Addition of carbon fibers into B4C infiltrated with molten silicon
Introduction
Refractory carbides are important industrial materials that have a number of attractive chemical and physical properties [1]. Among the carbides, boron carbide (B4C) is of particular interest. Currently it is extensively studied as a promising material for structural applications when low density, outstanding hardness, high stiffness, good chemical and mechanical properties are required [2]. It is used for armor plating, wear-resistant parts, neutron shielding, etc. [1], [2]. The main obstacles that restrict its wide application are high sintering temperature and relatively low fracture toughness. Due to strong covalent nature and low vacancy diffusion, fully dense B4C materials are obtained at high temperatures. Conventional pressureless sintering was demonstrated to work, but, for a higher efficiency of consolidation, effective are pressure-assisted methods such as hot pressing or spark plasma sintering [3]. Sintering aids and ultrafine raw powders are used to enhance the sinterability of B4C [3]. Coarse B4C powders are cheaper than fine powders, but the drawback is that they cannot be effectively densified by general sintering techniques. To overcome this issue, reaction-bonded B4C (RBBC) was developed [4].
The RBBC composites are fabricated by infiltration at 1450–1600 °C of molten silicon into a porous preform of B4C [5]. In the B4C preform, usually some amount of free carbon is added because the infiltration process is accompanied by chemical reactions which promote formation of B12(C,Si,B)3 and β-SiC phases. In the samples are also present unreacted B4C, C, and Si [6]. The main advantage of RBBC materials over sintered B4C is related to their capability to be produced into complex shapes and large products with (near-)zero shrinkage. Manufacturing costs are lower for RBBC when compared with other technologies and this is important for industrial production. On the other hand, RBBC materials show lower hardness (13–27 GPa) and flexural strength (250–420 MPa) [7], [8], [9], [10] than sintered B4C [11], [12], [13], [14].
The effect of starting materials and technological parameters on the microstructure and mechanical properties of RBBC composites has been studied by many researchers [6], [7], [10]. It has been shown that properties of RBBC composites can be controlled by the ratio of formed phases that depends among different parameters on the B4C particle size [8], [15], carbon and silicon content [16], [17], carbon source type [6], and different additives [9], [18]. Recently Zhou et al. [17] used polycarbosilane as an additional source of carbon and silicon to tune the properties of RBBC composites. It was found that mechanical properties drastically decreased due to residual Si agglomeration, when the content of polycarbosilane is above 5 wt%. The decrease of flexural strength with the increase of Si content was also observed by Chhillar et al. [16] and this was explained considering the increase of the critical flaw size. It is inferred that the amount of residual Si should be reduced and this can be done e.g. by addition of free carbon. Addition of more carbon will also modify the morphology of resulting β-SiC phase from a plate-like shape to a polygonal one [6].
In the present study, carbon fibers were added to B4C powder to tailor the properties of RBBC composites. Carbon fibers are commonly used in strengthening applications [19], but to the best of the authors’ knowledge they were not introduced in the RBBC materials. Carbon fibers play the role of a carbon source, but through their elongated nature they are also expected to influence processes during infiltration leading to novel microstructures with useful mechanical properties. Assessment of the carbon fiber (Cf) addition amount to RBBC materials shows a maximum increase of the bending strength from 355 ± 19 MPa up to 510 ± 27 MPa for the unadded and 10 wt% Cf-added samples, respectively.
Section snippets
Starting materials and samples preparation
Boron carbide powder (95 wt% B4C, 3 wt% C, 0.8 wt% Fe, 0.5 wt% Si, 0.7 wt% B2O3, JSC Zaporozhabraziv, Ukraine) with a particle size in the range of 1–50 μm and carbon fibers (density: 1.76 g/cm3, HTA 40, Toho Tenax Europe GmbH, Germany) with a diameter of 7 ± 1 µm were used as the starting materials. Fibers were cut into 5–8 mm long segments and thoroughly mixed with B4C powder. The mixture was homogenized by ball-milling for 30 min in ethanol using a plastic milling jar with B4C balls. The
Phase composition and reactions
In Fig. 2 and Table 1 are shown the XRD results of silicon infiltrated B4C/Cf composites. The XRD analysis indicates that all our samples contain B13C2 (PDF 01-074-4876), Si (PDF 27-1402), and β-SiC (PDF 1011031) phases as for a typical RBBC composite, but there are also notable differences. One is that the B12(B,C,Si)3 phase [6], is observed only in the samples without Cf (sample BC0) and with low Cf content (sample BC5). Thus, a higher amount Cf does not promote the formation of B12(B,C,Si)3
Conclusion
Carbon fibers 0–20 wt% were added to Si-infiltrated B4C. A higher amount of Cf promotes formation of more SiC at the expense of free-Si. SiC also develops a particular morphology. Namely, carbon fibers partially react with Si to form SiC-C fibers. The newly formed composite core (C)-shell (SiC) fibers were theoretically estimated to promote enhancement of the bending strength if their minimum weight percent is at least 13 wt%. Indeed, the experimentally measured bending strength maximizes to a
Acknowledgement
Ukrainian team acknowledges projects Nos. 0117U006427, 0117U004301 supported by Ministry of Education and Science of Ukraine. This article contains some of the results obtained under President's of Ukraine grant for competitive projects (F75/155-2018), State Fund for Fundamental Research. PB acknowledges MEC-UEFISCDI project POC 37_697 no. 28/01.09.2016 REBMAT, Romania.
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