Compressive strength is how much inward force a given area of material can withstand before failure. The force tries to compress the atoms closer together.

Mohs hardness is the difficulty of a material to be scratched. Abrasion resistance. However, when an object scratches another, there first is a point of contact where the harder material is forced into the surface of the other, and then lateral forces shear off a grove from the softer material. But at this point of contact, the forces are compressive, trying to force the atoms together, until one of the materials give way.

As such, I would expect materials with a high Mohs hardness to have a high compressive strength and vice versa. This holds for ceramics such as silicon carbide, aluminum oxide, and others which have both a high compressive strength and a high Mohs hardness.

Does this trend hold for most/all materials and to be expected for the above reasons? Are there any counterexamples? If so, what could explain a high Mohs hardness and low compressive strength or vice versa?

  • $\begingroup$ "The force tries to compress the atoms closer together." This is harmless; equitrixial compression can't damage uniform materials. The issue is that a nonequitriaxial stress state causes deviatoric stress that induces failure (by dislocation slip in ductile materials, for example, or crack growth in brittle materials). Many contact mechanics texts (e.g., Fischer-Cripps's Nanoindentation) discuss models relating hardness to strength. Yes, these are broadly correlated, with fracture toughness also being a factor. $\endgroup$ Jun 30 at 7:34


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