Comments on "An extended hardness limit in bulk nanoceramics" by J.A. Wollmershauser, B.N. Feigelson, E.P. Gorzkowski, C.T. Ellis, R. Goswami, S.B. Qadri, J.G. Tischler, F.J. Kub, R.K. Everett, Acta Materialia 69 (2014) 9-16

The recently suggested validity of the Hall-Petch relationship for transparent spinel ceramics with grain sizes down to 28 nm is discussed here regarding the equivalence of grain size and indentation size effects. The quantitative characterization of the samples investigated as transparent needs a correction of the authors' calculation of the theoretical transmittance. For fundamental physical reasons, this theoretical transmission of ceramics with a cubic crystal lattice does not exhibit an intrinsic grain size influence.