The Knoop test is done on an unfinished surface of the material, but the test is performed over a much smaller test area than the Brinell test, making it ideal for brittle materials such as ceramics.
The standard Vickers hardness test usually requires a test depth of at least 0.2 mm in order to provide accurate readings. The Knoop hardness test (Figure 10.2) is only capable of testing the hardness of materials less than about 0.05 mm in thickness (Figure 10.3), however it is recommended that non-ferrous materials be tested to a depth of 0.5 mm. This can be done by placing the Knoop indenter on the test specimen two-three times and averaging the hardness of the material at the maximum test depth.
Specimen thickness may be a factor in the choice of test method. The Vickers and Knoop tests can be used for a range of specimen thicknesses. The hardness of ferrous materials decreases with increasing specimen thickness and the Vickers hardness test should not be used for ferrous materials greater than 0.5 mm in thickness.
Other tests are available to provide hardness and elastic modulus values for materials. For example, the indentation test or “push-in” method using a 0.25 mm diameter cylinder, which is often used in the evaluation of plastics, may be used to measure the hardness and Young’s modulus of plastics.
The Mohs hardness test is widely used by industry. This is because it is quick and easy to use. However, this test does not provide any information about the metallurgical properties of a substance. It does not determine if a substance is ductile, brittle, ferritic, or austenitic.
Mohs hardness test is a quick, simple, and reliable test to determine the relative hardness of a mineral. However, it is not generally used on other materials such as metals and ceramics. Fortunately, there are other tests that can be used to determine the hardness of other materials. The tests that follow will show you how to test a variety of materials for hardness and how to use the results of the test. 827ec27edc