Jingzhe Fan(范靜哲), Weixia Shen(沈維霞), Zhuangfei Zhang(張壯飛),Chao Fang(房超), Yuewen Zhang(張躍文), Liangchao Chen(陳良超),Qianqian Wang(王倩倩), Biao Wan(萬彪), and Xiaopeng Jia(賈曉鵬)

Key Laboratory of Material Physics,Ministry of Education,School of Physics and Microelectronics,Zhengzhou University,Zhengzhou 450052,China

Keywords: boron carbide ceramics,conductivity,hardness,fracture toughness

1. 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 oxidizing atmospheres,as cathodes in high-temperature fuel cells and as electrodes for magnetohydrodynamic power generation.[1,2]Traditional conductive ceramic materials, whether oxide or nonoxide 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 B4C ceramic,[3-6]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 armor,and for controlling nuclear fission.[7,8]B4C ceramic also has high Young’s modulus (390-440 GPa) and low fracture toughness(2.16-2.52 MPa·m1/2).[9,10]Due to these characteristics,B4C ceramic is too difficult to sinter. B4C ceramic with high density can be obtained by pressureless sintering or hot pressing at above 2200?C,however these conditions not only enable the grains to grow too easily but also waste thermal energy,which greatly limits B4C ceramic application.[11,12]

TiB2is another super-hard material with high hardness(34 GPa) and excellent electrical conductivity (14.4 μΩ·cm).It can be combined with B4C to form a composite material that results in inhibition of grain growth, lower sintering temperature, and improved mechanical properties.[13-19]When sintered at high temperature, the overall performance of nanoscale Ti and B4C powders is weaker than that of pure B4C.[20]Additionally, Ti is easily oxidized 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, TiH2has been used to disperse Ti particles in the B4C ceramic matrix to synthesize TiB2,thereby optimizing its performance.[21]The fabrication of B4C-TiB2composite ceramics using B4C and TiH2as starting materials by the spark plasma sintering (SPS) method has not yet been reported. In the SPS process, the sintering is carried out at lower temperature and the process proceeds faster than conventional processes thanks to the use of pulse current for heating the powders to be sintered. Besides, spark discharges are ignited in the pores during a current pulse,which can efficiently remove the adsorbed gases and oxides from the powder particle surfaces,thereby facilitating the formation of active contacts between them.[22]The main aim of the present work is to synthesize B4C-TiB2composite ceramics and investigate the effect of TiH2powders on the B4C ceramic properties.

2. Materials and methods

B4C powder(99.5%, 1-10μm)and TiH2powder(99%,?325 mesh) were purchased from Shanghai Aladdin Biochemical Technology, Shanghai, China and used as the starting materials.[23]Figure 1 shows the microstructures of these two raw materials. Powders composed of B4C and TiH2(0,5,10,15,20 wt%)were mixed for more than 30 min 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 Fig.2. By using a two-stage heating process,TiH2[24-26]was first completely decomposed at 800?C for 10 min and then heated to 1400?C,1500?C,1600?C,1700?C,and 1800?C for 20 min,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.

Fig.2. The procedure of sample sintering.

After polishing the sintered B4C-TiB2composite 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) using Cu-Kα radiation (λ = 1.5406 ?A) and scanning electron microscopy (SEM; JSM-IT200(A), Japan).The elemental analysis of the B4C-TiB2composite ceramics was obtained by energy dispersive spectroscopy (EDS; JSMIT200(A), Japan). The hardness of the B4C-TiB2composite ceramics was measured by a Vickers hardness tester (KB5-BVZ, Germany) with an applied load of 0.98-49 N for 10 s on the polished surface. The electrical performance was measured by a Hall effect measurement system (ECOPIA/HMS-5500, Korea). The shear and longitudinal velocities were measured by an ultrasound measurement system (OWON,5072PR,China)to calculate Young’s modulus,and then fracture toughness was calculated based on the indentation crack length.

Fig.3. The temperature and displacement change at 1800 ?C with SPS.

3. Results and discussion

3.1. Density and phase compositions

The densities of B4C+x wt%TiH2(x=0,5,10,15,20)sintered at 1400?C,1500?C,1600?C,1700?C,and 1800?C are shown in Table 1. At 1900?C sintering, the sample began to melt out of the sintering range,and the mechanical and electrical properties of B4C+TiB2composite ceramics were the best when sintering at 1800?C. On this basis, 0-20 wt%TiH2were added to further optimize its performance.

Table 1. Density of monolithic B4C sintered from 1400 ?C to 1800 ?C.

The results showed that the pure B4C ceramic reached complete densification at 1800?C (the theoretical density of B4C is 2.51 g·cm?3). Figure 4 shows the density of B4C ceramics sintered with different TiH2content at 1800?C,[27]with increasing TiH2content, the density of the composite ceramic reached a maximum at 20 wt% TiH2. Both TiB2and C can reduce the sintering temperature and inhibit the grain growth. Because TiB2has a relatively low crystalline boundary diffusion coefficient, which promotes slow densification,[28]graphite(C)has a binding effect on the grain boundary, which can enhance grain bounding diffusing and fast densification. Therefore,the B4C-TiB2composite ceramics are densified well.

Fig.4. Density of B4C specimens as a function of (0-20) wt% TiH2 content at 1800 ?C.

Figure 5 shows the XRD analysis of the B4C-TiB2composite ceramics doped with different TiH2content and sintered at 1800?C for 20 min. With increasing TiH2content, the intensity of the TiB2diffraction peaks (around 44?) continued to increase and reached a maximum with 20 wt% TiH2.[29]Due to the decomposition of TiH2powder at 620?C via the process shown in Eq. (1), the temperature was maintained at 800?C for 10 min to ensure the complete formation of Ti.B4C is divided into B and C through the process shown in Eq.(2),the presence of element C in the XRD pattern also confirms this statement. Ti and B form TiB2through the process shown in Eq. (3). The results indicated that an appropriate ratio of B4C and TiH2could be completely converted into B4C-TiB2ceramics through the process shown in Eq.(4)after sintering by SPS in a high vacuum environment. In this experiment,TiB2was generated by the in situ reaction of B4C and TiH2,which can be described by the following three equations:

The overall reaction can be summarized as follows:

Fig.5. XRD patterns of the synthesized B4C+x wt% TiH2 (x=0, 5,10,15,20)composite ceramics.

Fig.6. Polished surfaces of B4C ceramics sintered with different amount of TiH2 at 1800 ?C.(a)B4C-0 wt%TiH2;(b)B4C-5 wt%TiH2;(c)B4C-10 wt%TiH2;(d)B4C-15 wt%TiH2;(e)B4C-20 wt%TiH2. (f)The enlarged view of B4C-20 wt%TiH2,the blue(A)and red(B)squares are EDS regions.

Figures 6(a)-6(f) show the images of the microstructure of B4C-TiB2composite ceramics containing different TiH2content. The content of second-phase TiB2increased with increasing TiH2content. It was supposed that Ti originally existed at the brighter areas, and then the melted Ti reacted with B to form TiB2in situ during sintering. Grain growth was not obvious in the B4C matrix as it was inhibited by the reaction of second-phase TiB2. When the content of TiH2was increased to 20 wt%, large-sized TiB2grains of about 3-4μm appeared with inhomogeneous distribution in the microstructure,as shown in Fig.6(e),these could aggregate into 30-40 μm grains. The reaction between Ti and B is highly exothermic in behavior and the heat generated helps to accelerate the formation of TiB2readily.The primary TiB2particles on the surface of B4C are appreciably free and movable and because of boron diffusion across boundary layer,TiB2particles grow with the primary ones forming agglomerates.[30]

Figure 7 shows the results of EDS of part A in Fig.6(f).Figure 7(a)is an enlarged image of A,and the elemental content and distribution of A are shown in Figs.7(b)-7(f). XRD results combined with EDS spectra confirmed the phase distribution and that the light grey phase[31]was TiB2,which has excellent electrical conductivity.

Figure 8 shows the EDS analysis of part B in Fig.6(f).Figure 8(a)is an enlarged image of B,and the elemental content and distribution of B are shown in Figs.8(b)-8(f). XRD results combined with EDS spectra confirmed that the phase distribution of the matrix was B4C,which has poor electrical conductivity.

Fig.7. The related EDS spectra of blue A in Fig.6(f)sintered with 20 wt%of TiH2 at 1800 ?C.(a)EDS test area image A;(b)titanium-K,boron-K;(c)boron-K;(d)titanium-K;(e)EDS spectra of image A;(f)element content of image A.

Fig.8. The related EDS spectra of red B in Fig.6 (f) sintered with 20 wt% of TiH2 at 1800 ?C. (a) EDS test area image B; (b) boron-K,carbon-K;(c)boron-K;(d)carbon-K;(e)EDS spectra of image B;(f)element content of image B.

3.2. Hardness and fracture toughness

The mechanical properties of B4C-TiB2were measured on the polished surface, as displayed in Fig.9.[32,33]The hardness-load curve of B4C-20 wt% TiH2is shown in Fig.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 HVvalue of ～31.4 GPa was obtained when the applied load exceeded 49 N. It may be because the hardness of TiB2generated by the reaction is 34 GPa, and the hardness of C produced by the reaction is poor, whose hardness is 1-2 GPa,[34,35]and the hardness of B4C is 31 GPa, which is between those of TiB2and C. However, the results indicated that due to very little C produced by the reaction, the hardness of B4C-TiB2composite ceramics did not decrease under the combined action of TiB2and C. Vickers hardness and fracture toughness results for the B4C-TiB2composite ceramics are shown in Fig.9(b). When doped with 20 wt% TiH2, the hardness of the sample was stable and the fracture toughness was a maximum of 8.5 MPa·m1/2. TiB2is a high-toughness material,and C also can be used as an additive to produce high-toughness ceramic,[36-39]so the fracture toughness of B4C-TiB2composite ceramics is improved.

Fig.9. Comparison of mechanical properties. (a)The measured Vickers hardness of B4C-20 wt% TiH2 up to 49 N load at 1800 ?C. (b)Effect of TiH2 addition on the hardness and fracture toughness of B4C at 1800 ?C.

To calculate the fracture toughness, an oscilloscope was used to measure the shear and longitudinal wave sound velocities and then the Poisson ratioμ was calculated using Eq.(5).Data for the Young’s modulus calculation was then obtained using Eq.(6). Finally,the fracture toughness of the B4C-TiB2composite ceramics was calculated according to Eq.(7).

The crack trace of the polished surface of the 20 wt%TiH2composite ceramic is shown in Fig.10. Crack bridging,crack deflection, crack bending, and crack forking occurred when a force was applied,and these cracks acted as tougheners for consuming energy. Crack bridging is helpful to improve fracture toughness.[40]The following three formulas were applied to calculate the fracture toughness:

The factors affecting fracture toughness are density,hardness, sound velocity, and so on. Under the condition of constant hardness, B4C-TiB2composite ceramic had the maximum density at 20 wt%TiH2doping,which can improve the fracture toughness. At the same time,grain size also affected the sound velocity of the B4C-TiB2composite ceramic. Under the action of various factors,the fracture toughness of the B4C-TiB2composite ceramic reached 8.7 MPa·m1/2.

Fig.10. SEM micrographs of the polished surfaces cracks of the B4C-TiB2 composite ceramics with 20 wt%TiH2.

3.3. Electrical conductivity

The electrical conductivity of the B4C+x wt%TiH2(x=0,5,10,15,20)composite ceramics at different sintering temperatures(1400?C,1500?C,1600?C,1700?C,and 1800?C)is shown in Fig.11. The electrical conductivity could be enhanced by increasing the sintering temperature and increasing the content of TiH2.When doped with 5 wt%or 10 wt%TiH2,the electrical conductivity of the B4C-TiB2composite ceramics suddenly increased. When TiH2was further increased to 15 wt%or 20 wt%,the electrical conductivity of the B4C-TiB2composite ceramics was two orders of magnitude higher than that of pure B4C. At room temperature, a maximum electrical conductivity of 114.9 S·cm?1was achieved with 20 wt%TiH2. Graphite is one type of carbon-based filler, due to being layered structure, its electrical conductivity is 104S/cm at room temperature.[41]Ti reacted with B to form TiB2, the layered structure of boron atoms similar to graphite and the outer electrons of Ti determine the outstanding electrical conductivity of TiB2,they can be used for improving the electrical conductivity of composite ceramics.

Fig.11. The electrical conductivity of B4C-(0,5,10,15,20)wt%TiH2 composite ceramics at different sintering temperature.

4. Conclusion

Completely densified B4C-TiB2composite ceramics with high conductivity, high strength, and high hardness properties were successfully obtained by the spark plasma sintering method starting from raw mixtures of B4C and TiH2powders.Using TiH2as a sintering aid improved the density of the B4C ceramics. During the sintering process, B4C reacted with Ti to form TiB2and a small amount of C, which successfully inhibited the growth of B4C and improved the electrical conductivity,Young modulus,and fracture toughness.