The Limited Role of Pit Formed by Microjet in Evolution of Cavitation Erosion in the Incubation Period

Stepwise cavitation erosion is used to study pits formed by microjets. The aim is to determine the role of the pit in the development of cavitation erosion damage, including geometrical features of the pitted area and pit counting. The study also identified the dominant feature in the development of cavitation damage. We used a vibratory cavitation apparatus to perform cavitation erosion damage on a stainless steel, SUS 304, in tap water with scanning electron microscopy (SEM) at 5-min intervals from t = 0 min to t = 45 min. We then analyzed and characterized the pits formed by impacting the surface with a microjet. These results showed that individual pits and clusters of pits are characterized by a black spot surrounded by plastic deformation, and both are irregular in shape. This suggests that the microjet is hitting the wear surface in an oblique direction. Over the course of the test time, neither the shape nor the size of the microjet pits and their clusters change; pit diameters approximately 2 μm or less. The area of pits and their clusters does not exceed 1% of the worn surface. All these features demonstrate that the pit and its clusters do not impact the evolution of cavitation damage. Our results also indicated that slip bands were caused by a shock wave. Slip bands are large (>10 μm) and account for about 30% of the wear surface. This confirms that slip bands are predominantly responsible for the development of cavitation; the predominant cavitation mechanism is fatigue due to multiple shock waves.