In article , Ed Huntress says...

Ahh but the question IS; can tungsten carbide tooling (since it is an
intimate "mixture" of tungsten, carbon, and cobalt), be correctly
referred to as metallic or a metal since the finished product has
metallic properties?

Neither one, Bob. It's a metal-matrix composite. Think about reinforced
epoxy. 'Same thing.

Cobalt is the binder, and makes up a good deal of the material.

The correct term is "Cemented Carbide" for these tools, which
is a pretty good name. Basically very hard particles in a binder
that is amenable to sintering. Really hard particles embedded
in soft, gooey stuff. The hard stuff cuts, the gooey stuff holds
it together.

Sintering is one of those 'magic' processes whereby materials
densifiy under the application of heat. Basically the deal is
that one can pick a system where the free energy goes down as
the free surface area goes down. There's a net free energy
loss as grains grow closer together.

I used to work for GTE and they did a lot of that sort of thing,
they developed new materials for, I think, Valenite. They had
one guy who's job it was to test new concoctions of cutting tools,
he had a *huge* lathe, and had stacks of steel bar stock, about
10 inch diameter, by six feet long, for testing the tools.

The bar stock had lengthwise slots in it, about a half inch wide,
along the entire length. I think they were interested in shock
loading the tools with interrupted cuts. g
They put the lathe out in a separate building because of the noise.
Anyway I bowled with the guy, in the mixed league there.

Cemented carbides don't stand up to really abrasive materials like
glass-epoxy, or carbon fiber, composites. What happens is the
whiskers of very hard carbon or glass tend to 'pick out' the relatively
soft cobalt binder, so the individual carbide grains lose their support
in the matrix, and eventually drop out.

The same thing happens in polycrystaline diamond to some degree,
as they also use cobalt metal as a binder. I'm honestly not sure
why PCD works so much better than cemented carbide, it may have to
do with a relatively higher ratio of diamond to cobalt compared
with cemented carbides.

Jim


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