SiC composites in air and combustion environment

Composites: Part A 31 (2000) 1015–1020 www.elsevier.com/locate/compositesa

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Oxidation behavior of three-dimensional SiC/SiC composites in air and combustion environment Laifei Cheng*, Yongdong Xu, Litong Zhang, Xiaowei Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 28 May 1998; received in revised form 3 September 1999; accepted 1 March 2000

Abstract A three-dimensional Hi-Nicalon/SiC composite was prepared, the mechanical properties and weight changes before and after oxidation in air and combustion environment were investigated and compared in the present paper. The combustion gas with a high flow rate and a larger amount of H2O accelerated the silica formation on the fibers and matrix, the oxidation channel could be sealed much earlier in combustion than in air, and the PyC interlayer was oxidized less. Weight loss of the composite in air was larger than that in combustion. The flexural strength did not decrease remarkably, in both air and combustion. Oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix besides removal of the interlayer, the fracture displacement was greatly increased after oxidation in combustion compared with those before and after oxidation in air. It was shown that the composite had an excellent oxidation resistance in a short period of exposure. This result was of very important significance in the development of a self-adaptable interlayer. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keyword: SiC/SiC composites

1. Introduction Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC) are one of the best promising structural materials for high temperature applications [1–3]. SiC based fibers, such as Nicalon, have a better resistance to oxidation. However, the tensile strength of the fibers decreases dramatically with a significant weight loss as the temperature is raised beyond 1200⬚C. The large amount of oxygen (10–14%) is responsible for the degradation of the fibers at high temperature. It is necessary to reduce the oxygen content of the fibers to improve the thermal stability of SiC/SiC. Recently, Hi-Nicalon with a low oxygen content (0.5% or less) was developed by Japan Nippon Carbon. With the oxygen content being decreased, silicon carbide fiber reinforced silicon carbide composites (Hi-Nicalon/ SiC) are expected to be used at above 1400⬚C, and are being paid more and more attention [4]. The oxidation behavior of ceramic matrix composites (CMC) is influenced cooperatively by the oxidation of the fiber, matrix and interphase, and related to the oxidation conditions. A number of studies have been conducted on the oxidation of SiC prepared by chemical vapor deposition * Corresponding author. Tel.: ⫹ 86-29-849127; fax: ⫹ 86-29-8491000. E-mail address: [email protected] (C. Laifei).

in various environments, including air and combustion. The oxidation behavior of SiC based fibers, including Nicalon and Hi-Nicalon, has been investigated in air. A few of studies have been carried out on the oxidation of SiC/SiC composites [5,6], but the oxidation behavior Hi-Nicalon/ SiC composites, especially three-dimensional and in combustion has not been reported up to now. Because the fibers and matrix have a better resistance to oxidation, the oxidation behavior of the Hi-Nicalon/SiC composite, especially the mechanical behavior, is very sensitive to the oxidation of the interphase. In the present paper, a threedimensional Hi-Nicalon/SiC composite was prepared, the mechanical properties and weight changes before and after oxidation in air and combustion environment were investigated and compared.

2. Experimental procedure 2.1. Fabrication of the composite Hi-Nicalon娃 silicon carbide fiber from Japan Nippon Carbon was employed. The fiber preform was prepared by the three-dimensional braid method. The cross-section dimension of the preform was 5 × 6 mm: The volume fraction of fibers was controlled in the range from 40 to 45%.

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Fig. 1. Schematic of high temperature wind tunnel.

The preforms were deposited with a pyrolysis carbon (PyC) and SiC by low pressure chemical vapor deposition (LPCVI). The hot zone of the CVI furnace is ⭋60 mm × 200 mm: The interfacial layer of PyC was deposited for 1 h at 870⬚C and 5 kPa with a C3H6 flow of 200 ml min ⫺1. The conditions for deposition of SiC matrix were as follows: the deposition temperature was 1100⬚C, pressure was 5 kPa, time was 20 h, flow of H2 was about, and the molar ratio of H2 and methyltrchlorosilane (MTS) was 10. The thickness of the interfacial layer was calculated to be about 0.1 mm by the weight gain after deposition of PyC. The weight gain after deposition of the SiC matrix was about 100%. The SiC coating were prepared on the specimens for 5 h to seal the open ends of the fibers after cutting from the prepared composite. 2.2. Measurements of the composite Specimens with a length of 40 mm were cut from the composite. The flexural strength and weight change of the specimens before and after oxidation were first measured, and then the fracture work and fracture displacement were determined by the steady stage of the load–displacement curves. The flexural strength was measured by the threepoint bending method with a span of 30 mm. 2.3. Oxidation tests Oxidation tests in dry air were conducted in a furnace heated with SiC rods at 1250⬚C. Oxidation tests in

combustion at 1250⬚C were conducted in a high temperature wind tunnel which had a nozzle of 170 mm in diameter. The maximum temperature at the flame center was 1350⬚C. Fig. 1 is the schematic of the high temperature wind tunnel. A special holder for specimens was designed and fabricated. In order to raise the efficiency of the oxidation tests, the holder was star shaped and five arms in it could be used. Fig. 2 is the schematic of the holder with specimens. The specimens were kept apart from each other by blocks of silicon nitride ceramic. The parameters of the oxidation tests are given in Table 1. The composition of the combustion gas has been listed in Table 2. 3. Results and discussion The density and open porosity of the SiC/SiC composite are given in Table 3. It could be clearly seen that they changed little. Fig. 3 depicts the microstructure of the SiC/SiC composite. It was not difficult to make out that the deposition of the SiC matrix around the fiber filaments was homogeneous and the thickness of the SiC layer was 5 mm less or more. This could be concluded more from the preform of Hi-Nicalon娃 than from that of T-300娃. Table 4 contains the weight changes and mechanical properties of the Hi-Nicalon/SiC composite before and after oxidation in air and combustion for 3 h. It could be found that the composite lost little in weight after oxidation in both air and combustion, the flexural strength did not have a remarkable decrease, no matter which environment the composite was oxidized in, and the fracture displacement was greatly increased after oxidation in combustion compared with those before and after oxidation in air. Consequently, the fracture work was also increased greatly. The fluctuation of the weight changes was considered to be caused by the holding and the surface oxidation of the specimens. The holding of the specimens led to different weight loss. Because the specimens had not been machined, different specimens had different surface roughness and then different weight gain after oxidation. These results reveal that the properties of the composite changed little after oxidation in air, and increased obviously after oxidation in

Fig. 2. Schematic of substrate holder: (a) the back; (b) the side; (c) the front.

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Table 1 Parameters of the oxidation tests Flow

Temperature

Pressure

Discharge

Rate

Distribution

Relative change

Entrance

Exit

0.991 kg/s

200 m/s

1209–1250⬚C

⬍ 5%

1:1 × 105 Pa

0.986 kg/cm 3

Table 2 Composition of the combustion gas Air (53.52%)

Pure combustion (46.48%)

O2

Remainder

CO2

H2O

13.27%

40.25%

33.21%

13.27%

combustion. This was exciting and very important for applications of this composite in advanced engines. If only favorable interlayers were prepared, could CMC exhibit a tough fracture behavior? Most often the pyrocarbon (PyC) interlayer was applied, and the CVD BN interlayer had been used in place of PyC recently for improving the oxidation resistance [7]. Unfortunately, PyC had very poor oxidation resistance and BN was not sufficiently better to be viable at above 1300⬚C. Up to now there are no demonstrated oxidation resistant interface approaches [8]. For C/ SiC composites, it is not difficult to understand the effect of interfacial oxidation on properties. For Nicalon/SiC composites, it was considered that the degradation of mechanical properties after short periods of exposure was due to the oxidation and removal of the PyC interlayer [9]. For the Hi-Nicalon/SiC composite investigated in the present paper, the PyC interlayer not only provided a sufficiently weak interface, but also could further weaken the interfacial binding after a relatively longer period of exposure in the oxidizing atmosphere. This result was of great significance in developing a self-adaptable interlayer, although properties of the Hi-Nicalon/SiC composite after much longer oxidation are still to be studied. A silica film would be formed on the CVD SiC matrix or the Hi-Nicalon fibers after oxidation in both air and combustion. Because the oxidation of SiC produced a weight gain and a volume expansion, the oxidation of the PyC interlayer should be responsible for the weight loss of the specimens. The oxidation channel of the PyC interlayer could be considered to start from the intersection of bundles, which was connected with the open interbundle porosity. Because there was a large amount of H2O in the combustion gas (Table 2), the molecule net of the silica was broken by

forming non-bridge SiOH groups. This decreased the silica viscosity and increased the diffusion of H2O in the silica [10,11]. For the oxidation rate of CVD SiC, there was a lineal stage and a parabolic stage. The former was controlled by the reaction rate, and the later was controlled by the diffusion rate of oxygen in the silica. The lineal stage was about 1 min short and was negligible compared with the parabolic stage when oxidized in air. The fast diffusion of H2O in the silica changed the controlling step of the oxidation rate from diffusion to reaction when oxidized in combustion and the lineal stage was prolonged for about 1 h [12,13]. The silica formation in combustion was much faster and had a lower viscosity than that formed in air, the oxidation channel could be sealed much earlier, and the PyC interlayer should be oxidized less (Fig. 4). The weight gain produced by silica formation in combustion was larger than that in air, and the weight loss caused by the interlayer oxidation was smaller. Consequently, the weight loss of the specimens in air should be larger than that in combustion, similar to that confirmed by the test result. At the same time, this result showed that influence of the erosion caused by the combustion gas with a high flow rate of 200/s on the weight loss could be neglected at the test temperature. Compared with the still air, the combustion gas decreased the boundary layer thickness on the specimens, promoted the inward diffusion of oxidizing gases and the outward

Fig. 3. Microstructures of the SiC/SiC composite as prepared.

Table 3 Density and open porosity SiC/SiC the composite Number

1

2

3

4

5

6

Average

Density (g/cm 3) Open porosity (%)

2.46 13

2.46 13

2.46 14

2.47 15

2.48 15

2.49 15

2.47 14

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Fig. 4. Schematic representation of SiC/C/SiC oxidation mechanisms.

Fig. 5. The fracture surfaces of the Hi-Nicalon/SiC after oxidation.

diffusion of products, and accelerated the silica formation on the fibers and matrix [14]. In the oxidation process, on the one hand the removal of the PyC interlayer weakened the interfacial binding; on the other hand, the fibers and matrix were bonded together by the silica formed so that the interfacial binding was strengthened. Because the oxidation channel was sealed much earlier, the interface of the fibers and matrix bonded by silica in combustion was much longer than that in air, and then the fracture displacement of the composite in combustion was much larger than that in air. The fact that the fracture displacement in air was little larger and that in combustion was much larger than that before oxidation showed that the weakened interface by oxidation in both air and combustion was much longer than that bonded. The reasonable explanation was that oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix, besides removal of the interlayer. This could be also confirmed by the fact that pull out of the fibers on fracture surfaces of specimens oxidized in combustion was much longer than that in air (Fig. 5). For improving the oxidation resistance of the SiC/SiC composites, the thickness of the PyC interlayer should not be larger than 0.2 mm [5]. In order to demonstrate the oxidation behavior of the PyC interfacial layer presented in this

paper, a Hi-Nicalon/SiC composite with a larger thickness of the PyC interlayer was prepared. Fig. 6 shows the SEM micrograph of the interlayer. After oxidation for 1 h in air (Fig. 7a), the PyC close to the section was oxidized out, and some fibers were bonded with the matrix by the silica formed. After oxidation for 7 h (Fig. 7b), more fibers were bonded with the matrix.

Fig. 6. SEM micrograph of the interlayer in the SiC/SiC composite as prepared.

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Table 4 Weight change and properties of Hi-Nicalon SiC/SiC composite before and after oxidation in air and combustion for 3 h Weight loss, properties

Condition

1

2

3

Average

Weight change (%)

As prepared In air In combustion

0 ⫺0.12 ⫺0.27

0 ⫺0.30 ⫺0.29

0 ⫺0.90 ⫺0.60

0 ⫺0.44 ⫺0.39

Flexural strength (MPa)

As prepared In air In combustion

583 504 542

596 579 543

605 656

590 563 581

As prepared In air In combustion

27 23 17

20 29 60

20 60

23 24 46

Fracture work (kJ/m 2)

Fracture displacement (mm)

As prepared in air In combustion

0.55 0.60 0.58

The thicker the PyC interlayer, the larger the length of the interlayer consumed by oxidation in both air and combustion. For Nicalon/SiC composites this meant that the fibers would be oxidized more strongly and lose strength more greatly. It was considered that the thickness of the PyC interlayer should be less than 0.1 mm in order to improve the oxidation resistance of the interlayer [6]. For the Hi-Nicalon/SiC composite, the interlayer could be thicker because the fibers retained strength much more than Nicalon fibers after oxidation. This could be favorable for keeping a weak interfacial binding after longer periods of exposure in both air and combustion, especially to prevent the fibers and matrix from being bonded by silica and producing brittle behavior after oxidation in air.

4. Conclusions 1. When the Hi-Nicalon/SiC composite prepared, had a higher density, a lower porosity and a better homogene-

0.61 0.65 1.03

0.74 1.37

0.58 0.66 0.99

ity, then it had a higher strength. Because the PyC interlayer was well deposited, the composite had a larger fracture displacement and fracture work. 2. The flexural strength did not have a remarkable decrease, no matter which environment the composite was oxidized in. It was shown that oxidation had little effect on strength of the fibers after short periods of exposure. 3. Compared with air, the larger amount of H2O in combustion changed the controlling step of the oxidation rate of SiC from diffusion to reaction when oxidized in combustion, and the silica formation was faster. The combustion gas with a high flow rate decreased the boundary layer thickness on the Hi-Nicalon/SiC composite, promoted the inward diffusion of oxidizing gases and the outward diffusion of products, and accelerated the silica formation on the fibers and matrix. Consequently, the oxidation channel could be sealed much earlier, and the PyC interlayer was oxidized less. 4. Because oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix

Fig. 7. SEM micrograph of the interlayer in the SiC/SiC composite after oxidation in air.

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besides removal of the interlayer, the fracture displacement and the fracture work were greatly increased after oxidation in combustion compared with those before and after oxidation in air.

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