Continuous microindentation of passivated surfaces in surface active media

Continuous microindentation tests on electropolished, single crystal Fe(3wt%Si) and Ni surfaces with thin passivation layers exhibit sharp discontinuities in the load-displacement behaviour. At the discontinuity, which occurs at a load of 1.8 mN for Fe(3wt%Si) and 0.1 mN for Ni, stresses below the indenter were close to the theoretical strength of the corresponding metal. Microindentation tests performed to lads less than the discontinuity point showed an elastic load-unload behavior. On removing the passive film with a NaCl or HCl solution, strengths are one to two orders of magnitude smaller in the presence of the chloride ions. On evaporation of the liquid, the load at the discontinuity returned to its initial value due to repassivation, indicative of a passivation oxide layer with varying thickness. This conclusion was verified by ellipsometry. Several suggestions as to the reasons for the deviation from continuum theory are presented.