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 g lass by using a spherical and a Berkovich indenter. The composite hardness as well as the effective elastic modulus were determined as a function of i ndentation depth for coatings of a thickness 0.5, 2 and 4 mu m. Using the B erkovich 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 whic h the coatings exhibited radial cracking, we measured a constant composite hardness over the whole depth range instead of the expected increase. The e lastic modulus, on the other hand, turned out to be insensitive to the radi al cracking. All results show the expected increase of the modulus with ind entation depth. The spherical indenter showed a much steeper rise of the ef fective elastic modulus with indentation depth than the (sharp) Berkovich i ndenter. By re-scaling the results with respect to contact area instead of indentation depth, the Berkovich and the sphere elastic moduli are comparab le. Only at the point where delamination occurred in the Berkovich indentat ions, the re-scaled results of the two indenter geometries start to deviate . (C) 2000 Elsevier Science S.A. All rights reserved.