The onset of gouging in high-speed sliding contacts
Hypervelocity gouging occurs in high speed sliding systems such as rocket sled test tracks, light gas guns, and railguns. Gouging takes the form of teardrop-shaped craters on the rail surface, and usually only occurs above a threshold speed which is dependent on the slider and rail material properties. In this dissertation, the onset of gouging was studied from three perspectives: application of existing modeling techniques developed for gouging and related fields of research, performing new high-speed experiments using a medium-caliber railgun, and analyzing rail microstructural evolution during gouge onset. A previous gouging model based on shock mechanics was extended, while other models based on mechanisms such as Rayleigh waves, bending waves, and shear band formation were ruled out. An effective Reynolds number approach from explosive welding research was applied to gouging with encouraging results. Based on similarities between gouging, explosive welding, and Kelvin-Helmholtz waves, a linear instability analysis was also performed. A total of 22 railgun experiments were performed to explore different aspects of gouging. Through these experiments, the effect of new slider materials, thin aluminum coatings, and macroscopic rail indentations on the gouging of copper alloy rails were examined. Results using new materials matched the existing models well, though galling damage to copper rails was often as severe as gouging. Gouging was delayed using electroplated aluminum coatings as thin as 2 μm, though this is not necessarily a robust solution. Macroscopic indentations were found to have negligible effect on the threshold velocity for gouging onset, though the morphology of the gouges was strongly affected. Both galling and gouging craters were shown to initiate at existing defects. This applied to both microscopic and macroscopic features. A consistent microscopic feature observed prior to galling and gouging were deformation bands that resembled persistent slip bands on the rail surface. Another consistent feature was the transfer of slider material to the rail prior to galling and gouging. This suggests that gouging may not be triggered by micro-impact events, but by instabilities associated with high-speed thermoplastic shear.