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

Low vapor pressure and several other outstanding properties make room-temperature ionic liquids attractive candidates as lubricants for machine elements in space applications. Ensuring sufficient liquid lubricant supply under space conditions is challenging, and consequently, such tribological systems may operate in boundary lubrication conditions. Under such circumstances, effective lubrication requires the formation of adsorbed or chemically reacted boundary films to prevent excessive friction and wear. In this work, we evaluated hydrocarbon-mimicking ionic liquids, designated P-SiSO, as performance ingredients in multiply alkylated cyclopentane (MAC). The tribological properties under vacuum or various atmospheres (air, nitrogen, carbon dioxide) were measured and analyzed. Thermal vacuum outgassing and electric conductivity were meas- ured to evaluate ‘MAC & P-SiSO’ compatibility to the space environment, including the secondary effects of radiation. Heritage space lubricants—MAC and perfluoroalkyl polyethers (PFPE)—were employed as references. The results corroborate the beneficial lubricating performance of incorporating P-SiSO in MAC, under vacuum as well as under various atmospheres, and demonstrates the feasibility for use as a multifunctional additive in hydrocarbon base oils, for use in space exploration applications.

The growing demand for sustainable lubricant solutions is driving the exploration of bio-based materials that deliver comparable performance to conventional, primarily fossil-based lubricant chemistries. This study focuses on squalane as a sustainable base oil, which can be derived from different renewable sources. A total of two squalane products were evaluated for thermal-oxidative stability and benchmarked against a polyalphaolefin, PAO 4, of the same total carbon number. Oils artificially altered in a closed reactor were sampled and subjected to conventional lubricant analyses, including infrared spectroscopy, to determine the changes due to autoxidation over time. For in-depth information, direct-infusion high-resolution mass spectrometry and gas chromatography coupled with triple quadrupole mass spectrometry were employed to identify degradation products from thermo-oxidative stress. The results revealed substantial variability in the stability of squalane products, suggesting that differences in raw materials and production processes have a major impact on their performance, including rheological properties. The degradation products of polyalphaolefin and squalane, identified through detailed mass spectrometry, were analyzed to understand their impact on conventional physicochemical properties. While polyalphaolefin predominantly generated carboxylic acids with short to medium chain lengths as degradation products, squalane oxidation produced carboxylic acids with medium to long chain lengths as well as several alcohols and ketones. Despite these differences, squalane demonstrates its potential as a non-fossil hydrocarbon base oil, as squalane products matched and even exceeded PAO 4 stability.

The growing demand for sustainable lubricant solutions is driving the exploration of biobased materials that can deliver comparable performance to conventional, primarily fossil-based lubricant chemistries. This study focuses on squalane as sustainable base oil, which can be derived from different renewable sources. A total of three squalane products were evaluated for ther-mal-oxidative stability and benchmarked against a polyalphaolefin PAO4 of the same total carbon number. Oil samples subjected to artificial alteration in a closed reactor underwent standard lub-ricant analyses, including infrared spectroscopy, to determine the conventional degradation changes over time. Additional in-depth information, by direct infusion high-resolution mass spec-trometry and gas chromatography coupled with triple quadrupole mass spectrometry were em-ployed to identify oxidation degradation products. The results revealed substantial variability in the stability of squalane products, suggesting that differences in raw materials and production processes have a profound influence on their performance, as well as rheological properties. Furthermore, detailed mass spectrometry analysis was applied to identify degradation products specific to poly-alphaolefin and squalane, respectively, and to understand their impact on the conventional phys-icochemical properties. While polyalphaolefin predominantly generated short-chain carboxylic acids as degradation products, squalane oxidation also produced several alcohols and ketones. Despite these differences, squalane demonstrates potential as a non-fossil hydrocarbon-based base oil, as squalane products matched and even outperformed stability of PAO4.