Thermal shock resistance refers to the ability of a material to withstand a sharp change in temperature without being destroyed. It can also be called thermal shock resistance or thermal stability. Since ceramic materials are often subjected to thermal shocks from fluctuations in ambient temperature during processing and service, the temperature changes are sometimes very drastic. For example, the boron nitride ceramics used in the water nozzles used for horizontal continuous casting in the metallurgical industry have to withstand a temperature change of nearly 800°C in an instant; the quartz fiber insulation tiles on the outer surface of the space shuttle need to withstand the high temperature of 1650°C caused by severe friction when entering the atmosphere. Therefore, heat resistance is an important property of ceramic materials, and “thermal shock” is also a common phenomenon that causes ceramic material damage.
What are the factors that affect thermal shock resistance?
(1) For the case of instantaneous fracture of ceramics due to thermal stress, the strength σ of the material can be known from the R and R’ factors. The main influencing factors are the elastic modulus E, thermal expansion coefficient α, and thermal conductivity R.
1. Improving the material strength σ is conducive to improving thermal shock resistance, while the elastic modulus E is large, and the elasticity is small. Under thermal shock conditions, it is difficult for the material to partially offset the thermal stress by deformation, which is not conducive to thermal shock resistance. In addition, if σ/E is increased, it is also beneficial to improve thermal shock resistance. For example, graphite has low strength, but because E is extremely small and the thermal expansion coefficient α is also small, its R is very high. Because of its high thermal conductivity and R’, it has excellent thermal shock resistance.
2. Thermal expansion coefficient α. At the same temperature, materials with small α produce small thermal stress and large R. For example, the σ of quartz glass is not high (109 MPa), its /E is slightly higher than that of ceramics, but its α is only 0.5×10-5/℃, which is one order of magnitude lower than that of general ceramics. The thermal stress factor R is as high as 3000, and R’ is also relatively high in ceramics. Therefore, quartz glass has good thermal shock resistance. Others such as cordierite ceramics, spodumene ceramics, and fused quartz ceramics have excellent thermal shock resistance due to their small α.
3. Thermal conductivity is R. When the thermal conductivity is high, the temperature gradient in the material will decrease, and the temperature difference stress will be small, which is conducive to improving thermal shock resistance. For example, the R values of BeO and Al2O3 ceramics are similar, but BeO has a much higher R than Al2O3 due to its large R value, and has excellent thermal shock resistance. In addition, ceramics such as graphite, boron carbide, silicon carbide, and hexagonal boron nitride have good thermal shock resistance due to their high R values. In addition, ceramics with high fracture toughness are conducive to improving thermal shock resistance. For example, partially stable phase transformation toughened ZrO2, although it has a high thermal expansion coefficient (about 12×106/℃) and medium strength, has good thermal shock resistance due to its high fracture toughness.
(2) For slow material damage such as thermal shock damage, the R™ and R” factors can be used to It is known that low σ value and large E value are more favorable, and the structure that can absorb fracture work in the microstructure is beneficial to thermal shock resistance. For example, polycrystalline aluminum titanate ceramics, due to the anisotropy of thermal expansion of aluminum titanate grains, spontaneously generate microcracks during the sintering cooling process. Although these tiny microcracks limit the strength of the material, they can provide an effective mechanism to absorb strain energy during thermal shock and prevent serious crack propagation. The influence of thermal expansion coefficient α and thermal conductivity is consistent with the above discussion. In addition, reducing the heat transfer coefficient h on the surface of ceramic products, such as maintaining a certain heating and cooling rate of the product, and not blowing air to cool the surface, to maintain slow heat dissipation and cooling, is an important measure to improve the quality and yield of ceramic products. Reducing the effective thickness of the product is also conducive to reducing thermal shock damage.