Stressed out

UK-US research into technique to force metal around corners and stretch it to shape could mean safer, lower cost manufacture of tiny mechanical devices. Stuart Nathan reports.

Forcing metal round corners could be a vital step in the manufacture of microelectromechanical systems (MEMS), according to researchers in the UK and US.

Engineers at the Universities of Southampton and Southern California believe that a technique called equal channel angular processing (ECAP) could give metals and alloys properties that make them especially suitable for use in tiny mechanical devices.

A billet of metal is pushed into a vertical channel which starts off straight but then turns through an angle of up to 90o . The billet is placed under pressure to force it through the angle, which causes stresses inside the metal. This action is repeated through several cycles, each one causing new strain.

By the end of the process, explained Southampton‘s Marco Starink, the strains will have changed the microstructure of the metal. ‘The key issue here is that ECAP has the ability to change the grain structure,’ he said, ‘and we use it to create grain sizes in the sub-micrometre range. We are way above nanometre-scale, but it’s certainly within the realms of nanotechnology.’

Previously, according to the team, the main method of making nanostructured metals has been to start from extremely fine ‘nano-powdered’ metal. These, however, are very costly, and their minute particle size can make them toxic, explosive or both.

ECAP processing offers similar results, but at a much lower cost and with less risk. The technique offer the ability to tailor the grain size and orientation within a billet, which can be done by varying the angle of the processing channel, or by rotating the billet between passes.

Nanostructured metals are interesting for MEMS fabrication because of one particular property, says Starink -deformability. ‘These micrometre-grain metals are easy to shape at high temperature -their ductility is much greater than before the grain size was reduced,’ he explained. ‘We can even get what’s known as superplastic deformation, where a small force can make the metal stretch by up to 1,000 percent. This give us the ability to make drastic changes in shape and curvature over very small distances, which could be very useful in MEMS manufacture.’

Currently, said Starink, the main materials for making MEMS are silicon, silicon dioxide, silicon nitride, and nickel. However, the Southampton team plans to work with aluminium and its alloys. ‘They have much better mechanical properties, and also greatly superior electrical and thermal conductivity,’ he said.

The Southampton project, due to begin in the next few weeks and run until 2009, aims to produce ultra-fine grade aluminium and shape it into microscopic devices such as heat exchangers and micro heat pipes by embossing patterns on to the metal. The team is working on an instrumented embossing rig, and will use techniques such as electron micro- scopy and flow modelling to under- stand how the devices will behave.

‘The project has the potential to provide a gateway for a more traditional industry to contribute to nanotechnology,’ said Starink. Rather than consolidating powders of metal to make structures, the techniques centre around ‘old-fashioned’ bending and pressing of, admittedly small, bulk metals. This, Starink said, could turn out to be a much faster and easier route to MEMS.