silica specimen onto the contact in order to image the distribution of dyed oil. Tests were performed using a range of texture geometries and orientations, under starved and fully-flooded lubrication conditions, with measurements being compared against those from a non-textured reference. Under limited oil supply conditions, the non-textured reciprocating contact sweeps oil towards the reversal points (TDC and BDC), leading to starvation and increased friction. This issue is alleviated by the presence of surface texturing, with each pocket transferring oil from the inlet to the outlet of the contact as it passes; the result being 33% lower friction and oil distributed evenly over the liner surface. Even under fully flooded conditions, starvation is shown to occur following each reversal, as the change in sliding direction causes the cavitated outlet to become the oil-deprived inlet. This proof of cavitation-reversal-starvation, which occurs for up to the first 5% of the stroke length, depending on the lubricant’s viscosity, corresponds to regions of high wear, measured in this study and on actual cylinder liners reported in the literature. This process is also counteracted by the presence of surface texture, with each pocket depositing oil into the cavitated region prior to reversal. Fluorescence data also provides insights into other mechanisms with which different textures geometries control friction. Grooves oriented parallel to sliding direction increase friction as they appear to connect the high pressure inlet with the low pressure outlet, leading to oil film collapse. Grooves oriented transverse to sliding direction produce localised cavitation inside each pocket, which supports the theory that texture draws lubricant into the contact through the ‘inlet suction’ mechanism. These findings can aid texture design by showing how pockets can be used in practice to simultaneously control oil consumption, and reduce friction and wear along the stroke. It should be noted that the lubricant transport mechanisms described above should also result from other types of depressions, such those produced by porous coatings (provided they are smaller than the contact area).

The aim of this study is to gain insights into the interactions between laser-textured surface pockets and friction and wear behaviour of an automotive piston-liner pairing. To do this, a recently developed, reciprocating, test apparatus was used to conduct wear tests under highly loaded conditions. Fused silica specimens with a range of pocket geometries were rubbed against a convex steel pad and the resulting friction and wear data were compared with those from a non-textured specimen. Contact conditions were set to remove the influence of initial surface roughness on texture behaviour. These tests showed that, as the specimen become worn and surface roughness increases, the contact progresses further into the mixed and boundary regime. This leads to a significant improvement in the relative performance of the textured specimens, showing reductions in friction of up to 70%, compared with the non-texture case. This is consistent with previous results that have shown texture to have the effect of boosting film thickness in the mixed lubrication regime. Surface texture was also shown to reduce the volume of wear, by up to 69% (corresponding to a change in wear coefficient from 2.67e−4 to 0.81e−4 [mm3/N m]). Another, important, finding is that both friction and wear reduce monotonically as the sum of the pocket volumes along the stroke increases. This may aid texture design, since it means that individual pocket width and depth values can largely be ignored, so long as the overall volume is maximised. The only exception to this trend is when the pockets are larger than the contact area. In this case, friction increases due to a collapse of the lubricant film, while wear reduction remains unaffected – a discrepancy which may suggest that pockets reduce wear and friction via separate mechanisms.

Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses

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.