Ultrasonic machining of glass substrates

Machining of dielectric materials, like glass, poses significant challenges because of their unique physical, mechanical, and chemical properties. Glass is a hard, brittle, and electrically non-conductive material with a high degree of transparency and chemical inertness [ 1 , 2 ]. Since these materials lack electrical conductivity, they cannot be machined using standard electrical discharge machining (EDM) or electrochemical machining (ECM) techniques, as both processes depend on conductive workpieces to facilitate material erosion [ 3 , 4 ]. Additionally, traditional mechanical machining methods, such as milling and drilling, often lead to undesirable outcomes, including excessive tool wear, chipping, micro-cracks, and surface defects due to glass's high brittleness and low fracture toughness [ 5–7 ]. To address these challenges, alternative machining methods have been developed, among which ultrasonic machining (USM) has emerged as a highly efficient solution for processing glass and other non-conductive materials. USM is a material removal process that relies on high-frequency ultrasonic vibrations (typically in the range of 20–40 kHz), operating without thermal, chemical, or electrical effects to generate localized stress concentrations, facilitating micro-fracture-based material removal [ 8–10 ]. This process employs an abrasive slurry, typically consisting of fine particles of boron carbide (B 4 C), silicon carbide (SiC), or aluminum oxide (Al 2 O 3 ), suspended in a liquid medium, which aids in material removal through a combination of impact erosion and mechanical abrasion. Also, the key advantages of USM in machining glass are its ability to achieve intricate geometries and high surface quality with minimal subsurface damage [ 11–13 ]. Unlike thermal-based machining methods, USM does not generate excessive heat, thereby preventing thermal stress-induced cracks and structural deformations, which are common issues when machining brittle materials. Furthermore, the process allows for the fabrication of micro-features, precision holes, and complex contours, making it particularly suitable for applications in microfluidics, biomedical devices, optical components, and micro-electromechanical systems (MEMS). The absence of tool wear on conductive electrodes, as seen in EDM, and the non-reliance on electrolytes, as required in ECM, further enhance the appeal of USM for machining glass and other brittle dielectric materials [ 14 , 15 ].