<P> The Knoop hardness test / kəˈnuːp / is a microhardness test--a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes . A pyramidal diamond point is pressed into the polished surface of the test material with a known (often 100g) load, for a specified dwell time, and the resulting indentation is measured using a microscope . The geometry of this indenter is an extended pyramid with the length to width ratio being 7: 1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge . The depth of the indentation can be approximated as 1 / 30 of the long dimension . The Knoop hardness HK or KHN is then given by the formula: </P> <Dl> <Dd> H K = load (kgf) impression \ area (mm 2) = P C p L 2 (\ displaystyle HK = (((\ textrm (load)) ((\ mbox (kgf)))) \ over ((\ textrm (impression \ area)) ((\ mbox (mm)) ^ (2)))) = (P \ over (C_ (p) L ^ (2)))) </Dd> </Dl>

Knoop hardness test is used to measure the hardness of