Understanding the Tribocorrosion Behavior of Engineered Surfaces
AdvisorMenezes, Pradeep L
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Wear and corrosion are the most common forms of degradation in automobiles, ships, aircraft, biomedical implants, industrial machinery, and other mechanical systems. The dependence of these mechanical systems on tribological contacts accounts for a global economic wear loss of US$300 billion USD (2017) annually. The losses from fiction and wear are estimated to be 73% and 27%, respectively. Additionally, the global cost of corrosion is estimated to be US$2,505 billion, which is equivalent to 3.4% of the global Gross Domestic Product (GDP 2013). Today, with advanced manufacturing and surface processing techniques, it is possible to render a surface functionally resistant to wear or corrosion. A major drawback of such functionalization is that the processing specifically enhances resistance to either wear or corrosion, however, the action of wear and corrosion can lead to a wear-corrosion synergistic degradation, which has been scarcely understood. When functional surfaces are in relative sliding motion in a lubricated environment, they undergo wear and corrosion over extended periods of operation. The oxide layer formed as a result of corrosion is mechanically removed during sliding, thereby exposing a fresh layer of metal to degrade by wear and corrosion. The cycle of synergism between wear and corrosion is called tribocorrosion. The onset of tribocorrosion causes material degradation to occur faster than the action of either wear or corrosion alone. Understanding and characterizing the mechanism of tribocorrosion in various mechanical systems is complicated due to the variability in surface characteristics, lubricants, and tribo-interface conditions. This has also made it challenging to develop a stable wear-corrosion synergism monitoring system and tribocorrosion resistant surfaces for oil and aqueous environments.In the present study, an experimental module to monitor tribocorrosion in-situ is designed and implemented to measure the tribocorrosion behavior of various surface characteristics, lubricants, and tribo-interface conditions. A focus is placed on understanding the mechanism of tribocorrosion and designing surfaces with enhanced tribocorrosion resistance. The effect of surface engineering methods, such as textures and coatings on tribocorrosion, are investigated. More specifically, the surface engineering methods include surface processing using laser shock peening (LSP) on steel and magnesium alloys, and nanocomposite coatings on surfaces using Nickel-Graphene. The environmental conditions include aqueous environments that simulate seawater conditions. The study investigates the mechanochemical and physicochemical behavior of surface characteristics after modification to understand the mechanism of tribocorrosion. The effect of LSP intensity on wear, corrosion, tribocorrosion, surface roughness, and surface energy is discussed, and phenomenological models are proposed. The observed correlation between surface roughness, surface energy, and wear-corrosion synergism in defining the tribocorrosion mechanism is also discussed.This study has enabled (1) understanding of the association between electrochemical and mechanical processes that affect the material and lubricant degradation, (2) application of innovative solutions to mitigate tribocorrosion in advanced manufacturing and material processing applications, and (3) design of material systems that are resistant to tribocorrosion. The study on the tribological behavior of LSP surfaces shows the coefficient of friction (COF) can be reduced by up to 83.25% depending on the applied laser intensity. From the results of this study, it was inferred that the surface roughening effects induced by the laser intensity plays a major role in defining the tribocorrosion behavior of LSP surfaces. The tribocorrosion studies that followed indicate that a change in surface roughness can drastically modify the wettability of the surface, more so in environments susceptible to corrosion. The wettability can be quantified into various components of surface energy. The surface energy was found to be the lowest at lower laser intensities. Lower interfacial surface energy showed decreased wettability, providing enhanced tribocorrosion resistance. However, higher laser intensities increased the surface roughening effect, causing an increase in the interfacial surface energy and wettability of the surfaces, and thereby decreasing the tribocorrosion resistance. Depending on the applied laser intensity, the surface roughness and the profile of the treated area can be precisely controlled, thereby providing a technique to tailor not just tribological properties, but also the tribocorrosion properties. Tribocorrosion studies have also been conducted for electrodeposited nickel-graphene (Ni/GPL) nanocomposite films on steel to understand the wear accelerated corrosion in contaminated oil-lubricated mediums. It was observed that the wear-corrosion synergism for steel with and without the Ni/GPL film was negligible in an uncontaminated oil (synthetic transmission oil). However, when the oil was contaminated, the wear-corrosion synergy was higher on steel than on steel with Ni/GPL. The enhanced resistance to wear corrosion synergy was attributed to the Ni/GPL having refined grains that results in minimal transport of corrosive contaminants, water, and oxygen that inhibits corrosion cracking or pitting to the substrate. Further, the presence of GPL in the film minimized the effects of wear, which resulted in enhanced resistance of Ni/GPL films to tribocorrosion. This study provides insight into the role of surface characteristics, lubricants, and tribo-interface conditions that define the synergism between wear and corrosion. The research will enable the effective utilization and design of a tribo-system that can maximize the tribological performance and inhibit tribocorrosion.