Preparation and mechanical properties of unidirectional boron nitride fibre reinforced silica matrix composites

Materials and Design 34 (2012) 401–405

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Preparation and mechanical properties of unidirectional boron nitride fibre reinforced silica matrix composites Duan Li 1,⇑, Chang-Rui Zhang 1, Bin Li 1, Feng Cao 1, Si-Qing Wang 1 State Key Laboratory of Advanced Ceramic Fibres & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, PR China

a r t i c l e

i n f o

Article history: Received 17 July 2011 Accepted 27 August 2011 Available online 1 September 2011 Keywords: A. Composites: ceramic matrix E. Mechanical G. X-ray analysis

a b s t r a c t The unidirectional BNf/SiO2 composites were prepared via sol–gel method, and the structure, composition and mechanical properties were studied. The results show that the composites consist of BN fibres and a-cristobalite matrix probably as well as the interface phases of Si3N4 and B2O3. The composites have a density of 1.70 g cm3 and an open porosity of 20.8%. The average flexural strength, elastic modulus and fracture toughness at room temperature are 51.2 MPa, 23.2 GPa and 1.46 MPa m1/2, respectively. The composites show a very plane fracture surface with practically no pulled-out fibres. The mechanical properties of BNf/SiO2 composites at 300–1000 °C are desirable, with the maximum flexural strength and residual ratio being 80.2 MPa and 156.8% at 500 °C, respectively, while it is a sharply reduced trend as for SiO2f/SiO2 composites. The high thermal stability of BN fibres and self-healing properties caused by the fused SiO2 and B2O3 enable the composites fine high-temperature mechanical properties. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction With fast pace of aerospace development, increasing attentions have been directed to high performance continuous ceramic fibre reinforced ceramic matrix composites (CMCs) for structural and functional applications. The silica fibre reinforced silica (SiO2f/ SiO2) composites are especially outstanding due to their excellent dielectric properties, good ablation resistance, fine thermal shock damage resistance and chemical stability [1–3]. However, a severe drawback to SiO2f/SiO2 composites is the significantly reduced tensile strength caused by the growth of the crystals when the composites are exposed to high temperatures [3–5]. Furthermore, the composites derived from the silica sol exhibit high porosity, which in turn leads to low density, strength and rain erosion resistance as well as the poor moistureproof ability, and this to a large extent limits their applications on the aircrafts of higher Mach numbers [6,7]. In recent decades, the continuous boron nitride (BN) fibres were successfully synthesized by various means [8–15]. As the fibrous form of BN, BN fibres similarly display desirable mechanical, dielectric and thermal properties. Its high thermal stability, fine dielectric properties over a wide temperature range, excellent thermal shock resistance and exceptional resistance to corrosion make

⇑ Corresponding author. E-mail addresses: [email protected] (D. Li), [email protected] (C.-R. Zhang), [email protected] (B. Li), [email protected] (F. Cao), whataboutduan@ hotmail.com (S.-Q. Wang). 1 Tel./fax: +86 731 84576433. 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.08.040

it one of the most brilliant high-temperature structural/functional materials [12–16]. The most significant characteristic of BN fibres, however, is that they hardly exhibit structural change and give little reduction in tensile strength when exposed to high temperatures [13,14], which is superior to silica fibres. Hence, in order to overcome the drawbacks of SiO2f/SiO2 composites, the preparation of boron nitride fibre reinforced silica based (BNf/SiO2) composites is considered, which may improve the mechanical properties especially at high temperatures. However, till now there have been few papers concerning such kind of materials. In this work, with silica sol and BN fibres as the raw materials, unidirectional BNf/SiO2 composites were manufactured via sol–gel method, and the structure, composition and mechanical properties of the composites were investigated. More importantly, the mechanical properties of BNf/SiO2 composites at high temperatures were studied in contrast to SiO2f/SiO2 composites obtained by our previous research. 2. Experiment 2.1. Raw materials BN fibres and silica fibres used in this study were produced by Shandong Research & Design Institute of Industrial Ceramics (Zibo, China) and Feilihua Quartz Glass Corporation (Jingzhou, China) respectively, with properties listed in Table 1. Silica sol was purchased from Xinyu Reagent Co. (Zhuozhou, China). The fibres were put in a mould unidirectionally with a fibre volume fraction of 35%.

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Table 1 Properties of BN fibre and silica fibre.

3. Results and discussion

Fibre

Density (g cm3)

Diameter (lm)

Tensile strength (GPa)

Elastic modulus (GPa)

3.1. Structure and composition

BN Silica

1.6–1.8 2.2

6–8 6–8

0.8–1.2 1.7

10–30 78

The infrared spectrum of BNf/SiO2 composites is illustrated in Fig. 1. The strong absorption peaks of N–H (3410 cm1), O–H (3050–3500 cm1), B–O (1590 cm1), Si–O (1100/480 cm1) and B–N (1410/800 cm1) are obvious [5,7,17]. A little broad peak at 950–1100 cm1 are also visible. Because of the similar location, the peak of Si–N–Si group may also be contained. Fig. 2 shows the XRD pattern for the composites. No crystalline phases are observed according to the broad peaks at about 20–30° and 40–70°, suggesting the amorphous composites. The peaks of hBN located at 26.2° (0 0 2), 42.8° (1 0 0) and the peak of a-cristobalite at 22.5° (1 0 1) are distinct. Besides, the peaks located at 23.5°, 26.0°, 42.8°, 52.0° and 62.5° probably belong to a-Si3N4, which may be formed during the preparation process [18]. However, the peaks of h-BN, a-Si3N4, a-cristobalite and B2O3 usually appear at similar locations (20–30°), therefore, all the peaks cannot be clearly distinguished. As is analyzed above, we can draw the conclusion that the BNf/ SiO2 composites mainly consist of h-BN, a-Si3N4, a-cristobalite and probably a little B2O3. The existence of Si3N4 may be due to the following reaction:

2.2. Preparation process BNf/SiO2 composites were prepared by sol–gel method according to the following steps. First, the obtained preform was dipped in the silica sol for 30 min in vacuum condition. Then the silica sol in the preform was dried at about 90 °C and turned into silica gel by heating the temperature up to 150 °C and holding for about 5 h. Finally, the preform was sintered in nitrogen atmosphere at appropriate temperature. The porosity of specimens was decreased by repeating the above infiltration-sintering procedure. SiO2f/SiO2 composites were synthesized by the same method according to our previous work.

2.3. Characterization An investigation of bondings was performed via Fourier transform infrared spectrometer (FT-IR, Avatar 360, Nicolet Instrument Corp., Wisconsin, USA) on discs pressed from composites powders mixed with KBr. X-ray diffractometer (XRD, D8 Advance, Bruker/ Axs Corp., Germany) was employed to examine the crystalline phase and its preferred orientation using Cu Ka radiation. The density and open porosity of the samples were measured by Archimedes’ method in distilled water at 20 °C. The flexural strength and elastic modulus were tested via the three-point bending test machine (WDW-100, Changchun Research Institute of Testing Machines, Jilin, China) with a support distance of 30 mm and a loading speed of 0.5 mm min1 at room temperature. The rectangular specimens were made with a dimension of 3 mm  4 mm  35 mm. Five specimens were tested to obtain the average strength and modulus. The high-temperature mechanical properties were tested by the same way with a heat rate of 10 °C min1 and a hold time of 10 min for each temperature. The fracture toughness KIC was measured using single edge notched beam (SENB) method with a span of 30 mm and a load speed of 0.05 mm min1 in air at room temperature. At least five specimens with a dimension of 30 mm  2.5 mm  5 mm were tested to obtain the average toughness. The edge notch with a thickness of about 2.5 mm was made. The fracture toughness was calculated from the following equations:

3PL 2

2bh

a1=2 f ða=hÞ

where Si–OH group is generated from [SiO4] group, which is prone to absorb H2O, and H–N group comes from the hydrolyzation of BN described as below:

ABANA þ HAOH ! HANA þ ABAOH

ð4Þ

And B2O3 is probably produced by the oxidation and hydrolyzation of BN, which may occur at the stage of infiltrating and drying BN fibres with silica sol. It can be explained like this:

4BNðsÞ þ 3O2 ðgÞ ! 2B2 O3 ðsÞ þ 2N2 ðgÞ

ð5Þ

2BNðsÞ þ 3H2 OðlÞ ! B2 O3 ðsÞ þ 2NH3 ðgÞ

ð6Þ

In the absence of moisture, BN experiences minimal oxidation up to 850 °C [13], and the B2O3 glass will be formed on its surface acting as an anti-oxidant protection layer to prevent BN from oxidation at elevated temperature [19,20]. The glassy form of high purity boric oxide has no definite melting point, and it begins to soften at about 325 °C and is fluid and pourable at about 500 °C [21,22]. At temperatures less than 1100 °C, B2O3 has a low vapor pressure and consequently volatilizes slowly. It is known that oxidation proceeds via diffusion of oxygen through the B2O3 layer,

ð1Þ

f ða=hÞ ¼ 1:93  3:07ða=hÞ þ 13:66ða=hÞ2  23:98ða=hÞ3 þ 25:22ða=hÞ4

ð3Þ 4

ð2Þ

where a is the notch thickness, b the width of rectangular bar specimens, h the height of rectangular bar specimens, L the support span, and P is the maximum load. The surfaces of the fractured test samples were examined in a Hitachi S-4800 scanning electron microscope (SEM). Prior to viewing, the surfaces were covered with a thin gold–palladium layer in a polaron sputtering chamber.

Intensity (a. u.)

K IC ¼

ASiAOH þ HANA ! ASiANA þ H2 O

4000

3000

2000

1000 -1

Wavenumbers / cm

Fig. 1. FT-IR spectrum of BNf/SiO2 composites.

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D. Li et al. / Materials and Design 34 (2012) 401–405 Table 2 Physical and mechanical properties of BNf/SiO2 composites (R.T.).

Intensity (a. u.)

h-BN α-cristobalite α-Si3N4

Fibre volume fraction (%)

Density (g cm3)

Open porosity (%)

Flexural strength (MPa)

Elastic modulus (GPa)

Fracture toughness (MPa m1/2)

35

1.70

20.8

51.2

23.2

1.46

0.05

0

10

20

30

40

50

60

70

0.04

80

Fig. 2. XRD pattern of BNf/SiO2 composites.

with the rate of conversion depending on the oxygen partial pressure and the surface area [21,22]. Otherwise, the nitrogen gas, which is produced in reaction (5), will provide an inert gas blanket for further protection against oxidation [19,20]. B2O3 will easily react with H2O to form metaboric acid (HBO2), and both of B2O3 and HBO2 are of high moisture sensitivity [23]. Consequently, BN containing B2O3 will induce significant moisture. Earlier researches [19,24,25] have shown that moisture sensitivity of hexagonal BN and its tendency to hydrolyze when exposed to water are intrinsic problems in the long term use of this material. The interlayer spacing d(0 0 2) is considered to be the crucial parameter to control the stability of h-BN [19]. Specimens which have large d(0 0 2) spacings (usually more than 3.40 Å) are significantly more sensitive than those are close to the theoretical value (3.33 Å). BN with a large d(0 0 2) spacing has a higher reactivity, which may be due to its less densely packed basal planes implying weaker atomic bonding [24,25]. In addition, with a smaller grain size, such kind of materials is usually vulnerable to moisture [19]. According to Bragg formula, d(0 0 2) spacing of h-BN can be calculated by the following equation:

d002 ¼

k 2 sin h

ð7Þ

where k is the wavelength of Cu Ka (0.154 nm), and h is the Bragg’s angle which can be obtained from the XRD pattern (2h = 26.2°). So the d(0 0 2) spacing of h-BN from our specimen is 3.40 Å, showing a little loose structure, so there will be entrapment of oxygen and moisture. On the other hand, crystallization could reduce the oxidation and hydrolyzation [19].

Load / kN

2θ / (°) 0.03

0.02

0.01

00

0.1

0.2

0.3

0.4

0.5

Displacement / mm Fig. 3. Load–displacement curve for BNf/SiO2 composites.

mechanical properties of CMCs to a large extent depend on the optimization of fibre/matrix bonding, which must have a moderate intensity to retain load transfer from the matrix to the fibres and allow crack deflection along the interface [26]. However, in the BNf/SiO2 composites, a strong interface bonding is formed due to the chemical reactions between the fibres and matrix during the preparation process. As is illustrated in reaction (3)–(6), the oxidation and hydrolyzation of BN will lead to a vast amount of –N–H and B2O3. –N–H will react with –Si–OH to form –Si–N–, and B2O3 is prone to absorb water converting to –B–OH, which contributes to the reaction with –Si–OH like this:

ASiAOH þ HOABA ! ASiAOABA þ H2 O

ð8Þ

Therefore, we deduce that Si3N4 and B2O3 are the two main interface phases leading to strong interface bondings of –Si–N– and – Si–O–B–, which can also be proved by the FT-IR spectrum shown in Fig. 1.

3.2. Mechanical properties at room temperature The properties of BNf/SiO2 composites at room temperature are shown in Table 2, while the load–displacement curve is illustrated in Fig. 3. The composites have a density of 1.70 g cm3 and an open porosity of 20.8%. The average flexural strength, elastic modulus and fracture toughness are 51.2 MPa, 23.2 GPa and 1.46 MPa m1/2, respectively. The composites show an elastic response in the beginning with displacement increasing linearly depending on the load and then a non-linear behaviour near the peak load followed by a sudden decreasing (see Fig. 3), which demonstrates a catastrophic and early failure mode. From the SEM micrographs shown in Fig. 4, we can see practically no pulled-out fibres with a very plane fracture surface. Besides, some micro-cracks between the fibres and matrix are also observed. The poor toughness are probably caused by the behaviour of fibre/matrix interface. In the continuous fibre reinforced CMCs, the fibre/matrix interface is dominant because the

3.3. Mechanical properties at high temperatures The mechanical properties of BNf/SiO2 composites at high temperatures in air are shown in Table. 3. As temperature increases, the flexural strength exhibits improvement from 300 °C to 1000 °C, and it reaches the top accounting for 80.2 MPa at 500 °C. Besides, the load–displacement curves at different temperatures show a little diversity (see Fig. 5). The curves at 300 °C, 500 °C and 700 °C demonstrate a similarly brittle fracture mode, while the 1000 °C curve displays a higher toughness with a gradually ladder-like decline after the peak load. It can be concluded that the vitreous SiO2 and B2O3 formed at high temperatures become a viscoelastic fluid, which is effective to fill up the micro-cracks avoiding propagation. Such self-healing property was also revealed by Udayakumar et al. [23] and Chen et al. [27]. Besides, the reduced trend of flexural strength above 500 °C is probably caused by the

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D. Li et al. / Materials and Design 34 (2012) 401–405

0.07 0.06

Load / kN

0.05 0.04 0.03 1000ºC

0.02 500ºC

0.01 300ºC 700ºC

0

0

0.1

0.2

0.3

0.4

Displacement / mm Fig. 5. Load–displacement temperatures.

curves

for

BNf/SiO2

composites

at

different

160

156.8%

Fig. 4. SEM micrographs of the fracture surface of BNf/SiO2 composites.

Residual strength (%)

140 120

(a) 151.2%

125.7%

100 80

85.6%

60

51.2%

40

48.9%

20 Table 3 Mechanical properties of BNf/SiO2 composites at high temperatures in air. Temperature (°C)

Flexural strength (MPa)

Elastic modulus (GPa)

25 300 500 700 1000

51.2 64.4 80.2 77.4 76.2

23.2 38.0 30.6 19.2 17.8

148.8%

(b) 18.2%

0

0

200

400

600

800

1000

Temperature / ºC Fig. 6. Residual flexural strength ratio of (a) BNf/SiO2 and (b) SiO2f/SiO2 composites at high temperatures in air.

4. Conclusions increase of strong bondings formed at higher temperatures, which are extremely detrimental to the fibre/matrix interface. In order to find the advantage of such composites, the curves for the residual flexural strength ratio of BNf/SiO2 composites at high temperatures compared to SiO2f/SiO2 composites fabricated in our previous work are drawn in Fig. 6. It should be noticed that both of the composites have the same unidirectional fibre volume fraction of 35%. With temperature increasing, the residual ratio of SiO2f/ SiO2 composites decreases sharply, dropping to the lowest 18.2% at 1000 °C, which is caused by the severe degradation of silica fibres at high temperatures [3–5]. In contrast, BNf/SiO2 composites display good high-temperature mechanical properties owing to the high thermal stability of BN fibres and self-healing properties, which make the composites promising for use in high-temperature structural situations. However, in order to enhance the toughness and strength of BNf/SiO2 composites, further work should be done to fabricate high performance BN fibres and optimize the fibre/matrix interfaces (e.g. by using the inorganic coatings).

With silica sol and BN fibres as the raw materials, unidirectional BNf/SiO2 composites were manufactured via sol–gel method, and the structure, composition and mechanical properties of the composites were investigated. The conclusions can be drawn as below: (1) The BNf/SiO2 composites consist of BN fibres and a-cristobalite matrix, probably as well as the interface phases of Si3N4 and B2O3. The oxidation and hydrolyzation of BN are dominated by the d(0 0 2) spacing of h-BN. The boron nitride fibres have a d(0 0 2) spacing of 3.40 Å. (2) The BNf/SiO2 composites have a density of 1.70 g cm3 and an open porosity of 20.8%. The average flexural strength, elastic modulus and fracture toughness at room temperature are 51.2 MPa, 23.2 GPa and 1.46 MPa m1/2, respectively. The composites show a brittle fracture mode with practically no pulled-out fibres and a very plane fracture surface. The strong interface bondings of –Si–N– and –Si–O–B– cause the poor toughness of the composites.

D. Li et al. / Materials and Design 34 (2012) 401–405

(3) The mechanical properties of BNf/SiO2 composites at 300– 1000 °C are desirable, with the maximum flexural strength and residual ratio being 80.2 MPa and 156.8% at 500 °C, respectively, while it is a sharply reduced trend as to SiO2f/ SiO2 composites. BNf/SiO2 composites display good hightemperature mechanical properties owing to the high thermal stability of BN fibres and self-healing properties caused by the fused SiO2 and B2O3.

Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 50902150 & 90916019) and the Graduate Innovation Foundation of the National University of Defense Technology (Grant No. S100103). The authors record sincere thanks to Mr. Cheng Zhiqiang, Mrs. Zhang Mingxia and Miss Tang Jie in Shandong Research & Design Institute of Industrial Ceramics for their raw materials.

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