G’day; an alternative to sputtering as a method of metal-coating
non metals is vacuum evaporation.
This is similar to sputtering in that it uses a vacuum, but in this
the amount of gas in the system is very very low; - a 'high vacuum’
indeed. The metal can be placed in a very small ‘boat’ shaped
piece of tungsten sheet, or a loose coil of 1mm tungsten or
molybdenum wire. Tiny pieces of gold, silver or even aluminium can
be hung in the coil or placed in the boat, and a fairly high
electric current is passed through the wire or boat to heat it to
white heat. In the virtual absence of gas molecules, the metal
melts and boils actually evaporating as well. As there are very
few molecules in the way, the atoms of evaporated metal shoot out in
straight lines, like light and will coat any properly prepared
surface in their flight path. Such a coating may be made as thin or
as thick as required. Half silvered mirrors with metal or other
coatings of precise thickness can be made in this way; it is really
a quite simple process.
I built and used such an apparatus about 56 years ago, making
aluminised mirrors for a special research purpose. Having made a
mirror, I then changed the coil for a clean tungsten 'boat’
containing pure silica, which I evaporated and coated my mirrors to
make them more durable. The special coatings on camera and other
lenses are made in this way. Just for fun, (and when the Boss wasn’t
there) I coated a 10 inch clock glass with pure copper and made a
convex mirror, which made the viewer look tanned and handsome(!?)
It hung on the wall of our house for years. Rather naughty - but
then so many fun things are. Aren’t they? –
Cheers for now,
JohnB of Mapua, Nelson NZ