EFFECTS OF APPLIED CURRENT-DENSITY AND POTENTIAL STEP ON THE STRESS GENERATION DURING ANODIC-OXIDATION OF TUNGSTEN IN 0.1M H2SO4 SOLUTION

Abstract

Stresses generated during the anodic oxidation of tungsten in 0.1 M H2SO4 solution were measured as a function of applied current density and potential step by using a beam deflection technique. The stresses which amounted to a few GPa were developed in the oxide film and were separated into the volume-generated stress and electrostrictive stress by measuring the transients in deflections upon interrupting the applied current densities. The deflection due to electrostriction was compressive and less than 10% of total deflection irrespective of applied current density and anodic oxidation time, indicating that the magnitude and sign of the stresses developed during the oxide growth are critically dependent upon the volume-generated stress. Compressive and tensile deflections due to the volume-generated stress were observed at the applied current density of 0.02 mA cm(-2) and 0.65 mA cm(-2), respectively. In contrast, the compressive to tensile deflection transition occurred with growing oxide film at various current densities between 0.05 and 0.10 mA cm(-2). The results are discussed with respect to the relevant reactions occurring at the metal-oxide interface during the oxide growth. Positive potential steps imposed upon the anodic oxide film on tungsten produce instantaneous tensile deflections followed by deflections in a compressive direction. From the experimental results it is suggested that the formation of oxygen vacancies at the metal-oxide interface and their subsequent annihilation by the inward movement of oxygen ion are closely related with the mechanism of build-up and relief of stresses developed during the anodic oxidation of tungsten.