Development of experimental verification techniques for non-linear deformation and fracture on the nanometer scale.

Abstract

This work covers three distinct aspects of deformation and fracture during indentations. In particular, we develop an approach to verification of nanoindentation induced film fracture in hard film / soft substrate systems ; we examine the ability to perform these experiments in harsh environments; we investigate the methods by which the resulting deformation from indentation can be quantified and correlated to computational simulations, and we examine the onset of plasticity during indentation testing.First, nanoindentation was utilized to induce fracture of brittle thin oxide films on compliant substrates . During the indentation, a load is applied and the penetration depth is continuously measured . A sudden discontinuity, indicative of film fracture, was observed upon the loading portion of the load-depth curve . The mechanical properties of thermally grown oxide films on various substrates were calculated using two different numerical methods . The first method utilized a plate bending approach by modeling the thin film as an axisymmetric circular plate on a compliant foundation . The second method measured the applied energy for fracture. The crack extension force and applied stress intensity at fracture was then determined from the energy measurements.Secondly, slip steps form on the free surface around indentations in most crystalline materials when dislocations reach the free surface . Analysis of these slip steps provides information about the deformation taking place in the material . Techniques have now been developed to allow for accurate and consistent measurement of slip steps and the effects of crystal orientation and tip geometry are characterized . These techniques will be described and compared to results from dislocation dynamics simulations.iii Finally, the stress required to cause fracture of passive films on stainless steels was studied using nanoindentation under in situ and ex situ conditions . For films formed at a passive potential, the alloy chemistry dominated the film strength . Increasing the electrolyte salt concentration from O .O1M NaCl to O .1M NaCl and changing the substrate on which the film was grown from 904L to 304 stainless steels reduced the applied tensile stress for fracture from 1 .74GPa to 1 .63GPa and from 1 .76GPa to 1 .63GPa, respectively . Trends in film strength as a function of environment are the same between in situ and ex situ testing, suggesting the two tests are both feasible methods of analyzing environmental effects on film strength.iv CONTENTS