Recent advances in green lubricants: Structure–property relationships, tribological performance, and sustainable applications

The transition toward sustainable manufacturing and carbon-neutral technologies has accelerated the development of environmentally benign lubricants capable of replacing conventional petroleum-based oils. Among these, bio-based lubricants derived from vegetable oils and essential oils have emerged as promising alternatives owing to their renewability, biodegradability, low toxicity, and excellent lubricity. However, their widespread industrial adoption remains constrained by poor oxidative stability, limited thermal resistance, un favourable low-temperature properties, and hydrolytic degradation. This review critically examines recent advances in green lubricants by establishing the relationships between molecular structure, physicochemical properties, lubrication mechanisms, and tribological performance. Particular emphasis is placed on the influence of fatty acid composition and molecular architecture on viscosity, oxidation stability, thermal behavior, film-forming ability, friction reduction, and wear resistance. Recent developments in advanced characterization techniques, chemical modification strategies, including transesterification, esterification, and epoxidation, and the incorporation of environmentally benign additives and functional nanomaterials are comprehensively discussed as effective approaches for overcoming the inherent limitations of bio-based lubricants. The review further analyzes the tribological mechanisms responsible for enhanced lubrication, including boundary film formation, tribo chemical reactions, rolling effects, mending effects, and protective tribo film development. In addition, the environmental benefits of bio-lubricants are evaluated from a life-cycle perspective, highlighting their potential to reduce carbon emissions while improving resource efficiency and supporting sustainable manufacturing. Emerging applications in automotive, metalworking, marine, agricultural, food-processing, and total-loss lubrication systems are also discussed. Finally, current challenges and future research directions are identified, emphasizing multifunctional nano-enhanced lubricants, molecular engineering, and intelligent formulation strategies. This review provides a comprehensive structure–property–performance framework for the rational design and industrial implementation of next-generation sustainable lubrication technologies.