99 results found
Bahshwan M, Myant CW, Reddyhoff T, et al., 2020, The role of microstructure on wear mechanisms and anisotropy of additively manufactured 316L stainless steel in dry sliding, Materials and Design, Vol: 196, ISSN: 0264-1275
Wear control, which relies on understanding the mechanisms of wear, is crucial in preserving the life of mechanical components and reducing costs. Additive manufacturing (AM) techniques can produce parts with tailored microstructure, however, little has been done to understand how this impacts the mechanisms of wear. Here we study the impact of initial grain arrangement and crystal orientation on the wear mechanisms of austenitic stainless steel (SS) in dry sliding contact. Specifically, the anisotropic sliding wear behavior of as-built, AM-ed 316L SS is compared against annealed, wire-drawn counterparts. We describe, in-detail, how the sliding wear mechanisms of delamination, abrasion, oxidation, and plastic deformation are attributed to the initial surface microstructure under different loading conditions using a number of techniques. This new understanding sheds light on how different AM-induced microstructures affect wear, thereby allowing for better utilization of this technology to develop components with enhanced wear properties.
Yu M, Shen L, Mutasa T, et al., 2020, Exact analytical solution to ultrasonic interfacial reflection enabling optimal oil film thickness measurement, Tribology International, Vol: 151, Pages: 1-10, ISSN: 0301-679X
The ultrasonic reflection from a lubricated interface has been widely analyzed to measure fluid film thickness, with different algorithms being applied to overcome measurement accuracy and resolution issues. Existing algorithms use either the amplitude or the phase angle of the ultrasonic interfacial reflection. In this paper, a new algorithm (named the “exact model – complex”) that simultaneously utilizes both the amplitude and the phase of the complex ultrasonic reflection coefficient is proposed and mathematically derived. General procedures for theoretical analysis in terms of measurement accuracy and uncertainty are proposed and applied to the new algorithm, the beneficial features of which (as compared to other existing algorithms) can be summarized as: 1) a direct calculation, instead of an iterative approximation, 2) guaranteed maximum measurement accuracy, and 3) acceptable measurement uncertainty. None of the existing methods have showed this combination of benefits. Moreover, two groups of raw data from previous experimental studies are utilized to further validate the practical feasibility of the new algorithm. Overall, the proposed “exact model – complex” algorithm fully exploits the potential of ultrasonic reflection for oil film thickness measurement, with an accurate and a convenient calculation suited to practical implementation.
Vlădescu S-C, Bozorgi S, Hu S, et al., 2020, Effects of beverage carbonation on lubrication mechanisms and mouthfeel, Journal of Colloid and Interface Science, ISSN: 0021-9797
Wen J, Dini D, Reddyhoff T, 2020, Design and optimization of a liquid ring thrust bearing, Tribology International, Vol: 149, 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.
Hu S, Cao X, Reddyhoff T, et al., 2020, Liquid repellency enhancement through flexible microstructures, Science Advances, Vol: 6, Pages: 1-7, ISSN: 2375-2548
Artificial liquid-repellent surfaces have attracted substantial scientific and industrial attention with a focus on creating functional topological features; however, the role of the underlying structures has been overlooked. Recent developments in micro-nanofabrication allow us now to construct a skin-muscle type system combining interfacial liquid repellence atop a mechanically functional structure. Specifically, we design surfaces comprising bioinspired, mushroom-like repelling heads and spring-like flexible supports, which are realized by three-dimensional direct laser lithography. The flexible supports elevate liquid repellency by resisting droplet impalement and reducing contact time. This, previously unknown, use of spring-like flexible supports to enhance liquid repellency provides an excellent level of control over droplet manipulation. Moreover, this extends repellent microstructure research from statics to dynamics and is envisioned to yield functionalities and possibilities by linking functional surfaces and mechanical metamaterials.
Hu S, Reddyhoff T, Puhan D, et al., 2020, Droplet manipulation of hierarchical steel surfaces using femtosecond laser fabrication, Applied Surface Science, Vol: 521, Pages: 146474-146474, ISSN: 0169-4332
Lu J, Reddyhoff T, Dini D, 2020, A study of thermal effects in EHL rheology and friction using infrared microscopy, Tribology International, Vol: 146, Pages: 106179-106179, ISSN: 0301-679X
Wen J, Reddyhoff T, Hu S, et al., 2020, Exploiting air cushion effects to optimise a superhydrophobic/hydrophilic patterned liquid ring sealed air bearing, Tribology International, Vol: 144, Pages: 106129-106129, ISSN: 0301-679X
Hu S, Cao X, Reddyhoff T, et al., 2019, Self-compensating liquid repellent surfaces with stratified morphology, ACS Applied Materials and Interfaces, Vol: 12, Pages: 4174-4182, ISSN: 1944-8244
Artificial liquid repellent surfaces have recently attracted vast scientific attention; however, achieving mechanical robustness remains a formidable challenge before industrialization can be realized. To this end, inspired by plateaus in geological landscapes, a self-compensating strategy is developed to pave the way for the synthesis of durable repellent surfaces. This self-compensating surface comprises tall hydrophobic structural elements, which can repel liquid droplets. When these elements are damaged, they expose shorter structural elements that also suspend the droplets and thus preserve interfacial repellency. An example of this plateau-inspired stratified surface was created by 3D direct laser lithography micro-nano fabrication. Even after being subjected to serious frictional damage, it maintained static repellency to water with a contact angle above 147 and was simultaneously able to endure high pressures arising from droplet impacts. Extending the scope of nature-inspired functional surfaces from conventional biomimetics to geological landscapes, this works demonstrates that the plateau-inspired self-compensating strategy can provide an unprecedented level of robustness in terms of sustained liquid repellency.
Ciniero A, Fatti G, Righi MC, et al., 2019, A combined experimental and theoretical study on the mechanisms behind tribocharging phenomenon and the influence of triboemission, Tribology Online, Vol: 14, Pages: 367-374, ISSN: 1881-218X
This work describes recent research into the mechanisms behind tribocharging and the influence of triboemission. The term tribocharging is a type of contact-induced electrification and refers to the transfer of charge between rubbing components. The term triboemission, on the other hand, refers to emission of electrons, ions and photons generated when surfaces are rubbed together. The understanding of tribocharging is of wide interest for several industrial applications and in particular the combination of tribocharging and triboemission may be important in lubricated contacts in the formation of boundary lubricant films. We report the use of a unique vacuum measurement system that enables to measure surface charge variations while simultaneously recording triboemission events during the sliding of a diamond tip on silica specimens. Results show for the first time that tribocharging and triboemission behavior are linked and depend on the surface wear. The contribution of contact-induced electrification to the charging of the surface is then described by means of density functional theory (DFT). Results give insight into the transfer of charge from the SiO2 amorphous surface (silica) to the C(111) surface (diamond ) and into the variation of charging during simulated sliding contact.
Reddyhoff T, 2019, LUBRICANT COMPOSITIONS, WO2019193368
Tufail K, Gangopadhyay AK, Skipp D, et al., 2019, A bore portion for receiving a reciprocating piston, US20200095951
Carpenter G, Bozorgi S, Vladescu S, et 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
Hu S, Cao X, Reddyhoff T, et 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.
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.
Vladescu S-C, Fowell M, Mattsson L, et 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
Geng Z, Shi G, Shao T, et al., 2019, Tribological behavior of patterned TiAlN coatings at elevated temperatures, SURFACE & COATINGS TECHNOLOGY, Vol: 364, Pages: 99-114, ISSN: 0257-8972
Hu S, Vladescu S-C, Puhan D, et al., 2019, Bi-Gaussian stratified theory to understand wettability on rough topographies, Surface and Coatings Technology, ISSN: 0257-8972
Hu S, Reddyhoff T, Puhan D, et 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.
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.
Vladescu S-C, Putignano C, Marx N, et 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
Hu S, Reddyhoff T, Wen J, et al., 2019, Characterization and simulation of bi-Gaussian surfaces induced by material transfer and additive processes, Tribology International, ISSN: 0301-679X
Puhan D, Nevshupa R, Wong J, et 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.
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.
Reddyhoff T, Underwood R, Sayles R, et 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.
Hili J, Pelletier C, Jacobs L, et 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.
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.
Vlădescu S-C, Ciniero A, Tufail K, et al., 2017, Optimisation of Pocket Geometry for Friction Reduction in Piston-Liner Contacts, Tribology Transactions, Pages: 00-00, ISSN: 1040-2004
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.
Vladescu S, Ciniero A, Tufail K, et 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
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