Experimental and theoretical evaluation of the laser-assisted machining of silicon nitride

This study focused on the experimental and theoretical evaluation of the laser assisted machining (LAM) of silicon nitride ceramics. A laser assisted machining facility was constructed whose main components consist of a CO$\sb2$ laser and a CNC lathe. Surface temperature histories were first measured and compared to a transient, three-dimensional numerical simulation for a rotating silicon nitride workpiece heated by a translating laser for ranges of the workpiece rotational and laser-translation speeds, as well as the laser beam diameter and power. Excellent agreement was obtained between the experimental and predicted temperature histories. Laser assisted machining experiments on silicon nitride ceramic workpieces were completed for a wide range of operating conditions. Data for cutting forces and surface temperature histories illustrated that the lower bound for the avoidance of cutting tool and/or workpiece fracture for LAM is defined by the YSiAlON glass transition temperature (920-970$\sp\circ$C). As temperatures near the cutting tool increase to values above the glass transition temperature range, the glassy phase softened, facilitating plastic deformation and, correspondingly, the production of semi-continuous or continuous chips. The silicon nitride machined workpiece surface roughness $(R\sb{a}=0.39\ \mu$m) for LAM at the nominal operating condition was nearly equivalent to a value associated with the grinding of silicon nitride using a diamond wheel $(R\sb{a}=0.2\ \mu$m). By examining the machined surfaces and chips, it was shown that LAM does not produce detectable sub-surface cracking or significant silicon nitride microstructure alteration, respectively. A transient, three-dimensional numerical heat transfer model of laser assisted machining was constructed, which includes a preheat phase and material removal, with the associated changes in the workplace geometry. Excellent agreement was obtained between the measured and predicted temperature histories. The strong influence of the temperature distribution near the cutting tool location on the cutting forces, the specific cutting energy, and the tool wear demonstrated the usefulness of the theoretical heat transfer model for predicting successful operating conditions for the LAM of silicon nitride ceramics