Of fiery sparks and glittering spots: melting-resolidification and spherical particle formation in abrasion

The curious occurrence of perfectly spherical particle debris when a steel substrate is slid against a hard abrasive was first documented by Robert Hooke in the seventeenth century. Similar observations now abound in other abrasion-type processes, from industrial grinding to sliding rock faults. The prevalent hypothesis, originally proposed by Hooke, is that these particles form due to high local temperatures, resulting in particle ejection, melting and resolidification. In this work, we revisit this hypothesis, using a model steel-abrasive contact, a combination of in situ and post-process investigations, and complementary analytical calculations. Our results reveal two primary findings-firstly, the temperature of particles ejected from the contact zone is far from the melting point, and secondly, exothermic surface oxidation plays a critical role in actually melting the particle. Melting is either complete or partial, leading to spherical particles or 'slivers', as described originally by Hooke. Finally, we confirm that resulting particle surface patterns are typical of rapid solidification from the melt. Apart from throwing light on a centuries' old curiosity, our results precisely quantify the melting-resolidification process, with implications for a variety of applications, ranging from abrasion and powder production to the formation of micrometeorite dust.

Automated summary

The occurrence of perfectly spherical particle debris during steel abrasion has been a curiosity since the seventeenth century. Previous hypotheses suggest that high local temperatures cause particle ejection, melting, and resolidification. However, our in-depth investigation reveals that the ejected particles are not near the melting point and surface oxidation actually plays a critical role in melting. Complete or partial melting results in spherical particles or 'slivers'. The resulting surface patterns confirm rapid solidification. This research sheds light on an age-old curiosity and provides important insights for various applications.