Indentation Curves of Surface Films

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Abstract
Various ball indentation curves of thin surface films have been obtained experimentally. The normal contact forces (range 10−5 — 10N) and the normal displacement (range 1 nm — 1500 nm) between a sphere and a flat plate are continuously investigated during the indentation process. The solids are made in soda-lime or sapphire (Al2O3) and covered by natural pollution layers or Polystyrene layers. The speed of the indentation displacement is kept constant during the experiment. For higher loads (> 0.5 N), the results agree with the classical Hertz theory, but for smaller loads, the disparity is important. The hypotheses proposed in the literature are analyzed: intermolecular forces, surface roughness, mechanical properties of solid surface films. According to our results, the mechanical properties of surface films connected with adhesion are responsible for the results presented.

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