A study of the abrasive waterjet micro-machining process for quartz crystals and impact erosion by high velocity micro-particles

Abstract

A comprehensive literature review on the development of abrasive waterjet (AWJ) machining technology and the understanding of particle impact erosion has been conducted. It has revealed that this technology possesses distinct advantages in performing micro-machining tasks over many other technologies, but further effort is required to enhance its cutting performance and understand the associated impact erosion process.

An experimental study using a pre-mixing AWJ, or abrasive slurry jet, to produce micro-channels on a quartz crystal has been undertaken to understand the machining process and performance, and the effect of process parameters. It shows that an increase in water pressure, particle concentration, abrasive particle size or jet impact angle, or a decrease in nozzle traverse speed is recommended to increase channel depth and material removal rate. By properly controlling the machining process, large wavy patterns can be minimised on the channel bottom surface. When a micro-particle impacts a quartz crystal, three types of impressions have been identified, namely craters, scratches and micro-dents, of which craters caused by brittle conchoidal fractures significantly contribute to material removal. Mathematical models for predicting the channelling performance have been developed.

A computational model for representing the impact process by a high velocity micro-particle on a quartz crystal has been developed using a discrete element method. It shows that micro-cracks on the target are initiated by high shear stresses and then median and lateral cracks are formed by both tensile and shear stresses. Material removal is mainly due to the propagation and intersection of micro-cracks which consume most of particle energy. A smaller impact angle with a lower particle velocity yields less subsurface damage to the target.

The single particle impact model has been extended to study multiple impact process incorporating a particle flow model. It shows that residual cracks can degrade the strength of substrate and facilitate material removal in subsequent impacts. A relatively large overlapping condition between successive particle impacts is more efficient in material removal in the second impact under both normal and oblique impact angles. A small jet impact angle with a fast nozzle traverse is recommended to minimise the subsurface damage.