- Email: [email protected]
Mullite fiber cloth-reinforced mullite composite fabricated via an optimized layer-by-layer assembly method
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Mullite fiber cloth-reinforced mullite composite fabricated via an optimized layer-by-layer assembly method Dazhao Liu, Mingyi Tan, Cheng Fang, Wenbo Han
⁎
National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, PR China
ARTICLE INFO
ABSTRACT
Keywords: Ceramic composites Laminated Mechanical properties Mullite Fibers Microstructure
This article reports the fabrication of fiber cloth-reinforced mullite composite via an optimized layer-by-layer (LbL) assembly method involving the infiltration and pyrolysis of fiber cloth with mullite precursor (PIP) followed by brushing matrix onto the treated fiber cloth and hot pressing. The influence of fiber content on the microstructure and mechanical properties of composite are investigated and the crack propagation is discussed to explain the toughening mechanisms. The composite with 30 vol% fiber content exhibited a high density 2.89 g/cm3, flexural strength 135.5 MPa and fracture toughness 4.13 MPa m1/2. After PIP pretreatment, the gaps existed in the fiber bundle are filled with mullite matrix which transform from mullite precursor, improving the density from 2.89 g/cm3 to 2.94 g/cm3, flexural strength from 135.5 MPa to 163.2 MPa and fracture toughness from 4.13 MPa m1/2 to 4.55 MPa m1/2. The optimized LbL assembly method realizes the creation of fiber clothreinforced mullite composites with high density and excellent mechanical properties.
1. Introduction Mullite ceramics, which exhibit outstanding mechanical properties, good thermal shock resistance, low thermal expansion coefficient, high creep resistance and good chemical and thermal stability, are applied as high-temperature structural materials [1–3]. However, fabricating strong and damag-tolerant composites is a challenge due to the intrinsic brittleness and low fracture toughness of ceramics. A large number of toughening measures are developed to fabricate mullite matrix composites with favorable mechanical properties such as introducing whiskers, chopped fibers, one dismension continuous fiber or fiber cloth. Among these measures, one dismension continuous fiber and fiber cloth reinforced mullite composite have enticed crucial scientific interest for high-temperature applications due to their remarkable toughening effect [4–7]. Recently, fiber reinforced mullite matrix composites have been fabricated via layer-by-layer (LbL) assembly [8–10]. However, the density and mechanical properties of composite obtained through LbL assembly are not high enough and should be enhanced to satisfy the requirements of specific applications. What's more, the microstructure of composite reinforced by one dismension continuous fiber are uncontrollable and the distribution of fiber filament or bundle in matrix was not uniform. The fabrication of one dismension continuous fiber reinforced composite is complex and the mechanical properties of composite were anisotropy, which is
⁎
depended on the cut direction: the flexural strength of sample cut along the fiber is much higher than the samples cut in other two directions, limiting the application of mullite matrix composite. In the present work, we choose fiber cloth as reinforced component to prepare mullite matrix composite with controllable structure and isotropic mechanical property via an improved LbL assembly method. During the process, fiber cloths are pretreated with mullite precursor. Then, fiber reinforced mullite matrix composite is fabricated via traditional LbL assembly method. 2. Experimental procedure 2.1. Matrix materials Mullite powder (> 99%, Northwest Institute for Non-ferrous Metal Research, China) was used as the matrix material. The matrix slurry was prepared using mullite powder, 0.5 wt% of polyacrylic acid (> 97%, Fuchen, China) as binder, and 0.2 wt% of polyacrylamide as dispersant. The mixture was stirred for 20 min and solid content of mixture was 60 wt% to keep good fluidity. 2.2. Preparation of SiC coating Aluminosilicate fiber cloth (Nextel 720, 3 M Corp., MN, USA) was
Corresponding author. E-mail address: [email protected] (W. Han).
https://doi.org/10.1016/j.ceramint.2018.09.167 Received 9 August 2018; Received in revised form 3 September 2018; Accepted 16 September 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, D., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.167
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 1. Flowchart of fabrication of fiber cloth-reinforced mullite composites via optimized LbL assembly.
Fig. 2. Surface and fractured micrographs of composites with 20 vol% fiber content in XZ direction: (a), (b) and YZ direction: (c), (d); (e) and (f) are the SEM images of fiber coating.
used as the reinforced component because of its excellent properties. In this study, the SiC coating was prepared on the mullite fiber by dip coating, and polycarbosilane (provided by the National University of Defense Technology) was used as the precursor. The fiber cloths were dipped into the precursor solution (0.01 mol/L) and ultrasonically vibrated for 10 min and dried in air. After the dipping process, all dipped fiber cloths were treated at 1400 °C for 1 h in a tubular furnace with argon atmosphere.
2.4. Composite processing Fig. 1 shows the manufacturing procedure of fiber cloth-reinforced composite. First, coated fiber cloth was cut into 36 mm × 36 mm and impregnated into the mullite precursor solution and pyrolyzed (PIP) at 1000 °C. Then, the treated fiber cloth was brushed with matrix slurry and manually stacked up and dried at 50 °C. Finally, the green body was hot press sintered for 1 h in vacuum at 1250 °C and the sintering pressure was 30 MPa. The obtained composite was named Mx (the x referred to the PIP cycle) and incised for mechanical property testing and microstructure observation. We also prepared the composite reinforced by fiber cloth, which was not treated with the mullite precursor, and untreated composite was named M0.
2.3. Preparation of mullite precursor The aluminum and silicon sources of mullite were aluminum nitrate nonahydrate (ANN, Fuchen, China) and tetraethoxysilane (TEOS, Fuchen, China), respectively. Biphase sol with molar ratio of Al/Si 3:1 was fabricated as follows: 75.3 g of ANN was dissolved in 200 mL deionized water and stirred at 50 °C for 24 h. Next, 208.33 g of TEOS dissolved in EtOH previously was dropped into the ANN solution and stirred at 40 °C for 96 h. The obtained precursor solution was used for preparing the composite. During the fabrication of mullite precursor, the PH of mixture was neutral (PH=7) and the hydrolysis of TEOS was promoted by keeping at 40 °C.
2.5. Characterization The morphology and microstructure of the composite were investigated using a focusing ion/electronic double-beam microelectron microscope (FIB/SEM, Helios NanoLab 600i, FEI, USA). The phases were analyzed via X-ray diffraction (XRD, Empyrean, PANalytical, Netherlands). The density of the composite was measured using the Archimedes method. The chemical reaction that formed mullite was 2
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 3. Surface micrographs and load displacement curves of composites with 20 vol% fiber content in XZ direction: (a), (b) and YZ direction: (c), (d).
Fig. 4. Surface micrographs of composites with different fiber content: (a), (b), (c) and (d) are referred to 10 vol%, 20 vol%, 30 vol% and 40 vol%, respectively.
3
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
3.2. Influence of fiber content on the microstructure and mechanical property of mullite matrix composite
Table 1 Density, relative density, and mechanical properties of composite with different fiber content. Sample
Fiber content （vol%）
Density (g/cm3)
Relative density (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
1 2 3 4
10 20 30 40
2.99 2.93 2.89 2.81
93.4 91.5 90.3 87.8
175.4 166.7 134.6 120.3
3.11 3.67 4.15 4.78
Fig. 4 shows the surface morphology of composites with different fiber contents, containing 10 vol%, 20 vol%, 30 vol% and 40 vol% fiber, respectively. It could be seen that with the increase of fiber content, the thickness of matrix layer between the fiber layers decreased continuously and the increase of fiber content also led to the increase of the gaps (in the fiber bundle) within the composite. The density and flexural strength of composite decreased due to the decrease of matrix and the increase of interstices in the materials, but the fracture toughness of the composite increased to a certain extent (as shown in Table 1). The composite with 10 vol% fiber content displayed highest relatively density 93.4%, flexural strength 175.4 MPa and lowest fracture toughness 3.11 MPa•m1/2. On the contrary, the composite with 40 vol% fiber content exhibited lowest relatively density 87.8%, flexural strength 120.3 MPa and highest fracture toughness 4.78 MPa m1/2. Therefore, it was necessary to select the fiber content reasonably according to the actual service environment of the composite to meet the application requirements. Fig. 5 shows the fractured images and three-point bending curves of composites with different fiber content. As shown in Fig. 5, the fractured morphology of composites with different fiber content were all concave and uneven, which was different from the fracture of pure ceramic. It was ascribed to the crack deflection, branching along the weak interface between the fiber and matrix or crack propagation termination as the crack tip extended into the interstitial space of the fiber bundle. With the increase of fiber content, the more such weak interface and gap, the more difficult the crack propagation would be. On the macro level, the flexural strength and density of the material decreased and fracture toughness increased. By comparing flexural curve of the composites with different fiber content, it could be concluded that the fracture mode changed from brittle fracture to damage tolerant behavior and the slope of the load-drop curve decreased following the increase of fiber content (Fig. 5b, d, f and h). It could be explained that the increase of fiber content resulted in more setback such as weak interface and gaps, inhibiting the propagation of crack and preventing the composite from devastation. On the whole, fiber content had an obviously influence on the microstructure of composite and led to the decrease of density and flexural strength and the improvement of fracture toughness.
characterized with a thermogravimetry (TG)/differential scanning calorimetry (DSC) instrument (Netzsch STA 449C, Germany) and Fourier transform infrared spectroscopy (FTIR, PerkinElmer 2000, USA) using KBr disks. Three-point bending test was performed on an Instron (USA) test machine with a crosshead speed of 0.5 mm/min and the size of samples tested on Instron (USA) test machine were 36 mm × 4 mm × 3 mm for three-point bending test and 22 mm × 4 mm × 2 mm for fracture toughness test. 3. Results and discussion 3.1. Characterization of the mullite matrix composite Fig. 2 shows the surface and fractured morphology of mullite matrix composite reinforced via 20 vol% fiber cloth, which was not impregnated into mullite precursor and pyrolyzed. Fig. 2a and b show the surface and fractured morphology of sample cut along the direction of XZ plane, and Fig. 2c and d were the surface and fractured morphology of sample cut along the direction of YZ plane. It could be seen from the SEM images that the fiber cloth distributed evenly in the mullite matrix, and the surface and cross section of the samples obtained from the two directions were similar and the thickness of the substrate between the fibers were also close. As shown in Fig. 2a and c, some uneven distributed gaps were observed, which was ascribed to intrinsic characteristic of fiber bundle and fiber coating. On one hand, small gaps existed inherently among the fiber monofilaments with circular section and it was difficult to introduce big mullite particles to fill these small gaps during the fabrication of composite. On the other hand, the fiber coating (Fig. 2e and f) inhibited the sintering between mullite fibers in the subsequent sintering process, protecting the fibers, so some gaps finally remained in the composite. Fig. 3 shows crack propagation of mullite composite with 20 vol% fiber content and corresponding three-point bending curve along X axis in XZ plane (XZ direction) and Y axis in YZ plane (YZ direction), respectively. Fig. 3a and b were surface morphology and three-point bending curve of sample cut in XZ direction and Fig. 3c and d were surface morphology and three-point bending curve of sample cut in YZ direction. It could be seen from the three-point bending curves of two directions that the fiber cloth reinforced composite materials exhibited a damage-tolerant fracture behavior, indicating that the introduction of fiber cloth could hinder the rapid propagation of crack in the matrix and consume the fracture energy, having an obviously toughening effect. As expected and shown in Fig. 3a and c, the crack propagation mode of the samples cut along XZ and YZ directions were similar during the fracture process. The crack deflected an angle at the weak interface between the fiber and mullite matrix or the gaps inside fiber bundles, consuming more fracture energy. It could be demonstrated that the interstitial space in the fiber bundle could also contribute to the fracture toughness while reducing the density and bending strength of the composite.
3.3. Characterization of the mullite precursor As the above content mentioned, there were several gaps in the composite, so the density and mechanical strength of composite was not high enough to meet the requirement. In this work, the interstitial space between single filaments of the fiber bundle were filled by the method of impregnation and pyrolysis with the mullite precursor, which would be discussed as following. According to the TG-DSC curve (Fig. 6), the precursor was constantly in weightlessness from room temperature to 500 °C, and total weight loss was 59.1%. The DSC curve at 161.9 °C had an obvious endothermic peak，which was mainly ascribed to the decomposition of aluminum nitrate and removal of organic constituents. Another peak at 980 °C was an exothermic peak, resulting from the crystallization of mullite. From the TG-DSC curve, we preliminarily believed that the precursor could be crystallized under 1000 °C [11–13]. Fig. 7 depicts the XRD patterns of precursor samples treated at different temperatures. The diffuse peak at 67.2° of 900 °C curve was the characteristic peak of Al-Si spinel, indicating the precursor started to transform into alumina-silica (Al-Si) spinel (6Al2O3·SiO2) [14]. After 1000 °C heat treatment, the diffraction peak at 26.4° of mullite phase appeared, which was ascribed to the transformation from Al-Si spinel phase to mullite phase. The XRD pattern of the precursor pyrolyzed at 1100 °C shows enhanced mullite diffraction peak, due to the increase of 4
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 5. Fractured surface micrographs and load displacement curves of composites with different fiber content: (a), (c), (e) and (g) are referred to 10 vol%, 20 vol%, 30 vol% and 40 vol%, respectively; (b), (d), (f) and (h) are corresponding load displacement curves.
5
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
From the FTIR spectra (Fig. 8) of precursor pyrolyzed at 900 °C, the absorption band at 1130 cm−1 corresponding to Si-O tetrahedra could be seen, which was similar to the standard FTIR spectra of sample containing Al-Si spinel and amorphous silica, indicating that the precursor changed into Al-Si spinel at 900 °C. With the increase in temperature, the absorption band at 1130 cm−1 broadened, resulting from the interaction effect of the Si-O keys in Al-Si spinel and amorphous silica. The spectrum absorption peak at 820 cm−1 of the 1000 °C curve was obvious, demonstrating that more 6 coordination of aluminum ion had been converted into 4 coordination at 1000 °C. As shown in Fig. 4, the sample sintered at 1200 °C gave absorption bands at 560, 730, 820, and 1130 cm−1. The 560 cm−1 absorption band in the spectra was caused by the vibration of AlO6 stretching mode. The absorption bands at 730 and 820 cm−1 were caused by the vibration of AlO4 stretching mode, and the Si-O absorption band was located at 1130 cm−1. After sintered at 1200 °C, mullite precursor had been completely transformed into mullite, and the Si-O absorption band at 1130 cm−1 became narrow and moved toward the Takanami number, indicating that the interaction effect of the Si-O keys in Al-Si spinel and amorphous silica had greatly weakened [15].
Fig. 6. TG-DSC curve of the mullite precursor from RT to 1200 °C.
3.4. Microstructure and mechanical properties of the mullite composite treated by the PIP method Fig. 9 shows the morphology of the fiber cloth in XY and XZ directions. The morphology of fiber cloth which was not treated via the PIP method shows that no mullite matrix existed between the fibers (Fig. 9a). Fig. (9c, e and g) depict the morphology of fiber cloth, which was infiltrated by mullite precursor and pyrolyzed at 1000 °C (temperature was determined according to the above contents: XRD, FTIR, and TG characterization) for 1 h. The PIP cycles were one, two, and three times, respectively. The gaps between the fibers were filled with mullite matrix after PIP, and the matrix content increased with the number of cycles. After three cycles, the fibers were completely coated with mullite matrix (Fig. 9g and h), leading to the improvement of mechanical properties and density of fiber cloth reinforced mullite composite. In this study, the mullite composite with 30 vol% fiber content was chosen as research object. Fig. 10 shows the surface morphology of the composites prepared by two methods (LbL and LbL assisted by PIP). Comparing the experimental results, there were some differences between the composites obtained through the two processes. As shown in Fig. 10a (composite named M0), several obvious gaps existed between the fiber cloth and matrix or in the fiber bundle. After infiltration and pyrolysis for three cycles, the fibers were completely surrounded by the mullite matrix, and during the subsequent sintering, the quasi-mullite transformed from the precursor would be sintered with brushed mullite matrix, obtaining a composite (named M3) with high density and excellent mechanical properties (Fig. 10c). Compared with M0, the composite M3 displayed an enhanced flexural strength 163.2 MPa, fracture toughness 4.55 MPa m1/2 and high relative density 91.8% (in Table 2). As shown in Fig. 11, the composite M3 exhibited highest relative density, second highest flexural strength and fracture toughness among the four composites. The highest relative density resulted from the three cycles PIP for filling the gaps in the fiber bundle, leading to improvement of flexural strength and fracture toughness.
Fig. 7. XRD patterns of production from mullite precursor at temperature of 900 °C, 1000 °C, 1100 °C, and 1200 °C.
Fig. 8. FTIR spectra of production from mullite precursor at temperature of 900 °C, 1000 °C, 1100 °C, and 1200 °C.
4. Conclusion
mullite phase content. As the temperature increased to 1200 °C, the precursor transformed into mullite phase completely. The temperature was much lower than the required temperature (1400 °C) of solid phase reaction to completely form mullite and close to the sol gel method, using the soluble aluminum salt and TEOS as raw materials.
In this study, the composite with high density and excellent mechanical properties was prepared by the optimized LbL assembly method. The fiber content had an obviously influence on the microstructure of composite, resulting in the decrease of density and flexural strength but the improvement of fracture toughness. The crack propagation was investigated. The results showed that the crack deflected or branched along the weak interface between the matrix and fiber, 6
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 9. Micrographs of the fiber cloth with different PIP cycles in two directions: (a), (c), (e) and (g) are referred to zero, one, two, and three cycles in XY direction; (b), (d), (f) and (h) are in XZ direction.
Fig. 10. SEM micrographs and load displacement curves of composites with 20 vol% fiber content: (a) and (b) composite with three PIP cycles pretreatment (M0); (c) and (d) composite without pretreatment (M3).
enhancing fracture toughness and the residual gaps in the bundle also hindered crack propagation. The density and mechanical properties of the composite could be improved effectively by pretreatment, in which the fiber cloth was infiltrated into the mullite precursor and pyrolyzed at 1000 °C. After PIP for three cycles, the relative density of the composite with 30 vol% fiber content increased from 90.3% to 91.8%,
resulting in high flexural strength (163.2 MPa) and fracture toughness (4.55 MPa•m1/2). The composite obtained through optimized LbL assembly method could satisfy the requirements of specific applications for high-temperature structural materials.
7
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
and National Natural Science Foundation of China (Nos. 11572105 and 51772061).
Table 2 Density, relative density, and mechanical properties of M0 and M3. Sample
Density (g/ cm3)
Relative density (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
M0 M3
2.89 2.94
90.3 91.8
134.6 163.2
4.13 4.55
References [1] B. Kanka, H. Schneider, Aluminosilicate fiber/mullite matrix composites with favorable high-temperature properties, J. Eur. Ceram. Soc. 20 (5) (2000) 619–623. [2] C. Kaya, E.G. Butler, A. Selcuk, et al., Mullite (Nextel™ 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties, J. Eur. Ceram. Soc. 22 (13) (2002) 2333–2342. [3] W. Krenkel, R. Naslain, H. Schneider, Processing and Characterisation of Mullite (Nextel™ 720) Fibre-Reinforced Mullite Matrix Composites from Hydrothermally Processed Mullite Precursors[M]// High Temperature Ceramic Matrix Composites, Wiley‐VCH Verlag GmbH & Co. KGaA, 2006, pp. 639–644. [4] K. Nubian, B. Saruhan, B. Kanka, et al., Chemical vapor deposition of ZrO2, and C/ ZrO2, on mullite fibers for interfaces in mullite/aluminosilicate fiber-reinforced composites, J. Eur. Ceram. Soc. 20 (5) (2000) 537–544. [5] W. Krenkel, R. Naslain, H. Schneider, Stability of Mullite-Precursor Versus Potential Fiber-Coating Material[M]// High Temperature Ceramic Matrix Composites, Wiley‐VCH Verlag GmbH & Co. KGaA, 2006, pp. 164–167. [6] B. Kanka, Ceramic fiber composite material: US, US7919039[P], 2011. [7] O. Reese B. Saruhan H. Schneider et al., Fabrication of Continuous Fiber-Reinforced Ceramics with a Nanosized Mullite Precursor[C]// Ht-Cmc. DLR, 1995. [8] S. Kang, L. Wang, J. Zhang, et al., Electroreductive coupling layer-by-layer (ERCLbL) assembly, Acs Appl. Mater. Interfaces 9 (2017) 37. [9] H. Qiu, Layer-by-layer self-assembly of ceramic particles for complex shape coating synthesis, J. Am. Ceram. Soc. 89 (4) (2010) 1180–1187. [10] J. Shi, L. Zhang, Z. Jiang, Facile construction of multicompartment multienzyme system through layer-by-layer self-assembly and biomimetic mineralization, Acs Appl. Mater. Interfaces 3 (3) (2011) 881. [11] O. Reese B. Saruhan B. Kanka et al. Fabrication of continuous fiber-reinforced ceramics with a nanosized mullite precursor, 1995. [12] S. Baitalik, N. Kayal, O.P. Chakrabarti, Properties of mullite-bonded SiC ceramics from mullite precursor sol coated SiC powder and crystallization kinetics of mullite formation, Mater. Res. Innov. 20 (2) (2016) 99–105. [13] L. Radonjić, Formation of sol-gel nanostructured mullite by additions of fluoride ion, J. Therm. Anal. Calorim. 79 (3) (2005) 487–492. [14] D. Roy, B. Bagchi, A. Bhattacharya, et al., A comparative study of densification of sol‐gel‐derived nano‐mullite due to the influence of iron, nickel and copper ions, Int. J. Appl. Ceram. Technol. 11 (6) (2014) 1054–1060. [15] B. Bagchi, S. Das, A. Bhattacharya, et al., Nanocrystalline mullite synthesis at a low temperature: effect of copper ions, J. Am. Ceram. Soc. 92 (3) (2009) 748–751.
Fig. 11. Properties comparison of composites with different fiber content: 20 vol%, 30 vol%, 40 vol% and 20 vol% (PIP for 3cycles).
Acknowledgements The authors are grateful for Project supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 11421091); Project supported by National Science Fund for Distinguished Young Scholars of China (Grant No. 51525201)
8
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Mullite fiber cloth-reinforced mullite composite fabricated via an optimized layer-by-layer assembly method Dazhao Liu, Mingyi Tan, Cheng Fang, Wenbo Han
⁎
National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, PR China
ARTICLE INFO
ABSTRACT
Keywords: Ceramic composites Laminated Mechanical properties Mullite Fibers Microstructure
This article reports the fabrication of fiber cloth-reinforced mullite composite via an optimized layer-by-layer (LbL) assembly method involving the infiltration and pyrolysis of fiber cloth with mullite precursor (PIP) followed by brushing matrix onto the treated fiber cloth and hot pressing. The influence of fiber content on the microstructure and mechanical properties of composite are investigated and the crack propagation is discussed to explain the toughening mechanisms. The composite with 30 vol% fiber content exhibited a high density 2.89 g/cm3, flexural strength 135.5 MPa and fracture toughness 4.13 MPa m1/2. After PIP pretreatment, the gaps existed in the fiber bundle are filled with mullite matrix which transform from mullite precursor, improving the density from 2.89 g/cm3 to 2.94 g/cm3, flexural strength from 135.5 MPa to 163.2 MPa and fracture toughness from 4.13 MPa m1/2 to 4.55 MPa m1/2. The optimized LbL assembly method realizes the creation of fiber clothreinforced mullite composites with high density and excellent mechanical properties.
1. Introduction Mullite ceramics, which exhibit outstanding mechanical properties, good thermal shock resistance, low thermal expansion coefficient, high creep resistance and good chemical and thermal stability, are applied as high-temperature structural materials [1–3]. However, fabricating strong and damag-tolerant composites is a challenge due to the intrinsic brittleness and low fracture toughness of ceramics. A large number of toughening measures are developed to fabricate mullite matrix composites with favorable mechanical properties such as introducing whiskers, chopped fibers, one dismension continuous fiber or fiber cloth. Among these measures, one dismension continuous fiber and fiber cloth reinforced mullite composite have enticed crucial scientific interest for high-temperature applications due to their remarkable toughening effect [4–7]. Recently, fiber reinforced mullite matrix composites have been fabricated via layer-by-layer (LbL) assembly [8–10]. However, the density and mechanical properties of composite obtained through LbL assembly are not high enough and should be enhanced to satisfy the requirements of specific applications. What's more, the microstructure of composite reinforced by one dismension continuous fiber are uncontrollable and the distribution of fiber filament or bundle in matrix was not uniform. The fabrication of one dismension continuous fiber reinforced composite is complex and the mechanical properties of composite were anisotropy, which is
⁎
depended on the cut direction: the flexural strength of sample cut along the fiber is much higher than the samples cut in other two directions, limiting the application of mullite matrix composite. In the present work, we choose fiber cloth as reinforced component to prepare mullite matrix composite with controllable structure and isotropic mechanical property via an improved LbL assembly method. During the process, fiber cloths are pretreated with mullite precursor. Then, fiber reinforced mullite matrix composite is fabricated via traditional LbL assembly method. 2. Experimental procedure 2.1. Matrix materials Mullite powder (> 99%, Northwest Institute for Non-ferrous Metal Research, China) was used as the matrix material. The matrix slurry was prepared using mullite powder, 0.5 wt% of polyacrylic acid (> 97%, Fuchen, China) as binder, and 0.2 wt% of polyacrylamide as dispersant. The mixture was stirred for 20 min and solid content of mixture was 60 wt% to keep good fluidity. 2.2. Preparation of SiC coating Aluminosilicate fiber cloth (Nextel 720, 3 M Corp., MN, USA) was
Corresponding author. E-mail address: [email protected] (W. Han).
https://doi.org/10.1016/j.ceramint.2018.09.167 Received 9 August 2018; Received in revised form 3 September 2018; Accepted 16 September 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, D., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.167
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 1. Flowchart of fabrication of fiber cloth-reinforced mullite composites via optimized LbL assembly.
Fig. 2. Surface and fractured micrographs of composites with 20 vol% fiber content in XZ direction: (a), (b) and YZ direction: (c), (d); (e) and (f) are the SEM images of fiber coating.
used as the reinforced component because of its excellent properties. In this study, the SiC coating was prepared on the mullite fiber by dip coating, and polycarbosilane (provided by the National University of Defense Technology) was used as the precursor. The fiber cloths were dipped into the precursor solution (0.01 mol/L) and ultrasonically vibrated for 10 min and dried in air. After the dipping process, all dipped fiber cloths were treated at 1400 °C for 1 h in a tubular furnace with argon atmosphere.
2.4. Composite processing Fig. 1 shows the manufacturing procedure of fiber cloth-reinforced composite. First, coated fiber cloth was cut into 36 mm × 36 mm and impregnated into the mullite precursor solution and pyrolyzed (PIP) at 1000 °C. Then, the treated fiber cloth was brushed with matrix slurry and manually stacked up and dried at 50 °C. Finally, the green body was hot press sintered for 1 h in vacuum at 1250 °C and the sintering pressure was 30 MPa. The obtained composite was named Mx (the x referred to the PIP cycle) and incised for mechanical property testing and microstructure observation. We also prepared the composite reinforced by fiber cloth, which was not treated with the mullite precursor, and untreated composite was named M0.
2.3. Preparation of mullite precursor The aluminum and silicon sources of mullite were aluminum nitrate nonahydrate (ANN, Fuchen, China) and tetraethoxysilane (TEOS, Fuchen, China), respectively. Biphase sol with molar ratio of Al/Si 3:1 was fabricated as follows: 75.3 g of ANN was dissolved in 200 mL deionized water and stirred at 50 °C for 24 h. Next, 208.33 g of TEOS dissolved in EtOH previously was dropped into the ANN solution and stirred at 40 °C for 96 h. The obtained precursor solution was used for preparing the composite. During the fabrication of mullite precursor, the PH of mixture was neutral (PH=7) and the hydrolysis of TEOS was promoted by keeping at 40 °C.
2.5. Characterization The morphology and microstructure of the composite were investigated using a focusing ion/electronic double-beam microelectron microscope (FIB/SEM, Helios NanoLab 600i, FEI, USA). The phases were analyzed via X-ray diffraction (XRD, Empyrean, PANalytical, Netherlands). The density of the composite was measured using the Archimedes method. The chemical reaction that formed mullite was 2
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 3. Surface micrographs and load displacement curves of composites with 20 vol% fiber content in XZ direction: (a), (b) and YZ direction: (c), (d).
Fig. 4. Surface micrographs of composites with different fiber content: (a), (b), (c) and (d) are referred to 10 vol%, 20 vol%, 30 vol% and 40 vol%, respectively.
3
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
3.2. Influence of fiber content on the microstructure and mechanical property of mullite matrix composite
Table 1 Density, relative density, and mechanical properties of composite with different fiber content. Sample
Fiber content （vol%）
Density (g/cm3)
Relative density (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
1 2 3 4
10 20 30 40
2.99 2.93 2.89 2.81
93.4 91.5 90.3 87.8
175.4 166.7 134.6 120.3
3.11 3.67 4.15 4.78
Fig. 4 shows the surface morphology of composites with different fiber contents, containing 10 vol%, 20 vol%, 30 vol% and 40 vol% fiber, respectively. It could be seen that with the increase of fiber content, the thickness of matrix layer between the fiber layers decreased continuously and the increase of fiber content also led to the increase of the gaps (in the fiber bundle) within the composite. The density and flexural strength of composite decreased due to the decrease of matrix and the increase of interstices in the materials, but the fracture toughness of the composite increased to a certain extent (as shown in Table 1). The composite with 10 vol% fiber content displayed highest relatively density 93.4%, flexural strength 175.4 MPa and lowest fracture toughness 3.11 MPa•m1/2. On the contrary, the composite with 40 vol% fiber content exhibited lowest relatively density 87.8%, flexural strength 120.3 MPa and highest fracture toughness 4.78 MPa m1/2. Therefore, it was necessary to select the fiber content reasonably according to the actual service environment of the composite to meet the application requirements. Fig. 5 shows the fractured images and three-point bending curves of composites with different fiber content. As shown in Fig. 5, the fractured morphology of composites with different fiber content were all concave and uneven, which was different from the fracture of pure ceramic. It was ascribed to the crack deflection, branching along the weak interface between the fiber and matrix or crack propagation termination as the crack tip extended into the interstitial space of the fiber bundle. With the increase of fiber content, the more such weak interface and gap, the more difficult the crack propagation would be. On the macro level, the flexural strength and density of the material decreased and fracture toughness increased. By comparing flexural curve of the composites with different fiber content, it could be concluded that the fracture mode changed from brittle fracture to damage tolerant behavior and the slope of the load-drop curve decreased following the increase of fiber content (Fig. 5b, d, f and h). It could be explained that the increase of fiber content resulted in more setback such as weak interface and gaps, inhibiting the propagation of crack and preventing the composite from devastation. On the whole, fiber content had an obviously influence on the microstructure of composite and led to the decrease of density and flexural strength and the improvement of fracture toughness.
characterized with a thermogravimetry (TG)/differential scanning calorimetry (DSC) instrument (Netzsch STA 449C, Germany) and Fourier transform infrared spectroscopy (FTIR, PerkinElmer 2000, USA) using KBr disks. Three-point bending test was performed on an Instron (USA) test machine with a crosshead speed of 0.5 mm/min and the size of samples tested on Instron (USA) test machine were 36 mm × 4 mm × 3 mm for three-point bending test and 22 mm × 4 mm × 2 mm for fracture toughness test. 3. Results and discussion 3.1. Characterization of the mullite matrix composite Fig. 2 shows the surface and fractured morphology of mullite matrix composite reinforced via 20 vol% fiber cloth, which was not impregnated into mullite precursor and pyrolyzed. Fig. 2a and b show the surface and fractured morphology of sample cut along the direction of XZ plane, and Fig. 2c and d were the surface and fractured morphology of sample cut along the direction of YZ plane. It could be seen from the SEM images that the fiber cloth distributed evenly in the mullite matrix, and the surface and cross section of the samples obtained from the two directions were similar and the thickness of the substrate between the fibers were also close. As shown in Fig. 2a and c, some uneven distributed gaps were observed, which was ascribed to intrinsic characteristic of fiber bundle and fiber coating. On one hand, small gaps existed inherently among the fiber monofilaments with circular section and it was difficult to introduce big mullite particles to fill these small gaps during the fabrication of composite. On the other hand, the fiber coating (Fig. 2e and f) inhibited the sintering between mullite fibers in the subsequent sintering process, protecting the fibers, so some gaps finally remained in the composite. Fig. 3 shows crack propagation of mullite composite with 20 vol% fiber content and corresponding three-point bending curve along X axis in XZ plane (XZ direction) and Y axis in YZ plane (YZ direction), respectively. Fig. 3a and b were surface morphology and three-point bending curve of sample cut in XZ direction and Fig. 3c and d were surface morphology and three-point bending curve of sample cut in YZ direction. It could be seen from the three-point bending curves of two directions that the fiber cloth reinforced composite materials exhibited a damage-tolerant fracture behavior, indicating that the introduction of fiber cloth could hinder the rapid propagation of crack in the matrix and consume the fracture energy, having an obviously toughening effect. As expected and shown in Fig. 3a and c, the crack propagation mode of the samples cut along XZ and YZ directions were similar during the fracture process. The crack deflected an angle at the weak interface between the fiber and mullite matrix or the gaps inside fiber bundles, consuming more fracture energy. It could be demonstrated that the interstitial space in the fiber bundle could also contribute to the fracture toughness while reducing the density and bending strength of the composite.
3.3. Characterization of the mullite precursor As the above content mentioned, there were several gaps in the composite, so the density and mechanical strength of composite was not high enough to meet the requirement. In this work, the interstitial space between single filaments of the fiber bundle were filled by the method of impregnation and pyrolysis with the mullite precursor, which would be discussed as following. According to the TG-DSC curve (Fig. 6), the precursor was constantly in weightlessness from room temperature to 500 °C, and total weight loss was 59.1%. The DSC curve at 161.9 °C had an obvious endothermic peak，which was mainly ascribed to the decomposition of aluminum nitrate and removal of organic constituents. Another peak at 980 °C was an exothermic peak, resulting from the crystallization of mullite. From the TG-DSC curve, we preliminarily believed that the precursor could be crystallized under 1000 °C [11–13]. Fig. 7 depicts the XRD patterns of precursor samples treated at different temperatures. The diffuse peak at 67.2° of 900 °C curve was the characteristic peak of Al-Si spinel, indicating the precursor started to transform into alumina-silica (Al-Si) spinel (6Al2O3·SiO2) [14]. After 1000 °C heat treatment, the diffraction peak at 26.4° of mullite phase appeared, which was ascribed to the transformation from Al-Si spinel phase to mullite phase. The XRD pattern of the precursor pyrolyzed at 1100 °C shows enhanced mullite diffraction peak, due to the increase of 4
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 5. Fractured surface micrographs and load displacement curves of composites with different fiber content: (a), (c), (e) and (g) are referred to 10 vol%, 20 vol%, 30 vol% and 40 vol%, respectively; (b), (d), (f) and (h) are corresponding load displacement curves.
5
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
From the FTIR spectra (Fig. 8) of precursor pyrolyzed at 900 °C, the absorption band at 1130 cm−1 corresponding to Si-O tetrahedra could be seen, which was similar to the standard FTIR spectra of sample containing Al-Si spinel and amorphous silica, indicating that the precursor changed into Al-Si spinel at 900 °C. With the increase in temperature, the absorption band at 1130 cm−1 broadened, resulting from the interaction effect of the Si-O keys in Al-Si spinel and amorphous silica. The spectrum absorption peak at 820 cm−1 of the 1000 °C curve was obvious, demonstrating that more 6 coordination of aluminum ion had been converted into 4 coordination at 1000 °C. As shown in Fig. 4, the sample sintered at 1200 °C gave absorption bands at 560, 730, 820, and 1130 cm−1. The 560 cm−1 absorption band in the spectra was caused by the vibration of AlO6 stretching mode. The absorption bands at 730 and 820 cm−1 were caused by the vibration of AlO4 stretching mode, and the Si-O absorption band was located at 1130 cm−1. After sintered at 1200 °C, mullite precursor had been completely transformed into mullite, and the Si-O absorption band at 1130 cm−1 became narrow and moved toward the Takanami number, indicating that the interaction effect of the Si-O keys in Al-Si spinel and amorphous silica had greatly weakened [15].
Fig. 6. TG-DSC curve of the mullite precursor from RT to 1200 °C.
3.4. Microstructure and mechanical properties of the mullite composite treated by the PIP method Fig. 9 shows the morphology of the fiber cloth in XY and XZ directions. The morphology of fiber cloth which was not treated via the PIP method shows that no mullite matrix existed between the fibers (Fig. 9a). Fig. (9c, e and g) depict the morphology of fiber cloth, which was infiltrated by mullite precursor and pyrolyzed at 1000 °C (temperature was determined according to the above contents: XRD, FTIR, and TG characterization) for 1 h. The PIP cycles were one, two, and three times, respectively. The gaps between the fibers were filled with mullite matrix after PIP, and the matrix content increased with the number of cycles. After three cycles, the fibers were completely coated with mullite matrix (Fig. 9g and h), leading to the improvement of mechanical properties and density of fiber cloth reinforced mullite composite. In this study, the mullite composite with 30 vol% fiber content was chosen as research object. Fig. 10 shows the surface morphology of the composites prepared by two methods (LbL and LbL assisted by PIP). Comparing the experimental results, there were some differences between the composites obtained through the two processes. As shown in Fig. 10a (composite named M0), several obvious gaps existed between the fiber cloth and matrix or in the fiber bundle. After infiltration and pyrolysis for three cycles, the fibers were completely surrounded by the mullite matrix, and during the subsequent sintering, the quasi-mullite transformed from the precursor would be sintered with brushed mullite matrix, obtaining a composite (named M3) with high density and excellent mechanical properties (Fig. 10c). Compared with M0, the composite M3 displayed an enhanced flexural strength 163.2 MPa, fracture toughness 4.55 MPa m1/2 and high relative density 91.8% (in Table 2). As shown in Fig. 11, the composite M3 exhibited highest relative density, second highest flexural strength and fracture toughness among the four composites. The highest relative density resulted from the three cycles PIP for filling the gaps in the fiber bundle, leading to improvement of flexural strength and fracture toughness.
Fig. 7. XRD patterns of production from mullite precursor at temperature of 900 °C, 1000 °C, 1100 °C, and 1200 °C.
Fig. 8. FTIR spectra of production from mullite precursor at temperature of 900 °C, 1000 °C, 1100 °C, and 1200 °C.
4. Conclusion
mullite phase content. As the temperature increased to 1200 °C, the precursor transformed into mullite phase completely. The temperature was much lower than the required temperature (1400 °C) of solid phase reaction to completely form mullite and close to the sol gel method, using the soluble aluminum salt and TEOS as raw materials.
In this study, the composite with high density and excellent mechanical properties was prepared by the optimized LbL assembly method. The fiber content had an obviously influence on the microstructure of composite, resulting in the decrease of density and flexural strength but the improvement of fracture toughness. The crack propagation was investigated. The results showed that the crack deflected or branched along the weak interface between the matrix and fiber, 6
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
Fig. 9. Micrographs of the fiber cloth with different PIP cycles in two directions: (a), (c), (e) and (g) are referred to zero, one, two, and three cycles in XY direction; (b), (d), (f) and (h) are in XZ direction.
Fig. 10. SEM micrographs and load displacement curves of composites with 20 vol% fiber content: (a) and (b) composite with three PIP cycles pretreatment (M0); (c) and (d) composite without pretreatment (M3).
enhancing fracture toughness and the residual gaps in the bundle also hindered crack propagation. The density and mechanical properties of the composite could be improved effectively by pretreatment, in which the fiber cloth was infiltrated into the mullite precursor and pyrolyzed at 1000 °C. After PIP for three cycles, the relative density of the composite with 30 vol% fiber content increased from 90.3% to 91.8%,
resulting in high flexural strength (163.2 MPa) and fracture toughness (4.55 MPa•m1/2). The composite obtained through optimized LbL assembly method could satisfy the requirements of specific applications for high-temperature structural materials.
7
Ceramics International xxx (xxxx) xxx–xxx
D. Liu et al.
and National Natural Science Foundation of China (Nos. 11572105 and 51772061).
Table 2 Density, relative density, and mechanical properties of M0 and M3. Sample
Density (g/ cm3)
Relative density (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
M0 M3
2.89 2.94
90.3 91.8
134.6 163.2
4.13 4.55
References [1] B. Kanka, H. Schneider, Aluminosilicate fiber/mullite matrix composites with favorable high-temperature properties, J. Eur. Ceram. Soc. 20 (5) (2000) 619–623. [2] C. Kaya, E.G. Butler, A. Selcuk, et al., Mullite (Nextel™ 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties, J. Eur. Ceram. Soc. 22 (13) (2002) 2333–2342. [3] W. Krenkel, R. Naslain, H. Schneider, Processing and Characterisation of Mullite (Nextel™ 720) Fibre-Reinforced Mullite Matrix Composites from Hydrothermally Processed Mullite Precursors[M]// High Temperature Ceramic Matrix Composites, Wiley‐VCH Verlag GmbH & Co. KGaA, 2006, pp. 639–644. [4] K. Nubian, B. Saruhan, B. Kanka, et al., Chemical vapor deposition of ZrO2, and C/ ZrO2, on mullite fibers for interfaces in mullite/aluminosilicate fiber-reinforced composites, J. Eur. Ceram. Soc. 20 (5) (2000) 537–544. [5] W. Krenkel, R. Naslain, H. Schneider, Stability of Mullite-Precursor Versus Potential Fiber-Coating Material[M]// High Temperature Ceramic Matrix Composites, Wiley‐VCH Verlag GmbH & Co. KGaA, 2006, pp. 164–167. [6] B. Kanka, Ceramic fiber composite material: US, US7919039[P], 2011. [7] O. Reese B. Saruhan H. Schneider et al., Fabrication of Continuous Fiber-Reinforced Ceramics with a Nanosized Mullite Precursor[C]// Ht-Cmc. DLR, 1995. [8] S. Kang, L. Wang, J. Zhang, et al., Electroreductive coupling layer-by-layer (ERCLbL) assembly, Acs Appl. Mater. Interfaces 9 (2017) 37. [9] H. Qiu, Layer-by-layer self-assembly of ceramic particles for complex shape coating synthesis, J. Am. Ceram. Soc. 89 (4) (2010) 1180–1187. [10] J. Shi, L. Zhang, Z. Jiang, Facile construction of multicompartment multienzyme system through layer-by-layer self-assembly and biomimetic mineralization, Acs Appl. Mater. Interfaces 3 (3) (2011) 881. [11] O. Reese B. Saruhan B. Kanka et al. Fabrication of continuous fiber-reinforced ceramics with a nanosized mullite precursor, 1995. [12] S. Baitalik, N. Kayal, O.P. Chakrabarti, Properties of mullite-bonded SiC ceramics from mullite precursor sol coated SiC powder and crystallization kinetics of mullite formation, Mater. Res. Innov. 20 (2) (2016) 99–105. [13] L. Radonjić, Formation of sol-gel nanostructured mullite by additions of fluoride ion, J. Therm. Anal. Calorim. 79 (3) (2005) 487–492. [14] D. Roy, B. Bagchi, A. Bhattacharya, et al., A comparative study of densification of sol‐gel‐derived nano‐mullite due to the influence of iron, nickel and copper ions, Int. J. Appl. Ceram. Technol. 11 (6) (2014) 1054–1060. [15] B. Bagchi, S. Das, A. Bhattacharya, et al., Nanocrystalline mullite synthesis at a low temperature: effect of copper ions, J. Am. Ceram. Soc. 92 (3) (2009) 748–751.
Fig. 11. Properties comparison of composites with different fiber content: 20 vol%, 30 vol%, 40 vol% and 20 vol% (PIP for 3cycles).
Acknowledgements The authors are grateful for Project supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 11421091); Project supported by National Science Fund for Distinguished Young Scholars of China (Grant No. 51525201)
8