Knoop
The Knoop hardness test is a microhardness indentation method that uses an elongated rhombohedral diamond indenter and measures the long diagonal of the indent to calculate hardness. It is used especially for thin coatings, thin sections, brittle materials, and case-depth work because the shallow, narrow indent minimizes cracking and substrate damage.
On the shop floor, Knoop testing is used when a hardness reading must come from a small heat-affected zone, nitrided layer, carburized case, plated surface, ceramic insert, glass component, or thin coating without crushing through to the base material. The specimen is mounted, ground, and polished to a mirror finish, then tested with a low load on a microhardness tester; the operator measures the long diagonal optically and calculates hardness from the load and diagonal length. This matters because a Knoop trace can be placed near an edge or across a cross-section to verify whether a process produced the intended hardness gradient, coating integrity, or case depth before parts are released. The method is standardized in ASTM E384 and ISO 4545, with low test-force ranges from grams-force up to about 1–2 kgf, making it suitable for micro-scale evaluation rather than bulk hardness mapping.
Why use Knoop instead of Vickers?
Knoop produces a more elongated, shallower indent, making it better for thin layers and brittle materials where cracking or substrate influence must be minimized.
What is measured in Knoop testing?
Only the long diagonal of the indent is measured optically; the hardness number is then computed from the applied load and that diagonal.
What is the basic equation?
Commonly expressed as HK = 14.229 × F / L², where F is load and L is the long diagonal length.