Ultrasonic cavitation shows a great potential in various industrial applications such as sonochemistry, food processing, ultrasonic cleaning, and surface treatments. These applications have the advantages of high temperatures or high pressure due to the collapse of cavitation bubbles. In surface treatments, the collapse of bubbles occurs near workpiece surfaces and creates micro-jets which lead to high impact forces. As one of these surface treatment processes, ultrasonic cavitation peening requires a small gap between the vibration source and the treated surface to obtain the maximum impact force. Due to these small gaps, the growth and collapse of cavitation bubbles are affected, which result in the changes of impact forces. Therefore, the investigation of the impact loads caused by ultrasonic cavitation bubbles in small gaps is the focus of this contribution. A theoretical model taking into consideration bubble interactions is utilized to estimate the optimal standoff distance at which the largest impact forces occur. Then, experimental investigations are carried out. A piezoelectric sensor with a titanium alloy cover is used to record the number of impacts and their amplitudes. The recorded signals are then processed in time and frequency domains. The experimental results show that large impact loads are generated when the gap width is in the range of 0.5-0.8 mm. It is also found that the maximum working efficiency occurs in this range
