Imperial College London

Dr Tom Reddyhoff

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 3840t.reddyhoff Website

 
 
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Location

 

670City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

88 results found

Carpenter G, Bozorgi S, Vladescu S, Forte A, Myant C, Potineni R, Reddyhoff T, Baier Set al., 2019, A study of saliva lubrication using a compliant oral mimic, Food Hydrocolloids, Vol: 92, Pages: 10-18, ISSN: 0268-005X

Due to ethical issues and the difficulty in obtaining biological tissues, it is important to find synthetic elastomers that can be used as replacement test media for research purposes. An important example of this is friction testing to understand the mechanisms behind mouthfeel attributes during food consumption (e.g. syrupy, body and clean finish), which requires an oral mimic. In order to assess the suitability of possible materials to mimic oral surfaces, a sliding contact is produced by loading and sliding a hemispherical silica pin against either a polydimethyl siloxane (PDMS), agarose, or porcine tongue sample. Friction is measured and elastohydrodynamic film thickness is calculated based on the elastic modulus of the samples, which is measured using an indentation method. Tests were performed with both saliva and pure water as the lubricating fluid and results compared to unlubricated conditions.PDMS mimics the tongue well in terms of protein adhesion, with both samples showing significant reductions in friction when lubricated with saliva versus water, whereas agarose showed no difference between saliva and water lubricated conditions. This is attributed to PDMS's OSi(CH3)2- group which provides excellent adhesion for the saliva protein molecules, in contrast with the hydrated agarose surface. The measured modulus of the PDMS (2.2 MPa) is however significantly greater than that of tongue (3.5 kPa) and agarose (66–174 kPa). This affects both the surface (boundary) friction, at low sliding speeds, and the entrained elastohydrodynamic film thickness, at high speeds.Utilising the transparent PDMS sample, we also use fluorescence microscopy to monitor the build-up and flow of dyed-tagged saliva proteins within the contact during sliding. Results confirm the lubricous boundary film forming nature of saliva proteins by showing a strong correlation between friction and average protein intensity signals (cross correlation coefficient = 0.87). This demonstrates

Journal article

Hu S, Cao X, Reddyhoff T, Puhan D, Huang W, Shi X, Peng Z, Dini Det al., 2019, Three-dimensional printed surfaces inspired by bi-Gaussian stratified plateaus, ACS Applied Materials and Interfaces, Vol: 11, Pages: 20528-20534, ISSN: 1944-8244

Wettability of artificial surfaces is attracting increasing attention for its relevant technological applications. Functional performance is often achieved by mimicking the topographical structures found in natural flora and fauna; however, surface attributes inspired by geological landscapes have so far escaped attention. We reproduced a stratified morphology of plateaus with a bi-Gaussian height distribution using a three-dimensional direct laser lithography. The plateau-inspired artificial surface exhibits a hydrophobic behavior even if fabricated from a hydrophilic material, giving rise to a new wetting mechanism that divides the well-known macroscopic Wenzel and Cassie states into four substates. We have also successfully applied the plateau-inspired structure to droplet manipulation.

Journal article

Geng Z, Puhan D, Reddyhoff T, 2019, Using acoustic emission to characterize friction and wear in dry sliding steel contacts, Tribology International, Vol: 134, Pages: 394-407, ISSN: 0301-679X

© 2019 Acoustic emission (AE) was recorded during tribological tests on 52,100 steel specimens under different loads. AE signals were transformed to the frequency domain using a Fast Fourier Transform and parameters such as power, RMS amplitude, mean frequency, and energy were analyzed and compared with friction coefficient and wear volume measurements. Results show that certain acoustic frequencies reflect friction while others reflect wear. If frequencies are chosen optimally, AE and friction signals are highly correlated (Pearson coefficients >0.8). SEM and Raman analysis reveal how plastic deformation and oxide formation affect friction, wear and AE simultaneously. AE recordings contains more information than conventional friction and wear volume measurements and are more sensitive to changes in mechanism. This all demonstrates AE's potential as a tool to monitor tribological behavior.

Journal article

Vladescu S-C, Fowell M, Mattsson L, Reddyhoff Tet al., 2019, The effects of laser surface texture applied to internal combustion engine journal bearing shells - An experimental study, TRIBOLOGY INTERNATIONAL, Vol: 134, Pages: 317-327, ISSN: 0301-679X

Journal article

Geng Z, Shi G, Shao T, Liu Y, Duan D, Reddyhoff Tet al., 2019, Tribological behavior of patterned TiAlN coatings at elevated temperatures, SURFACE & COATINGS TECHNOLOGY, Vol: 364, Pages: 99-114, ISSN: 0257-8972

Journal article

Hu S, Vladescu S-C, Puhan D, Huang W, Shi X, Peng Zet al., 2019, Bi-Gaussian stratified theory to understand wettability on rough topographies, Surface and Coatings Technology, ISSN: 0257-8972

Journal article

Hu S, Reddyhoff T, Puhan D, Vladescu S-C, Huang W, Shi X, Dini D, Peng Zet al., 2019, Bi-Gaussian stratified wetting model on rough surfaces, Langmuir, Vol: 35, Pages: 5967-5974, ISSN: 0743-7463

Wetting mechanisms on rough surfaces were understood from either a monolayer or a multiscale perspective. However, it has recently been shown that the bi-Gaussian stratified nature of real surfaces should be accounted for when modeling mechanisms of lubrication, sealing, contact, friction, acoustic emission, and manufacture. In this work, a model combining Wenzel and Cassie theories was put forward to predict the static contact angle of a droplet on a bi-Gaussian stratified surface. The model was initially applied to numerically simulated surfaces and subsequently demonstrated on hydrophilic steel and hydrophobic self-assembled monolayer specimens with preset bi-Gaussian stratified topographies. In the Wenzel state, both the upper and the lower surface components are fully wetted. In the Cassie state, the upper component is still completely wetted, while the lower component serves as gas traps and reservoirs. By this model, wetting evolution was assessed, and the existence of different wetting states and potential state transitions was predicted.

Journal article

Vladescu S-C, Putignano C, Marx N, Keppens T, Reddyhoff T, Dini Det al., 2019, The percolation of liquid through a compliant seal - an experimental and theoretical study, Journal of Fluids Engineering, Vol: 141, Pages: 031101-031101, ISSN: 0098-2202

Journal article

Reddyhoff T, Schmidt A, Spikes H, 2019, Thermal conductivity and flash temperature, Tribology Letters, Vol: 67, Pages: 22-22, ISSN: 1023-8883

The thermal conductivity is a key property in determining the friction-induced temperature rise on the surface of sliding components. In this study, a Frequency Domain Thermoreflectance (FDTR) method is used to measure the thermal conductivity of a range of tribological materials (AISI 52100 bearing steel, silicon nitride, sapphire, tungsten carbide and zirconia). The FDTR technique is validated by comparing measurements of pure germanium and silicon with well-known values, showing discrepancies of less than 3%. For most of the tribological materials studied, the thermal conductivity values measured are reasonably consistent with values found in the literature. However the measured thermal conductivity of AISI 52100 steel (21 W/mK) is less than half the value cited in the literature (46 W/mK). Further bulk thermal conductivity measurements show that this discrepancy arises from a reduction in thermal conductivity of AISI 52100 due to through-hardening. The thermal conductivity value generally cited and used in the literature represents that of soft, annealed alloy, but through-hardened AISI 52100, which is generally employed in rolling bearings and for lubricant testing, appears to have a much lower thermal conductivity. This difference has a large effect on estimates of flash temperature and example calculations show that it increases the resulting surface temperatures by 30 to 50%. The revised value of thermal conductivity of bearing steel also has implications concerning heat transfer in transmissions.

Journal article

Hu S, Reddyhoff T, Wen J, Huang W, Shi X, Dini D, Peng Zet al., 2019, Characterization and simulation of bi-Gaussian surfaces induced by material transfer and additive processes, Tribology International, ISSN: 0301-679X

Journal article

Wen J, Dini D, Reddyhoff T, 2019, Design and optimization of a liquid ring thrust bearing, Tribology International, ISSN: 0301-679X

Liquid menisci at millimeter length scales and smaller exhibit large Laplace pressures. To utilise these effects, liquid ring bearings have recently been developed, which consist of liquid rings confined between alternate superhydrophobic and hydrophilic patterns. We present a detailed experimental and theoretical performance analysis of such bearings. For a single, 100 μm thickness, liquid ring, the maximum supporting force is 0.13 N, which decreases with increasing the ring misalignment. The frictional torque increases linearly with rotational speed until a critical Reynolds number is reached. Above this, an instability occurs due the concave liquid ring meniscus, which further increases friction. These results show how liquid ring bearings can be optimised.

Journal article

Puhan D, Nevshupa R, Wong J, Reddyhoff Tet al., 2019, Transient aspects of plasma luminescence induced by triboelectrification of polymers, Tribology International, Vol: 130, Pages: 366-377, ISSN: 0301-679X

Transient electric gas discharges that occur around sliding interfaces during contact electrification of polymers were studied at millisecond timescales and with micrometre resolution. Deduced vibrational temperatures indicate cold plasma resulting from positive corona discharge. At millisecond timescales, previously unseen rapid discharge events are observed, and modelling suggests that these result from streamer development, triggered by electron emission from the polymer surface. Those which occur over a period of several seconds are shown to be caused by competition between charge generation and the formation of polymer films. The findings explain the interplay between charging and plasma generation and their dependence on wear processes.

Journal article

Hartinger M, Reddyhoff T, 2018, CFD modeling compared to temperature and friction measurements of an EHL line contact, Tribology International, Vol: 126, Pages: 144-152, ISSN: 0301-679X

In this paper, predictions from CFD modeling are compared against measurements of surface temperatures and friction for an EHL line contact lubricated with the fluid Santotrac 50. Two slide-to-roll-ratios (SRR), 50% and 100%, and entrainment velocities ranging from 0.211 to 1.13 m/s are considered. Very good agreement is shown for the 50% SRR cases, with only a 3% deviation in friction coefficient values. At 100% SRR, the deviation in friction increases to 3–7% which is attributed to deficiencies in the modeling approach with regard to shear-thinning. The temperature profiles agree reasonably well at 50% SRR and show larger deviations at 100% SRR. For all cases, the formation of a shear-band in the center of the fluid film is predicted. This is very pronounced for 100% SRR, although likely to be over-estimated by this CFD-approach. The data presented here serve as a basis from which further refinements in the modeling and measurements shown can be made.

Journal article

Reddyhoff T, Underwood R, Sayles R, Spikes Het al., 2018, Temperature measurement of debris particles in EHL contacts, Surface Topography: Metrology and Properties, Vol: 6, ISSN: 2051-672X

Dents caused by entrained debris are now the main cause of fatigue failure in rolling element bearings. It is therefore important to be able to understand and predict the deformation behaviour of particles in elastohydrodynamic contacts. This paper describes a new method to study debris entrainment. This uses a sensitive infrared microscope to map the temperature of a contact between a steel ball and coated sapphire disc as lubricant dispersed with bearing dust is entrained. Full-field thermal maps were acquired at a sufficient rate to monitor the deformation of a single particle on its journey through the contact.Under the low-speed, high-sliding conditions studied, the temperature rise increases from when the particle is trapped by the inlet to reach a peak near the contact centre, where shearing is a maximum. Under these conditions, temperature rises are typically of the order of 10 °C, which is significantly lower than has been predicted theoretically. Even lower temperature rises were observed under pure rolling conditions, since minimal shearing occurs.Experimental results are also compared with existing models used to predict particle behaviour. Measured radiation distributions confirm qualitatively the ductile particle deformation mechanisms originally proposed by Hamer et al.

Journal article

Hili J, Pelletier C, Jacobs L, Olver A, Reddyhoff Tet al., 2018, High-Speed Elastohydrodynamic Lubrication by a Dilute Oil-in-Water Emulsion, Tribology Transactions, Vol: 61, Pages: 287-294, ISSN: 1040-2004

© 2018 Society of Tribologists and Lubrication Engineers. When a concentrated contact is lubricated at low speed by an oil-in-water emulsion, a film of pure oil typically separates the surfaces (stage 1). At higher speeds, starvation occurs (stage 2) and the film is thinner than would be expected if lubricated by neat oil. However, at the very highest speeds, film thickness increases again (stage 3), though little is known for certain about either the film composition or the mechanism of lubrication, despite some theoretical speculation. In this article, we report the film thickness in a ball-on-flat contact, lubricated by an oil-in-water emulsion, at speeds of up to 20 m/s, measured using a new high-speed test rig. We also investigated the sliding traction and the phase composition of the film, using fluorescent and infrared microscopy techniques. Results show that, as the speed is increased, starvation is followed by a progressive change in film composition, from pure oil to mostly water. At the highest speeds, a film builds up that has a phase composition similar to the bulk emulsion. This tends to support the “microemulsion” view rather than the “dynamic concentration” theory.

Journal article

Lu J, Reddyhoff T, Dini D, 2017, 3D Measurements of Lubricant and Surface Temperatures Within an Elastohydrodynamic Contact, Tribology Letters, Vol: 66, ISSN: 1023-8883

We present an infrared microscopy technique, capable of measuring the temperature of both the bounding surfaces and the oil film in an elastohydrodynamic contact. This technique can, for the first time, spatially resolve the oil film temperature in three dimensions. The contact is produced by loading a steel ball against a sapphire disc, and the film is viewed using an infrared microscope focussing through the disc. Two band pass filters are used to isolate the radiation from the oil film, and Planck’s law is applied to data obtained at a known temperature as part of the calibration procedure. The proposed technique requires the emissivity of the oil film to be measured, which is acquired in situ and is shown to vary strongly as a function of thickness and temperature. The technique is validated under pure rolling conditions, when the temperature of the oil film is equal to the controlled lubricant reservoir temperature, and also compared to an equation commonly used to predict average film temperatures, confirming the value of the unknown constant. The technique is then used to gain insights into the thermal/rheological behaviour within a contact. This is important since the temperature of elastohydrodynamic contacts is critical in determining friction and hence the efficiency of machine components and this technique enables much needed validation and provides input data for CFD and numerical simulations.

Journal article

Vlădescu S-C, Ciniero A, Tufail K, Gangopadhyay A, Reddyhoff Tet al., 2017, Optimisation of Pocket Geometry for Friction Reduction in Piston-Liner Contacts, Tribology Transactions, Pages: 00-00, ISSN: 1040-2004

Journal article

ciniero A, Le-Rouzic J, Reddyhoff T, 2017, The Use of Triboemission Imaging and Charge Measurements to Study DLC Coating Failure, Coatings, Vol: 7, ISSN: 2079-6412

We present a study on the simultaneous evolution of the electron emission and surface charge accumulation that occurs during scratching tests in order to monitor coating failure. Steel discs coated with a diamond-like-carbon (DLC) film were scratched in both vacuum (~10−5 Torr) and atmospheric conditions, with electron emission and surface charge being measured by a system of microchannel plates and an electrometer, respectively. The results highlight a positive correlation between emission intensity values, surface charge measurements and surface damage topography, suggesting the effective use of these techniques to monitor coating wear in real time.

Journal article

Vladescu S, Ciniero A, Tufail K, Gangopadhyay A, Reddyhoff Tet al., 2017, Looking into a laser textured piston ring-liner contact, Tribology International, Vol: 115, Pages: 140-153, ISSN: 1879-2464

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

Journal article

Profito FJ, Vladescu S-C, Reddyhoff T, Dini Det al., 2017, Experimental validation of a mixed-lubrication regime model for textured piston-ring-liner contacts, Materials Performance and Characterization, Vol: 6, Pages: 112-129, ISSN: 2165-3992

Recent experiments have shown that automotive piston-liner friction may be reduced by up to 50 % if the surface of the liner is laser textured with certain configurations of micro-pockets. It is important to model this behavior to understand and optimize the friction reduction mechanisms that are occurring. However, until now, very few models that predict the lubrication performance of textured surfaces have been successfully validated against experimental data. This is because of the requirement for them to: (1) reproduce experimental configurations with a certain degree of fidelity, (2) conserve mass properly, and (3) account for transient, boundary lubrication conditions. To address this, the current paper presents a comparison between the results from a numerical model, which fulfils these criteria, and an experimental test rig operating under the same conditions. The mathematical modeling is based on the averaged Reynolds’ equation with Patir and Cheng’s flow factors and the p − θ Elrod–Adams mass-conserving cavitation model. Simultaneously to the fluid flow solution, the contact pressures that arise from the asperity interactions are also included into the calculations through the well-known stochastic Greenwood and Tripp model for rough contacts. The experimental data is produced using a reciprocating tribometer, whose contact conditions are closely controlled and accurately mimic those found in an automotive piston–liner conjunction. Data is presented in terms of friction force versus stroke angle, and the similarities and differences between the model and experiment are discussed.

Journal article

Vladescu S, Reddyhoff T, Olver A, Pegg IGet al., 2017, Bearing interface with recesses to reduce friction, US20170009886 A1

A bearing interface of an apparatus, the apparatus having a first element and a second element configured to move relative to each other during operation of the apparatus, the first element comprising a first bearing surface configured to engage at least a portion of a second bearing surface of the second element thereby defining a contact zone between the first bearing surface and the second bearing surface, the first bearing surface having at least one recess indented into the first bearing surface, wherein the dimension of the recess in ...

Patent

Ciniero A, Le Rouzic J, Baikie I, Reddyhoff Tet al., 2017, The origins of triboemission - correlating electron emission with surface damage, Wear, Vol: 374-375, Pages: 113-119, ISSN: 1873-2577

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 (10−5 Torr), 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.

Journal article

Wang P, Zhang J, Spikes HA, Reddyhoff T, Holmes ASet al., 2016, Development of hydrodynamic micro-bearings, 16th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2016), Publisher: IOP Publishing, ISSN: 1742-6588

This paper describes the modelling and testing of mm-scale hydrodynamic bearings which are being developed to improve the efficiency of a cm-scale turbine energy harvester, whose efficiency was previously limited by poorly lubricated commercial jewel-bearings. The bearings were fabricated using DRIE and their performance was assessed using a custom built MEMS tribometer. Results demonstrate that acceptably low friction is achieved when low viscosity liquid lubricants are used in combination with an appropriate choice of friction modifier additive. Further reduction in friction is demonstrated when the step height of bearing is adjusted in accordance with hydrodynamic theory. In parallel with the experiments, hydrodynamic lubricant modelling has been carried out to predict and further optimize film thickness and friction performance. Modelling results are presented and validated against experimental friction data.

Conference paper

Profito FJ, Vladescu S, Reddyhoff, Dini Det al., 2016, Transient experimental and modelling studies of laser-textured micro-grooved surfaces with a focus on piston-ring cylinder liner contacts, Tribology International, Vol: 113, Pages: 125-136, ISSN: 1879-2464

This paper presents a comparison between the results from numerical modelling and experiments to shed light on the mechanisms by which surface texture can reduce friction when applied to an automotive cylinder liner. In this configuration, textured features move relative to the piston-liner conjunction and to account for this our approach is to focus on the transient friction response to individual pockets as they pass through, and then leave, the sliding contact. The numerical approach is based on the averaged Reynolds’ equation with the Patir & Cheng’s flow factors and the p-θ Elrod-Adams mass-conserving cavitation model. The contact pressures that arises from the asperity interactions are solved simultaneously to the fluid flow solution using the Greenwood and Tripp method. The experimental data is produced using a pin-on-disc set up, in which laser textured pockets have been applied to the disc specimen. Under certain conditions in the mixed and boundary lubrication regimes, both model and experimental results show i) an increase in friction as the pocket enters the contact, followed by ii) a sharp decrease as the pocket leaves the contact, and then iii) a gradual decay back to the pre-entrainment value. From the evidence obtained for the first time from the proposed combined modelling and experimental investigation conducted under carefully controlled conditions, we suggest that these three stages occur due to the following mechanisms: i) a reduction in fluid pressure due to the increased inlet gap, ii) inlet suction as the cavitated fluid within the pocket draws lubricant into the contact, and iii) film thickness decay as oil is squeezed out of the contact. The interplay of these three mechanisms is shown to control the response of micro-textured surfaces und

Journal article

wang P, Reddyhoff T, 2016, Wall slip in an EHL contact lubricated with 1-dodecanol, Tribology International, Vol: 113, Pages: 197-205, ISSN: 1879-2464

Astudy was conducted ofthe factors that cause anomalous EHDfilm shapes in 1-dodecanol lubricated contacts –i.e.a thickened central plateau and an exaggerated outlet constriction. This has ledto the hypothesisthat, in the region wherepressuresexceed~0.44 GPa, dodecanol experiences slip, which reduces the average lubricant velocityand causesan elevated film thickness in order to maintain flow continuity. The analysis suggests that slip mustbeoccurringat, or closeto, the faster moving surface, since a discontinuityin the flowmidway through thethickness of the film wouldnot affectthe average speedof the lubricant. Based on this hypothesis, an equation has beenderived to predict the anomalous central film thickness as a function of applied slide-roll ratio and this has beenvalidated by comparison with experimentally measured central film thickness values. Further support of this hypothesis is provided by traction test results showing a discontinuity occurring close tothe onset of slip.

Journal article

Vladescu S-C, Olver AV, Pegg IG, Reddyhoff Tet al., 2016, Combined friction and wear reduction in a reciprocating contact through laser surface texturing, Wear, Vol: 358-359, Pages: 51-61, ISSN: 0043-1648

Journal article

Vladescu S-C, Medina S, Olver AV, Pegg IG, Reddyhoff Tet al., 2016, The Transient Friction Response of a Laser-Textured, Reciprocating Contact to the Entrainment of Individual Pockets, Tribology Letters, Vol: 62, ISSN: 1573-2711

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

Journal article

Putignano C, Reddyhoff T, Dini D, 2016, The influence of temperature on viscoelastic friction properties, Tribology International, Vol: 100, Pages: 338-343, ISSN: 0301-679X

Viscoelastic friction strongly depends on temperature, which determines the material stiffness and, therefore, given a constant load, the volume that is deformed and dissipates energy. We compare the results obtained by a numerical approach introduced by Carbone and Putignano (2013) [1] with measurements that separate viscoelastic losses from Coulomb contribution. This is done for a range of temperatures. We show that viscoelastic friction curves for different temperatures can be arranged into a single master curve using a frequency shift coefficient, which can be found from the characterization of the viscoelastic material response. This shows that it is possible to accurately (a) use dynamic material analysis data to extrapolate viscoelastic friction measurements to values outside the tested range, and (b) use a tribometer to obtain fundamental viscoelastic material properties.

Journal article

Vladescu S, medina S, olver A, Pegg I, Reddyhoff Tet al., 2016, Lubricant film thickness and friction force measurements in a laser surfacetextured reciprocating line contact simulating the piston ring – linerpairing, Tribology International, Vol: 98, Pages: 317-329, ISSN: 1879-2464

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 <50 nanometres to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20 nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing

Journal article

Leong JY, Zhang J, Sinha SK, Holmes A, Spikes H, Reddyhoff Tet al., 2015, Confining Liquids on Silicon Surfaces to Lubricate MEMS, Tribology Letters, Vol: 59, ISSN: 1573-2711

Liquid lubrication may provide a solution tothe problem of high friction and wear in micro-electromechanicalsystems. Although the effectiveness of thisapproach has been demonstrated in laboratory-based frictiontests, practical constraints prevent it from being appliedin commercial devices. The main problem is how toposition the lubricant on a silicon surface in order to limitspreading and evaporation. This paper describes twotechniques to address this issue. First, low concentrationsof additives are used to promote autophobic behaviour.Tests’ results show that certain concentrations of bothmultiply alkylated cyclopentane and amine additives areeffective in halting the spread of hexadecane on silicon,and, in the latter case, cause the hexadecane drop to subsequentlyretract. The second approach involves applying amicro-contact printing technique previously used on goldsurfaces. Here, silicon surfaces are coated with octadecyltrichlorosilanemono-layers that are then selectively removed,using oxygen plasma, to leave regions ofcontrasting surface energy. Results from spin tests showthat surfaces treated in this way can anchor 1 ll drops ofhexadecane and water when forces of up to 22 and 230 lN,respectively, are applied.

Journal article

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