Tribology Properties Of Crop Leaf-surface Wax As Lubricant Additives

Environmental friendly and degradable lubricant has been a significant research orientation for the purpose of reducing environmental pollution, and some plant constituent has been utilized as addictive into lubricant in some ways by researchers. It is known plant leaves are covered with thin wax layer on the surface of the leaves, which is called plant leaf-surface wax or plant epicuticular wax. Plant leaf-surface wax mixed of organic material mainly contained alkanes, alcohols, esters, and acid, which are insoluble in water and some organic solvent. From this point, we can infer that using the plant leaf-surface wax as lubricant additive is possibly. We selected four kinds of crops leaves as the research object, and the tribological properties of crop leaf-surface wax used as additives were evaluated on MFT-R4000 reciprocating friction and wear tester. The tests were done on three kinds of friction pairs, and the morphologies and wear volume of the worn surfaces were measured by a surface profilometer. The morphologies and element constitution of the worn surfaces were observed and examined using scanning electron microscope(SEM) and Energy Dispersive Spectrometer(EDX), respectively.Results showed that the poly-alpha-olefin (PAO) containing alfalfa or sorghum leaf-surface wax (ALW OR SLW) have excellent tribological properties on steel/aluminum or steel/steel friction pair; the poly-alpha-olefin (PAO) containing wheat leaf-surface wax (CWL)have excellent tribological properties on steel/copper friction pair. As to friction coefficient, alfalfa leaf-surface wax used as additives showed lower and more stable friction coefficient on steel/aluminum friction pair. The average friction coefficient reduced by 1/2 compared with the base oil, so its antifriction property is better than that of base oil and closed to the MoDTC. Compared with PAO or samples contained MoDTC, glycerin, and stearic acid, the wear volume of alfalfa leaf-surface wax sample reduced by 10%. Obviously, alfalfa leaf-surface wax possesses uperior anti-wear performance comparing to MoDTC and stearic acid, and is similar to that of glycerol. Wheat leaf-surface wax used as additives showed lower and more stable friction coefficient on steel/copper friction pair. The average friction coefficient reduced by 1/4 compared with the base oil, so its antifriction property is better than that of base oil and closed to the MoDTC. Compared with PAOor samples contained MoDTC, glycerin, and stearic acid, the wear volume of wheat leaf-surface wax sample reduced by 70% nearly. Obviously, wheat leaf-surface wax possesses uperior anti-wear performance comparing to MoDTC and stearic acid, and is similar to that of glycerol. The SEM results showed that samples with crop leaf-surface wax have fewer scratches, shallower grinding crack and smaller edge roughness compared to base oil.Gas chromatography-mass spectrometry (GC-MS)method was adopted to analysis the alfalfa and wheat leaf-surface wax. From the analysis results, we can know that the wax is mainly composed by fatty alcohol and fatty acid. Thereinto, fatty alcohol content of the leaf-surface wax is about 50%, the carbon chain length mainly ranges between C20～C30 and centers on C26 and C27, and the majority of alcohols are straight chain fatty alcohol, and some are with annular structure. The similar chemical structures were found by ToF-SIMS IV on the worn surface.Meanwhile, it also included alkane and the corresponding content is about 19%, and the carbon chain length ranges between C21 ～C30. And fatty acid of the wheat leaf-surface wax is about 2.6%.The tests also showed most carbon chains own odd number of carbon atoms. The alcohols, acids and other natural ingredients mixture of wheat leaf-surface wax can form a physical adsorption tribofilm on the worn surface. The esterification reaction also performed on the worn surface. The physical and chemical adsorption film is the basicreason of improving the anti-wear and anti-friction propertieson the friction pair. It is known that the better tribological property is partly attributed to the physical film and chemical adsorption reactions between the sliding surfaces.The results of the crop leaf-surface wax used as lubricant additive provides the theoretical supports for the research of the environment friendly lubricant, and also have a great significance for crop leaves utilization comprehensively and economically.