Establishing a common Rockwell hardness scale using geometrically calibrated standard diamond indenters

Recently developed microform measurement techniques have reduced the measurement uncertainties in the geometry of Rockwell diamond indenters. In this paper, we describe several intercomparisons to determine if tightly controlled indenter geometry can improve the consistency of Rockwell C hardness (HRC) measurements. First, using a common indenter, five national laboratories obtained a total performance variation range of (-0.19 to +0.16) HRC on a set of hardness blocks with nominal hardness values ranging from 30 HRC to 60 HRC. Second, a set of 11 indenters with tightly controlled geometrical parameters produced a performance variation range of (-0.17 to +0.23) HRC from 25 HRC to 60 HRC when tested in a single deadweight hardness machine. The consistent geometries of these indenters were verified both with a stylus instrument and a laser interferometer. Third, a similar intercomparison of eight indenters provided by NIST and NRLM and calibrated by a stylus instrument yielded a variation range of +0.19 HRC from 25 HRC to 60 HRC. These results support the feasibility of establishing a worldwide unified Rockwell C hardness scale using geometrically calibrated standard indenters with consistent geometry and hardness performance. It should be possible to establish a common Rockwell C hardness scale with an expanded uncertainty of approximately +0.2 HRC and without significant bias with respect to an ideal scale.