Preparation and properties of B 4 C-TiB 2 ceramics prepared by spark plasma sintering

: By doping titanium hydride (TiH 2 ) into boron carbide (B 4 C), a series of B 4 C + x wt% TiH 2 (x = 0, 5, 10, 15 and 20) composite ceramics were obtained through spark plasma sintering (SPS). The effects of sintering temperature and the amount of TiH 2 additive on the microstructure, mechanical and electrical properties of the sintered B 4 C-TiB 2 composite ceramics were investigated. Powder mixtures of B 4 C with 0  20 wt% TiH 2 were heated from 1400 to 1800 °C for 20 minutes under 50 MPa. The results indicated that higher sintering temperatures contributed to greater ceramic density. With increasing TiH 2 content, titanium diboride (TiB 2 ) formed between the TiH 2 and B 4 C matrix. This effectively improved Young’s modulus and fracture toughness of the composite ceramics, significantly improving their electrical properties: the electrical conductivity reached 114.9 S·cm  1 at 1800 °C when x = 20. Optimum mechanical properties were obtained for the B 4 C ceramics sintered with 20 wt% TiH 2 , which had a relative density of 99.9 ± 0.1%, Vickers hardness of 31.8 GPa and fracture toughness of 8.5 MPa·m 1/2 . The results indicated that the doping of fine Ti particles into the B 4 C matrix increased the conductivity and the fracture toughness of B 4 C.


Introduction
With developments in science and technology, the research and application of ceramic composite materials have attracted increasing attention. Furthermore, due to the electrical conductivity of ceramics, they can be widely used as high-temperature electrothermal elements in oxidising atmospheres, as cathodes in high-temperature fuel cells (HTFC) and as electrodes for magnetohydrodynamic (MHD) power generation [1][2] .
Traditional conductive ceramic materials, whether oxide or non-oxide ceramic materials, have low conductivity at room temperature. Therefore, the synthesis of conductive super-hard materials at room temperature would not only be of great scientific significance but also of great value for expanding the applications of ceramic materials.
The third hardest material known to man, black diamond, or B 4 C ceramic [3][4] , is an attractive ceramic because of its excellent properties such as low density (2.51 g·cm 3 ), high melting point (2447 °C), high hardness (2931 GPa), outstanding chemical stability and excellent absorption neutron cross-section (600 b). It has many applications, such as abrasive cutting, as a coating material, for light-weight armour and for controlling nuclear fission [5][6] . B 4 C ceramic also has high Young's modulus (390440 GPa) and low fracture toughness (2.162.52 MPa·m 1/2 ) [7][8] . Due to these characteristics, B 4 C ceramic is too difficult to sinter. However, B 4 C ceramic with high density can be obtained by pressureless sintering (PS) or hot pressing (HP) at above 2200 °C, these conditions not only enable the grains to grow too easily but also waste thermal energy, which greatly limits B 4 C ceramic application [9][10] . TiB 2 is another super-hard material with high hardness (34 GPa) and superconductivity (14.4 μΩ·cm). It can be combined with B 4 C to form a composite material that results in inhibition of grain growth, lower sintering temperature and improved mechanical properties [11][12][13][14][15][16][17] . When sintered at high temperature, the overall performance of nanoscale Ti and B 4 C powders is weaker than that of pure B 4 C [18] .
Additionally, Ti is easily oxidised into Ti-O phase, or Ti-B-O and Ti-C-O solid solutions.
Nanoscale Ti powers are usually unstable, so they are too difficult to handle. Therefore, TiH 2 has been used to disperse Ti particles in the B 4 C ceramic matrix to synthesise TiB 2 , thereby optimising its performance [19] . In addition, the fabrication of B 4 C-TiB 2 composite ceramics using B 4 C and TiH 2 as starting materials by the spark plasma sintering (SPS) method has not yet been reported. The main aim of the present work was to synthesise B 4 C-TiB 2 composite ceramics and investigate the effect of TiH 2 powders on the B 4 C ceramic properties.

. Materials and methods
B 4 C powder (99.5%, 110 μm) and TiH 2 powder (99%, 325mesh) were purchased from Shanghai Aladdin Biochemical Technology, Shanghai, China and used as the starting materials [20] . Figure 1 shows the microstructures of these two raw materials.
Powders composed of B 4 C and TiH 2 (0, 5, 10, 15, 20 wt%) were mixed for more than 30 minutes in an agate mortar and then put into a graphite die (40 mm in depth and 13.1 mm in diameter). The die was lined with graphite paper to separate the powders from graphite mould. The mixtures were sintered by SPS (LABOX-325R, Sinter Land Inc, Japan) under a uniaxial pressure of 50 MPa in a vacuum. A schematic diagram of the sample preparation process and SPS is shown in Figure 2. Using a two-stage heating process, TiH 2 [21][22][23] was first completely decomposed at 800 °C for 10 minutes and then heated to 1400, 1500, 1600, 1700 and 1800 °C for 20 minutes, each, at a heating rate of 100 °C·min 1 . Figure 3 shows the temperature and displacement changes of the sample during 1800 °C sintering, revealing that the sample had a positive displacement and indicating that the sample was gradually densified.
After polishing the sintered B 4 C-TiB 2 composite ceramics, their density was determined using the Archimedes displacement method. The microstructures of the composite ceramics were characterised by X-Ray diffraction (XRD; X-pert, Japan)

Density and Phase compositions
The densities of B 4 C + x wt% TiH 2 (x = 0, 5, 10, 15, 20) sintered at 1400, 1500, 1600, 1700 and 1800 °C are shown in Table 1. The results showed that the pure B 4 C ceramic reached complete densification at 1800 ℃ (the theoretical density of B 4 C is 2.51 g·cm 3 ). Figure 4 shows the density of B 4 C ceramics sintered with different TiH 2 content at 1800 °C [24] , with increasing TiH 2 content, the density of the composite ceramic reached a maximum at 20 wt% TiH 2 . Both TiB 2 and C can reduce sintering temperature and inhibit grain growth. Because TiB 2 has relatively low crystalline boundary diffusion coefficient, which promotes slow densification [25] , graphite (C) has a binding effect on grain boundary, which can enhance grain bounding diffusing and fast densification [26] .
Therefore, the B 4 C-TiB 2 composite ceramics are densified well. Figure 5 shows the XRD analysis of the B 4 C-TiB 2 composite ceramics doped with different TiH 2 content and sintered at 1800 °C for 20 minutes. With increasing TiH 2 content, the intensity of the TiB 2 diffraction peaks (around 44°) continued to increase and reached a maximum with 20 wt% TiH 2 [27] . Due to the decomposition of TiH 2 powder at 620 °C via the process shown in Equation (1), the temperature was maintained at 800 °C for 10 minutes to ensure the complete formation of Ti. B 4 C is divided into B and C through the process shown in Equation (2), the presence of element C in the XRD pattern also confirms this statement. Ti and B form TiB 2 through the process shown in Equation (3), The results indicated that an appropriate ratio of B 4 C and TiH 2 could be completely converted into B 4 C-TiB 2 ceramics through the process shown in Equation (4) after sintering in SPS in a high vacuum environment. In this experiment, TiB 2 was generated by the in situ reaction of B 4 C and TiH 2 , which can be described by the following three equations: The overall reaction can be summarised as follows: grains of about 34 μm appeared with inhomogeneous distribution in the microstructure, as shown in Figure 6 (e), these could aggregate into 3040 μm grains. The reaction between Ti and B is highly exothermic in behaviour and the heat generated helps to accelerate the formation of TiB 2 readily. The primary TiB 2 particles on the surface of B 4 C are appreciably free and movable and because of boron diffusion across boundary layer, TiB 2 particles produces growth with the primary ones formed agglomerates [28] . Figure 7 shows the results of energy-dispersive X-ray spectroscopy (EDS) of part A in Figure 6 (f). Figure 7 (a) is an enlarged image of A, and the elemental content and distribution of A are shown in Figure 7 (b)(f). XRD results combined with EDS spectra confirmed the phase distribution and that the light grey phase [29] was TiB 2 , which has excellent electrical conductivity. Figure 8 shows the EDS analysis of part B in Figure 6 (f). Figure 8 (a) is an enlarged image of B, and the elemental content and distribution of B are shown in Figure 8 (b)(f). XRD results combined with EDS spectra confirmed that the phase distribution of the matrix was B 4 C, which has poor electrical conductivity.

Hardness and fracture toughness
The mechanical properties of B 4 C-TiB 2 were measured on the polished surface, as displayed in Figure 9 [30][31] . The hardness-load curve of B 4 C20 wt% TiH 2 is shown in Figure 9(a), revealing the obvious decrease in Vickers hardness (Hv) with increasing applied force, which was primarily attributed to the indentation size effect. An asymptotic Hv value of ~31.4 GPa was obtained when the applied load exceeded 5 N. It may be because the hardness of TiB 2 generated by the reaction is 34 GPa, and the hardness of C produced by the reaction is poor, which hardness is 1~2 GPa [32][33] , the hardness of B 4 C is 31 GPa, which hardness is between TiB 2 and C. However, the results indicate that due to very little C produced by the reaction, the hardness of B 4 C-TiB 2 composite ceramics does not decrease under the combined action of TiB 2 and C. Vickers hardness and fracture toughness results for the B 4 C-TiB 2 composite ceramics are shown in Figure 9(b). When doped with 20 wt% TiH 2 , the hardness of the sample was stable and the fracture toughness was a maximum of 8.5 MPa·m 1/2 . TiB 2 is a high-toughness material, and C also can be used as an additive to produce high-toughness ceramic [34][35][36][37] , so the fracture toughness of B 4 C-TiB 2 composite ceramics are improved.
To calculate fracture toughness, an oscilloscope was used to measure the shear and longitudinal wave sound velocities and then the Poisson ratio μ was calculated using Equation (5). Data for the Young ' s modulus calculation was then obtained using Equation (6). Finally, the fracture toughness of the B 4 C-TiB 2 composite ceramics was calculated according to Equation (7).
The crack trace of the polished surface of the 20 wt% TiH 2 composite ceramic is shown in Figure 10. Crack bridging, crack deflection, crack bending and crack forking occurred when force was applied, and these cracks acted as tougheners for consuming energy. Crack bridging is helpful to improve fracture toughness [38] . The following three formulas were applied to calculate fracture toughness:

Electrical conductivity
The electrical conductivity of the B 4 C + x wt% TiH 2 (x = 0, 5,10,15,20) composite ceramics at different sintering temperatures (1400, 1500, 1600, 1700 and 1800 °C) is shown in Figure 11. The electrical conductivity could be enhanced by increasing the sintering temperature and increasing the content of TiH 2 . When doped with 5 wt% or 10 wt% TiH 2 , the electrical conductivity of the B 4 C-TiB 2 composite ceramics suddenly increased. When TiH 2 was further increased to 15 wt% or 20 wt%, the electrical conductivity of the B 4 C-TiB 2 composite ceramics was two orders of magnitude higher than that of pure B 4 C. At room temperature, a maximum electrical conductivity of 114.9 S·cm 1 was achieved with 20 wt% TiH 2 . Graphite is one type of carbon-based filler, due to being layered structure, its electrical conductivity is 10 4 S/cm at room temperature [39] .
Ti reacted with B to form TiB 2 , the layered structure of boron atoms similar to graphite and the outer electrons of Ti determine the outstanding electrical conductivity of TiB 2， they can be used for improving the electrical conductivity of composite ceramics.

Conclusions
Completely densified B 4 C-TiB 2 composite ceramics with high conductivity, high strength and high hardness properties were successfully obtained by the spark plasma sintering method starting from raw mixtures of B 4 C and TiH 2 powders. Using TiH 2 as a sintering aid improved the density of the B 4 C ceramics. During the sintering process,