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

Additive Manufacturing (AM) has increasingly gained attention as a tool for the fabrication of complex biomedical components, due to the flexibility of the technique for accounting to the patient individuality. Additive manufacturing techniques, like laser beam melting, often result in highly anisotropic microstructures that greatly differ from those obtained in conventionally manufactured alloys. This study evaluates the potential of AM manufactured CoCrMo for body implants as an alternative to the wrought CoCrMo, especially considering tribocorrosion performance in buffered fluid. Its biocompatibility is also assessed via in-vitro cytotoxicity assays. The results show that both materials have a comparable tribocorrosion performance, independently of the manufacturing process, despite their radically different initial microstructure. This results from the microstructural convergence arising from the plastic deformation imparted by sliding motion. While the initially elongated grains of the AM CoCrMo tend to grain refinement, the microstructure of the wrought CoCrMo undergoes grain coarsening, resulting in a similar final grain size detected after the tribocorrosion experiments. The addition of albumin to the phosphate buffer testing fluid, simulating body fluid applications, reduces the grain refinement, particularly under constant 0.21 V, due to lower shear stresses caused by the lower coefficient of friction. Therefore, the initial dissimilarity found in the untested microstructure between the materials does not affect the wear rate nor lead to an increased metal release. As the cytotoxicity is neither impaired by the manufacturing process, the use of AM CoCrMo could be recommended on those biomedical applications requiring wear resistance in body fluid environment.

Hemiarthroplasty consists of the replacement of local cartilage defects by a partial implant and provides a less aggressive alternative to total joint replacement. The material frequently selected nowadays for partial implants is CoCrMo alloy, which results in a sliding contact between articular cartilage and the alloy. Since the geometry of the implant needs to be tailored to the patient, partial implant technology would greatly profit from novel additive manufacturing techniques. This study examines the feasibility of using additive manufacturing techniques on partial implants made of CoCrMo alloys, with a particular focus on the impact of manufacturing technique on mechanical stimulation, cartilage analysis, and friction performance. To this end, in vitro biotribological experiments were performed between CoCrMo samples and bovine articular cartilage using PBS as simulated body fluid. Key findings reveal significant changes in microstructure between laser beam melted (LBM) and wrought CoCrMo alloys, despite having a comparable elemental composition. The coefficient of friction (CoF) measured between bovine articular cartilage and the CoCrMo specimens during biotribocorrosive testing revealed no significant differences resulting from the manufacturing techniques, even though wrought CoCrMo resulted in a higher reproducibility. Conventionally produced CoCrMo also exhibited a more anodic open circuit potential during the experiments, likely due to the significant differences in microstructure that affect corrosion resistance. The tested cartilage samples showed a slight increase in MMP13 (Matrix Metalloproteinases - degradative enzymes) in comparison to the controls, indicating potential remodeling effects, especially for the LBM CoCrMo alloy. Additionally, the metabolic activity in the cartilage specimens increased due to mechanical stimulation. No cracks or fissures were detected in histological imaging thus highlighting that the cartilage samples were not damaged during harvesting or testing. These findings indicate the possibility of an equivalent use of additive manufactured CoCrMo, enabling patient-specific surgeries and encourage further research to explore the long-term impact of corrosion stability on implant longevity and functionality.

The objective of this research is to identify a bio-based friction modifier (FM) with tribological performance comparable to conventional FMs. Promising alternatives to conventional FMs, such as the FMs derived from natural sources, including rapeseed and salmon oil, were selected. Increasing concerns about crude oil prices, environmental impact, and the depletion of fossil resources have further fueled the search for renewable, biodegradable, and environmentally friendly raw materials for lubricants Tribological tests were conducted using a rheometer under non-conformal contact. The normal force, temperature, and sliding speed were varied to simulate conditions such as those found in a food extruder. To simulate cold extrusion applications, water and bio-based FM mixtures were used. The best-performing bio-based FMs were then mixed with a polyalphaolefin to simulate warm extrusion conditions. The results were compared to those obtained from mixtures of a polyalphaolefin and selected conventional FMs. The main finding of this study demonstrated that rapeseed and salmon oils, with a peak coefficient of friction (COF) of 0.16, are the best-performing bio-based FMs for reducing friction. When mixed with distilled water for cold extrusion (case 1) and with polyalphaolefin for warm extrusion (case 2), they performed similarly to the conventional FM, tallow amine, also with a maximum COF of 0.16, and significantly better than polyalphaolefin alone (maximum COF of 0.25). Consequently, rapeseed and salmon oils are suitable bio-based FM candidates to replace conventional FMs in food-grade lubrication.