Hardness Testers Information
Hardness testers measure a material's resistance to indentation. Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth or projected area of the indentation. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth of the area of the indentation using one of many different test methods.
Hardness testers use one of several types of scales for determining the hardness of a sample. These include Brinell, dynamic rebound, coating or nanoindentation, Rockwell, Vickers and Knoop, and ultrasonic. Methods include macro, micro, and superficial testing.
- Brinell hardness testers are widely used on castings and forgings. This method applies a predetermined test force to a carbide ball of fixed diameter that is held for a predetermined time and then removed. The diameter of the indentation width is measured twice—usually at right angles to each other and averaged. A formula or chart is then used to convert the averaged measurements to a Brinell hardness number. Test forces usually range from 500 kilograms to 3,000 kilograms (occasionally down to 1kg in less frequently used tests).
- In dynamic rebound, or impact hardness testers, a hammer or diamond tipped probe is dropped onto a sample and the rebound height or velocity change is measured and converted into a hardness reading. The rebound height increases with increasing hardness. These tests are less destructive than conventional static indentation tests and applied where even a small indent on a surface cannot be tolerated (e.g., forged rolls for printing).
- Coating hardness testers use indentation, scratching, or rubbing tests to evaluate the hardness or wear resistance of thin films of paint, sealants, adhesives, vapor deposits, CVD/PVD deposits, or plated layers.
- The Rockwell test method is defined in ASTM E-18 and is the most commonly used hardness tester operation method since it is generally easier to perform and more accurate than other types of hardness testing. Rockwell testers can be used on all metals except in conditions where the test metal structure or surface conditions would introduce too much variation, where the indentations would be too large for the application, or where the sample size or shape prohibits its use. The Rockwell tester method measures the permanent depth of indentation produced by a force on an indenter.
- Vickers and Knoop hardness testers can be used for micro and macro hardness testing. Typically, loads are very light, ranging from a few grams to one or several kilograms, although “macro” Vickers loads can range up to 30 kg or more. The microhardness testing operation, according to ASTM E-384, specifies a range of loads between 1 g to 1,000 g. There are two types of indenters: a square base pyramid shaped diamond for testing in a Vickers tester and a narrow rhombus shaped indenter for a Knoop tester. The micro-hardness methods are used to test on metals, ceramics, and composites—almost any type of material.
- In ultrasonic hardness testers, a probe tipped with an indenter is piezoelectrically resonated at an ultrasonic frequency. The probe is held against the sample with a spring and a small indentation is made. The frequency of the probe changes in proportion to the contact area of the indentation. The tester measures the frequency change, then calculates and displays the equivalent hardness value.
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