Aqueous Lubrication of SiC and Si3N4 Ceramics Aided by a Brush-like Copolymer Additive, Poly(L-lysine)-graft-poly(ethylene glycol)

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We have examined the adsorption properties of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)—a brush-like polymer—on Si3N4 and SiC surfaces and determined its impact on the aqueous lubrication of Si3N4 and SiC at various speeds and applied loads. The addition of PLL-g-PEG in aqueous solution reduces the interfacial friction forces significantly for self-mated sliding contacts of these two ceramics, as compared to lubrication with water or buffer solution alone. For SiC, the improved lubricating performance by addition of PLL-g-PEG was apparent for all tested speeds (from 1.4 to 185 mm/s under 2 N load). For Si3N4, the effect was more apparent in the slow-speed regime (≤20 mm/s under 2 N load) than in the high-speed regime (>100 mm/s), where extremely low coefficients of friction (μ ≤ 0.006) are readily achieved by aqueous buffer solution alone. It was further observed that the optimal lubricating effect with Si3N4 is achieved when the tribopairs are first run-in in polymer-free aqueous buffer to render the sliding surfaces smooth, after which the PLL-g-PEG copolymer is added to the buffer solution.

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