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JAROSLAV MENCIK and MICHAEL V. SWAIN CSIRO Division of Applied Physics, Lindfield NSW 2070, and Department of Mechanical and Mechatronic Engineering University of Sydney, NSW 2006 Australia

ABSTRACT Platinum films of thickness 0.5 - 1.2 - 3.0 pmn, deposited by magnetron sputtering onto glass and silicon, were investigated using ultra-microindentation. The tests were done with a pointed (Berkovich) indenter. The values of the hardness and elastic modulus of the composite "film + substrate" and the substrates alone were obtained for various loads and depths of penetration, and used to evaluate the hardness and elastic modulus of the films. The use of various film thicknesses and substrates has enabled us to investigate in more detail the influence of the contact radius/contact depth compared to the film thickness as well as the substrate properties on the total indentation behaviour. Various approaches to the data evaluation are discussed in the paper.

INTRODUCTION

Mechanical properties of thin surface layers can be investigated using ultra-microindentation systems which press a hard indenter into the specimen, and follow the force-displacement response. The measured data (load vs. displacement) are then used to calculate the contact area under the indenter, mean contact pressure (hardness), elastic modulus and other quantities. The basic theory and formulae for testing of homogeneous materials is well known[I-6]. The determination of hardness and elastic modulus of thin films, deposited on various substrates, is more difficult, as it is the response of the system, "film + substrate". This response changes from that corresponding to the film alone if the indenter has penetrated only slightly into the film, to that corresponding to the substrate for depths of penetration large compared to the film thickness. The response can, in principle, be expressed as a function of the properties of the film and substrate and of the relative penetration of the indenter, and such a function is then used for the estimation of the hardness or elastic modulus of the film from the measured values. The relative penetration can be characterised by the ratio h/t or a/t, where h is the plastic (or contact) depth of penetration, t is the film thickness, and a is the characteristic size (e.g. mean radius) of the contact area (Fig. 1). Since there is a functional relationship between h and a (for example, a = const.h for an ideally sharp pointed indenter, or a = 4(R.h) for a spherical indenter with radius R) the mean contact pressure maybe determined. However, it is considered that the contact radius is more useful, particularly for hard films on softer substrates, as it approximately corresponds to the radius of the core of highly stressed material under hydrostatic pressure. On the other hand, the contact depth is useful especially for soft films on hard substrates as it indicates whether the indenter has penetrated into the substrate or not.

729 Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society

t Figure 1. Schemat