Destabilisation of nanoporous membranes through GB grooving and grain growth

We have used a phase field model to study destabilization of cylindrical pores in a polycrystalline membrane; a key feature in the model is that it incorporates surface diffusion as the mechanism for mass transport. Using a model system in which a cylindrical pore runs through a material in which all the grain boundaries (GBs) are perpendicular to the pore axis, we identify two elementary mechanisms for pore failure. The first one is based just on grain boundary (GB) grooving, which causes a circular trench at the groove, and a constriction of the pore on either side of the GB; as the groove deepens, the constriction narrows, and eventually closes the pore. Pore closure through this mechanism is possible only when the grain size exceeds a critical size (below which the open pore surface acquires an inverse-bamboo morphology), and therefore, it is controlled by grain growth kinetics. In the second mechanism, the groove profiles of unequal sized grains is such that the curvature differences trigger a coarsening-like process in which atoms from the surface of smaller grains are transported to that of larger grains, causing an ever-narrowing constriction there. A simplified model that incorporates these two mechanisms acting in parallel is used to rationalize our observations of pore failure in polycrystalline systems. (C) 2017 Elsevier B.V. All rights reserved