Effect of inhomogeneous intrinsic stresses on the cracking of layered brittle coatings

O. Jørgensen, A. Horsewell, Bent F. Sørensen, P. Leisner

    Research output: Contribution to journalJournal articleResearchpeer-review

    Abstract

    The cracking and spalling of layered chromium coatings on steel substrates, which is caused by residual stresses developed during fabrication, is studied. 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 release of free volume in the DC-layers following electrodeposition. It is found that a unique misfit strain quantifies this contraction independently of layer and substrate configuration. Successive deposition and contraction of surface layers produces a bending moment in the coating whenever the substrate is compliant. Eventually, the strain energy stored in the coating and compliant substrate may be released by the propagation of cracks. The observed fracture process is qualitatively divided into cracks channelling in the coating and debonding cracks running in the interface between the coating and the substrate. The fracture mode is highly influenced by the bending moment. For thin, compliant substrates, the residual bending moment causes early spallation of the coating. On thick, non-compliant substrates however, thick coatings can be formed. The analysis, in combination with the observation of channel crack arrest in these thick coatings, shows that the multilayered chromium coating is far tougher than the pure DC-chromium. Scanning electron microscopy reveals that this type of crack, which appears to be a channel crack on a macroscopical scale, is composed of terraces at the level of the individual layers in the coating; this type of crack is denoted a 'terrace crack'. The crack surface on the microscopical scale strongly reflects the stepwise distribution of through-thickness alternating compressive and tensile stresses in the layers. The crack surface, newly-exposed by the through-thickness crack, will display free-edge effects, and therefore delaminations with associated friction between each PR- and DC-layer. We propose that this terrace crack requires substantially more energy for its formation than a macroscopically similar channel crack in a homogenous layer with a homogenous stress distribution.
    Original languageEnglish
    JournalKey Engineering Materials
    Issue number116-117
    Pages (from-to)351-370
    ISSN1013-9826
    Publication statusPublished - 1996

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