Vickers hardness of NiW coating as a potential replacement for Cr-VI: A methodology to consider size effect and tip defect in classical microindentation

Abstract

Hard chromium coatings are widely used due to their excellent mechanical properties. However, they are historically obtained from hexavalent chromium (Cr-VI) solutions, which are classified as Carcinogenic Mutagenic Reprotoxic (CMR) material due to their very high toxicity. NiW coating is studied in this work as a possible replacement for Cr-VI coating. The substrate used is ST37 steel. The conventional commercial Cr-VI coating is considered here as reference material in terms of hardness.Usually in classical microindentation, the tip defect is always neglected and the indentation size effect (ISE) is studied disjointedly. However, these two aspects can interact together into the hardness measurement. Based on Vickers hardness measurements on steel substrate and both on a cross-section and on the top surface of the Cr-VI coating, 50 μm of thickness, we propose a discussion on the indentation size effect and the influence of the indenter tip defect on the hardness measurement. To validate our approach, the tip defect length was previously determined by SEM observation. Its value allows concluding that the apparent indentation size effect on the Cr-VI coating is only due to the tip defect. However, the corrected Vickers hardness for the steel substrate reveals always a residual ISE. This hardness-load dependence of steels is attributed to the strain hardening generated by the surface preparation.Starting from the methodology developed on the quasi-massive materials, we have corrected the raw indentation data obtained for the NiW coatings and we have applied the model of Jönsson and Hogmark to separate the contribution of the substrate on the hardness measurement to determine the hardness of the coating only. A discussion on the value of the constant involved in their model confirms that it should be cautiously considered. As a result, the macrohardness of the Cr-VI coating is determined around 8.0 GPa whereas for the NiW coating it is found to be from 4.5 to 5 GPa depending on the deposition parameters.