Mechanical properties, i.e. hardness and elastic modulus, of amorphous SiO2 and Si3N4 PE-CVD coatings have been studied for two coating thicknesses (0.1 and 1.0 mu m) and two substrate materials (fused silica, i.e. Herasil, and borosilicate glass, i.e., BK7) using low load instrumented indentation. The coating systems are being considered for possible use as reference materials for thin film mechanical property test methods. Single layers of SiO2 and Si3N4 and a multilayer stack consisting of five double layers of SiO2/Si3N4 (individual layer thickness : 0.1 mu m) were investigated on both substrate materials. A special plasma pre-treatment of the substrates prior to deposition ensured that coating adhesion exceeded inner film stress for all systems considered. The applied indentation load ranged from 700 down to 0.1 mN and resulted in indentation depths from more than 1 mu m to about 15 nm. The influence of coating thickness and the effect of the substrate on the measurement of hardness and elastic modulus of the films an discussed, with emphasis on the effects of indentation depth vs. coating thickness on the observed coating properties. One coating-substrate system (1.0 mu m Si3N4 on Herasil) out of ten was found to be above a critical threshold for tensile cracking of the coating. In this system, termed 'overcritical'. tensile cracks occurred both prior to, and as a result of, indentation, indicating that Berkovich indentation may also be used to probe residual film stress.