Indentation and Finite Element Modelling Investigations of the Indentation Size Effect in Aluminium Coatings on Borosili
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Indentation and Finite Element Modelling Investigations of the Indentation Size Effect in Aluminium Coatings on Borosilicate Glass Substrates I.Spary1,2, N.M.Jennett1 and A.J.Bushby2 1 2
NPL Materials Centre, National Physical Laboratory, Teddington, TW11 0LW, UK Centre for Materials Research, Queen Mary, University of London, E1 4NS, UK
ABSTRACT Nanoindentation is one of the few available methods and the most commercially widespread technique for investigating the elastic and plastic properties of small volumes such as thin films. Quantitative methods for obtaining the indentation (plane strain) modulus and hardness of a coating have been published and finite element models (FEM) of the elastic-plastic response of indentation have been developed. Comparison of the FEM output with actual indentation data has shown that, as the indentation size reduces, the apparent yield stress of the material increases. We have shown that the increase in yield stress is predictable and falls on a master curve (MRS Symp. Proc., Vol 788, p123, 2003). Predictions have been tested and agree for a range of metals (Cu, Al, W, Ir). This points to there being a fundamental length scale for dislocation-based deformation and raises the question as to whether the yield stress of thin films may be altered by reducing thickness. This study therefore investigates the indentation response of Al coatings on Borosilicate glass as a function of coating thickness and indentation depth. FEM of the indentation contact will be compared with indentation data and AFM measurements of the surface profile to investigate the relative contributions of the indentation size effect and the effect of hardening due to the additional constraint of substrate proximity to the plastic zone.
INTRODUCTION It is now widely recognised that the rules governing plasticity at small length scales are different to the criteria successfully applied to the uniaxial loading of bulk material [1,2]. Size effects in the plastic deformation of thin metallic films result in behaviour that is not well described by conventional plasticity theory. We have recently shown [1] that the indentation size effect can be simulated for spherical indentation by increasing the initial yield stress in the material by a factor that depends on the radius of the indenter. We have also shown that metals with very different elastic and plastic behaviour appear to follow a common relationship between the increase in the initial yield stress (above its uniaxial value) and the indenter radius [1]. This material independent behaviour strongly suggests that there is a fundamental length scale involved. Theories suggest that there is a critical volume over which energy changes occur upon the generation of a dislocation [2,3,4]. Decreasing the radius of an indenter changes the spatial distribution of strain energy. The smaller the radius, the steeper the strain energy density gradient becomes, such that higher indentation pressures are necessary to generate sufficient energy, when integrated over the
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