Determination of the elastic modulus and hardness of sol-gel coatings on glass: influence of indenter geometry

Indentations have been carried out on methyltrimethoxysilane coated float glass by using a spherical and a Berkovich indenter. The composite hardness as well as the effective elastic modulus were determined as a function of indentation depth for coatings of a thickness 0.5, 2 and 4 µm. Using the Berkovich indenter, the coating exhibited radial cracking (only for the two thicker coatings), delamination and chipping, whereas no cracking occurred during indentations using a spherical indenter. For the experiments in which the coatings exhibited radial cracking, we measured a constant composite hardness over the whole depth range instead of the expected increase. The elastic modulus, on the other hand, turned out to be insensitive to the radial cracking. All results show the expected increase of the modulus with indentation depth. The spherical indenter showed a much steeper rise of the effective elastic modulus with indentation depth than the (sharp) Berkovich indenter. By re-scaling the results with respect to contact area instead of indentation depth, the Berkovich and the sphere elastic moduli are comparable. Only at the point where delamination occurred in the Berkovich indentations, the re-scaled results of the two indenter geometries start to deviate.