Path Geometry
Mass transfer processes through a network of small scale fractures allow gases and liquids to bypass the bulk lattice of a protective coating or structural component. Movement through micro-cracking transport occurs when the density or connectivity of these defects exceeds a critical threshold for percolation. This mechanism often leads to the premature degradation of the underlying substrate by providing a path for corrosive species or oxidizing agents.
Permeability Threshold
Systems remain sealed until the mechanical stress exceeds the fracture toughness of the material. Once a connected path forms, the permeability of the layer increases. Monitoring this change provides a way to detect the onset of mechanical failure before it is visible to the naked eye.
Environmental Constraint
Temperature changes can open or close these transport paths due to the different expansion rates of the coating and the substrate. In some cases, the reaction products of the diffusing species may fill the cracks and slow down further transport. This self healing behavior depends on the volume change associated with the formation of the new phase.
Flow Mechanism
Gas movement through these cracks is governed by the width and tortuosity of the fracture network. Narrow channels restrict the movement of larger molecules while allowing smaller atoms to pass through via diffusion.
