Ultrasonic assistent lapping fot the manufacture of complex and high surface quality microstructures on brittle materials
Zusammenfassung der Projektergebnisse
The overall long term goal of this project was to establish a flexible and efficient micromachining technique to create microstructures in brittle materials to meet the increasing need for such structures. Micro ultrasonic assisted lapping (USAL) has been shown to be an economic and flexible alternative to current micromachining technologies such as lithography, ion beam or laser based micromachining techniques. Micro USAL is especially useful for the small batch production and prototyping of micro-parts and microstructures in brittle materials. The research performed for this project has resulted in a number of scientific findings that have advanced the understanding of micro USAL. These findings are summarized below. The material removal that occurs during micro USAL was found to be very strongly dependent on the depth of cut at which the experiments were performed. Depths of cut less than approximately 1 μm resulted in mostly ductile removal and depths of cut greater than about 5 μm resulted in mostly brittle fracture. For depths of cut in the range between 1 μm and 5 μm material removal occurred with a combination of the brittle fracture and ductile plastic deformation. In general it was found that tools made of high speed steel exhibited slightly more wear than tools made of cemented tungsten carbide. Diamond coated tools were found to give nearly zero wear but they left the surface very rough. This was caused by the roughness of the diamond coating itself and improvements in the roughness of the coating would lead to decreases in the roughness of the machined surface. Subsurface cracks were found to extend about 5 μm beneath the surface when silicon was machined with a depth of cut of about 1 μm. Investigation by confocal Raman microscopy showed that the amount of amorphous silicon created in the near surface during machining was strongly influenced by the size of the abrasives in the slurry. When the abrasives were 0.25 μm there was more amorphous silicon created than with the 0.1 μm abrasive. Automated surface detection and process control was achieved by the use of an acoustic emission (AE) sensor. Surface detection with the AE sensor significantly reduced the number of broken tools during surface detection. The AE sensor was also used in a feedback loop in an effort to maintain a constant depth of machining that accounted for decreases in tool length caused by wear. A feedback control system was designed an implemented for both linear and curved grooves. The feedback system for the curved grooves used only proportional gain and was found to be very stable. The achievable surface roughness for all cutting cases was found to be dependent on many factors. For a deep depth of cut diamond slurries were found to produce rougher surfaces than alumina slurries of the same size abrasive. The size of the abrasive particles was found to influence the roughness as well where smaller abrasives resulted in better surface finish. At shallow depths of cut surface finishes in the range of 5 – 30 nm Sa were achieved which is significantly better than previously reported results in the literature. This level of surface roughness was also obtained with a chemical slurry. Example structures such as spiral patterns and cylindrical holes were generated in silicon with and without feedback control. All of these results further our understanding of micro USAL and advance its use as an alternative micromachining technique. The use and development of the acoustic emission sensor for controlling the machining was a particularly important result that should hasten the adoption of micro USAL by industry. The knowledge gained in this project about the successful integration of micro USAL and in-process monitoring could be extended to other micro-machining techniques such as micro-milling and micro-grinding. Another possible extension of this work is to ultrasonic assisted electrochemical lapping which could be used on hardened steels for example. An application for this would be in the machining of structured molds for the generation of complex optical elements as in DFG project “Prozessketten zur Replikation komplexer Optikkomponenten”. The overall objective of this project is to lay the scientific foundations for the deterministic and economical mass production of optical elements with complex geometry.
Projektbezogene Publikationen (Auswahl)
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Advances in Micro Ultrasonic Assisted Lapping of Microstructures in Hard-Brittle Materials: a Brief Review and Outlook. International Journal of Machine Tools and Manufacture, Vol. 45, No. 7-8, 2005, 881-890
C. Zhang, R. Rentsch, E. Brinksmeier
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AE-based Process Monitoring and Control of Micro Ultrasonic Assisted Lapping (u-USAL). Annals of WGP, Vol. XIII/1, 2006, 27-30
C. Zhang, R. Rentsch, E. Brinksmeier
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Micro USAL Technique for the Manufacture of High Quality Microstructures in Brittle Materials. Precision Engineering, Vol. 30, No. 4, 2006, 362-372
C. Zhang, E. Brinksmeier, R. Rentsch
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Micro-ultrasonically Assisted Lapping of Brittle Materials. Proceedings of 7th International Conference of the European society for precision engineering and nanotechnology, Bremen, Germany, 2007, 257 - 260
M.J. Klopfstein, O. Riemer, E. Brinksmeier
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Micromachining of Brittle Materials Using Ultrasonic Assisted Lapping. Proceedings of the American Society for Precision Engineering Spring Topical Meeting, Chapel Hill, North Carolina, 2007, 87-91
M.J. Klopfstein, O. Riemer, E. Brinksmeier
