Indentation hardness of diamond single crystals, nanopolycrystal, and nanotwinned diamonds: A critical review

In this critical review, indentation hardness data obtained during the past 80 years using Vickers, Knoop, and Berkovich diamond pyramidal indenters are presented. The data show a very wide variation of values, ranging from ~60 GPa to 6000 GPa. To rationalize the data, it has been argued, using theoretical grounds, that values higher than ~230 GPa are most unlikely. Moreover, it has been emphasized that a recently reported Vickers hardness of 200 GPa for nanotwinned diamonds, and claimed as being unprecedented, has been shown to be an incorrect claim. This emphasis is made as several previous authors had reported higher Vickers hardness values. It is suggested that the most suitable indenter for measuring the indentation hardness of diamond single crystals, diamond nanopolycrystals and nanotwinned polycrystals is the Knoop indenter, as it is more robust than the other two. The most likely highest Knoop hardness values of high purity, type IIa, diamond single crystals, and ultrafine nanopolycrystals are in the range ~130 to ~145 GPa. The upper value is Meyer's hardness obtained using a Berkovich indenter. It has been suggested that the cause of the large variation in the hardness data reported in the literature is due to the difficulty in the measurement of the indentation dimensions, fracture of diamond sample and indenter tip, and the formation of unclear indentations.