Cobalt's Other Job: Holding Cutting Tools and Turbines Together

Batteries dominate the headlines about cobalt demand, but roughly a fifth of global cobalt still goes into two much older, purely mechanical applications, and both depend on real, specific properties that are hard to replace, properties that show up as numbers an engineer actually designs against, not just as qualitative description.

Binding tungsten carbide cutting tools

Cemented tungsten carbide, the material behind most industrial cutting tools and drill bits, is not a single substance but a composite: hard tungsten carbide grains held together by a metal binder, and cobalt is that binder in the overwhelming majority of commercial grades. Cobalt was chosen specifically because it wets and bonds to tungsten carbide grains exceptionally well during sintering, and because the resulting composite combines the carbide’s hardness with real fracture toughness: a cutting tool that was only as hard and brittle as pure tungsten carbide would shatter in use rather than cut.

That hardness-versus-toughness trade-off is the entire design problem in WC-Co, and it runs directly with cobalt content:

6% Co grade

1750 HV

10% Co grade

1500 HV

15% Co grade

1250 HV

Typical Vickers hardness (HV) for cemented tungsten carbide grades of increasing cobalt binder content. Fracture toughness runs the opposite direction: the 15% Co grade is tougher but softer, the 6% Co grade is harder but more brittle. Figures are representative industry ranges, not a single manufacturer's exact specification.

Why grain structure controls this trade-off at all traces back to a relationship that shows up across almost every polycrystalline metal and composite, not just cemented carbide:

\sigma_y = \sigma_0 + \frac{k}{\sqrt{d}}

The Hall-Petch relationship. σy is the yield strength, σ0 and k are material constants, and d is the average grain size. Smaller grains (smaller d) mean more grain boundaries per unit volume, and grain boundaries resist the dislocation motion that lets a metal deform, so finer-grained material is stronger. The same logic governs how fine or coarse the WC grains and cobalt binder channels are in a cemented carbide, not only pure metals.

Strength at extreme temperature

Cobalt-based superalloys retain useful mechanical strength at temperatures where most metals would soften or creep: gradually deform under sustained load well below their melting point, a failure mode that matters enormously in gas turbine components exposed to extreme heat for thousands of hours. The rate of that creep deformation is itself described by a standard power-law relationship:

\dot{\varepsilon} = A\,\sigma^{n} e^{-Q/RT}

Norton's power-law creep equation. ε̇ is the steady-state creep strain rate, σ the applied stress, n the stress exponent (typically 3 to 8 for metals), Q the activation energy for creep, and T the absolute temperature. The exponential term means creep rate rises sharply as a component's operating temperature climbs toward the alloy's limit, which is exactly why a turbine designer treats a few tens of degrees of margin as a real engineering budget, not a rounding error.

Cobalt’s own high melting point and the specific strengthening mechanisms available in cobalt-rich alloys (carbide precipitation along grain boundaries, which physically blocks the grain-boundary sliding that drives creep) give these alloys real high-temperature performance, used in some jet engine and industrial turbine components where nickel-based superalloys, more common overall, are not the optimal choice.

Sources: cemented carbide hardness ranges and superalloy strengthening mechanisms draw on standard materials engineering references (ASM Handbook); the Hall-Petch relationship and Norton creep law are standard mechanical metallurgy; production context references the demand data elsewhere on this site.