Indentation Velocity Effect on Martens Hardness Measurement

The Martens Hardness (HM) is an important parameter characterising the elasticplastic properties of the to be investigated sample material which is derived from the instrumented indentation test. At present the standardisation of the instrumented indentation test in the framework of ISO/DIS 14577-1, -2, -3 is underway [1]. This standard addresses the macro-, microand nanorange of the indentation test. The peculiarities of the nanoindentation test when measuring samples with thin coatings (coating thickness d < 2 μm) will be considered in ISO/CD 14577-4 for which inputs came from CEN TC 184/WG 5 [2] and from the EU-project „INDICOAT“ [3]. Martens Hardness presents a number of advantages, but, as any newly defined method, requires a general analysis of influence quantities to determine the sensitivity coefficients necessary for the uncertainty evaluation. Indentation velocity was found to be one of the main influence quantities for Rockwell and Vickers scales, therefore its effect was evaluated in a previous work that indicated an effect much higher than expected. In that work some warning was given, because the analysis was based on the results obtained on a single Hardness Standard Machine, moreover based on a simple experimental plan that did not guarantee any separation of the effects of time and velocities. In the present work these drawbacks are overcome. The analysis is based on an experimental plan that takes into account the load increasing time, the velocity of the initial part of indentation and the velocity of the last part of indentation, that is for the Rockwell and the Vickers method the most important influence factor. Moreover, following the resolution adopted within the recent ISO TC 164/SC3 meeting during which Martens Hardness was extended to cover from nano to macro ranges, experiments are performed on each of these ranges and with different machines, delivering in that way more significant results.