Nuria Espallargas

Polyoxometalate ionic liquids (POM-ILs) represent a class of multifunctional materials combining the structural tunability of polyoxometalates (POMs) with the versatile properties of ionic liquids (ILs). In this work, a series of POM-ILs were designed, synthesized, and evaluated as lubricant additives and as plain lubricants to demonstrate the relationship between their molecular structures and tribological performance. The influence of both the cationic moiety (ammonium vs phosphonium) and the anion structure (SiW11 vs SiW9) was investigated on two metallic substrates, AISI 316L stainless steel and AISI 52100 bearing steel, under boundary lubrication conditions. Comprehensive characterization combining friction and wear measurements, QCM-D adsorption analysis, and surface and subsurface chemical and structural analysis (XPS, SEM, and FIB-SEM) revealed that the POM-IL molecular structure critically influences friction and wear behavior. Increasing the lacunarity and negative charge density of the POM enhanced antiwear performance and reduced friction, likely through the formation of viscoelastic layers that promote surface separation. In addition, the base lubricant (PAO8) was found to be detrimental to the POM-IL surface interactions, reducing their efficiency as additives compared to their plain form. The results obtained in this work demonstrate that POM-ILs offer an effective alternative to conventional antiwear additives such as ZDDP, combining high molecular designability, strong substrate activity, and enhanced antiwear and friction-reducing behavior. These insights establish a structure-to-function relationship and highlight the potential of molecularly engineered POM-ILs for next-generation lubrication systems.

Environmentally acceptable lubricants (EALs) are increasingly required in high-risk oil-to-sea interfaces, yet their boundary lubrication performance remains difficult to predict because it depends strongly on both lubricant chemistry and metal substrate reactivity. In this work, we compare the tribological response of stainless steel (AISI 316L) and bearing steel (AISI 52100) lubricated with a low-viscosity polyalphaolefin (PAO4), a polar pentaerythritol polyol ester (PE), and their 1:1 blended base oil, formulated with an environmentally acceptable ionic liquid, tributyl(ethyl)phosphonium diethyl phosphate (PEP), and with ZDDP as a benchmark additive. Ball-on-disc tests under identical boundary lubrication conditions show systematically lower friction and wear on bearing steel than on stainless steel, evidencing a strong substrate dependence. On stainless steel, a Cr-rich passive film limits additive-driven tribochemistry, and wear is therefore governed by base-oil polarity and additive-base lubricant competition for surface sites, which promotes abrasion when polarity increases. In contrast, the Fe-dominated chemistry of bearing steel enables tribochemical film formation, yielding lower wear across formulations. PEP delivers the most robust antiwear response, including cases of no measurable wear, and its performance is controlled by both adsorption and dispersion state: in PAO4, PEP forms an emulsion that deposits a thick viscoelastic layer suppressing wear but increasing friction, whereas in the base oil blend and PE, reduced emulsion stability and stronger surface competition shift the response toward phosphate-rich tribofilms and abrasive contributions. QCM-D reveals stronger/less reversible adsorption for PEP than for ZDDP, while ToF-SIMS confirms phosphorus-rich films as central to PEP protection and mixed P/S chemistry for ZDDP on Fe-reactive surfaces. These findings underline that the EAL formulation must be codesigned with substrate chemistry and base lubricant polarity to ensure reliable boundary performance.

The MORE project (Next Generation Marine Materials for Resilient Offshore Renewable Energy Devices) focuses on addressing multi-degradation challenges in offshore renewable energy (ORE) devices through the development of an advanced methodology for material selection and validation. Offshore environments subject critical ORE components to simultaneous wear, corrosion, and fatigue, leading to multi-degradation, which is a primary cause of unexpected failure. This paper presents the initial results of the MORE work about two complementary test rigs designed for multi-degradation analysis: a small-scale multi-degradation (SSMD) rig and a laboratory-scale multi-degradation (LSMD) pilot test rig. Two SSMD rigs provide a controlled environment for simulating and isolating the fundamental mechanisms of tribocorrosion and fatigue at the micro-scale, facilitating rapid, low-cost material screening. In contrast, the LSMD rig replicates real-world offshore conditions to validate the scalability and industrial applicability of materials previously tested at small-scale. The MORE project’s integrated approach to multi-degradation testing represents a significant advancement over conventional qualification methods by offering a faster, more reliable pathway for material validation. By harmonizing small- and large-scale testing, MORE aims to deliver a validated, scalable methodology for predicting the long-term behavior of critical ORE components, contributing to the advancement of resilient renewable energy systems. The project’s ongoing work will culminate in the creation of industry guidelines and recommended practices, ensuring broad adoption by developers, manufacturers, and policymakers in the offshore energy sector.

The development of environmentally acceptable lubricants and lubricant additives has become a focal point within tribology due to increasing regulatory and sustainability demands. In this context, low-viscosity lubricants are gaining attention for their potential to reduce energy losses. However, their performance under a boundary lubrication regime, where thinner oil film build-up is present, requires more efficient boundary additives. This work evaluates a polyoxometalate-ionic liquid (POM-IL) as a multifunctional boundary additive in a low-viscosity polyalphaolefin-based lubricant, comparing its performance to zinc dialkyldithiophosphate (ZDDP) and a halogen-containing ionic liquid (IL). Tribological tests on AISI 316L stainless steel and AISI 52100 bearing steel revealed that while ZDDP showed substrate-independent adsorption and tribological performance, the IL-based additives had substrate-dependent behaviour. Strong chemisorption was consistent for both IL-based additives, yet their anti-wear and friction-reducing properties differed, showing evidence for the presence of a combined mechanism that includes both strong adsorption and tribochemical reactions. Additionally, the interaction between POM-ILs’ negatively charged surfaces, W atoms, and Cr(III) in 316L was identified as a key factor in their performance. Notably, significant work-hardening was observed in 316L lubricated with POM-IL-containing blends, further enhancing its anti-wear properties. These findings emphasize the role of substrate chemistry in boundary lubricant additive performance in low-viscosity lubricants, offering insights for the development of more efficient multifunctional boundary lubrication solutions.

The pressing need to develop reliable and cost-effective Environmentally Acceptable Lubricants (EALs) is a challenge to the entire lubrication field in academia and industry. In this work, we propose blending low-viscosity polyalpholefins (PAOs) and synthetic esters as base fluids, as a strategy to formulate cost-effective EALs. Pentaerythritol polyol ester (PEPE) and 2-ethylhexyl palmitate (2-EHP) are the synthetic esters chosen. Tributyl(ethyl)phosphonium diethyl phosphate (PEP) ionic liquid, dialkyl dithiophosphate (DDP), and alkoxylated long chain ether phosphate (AEP) have been chosen as lubricant additives. Two reference lubricant additives were also tested for comparison, i.e., hexadecanoic acid (palmitic acid, C16) as friction modifier, and primary zinc dialkyl dithiophosphate (ZDDP) as anti-wear additive. Stainless steel AISI 316L disks were tested against alumina balls using a ball-on-disk tribometer to evaluate the performance of the formulated lubricants. All additives improved the performance of plain PAO, while in the case of esters competition with the additives was observed due to their high polarity. The blends performed better than the lubricants with a single base fluid. ToF–SIMS studies showed richer sulfur and phosphorus-based tribofilms for those samples lubricated with blends than with single base fluids. The blends also helped improving the solubility of the ionic additives with respect to plain PAO. Overall, the results confirmed the better performance of blends as a strategy for formulating cost-effective EALs and for improving the solubility of ionic liquids in non-polar base fluids.

Lubricants are complex mixtures of chemicals that help machines function at the right level of friction and wear. Lubricant formulation methods are based on empirical experience of chemical substances that have been used as lubricants for decades. In the last years, the discussion about their environmental problem has triggered new legislations resulting in the search for Environmentally Acceptable Lubricants, which should be biodegradable, minimally toxic, and nonbioaccumulative. Finding new chemicals that comply with these three criteria is a long and expensive process that can be boosted by machine learning (ML). In this paper, we are addressing toxicity prediction with machine learning models by exploring the application of ensemble learners to chemicals having imbalanced data distribution. We investigated the effectiveness of sampling techniques to balance the data and improve the performance of the ensemble learning model. The model can predict toxicity for nonundersampled groups, which in our case corresponds to the moderately to highly toxic groups. The results of this work are useful for lubricant formulators since regulations accept moderate-to-highly toxic chemicals in lubricants if their concentration is below 20 wt %.

The focus on energy efficiency to move towards a more sustainable use of resources has intensified efforts to minimize friction and wear in mechanical systems, which account for 23% of the world's energy consumption.

The tribological performance of four different thermal spray coatings has been tested against five different polymer seal materials in a fully formulated water-based lubricant. Water-based lubricants have been proposed for a marine application due to their environmental acceptability and their potential benefits when they are used in oil-to-sea interfaces minimizing the issues related to seawater contamination. The effect of normal load and speed on friction was studied for all seal-coating candidates. Hydrogenated acronitrile–butadiene rubber (HNBR) and ethylene–propylene–diene rubber (EPR) seals resulted in higher coefficient of friction (CoF) compared to ultra-high molecular weight polyethylene (UHMWPE), aliphatic polyketone (PK) and synthetic woven fabric impregnated with phenolic resin (SWF) plastic seals. This was attributed to the higher real contact area generated by the softer rubber seals compared to the harder plastic seals at the same normal load. From the point of view of the tribosurfaces tested in this work (hardened steel, WC-CoCr, Cr₃C₂-NiCr and SiC-YAG), friction and wear were controlled by two different mechanisms depending on the type of tribosurface. For metallic surfaces, the friction modifiers in the lubricant were adsorbed on the metals and controlled the frictional performance. The hardened steel (100% metallic surface) showed the lowest CoF, followed by the two cermet coatings (21-23 vol.% of metallic binder). The SiC-YAG coating (ThermaSiC) showed the best friction and wear performance due to the formation of a hydrated film on the SiC phase (77 vol.% of the surface) despite not having any metal matrix.

Carboxylic acids make up a well-known group of organic friction modifiers (OFMs). OFMs can present different types of polar heads that can eventually lead to different surface adsorption properties and tribological responses. Therefore, the goal of this work is to study the effects of the polar head type on the frictional and wear performances of carboxylic acids in a water-based lubricant. Lauric acid (C12) was chosen as the reference OFM, and methyl laurate and monolaurin were chosen for the comparison. Sliding friction tests were performed on stainless steel against alumina balls under boundary lubricating conditions. The effect of the adsorbed layers and the tribofilm formation was studied by varying the initial maximum hertzian contact pressure, i.e., tests were performed at 1.97 and 0.66 GPa. At the lowest contact pressure, not enough load is applied to obtain enough plastic deformation on the asperity contacts. In this case, a combination of asperity contacts and a thick fluid film formation results in a lack of tribofilm formation, whereas at the highest contact pressure, tribofilms are formed in the asperity contact through tribochemical reactions. Methyl laurate showed no adsorption on the surface, and it was not tested further. C12 and monolaurin showed good adsorption, but the adsorbed layers had different viscoelastic properties. Micro and macrotribological tests showed good frictional behavior for C12 at 0.5 wt % concentration due to the good viscoelastic properties of its adsorbed layer. The adsorbed layer of monolaurin did not show good friction-reducing ability during the micro tribological tests due to its poorer viscoelastic properties. However, the macro tribological tests revealed that monolaurin forms a robust tribofilm protecting the surface from wear and efficiently reducing friction at a concentration of 0.5 wt % resulting in the lowest wear and friction values as observed in this study.