Towards understanding the ultrasonic vibration-assisted diamond cutting of ZnO crystals: Surface integrity and tool wear mechanism

Zinc oxide (ZnO) is a promising material for optics and optoelectronics. However, its machining methods are limited, with polishing being virtually the only available technique, which restricts the fabrication of complex shapes. In this study, for the first time, ultraprecision diamond cutting of single-crystal ZnO was explored. Conventional cutting (CC) and ultrasonic vibration-assisted cutting (UVC) experiments were performed in dry and oil mist lubrication environments. It was found that under dry conditions, both CC and UVC resulted in surface fractures owing to the oxidation-induced chemical tool wear. Using oil mist lubrication significantly reduced tool wear, particularly for UVC. As a result, surface roughness was reduced to 1.9 nm Sa. Cutting in the [11−20] direction was more likely to achieve crack-free surfaces on ZnO than cutting in the [1−100] direction under CC; however, the machinability in cutting in the [1−100] direction was improved by applying UVC. Both CC and UVC processes activated basal and pyramidal slip systems in the subsurface region, accompanied by a high density of dislocations located immediately beneath the surface. UVC further promotes polycrystallization and pyramidal slip activity in the subsurface. A microlens array was fabricated with a surface form error of less than 49 nm peak-to-valley and a surface roughness of less than 2.2 nm Sq by integrating slow tool servo diamond turning with ultrasonic vibration assistance under oil mist lubrication. This study reveals the fundamental cutting characteristics of ZnO, provides guidance for ultraprecision cutting of brittle oxide crystals, and validates a potential solution for fabricating micro-structures on ZnO surfaces.