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United States Department of Agriculture

Agricultural Research Service

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Research Project: X-Ray Photoelectron Spectroscopy Investigation of Tribochemical Properties of Biolubricants

Location: Bio-oils Research Unit

2013 Annual Report


1a.Objectives (from AD-416):
The objective of this agreement is to conduct in depth understanding of the tribochemical reactions that occur during lubrication with biobased lubricants. Such investigation will allow for elucidating the reactants, the products, and the reaction mechanism occuring during a biolubrication process. Understanding of tribochemical reactions will allow for designing and synthesizing new biobased lubricant ingredients with superior oxidation stability, cold flow, and a number of tribological properties.


1b.Approach (from AD-416):
Model biobased ingredients will be used in extreme pressure tribological investigations on a 4-ball tribometer. The wear surfaces from such investigation will be analyzed by x-ray photoelectron spectroscopy (XPS). The analytical results will be used to construct hypothesis of potential reaction mechanisms at the tribological interface. The hypothesis will be refined further with additional carefully designed experiments.


3.Progress Report:

ARS scientists in the Bio-Oils Research Unit at the USDA-ARS National Center for Agricultural Utilization Research (NCAUR), Peoria, Illinois, in collaboration with scientists at the University of Illinois, conducted a preliminary investigation on the use of X-ray photoelectron spectroscopy (XPS) to study the tribochemistry of biobased oils. In this investigation, soybean oil (SO), with and without 20% zinc dialkyl dithio phosphate (ZDDP), was used in tribological tests at various loads on a 4-ball tribometer. The chemical characteristics and compositions of the tribofilms generated during the test on the steel balls were analyzed by XPS. The results showed that the tribofilm on the steel surface lubricated by SO with 20% ZDDP contained peaks corresponding to the following: carbon (C1s), oxygen (O1s), sulfur (S20), phosphorus (P2p), and zinc (Zn3p). The XPS peaks at binding energies in the carbon region were identifiable as those of carbon (C) and carbon-hydrogen (C-H) in SO and ZDDP structures. The result indicated that, at 320 Kg load, ZDDP molecules adsorbed on the metal surface and that zinc was involved in tribofilm formation on the steel surface during the friction process.

In the region of peaks corresponding to oxygen, three sets of peaks were identifiable: low binding energy peaks assigned to inorganic oxygen on the steel surface (such as ferric and ferrous oxides); high binding energy peaks assigned to inorganic oxygen in SO and ZDDP (such as esters, etc.); intermediate binding energy peaks assigned to metal-organic complex materials. It is proposed that the metal-organic complex materials with intermediate oxygen binding energies correspond to the tribofilms generated by adsorption and in situ tribochemical reaction between the molecules in the SO with ZDDP lubricant and metallic iron. The generation of such organo-phosphate iron complex tribofilm on the surface, capable of lowering friction and also protecting the metal from wear, is responsible for the good tribological properties of ZDDP. More work is underway to clearly understand and confirm the mechanism of biobased lubricant performance under extreme lubrication conditions.


Last Modified: 10/25/2014
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