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SiO2 composites after thermal shock and fatigue tests
Journal Pre-proof Microstructure and mechanical property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests Cao Jie, Xiang Yang, Wang Hua-qiong, Zhu Cheng-xin, Wang Yi, Li Guang-de, Wen Jin, Cao Feng PII:
S2452-2139(19)30159-7
DOI:
https://doi.org/10.1016/j.coco.2019.11.002
Reference:
COCO 271
To appear in:
Composites Communications
Received Date: 23 June 2019 Revised Date:
31 October 2019
Accepted Date: 1 November 2019
Please cite this article as: C. Jie, X. Yang, W. Hua-qiong, Z. Cheng-xin, W. Yi, L. Guang-de, W. Jin, C. Feng, Microstructure and mechanical property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests, Composites Communications, https://doi.org/10.1016/j.coco.2019.11.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Microstructure and mechanical property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests Cao Jie 1, Xiang Yang2∗, Wang Hua-qiong3, Zhu Cheng-xin2, Wang Yi4, Li Guang-de4, Wen Jin5, Cao Feng2∗ 1 Beijing Aerospace Technology Institute, Beijing 100074, China 2 Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan, 410073, P. R China 3 Aerospace research institute of special material and processing technology, Beijing 100074, China 4 Unit 96901 of People’s Liberation Army, Beijing 100094, P. R China 5 Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, Hunan University of Humanities, Science and Technology, Loudi Hunan 417000, P. R China
Abstract: To meet the operating requirements of hypersonic flight vehicles in the atmosphere environment, microstructure and mechanical property of 2.5D Al2O3f/SiO2 composites by thermal shock and fatigue were investigated and compared. After 100000 fatigue tests, cracks hardly formed on the sample surface, and the interfacial bonding or fiber property was barely affected. However, after thermal shock and fatigue tests, the composites underwent typical brittle fracture and had the lowest fracture strength. The mass loss was less than 1%, and the flexural strength reached to 55.6 MPa, with the retention ratio of only 51.1%. The mechanical property degradation of the composites after tests was closely related to microstructure evolution. ∗
Corresponding author: Tel: +86 731 84575163; Fax: +86 731 84576578
E-mail address: [email protected] (Dr. Xiang Yang). ∗
Corresponding author: Tel: +86 731 84575162; Fax: +86 731 84576578
E-mail address: [email protected] (Prof. Cao Feng).
1
Key words: Al2O3f/SiO2; Mechanical property; Fatigue; Thermal shock 1. Introduction Advances in aerospace technology have raised the demand for structural materials which show superior mechanical property under high pressure and high temperature[1,2]. Continuous fiber-reinforced ceramics matrix composites (CMCs) capable of excellent strength and fracture toughness at high temperatures, attract attention as candidate materials for the applications[3-14]. Advanced reusable space launch vehicles will likely incorporate CMCs in critical propulsion components[15]. However, these applications require exposure to oxidizing environments. Therefore the thermodynamic stability and oxidation resistance of CMCs are vital issues. Accordingly, silica ceramics can be remarkably toughened by introducing high-strength fibers as reinforcements [16-20]. At present, oxide fibers, such as SiO2 fiber and Al2O3 fiber, are commonly employed as reinforcements for silica ceramics. Silica matrix composites reinforced by these fibers (SiO2f/SiO2 [17-19], Al2O3f/SiO2 [20]) are in all-oxide natures, so oxidations at extremely high temperatures can be better resisted. Owing to low densification temperature (<1000°C), small shrinkage, reduced drying stress and near-stoichiometric composition in a matrix, Sol-Gel process is one of the most convenient, effective and economical techniques for producing high-quality composites [21-26]. In many potential applications, CMCs will be subject to fatigue loading under a wide range of frequencies. The applications of Al2O3f/SiO2 composites are largely limited by thermal stabilities. Until now, however, the thermal stabilities of Al2O3f/SiO2 composites after thermal shock and fatigue tests are still unknown. The objective of this study is to investigate the microstructure and mechanical property of 2.5D Al2O3 fiber-reinforced SiO2 (Al2O3f/SiO2) by thermal shock and fatigue. The fracture mechanism was clarified by observing the fractural surface and cross-section with scanning electron microscopy (SEM).
2
2. Experimental Procedures 2.1 Processing of Al2O3f/ SiO2 composites 2.5D Al2O3 fiber reinforcements（Nextel 440, DF11, from 3M Co., USA）, which were finished by Z-stitching stacked fabrics with Al2O3 fiber yarn in a 10 mm×10 mm space. The volume fraction of the fibers was 40.0%. Diphasic SiO2 sol were used as the precursors SiO2. First, the reinforcements were placed in a closed container, and then that was evacuated to 0.1 Pa. SiO2 sol was sucked into the container, and the pressure was maintained at 0.1 Pa during the entire process for 8 h. The composites were dried at 150°C for 2 h, and then sintered at 800 °C for 1 h to remove the coupling agent and bounded water. The process was repeated ten cycles. The density of the composites was measured by Archimedes' method. 2.2 Flexural strength tests The samples were cut into 90 mm × 15 mm × 2.5 mm and polished for three-point bending test in a CSS-44050 (Changchun Research Institute of Testing Machines, Jilin, P. R. China) universal testing machine, with a cross head speed of 0.5 mm/min and a span of 40 mm. Five specimens were used for all the mechanical tests. 2.3 Fatigue tests The samples were cut into 90 mm × 15 mm × 2.5 mm and polished for fatigue tests in a INSTRON8501-5kN universal testing machine, with a cross head speed of 0.5 mm/min, load-up condition of 6Hz, a span of 40 mm, , a stress level of 92%, a goal service life of 105,σmin=70.5MPa, stress ratio=0.5. Five specimens were used for all the mechanical tests. 2.4 Thermal shocks tests The thermal shock tests were conducted by alternating the specimens quickly 3
between 1200℃ in static air and room temperature. First, each specimen was held in the tube furnace, which was preheated to 1200℃ for 10 minutes, and then the sample was cooled to room temperature for 10 minutes. After the temperature of the specimen dropped to room temperature for 10 minutes, it was immediately sent back to the furnace, temperature of which was held at 1200℃, for the next heating-cooling cycle. Such a heating-cooling cycle indicated that the specimens were thermally shocked for one time. In order to reveal the thermal shock resistance of the composites, at least Five specimens were investigated at prescribed number of 20 cycles. 2.5 Characterization The density of the composites was measured by Archimedes' method. The samples were weighed after tests by an electronic balance with a sensitivity of ±0.001 g. Microstructure of Al2O3f/SiO2 composites was characterized by SEM (quanta450 and quanta220, FEI). 3. Results and discussion 3.1 Microstructure of Al2O3f/ SiO2 composites 2.5D Al2O3f/SiO2 composites were fabricated by sol-gel process, with the density of 1.60 g/cm3 and open porosity of 16%. SEM image of Al2O3f/SiO2 composite surface (Fig. 1(a)) showed that Al2O3 fibers were covered by the SiO2 matrix, without obvious pores. Fig. 1(b) exhibited that the pores inside fiber bundles of the composite cross-section were filled with dense SiO2 matrix. The results of TG-DSC showed the oxidation stability and the thermal stability of the material. The existence of Al2O3 fibers suggested that no reaction occurred, so the composites had high chemical and thermal compatibilities. Given that the fibers hardly deboned from the matrix, the bonding between them was tight. Thus, Al2O3f/SiO2 composites had been fabricated efficiently by sol-gel method.
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3.2 Mechanical property of Al2O3f/ SiO2 composites Table 1 shows the mechanical properties of Al2O3f/SiO2 composites. The flexural strength was (108.8 ± 14.8) MPa, and the flexural modulus was (8.5 ± 1.0) GPa. The samples displayed a typical quasi-ductile response (Fig. 2). The fracture surface had long fiber pull-out, which can be ascribed to the weak fiber/matrix interface, being significantly associated with the quasi-ductile fracture behavior and high flexural strength of the composites. The weak interface formed because firstly, the SiO2 matrix reduced the mobility of fiber grain boundary, thus suppressing SiO2 diffusion outwards and preventing the chemical reaction between fibers and matrix [27]. Secondly, the fabricated matrix was weak [28]. Fatigue tests were conducted at the frequencies of 6Hz. This fatigue limit was based on the run-out condition of 105 cycles, approximate number of loading cycles expected in aerospace applications at 1200°C [2]. Fracture surfaces of several specimens were examined to gain a better understanding of the effects of the thermal shock and fatigue on the damage and failure mechanisms in the porous matrix. After the fatigue tests, the sample surface almost had no cracks in the macro before and after the tests(Fig. 3 and Fig. 4). As presented in Fig. 5, the cross-section of the composites after tests had long fiber pull-out. The microstructures of the composites before and after fatigue tests were basically identical (Fig. 2 and Fig. 5). The fiber pullout was extensive and the variation in pull-out length was considerable. It was recognized that the increase in the spatial correlation in the fiber failure locations was among the main manifestations of the matrix densification. After thermal shock and fatigue tests. After tests, few cracks formed on the sample surface (Fig. 6). SEM micrographs of the aforementioned fracture surfaces shown in Fig. 7, the fracture surface was dominated by planar regions of coordinated fiber failure, indicative of matrix densification due to additional sintering. A brushy fracture surface with extensive fiber pullout demonstrates that mechanical cycling had successfully counteracted the effects of matrix densification by maintaining matrix porosity at the level sufficient to enable crack-deflecting 5
behavior and fiber pullout. There was neither obvious oxidation of Al2O3 fibers nor chemical reaction between the fibers and SiO2 matrix after tests. The microstructure transformation of Al2O3f/SiO2 composites was responsible for the mechanical property changes after oxidation during thermal shock. As a result, Al2O3f/SiO2 composites exhibited decreased damage tolerance, brittle fracture behavior and a short lifetime. 3.3 Mechanical property of Al2O3f/ SiO2 composites after tests The fatigue test results of Al2O3f/SiO2 composites are shown in Fig. 3 and Table 1. After tests, the flexural strength was (98.4 ±15.4) MPa, with the retention ratio of 90.4%. The flexural modulus was (8.5±1.3) GPa. Therefore, the as-studied Al2O3f/SiO2 composites were stable after tests. Table 2 shows the properties of Al2O3f/SiO2 composites after fatigue tests. After 100000 tests, the fatigue tests had little effect on interfacial bonding or fiber properties. Since oxide/oxide CMCs are designed to work in high-temperature environments, it is of vital importance to evaluate the mechanical properties of composites at high temperatures [16, 29]. Thus, the composite thermal stability was further evaluated by thermal shock tests. The samples were heated at 1200°C in air, and then cooled to room temperature. Each sample in the tube furnace was preheated to 1200°C within 10 min and thereafter cooled to room temperature in 240 min. Table 3 lists the masses of Al2O3f/SiO2 composites before and after thermal shock tests. After tests, the mass loss was less than 1%, no obvious mass loss after thermal shock tests. However, the flexural strength reached 55.6 MPa, with the retention ratio of only 51.1%. The flexural strength loss of Al2O3f/SiO2 composites can be attributed to the strength loss of fibers during thermal shock, and then the strength loss of fibers lead to the changed the mechanical properties. Table 4 summarizes the properties of Al2O3f/SiO2 composites after thermal shock and fatigue tests. The composites underwent typical brittle fracture, with the lowest fracture strength (55.6 MPa).The microstructure transformation of Al2O3f/SiO2
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composites was responsible for the mechanical property changes after oxidation during thermal shock. After thermal shock, cracks appeared on the surface due to the difference between the thermal expansion coefficients of fibers and matrix. A sudden temperature change produced tensile stress in the matrix, which then accumulated and reached the critical value, finally giving cracks in the matrix. 4. Conclusions Microstructure and mechanical property of 2.5D Al2O3f/SiO2 composites by thermal shock and fatigue were investigated. The main results are summarized as follows: 2.5D Al2O3f/SiO2 composites were efficiently fabricated by sol-gel method. The composites showed quasi-ductile fracture behavior and high strength ((108.8 ± 14.8) MPa). After 100000 fatigue tests, there were no obvious cracks on the sample surface. The fatigue tests barely affected interfacial bonding or fiber properties. After thermal shock and fatigue tests, the composites were subjected to typical brittle fracture, with the lowest fracture strength, indicating that high temperature predominantly affected the composite mechanical properties. The mechanical performance degradation of the composites after thermal shock and fatigue tests was closely related to their microstructure evolution. Acknowledgements The authors are grateful to National Science Foundation of China (51602347) and Hunan Natural Science Foundation (2019JJ50282) for financial support. References [1] H.Ohnabe, S.Masaki, M.Onozuka, K.Miyahara , T.Sasa . Potential application of ceramic matrix composites to aero-engine components. Composites: Part A 1999;30:489-96. [2] M.B. Ruggles-Wrenn a, G. Hetrick, S.S. Baek. Effects of frequency and environment on fatigue behavior of an oxide-oxide ceramic composite at 1200°C. International Journal of 7
Fatigue 30 (2008) 502-516. [3] C.G. Levi, J.Y.Yang, B.J.Dalgleish, F.W.Zok, A.G.Evans, Processing and performance of a nall-oxide ceramic composite, J.Am.Ceram.Soc. 81 (1998) 2077–2086. [4] D.H. Ding, W.C.Zhou, F.Luo, M.L.Chen, D.M.Zhu, Mechanical properties and oxidation resistance of SiCf/CVI-SiC composites with PIP-SiC interphase, Ceram.Int.38 (2012) 3929–3934. [5]
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[10] J. Wu, F.R.Jones, P.F.James, Continuous fibre reinforced mullite matrix composites by sol–gel processing :part I fabrication and microstructures, J. Mater.Sci.32 (1997) 3361–3368. [11] Y.Z. Zhu, B.B.Pei, M.Yuan, Y.Liu, Z.R.Huang, Microstructureand properties of Cf/SiC composites with thin SiCN layer as fiber-protecting coating, Ceram.Int.39 (2013) 7101–7106. [12] J. Wu, F.R.Jones, P.F.James, Continuous fibre reinforced mullite matrix composites by sol–gel processing: part II properties and fracture behaviour, J.Mater.Sci.32 (1997) 3629–3635. [13] F.W. Zok, Developments in oxide fiber composites, J.Am.Ceram.Soc. 89 (2006) 3309–3324. [14] J.Y. Yang, J.H.Weaver, F.W.Zok, Processing of oxide composites with three-dimensional fiber architectures, J.Am.Ceram.Soc.92(2009) 1087–1092. 8
[15] S.Schmidt, S.Beyer, H.Knabe, H.Immich , R.Meistring, A.Gessler. Advanced ceramic matrix composite materials for current and future propulsion technology applications. Acta Astronaut 2004;55:409-420. [16] Y. Xiang, Q. Wang, Z.H. Peng, F. Cao, High-temperature properties of 2.5D SiO2f/SiO2 composites by sol-gel, Ceram. Int. 42 (2016) 12802-12806. [17] L.W. Yang, J.Y. Wang, H.T. Liu, R. Jiang, H.F. Cheng, Sol-gel temperature dependent ductile-to-brittle transition of aluminosilicate fiber reinforced silica matrix composite, Compos. Part B 119 (2017) 79-89. [18] Y.Q. Gao, L.H. Zhang, J. Chen, X.W. Wang, H.F. Cheng, Preparation and properties of an oxide fiber-reinforced silica matrix composite, Int. J. Appl. Ceram. Technol. 14 (2017) 1041-1048. [19] Y.Q. Gao, L.F. Zhang, X.W. Wang, J. Chen, H.F. Cheng, Interfacial characterization of an oxide fiber-reinforced silica matrix composite containing a single layer pyrocarbon interphase, Ceram. Int. 42 (2016) 6504-6509. [20] D. Li, C.R. Zhang, B. Li, F. Cao, S.Q. Wang, Preparation and mechanical properties of unidirectional boron nitride fiber reinforced silica matrix composites, Mater. Des. 34 (2012) 401-405. [21] Y. Wang, H.T. Liu, H.F. Cheng, J. Wang, Effective fugitive carbon coatings for the strength improvement of 3D Nextel™ 440/alumino silicate composites, Mater. Lett. 126 (2014) 236–239. [22] Y. Wang, H.F. Cheng, J. Wang, Effects of the single layer CVD SiC interphases on mechanical properties of mullite fiber-reinforced mullite matrix composites fabricated via a sol-gel process, Ceram. Int. 40 (3) (2014) 4707–4715. [23] Y. Wang, H.T. Liu, H.F. Cheng, J. Wang, Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites, Ceram. Int. 39 (8) (2013) 9229–9235. [24] Y. Xiang, Q. Wang, F. Cao, Y.W. Ma, D.L. Quan, Sol-gel process and high temperature property of SiO2/ZrO2-SiO2 composites, Ceram. Int. 43 (1) (2017) 854–859. [25] Q. Wang, F. Cao, Y. Xiang, Z.H. Peng, Effects of ZrO2 coating on the strength improvement of 2.5D SiCf/SiO2 composites, Ceram. Int. 43 (1) (2017) 884–889. 9
[26] H.T. Liu, Q.S. Ma, W.D. Liu, Mechanical and oxidation resistance properties of 3D carbon fiber-reinforced mullite matrix composites prepared by sol-gel process, Ceram. Int. 40 (5) (2014) 7203–7212. [27] M. Schmückerw, P. Mechnich, Improving the microstructural stability of NextelTM 610 alumina fibersembedded in a porous alumina matrix, J. Am. Ceram. Soc. 93 (2010) 1888-1890. [28] F.W. Zok, C.G. Levi, Mechanical properties of porous-matrix ceramic composites, Adv. Eng. Mater. 3 (2001) 15-23. [29] X.Yang, C. Feng, C. Jie, C. Xiao-na, X. Qiang. Effects of high-temperature annealing on microstructure and mechanical property of SiO2f/SiO2 composites. Vacuum (2017) 144:1-7.
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Figures Caption Fig.1 The surface and cross-section of Al2O3f/SiO2 composites Fig.2 The cross-section of Al2O3f/SiO2 composites after flexural strength test Fig.3 The Al2O3f/SiO2 composites under fatigue tests Fig.4 The Al2O3f/SiO2 composites before and after fatigue tests Fig.5 The cross-section of Al2O3f/SiO2 composites after fatigue tests Fig.6 The surface of Al2O3f/SiO2 composites after thermal shock and fatigue tests Fig.7 The cross-section of Al2O3f/SiO2 composites after thermal shock and fatigue tests
Tables Caption Table 1 Mechanical property of Al2O3f/ SiO2 composites Table 2 The properties of Al2O3f/ SiO2 composites after fatigue tests Table 3 The mass of Al2O3f/SiO2 composites before and after thermal shock tests Table 4 The property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests
Highlights (1) 2.5D Al2O3f/SiO2 composites had been fabricated by sol-gel method efficiently. (2) 2.5D Al2O3f/SiO2 composites were tested by 100000 fatigue tests. (3) The fatigue tests had little effect on the interfacial bonding and fibers property. (4) The degradation of mechanical property after thermal shock and fatigue tests, indicating major influence of high temperature on the mechanical property of Al2O3f/SiO2 composites.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
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Table 1 Mechanical property of Al2O3f/ SiO2 composites Samples
Flexual strength (MPa)
Flexual modulus (GPa)
Befores Tests
108.8±14.8
8.5±1.0
After fatigue tests
98.4±15.4
8.5±1.3
After thermal shocks tests and fatigue tests
65.6±12.4
8.1±1.0
Table 2 The properties of Al2O3f/ SiO2 composites after fatigue tests Samples
Load-up condition (Hz)
Stress (MPa)
Load(N)
P1-6
60.76
P1-7
68.05
P1-8
6.0
64.86
60.34
P1-9
65.63
P1-10
62.20
Goal service life
Results
100000
No obvious cracks on the surface of samples
Table 3 The mass of Al2O3f/SiO2 composites before and after thermal shock tests Samples
Before test (g)
First test (g)
Second test (g)
P2-1 P2-2 P2-3 P2-4 P2-5
5.7626 5.7447 5.7108 5.8178 5.4166
5.7191 5.7070 5.6762 5.7797 5.3800
5.7188 5.7008 5.6700 5.7730 5.3794
Table 4 The property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests Samples
Load-up condition (Hz)
Stress (MPa)
Load(N)
P2-1
69.70
P2-2
67.78
P2-3
6.0
64.86
66.70
P2-4
67.08
P2-5
65.20
Goal service life
Results
100000
No obvious cracks on the surface of samples
(1) 2.5D Al2O3f/SiO2 composites had been fabricated by sol-gel method efficiently. (2) 2.5D Al2O3f/SiO2 composites were tested by 100000 fatigue tests. (3) The fatigue tests had little effect on the interfacial bonding and fibers property. (4) The degradation of mechanical property after thermal shock and fatigue tests, indicating major influence of high temperature on the mechanical property of Al2O3f/SiO2 composites.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
1
S2452-2139(19)30159-7
DOI:
https://doi.org/10.1016/j.coco.2019.11.002
Reference:
COCO 271
To appear in:
Composites Communications
Received Date: 23 June 2019 Revised Date:
31 October 2019
Accepted Date: 1 November 2019
Please cite this article as: C. Jie, X. Yang, W. Hua-qiong, Z. Cheng-xin, W. Yi, L. Guang-de, W. Jin, C. Feng, Microstructure and mechanical property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests, Composites Communications, https://doi.org/10.1016/j.coco.2019.11.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Microstructure and mechanical property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests Cao Jie 1, Xiang Yang2∗, Wang Hua-qiong3, Zhu Cheng-xin2, Wang Yi4, Li Guang-de4, Wen Jin5, Cao Feng2∗ 1 Beijing Aerospace Technology Institute, Beijing 100074, China 2 Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan, 410073, P. R China 3 Aerospace research institute of special material and processing technology, Beijing 100074, China 4 Unit 96901 of People’s Liberation Army, Beijing 100094, P. R China 5 Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, Hunan University of Humanities, Science and Technology, Loudi Hunan 417000, P. R China
Abstract: To meet the operating requirements of hypersonic flight vehicles in the atmosphere environment, microstructure and mechanical property of 2.5D Al2O3f/SiO2 composites by thermal shock and fatigue were investigated and compared. After 100000 fatigue tests, cracks hardly formed on the sample surface, and the interfacial bonding or fiber property was barely affected. However, after thermal shock and fatigue tests, the composites underwent typical brittle fracture and had the lowest fracture strength. The mass loss was less than 1%, and the flexural strength reached to 55.6 MPa, with the retention ratio of only 51.1%. The mechanical property degradation of the composites after tests was closely related to microstructure evolution. ∗
Corresponding author: Tel: +86 731 84575163; Fax: +86 731 84576578
E-mail address: [email protected] (Dr. Xiang Yang). ∗
Corresponding author: Tel: +86 731 84575162; Fax: +86 731 84576578
E-mail address: [email protected] (Prof. Cao Feng).
1
Key words: Al2O3f/SiO2; Mechanical property; Fatigue; Thermal shock 1. Introduction Advances in aerospace technology have raised the demand for structural materials which show superior mechanical property under high pressure and high temperature[1,2]. Continuous fiber-reinforced ceramics matrix composites (CMCs) capable of excellent strength and fracture toughness at high temperatures, attract attention as candidate materials for the applications[3-14]. Advanced reusable space launch vehicles will likely incorporate CMCs in critical propulsion components[15]. However, these applications require exposure to oxidizing environments. Therefore the thermodynamic stability and oxidation resistance of CMCs are vital issues. Accordingly, silica ceramics can be remarkably toughened by introducing high-strength fibers as reinforcements [16-20]. At present, oxide fibers, such as SiO2 fiber and Al2O3 fiber, are commonly employed as reinforcements for silica ceramics. Silica matrix composites reinforced by these fibers (SiO2f/SiO2 [17-19], Al2O3f/SiO2 [20]) are in all-oxide natures, so oxidations at extremely high temperatures can be better resisted. Owing to low densification temperature (<1000°C), small shrinkage, reduced drying stress and near-stoichiometric composition in a matrix, Sol-Gel process is one of the most convenient, effective and economical techniques for producing high-quality composites [21-26]. In many potential applications, CMCs will be subject to fatigue loading under a wide range of frequencies. The applications of Al2O3f/SiO2 composites are largely limited by thermal stabilities. Until now, however, the thermal stabilities of Al2O3f/SiO2 composites after thermal shock and fatigue tests are still unknown. The objective of this study is to investigate the microstructure and mechanical property of 2.5D Al2O3 fiber-reinforced SiO2 (Al2O3f/SiO2) by thermal shock and fatigue. The fracture mechanism was clarified by observing the fractural surface and cross-section with scanning electron microscopy (SEM).
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2. Experimental Procedures 2.1 Processing of Al2O3f/ SiO2 composites 2.5D Al2O3 fiber reinforcements（Nextel 440, DF11, from 3M Co., USA）, which were finished by Z-stitching stacked fabrics with Al2O3 fiber yarn in a 10 mm×10 mm space. The volume fraction of the fibers was 40.0%. Diphasic SiO2 sol were used as the precursors SiO2. First, the reinforcements were placed in a closed container, and then that was evacuated to 0.1 Pa. SiO2 sol was sucked into the container, and the pressure was maintained at 0.1 Pa during the entire process for 8 h. The composites were dried at 150°C for 2 h, and then sintered at 800 °C for 1 h to remove the coupling agent and bounded water. The process was repeated ten cycles. The density of the composites was measured by Archimedes' method. 2.2 Flexural strength tests The samples were cut into 90 mm × 15 mm × 2.5 mm and polished for three-point bending test in a CSS-44050 (Changchun Research Institute of Testing Machines, Jilin, P. R. China) universal testing machine, with a cross head speed of 0.5 mm/min and a span of 40 mm. Five specimens were used for all the mechanical tests. 2.3 Fatigue tests The samples were cut into 90 mm × 15 mm × 2.5 mm and polished for fatigue tests in a INSTRON8501-5kN universal testing machine, with a cross head speed of 0.5 mm/min, load-up condition of 6Hz, a span of 40 mm, , a stress level of 92%, a goal service life of 105,σmin=70.5MPa, stress ratio=0.5. Five specimens were used for all the mechanical tests. 2.4 Thermal shocks tests The thermal shock tests were conducted by alternating the specimens quickly 3
between 1200℃ in static air and room temperature. First, each specimen was held in the tube furnace, which was preheated to 1200℃ for 10 minutes, and then the sample was cooled to room temperature for 10 minutes. After the temperature of the specimen dropped to room temperature for 10 minutes, it was immediately sent back to the furnace, temperature of which was held at 1200℃, for the next heating-cooling cycle. Such a heating-cooling cycle indicated that the specimens were thermally shocked for one time. In order to reveal the thermal shock resistance of the composites, at least Five specimens were investigated at prescribed number of 20 cycles. 2.5 Characterization The density of the composites was measured by Archimedes' method. The samples were weighed after tests by an electronic balance with a sensitivity of ±0.001 g. Microstructure of Al2O3f/SiO2 composites was characterized by SEM (quanta450 and quanta220, FEI). 3. Results and discussion 3.1 Microstructure of Al2O3f/ SiO2 composites 2.5D Al2O3f/SiO2 composites were fabricated by sol-gel process, with the density of 1.60 g/cm3 and open porosity of 16%. SEM image of Al2O3f/SiO2 composite surface (Fig. 1(a)) showed that Al2O3 fibers were covered by the SiO2 matrix, without obvious pores. Fig. 1(b) exhibited that the pores inside fiber bundles of the composite cross-section were filled with dense SiO2 matrix. The results of TG-DSC showed the oxidation stability and the thermal stability of the material. The existence of Al2O3 fibers suggested that no reaction occurred, so the composites had high chemical and thermal compatibilities. Given that the fibers hardly deboned from the matrix, the bonding between them was tight. Thus, Al2O3f/SiO2 composites had been fabricated efficiently by sol-gel method.
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3.2 Mechanical property of Al2O3f/ SiO2 composites Table 1 shows the mechanical properties of Al2O3f/SiO2 composites. The flexural strength was (108.8 ± 14.8) MPa, and the flexural modulus was (8.5 ± 1.0) GPa. The samples displayed a typical quasi-ductile response (Fig. 2). The fracture surface had long fiber pull-out, which can be ascribed to the weak fiber/matrix interface, being significantly associated with the quasi-ductile fracture behavior and high flexural strength of the composites. The weak interface formed because firstly, the SiO2 matrix reduced the mobility of fiber grain boundary, thus suppressing SiO2 diffusion outwards and preventing the chemical reaction between fibers and matrix [27]. Secondly, the fabricated matrix was weak [28]. Fatigue tests were conducted at the frequencies of 6Hz. This fatigue limit was based on the run-out condition of 105 cycles, approximate number of loading cycles expected in aerospace applications at 1200°C [2]. Fracture surfaces of several specimens were examined to gain a better understanding of the effects of the thermal shock and fatigue on the damage and failure mechanisms in the porous matrix. After the fatigue tests, the sample surface almost had no cracks in the macro before and after the tests(Fig. 3 and Fig. 4). As presented in Fig. 5, the cross-section of the composites after tests had long fiber pull-out. The microstructures of the composites before and after fatigue tests were basically identical (Fig. 2 and Fig. 5). The fiber pullout was extensive and the variation in pull-out length was considerable. It was recognized that the increase in the spatial correlation in the fiber failure locations was among the main manifestations of the matrix densification. After thermal shock and fatigue tests. After tests, few cracks formed on the sample surface (Fig. 6). SEM micrographs of the aforementioned fracture surfaces shown in Fig. 7, the fracture surface was dominated by planar regions of coordinated fiber failure, indicative of matrix densification due to additional sintering. A brushy fracture surface with extensive fiber pullout demonstrates that mechanical cycling had successfully counteracted the effects of matrix densification by maintaining matrix porosity at the level sufficient to enable crack-deflecting 5
behavior and fiber pullout. There was neither obvious oxidation of Al2O3 fibers nor chemical reaction between the fibers and SiO2 matrix after tests. The microstructure transformation of Al2O3f/SiO2 composites was responsible for the mechanical property changes after oxidation during thermal shock. As a result, Al2O3f/SiO2 composites exhibited decreased damage tolerance, brittle fracture behavior and a short lifetime. 3.3 Mechanical property of Al2O3f/ SiO2 composites after tests The fatigue test results of Al2O3f/SiO2 composites are shown in Fig. 3 and Table 1. After tests, the flexural strength was (98.4 ±15.4) MPa, with the retention ratio of 90.4%. The flexural modulus was (8.5±1.3) GPa. Therefore, the as-studied Al2O3f/SiO2 composites were stable after tests. Table 2 shows the properties of Al2O3f/SiO2 composites after fatigue tests. After 100000 tests, the fatigue tests had little effect on interfacial bonding or fiber properties. Since oxide/oxide CMCs are designed to work in high-temperature environments, it is of vital importance to evaluate the mechanical properties of composites at high temperatures [16, 29]. Thus, the composite thermal stability was further evaluated by thermal shock tests. The samples were heated at 1200°C in air, and then cooled to room temperature. Each sample in the tube furnace was preheated to 1200°C within 10 min and thereafter cooled to room temperature in 240 min. Table 3 lists the masses of Al2O3f/SiO2 composites before and after thermal shock tests. After tests, the mass loss was less than 1%, no obvious mass loss after thermal shock tests. However, the flexural strength reached 55.6 MPa, with the retention ratio of only 51.1%. The flexural strength loss of Al2O3f/SiO2 composites can be attributed to the strength loss of fibers during thermal shock, and then the strength loss of fibers lead to the changed the mechanical properties. Table 4 summarizes the properties of Al2O3f/SiO2 composites after thermal shock and fatigue tests. The composites underwent typical brittle fracture, with the lowest fracture strength (55.6 MPa).The microstructure transformation of Al2O3f/SiO2
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composites was responsible for the mechanical property changes after oxidation during thermal shock. After thermal shock, cracks appeared on the surface due to the difference between the thermal expansion coefficients of fibers and matrix. A sudden temperature change produced tensile stress in the matrix, which then accumulated and reached the critical value, finally giving cracks in the matrix. 4. Conclusions Microstructure and mechanical property of 2.5D Al2O3f/SiO2 composites by thermal shock and fatigue were investigated. The main results are summarized as follows: 2.5D Al2O3f/SiO2 composites were efficiently fabricated by sol-gel method. The composites showed quasi-ductile fracture behavior and high strength ((108.8 ± 14.8) MPa). After 100000 fatigue tests, there were no obvious cracks on the sample surface. The fatigue tests barely affected interfacial bonding or fiber properties. After thermal shock and fatigue tests, the composites were subjected to typical brittle fracture, with the lowest fracture strength, indicating that high temperature predominantly affected the composite mechanical properties. The mechanical performance degradation of the composites after thermal shock and fatigue tests was closely related to their microstructure evolution. Acknowledgements The authors are grateful to National Science Foundation of China (51602347) and Hunan Natural Science Foundation (2019JJ50282) for financial support. References [1] H.Ohnabe, S.Masaki, M.Onozuka, K.Miyahara , T.Sasa . Potential application of ceramic matrix composites to aero-engine components. Composites: Part A 1999;30:489-96. [2] M.B. Ruggles-Wrenn a, G. Hetrick, S.S. Baek. Effects of frequency and environment on fatigue behavior of an oxide-oxide ceramic composite at 1200°C. International Journal of 7
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[26] H.T. Liu, Q.S. Ma, W.D. Liu, Mechanical and oxidation resistance properties of 3D carbon fiber-reinforced mullite matrix composites prepared by sol-gel process, Ceram. Int. 40 (5) (2014) 7203–7212. [27] M. Schmückerw, P. Mechnich, Improving the microstructural stability of NextelTM 610 alumina fibersembedded in a porous alumina matrix, J. Am. Ceram. Soc. 93 (2010) 1888-1890. [28] F.W. Zok, C.G. Levi, Mechanical properties of porous-matrix ceramic composites, Adv. Eng. Mater. 3 (2001) 15-23. [29] X.Yang, C. Feng, C. Jie, C. Xiao-na, X. Qiang. Effects of high-temperature annealing on microstructure and mechanical property of SiO2f/SiO2 composites. Vacuum (2017) 144:1-7.
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Figures Caption Fig.1 The surface and cross-section of Al2O3f/SiO2 composites Fig.2 The cross-section of Al2O3f/SiO2 composites after flexural strength test Fig.3 The Al2O3f/SiO2 composites under fatigue tests Fig.4 The Al2O3f/SiO2 composites before and after fatigue tests Fig.5 The cross-section of Al2O3f/SiO2 composites after fatigue tests Fig.6 The surface of Al2O3f/SiO2 composites after thermal shock and fatigue tests Fig.7 The cross-section of Al2O3f/SiO2 composites after thermal shock and fatigue tests
Tables Caption Table 1 Mechanical property of Al2O3f/ SiO2 composites Table 2 The properties of Al2O3f/ SiO2 composites after fatigue tests Table 3 The mass of Al2O3f/SiO2 composites before and after thermal shock tests Table 4 The property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests
Highlights (1) 2.5D Al2O3f/SiO2 composites had been fabricated by sol-gel method efficiently. (2) 2.5D Al2O3f/SiO2 composites were tested by 100000 fatigue tests. (3) The fatigue tests had little effect on the interfacial bonding and fibers property. (4) The degradation of mechanical property after thermal shock and fatigue tests, indicating major influence of high temperature on the mechanical property of Al2O3f/SiO2 composites.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
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Table 1 Mechanical property of Al2O3f/ SiO2 composites Samples
Flexual strength (MPa)
Flexual modulus (GPa)
Befores Tests
108.8±14.8
8.5±1.0
After fatigue tests
98.4±15.4
8.5±1.3
After thermal shocks tests and fatigue tests
65.6±12.4
8.1±1.0
Table 2 The properties of Al2O3f/ SiO2 composites after fatigue tests Samples
Load-up condition (Hz)
Stress (MPa)
Load(N)
P1-6
60.76
P1-7
68.05
P1-8
6.0
64.86
60.34
P1-9
65.63
P1-10
62.20
Goal service life
Results
100000
No obvious cracks on the surface of samples
Table 3 The mass of Al2O3f/SiO2 composites before and after thermal shock tests Samples
Before test (g)
First test (g)
Second test (g)
P2-1 P2-2 P2-3 P2-4 P2-5
5.7626 5.7447 5.7108 5.8178 5.4166
5.7191 5.7070 5.6762 5.7797 5.3800
5.7188 5.7008 5.6700 5.7730 5.3794
Table 4 The property of Al2O3f/ SiO2 composites after thermal shock and fatigue tests Samples
Load-up condition (Hz)
Stress (MPa)
Load(N)
P2-1
69.70
P2-2
67.78
P2-3
6.0
64.86
66.70
P2-4
67.08
P2-5
65.20
Goal service life
Results
100000
No obvious cracks on the surface of samples
(1) 2.5D Al2O3f/SiO2 composites had been fabricated by sol-gel method efficiently. (2) 2.5D Al2O3f/SiO2 composites were tested by 100000 fatigue tests. (3) The fatigue tests had little effect on the interfacial bonding and fibers property. (4) The degradation of mechanical property after thermal shock and fatigue tests, indicating major influence of high temperature on the mechanical property of Al2O3f/SiO2 composites.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
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