Energy requirements for ablation or decomposition of ceramics during CO2 and Nd:YAG laser machining

Abstract

In order to asses a priori the outcome of a laser machining process using either simplistic ad hoc or sophisticated models, knowledge is needed ofthe energy required to remove a unit mass of material. Since laser-material interactions involve vaporization, decomposition, ejection of fragments, plasma initiation and expansion, etc., extrapolating enthalpy data from JANAF tables will result in significant uncertainty. In this paper, an experimental setup to measure this “heat of removal” is described.The apparatus mainly consists of an integrating sphere for the sample and a second integrating sphere for reference. The specimen is mounted at the center of the sample sphere, and is heated by a CO2 or Nd:YAG laser, whose beam is split, with a part of the beam going into the reference sphere, to monitor the temporal laser power. The energy reflected by the specimen inside the sample sphere is recorded by a second detector. Combining these data, the total absorbed energy may be deduced.For this type of measurement, energy losses by convection and radiation are generally negligible, while conduction losses may be substantial. In order to minimize conduction losses, the specimens are manufactured to a thickness of less than 500μm. Still, conduction losses may account for up to 10-20 percent of the total laser energy for CW and conventionally pulsed lasers even for such thin samples. Thus, a numerical model is also employed to estimate the conduction losses and to correct the measured value of the “heat of removal.”Experimental data have been obtained for CO2 and Nd:YAG laser irradiation on graphite, hot-pressed silicon nitride and sintered α-silicon carbide.In order to asses a priori the outcome of a laser machining process using either simplistic ad hoc or sophisticated models, knowledge is needed ofthe energy required to remove a unit mass of material. Since laser-material interactions involve vaporization, decomposition, ejection of fragments, plasma initiation and expansion, etc., extrapolating enthalpy data from JANAF tables will result in significant uncertainty. In this paper, an experimental setup to measure this “heat of removal” is described.The apparatus mainly consists of an integrating sphere for the sample and a second integrating sphere for reference. The specimen is mounted at the center of the sample sphere, and is heated by a CO2 or Nd:YAG laser, whose beam is split, with a part of the beam going into the reference sphere, to monitor the temporal laser power. The energy reflected by the specimen inside the sample sphere is recorded by a second detector. Combining these data, the total absorbed energy may be deduced.For this type of measurem...