Investigating Tempo-Oxidized Cellulose Nanofiber Lubricating Greases Using Microscopy, Rheology, and Tribology
Date
2024-05-01
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Abstract
Lubricating grease plays a significant role limiting the negative effects of friction and wear present in moving components across a multitude of industries and applications. However, current materials commonly used to formulate greases often pose environmental risks during operation and disposal efforts. Additionally, due to the complex colloidal structures that form the backbone of grease, a detailed understanding of underlying lubricating mechanisms remains elusive, especially if considering greases composed of nanoparticle constituents. This thesis focuses on using sustainable materials such as cellulose nanofibers and organically modified nanoclay to formulate lubricating grease. Special detail is given concerning formulation failures and successes highlighting the role of surface modification using oleic acid and interfacial stabilization from clay particles. The performance of these greases are systematically characterized using two forms of microscopy, laser scanning confocal microscopy and cryogenic scanning electron microscopy, to understand microstructure networking behavior that ultimately governs their response under mechanical stress. Rheological characterization is performed to quantitatively evaluate grease behavior under dynamic stress/strain. Lastly, tribological characterization is performed to evaluate friction and wear performance of formulated samples. Care has been taken to link all three avenues of characterization to help provide a more detailed understanding of underlying mechanisms governing these specific grease systems. It was observed that most greases are composed of networks between tightly held spheroid bundles of cellulose and oleic acid while clay interfacially adsorbs at the interface and prevents large scale aggregates from forming. However at high concentrations of cellulose and oleic acid (>10% and 0.75 OA:TOCN ratio, respectively), the dispersion effect is so great that agglomerates form bundle-like structures which significantly alter rheological and tribological responses. Rheological flow curves and amplitude sweeps indicate two main competing factors, aggregate size and number of interfacial connections, are responsible for differences in network dynamics. Overall morphology also plays an important role indicated by the high storage moduli obtained for fiber-like samples (≈ 35 kPa). Stribeck and steady sliding wear curves revealed excellent boundary lubrication coefficient of friction performance in the range of 0.05-0.10 for all samples courtesy of both oleic acid and nanoparticle surface activity/tribofilm generation. It also revealed interesting high wear behavior at high relative concentrations of oleic acid. For example, wear scar diameter roughly increases 6x at 0.75 OA:TOCN ratio compared to both 0.25 and 0.50 ratios.
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Keywords
Tribology, Rheology, Confocal Microscopy, Grease, Sustainable
Citation
Uhryn, J. J. L. (2024). Investigating tempo-oxidized cellulose nanofiber lubricating greases using microscopy, rheology, and tribology (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.