Development of a split-beam method for the improved laser machining of unsupported ceramics

Most ceramics cutting occurs in essentially the same way, first by making a scratch on the surface with a diamond or wheel cutter and then snapping it along the scratch. Although not visible to the naked eye, the cut edge is often full of tiny chips and defects. These are the starting point of microscopic cracks which spread across the cut edge. Any of these micro-cracks can be the beginning of fracture if the ceramics is subjected to any combination of mechanical or thermal stresses.; The use of lasers for processing ceramics is by no means new, but, lasers are particularly advantageous for the precision machining of ceramics by virtue of their low force signature and cost effectiveness. However, the full potential of lasers has yet to be realized because of the fracture problems inherent in the brittle nature of ceramics and the associated higher manufacturing and environmental costs. One of the most vexing of these problems is the mixed-mode nature of early fractures and the separation burrs and chips that result, especially when cutting unsupported ceramics.; The objective of this project was to develop unique laser machining methods that hold the promise of improving quality and throughout by stalling premature fractures and eliminating separation burrs and chips. Of particular interest to this study was the concept of pre-scoring which uses simultaneous dual-beam cutting to preordain fracture paths. In order to investigate and develop the concept of pre-scoring, a customized dual-beam laser system was designed and built to perform a series of experiments. First, the research determined the linear relationship between power and groove depth. Among these experiments, a study of groove morphology found that the recast layer formed during pre-scoring induced new and severe flaw populations which reduced the strength of ceramics and increased the possibility of fractures during laser cutting. Next, two series of experiments with and without a pre-score were run and compared. Results showed that those samples with a pre-score had many advantages over samples without a pre-score in terms of overall cut quality. The statistic nature of ceramics was also considered in these experiments. The ranges of groove depths to efficiently control fracture were determined, and SEM assessed the cut quality and analyzed the fracture initiation sites. Meanwhile, FEA modeling provided an understanding of the thermal transients and stresses caused by the dual-beam machining at different laser spacings, which was helpful in guiding the practical implementation of the concept of pre-scoring. Finally, two additional methods, offset alignment and angled pre-scoring, were explored to further implement and extend the applications of the concept. Based on the experiments and modeling, the pre-score concept was shown to markedly improve the cut quality and eliminate mixed-mode fractures.