Six Polymorphs of Sodium Chloride upon Depth-Sensing Macroindentation with Unusual Long-Range Cracks Requiring 30 N Load

Abstract

A sodium chloride single crystal is depth-sensing indented with a Vickers indenter at a Zwick/Roell ZHV Zwicky Z2.5 macro region instrument, together with Stephanie Rosner from Zwick GmbH & Co KG, Ulm, Germany. Normal forces up to 50 N load and 120 μm depth are applied, to experimentally secure the onsets of known and to locate new phase-changes with direct linear regression analyses, avoiding iterations. The author's physically deduced universal eqns. (1-7) are used for the data analyses. Four sharp phase-change onsets could thus be experimentally located, revealing four new polymorphs of NaCl in addition to the long known fcc and bcc polymorphs. Their relation to the three theoretically predicted higher pressure crystal structures in Reference is thus not clear. The predicted metallic character is unclear: no color formation or metallic reflection was observed by the author up to 50 N loads. No cracking of any type occurs at the indenter tip, but a new type of long-range cracking at the 30 N ranges occurred, and its highly resolved microscopy reveal two-step nucleation from a polymorphs’ interface exit. Inverted 3D microscopy of the residual impressions of the author with his Keyence VHX 100 microscope reveals crystallization of halite cubes upon unloading. The physical hardness increases (factor 5.9), the physical indentation modulus decreases (factor 2.3) upon load. The indentation phase-transformation energies cover 7 powers of 10. These data question the reliability of non-depth-sensing ISO/ASTM Standards of industrial Vickers, Brinell, and Rockwell, etc hardness parameters, as they cannot consider inevitable multiple phase-changes at very high loads, do not characterize the pristine rather than phase-transformed industrial material (including super-alloys), and miss the permissible pressure stress for avoiding phase changes. The risk of failure by nucleation of cracks at polymorph interfaces requires depth-sensing. Deviations from the mathematically required 5/4 ratio of applied-work over indentation-work are a rapid means to reveal phasechanges from old published loading curves by their graphical integration.