Lubrication for aluminum-steel contacts remains a critical yet underexplored area in tribology, especially under boundary conditions. This study examines three environmentally friendly choline-based protic ionic liquids (PILs): choline glycine [CHO][GLY], choline alanine [CHO][ALA], and choline lysine [CHO][LYS], as neat lubricants and as 1 wt% additives in a fully formulated oil to determine how molecular structure governs friction, wear, and tribochemical film formation at the aluminum-steel interface. Reciprocating sliding tests were conducted using a ball-on-disk configuration at varying loads. Friction coefficients were recorded in real time, while post-test wear characterization measurements were performed to assess tribochemical transformations. The results revealed that, when used as neat lubricants, the structure of the anion strongly influences friction-reducing and anti-wear performance. The short alkyl chain in the anions of [CHO][GLY] and [CHO][ALA] does not lead to the formation of an effective boundary layer, and the resulting hard/hard contact between sliding asperities leads to the pronounced formation of carbonaceous, disordered films. In contrast, the long alkyl chain in the anions of [CHO][LYS] leads to the generation of a boundary film upon confinement that reduces wear, while minimizing hard/hard contact and the tribochemical reaction of the PIL. As oil additives, all PILs improved wear resistance relative to the base oil, with reductions of up to ∼33 % at high loads, accompanied by more stable friction responses. The findings provide a pathway for designing task-specific lubricants for lightweight metal interfaces, with implications for the automotive, aerospace, and manufacturing sectors, which are focused on energy efficiency and environmental sustainability.
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