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Low-temperature densification and mechanical properties of monolithic mullite ceramic
Journal Pre-proof Low-temperature densification and mechanical properties of monolithic mullite ceramic Dazhao Liu, Kaixuan Gui, Jianzhou Long, Yu Zhao, Wenbo Han, Gang Wang PII:
S0272-8842(20)30302-3
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.282
Reference:
CERI 24214
To appear in:
Ceramics International
Received Date: 24 December 2019 Revised Date:
23 January 2020
Accepted Date: 30 January 2020
Please cite this article as: D. Liu, K. Gui, J. Long, Y. Zhao, W. Han, G. Wang, Low-temperature densification and mechanical properties of monolithic mullite ceramic, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.01.282. 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. © 2020 Published by Elsevier Ltd.
Low-temperature densification and mechanical properties of monolithic mullite ceramic Dazhao Liua, Kaixuan Guia1, Jianzhou Longa, Yu Zhaoa, Wenbo Hanb, Gang Wanga1 a
Anhui Key Laboratory of High-Performance Non-ferrous Metal Materials, Anhui Polytechnic University, Wuhu 241000, China b
Science and Technology on Advanced Composites in Special Environment Laboratory, Harbin Institute of Technology, Harbin 150001, P.R. China
Abstract Dense mullite ceramics with excellent mechanical properties were produced using three kinds of mullite powders, namely micro-, submicro-, and nano-powders. The influence of sintering temperature on the microstructural and mechanical properties of the produced mullite ceramics was investigated. As the sintering temperature increased from 1200 to 1500 ºC, the relative density of the mullite ceramic produced using micro-powder varied from 65.3 to 99.2%, leading to an increase in the flexural strength and fracture toughness from 87.6 to 245.7 MPa, and 1.4 to 2.5 MPa·m1/2, respectively. The densification temperature of mullite ceramics significantly reduced when submicro- and nano-powders were used as raw materials. The relative density and mechanical properties of mullite ceramics fabricated using submicro- or nano-powders increased at first, then decreased following an increase in the sintering temperature. The mullite ceramic produced using submicro-powder became compact after being sintered at 1200 ºC, and exhibited a high flexural strength of 254.7 MPa and fracture toughness of 2.9 MPa·m1/2, indicating that low-temperature densification of mullite ceramics can be realised by enhancing the sintering activity of the mullite powder.
Keywords: Monolithic ceramic; Mechanical properties; Mullite; Microstructure
1
Corresponding author: Tel/fax: +86 553 2871252 E-mail address: [email protected] (G. Wang)
1. Introduction The intrinsic brittleness of monolithic mullite ceramic had drastically limited its application in the field of astronautics [1]. The introduction of continuous fibre was considered to be an effective method for improving the toughness and thermal shock resistance of mullite ceramics, satisfying the demand of mullite ceramics-related engineering applications [2-5]. Fibres, including carbon fibre and oxide fibre, had attracted worldwide interests due to their outstanding properties in a high temperature environment [6-9]. Mullite composite reinforced with three-dimensional carbon fibre fabric exhibited a high flexural strength of 277.6 MPa [10]. However, the microstructure and mechanical properties of carbon fibre fabric-reinforced mullite composite would degrade when served under an oxidising environment, due to the oxidation and damage of the carbon fibre [11, 12]. Oxide fibres displayed excellent mechanical properties, resistance to oxidation, and chemical compatibility with mullite, making them a promising candidate for toughening the mullite ceramics [13-16]. Volkmann investigated the influence of heat treatments on the mechanical performance of NextelTM 610 fibres reinforced mullite composite [17]. The mechanical properties decreased with an increase in the temperature because of the degradation of fibre properties, resulting from grain growth and a high interfacial adhesion between fibre and matrix [18]. During the fabrication of continuous oxide fibre-reinforced mullite matrix composite, the oxide fibre suffered from grain growth and creeps under high temperature. The fibre and mullite matrix might form an oxide solid solution under high temperature sintering, increasing the bonding force between the fibre and matrix and significantly reducing the toughening effect of the fibre [19-23]. Therefore, it was necessary to limit the damage of the oxide fibre caused by the high temperature treatment or mechanical friction. The conventional technique to regulate this damage was by coating the fibre surface [24-26]. Besides fibre coating, reducing the sintering temperature was another effective and cost-efficient method. The conventional methods to reduce the sintering temperature includes introduction of sintering assistant, reaction sintering, and refinement of particle size [27]. The introduction of sintering auxiliaries led to the degradation of high-temperature properties of the composites, while reactive sintering resulted in fibre damage. Hence, particle size refinement was determined to be a suitable method for reducing the sintering temperature [28]. In this paper, three kinds of mullite powders, including
micro-, submicro-, and nano-powders, were obtained through physical or chemical methods, and the fabrication of monolithic mullite ceramics with the use of these powders as raw materials were studied. The microstructure and mechanical properties of mullite ceramics were investigated and low-temperature densification of the mullite ceramic was realised, which provided technical support for preparing continuous fibre-reinforced mullite matrix composites with high fracture toughness and excellent thermal shock resistance.
2. Experimental methods 2.1 Mullite ceramics processing Mullite powder (>99%, Northwest Institute for Non-ferrous Metal Research, China) with particle size of 20~70 µm was used as the raw material for the experiment. In this study, micro-powders with a regular shape were obtained via ball mill for 24 h and nano-powders were acquired through vertical superfine ball mill. The sol-gel method was used to fabricate the submicro-powders. The aluminium and silicon sources of mullite were aluminium nitrate nonahydrate (ANN, Fuchen, China) and tetraethoxysilane (TEOS, Fuchen, China), respectively. The details about preparation of mullite precursor were described elsewhere [29]. The submicro-powders used for our study were obtained by pyrolyzing the precursor at 900 ºC, and maintaining the high sintering activity of the submicro-powder. Mullite ceramics were fabricated at different temperatures, and the ones produced using micro-, nano-, and submicro-powders were named MMC, NMC and SMC, respectively. 2.2 Characterisation The microstructures of the mullite ceramics were characterised by focusing ion/electron dual-beam micro electron microscope (FIB/SEM, HELIOS NanoLab 600i, FEI, USA). Archimedes’ method was adopted to measure the material density. The flexural strength and fracture toughness tests were conducted on the Instron test machine with crosshead speed of 0.5 and 0.05 mm/min, respectively. 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. All the test samples were obtained from the same batch, and each value represented an average measurement for five specimens.
3. Results and discussion 3.1 Morphology of mullite powders
The original and pre-treated mullite powders were shown in Fig. 1. After milling for 24 h, the size of the mullite powder reduced below 5 µm (Fig. 1(b)). Submicro-mullite powders produced via the sol-gel process were depicted in Fig. 1(c). The powder had a regular shape with concentrated size distribution. Compared to the raw powder, the size of mullite powder treated with ultrafine ball-milling for 10 h decreased from micro- to nano-size with uniform morphology (Fig. 1(a) and 1(d)). Due to the high surface energy, agglomeration of the nano-powder was observed. 3.2 Fabrication of mullite ceramics produced with micro-powder (MMC) The influence of temperature and holding time on the relative density of MMCs were investigated (Fig. 2). As shown in Fig. 2, the MMC density exhibited a rising trend with the increase in the sintering temperature. The relative density of MMCs sintered at different temperatures varied from 65.3 to 99.2%. The micro-powders with a large size had a high degree of crystallisation, increasing the densification difficulty; thus, the density of MMC prepared at 1200 ºC was just 65.3%, and the MMC remained compact until the temperature increased to 1500 ºC. It could also be seen from Fig. 2 that the holding time had minimum influence on the density when the sintering temperature was 1200 ºC. Only after the sintering temperature reached 1300 ºC, the density of MMC increased with an increase in the holding time. The microstructure of MMC prepared at different sintering temperatures was characterised (Fig. 3). As shown in Fig. 3, there were several irregular pores in the mullite ceramic sintered at 1200 ºC with no obvious sintering between the mullite powders, indicating that the low sintering temperature could not provide sufficient energy for diffusion and rearrangement of the atoms. Even after the sintering temperature rose to 1300 ºC, numerous pores existed inside the ceramic with a relative density of only 76.9%. When the sintering temperature increased further, the mullite powders became sintered, and the pores within the ceramic gradually disappeared. When MMC was sintered at 1500 ºC, it was almost dense. The fracture surfaces of MMCs prepared at different temperatures were shown in Fig. 4. It could be seen that the number of pores in mullite ceramics gradually decreased as the temperature rose. The fracture surface of MMC prepared at 1200 ºC was rough, and the mullite ceramic displayed intergranular fracture behaviour, indicating that the adhesion strength between the incomplete sintered grains was low. From Fig. 4(c) and 4(d), it could be noted that the fracture morphology was relatively flat and the mode transformed into trans-granular fracture. Based on the above-mentioned studies, it
could be concluded that the main process parameters affecting the relative density and microstructure of MMC were sintering temperature and holding time. 3.3 Fabrication of mullite ceramics produced with nano-powder (NMC) Fig. 5 illustrates the influence of temperature and holding time on the relative density of NMC. The relative density of NMC sintered at 1100 ºC reached beyond 95% and increased to 98.7% as the sintering temperature ascended up to 1200 ºC. After being sintered at 1300 ºC for 1 h, the NMC was complete densification. Compared to MMC, the sintering temperature of NMC has decreased by 200 ºC. It could be mainly attributed to the high sintering activity of the nano-powder with large specific surface area and high surface energy. The results showed that using nano-powder obtained by ultra-fine ball milling could effectively reduced the densification temperature of mullite ceramics and realised low temperature densification. It could also be noted that the relative density of NMC produced at 1400 ºC was slightly lower than that produced at 1300 ºC. It was ascribed to the growth of mullite grains induced by excessive sintering temperature, resulting in generation of pores and decrease in relative density. The relative density of NMC sintered at 1100 ºC gradually increases with an increase in holding time. The holding time has minimum effect on the density of mullite ceramic as sintering temperature increased beyond 1200 ºC and a prolonged holding time led to the growth of grain. The microstructures of NMCs produced at different sintering temperatures were characterised and SEM images were shown in Fig. 6. There were only a few irregular pores inside the mullite ceramic sintered at 1100 ºC, indicating that the sintering temperature could lead to the diffusion and rearrangement of atoms. When the sintering temperature increased to 1300 ºC, the NMC became completely compact. Fig. 7 shows the fracture surfaces of NMCs produced at different sintering temperatures. The fracture morphologies of NMCs obtained at the four sintering temperatures were relatively uniform and the fracture mode was trans-granular fracture. 3.4 Fabrication of mullite ceramics produced with submicro-powder (SMC) The submicro-mullite powder was produced by using the sol-gel method, and the sintering activity of the powder could be adjusted by controlling the composition and pyrolysis temperature; thus, affecting the subsequent sintering process and the final density of mullite ceramics. The relative density of mullite ceramic sintered at 1000 ºC reached beyond 85% (Fig. 8). In accordance with results of previous studies, the
mullite precursor after heat treatment at 1000 ºC was composed of mullite phase and Al-Si spinel phase [30-32]. The atomic arrangement of Al-Si spinel phase was unstable and the atomic diffusion required low energy. Therefore, SMC with a higher density could be obtained at 1000 ºC. However, the relative density of SMC increased just to 93.5% when the sintering temperature was increased to 1100 ºC. It could be because that some of Al-Si spinel powders might have been directly sintered, while the Al-Si spinel powders were first transformed into mullite powders and then made to undergo incomplete sintering at 1100 ºC, resulting in a reduced increase in relative density. The sintering temperature of 1200 ºC was high enough to obtain a completely dense mullite ceramic; hence, achieving low temperature densification. The microstructures of SMCs produced at different sintering temperatures was characterised. As shown in Fig. 9, mullite ceramics sintered at 1000 ºC and 1100 ºC possessed a high density, indicating that a low sintering temperature could prompt the atoms to diffuse and rearrange. When the sintering temperature rose to 1200 ºC, SMC underwent completely densification. Fig. 10 depicts the fracture surfaces of SMCs produced at different sintering temperatures. The fracture surfaces were relatively flat and the mullite ceramics displayed a trans-granular fracture mode. 3.5 Mechanical properties of mullite ceramics The mechanical properties of MMCs were shown in Fig. 11. The flexural strength of MMC sintered at different temperatures gradually increased with the rise in the sintering temperature. The MMC fabricated at 1200 ºC had the lowest flexural strength of 87.6 MPa and fracture toughness of 1.4 MPa·m1/2, while that sintered at 1500 ºC exhibited the highest flexural strength of 245.7 MPa and fracture toughness of 2.5 MPa·m1/2. As per microstructural analysis of MMC, it was ascribed to the increase in relative density of MMC following the increase in the sintering temperature. The mechanical properties of NMC were shown in Fig. 12. It could be noted that the flexural strength of NMC first increased, followed by a decrease with an increase in the temperature. The flexural strength and fracture toughness of the NMC prepared at 1100 ºC were 212.2 MPa and 2.4 MPa·m1/2, respectively, while the highest flexural strength of 251.2 MPa and fracture toughness of 2.8 MPa·m1/2 were obtained at 1300 ºC. When the sintering temperature exceeded 1300 ºC, the mechanical properties of NMC decreased due to the growth of mullite grains. Fig. 13 depicts the mechanical properties of SMCs. With respect to the increase in the
sintering temperature, the variation trend in terms of mechanical properties of SMC was quite similar to that of NMC. The sintering activity of submicro-powder was higher than micro-, and nano-powders. The flexural strength of SMC sintered at 1000 ºC was 198.3 MPa, and then increased to 218.1 MPa (1100 ºC). The flexural strength and fracture toughness of SMC sintered at 1200 ºC rose to 254.7 MPa and 2.9 MPa·m1/2, respectively, due to an increase in the density and reduction of internal gaps. The flexural strength decreased as the temperature increased to 1300 ºC, and it was ascribed that a further increase in temperature would result in the growth of mullite grains. It could be determined that the flexural strength and fracture toughness of SMC were higher than those of MMC and NMC, but the fracture mode still remained brittle.
4. Conclusions Three kinds of mullite powders, namely micro-, submicro- and nano-powders were used as the raw material to produce mullite ceramics. The influence of sintering temperature on the microstructural and mechanical properties of mullite ceramics was investigated and optimised. A compact MMC with flexural strength of 245.7 MPa and fracture toughness of 2.5 MPa·m1/2 was obtained at 1500 ºC. NMC sintered at 1300 ºC was completely dense, exhibiting flexural strength of 251.2 MPa and fracture toughness of 2.8 MPa·m1/2. SMC underwent densification at 1200 ºC, significantly reducing the sintering temperature. The flexural strength and fracture toughness of SMC was determined to be 254.7 and 2.9 MPa·m1/2, respectively, and the continuous increase in sintering temperature led to the rapid growth of mullite grains, resulting in the reduction of mechanical properties. The optimisation of mullite powder was found to be an effective measure in reducing the sintering temperature.
Acknowledgements The authors are grateful for Talent Project of Anhui Province (Z175050020001); Talent Project of Anhui Polytechnic University; Pre-research Project of National Natural Science Foundation of Anhui Polytechnic University (2019yyzr05); University Synergy Innovation Program of Anhui Province (GXXT-2019-015) and National Nature Science Foundation of China (No. 11572105).
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three-dimensional braided carbon fiber reinforced mullite composites with different sols as raw materials, Materials Science Forum. 816 (2015) 27-32. [13] Stoll E, Mahr P, H. G. Krüger, et al. Fabrication technologies for oxide-oxide ceramic matrix composites based on electrophoretic deposition, Journal of the European Ceramic Society. 26 (2006) 1567-1576. [14] Ruh R, Mazdiyasni K S, Mendiratta M G. Mechanical and microstructural characterization of mullite and mullite-SiC-whisker and ZrO2-toughened-mullite-SiCwhisker composites, ChemInform. 71 (1988) 503-512. [15] Boccaccini A R, Atiq S, Boccaccini D N , et al. Fracture behaviour of mullite fibre reinforced-mullite matrix composites under quasi-static and ballistic impact loading, Composites Science and Technology. 65 (2005) 325-333. [16] Bao Y, Nicholson P S. AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites, Journal of the European Ceramic Society. 28 (2008) 3041-3048. [17] Volkmann E, Tushtev K, Koch D, et al. Assessment of three oxide/oxide ceramic matrix composites Mechanical performance and effects of heat treatments, Composites Part A Applied Science and Manufacturing. 68 (2015) 19-28. [18] Martin Schmücker, Mechnich P. Microstructural coarsening of Nextel 610 fibers embedded in alumina-based matrices, Journal of the American Ceramic Society. 91 (2008) 1306-1308. [19] Armani C J, Marina B. Ruggles￤renn, Fair G E , et al. Creep of Nextel (TM) 610 Fiber at 1100 degrees C in Air and in Steam, International Journal of Applied Ceramic Technology. 10 (2013) 276-284. [20] Almeida R S M, Bergmüller, Eduardo L, Lührs, Hanna, et al. Thermal exposure effects on the long-term behavior of a mullite fiber at high temperature, Journal of the American Ceramic Society. 100 (2017) 4101-4109. [21] Gurauskis J, A.J. Sánchez-Herencia, C. Baudín. Al2O3/Y-TZP and Y-TZP materials fabricated by stacking layers obtained by aqueous tape casting, Journal of the European Ceramic Society. 26 (2006) 1489-1496. [22] Licciulli A, Chiechi A, Fersini M, et al. Influence of zirconia interfacial coating on alumina fiber-reinforced alumina matrix composites, International Journal of Applied Ceramic Technology. 10 (2013) 251-256. [23] Bao Y, Nicholson P S. AlPO4 coating on alumina/mullite fibers as a weak
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Figure captions Fig. 1 SEM images of mullite powders: (a) original mullite powder; (b) ball-milling for 24h; (c) prepared via sol-gel method; (d) ultrafine ball-milling for 10h. Fig. 2 The influence of sintering temperature and holding time on the relative density of MMC Fig. 3 Surface morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d). Fig. 4 Fracture morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d). Fig. 5 The influence of sintering temperature and holding time on the relative density of NMC Fig. 6 Surface microstructure of NMCs fabricated at different temperature: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d). Fig. 7 Fracture surfaces of NMCs fabricated at different temperatures: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d). Fig. 8 The influence of sintering temperature and holding time on the relative density of SMC Fig. 9 Surface microstructure of SMCs fabricated at different temperatures for 1h: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d). Fig. 10 Fracture morphology of SMCs fabricated at different temperatures: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d). Fig. 11 Flexural strength and fracture toughness of MMC Fig. 12 Flexural strength and fracture toughness of NMC Fig. 13 Flexural strength and fracture toughness of SMC
Figures
Fig. 1 SEM images of mullite powders: (a) original mullite powder; (b) ball-milling for 24h; (c) prepared via sol-gel method; (d) ultrafine ball-milling for 10h.
Fig. 2 The influence of temperature and holding time on the relative density of MMC
Fig. 3 Surface morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d).
Fig. 4 Fracture morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d).
Fig. 5 The influence of sintering temperature and holding time on the relative density of NMC
Fig. 6 Surface microstructure of NMCs fabricated at different temperature: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d).
Fig. 7 Fracture surfaces of NMCs fabricated at different temperatures: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d)
Fig. 8 The influence of sintering temperature and holding time on the relative density of SMC
Fig. 9 Surface microstructure of SMCs fabricated at different temperatures for 1h: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d).
Fig. 10 Fracture morphology of SMCs fabricated at different temperatures: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d)
Fig. 11 Flexural strength and fracture toughness of MMC
Fig. 12 Flexural strength and fracture toughness of NMC
Fig. 13 Flexural strength and fracture toughness of SMC
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(20)30302-3
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.282
Reference:
CERI 24214
To appear in:
Ceramics International
Received Date: 24 December 2019 Revised Date:
23 January 2020
Accepted Date: 30 January 2020
Please cite this article as: D. Liu, K. Gui, J. Long, Y. Zhao, W. Han, G. Wang, Low-temperature densification and mechanical properties of monolithic mullite ceramic, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.01.282. 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. © 2020 Published by Elsevier Ltd.
Low-temperature densification and mechanical properties of monolithic mullite ceramic Dazhao Liua, Kaixuan Guia1, Jianzhou Longa, Yu Zhaoa, Wenbo Hanb, Gang Wanga1 a
Anhui Key Laboratory of High-Performance Non-ferrous Metal Materials, Anhui Polytechnic University, Wuhu 241000, China b
Science and Technology on Advanced Composites in Special Environment Laboratory, Harbin Institute of Technology, Harbin 150001, P.R. China
Abstract Dense mullite ceramics with excellent mechanical properties were produced using three kinds of mullite powders, namely micro-, submicro-, and nano-powders. The influence of sintering temperature on the microstructural and mechanical properties of the produced mullite ceramics was investigated. As the sintering temperature increased from 1200 to 1500 ºC, the relative density of the mullite ceramic produced using micro-powder varied from 65.3 to 99.2%, leading to an increase in the flexural strength and fracture toughness from 87.6 to 245.7 MPa, and 1.4 to 2.5 MPa·m1/2, respectively. The densification temperature of mullite ceramics significantly reduced when submicro- and nano-powders were used as raw materials. The relative density and mechanical properties of mullite ceramics fabricated using submicro- or nano-powders increased at first, then decreased following an increase in the sintering temperature. The mullite ceramic produced using submicro-powder became compact after being sintered at 1200 ºC, and exhibited a high flexural strength of 254.7 MPa and fracture toughness of 2.9 MPa·m1/2, indicating that low-temperature densification of mullite ceramics can be realised by enhancing the sintering activity of the mullite powder.
Keywords: Monolithic ceramic; Mechanical properties; Mullite; Microstructure
1
Corresponding author: Tel/fax: +86 553 2871252 E-mail address: [email protected] (G. Wang)
1. Introduction The intrinsic brittleness of monolithic mullite ceramic had drastically limited its application in the field of astronautics [1]. The introduction of continuous fibre was considered to be an effective method for improving the toughness and thermal shock resistance of mullite ceramics, satisfying the demand of mullite ceramics-related engineering applications [2-5]. Fibres, including carbon fibre and oxide fibre, had attracted worldwide interests due to their outstanding properties in a high temperature environment [6-9]. Mullite composite reinforced with three-dimensional carbon fibre fabric exhibited a high flexural strength of 277.6 MPa [10]. However, the microstructure and mechanical properties of carbon fibre fabric-reinforced mullite composite would degrade when served under an oxidising environment, due to the oxidation and damage of the carbon fibre [11, 12]. Oxide fibres displayed excellent mechanical properties, resistance to oxidation, and chemical compatibility with mullite, making them a promising candidate for toughening the mullite ceramics [13-16]. Volkmann investigated the influence of heat treatments on the mechanical performance of NextelTM 610 fibres reinforced mullite composite [17]. The mechanical properties decreased with an increase in the temperature because of the degradation of fibre properties, resulting from grain growth and a high interfacial adhesion between fibre and matrix [18]. During the fabrication of continuous oxide fibre-reinforced mullite matrix composite, the oxide fibre suffered from grain growth and creeps under high temperature. The fibre and mullite matrix might form an oxide solid solution under high temperature sintering, increasing the bonding force between the fibre and matrix and significantly reducing the toughening effect of the fibre [19-23]. Therefore, it was necessary to limit the damage of the oxide fibre caused by the high temperature treatment or mechanical friction. The conventional technique to regulate this damage was by coating the fibre surface [24-26]. Besides fibre coating, reducing the sintering temperature was another effective and cost-efficient method. The conventional methods to reduce the sintering temperature includes introduction of sintering assistant, reaction sintering, and refinement of particle size [27]. The introduction of sintering auxiliaries led to the degradation of high-temperature properties of the composites, while reactive sintering resulted in fibre damage. Hence, particle size refinement was determined to be a suitable method for reducing the sintering temperature [28]. In this paper, three kinds of mullite powders, including
micro-, submicro-, and nano-powders, were obtained through physical or chemical methods, and the fabrication of monolithic mullite ceramics with the use of these powders as raw materials were studied. The microstructure and mechanical properties of mullite ceramics were investigated and low-temperature densification of the mullite ceramic was realised, which provided technical support for preparing continuous fibre-reinforced mullite matrix composites with high fracture toughness and excellent thermal shock resistance.
2. Experimental methods 2.1 Mullite ceramics processing Mullite powder (>99%, Northwest Institute for Non-ferrous Metal Research, China) with particle size of 20~70 µm was used as the raw material for the experiment. In this study, micro-powders with a regular shape were obtained via ball mill for 24 h and nano-powders were acquired through vertical superfine ball mill. The sol-gel method was used to fabricate the submicro-powders. The aluminium and silicon sources of mullite were aluminium nitrate nonahydrate (ANN, Fuchen, China) and tetraethoxysilane (TEOS, Fuchen, China), respectively. The details about preparation of mullite precursor were described elsewhere [29]. The submicro-powders used for our study were obtained by pyrolyzing the precursor at 900 ºC, and maintaining the high sintering activity of the submicro-powder. Mullite ceramics were fabricated at different temperatures, and the ones produced using micro-, nano-, and submicro-powders were named MMC, NMC and SMC, respectively. 2.2 Characterisation The microstructures of the mullite ceramics were characterised by focusing ion/electron dual-beam micro electron microscope (FIB/SEM, HELIOS NanoLab 600i, FEI, USA). Archimedes’ method was adopted to measure the material density. The flexural strength and fracture toughness tests were conducted on the Instron test machine with crosshead speed of 0.5 and 0.05 mm/min, respectively. 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. All the test samples were obtained from the same batch, and each value represented an average measurement for five specimens.
3. Results and discussion 3.1 Morphology of mullite powders
The original and pre-treated mullite powders were shown in Fig. 1. After milling for 24 h, the size of the mullite powder reduced below 5 µm (Fig. 1(b)). Submicro-mullite powders produced via the sol-gel process were depicted in Fig. 1(c). The powder had a regular shape with concentrated size distribution. Compared to the raw powder, the size of mullite powder treated with ultrafine ball-milling for 10 h decreased from micro- to nano-size with uniform morphology (Fig. 1(a) and 1(d)). Due to the high surface energy, agglomeration of the nano-powder was observed. 3.2 Fabrication of mullite ceramics produced with micro-powder (MMC) The influence of temperature and holding time on the relative density of MMCs were investigated (Fig. 2). As shown in Fig. 2, the MMC density exhibited a rising trend with the increase in the sintering temperature. The relative density of MMCs sintered at different temperatures varied from 65.3 to 99.2%. The micro-powders with a large size had a high degree of crystallisation, increasing the densification difficulty; thus, the density of MMC prepared at 1200 ºC was just 65.3%, and the MMC remained compact until the temperature increased to 1500 ºC. It could also be seen from Fig. 2 that the holding time had minimum influence on the density when the sintering temperature was 1200 ºC. Only after the sintering temperature reached 1300 ºC, the density of MMC increased with an increase in the holding time. The microstructure of MMC prepared at different sintering temperatures was characterised (Fig. 3). As shown in Fig. 3, there were several irregular pores in the mullite ceramic sintered at 1200 ºC with no obvious sintering between the mullite powders, indicating that the low sintering temperature could not provide sufficient energy for diffusion and rearrangement of the atoms. Even after the sintering temperature rose to 1300 ºC, numerous pores existed inside the ceramic with a relative density of only 76.9%. When the sintering temperature increased further, the mullite powders became sintered, and the pores within the ceramic gradually disappeared. When MMC was sintered at 1500 ºC, it was almost dense. The fracture surfaces of MMCs prepared at different temperatures were shown in Fig. 4. It could be seen that the number of pores in mullite ceramics gradually decreased as the temperature rose. The fracture surface of MMC prepared at 1200 ºC was rough, and the mullite ceramic displayed intergranular fracture behaviour, indicating that the adhesion strength between the incomplete sintered grains was low. From Fig. 4(c) and 4(d), it could be noted that the fracture morphology was relatively flat and the mode transformed into trans-granular fracture. Based on the above-mentioned studies, it
could be concluded that the main process parameters affecting the relative density and microstructure of MMC were sintering temperature and holding time. 3.3 Fabrication of mullite ceramics produced with nano-powder (NMC) Fig. 5 illustrates the influence of temperature and holding time on the relative density of NMC. The relative density of NMC sintered at 1100 ºC reached beyond 95% and increased to 98.7% as the sintering temperature ascended up to 1200 ºC. After being sintered at 1300 ºC for 1 h, the NMC was complete densification. Compared to MMC, the sintering temperature of NMC has decreased by 200 ºC. It could be mainly attributed to the high sintering activity of the nano-powder with large specific surface area and high surface energy. The results showed that using nano-powder obtained by ultra-fine ball milling could effectively reduced the densification temperature of mullite ceramics and realised low temperature densification. It could also be noted that the relative density of NMC produced at 1400 ºC was slightly lower than that produced at 1300 ºC. It was ascribed to the growth of mullite grains induced by excessive sintering temperature, resulting in generation of pores and decrease in relative density. The relative density of NMC sintered at 1100 ºC gradually increases with an increase in holding time. The holding time has minimum effect on the density of mullite ceramic as sintering temperature increased beyond 1200 ºC and a prolonged holding time led to the growth of grain. The microstructures of NMCs produced at different sintering temperatures were characterised and SEM images were shown in Fig. 6. There were only a few irregular pores inside the mullite ceramic sintered at 1100 ºC, indicating that the sintering temperature could lead to the diffusion and rearrangement of atoms. When the sintering temperature increased to 1300 ºC, the NMC became completely compact. Fig. 7 shows the fracture surfaces of NMCs produced at different sintering temperatures. The fracture morphologies of NMCs obtained at the four sintering temperatures were relatively uniform and the fracture mode was trans-granular fracture. 3.4 Fabrication of mullite ceramics produced with submicro-powder (SMC) The submicro-mullite powder was produced by using the sol-gel method, and the sintering activity of the powder could be adjusted by controlling the composition and pyrolysis temperature; thus, affecting the subsequent sintering process and the final density of mullite ceramics. The relative density of mullite ceramic sintered at 1000 ºC reached beyond 85% (Fig. 8). In accordance with results of previous studies, the
mullite precursor after heat treatment at 1000 ºC was composed of mullite phase and Al-Si spinel phase [30-32]. The atomic arrangement of Al-Si spinel phase was unstable and the atomic diffusion required low energy. Therefore, SMC with a higher density could be obtained at 1000 ºC. However, the relative density of SMC increased just to 93.5% when the sintering temperature was increased to 1100 ºC. It could be because that some of Al-Si spinel powders might have been directly sintered, while the Al-Si spinel powders were first transformed into mullite powders and then made to undergo incomplete sintering at 1100 ºC, resulting in a reduced increase in relative density. The sintering temperature of 1200 ºC was high enough to obtain a completely dense mullite ceramic; hence, achieving low temperature densification. The microstructures of SMCs produced at different sintering temperatures was characterised. As shown in Fig. 9, mullite ceramics sintered at 1000 ºC and 1100 ºC possessed a high density, indicating that a low sintering temperature could prompt the atoms to diffuse and rearrange. When the sintering temperature rose to 1200 ºC, SMC underwent completely densification. Fig. 10 depicts the fracture surfaces of SMCs produced at different sintering temperatures. The fracture surfaces were relatively flat and the mullite ceramics displayed a trans-granular fracture mode. 3.5 Mechanical properties of mullite ceramics The mechanical properties of MMCs were shown in Fig. 11. The flexural strength of MMC sintered at different temperatures gradually increased with the rise in the sintering temperature. The MMC fabricated at 1200 ºC had the lowest flexural strength of 87.6 MPa and fracture toughness of 1.4 MPa·m1/2, while that sintered at 1500 ºC exhibited the highest flexural strength of 245.7 MPa and fracture toughness of 2.5 MPa·m1/2. As per microstructural analysis of MMC, it was ascribed to the increase in relative density of MMC following the increase in the sintering temperature. The mechanical properties of NMC were shown in Fig. 12. It could be noted that the flexural strength of NMC first increased, followed by a decrease with an increase in the temperature. The flexural strength and fracture toughness of the NMC prepared at 1100 ºC were 212.2 MPa and 2.4 MPa·m1/2, respectively, while the highest flexural strength of 251.2 MPa and fracture toughness of 2.8 MPa·m1/2 were obtained at 1300 ºC. When the sintering temperature exceeded 1300 ºC, the mechanical properties of NMC decreased due to the growth of mullite grains. Fig. 13 depicts the mechanical properties of SMCs. With respect to the increase in the
sintering temperature, the variation trend in terms of mechanical properties of SMC was quite similar to that of NMC. The sintering activity of submicro-powder was higher than micro-, and nano-powders. The flexural strength of SMC sintered at 1000 ºC was 198.3 MPa, and then increased to 218.1 MPa (1100 ºC). The flexural strength and fracture toughness of SMC sintered at 1200 ºC rose to 254.7 MPa and 2.9 MPa·m1/2, respectively, due to an increase in the density and reduction of internal gaps. The flexural strength decreased as the temperature increased to 1300 ºC, and it was ascribed that a further increase in temperature would result in the growth of mullite grains. It could be determined that the flexural strength and fracture toughness of SMC were higher than those of MMC and NMC, but the fracture mode still remained brittle.
4. Conclusions Three kinds of mullite powders, namely micro-, submicro- and nano-powders were used as the raw material to produce mullite ceramics. The influence of sintering temperature on the microstructural and mechanical properties of mullite ceramics was investigated and optimised. A compact MMC with flexural strength of 245.7 MPa and fracture toughness of 2.5 MPa·m1/2 was obtained at 1500 ºC. NMC sintered at 1300 ºC was completely dense, exhibiting flexural strength of 251.2 MPa and fracture toughness of 2.8 MPa·m1/2. SMC underwent densification at 1200 ºC, significantly reducing the sintering temperature. The flexural strength and fracture toughness of SMC was determined to be 254.7 and 2.9 MPa·m1/2, respectively, and the continuous increase in sintering temperature led to the rapid growth of mullite grains, resulting in the reduction of mechanical properties. The optimisation of mullite powder was found to be an effective measure in reducing the sintering temperature.
Acknowledgements The authors are grateful for Talent Project of Anhui Province (Z175050020001); Talent Project of Anhui Polytechnic University; Pre-research Project of National Natural Science Foundation of Anhui Polytechnic University (2019yyzr05); University Synergy Innovation Program of Anhui Province (GXXT-2019-015) and National Nature Science Foundation of China (No. 11572105).
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Figure captions Fig. 1 SEM images of mullite powders: (a) original mullite powder; (b) ball-milling for 24h; (c) prepared via sol-gel method; (d) ultrafine ball-milling for 10h. Fig. 2 The influence of sintering temperature and holding time on the relative density of MMC Fig. 3 Surface morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d). Fig. 4 Fracture morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d). Fig. 5 The influence of sintering temperature and holding time on the relative density of NMC Fig. 6 Surface microstructure of NMCs fabricated at different temperature: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d). Fig. 7 Fracture surfaces of NMCs fabricated at different temperatures: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d). Fig. 8 The influence of sintering temperature and holding time on the relative density of SMC Fig. 9 Surface microstructure of SMCs fabricated at different temperatures for 1h: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d). Fig. 10 Fracture morphology of SMCs fabricated at different temperatures: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d). Fig. 11 Flexural strength and fracture toughness of MMC Fig. 12 Flexural strength and fracture toughness of NMC Fig. 13 Flexural strength and fracture toughness of SMC
Figures
Fig. 1 SEM images of mullite powders: (a) original mullite powder; (b) ball-milling for 24h; (c) prepared via sol-gel method; (d) ultrafine ball-milling for 10h.
Fig. 2 The influence of temperature and holding time on the relative density of MMC
Fig. 3 Surface morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d).
Fig. 4 Fracture morphology of MMCs fabricated at different temperatures: 1200 ºC (a); 1300 ºC (b); 1400 ºC (c); 1500 ºC (d).
Fig. 5 The influence of sintering temperature and holding time on the relative density of NMC
Fig. 6 Surface microstructure of NMCs fabricated at different temperature: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d).
Fig. 7 Fracture surfaces of NMCs fabricated at different temperatures: 1100 ºC (a); 1200 ºC (b); 1300 ºC (c); 1400 ºC (d)
Fig. 8 The influence of sintering temperature and holding time on the relative density of SMC
Fig. 9 Surface microstructure of SMCs fabricated at different temperatures for 1h: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d).
Fig. 10 Fracture morphology of SMCs fabricated at different temperatures: 1000 ºC (a); 1100 ºC (b); 1200 ºC (c); 1300 ºC (d)
Fig. 11 Flexural strength and fracture toughness of MMC
Fig. 12 Flexural strength and fracture toughness of NMC
Fig. 13 Flexural strength and fracture toughness of SMC
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: