B4C-TiB2 Composite Ceramics with Ultra-High Fracture Toughness Fabricated by Spark Plasma Sintering

B 4 C-TiB 2 composite ceramics with ultra-high fracture toughness were successfully prepared via spark plasma sintering using B 4 C and 30 vol.% Ti 3 SiC 2 as raw materials at different sintering temperatures. The results show that compared with pure B 4 C ceramics sintered by SPS, the �exural strength and fracture toughness are signi�cantly improved, especially the fracture toughness has been improved by leaps and bounds. When the sintering temperature is 1900 ℃ , the B 4 C-TiB 2 composite ceramic has the best comprehensive mechanical properties: hardness, bending strength and fracture toughness are 27.28 GPa, 405.11 MPa and 18.94 MPa·m 1/2 , respectively. The main two reasons for the ultra-high fracture toughness are the formation of TiB 2 three-dimensional network covering the whole composites, and the existence of lamellar graphite at the grain boundary.


Introduction
Boron carbide is widely used in wear-resistant parts, armor protection and aerospace [1][2][3] .These excellent properties are due to the strong covalent bond, but this structure also leads to the low self-diffusion coe cient of B 4 C. Low self-diffusion ability, high melting point and the oxygen-rich layer (B 2 O 3 ) on the surface of B 4 C particles make it di cult to sinter B 4 C compactly, which greatly limits its application [4] .
There are three common methods to obtain high density B 4 C ceramics: increasing sintering temperature, applying high pressure and using additives.Increasing sintering temperature and pressure has great requirements for the equipment, so the use of additives has become the most convenient method.Metal additives are very effective for the densi cation of B 4 C ceramics, but the hardness of B 4 C ceramics decreases obviously with the soft metal phase [5] .Ti 3 SiC 2 can react with B 4 C to form the second phase with high hardness, i.e.TiB 2 and SiC, which can achieve the second phase toughening effect at the same time of promoting sintering.Meanwhile, the reduction of hardness can be controlled in a small range.
The traditional hot pressing sintering of B 4 C needs to be kept above 2000 ℃ for more than 1h [3] .SPS (Spark Plasma Sintering) is a kind of low-temperature rapid sintering process which is widely concerned.
The combination of mechanical pressure, electric eld and thermal eld can enhance the bonding and densi cation of particles [6][7][8] .In this study, B 4 C-TiB 2 composite ceramics with high toughness were prepared by SPS process (using 30 vol.% Ti 3 SiC 2 + B 4 C mixed powder as raw materials).In addition, the effect of sintering temperature on the microstructure and properties of B 4 C ceramics was also studied.

Experimental Procedure
Commercially available B 4 C powders (purity 99.9%, 1 µm, 4.53 g/cm 3 , Nangong Naiyate Alloy Welding Material Co., Ltd), Ti 3 SiC 2 powders (purity 99.9%, < 74 µm, 4.53 g/cm 3 ,Nanjing Mingchang New Material Co., Ltd) were used as raw materials.The scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns of the as-received powders of B 4 C and Ti 3 SiC 2 are shown in Fig. 1.It can be observed from SEM images that B 4 C particles have sharp edge and ladder-like surface undulation, which is a typical transgranular fracture appearance during the particle crushing process; Ti 3 SiC 2 particles have larger grain size and obvious lamellar structure.It can be seen from the XRD images that the two kinds of powders are relatively pure and almost no oxide exists (the content of oxide is too small to be detected in XRD).30 vol.% Ti 3 SiC 2 -B 4 C powders were mixed for 24 h through a small vertical mixer at 80 r/min without adding solvent.The specimens were prepared by SPS equipment (HP D 25/4-SD, FCT Systeme GmbH, Germany) in vacuum with 35 MPa mechanical pressure at 1800 ℃, 1850 ℃, 1900 ℃ and 1950 ℃ for 5 min.The heating rate was 100 ℃/min and the cooling rate was 50 ℃/min.The SPS method was used to prepare the sample under vacuum environment with 35 MPa mechanical pressure at 1800 ℃, 1850 ℃, 1900 ℃ and 1950 ℃ for 5 min.the heating rate was 100 ℃/min and the cooling rate was 50 ℃/min.Absolute density of B 4 C-TiB 2 composite ceramics were determined using the Archimedes method.
Hardness was measured by a Vickers-indentation tester (Shimadzu, HMV-2TADW E, Japan) at 9.81 N load with a holding time of 15 s on the polished surface.Flexural strength was determined by three-point bending test with the span of 30 mm and the loading speed of 0.5 mm/min, and the specimens used in the test were 3 × 4 × 35 mm bars.SENB method was used to determine the fracture toughness of the specimens, with dimensions of 2 × 4 × 20 mm (with 2 mm high notch).The microstructures of the composite ceramics were characterized by X-Ray powder diffraction (XRD, X′ Pert PRO-MPD, Holland Panalytical, Netherlands), scanning electron micro-scope (SEM, S-4800N, Hitachi, Japan), transmission electron microscope (TEM, JEM-2100, JEOL, Japan) and energy dispersive spectrometer (EDS, INCA, OXFORD INSTRUMENTS, England).When the temperature is above 1200 ℃, the following reactions occur [9] :

Results And Discussion
The reaction ( 1) and ( 2) ended at 1600 ℃, on the basis of the above results, the overall reaction in the system can be described as the following reaction [9] TiC does not exist in the nal product, appearing as an intermediate product during the whole sintering process.According to the XRD test results, the content of each phase is shown in Table 1.It can be seen that the composition of the composite ceramics changes little when sintered at different sintering temperatures, which is mainly composed of TiB 2 and B 4 C, with a small amount of SiC and C. When the temperature rise, the content of SiC decreases and the content of C increases, which may be due to the slight evaporation of silicon in the sintering process.It can be inferred that the evaporation of Si also increases with the increase of temperature.in the gure respectively [10] .In Fig. 4, the dark gray at area is the B 4 C matrix, and the light gray rough area is TiB 2 particles, which indicates that in B 4 C-TiB 2 composite ceramics, the fracture mode of B 4 C phase and TiB 2 phase is the transgranular fracture and intergranular fracture respectively.and TiB 2 (B 4 C: 4.5×10 -6 k -1 ; TiB 2 : 8.1×10 -6 k -1 ) [11] , there is large residual stress at the interface of the two phases, which induces the crack de ection along the grain boundary and prolongs the crack propagation path, which greatly improves the toughness.The nano TiB 2 particles embedded in the B 4 C matrix can introduce internal stress, which will strengthen the B 4 C matrix by lattice distortion effect, and can also nail the dislocations and hinder their movement.In addition, as shown in Fig. 5(c), the nano TiB 2 and SiC particles at the grain boundary of B 4 C can also strengthen the grain boundary and prevent the crack growth.Fig. 5(b) is the BSE image of TiB 2 -SiC aggregates, in which the dark gray, medium gray and light gray phases are B 4 C, SiC and TiB 2 , respectively.In this multiphase mixing region with a large amount of SiC, there is adverse stress effects that have a negative effect on exural strength [12] .However, the TiB 2 -SiC aggregates can expand the crack propagation path and change the crack propagation direction to consume the crack growth energy, which is helpful to improve the toughness.The crack propagation path in this area is shown in Fig. 5(d).3 shows the mechanical properties of B 4 C-TiB 2 composite ceramics prepared by different starting materials and sintering methods in recent two years [13][14][15][16][17] .Compared with these works, the exural strength (405.11MPa) of the simple that we sintered by SPS at 1900 ℃ is in the middle to a low level, but the fracture toughness (18.94 MPa•m 1/2 ) is much higher than the fracture toughness shown in the table, which can be described as a leap forward improvement.By making an in-depth comparative study of the differences between our work and the work of other researchers, we nd that there are two fundamental reasons for the excellent fracture toughness of our B 4 C-TiB 2 composite ceramics: The rst is the formation of a three-dimensional network of TiB 2 in the composites.In the area as shown in Fig. 5(f), B 4 C and TiB 2 are occluded and interlaced, and TiB 2 forms a network structure in the B 4 C matrix.In the low magni cation backscatter image Fig. 5(e), we can nd out that this network structure accounts for a large proportion of the whole material composition, and it does not exist in isolation.Links are formed between each small network, connecting a larger network structure covering the whole composite as a whole.At the same time, this network divides B 4 C concentrated area and surrounds each one, so that there is no large area of continuous B 4 C phase in the material, which is very unfavorable to the toughness of the composite.With higher interlacing degree of TiB 2 and B 4 C phases, the cracks need to bypass more multiple two-phase interfaces, change the direction for more times, and disperse into more small cracks in the process of extension.Therefore, the overall three-dimensional network structure greatly improves the fracture toughness of the B 4 C-TiB 2 composite ceramics.Another reason is the existence of C, which exists in the form of graphite.With the increase of reaction temperature, the content of C increased, and its effect on the properties of the materials cannot be ignored.The graphite formed by the reaction of Ti 3 SiC 2 and B 4 C exists in the grain boundary in the form of bands, as shown in the TEM image Fig. 6.The existence of the graphite layer reduces the bonding strength of the interface, which has an adverse effect on the hardness and strength of the composite ceramics [18] .But on the other hand, its existence can limit grain growth.In the cooling process, microcracks are produced under the effect of interfacial stress produced by different thermal expansion coe cients, and according to the mechanism of microcrack toughening, it is bene cial to improve the toughness of the material.Combined with the data in Table 1 and Fig3(b), it can be found out that the fracture toughness of the material is positively related to the graphite content before the abnormal grain growth occurs at 1950 ℃

Conclusions
The ultra-high toughness and full density B 4 C-TiB 2 composite ceramics were prepared by SPS with the addition of 30 vol.%Ti 3 SiC 2 .Sintering temperature has a great in uence on its microstructure and mechanical properties, mainly re ected in the sensitivity of density and grain size to temperature.The change of sintering temperature has little effect on the hardness of B 4 C-TiB 2 composite ceramics.With the increase of sintering temperature, both bending strength and fracture toughness increase and then decrease.When the sintering temperature is 1900 ℃, the B 4 C-TiB 2 composite ceramic has the maximum relative density and kept the small grain size, and obtain the best comprehensive mechanical properties: hardness, bending strength and fracture toughness are 27.28GPa, 405.11MPa and 18.94 MPa•m 1/2 , respectively.The fracture toughness of B 4 C-TiB 2 composite ceramics has been greatly improved at the expense of some hardness and bending strength.In B 4 C-TiB 2 composite ceramics, the fracture mode is a mixture of transgranular fracture and intergranular fracture.The leap of fracture toughness is attributed to the formation of the TiB 2 three-dimensional network in the B 4 C matrix, and the layered graphite exists at the grain boundary.

Fig. 2
Fig.2 XRD patterns of B 4 C-TiB 2 ceramic composites sintered at different temperatures

Fig. 3 (Fig. 3 (
Fig.3(b) shows the variation of exural strength and fracture toughness of B4C-TiB2 ceramics with sintering temperature.Simple and BT1850 have similar exural strength and fracture toughness, and the properties of BT1900 are obviously higher than the former two.This change trend is consistent with the relative density in Fig.3(a).Fig.4(a)-(d) are the SEM pictures of fracture appearance of samples BT1800 to BT1950 in turn.Fig.4(b) shows there are some large-size B 4 C grains in the samples sintered at 1850 ℃, resulting in a small decrease in the exural strength of the samples sintered at 1800 ℃.The comparison of Fig.4(a)-(c) shows that the samples sintered at 1800 ℃, 1850 ℃ and 1900 ℃ have similar grain sizes (excluding a small part of large grains in Fig.5(b)), so the signi cant increase of density at 1900℃ is the reason for the improvement of exural strength and fracture toughness of sample BT1900.Compared with BT1900, the bending strength and fracture toughness of BT1950 are decreased, especially the latter.The main reason for that is the obvious grain growth of B 4 C as shown in Fig.5(d), and the secondary reason is the slight decrease of the relative density as shown in Fig.3(a).

Fig. 4 (
Fig.4(f) is the BSE image of the ceramics sintered at 2000 ℃.It can be clearly seen that if the temperature is further increased, the grains grow furthermore, which is not conducive to the integrated mechanical properties of the B 4 C-TiB 2 composite ceramic.Moreover, when the sintering temperature is 2000 ℃, the raw materials reacted with graphite mold and damage it.So, the bending strength and fracture toughness reach the maximum values of 405.11MPa and 18.94 MPa•m 1/2 when the sintering temperature is 1900 ℃.Compared with the fracture appearance of pure B 4 C ceramic (Fig.4 (e)), the fracture surface of the simples with additive Ti 3 SiC 2 (Fig.4(a)-(d)) is much rougher, showing a mixed fracture mode.Intergranular fracture and transgranular fracture correspond to the rough and at surface

Fig. 3 Fig. 4 (
Fig.3 Relative density and mechanical properties of the B 4 C-TiB 2 composite ceramics sintered at different temperatures

Fig. 5
Fig.5 BSE images of different structures of the B 4 C-TiB 2 composite ceramic sintered at 1900 ℃

Figures
Figures

Figure 4 a
Figure 4

Table 1
contents of different phases in B 4 C-TiB 2 composite ceramics sintered at different temperatures

Table 2
Properties of ceramics prepared by different sintering processes

Table 2
to BT1950 is slightly higher than that of pure B 4 C ceramic sintered by SPS.Because of the slight evaporation of silicon, the density of B 4 C-TiB 2 composite ceramic is even slightly higher than the theoretical density.
shows the mechanical properties of the samples prepared by different sintering processes.The sample B1800 is pure B 4 C ceramic sintered by SPS as comparative group, whose hardness, bending strength and fracture toughness are 33.5 GPa, 224.43 MPa and 5.96 MPa•m 1/2 , respectively.The density of simples BT1800

Table 3
Comparison of the properties of the B 4 C-TiB 2 composite ceramics reported in recent years and