Stress-Strain Behaviour of thin films using a Spherical Tipped Indenter
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STRESS-STRAIN BEHAVIOUR OF THIN FILMS USING A SPHERICAL TIPPED INDENTER
T.J.Bell, J.S.Field and M.V.Swain CSIRO Division of Applied Physics Lindfield, NSW 2070 Australia
ABSTRACT The stress-strain behaviour of metallic and polymer thin films are measured using a spherical indenter. A modification of the Hertzian analysis enables the elastic to plastic transition to be analyzed so that the variation of mean pressure versus depth can be calculated. The stress-strain behaviour is then calculated from the mean indentation strain using a relationship proposed by Tabor/l]. Observations of high precision load displacement data generated with a UMIS-2000 instrument are analyzed on the basis of the simple analysis developed. The measured values are compared with simulated data for the same materials. INTRODUCTION The determination of the mechanical properties of thin films is an important parameter in the characterization of these materials. To date, the options available for such characterization have been limited to pointed indenters and scratch tests. Such tests are measuring a wealth of elastic/plastic/adhesion information but unfortunately in a manner that does not enable the various components to be readily identified. These limitations have seriously hindered the mechanical property optimization of thin films in an analogous manner to only using strength as a basis for bulk material selection. The development of ultra micro-hardness indenting systems has enabled the determination of hardness of very thin films with the criteria that the depth of indentation be less than about 30 of the film thickness. Most evaluation of such measurements has been conducted with a Berkovich indenter in order to minimize indenter tip chisel effects with other indenter geometries. However the Berkovich indenter still suffers from a finite but an exceptionally difficult to define or measure tip bluntness. In addition pointed indenters produce a virtually constant plastic strain impression and there is the additional problem of assessing the elastic modulus from the continuously varying unloading slope. Such problems have invariably led to plots of normalised hardness when comparing thin films prepared by slightly different methods. Spherical indentation overcomes many of the problems associated with pointed indenters. Hertz[2], more than a century ago proposed that hardness be measured with a spherical indenter as one is then able to follow the transition from elastic to plastic behaviour and thereby define the yield stress. The availability of very high precision indenting systems coupled to modern computing facilities now allows the detection of the onset of plastic flow and the analysis of such data. In this paper a brief outline of the Hertzian contact problem and its extension to elastic-plastic indentation is presented. This is followed by observations of force-displacement measurements on bulk and thin films of metals and polymers. These observations are then analyzed by a software package developed on the basis of the theoretic
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