Determining the fracture resistance of B4C-NanoSiB6 nanocomposite by Vickers indentation method and exploring its mechanical properties

https://doi.org/10.1016/j.ijrmhm.2017.07.009Get rights and content

Highlights

  • Influences of adding nanoSiB6 reinforcements with different wt. % on mechanical properties and sinterability of B4C.

  • The many equations have been used for the estimation ofKIFR in nanocomposites.

Abstract

In this research, the effects of NanoSilicon hexaboride (NanoSiB6) additive, as a sintering aid, on mechanical properties and the sinterability of Boron Carbide (B4C) have been investigated. In addition, the results of several equations used for determining the Indentation fracture resistance (KIFR) by Vickers indentation test have been evaluated. For this purpose, 0, 2, 4, 6 and 8 wt% NanoSiB6 additive have been added to B4C in order to improve its sinterability at temperatures 2050, 2150 and 2250 °C. The findings indicate that by adding NanoSiB6, up to 4 wt%, to B4C, its properties such as relative density, Young's modulus, microhardness and KIFR improve as the sintering temperature rises; however, these properties diminish with the further increase of the mentioned sintering aid. Also, a comparison between KIFR values shows the closeness of the results obtained by different equations and the satisfactory accuracy of the equation for determining KIFR by crack area method compared to the results of other equations; with a difference of < 20% between the two results.

Introduction

Boron Carbide (B4C) has attracted a great deal of attention for its extraordinary hardness, low density, high melting point, good thermal resistance, good resistance to abrasion, providing a large surface area for neutron adsorption and also for its chemical stability [1], [2], [3]. After diamond and cubic boron nitride (CBN), B4C has been ranked as the third hardest material known [4], [5]. B4C ceramics have been extensively used in various types of abrasives, boride compounds, chemical chambers made for acids and bases, and in thermocouples [6], [7], [8].

The existence of covalent bonds between carbon and boron atoms in B4C has made it a relatively neutral material. This characteristic has caused a limitation in the sinterability of this ceramic [6], [7], [8], [9], [10]. Moreover, B4C has a low fracture toughness; and these factors restrict the use of this material [11]. One way of improving the sintering behavior of B4C and enhancing its mechanical properties is to add a secondary phase to it as a sintering aid. To this end, various sintering aids such as ZrO2 [6], SiC [12], HfB2 [13], ZrB2 [14], TiB2 [15], WC [16] and C [17] have been used, with beneficial effects on the sinterability and physical and mechanical properties of boron carbide.

NanoSiB6 is very similar to B4C in terms of microstructure and physical and mechanical properties. The Vickers hardness of this ceramic is about 22 GPa. This material enjoys a high chemical resistance and low density and it is also proclaimed as a suitable compound for impact-resistant material [18], [19]. Since the melting temperature of SiB6 is 1850 °C, another important advantage of this additive is its lower melting point relative to B4C [20]. By adding NanoSiB6 to B4C, as a sintering aid, and using the Liquid Phase Sintering method at a temperature higher than the melting point of NanoSiB6, this material can play a key role in improving the sinterability of boron carbide. These advantages make NanoSiB6 a suitable sintering aid for B4C.

In many ceramics, the amount of fracture toughness (KIC) can be estimated by measuring the indentation fracture resistance (KIFR). One of the indenters by means of which one can determine the amount of fracture resistance is Vickers indentation. Generally, two models of fractures called Palmqvist carck system and radial/median/half penny crack system are hypothesized in Vickers indentation fracture (VIF) method [21]. Fig. 1 shows these two models and their differences. In loadings below the indenter, Palmqvist crack system model is used and in loadings over the indenter, median crack system model is formed [22], [23].

In this research, the effects of adding various amounts of NanoSiB6 sintering aid (0, 2, 4, 6 and 8 wt% NanoSiB6) to B4C on the sintering behavior and mechanical properties of boron carbide such as relative density, hardness, Young's modulus and especially fracture toughness at sintering temperatures of 2050, 2150 and 2250 °C have been investigated. Various equations developed for the Vickers indentation test method have been used to get KIFR values; the results of these equations have been compared, and the scattering in results of the equation used for determining KIFR by crack trace area method has been evaluated.

Section snippets

Materials and experimental procedures

The raw materials used to make the considered nanocomposite include Micrometre-sized B4C powder with a purity above 96% (resulting from the research work of Baharvandi et al. [24]) and also NanoSiB6 powder with a purity above 99% (Kojundo Chemical Lab Co., Ltd., Japan). The chemical compositions and the XRD patterns of the mentioned powders can be observed in Table 1 and Fig. 2, respectively. Fig. 3 shows the microstructural images of B4C and NanoSiB6 powders obtained by a Scanning Electron

Relative density, microhardness and elastic modulus values of specimens

The relative density of a specimen is obtained by knowing its theoretical density. Considering the theoretical density of B4C (ρB4C = 2.52 g/cm3) and the theoretical density of SiB6 (ρSiB6 = 2.43 g/cm3 [20]), the theoretical density of composite samples (ρcom) can be obtained from Eq. (18).ρcom.=VB4C×ρB4C+VSiB6×ρSiB6

In the above equation, VB4C is the percent volume of boron carbide and VSiB6 is the percent volume of silicon hexaboride in various composite specimens. Fig. 4 shows the relative density

Microstructures

Fig. 9 shows the SEM microstructure images of some specimens. A close examination of Fig. 9-a and -b reveals that the specimen sintered at 2050 °C in the absence of a sintering aid has a great deal of porosity; and although these pores have been reduced in number by raising the sintering temperature up to 2250 °C, the amount of porosity is still high. SEM images in Fig. 9-c and -d indicate a considerable reduction of porosity in B4C-4 wt% NanoSiB6 composite specimens and show that the highest

Conclusions

The important issues investigated in this research were the effects of increasing the sintering temperature and adding various portions of sintering aid NanoSiB6 on the mechanical properties of B4C. Also, the scattering in results of an equation for determining KIFR was evaluated by examining the indentation zone and crack marks. The following conclusions were reached:

  • By increasing the sintering temperature from 2050 to 2250 °C, the sintering process improves in all the examined specimens and

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