Geometric Measurement Comparisons for Rockwell Diamond Indenters

− In the uncertainty budget of Rockwell C hardness (HRC) tests, geometric error of the Rockwell diamond indenter is a major contributor. The geometric calibration of Rockwell diamond indenters has been a key issue for Rockwell hardness standardization. The National Institute of Standards and Technology (NIST) developed a microform calibration system based on a stylus instrument for the geometric calibration of Rockwell diamond indenters. Using that system, a NIST master standard Rockwell diamond indenter No. 3581 was established in 1995, by which approximately 300 standard reference material (SRM) Rockwell hardness blocks of HRC scale were calibrated for implementation of the HRC scale in the United States. This indenter has been re-calibrated in 1997, 2005 and 2007. The calibration results have shown both high stability for the NIST standard Rockwell diamond indenter and high reproducibility for the NIST microform calibration system. After more than fifteen years of service, the stylus instrument was recently replaced by a new one. In order to test the measurement agreement between the two stylus instruments, another NIST master Rockwell diamond indenter No. 101 has been recently calibrated by the new instrument and the calibration result shows good agreement with the previous results.