23rd International Colloquium Tribology - January 2022 27 Tribological optimisation for sustainable lubrication design Prof. Roland Larsson Division of Machine Elements, Luleå University of Technology, Sweden Corresponding author: roland.larsson@ltu.se Marcus Björling Division of Machine Elements, Luleå University of Technology, Sweden Yijun Shi Division of Machine Elements, Luleå University of Technology, Sweden Anders Pettersson Division of Machine Elements, Luleå University of Technology, Sweden 1. Introduction As with most other product development processes, the lubrication design has followed an optimisation procedure with two main goals, the best possible performance, and the lowest possible cost. In some cases, additional constraints have been added such as adequate safety and high efficiency. Over the past decades, sustainability has become increasingly important, but not a constraint on the optimisation. It has only been a profitable addition when it has been possible to claim that the product is made of renewable materials or produced in a sustainable way. Without sacrificing performance. But what happens if sustainability or circularity become constraints that must be considered? What happens if the product must be made from fossil-free, renewable materials and/ or if it must be possible to re-manufacture or re-use it? Then it may not be possible to get as good performance as before. 2. Circular economy Circular economy is a way to keep materials in the use-loop as long as possible. Ideally, the materials should be in use forever and no virgin material should be needed to be added [1]. In lubrication this is not possible since there is always a certain loss due to spill and evaporation. Two ways to obtain sustainability are possible, to re-condition and re-use the lubricants over and over again, or to use lubricants made from renewable, fossil-free, raw materials. In the latter case, it may not be possible to achieve as good performance as with petroleum-based oils without re-optimising the entire tribo-system. Today’s lubricants and tribo-systems have been optimised for more than a hundred years. When we start using completely new base fluids, we must redo the tribo-optimisation to get as high performance as today. In this presentation we will discuss how this can be done. We will compare petroleum-based oils, natural oils, and water-based fluids from a cost and sustainability perspective. 3. Base fluid candidates A vast majority of today’s lubricants are made from petroleum, i.e., mineral oils. The reason is the very good availability of hydrocarbon fractions of suitable viscosity when fuels are produced from the crude oil lower viscosity fractions. The raw material cost for mineral oils, consequently, also become very low due to the large volumes of crude oil. Most other fluids with similar viscosity would work as lubricants when it comes to machine components such as plain bearings. When the contact forces increase, however, oils have the good advantage of being strongly pressure sensitive. The viscosity increases dramatically, and the oil becomes solid-like in lubricated contacts such as those found in ball bearings or in between gear teeth. The pressure-sensitivity is normally described by the pressure-viscosity coefficient, defined as: where is dynamic viscosity and is pressure. Many fossil-free alternatives to mineral oils have pressure-viscosity coefficients on a similar level as mineral oils. That is in the range 15-20 GPa -1 . Some examples of alternative, fossil-free oil candidates are synthetic esters and other synthetic hydrocarbons made from renewable sources [2][3][4][5]. The cost for fluids like this is very much higher than for mineral oils. Another problem is that they are often made from natural oil plants such as rapeseed or sunflower. Such oils are also used as food and there will thus be a competition with the supply of food to an increasing world population. 28 23rd International Colloquium Tribology - January 2022 Tribological optimisation for sustainable lubrication design Another category of base fluid candidates are water-based compounds. Or more correctly, water-soluble compounds. The advantage with such water-based lubricants (WBL) would be that it is possible to make them non-toxic, fire-resistive, and easy to clean surfaces contaminated by the lubricants. Typical fluids are poly-alkylene glycols (PAG) and glycerol [6]. In this presentation we are focussing on the latter type. Glycerol is produced in relatively large volumes but far from the volumes of mineral oils. The problem is that all mentioned fossil-free candidates are not produced in sufficient amount, and they all make use of natural oils. Glycerol is, for example, a waste product from biodiesel (RME) production using natural oils as raw material. Still, glycerol is a low-cost product. Glycerol has a disadvantage in having a low pressure-viscosity coefficient. It is around 5 GPa -1 [7]. Other water-soluble base fluids have the same problem. The difference in may sound little but implies that viscosity at 1 GPa may be some 3 orders of magnitude lower than for mineral oils and, as explained below, this will make the lubricating film thinner. One advantage with low is, however, that friction in full film, elastohydrodynamic lubrication (EHL) is extremely low, up to 4-5 times lower than for mineral oils [7]. If we can design for full-film lubrication then WBLs should have excellent friction behaviour in, for example, gears. 4. Re-design for WBL As mentioned, tribo-systems with oils are optimised over more than hundred years. Just to replace them without changing anything else in the tribo-system can hardly be done and most comparisons between the performance of mineral oils and WBL will show that the oil will perform better, at least in terms of wear performance. To make it possible to obtain as good lubricating performance in EHL, for the WBL, as for the oil we need to compensate for the low . The Hamrock-Dowson equation [8] can be used for this purpose. Film thickness is proportional to pressure-viscosity coefficient and viscosity in the following way: If the is assumed to be 18 for the oil and 5 for the WBL then viscosity must increase by (18/ 5)0.49/ 0.68 ≈ 2.5 times to maintain the same film thickness. Another alternative is to keep the film parameter [9] constant, then surface roughness must be reduced to approximately half the value compared to the oil lubricated contact. But the film parameter is criticised as a measure of lubrication quality [10] and the film parameter presented by Hansen et al. shows better accuracy [11]: where is the surface roughness reduced peak height, is the central film thickness, and is a number smaller than 1 and a function of asperity summit radius. The larger summit radius, the larger . Transition to full film lubrication takes place when >1. Consequently, by optimising roughness with large asperity summit radii it would then be possible to obtain the same as for an oil lubricated case. 5. Mixed and boundary lubrication The base oil is providing the hydrodynamic functionality of the lubricant but its high performance under severe conditions is controlled by additives. The chemistry of WBL is completely different than for oils so a new family of tribo-improvers must be developed. There are some few (public) attempts made, for example using myoinositol [6]. Zapata et al. [12] made a study on micro-pitting under mixed lubrication conditions. They showed that the WBL gave rise to less micro-pitting since the wear was much more pronounced than for the oil. The fractured top-layer was, therefore, continuously worn off and kept fatigue effects to a minimum Furthermore, it might be necessary to apply coatings since film formation is less efficient with low lubricants. 6. Conclusions and remarks There is no single fossil-free alternative to mineral oils as lubricant base fluids. Instead, we may see a big variety of base fluids, both oily ones and water-soluble ones. Water-based or water-soluble base fluids (WBL) have an advantage in being non-toxic, fire-resistive, and water washable. Potentially they also have significantly better friction performance. But they have poorer film forming properties than oils. This can be compensated by using higher viscosity, smoother surfaces, optimised topography, and/ or coated surfaces. In future lubricated systems we will need to design with sustainability as a demand and this will make it necessary to re-design for the use of WBL. An important question to raise (even if it is not the scope of this presentation) is if we need to design the system at the edge of its performance? Is downsizing always the right thing to do? The life-cycle cost (economically and sustainably) may be lower if we oversize the system and reduce the risk of wear to a minimum. 23rd International Colloquium Tribology - January 2022 29 Tribological optimisation for sustainable lubrication design References [1] Walter R. Stahel, W. R., ”The circular economy“, Nature, 531, 435-438 (2016). [2] Rubin, B., Glass, E.M., ”The air force looks at synthetic lubricants“, 1950, SAE Technical Papers [3] Pettersson, A., ”High-performance base fluids for environmentally adapted lubricants“, 2007, Tribology International, 40(4), pp. 638-645. [4] www.novvi.com [5] www.biosynthetic.com [6] Matta C., Joly-Pottuz L., De Barros Bouchet, M.I., Martin J.M., Kano, M., Zhang Q., Goddard W.A., ”Superlubricity and tribochemistry of polyhydric alcohols”, Physical Review B - Condensed Matter and Materials Physics, 78 (828), 2008. [7] Shi, Y., Minami, I., Grahn, M., Björling, M., Larsson, R., ”Boundary and elastohydrodynamiclubrication studies of glycerol aqueous solutions as green lubricants“, Tribology International, 69 (2014) 39-45. [8] Hamrock, B. J.and Dowson, D., ”Isothermal elastohydro-dynamic lubrication of point contacts, part I, theoreticalformulation“, ASME J. Lubr. Tech., 1976, 98, 223-229. [9] Tallian, T.E.: ”On competing failure modes in rolling contact“, ASLE Trans., 10, 418-439 (1967). [10] Cann, P., Ioannides, E., Jacobson, B., Lubrecht, A.A.: ”The lambda ratio - a critical re-examination“, Wear, 175, 177-188 (1994). [11] Hansen, J., Björling, M., Larsson, R., ”A New Film Parameter for Rough Surface EHL Contacts with Anisotropic and Isotropic Structures”, Tribology Letters (2021) 69: 37. [12] Zapata, J.G., Björling, M., Shi, Y., Prakash, B., Larsson, R., “ Micropitting performance of Glycerol-based lubricants under rolling-sliding contact conditions“, accepted for publication in Tribology International.