Synthesizing in situ Friction and Wear with ex situ Surface Metrology to Provide Post-mortem Tribological Analysis: Experiments and Modeling
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Surface wear is estimated to result in upwards of 70% of material failure in the US with costs of over 300 billion dollars per annum. Tribology is defined as the study of friction, lubrication, and wear, and while all three of these sub-disciplines stem from interactions at the asperity scale, studies involving the mechanics of industrial interfaces will often ignore their interdependence. This work makes use of a synthesis of in situ tribological data with ex situ surface characterization, in order to elucidate the mechanics of friction and wear in a number of industrial interfaces each with its own objective. Section 1 focuses on using this technique to elucidate the mechanics of protective solid lubricants. Detailed experiments were conducted to study the formation and depletion of self-replenishing powder transfer films in both single component and composite forms. Based on the synthesis of ex situ and in situ findings, the primary wear mechanisms at each interface were described as abrasive and transfer film phenomena were described in a new way using a quasi-hydrodynamic approach. In addition to transfer films, hard tribological coatings were also studied for their ability to mitigate impact damage. Coefficient of restitution results were combined with investigations ex situ of the impact sites and compared to nanoindentation results of coating properties. Through synthesizing these results, it was found that more elastic coatings worked best on stiffer substrates, while harder, more brittle coatings worked best on soft substrates. In Section 2, the Section 1 findings were crucial in the development of a numerical model which was presented using abrasive formulations for the wear of soft surfaces and preferential patterning. Quantitative agreement for modeling friction and surface evolution, as well as qualitative agreement for wear trends were provided for experimental values from previous studies. In Section 3, this technique was used to study powder rheology as it applies to flows in the additive manufacturing process. Rheological characterization was conducted for stress states akin to spreading and hopper flow on an FT4 powder rheometer, while morphological characterization was performed in collaboration with the Material Science Department at Carnegie Mellon using scanning electron microscopy. By analyzing the results in concert, it was found that morphology proved to be more important than material type or manufacturing process in governing flow properties. Spreading-like states were found to be most sensitive to factors affecting particle rolling, while hopper-like states were found to be most sensitive to factors affecting particle cohesion. In Section 4, the interactions of single cutters were explored for rock substrates found during drilling for oil, gas, and geothermal heat. Experiments were conducted first for O1 tool steel buttons on Carthage Marble. Cutting with this type cutter was found to produce rough surfaces which would lead to an increase of friction force in the cut. Experiments for dry and lubricated cutting with industrial polycrystalline diamond compact cutters were performed for Carthage Marble, Nugget Sandstone, and Mancos Shale on a retrofitted UMT-3 Tribometer from Bruker. Cutting was found to produce smoother topographies and a decrease in friction. Lubricants were found to possess both a lubricating effect which would reduce COF as well as a weakening effect which would enhance rate of penetration and the load at which cutting would commence. Scraping was found to produce a scalloped topography similar to “bit-bounce.” Industrial drilling fluids or “muds” were also evaluated and ranked using a figure of merit proposed within this work. By normalization with the dry scenario, muds performance in terms of friction and rate of penetration could be combined to provide ranking of a given mud for a given rock type. Overall it was found that implementing both in situ tribological data and ex situ surface metrology was an extremely effective way to recreate the mechanics present in industrial interfaces which are difficult to observe otherwise.