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Testers which measure the ability of solid materials to resist plastic deformation or fracture when a force or load is applied to them. Common types of hardness tests include scratch, indentation, and rebound hardness. Scratch hardness is the measure of the resistance of a material to scratching of its surface. Scratch hardness testers usually consist of some type of an indenter with a very hard tip (e.g., tungsten carbide, diamond), a load or applied force, and a scale to measure the pressure applied by the load or force on the indenter to scratch the surface of a material. Scratch hardness is commonly measured on the Mohs scale. Indentation hardness is a measure of the resistance of a material to permanent plastic deformation due to a constant compression load from a sharp object. Indentation hardness testers typically feature an indenter made of a very hard material (e.g., diamond, steel, tungsten carbide) to which a load or force is applied, a platform to hold in place the material to be tested, a load to apply the force to the indenter, and various measuring mechanisms to measure the indentation made in the test material by the indenter according to different hardness scales (e.g., Brinell, Knoop, Rockwell, Shore, Vickers, etc.). Rebound hardness, also known as dynamic hardness, measures the height of the bounce of a hard-tipped hammer dropped from a fixed height onto a material. Rebound hardness testers usually have a diamond tipped hammer and a measuring scale to measure the height of the bounce caused by the return energy of the tester according to the hardness of the test material. Some hardness testers are interfaced with computers that aid in selecting the test parameters for the hardness test and in presenting results. Some hardness testers can be pocket-sized or be designed to fit in hard to reach locations; many use electronic components to calculate hardness. Hardness testers that make microscopic indentations on delicate or brittle materials and use a microscope to make precision measurements of the indentations (microhardness testers) are also available. Hardness testers are used in laboratories and sometimes in the field to ascertain the hardness of materials used in many different applications; they may have clinical applications in testing the hardness of dental materials and other materials used in clinical engineering.
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