Effect of sintering temperature on microstructure and mechanical properties of boron nitride whisker reinforced fused silica composites

Journal Pre-proof Effect of sintering temperature on microstructure and mechanical properties of boron nitride whisker reinforced fused silica composites Wenjiu Duan, Zhihua Yang, Delong Cai, Junwu Zhang, Bo Niu, Dechang Jia, Yu Zhou PII:

S0272-8842(19)33131-1

DOI:

https://doi.org/10.1016/j.ceramint.2019.10.257

Reference:

CERI 23325

To appear in:

Ceramics International

Received Date: 1 July 2019 Revised Date:

14 October 2019

Accepted Date: 27 October 2019

Please cite this article as: W. Duan, Z. Yang, D. Cai, J. Zhang, B. Niu, D. Jia, Y. Zhou, Effect of sintering temperature on microstructure and mechanical properties of boron nitride whisker reinforced fused silica composites, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.257. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Effect of sintering temperature on microstructure and mechanical properties of boron nitride whisker reinforced fused silica composites Wenjiu Duana,b, Zhihua Yanga,b,c, Delong Caia,b,*, Junwu Zhangd, Bo Niua,b, Dechang Jiaa,b,c,*, Yu Zhoua,b a

Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 15000, China b Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin, 150001, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China d China Academy of Launch Vehicle Technology, Beijing, 100076, China

Abstract: Novel BN whisker reinforced SiO2 matrix composites were achieved via hot-pressing at various temperatures, oriented to an enhanced mechanical properties of fused silica. Near full dense BNW/SiO2 composite sintered at 1350

presents

optimal mechanical properties with bending strength of 124.8±3.4 MPa and fracture toughness of 1.76±0.32 MPa·m1/2. The improved bending strength is mainly ascribed to the strengthening effects of BNW and crystallized nanosized α-cristobalite. Indentation crack paths indicate that the crack deflection and interface debonding are the main toughening mechanisms when BNW/SiO2 composite is dense. The desirable dielectric properties (dielectric constant of 3.93 and loss tangent of 0.46×10-3), the tiny mean thermal expansion coefficient (1.40×10-6 K-1) and thermal diffusivity (1.16 mm2/s) suggest that the BNW/SiO2 composite is definitely a promising high temperature electromagnetic wave transparent material.

Keywords: Boron nitride whisker; Fused silica; Sintering temperature;

*

Corresponding author: Tel.: +86 451 86418792; E-mail: [email protected] (Delong Cai); [email protected] (Dechang Jia). 1

Whisker-matrix interface; Strengthening and toughening mechanisms

1. Introduction Fused silica has attracted widespread attention as essential candidate material for advanced thermal protection applications, especially as an irreplaceable radome material, due to its intrinsic thermal stability, superior thermal shock resistance and dielectric properties [1, 2]. However, the relatively poor mechanical strength and high micro-crack susceptibility of monolithic fused silica limit its widespread applications [3]. To overcome the drawbacks, considerable fused silica matrix composites have been developed by adding various reinforcements, such as Si3N4p, BNP, AlNp, SiO2f, Si3N4f, and BNf [4-11]. Unfortunately, two obstacles still exist such as fabrication of large parts with high reliability and low cost, and excellent balance between reliability and strength. Among the aforementioned reinforcements, h-BN is an attractive radar window material owing to its extremely low dielectric constant of 5.16, extremely high sublimation temperature of approximately 3000

, and good machinability.

Consequently, boron nitride nanotubes (BNNTs) and nanosheets (BNNSs) are regarded as ideal strengthening agents due to high elastic modulus and tensile strength, they improved the mechanical properties of fused silica to some extent [12, 13]. Limited improvement of the mechanical properties is mainly ascribed to the obstacle that the homodisperse of BNNTs and BNNSs with high content remain extreme difficulty. Furthermore, the large-scale synthesis of BNNTs and BNNSs restrict fabrication of large parts. To circumvent these obstacles, it is still imperative to 2

explore a novel fused silica matrix composite with excellent comprehensive performances by simple preparation technology. Adding whiskers with large aspect ratio seems to be a valid solution, because whiskers can consume fracture energy via the crack deflection, whisker bridging and whisker pull-out similar to fibers [14-16], and undergo a higher preparation temperature to fabricate a denser composite compared to fibers. Furthermore, aggregation can be avoid via increasing the diameter of whisker to reduce the specific surface area. However, to the best of our knowledge, BNW/SiO2 composite has never been reported so far. In present work, we explore initially the feasibility of producing fused silica-based composites reinforced by BNW of micron diameter via hot pressing sintering. Effect of sintering temperatures on phase composition, microstructure, mechanical and dielectric properties of the BNW/SiO2 composite are investigated. Especially, the strengthening and toughening mechanisms associated with BNW are revealed to obtain an essential relevancy between microstructure and mechanical properties.

2. Experimental details 2.1. Fabrication of BNW/SiO2 composites The flocculent BNW was synthesized by modified precursor method reported elsewhere [17]. Briefly, 0.05 mol melamine (99.5% purity, Tianda Chemical Reagent Co., Ltd, China) and 0.15 mol boric acid (99.5% purity, Tianda Chemical Reagent Co., Ltd, China) were dissolved in 95

deionized water of 400 mL. The white whisker

precursors precipitated from the solution were dried at 60

under atmosphere, and

then calcined at target temperature. Fig. 1(a) illustrates XRD pattern and inset SEM 3

image of BNW, low crystallinity is speculated from the broad (002), (100), (101) and (110) diffraction peaks [18], especially, the unsolved (100) and (101) peaks reveal its turbostratic feature [19]. The statistical diameter of BNW is approximately 4 µm, and the aspect ratio is more than 20, which is consistent with inset SEM image of BNW, suggesting the whisker meets requirements of large diameter and aspect ratio. Fig. 1(b) shows morphology of commercial fused silica powder (99.5% purity, d50=3.3 µm, Lianyungang Guangyu Quartz Products Co., Ltd, China). Firstly, the flocculent BNW was divided into small pieces in alcohol, then BNW suspension liquid was stirred in the ice-water bath for 2 h with ultrasonic assistance to prepare the BNW slurry, the stirring rate was about 900 rpm. Then powder mixture of BNW/SiO2 in the mass ratio of 1:19 was blended via conventional wet ball-milling processing. Subsequently, the sieved powder mixture was hot-pressed for 10 min with a pressure of 30 MPa at 1150 rate of 25

, 1250

, and 1350

in N2 atmosphere, respectively. A heating

/min was adopted and sintered samples were cooled inside the furnace to

room temperature. 2.2. Characterization of specimens The statistical aspect ratio of the BNW was quantitatively estimated by counting their diameters and lengths applying image analysis software to measure 100 whiskers (Image-Pro Plus6.0, Media Cybernetics, USA). According to GB/T 25995-2010, bulk density and apparent porosity were determined by the Archimedes method using distilled water as the medium. Phase composition was identified via X-ray diffractometer with a scanning speed of 4 °/min (XRD, D/max-γB, Ricoh, Japan). 4

Scanning electron microscopy (SEM, Helios Nano lab 600i, FEI Co., USA) was employed to characterize the crack propagation paths produced via Vickers hardness tester (HVS 30) with load of 5 kg and microstructures, the sample was dry at 60

for

24 h and then sprayed gold for 180 s for better imaging. The fine whisker-matrix interface was characterized by transmission electron microscopy (TEM, Talos F200x, FEI Co., USA) coupled with energy-dispersive X-ray spectra (EDS) using samples prepared via ion milling. Slices of 50 µm prepared by double-sided mechanical thinning and polishing were inserted into the copper rings, ion thinning instrument (GATAN 695) was used for the further thinning. The accelerating voltage of argon ion of 5 kV, the beam density of 4.5 mA and the initial sweep angle of 6 ° were adopted. When a micro-hole appeared in the center of the sample, the sweep angle and accelerating voltage were adjusted to 3 ° and 3 kV, respectively. Then the sample was further reduced to 20 µm. According to high Q cavity method, Each specimens of Φ 18 mm×1 mm were examined four times by RF impedance/material apparatus (PNA N5230A, Agilent, USA) to acquire room-temperature dielectric constant (ε) and loss tangent (tan δ) in the frequency range of 20-40 GHz. Specimens of 4 mm×4 mm×25 mm were examined in Ar atmosphere with heating rate of 5

/min by thermal

expansion instrument (NETZSCH DIL402C, Germany) to acquire thermal expansion coefficient (TEC) at RT~1000 °C. The room-temperature thermal diffusivities (α) of composites with a size of Φ 12.65 mm×2.5 mm were obtained by LFA laser instrument (NETZSCH LFA427, Germany). Average three-point flexural strength and elastic modulus were performed using 5

five rectangular samples (3 mm × 4 mm × 36 mm), the crosshead rate and span length maintained in 0.5 mm/min and 30 mm, respectively. Average fracture toughness was obtained using five beam bars (2 mm × 4 mm × 20 mm) with a notched depth of 2 mm, the crosshead rate and span length were set as 0.05 mm/min and 16 mm, respectively. 3. Results and discussion 3.1. Bulk density and mechanical properties Bulk density and apparent porosity of the BNW/SiO2 composites sintered at various temperatures are presented in Fig. 2. As indicated, apparent porosity decrease dramatically, indicating that sintering temperature has a positive effect on densification. Notably, near full dense BNW/SiO2 composite with bulk density of 2.17 g/cm3 was obtained at the sintering temperature of 1350

.

Fig. 3 presents the room-temperature mechanical properties of the BNW/SiO2 composites. The flexural strength, fracture toughness and elastic modulus are positively correlated with sintering temperature. The noteworthy increase of mechanical properties when sintering temperature elevate is probably due to the increase of relative density. Near full dense BNW/SiO2 composite sintered at 1350 exhibits optimal comprehensive mechanical properties with flexural strength of 124.8±3.4 MPa and fracture toughness of 1.76±0.32 MPa·m1/2, which are 2.1 and 3.3 times higher than those of monolithic fused silica synthesized by same preparation process, respectively [5]. A comparison of mechanical properties of the BNW/SiO2 composite with other fused silica-based ceramics is shown in Table 1. Expectedly, the 6

BNW/SiO2 composite prepared in our work shows better comprehensive mechanical properties than previous reported similar materials. The possible strengthening mechanisms will be discussed in Section 3.2. 3.2. Phase composition and microstructure The aforementioned relative density evolution of the BNW/SiO2 composites can be clearly uncovered by the surface morphology. Fig. 4(a) and Fig. 4(b) exhibit a typical porous composite, considerable pores and homogeneous dispersed BNW marked by red arrows can be clearly observed. The SiO2 particles retain original geometrical shape and pile together loosely, ascribed to metallurgical bonding under insufficient sintering temperature. When sintering temperature increase to 1250

, the BNW/SiO2

composites seem denser with a small amount of pores (Fig. 4(c and d)). The shape of whiskers remain intact when sintering temperature were 1150

and 1250

. As

expected, the best densified microstructure can be achieved when composite was sintered at 1350

(Fig. 4(e and f)). Some abnormal microstructures such as the pits

with micron diameter and undifferentiated contrast between whisker and matrix (Fig. 4(e and f)) are probably attributed to the shedding of SiO2 particles and BNW during high speed polishing, because the smooth and dense walls of pits are different from pores exhibited in Fig. 4(b). Fig. 5 shows the fractographs of the BNW/SiO2 composites sintered at different temperatures. A porous composite with conspicuous pull-out whiskers can be observed (Fig. 5(a)). As the sintering temperature increase, the fracture surfaces seem to be much denser and more flat, a near full dense BNW/SiO2 composite is shown in 7

Fig. 5(e). The densification mechanism of the BNW/SiO2 composite can be concluded that metallurgical bonding tends toward viscous flow under a higher sintering temperature, viscous flow cooperated with high press can eliminate pores rapidly and facilitate densification. It is worth noting that the strong interface bonding tends to incur undesirable BNW pull-out length, which is similar to the BNf/SiO2 composites [11]. However, a distinctive wavy fracture surface emerges owing to the curved crack propagation paths (Fig. 5(e)), which is conducive to a relatively satisfactory mechanical properties when the composite is extremely dense. The lacerated fractographs of the BNW in high magnification are rough, as shown in Fig. 5(b, d and f), rough lacerated fracture are desired owing to consuming more fracture energy. Fig. 6 depicts the XRD patterns of the BNW/SiO2 composites. As expected, the major phase is amorphous silica for all samples, while minor turbostratic BN could still be detected, signifying that the devitrification temperature of the fused silica cooperated with BNW is higher than that of previous reports [2, 20]. The higher devitrification temperature is beneficial to the better mechanical properties and thermal shock resistance, because that can maintain none crystallization of α-cristobalite, while improving the density of the matrix. None crystallization of α-cristobalite is very essential in maintaining favorable thermal shock resistance owing the thermal expansion coefficient (19.4×10-6 K-1) of α-cristobalite is much higher than that (~0.54× 10-6 K-1) of fused silica. For a thorough insight into the strengthening mechanism, the whisker-matrix interfaces of BNW/SiO2 composite were further characterized by TEM and HRTEM. 8

The clear whisker-matrix interfaces without micro crack are desirable, as shown in Fig. 7(a and c). The tight interface bonding means the excellent sinterabilities of the two materials, which is requisite to transfer load from the matrix to the reinforcing whiskers. However, the strong interface bonding is adverse to interface debonding, resulting in a short length of whisker pull-out. It is consistent with the fractographs in Fig 5(f). The bright and dark region correspond to matrix and turbostratic BNW, respectively, as identified by selective area electron diffraction (Fig. 7(A and B)). Although the XRD patterns indicate non crystallization of α-cristobalite for all samples, when the composite sintered at 1350

, the amorphous halo tends toward

the diffraction ring, some tiny equiaxed dots in dark contrast approximately 5 nm are observable, identified as nanosized α-cristobalite (Fig. 7(d)). As reported, a small quantity of nanosized α-cristobalite crystallites could promote the bending strength [21]. In addition, EDS elemental maps combined with the line scanning for Si, O, B and N are presented in Fig 7(e). The distributions of element on both sides of the interface are extremely distinct. There is no evidence indicating element diffusion and interface reaction, confirming that the BNW possess of desirable thermostability, which is indispensable for BNW as effective reinforcements. E. Y. Sun et al. pointed out that inevitable oxygen diffusion will induce the deterioration of mechanical properties, owing the fiber are tightly surrounded by oxide matrix during sintering samples in spite of inert atmosphere [22]. And similar oxidation and diffusion will take place in the silica matrix composites although heat treatment temperature is low [9]. In this 9

work, although the sintering temperature of 1350

is much higher than those heat

treatment temperatures previously mentioned [9, 22], a short sintering holding time shouldn’t be neglect for no atomic diffusion and interface reaction. From the above results, it is concluded that BNW play effective reinforcements without interface reaction, besides, moderate nanosized α-cristobalite crystallites contribute to the improvement of bending strength. 3.3. Toughening mechanisms Fig.8 depicts three typical fracture photographs of the whisker reinforced ceramics, which are whisker pull-out, whisker attached with matrix and pit left after whisker pull-out. When length of whisker is greater than critical length, the whisker will break once the stress meets the breaking strength of whisker. Subsequently, the interface debonding and whisker pull-out will occur. The whisker breaking, interface debonding and whisker pull-out consuming plenty of fracture energy can retard crack propagation [23-25]. The above mechanisms are the main toughening mechanisms of BNW reinforced fused silica ceramic. For purpose of completely understanding the toughening mechanisms of the BNW/SiO2 composites, it is indispensable to characterize Vickers’ hardness indentation crack paths. As illustrated in Fig. 9(a, b and c), all cracks propagate in a clearly zigzagged way. When the crack propagates from the matrix to the whisker, supposing that the propagation energy is pretty sufficient to overcome the obstruction created by the whisker, the crack front will move forward. A situation is the crack changes propagating direction and propagates along the weak bonding interface 10

between BN whisker and matrix, as shown in Fig. 9(a). Another situation is the crack propagates directly through the whisker owing to a strong interface bonding, as illustrated in Fig. 9(b). The crack tip stress are more than break strength of whisker and can’t be released in time by interface debonding, which is also responsible for crack propagating directly through the whisker. Crack branching and crack bridging are prevalent as a weak bonding interface exists in porous composite, as indicated in Fig. 9(a1 and a2). Dissipation of energy induced by crack branching and crack closure caused by whisker bridging are the main toughening mechanisms. We can obviously observe the crack propagation terminates when the crack tip reaches the vicinity of whisker (Fig. 9(b1)), which is called ‘crack arrest’. This exhilarating ‘crack arrest’ phenomenon can inhibit the further extension of the crack, consequently, the fracture toughness of the fused silica material will be improved, as reported by Islam et al. [26]. Fig. 9(c1) shows an interesting interlocking, where coarser fused silica particle induces the crack deflection and creates a frictional crack bridging. Moreover, the crack deflection along with interface debonding are shown in Fig. 9(b2 and c2). On the basis of above results, it can be concluded that crack deflection, crack branching and whisker bridging are the main toughing mechanisms of porous composites, crack deflection and interface debonding are the main toughing mechanisms when the composite is dense. 3.4. Dielectric properties and thermal properties As critical performance parameters for wave-transparent ceramic, ε and tan δ of the 11

BNW/SiO2 composites are presented in Fig. 10, where ε shows a positive correlation with sintering temperature, inversely, tan δ shows a negative correlation. According to the Lichtenecker's mixture law [27], ε of a random distributed three-phase composite can be estimated as the following equation: ln = Where,

· ln

+

· ln

+

· ln

denotes the dielectric constant of BNW/SiO2 composite,

(1) ,

represent the volume fractions of SiO2, BNW and pores, respectively.

and

represent the ε of fused silica (3.81), BNW (4.5) and pores (1),

and

is approximately 5%, the ε of BNW/SiO2 composite sintered in

respectively. 1350

,

is calculated as 3.84, which is well consistent with test result. The effects of

sintering temperature on ε and tan δ are mainly depend on porosity controlled by sintering temperature, as demonstrated in Fig. 2. The variations of dielectric properties with porosity can be described as follows [28, 29]. = tan Here,

(2)

= tan

+

denotes the dielectric constant of the fully dense material,

porosity. tan

and tan

(3) is the

represent loss tangent of the porous and ideal dense

composite, respectively. A and n are constants. Evidently, ε of BNW/SiO2 composite increases and tan δ reduces with the decrease of porosity. Besides dielectric properties, TEC and α are important for structural/functional ceramic material. Fig. 11 illustrates the thermal expansion curve of each sample from room temperature to 1000 °C, the annexed table gives the calculated average TECs 12

and α. The average TEC and α rise as sintering temperature elevate, and reach to tiny value of 1.40×10-6 K-1 and 1.16 mm2/s when composite was sintered at 1350

. The

low TEC and α ensure BNW/SiO2 composite suitable for high temperature where excellent thermal shock resistance and thermal insulation property are needed [30].

4. Conclusions In the present work, a novel BNW reinforced fused silica composites were prepared by means of hot pressing sintering at different sintering temperatures. The microstructures, bulk density, mechanical properties and dielectric properties are exceedingly dependent on the sintering temperature. The BNW/SiO2 composite tends to be compact with increasing hot-press temperature. When the composite was sintered at 1350

, it possesses maximum bending strength of 124.8±3.4 MPa and

fracture toughness of 1.76±0.32 MPa·m1/2. The improvement of mechanical properties suggests BNW is a credible and novel reinforcement for fused silica ceramic. The explicit whisker-matrix interface without atom mutual diffusion demonstrates that the BNW exhibits a desirable thermostability as sintering temperature reaches up to 1350

. The composite with a low dielectric constant of 3.93, a loss tangent of 0.46×

10-3, and a tiny mean TEC of 1.40×10-6 K-1 is definitely a promising candidate for high temperature electromagnetic wave transparent material.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51621091, 51225203, and 51672060) and China Postdoctoral Science Foundation Funded project (2017M621264 and 2018T110294). 13

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Figure and table captions Fig. 1. The XRD pattern with inset SEM image of (a) BNW and (b) fused silica. Fig. 2. Effects of sintering temperature on (a) bulk density and (b) apparent porosity. Fig. 3. Effects of sintering temperature on mechanical properties: (a) flexural strength; 17

(b) fracture toughness; (c) elastic modulus. Fig. 4. SEM microstructure of the BNW/SiO2 composites sintered at different temperatures: (a & b) 1150

; (c & d) 1250

; (e & f) 1350

. The red arrows

indicating BNW. Fig. 5. SEM fractographs of the BNW/SiO2 composites sintered at different temperatures: (a & b) 1150

; (c & d) 1250

; (e & f) 1350

. The red arrows

indicating BNW. Fig. 6. XRD patterns of the BNW/SiO2 composites. Fig. 7. Bright field TEM images and corresponding SAED patterns and HRTEM images of interface in the BNW/SiO2 composites sintered at different temperatures: (a-b) 1250

; (c-d) 1350

; (e) Atomic scale EDS elemental maps coupled with line

scanning of area e in (c). Fig. 8. Typical fracture photographs of the BNW/SiO2 composite sintered at 1250

:

(a) whisker pull-out; (b) whisker attached with matrix; (c) pit left after whisker pull-out. Fig. 9. SEM morphologies of Vickers’ hardness indentation crack paths of the BNW/SiO2 composites sintered at different temperatures: (a, a1 & a2) 1150 & b2) 1250

; (c, c1 & c2) 1350

; (b, b1

.

Fig. 10. Dielectric properties of the BNW/SiO2 composites: (the table lists the mean dielectric constants and loss tangents). Fig. 11. Thermal expansion curves (dL/L0) of the BNW/SiO2 composites: (the table lists the mean thermal expansion coefficients and thermal diffusivities). 18

Table 1. Comparison of mechanical properties of the BNW/SiO2 composite sintered at 1350

with other fused silica-based ceramics.

19

Table 1. Comparison of mechanical properties of the BNW/SiO2 composite sintered at 1350

with other fused silica-based ceramics.

Materials

Preparation method

Bending strength (MPa)

Fracture toughness (MPa m1/2)

Elastic modulus (GPa)

Reference

5 vol.% Si3N4/SiO2

Hot pressing

95.4

1.22

57.0

[4]

15 vol.% BNP/SiO2

Gel casting

101.5±4.3

1.57±0.04

61.3±2.4

[2]

5 wt.% BNNTS/SiO2

Hot pressing

120.5±2

1.21±0.1

-

[12]

35 vol.% BNf/SiO2

Sol-gel

51.2

1.46

23.2

[11]

0.5 wt.% BNNs/SiO2

Hot pressing

100.8±7.6

1.84±0.25

-

[13]

5 wt.% BNW/SiO2

Hot pressing

124.8±3.4

1.76±0.31

69.2±2.0

This work

1

Fig. 1. The XRD pattern with inset SEM image of (a) BNW and (b) fused silica.

Fig. 2. Effects of sintering temperature on (a) bulk density and (b) apparent porosity.

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Fig. 3. Effects of sintering temperature on mechanical properties: (a) flexural strength; (b) fracture toughness; (c) elastic modulus.

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Fig. 4. SEM microstructure of the BNW/SiO2 composites sintered at different temperatures: (a & b) 1150

; (c & d) 1250

; (e & f) 1350

indicating BNW.

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. The red arrows

Fig. 5. SEM fractographs of the BNW/SiO2 composites sintered at different temperatures: (a & b) 1150

; (c & d) 1250

; (e & f) 1350

indicating BNW.

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. The red arrows

Fig. 6. XRD patterns of the BNW/SiO2 composites.

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Fig. 7. Bright field TEM images and corresponding SAED patterns and HRTEM images of interface in the BNW/SiO2 composites sintered at different temperatures: (a-b) 1250

; (c-d) 1350

; (e) Atomic scale EDS elemental maps coupled with line scanning of area e in (c).

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Fig. 8. Typical fracture photographs of the BNW/SiO2 composite sintered at 1250 (a) whisker pull-out; (b) whisker attached with matrix; (c) pit left after whisker pull-out.

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:

Fig. 9. SEM morphologies of Vickers’ hardness indentation crack paths of the BNW/SiO2 composites sintered at different temperatures: (a, a1 & a2) 1150 & b2) 1250

; (c, c1 & c2) 1350

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.

; (b, b1

Fig. 10. Dielectric properties of the BNW/SiO2 composites: (the table lists the mean dielectric constants and loss tangents).

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Fig. 11. Thermal expansion curves (dL/L0) of the BNW/SiO2 composites: (the table lists the mean thermal expansion coefficients and thermal diffusivities).

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: