Epoxy filled with micro spheres for thickness control

OK follow-up, I ordered these from McM-C

George H.

Yep, loading the epoxy with microballoons or microspheres (the balloons are less dense, don't settle to the bottom) would achieve spacing. Epoxy, though, would cure with unrelieved tensile stress, and the microspheres would make the spacing constant, and thermal cycling would modulate the stress: your epoxy might microfracture.

A woven mat of fiberglass would achieve spacing, too, while remaining somewhat compliant.

We tried the beads for a magnetic structure under high sheer and they failed constantly. Ended up using 0.5oz/yd random kevlar mat for spacing, and a semi rigid epoxy for thermal expansion.

Mark

I wonder whether loading the epoxy with a substantial percentage of a very fine ceramic powder might be beneficial. This would reduce the amount of epoxy present and break it up into an irregular spacing pattern. The ceramic powder would match the TCOE of ceramic spacing beads fairly well, and would increase thermal conductivity somewhat.

Sticking with more compliant materials (Kevlar and a semi-rigid epoxy) certainly has advantages, especially if the materials you are bonding have different TCOEs.

We tried a number of filler materials, the best one we found was aluminum powder. It did really well in the overlap shear test but in the actual prototypes it still fractured before the parts with the kevlar matt. Just my two cents on it. It is kind of an extreme case with 300-500F ambient, Ni plated NdFeB to Ni plated steel, Ni plated steel to anodized aluminum bonds. Using just epoxy with no filler the bond line would be squished too thin under the force of the magnet and the result would be like 50% of what we got in the overlap shear test.

Mark