Why do coarse grains help some refractories resist cracking?

In the context of industrial materials, coarse grains are incorporated into the microstructure of refractories to improve their resistance to thermal shock.

When a refractory material is rapidly cooled, the surface layers contract against the inner layers, creating tensile stress that can cause cracks to propagate. While a coarse microstructure—often featuring filler grains on the scale of millimeters—may reduce the overall load-bearing strength of the material, these large grains serve a critical function: they act to blunt the tips of developing cracks. By arresting the progression of these cracks, the coarse grains help the material withstand the stresses of rapid temperature changes.1