Synergistic hydration lubrication: design and tribological performance of a functional composite for water-lubricated bearings

To address severe friction and wear of water lubricated bearings under boundary lubrication caused by the low viscosity of water, a functional composite was developed by grafting a hydrophilic layer onto Si 3 N 4 /PU and incorporating load bearing hydrogel particles. The design utilizes the hydration lubrication of the surface hydrophilic layer in the initial friction stage and the sustained hydration lubrication of exposed hydrogel particles after wear, thereby maintaining an improved hydration lubrication state throughout the entire friction process. Systematic materials tests (contact angle, compression, and reciprocating friction) were conducted to investigate the effects of hydrogel content, while bearing-level bench tests under various specific pressures were carried out with pure PU bearings as a reference to verify the practical tribological performance. The composite with 0.5 wt% hydrogel showed optimal performance. Under 40 N, its friction coefficient (0.23) and wear rate (8.5 × 10 −6 mm 3 /(N·m)) were reduced by 19% and 53.1% compared with the hydrogel-free composite. Under 0.3 MPa, the composite bearing reduced friction coefficient and wear volume by 12.1% and 65% relative to pure PU (0.19 and 0.8 mm 3 ). Notably, when specific pressure increased from 0.3 to 0.7 MPa, the composite's average friction coefficient further decreased by 59.9%, showing a continuous declining trend with increasing load, indicating excellent load-adaptive capacity. This study provides an effective approach for improving the tribological performance of water-lubricated bearing materials under boundary lubrication.