Influence of dip-coated MgO interphase on the mechanical and dielectric properties of 2D-SiC_f/SiC composites

Abstract

This work focused on the use of the dip-coating process to prepare magnesium oxide (MgO) coatings on silicon carbide fibers (SiC_f) as an interfacial system for SiC_f/SiC composites. The MgO coatings were prepared on KD-I SiC_f using thermal decomposition of magnesium acetate tetrahydrate (C₄H₆MgO₄·4H₂O) in a vacuum furnace. The effect of the amount of C₄H₆MgO₄·4H₂O on the coating's microstructure and morphology, mechanical and dielectric properties of SiC_f/SiC composites were investigated. The Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction results show that the coating's surface is relatively smooth and uniform as the content of C₄H₆MgO₄·4H₂O reaches 3 wt%. The SiC_f/SiC composites containing an 0.18 μm thick MgO interphase show improved mechanical properties in terms of flexural strength and failure displacement. Also the complex permittivity for composites with an MgO interphase is a little lower than the as-received composites because of the low electric conductivity of MgO. The effective microwave absorbing bandwidth (below −10 dB) for composites is in the range 8.8–11.5 GHz, and the minimal reflection loss reached −12.56 dB.

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1. Introduction

In recent years, silicon carbide (SiC) materials have attracted considerable attention because of their characteristics of high corrosion resistance, high thermal conductivity and good thermal shock resistance.^1,2 In addition, because of their excellent semiconductive nature and relatively stable dielectric characteristics, SiC materials have been considered as a promising microwave absorbing material.^3–6 However, the utilization of such materials is limited by their low toughness with a value of approximately 3–4 MPa m^1/2. In comparison to monolithic SiC ceramic, SiC fibers (SiC_f)/SiC composites can offer improvements in damage tolerance and strain-to-failure.^7–9 This is achieved when debonding of the fiber–matrix interface and fiber pullout are activated, which is facilitated by a fiber coating that provides an interfacial layer that debonds fairly readily during loading of the composite.^10–13 In a number of SiC_f/SiC composites, the reinforcing fibers are usually coated with layered structures such as pyrolytic carbon (PyC) and/or boron nitride (BN). However, the PyC is not appropriate for absorption of microwaves because of its high electrical conductivity, leading to strong reflection of electromagnetic waves.¹⁴ Although the BN with a low dielectric constant is regarded as an appropriate material for microwave absorption, such an interphase was conventionally prepared via a chemical vapor deposition process with some environmentally harmful precursors, and the reactants or gaseous products may affect the substrate materials at deposition.¹⁵ Furthermore, PyC and BN cannot undergo high temperature oxidation under a high moisture environment and this results in the degradation of the mechanical properties of the SiC_f/SiC composite.^16,17 Utilization of the oxide interphase can effectively improve the mechanical properties of the composites in a high temperature oxidative environment. Igawa et al.^18,19 prepared silicon dioxide–magnesium oxide (SiO₂–MgO) coatings on SiC_f using a sol–gel method, and the tensile strength of the coated fiber was retained at 85% (or higher) of the original fiber after heating below 1300 °C and the SiC_f/SiC composites with such an interphase exhibited ductile fracture behavior in the three-point bending test. However, up to now, reports on the effect of the single layer dip-coated MgO interphase on the mechanical and dielectric properties of SiC_f/SiC composites are rare.

In this work, MgO was coated on SiC_f as a fiber-matrix interphase of the SiC_f/SiC composites by use of a dip-coating process through the thermal decomposition of magnesium acetate tetrahydrate (C₄H₆MgO₄·4H₂O). The microstructure of the prepared MgO coatings was characterized, and the effects of the MgO interphase on the mechanical and dielectric properties of the SiC_f/SiC composites were investigated.

2. Experimental procedure

The two-dimensional (2D) SiC fiber cloths with the volume fraction of 30% (National University of Defense Technology, China) were used as reinforcement. The poly(carbosilane) (PCS) with a molecular weight of ∼1800 and a softening point of ∼180 °C was used as the precursor for the SiC matrix, which was provided by the National University of Defence Technology (China). C₄H₆MgO₄·4H₂O was dissolved in deionized water (H₂O) and was used as the source of the MgO coatings using the amounts of each chemical as given in Table 1.

Table 1 Composition of various magnesium acetate tetrahydrate solutions

Number	C4H6MgO4·4H2O/g	H2O/g	C4H6MgO4·4H2O ratio
1	0	0	0
2	2	198	1%
3	4	196	3%
4	6	194	5%

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The fabrication process of SiC_f/SiC composites with an MgO interphase is shown in Fig. 1. The SiC fiber cloths (45 mm × 45 mm) were heated to 700 °C in a vacuum for desizing, and were then dipped in the C₄H₆MgO₄ solution for 30 min and then dried in an oven. After heating to 400 °C in a vacuum furnace, the MgO coated SiC fibers were obtained. The dip coating process is shown in Fig. 2. The coated SiC fiber cloths were brushed with PCS solution on both sides. Several coated fiber cloths were stacked layer by layer in a metal mold followed by hot pressing at 240 °C for 2 h so that the layers were bonded by the viscous flow of the molten PCS from the brush. The approximate thickness of the samples was 2.3 mm. Finally, the hot pressed SiC fiber cloths were pyrolyzed at 1000 °C for 2 h in vacuum furnace and the green compacted cloth was obtained. Because of the volume shrinkage and the low yield in the conversion process of the PCS precursor to the SiC ceramic, it was necessary to repeat the process of infiltration and pyrolysis until the weight gain of the composites was less than 1%.

Fig. 1 Fabrication process of SiC_f/SiC composites with MgO interphase.

Fig. 2 The dip-coating process for SiC fibers in C₄H₆MgO₄·4H₂O solution.

The phases of coating layers were determined using X-ray diffraction (XRD; X'pert PRO MPD, PANalytical, Almelo, Netherlands) and Fourier transform infrared spectroscopy (FT-IR; Nexus 670, Nicolet). The morphology of the coated fibers and the fracture surfaces of the SiC_f/SiC composites were characterized using scanning electron microscopy (SEM, SUPRA 55, Zeiss, Germany).

The open porosities of the SiC_f/SiC composites were measured using Archimedes' method which used distilled H₂O as an immersion medium. The bending strength of the composites was characterized using a three-point bending test with a cross head speed of 0.5 mm min⁻¹ loaded on samples at room temperature, following the guidelines of ASTM standard C1341, using five specimens. The bending strength, σ, was calculated using the following equation:

σ = 3FL/2bh² (1)

where F is the load of deflection in the test and L is the support span. b and h are the width and thickness of the specimen, respectively.

The complex permittivity of the SiC_f/SiC composites was measured at the frequency between 8.2 GHz and 12.4 GHz using the waveguide method (E8362B vector network analyzer, Keysight Laboratories).

3. Results and discussion

3.1 Microstructure and morphology of the MgO coating

The growth mechanisms of MgO on SiC fibers were shown to be as follows: firstly, the C₄H₆MgO₄·4H₂O was attached to the SiC_f in the dipping process. After the SiC_f were heated to 150 °C, the H₂O contained in the acetate evaporated completely and pure C₄H₆MgO₄ was left on the fibers' surface. As the heating temperature was increased to 400 °C and then kept at this temperature for 1 h, the C₄H₆MgO₄ decomposed into MgO coated on the SiC_f.^20,21

Mg(CH₃COO)₂·4H₂O → Mg(CH₃COO)₂ + 4H₂O (2)

Mg(CH₃COO)₂ → MgO + CO₂ + CO + C₂H₆ (3)

The XRD patterns of C₄H₆MgO₄·4H₂O are shown in Fig. 3(a). The strong, low angle reflection at 10° and a broadband reflection in the region of 23–44° with many small peaks are attributed to C₄H₆MgO₄. The XRD pattern of the prepared MgO products is shown in Fig. 3(b), the strong peaks at 36.8°, 42.9° and 62.3° typically correspond to (111), (200) and (220) reflections, respectively, of the cubic structure MgO (JCPDS number 45-0946). The narrow diffraction peaks indicate that the C₄H₆MgO₄ is completely converted to the single form of MgO.^22–24

Fig. 3 XRD patterns of C₄H₆MgO₄·4H₂O (a) and the MgO products obtained from the decomposition of C₄H₆MgO₄·4H₂O (b).

Infrared radiation (IR) analysis was used to monitor the changes occurring in the chemical bonds during the decomposition and the target structure was identified. The IR spectra of C₄H₆MgO₄·4H₂O is presented in Fig. 4(a). Three strong absorption peaks appear at 1652 cm⁻¹, 1554 cm⁻¹ and 1402 cm⁻¹, which are assigned to the stretching vibration of the C–O. The bands corresponding to the rocking vibration modes of methyl in acetate are observed at 1126 cm⁻¹ and 1024 cm⁻¹. The peaks at 942 cm⁻¹, 665 and 622 cm⁻¹ are attributed to C–C stretching, COO– deformation and rocking, respectively.²⁵ In Fig. 4(b), there is no characteristic peak assigned to C–C stretching in the region of 500–1000 cm⁻¹. The strong stretching appears at 1400 cm⁻¹ and 1632 cm⁻¹ and this corresponds to the Mg–O bond. It is revealed that no organic groups exist in the decomposed product of C₄H₆MgO₄.²⁶

Fig. 4 FT-IR spectra of C₄H₆MgO₄·4H₂O (a) and the product of decomposition from C₄H₆MgO₄·4H₂O at 400 °C (b).

The morphologies of the MgO coated SiC_f prepared with different amounts of C₄H₆MgO₄·4H₂O solution are shown in Fig. 5. When fiber is treated with 1 wt% C₄H₆MgO₄·4H₂O solution, fragmented MgO products appear on the SiC_f surface. As the amount of C₄H₆MgO₄·4H₂O reaches 3 wt%, the smooth coatings are uniform in the radial direction of the fibers. Fig. 4(e) and (f) show that some coatings peel off and the surface of the fibers was exposed because of the external force. However, as the MgO precursor increases to 5 wt% in solution, the fiber surface assembled with irregularly shaped MgO products into barbed-like structure defects, which could reduce the tensile strength of the SiC_f and hamper the binding between the matrix and the SiC_f.

Fig. 5 Surface morphologies of MgO coated on SiC fibers in different concentrations of C₄H₆MgO₄·4H₂O solutions: (a) and (b) without coating; (c) and (d) 1 wt%; (e) and (f) 3 wt%; (g) and (h) 5 wt%.

To further confirm the distribution of MgO products on the surface of the fibers, SEM-energy dispersive X-ray spectroscopy (EDS) line scanning was used for characterization (Fig. 6). The yellow arrow indicates the area of interest and direction for the EDS line scanning. The signal of Mg stays nearly constant at the two outside edges of the fiber and has a low value across the fiber, which confirms that the MgO products are formed on the outer surface of the SiC fibers. In order to investigate the interaction of the SiC fibers and MgO coatings, the fibers were dipped in a 3 wt% coating solution followed by heating to 400 °C and 1000 °C in air for 1 h. Fig. 7 shows the XRD patterns of the MgO coated SiC fibers after the dipping and heating process. For comparison, an uncoated SiC fiber is also shown in Fig. 7. The XRD patterns of uncoated SiC fibers show three peaks around the 2θ of 35.8°, 60.1° and 71.8° which correspond to the (111), (220) (311) reflections of β-SiC, respectively.^18,19 The small peaks attributed to MgO are observed around the 2θ of 42.9° and 62.7° for the samples heated at 400 °C, and these peaks remain nearly constant with the increase of the temperature to 1000 °C. No change of the peak shape and position of the MgO coated SiC fiber were observed in the present work. This suggests that the MgO coatings are stable under these conditions.

Fig. 6 The EDS line scanning of the MgO coated SiC fiber.

Fig. 7 XRD patterns of uncoated fiber (a) and an MgO coated fiber after heating to 400 °C (b) and 1000 °C (c).

3.2 Effect of MgO interphase on mechanical properties of SiC_f/SiC composites

The mechanical properties of SiC_f/SiC composites with MgO interphase are shown in Table 2. The densities and open porosities of the SiC_f/SiC composites with and without interphase are close to each other, whereas the flexural strengths are notably different. Fig. 8 shows the typical stress–displacement curves of the SiC_f/SiC composites with and without MgO interphase. The SiC_f/SiC composite without interphase exhibits an earlier catastrophic failure mode, whereas the SiC_f/SiC composites with the MgO interphase show toughened fracture behavior with a gradual decrease in load after the point of maximum load. The MgO coated fibers act as the reinforcement in the SiC_f/SiC composites for load transmission. Thus, the stability of the coated fibers has a direct effect on the mechanical properties of the composites. The flexural strength of the composites increases with the content of the C₄H₆MgO₄·4H₂O increase in dipping solution, and the maximum of 256 MPa is obtained for a composite treated with 3 wt% MgO precursor solution, also the failure displacements vary from 0.13 mm to 0.31 mm, which compared favorably to that of composites without interphase. The improved flexural strength of the composites suggests that the MgO coated fibers are stable enough to provide an effective bearing capacity for the composites. However, the value of flexural strength decreased to 150 MPa for the composites treated with 5 wt% MgO precursor. The stress reduction can be ascribed to the higher stress concentration associated with the rougher fiber surface caused by excess product of the acetate accumulating on the fibers' surface during the dip-coating process, which leads to tremendous physical damage to the SiC fiber and makes the fiber reinforced mechanism faulty.²⁷ Fig. 9 shows the fracture morphologies of SiC_f/SiC composites with and without MgO interphase. The composite without interphase shows hardly any fiber pull-out. The reason for that is considered to be mainly because a strong interfacial bonding can hardly allow crack deflection to happen at the interface between the fibers and matrix. In addition, some elements would migrate from the matrix into the fibers to form a strong contact during the precursor infiltration and pyrolysis (PIP) process and this causes a deterioration of the mechanical properties of the composites. In contrast, composites containing MgO interphase exhibit evident fiber pull-out behaviors. The fiber pull-out behavior is observed preferentially for a sample treated with a 3 wt% dipping solution, the residues are found adhered to the pull-out fibers, which indicates that a large amount of elastic and plastic energy was consumed during the failure process. The results indicate that the composites treated with 3 wt% MgO precursor solution have a better fracture toughness than other composites, which were verified in Table 2 and the curves in Fig. 8. The average length of pull-out fibers is about 12 μm. Whereas, the fiber pull-out behavior is not observed for the composites treated with a 5 wt% MgO precursor, which may be attributed to the fact that there is more resistance to debonding and sliding at the irregularly interface. The flexural strengths of SiC_f/SiC composites with various interphases are summarized in Table 3 for comparison. As shown in the Table 3, the mechanical behaviors of SiC_f/SiC composites are dependent on the interphase type and thickness.

Table 2 Mechanical properties of the fabricated SiC_f/SiC composites

Sample	Density/g cm^−3	Porosity/%	Flexural strength/MPa	Failure displacement/mm
a Numbers in the parentheses represent standard deviations for flexural strength.b Numbers in the parentheses represent standard deviations for displacement.
a	2.25	11.5	137 (10^a)	0.13 (0.01^b)
b	2.20	11.7	192 (13)	0.22 (0.02)
c	2.24	11.3	253 (12)	0.31 (0.02)
d	2.21	11.8	153 (13)	0.20 (0.01)

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Fig. 8 Stress–displacement curves of SiC_f/SiC composites without and with MgO interphase prepared in different mass ratio of C₄H₆MgO₄·4H₂O solutions: (a) without interphase; (b) 1 wt%; (c) 3 wt%; (d) 5 wt%.

Fig. 9 Fracture morphologies of SiC_f/SiC composites without and with MgO interphase prepared in different concentrations of C₄H₆MgO₄·4H₂O solutions: (a) without interphase; (b) 1 wt%; (c) and (d) 3 wt%; (e) 5 wt%.

Table 3 Comparison of the flexural strength of the SiC_f/SiC composites

SiCf/SiC composites	Interphase type	Thickness of interphase/μm	Flexural strength/MPa	References
a Numbers in the parentheses represent standard deviations for flexural strength.
1	MgO	0.185	256 (10^a)	This work
2	BN	0.20	278 (20)	27
3	SiC	2	220 (10)	31
4	PyC	0.06	190 (25)	32
5	ZnO and SiO2	0.3–0.4	330 (20)	33

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3.3 Effect of MgO interphase on the dielectric properties of SiC_f/SiC composites

Real part (ε′) and imaginary part (ε′′) of complex permittivity for composites with and without MgO interphase are shown in Fig. 10. The complex permittivity decreases with increasing frequency in the range of 8.2–12.4 GHz (X band) for all the composites. The values of ε′ and ε′′ for the composites with the MgO interphase vary in the range of 11.4–i8 which are a little smaller than the results obtained without interphase, and which vary in the range of 11.5–i8.5. For SiC materials, the real part of complex permittivity is related to relaxation polarization and the imaginary part is regulated by electrical conductivity according to the Debye theory.²⁸

Fig. 10 Real part (a) and imaginary part (b) of complex permittivity for SiC_f/SiC composites with and without MgO interphase.

The free electrons gathered in the fiber/matrix interfaces to form the space-charge layer under an external electric field, which can effectively enhance the polarization and energy dissipation, result in a relative high ε′ value. In addition, the PIP–SiC matrix is incompletely crystalline with excess carbon, which can form a relatively continuous conductive network and lead to a high ε′′ value.^29,30 Consequently, the SiC_f/SiC composites without interphase exhibit a relatively high complex permittivity. In contrast, the imaginary part of complex permittivity for composites with MgO interphase is a little lower, which indicates the low electrical conductivity of MgO may have an impact on the complex permittivity of the composites.

According to the transmission line theory, the reflection loss (R_L) of SiC_f/SiC composite with and without MgO interphase can be calculated through a computer simulation with eqn (4)–(6).

R_L = 20 log|(Z_in − Z₀)/(Z_in + Z₀)| (4)

where Z_in and Z₀ are the input impedances of absorber and air. ε_r and μ_r are the relative complex permittivity and permeability of absorber. f is the microwave frequency, d is the thickness of the absorber and c is the velocity of light.

The calculated reflection losses in the X-band frequency range are shown in Fig. 11. It is observed that the minimal reflection loss moves towards a lower frequency, and as well the effective absorption bandwidth (below −10 dB) narrows with an increase in the absorber thickness, which causes the capacity of composites to deteriorate or dissipate the microwave energy in the X-band. The SiC_f/SiC composites without interphase exhibits 2 GHz of −10 dB absorbing bandwidth with the appropriate matching thickness of 2.3 mm, and the minimal value reaches −11.1 dB at 10.2 GHz. This can be explained by the fact that the composites exhibit high complex permittivity, leading to impedance mismatch with air for the sample even with the change of the thickness. To a great extent, electromagnetic waves cannot enter the samples, displaying limited absorption performance, whereas the SiC_f/SiC composites containing MgO interphase exhibit a more effective microwave absorbing capability when the thickness is the same as that of the sample without interphase. The absorption bandwidth of the composites with interphase below −10 dB broadened to 2.7 GHz and the minimum reflection loss of −12.56 dB is observed at 10.15 GHz, which indicates that the low electrical conductivity of the MgO coating may contribute to the improvement of the impedance match between composites and air, broadening the absorption bandwidth.

Fig. 11 Reflection loss of SiC_f/SiC composites: (a) without interphase; (b) with MgO interphase.

4. Conclusion

Magnesium oxide coatings were prepared on KD-I SiC fibers using a dip-coating method using C₄H₆MgO₄·4H₂O as precursor followed by thermal decomposition in vacuum. The effect of concentration of C₄H₆MgO₄·4H₂O in the dipping solution, on the coating microstructure and morphology, mechanical and dielectric properties of SiC_f/SiC composites were investigated. When fibers were coated with 3 wt% C₄H₆MgO₄·4H₂O, the composites exhibited higher flexural strength and improved failure displacement. Because of the low electrical conductivity of MgO, the composites with MgO interphase show a lower complex permittivity, and also the effective absorption bandwidth (below −10 dB) is in the range of 8.8–11.5 GHz, and the calculated reflection loss minimum of −12.56 dB was obtained at 10.15 GHz.

Acknowledgements

This work was supported by Chinese National Natural Science Foundation (Grant No. 51072165) and financially supported by the fund of the State Key Laboratory of the Solidification Processing in NWPU, No. KP201307.

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