Hardness & Microhardness Testers
New Hampshire Materials Lab specializes in hardness testing of incoming materials and finished products. Materials tested include metals and alloys, polymers, ceramics, and composites. Hardness tests include Rockwell, Knoop, Vickers, Durometer, and Barcol.
Microhardness (Knoop and Vickers)
Microhardness testing is typically used for parts, wire, and foil samples that are too small or too thin for conventional hardness testing. This testing uses low loads, typically 10 to 1000 grams. A highly polished, pyramid shaped diamond is used as an indenter. The diamond may be a rhombic based pyramid (Knoop microhardness) or a square based pyramid (Vickers microhardness). With Knoop microhardness tests, the depth of penetration of the indenter is about 1/30th of the length of the indentation produced.
Both hardness scales are based on formulas that include the load used and the indentation length produced by the test, which are typically are in lengths between 20 and 200 micrometers. With a given load, the Vickers indenter penetrates roughly twice as far into the material being tested when compared with the Knoop indenter. Accordingly, the Vickers tests are less sensitive to small differences in the surface condition.
Accurate measurement of either the Vickers or the Knoop indentation requires a good surface finish that results from standard metallographic grinding and polishing sequences. The smaller the indentation, the more critical the surface finish becomes. Furthermore, the hardness number determined in the microhardness tests is load dependent. This load dependency occurs with loads of 500 gf or less for Knoop hardness values and loads of 100 gf or less for Vickers hardness values. In most cases, the apparent Knoop microhardness decreases with increasing load while the apparent Vickers hardness increases with increasing loads.
For more information on microhardness testing, please feel free to contact our office.