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 exami ned. The chromium coating, formed as a multilayer by alternating elect roplating utilizing direct current (DC) and periodic current reversal (PR), is in a state of biaxial tensile stress due to a volume contract ion in the successive DC layers which occurs during deposition. A unif orm biaxial misfit strain idealizes this layerwise contraction. The st ate of stress in the multilayer is modelled using laminate theory. Spe cial emphasis is given to the influence of the substrate flexibility o n the stress build-up. It is shown that the flexibility of the substra te produces an equal biaxial bending moment in the coating. At a criti cal coating thickness, the chromium multilayer cracks and spalls off t he substrate. The radius of curvature of detached coating fragments pr ovides a measure of the size of the bending moment and, indirectly, of the misfit strain. The observed fracture mechanism is qualitatively d ivided into cracks channelling in the coating and debonding cracks run ning in the interface between the coating and the substrate. Long crac k asymptotic solutions for the two distinct crack types are presented. The fracture analyses of the multilayered chromium coating show the f unctional dependence of relative layer and substrate thicknesses and f lexibility on the energy release rate for crack propagation.