Reducing friction and wear with alkyl gallate additives in water-based lubricants

Environmental concerns have made the development of non-flammable, high-specific heat capacity, and high-performance lubricants an urgent priority, driving an increased demand for aqueous-based formulations. A key challenge for their widespread application is achieving low friction across a broad range of speeds. A low-viscosity system composed of polyalkylene glycol (PAG), water, and diethylene glycol offers superlubricity (i.e., friction coefficient <= 0.01) at rolling speeds above 150 mm/s; however, significant friction remains at lower rolling or sliding speeds. This limitation can be addressed by introducing eco-friendly and non-toxic gallate molecules. For example, adding 1 % lauryl gallate stabilizes the friction coefficient at approximately 0.04 with no measurable wear. To activate the anti-friction and anti-wear properties of gallates, the molecules must have alkyl chains with eight or more carbon atoms. Large-scale molecular dynamics simulations have been conducted to explore the key mechanisms by which gallate molecules achieve such superior lubricity. This is made possible through innovative machine learning techniques that enable simulations with Density Functional Theory (DFT) accuracy, allowing the modeling of large systems over extended timescales. Simulations reveal that the superior lubricity of gallates results from the strong anchoring of molecular patches that chemisorb onto the iron surface in specific orientations, enabling the alkyl chains to form an inert cushion at the steel/steel interface. Lubrication occurs thanks to this chemical inert buffer region, which effectively separates the metal surfaces realizing a beneficial friction and wear reducing tribofilm, with a clear dependence on the chain length. These findings by a combined experimental-computational approach provide valuable insights for the development of sustainable lubricants, advancing the field of green tribology.