Review on the properties of hexagonal boron nitride matrix composite ceramics
Introduction
Hexagonal Boron nitride (h-BN) is well known to be an important engineering ceramic. It has crystal structure analog of graphite. That is, within each layer, boron and nitrogen atoms are bound together by strong sp2 covalent bonds, and the adjacent layers are integrated by weak van der Waals forces [1], [2], [3], [4], [5]. This special crystal structure provides h-BN a series unique combination of properties (see Table 1) [6], [7], [8], [9], [10], [11], [12], including low dielectric coefficient, low loss tangent, extremely high sublimation temperature of about 3000 °C (non-oxidizing atmosphere), excellent thermal shock resistance, and desirable machinability [12], [14].
However, there exist some problems that limit the application of h-BN material, such as low strength and poor sintering properties. As the strong covalence of B-N bond has a rather low self-diffusion coefficient, it is difficult to obtain dense materials even if they are sintered under high temperature (>2000 °C) or assisted by pressure. In tradition, only several kinds of oxides, such as SiO2 and B2O3, are used as sintered additives, and the mixed powders are often sintered without pressure-assistance. As a result, the h-BN ceramics show low relative density and poor properties, which constraint their application. While in most cases, h-BN are used as additives to adjust the properties of composite ceramics, for example, increasing thermal conductivity and improving processability [8], [11], [12], [13].
In recent years, advanced sintering technologies (hot pressing sintering (HPS), hot isostatic pressing sintering (HIPS), Spark plasma sintering (SPS), etc.) and new types of sintering aid or reinforced phases (Al2O3, ZrO2, CaO, Sialon, Si3N4, AlN, SiC, YAG, Y2SiO5, mullite, etc.) are applied to manufacture h-BN matrix composite ceramics, which render significantly improved performances for these materials in terms of mechanical, thermal and electric fields [5], [6], [7], [8], [9], [10], [11], [12], [13]. Thus, h-BN matrix composite ceramics with various properties have been commonly used in many high technology fields under extreme service environment, such as non-ferrous metal industry, chemical engineering, lubricating materials, high-temperature furnaces, and thermal protection systems [8], [9], [14], [15], [16], [17].
In this paper, we focus on the homogeneous h-BN matrix composite ceramics that manufactured by sintering ceramic powders, recent progress of mechanical properties, thermal shock resistance, ablation resistance, ion erosion resistances, molten metal erosion resistance and anisotropic properties of novel h-BN matrix composite ceramics are reviewed. The damage mechanisms under extreme environment are discussed. In addition, the future developing directions of h-BN and h-BN matrix composite ceramics are pointed out.
Section snippets
Fabrication methods
As one of the strongest covalently bonded ceramic materials, BN-based materials are difficult to be sintered due to the low coefficients of atomic diffusion and anisotropic plate-liked structure. Therefore, sintering aids are usually incorporated and the sintering had to be conducted at high temperature under pressure. The typical fabrication methods for h-BN matrix composite ceramics include pressureless sintering, hot pressing sintering, spark plasma sintering, and hot isostatic pressing
Mechanical properties
Typically, pure h-BN ceramic shows relative lower mechanical properties due to the poor sintering activity and low density. Therefore, the sintering additives and secondary phases with strengthening effects are often used to improve its properties, and the relevant preparation process is developed to sintering h-BN matrix composite ceramics with various performances. Table 2 summarized the mechanical properties of h-BN matrix composite ceramics manufactured by different researchers. It can be
Thermal shock resistance and damage mechanisms
Since h-BN matrix composite ceramics are often used under high temperature environments with rapid heating and cooling process, such as high temperature crucible, casting shell, and radome of reentry vehicle, the excellent thermal shock resistances are required to ensure reliability [37].
For most of ceramics, the residual strengths after thermal shock are lower than their original values, while some of h-BN composite ceramics show the inverse results. For h-BN-SiO2 composite ceramic, the
Ablation resistance and damage mechanisms
Ablation resistance is also an important property when the materials are used as high temperature thermal protection components, such as radome and antenna window [40]. During the ablation process, the surface materials will be melted, disappeared or deformed under the effect of high-temperature gas. h-BN ceramic with excellent high-temperature and thermal shock resistance can keep the reliability under rapid heating environments, but its anti-erosion ability under high speed gas flow is
Ion erosion resistances and damage mechanisms
Due to its suitable secondary electron emission coefficients, h-BN ceramic has been considered as the best candidate material for Hall thruster channel components. In addition, both the insulation properties and thermal shock resistance of h-BN ceramic can meet the requirement of service conditions of Hall thruster. Furthermore, h-BN ceramic can also be made into the designed shapes. However, the high-speed Xe ions that accelerated by the electric field will impact on the channel walls, which
Molten metal erosion resistance and damage mechanism
h-BN and h-BN matrix composite ceramics combine properties for application in the metal metallurgy industry [8]. Molten metal corrosion has been recognized to be one of the serious problems for the application of liquid metal/alloy at high temperature systems [60], [61]. Recently, h-BN and h-BN matrix composite ceramics against molten metal have been investigated widely, especially against molten steel, since h-BN matrix composite ceramics have been used as containment material (side-dams) in
Textured h-BN composite ceramics with anisotropic properties
The formation of textured h-BN composite ceramic is another important approach for the thermal protection and directional heat leading or dissipation. Due to the lamellar structure and eminent anisotropic properties of h-BN grain, once the c-axis of h-BN grains are arranged along the same direction, textured h-BN bulk ceramic with laminated microstructures are fabricated. Its mechanical, thermal and electrical properties are anisotropic, which offers a unique opportunity to optimize the
Conclusions and outlooks
Recently, h-BN matrix composite ceramics have been used as key materials in many fields and great endeavor has been made to improve the sintering process in order to optimize their performance. Now that the most of existing materials are manufactured by hot-press sintering, their performances need to be further improved by optimizing the parameters during the treatment.
For a period of time in the future, the research would focus on the development of new sintering methods, improving and
Acknowledgements
This work was supported by National Natural Science Foundation of China (51225203, 51321061, 51372050) and Heilongjiang Postdoctoral Scientific Research Development Fund (LBH-Q14082).
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