Instability of Polycrystalline Bridges that Span Cracks in Powder Films Densified on a Substrate

The opening of cracks within a powder film is described as the film densifies on a substrate that constrains biaxial shrinkage. Polycrystalline ligaments that bridge the crack fail by an instability mechanism known as “desintering.” This phenomenon is produced by one of two effects. At lower temperatures, where the porous material still has a high driving potential for densification, the polycrystalline bridges extend and deform with increasing crack‐opening displacement. Deformation occurs by combined grain rearrangement and desintering (separation of previously sintered grains) without coarsening. The bridge finally separates when the last, linking grain pair desinters. At higher temperatures, when the film is nearly dense and the grains within the remaining bridges near the crack tip have increased their size by coarsening, bridge failure occurs without extension. In this case coarsening of the grains promotes the instability, viz., desintering.