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

Active-passive potential steps have been applied during continuous microind entation into 300 mu m thick Fe3%Si sheets exposed to 1 M H2SO4. First, sam ples were allowed to deflect in the indentation direction whereby the inden ter penetration changed due to local metal dissolution and sample deflectio n resulting from passive film stress induced bending. Samples were then con strained to eliminate stress induced deflection. A distinct difference betw een the indentation curves for the above two types of tests allowed separat ion of the effects of film stress and local dissolution. Indenter tip displ acement correlated with the current behavior and was consistent with the sa lt film evolution and resulting electrostrictive film stress. A theoretical model allows estimates of the time dependent film thickness (3.5 mu m at m aximum) and electrostrictive film stress (330 MPa at maximum). (C) 1998 Els evier Science Ltd. All rights reserved.