User:Shabnam Seyidova/sandbox

Indentation hardness of diamond single crystals, nanopolycrystal, and nanotwinned diamonds: A critical review Right from the outset it should be stated that this review is primarily about indentation hardness values determined at room temperature and 1 atm. Indentation techniques are very powerful for determining the mechanical properties of bulk solids, thin solid films, and solids available only in mm sizes. One aspect of these techniques is the measurement of indentation hardness of test solids in which the indenter causes plasticity. Diamond is the hardest natural solid, and pyramidal indenters, also made of diamond, have been used for determining its indentation hardness. The three most commonly used pyramidal indenters are: Vickers, Knoop , and  Berkovich. Vickers indenter is a squarebased pyramid with an angle of 136° between its two opposite faces, whereas the Knoop indenter is a rhombus-based pyramid, with a ratio of ~7 between its long axis and the short axis. The Berkovich indenter is an equilateral triangle-based pyramid, with an angle of 65° between its axis of symmetry and an inclined face of the pyramid. A particularly interesting characteristic of the Berkovich indenter is that for a given height of the pyramid the total surface area of the three inclined surfaces is the same as the total surface area of the four inclined faces of a Vickers pyramid of the same height. An important point to note about the 4-sided pyramidal indenters is that there is always a ridge on their apex because the four inclined sides do not meet at a point in practice. On the other hand, in the case of a Berkovich indenter its three inclined sides can be made to meet and to give a definite point. Since the late 1930s several measurements of the indentation hardness of diamond single crystals and polycrystals have been published. The reported hardness values vary by more than an order of magnitude. So, it is the purpose of this critical review to discuss and to rationalize these reported hardness data, and to suggest methods for improvement. In particular sound theoretical arguments have been used to show the maximum possible Vickers hardness value of diamond.

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