Interaction between DLC and lubricants to develop recyclable low SAPS formulated oils

Diamond-like carbon (DLC) coatings doped with Ti, Mg, and Co (separately) were developed using non-reactive direct current magnetron sputtering (DCMS). The coatings were fabricated in conventional monolayer and new multilayer microstructures, employing a novel methodology by controlling the rotational speed of the substrate holder. Three mono- and multilayer DLC coatings doped with Ti and Mg, each with analogous dopant concentrations, were produced. The Co-doped DLC coatings were only developed with the multilayer microstructure. The multilayer coatings demonstrated significant improvements in mechanical properties, approaching those of undoped DLC, while simultaneously retaining the advantages introduced by the dopant elements. After addressing the mechanical aspects of the coatings, their tribological performance was evaluated for monolayer and multilayer Ti-DLC coatings and multilayer Mg-DLC and Co-DLC coatings. The tribological tests were conducted under lubrication with polyalphaolefin 4 (PAO 4) base oil and its formulations containing two organic, ashless, and sulphur-free (low SAPS) anti-wear (AW) and extreme-pressure (EP) additives: Duraphos® 178 (C12–14 - alkyl amine isooctyl phosphate) and Duraphos® OAP (octyl dihydrogen phosphate). These additives varied in phosphorus content (ester group) and nitrogen content (amine group). The multilayer coatings consistently exhibited superior tribological performance. Specifically, formulations of Duraphos® OAP significantly reduced friction, achieving superlubricity conditions, while formulations of Duraphos® 178 were effective in minimising wear. The study confirmed that mechanical properties of M-DLC coatings can be effectively controlled and enhanced by simply adjusting the rotational speed of the substrate holder in front of the targets, enabling the formation of alternating layers of DLC and M-DLC without employing a complex process. Additionally, the eco-friendly additives, used in a non-hazardous base oil, demonstrated a synergistic effect in reducing wear and friction for these coatings. This represents a significant step towards energy and resource conservation, aligning with the principles of Green Tribology to promote a sustainable future.