A wide variety of applications verify the great potential of high-performance ceramics for components with special requirements. For example, hip joint endoprostheses on aluminum oxide or zirconium oxide bases, components for slide bearings and burners of silicon carbide, as well as ceramic components for roller bearings or valves are made of silicon nitride [Spur 1989, Pattimor 1998, Popp 1998]. Extension of the market share for ceramic components is often opposed by the difficulties of manufacture with respect to achievable component quality and economic efficiency. Manufacturing costs arise mainly in grinding, honing, lapping, and polishing. High costs result from relatively inefficient technologies for machining of brittle-hard materials [Uhlmann 1998]. This demonstrates the need to provide economically efficient machining methods for ceramic workpieces. In addition, there is a lack of suitable strategies for economic manufacture of complex geometries such as bores, holes, grooves, spherical surfaces, and sculptured surfaces [Uhlmann and Holl 1998a].
Although ultrasonic lapping and electrodischarge machining (EDM) processes are suitable for manufacture of these geometries, there are significant disadvantages. Only electrically conductive ceramics such as SiSiC can be machined with EDM methods, and there are technological limits to ultrasonic lapping due to the small material removal rate, the high wear of the forming tools, and unsatisfactory accuracy. Therefore, suitable manufacturing methods for highly accurate and economic machining of ceramic materials have been developed over the past few years. The development of hybrid manufacturing processes on the basis of existing methods opens up new avenues.
In ultrasonic lapping, a forming tool oscillates with ultrasonic frequency and thrusts loose abrasive lapping grains into the surface of the workpiece, thus removing material. Based on this process, ultrasonic action has been superimposed onto conventional machining kinematics in several manufacturing processes over the past few years. In nearly all cases, process results have improved [Drozda 1983, Nankov 1989, Prabhakar, Ferreira, and Haselkorn 1992, Pei, Prabhakar, and Haselkorn 1993, Suzuki et al. 1993, Westkamper and Kappmeyer 1994, Pei and Ferreira 1999].
In industry, ultrasonic lapping and ultrasonic-assisted grinding have been applied so far for finishing brittle-hard materials. Due to high material removal rates and the freedom of geometrical configuration, this method is likely to have a wide range of application possibilities in the future. Figure 20.1 shows the advantage of ultrasonic-assisted grinding in comparison to conventional finishing methods with respect to an increase of the material removal rate in the machining of aluminum oxide through the superposition of grinding kinematics and an ultrasonic frequency.