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Friction surfacing onto ceramics

Connect, no. 161, January/February 2009, p.8

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A radical new advance in friction surfacing has been invented at TWI.

For years friction surfacing has been used to apply a metal coating onto a metal substrate. TWI has now pioneered the deposition of metal coatings onto ceramic substrates, a process previously considered impossible.

An aluminium consumable some 3mm in diameter is rotated at many thousands of revolutions per minute while it is pressed against a wafer-thin ceramic substrate. Once the aluminium is known to be plastic, at approximately 300°C, the rotating consumable is traversed across a ceramic token under sustained pressure.

TWI has laid tracks of aluminium less than 50µm thickness onto an alumina substrate. It is a solid phase process. No melting takes place, only plasticising of the material.

From examination of highly magnified cross sections of the joints between the aluminium and the ceramic there is some evidence of mechanical keying taking place. But from adhesion tests it is also evident that excellent bond strengths are achievable.

Although total failure eventually ensues, it occurs at the adhesive joint, rather than between the aluminium coating and the substrate.

'Clearly, there are more questions than answers just now' says project leader Abbas Mirlashari. 'What we can say for sure is that it works, and has enormous potential in several disparate technologies. The key thing to identify first is the range of consumables that you can put onto these substrates. We know for instance that it also works for aluminum nitride and silicon carbide substrates, and the technique can be used to lay copper tracks.'

Friction surfacing is a unique way of metallising ceramics. Potential uses include heat sinking applications on temperature sensitive substrates, and as tracks for providing electrical conductivity.

Future work at TWI will explore a range of deposition materials and substrates now possible. 'The technology offers huge potential in thermal management for electronics, medical and automotive industries and we are keen to exploit this' said Mirlashari.

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