Tribochemical stabilization of high-viscosity natural esters by a deep eutectic solvent on AISI 316L stainless steel under starved and flooded sliding conditions.

The tribological performance of natural esters in boundary-lubricated contacts is often limited by elevated viscosity, which impedes debris evacuation and promotes third-body abrasive wear under oil-starved conditions. This work evaluates a biodegradable deep eutectic solvent (DES) of menthol and decanoic acid as a tribochemical additive (0.1 wt.%) in PAO 4, sunflower oil, and castor oil. Wrought AISI 316L stainless steel was tested on a ball-on-disc tribometer under starved (100 µL) and fully flooded (60 mL) unidirectional sliding conditions (8 N, 0.05 m/s).

Under starved conditions, neat castor oil produced a friction coefficient of 0.172 and a 937 µm wear track width, showing debris retention. Adding 0.1 wt.% DES reduced friction to 0.126, wear track width to 621 µm, and the specific wear rate by approximately 70% (from 21.57 × 10⁻⁵ to 6.50 × 10⁻⁵ mm³ N⁻¹ m⁻¹). EDX analysis of the DES-treated track indicated a uniform surface film formed via reaction between the decanoic acid and the 316L passive layer.

Under fully flooded conditions, the DES-castor oil formulation maintained stable boundary lubrication (µ = 0.090) with no progressive surface degradation. However, DES incorporation into non-polar PAO 4 yielded poor performance across both regimes due to limited miscibility. Results demonstrate that base oil polarity governs the tribochemical efficacy of this DES on AISI 316L, and chemical film formation successfully offsets the debris-entrapment limitations of high-viscosity natural esters.