Trans. Nonferrous Met. Soc. China 30(2020) 463-471

Effects of SiC interfacial coating on mechanical properties of carbon fiber needled felt reinforced sol-derived Al₂O₃ composites

Kuan-hong ZENG, Qing-song MA, Xing-yu GU

Science and Technology on Advanced Ceramic Fibers & Composites Laboratory, National University of Defense Technology, Changsha 410073, China

Received 2 July 2019; accepted 5 December 2019

Abstract: 3D carbon fiber needled felt and polycarbosilane-derived SiC coating were selected as reinforcement and interfacial coating, respectively, and the sol-impregnation-drying-heating (SIDH) route was used to fabricate C/Al₂O₃ composites. The effects of SiC interfacial coating on the mechanical properties, oxidation resistance and thermal shock resistance of C/Al₂O₃ composites were investigated. It is found that the fracture toughness of C/Al₂O₃ composites was remarkably superior to that of monolithic Al₂O₃ ceramics. The introduction of SiC interfacial coating obviously improved the strengths of C/Al₂O₃ composites although the fracture work diminished to some extent. Owing to the tight bonding between SiC coating and carbon fiber, the C/SiC/Al₂O₃ composites showed much better oxidation and thermal shock resistance over C/Al₂O₃ composites under static air.

Key words: alumina; carbon fiber reinforcement; interfacial coating; mechanical properties; oxidation resistance; thermal shock resistance

1 Introduction

Owing to high hardness, high strength and excellent thermal and chemical stabilities, alumina (Al₂O₃) ceramic is a desirable candidate for friction and wear applications under high velocity and heavy load conditions [1,2]. Unfortunately, the brittle fracture behavior of monolithic Al₂O₃ ceramic is disadvantageous to the friction and wear properties. Therefore, it is necessary to enhance the fracture toughness of monolithic Al₂O₃ ceramic.

Continuous fibers have been considered as the best reinforcement for toughening monolithic ceramics. So far, much attention on oxide fiber reinforced Al₂O₃ composites have been paid to [3-6]. On the contrary, the reports concerning SiC fiber or C fiber reinforced Al₂O₃ composites were rarely found [7,8]. In the study of COLOMBAN and WEY [8], three-dimensional (3D) carbon fiber needled felt reinforced Al₂O₃ (C/Al₂O₃) composites were fabricated through slurry infiltration and heat treatment, followed by alkoxide solution infiltration and pyrolysis for further densification. However, the flexural strength was low (~120 MPa). Further- more, this technique is not suitable to manufacture large-size components with complex shape because of the sedimentation of slurry.

For the densification of 3D fiber preforms like the 3D fiber needled felt, vapor infiltration and liquid impregnation are preferable in order to achieve uniform distribution of matrix. If liquid impregnation is adopted, the choice of starting materials is important. The route from inorganic salt or alkoxide solution via sol-gel to Al₂O₃ ceramics has very low fabrication efficiency. Recently, the sol-impregnation-drying-heating (SIDH) route using the sol with high solid content as raw material has been employed to fabricate 3D fiber preform reinforced oxide ceramic composites [9-15]. Owing to the high solid content and the nano-sized colloidal particle, the SIDH route can improve fabrication efficiency of the route from solution via sol-gel to oxide ceramics and retain its advantages of low processing temperature and homogeneous distribution of oxide matrix.

In previous studies, we fabricated 3D carbon fiber preforms reinforced mullite [12,16], YAG (yttrium aluminum garnet, Y₃Al₅O₁₂) [14,15] through the SIDH route. In addition, the Al₂O₃ composites reinforced with the laminated and stitched carbon fiber cloth were also prepared by the SIDH route [13]. It was indicated that the characteristics of sol and the structure of fiber preform had great influence on the processing and mechanical properties of 3D fiber composites. Therefore, the processing of SIDH route should not be simply copied for different 3D fiber preform reinforced oxide composites.

In addition to densification technologies, interface is very important for fiber reinforced ceramic matrix composites. For oxide fibers/Al₂O₃ composites, several kinds of interfacial coatings have been developed to avoid the formation of chemically bonded interface [3-6,9,10]. For SiC/Al₂O₃ composites, BN/SiC double-layer interfacial coating was prepared by chemical vapor deposition to protect SiC fiber from the oxidation [7] because the Al₂O₃ matrix was obtained by the oxidation of liquid Al. However, the effects of interfacial coating on the performance of C/Al₂O₃ composites were less investigated.

Aiming at the friction and wear applications and from the experience of C/C, C/SiC or C/C-SiC brakes [17-19], 3D carbon fiber needled felt reinforced Al₂O₃ composites with SiC coating as interphase were fabricated through the SIDH route in this work. The processing and the effect of interfacial coating on mechanical properties of C/Al₂O₃ composites under various environments were discussed.

2 Experimental

2.1 Materials and processing

The Al₂O₃ sol used in this study was the same as that in previous work [13]. The solid content, pH, colloid particle diameter and viscosity of Al₂O₃ sol were 30 wt.%, 2.7, 20-30 nm and 12-13 mPa·s, respectively. The thermal analysis, phase evolution and sintering shrinkage of the Al₂O₃ gel powder were studied in our previous work [13]. The 3D carbon fiber needled felt consisted of T300 carbon fiber plain cloth and T700 short-cut carbon fiber web. One unit-layer was composed of a ply of 0° plain cloth and a ply of short-cut fiber web, stacked with 90° plain cloth ply and then another short-cut fiber web ply. After the desired thickness was reached, needling process was performed to keep adjacent units together with carbon fibers carried. The obtained 3D carbon fiber needled felt had a fiber volume fraction of 24%.

Prior to the preparation of C/Al₂O₃ composites, the 3D carbon fiber needled felt was de-sized at 1400 °C [20]. Then, the felt was impregnated with the Al₂O₃ sol under vacuum. After being soaked in the sol for 4 h, the felt was dried at 200 °C for 4 h in air, followed by heat treatment at 1100 °C for 1 h under the protection of high purity Ar. The cycle of impregnation-drying-heat treatment was repeated 33 times to obtain C/Al₂O₃ composites. To study the effect of interfacial coating on properties of C/Al₂O₃ composites, SiC coating was prepared on the surface of carbon fibers after de-sizing. Considering the homogeneousness and reliability of coating, especially in case of large-size component with complex shape, polycarbosilane (PCS) pyrolysis was employed to prepare SiC coating. The de-sized carbon fiber needled felt was impregnated with PCS/xylene solution under vacuum. The mass fraction of PCS in solution was 5%. After being soaked in PCS/xylene solution for 4 h, the felt was taken out and dried at 80 °C for 4 h in air, followed by pyrolysis at 1600 °C for 1 h under the protection of high purity Ar. The cycle of PCS solution impregnation and pyrolysis was repeated 3 times to obtain SiC coating. Then, the Al₂O₃ matrix was introduced by the above-mentioned SIDH procedure. The obtained composites were named C/SiC/Al₂O₃ composites.

The as-fabricated C/Al₂O₃ and C/SiC/Al₂O₃ composites were oxidized and thermally shocked under static air. The oxidation was carried out at 400, 600, 800 and 1000 °C for 30 min, respectively. The thermal shock was performed from 400, 600, 800 and 1000 °C to room temperature, respectively. The muffle was heated to preset temperature. Then, the sample was put into muffle. After being soaked for 10 min, the sample was moved out of muffle. After being cooled to room temperature, the sample was put into muffle again. The thermal shock was repeated 10 times.

2.2 Characterization methods

According to the density mixing law, the theoretic density (ρ_T) of C/Al₂O₃ composites was calculated from the equation of ρ_T=V_f·ρ_f+V_m·ρ_m, where V_f and V_m are the volume fractions of fiber (24%) and matrix (76%), and ρ_f and ρ_m are the true densities of fiber (1.76 g/cm³) and Al₂O₃ matrix (3.90 g/cm³), respectively. The apparent density (ρ_a) was obtained from the mass-to-volume ratio. Thus, total porosity was equal to 1-(ρ_a/ρ_T).

The as-fabricated composites were cut into the samples with a size of 70 mm (length) × 5 mm (width) × 4 mm (height) for the flexural strength test. The flexural strength was measured by three-point bending test with a cross-head speed of 0.5 mm/min and a span-to-height ratio of 15:1. Fracture work was calculated from the formula of W=A_C/(B·H) [21], where A_C is the characteristic area of fracture curve, which refers to the area under load–displacement curve above 90% stress; H and B are the thickness and width of the sample, respectively. The samples for interlaminar shear strength test had the size of 30 mm (length) × 5 mm (width) × 4 mm (height). Short beam method was employed to measure interlaminar shear strength with a cross-head speed of 0.5 mm/min and a span-to-height ratio of 5:1. The samples with a size of 12 mm × 4 mm × 4 mm were cut from the as-prepared composites for the compressive strength test. The compressive strength test was carried out on Z direction of sample. The Z direction was perpendicular to carbon fiber cloth. The composites were machined into dog-bone shaped samples for tensile strength test. Five specimens were tested to obtain average strength.

Mass loss and flexural strength retention after oxidation and thermal shock were recorded to characterize the oxidation resistance and thermal shock resistance of the composites. Scanning electron microscopy (SEM, Quanta FEG 250) was employed to observe the microstructure of the composites.

3 Results and discussion

3.1 Effects of SiC interfacial coating on mechanical properties of C/Al₂O₃ composites

The apparent density of C/Al₂O₃ composites was measured to be 2.68 g/cm³ from the mass-to- volume ratio. According to the density mixing law, the theoretical density was computed to be 3.39 g/cm³. Thus, the total porosity is calculated to be 20.8%. Figure 1 shows the cross-sections of C/Al₂O₃ composites and Al₂O₃ matrix. As shown in Fig. 1(a), carbon fiber preform was well filled by Al₂O₃ matrix, and the cross-section was relatively dense except several large pores. Due to the low viscosity and the nano-sized colloid particles, it is easy for Al₂O₃ sol to diffuse into carbon fiber needled felt in early cycles. With the increase of density, the diffusion channels became narrow and wandering. Thus, some voids were not occupied by Al₂O₃ matrix. When the diffusion channels were completely clogged, the closed pores formed. Besides the large closed pores, as shown in Figs. 1(b) and (c), the composites contained some small pores and some microcracks. The microcracks resulted from the under-sintering of Al₂O₃ matrix at 1100 °C and the thermal mismatch between carbon fiber and Al₂O₃ matrix. The small pores inside fiber bundle could also be attributed to the diffusion of sol.

Fig. 1 Cross-sectional morphologies of C/Al₂O₃ composites (a, b) and Al₂O₃ matrix (c)

In comparison with the carbon fiber needled felt reinforced YAG (C/YAG) composites [15] which were also fabricated by SIDH route, the vertical cracks in fiber web and the gaps between fiber cloth and fiber web were not observed in this study. This phenomenon is related with the state of thermal stress. On one hand, the discrepancy of thermal expansion coefficients between carbon fiber and Al₂O₃ is less than that between carbon fiber and YAG. On the other hand, the heat treatment was carried out at 1100 °C in this study whereas the highest temperature of heat treatment was 1600 °C for the fabrication of C/YAG composites. At 1100 °C, the Al₂O₃ matrix exhibited a sintering linear shrinkage of ~13% [13]. The linear shrinkage of YAG at 1600 °C was ~23% [14]. Due to the smaller thermal mismatch between carbon fiber and Al₂O₃ matrix and the lower linear shrinkage, the thermal stress in C/Al₂O₃ composites was lower than that in C/YAG composites.

Table 1 lists the mechanical properties of C/Al₂O₃ and C/SiC/Al₂O₃ composites. The flexural strength, fracture work and compressive strength of C/Al₂O₃ composites were 155.9 MPa, 6772.2 J/m² and 274.2 MPa, respectively, which could meet the demands of brake application. Moreover, considering the high total porosity of 20.8%, the mechanical properties of C/Al₂O₃ composites are favorable. Figures 2 and 3 show the fracture surface and the stress-strain curve of C/Al₂O₃ composites, respectively. Extensive fiber pull-out and long pull-out length were observed clearly. The flexural strain was as high as 2.5%. These results indicated that the C/Al₂O₃ composites exhibited non- catastrophic fracture behavior and the fracture toughness of monolithic Al₂O₃ ceramics was enhanced notably.

Table 1 Mechanical properties of C/Al₂O₃ and C/SiC/ Al₂O₃ composites

Fig. 2 Morphologies of fracture surfaces for C/Al₂O₃ composites

Fig. 3 Stress-strain curves of C/Al₂O₃ and C/SiC/Al₂O₃ composites

Fiber reinforced ceramic matrix composites are often fabricated at high temperature. Sometimes, high pressure is also required. Therefore, it is very likely to form strong interfacial bonding. Once the strong interfacial bonding belongs to the chemical bonding, the mechanical properties of composites are affirmatively low because fibers are destroyed by chemical reaction. In this study, the heat treatment during fabrication of C/Al₂O₃ composites was performed at 1100 °C without pressure. In this case, the formation of strong chemical bonding was impossible. As mentioned above, the Al₂O₃ matrix was porous and under-sintered. The thermal stress derived from the mismatch of thermal expansion between carbon fiber and Al₂O₃ matrix could be released. Thus, strong physical bonding was also difficult to create. As a result, C/Al₂O₃ composites showed high fracture work and flexural strain. In our previous study [13], the laminated and stitched carbon fiber cloth reinforced Al₂O₃ composites were fabricated at 1400 °C and had a total porosity of 15.5%. Due to the higher processing temperature and lower total porosity, the interfacial bonding was relatively strong, resulting in lower flexural strain and fracture work (3259.6 J/m²).

The morphology of SiC coating is shown in Fig. 4. It is shown that carbon fiber was well coated by SiC. The SiC coating was dense and smooth, and its thickness was about 250 nm. The microstructure of C/SiC/Al₂O₃ composites is displayed in Fig. 5. In comparison with C/Al₂O₃ composites (Fig. 1), the quantity and size of pores in C/SiC/Al₂O₃ composites obviously decreased. Thus, it is found from Table 1 that the C/SiC/Al₂O₃ composites had lower total porosity. By comparing Fig. 5(b) with Fig. 1(b), it is evident that the intra-bundle density of C/SiC/Al₂O₃ composites was higher than that of C/Al₂O₃ composites. As compared with the water- based Al₂O₃ sol, it is easier for the PCS/xylene solution to diffuse into intra-bundle space. In addition, SiC coating has a thermal expansion coefficient between that of carbon fiber and Al₂O₃. As a result, the interfacial microcracks were reduced.

Fig. 4 SEM images of surface (a) and cross-section (b) of SiC-coated carbon fiber

Fig. 5 SEM images of cross-section of C/SiC/Al₂O₃ composites

Due to the good affinity between PCS and carbon fiber and the relatively smaller thermal expansion discrepancy between SiC and carbon fiber, stronger interfacial bonding was created for C/SiC/Al₂O₃ composites. In Fig. 6, fiber pull-out was not as extensive as that of C/Al₂O₃ composites. And the surface of pull-out fibers was smoother, which was due to the debonding from the SiC/ Al₂O₃ interface. Accordingly, as listed in Table 1, the fracture work of C/SiC/Al₂O₃ composites was lower than that of C/Al₂O₃ composites. At the same time, it is found from Fig. 3 that the flexural strain of C/SiC/Al₂O₃ composites was about 1.5%, which was less than that of C/Al₂O₃ composites. However, the strong interfacial bonding still belonged to physical bonding. In this situation, the fiber strength was not obviously impaired. Moreover, the strong interfacial bonding is beneficial to the load-transfer from matrix to carbon fibers. Consequently, the flexural strength, interlaminar shear strength, compressive strength and tensile strength were promoted owing to the high strength of carbon fibers. Especially, the interlaminar shear strength, compressive strength and tensile strength were increased to a great extent.

Fig. 6 Morphologies of fracture surfaces for C/SiC/Al₂O₃ composites

3.2 Effects of SiC interfacial coating on oxidation and thermal shock resistance of C/Al₂O₃ composites

The mass loss and flexural strength retention ratio of C/Al₂O₃ and C/SiC/Al₂O₃ composites after oxidation and thermal shock are presented in Tables 2 and 3, respectively. For the C/Al₂O₃ composites, mass loss and flexural strength loss were detected from 400 °C because of their porous microstructure (Fig. 1). With the elevation of temperature, mass loss and flexural strength loss increased. Since the Al₂O₃ matrix is immune to oxidation, the mass loss can be ascribed to the oxidation of carbon fibers. As shown in Fig. 7(a), the interface gaps resulting from the oxidation of carbon fibers were visible. After oxidation, the sample was cut. Figure 7(b) shows the optical image of the side face. The surface of sample was white and the interior was black, indicating that the oxidation occurred from surface to interior. In addition, the white layer thickened from 800 to 1000 °C, implying that the oxidation depth increased with the elevation of temperature.

Table 2 Oxidation resistance of C/Al₂O₃ and C/SiC/ Al₂O₃ composites

Table 3 Thermal shock resistance of C/Al₂O₃ and C/SiC/Al₂O₃ composites

Fig. 7 SEM image of cross-section of C/Al₂O₃ composites after oxidation at 1000 °C (a) and optical images of side faces of C/Al₂O₃ (b) and C/SiC/Al₂O₃ (c) composites after oxidation

It is noticed that the mass loss and flexural strength loss after thermal shock were much higher than those after oxidation. Firstly, 10 times of thermal shock in air corresponded to static oxidation for 100 min, whereas the static oxidation was carried out only for 30 min. Secondly, the thermal stress was enlarged during thermal shock, creating more diffusion channels for oxygen and impairing the load-bearing capacity of matrix. By comparing Figs. 7(a, b) with Figs. 8(a, b), the severer oxidation of carbon fibers during thermal shock was very apparent.

Fig. 8 SEM image of cross-section of C/Al₂O₃ composites after thermal shock at 1000 °C (a) and optical images of side faces of C/Al₂O₃ (b) and C/SiC/Al₂O₃ (c) composites after thermal shock

In addition, it seems strange that the flexural strength retention ratio was increased with the elevation of temperature from 600 to 1000 °C, whereas the mass loss was on the contrary. In our previous studies [15,22], this phenomenon was considered to be related with the state of microcracks during static oxidation and thermal shock. The C/Al₂O₃ composites were fabricated at 1100 °C and had some microcracks when the composites were cooled to room temperature due to the thermal mismatch between carbon fiber and Al₂O₃ matrix. When static oxidation and thermal shock were performed at 1000 °C, these micro- cracks could be better healed. The closure of microcracks could enhance the load-bearing capacity of matrix to a certain extent, which was responsible for the higher flexural strength retention ratio at 1000 °C. Of course, the microcracks could not be entirely closed. With the elevation of temperature, the oxidation of carbon fibers became serious. Therefore, the mass loss still increased from 600 to 1000 °C.

By comparison, the positive effects of SiC interfacial coating on the oxidation and thermal shock resistance of C/Al₂O₃ composites are significant. After static oxidation at 800 and 1000 °C, no flexural strength degradation was observed for the C/SiC/Al₂O₃ composites. After thermal shock at 800 and 1000 °C, the flexural strength ratios were above 80%. In Figs. 7 and 8, it is clear that the oxidation degrees of C/SiC/Al₂O₃ composites were much lower than those of C/Al₂O₃ composites. As mentioned above, SiC coating was tightly bonded to carbon fibers. Thus, carbon fibers can be effectively protected by the viscous SiO₂ derived from the oxidation of SiC coating during static oxidation and thermal shock. However, there is thermal mismatch between carbon fiber and SiC coating. Microcracks were probable to form at fiber/coating interface during thermal shock, leading to larger mass loss than that during static oxidation. With regard to the mass loss under static oxidation, it may be caused by the attack of oxygen from the end of carbon fiber. This oxidation mode exhibited the characteristics of large range and low degree. Namely, many carbon fibers were oxidized to a very low degree. As a result, the flexural strength almost did not decrease.

4 Conclusions

(1) 3D carbon fiber needled felt reinforced Al₂O₃ composites have been fabricated through the SIDH route using the Al₂O₃ sol with a high solid content as raw materials. Thanks to the reinforcement of 3D carbon fiber needled felt, the fracture work of C/Al₂O₃ composites was as high as 6772.2 J/m² which was much higher than that of monolithic Al₂O₃ ceramics. Although the total porosity reached 20.8%, the C/Al₂O₃ composites showed favorable strengths.

(2) The fracture work decreased to 4828.3 J/m² by introducing polycarbosilane-derived SiC interfacial coating, but the flexural strength, interlaminar shear strength, compressive strength and tensile strength were increased to 168.1, 21.2, 468.8 and 90.2 MPa, by 94.5%, 71.0% and 42.3%, respectively.

(3) Due to the protection of SiC coating, the oxidation and thermal shock resistance of C/Al₂O₃ composites were enhanced notably. The flexural strength of C/SiC/Al₂O₃ composites kept unchanged after oxidation at 1000 °C for 30 min. After 10 times of thermal shock from 1000 °C to room temperature under static air, the C/SiC/Al₂O₃ composites retained 83.1% of original flexural strength. In all, C/SiC/Al₂O₃ composites are promising candidates for friction and wear applications.

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SiC界面涂层对碳纤维针刺毡增强Al₂O₃复合材料力学性能的影响

曾宽宏，马青松，顾星宇

国防科技大学 新型陶瓷纤维及其复合材料重点实验室，长沙 410073

摘 要：分别采用3D碳纤维针刺毡为增强体以及聚碳硅烷(PCS)衍生SiC涂层为界面相，通过溶胶-浸渍-干燥-热处理 (SIDH) 技术制备C/Al₂O₃复合材料，研究SiC界面涂层对C/Al₂O₃复合材料力学性能、抗氧化性能和抗热震性能的影响。结果表明，C/Al₂O₃复合材料的断裂韧性显著优于Al₂O₃单体陶瓷，引入SiC界面涂层后，尽管断裂功有一定程度下降，但C/Al₂O₃复合材料的强度得到明显提高；得益于SiC涂层和C纤维之间的强结合，C/SiC/Al₂O₃复合材料在静态空气中表现出明显优于C/Al₂O₃复合材料的抗氧化和抗热震性能。

关键词：氧化铝；碳纤维增强体；界面涂层；力学性能；抗氧化性能；抗热震性能

(Edited by Bing YANG)

Corresponding author: Qing-song MA; Tel: +86-731-87007601; Fax: +86-731-84576578; E-mail: nudtmqs1975@163.com

DOI: 10.1016/S1003-6326(20)65226-8