Cérarniques techniques — Méthode d’essai de dureté des cérarniques monolithiques a tern pérature ambiante.
4.6.12 Measure the length of the two diagonals to within 0,2 pm for diagonals less than 50 pm, or to within 0,5 lIm for diagonals equal to or more than 50 pm. The arithmetical mean of two readings shall be taken for the calculation of the Vickers hardness. If the difference of the two diagonals is more than 5 % of the mean value (see Figure 4), the result shall be rejected, and a check made of the parallelism and flatness of the test piece. and of the alignment of the indenter. Follow the manufacturer’s instructions very carefully, with regards to the proper usage of the measuring crosshairs. Figure 5 is provided for guidance. The use of optical methods to enhance contrast (like Nomarski interferences) is not permitted.
4.6.13 At least five valid indentations shall be made for obtaining a mean result in accordance with this document.
4.6.14 Calculate the Vickers hardness, HV, for each valid indentation, using the formula in Table 1.
Calculate the mean hardness for all valid indentations and the standard deviation. The calculated Vickers
hardness shall be expressed with three significant numbers (e.g. 15,4 GPa HV 9,807 N or 1540 HV 1)
4.6.15 Alternatively, see ISO 6507-4 for conversion tables for use in tests made on flat surfaces.
4.7 Accuracy and uncertainties
The principal errors arising in a Vickers hardness test on advanced monolithic technical ceramics vary in magnitude according to the size of the indentation, and thus the indentation force used. The Vickers diamond geometry was originally chosen because natural cleavage planes of the diamond were employed. Variations in geometry between indenters are therefore small, and can usually be ignored except when indentations are of less than 20 pm diagonal length where the tip and edges near the tip may be variable between indenters. In particular, the edges may have flats up to 1 pm across on them, and this has the effect of cutting the corners off the indentation. The error that this introduces is insignificant if the indentation is larger than about 30 pm, but increases rapidly in importance as the size is reduced.
Determination of the diagonal lengths using cross-wires or other device attached to the instrument relies on the operator positioning them at the “true opposing corners of the indentation. There is a subjective element in performing this task which increases with poor optical contrast and reducing size of the indentation. The possible errors can be reduced by experience, and by consistent use of high-hardness, preferably ceramic or hardrnetal, test blocks to familiarize the eye at the start of measurement sessions. In this way, any systematic measurement bias can be reduced. In a round-robin exercise on high-alumina ceramicsl4l, it was found that when two individuals measure the same set of indentations on different measurement equipment, a poor correlation was obtained unless the true sizes of the indentations varied by more than ii pm. It follows that, discounting differences between machines, it cannot be guaranteed that any two observers will agree that one material is significantly harder than another unless the average indentation sizes are systematically smaller by more than 1 pm. ISO 14705-2016 pdf free download.