Is the hardness of material harder than diamond reliable?

With the development of new synthesis methods and chemistries, a number of new superhard materials have been reported to be harder than diamond. While such materials are highly desirable due to their wide-ranging applications, there are some inherent uncertainties in the methods utilized to determine and define the hardness of such materials. In this paper, we employed the standard Vickers diamond indenter and substitute indenters with the same shape to measure the hardness of nine ceramics and superhard materials within well-defined criteria and methodology, for the assessment of consistency in the hardness testing. The findings and the developed testing method in the current study have broad implications in characterizing new and emerging superhard materials, leading to new discoveries.