In the last few years, there is a general shift observable toward greener lubrication, fueled amongst others by policy initiatives such as the European Green Deal in consistency with the UN Sustainable Development Goals. At least 70 vol% of a lubricant is composed of a specific base oil, the rest is a variation of additives altering the lubricant properties (enhancing or suppressing existent base oil properties or adding new properties) to be operational for a particular field of application. So, in terms of sustainability, biodegradability, bioaccumulation, and toxicity the type of base oil plays a major role, which makes environmentally harmful petroleum-based lubricant formulations highly problematic for future applications. Hence, this leads to an ever-growing demand of environmentally friendly lubricant alternatives. Within the scope of this review lies the investigation of bio-based, bio-derived, and other sustainable lubricant components that could serve as promising replacements for conventional petroleum-based formulations, in accordance with the principles of green chemistry and tribology. As recycling is embraced by the term sustainability, waste-derived components of non-biological origin are also included in this work. An overview of studies on the tribological performance such as friction and wear properties of these sustainable and benign lubricant components is given. GRAPHICAL ABSTRACT

Ammonia is a prospective zero-carbon-emission fuel for use in large marine diesel engines. Current research focuses on several technical aspects, such as injection strategies or exhaust gas aftertreatment options, but investigations regarding the impact of ammonia on engine oil degradation are largely absent from the literature. This study provides a methodology with which to evaluate this phenomenon via artificial oil alteration. By using an admixture of various contaminations to air, such as ammonia and its partial combustion product nitrogen dioxide, their respective impacts on chemical oil degradation were assessed. Subsequently, the lubricating performance of altered oils was investigated, with a focus on corrosion properties, deposit formation, and load-bearing capability. Although the application of a stoichiometric ammonia–air mixture resulted in less pronounced thermo-oxidative degradation compared to alteration with neat air, static and dynamic deposit formation as well as corrosion properties and load-bearing capability were severely impacted by the presence of ammonia. On the contrary, nitrogen dioxide contamination resulted in higher oxidation and acidification of the oil, but altered samples performed considerably better than ammonia-altered aliquots in terms of coking tendencies, corrosivity, and load bearing.

During their service, engine oils suffer from various influencing parameters such as thermo-oxidative stress and nitration, hence, the accumulation of degradation products and the entry of contaminants. Accordingly, ICEs need to be able to operate satisfactorily, especially with a degraded lubricant, making it highly recommendable to use such oils for component testing in ICE development. Thus, a new nitrative thermo-oxidative ageing method is presented for closer-to-reality simulation of engine oil alteration with the intention to provide reproducibly aged oils for subsequent bench testing. With this method, a target used oil from field application was replicated and the comparability of oil condition in the lab vs. field regarding oxidation, nitration, additive depletion, and acidification amongst others was verified by conventional and advanced analyses. Special focus was laid on the identification of nitration products, proving them to be predominantly oxidized aromatic species or organophosphates. The presented method gives valuable benefit for the closer-to-reality ageing of engine oils in reasonable time frames with moderate costs and, hence, for the provision of test oils for ICE bench testing enabling rapid engine component assessment.

As lubricants with a high thermo-oxidative stability such as synthetic esters are gaining more importance in the lubricant market, a detailed knowledge regarding their oxidative degradation behaviour is of high importance. In order to reveal their degradation products and processes, a novel approach combining artificial alteration, isotope labelling based on oxidation with ¹⁶O₂ and ¹⁸O₂, and mass spectrometry (MS), was applied to a bis(2-ethylhexyl) adipate base oil. The degradation products such as 2-ethylhexanol and its monoesters with short-chain fatty acids pinpointed the C–O ester bond as the site prone to oxidative attack, allowing the collection of information about the oxidation mechanisms. Furthermore, the influence of the antioxidant (AO) 4,4′-methylene-bis(2,6-di-tert-butylphenol) as an additive on the oxidation behaviour and resulting products was studied: blends containing AO showed a remarkably higher resistance against oxidation. However, similar degradation products were obtained after AO depletion and without AO. AO cleavage occurred at the carbon atom that bridges the phenols to give 2,6-di-tert-butyl-p-benzoquinone and 3,5-di-tert-butyl-4-hydroxybenzoic acid. By applying the isotope labelling approach, sites of preferential oxidative cleavage and hence differentiation of the origin of oxygen atoms—either from the atmosphere or from base oil components—can be unambiguously related in oxygen-containing base oils, as well as in blends with additives.