Effect of the indentation depth on the evaluation of mechanical properties of thin films

Hardness measurement of thin films is a major topic. At the nanoscale the interplay between substrate and coating during the indentation process is not well known. Atomic force microscopy nanoindentation has been used to address this problem. We prepared a set of five chromium nitride films (100-500 nm thick) deposited on a steel Substrate. Using nanoindentation we measured the hardness of these coatings. varying the depth of indentation and thus observing the effect of the Substrate-coating interaction oil the hardness evaluation. The results show that in this case (hard film on soft substrate) it is not trivial to have measurements not affected by the Substrate even at very small depths of indentation. Focused ion beam cross-sections on the indentation and numerical Simulations confirm these results showing a relevant contribution of the substrate to the hardness evaluation.