1.1 High temperature strength of ceramics The high temperature resistance of structural ceramics is generally better, usually below 800°C, and temperature has little effect on the strength of ceramic materials. Compared with covalent bond ceramics, ion-bonded ceramics have poorer high temperature resistance. Generally speaking, in the lower temperature range, the fracture failure of ceramics is brittle behavior, that is, there is no plastic deformation, and the ultimate strain is very small, and it is very sensitive to small defects. However, in the high temperature zone, ceramics can produce small plastic deformation before fracture, the ultimate strain is greatly increased, and there is a small amount of elastoplastic behavior. In addition, the sensitivity of strength to defects varies greatly. The boundary between the low temperature zone and the high temperature zone that produces this material property change is usually called the brittle ductility transition temperature. The brittle ductility transition temperature is closely related to the ceramic chemical composition and the type of valence bond. Relevant, but also related to the microstructure of the ceramic, the composition of the grain boundary phase, especially the composition and content of the grain boundary glass phase. At high temperatures, above the brittle ductility transition temperature, the strength of most ceramic materials will decrease. For ion-bonded MgO ceramics, the brittle ductility transition temperature is very low, and the strength decreases with the increase of temperature almost from room temperature. The brittle ductility transition temperature of Al2O3 is about 900°C, the brittle ductility transition temperature of hot-press sintered Si3N4 is about 1200°C, and SiC ceramics can often withstand high temperatures of 1600°C.
At high temperatures, the strength of most ceramic materials decreases with increasing temperature. Figure 1-33 shows the change in bending strength of some typical structural ceramics with temperature. However, some ceramics have a rebound strength near the brittle ductility transition temperature, such as silicon carbide and mullite ceramics. This phenomenon is related to the viscous effect of the glass phase in ceramics, that is, when approaching the brittle ductility transition temperature, the strength of the glass phase has not yet decreased but the viscosity is just reduced to relax the concentrated stress at the crack tip, which improves the resistance to crack growth. At this time, the influence of micro-cracks is minimized.
Figure 1-33 The influence of temperature on the bending strength of structural ceramic materials
Zirconia toughened alumina ceramics (ZTA) has the following characteristics in terms of its strength changing with temperature: in the range of normal temperature to 300C, the strength of various ZTA ceramic materials decreases by an average of 30%; in the high temperature zone of 800-1400°C, the strength decreases About 40%, and in the middle temperature section, the intensity changes little, as shown in Figure 1-34. If ZTA is compared with Al2O3, ZTA material is not suitable for high temperature occasions, and its strength decline is much more serious than that of Al2O3. The high-temperature strength of carbide and nitride ceramics is relatively high. For example, some hot-pressed and atmospheric sintered or recrystallized silicon carbide ceramics maintain their flexural strength at 1500°C.
Table 1-21 lists the service temperature of typical structural ceramic materials.
It can be seen that the long-term use temperature under load is very different from the short-term use temperature under no load. The former is lower than the latter by several hundred degrees (Morrell, 1989). Such as refractory high-purity Al2O3 ceramics, the long-term use temperature is only 1400°C under load, and the short-term use temperature reaches 1900°C under no load; the hot-pressed dense sintered SiC ceramics, the long-term use temperature under load is 1500C, The short-term use temperature under no load is 2100°C. In addition, different ceramic materials have different creep temperatures under load. Covalently bonded SiC and Si3N4, non-oxide ceramics, and non-oxide ceramics usually appear at a creep temperature above 1600 °C, and ionically bonded oxide ceramics appear to creep. The temperature of change is usually around 1000°C.