This force was determined by using the image method. The elastic interaction of a point defect with the surface layer was analyzed in terms of the induced interaction energy and the force which acted on the defect. The point defect was treated as a center of dilatation. The surface layer was assumed to be perfectly bonded to a semi-infinite substrate, and both were treated as isotropic elastic solids; but with differing elastic constants. It was found that there was no stable equilibrium position for the point defect in the surface layer, and that it was attracted either to the free surface or to the interface. When the point defect was in the substrate, it was attracted to the interface if the surface layer was softer than the substrate. It was attracted to an equilibrium position in the substrate when the surface layer was stiffer than the substrate. The equilibrium position was a function of the elastic constants and the layer thickness.

H.Yu, S.C.Sanday, D.J.Bacon: Journal of Applied Mechanics, 1996, 63[4], 1042-5