Hardness measurement methods for rubber and plastics are described in the standards ISO 7619-1, ISO 7619- 2, ISO 868 and ISO 48. Rising requirements for the accuracy of this hardness measurement have led to the result that the users of rubber hardness testers and also metrology experts who deal with their calibration need a calibration method to check the overall function of the testers acknowledged by a standard. Therefore extensive experiments were conducted in a cooperation between the Physikalisch - Technische Bundesanstalt (PTB) in Braunschweig (Germany) and the ZAG Ljubljana (Slovenia). The results have been used as a basis for the elaboration of the relevant international standard ISO 18898 which was published in 2006. In this standard direct verification methods are laid down. An indirect verification method using rubber hardness reference blocks, which is also applicable for the verification of hardness testers, is presented in this paper. At present hardness reference blocks are available only for some of the hardness scales which are used for rubber materials. Suchwise, Shore A, IRHD N and IRHD M reference blocks are available in the market. During the verification process the hardness tester is calibrated by using rubber hardness reference blocks. The evaluation of the repeatability and the error of the hardness testers including the uncertainty of measurement of the calibration results is given in this paper.

In order to perform traceable hardness measurements using Rockwell Hardness Indenters, the indenter must conform to the requirement of ISO 6508-3 standard. The standard defines specifications for the geometric parameters for this type of indenter. Conformance to this standard is of particular importance to all calibration laboratories which operate in accordance with ISO/IEC 17025. At present there is no facility in South Africa with the capability of calibrating indenters for conformance to ISO 6508-3. This situation is being addressed through a series of studies currently being conducted at the National Metrology Institute of South Africa. The following parameters are calibrated using the µCMM. ISO 6508-3 specifies a radius of 200 µm with an uncertainty of ± 5 µm for Rockwell Hardness Indenter tips. Also specified in the ISO standard is the cone perpendicularity between the axis of the diamond cone to the axis of the indenter holder, with a specification of ± 0,03°. The last part of the ISO standard which was investigated states: “The surface of the cone and the spherical tip shall blend in a truly tangential manner.” All these parameters were measured and associated uncertainties calculated to prove conformance with ISO 6508-3 for the calibration of hardness indenters.

The Rockwell hardness test is commonly used and accepted by many industrial users, but the hardness value requires conversion between scales because the geometry of the indenters and the load ranges are different. On the other hand, hardness calculation using pyramidal indenters is quite simple and can be applied to any load range, though it is much more complicated to calibrate the frame compliance and the truncation of indenters in nanoindentation measurement. For this reason, nanoindentation measurement is not yet fully industrially friendly. In order to improve the above situation, newly developed industrial hardness test methods and a new concept of “Equivalent indentation depth (or indentation depth index)” were proposed and investigated in this paper. These methods are based on the principle of “similarity” of Vickers and other pyramidal indenters, and take advantage of the industrial and practical usability of the Rockwell hardness test. Experimental results that covered macroscopic through nanoscopic ranges show that the methods can be applied to a wide range of test loads. One of the expected advantages of the methods of performing nanoindentation and other instrumented indentation tests (ISO 14577) is that the hardness is not significantly influenced by the frame compliance or the truncation of indenters. Other advantages over conventional hardness tests are also discussed in this paper. More experimental work is necessary to confirm and establish the new methods.

Brinell hardness measurements are widely used at industrial level. For the calculation of Brinell hardness values, the measurement of the diameter of indentations is necessary. In practice, the measurement of the image created by the optical systems used for the magnification of the indentation is usually carried out. The dimension of the indentation image depends by the optical system that, in practice, transform the real indentation in image using properties of light reflection. The paper describe the effect of this influence parameter in Brinell hardness measurements in experiments carried out at hardness laboratory of Istituto Nazionale di Ricerca Metrologica (INRIM) (formerly Istituto di Metrologia “G. Colonnetti” – IMGC) with application to the data obtained at international comparison at the National Metrology Institutes level. Moreover, some methods for its evaluation and possible correction will be proposed.

Significant measurement differences have been continually observed worldwide in Brinell hardness tests, even in the secondary calibration laboratories that calibrate test block reference standards. The main cause of this problem is the edge of the indentation is not a distinct boundary, but is instead a curved surface from either material piling up (pile-up) or sinking in (sink-in) caused by plastic flow of the material surrounding the ball indenter. This makes it difficult to clearly resolve the edge of the indentation and thus determine the indentation diameter. In this research, Brinell hardness indentations were made using various indentation forces and ball indenter sizes. Using a confocal microscope, the indentations were measured in three dimensions from which the indentation profiles were determined. Additionally, finite element analysis (FEA) models were developed for studying the location of contact points at indentation pile-up edges. From both microscope measurements and FEA simulations, the difference between the measured indentation diameter and the actual contact diameter was determined for each indentation.