Cutting and Shaping of Hard Ceramics with a CO2-Laser

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Extremely hard ceramics such as silicon nitride and silicon carbide have generated considerable interest in recent years as potential materials for many high-temperature engineering applications. In their dense high-strength forms, these materials are being proposed for use as structural materials in power-generating gas turbines to increase their efficiency by allowing gas inlet temperatures to about 1400oC. These materials are also being considered for automotive gas turbines, small thermal reactors, roller bearings, and advanced combustion chambers. An important drawback to silicon compound ceramics is their extreme hardness, which makes their processing and shaping an extremely difficult, tedious and expensive procedure. Preliminary work has already shown that vastly higher removal rates are possible with a CO2 laser as compared to the conventional diamond grinding. To demonstrate the feasibility of laser shaping of hard-to-machine materials, a multi- disciplinary research effort has been undertaken to study the basic mechanisms governing the interaction between a high- powered CO2-laser beam and silicon compound ceramics. The research program is looking at the heat transfer, material property and manufacturing aspects of the laser-shaping process. A heat transfer model is being established to predict shape and depth of single, multiple-pass and overlapping grooves, material removal rates, surface roughness, etc. Material properties relevant for the model are being measured, including surface reflectivity (as function of temperature, angle and beam polarization), surface temperatures and ejected material properties (particle size and number density). Material characteristics of the ceramics before and after laser treatment are being investigated, such as microstructure, chemical composition of elements on the surface, flexure strength an thermal shock resistance. Laser machining is being carried out with variable operating conditions, and measurements are being made to determine quality, uniformity, reproducibility and predictability of the process. Machining results from single and overlapping grooves are being used to validate the heat transfer model.