Abstract: boride high temperature ceramics have high melting point, high thermal conductivity, high hardness, excellent chemical stability and good resistance to high temperature oxidation. Therefore, in the aerospace field, it is applied to the rocket nozzle and can withstand 1600 ℃ high temperature and erosion wear. In the aviation field, it can replace the traditional superalloy as turbine blade material. In the field of nuclear industry, it can be used as barrier material and control material of atomic reactor. In the traditional industrial field, it can be used to make thermocouple protection tube resistant to high temperature oxidation, and can also replace the traditional graphite to improve the service life of the crucible. For its various applications, the preparation process of boride high-temperature ceramics is also different. This paper will analyze the current situation of the preparation process of boride high-temperature ceramics.

Most high temperature ceramic materials have strong covalent bonds and are difficult to sinter and densify, so most of them need to be prepared under high temperature and high pressure. At present, the common Sintering Densification methods of boride high-temperature ceramic materials include: hot pressing sintering method, reaction sintering method, discharge plasma sintering method, and another pressureless sintering method.

The reduction of particle size can reduce the sintering temperature and pressure, but too small will lead to oxidation reaction

Generally, the preparation temperature of pure boride high-temperature ceramics is above 2000 ℃ and the pressure is 20 ~ 30MPa, so as to realize Sintering Densification. Later, it was found that reducing the particle size of the particles to be sintered can reduce the sintering temperature and pressure in a certain range. When the average particle size of zirconium boride is reduced to 2 μ m. The fully dense boride ceramics only need to be sintered at 1900 ℃ and 32Mpa for 45min. If the particle size is too small, oxidation reaction is easy to occur, and oxide is generated to hinder the diffusion of sintering materials.

In addition, by adding metal additives such as aluminum, chromium and nickel or ceramic additives such as silicon carbide, the sintering temperature can be reduced and the compactness can be improved. Harbin Institute of technology and Tsinghua University have used hot pressing sintering method to prepare high-performance composite ceramics in this way.

Reactive sintering has the advantages of Sintering Densification and in-situ synthesis

The chemical reaction of raw materials produces new products with stable thermodynamics, and Sintering Densification is carried out at the same time. This process is reactive sintering. This saves time and improves productivity, but the disadvantage is that the reaction process is not easy to control. Reaction sintering is used to prepare zirconium boride, hafnium boride and silicon carbide high temperature composite ceramics because it has the advantages of Sintering Densification and in-situ synthesis. The reaction formula is as follows:

2Zr+Si+B4C→2ZrB2+SiC

(2 + x)Hf + (1 - x)Si + B4C→2HfB2 + (1 - x)SiC + xHfC

To a certain extent, the silicon carbide produced by in-situ reaction sintering reduces the temperature required for sintering and affects the microstructure of the product. The ordinary reaction sintering temperature is 2100 ℃, the reaction sintering temperature is only 1650 ℃, and the average particle size of ordinary reaction sintering is 12 μ m. The average grain diameter is now 2 μ m。

Spark plasma sintering technology appeared late, but it is widely used

Spark plasma sintering process appeared later. Firstly, metal or other powders were loaded into the mold, and the sintered powder was acted on by electrified electrode and die punch, and then high-performance powder metallurgy was prepared through electrode activation, high-temperature deformation and final cooling. At present, this method is widely used in the densification sintering of ultra-high temperature ceramics.

The Institute of silicate, Chinese Academy of Sciences and others used spark plasma sintering technology to prepare ZrB2 SiC composites with relative density of 98.5% by using Zr, B4C and Si powder as starting materials at 1450 ℃ and 30 MPa.

Pressureless sintering is more economical but requires more

Compared with hot pressing sintering, pressureless sintering has higher efficiency and more economy. Sintering can be promoted by refining particles and adding additives. Previous studies believe that single-phase pure boride can not complete densification sintering under pressureless environment, but researchers at home and abroad have prepared composite ceramics with high density under pressureless environment through experiments, but the cost is to add sintering additives. In recent research, foreign researchers prepared ZrB2 ceramics with a density of more than 95% under the condition of pressureless sintering. However, this Sintering Densification can only be completed by adding a variety of additives such as AlN (15wt.%), BN (5wt.%) and SiC (5wt.%). In short, this method requires higher additives for sintering. See Table 1 for the comparison of advantages and disadvantages of Sintering Densification methods of four boride high temperature ceramic materials.

Table 1 Comparison of advantages and disadvantages of Sintering Densification methods of boride high temperature ceramic materials

(source: public data sorting)

Epilogue

Boride high temperature ceramics have high melting point, high thermal conductivity, high hardness, excellent chemical stability and good high temperature oxidation resistance. It is widely used in aerospace, nuclear industry and other fields. At present, the common Sintering Densification methods of boride high temperature ceramic materials include hot pressing sintering, reaction sintering, discharge plasma sintering and pressureless sintering. Reducing the particle size of hot pressing sintering method can reduce the sintering temperature and pressure, but if it is too small, oxidation reaction will occur. Reactive sintering method has the advantages of in-situ synthesis and Sintering Densification, saves time and improves productivity, but the disadvantage is that the reaction process is not easy to control. Spark plasma sintering technology appeared late, but it is widely used. Pressureless sintering is relatively economical, but there are more requirements for additives.