During service, refractory materials are frequently exposed to rapid temperature fluctuations. These sudden changes result in steep temperature gradients and corresponding thermal stresses within the material, which may lead to the development of cracks, spalling, or even catastrophic failure. This phenomenon is one of the leading causes of refractory lining damage in industrial furnaces.
The ability of a refractory material to resist sudden temperature variations without breaking is defined as its thermal shock resistance. Thermal shock not only causes the deterioration of mechanical strength but also induces the formation and growth of cracks, accelerates material spalling, and intensifies corrosion by molten substances. These processes significantly shorten the service life of refractory linings. Thermal shock resistance depends on both the mechanical and thermal properties of the refractory material and is influenced by the magnitude and distribution of thermal stresses, geometric shape, testing conditions, and microstructure. In high-temperature applications, thermal shock resistance is considered a critical indicator of performance. Generally, the higher the thermal shock resistance, the better the service reliability.
This type of damage is described as thermal shock fracture. Based on thermoelastic theory, when the thermal stress caused by a temperature gradient exceeds the material’s inherent strength, an immediate fracture will occur. The maximum thermal stress generated at the surface under sudden cooling conditions is given by the following formula:
σmax = EαΔT / (1 – μ)
Where:
The critical thermal shock temperature difference ΔTc, which causes failure, can be derived by balancing the maximum thermal stress with the fracture strength σf:
This thermal shock resistance parameter R is used to measure a material’s capability to withstand rapid temperature shifts. The larger the value of R, the better the material resists thermal shock.
This theory focuses on the formation of new cracks but does not consider the growth of existing ones. According to this approach, materials with higher strength, lower thermal expansion, lower elastic modulus, and smaller Poisson’s ratio perform better under thermal shock. However, in practice, high-density and high-strength refractories are often prone to explosive spalling, suggesting this model is insufficient. Thus, in the study of refractory thermal shock behavior, crack propagation and pre-existing flaws must also be considered.
This alternative model, known as the thermal shock damage theory, uses the balance between the thermoelastic strain energy (UE) and fracture surface energy (US) as the criterion for evaluating damage. Under cyclic thermal shock, surface cracks form and grow, eventually leading to disintegration and failure.
The more cracks generated by thermal stress, the larger the total crack area. Conversely, the smaller the total crack area, the better the material’s resistance to thermal damage. The inverse of crack area can be used as a damage resistance parameter:
Where:
For materials with similar γ values, R”’ is inversely proportional to the elastic strain energy. While thermoelastic models emphasize crack nucleation, this theory focuses on crack propagation. It’s important to note that crack growth does not always result in complete fracture. When all elastic energy is transformed into new crack surface energy, further propagation is halted.
Surface cracks alone don’t cause failure—thermal spalling and internal rupture are the main risks. Increasing porosity shortens crack lengths and promotes the formation of an interconnected crack network, which requires more energy to propagate. As a result, the material becomes more resistant to thermal shock. Studies suggest an optimal porosity range of 13–20% for this purpose.
To increase crack density and achieve a well-distributed crack network, the raw material’s particle size must be optimized. For example, in MgO-Cr₂O₃ bricks, keeping particles >1 mm at 3–8 wt% enables the product to withstand 25 water quenching cycles from 1100°C without spalling. Raw materials with low thermal expansion (e.g., fused silica, aluminum titanate, cordierite, Molochite) or high thermal conductivity (e.g., silicon carbide, silicon nitride, graphite, boron nitride, Sialon) can significantly improve thermal shock resistance.
Using materials with different expansion rates or volume-change effects due to phase transformation can induce microcracks that absorb thermal energy and prevent catastrophic damage. For instance, adding 6–9 wt% ZrO₂ to alumina refractories and maintaining 14% porosity significantly improves performance. Similarly, adding minerals like andalusite, sillimanite, and kyanite into the matrix of dense fireclay bricks used in coke dry quenching ovens increases the thermal shock resistance from 3–5 cycles to 10–15 cycles. These materials are also widely used in kiln furniture to enhance durability.
Fibers and whiskers enhance refractory fracture toughness through multiple mechanisms including:
Common fiber reinforcements include aluminosilicate fiber, mullite and zircon-mullite fibers, Al₂O₃ fiber, heat-resistant steel fiber, SiC whiskers, and Si₃N₄ whiskers. These additives significantly enhance the thermal shock resistance without severely compromising mechanical strength.
Thermal shock resistance in refractory materials is a complex phenomenon that still lacks a unified and comprehensive theoretical model. Therefore, any practical improvements must combine theoretical insights with experimental data and field experience. Whether through pore engineering, grain size optimization, phase-transition additives, or advanced fiber reinforcements, improving thermal shock performance requires a holistic and application-specific approach.
At Firebird New Materials Co., Ltd., we specialize in providing high-performance refractory solutions tailored to withstand the most demanding thermal shock environments. With decades of experience and advanced technical expertise, we help our clients improve operational stability, reduce maintenance costs, and extend furnace service life. Contact us today to learn how we can help optimize your refractory system.
