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:
knoop hardness test is used to measure the hardness of
very brittle materials or thin sheets