Visualizing, quantifying, and understanding nanowear of multi-asperity contacts

Wear takes place across the scale of contacts, involving different mechanisms, making predictive understanding difficult to build. At the macroscopic scale, the famous empirical Archard law predicts a linear relationship between the volume of material worn, the applied load and slid distance, normalized by the hardness of the softest material at the interface. This law describes wear extremely well in the context of debris formation, covering a wide range of wear mechanisms (fatigue, fracture, fretting, etc.) causing the softer material to wear down. For single asperity contacts, tribochemical (or stress-assisted) processes have been found to be the dominant mechanism driving wear. Derived from the Arrhenius equation governing chemical reaction rates, such processes describe the removal of single (or groups of) atoms by the supply of energy in the form of stress that enables the breaking of covalent bonds at the interface. On the one hand, attempts have been made to scale down the Archard law to single asperity context to understand how contact junction size is crucial in the transition from gradual to fracture type wear. On the other hand, efforts are made to observe tribochemical wear at multi-asperity contacts. In this thesis, we have experimentally studied promising systems to bridge the small-scale single asperity contact understanding to the large multi-asperity contact empirically understood. Our results indicate that tribochemical wear on hard, multi-asperity contacts can be detected but also understood (and manipulated) by changing the environmental conditions.