Cermet

Cermet is a cutting tool material that bridges the gap between the two families this wiki treats — the tough, edge-tough carbide on one side and the hard, brittle ceramics on the other — and it earns its name from what it is: a ceramic-metal composite. Cermet tools are made largely of hard ceramic compounds — chiefly titanium carbide and titanium nitride — bound with a small amount of metal, usually nickel or cobalt, and the blend gives them a character all their own. They are harder and more heat-resistant than carbide, able to cut steel at speeds that carbide cannot hold; and they are tougher and more practical than the full ceramics, able to take the light, steady cuts of finishing that a ceramic edge would find too demanding. The result is the material of the fine cut on steel: the cermet insert that finishes a shaft or a bore at high speed, holding size and leaving a smooth surface, where carbide wears too fast and ceramic is too fragile. This entry sets out what cermet is, what it does well and where it gives way to the tools around it.

What cermet is

Cermet is best understood as the refined relative of carbide. Ordinary cemented carbide is tungsten carbide grains in a cobalt binder; a cermet takes the same idea and changes both parts. Its hard phase is the titanium-based ceramics — titanium carbide (TiC) and titanium nitride (TiN), often together as a titanium carbonitride — and its binder is a smaller fraction of metal, nickel and molybdenum rather than the cobalt of carbide. The making is the same powder-and-sinter route: the fine ceramic powders are pressed with the metal binder into insert shapes and sintered at high temperature into a dense, hard body. The composition is what gives cermet its place between the materials. Titanium carbide and nitride are harder than tungsten carbide and far more resistant to heat and to the chemical wear of cutting steel, so a cermet edge keeps its hardness where a carbide edge softens; but the ceramic content also makes the material more brittle than carbide, and the cermet’s whole art lies in managing that brittleness by using it where the cut is light and steady.

The character of the cermet edge

What the cermet’s composition produces is an edge with a distinctive set of strengths, and they are exactly the strengths of the finish cut. The first is heat resistance: cermet holds its hardness at the high temperatures of fast cutting, so it cuts steel at speeds well above the carbide range, removing metal in the finishing pass faster than carbide would dare. The second is wear resistance of the right kind: titanium-based ceramics are chemically stable against steel, so the cermet edge resists the cratering and the diffusion wear that hot steel chips inflict on a carbide rake face, and it resists the built-up edge that blurs a carbide edge at low speed. The third follows from the first two: a cermet edge keeps its form, and a cermet cutting tool holding its form at high speed leaves the fine surface finish — the smooth, consistent finish of a good finishing pass, often without the need for a separate polish — that makes cermet the material of choice for finish turning and finish boring of steel. On the right cut, a cermet insert outlasts carbide and finishes better while cutting faster.

Where cermet earns its keep

The cermet’s strengths point it to a particular work, and the work is the finishing of steel and cast iron at speed. The classic cermet application is the finish turning of a steel shaft or the finish boring of a steel bore: a light cut, a fine feed, a high speed, and the cermet insert holding the size and leaving the finish through a long run of parts. The material suits the steels that a shop cuts most — the carbon and alloy steels, the tool steels in their softer states, the cast irons — and it shines where the finish and the size matter and the cut is steady. It is chosen for the operation that a carbide insert would do at moderate speed with more wear and a coarser finish, and where a ceramic insert would be too demanding in its setup. The cermet insert typically runs with the tool geometry that finishing wants — a positive rake and a good nose radius — on a rigid machine with a rigid setup, because a fine, high-speed finish cut forgives nothing.

The cermet’s weaknesses

The cermet’s place is defined as much by its weaknesses as its strengths, and the wise user knows them. Because the cermet is largely ceramic, it is brittle: it cannot take interrupted cuts — a keyway, a hole, a scale-ridden forging that hammers the edge — where carbide shrugs and cermet chips. It is vulnerable to the notch wear this wiki treats under tool wear, the groove cut into the edge at the depth-of-cut line by the work-hardened skin of the previous pass, and to the micro-chipping that ends a fine edge early. And its chemistry, so good against plain steel, is less happy against the gummy or the reactive alloys — the stainless steels and the high-temperature alloys that this wiki treats under materials — where the titanium-based cermet can react and wear where a coated carbide or a tougher grade serves better. The cermet is thus not a general-purpose tool but a specialist, and it gives its best only within its bounds: light, steady, fast cuts on steel, with a rigid machine and a clean edge of cut.

Cermet among the tool materials

Cermet completes the ladder of tool materials that this wiki’s cutting-tool entries describe, sitting precisely between carbide and the ceramics. It is harder and faster than carbide but less tough, so it takes the cuts carbide cannot hold at speed but yields where carbide endures; it is tougher and more forgiving than the full ceramics but less extreme in speed, so it does the work a ceramic would find too demanding. In practice the modern shop keeps all three, reaching for the carbide for the rough and the interrupted, the cermet for the fast finish on steel, and the cubic boron nitride or the ceramics for the hard and the hot. The cermet’s place — the fine, fast, steady cut on the common steels — is a narrow place, but it is one of the most common places in machining, which is why the cermet insert is standard equipment in the finishing of turned parts, holding its edge and its finish at speeds that carbide cannot match, and doing quietly the work that makes the part’s last surface its best.