Knowledge

Micropitting is a type of surface fatigue damage that occurs in rolling-sliding contacts operating under thin oil film conditions. It is caused by stress fluctuations, brought about by surface asperity interactions, which lead to initiation and propagation of numerous surface fatigue cracks and subsequent loss of material. Despite its increasing importance to gear and bearing reliability, the mechanisms of micropitting are poorly understood. This is particularly the case concerning the effects of friction on micropitting which are difficult to study under controlled conditions. This is because it is difficult to isolate the friction effects from other influential factors, in particular from the build-up of any anti-wear tribofilm and its subsequent effect on the running-in of counterface roughness that is known to strongly affect micropitting through its influence on severity of asperity stresses. This paper presents new data on the impact of friction on micropitting obtained using a new test methodology. Micropitting tests were conducted using a ball-on-disc MTM rig with the additional functionality to continuously monitor the growth of tribofilm during the test. Friction was varied by using custom-made oils containing different concentrations of MoDTC. Crucially, the effect of friction was isolated from the effect of counterface roughness running-in by introducing the MoDTC blend only after the running-in period was completed with a ZDDP solution alone. This approach eliminates the influence of MoDTC on ZDDP anti-wear tribofilm growth in early stages and hence ensures the same running-in takes place in each test. This gives similar asperity pressure history, regardless of the amount of MoDTC present.

Four nonhalogenated ionic liquids (ILs) based on the same phosphonium cation are investigated in terms of the anion suitability for enhancing the lubricity of a biodegradable oil. For all test conditions, typical for industrial machine components, the lubrication is shown to be governed by nonsacrificial films formed by the physisorption of ionic species on the tribo-surfaces. The anionic structure appears to have an important role in the formation of friction modifying films. The orthoborate ILs exhibit the formation of robust ionic boundary films, resulting in reduced friction and better wear protection. On the contrary, the surface adsorption of phosphinate and phosphate ILs appears to antagonistically disrupt the intrinsic lubrication properties of the biodegradable oil, resulting in high friction and wear. Through additional investigations, it is postulated that the higher dissociation of orthoborate ILs in the biodegradable oil allows the formation of hierarchical and electrostatically overscreened layer structures with long-range order, whereas the ILs with phosphate and phosphinate anions exhibit low dissociation in biodegradable oil, possibly due to the ion pairs being surrounded by a hydrocarbon halo, which presumably results in weak adsorption to form a mixed interfacial layer with no long-range order.

Hybrid lubrication is a promising candidate for spacecraft applications. The methodology consists of the application of a viscous fluid medium (typically a low vapour pressure oil) on top of a thin-film solid lubricant coating (i.e. sputtered MoS2), and when performed correctly can result in an extension in lubricant lifetime, greater than the sum of each of the constituent lubricants alone, without an associated increase in friction.

Hybrid lubrication can also act to protect the underlyingMoS2 during moist environment running (for example ground testing), which can reduce AIT costs and would make hybrid lubrication attractive for high load, low lifetime applications. In addition, hybrid lubrication is potentially attractive to mechanisms engineers as the low fluid volumes applied, theoretically allow one to consider hybrid lubrication for applications where fluid lubricant alone may not be applicable, such as at high or low temperature.

This paper shall present the recent investigations by ESTL into the behaviour of hybrid lubrication, leading to recommendations for use in spacecraft mechanism applications.

In this paper, an experimental simulation method was used for evaluating the tribofilm formation in rolling/sliding contact at different points in the line of action. A ball-on-disc test method was employed by which the pressure and slide to roll ratio of gear contact could be simulated. In order to reach a general conclusion, four different oils and two surface roughness were involved in the experiments. The tribofilm evolution was captured using spacer layer interferometry method, and the correlation of tribofilm with the location at the line of action was studied. Results showed that there is a threshold pressure for the tribofilm formation around which the tribofilm growth rate is maximum. Above this threshold pressure, the tribofilm formation is not stable, and the wear is dominant. Below this threshold pressure, the tribofilm growth rate rises by increasing the pressure and the gear contact is safely protected by a stable tribofilm.

One commonly used lubricant in rolling bearings is grease, which consists of base oil, thickener and small amounts of additives. Commercial greases are mostly produced from petrochemical base oil and thickener. Recently, the development of base oils from renewable resources have been significantly focused on in the lubricant industry. However, to produce an entirely bio-based grease, the thickener must also be produced from renewable materials. Therefore, this work presents the design and evaluation of three different bio-based polymer thickener systems. Tribological tests are performed to characterize lubrication properties of developed bio-based greases. The effect of thickener type on film thickness and friction behavior of the produced bio-based greases is evaluated on a ball-on-disc tribometer. Moreover, the results are compared to a commercial petrochemical grease chosen as benchmark.

In this work, two different 2D materials, molybdenum disulfide nanoplatelets (MSNP) and graphene nanoplatelets (GNP), prepared by electrochemical exfoliation, were used as additives to prepare nanolubricants. The tribological behaviour of the nanolubricants was evaluated under two configurations (pure sliding and rolling/sliding) using two different tribometers: an Universal Macro Materials Tester (UMT-3) and a Mini Traction Machine (MTM2). Wear volume was determined, after the sliding tests, in a confocal microscope (Leica DCM 3D) and the worn surface was analyzed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Raman microscopy. Lubrication mechanisms of GNP and MSNP dispersed in an engine oil for improving its antifriction and antiwear capabilities are proposed. The traction coefficient determination was performed at a 50% of slide-to-roll ratio and at different temperatures. The results showed that the nanolubricants formulated with both types of additives, in their lowest concentration, improved friction and wear in sliding tests, compared to neat engine oil. In addition, only the nanolubricants with the MSNP nano additive at loadings of 0.05 and 0.2 wt% showed friction reductions compared to the commercial engine oil under the rolling/sliding tests.

Micropitting is a type of surface fatigue damage that occurs in rolling-sliding contacts operating under thin oil film conditions. Application of black oxide (BO) coating to steel rubbing surfaces has been suggested as a potential approach to alleviate micropitting. This paper confirms that BO coatings can prevent micropitting and identifies the predominant mechanism by which this occurs. Micropitting tests were carried out using ZDDP solutions in a ball on disc tribometer. Micropitting was preferentially generated on the smooth balls and this was completely prevented by applying a BO coating to the rougher discs, regardless of whether the balls were coated or not. In contrast, when the rough discs were not BO-coated, micropitting was consistently generated on both BO-coated and uncoated balls. BO coating has about one quarter the hardness of the steel used and was found to be very rapidly removed from the surface asperity peaks at the onset of rubbing, despite the presence of ZDDP. This resulted in an almost immediate and very large reduction of the surface roughness of the discs and this prevented high asperity stresses that would normally initiate and propagate the surface fatigue cracks leading to micropitting. Parallel measurement showed that BO did not suppress tribofilm growth, so the ZDDP was able to protect against adhesive wear while not promoting micropitting. The insights presented here can help with the design of components and lubricants that are effective in controlling both sliding wear and micropitting.

Wear is a type of surface damage commonly observed in industrial components in relative motion and in contact with other solid surfaces. The majority of wear occurs progressively in a given contact starting from an initial running-in period followed by a steady-state period. Being able to accurately classify the running-in and steady-state periods allow reducing significant production or damage costs of complex machines, in particular when the load varies during operation. Production cost can be addressed by optimizing the running-in time. In contrast, significant damages can takes place if the machine are of set to full production capacity before the running-in time is finished. To address these two problems, we use a real-time monitoring system to differentiate between running-in and steady-state periods as well as classify the loading conditions simultaneously based on AE signals using a multi-label Convolutional Neural Network (CNN). Reciprocating sliding tests are performed at two loads (200 and 500 g). The tribopair used is a steel ball sliding against steel plates under dry conditions. The tribotest is divided into two different states, running-in and steady-state based on the obtained friction curves. A pico-acoustic sensor is attached on the steel plate's surface, the fix body, to acquire AE signals during the friction test. Raw AE signals are processed and directly analyzed using a multi-label CNN to simultaneously classify the running-in and steady-state periods as well as the loading conditions. This machine learning method accurately classifies the running-in and steady-state as well as the loading conditions with a 99% average accuracy.

To meet the need for oil-compatible friction modifier additives that can significantly reduce energy consumption in the boundary-lubrication regime, a macromolecular design approach has been taken. The aim was to produce a lubricious polymer film on the sliding surfaces. A series of readily functionalizable block copolymers carrying an oleophilic poly(dodecyl methacrylate) block and a functionalizable poly(pentafluorophenyl methacrylate) block of various lengths was synthesized by means of reversible addition-fragmentation chain-transfer (RAFT) polymerization. The poly(pentafluorophenyl methacrylate) block was used to attach surface-active nitrocatechol anchoring groups to the polymer. The friction-reduction properties of these polymers were assessed with 0.5 wt% solutions in hexadecane by means of rolling-sliding macroscopic tribological tests. Block copolymers with roughly equal block lengths and moderate molecular weights were significantly more effective at friction reduction than all other architectures investigated. They also displayed lower friction coefficients than glycerol monooleate—a commercially used additive. The film-formation ability of these polymers was examined using a quartz-crystal microbalance with dissipation (QCM-D), by monitoring their adsorption onto an iron oxide-coated QCM crystal. The polymer with highest lubrication efficiency formed a thin film of ~ 17 nm thickness on the crystal, indicating the formation of a polymer brush. Interferometric rolling-sliding experiments with the same polymer showed a separating film thickness of ~ 20 nm, which is consistent with the QCM-D value, bearing in mind the compression of the adsorbed layers on the two sliding surfaces during tribological testing.