Role of dielectric fluid and concentrator material in microwave drilling of borosilicate glass

Microwave drilling is electromagnetic energy-based machining process in which microwave radiation at 2.45 GHz is concentrated into a narrow region using a thin metallic concentrator. A high electric field region gets developed around the concentrator tip which ionizes the dielectric media around to form plasma. Heat released from the plasma removes the target material in the vicinity through melting and ablation. However, limited control over the heat during the plasma discharge results in thermal damage of the target material. Thermal damage is more in the materials with poor thermal conductivity. Therefore, materials like glass experience thermal damage as well as frequent cracking. Thus, control over the plasma is critical for quality output of the process. The current work presents a new approach to minimize the plasma induced defects like crack, heat affected zone (HAZ), overcut and taper during microwave drilling of borosilicate glass at 2.45 GHz and 700 W. The study was carried out while drilling 1.3 mm thick glass plate using 0.6 mm concentrator at 700 W in a domestic applicator. Effects of concentrator material, dielectric medium and immersion depth on hole characteristics were studied in terms of HAZ, cracks, taper and overcut to obtain the optimum drilling condition. Mechanism of microwave drilling in the presence of a dielectric medium has been explained. The results revealed that the dielectric constant of dielectrics and electric conductivity of the concentrator materials affect the plasma shape and intensity whereas thermo-physical properties like viscosity and thermal diffusivity affect the confinement of plasma into a narrow zone. On the other hand, it was found that higher immersion depth reduces defects like crack, thermal damage due to low-temperature gradient on the workpiece surface and better heat dissipation from the surface of the workpiece. The best result was obtained with graphite concentrator in transformer oil dielectric at an immersion depth of 35 mm.