Polymer derived tribofilm on silicon-doped diamond-like carbon coatings

Metal-free polymer additives used in conjunction with diamond-like carbon (DLC) coatings offer an environmentally sustainable approach for reducing friction and wear under boundary lubrication. In this study, a functionalized block copolymer, poly(lauryl methacrylate)-block-poly(2-dimethylaminoethyl methacrylate) (PLMA-b-PDMAEMA), is investigated as a friction modifier for silicon-doped DLC (Si-DLC) coatings. Tribological tests reveal that this copolymer significantly lowers friction compared to both a PLMA homopolymer and the base oil alone. ToF-SIMS, synchrotron-based XPS, and cross-sectional FIB-TEM analyses confirm the in-situ formation of a robust tribofilm, with a thickness of approximately 5–12 nm. Depth-resolved chemical analysis reveals a stratified structure in which the PDMAEMA segment chemisorbs onto reactive silicon sites via N–Si bonds, while the PLMA alkyl chains form a low-shear, oleophilic outer interface. The tribofilm remains intact after sliding under a Hertzian contact pressure of approximately 1 gigapascal, indicating excellent mechanical resilience. These findings demonstrate that molecular-level design of copolymers enables strong substrate anchoring and friction reduction without the use of metals, phosphorus, or sulfur. The mechanistic insights gained from this work provide design principles for next-generation, sustainable lubricant additives capable of forming self-assembled, nanoscale tribofilms on reactive DLC surfaces under demanding conditions.