Non-Destructive Measurement of the Density of Thin Coatings by a Combination of Instrumented (Nano) Indentation and Acou
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Non-Destructive Measurement of the Density of Thin Coatings by a Combination of Instrumented (Nano) Indentation and Acoustical Techniques. Nigel M. Jennett, Giles Aldrich-Smith and Antony S. Maxwell. National Physical Laboratory, Teddington, Middlesex, TW11 0LW, U.K. ABSTRACT Nanoindentation is used to provide a measurement of coating plain strain modulus. The coating properties are then measured by surface acoustic wave spectroscopy (SAWS). It is shown that, by exploiting the relative insensitivity of the SAWS acoustical model to Poisson’s ratio, it is possible to measure either the thickness or the density of a thin coating given the input of the other. Furthermore, the indentation and acoustical tests are non-destructive and the resulting value of thickness or density is of high precision. A case study is presented for the measurement of density for a 250 nm thick nano-crystalline Niobium coating on a Si (001) substrate. The density is found to be ρ = 8.4 ± 0.3 g cm-3, about 2 % below bulk values but the modulus is found to be only 85 GPa (for a Poisson’s ratio of 0.4), which is 18 % below bulk. INTRODUCTION Nanoindentation is one of the very few techniques that can measure both the elastic and plastic properties of very small volumes of materials. Nanoindentation protocols from the EC project ‘INDICOAT’ [1] are now available to determine modulus with a low uncertainty for thin coatings (~200 nm) even when the normal indentation response includes a significant component due to the substrate. The combination of nanoindentation with acoustical measurements (impact resonance and surface acoustic wave spectroscopy, SAWS) has already been shown to give a self-consistent and self-validating set of mechanical properties for thin coatings, assuming that the nanoindentation result is the equivalent isotropic plain strain modulus (a composite of Young’s modulus and Poisson’s ratio) of the coating [2]. Classic acoustical theory can model the data obtained from SAWS but requires the input parameters of elastic modulus, Poisson’s ratio and density for all components in the measurement volume of the technique as well as the thickness of any coating [3-9]. When measuring a thinly coated substrate, the properties of the substrate are usually well known, or can be readily obtained by other methods such as pulse-echo ultrasonics. Thus the input of up to three parameters is required to obtain the fourth for a coating. In this paper, nanoindentation is used to provide a measurement of modulus coupled with Poisson’s ratio. It is shown that, by exploiting the relative insensitivity of the acoustical model to Poisson’s ratio, it is possible to measure either the thickness or the density of a thin coating given the input of the other. Furthermore, the indentation and acoustical tests are non-destructive and the resulting value of thickness or density is of high precision. EXPERIMENTAL The sample used in these experiments was a 250 nm thick Nb film sputtered onto a singlecrystal Si (001) substrate using room-temperature DC magne
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