Abstract
Leaf-surface wax extracted from three kinds of plants, Hoya carnosa (L.f.) R.Br, Allium fistulosum var. giganteum Makino and Solanum melongena L., was used to verify the tribological properties of leaf-surface wax as environmentally friendly lubricant additives. The tribological properties of plant leaf extracts as lubricant additives to the base oil PAO for an aluminum-on-steel contact were evaluated using a MFT-R4000 reciprocating friction and wear tester. The morphologies of wear scars on an aluminum block were observed using a scanning electron microscope. A general trend of friction coefficients for the contact of aluminum/steel was as following: H. carnosa S. melongena A. fistulosum S. melongena > H. carnosa > A. fistulosum >PAO> MoDTC . As compared with MoDTC, three plant additives decreased the friction coefficient and increased wear resistance effectively, thus showing good friction reduction property, which might be related to the alcohol, ester and acid components contained in leaf-surface wax. The SEM results showed that plant leaf extracts as lubricant additives exhibited a lower wear and smooth worn surface as compared with PAO and MoDTC. We examined the anti-wear and friction reduction mechanisms of the plant additives by X-ray photoelectron spectroscope analysis of the worn surface of the aluminum alloy, taking S melongena as an example. The data showed that leaf-surface wax on the worn surface might exist in two forms, Al2O3 and the complexes of ethanediol and propanetriol. Our results indicated that all three plant leaf-surface waxes possessed excellent anti-wear and friction reduction properties in an aluminum-on-steel system, and were promising environmentally friendly lubricant additives.
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