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Since the 1960s, researchers mainly from the United States have conducted comprehensive research on the properties of three typical ultra-high temperature ceramic materials: borides, carbides, and nitrides, mainly including basic data on thermal, mechanical, and electrical properties. Most of these data come from hot-pressed sintered samples. Although the performance of the samples obtained is not at the current highest level due to factors such as sintering process and initial powder raw materials used, these data are of great benefit for understanding these three types of ultra-high temperature ceramics.

1. Mechanical properties

The elastic modulus, bending strength, Poisson’s ratio, and hardness of boride and carbide ultra-high temperature ceramics are shown in Table 12-2. This includes ZrB2 and HfB2 composite ceramics with SiC as an additive. From Table 12-2, it can be seen that these ceramics have high hardness because they all have strong covalent bonds. In addition, the hardness value of the same material varies within a certain range, which may be due to differences in material grain size and porosity due to different preparation processes. Similarly, ZrB2 and HfB2 have high elastic moduli, with elastic moduli reaching 500 GPa for both single-phase HfB2 and ZrB2 ceramics, as well as HfB2/SiC and ZrB2/SiC composite ceramics. The elastic modulus of HfB2, ZrB2, HfB2/SiC, ZrB2/SIC composite ceramics varies with temperature, and only shows a significant decrease above 800 ℃. The bending strength of HfB2, ZrB2, and their SiC containing composite ceramics is approximately 400-500 MPa. Typically, specimens with smaller grain sizes have higher strength, with a grain size of approximately 3 μ The room temperature bending strength of ZrB2-30% (vol) SIC composite material with m can reach 100, and the temperature decrease is relatively gentle, indicating that the addition of SiC is beneficial for the high-temperature strength of ZrB2 ceramics.

2. Thermal performance

The thermal expansion coefficients and thermal conductivity of single-phase ceramics containing borides, carbides, and nitrides, as well as HfB2-20% (vol) SiC and ZrB2-20% (vol) SiC composite ceramics, at different temperature ranges, are shown in Table 12-3. Overall, the thermal expansion coefficients of these materials will correspondingly increase with increasing temperature. The thermal expansion coefficients of HfB, HfB2-20% (vol) SiC, and SiC ceramics vary with temperature. It is obvious that the thermal expansion coefficient of HfB2 composite ceramics with the addition of SiC is less than that of single-phase HfB2 ceramics with temperature variation, indicating that the addition of SiC is beneficial for reducing the thermal expansion coefficient of HfB and ceramics at high temperatures.

Boride ceramics such as HfB2, HfB2-20% (vol) SiC, ZrBr20% (vol) SiC all have high thermal conductivity, which is significantly higher than that of carbides and nitrides. The variation of thermal conductivity with temperature for HfB, HB-20% (vol) SiC, HfC.8, and HfNo.g2. Although the thermal conductivity of boride ceramics (HfB2, HfB-20% (vol) SiC) decreases to some extent with increasing temperature, their thermal conductivity is much higher than that of carbides (HfC.s.) and heliides (HfNon). We know that high thermal conductivity helps to reduce the thermal gradient inside the component, thereby reducing the thermal stress inside the material, which is very beneficial for the front-end components of aerospace vehicles.

3. Electrical performance

The resistivity of boride and carbide ultra-high temperature ceramics is shown in Table 12-4. These two materials have the following characteristics: ① The resistivity of boride ceramics is much lower than that of carbide ceramics, such as HfB2 and ZrB2, which have resistivity of 11X10-6 at high temperatures Cm and 12.1X10-6 n. cm, while the resistivity of HfC and ZrC at room temperature is 109 X10-6 n. cm and 63X10-6 n. cm, respectively; ② After adding SiC to HfB2 and ZrB2, the resistivity decreased, from 11X10-6 n. cm to 9.6X10-6 n. cm, and from 12.1X10-6 n. cm to 10.2X 10-6n ● cm, respectively As the temperature increases, the resistivity significantly increases, such as ZrB, with a room temperature resistivity of 12 1X10-6 Q. cm, increased to 44X10-5 n. cm at 1000C.