Hardness testers are a widely used tool to characterize the properties of a given material and to determine whether it is appropriate for a particular application. Most of the different types of hardness tests involve the use of a specifically shaped indenter that is harder than the material being tested and which is pressed onto the material using a certain amount of force. The size of the resulting indentation is then measured and compared to a hardness scale in order to derive the hardness value.
Metals are primarily tested for hardness for two reasons; to research their characteristics prior to the manufacturing process and to confirm that they are of a suitable hardness after heat treatment to a desired specification. The test results can also be used to determine if the material is susceptible to cracking due to environmental factors. Conventional hardness testing machines require that the sample be brought to the device, but portable testers have been developed which permit on-site hardness measurements.
The Brinell method uses a steel ball indenter to mark hardness testers the surface of the sample. It is primarily used to test large parts of low to medium hardness and is the preferred method for testing rough samples that are difficult to handle with other methods. The Rockwell and Vickers methods are both static indentation tests with a direct readout for production control, and they are typically used for metallic materials. The microhardness test methods, such as the Knoop and microcharacter, employ narrow diamond shaped indenters to make very small impressions in the sample and require an optical measurement of the diagonal lengths of the indent to determine the hardness.
A number of different factors can affect the hardness test result, and the quality of the surface preparation is usually a key factor. The test method and load used will influence the depth of the indent left by the indenter, and it is important that the standards are followed closely to avoid a misinterpretation of the test results.
The most important thing to remember when selecting a hardness testing system is that the test method must be selected based on the expected hardness of the material, not the type of the indenter. Different test methods use differently shaped indenters and produce dissimilar responses, and conversion tables must be developed in order to translate results from one scale to another. If possible, it is recommended that the highest load/force for the largest indent be used to obtain the most accurate result.