We will implement dynamical real-time mechanical testing of industrially relevant tribosystems in-situ in electron microscopes to evaluate time-resolved frictional phenomena. The teams in Leeds and Sheffield have been leading in this area through their work on:. The core methodology here will be tribological testing of material contacts inside a Scanning Electron Microscope SEM.
Friction reduction through biologically inspired scale-like laser surface textures
A variety of electron microscopy probes have been developed over the last 10 years. One of the most promising for multi-scale frictional phenomena is in-situ SEM nanoindentation where a sharp probe is brought into contact with another surface.
Such methodologies are a challenge due to requirements including instrumentation size minimisation to fit inside the SEM chamber whilst maintaining instrumentation stiffness , geometry optimization to get coincidence of the triboprobe, sample and electron beam, positioning resolution and load range. Commercial systems have recently available optimised for SEM-scale mechanics, which this project will exploit for dynamical tribology.
Challenge 4 - Particles and 2 nd Phase. Project 4 - Advanced microscopy for examination of nanoscale tribological contacts. This demonstrates that, at least over a certain range of sliding speeds, these biologically inspired surface morphologies have the same potential for reducing friction forces as standard surface textures, which have already been optimized for several decades . Running-in is a difficult subject to understand [29,30] and it is known that the sliding speed can have a deciding influence on run-in . Answering this question will require run-in experiments, e.
Another way to understand why these textures demonstrate a non-typical Stribeck curve behaviour for the lubricated experiments might be found in the fact that, compared to classical surface textures, the debris formed during laser texturing was not removed. In fact the scale-like structures may be viewed as the debris.
Therefore, the minimum thickness for the oil film to fully separate the two contacting bodies might need to be higher compared to round dimples. For even slower sliding, a more classical mixed lubrication contact is formed, as it is also encountered for samples textured with dimples . The discs therefore became slightly rougher during the experiments. At the same time, surface profilometry did not reveal any major scratches or signs of wear. Polyetheretherketone PEEK is a polymer widely employed in tribological applications, and therefore an excellent choice for testing the frictional properties of a laser-generated, biologically inspired, scale-like surface morphology when paired against a polymeric counter body.
As these are the results for an unlubricated contact, it has to be pointed out that the trends observed in this diagram cannot be explained by the transition from mixed to hydrodynamic lubrication. For the polymeric counter body investigated here, the scale-like surface morphology reduced friction over the entire range of sliding speeds tested. The maximum reduction in the friction coefficient for the steel-on-polymer contact therefore is significant, but less compared to the steel-on-steel one.
No transition from a beneficial to a detrimental effect by introducing a scale-like surface morphology for smaller sliding speeds was observed. For tribological applications, a wide range of ceramics has been experimentally tested in contacts involving laser surface textures. Aluminium oxide is a common material in those investigations [35,36]. Therefore, and because Al 2 O 3 can be considered as the model engineering ceramic, it was chosen as the counter body material for testing the performance of the scale-like surface morphology in a steel-on-ceramic contact.
This suggests that for an unlubricated steel-on-ceramic contact, texturing the steel surface with a scale-like morphology is beneficial only for low sliding speeds, and might be detrimental for higher ones. It should be noted that the absolute values for the friction coefficient for the steel-on-ceramic contact are significantly larger compared to the steel and polymer counter bodies presented above. This might be due to the significantly larger surface roughness of the aluminium oxide discs compared to the PEEK and Cr6 ones.
Unfortunately, it is experimentally very challenging to prepare all three counter body materials to the same surface roughness. When testing the dry friction performance of biologically inspired surface morphologies, Baum et al. This was explained with a decrease in apparent contact area when roughness increased. We did not observe this effect in our data, but it should be noted that we did not systematically change the roughness of one contacting material. This could be another factor explaining this apparent difference. For the lubricated contact, the frictional behaviour is similar compared to what was discussed above for the steel-on-steel contact.
The efficiency of laser surface texturing strongly depends on the sliding speed regime.
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This might again be explained by the fact that once the thickness of the oil film is no longer sufficient to fully separate both bodies, solid—solid contacts are formed. Especially with a brittle, stiff ceramic, such contacts are expected to result in a very pronounced friction increase compared to the hydrodynamic regime.
The fact that the friction increased for the textured surfaces when pairing them in slow moving, lubricated contacts against ceramics or metals might additionally be explained by the minimal influence of micro hydrodynamic effects at low sliding speeds. These effects manifest themselves in friction forces that are closer to what one might expect for unlubricated contacts.
For faster sliding, lubricated contacts, the scale-like surface texture therefore allows for a significant reduction in friction forces, similar to what has been reported for the maximum efficiency of adding round dimples to steel-on-ceramic contacts .
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No signs of wear were detected by optical profilometry. When testing the tribological performance of round dimples created by laser surface texturing, considerable size effects for dry and lubricated experiments, as well as for static and dynamic friction were reported [15,16,27,28]. This is true for the dimple diameter [15,27,28] , as well as for the dimple depth [16,28]. Size effects are a common phenomenon in materials science and have been found to influence mechanical and magnetic as well as surface properties like adhesion .
We therefore experimentally investigated whether such an effect might also exist for the friction properties of a laser-generated biologically inspired scale-like surface morphology. The reason for choosing this pattern compared to the ones employed when investigating the effect of different counter bodies is that the offset scales can be manufactured with higher precision with respect to the scale diameter over a larger range in diameters compared to the ones tested above.
Additionally, we wanted to probe the existence of such a size effect against one of the most well defined counter body materials available: polished sapphire discs. The experiments were conducted without lubrication and a steel-on-ceramic contact with sapphire as counter body material.
Please note that the abscissa is not to scale. Compared to the untextured control, the friction coefficient was reduced by a factor of two for the largest scales tested. Explaining these results is currently difficult.
We attempted to follow the argumentation of Baum et al. When doing so, we neglected that the local contact pressure will increase with decreasing contact area assuming a constant normal load and the complication that arguments based on indentation depth do change for a sliding contact e. We made use of the contact mechanics solver developed and provided by Pastewka .
This program allows uploading white light profilometry images and analysing the contact mechanics. Profilometry images of all four scale sizes were taken and attention was focused on the ratio of the true to the projected contact area for a contact with a sapphire counter body. This analysis showed that, in agreement with what was postulated by Baum et al. Size and scaling effects in biologically inspired surface textures are well known to exists, for example, in fibrillary adhesives inspired by the hierarchical hairy structures found on the feet of certain lizards and insects [39,40,45].
There are four effects that are classically used to argue why laser surface texturing has a beneficial influence on tribological properties — the trapping of wear debris  , changes in the contact angle  , the storage of lubricant  and an additional micro-hydrodynamic pressure build-up effect  — however, these seem not to be able to explain our results presented here.
Wear is negligible in our experiments see below , changes in contact angle between the differently textured surfaces are expected to be minimal, and there cannot be any storage of lubricant or micro-hydrodynamic pressure build-up as these experiments were performed under dry sliding. Understanding and exploiting this size effect phenomenon therefore will be the focus of future research.
These investigations will mainly involve a detailed modelling of the contact mechanics for each surface morphology. We will also investigate whether even larger scale-like structures further decrease friction, or upon which limit in scale size the friction coefficient increases again. It will also need to be tested whether a similar size effect exists for lubricated contacts and for counter body materials other than sapphire.
Attempting a holistic examination of all of the results presented above, it becomes apparent that the texturing effect on friction of a bearing steel surface with biologically inspired scale-like surface morphology strongly depends on the exact circumstances of the sliding contact. This is in agreement with what was reported for polymeric bioinspired surface morphologies .
Among the factors that need to be considered are: contact lubrication conditions, sliding speed and the counter body material. Certainly, future experiments will reveal that even more parameters need to be considered, for example, reciprocating versus unidirectional sliding, temperature, and the viscosity of the lubricant. For each tribological application, the surface textures must be optimized individually. At the same time, our results strongly suggest that bioinspired morphologies have at least the same potential for reducing the friction coefficient as standard laser-generated surface morphologies, i.
Taking into account that the latter ones have been optimized for decades and the scale-like surface textures only recently have emerged as a research focus, it is quite likely that their full potential for positively influencing tribological properties has not yet been reached. This is especially true as wear was not the focus of this study, but will be investigated in more detail in the future. This image demonstrates that wear of these surface morphologies is negligible, in agreement with previous results  and with the good antierosion performance of surface textures inspired by scorpion armour .
Future research will involve thoroughly quantifying wear rates. In the case of lubricated contacts, we will also involve fluid dynamics modelling to understand and optimize the tribological behaviour of these surfaces. For dimpled surface morphologies, such simulations have recently been able to explain the beneficial effect of laser surface texturing [27,50]. An intriguing result of these simulations is that the depth of the surface textures is expected to have a stronger influence as compared to their lateral size. This result, that was verified experimentally  , demonstrates that mechanisms explaining classical round dimples can unfortunately not simply be transferred to scale-like surface textures.
In our experiments, the variation in depth was below one micrometre. These surface textures themselves need to be modelled as realistically as possible, requiring future elaborate fluid dynamics simulations.
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As far as the unlubricated experiments are concerned, the concept developed by Bowden and Tabor  points out that most frictional energy dissipation is due to plastic deformation of the subsurface layer. Thus, the bulk properties of the softer surface determines the friction and wear properties of the entire tribological system . The coefficient of friction in a sliding system accordingly is expressed as the ratio of the shear strength to the yield pressure of the softer metal .
Consequently, the tribological properties of the surface are strongly influenced by the subsurface material and the subsurface microstructure is strongly influenced by the plasticity and the nature of the corresponding dislocation activity under a tribological load . The microstructure under the contact therefore undergoes drastic and complex changes during sliding, which in themselves strongly depend on the nature of the tribological loading conditions .
Friction reduction through biologically inspired scale-like laser surface textures
We have studied such phenomena in detail recently with high purity copper [53,54] and a pearlitic steel  as model materials. Whether and how the microstructure under the sliding contact changes in the tribological systems investigated here will be an interesting task for future research. It is expected that only small microstructural change will be found on the side of the textured bearing steel when it is paired with a much softer material like PEEK, whereas significant changes should be seen when the textured Cr6 samples are investigated after being run against alumina.
At the same time, the mechanical properties of both contact materials need to be characterized after the experiments to reach a more thorough understanding of where and how frictional energy is dissipated.
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