A composite, not a single compound
People often say “tungsten carbide” when they mean a cemented carbide. The distinction is useful. Tungsten carbide, WC, is a hard compound of tungsten and carbon. A cemented carbide is a composite made of tungsten carbide grains held together by a metal binder, most commonly cobalt. The ceramic-like grains supply hardness and wear resistance, and the metallic binder supplies toughness and holds everything in place.
The result is a material that behaves very differently from either part alone. Pure tungsten carbide is hard but brittle and hard to form. Cobalt on its own is a fairly soft metal. Together they make something both hard and reasonably tough, which is why cemented carbide is used so widely in tooling.
How it is made
The process belongs to powder metallurgy:
- Tungsten carbide powder and a binder metal powder are milled together, often with an organic wax to help shaping.
- The powder mix is pressed into a shape, or injected or extruded, and it is then lightly heated to remove the wax.
- The shape is sintered at a temperature at which the binder melts but the carbide grains stay solid. This is called liquid-phase sintering, and it is a key example of the sintering process at work.
- The binder flows around the grains and pulls them together, shrinking the part considerably and leaving a dense material with very little porosity.
- The part is finished by grinding, usually with diamond wheels, because ordinary abrasives are too soft to do it efficiently.
Cemented carbide emerged in Germany in the early twentieth century as a replacement for diamond in wire-drawing dies and quickly spread to cutting tools.
The balance of grain size and binder
Two settings control the properties of a grade:
- Binder content. More cobalt means more toughness and less hardness and wear resistance. Grades for heavy impact or mining may contain a larger share of binder, while grades for finishing cuts contain less. This binder phase concept also appears in diamond compacts.
- Carbide grain size. Finer grains typically give higher hardness and edge strength, while coarser grains generally give more toughness and shock resistance. Grain sizes range from sub-micron to several microns or more.
Because of this, one family of materials covers a wide range of uses. A manufacturer supplies many grades, each placing a different point on the hardness and toughness trade-off. A grade that is excellent for one job can be poor for another.
Additions of other carbides, such as titanium carbide and tantalum carbide, are common in grades meant for cutting steel. They improve resistance to the chemical wear that occurs at high temperature. Nickel and other binders are sometimes used in place of cobalt where corrosion resistance or other properties matter.
Where cemented carbide is used
- Metal cutting inserts and end mills. Carbide is the default material for a large share of machining. Inserts are often coated, using methods such as chemical vapor deposition or physical vapor deposition, to improve wear resistance.
- Mining and drilling. Carbide buttons and tips are set into drill bits and cutting tools used on rock and concrete.
- Wear parts. Nozzles, valve seats, rolls, dies and punches rely on carbide’s resistance to abrasion.
- Wire drawing and forming. Dies for drawing metal wire are a long-standing use.
- Woodworking and other tools. Saw tips and router bits for abrasive wood-based products stay sharp far longer with carbide.
How it relates to superabrasives
Cemented carbide is not superhard in the strict sense, because its hardness is well below that of diamond and cubic boron nitride. It is, however, much tougher than either. That is why it is a common substrate for superabrasive products: polycrystalline diamond layers are often bonded to a carbide base, which supplies support and toughness. It is also a typical workpiece for diamond grinding, since the abrasive must be harder than the thing it cuts.
Where it competes with ceramics, the tougher carbide wins in interrupted or shock-loaded work, whereas ceramics may outlast it in steady, hot cutting.
Limits
- Density. Cemented carbide is heavy, with a density well above that of steel, which can matter for rotating or hand-held parts.
- Binder weaknesses. The cobalt binder is the weak link in some environments. It may corrode in certain liquids or be leached out, leaving a weakened skeleton of grains.
- Heat. At very high cutting temperatures, the binder softens and the carbide may react with steel, causing crater wear. This is one of the tool wear mechanisms that coatings are designed to reduce.
- Supply and cost. Tungsten and cobalt are both relatively costly raw materials. Recycling carbide scrap is common for that reason.
Why it stays dominant
Cemented carbide offers a middle position that suits most jobs: harder and more heat-resistant than tool steel, and much tougher than ceramics or superabrasives. For work where those other materials are too brittle or too expensive, it remains the general-purpose choice.