Friction, Scuffing and Transfer Layer Formation in Lubricated Sliding of EN31 Steel and Tungsten Carbide (WC): Surface Topography Effects

Surface topographies play a critical role in controlling friction, surface damage and transfer layer formation in engineering applications; hence understanding this is of great importance. In this work, experimental studies were carried out to understand the influence of surface topography on friction, scuffing and transfer layer formation in completely immersed lubricated sliding interactions. For this, sliding experiments were carried out in sphere on flat configuration using EN31 steel flats and Tungsten Carbide pin countersurface. Perpendicular and parallel surface topographies were induced onto the steel flats. Experiments were conducted at high normal loads of 1000N, 2000N and 3000N. The results show that Surface topography has a significant influence on the frictional response. When the topography directionality was perpendicular to the sliding direction, scuffing was observed only at a high load of 3000N. A ‘peak friction’ was also observed during the occurrence of scuffing. When the directionality in topography was parallel to sliding direction, scuffing and surface damage occurred from 2000N itself, accompanied by a high amount of transfer layer formation. This can be attributed to the directionality of parallel topography, which displaces away the lubricant during sliding interaction, creating metal-to-metal contact and hence leading to scuffing and higher transfer layer formation.

Design of bidirectional frictional behaviour for tactile contact using ellipsoidal asperity micro-textures

Tactile perception and friction can be modified by producing a deterministic surface topography. Change of surface feature arrangement and texture symmetry can produce an anisotropic frictional behaviour. It is generally achieved through skin hysteresis by promoting its deformation. This work investigates whether a bidirectional friction can be created with microscale ellipsoidal asperity textures, thus relying on the adhesive component of friction. For this purpose, four textured samples with various asperity dimensions were moulded with a silicone rubber having an elastic modulus comparable to that of the skin. Coefficient of friction measurements were conducted in-vivo in two sliding directions with a range of normal loads up to 4 N. Finite element method (FEM) was used to study elastic deformation effects, explain the observed friction difference, and predict surface material influence. Measurements performed perpendicular to the asperity major radii showed consistently higher friction coefficients than that during parallel sliding. For the larger asperity dimensions, a change of the sliding direction increased friction up to a factor of 2. The numerical analysis showed that this effect is mostly related to elastic asperity deflection. Bidirectional friction differences can be further controlled by asperity dimensions, spacing, and material properties.

Experimental Investigation of Transfer of PTFE on Bearing Steel

Polytetrafluoroethylene (PTFE) lubricant is commonly applied for dry contact due to its low friction properties. However, low strength properties can lead to short service-life due to high wear rate, especially under high contact load. The method to add PTFE into a solid contact during operation as a transfer layer has been one of the major attempts in this field. This study aims to investigate the influence of operating parameters, i.e., revolution cycle, sliding speed and applied load, on coverage area of transfer PTFE on the bearing steel (AISI 52100) disc. The experiments were performed on the modified pin-on-disc apparatus using a unidirectional ground disc. The areas with disc grinding direction parallel (parallel morphology) and perpendicular (perpendicular morphology) to the pin sliding direction were both examined. The ascending of transfer coverage area with an increasing revolution cycle within the first 1000 cycle was observed on the area with a sliding direction perpendicular to the disc grinding direction while the descending of transfer coverage area was found on the parallel case. The further increase in the revolution cycle led to only a small change in the transfer coverage area. With more revolution cycles, the pin wear rate increased as a decrease in transfer coverage area formed on the counter-face. Research suggested that the amount of transfer coverage area decreased with increasing sliding speed. However, it could be increased by increasing the applied load.

Closure to Discussion -Fabrication and Testing of Bioinspired Surface Designs for Friction Reduction at the Piston Ring and Liner Interface- [DOI: 10.1115/1.4050795] (2021, ASME J. Tribol., 143(5): 051109)

We thank the discussant for their interest in our manuscript and their very helpful remarks. Existing tribological studies of biomimetic surfaces were mostly focused on dry friction and biological surfaces are highly deformable. Therefore, the learnings on the effects of textures may not be directly translate to fully lubricated interfaces. Nonetheless, we agree that we can still learn much from these studies. Investigating additional orientations of the elongated hexagon could possibly improve the frictional response of the lubricated surfaces. Given that existing literature indicates that orienting the hexagons with two edges perpendicular to the sliding direction yields lower friction than in the case of edges parallel to the sliding direction [1], the experimental conditions in the manuscript could be the worst-case scenario and thus a lower bound for frictional improvement. Additionally, the hexagon was designed based not only on the design of the frog toe (and other natural hexagonal surfaces such as snakes) but also on existing industrial piston cylinder liner designs, where the crossing grooves are oriented nearly perpendicular to the sliding direction [2, 3]. Hence, our tested design is an extension of that existing technology with learning from nature. However, the suggestions to expand the experiment with additional hexagonal orientations is well received and will be considered for future work.

Ruler elements in chromatin remodelers set nucleosome array spacing and phasing

Arrays of regularly spaced nucleosomes dominate chromatin and are often phased by alignment to reference sites like active promoters. How the distances between nucleosomes (spacing), and between phasing sites and nucleosomes are determined remains unclear, and specifically, how ATP-dependent chromatin remodelers impact these features. Here, we used genome-wide reconstitution to probe how Saccharomyces cerevisiae ATP-dependent remodelers generate phased arrays of regularly spaced nucleosomes. We find that remodelers bear a functional element named the ‘ruler’ that determines spacing and phasing in a remodeler-specific way. We use structure-based mutagenesis to identify and tune the ruler element residing in the Nhp10 and Arp8 modules of the INO80 remodeler complex. Generally, we propose that a remodeler ruler regulates nucleosome sliding direction bias in response to (epi)genetic information. This finally conceptualizes how remodeler-mediated nucleosome dynamics determine stable steady-state nucleosome positioning relative to other nucleosomes, DNA bound factors, DNA ends and DNA sequence elements.

The role of surface topography and normal load in the initiation of ratchetting-peak friction, seizure, scuffing, and elastic shakedown

Surface topography is a critical parameter that can influence friction and wear in engineering applications. In this work, the influence of surface topography directionality on seizure and scuffing initiation during tribological interactions is explored. For this, unidirectional sliding wear experiments were carried out in immersed lubrication conditions for various normal loads. The tribological interactions were studied using EN31-60 HRC flats and SAE52100-60HRC pins in a sphere on flat configuration. The results show that, in some cases, the sliding interactions in the initial cycles lead to a high friction coefficient of up to ∼0.68 in lubricated conditions, which was termed as ‘peak friction’, and this was accompanied by scuffing. The existence of peak friction was found to be dependent on surface topography directionality, especially when the directionality in topography was parallel to the sliding direction. Continuous ratchetting was found to be the cause of peak friction which was accompanied by seizure and scuffing. When the topography directionality was perpendicular or independent of sliding direction, elastic shakedown occurred at earlier cycles and prevented peak friction initiation, scuffing and also facilitated for higher steady-state friction values.

Deformation due to sliding of single and woven carbon tows in dry and epoxy-lubricated conditions

This experimental work focuses on the evaluation of deformation mechanisms due to sliding between carbon fiber tows with a flat tool in dry and lubricated with liquid resin conditions. The experiments were carried out on manually woven and single tows. The effect of angle between tow axes and sliding direction was also studied. The topography of the tows in contact with a sliding transparent glass plate was measured with a 3D optical microscope before and after sliding. These measurements revealed a decrease of roughness with sliding in all tested conditions, a contraction of lubricated single tows in perpendicular to sliding orientation, and high residual displacements in lubricated woven tows in 0°/90° orientation and dry single tows in perpendicular to sliding orientation.

Effect of simulated tennis steps and slides on tread element friction and wear

In hard court tennis, players change direction by either stepping or sliding. The shoe–surface friction during these movements is crucial to player performance. Too little friction when stepping may result in a slip. Too much friction when attempting to slide could cause the player to move only a short distance, or to fail to slide. To understand the influence of tread design on shoe–surface friction in tennis, experiments were performed on individual shoe tread elements that replicated the tribological conditions typically experienced during hard court step and slide movements. Tread element orientation had no effect on the static friction in step movements, but longer tread elements (in the sliding direction) had 9% lower dynamic friction during slide movements (p < 0.001). The friction between tennis shoe tread and hard court tennis surfaces is also shown to be influenced by the tread’s sliding history, and the wearing pattern that forms on the surface of the rubber.

Nanoscale Simulations of Wear and Viscoelasticity of a Semi-Crystalline Polymer

We investigate the underlying tribological mechanisms and running-in process of a semi-crystalline polymer using molecular-dynamics simulations. We subject a slab of simulated polyvinyl alcohol to a sliding contact asperity resembling a friction force microscope tip. We study the viscoelastic response of the polymer to the sliding and show both plastic and elastic contributions to the deformation, with their relative strength dependent on the temperature. As expected, the elastic deformation penetrates deeper into the surface than the plastic deformation. Directly under the tip, the polymer has a tendency to co-axially align and form a layered structure. Over time, the plastic deformation on and near the surface builds up, the friction decreases, and the polymers in the top layer align with each other in the sliding direction (conditioning).

Influence and Improvement of Hydraulic Power on Spool Valve Reversing

This paper analysed the effect of steady-state flow force and transient flow force to sliding direction valve, and two examples were given to illustrate adverse consequences caused by excessive fluid power, put forward the compensation measures. The effect of flow force should be considered when designing the hydraulic system in order to make the hydraulic system work more stable.