Rockwell hardness testing measures the hardness of metallic and polymer materials. Although hardness tests do not give a direct measurement of any performance properties, hardness scores correlate with strength, wear resistance, and other properties. Hardness tests are widely used for material evaluation due to its simplicity and low cost relative to direct measurement of many properties. The NIST Microform Calibration System was established in 1994. With the world's lowest uncertainty for the dimensional calibration of Rockwell diamond indenters, a group of indenters were calibrated in 1995 and characterized by high uniformity for both their micro-' form geometries and hardness performances. One of them, the No. 3581 indenter, was selected as the NIST primary Rockwell diamond indenter for establishing and maintaining the U.S. National Rockwell C Hardness Scale. A metrological approach was proposed by NIST to establish a worldwide unified Rockwell hardness scale.
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