Oil-lubricated Contacts Research Articles

Influence of displacement amplitude on fretting-induced friction and wear of steel in oil-lubricated contact

Many fretting-prone contacts are surrounded by oils, although they are not intended to lubricate them. To study the fretting behavior of contacts whose edge is exposed to engine oil, self-mated 34CrNiMo6 +QT steel was employed with a large annular flat-on-flat contact. A displacement-controlled loading was tested in a range, encompassing stick and gross sliding. No stick-to-slip transition with displacement amplitude was observed up to a tangential-to-normal traction ratio of 1.6, compared to that of 0.5 in dry contact. Beyond that, a typical peak-to-stabilized friction curve was reached in oil with a steady-state value of roughly 0.4, lower than that of dry contact. Adhesive wear existed as the dominant wear mechanism, and the severity of adhesion increased with higher loading.

Wear Behaviors of Stainless Steel and Lubrication Effect on Transitions in Lubrication Regimes in Sliding Contact

The wear behavior of AISI304 stainless steel was investigated under dry, water-, and oil-lubricated conditions. A block-on-disk wear test was conducted in this work, since the test conditions could be controlled easily. For oil-lubricated contact, a significant amount of thin and elongated cutting chip-like debris was observed. This is attributed to the high lubricating effect of oil. Strain-induced martensitic (SIM) transformation was observed for all AISI304 blocks after the wear test, while AISI304 consisted of a single γ-austenite phase prior to the wear test. The Stribeck curve and the corresponding lubrication regimes were also considered to explain the wear behaviors and lubrication effect of AISI304. In comparison to the dry or water-lubricated conditions, which fall in the boundary lubrication regime at a low rotation speed, it is considered that the high viscosity of the oil-based lubricant causes the lubrication condition to enter the mixed lubrication regime early at a lower speed, thus reducing the specific wear rate over the 100–300 rpm range.

Guest Editorial: Special Issue on Magnetically Levitated Motor Systems

Magnetically levitated motor systems based around bearingless motors and generators and magnetic bearings offer a potential step change improvement in motor system lifetime and efficiency, solve critical challenges related to oil-lubricated contact bearings, and feature prognostic capabilities for condition-based maintenance. Acceptance of this technology has been historically limited by challenges related to cost, design and control complexity, manufacturing issues, and low force density. However, recent advances in supporting technologies enable new approaches to address these challenges.

The multiple roles of a chemical tribofilm in hydrogen uptake from lubricated rubbing contacts

A newly developed in-situ technique has been employed to investigate the effect of lubricant additives and water contamination in oil on hydrogen intrusion into the steel. Hydrogen in oil-lubricated contacts is generated as a result of tribochemical reactions. The hydrogen uptake measurement results indicated that tribofilm formation can impede hydrogen ingress, in the substrate, by impeding the formation of fresh metal surfaces. This reduces the generation of hydrogen atoms by preventing the decomposition of hydrocarbon molecules. A uniform tribofilm across the wear track also acts as a physical barrier for hydrogen permeating through the tribo-contact. The water contamination in the oil negatively influenced tribofilm properties and therefore led to higher permeation of hydrogen into the steel.

Tribofilms Forming in Oil-Lubricated Contacts

The subject of the present paper is the characterization of third bodies of run-in systems. By means of continuous friction and wear measurement, lubricated steel-steel and steel-aluminum contacts were evaluated. Microstructure, chemical composition and response of the materials to shear were analyzed by XPS/AES and focused ion beam technique. After a proper running-in, both systems developed a third body. The third body differs significantly from the base materials. In addition to adapted microstructure and near-surface chemistry, the third body exhibited a substructure characterized by a near-surface zone that accommodates shear and a second, deeper region that ensures strengthening.

The Effect of Wetting and Surface Energy on the Friction and Slip in Oil-Lubricated Contacts

This work shows the influence of solid–liquid interactions between engineering surfaces (steel and several types of DLC coatings) and lubricating oil (polyalphaolefin, PAO) on the coefficient of friction in the elastohydrodynamic lubrication (EHL) regime. Specifically, it confirms that the spreading parameter, rather than the contact angle, is the relevant parameter to evaluate the wetting behaviour of these surfaces with oils. Both the spreading parameter and the surface energy correlate very well with the friction in the EHL regime and can predict its behaviour. In particular, the polar component of the surface energy was found to correlate almost perfectly with the friction behaviour (a Pearson’s linear correlation coefficient of 0.999). By tailoring the wetting and surface energy—achieved by varying the DLC/DLC contacts with different types of DLC coatings—the coefficient of friction in the EHL regime was reduced by more than 30 % compared to steel/steel contacts. Poor wetting of the DLC coatings with a low surface energy is reflected in low values of the spreading parameter, which indicates easier slip of the lubricant over the solid surface due to shear action, and this leads to a lower viscous friction. A “Slip-inducing interaction model based on surface forces” is presented to explain why oil slip is promoted, particularly at surfaces with a low polar surface energy. The model suggests that a small number of permanent polar interactions, i.e. a larger proportion of intermittent dispersive interactions, results in less adhesive interactions between the predominantly non-polar liquid (oil) and the low polar surface (DLC), which enables easier slip at the solid–liquid interface.

Performance Characteristics of Jet Fuel in Heavily Loaded Contacts

Friction coefficient, bearing performance (i.e., torque generation and temperature rise), and rolling contact fatigue of steel and ceramic (silicon nitride) materials operating in the presence of JP-8+100 jet fuel and Mil-L-23699 turbo jet oil were investigated. A ball-on-disk test machine was used to measure the coefficient of friction of a steel ball on a steel disk operating under various loads and slide-to-roll ratios when lubricated with JP-8+100 jet fuel. The results indicate that the fuel provides an extremely low friction coefficient under the conditions considered in this investigation. A three-ball-on-rod rolling contact fatigue testing machine was used to determine the fatigue life for different combinations of materials (i.e., silicon nitride and steels) and lubricants. As expected, the Weibull analysis indicates that the rolling fatigue life operating in the presence of jet fuel was less than that of an oil-lubricated contact. A bearing test machine was also designed, developed, and used to obtain the friction (torque) and the temperature characteristics of bearings operating in jet fuel and oil. The result from the bearing rig indicates that the fuel has a lower generated torque and temperature rise than oil.

The plasma generated and photons emitted in an oil-lubricated sliding contact

Intensive work has long been going on to find out the unknown origin that sets off curious tribo-physicochemical phenomena and that causes various kinds of problems in oil-lubricated sliding contacts in mechanical and processing systems. The strange tribochemical reaction is one of the such curious chemical phenomena observed in the degradation of perfluoropolyether (PFPE) lubricating oil film in a hard disk drive. Plasma (triboplasma) (Nakayama and Mirza 2006 Tribol. Trans. 49 17) would be one of the most probable origins of the problems if it were generated sufficiently intensely in oil-lubricated sliding contacts, as it is in such a highly energetic state. The generation of plasma was predicted in both dry and oil-lubricated sliding (Nakayama 1997 Japan. J. Tribol. 42 1077, Nakayama 2004 Surf. Coat. Technol. 188–189 599). However, plasma generation in industrially important oil-lubricated contacts has not yet been proven, though it has been found in dry sliding (Nakayama and Nevshupa 2002 J. Phys. D: Appl. Phys. 35 L53). Here the author reports the discovery of the predicted plasma generated in the rear part of an oil-lubricated sliding contact, together with a new finding on photon emission phenomena from inside and outside of the sliding contact in the tribosystem of a diamond pin sliding against a sapphire disc. The plasma in oil-lubricated sliding has a simple ellipse shape, while that in dry sliding has a horseshoe pattern. Visible and infrared (IR) photons are emitted from oil-lubricated sliding contact, while ultraviolet (UV), visible and IR photons are emitted from the region of plasma generation as well as from dry sliding. The UV photon spectra emitted under oil-lubrication completely coincided with those in dry sliding and air discharge, demonstrating that plasma is generated by air discharge even under oil-lubrication. Two kinds of new concepts have been proposed. One is plasma generation by discharge of air through bubble formation in decompressed oil in the rear part of the sliding contact. The other is photon emission from surface atoms and oil molecules excited by the attack of electrons accelerated in the intense tribocharge-induced electric field of the asperity gap inside the sliding contact under dry and oil-lubricated slidings. The size and intensity of the plasma and photon emission from the sliding contact under oil-lubrication increases greatly with the applied normal force and the sliding velocity.

Real-time monitoring of wear debris using electrostatic sensing techniques

In this article, electrostatic charge sensing technology has been used to monitor adhesive wear in oil-lubricated contacts. Previous work in this area using FZG gear wear rig and pin-on-disc tribometers demonstrated that ‘precursor’ charge events may be detected prior to the onset of scuffing. Possible charging mechanisms associated with the precursor events were identified as tribocharging, surface charge variation, exo-emissions, and debris generation. This article details tests carried out to investigate the contribution of wear debris. Tests were carried out on a modified pin-on-disc rig using a sliding point contact and fitted with electrostatic sensors, one of which monitored the disc wear track and the other the disc surface just outside the wear track. Baseline tests used mild wear conditions with no seeded particles added to the entrained lubricant, whereas the high wear tests entrained seeded steel particles into the contact to promote wear. The wear debris produced dynamic charge features and the overall charging activities are directly related to the wear rate (i.e. charging levels increase with increasing wear rate). There appears to be a link between the net volume loss and the charge levels, relating charge directly to the increasing rate of debris production. Wear debris due to natural wear produced positive dynamic charge features, whereas debris from the seeded tests produced negative dynamic charge features. The polarity of the charge on debris is thought to depend on which charging and wear mechanism is predominant.

The transition between fully flooded and starved regimes in EHL

Lubricant starvation of the contact can occur in high-speed oil or grease lubricated bearings resulting in reduced elastohydrodynamic (EHL) film thickness (Lubricat. Sci. 11 (1999) 227). To achieve optimum bearing performance and component life, it is obviously desirable to be able to predict when starvation will occur and the resulting level of surface protection. For both oils and greases, the transition between fully flooded and starved behaviour is determined by lubricant loss and replenishment of the track (ASME Trans. J. Tribol. 120 (1998) 126; Cann PME, Chevalier F, Lubrecht AA. Track depletion and replenishment in a grease lubricated point contact: a quantitative analysis. Proceedings of the 23rd Leeds–Lyon Symposium on Tribology. 1996. p. 405–14). The current paper develops a criterion for the fully flooded and starved transition of an oil-lubricated contact based on four primary factors controlling lubrication level; these are volume of oil, contact dimensions, oil viscosity and speed. Experiments have been carried out using an optical EHL device to investigate the effect of each of these parameters on starvation. The results show how the boundary between the fully flooded and the starved regime and the film thickness in the starved regime depend on these parameters. A single dimensionless parameter (SD), based on replenishment local to the contact, has been established between the operating parameters and the transition from the fully flooded to starved regime. In this paper, the starvation criterion has been developed for an oil-lubricated contact with a fixed volume of oil present. The next stage is to extend the analysis to grease and the prediction of lubrication failure in bearings.

The effects of surface defects on the fatigue of water-and oil-lubricated contacts

A study into effects of surface defects on the rolling contact fatigue of brass and rail steel has been undertaken on a twin-disc rolling-sliding test machine with both oil and water lubrication. Furrows and dents were artificially introduced into the disc surfaces, and surface microcracks and pits were monitored by means of surface replication. The results showed that artificial dents only reduce the fatigue life of the contact with oil, but not water lubrication. With oil lubrication the fatigue failure initiates at the surface defect. However, with water as a lubricant the whole of the surface undergoes cracking with the defect having no preferential effect. The possible mechanisms behind this behaviour are discussed in this paper.

Interactions in silicon nitride ceramics vs. steel contact under fretting conditions

The wear of silicon nitride and bearing steel under oil-lubricated fretting conditions was investigated and the mechanisms involved studied. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and Auger spectroscopy (AES) were employed to analyse the surface layers of the wear scars. The increased temperature at the contact of ceramic balls and steel plates induced tribochemical reactions of the material with chemical species from the environment. In contrast to the tribolayer created under non-lubricated conditions where oxidation of silicon nitride and steel is a wear determining mechanism, at oil-lubricated contacts carbonisation of steel due to oil decomposition takes place.

Coatings tribology—contact mechanisms and surface design

The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.

Soap-Thickener Induced Local Pressure Fluctuations in a Grease-Lubricated Elastohydrodynamic Point Contact

In several experimental investigations of grease-lubricated elastohydrodynamic (EHD) contacts indications of soap-thickener formations that enter the contact area have been reported, for example by Kageyama et al. (1), Cann and co-workers (2-4) and Åström and co-workers (5, 6). While passing through a contact such soap-thickener lumps significantly disturb the film thickness by locally increasing the surface deformation. These film-thickness fluctuations must be accompanied by pressure fluctuations, knowledge of which is essential to increase insight in the phenomena determining service life and emitted noise of grease-lubricated contacts. In this paper the authors present a combined experimental/numerical approach to generate insight into such pressure fluctuations. From a fringe pattern obtained with optical interferometry (ball-on-disc apparatus) a film-thickness map is created employing image analysis. This map serves as input to a numerical algorithm for the calculation of the pressure from force balance and elastic deformation theory. Consequently, no assumptions about the rheological behaviour of grease can be made. The method was first tested out on an oil-lubricated contact. This test gave essential insight into the accuracy of the method proposed here and in the magnitude of surface texture induced pressure fluctuations. Subsequently the approach was successfully used to estimate the pressure variations resulting from soap-thickener formations in a grease-lubricated contact (between the same ball and disc).