Advanced Lubricant Additives of Dialkyldithiophosphate (DDP)-Functionalized Molybdenum Sulfide Nanoparticles and Their Tribological Performance for Boundary Lubrication

Advanced lubricants can improve productivity through energy saving and reliability of engineered systems, and there is a growing interest in application of nanostructured lubrication materials stimulated by offering valuable properties and exhibiting novel lubrication behaviours. The hybrid chemo-mechanical milling process of molybdenum sulfide with second component based on phosphorous element groups was used as a synthetic approach to prepare multi-component nanoparticle additives. Their addition to base oil could provide advanced lubrication by: (1) supplying nanosized lubricating agents which reduce friction and wear at the asperities contact zone, (2) enabling strong tendency to adsorb on the surfaces, (3) reacting with the surface, forming lubricating tribofilms to sustain high loads and high temperatures, and (4) enabling all these at minimal cost and less environmental impact by reduction or substitution of toxic additives in formulated oils. The authors have successfully synthesized layered inorganic solid nanoparticles based on molybdenum sulfide which were functionalized with dialkyldithiophosphate groups. The structural properties of novel additives were characterized using TEM and particle size analysis. These additives have demonstrated superior tribological properties under harsh friction conditions and their tribological behaviors were compared using Four-ball and Block-on-ring tests. The formed tribofilms in the wear scars were studied using XPS, Auger and TOF-SIMS techniques. Addition of functionalized MoS2 nanoparticles significantly improves anti-wear properties of base oil, and there is a friction reduction in comparison with samples of base oil, conventional MoS2, or non-modified MoS2 nanoparticles.