Tool Material Families

The substrate is what survives at the edge: HSS, carbide, cermet, ceramic, PCBN, PCD. They trade hardness for toughness along one spectrum — harder lasts longer at speed but breaks easily, tougher forgives abuse but cannot run fast. This page places each family and tells you when you reach for it.

Concept

Tool materials are ordered by their ability to stay hard at the temperature of the cut (hot hardness) and their toughness (resistance to fracture). The central trade-off: the harder the material, the faster it can cut, but the more brittle it is.

• HSS (high-speed steel) — tough, cheap, resharpened; low speed.
• Cemented carbide — the workhorse; balanced hardness/toughness, used coated or uncoated.
• Cermet — harder, sharper, for high-speed finishing of steel.
• Ceramic — very hot-hard, for high-speed and hardened steel, brittle.
• PCBN (cubic boron nitride) — hardest after diamond, for hard turning >45 HRC.
• PCD (polycrystalline diamond) — hardest, for non-ferrous/abrasive non-metals; never on steel.

Why It Matters

Picking a substrate is the first materials decision after you know the workpiece. Running carbide at HSS speeds wastes productivity; running ceramic in an interrupted cut snaps it. PCD on steel chemically diffuses and destroys the edge — an expensive mistake. Knowing which family fits the material and hardness window prevents both scrap and broken tooling.

How It Works

Coatings (page 12) are applied on top of carbide to extend this window; they do not change the underlying substrate's basic toughness.

Worked Example

Three jobs. (1) Finish-turning 55 HRC hardened steel: carbide will wear fast, but PCBN or ceramic can hard-turn it. (2) Milling abrasive high-silicon aluminum: uncoated carbide abrades quickly, PCD lasts far longer. (3) A form tool on an old manual lathe: HSS is cheap and resharpened, and the low speed does not demand hot hardness. The substrate choice follows the material and hardness, not the other way around.

Choosing a tool-material family

The broad order of hardness and heat resistance, from toughest-but-softer to hardest-but-brittle, is: high-speed steel → cemented carbide (the workhorse) → ceramics → PCBN/PCD. Roughing and interrupted cuts need toughness, so carbide with a tough grade wins; high-speed finishing of hardened or abrasive materials pushes you toward PCBN or ceramic. Coating (page 12) on a carbide substrate is the everyday upgrade that lets you run faster and last longer without buying a superhard tool. Most CNC machining uses coated carbide because it balances speed, wear resistance and toughness across steel, cast iron and stainless. Reach for the exotic families only when the material or operation specifically demands them — as page 11 spells out — because they are brittle, expensive and unforgiving.

Why coated carbide dominates the shop

If carbide is the workhorse, coated carbide is why it stays that way. A thin CVD or PVD film (page 12) on a carbide substrate raises hot hardness and lowers friction, so the same edge runs faster and resists crater and flank wear without losing the substrate's toughness. That is the everyday upgrade: instead of jumping from HSS to a brittle ceramic, most CNC operations step from uncoated to coated carbide, gaining productivity while keeping interruption tolerance. The decision sequence on the shop floor is therefore almost always: start with a coated-carbide grade matched to the material group, and only move to cermet/ceramic/PCBN when the material hardness, speed requirement or abrasiveness specifically exceeds what coated carbide can give — which is exactly the hard-turned 55 HRC and abrasive-aluminum cases on this page. Buying PCD or PCBN "just in case" for ordinary steel roughing is the mirror mistake: an expensive brittle tool in a job coated carbide does better.

Why PCD on steel is a chemistry problem, not a hardness problem

It is tempting to think "diamond is the hardest, so it should cut anything" — but PCD on steel fails fast for a chemical reason, not a mechanical one. At cutting temperature, carbon (the diamond edge) diffuses into iron, which is a strong carbon solvent; the edge steadily dissolves into the chip rather than staying sharp. This is why the superhard, longest-lasting tool for aluminum and composites is essentially useless on steel, and why the rule "PCD never on ferrous metal" is absolute rather than a tuning preference. The matching superhard material for hardened steel is PCBN, which is not carbon and does not suffer that iron reaction. The lesson generalizes: tool-material selection is not ordered by hardness alone but by chemical compatibility with the workpiece at cutting temperature. Aluminum loves PCD because it does not react and because PCD resists the built-up edge that gums carbide; steel needs carbide, cermet, ceramic or PCBN, none of which diffuses into it. When a PCD edge mysteriously self-destructs on a ferrous cut, this is the mechanism.

Why hot hardness, not room hardness, decides speed

The reason a ceramic or PCBN edge can run so much faster than HSS is not how hard it sits on a bench at room temperature — it is how hard it remains at the temperature of the cutting edge, which easily reaches several hundred to over a thousand degrees Celsius in a high-speed cut. HSS is hard enough at room temperature to machine steel, but it softens sharply once the edge gets hot, which is exactly why it is limited to low cutting speeds: push vc up and the edge reaches its tempering temperature and collapses. Carbide keeps its hardness to a much higher temperature, and ceramic and PCBN higher still, so they can run the high surface speeds that productivity demands. This reframes the whole spectrum on this page: you are not ranking materials by static hardness but by how much of their hardness they retain while red-hot under load. It also explains why "the edge got hot and deformed" is a hot-hardness failure (page 25, plastic deformation), and why raising speed without a material that survives the heat does not buy productivity — it just softens the tool faster.

Why a brittle tool needs a rigid machine to work

Moving right along the spectrum toward ceramic, PCBN or PCD buys hot hardness and speed, but it also buys zero tolerance for vibration. A tough carbide edge absorbs a small chatter or a light interruption by flexing and wear; a brittle superhard edge has no plastic give, so the same vibration or shock that carbide shrugs off cracks or snaps it. This is why the exotic families are not drop-in upgrades: they only pay off on rigid, well-damped machines with short overhangs, stable workholding and uninterrupted cuts — the conditions of pages 21–22. On a flimsy knee mill or a long boring bar, a ceramic insert chips on the first pass and the operator wrongly concludes "ceramic doesn't work." The selection logic therefore pairs the material family with the machine: toughness-demanding setups stay on carbide, and superhard grades are reserved for rigid setups where their speed advantage can actually be used. It also explains the recurring diagnostic pattern — a brittle edge that chips wants more rigidity and a tougher grade, not a harder one; the hardness was never the limiting factor.

Why there is no single "best" tool material

The spectrum on this page is a trade axis, not a ranking, and that is why no one family wins every job. The far-right materials (PCD, PCBN, ceramic) are the fastest and hottest-hard, but they are brittle, expensive, and demand a rigid machine and uninterrupted cut; the far-left HSS is the toughest and cheapest, but it cannot run fast enough for modern CNC. A "best tool" only exists once the job's constraints are fixed: the workpiece chemistry and hardness, the interruption (roughing vs finishing), the machine rigidity, and whether the marginal speed gain justifies the brittle tool's cost and risk. This is why a sensible shop stocks coated carbide as the default and reaches for a superhard grade only for the specific hard-turning or abrasive job where carbide genuinely cannot keep up — and why the page's worked example deliberately covers three opposite jobs rather than one. Picking the substrate is the first place the causal chain on page 01 branches; choose a material that survives the heat and the shock of the actual cut, and you avoid both the over-soft HSS failure and the over-brittle ceramic snap.

Why HSS still survives in the modern shop

It is easy to write off HSS as obsolete, given that carbide runs many times faster, but it survives for good engineering reasons. Its extreme toughness lets it take forms, interruptions and clamping abuse that would snap carbide, and it is ground in-house to special profiles that would be expensive to make as indexable carbide. On low-speed or manual machines, where the spindle simply cannot reach carbide's surface speeds anyway, hot hardness is irrelevant and HSS's cheapness and regrindability win. It also tolerates a clumsy setup: a form tool or a tap taking a small interrupted cut flexes rather than breaks. The lesson is not "use HSS everywhere" but that the family choice follows the machine and the geometry: a special form tool, a low-speed tapping operation, or a job where the spindle is the bottleneck rather than the edge, is exactly where HSS still makes sense, and reaching for a brittle superhard tool there would be both overkill and fragile. The spectrum is matched to the actual speed and shock, not to a modernity contest.

Why cermet sits between carbide and ceramic

Cermet is the family shops most often skip past, yet it occupies a deliberate middle position. It is harder and sharper than conventional carbide and holds a fine edge at high speed, which makes it good for high-speed finishing of steel where a clean, consistent surface matters; but it is more brittle than carbide, so it does not tolerate the interruptions and heavy roughing that carbide handles. Think of it as the finishing specialist rather than a general upgrade: on a light, continuous steel finishing pass it can hold a sharp edge longer and give a better, more predictable finish, but the moment the cut is interrupted or takes heavy stock, carbide's toughness is the safer bet. This is why it appears between the two on the spectrum rather than replacing carbide — it trades a little of carbide's forgiveness for edge sharpness and hot hardness in a narrow finishing window. When a job is a continuous steel finish at high speed and a consistent surface is the target, cermet is worth the catalog look; for roughing or interrupted work, stay on carbide.

Choosing a tool material starts with the load the edge will see. A roughing cut on steel wants toughness to survive interruptions; a finishing cut on hard material wants hot hardness to hold the edge at temperature. Matching the family to the operation, rather than to the hardest grade in the drawer, is what avoids both chipping and fast flank wear.

A useful shop rule of thumb is to match the toughness of the edge to the harshness of the entry, not to the material name. Interrupted cuts and scale want toughness; continuous finishing on hard material wants hot hardness. Choosing by material alone ignores the entry condition that actually breaks the edge.

Common Mistakes

• Using PCD on steel. Diamond diffuses with iron at temperature and the edge fails. Fix: PCD is for non-ferrous only.
• Expecting HSS to run at carbide speeds. It softens and dulls instantly. Fix: reserve HSS for low-speed/form work.
• Using brittle ceramic/PCBN in interrupted cuts. They chip. Fix: use tough carbide for interruption; save superhard grades for continuous cuts.

Worked family choice. Walk a job through the families before reaching for a catalog. A general steel pocket on a VMC, moderate speed, interrupted cuts → coated cemented carbide is the default: tough enough for interruption, cheap per edge, and matched to P-steel speeds. The same pocket in hardened steel above about 45 HRC pushes carbide toward rapid wear, so a cermet or PCBN finishing edge becomes worth the higher cost. Aluminum, by contrast, is machined fast and gummy: a polished carbide or PCD edge resists built-up edge where a heat-coated grade would actually weld. Superalloys (group S) live at low speed and high heat, favouring ceramic or coated grades with strong edge prep. The point is that the family is chosen by the combination of material, speed and interruption — not by the single word "hardest," which would over-specify every job and waste money.

Practice