Components for high performance

Ceramic Screws – Corrosion and Heat Resistance

Doctors bind the bone fragments with splints to ensure the bones knit themselves back together correctly. If metal screws are used to hold the splint in place, there is a risk of intolerance, which is why many doctors would prefer ce­ ramic screws. Longterm implants present a similar prob­lem. Even if something as small as one tiny screw contains metal material, it will be magnetic; this excludes all future use of computer and magnetic resonance imaging diag­ nostics. Outside of the hospital, ceramic screws are also a good alternative for chemical, electrical and thermal ap­plications because they are electrically isolating and hold up well to immersion in acids and lye. Ceramic screws can also withstand temperatures over 1000 °C, while their me­tallic counterparts soften at around 500 °C. Industrial fur­naces are almost entirely made of ceramic parts because of the high temperatures they must tolerate – except for the screws. Here ceramic screws could finally make the tech­ nologic leap to all­ceramic solutions. But until now manufactures have been sceptical for an un­derstandable reason: ceramics are notoriously brittle. Al­though some ceramics have a load­bearing capacity close to that of steel, once the material has been processed into the final screw form, it is estimated that only about 10 – 20 % of the original strength remains. Until now, screw manufactur­ers did not know exactly what load they could support. Using a screw test rig and simulations, researchers at IWM in Freiburg/DE and their colleagues at the Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Dresden/DE and the Institute for Machine Tools and Factory Management IWF at the Technische Universität Berlin/DE have devoted themselves to the question how much stress ceramic screws can re­ally withstand. The project is funded by the German Federal Ministry for Economic Af­fairs and Energy (BMWi) and the German Federation of Industrial Research Associa­tions (AiF). Researchers are also optimizing the screw design. The challenge is that load capacity varies greatly even among ceramic screws of the same design; while one screw can tolerate a great deal, another breaks much sooner. The load on the screws is there­ fore limited by the stress that the weakest among them can withstand. The ceramic’s composition is the deciding factor – if the tiny grains that make up the substance bond incorrectly during manufacture, small cracks develop which can later cause the material to fail. Researchers have now op­timized the manufacturing process so that such cracks no longer occur in any of the numerous process steps. The researchers have also used the test rig to test the stress resistance of ceramic screws manufactured in their own labora­tories. Their load­-bearing capacity exceeds that of their steel counterparts by between 30 and 35 %.

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