THE CRACKING AND SPALLING OF MULTILAYERED CHROMIUM COATINGS

Cracks in a chromium coating on a steel substrate which are caused by residual stresses developed during an electroplating process are examined. The chromium coating, formed as a multilayer by alternating electroplating utilizing direct current (DC) and periodic current reversal (PR), is in a state of biaxial tensile stress due to a volume contraction in the successive DC layers which occurs during deposition. A uniform biaxial misfit strain idealizes this layerwise contraction. The state of stress in the multilayer is modelled using laminate theory. Special emphasis is given to the influence of the substrate flexibility on the stress build-up. It is shown that the flexibility of the substrate produces an equal biaxial bending moment in the coating. At a critical coating thickness, the chromium multilayer cracks and spalls off the substrate. The radius of curvature of detached coating fragments provides a measure of the size of the bending moment and, indirectly, of the misfit strain. The observed fracture mechanism is qualitatively divided into cracks channelling in the coating and debonding cracks running in the interface between the coating and the substrate. Long crack asymptotic solutions for the two distinct crack types are presented. The fracture analyses of the multilayered chromium coating show the functional dependence of relative layer and substrate thicknesses and flexibility on the energy release rate for crack propagation.