Hole formation and thermal damage in laser drilling of oxide-oxide ceramic matrix composites

Trepanning drilling using millisecond pulsed fibre lasers offers an industrially viable route for producing millimetre-scale holes in oxide-oxide ceramic matrix composites (Ox-Ox CMC). However, the underlying material removal behaviour governing hole formation and thermal damage is not yet well understood, hampering the use of these advanced materials across the engineering sector, from aero-engines to power generation gas turbines. This study investigates the characteristics of trepanning laser drilling of alumina-based ceramic matrix composites comprising alumina fibres embedded in an alumina matrix. Experiments were conducted on 6 mm thick material using a millisecond pulsed fibre laser. The results show that material removal occurs predominantly through a fusion-based mechanism, governed by melt generation, expulsion efficiency and resolidification along the hole walls. Pulse energy, pulse duration and frequency jointly define the thermal regime of the process, and stable melt-driven drilling was maintained across most conditions examined. Delamination was observed at the combined highest pulse energy and pulse duration tested, consistent with excessive cumulative thermal input and was not a general feature of the drilling process. Oxygen assist gas enhances melt expulsion and stabilises the drilling process without introducing adverse chemical effects, consistent with the fully oxidised nature of the alumina constituents. Under the process conditions investigated, hole taper below 0.8°, surface roughness near 4 µm and thermal damage thickness of approximately 40–55 µm are achieved using pulse energies of 20–30 J, pulse durations of 1.0–1.5 ms, repetition rates of 40–50 Hz and trepanning speeds of 150–200 mm/min with oxygen assist gas.