Knoop hardness test method (Microhardness test method) is applicable to extremely thin metals, plated surfaces, very hard and brittle materials, carburized and nitrided surfaces and whenever applied load must be kept under 3600 gm.
The load is applied by a diamond pyramid indentor with long but short in depth diagonal in ratio 7:1 (shallow indentation), longitudinal angle 172° 30′, traverse angle is 130°. In this test, the diamond pyramid is indented on the specimen and length of diagonal is measured under a microscope. The Knoop hardness number (KHN) is calculated from the following relation:
where ‘P’ is applied a load in kg, ‘A’ is uncovered projected area of indentation in mm2, ‘L’ is the length of the long diagonal in mm and ‘C’ is a constant related to the length of a projected area for each indentor supplied by the manufacturer.
One can calculate the Knoop hardness from the projected area of the impression or indentation rather than the area of contact as in the case of indentation tests.
We must note that micro-hardness require extra care in all stages of testing. A good polishing of the surface is required, i.e. the surface of the specimen must be prepared carefully.