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April 2nd, 2012

Removing distortion from thin ceramics with shot peening

Published on April 2nd, 2012 | By: Eileen De Guire

Shot is fired from a blasting gun at a ceramic leaf spring to correct its shape or cause specific warping as desired. Credit: Felicitas Gemetz; Fraunhofer IWM.

Ceramic materials are the go-to materials for applications in corrosive, high-temperature environments. However, shaping ceramics, especially thin parts such as leaf springs or lightweight mirrors is not simple and is often time-consuming and expensive. For example, finishing fired ceramic components by machining usually is done with costly diamond tooling. The machining process itself introduces residual stresses and distortions. Add in the problems inherent to machining brittle materials, and the cost of machining becomes prohibitive.

Metallurgists use shot peening to improve the fatigue life of metal components by inducing compressive stresses in the peened surface. The deformation caused by the shot also cold works the surface layer and hardens it.

Researchers at the Fraunhofer Institutes for Mechanics of Materials IWM in Freiburg and for Production Systems and Design Technology IPK in Berlin turned to shot peening to remove distortion from thin, even ceramic plates. In the new release, IWM business leader Wulf Pfeiffer says, “Shot peening is common practice for working metals, but the technique has never been used on ceramics because they are so brittle – they could shatter, like a china plate being hit with a hammer. This meant that we had to adapt the method to the material with great precision.”

Three factors proved to be critical. First, shot size mattered. It the pellets were too large, they would destroy the ceramic surface. Second, shot speed must be kept within a proper range. Too slow and the surface does not alter enough, but too fast, and the surface will be damaged. Third, the duration needs to be controlled. Researchers found “it is important not to bombard the same spot too often with too much shot.”

The Fraunhofer team has applied the technique to several prototypes, including a ceramic leaf spring and a concave mirror, and they have worked out enough of the process parameters to use it in series production. For the next steps, the IWM team is working on multi-axis simulation, and the IPK team is working on developing robotic automation

The news release does not say what materials have been successfully shot peened, nor is anything said about the resulting microstructure of the surface and possible effects on properties. It would be nice to know, for example, if the surface amorphizes during peening, which could change the surface properties dramatically.


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