The Concept of a Misfit Strain
An important concept in layered (and other) systems is that of a misfit strain - ie a difference between the stress-free dimensions of two or more constituents that are bonded together. It is relevant to composite materials and also to macroscopic systems such as two or more components that are bolted or welded together in some way. In general, this strain is a tensor, with principal axes and three principal values. For a layered system, however, the focus is on a single (in-plane) direction, so that the strain can be treated as a scalar.
A simple type of misfit strain is that arising from differential thermal expansion (in a 2-layer system). In general, one layer will expand more during heating than the other (in the direction concerned). If the two layers were not bonded together, then they would behave as shown in the figure below. The misfit strain (in the x-direction) is given by the product of the difference in expansivity between the two constituents and the temperature change. It is often written as Δε. The fact that the two layers are actually bonded together leads to creation of internal stresses and strains, and to changes in the shape of the system - see the next page.
Sources of Misfit Strains
What are effectively misfit strains can arise in a number of ways. One of the simplest is differential thermal contraction, but anything that creates a difference between the (in-plane) stress-free dimensions of a substrate and a coating (or a surface layer of a substrate) has a similar effect. These include phase changes, plastic deformation, creep etc, as well as phenomena, such as atomic bombardment, that can create stresses during formation of a coating.
- \(\Delta \alpha \Delta T\)
- Phase transformations (eg solidification, resin curing, martensitic transformations)
- Plastic deformation (eg shot peening)
- (Such deformation can also modify existing Δε values.)