Microindentation method for in situ stress measurements in precipitated iron sulfate FILMS

Active-passive potential steps have been applied during continuous microindentation into 300 μ m thick Fe3%Si sheets exposed to 1 M H 2 SO 4 . First, samples were allowed to deflect in the indentation direction whereby the indenter penetration changed due to local metal dissolution and sample deflection resulting from passive film stress induced bending. Samples were then constrained to eliminate stress induced deflection. A distinct difference between the indentation curves for the above two types of tests allowed separation of the effects of film stress and local dissolution. Indenter tip displacement correlated with the current behavior and was consistent with the salt film evolution and resulting electrostrictive film stress. A theoretical model allows estimates of the time dependent film thickness (3.5 μ m at maximum) and electrostrictive film stress (330 MPa at maximum).