A short video on thermal shock, the sudden temperature change that cracks stiff and brittle materials.
Heat a ceramic slowly and it expands.
Heat or cool it suddenly and it may crack.
Thermal shock is the cracking of a material when its surface heats or cools much faster than its interior, so that the two parts try to expand by different amounts. Stiff and brittle materials, which includes most dense ceramics, are the most prone to it. The Blasch video shows a thermal shock test of Blasch ceramics and explains why the structure of the cast part matters as much as its composition.
Ceramic parts are brought to temperature and then exposed to a sudden change, the way a furnace part sees it at a trip, a restart or when cold air or liquid reaches a hot surface. The video then compares the condition of the parts after the shock.
When the surface cools faster than the core, it tries to shrink while the core holds it back. The stress that builds up depends on how much the material expands with heat, how stiff it is and how quickly heat moves through it. A stiff, brittle part cannot give way, so the stress goes straight into a crack.
Blasch improves thermal shock resistance through the structure of the part rather than only the choice of grade:
Thermal shock is one of the tests Blasch runs in its own laboratory in Albany, alongside strength, abrasion, acid resistance, creep, corrosion and flame resistance. Every batch is checked for strength, thermal shock and dimensions.
Two parts with the same composition can behave completely differently in a thermal shock, because what decides it is how the grains and pores are arranged, and that is set when the part is cast.
If a ceramic part in your plant cracks at trips or restarts while its material data look fine, thermal shock is the likely cause. Ask for the thermal shock behavior of the actual cast part, not only the data sheet of the grade.
How Blasch tests and models parts is described on the page for CFD, FEA and in-house testing, and the casting method on the page for the Blasch net shape forming process. The materials themselves are listed under engineered ceramic materials, and a test on ferrules is shown in the ultra high alumina thermal test.
Cracking caused by a sudden temperature change, when the surface and the interior of a part expand or contract by different amounts at the same time.
Stiff, brittle and dense materials. Their structure cannot absorb the stress between a hot core and a cooled surface, so it releases as a crack.
Yes, through the structure of the part: particle packing, the binder system and a network of fine interconnected pores all reduce the risk of a through crack.
Describe the part, the temperatures and how fast they change. A Blasch engineer suggests a grade and a shape.
