Tungsten-carbide
HOkay, sometimes I lose track of the confusion that's propagated about this material, and the common uses of the term "ceramic," as well as the vague terms used in metallurgy to describe WC and TaC, which are interstitial carbides. On top of it all, the metalworking- and other industrial fields have added their own jargon. This is going to be one of my lengthy dissertations, so I'll apologize in advance and warn off anyone who doesn't really give a damn. On the other hand, they may find this interesting, because it's the result of research I did on the history of carbide tools in
1977, plus years of exposure as Materials Editor of American Machinist, and more recently as Tooling Editor of Machining magazine. You won't see it put together quite this way anywhere else, I believe.The term "ceramic" is old and vague as hell. It can include any non-metallic material, or compound of a metal and a non-metallic material, that's hard and brittle. You will sometimes see plain sand- and lime-putty mortar, which has a compression strength of only 75 - 350 psi, described as a "ceramic" in the building-restoration literature. This is not as ridiculous as it sounds; when lime putty (hydrated lime, Ca(OH2)) picks up CO2 from the atmosphere, it becomes CaCO3 -- limestone. That fills the bill: non-metallic and somewhat hard, somewhat brittle. As reconstituted limestone goes, lime mortar pretty lousy, but it's still within the definition. The definition of "ceramic" actually is that broad. Fine-clay dishware is generally described as "ceramic," as another example.
There is no absolute technical definition, although engineers of advanced ceramics from around the world got together in 1993 and wrote one for their own use. It doesn't apply to anything we're interested in. It applies only to the newest, most advanced engineering ceramics. In Europe, engineers define plain glass as a ceramic. We're not interested in that definition, either.
What we're interested in is something that will help us sort out the often contradictory uses of the term in industry. Here's a good one from Encyclopedia Britannica:
"Industrial ceramics are commonly understood to be all industrially used materials that are inorganic, nonmetallic solids. Usually they are metal oxides (that is, compounds of metallic elements and oxygen), but many ceramics (especially advanced ceramics) are compounds of metallic elements and carbon, nitrogen, or sulfur."
There's tungsten carbide for you: a metal (tungsten) compounded with carbon. It's hard, it's brittle, it's refractory, and it doesn't even melt like a metal. That's a ceramic. The bonding of W and C fit well within the more technical chemical definitions, which allow for both ionic and covalent bonds, plus various combinations thereof.
WC is a particular type of ceramic: an interstitial carbide. Here's Britannica again:
"Interstitial carbides are derived primarily from relatively large transition metals acting as a host lattice and the small carbon atoms occupying interstices of the close-packed metal atoms...Several have industrial importance, including tungsten carbide (WC) and tantalum carbide (TaC), which are used as high-speed cutting tools because of their extreme hardness and chemical inertness."
Now for some background on the confusing, often contradictory uses of the terms.
Back in the 1920s, some Germans came up with a way to make tools out of this hard stuff by mixing some WC powder and some cobalt powder, and then pressing and sintering them together into a composite material. It was NOT a compound. The carbide particles remained distinct in the composite. It was something like concrete in that regard.
At about the same time, Philip McKenna (Kennametal) was experimenting with WC for cutting tools, too, trying to make them out of solid WC. You do this by taking straight WC powder and pressing it, and then sintering it at extremely high temperatures. I think WC sinters at around 4700 deg. F. Very difficult, and quite expensive (what do you make the pressing tools out of, for example?). He made the WC into billets and then sliced them into cutting tools with diamond saws.
McKenna had some success but the result was too brittle and too expensive. It *was* capable of handling higher temperatures than the German stuff, and it was harder. But it was a no-go commercially. (Aside: the US Army is making big pieces of solid tungsten carbide again, much like McKenna's billets he made in the 1920s. Only the Army is using it for artillery shells.)
So McKenna reverted to the German method, and these carbide/cobalt (and sometimes nickel) composites became available around the world, and were commonly called "cemented carbides." The cement, also known as the matrix, is the cobalt.
First terminology problem: People got sloppy and started dropping the "cemented" part of "cemented carbide." It was just "carbide," or "tungsten carbide." Only it wasn't, really. It was some tungsten carbide mixed with some metal and sintered into a composite material.
Second terminology problem: Metallurgists are more contrary than chemists, and they sometimes call tungsten carbide "metallike," or even a "metal." Chemists object. Metallurgists don't care. Machinists wind up confused. It's metallike, only it allows almost no slippage of crystals along slip planes: it has virtually no ductility, in other words. It behaves like a ceramic. It fits the definitions of ceramics that chemists and other scientists use. Metallurgists still don't care. They have their own little world.
Third terminology problem: As newer ceramics came into play in the emerging world of "hard materials," they, too, got mixed with metal binders or matrices to form composites, and were given the name "cermets." Snappy, descriptive, and neat. However, somebody forgot about tungsten-carbide/cobalt composites. They're cermets, too. Marketing people in the cutting tool business didn't care. They had a snappy new name to apply to their new products, and they weren't going to let lowly old tungsten carbide horn in on the action. Machinists wind up confused. Tool companies don't care.
So, does that help? There are no WC tools on the market that I know of. They're all cemented, metal-matrix composites made of ceramic WC, refractory metals, and sometimes odds and ends of other metals and ceramic compounds. A pure mass of WC is a pretty rare thing, and is not used for cutting tools today. It never was, except for some experiments.
'Sorry for the length. I didn't have time to write it shorter.
Ed Huntress