Fabrication and Mechanical Properties of Boron Nitride Nanotube Reinforced Boron Carbide Ceramics

A series of BNNTs/B 4 C composite ceramic were prepared by the spark plasma sintering (SPS) technology using boron carbide (B 4 C) powders as the matrix and boron nitride nanotubes (BNNTs) as the toughening phase. The XRD, SEM, TEM and HR-TEM were used to characterize the B 4 C samples. The inuence of sintering temperature, BNNTs content and matrix particle size on the microstructures and mechanical properties of B 4 C composite ceramics, as well the toughening mechanism were investigated in detail. The experimental results showed that changing the particle size of the powder, increasing the sintering temperature and adding BNNTs could signicantly improve the mechanical properties of the material. The ceramic samples obtained by adding 5wt.% BNNTs content sintered at 1750 ℃ displayed the best mechanical properties. Its relative density, microhardness and fracture toughness respectively were 99.41%, 32.68 GPa and 6.87 Mpa·m 1/2 , respectively. In particular, the fracture toughness value of the BNNTs/B 4 C composite ceramic was 54.59% higher than that of B 4 C ceramics without BNNTs.


Introduction
Boron carbide (B 12 C 3 or B 4 C) is a kind of oxide light solid material, its single hexahedron diamond crystal cell contains 15 atoms (composition B 11 C icosahedron and a linear C-B-C three atomic chain, both by covalent bond connection, form a stable structure) [1,2]. The highly stable covalent bond between B and C atoms in B 4 C and its special crystal structure make B 4 C have many excellent physical and chemical properties. For example, low density (2.52 g·cm − 3 ), high hardness (> 30 Gpa, second only to diamond and cubic boron nitride), high melting point (2450℃), high temperature wear resistance (1400℃ to 0.05), low thermal expansion coe cient (about 5.73 × 10 − 6 /℃) and the thermoelectric performance, thermal neutron absorption ability, etc [3][4][5][6]. These properties make B 4 C have broad application prospects in high-performance engineering ceramics, cutting tools, composite armor, body armor and other national defense and military industries. It is an important strategic material material in today's national economy and national defense construction [7,8]. However, the high covalent bonds of B 4 C content(> 90%) and high melting point, resulting in B 4 C ceramic sintering di culties and poor toughness (2)(3)(4) Mpa·m 1/2 ),these make the mechanical properties unable to be further improved, and greatly limits the application range of B 4 C ceramics as structural ceramics [9].
At present, particle toughening and whisker ( ber) toughening are effective methods to improve fracture toughness of ceramics. Baris Yavas et al used SPS technology to prepare CNTs/B 4 C ceramic composites with carbon nanotubes (CNTs) as toughening phase [10]. The results showed that adding CNTs or increasing the heating rate can improve the fracture toughness of B 4 C ceramics. Recent years studies have found that BNNTs have better comprehensive mechanical properties, chemical stability and oxidation resistance than CNTs, making them an ideal toughening material [11,12]. In 2018, Li used hotpressing sintering technology to study silicon nitride (Si 3 N 4 ) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated. The results showed that BNNTs can enhance the fracture toughness of Si 3 N 4 dramatically, which increases from 7.2 Mpa·m 1/2 (no BNNTs) to 10.4 Mpa·m 1/2 (0.8 wt.% BNNTs) [13]. Zeng Xiaojun et al studied the microstructure and mechanical properties of BNNTs/B 4 C composite ceramics by hot-pressing sintering process, and the results showed that the bending strength and fracture toughness of the composite with 1.5wt.% BNNTs increased by 28% and 31.5%, respectively [14,15].
In this paper, the high-activity B 4 C micro-nano powders was used as the matrix and the BNNTs/B 4 C composite ceramics were prepared by SPS low-temperature rapid sintering technology. The in uence of the sintering temperature, BNNTs content and matrix particle size on the microstructures and mechanical properties of B 4 C composite ceramics, as well the toughening mechanism were also investigated in detail.

Experimental Reagents
The commercially available micron-sized B 4 C powders were purchased from Mudanjiang Boron Carbide Co., PR China (Particle size of about 3.5 µm, purity > 98%, Fig. 1(a)), homemade B 4 C nano-powders Finally, the obtained mixture was subjected to pickling (removal of impurities) water washing, ethanol washing, suction ltration and vacuum drying and storage.
SPS sintering of the mixture: taking the mixture out of the vacuum drying oven, weighing an appropriate amount of the mixture and grinding for 15 min, putting the ground mixture into a graphite mold with a diameter of 15 mm for SPS sintering. The sintering temperature was 1700 °C, the heating rate was 200 °C/min, the sintering pressure was 30 MPa, and the holding time was 5 min. After the sintering, the graphite layer on the surface of the sintered sample was removed, then polished, ultrasonically cleaned and dried to perform related test and characterization. The experimental technical parameters of the samples are shown in Table 1

Characterization
The relative density of the sintered sample was measured by the Archimedes method. The fracture morphology of ceramic samples with different ceramic matrix and different contents of BNNTs is shown in Fig. 3. It could be seen from Fig. 3 that all the sample sections showed low porosity and high density. It could be seen from Fig. 3(e) that there were some pores of about 0.5 µm and a small amount of white impurity particles (shown in the white wire frame) on the cross-section of C-B 4 C ceramic.
It was also found that the section of the C-B 4 C sample was at, and it was speculated that the main fracture mode was transgranular fracture. When adding 5wt.% BNNTs, the C-B 4 C-5wt.% BNNTs sample appeared some intergranular fractures ( Fig. 3(g)), indicating that the addition of a certain amount of  h)). This kind of agglomeration was equivalent to micron-sized defects, and this loose agglomerate will also produce more void defects at the junction of the nanotube and the matrix, which will hinder the densi cation of the matrix [14,19].
It can be seen from Fig. 4(a) that when the content of BNNTs was the same, the B 4 C-based ceramic composite material with H-B 4 C nano-powders as the matrix had a higher relative density than the B 4 Cbased ceramic composite material with C-B 4 C powders as the matrix. The reason may be that the particles of H-B 4 C nano-powders are much smaller than micron-sized C-B 4 C powders particles, under the same conditions, the smaller the particle size of the raw material, the more conducive to obtaining highdensity ceramic samples. When C-B 4 C was used as the matrix, the relative density of the B 4 C-BNNTs ceramic composite material increased accordingly with the increase of BNNTs content. The reason is that because BNNTs have a relatively small particle size, in the ceramic sintering process, BNNTs are easy to ll the gaps between B 4 C micron grains. However, when H-B 4 C was used as the matrix, the relative density of B 4 C-BNNTs ceramic composite material decreased slightly with the increase of BNNTs content [20], and the relative density of the four was close to the theoretical density.
It can be seen from Figs. 4(b) that under the same conditions, H-B 4 C as a matrix had a higher hardness than C-B 4 C. The reason may be that the particle size of H-B 4 C powders are nanometer, while the particle size of C-B 4 C powders are micrometer. Under the same conditions, the smaller the particle size of the raw material, the more conducive it is to obtain ceramic samples with high density and high hardness. When the matrix was the same, with the increase of BNNTs content, the hardness of the ceramic composite material gradually decreased. The reason may be as the content of BNNTs continued to increase, the possibility of nanotube agglomeration became higher, and the defects and matrix pores introduced by agglomerations will increase, which will reduced the continuity and density of the ceramic matrix, which will eventually lead to the hardness of the ceramic material decreases [21].
It can be seen from Fig. 4(c) that whether the ceramic matrix is C-B 4 C or H-B 4 C, as the content of BNNTs increased, the fracture toughness of the composite ceramics rst increased and then decreased. When the content of BNNTs was 5wt.%, the fracture toughness of C-B 4 C-5wt.%BNNTs and H-B 4 C-5wt.%BNNTs ceramics were both the best, respectively 4.31 Mpa·m 1/2 and 5.92 Mpa·m 1/2 . The results showed that adding an appropriate amount of BNNTs could effectively improve the fracture toughness of B 4 C ceramics. The reason is that BNNTs are uniformly distributed on the grain boundaries and grains of the B 4 C matrix. During the crack propagation process, the excellent mechanical properties of the nanotubes can effectively prevent the further propagation of the crack, thereby improving the fracture toughness of the ceramic [14,[22][23][24]. However, with the further increase of the BNNTs content, the agglomeration of the nanotubes continued to increase, which caused the pores around the nanotubes to increase, which easily induced crack propagation and reduced the toughness of the composite material. Thus, the optimum BNNTs content for B 4 C composite ceramics in our study is determined to be 5wt.%. In addition, it was found that H-B 4 C has better mechanical properties than C-B 4 C under the same conditions.

The in uence of sintering temperature
The mechanical properties of superhard structure ceramics are directly determined by its microstructure, which is affected by the sintering temperature. Figure 5 illustrates the SEM patterns of H-B 4 C-5wt.% BNNTs samples section at different temperatures. It can be seen from Fig. 5(a) that at low temperatures, there existed more pores and pits in sintered samples, and the powder particles failed to combine with each other to form obvious grain boundaries. With the increase of the sintering temperature, the sample grains tended to fuse, and the grain size increased gradually, the pores reduced and closed, and the density increased. When the sintering temperature reached 1750 °C, the sample was almost completely sintered. The mainly reason is as the sintering temperature increases, the process of surface diffusion and interface diffusion mass transfer speeds up, the density increases, and the pores are continuously eliminated. When the sintering temperature was 1800℃, the sample section was uneven, which may be due to the high temperature, the grain boundary migration rate was greater than the pore migration rate, the grain size increased signi cantly, and small closed pores were formed inside the grains [15].
It can be seen from Fig. 6 that with the increase of the sintering temperature, the change trend of the microhardness of the H-B 4 C-5 wt% BNNTs ceramic sample was the same as the change trend of the relative density of the B 4 C ceramic, indicating that the particle rearrangement of the B 4 C ceramic mixed with BNNTs was enhanced in the sintering process. With the increase of the sintering temperature, the driving force of B 4 C sintering continued to increase, and the continuous growth of crystal grains increased the sintering densi cation of ceramics, which made the microhardness increase, gradually. When the sintering temperature was 1750℃, the microhardness and relative density of the sintered sample were the largest, which are 99.14% and 32.68 GPa, respectively. As the temperature continued to rise, the particle size grew rapidly, and more pore defects were produced, which caused a decrease in the density and hardness of the composite material.
It can be seen from Fig. 6(b) that the trend of the fracture toughness of the composite material with the sintering temperature was similar to that of the microhardness. When the temperature was lower than 1750℃, the fracture toughness of the ceramic continued to increase as the temperature rose. When temperature reached 1750 °C, the fracture toughness was the largest at 6.87 Mpa·m 1/2 . Thus, the optimum sintering temperature for H-B 4 C-5wt.% ceramics in our study is determined to be 1750℃.
The reason is that when the sintering temperature increases, the bonding strength of the heterogeneous interface between BNNTs and the B 4 C ceramic matrix increases. When the crack extends to the surface of the nanotube, the crack propagation path or crack growth energy is increased through crack de ection, bridging and pull-out effects. As the temperature continued to rise, the fracture toughness of ceramic materials decreased signi cantly. The reason is the B 4 C grains grow rapidly, and the toughening effect of BNNTs is di cult to offset the abnormal growth of B 4 C grains and the abnormal interface strength reduction, which leads to a signi cant reduction in the fracture toughness of the composite material [20]. Figure 7 shows the microstructure of the BNNTs/B 4 C composites. From Fig. 7(a), we can nd that there is a relatively long groove (marked by the frame) formed between the crystal planes due to the pulling out of the nanotube. In addition, the nanotubes marked by the arrows in the Figure shown a clear hollow structure and the phenomenon of cracking occur, which was caused by the force during the cracking process. It could be inferred from this phenomenon that the mode of strengthening and toughening the B 4 C ceramic was the ber pull-out effect. In Fig. 7(c), BNNTs was connected between the two grain boundaries. It was speculated that the mechanism of strengthening and toughening the ceramic was the ber bridging mechanism.