Drilling a 2-inch in Diameter Hole in Granites Submerged in Water by CO2 Lasers

Abstract

Abstract As part of a fundamental study of rock drilling by laser irradiation, this paper describes laser drilling of a 2-inch-in-diameter hole in granites submerged in water. In the air, fused rock produced by laser irradiation reflects or absorbs successively irradiated laser beams, thereby refraining laser heat from being transmitted into the rock. This absorption leads both to a temperature rise of the fused rock and a further increase in absorption of laser radiation by the fused rock, causing the rock destruction process to cease completely. From consideration of the lack of success in past laser rock drilling methods, we arrived at the idea that laser-induced mechanical forces could be utilized to generate a hole in rocks by laser irradiation in water. A granite specimen was submerged at 50mm from a water-surface and then 10.6 pm carbon dioxide laser (CO2) beams which have a high absorption coefficient in water were intermittently irradiated. The laser beams induced underwater shock waves, bubble formations, and micro-water jet formations upon bubble collapse. High-speed video observations revealed the generation of an initial cone-shaped cavity in the water, propagation of the laser beams through the cavity and eventual ablation of the granite surface. The laser beams locally melt the granite surface to form small glassy beads, which are readily removable by mechanical methods. Repetition of this procedure demonstrated that a high-power laser beam propagating in water can be used to generate a hole in rocks. The laser beam irradiation indeed drilled the granite specimen submerged in the water. A 2-inch-in-diameter hole in the underwater granites was successfully generated by two methods that were melting and spalling by the laser. Melting the granite to generate a 2-inch-in-diameter hole requires energy almost 3 times as much as spalling the granite to generate the same size hole.