Progress on B₄C–SiC Ceramics Prepared by Reaction Sintering

Boron carbide-silicon carbide (B₄C-SiC) ceramics have the performance of B₄C and SiC, which have low density, high melting point, high hardness, high elastic modulus, good wear resistance, high thermal tolerance, excellent chemical stability, etc.. Compared with B₄C ceramics, B₄C–SiC ceramics possess a higher fracture toughness and a lower cost. Compared with SiC ceramics, B₄C–SiC ceramics have a higher hardness and a lower density. B₄C–SiC ceramics can be widely used in modern high-technology industries, such as aerospace, military, mechanical and automotive engineering, chemical industry, nuclear energy, high-temperature thermoelectric conversion, tribology, aviation, etc. due to their outstanding properties. In particular, the materials with the combination of high hardness and low density are more desirable than the materials without this characteristic. B₄C–SiC ceramics are suitable for use as lightweight structural materials and rotating tribo-components.

Despite numerous advantages, B₄C–SiC ceramics are difficult to sinter to obtain high densification because of the high covalent bond ratios of both B₄C and SiC. The existing preparation methods of B₄C–SiC ceramics mainly include pressureless sintering, hot-press sintering, and spark plasma sintering. Although these methods can produce dense B₄C–SiC ceramics with a good performance, the sintering temperature is relatively high. In order to reduce the sintering temperature of B₄C–SiC ceramics, the reaction sintering method is developed to produce B₄C–SiC ceramics. The preparation method of reaction sintered B₄C–SiC ceramics is similar to that of SiC ceramics produced by reaction sintering, in which the molten Si with an appropriate fluidity infiltrates into a porous green/partially sintered body composed of only B₄C, the mixture of B₄C and free C, B₄C–SiC–C, or B₄C–SiC powders at high temperatures. The molten Si infiltrated reacts with C either from free C or B₄C to form SiC, the reaction leads to a volume expansion occupying partial pores in the preform. The remaining excess pores in the preform are filled with molten Si after the reaction, making the reaction sintered B₄C–SiC ceramics dense. The preparation of reaction sintered B₄C–SiC ceramics is actually an in-situ chemical reaction. For the characteristics of reaction sintered B₄C–SiC ceramics, B₄C, SiC formed, and residual Si particles can interconnect into a uniform and intense three-dimensional network at low sintering temperatures without any applied pressure, and near-net shaped products with zero shrinkage can be produced. Also, fine reactive starting powders capable of being densified are not required, thus reducing the cost of raw materials.

The preparation method of reaction sintering is different from other preparation methods to obtain B₄C–SiC ceramics. The microstructure of reaction sintered B₄C–SiC ceramics has its own characteristics. The core-rim structure occurs in the reaction sintered B₄C–SiC ceramics. Firstly, B₄C cores surrounded by a thick B₁₂(B, C, Si)₃ envelope are formed, and there is an intense bonding between B₄C and B₁₂(B, C, Si)₃. Until now, there is still a heated debate on the formation mechanisms responsible for B₁₂(B, C, Si)₃ rim. The core-rim structure of B₄C is formed in B₄C–SiC ceramics produced via a conventional reaction sintering route. However, there is no core-rim structure of B₄C in the reaction sintered B₄C–SiC ceramics prepared by microwaves assisted processing method in Ar–H₂ atmosphere. Secondly, When the preform containing α-SiC is used to prepare the reaction sintered B₄C–SiC ceramics, the core-rim structure of primary α-SiC surrounded by secondary β-SiC rim also appears besides the core-rim structure of B₄C surrounded by B₁₂(B, C, Si)₃ rim., SiC grains with different morphologies, i.e., plate-like shape and polygonal shape, can be in-situ generated via depending on the carbon source.

The mechanical properties of reaction sintered B₄C–SiC ceramics are crucial for their applications, which are affected by many factors, such as phase composition and content, microstructure, raw materials, sintering temperature, etc.. The ratio of B₄C to SiC affects the mechanical properties of reaction sintered B₄C–SiC ceramics. Moreover, there is some residual phases in the resulting B₄C–SiC ceramics, eventually in the presence of excessive C or Si. The addition of excessive C in the green body will lead to insufficient Si to completely react with C, forming residual C. In contrast, residual Si is formed when free C added is insufficient or Si is excessive. The presence of C or Si is not conducive to the mechanical properties of reaction sintered B₄C–SiC ceramics. The mechanical properties of reaction sintered B₄C–SiC ceramics are related to their microstructure. The morphology and size of grains as well as the characteristics of grain boundaries directly determine the mechanical properties of reaction sintered B₄C–SiC ceramics. The kinds and purity of raw materials and the sintering temperature also have an impact on the mechanical properties of reaction sintered B₄C–SiC ceramics.

The preparation of B₄C–SiC ceramics via reaction sintering is a low-cost method, and this route is suitable for large-sized and complex-shaped products. Although significant progress is achieved in the preparation of reaction sintered B₄C–SiC ceramics, some challenges still remain in the scientific research and practical application. The reaction sintered B₄C–SiC ceramics have reached the industrial production stage; however, the residual Si phase with low hardness and stiffness contained in the B₄C-SiC ceramics can lead to a decrease in the performance, restricting their applications in the high-temperature field. New methods and processes still need to be explored to produce low Si or Si free reaction sintered B₄C–SiC ceramics. The formation mechanism of the microstructure of reaction sintered B₄C–SiC ceramics and its effect on the mechanical properties of B₄C-SiC ceramics are unclear yet, and the further in-depth exploration is needed through theoretical calculations and characterization techniques. The existing investigation for reaction sintered B₄C–SiC ceramics mostly focuses on conventional mechanical properties evaluation, and other mechanical properties are seldom reported. The impact strength and tribological properties of reaction sintered B₄C–SiC ceramics also need to be investigated deeply.