Abstract

The hardness of optical coatings is an important mechanical property, of particular interest if coatings are used under severe environmental conditions, such as encountered in military or ophthalmic applications. Hardness measurements are difficult to perform on optical coatings with standard microhardness testers which are used in connection with optical microscopes, since the minimal indentation loads of 0.5 to 2 g-wt (5 to 20 mN) cause imprints with penetration depths of about or even exceeding the coating thickness [1], thus measuring the hardness of the substrate rather than that of the coating. A Knoop diamond indentor with its pyramidal shape on a rhombic base (diagonal ratio 1:7) and an edge angle of 172°3′ causes a shallow penetration only 1/30 of the long diagonal deep. The more common Vickers pyramid (on a square base) with an edge angle of 148° has a penetration depth of 1/7 of the long diagonal. If the penetration depth in a thin film should not exceed 15-20% of its thickness in order to avoid the influence of the substrate hardness, the imprint diagonals range from about 0.06 μm downwards for the Vickers indentor and from about 0.25 μm down for the Knoop pyramid, e.g. for a single TiO₂ quarterwave layer of 550 nm center wavelength. Such tiny marks can only be observed and measured in a scanning electron microscope (SEM). In earlier attemps to measure ultra-microhardness (UMH), the penetration depth of the indentor was measured directly with a capacitor bridge during indentation.[2], thus avoiding the need of a high magnification for measuring the imprint diagonals. However, this method may be incorrect because not only plastic deformation contributes to the result as defined for microhardness testing, but also elastic forces acting on the indentor tip.

© 1984 Optical Society of America