Aqueous Lubrication of SiC and Si3N4 Ceramics Aided by a Brush-like Copolymer Additive, Poly( l -lysine)- graft -poly(ethylene glycol)
Hartung, Whitney ; Rossi, Antonella ; Lee, Seunghwan ; Spencer, Nicholas
In: Tribology Letters, 2009, vol. 34, no. 3, p. 201-210
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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 185mm/s under 2N load). For Si3N4, the effect was more apparent in the slow-speed regime (≤20mm/s under 2N load) than in the high-speed regime (>100mm/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