Study on the surface formation mechanism in scratching test with different ultrasonic vibration forms

Rotary ultrasonic machining (RUM) has been considered as an effective approach in the manufacturing of advanced materials such as ceramic matrix composite (CMC). However, the distinct removal mechanisms in RUM with different vibration forms has not been elucidated, which hinders the adoption of RUM technology in the field of material processing. In this research, the ultrasonic-assisted scratching tests of CMC were conducted to reveal the fundamentals of material removal and surface formation. Based on the kinematics of RUM processes, the vibration effects exerted in cutting areas were clarified as impact-separation and reciprocating-polishing, thus two types of diamond indenters have been specially designed for the intermittent and continuous scratching tests, respectively. The scratching force, friction factor and grooves morphology have been analyzed and compared with that of the conventional scratching without vibration. For the intermittent scratching with vertical vibration, it was found that the high frequency impact-separation between indenter and workpiece increased the critical penetration depth of ductile-brittle transition, which can help restrain the cracking defects of CMC. For the continuous scratching with tangential vibration, the overlapping rate was firstly proposed to evaluate the effect of reciprocating polishing. It was shown that higher overlapping rate and larger dynamic shear angle can be achieved by matching the scratching velocity with vibration frequency, which is conducive to obtaining even fiber fracture and better surface integrity. Due to the minimal volume of material removed in each vibration cycle, the scratching resistance was significantly lowered with smaller fluctuation under both vibration forms. Moreover, the diminishing of vibration effectiveness at certain scratching parameters was also discussed. This research work brings in a better understanding on the material removal mechanisms under different effects of ultrasonic vibration, which can provide a guidance for the design and optimization of the vibration assisted processing of advanced materials.