Absorptivity Transition in the 1.06 μm Wavelength Laser Machining of Structural Ceramics

This study presents absorptivity transitions in two-dimensional, low aspect ratio laser machining (cutting) of structural ceramics such as alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), and magnesia (MgO) using a 1.06 μm wavelength-pulsed Nd:YAG laser. Because temperature measurement at high temperatures is difficult, thermocouples were used to measure temperatures in the low-temperature regime (700–1150 K). A thermal model was then iterated to obtain trends in absorptivity variation below the phase transition temperature for different ceramics. Following this, measured machined depths were used as a benchmark to predict absorptivity transitions at higher temperatures (>1150 K), using the developed thermal model. For temperatures below the phase transition, because of intraband absorption, the absorptivity decreases with an increase in temperature until the surface temperature reaches the melting point (in the case of Al2O3, Si3N4, and SiC) and the vaporization temperature (in the case of MgO). The absorptivity then continues to follow an increasing trend with increasing temperature because the physical entrapment of the laser beam in the cavity evolved during machining of the desired depth in the ceramic. Such a study would provide an efficient control over the input energy required to machine to a certain depth.