Progress in pressureless sintering of boron carbide ceramics

Abstract

Boron carbide (B₄C) ceramics has many outstanding performance, such as extremely high hardness, low density, high melting point, high elastic modulus, high thermoelectromotive force, high chemical resistance, high neutron absorption cross section, high impact and excellent wear resistance. Therefore, B₄C ceramics can be used in various industrial applications, such as lightweight ceramic armour, high temperature thermocouples, neutron absorber, reactor control rods in nuclear power engineering, polishing media for hard materials, abrasive media for lapping and grinding, and wear resistant components (blasting nozzles, die tips and grinding wheels). Pressureless sintering is the method with industrialised application value for B₄C ceramics, however, it is impossible to sinter pure B₄C ceramics to high densities without additives by pressureless sintering. So sintering additives must be used to promote the densification of B₄C ceramics. The different sintering additives used to promote the densification of boron carbide will be described in this review, including carbon additives, metallic additives, oxide additives, non-oxide additives, combined additives and rare earth oxide additives. Finally, the recent research trends for sintering methods and sintering additives of B₄C ceramics will also be proposed.

Keywords

pressureless sintering additive relative density mechanical property

Introduction

Boron carbide (B₄C) ceramics has many outstanding performance, such as extremely high hardness (Vickers hardness: 3770 kg mm⁻², the only substances harder than B₄C are diamond and c-BN), high melting point (2427°C), low density (2.52 g cm⁻³), high elastic modulus (450 GPa), high thermoelectromotive force, high chemical resistance, high neutron absorption cross section, high impact and excellent wear resistance [1 -3]. Because of the valuable properties, B₄C ceramics is useful for various industrial applications both at room and high temperatures. B₄C ceramics can be used as lightweight ceramic armour, high temperature thermocouples, neutron absorber [2], reactor control rods in nuclear power engineering, polishing media for hard materials, abrasive media for lapping and grinding, and wear resistant components (blasting nozzles, die tips and grinding wheels).

However, it is impossible to sinter pure B₄C ceramics to high densities without additives or high external pressure owing to low self-diffusion coefficient, the strong high covalent bonding of B–C, high resistance to grain boundary sliding, high melting temperature, existence of surface oxide layer and absence of plasticity. It is reported that the covalent bond ratio of boron carbide is up to 93.94%, which is higher than other ceramics, such as SiC (88%), Al₂O₃ (33%), ZrO₂ (33%) [4]. Pure B₄C ceramics cannot be densified more than 80% of theoretical density even at temperature over 2300°C. Boron carbide of the specified atomic ratios, of which density is up to at least 90% of theoretical density, could only be obtained near the melting point of B₄C. Although fully densified B₄C ceramics can be fabricated by means of hot pressing at above 2100°C [5], disadvantages are that directional change of physical performance perpendicular and parallel to the hot pressing direction, high cost and limit to only relatively simple shapes. Whereas pressureless sintering can avoid the costly diamond machining required to form complicated shapes and the method of pressureless sintering is suitable for large-scale production. Pressureless sintering is a conventional method by sintering products at atmospheric pressure (0.1 MPa), which is the simplest sintering method. So many researchers have tried to study sintering B₄C ceramics without press but with addition of different kinds of sintering additives and then it was proved that sintering additives can promote the densification of boron carbide under a normal sintering process.

The different sintering additives used to promote the densification of boron carbide will be described here, including carbon additives, metallic additives, oxide additives, non-oxide additives, combined additives and rare earth oxide additives. And the recent research trends for sintering methods and sintering additives of B₄C ceramics will also be proposed.

Carbon additives

In order to reduce the sintering temperature for B₄C ceramics and improve the relative density and strength of B₄C ceramics, carbon can be used as additives. The reason is that the eutectic temperature for B₄C–C presented in phase diagram is about 2375°C, which is lower than melting point of B₄C. However, carbon-rich B₄C fabricated with an excess carbon of 2 wt-% could not be densified to higher than 85% of theoretical density at a temperature of up to 2200°C, because the free C in carbon-rich B₄C is in the form of non-active graphite. Therefore, carbon should be introduced from outside to B₄C. Usually, carbon can be provided in the form of carbonaceous precursor and amorphous carbon. C can also be introduced by in situ chemical reaction between metal carbide and B₄C. Recently, it was reported that carbon can be introduced into B₄C ceramics in the form of graphene platelets.

Carbon generated from organism

One method of introducing carbon to B₄C ceramics is pyrolysis of an organic precursor, which will coke to amorphous carbon at up to 1000°C.

Bougoin et al. [6] prepared a B₄C ceramics with an addition of a phenol formaldehyde resin (9 wt-%), of which relative density can reach 95% of the theoretical density. Schwetz and Grellner [7] proposed addition of 3 wt-% phenolic resin to fabricate B₄C ceramics with a density of more than 95% of the theoretical density after sintered at temperature of 2150°C. While for pure B₄C ceramics, even using submicron powders, a densification higher than 90% of the theoretical density can be achieved only at high temperature of 2300°C. They claimed that C acts as an inhibitor of surface-to-surface matter transport, therefore, densification is improved at lower sintering temperatures via grain boundary and/or lattice diffusion. But the flexural strength of the ceramics is only 353 ± 30 MPa (four points), which is lower than that of hot-pressed B₄C (480 ± 40 MPa). Li et al. [8] used phenol formaldehyde resin as the sintering additive and binder, a B₄C ceramics with the sintered density of 2.0 g cm⁻³ is obtained by adding 3 wt-% carbon. The chemical content and other technical indexes can meet the requirement of the B₄C pellets used in nuclear reactors. Wang et al. [9] researched the influences of glucose, stearic acid and phenolic resin on sintered density of B₄C. They found that glucose has the best effect on promoting the densification of B₄C ceramics. A sintered density of 2.06 g cm⁻³ was obtained by adding 3 wt-% glucose after sintered at 2270°C. Yuan et al. [10] reported that relative density and hardness of B₄C ceramics with the addition of 4 wt-% organic glucose sintered at 2200°C were higher than those of pure B₄C ceramics sintered at 2250°C.

Though sintering proceeds very satisfactory, there are some disadvantages which organism route suffers from: (1) hard agglomerates produced because of polymerisation reactions in the organism; (2) environmental pollution caused by pyrolysis products; (3) insufficient flow behaviour of particles into pressing dies; (4) the fairly complex handing of the organism-doped powders and (5) small amounts of free carbon contained in the final products, which must be avoided in nuclear applications.

Amorphous carbon

In order to solve the difficulties which organic carbon source suffers from, many research on the direct blending of B₄C with amorphous carbon have been investigated.

Sukuzi et al. [11] used carbon black as a sintering additive, sintered B₄C with 27.7 wt-% C additions up to 93.7% of the theoretical density after sintering at 2200°C, and found the formation of a liquid of eutectic composition. Schwetz and Vogt [12] discovered a method of fabricating dense-shaped pure B₄C ceramics by using a carbon-containing component consisting of amorphous carbon (such as carbon black, acetylene black) at sintering temperature of from 2100°C to 2200°C. Advantageously, a free carbon content from 0.5 wt-% to 7 wt-%, especially about 3 wt-% is used. The relative density and flexural strength of such articles can be up to 96% and 450 MPa (three points), respectively. The final product didn't contain free carbon in the form of graphite. The uniform fine-grained microstructure was found in the ceramics prepared by B₄C submicron powder together with additional carbon (Figure 1a). Whereas the coarse-grained microstructure was found in the ceramics prepared by B₄C submicron powder but with no added carbon (Figure 1b). This method, however, has strict requirement for the specific surface area of B₄C powder (10–50 m² g⁻¹). Sigl and Schwetz [13] also reported that using submicronic B₄C powders (carbon powders as a sintering additive) can decrease the sintering temperature to 2120–2150°C. Both of the cost of such fine powders stated above is very high. Lee and Speyer [14] stated that carbon-doped (3 wt-%) B₄C ceramics had a higher relative density (97% of theoretical density) than that of undoped B₄C ceramics (90% of theoretical density) after sintering at 2250°C, and had a smaller average grain size (2.03 μm) than that of undoped B₄C ceramics (2.32 μm). The smaller grain size of the higher density ceramics can contribute to higher hardness, thereby the B₄C ceramics with the addition of carbon showed a higher Vickers hardness (23.5 GPa) than that of B₄C ceramics without the addition of carbon. Although the existence of graphite was thought to decrease hardness, the smaller average grain size of the doped ceramics may have been a compensating factor. Dole et al. [15, 16] observed that B₂O₃ liquid formed at high temperature, which existed in B₄C, can provide a rapid diffusion path along granule surfaces, which can facilitate particle coarsening. And carbon additives could eliminate B₂O₃ from B₄C compacts, thereby the B₄C ceramics with the addition of C had not undergone much coarsening, whereas highly coarse grains were found in the B₄C ceramics without the addition of C. Speyer and Lee [17] also suggested that the addition of carbon can effectively eliminate B₂O₃ coatings by the reaction between B₂O₃ and carbon, and thus uncoated B₄C particle can contact with each other resulting in sintering to initiate at a lower temperature. They also reported that H₂ gas had a good effect on the elimination of B₂O₃, but resulted in facilitating coarsening and stalling the onset of sintering.

Figure 1

Microstructures of specimens [12]: (a) B₄C submicron powder with additional carbon and (b) B₄C submicron powder with no added carbon.

When the method of adding carbon black is used, incomplete densification of B₄C ceramics usually can be caused by inhomogeneity of initial B₄C and carbon black particulate dispersions. In order to solve this problem, Schwetz et al. [18] developed an injection moulding method to disperse carbon black (20 nm) in B₄C powders. Nanosized carbon black particles homogeneously distributed on the surface of the B₄C grains and at B₄C–B₄C grain boundaries, resulting in very fine-grained microstructure. The relative density of the injection moulded B₄C–C ceramics reached about 95% of the theoretical density, then the ceramics was subjected to a post-densification to a relative density of 96–99% of the theoretical density by hot isostatic pressing. Although the method developed can improve dispersing carbon black in B₄C powders, post-hot isostatic pressing is necessary when a post-densification is expected.

Organism route can be substituted by amorphous carbon route, which was expected to prepare well-densified B₄C ceramics, however, uniform distribution of ultra-fine amorphous C in a B₄C slurry is a delicate process, if handled improperly, can result in inadequate densification. Although injection moulding method can disperse amorphous carbon well in B₄C powders, the relative density of the B₄C ceramics is only about 95% of theoretical density. The relative density can only be post-hot isostatic pressing to a relative density of 96–99%.

Carbon generated by in situ reaction from metal carbide

In order to solve the deficiencies of introducing carbon into B₄C ceramics stated above, there is another method of introducing elemental carbon into B₄C powders that is in situ reaction of transition metal carbides with B₄C generating carbon.

Sigl [19] used TiC as a sintering additive, elemental carbon and TiB₂ both of which are able to aid the sintering process were generated via the in situ chemical reaction of TiC with B₄C. Relative density of B₄C–TiC composite ceramics exceeds 93% of theoretical density at temperatures between 2150°C and 2200°C, and nearly completely densification can be obtained by subsequent hot isostatic pressing. The flexural strength of B₄C–TiC is found to decline from ∼500 MPa to ∼300 MPa with increasing fracture toughness from ∼2.9 MPa to ∼4.2 MPa. Niihara [20] stated that the flexural strength of the B₄C ceramics can be increased by doping TiC, however, the flexural strength of the B₄C ceramics with TiC is lower than that of the B₄C ceramics with the addition of SiC when atmosphere temperature is more than 1100°C.

Metal carbides, unlike organic precursors or amorphous carbon, present less problems with dispersability of the sintering additive and flow behaviour of spray-dried particles into pressing dies. Moreover, the cost of TiC is low, and TiB₂ synthesized via in situ reaction is expected to promote densification similar to the support effects that have been found in B₄C–TiB₂ composite ceramics. However, relative density of B₄C ceramics is relatively low, and post-hot isostatic pressing was required to further enhance the densification.

Graphene platelets

Graphene platelets can be used as reinforcement fillers to improve the properties for ceramics [21 -23]. Kovalčíková et al. [24] reported that graphene platelets can enhance the mechanical properties of B₄C ceramics by hot pressing sintering. Tan et al. [25] suggested that adding graphene platelets into B₄C ceramics can increase electrical conductivity of the ceramics. However, there is little research on improving properties of B₄C ceramics by adding graphene platelets via pressureless sintering. In order to research the effects of graphene platelets on the performance of B₄C ceramics by pressureless sintering, Gao et al. [26] prepared B₄C ceramics with the addition of 4 wt-% (Al₂O₃+Dy₂O₃), 15 wt-% phenolic resin and different content of graphene platelets by liquid state pressureless sintering. The results showed that the B₄C ceramics with 0.8 wt-% graphene platelets separately had the best performance, relative density was 96.1% of the theoretical density, Vickers hardness, fracture toughness and flexural strength of the ceramics can reach 29.1 ± 0.5 GPa, 5.7 ± 0.1 MPa•m^1/2 (SENB) and 383.9 MPa, respectively. The fracture toughness obtained by this method is significantly higher than that by other methods. The improvement of fracture toughness was attributing to the pull-out of graphene platelets took place between intimal slices of graphene platelets after fracture occurring at the outermost graphene layers, which resulted in leaving a hole in the matrix. The pull-out of graphene platelets needs to overcome the bonding force between B₄C substrate and graphene as well as the resistance from graphene layers, which can absorb more rupture energy to inhibit the continuing propagation of cracks, and thus the fracture toughness is improved. However, the flexural strength obtained by this method is not too high.

The results of relative density, sintering temperature and mechanical properties of the B₄C ceramics with carbon additives by pressureless sintering are given in Table 1. In summary, the contribution of carbon additives to densification of B₄C is the suppression of grain growth or the formation of an eutectic melt in the system B₄C–C. The densification of B₄C ceramics is because of the increased surface energy resulting from the elimination of oxide layers by the reaction between C and B₂O₃. Although there are also some problems when using carbon as sintering additive for B₄C ceramics, such as environmental problems caused by decomposition of organic species, the problem of homogeneous dispersion of amorphous carbon, the problem of flexural strength of the ceramics with the addition of TiC or graphene platelets needing to be improved, it is still the most widely used standard additive for densification of B₄C ceramics in industry at present.

Table 1

Relative density, sintering temperature and mechanical properties of the B₄C ceramics with carbon additives by pressureless sintering.

Carbon additives	Relative density (%)	Sintering temperature (°C)	Atmosphere pressure	Flexural strength (MPa)	Fracture toughness (MPa•m^1/2)	Vickers hardness (GPa)	Elastic modulus (GPa)
Phenolic resin [7]	95	2150	Ambient or 0.1 mbar	353 ± 30 (four points)	3.2 ± 0.2 (SENB)	–	372
Carbon black [12]	96	2150	Ambient or vacuum	450 (three points)	–	–	–
TiC [19]	>93	2150–2200	10 mbar	–	–	–	–
Graphene + Al₂O₃/Dy₂O₃+phenolic resin [26]	96.1	2050	Ambient	383.9	5.72 ± 0.1 (SENB)	29.1 ± 0.5	–

‘–’ represent that the data were not offered in the corresponding literatures.

Metallic additives

Metallic additives can promote liquid phase sintering for B₄C ceramics, thereby different kinds of metal usually are added into B₄C ceramics as sintering additives. The metallic sintering additives can be introduced into B₄C ceramics by two methods: (1) it can be combined with the powdered B₄C compound to form a mixture which is cold pressed and then sintered and (2) it can be infiltrated into cold pressed compact of the B₄C compound while in the molten state or vapour phase. In order to achieve fully impregnated composites, good wetting between B₄C and metallic additives is required. Because of effectiveness in improving the densification of B₄C ceramics and light weight, metallic additives are usually chosen as sintering additives in the manufacturing of B₄C ceramics.

Aluminium (Al)

Aluminium is a metal with low density, ductile and non toxic. It has been reported that B₄C–Al composites can offer a combination of high toughness and high hardness in a lightweight structure [27]. Therefore Al is usually used as an additive to promote densification of B₄C ceramics at lower temperature.

Mashhadi et al. [28] researched the influence of small Al addition on B₄C ceramics. They stated that adding Al to the B₄C ceramics can increase densification of the ceramics, and the relative density of 94% was obtained by adding 4 wt-% Al when sintering temperature was 2050°C or 2150°C. Al₃BC and AlB₂ crystalline phase were formed, which existed within grain boundaries. While depending on the amount of Al addition and the process conditions, other several phases such as AlB₂, AlB₁₂, Al₄C₃, AlB₁₂C₂, AlB₂₄C₄, AlB₄₀C₄, AlB₄₈C₂, Al₃BC, Al₃B₄₈C₂ and Al₄B_1–3C₄ can be also formed when B₄C and Al react [29]. But in their work, Al₄C₃ phase is not be detected, which is thought to cause a substantial decrease of the mechanical properties. The fracture mode of B₄C ceramics, after Al addition, changed from intergranular fracture to transgranular fracture, which is the reason for flexural strength of B₄C ceramics with 4 wt-% Al addition up to 270 ± 15 MPa (four points). In addition, fracture toughness, hardness and elastic modulus of sintered B₄C ceramics also show an increase after Al addition. Fracture toughness is up to 5.5 ± 0.1 MPa•m^1/2 (IF) when the amount of Al is 3 wt-%. Though the hardness of Al is lower than that of B₄C matrix, the increased relative density of B₄C ceramics is because of the addition of Al. Therefore, the addition of Al shows a positive effect on hardness. Bhattacharya and Petrovic [30] stated that fracture toughness of B₄C ceramics can be increased by Al inclusions, and the mechanisms are crack bridging, plastic deformation of aluminium particles and thermal residual stress. Whereas in the research of Mashhadi et al. stated above, the mechanism for toughness increase is the interaction of cracks with Al or crack deflection by microcracks, caused by thermal expansion mismatch between Al and B₄C, around Al compounds phase (Figure 2).

Figure 2

Microstructure of B4C–4 wt-% Al ceramics sintered at 2050°C [28].

Although B₄C ceramics with the addition of Al displays good densification, the properties of B₄C/Al composites depend on densification temperature. The sintering temperature of B₄C above 1800°C results in particle surface passivation and prevents the reaction with Al. And B₄C/Al composites usually show significantly different phase compositions even using B₄C produced by different suppliers.

Silicon (Si)

Silicon with light weight is a metalloid type additive which can promote the densification of B₄C ceramics effectively. Stibbs and Thompson [31] used Si as a sintering additive which can be introduced into B₄C by combining with the powdered B₄C compound to form a mixture which is cold pressed and then sintered, or impregnating into cold pressed compacts of B₄C compound while in the molten state or vapour phase. The relative density of the B₄C compound containing 10 wt-% Si and 5 wt-% Si can be up to 99.2% and 100% of the theoretical maximum density after sintered at a temperature of about 2100°C, respectively. Although a highly dense, sintered Si-containing body can be obtained, cold-pressed and sintered articles are not necessarily of the same strength and impact resistance as hot-pressed B₄C articles. Cai et al. [32] investigated the effect of Si addition on properties of a B₄C ceramics. They found that Si can be solid soluted into B₄C and thus B₁₂ (B,C,Si)₃ is formed. Doping Si into B₄C is some like that of increasing B concentration in B _x C. The thermoelectric properties of the B₄C ceramics can be improved by adding Si into B₄C. While if the carbon-riched boron carbide is used as the starting materials, the result is different. Wei et al. [33] stated that when Si was added into B₄C ceramics, Si reacted with free carbon originating in the B₄C and solid soluted into B₄C structure. However, the main crystal phase of the ceramics is B₄C and SiC.

Al–Si

Both Al and Si can be used as sintering additives for the pressureless sintering of B₄C ceramics. So some researchers studied the effect of Al–Si binary additives on the densification for pressureless sintering of B₄C ceramics.

Xu et al. [34] used Al–Si binary metallic additives to enhance the densification of B₄C ceramics and found that most of the lattice space of the B₄C grains is occupied by the Al atoms, thereby there is a little residual space for Si atoms to enter into in the B₄C lattice. So other Si atoms remain at the B₄C grain boundaries, where SiC is formed by the reaction of Si and B₄C, which can hinder grain growth. In other words, Al–Si binary additives can produce a refined microstructure in contrast to individual Al or Si additive. The relative density of the B₄C ceramics with 5 wt-% Al–Si binary additives with molar ratio of 9:1 reaches 95.7% after sintering at 2250°C. The flexural strength, fracture toughness and Vickers hardness can reach 283.3 ± 33.3 MPa (three points), 2.9 ± 0.4 MPa•m^1/2 (IF) and 31.5 ± 0.8 GPa, respectively.

Frage et al. [35] incorporated Al–Si alloy into B₄C ceramics by the infiltration technique. This method makes molten Al–Si alloy penetrate into B₄C ceramics by the capillary force between the infiltrants and the B₄C matrix, or by the external pressure used on the molten infiltrants in a temperature range from 600°C to 1500°C. This technique avoids the formation of Al₄C₃ phase, which is expected to be harmful for mechanical properties, when only molten Al was infiltrated into B₄C ceramics. The presence of Si can suppress the formation of Al₄C₃, accelerate the penetration of the molten alloy and improve the wetting. The hardness of the infiltrated composites is increased to 25.5 ± 1.8 GPa, however, this value is not too high. Moreover, no mention is made of the other mechanical properties (flexural strength, fracture toughness). Kumazawa et al. [36] researched the effect of sintering aid gases of Al gas and Si compound gas produced from SiC on densification of B₄C. Sintering of B₄C ceramics can be accelerated by the sintering aid gas, of which relative density is up to 97.4% of theoretical density (a bulk density of 2.455 g cm⁻³) after being sintered at 2226°C. Al gas, a conduit for densification, can be used as a reducing agent for boron oxide on B₄C raw materials, which can lead to increased densification because removal of oxides prevents coarsening at intermediate temperatures [15]. SiC and Al₄SiC₄ phases were formed in grain boundaries after sintering with Al gas and Si compound gas, which contributed to the formation of liquid phase at high temperature. Although Al gas and Si compound gas can enhance the sinterability of B₄C ceramics, they also lead to the formation of grain boundary phases and a variety of precipitates, which influence the strength and fracture toughness. Miyazaki et al. [37] also demonstrated that gaseous Al and Si species can infiltrate into the green compacts and react with B₄C, and the densification of B₄C ceramics is caused by the liquid phase. But what is different from the result proposed above by Kumazawa et al. is that besides SiC and Al₄SiC₄ phases, Al₄C₃ phase was formed in the reaction products. They proposed that gaseous Al came directly from the Al powder, but gaseous Si didn't come directly from the SiC powder because the sublimation temperature of SiC is higher than 2000°C. It was thought that gaseous Si was generated by the reaction between SiC and Al [38]:

Titanium (Ti)

Ti can react with B₄C, and new phases are formed: (1) (2)

The absolute value of the standard free energy change, according to the thermodynamic data for elementary reactions, for Reaction [1] is much higher than that for Reaction [2] in the range of 1000–1600°C. Thereby the in situ production of TiB₂ and C will be formed, either of which is beneficial to the densification of B₄C ceramics.

Levin et al. [39] added Ti to B₄C ceramics, and they found that Ti has little effect on the densification of coarse B₄C powders at 2100°C but enhances the densification of fine B₄C powders at 2190°C. The relative density of B₄C ceramics with Ti additive reached about 87% of the theoretical density after sintering at 2190°C, and the flexural strength reached about 150 ± 11 MPa (three points).

Boron (B)

Liu et al. [40] used B as a sintering additive for B₄C ceramics and found that B can promote the densification of B₄C ceramics. Flexural strength, fracture toughness, hardness and elastic modulus were improved by doping B into boron carbide. Grabchuk and Kislyi [41] reported that B can react with C in the carbon-rich B₄C, resulting in the formation of columnar boron carbide on the surfaces of primary boron carbide particles. The addition of B can both promote the densification and enhance the microhardness of B₄C ceramics.

The results of relative density, sintering temperature and mechanical properties of the B₄C ceramics with metallic additives by pressureless sintering are given in Table 2. In summary, metallic additives can provide a medium for liquid phase sintering at high temperatures, which can accelerate the liquid solid mass transfer process, and thus significantly improve the sinterability of B₄C ceramics. Liquid phase sintering is thought to be an effective method to promote the densification of B₄C ceramics, however, some metallic carbide phases formed by reaction between metallic additives and B₄C powders at the grain boundaries generally have negative effect on mechanical properties of B₄C ceramics. The improvement of mechanical properties, especially for the flexural strength, needs to be further studied. And either very high sintering temperatures or a large addition of metallic additives is required for full densifications of B₄C ceramics. Moreover, some metallic additives facilitated the exaggerated grain growth, which is expected to be the reason for deterioration of mechanical properties of B₄C ceramics.

Table 2

Relative density, sintering temperature and mechanical properties of the B₄C ceramics with metallic additives by pressureless sintering.

Metallic additives	Relative density (%)	Sintering temperature (°C)	Atmosphere pressure	Flexural strength (MPa)	Fracture toughness (MPa•m^1/2)	Vickers hardness (GPa)	Elastic modulus (GPa)
Al [28]	94	2050–2150	Ambient	270 ± 15 (four points)	5.5 ± 0.1 (IF)	34 ± 3	325 ± 21.9
Si [31]	99.2–100	2100	Ambient	–	–	–	–
Al–Si [34]	95.7	2250	Ambient	283.3 ± 33.3 (three points)	2.9 ± 0.4 (IF)	31.5 ± 0.8	384.0 ± 14.5
Al–Si (gas) [36]	97.4	2226	Vacuum	–	–	–	–
Ti [39]	87	2190	Vacuum	150 ± 11 (three points)	–	–	–

‘–’ represent that the data were not offered in the corresponding literatures.

Oxide additives

It is reported that a small quantity of oxide additives are very effective in promoting the densification of non-oxide ceramics [42]. As another category of sintering additive for B₄C ceramics by pressureless sintering, some oxides, especially transition metal oxides, were found to be effective on densifying B₄C ceramics either by solid-state sintering or by liquid phase sintering techniques.

Al₂O₃

Lee and Kim [43] reported that the addition of Al₂O₃ enhanced the densification of B₄C ceramics, of which relative density could be up to 96% of the theoretical density after sintered at 2150°C when the addition of Al₂O₃ was 3 wt-%. B₄C can react with Al₂O₃ as follows (a small amount of free carbon existed in boron carbide powder): Several binary and ternary phases, such as AlB₂, Al₄C₃, AlB₁₂C₂ and AlB₂₄C₄, are included in the B–C–Al system. But only AlB₁₂C₂ phase is detected when Al₂O₃ is added into B₄C ceramics, which is different from the result when Al was added into B₄C ceramics as stated in the ‘Aluminium’ section. Decreasing of the diffusion energy barrier for densification by A1B₁₂C₂ and good wetting of Al₂O₃ melt on B₄C (Figure 3) promoted the densification of B₄C ceramics. However, no mention is made of the mechanical properties. Kim et al. [44] stated that adding Al₂O₃ into B₄C ceramics not only can increase relative density of the ceramics but also can improve flexural strength, fracture toughness, hardness and elastic modulus. Increase in densification was because of the increased mobility of elements through Al₂O₃ near the melting temperature or phases (Al₂O₃ or AlB₁₂C₂) at the grain boundaries. These authors also suggested that the residual stress formed from the thermal expansion mismatch between Al₂O₃ and B₄C, which has a negative effect on other mechanical properties, was expected to be in favour of the fracture toughness. Jiang et al. [45] suggested that Al₂O₃ can provide liquid phase diffusion and pinning effects for B₄C sintering, and the sintering additive can not only hinder the abnormal growth of B₄C grains but also significantly reduce the porosity of the ceramics.

Figure 3

Optical micrograph showing good wetting of liquid phase on B₄C grains [43].

TiO₂

Skorokhod et al. [46] claimed that the relative density of B₄C ceramics is up to higher than 99% by sintering B₄C with TiO₂ and C in the temperature range of 1900–2050°C. The novel feature of the technique is that the TiO₂ added into B₄C ceramics can be transformed into TiB₂ phase in situ. The conversion of TiO₂ into TiB₂ during sintering is excepted to be beneficial to improve both densification and mechanical properties of B₄C ceramics. It was also found that fine TiB₂ particles are formed at the boundaries of B₄C grains, which can limit the grain growth of B₄C matrix, and these TiB₂ particles promote mass transfer in the aggregate, thereby, sintering temperature is lowered. The sintering aptitude of B₄C/TiO₂ composite ceramics is higher than that of B₄C/TiB₂ composite ceramics formed from initial mixtures directly. C acted as a reducing agent for TiO₂. The flexural strength and the fracture toughness for a B₄C–15 vol.-% TiB₂ composite ceramics are 513 ± 21 MPa (four points) and 3.7 ± 0.2 MPa•m^1/2 (SENB), respectively, which are higher than those of pure B₄C ceramics obviously [47] and than the value for B₄C ceramics with a carbon addition (about 350 MPa). Skorokhod et al. [48, 49] also found that the sintering process of B₄C/TiO₂ ceramics is controlled by coarsening of TiB₂ formed in situ and by grain boundary diffusion, and heating rate has no visible effect on densification. TiB₂ particle coarsening was attributed to grain boundary diffusion, and the coarsened TiB₂ can improve the mechanical properties of B₄C ceramics because of thermoelastic residual compressive stresses. Levin et al. [50] added 40 wt-% TiO₂ into B₄C ceramics to promote sintering. They suggested that TiO₂ and B₄C can react as follows: Relative density is up to 95% of the theoretical density after sintering at 2190°C. The flexural strength and hardness can be increased with the addition of TiO₂. Different from the study of Skorokhod et al. stated above, TiO₂ also can be reduced by C originating from the B₄C phase in the absence of an extraneous carbon addition. Increase in the densification was attributed to the formation of sub-stoichiometric boron carbide. The mixture of TiB₂ and B₄C_1–x sinters at a higher rate than the monolithic B₄C. Sub-stoichiometric carbide can revert to stoichiometric composition by a treatment for 2 h at 2000°C in a carburising atmosphere. Moreover, TiB₂ particles formed by the reaction between B₄C and TiO₂ have an effect on inhibiting the growth of the B₄C grains. Although the B₄C ceramics prepared by this method can acquire better densification and mechanical properties, the sintering temperature is near 2200°C, higher than indicated by Skorokhod et al. stated above, and stoichiometric composition of B₄C can be obtained only by a post-sintering treatment of 2000°C × 2 h. In addition, TiO₂ concentration of 40 wt-% is required for satisfactory densification.

ZrO₂

Besides TiO₂, B₄C–TrB₂ (Tr is a transition element) composites also can be formed by the reaction between some other transition element oxides and B₄C, which are expected to have better mechanical properties than monolithic B₄C. Goldstein et al. [51] researched the behaviour of B₄C–ZrO₂ (yttria-stabilized zirconia polycrystals) mixtures during sintering in a temperature range of 1900°C–2200°C. B₄C composite ceramics with the addition of 30 wt-% ZrO₂ allows a sintered density of ≥97% theoretical density to be attained. The fine ZrO₂ grains are transformed to larger ZrB₂ grains during sintering, and B₄C–ZrB₂ system can be formed by in situ reaction, which is similar to B₄C–TiB₂ system that is formed by in situ reaction of B₄C and TiO₂. The average size of ZrB₂ formed by in situ reaction is 5 μm, which is similar to the size of ZrB₂ synthesised via carbothermal reduction by Xie et al. [52]. The Vickers hardness of B₄C–ZrO₂ ceramics can be 30–33 GPa. Baharvandi et al. [53] also added 30 wt-% ZrO₂–3 wt-% Y₂O₃ into B₄C ceramics, and they observed that both the size of grains and porosity of the ceramics were reduced with the addition of ZrO₂. The relative density increased to 98.5% from 75% of an undoped B₄C ceramics. The fracture toughness and the flexural strength can reach 3.2 ± 0.16 MPa•m^1/2 (IF) and 340 ± 18 MPa (four points), respectively. The addition of excess ZrO₂ leaded to a decrease in hardness, which was attributed to the formation of less hard ZrB₂ phase. Roy et al. [54] compared the effect of addition of C, TiB₂ and ZrO₂ on the densification of B₄C ceramics. They found that the addition of ZrO₂ was the most effective in achieving a higher relative density of B₄C ceramics. The relative density of B₄C–5 wt-% ZrO₂ ceramics can be up to 93% even at a relatively lower sintering temperature of 2275°C. They also found the formation of ZrB₂ which distributed uniformly throughout the matrix. However, they observed a different result from Baharvandi et al. stated above that Knoop hardness of the B₄C ceramics with 5 wt-% ZrO₂ exhibited a value of 32 GPa, which is higher than that of B₄C ceramics without the addition of ZrO₂ (24–25 GPa).

High-alumina talc

Although small amounts of oxides such as Al₂O₃, TiO₂ and ZrO₂ are very effective in promoting the densification of B₄C ceramics, these materials must have high purity and be very fine to enhance sinterability. Therefore, these powders are usually very costly. In order to develop a low cost additive, Baharvandi et al. [55] used high purity B₄C as a raw material, high alumina talc (26.62 wt-% Al₂O₃: 47.78 wt-% SiO₂: 25.6 wt-% MgO) as a sintering additive, researched the effect of high-alumina talc on densification of B₄C ceramics. The results showed that SiC, MgB₂ and Al₂O₃ are generated by the in situ chemical reaction between talc and B₄C, which all aided to promote the densification. Different from Lee and Kim's studies stated in ‘Al₂O₃’ section, AlB₁₂C₂ phase formed from the reaction between B₄C and Al₂O₃, which can improve the sinterability of B₄C by decreasing the diffusion energy barrier, was not detected in Baharvandi's studies. B₄C ceramics with the addition of 30 wt-% talc can be sintered up to 98% of theoretical density at temperature of 2150°C. The toughness of B₄C based ceramics increases with the increasing of talc up to 30 wt-%. The maximum hardness is obtained when the content of talc is 25 wt-%. The diminished hardness of B₄C based ceramics with more than 25 wt-% talc is because of the presence of fewer hard phases and the thermal expansion mismatch between the formed phases.

Al₂O₃+Y₂O₃

Al₂O₃+Y₂O₃ have been successfully used as additives, which promote liquid state sintering, to the sintering of SiC ceramics [56, 57] and Si₃N₄ ceramics [58, 59]. In order to research the liquid state sintering of B₄C ceramics, Li and Li [60] added Al₂O₃ and Y₂O₃ into B₄C ceramics, and they demonstrate that B₄C ceramics can be sintered in liquid phase by using both Al₂O₃ and Y₂O₃ as sintering additives. The relative density of the ceramics is 97.7% of the theoretical density after sintering at 1950°C when the addition of Al₂O₃ and Y₂O₃ is up to 15 wt-%. The flexural strength can reach 565 MPa (four points) when the addition of sintering additives is 17 wt-%. They also found that the fracture toughness of the B₄C ceramics is almost about 4.2 MPa•m^1/2 (IF), which is unrelated to the amount of liquid phase. That means Al₂O₃ and Y₂O₃ as a kind of sintering additives of liquid phases have no effect of toughening on B₄C ceramics. Wu et al. [61] also stated that the addition of Al₂O₃ and Y₂O₃ can promote the liquid phase sintering of B₄C ceramics, and microhardness reached up to 30.2–31.0 GPa.

The results of relative density, sintering temperature and mechanical properties of the B₄C ceramics with oxide additives by pressureless sintering are given in Table 3. In summary, the significant increase of densification of B₄C ceramics by oxide additions as well as the improved fracture toughness and flexural strength of the final product underline the merit of oxide additives. Oxide additives can improve the densification of B₄C ceramics either by solid phase sintering or by liquid phase sintering. However, oxide additives are rarely used to promote sintering of B₄C ceramics in practical industrial application because oxide systems are chemically unstable with B₄C, most of oxide additives can react with B₄C. The compounds formed during sintering may determine the unique properties of B₄C ceramics. Moreover, full densification of B₄C ceramics is usually attained with a large of oxide additions or extremely high sintering temperatures.

Table 3

Relative density, sintering temperature and mechanical properties of the B₄C ceramics with oxide additives by pressureless sintering.

Oxide additives	Relative density (%)	Sintering temperature (°C)	Atmosphere pressure	Flexural strength (MPa)	Fracture toughness (MPa•m^1/2)	Vickers hardness (GPa)	Elastic modulus (GPa)
Al₂O₃ [43]	96	2150	Ambient	–	–	–	–
TiO₂+C [46]	99	1900–2050	Ambient	513 ± 21 (four points)	3.7 ± 0.2 (SENB)	–	–
TiO₂ [50]	95	2190	Vacuum	–	–	–	–
ZrO₂ [53]	98.5	2150	Ambient	340 ± 18 (four points)	3.2 ± 0.16 (IF)	26–29	–
High-alumina talc [55]	98	2150	Ambient	–	2.8 (IF)	27	–
Al₂O₃+Y₂O₃ [60]	97.7	1950	Vacuum	565 (four points)	4.2 (IF)	–	–

‘–’ represent that the data were not offered in the corresponding literatures.

Non-oxide additives

Using non-oxide as sintering additives is another route to prepare dense B₄C ceramics. Borides, carbides and fluorides are often used as sintering additives for B₄C ceramics.

Borides

CrB₂

Liquid phase sintering is thought to be an alternative method to promote the sinterability of B₄C ceramics. The eutectic of the B₄C–CrB₂ system in the phase diagram is 2150°C, therefore CrB₂ is thought to be an effective additive for the promotion of liquid phase sintering of B₄C ceramics. Yamada et al. [62] fabricated a high relative density of 98.1% B₄C based ceramic composites with 20 mol-% CrB₂ by pressureless sintering at the temperature of 2030°C. The addition of CrB₂ promoted the sinterability of B₄C ceramics because of the formation of B₄C–CrB₂ eutectic liquid phase. Moreover, it was reported that Cr can diffuse into the grains of B₄C, which may result in the densification [63]. The flexural strength and fracture toughness can be up to 525 MPa (four points) and 3.7 MPa m^1/2 (SENB), respectively. The increase in fracture toughness is because of the formation of microcracks and the deflection of propagating cracks resulting from residual stress generated because of the thermal expansion mismatch of CrB₂ and B₄C. The grains showed transgranular fracture (Figure 4).

Figure 4

Fractured surface of B₄C–20 mol-% CrB₂ specimen sintered at 2030°C [62].

TiB₂

In order to solve the complicated processing route needed in situ method when TiO₂ is used as a sintering additive, Baharvandi and Hadian [64] directly used TiB₂ as a sintering additive. They found that the addition of TiB₂ can reduce porosity and inhibit grain growth of B₄C ceramics. The relative density of B₄C ceramics with the addition of 30 wt-% TiB₂ can reach 98.5% of theoretical density after sintering at 2150°C. The improved densification of B₄C ceramics through adding TiB₂ into B₄C matrix is due to point defects caused by different thermal expansion coefficients between B₄C and TiB₂. At the same time, the fracture toughness also reach the highest value of 3.4 ± 0.1 MPa•m^1/2 (IF). However, both flexural strength and hardness increase first and then decrease with the increasing of the addition of TiB₂. Both flexural strength and hardness reach the highest value when the addition of TiB₂ is 15 wt-%, the values are 345 ± 10 MPa (four points) and 31 ± 1 GPa, respectively. Srivatsan et al. [65] demonstrated that the addition of TiB₂ did certainly improve microhardness of the B₄C ceramics. Zorzi et al. [66] also discovered that the addition of TiB₂ improved both microhardness and wear resistance of B₄C ceramics.

Carbides

WC

Zakhariev et al. [67] reported that the addition of WC increased the densification and the microhardness of B₄C ceramics during pressureless sintering at temperatures from 2200°C to 2250°C. And the shear modulus, Young's modulus and flexural strength of B₄C–10 wt-% WC composite ceramics are close to those of hot-pressed B₄C ceramics. However, the mechanical characteristics decrease with the further increasing of the amount of WC because the free carbon is formed in the chemical interaction between the two carbides during the sintering. The composition of B₄C ceramics obtained by adding WC was three phase: B₄C–W₂B₅–C. The amount of eutectic phase increased with the amount of the sintering additive.

Be₂C

Prochazka [68] used submicron B₄C powder as a raw material and submicron Be₂C powder as a sintering additive to produce dense B₄C ceramics. The author observed that B₄C ceramics could be densified to a limited degree (72% of theoretical density) only on sintering submicron B₄C powder at temperature of 2260°C, which means the particle size of submicron alone is not of decisive importance, whereas the relative density of the B₄C ceramics with the addition of 1 wt-% Be₂C can reach 94% of the theoretical density after sintering at 2280°C. However, the effect of the addition of Be₂C on mechanical properties was not reported in this study. Moreover, Be₂C is highly toxic, and the presence of Be₂C prevents B₄C from use in the field of nuclear technology.

Cr₃C₂

Li et al. [69] used Cr₃C₂ as a sintering additive, and Cr₃C₂ reacts with B₄C as follows:

The in situ formed CrB₂ produced liquid phase that could improve the densification of B₄C ceramics, which was in agreement with the report that liquid phase can be obtained at 1875°C in the B₄C–CrB₂–C system [70]. Meanwhile, the graphite generated can remove surface B₂O₃ layer, which can also help to improve the densification of B₄C. The relative density of B₄C ceramics reached 93% when the content of Cr₃C₂ was 30 wt-% after sintering at 2070°C, meanwhile, the flexural strength of B₄C–30 wt-% Cr₃C₂ can reach 440 ± 18 MPa (three points). Although the relative density can reach about 95% when the B₄C ceramics sintered after 2100°C, the coarse grains began to appear at 2080°C, which can decrease the flexural strength sharply. From the effect of improving the densification of B₄C ceramics, the effect of CrB₂ formed in situ is not as good as directly adding CrB₂ aforementioned in ‘CrB₂’ section.

Fluoride

Kanno et al. [71] researched the effect of fluorides on densification of B₄C ceramics. They pointed out that MgF₂ did not show any significant densification effect on B₄C. While the B₄C ceramics with the addition of 1 wt-% AlF₃ can obtain the relative density of 86% of the theoretical density after sintering at 2200°C. It is interesting to note that, however, the effect of AlF₃ on densification of B₄C is weaker than that of Al or Al₂O₃ on densification of B₄C stated in sections ‘Aluminium’ and ‘Al₂O₃’, respectively.

The results of relative density, sintering temperature and mechanical properties of the B₄C ceramics with non-oxide additives by pressureless sintering are given in Table 4. In summary, some of non-oxide additives can promote the densification of B₄C ceramics. Besides easier densification, B₄C ceramics also can be toughened because of cracks deflection, cracks bridging as well as even controlled microcracking caused by the addition of some non-oxide additives. However, either high temperatures or a large amount of non-oxide additives is required for high densification of B₄C ceramics. Moreover, some of non-oxide additives cannot show good densification effect on B₄C ceramics, and B₄C ceramics with good densification effect non-oxide additives cannot show good mechanical properties. Thereby, non-oxide additives have not been widely applied in actual industrial production.

Table 4

Relative density, sintering temperature and mechanical properties of the B₄C ceramics with non-oxide additives by pressureless sintering.

Non-oxide additives	Relative density (%)	Sintering temperature (°C)	Atmosphere pressure	Flexural strength (MPa)	Fracture toughness (MPa•m^1/2)	Vickers hardness (GPa)	Elastic modulus (GPa)
CrB₂ [62]	98.1	2030	Ambient	525 (four points)	3.7 (SENB)	–	–
TiB₂ [64]	98.5	2150	Vacuum	345 ± 10 (four points)	3.4 ± 0.1 (IF)	31 ± 1	–
WC [67]	–	2200–2250	Vacuum	340 (three points)	–	53	527.3
Be₂C [68]	94.0	2280	Ambient or vacuum	–	–	–	–
Cr₃C₂ [69]	93.0	2070	Ambient	440 ± 18 (three points)	–	–	–
AlF₃ [71]	86.0	2200	Ambient	–	–	–	–

‘–’ represent that the data were not offered in the corresponding literatures.

Combined additives

Besides the single kind of additives mentioned above, sometimes two or more than two additives can also be used together to promote the sintering of B₄C ceramics. Various additive combinations have been tried by researchers.

Two organic precursors

Bougoin and Thevenot [72] used phenolic resin and polycarbosilane as the precursor of C and SiC, respectively, the B₄C ceramics with the addition of both phenolic resin and polycarbosilane was prepared. The ceramics prepared had a relative density more than 92% theoretical density at the sintering temperature of 2175°C. Though the relative density is not too high, it was possible to remove free C, which can carburise the metallic cladding materials and must be avoided in nuclear applications, from the resulting materials. The advantages of preparing B₄C–SiC composites by in situ method are that: (1) very homogeneous B₄C–SiC mixtures can be obtained without milling, during which pollution is usually caused; (2) polycarbosilane acts as a plasticiser as well as a temporary binder during the process of pressing. However, the disadvantage of this method is that coarse-grained α-SiC is formed from β-SiC (produced from polycarbosilane) during sintering.

Non-metal + metal

Some oxide additives or non-oxide additives stated above can promote the sintering of B₄C ceramics, and some metallic additives can also improve the densification of B₄C ceramics because of the generation of liquid phases, which accelerate the liquid solid mass transfer process. Therefore, non-metallic additives and metallic additives can be combined to use as additives to promote the densification of B₄C ceramics.

TiB₂ + Al

Mashhadi et al. [73] researched the influence of the addition of TiB₂ and Al on the densification behaviour of B₄C ceramics. It was shown that both TiB₂ and Al additions to B₄C ceramics provided fine and dense microstructure, thereby the relative density of B₄C–30 wt-%TiB₂–5 wt-%Al composite ceramics can reach 99% after sintering at 2050–2150°C. Flexural strength, fracture toughness, hardness and elastic modulus can reach 450 MPa (four points), 6.2 MPa•m^1/2 (IF), 35 GPa and 500 GPa, respectively. These values represent a significant increase, considering that the fracture toughness and the flexural strength of pressureless, sintered B₄C ceramics with other sintering additives rarely exceed 5.0 MPa•m^1/2 and 400 MPa, respectively. For the fracture toughness of 6.2 MPa•m^1/2, the value is almost identical to that of B₄C ceramics prepared with the addition of TiO₂ and C via hot pressing (6.1 ± 0.7 MPa•m^1/2 (IF)) [74]. The authors thought that TiB₂ does not undergo any change and only acts as grain growth inhibitor during the sintering. While Al reacted with B₄C as followings:

Al₈B₄C₇ phase can be detected either by XRD or by SEM (Figure 5). The effect of adding both TiB₂ and Al simultaneously is significantly better than that of adding TiB₂ or Al separately to promote the densification of B₄C ceramics.

TiB₂ + Fe

TiB₂ can densify the B₄C ceramics as stated in ‘TiB₂’ section and have high wear and corrosion resistance [75]. Fe has a relative low wetting angle and can react with B₄C to form iron boride which exists as a liquid phase at the sintering temperature [76]. Based on these, TiB₂ and Fe were selected by Kim and Kim [77] as a kind of combination of additives. They observed that the ceramics can obtain the maximum densification (97% of the theoretical density) when the composition of the ceramic was B₄C–10 wt-% TiB₂–1 wt-% Fe after sintering at 2175°C. TiB₂ is effective in limiting the grain growth of B₄C, and Fe-rich grain boundary phase forms a liquid phase and promotes the densification of B₄C based ceramics. Although the ceramics with a composition of B₄C–10 wt-% TiB₂–1 wt-% Fe can acquire a higher density after sintering at 2175°C, the flexural strength is only about 250 MPa (three points). The flexural strength of the ceramics with a composition of B₄C–20 wt-% TiB₂–1 wt-% Fe can be up to about 420 ± 27 MPa (three points) after sintering at 2150°C, and relative density of the ceramics with this composition is about 95% of theoretical density.

Figure 5

Fractured surface of B₄C–10 wt-% TiB₂–5 wt-% Al specimen sintered at 2050°C [73].

Other combinations

Ma et al. [78] added the composite mixture of TiB₂, Ni and C (phenolic resin) into B₄C ceramics, and pores could be filled by liquid Ni formed at high temperature. The relative density of the composite ceramics with the addition of 1.3 wt-% Ni can reach 98% of the theoretical density after sintering at 2100°C, flexural strength, fracture toughness and Vickers hardness can reach 330 MPa (three points), 5.5–6 MPa (SENB) and 20.4 GPa, respectively. Weaver [79] prepared dense B₄C based composites by sintering B₄C with SiC and Al. A density of 2.41 g cm⁻³, which corresponds to a relative density of 94% of theoretical density, was obtained for a B₄C–10 wt-% SiC–3 wt-% Al composite sintered at temperatures from 2050°C to 2150°C. However, no information is given regarding their mechanical properties (flexural strength, fracture toughness, or hardness). Moers et al. [80] proposed B₄C can be able to sinter by adding one or more carbides, such as silicon carbide, vanadium carbide, tungsten carbide, molybdenum carbide, titanium carbide, hafnium carbide and zirconium carbide, and one or more metals, such as Ni, Co, Fe and Cr, after heating at a temperature of 2200°C to 2300°C. They concluded that the amount of carbides and metals added must not together exceed 25% of the total mixture, otherwise the hardness of the final product is unsatisfactory. However, no data on the density of the finished product were reported. Gu et al. [81] reported that the relative density and mechanical properties of B₄C ceramics prepared by adding B₂O₃ and Si were significantly improved. Zhang et al. [82] found that composite sintering additives of TiC and Si can effectively improve the densification of the B₄C ceramics, thereby resulting in superior hardness.

Non-metal + non-metal

High purity raw materials, which are made by energy-intensive processes or by other costly methods, are often chosen as the starting materials for B₄C based ceramics. Generally speaking, minimising the impurity contents of raw materials is able to maximise the properties of the final ceramic products. In order to reduce cost of the starting ceramic powders for B₄C ceramics, Cameron [83] prepared the B₄C ceramics containing SiC and MgAl₂O₄ from low cost raw materials. The magnesium silicate (talc, vermiculite or mica), B₂O₃, Al and C were chosen as raw materials, B₄C–SiC–MgAl₂O₄ composite ceramics can be formed by in situ reaction according to the equation: But there is no mention about density and mechanical properties for the prepared B₄C ceramics. In addition, Prochazka and Coblenz [84] reported that mixed additives of SiC and C can also be used as sintering additives to enhance the sinterability of B₄C ceramics, and the relative density reached at least about 85% at sintering temperatures from 2050°C to 2180°C. The B₄C ceramics containing SiC can also improve its oxidation resistance [85].

The results of relative density, sintering temperature and mechanical properties of the B₄C ceramics with combined additives by pressureless sintering are given in Table 5. In summary, combined additives can integrate the advantage of each additive, the densification and the physical performance of B₄C ceramics can be further enhanced by a combination of some additives. Although B₄C ceramics with the combined additives cannot acquire high density or good mechanical properties unless sintered at temperatures above 2000°C, combined additives have shown better effect on promoting the densification of B₄C ceramics comparing with single additives.

Table 5

Relative density, sintering temperature and mechanical properties of the B₄C ceramics with combined additives by pressureless sintering.

Combined additives	Relative density (%)	Sintering temperature (°C)	Atmosphere pressure	Flexural strength (MPa)	Fracture toughness (MPa•m^1/2)	Vickers hardness (GPa)	Elastic modulus (GPa)
Polycarbosilane + phenolic resin [72]	≥92.0	2175	Ambient	–	–	–	–
TiB₂+Al [73]	99.0–100.0	2050–2150	Ambient	450 (four points)	6.2 (IF)	35	500
TiB₂+Fe [77]	>95.0	2150–2175	Ambient	420 ± 27 (three points)	–	–	–
TiB₂+Ni + C [78]	98.0	2100	Ambient	330 (three points)	5.5–6 (SENB)	20.4	–
SiC + Al [79]	94.0	2050–2150	Ambient	–	–	–	–

‘–’ represent that the data were not offered in the corresponding literatures.

Rare earth oxide additives

Rare earth oxides are only used as sintering additives for B₄C ceramics in recent years. The research about using rare earth oxides to promote densificaiton of B₄C ceramics is limited. Goldstein et al. [86] used La₂O₃ and Y₂O₃ as sintering additive to densify B₄C ceramics, respectively. The relative density of the ceramics can reach more than 95% of the theoretical densify after sintering 2180°C. The rare earth oxides, as sintering additives, are more effective than transition metal oxides. When 8 vol.-% Y₂O₃ was added into B₄C ceramics, the ceramics has a density of 2.7 g cm⁻³, which is most lightweight B₄C containing ceramics which can be densified to 97.5% of the theoretical densification by pressureless sintering. Thereby, the B₄C ceramics with the addition of Y₂O₃ can be used as lightweight ceramic armour in nuclear applications. Mu et al. [87] suggested that the addition of La₂O₃ can enhance the density and lower the sintering temperature of B₄C ceramics because LaAlO₃ phase generated form the reaction between B₄C and La₂O₃ has better surface coalescent with B₄C. Wei et al. [88] researched the effects of the addition of other rare earth oxides (Dy₂O₃, Eu₂O₃ and Sm₂O₃) on the densification and mechanical properties of B₄C ceramics, however, they came up with different results from Goldstein et al. and Mu et al. stated above. These authors suggested that only adding Dy₂O₃ to promote the densification of B₄C ceramics is limited, and only adding Eu₂O₃ or Sm₂O₃ has no effect on improving the densification of B₄C ceramics. However, adding the composite mixture of both 4 wt-% rare earth oxides and 18 wt-% phenolic resin is found to be excellent in improving the densification of B₄C ceramics. Among them, Dy₂O₃ has the best effect on promoting the density and improving the flexural strength of B₄C ceramics. When 18 wt-% phenolic resin and 4 wt-% Dy₂O₃ was added into B₄C ceramics after sintering at 2040°C, relative density and flexural strength can be increased to 96.6% and 358 ± 6 MPa (three points), respectively. For Eu₂O₃, relative density and flexural strength are 96.3% and 325 ± 12 MPa (three points), respectively, when the ceramics were sintered at 2080°C. And for Sm₂O₃, relative density and flexural strength are 96.0% and 303 ± 20 MPa (three points), respectively, when the ceramics were sintered at 2040°C. All of these results are better than that of the B₄C ceramics with only adding 18 wt-% phenolic resin, of which relative density and flexural strength are 96.2% and 234 ± 22 MPa (three points), respectively, when the ceramics were sintered as high as 2160°C. They attribute the increase in the densification of the ceramics to the formation of DyB₂C₂ and Dy₂C₃ phase because of the reaction between DyB₄ (generated from the reaction between B₄C and Dy₂O₃) and pyrolytic carbon, which can accelerate the diffusion between B and C atoms resulting in the promotion of the densification of B₄C ceramics. And the increase in strength is attributed to restriction of grain growth of B₄C particles by rare earth particles dispersing in intergranular (Figure 6). But there is no explanation for lowered sintering aptitude of B₄C ceramics in the presence of only rare earth oxide addition. Sairam et al. [89] also demonstrated that only using Eu₂O₃ as a sintering additive has a negative effect on densification of B₄C ceramics, and they gave an explanation for the mechanism. They thought the reasons are that: (1)the reaction between transition metal oxides and B₄C is only by one step and no borates are formed. However, Eu₂O₃ reacts with B₄C by the following equations:

Figure 6

SEM of specimens sintered at 2040°C for 2 h [88]: (a) B₄C + 18 wt-% phenolic resin and (b) B₄C + 18 wt-% phenolic resin + 4 wt-% Dy₂O₃.

Eu₂O₃ first converts to borates and then to borides. The conversion of Eu₂O₃ is more difficult and slower in comparison to transition metal oxides; (2)the release of CO gas turns to an additional burden to the densification, which does not exist in case of transition metal oxide additions with B₄C. In addition, Xu et al. [90] reported that adding Gd₂O₃ into B₄C ceramics can improve the hardness of the ceramics, but decrease the flexural strength of the B₄C ceramics.

The results of relative density, sintering temperature and mechanical properties of the B₄C ceramics with rare earth oxide additives by pressureless sintering are given in Table 6. The huge neutron absorption cross-sections is one of the characteristics of some rare earth elements [91], the advantage of adding rare earth into B₄C ceramics as sintering additives can not only promote the sintering, but improve the neutron absorption performance. However, whether the rare earth oxides can densify the B₄C ceramics or not has not yet reached a unified conclusion until now. It may also be that different rare earth oxides have different effects on the densification of B₄C ceramics. And so far, for rare earth oxides, neither the mechanism of increasing the densification of B₄C ceramics nor the mechanism of reducing the densification of B₄C ceramics is completely make clear. Therefore, more research is needed on the application of rare earth oxide as sintering additives for B₄C ceramics.

Table 6

Relative density, sintering temperature and mechanical properties of the B₄C ceramics with rare earth oxide additives by pressureless sintering.

Rare earth oxide additives	Relative density (%)	Sintering temperature (°C)	Atmosphere pressure	Flexural strength (MPa)	Fracture toughness (MPa•m^1/2)	Vickers hardness (GPa)	Elastic modulus (GPa)
Y₂O₃ [86]	97.5	2180	Ambient	160–200 (four points)	–	27–30	–
La₂O₃ [86]	95	2180	Ambient	–	–	–	–
Dy₂O₃+C [88]	96.6	2040	Ambient	358 ± 6 (three points)	–	–	–
Eu₂O₃+C [88]	96.3	2080	Ambient	325 ± 12 (three points)	–	–	–
Sm₂O₃+C [88]	96.0	2040	Ambient	303 ± 20 (three points)	–	–	–

‘–’ represent that the data were not offered in the corresponding literatures.

Recent research trends

Recently, spark plasma sintering (SPS), pulsed electric current sintering (PECS) and plasma pressure compaction (P²C) methods are gradually applied to the sintering of B₄C ceramics. The application of SPS, PECS or P²C can reduce the sintering temperature, shorten holding time, and improve the efficiency as sintering the B₄C ceramics. Many researchers proposed that B₄C ceramics added different additives prepared by SPS can acquire better relative density and mechanical properties than that of B₄C ceramics prepared by pressureless sintering or by hot pressing. Srivatsan et al. [65] found that the addition of TiB₂ into B₄C ceramics did certainly improve microhardness of the ceramics by spark plasma sintering. Sun et al. [92] used CeO₂ as sintering additive, and the resulting B₄C ceramics exhibits a high relative density of 96.7%. Pan et al. [93] used B, C (graphite) and WC as raw materials, and W₂B₅ reinforced B₄C ceramics was prepared via in situ reaction by spark plasma sintering. The relative density of the ceramics can be close to completely dense (99.8% of the theoretical density), the fracture toughness and Vickers hardness can reach 11.9 ± 0.2 MPa•m^1/2 (SENB) and 30.2 ± 0.7 GPa, respectively. Eqtesade et al. [94] combined pressureless spark plasma sintering with robocasting for the first time to prepare B₄C ceramics. The relative density and hardness of B₄C ceramics prepared reach 95 ± 3% and 27 ± 1.5 GPa, respectively, after sintering at 2100°C. Liu et al. [95] reported that relative density, Vickers hardness, fracture toughness and flexural strength of B₄C–41 vol.-% TiB₂ composite ceramics produced by reactive pulsed electric current sintering (1900°C × 7 min) can be up to 97.9%, 28.3 ± 1.4 GPa, 4.4 ± 0.5 MPa•m^1/2 (IF), 891 ± 32 MPa (three points), respectively. Ren et al. [96] used B₄C and Ti as starting materials, and core–shell B₄C–TiB₂&TiC powder composites were prepared by molten salt method without co-ball milling. The relative density of B₄C–TiB₂ interlayer ceramic composites can reach 98% by subsequent pulsed electric current sintering (1700°C × 10 min). The fracture toughness, however, is not too high, of which value is only 4.38 ± 0.14 MPa•m^1/2 (IF). To further increase the densification and fracture toughness of B₄C ceramics, Ren et al. [97] used Al₃BC as a sintering additive to improve the sinterability of B₄C by pulse electric current sintering. The results showed that the relative density, fracture toughness, Vickers hardness, and elastic modulus reached as high as 100%, 6.32 ± 0.07 MPa•m^1/2 (IF), 37.0 ± 5.0 GPa, and 495 ± 53.0 GPa, respectively, when the addition amount of Al₃BC was 18 wt-%. The fracture mode of a mixture of intergranular and transgranular modes, which is beneficial to improve fracture toughness can be formed from the single transgranular mode by the good dispersion of Al₃BC. Rubino et al. [98] reported that relative density, fracture toughness and hardness of B₄C ceramics sintered by plasma pressure compaction (1750°C × 5 min) can be up to 99.2%, 3.6 MPa•m^1/2 and 28 GPa, respectively. In addition, rapid heating is a new sintering method for B₄C ceramics put forward in recent years. Rapid heating can minimise the time over which coarsening could occur by evaporation and condensation of B₄C. Lee and Speyer [99] utilised rapid heating method to sinter and densify B₄C ceramics. The relative density of the B₄C ceramics without and with the addition of carbon (3 wt-%) can reach 92.76% and 98.65% of theoretical density, respectively.

Furthermore, some newly developed sintering additives have been utilised for densification and enhancing the mechanical properties of B₄C ceramics by hot pressing. For example, Ma et al. [100] added 2 wt-% SiB₆ into B₄C ceramics. The B₄C ceramics with the addition of SiB₆ can be completely densified, the bulk density, flexural strength and Vickers hardness of the B₄C ceramics are 2.515 g cm⁻³ (99.5% of the theoretical density), 426.6 ± 35.6 MPa (three points), 31.2 ± 0.9 GPa, respectively. Sairam et al. [101] used HfO₂ as a sintering additive, and HfB₂ was a reaction product in the resulting materials. Relative density can reach 100% of the theoretical density. Transgranular fracture, crack deflection, crack arrest and crack bridging results in high fracture toughness (4–7 MPa•m^1/2 (IF)). Grigor'ev et al. [102] used the composite mixture of V₂O₅ and carbon black as sintering additives, B₄C–VB₂–C composite ceramics was prepared by reaction sintering with hot pressing. The fracture toughness, flexural strength and Vickers hardness can reach about 6 ± 1.3 MPa•m^1/2 (IF), 600 ± 51 MPa (three points) and 42 ± 7 GPa, respectively. Sun et al. [103] used both TiO₂ and Al as sintering additives to improve the sinterability of B₄C ceramics. The relative density, flexural strength, fracture toughness and Vickers hardness can reach 97.5 ± 0.3%, 540 ± 14 MPa (three points), 5.1 ± 0.2 MPa•m^1/2 (IF) and 28.2 ± 1.0 GPa, respectively. The enhanced fracture toughness was because of the formation of microcracks and deflection of cracks, and the increased flexural strength was because of the change of fracture mode. Zhang et al. [82] used both TiC and Si as sintering additives, and B₄C–TiB₂–SiC ceramics was prepared by in situ process. The relative density of B₄C composite ceramics can be close to complete dense (98.9–99.4%), and the flexural strength, fracture toughness and hardness can reach 380–570 MPa (three points), 5.3–6.5 MPa•m^1/2 (SENB), 33.1–36.2 GPa, respectively. In future research, these additives can be tried to apply to prepare B₄C ceramics by pressureless sintering.

Recently, in order to solve the problems that mixtures of the required dispersion (<1 μm) can't be prepared even by high-energy ball milling and the nanoparticles are uneven distributed because of agglomeration of particles during mixing by using B₄C, SiC, and TiB₂ as raw materials to produce B₄C–SiC–TiB₂ composite, Kotsar et al. [104] used H₃BO₃, SiO₂, TiO₂, and C as starting materials, B₄C–SiC–TiB₂ composite was prepared in situ by the reaction as follows:

The synthesised powders are nanoparticles of average diameter 40 nm and conglomerations of B₄C crystals of size < 1 μm after sintering at 1600°C, and agglomerates of submicron- and nanosized particles can be completely destroyed.

Conclusion

B₄C ceramics have been successfully used in various industrial applications. The pressureless sintering realises wide applications in a low cost way. Some sintering additives have been proved to have a strong ability to increase pressureless sintered density and mechanical properties of B₄C ceramics. Although some sintering additives can promote the densificaiton of B₄C ceramics, there are still some problems existed in the application of additives to the densification of B₄C ceramics: (1) some the second phases formed because of the addition of sintering additives often deteriorate the mechanical properties of the B₄C ceramics. (2) Although the addition of some sintering additives from an extraneous source can promote the sinterability of B₄C ceramics, either a large addition of additives or very high sintering temperatures are required for completely densifications of B₄C ceramics. Moreover, good sintering effect can be realised only when very fine size initial boron carbide powders are used. (3) Only a few reports mentioned the mechanical properties of B₄C ceramics prepared by pressureless sintering. The transport mechanism, especially for rare earth oxides, and the influence of impurities on the sintering of B₄C ceramics have not been researched in detail. (4) Carbon additives, being most effective and superseding all competitors, are now used to improve the sinterability of B₄C ceramics on an industrial scale. However, the mechanical properties of pressureless sintered B₄C ceramics with carbon additives needs to be further improved. Furthermore, usually above 2200°C for ceramics with a relative density more than 97% of theoretical density is required for most practical uses, and a highly homogeneous dispersion of carbon additives is difficult to achieve in practice.

Some new technologies, such as rapid heating, spark plasma sintering and pulsed electric current sintering, are other routes to produce dense B₄C ceramics, however, these new technologies are not suitable for industrial applications because of either limited simple shapes or much more cost. B₄C ceramics prepared by pressureless sintering have the lowest cost, and different kinds of B₄C products with complex shape and/or large size can also be fabricated by pressureless sintering. Therefore, pressureless sintering is the method with industrialised application value, and the research of sintering additives for pressureless sintering is of practical significance.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

References

Kumazawa

, Kiyoto

, Matsuoka

炭化ホウ素セラミックスの常圧焼結

[Pressureless sintering of boron carbide ceramics]. Ceramics. 2014;49(2):122–125. Japanese.

Reinmuth

, Lipp

, Knoch

, Borhaltige keramische neutronenabsorberwerkstoffe. J Nucl Mater. 1984;124:175–184.

Mashhadi

, Saraf MR , Baghshahi

Synthesis of boron carbide powder via carbothermic reaction of boron oxide. Int J Eng Sci. 2005;16(1):9–14.

Wang

, Yang

, Zhang

碳化硼烧结动力学和烧结机制

[Study on the mechanism and kinetic of boron carbide sintering]. J Cent South Univ Technol. 1999;30(5):505–508. Chinese.

Kuzenkova

, Kislyi

, Grabchuk

, The structure and properties of sintered boron carbide. J Less-Common Met. 1979;67(1):217–223.

Bougoin

, Thevenot

, Dubois

, Synthèse et caracterisation de ceramiques denses en carbure de bore. J Less-Common Met. 1985;114(2):257–271.

Schwetz

, Grellner

The influence of carbon on the microstructure and mechanical properties of sintered boron carbide. J Less-Common Met. 1981;82:37–47.

, Liu

, Li

核反应堆用碳化硼芯块常压烧结工艺研究

[Study on pressureless sintering of B₄C pellets used in nuclear reactors]. Powder Metall Technol. 2017;35(1):53–56. Chinese.

Wang

, Yin

, Fang

YC.

用掺碳活化烧结技术制取碳化硼材料

[Boron carbide material fabricated by carbon-doping activated sintering]. Chin J Nonferrous Met. 2002;12(6):1210–1213. Chinese.

10.

Yuan

, Jiang

, Zheng

YJ.

葡萄糖助剂对无压烧结碳化硼性能的影响

[Effect of glucose additives on the properties of pressureless-sintered boron carbide]. J Mudanjiang Normal Univ. 2016;3:36–38. Chinese.

11.

Suzuki

, Hase

, Maruyama

炭化ホウ素の焼結に及ぽす炭素の影響

[Effect of carbon on sintering of boron carbide]. Ceram Soc Jpn. 1979;87(8):430–433. Japanese.

12.

Schwetz

, Vogt

Elektroschmelzwerk Kempten Gmbh, Munich, Fed, Rep. of Germany. Process for the production of dense sintered shaped articles of polycrystalline boron carbide by pressureless sintering. United States patent US 4,195, 066.1980 Mar 25.

13.

Sigl

, Schwetz

KA.

Pressureless sintering of boron carbide with carbon black. In: Ziegler

, Hausner

, editor. Proceedings of Euro-ceramics II. Köln: Deutsche Keramische Gesellschafe e.V.; 1991. Vol. 1, p. 171–182.

14.

Lee

, Speyer

RF.

Hardness and fracture toughness of pressureless-sintered boron carbide (B₄C). J Am Ceram Soc. 2002;85(5):1291–1293.

15.

Dole

, Prochazka

, Doremus

RH.

Microstructural coarsening during sintering of boron carbide. J Am Ceram Soc. 1989;72(6):958–966.

16.

Dole

, Prochazka

Densification and microstructure development in boron carbide. Ceram Eng Sci Proc. 1985;6(7–8):1151–1160.

17.

Speyer

, Lee

Advances in pressureless densification of boron carbide. J Mater Sci. 2004;39(19):6017–6021.

18.

Schwetz

, Sigl

, Pfau

Mechanical properties of injection molded B₄C–C ceramics. J Solid State Chem. 1997;133:68–76.

19.

Sigl

LS.

Processing and mechanical properties of boron carbide sintered with TiC. J Eur Ceram Soc. 1998;18(11):1521–1529.

20.

Niihara

Sumitomo Electric Industries, Inc., Japan. Process for forming a sintered composite boron carbide body. United States patent US 5,637,269. 1997 Jun 10.

21.

, Chen

, Zhang

陶瓷/石墨烯块体复合材料的研究进展

[Recent progress in ceramic/graphene bulk composites]. J Inorg Mater. 2014;29(3):225–236. Chinese.

22.

Zhao

, Sun

, Wang

, Microstructure and anisotropic mechanical properties of graphene nanoplatelet toughened biphasic calcium phosphate composite. Ceram Int. 2013;39(7):7627–7634.

23.

, Zhang

, Gong

, Enhanced fracture toughness of pressureless-sintered SiC ceramics by addition of graphene. J Mater Sci Technol. 2016;32(7):633–638.

24.

Kovalčíková

, Sedlák

, Pawel

, Mechanical properties of boron carbide + graphene platelet composites. Ceram Int. 2016;42(1):2094–2098.

25.

Tan

, Zhang

, Peng

Electrically conductive graphene nanoplatelet/boron carbide composites with high hardness and toughness. Scr Mater. 2016;114:98–102.

26.

Gao

, Jing

, Yu

, Graphene platelets enhanced pressureless-sintered B₄C ceramics. R Soc Open Sci. 2018;5(4):1–7.

27.

Halverson

, Pyzik

, Aksay

IA.

Processing and microstructural characterization of B₄C–Al cermets. Ceram Eng Sci Proc. 1985;6(7–8):736–744.

28.

Mashhadi

, Nassaj

, Sglavo

VM.

Pressureless sintering of boron carbide. Ceram Int. 2010;36(1):151–159.

29.

Pyzik

, Aksay

IA.

Multipurpose boron carbide–aluminium composite and its manufacture via the control of the microstructure. United States patent US 4,702,770. 1987 Oct 27.

30.

Bhattacharya

, Petrovic

JJ.

Ductile phase toughening and R-curve behaviour in a B₄C–Al cermet. J Mater Sci. 1992;27(8):2205–2210.

31.

Stibbs

, Thompson

United States Borax & Chemical Corporation, Los Angeles, Calif. Cold-pressed compositions. United States patent US 3,749,571. 1973 Jul 31.

32.

Cai

, Nan

, Min

XM.

The effect of silicon addition on thermoelectric properties of a B₄C ceramic. Mater Sci Eng B. 1999;67(3):102–107.

33.

Wei

, Zhang

, Gong

HY.

热压烧结工艺制备SiC/B₄C复合材料

[SiC/B₄C composites prepared by hot pressing]. Bull Chin Ceram Soc. 2009;28(2):249–252. Chinese.

34.

, Zeng

, Zhang

GJ.

Pressureless sintering of boron carbide ceramics with Al–Si additives. Int J Refract Metals Hard Mater. 2013;41:2–6.

35.

Frage

, Levin

, Frumin

, Manufacturing B₄C–(Al,Si) composite materials by metal alloy infiltration. J Mater Process Technol. 2003;143–144(1):486–490.

36.

Kumazawa

, Honda

, Zhou

, Pressureless sintering of boron carbide ceramics. J Ceram Soc Jpn. 2008;116(12):1319–1321.

37.

Miyazaki

, Zhou

, Hyuga

, Microstructure of boron carbide pressureless sintered in an Ar atmosphere containing gaseous metal species. J Eur Ceram Soc. 2010;30(4):999–1005.

38.

Iseki

, Kameda

, Maruyama

Interfacial reactions between SiC and aluminium during joining. J Mater Sci. 1984;19(5):1692–1698.

39.

Levin

, Frage

, Dariel

MP.

The effect of Ti and TiO₂ additions on the pressureless sintering of B₄C. Metall Mater Trans A. 1999;30(12):3201–3210.

40.

Liu

, Tan

, Zhang

掺硼改性碳化硼陶瓷的研究

[Study on boron-doped B₄C ceramics]. China Ceramic Industry. 2007;14(5):1–5. Chinese.

41.

Grabchuk

, Kislyi

PS.

Sintering of boron carbide containing small a mounts of free carbon. Sov Powder Metall Met Ceram. 1975;14(7):538–541.

42.

Pyzik

, Beaman DR.

Microstructure and properties of self-reinforced silicon nitride. J Am Ceram Soc. 2010;76(11):2737–2744.

43.

Lee

, Kim

CH.

Pressureless sintering and related phenomena of Al₂O₃-doped B₄C reaction. J Mater Sci. 1992;27(23):6335–6340.

44.

Kim

, Koh

, Kim

HE.

Densification and mechanical properties of B₄C with Al₂O₃ as a sintering aid. J Am Ceram Soc. 2000;83(11):2863–2865.

45.

Jiang

, Fu

, Huang

添加氧化铝烧结助剂的碳化硼陶瓷无压烧结工艺研究

[Pressureless sintering of boron carbide with an addition of alumina]. Powder Metall Technol. 2017;35(6):462–468. Chinese.

46.

Skorokhod

, Vlajic

, Krstic

VD.

Mechanical properties of pressureless sintered boron carbide containing TiB₂ phase. J Mater Sci Lett. 1996;15(8):1337–1339.

47.

Francois

Boron carbide – a comprehensive review. J Eur Ceram Soc. 1990;6(4):205–225.

48.

Skorokhod

, Vlajić

, Krstić

VD.

Pressureless sintering of B₄C–TiB₂ ceramic composites. Mater Sci Forum. 1998;282–283:219–224.

49.

Levin

, Frage

, Dariel

MP.

A novel approach for the preparation of B₄C-based cermets. Int J Refract Met Hard Mater. 2000;18(2):131–135.

50.

Skorokhod

, Krstic

VD.

Processing, microstructure, and mechanical properties of B₄C–TiB₂ particulate sintered composites. pressureless sintering and microstructure evolution. Powder Metall Met Ceram. 2000;39(7–8):414–423.

51.

Goldstein

, Geffen

, Goldenberg

Boron carbide–zirconium boride in situ composites by the reactive pressureless sintering of boron carbide–zirconia mixtures. J Am Ceram Soc. 2001;84(3):642–644.

52.

Xie

, Ma

, Gao

, Influence of SiC on phase and microstructure of ZrB₂ powders synthesized via carbothermal reduction at different temperatures. Ceram Int. 2018;44(8):8795–8799.

53.

Baharvandi

, Hadian

, Abdizadeh

, Investigation on addition of ZrO₂–3 mol% Y₂O₃ powder on sintering behavior and mechanical properties of B₄C. J Mater Sci. 2006;41(16):5269–5272.

54.

Roy

, Subramanian

, Suri

AK.

Pressureless sintering of boron carbide. Ceram Int. 2006;32(3):227–233.

55.

Baharvandi

, Hadian

, Abdizade

, Investigation on addition of talc on sintering behavior and mechanical properties of B₄C. J Mater Eng Perform. 2006;15:280–283.

56.

Liang

, Yao

, Zhang

, The effect of TiC on the liquid phase sintering of SiC ceramics with Al₂O₃ and Y₂O₃ additives. Key Eng Mater. 2014;602–603:197–201.

57.

Liang

, Yao

, Huang

, The relationship between microstructure and flexural strength of pressureless liquid phase sintered SiC ceramics oxidized at elevated temperatures. Ceram Int. 2016;42(11):13256–13261.

58.

Jeong

, Tatami

, Iijima

, Fabrication of Si₃N₄ ceramics by post-reaction sintering using Si–Y₂O₃–Al₂O₃ nanocomposite particles prepared by mechanical treatment. Ceram Int. 2016;42(10):11554–11561.

59.

, Tan

, Guo

, Preparation of Si₃N₄–ZrSi₂–ZrN–BN ceramic by reactive hot pressing and its processing properties. J Chin Ceram Soc. 2017;45(6):823–828.

60.

, Li

WX.

碳化硼陶瓷的液相烧结试验研究

[Liquid phase sintering of boron carbide]. J Harbin Univ Sci Technol. 2002;7(2):73–75. Chinese.

61.

, Jiao

, Xie

液相烧结碳化硼陶瓷的实验研究

[Study on the experiment of liquid phase sintering boron carbide ceramics]. China Ceramics. 2015;51(11):68–70. Chinese.

62.

Yamada

, Hirao

, Yamauchi

, Densification behaviour and mechanical properties of pressureless-sintered B₄C-CrB₂ ceramics. J Mater Sci. 2002;37:5007–5012.

63.

Marek

, Dudnik

, Makarenko

, Physical properties of boron carbide alloys with vanadium and chromium additives. Porosh Metall. 1975;14(2):54–56.

64.

Baharvandi

, Hadian

AM.

Pressureless sintering of TiB₂–B₄C ceramic matrix composite. J Mater Eng Perform. 2008;17(6):838–841.

65.

Srivatsan

, Guruprasad

, Black

, Influence of TiB₂ content on microstructure and hardness of TiB₂-B₄C composite. Powder Technol. 2005;159(3):161–167.

66.

Zorzi

, Perottoni

, Jornada

JAHD.

Hardness and wear resistance of B₄C ceramics prepared with several additives. Mater Lett. 2005;59(23):2932–2935.

67.

Zakhariev

, Radev

Properties of polycrystalline boron carbide sintered in the presence of W₂B₅ without pressing. J Mater Sci Lett. 1988;7(7):695–696.

68.

Prochazka

General Electric Company, Schenectady, N.Y. Dense sintered boron carbide containing beryllium carbide. United States patent US 4,005,235. 1977 Jan 25.

69.

, Jiang

, Zhang

, Pressureless sintering of boron carbide with Cr₃C₂ as sintering additive. J Eur Ceram Soc. 2014;34(5):1073–1081.

70.

Rogl

Refractory metal systems: phase diagrams crystallographic and thermodynamic data. In: Effenberg

, Ilyenko

, editors. Boron-carbon-chromium. Stuttgart: Springer Materials the Landolt-Bornstein Database; 2009. p. 357–383.

71.

Kanno

, Kawase

, Nakano

Additive effect on sintering of boron carbide. Yogyo Kyokai Shi. 1987;95(11):1137–1140.

72.

Bougoin

, Thevenot

Pressureless sintering of boron carbide with an addition of polycarbosilane. J Mater Sci. 1987;22(1):109–114.

73.

Mashhadi

, Nassaj

, Mashhadi

, Pressureless sintering of B₄C–TiB₂ composites with Al additions. Ceram Int. 2011;37(8):3229–3235.

74.

Skorokhod

, Krstic

VD.

High strength-high toughness B₄C–TiB₂ composites. J Mater Sci Lett. 2000;19(3):237–239.

75.

Watanahe

, Shoubu

Mechanical properties of hot-pressed TiB₂–ZrO₂ composites. J Am Ceram Soc. 1985;68(2):34–36.

76.

Panasyuk

, Oreshkin

, Maslennikova

VR.

Kinetics of the reactions of boron carbide with liquid aluminum, silicon, nickel, and iron. Sov Powder Metall Met Ceram. 1979;18(7):487–490.

77.

Kim

, Kim

CH.

Pressureless sintering and microstructural development of B₄C–TiB₂ composites. Adv Ceram Mater. 1988;3(1):52–55.

78.

, Yu

, Guo

, Pressureless densification and properties of TiB₂–B₄C composite ceramics with Ni as additives. Micro Nano Lett. 2018;13(7):1–4.

79.

Weaver

GQ.

Norton company, worcester, mass. Sintered high density boron carbide. United States patent US 4,320,204. 1982 Mar 16.

80.

Moers

, Schroeter

, Wolff

Fried. Krupp Aktiengesellschaft, Essen-on-the-Ruhr, Germany. Hard alloys. United States patent US 1,973,441. 1931 Apr 24.

81.

, Wang

, Xu

, B₂O₃和Si粉的添加对碳化硼陶瓷性能的影响 [Effects of B₂O₃ and Si on the properties of boron carbide]. J Ceram. 2016;37(4):329–333.

82.

Zhang

, Zhang

, Wang

, Microstructure and mechanical properties of B₄C–TiB₂–SiC composites toughened by composite structural toughening phases. J Am Ceram Soc. 2017;100(7):3099–3107.

83.

Cameron

CP.

W.R. Grace & Co.-Conn., New York, NY. Ceramic composites containing spinel, silicon carbide, and boron carbide. United States patent US 5,120,681. 1992 Jun 09.

84.

Prochazka

, Coblenz

WS.

General electric company. Silicon carbide-boron carbide sintered body. United States patent US 4,081,284. 1978 Mar 28.

85.

Kobayashi

, Maeda

, Sano

, Formation and oxidation resistance of the coating formed on carbon material composed of B₄C–SiC powders. Carbon. 1995;33(4):397–403.

86.

Goldstein

, Yeshurun

, Goldenberg

B₄c/metal boride composites derived from B₄C/metal oxide mixtures. J Eur Ceram Soc. 2007;27(2):695–700.

87.

, Tang

, Zhang

稀土氧化物对碳化硼陶瓷性能的影响

[Influence of the rare-earth oxide on the properties of boron carbide ceramics]. Powder Metall Technol. 2008;26(3):187–191. Chinese.

88.

Wei

, Zhang

, Gong

, The effect of rare-earth oxide additives on the densification of pressureless sintering B₄C ceramics. Ceram Int. 2013;39(6):6449–6452.

89.

Sairam

, Vishwanadh

, Sonber

, Competition between densification and microstructure development during spark plasma sintering of B₄C–Eu₂O₃. J Am Ceram Soc. 2018;101:2516–2526.

90.

, Wu

, Bian

GdB₆对热压烧结B₄C材料性能的影响

[Influence of GdB₆ on performance of hot-pressing sintering B₄C material]. Chinese Rare Eeaths. 2005;26(4):10–14. Chinese.

91.

Celli

, Grazzi

, Zoppi

A new ceramic material for shielding pulsed neutron scattering instruments. Nuclear Instrum Methods Phys Res A. 2006;565(2):861–863.

92.

Sun

, Li

, Wang

, B₄c/CeB₆ composite ceramic prepared by spark plasma sintering. Rare Met Mater Eng. 2016;45(8):2071–2074.

93.

Pan

, Li

, Zhang

, Effect of graphite content on properties of B₄C–W₂B₅ ceramic composites by in situ reaction of B-Gr-WC. J Am Ceram Soc. 2018;101:3617–3626.

94.

Eqtesadi

, Motealleh

, Perera

, Fabricating geometrically-complex B₄C ceramic components by robocasting and pressureless spark plasma sintering. Scr Mater. 2018;145:14–18.

95.

Liu

, Wang

, Li

, Densification of high-strength B₄C–TiB₂ composites fabricated by pulsed electric current sintering of TiC-B mixture. Scr Mater. 2017;135:15–18.

96.

Ren

, Deng

, Wang

, Synthesis and properties of conductive B₄C ceramic composites with TiB₂ grain network. J Am Ceram Soc. 2018;101:1–7.

97.

Ren

, Deng

, Wang

, Densification and mechanical properties of pulsed electric current sintered B₄C with in situ synthesized Al₃BC obtained by the molten-salt method. J Eur Ceram Soc. 2017;37(15):1–8.

98.

Rubino

, Pisaturo

, Senatore

, Tribological characterization of SiC and B₄C manufactured by plasma pressure compaction. J Mater Eng Perform. 2017;26(7):1–12.

99.

, Zhang

, Kan

六硼化硅(SiB₆)添加剂对B₄C陶瓷致密化与力学性能的影响

[Densification and mechanical properties of boron carbide ceramics with addition of silicon hexaboride]. J Inorg Mater. 2008;23(6):1175–1178. Chinese.

100.

Lee

, Speyer

RF.

Pressureless sintering of boron carbide. J Am Ceram Soc. 2003;86(9):1468–1473.

101.

Sairam

, Sonber

, Murthy

TSRC

, Development of B₄C–HfB₂ composites by reaction hot pressing. Int J Refract Met Hard Mater. 2012;35(1):32–40.

102.

Grigor'ev

, Koval'chuk

, Zaporozhets

, Synthesis and physicomechanical properties of B₄C–VB₂ composites. Powder Metall Met Ceram. 2006;45(1–2):47–58.

103.

Sun

, Liu

, Duan

CY.

Effect of Al and TiO₂ on sinterability and mechanical properties of boron carbide. Mater Sci Eng A. 2009;509(1):89–93.

104.

Kotsar’

, Danilovich

, Ordan'yan

, Combined carbothermal synthesis of powders in the B4C–SiC–TiB₂ system. Refract Ind Ceram. 2017;58(2):174–178.

Progress in pressureless sintering of boron carbide ceramics – a review

Abstract

Keywords

Introduction

Carbon additives

Carbon generated from organism

Amorphous carbon

Carbon generated by in situ reaction from metal carbide

Graphene platelets

Metallic additives

Aluminium (Al)

Silicon (Si)

Al–Si

Titanium (Ti)

Boron (B)

Oxide additives

Al2O3

TiO2

ZrO2

High-alumina talc

Al2O3+Y2O3

Non-oxide additives

Borides

CrB2

TiB2

Carbides

WC

Be2C

Cr3C2

Fluoride

Combined additives

Two organic precursors

Non-metal + metal

TiB2 + Al

TiB2 + Fe

Other combinations

Non-metal + non-metal

Rare earth oxide additives

Recent research trends

Conclusion

Footnotes

References

Al₂O₃

TiO₂

ZrO₂

Al₂O₃+Y₂O₃

CrB₂

TiB₂

Be₂C

Cr₃C₂

TiB₂ + Al

TiB₂ + Fe