Tribological properties and synergetic self-lubricating mechanism of PS-PAO based nanocapsules with MoDTC addtives

With development of encapsulation technology, analysis of dependent nano-scaled microcapsule is significant to intensively understand self-lubrication behavior. In this paper, polystyrene (PS)-poly alpha olefin (PAO) capsules were synthesized with PAO as core and PS as shell. The diameter, shell thickness and encapsulated rate of PS-PAO4 were 381.51 ± 25.44 nm, 40 nm and 43.29 wt.%. The friction coefficient (COF) of PS-PAO4 by SRV friction test was 0.089, which played intermediate between PS nanosphere and PAO oil. Comprehensive analysis with Hamrock-Dowson equation and classical Stribeck curve manifested that the PS-PAO nanocapsules and PS nanospheres stayed in mixed lubrication regime, and PAO oil stayed in the boundary lubrication regime. Due to the absence of lubrication, PS dry friction and asperities contact status were more significant to determine the lubrication state of PS-PAO4 sample. The influence factors including load, operating temperature, PAO types and molybdenum dithiocarbamate (MoDTC) addtives were respectively investigated. A three-stage decline curve of PS-PAO4/MoDTC was found with final COF of 0.015, which the COF value decreased by 83.15% compared by PS-PAO4. The synergetic self-lubricating mechanism was mainly determined by asperity contacts, PS debris grinding, FeS tribofilm formation of MoDTC additives and counterparts, transfer film and third body formation by PS, MoDTC, PAO oil and counterparts. This work provides intensive understanding of nanocapsules for mixed lubrication regime and is expected to apply in ultrathin coating, liquid lubricant additives and functional polishing abrasives under heavy-duty and low speed conditions.