Tribological Interface Improvement through Lubricant Additive Innovation, Surface and Material Modification


Increasingly high global energy consumption demands effective approaches to high energy efficiency and, at the same time, paths to reduced release of carbon dioxide, a primary greenhouse gas behind global climate change. Friction reduction is a vital aspect towards making energy systems more efficient and one of the most crucial factors affecting energy and environmental sustainability. A tribological interface refers to an interface under surface contact and relative motion. Friction is a vital property of such an interface and a low-shear environment at the interface is favorable for friction reduction. The research presented in this thesis is on achieving such low-shear interfaces through discovery of promising lubricant additives, surface and material modification approaches by identifying key gaps in the current understanding of the major lubrication technologies and solving problems with modeling and experimental approaches.The work on lubricant additives is aimed at overcoming deficiencies of deposited low-friction coatings by self-generated and replenishable lubricious anti-friction carbon-film product (termed as carbon tribofilms) on a lubricated rubbing surface. One of the main objectives of this portion of the research is to accurately identify the chemical nature of such carbon tribofilms responsible for boundary lubrication, which have been ubiquitously characterized as diamond-like carbon (DLC), or graphitic, or polymeric in nature. A comprehensive investigation involving both experimental and computation techniques shows that such tribofilms are high molecular-weight friction polymers although they may resemble some features associated with DLC, which can be deceptive. Building upon the investigation of the nature of tribofilms is the analysis on the major factors that influence their generation. Possible lubricant additive molecules with differences in relative stability and adsorption strength on the steel surfaces are explored and inferences are drawn about the parameters affecting the kinetics of tribochemical reactions and hence, the friction-wear reduction abilities. The experimental and computational analyses reveal that strong adsorption and high instability of the lubricant additive molecules facilitates tribofilm formation which, in turn, alters the material and tribochemical nature of an interface. The first piece of work on the surface modification improves the compatibility of DLC surfaces with the polar head groups of friction modifier molecules. This is accomplished by subjecting DLC surfaces to an acidic treatment. Water contact angle measurements reveal increased hydrophilicity of DLC surfaces after the treatment. As a result, increased uptake of friction modifier molecules is observed on such treated surfaces, translating to a lower coefficient of friction obtained from micro-scale tests. The second surface modification method enhances interfacial tribological performance via topographical changes in the form of a micron sized pores which are random in their distribution as well as in their shape and size. A modeling system is developed that combines a microscopic single-pore computational fluid dynamic analysis with a macroscopic hydrodynamic model in order to capture the friction behavior of surfaces with pores and grooves, validated by bench reciprocating tests. The analysis revealed that surface pores help in friction reduction under starved lubrication conditions although they offer no benefit under flooded lubrication conditions. The material modification approach aims at replacing typical engineering steels (such as AISI 52100) with specially designed dual-functional steels. The dual-functional steels are not only strong in mechanical properties, but their surface chemistry is so formulated that they also have the ability to furnish wear-protective lubricious tribofilms. Such steels contain selected elements that can catalytically yield tribofilms from the interaction with base oil and/or special additives. The concept is shown by using commercially available steels with potential catalytic elements in their composition. The results show that chromium is most potent in its ability to produce carbon tribofilms as demonstrated by the lowest friction and wear in a steel with maximum Cr content.

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