Boron carbide ceramics are important structural ceramic materials with high hardness and wear resistance in new ceramics. Because boron and carbon are non-metallic elements, and the atomic radius is close, its combination is different from the general gap compounds, so boron carbide ceramics have high melting point, ultra-high hardness, low density, wear resistance and corrosion resistance and many other unique excellent performance, in the national defense, nuclear energy, aerospace, machinery, wear technology and other fields, is increasingly showing its broad development and application prospects. At present, the research depth and intensity of boron carbide ceramics are increasing. In addition to the emergence of new methods for the synthesis of high purity and ultra-fine boron carbide powder, researchers are more committed to developing advanced and practical sintering technology research, among which density and porosity defects are important factors affecting the performance index of boron carbide ceramics. Therefore, by improving the sintering technology to further increase the density of boron carbide, it can effectively promote the application of boron carbide ceramics in many high technology fields.
Boron carbide ceramics
Boron carbide (B4C) was discovered in the mid-19th century from a by-product of the preparation of metal borides. The comparative and systematic study of borides did not really begin until the 1930s. The chemical composition of boron carbide is very complex and can range from a low B/C molar ratio all the way up to the formation of B51C.
Crystal morphology and crystal structure of boron carbide ceramics
The structure of boron carbide can be described by a Rhombotetrahedron lattice, whose top Angle is composed of regular boron icosahedrons along the diagonal direction of space (see Figure 1), belonging to the R m space group (see Figure 2). There are 15 atoms in each cell, containing 12 icosahedral sites. There are three atomic positions in the C-axis of the linear chain (the longest diagonal direction), where the c atom easily replaces the B atom to form B12C3, which is B4C in strict stoichiometric ratio. Because C atom can also replace B atom on B icosahedron and c axis chain, the homogeneous region of boron carbide compound is B 4.0c ~B 10.5c, that is, the content range of C in boron carbide is 8.82%~20.0% (atomic fraction).
Figure 1 Structure of boron icosahedron Figure 2 Crystal structure of boron carbide
It is now generally accepted that boron carbide has only one binary phase, consisting of B13C2±x. The binary phase finally melts at 2450℃ with 18.5% C content, as shown in FIG. 3.
FIG. 3 Phase diagram of B-C binary system
Characteristics of boron carbide ceramics
Boron carbide is a superhard material with a hardness of 3000 kg/mm2, second only to diamond and cubic boron nitride among known materials. Low density, only 2.52g /cm3, is one third of steel; The elastic modulus is high, 450 GPa; High melting point, about 2450℃; Its thermal expansion coefficient is low, high thermal conductivity. In addition, boron carbide has good chemical stability, acid and alkali resistant corrosion resistance, at room temperature does not react with acid and alkali and most inorganic compound liquid, only in hydrofluoric acid-sulfuric acid, hydrofluoric acid-nitric acid mixture has slow corrosion; And most of the molten metal does not wet, do not have action. Boron carbide also has good neutron absorption capacity. The high neutron absorption cross section of B compounds is derived from the B isotope 10B, which in nature makes up about 20% of the element B content.
Table 1 Basic properties of B4C
Densification sintering technique of boron carbide ceramics
Densification sintering of pure boron carbide is extremely difficult. This is because the number of covalent bonds reached 93.94%, higher than other structural ceramics, such as SiC(88%), Si3N4(70%), etc., so that the elimination of pores in boron carbide, grain boundary and volume diffusion mass transfer mechanism needs to be carried out above 2000℃. At low temperatures, surface diffused nuclear evaporation-reagglutination is the main diffusion mechanism, resulting in neck, pore aggregation, and grainization (reducing specific surface area), respectively, or due to the spontaneous growth of gas phase particles, but the low surface energy of particles prevents the rearrangement or shape adjustment of comparable particles. Without mechanical or chemical activation, Even submicron powders do not reach full densification. So the sintering of boron carbide must use effective additives or pressure sintering.
Pure B4C compaction is difficult, ordinary B4C powder at 2250-2300℃ atmospheric pressure sintering, can only reach 80%-87% relative density, and sintering at such a high temperature, the grain will be coarsened and grow rapidly, is not conducive to the exclusion of pores, will cause a large number of residual pores so that the material density is limited. The results show that the most important conditions for the densification of pure boron carbide sintering are the use of ultra-fine powder ≤3μm with low oxygen content and temperature range of 2250-2350 ℃.
At present, the non-pressure sintering of B4C powder is mainly through additives to remove the oxide layer on the surface of B4C, and to increase the density of defects or dislocation to improve the activation of grain boundaries and volume diffusion, so as to obtain a high density (95-98%) at slightly lower temperature (2100 -- 2200℃).
Boron carbide ceramic sintering additives commonly used
Hot pressing sintering
The non-pressure sintering of B4C can produce complex products, but it often results in overgrowth of grain and porosity of 3-7Vol.%, so the strength and toughness of the material are low. 300 MPa, KIC≤3 MPa·m1/2). The B4C ceramics with higher density and better mechanical properties can be obtained by hot pressing sintering technology. Table 2 lists the mechanical properties of hot pressed sintered pure boron carbide and non-pressed sintered boron carbide with C as the sintering agent.
Mechanical properties of hot pressing sintered pure boron carbide and non-pressing sintered boron carbide with C as sintering agent
Due to the effect of pressure at high temperature, the particles are rearranged and plastic flow occurs, leading to grain boundary slip, strain-induced twin, creep and volume diffusion. High density and high strength B4C ceramics can be obtained by the combined action of these mechanisms. FIG. 4 shows a large number of experimental data on the relationship between temperature, pressure and density during hot pressing sintering of pure B4C. The lines in the figure are the corresponding conditions leading to pore closure.
The relationship between density of pure B4C and hot pressing sintering temperature and pressure (continuous line is the separation condition leading to pore closure)
It can be seen that, compared with non-pressing sintering, hot pressing sintering has the following advantages: (1) Hot pressing, because the powder is in a thermoplastic state, deformation resistance is small, easy to plastic flow and densification, the required molding pressure is only 1/10 of the cold pressing method; (2) Due to the simultaneous heating and pressure, help powder particles contact and diffusion, flow and other mass transfer process, reduce sintering temperature and shorten sintering time, inhibit the growth of grain; ③ Hot pressing method is easy to obtain sintered bodies with theoretical density and porosity close to zero, easy to obtain fine grain structure, easy to get good mechanical and electrical properties of products; (4) Can produce more complex shape, more accurate size products; ⑤ Powder size and hardness have little influence on the hot pressing process, so some hard and brittle materials can be pressed.