thermal-expansion microcracking

Imagine a mosaic where each tile is made of a slightly different material, all cemented together, and then the whole panel is cooled. Tiles that want to shrink a lot pull on tiles that want to shrink little, and the grout between them is put under stress; cool far enough and the grout cracks. A polycrystalline ceramic made of anisotropic grains does exactly this to itself on cooling from the kiln. The result is a network of tiny cracks along the grain boundaries — thermal-expansion microcracking — cracks that form spontaneously with no external load at all.

The driving force is the residual stress set by the expansion mismatch between neighbouring grains, of order (delta alpha) times (delta T) times E, where delta alpha is the difference in expansion coefficient, delta T the cooling range, and E the elastic modulus. The elastic energy stored this way scales with grain size, while the energy needed to open a grain-boundary crack does not. So there is a critical grain size: below it, fine grains hold together, and above it, the stored energy is enough to pop microcracks open. This is why a coarse-grained titania, graphite, or aluminium titanate microcracks while a fine-grained version of the same material stays sound. Anisotropy sets whether it can happen; grain size sets whether it does.

Microcracking cuts both ways. It lowers strength, stiffness and thermal conductivity, because cracks are weak spots that also scatter phonons — genuinely a defect if you wanted a strong dense part. But it can be a gift: the crack network lets the body accommodate expansion mismatch by opening and closing rather than by building stress, which drives the bulk expansion toward zero and blunts and arrests any new crack. That is precisely how aluminium titanate and cordierite-based bodies achieve their extraordinary thermal-shock resistance, and why microcracked ceramics are the material of choice for diesel particulate filters and honeycomb catalyst supports that must survive violent heating.