High-temperature environment will reduce most of the properties of ceramics, such as strength, hardness, thermal conductivity, etc. Among them, the change of strength in a high-temperature environment is closely related to use, and the creep of materials at high temperature is related to it. Therefore, the high temperature creep and high temperature strength change of ceramic materials are the most important high temperature properties of structural ceramics.
The high temperature resistance of structural ceramics is generally good, usually below 800℃, and the temperature has little effect on the strength of ceramic materials. Compared with covalent-bonded ceramics, ionic-bonded ceramics have poorer high-temperature resistance. Generally speaking, within the lower temperature range, the fracture of ceramics exhibits brittle behavior, that is, there is no plastic deformation, and the limit strain is very small, and it is very sensitive to small defects. However, in the high temperature range, ceramics can produce small plastic deformation before fracture, the limit strain is greatly increased, and there is a small amount of elastic-plastic 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-ductile transition temperature. The brittle-ductile transition temperature is closely related to the chemical composition and valence bond type of the ceramic, and is also related to the microstructure of the ceramic, the grain boundary phase composition, and especially the grain boundary glass phase composition and content. At high temperatures, above the brittle-ductile transition temperature, the strength of most ceramic materials will decrease. For ion-bonded MgO ceramics, the brittle-ductile transition temperature is very low, and the strength decreases with the increase of temperature, almost from room temperature. The brittle-ductile transition temperature of Al2O3 is about 900℃, the brittle-ductile transition temperature of hot-pressed Si3N4 is about 1200℃, and SiC ceramics can often withstand high temperatures of 1600℃.
At high temperatures, the strength of most ceramic materials decreases with increasing temperature. However, some ceramics have a rebound in bending strength near the brittle-ductile transition temperature, such as silicon carbide and mullite ceramics. This phenomenon is related to the viscosity effect of the glass phase in the ceramic, that is, when approaching the brittle-ductile transition temperature, the strength of the glass phase has not yet decreased, and the viscosity is just reduced to relax the concentrated stress at the crack tip, thereby improving the resistance to crack propagation. At this time, the impact of microcracks is minimized.
Zirconia toughened alumina ceramics (ZTA) have the following characteristics of strength changes with temperature: in the range of room temperature to 300℃, the strength of various ZTA ceramic materials decreases by an average of 30%; in the high temperature zone of 800~1400℃, the strength decreases by about 40%, while in the intermediate temperature range, the strength changes little. If ZTA is compared with Al2O3, ZTA materials are not suitable for high-temperature occasions, and their strength decreases much more seriously than Al2O3. Carbide and nitride ceramics have high temperature strength. For example, some hot-pressed and normal pressure sintered or recrystallized silicon carbide ceramics have a bending strength that does not decrease at 1500℃. In addition, the recently developed ultra-high temperature ceramics represented by ZrB2 and HfB2 can be used at temperatures of 2000℃ or even higher.
Studies have shown that the long-term use temperature under load is very different from the short-term use temperature under no load, and the former can be several hundred degrees lower than the latter (Morrell, 1989). For example, the long-term use temperature of refractory high-purity Al2O3 ceramics under load is only 1400℃, while the short-term use temperature under no load reaches 1900℃; the long-term use temperature of hot-pressed densely sintered SiC ceramics under load is 1500℃, while the short-term use temperature under no load is 2100℃. In addition, different ceramic materials have different creep temperatures under load. The creep temperature of covalently bonded SiC and Si3N4 non-oxide ceramics is usually above 1600℃, and the creep temperature of ionically bonded oxide ceramics is usually around 1000℃.