Numerical study of the scratch-closing behavior of coatings containing an expansive layer

Recently, self-healing coatings using expansive phases to autonomously close artificial scratches have been developed and studied. The level of scratch closure achieved by these phases was found to be dependent on the environmental conditions. One of these self-healing coating systems employs a clay interlayer as the expansive phase and a topcoat to protect the expansive phase from the environment when the coating system is intact. Although the scratch closure has been demonstrated experimentally, not much is known about the relation between the expansion of the active layer and the degree of scratch closure. In this work, a simplified mathematical model has been established to evaluate the scratch closing behavior of a bi-layered coating system as a function of different material and geometric parameters. From the parameter sensitivity analysis we conclude that for a given expansion of the active layer the mechanical stiffness of each layer plays a key role in the scratch closure. The relative importance of the bulk stiffness (Youngʹs Modulus) or the effective stiffness (layer thickness times Youngʹs Modulus) has also been studied and found to influence the crack closure in a different manner. The type of behavior depends (i) on the ratio between the bulk stiffness of the layers (EA/EB); (ii) the ratio between the thickness of the layers (tA/tB); but also (iii) on the absolute thickness of each layer (tA, tB). Furthermore, this research also revealed the large influence of the debonded length between the coating and the substrate in the vicinity of the scratch on the subsequent scratch closing behavior. The results obtained in this research can be used as a benchmark in the development of future scratch closing and self-healing coatings using interlayer of an expansive phase.