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ZrO2-induced crack-healing mechanism of ZrB2–SiC–Graphite composite in high temperature atomic oxygen environment
Author’s Accepted Manuscript ZrO2-induced crack-healing mechanism of ZrB2SiC-Graphite composite in high temperature atomic oxygen environment Jiahong Niu, Hua Jin, Songhe Meng, Qingxuan Zeng, Zujun Peng www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(15)02314-7 http://dx.doi.org/10.1016/j.ceramint.2015.12.031 CERI11826
To appear in: Ceramics International Received date: 8 October 2015 Revised date: 14 November 2015 Accepted date: 1 December 2015 Cite this article as: Jiahong Niu, Hua Jin, Songhe Meng, Qingxuan Zeng and Zujun Peng, ZrO2-induced crack-healing mechanism of ZrB2-SiC-Graphite composite in high temperature atomic oxygen environment, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.12.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
ZrO2-induced crack-healing mechanism of ZrB2-SiC-Graphite composite in high temperature atomic oxygen environment Jiahong Niu, Hua Jin*, Songhe Meng, Qingxuan Zeng, Zujun Peng Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, China Abstract The ZrO2-induced crack-healing mechanism of ZrB2-SiC-Graphite (ZSG) composite is investigated under high temperature atomic oxygen atmosphere, which characterizes the real condition of hypersonic flight. This differs greatly from the borosilicate glass-induced crack-healing mechanism in previous studies. The cracks are healed limitedly in high temperature atomic oxygen condition with a 39.09% maximum improvement in residual strength compared to the pre-cracked specimens. The crack-healing is induced by ZrO2, and the crack-healing mechanism involves three main aspects: A. local particle rearrangement, which is induced by expansion from the chemical reaction (ZrB2→ZrO2), the phase transition
(t-ZrO2→m-ZrO2),
and
the
thermal
expansion.
B.
the crack volume filling by ZrO2 driven by the liberation of the reaction gases. C. Diffusion controlled sintering around the ZrO2 sintering temperature. The spontaneous crack-healing ability in high temperature atomic oxygen atmosphere is helpful to improve the reliability of ZSG composites in service. Keywords: A. Hot pressing; B. Composites; E. Structural applications Corresponding author: Hua Jin Email: [email protected] Tel/Fax: +86-451-86402432
1
1 Introduction ZrB2-SiC-Graphite (ZSG) composite is widely known by its significant performance enhancement in fracture toughness (~6.1 MPa·m1/2), thermal shock resistance, as well as its oxidation resistance among ZrB2 based ultra-high temperature ceramics [1-7]. Nevertheless, the intrinsic brittleness derived from structural ceramics is a main drawback, making it sensitive to the presence of surface flaws. This may result in performance degradation or even a catastrophic failure [8-9]. The surface flaws are inevitably introduced during the machining process of the ceramic matrix composites. Therefore, it would be viable if damages can be healed autonomously during service, given the potential extended application of the ZSG composites at high temperatures. To this end, the crack-healing mechanism of the different types of ceramic materials have been tirelessly studied [10-14], and there are three main crack-healing mechanisms in the literature, which is summarized in the following text. (A) Thermal diffusion- in early 1976, the thermal diffusion-induced crack-healing mechanism and consequent strength recovery behavior in thermally shocked alumina was discovered when it was annealed at the approaching sintering temperature [12-14]. (B) Phase transition- volume expansion caused by the phase transition that contributes to the facilitation of crack-healing and track-tip blunting. Therefore, it is thought to be a main mechanism of crack-healing. Typically, ZrO2 undergoes the phase transition from the tetragonal phase to the monoclinic phase, which could induce volume expansion at 950 °C [15]. (C) Surface oxidation- research on crack-healing caused by surface oxidation mainly addresses the ZrO2/SiC composites, Si3N4 matrix composites, and ternary carbides Zr2Al4C5, because their micro cracks can be effectively healed with high-temperature
2
oxidation products such as SiO2 and Al2O3 [16-18]. In the past decades, more focus has been placed on the crack healing mechanism driven by surface oxidation. The thought is the oxidation mechanism exhibits better at healing efficiency than the diffusion mechanism, which relies on oxidation products filling, as opposed to the slower mass flux. Unfortunately, more crack-healing mechanisms were studied in air atmosphere. In fact, the oxidation environment differs substantially between the application and the conditions examined in previous studies. During the hypersonic flight, the structural components are generally exposed to low pressure convective flows with significant concentrations of atomic oxygen instead of air atmosphere, due to real gas effects result from shock waves in hypersonic flows [19-20]. Differences between the real service environments and previous studies are primarily reflected in two aspects: higher service temperature and different atmosphere (low pressure, atomic oxygen atmosphere). Different service environments can result in a great difference between the oxidation behaviors of ceramic materials, especially for ultra-high temperature ceramics (UHTCs). Thus, crack-healing mechanism and the degree of strength recovery are also greatly altered. For ZrB2-based UHTCs containing SiC, the oxidation product used to heal crack is typically the borosilicate glass, due to the passive oxidation of SiC [21]. However, when the temperature reaches 1600 °C, it is easier to conduct active oxidation with amounts of SiO gas volatilizing. As a result, the crack-healing temperature in the previous studies were typically below 1400 °C in order to avoid active oxidation of SiC [21]. In addition, the tendency to conduct active oxidation for SiC gradually increases when the oxygen partial pressure reduces. Additionally, the atomic oxygen significantly enhances the oxidation of UHTCs. Therefore oxidation under lower oxygen partial pressures and higher temperatures
3
will result in evaporation of SiO2 and B2O3, leaving behind a single outer layer of ZrO2 in the
simulated
hypersonic
flight
environment
[22-24].
Under
such
conditions,
the borosilicate glass-induced crack-healing mechanism may be not appropriate, but there are no reports currently addressing the ZrO2-induced crack-healing mechanism of the ZSG composites in high temperature atomic oxygen environment. In this paper, the pre-cracked specimens of ZSG ceramic were exposed to high temperatures in an atomic oxygen environment to heal the crack, followed by the flexural strength and ZrO2 formation processing being characterized in an attempt to understand the mechanism of the ZrO2-induced crack-healing of ZSG ceramic. 2 Experimental procedure Commercially available ZrB2 powder (2 μm, >99.5%, Northwest Institute for non-ferrous metal research, China), SiC (1 μm, >99.5%, Weifang Kaihua Micro-powder Co., Ltd., China) and the graphite flake (mean diameter and thickness are 15 μm and 1.5 μm, respectively, >99%, Qingdao Tiansheng graphite Co., Ltd., China) were used as raw powders. The powder mixtures of ZrB2 plus 20 vol.% SiC plus 15 vol.% graphite flake (the ZSG composites) were ball-mixed for 20 h in a polyethylene bottle with ethanol using ZrO2 balls and dried in a rotary evaporator. The resulting powder mixtures were then hot-pressed at 1900°C for 1 h under a uniaxial load of 30 MPa in Ar atmosphere. The sintered plate was cut into 3 mm×4 mm×36 mm bars with the tensile surface perpendicular to the hot-pressing direction. The surface was ground and polished with diamond slurries down to a 1 μm finish. The edges of all the specimens were chamfered to minimize the effect of stress concentration due to machining flaws. The surface cracks were made at the center of the tension surface of the bending specimen by Vickers’ indentation, with a load of 50 N and
4
holding time of 30 s. The similar surface shapes and cross-sectional shapes of the crack and indentation are referenced in other papers [8], and the indentation crack length was about 80 μm. The low-pressure, atomic oxygen oxidation of the ZSG bars was carried out to study the crack-healing ability in this condition. The reaction chamber consisted of O2 and Ar with the ratio of 4:1, and the total pressure of the chamber was controlled at 50 Pa by the pressure regulator and mass flow meter. A microwave generator was used to generate the atomic oxygen with a frequency of 2.45 GHz and a working power of 1000 W. Finally the specimens were heated to 1200 °C, 1400 °C and 1600 °C respectively for 30 minutes by a high frequency induction device at the 50 Pa atomic oxygen environment. To minimize the effect of thermal shock, the heating rate was kept as low as 20 °C/s. Crack-healed surfaces and fracture patterns of specimens were analyzed by scanning electron microscopy (SEM; Quanta 200FEG, FEI, American). The phase composition was characterized by grazing incidence X-ray diffraction (GXRD; Empyrean, Panalytical, Holland). The flexural strength of crack-healing specimens under different temperature was measured by three-point bending, using a 30 mm span and a crosshead speed of 0.5 mm·min-1. The strength of polished specimens and pre-cracked specimens were also measured for comparison. 3 Results and discussion The microstructure of specimens after oxidized provides an insight into the most important details of the crack change as shown in Fig.1. From Fig.1a, the specimen is oxidized slightly, and the crack morphology remains unchanged approximately. When the temperature is increased to 1400 °C, visual crack-healing behavior emerges. As seen in Fig.1b, the depth of the crack is reduced very obviously, though the crack is not healed completely, and a clear crack trace is present. This indicates that the crack-healing
5
originates from the bottom of the crack and then extends towards to the surface. Moreover, the width of the residual crack does not keep a regular change, which is referred to as an uneven crack-healing behavior. Fig.1c shows the surface microstructure of the specimens oxidized at 1600 °C. From Fig.1c, no radial cracks are observed around the Vickers indented zone. In other words, the cracks have been healed completely. It can be concluded from the above observations that the cracks are indeed healed in high temperature atomic oxygen, and the crack-healing ability is enhanced with the increase of temperature. No obvious glass phases are observed to cover the surface of the specimens, which may be due to the strong volatility of the glass phases in these conditions from oxidation. Therefore, the crack-healing should be induced by the other oxidation product, most likely ZrO2, with the oxidation behavior of the ZrB2-based ceramics under simulated hypersonic flight environment considered [22-24]. Furthermore, X-ray diffraction analysis was applied to the surface of oxidized specimens (as shown in Fig.2) to identify which oxidation product influenced the crack-healing ability of the specimens in high temperature atomic oxygen environment. Phase analysis confirms that the main phases contain ZrB2, SiC, and graphite in original material. Then, the diffraction peaks of m-ZrO2 are observed after the specimens are oxidized at 1200 °C. Note that the diffraction peaks of graphite disappear due to the prior rapid oxidation, while the diffraction peaks intensity of the other initial phases, namely ZrB2 and SiC, are still higher than the m-ZrO2. This indicates that ZrB2-SiC-Graphite composites suffers mild oxidation. When the healing temperature reaches 1400 °C, the diffraction peaks of m-ZrO2 get sharper and another polymorph of ZrO2, namely t-ZrO2, appears. Moreover, few weak diffraction peaks for the substrate are seen, indicating that the
6
oxidation of ZrB2-SiC-Graphite composites become more severe. However, when the oxidized temperature increases to 1600 °C, the oxidation products do not continue the previous variation trend. Here, the diffraction peaks of m-ZrO2 get sharper further, while the diffraction peaks of t-ZrO2 appearing lower. Furthermore, a new phase of ZrO appears, and the primary initial phases, ZrB2 can be observed again, although it exhibits minor intensity. ZrB2-SiC-Graphite may be oxidized in dissociated oxygen by the reactions by consulting the oxidation of ZrB2-SiC-Graphite in molecule oxygen as follows [8]: ZrB2+5O(g)→ZrO2(s)+B2O3(l)
(1)
SiC+3O(g)→SiO2(l)+CO(g)
(2)
C+O(g)→CO(g)
(3)
SiC+2O(g)→SiO (g)+CO(g) 2370°C
1170°C m-ZrO2
950°C
(4)
t-ZrO2
2370°C
c-ZrO2
(5)
Formation of ZrO2 is attributed to the oxidation of ZrB2 (Reaction 1), and all ZrO2 phases refer to t-ZrO2 when the specimens are heated to 1200 °C, 1400 °C, and 1600 °C in accordance with Reaction 5. With the temperature reduced to room temperature, t-ZrO2 is transformed to m-ZrO2 during cooling. However, high t-ZrO2 content, due to severe oxidation, results in a certain amount of t-ZrO2 still being reserved to the room temperature, such as the microstructure after the oxidation at 1400 °C and 1600 °C. When the temperature gets between 1400 °C and 1600 °C in atomic oxygen environment, there are no glass phases observed in Fig.1 to cover the oxidation surface. This is caused by volatilization of B2O3 and active oxidation of SiC (Reaction 4), which is similar in reference [17]. Thus, more atomic oxygen reacts with SiC preferentially, which results in that a few
7
initial ZrB2 still is being present. Similarly, the existence of ZrO is thought to be due to the lack of atomic oxygen content as well. Therefore, ZrO2 mainly covers the surface of the specimens after oxidized. ZrO2-induced crack-healing mechanism of the ZSG composite in atomic oxygen environment is affirmed by combining the SEM of the crack-healing specimens. The flexural strength of the original specimens and the pre-cracked specimens before and after oxidation can characterize the ZrO2-induced crack-healing ability properly. The test results are shown in Fig.3. The strength of as-sintered and polished specimens is about 480 MPa. The indentation cracks can reduce the flexural strength to 194.47 MPa, which corresponds to ~60% reduction. This indicates that the ZSG composite is indeed very sensitive to the cracks. After oxidized in 50 Pa atomic oxygen at different temperatures for 30 minutes, the flexural strength changes greatly, which is mainly due to the temperature. When the pre-cracked specimen is oxidized at 1200 °C, the flexural strength has little variation compared to the unoxidized pre-cracked specimen, which corresponds to the crack morphology at 1200 °C. Then, as the healed temperature increases to 1400 °C and 1600 °C, the average flexural strength of crack-healing specimens increase to 218 MPa and 270.27 MPa, which means 12.15% and 39.09% improvement, respectively. Unfortunately the bending strengths of all the treated specimens are still lower than the strength of as-sintered
and
polished
specimens
(480
MPa).
These
results
suggest
that
ZrB2-SiC-Graphite composite presents limited crack-healing ability from 1200 °C to 1600 °C in 50 Pa atomic oxygen environment. Moreover, the fracture pattern of pre-cracked specimens is analyzed by SEM (Fig.4). As shown in Fig.4a-d, all the oxidized pre-cracked specimens still retain distinct
8
indentation. The difference reflects that pre-cracked specimens and 1200 °C-oxidized pre-cracked specimens fractured along the diagonal line of Vickers indentation. It indicates that the pre-cracks have not been healed within these two conditions. While the fracture of the pre-cracked specimens oxidized at 1400 °C and 1600 °C occurs at the locations of the maximum sectional area, rather than the along the diagonal line of Vickers indentation exactly, which results from random distribution of graphite soft phase leading to partial collapse. As a result, the blurry creak-healing trace can still be seen in Fig.4c and Fig.4d. These are just able to coincide with the increase of flexural strength and the crack-healing observed in SEM. In consequence, the ability of the crack-healing was verified further. Based on the observations above, the mechanism of creak-healing is analyzed briefly and ZrO2-induced crack-healing schematic of the ZSG composite is shown in Fig.5. The crack-healing is induced mainly by the ZrO2 in high temperature atomic oxygen environment. The important mechanisms governing crack healing in ZSG composite ceramics are summarized into three aspects as follows. A. Local particle rearrangement which is induced by expansion from the chemical reaction (ZrB2→ZrO2), the phase transition (t-ZrO2→m-ZrO2) and the thermal expansion. As described above, oxidation under higher temperatures atomic oxygen environment will result in evaporation of SiO2 and B2O3, leaving behind a single outer layer of porous ZrO2 in the surface. It is known that the oxidation reaction from ZrB2 to t-ZrO2, the phase transition from t-ZrO2 to m-ZrO2, and the thermal expansion [25-26] can lead to the volume expansion. In fact, the volume expansion can rearrange local porous ZrO2 particle, and then finally blunt the track-tip, and fill the pores, as is represented in Fig.5. Additionally, it also contributes to narrowing of the crack greatly. The volume expansion degrees can be
9
calculated by the density “ρ”. The densities of ZrB2, t-ZrO2 and m-ZrO2 are 6.104 g·cm-3, 6.134 g·cm-3 and 5.817 g·cm-3, respectively, which are taken from the XRD. Now, all the phases are assumed to be number of moles “n”, owing to the same Zr atomic quantities, then their mass “m” can be acquired by the product of molar mass “M” and “n”. As a result, the volume ratio of ZrB2, t-ZrO2 and m-ZrO2 is calculated to be 18.486 : 20.088 : 21.183 by m/ρ. In other words, the conversion from ZrB2 to t-ZrO2 leads to a 8.67% volume expansion, and the conversion to m-ZrO2 further increase with a 14.59% volume expansion, which is a considerable value to promote the track-tip blunting and narrowing of the crack. Considering the inverted cone shape of the crack, the crack healing derived from volume expansion of ZrO2 initiates at the bottom of the crack and then continues up the whole crack gradually. The expansion degrees of the ZrO2 on the either side of the crack specimens also become different, and thus, the width of the healing crack do not keep a regular change. This is due to the uneven distribution of initial phases which means the uneven distribution of pores after the oxidation of SiC and graphite. B. The crack volume filling by ZrO2. Oxide evaporation (B2O3 and SiO2) and formation of gas (CO and SiO) also play a positive role in improving ZrO2-induced crack-healing behavior, and it is very different compared to the borosilicate glass phase-induced crack-healing mechanism, in which the evolved gas is thought to be detrimental. The gas produced by the oxidation of the internal initial phases of the crack can separate these ZrO2 particles easily, because of the smaller binding force among the newly generated ZrO2 particles. Then, the separated particles “flow” to the inward of the crack naturally, which is depicted by red dots in Fig.5. Thus, the crack opening space is filled gradually. The crack-healing derived from the ZrO2 filling the crack opening space
10
starts from the bottom of the crack as well. In addition, when the pre-cracked specimens are oxidized at 1600 °C, a small number of ZrO is produced with a larger density (7.22 g·cm-3), which can lead to a smaller expansion. But more gas which is caused by active oxidation of SiC makes up for the deficiencies. Finally, the pre-cracked specimens oxidized at 1600 °C possesses the best crack-healing ability. C. Diffusion controlled sintering. According to the reaction 1, ZrO2 as the crack-healing phase is derived from the oxidation of ZrB2. At the initial stage of the oxidation, so-called “oxide nuclei” is formed at the surface of each phases [27] and the density of nuclei depends on the oxidation temperature and oxygen pressure. Then, with the temperature rising and oxidation product ZrO2 content increasing, these nuclei may grow together to form a continuous surface film of oxide. Particularly when the temperature is close to the ZrO2 sintering temperature (~1500°C), the crack regression by diffusional controlled sintering becomes an effective crack-healing mechanism. 4 Conclusions Crack-healing of ZrB2-SiC-Graphite composite in high temperature atomic oxygen environment was carried out. The crack can heal gradually with the increasing temperature in high temperature atomic oxygen environment. The oxidation product to induce the crack-healing is inferred to be ZrO2 by the SEM and XRD analysis. The ZrO2-induced crack-healing mechanism is summarized into three aspects: A. Local particle rearrangement which is induced by expansion from the chemical reaction (ZrB2→ZrO2), the phase transition (t-ZrO2→m-ZrO2) and the thermal expansion. B. The crack volume filling by ZrO2 driven by the liberation of the reaction gas. C. Diffusional controlled sintering around ZrO2 sintering temperature. In the end, the residual strength of the crack-healing specimens
11
verifies ZrO2-induced crack-healing ability further. The maximum strength of ZrO2-induced crack-healing specimens (oxidized at 1600 °C) increases by 39.09% when compared to the pre-cracked specimens, although it is still lower than the initial strength. Therefore, the results presented in this paper indicate that the ZSG composite has limited crack-healing ability in high temperature atomic oxygen atmosphere, which is induced by ZrO2. The good crack-healing ability, in the high temperature atomic oxygen atmosphere, could improve the reliability of ZSG composite components in service, as it could heal defects and flaws autonomously. Our findings may also provide a new insight into the design of materials used in high temperature. Acknowledgements This work is supported by National Natural Science Foundation of China (11502058) and China Postdoctoral Science Foundation Funded Project (2015M571403). References [1] M. Khoeini, A. Nemati, M. Zakeri, M. Tamizifard, H. Samadie, Comprehensive study on the effect of SiC and carbon additives on the pressureless sintering and microstructural and mechanical characteristics of new ultra-high temperature ZrB2 ceramics, Ceram. Int. 41 (2015) 11456-11463. [2] M.S. Asl, M.G. Kakroudi, R.A. Kondolaji, H. Nasiriet, Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2-SiC composite, Ceram. Int. 41 (2015) 5843-5851. [3] H. Jin, M.S. Meng, Q. Yang, Y.W. Zhu, Thermal shock resistance of a ZrB2-SiC-graphite composite in low oxygen partial pressure environment, Ceram. Int. 39 (2013) 5591-5596.
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[4] X.H. Zhang, Z. Wang, P. Hu, W.B. Han, C.Q. Hong, Mechanical properties and thermal shock resistance of ZrB2-SiC ceramic toughened with graphite flake and SiC whiskers, Scr. Mater. 61 (2009) 809-812. [5] S.R. Levine, E.J. Opila, M.C. Halbig, J.D. Kiser, M. Singh, J.A. Salem, Evaluation of ultra-high temperature ceramics for aero propulsion use, J. Eur. Ceram. Soc. 22 (2002) 2757-2767. [6] H. Jin, M.S. Meng, Y.W. Zhu, Y.J. Zhou, Effect of environment atmosphere on thermal shock resistance of the ZrB2-SiC-graphite composite, Mater. Des. 50 (2013) 509-514. [7] S. Zhou, Z. Wang, W. Zhang, Effect of graphite flake orientation on microstructure and mechanical properties of ZrB2-SiC-graphite composite, J. Alloys Compd. 485 (2009) 181-185. [8] Z. Wang, Q. Qu, Z.J. Wu, G.D. Shi, Effect of oxidation at 1100 °C on the strength of ZrB2-SiC-graphite ceramics, J. Alloys Compd. 509 (2011) 6871-6875. [9] Z. Zhao, L. Zhang, J. Zheng, H. Bai, S. Zhang, B. Xu, Microstructures and mechanical properties of Al2O3/ZrO2 composite produced by combustion synthesis, Scr. Mater. 53 (2005) 995-1000. [10] F. Tavangarian, G. Li, Crack healing and strength recovery in SiC/spinel nanocomposite, Ceram. Int. 41 (2015) 8702-8709. [11] J. Lin, Y. Huang, H. Zhang, Crack healing and strengthening of SiC whisker and ZrO 2 fiber reinforced ZrB2 ceramics, Ceram. Int. 40 (2014) 16811-16815. [12] T.K. Gupta, Crack healing in thermally shocked MgO, J. Am. Ceram. Soc. 58 (1975) 143-143. [13] F.F. Lange, K.C. Radford, Healing of surface cracks in polycrystalline Al2O3, J. Am.
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Ceram. Soc. 53 (1970) 420-421. [14] T.K. Gupta. Crack healing and strengthening of thermally shocked alumina. J. Am. Ceram. Soc. 59 (1976) 259-262. [15] L. Jun, Z.X. Zheng, H.F. Ding, Z.H. Jin, Preliminary study of the crack healing and strength recovery of Al2O3-matrix composites, Fatigue Fract. Eng. Mater. Struct. 27 (2004) 89-97. [16] G.Q. Chen, R.B. Zhang, X.H. Zhang, L. Zhao, W.B. Han, Oxidation-induced crack healing in Zr2Al4C5 ceramic, Mater. Des. 30 (2009) 3602-3607. [17] K.W. Nam, J.R. Hwang. The crack healing behavior of ZrO2/SiC composite ceramics with TiO2 additive, J. Mecha. Sci. Technol. 26 (2012) 2093-2096. [18] K. Houjou, K. Ando, S.P. Liuc, S. Sato, Crack-healing and oxidation behavior of silicon nitride ceramics, J. Eur. Ceram. Soc. 24 (2004) 2329-2338. [19] J. Marschall, D.A. Pejakovic, W.G. Fahrenholtz, G.E. Hilmas, S.M. Zhu, J. Ridge, D.G. Fletcher, C.O. Asma, J. Thomel, Oxidation of ZrB2-SiC ultrahigh-temperature ceramic composites in dissociated air, J. Thermophys Heat Transfer 23 (2009) 267-278. [20] F. Monteverde, R. Savino, M.D.S. Fumo, A.D. Maso, Plasma wind tunnel testing of ultra-high temperature ZrB2-SiC composites under hypersonic re-entry conditions, J. Eur. Ceram. Soc. 30 (2010) 2313-2321. [21] X.H. Zhang, L. Xu, S.Y. Du, W.B. Han, J.C. Han. Preoxidation and crack-healing behavior of ZrB2-SiC ceramic composite, J. Am. Ceram. Soc. 91 (2008) 4068-4073. [22] N. Li, P. Hu, X.H. Zhang, Y.Z. Liu, W.B. Han, Effects of oxygen partial pressure and atomic oxygen on the microstructure of oxide scale of ZrB2-SiC composites at 1500 °C, Corros. Sci. 73 (2013) 44-53.
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[23] J. Marschall, D.A. Pejakovic, W.G. Fahrenholtz, G.E. Hilmas, F.Panerai, O. Chazot, Temperature
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Figure Captions: Fig.1. SEM micrographs of healed specimen surface: (a) 1200 °C, 30min; (b) 1400 °C, 30min; (c) 1600 °C, 30min, and their high-magnification SEM images. Fig.2. Patterns of XRD of ZrB2-SiC-Graphite at different temperatures under atomic oxygen. Fig.3. Effect of crack-healing environment on the strength recovery behavior. Fig.4. Fracture morphologies of (a) pre-cracked specimen and specimen after oxidized at
15
(b)1200 °C; (c) 1400 °C and (d) 1600 °C in atomic oxygen. Fig.5. ZrO2-induced crack-healing mechanism schematic of the ZSG composite.
25μm
5μm
25μm
5μm
25μm
5μm
Fig.1. SEM micrographs of healed specimen surface: (a) 1200 °C, 30min; (b) 1400 °C, 30min; (c) 1600 °C, 30min, and their high-magnification SEM images.
16
Fig.2. Patterns of XRD of ZrB2-SiC-Graphite at different temperatures under atomic oxygen.
17
Fig.3. Effect of crack-healing environment on the strength recovery behavior.
18
100μm
100μm
100μm
100μm
Fig.4. Fracture morphologies of (a) pre-cracked specimen and specimen after oxidized at (b)1200 °C; (c) 1400 °C and (d) 1600 °C in atomic oxygen.
Local particle rearrangement
Oxidized
Oxidation product filling the crack
Track-tip blunting
Fig.5. ZrO2-induced crack-healing mechanism schematic of the ZSG composite.
19
PII: DOI: Reference:
S0272-8842(15)02314-7 http://dx.doi.org/10.1016/j.ceramint.2015.12.031 CERI11826
To appear in: Ceramics International Received date: 8 October 2015 Revised date: 14 November 2015 Accepted date: 1 December 2015 Cite this article as: Jiahong Niu, Hua Jin, Songhe Meng, Qingxuan Zeng and Zujun Peng, ZrO2-induced crack-healing mechanism of ZrB2-SiC-Graphite composite in high temperature atomic oxygen environment, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.12.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
ZrO2-induced crack-healing mechanism of ZrB2-SiC-Graphite composite in high temperature atomic oxygen environment Jiahong Niu, Hua Jin*, Songhe Meng, Qingxuan Zeng, Zujun Peng Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, China Abstract The ZrO2-induced crack-healing mechanism of ZrB2-SiC-Graphite (ZSG) composite is investigated under high temperature atomic oxygen atmosphere, which characterizes the real condition of hypersonic flight. This differs greatly from the borosilicate glass-induced crack-healing mechanism in previous studies. The cracks are healed limitedly in high temperature atomic oxygen condition with a 39.09% maximum improvement in residual strength compared to the pre-cracked specimens. The crack-healing is induced by ZrO2, and the crack-healing mechanism involves three main aspects: A. local particle rearrangement, which is induced by expansion from the chemical reaction (ZrB2→ZrO2), the phase transition
(t-ZrO2→m-ZrO2),
and
the
thermal
expansion.
B.
the crack volume filling by ZrO2 driven by the liberation of the reaction gases. C. Diffusion controlled sintering around the ZrO2 sintering temperature. The spontaneous crack-healing ability in high temperature atomic oxygen atmosphere is helpful to improve the reliability of ZSG composites in service. Keywords: A. Hot pressing; B. Composites; E. Structural applications Corresponding author: Hua Jin Email: [email protected] Tel/Fax: +86-451-86402432
1
1 Introduction ZrB2-SiC-Graphite (ZSG) composite is widely known by its significant performance enhancement in fracture toughness (~6.1 MPa·m1/2), thermal shock resistance, as well as its oxidation resistance among ZrB2 based ultra-high temperature ceramics [1-7]. Nevertheless, the intrinsic brittleness derived from structural ceramics is a main drawback, making it sensitive to the presence of surface flaws. This may result in performance degradation or even a catastrophic failure [8-9]. The surface flaws are inevitably introduced during the machining process of the ceramic matrix composites. Therefore, it would be viable if damages can be healed autonomously during service, given the potential extended application of the ZSG composites at high temperatures. To this end, the crack-healing mechanism of the different types of ceramic materials have been tirelessly studied [10-14], and there are three main crack-healing mechanisms in the literature, which is summarized in the following text. (A) Thermal diffusion- in early 1976, the thermal diffusion-induced crack-healing mechanism and consequent strength recovery behavior in thermally shocked alumina was discovered when it was annealed at the approaching sintering temperature [12-14]. (B) Phase transition- volume expansion caused by the phase transition that contributes to the facilitation of crack-healing and track-tip blunting. Therefore, it is thought to be a main mechanism of crack-healing. Typically, ZrO2 undergoes the phase transition from the tetragonal phase to the monoclinic phase, which could induce volume expansion at 950 °C [15]. (C) Surface oxidation- research on crack-healing caused by surface oxidation mainly addresses the ZrO2/SiC composites, Si3N4 matrix composites, and ternary carbides Zr2Al4C5, because their micro cracks can be effectively healed with high-temperature
2
oxidation products such as SiO2 and Al2O3 [16-18]. In the past decades, more focus has been placed on the crack healing mechanism driven by surface oxidation. The thought is the oxidation mechanism exhibits better at healing efficiency than the diffusion mechanism, which relies on oxidation products filling, as opposed to the slower mass flux. Unfortunately, more crack-healing mechanisms were studied in air atmosphere. In fact, the oxidation environment differs substantially between the application and the conditions examined in previous studies. During the hypersonic flight, the structural components are generally exposed to low pressure convective flows with significant concentrations of atomic oxygen instead of air atmosphere, due to real gas effects result from shock waves in hypersonic flows [19-20]. Differences between the real service environments and previous studies are primarily reflected in two aspects: higher service temperature and different atmosphere (low pressure, atomic oxygen atmosphere). Different service environments can result in a great difference between the oxidation behaviors of ceramic materials, especially for ultra-high temperature ceramics (UHTCs). Thus, crack-healing mechanism and the degree of strength recovery are also greatly altered. For ZrB2-based UHTCs containing SiC, the oxidation product used to heal crack is typically the borosilicate glass, due to the passive oxidation of SiC [21]. However, when the temperature reaches 1600 °C, it is easier to conduct active oxidation with amounts of SiO gas volatilizing. As a result, the crack-healing temperature in the previous studies were typically below 1400 °C in order to avoid active oxidation of SiC [21]. In addition, the tendency to conduct active oxidation for SiC gradually increases when the oxygen partial pressure reduces. Additionally, the atomic oxygen significantly enhances the oxidation of UHTCs. Therefore oxidation under lower oxygen partial pressures and higher temperatures
3
will result in evaporation of SiO2 and B2O3, leaving behind a single outer layer of ZrO2 in the
simulated
hypersonic
flight
environment
[22-24].
Under
such
conditions,
the borosilicate glass-induced crack-healing mechanism may be not appropriate, but there are no reports currently addressing the ZrO2-induced crack-healing mechanism of the ZSG composites in high temperature atomic oxygen environment. In this paper, the pre-cracked specimens of ZSG ceramic were exposed to high temperatures in an atomic oxygen environment to heal the crack, followed by the flexural strength and ZrO2 formation processing being characterized in an attempt to understand the mechanism of the ZrO2-induced crack-healing of ZSG ceramic. 2 Experimental procedure Commercially available ZrB2 powder (2 μm, >99.5%, Northwest Institute for non-ferrous metal research, China), SiC (1 μm, >99.5%, Weifang Kaihua Micro-powder Co., Ltd., China) and the graphite flake (mean diameter and thickness are 15 μm and 1.5 μm, respectively, >99%, Qingdao Tiansheng graphite Co., Ltd., China) were used as raw powders. The powder mixtures of ZrB2 plus 20 vol.% SiC plus 15 vol.% graphite flake (the ZSG composites) were ball-mixed for 20 h in a polyethylene bottle with ethanol using ZrO2 balls and dried in a rotary evaporator. The resulting powder mixtures were then hot-pressed at 1900°C for 1 h under a uniaxial load of 30 MPa in Ar atmosphere. The sintered plate was cut into 3 mm×4 mm×36 mm bars with the tensile surface perpendicular to the hot-pressing direction. The surface was ground and polished with diamond slurries down to a 1 μm finish. The edges of all the specimens were chamfered to minimize the effect of stress concentration due to machining flaws. The surface cracks were made at the center of the tension surface of the bending specimen by Vickers’ indentation, with a load of 50 N and
4
holding time of 30 s. The similar surface shapes and cross-sectional shapes of the crack and indentation are referenced in other papers [8], and the indentation crack length was about 80 μm. The low-pressure, atomic oxygen oxidation of the ZSG bars was carried out to study the crack-healing ability in this condition. The reaction chamber consisted of O2 and Ar with the ratio of 4:1, and the total pressure of the chamber was controlled at 50 Pa by the pressure regulator and mass flow meter. A microwave generator was used to generate the atomic oxygen with a frequency of 2.45 GHz and a working power of 1000 W. Finally the specimens were heated to 1200 °C, 1400 °C and 1600 °C respectively for 30 minutes by a high frequency induction device at the 50 Pa atomic oxygen environment. To minimize the effect of thermal shock, the heating rate was kept as low as 20 °C/s. Crack-healed surfaces and fracture patterns of specimens were analyzed by scanning electron microscopy (SEM; Quanta 200FEG, FEI, American). The phase composition was characterized by grazing incidence X-ray diffraction (GXRD; Empyrean, Panalytical, Holland). The flexural strength of crack-healing specimens under different temperature was measured by three-point bending, using a 30 mm span and a crosshead speed of 0.5 mm·min-1. The strength of polished specimens and pre-cracked specimens were also measured for comparison. 3 Results and discussion The microstructure of specimens after oxidized provides an insight into the most important details of the crack change as shown in Fig.1. From Fig.1a, the specimen is oxidized slightly, and the crack morphology remains unchanged approximately. When the temperature is increased to 1400 °C, visual crack-healing behavior emerges. As seen in Fig.1b, the depth of the crack is reduced very obviously, though the crack is not healed completely, and a clear crack trace is present. This indicates that the crack-healing
5
originates from the bottom of the crack and then extends towards to the surface. Moreover, the width of the residual crack does not keep a regular change, which is referred to as an uneven crack-healing behavior. Fig.1c shows the surface microstructure of the specimens oxidized at 1600 °C. From Fig.1c, no radial cracks are observed around the Vickers indented zone. In other words, the cracks have been healed completely. It can be concluded from the above observations that the cracks are indeed healed in high temperature atomic oxygen, and the crack-healing ability is enhanced with the increase of temperature. No obvious glass phases are observed to cover the surface of the specimens, which may be due to the strong volatility of the glass phases in these conditions from oxidation. Therefore, the crack-healing should be induced by the other oxidation product, most likely ZrO2, with the oxidation behavior of the ZrB2-based ceramics under simulated hypersonic flight environment considered [22-24]. Furthermore, X-ray diffraction analysis was applied to the surface of oxidized specimens (as shown in Fig.2) to identify which oxidation product influenced the crack-healing ability of the specimens in high temperature atomic oxygen environment. Phase analysis confirms that the main phases contain ZrB2, SiC, and graphite in original material. Then, the diffraction peaks of m-ZrO2 are observed after the specimens are oxidized at 1200 °C. Note that the diffraction peaks of graphite disappear due to the prior rapid oxidation, while the diffraction peaks intensity of the other initial phases, namely ZrB2 and SiC, are still higher than the m-ZrO2. This indicates that ZrB2-SiC-Graphite composites suffers mild oxidation. When the healing temperature reaches 1400 °C, the diffraction peaks of m-ZrO2 get sharper and another polymorph of ZrO2, namely t-ZrO2, appears. Moreover, few weak diffraction peaks for the substrate are seen, indicating that the
6
oxidation of ZrB2-SiC-Graphite composites become more severe. However, when the oxidized temperature increases to 1600 °C, the oxidation products do not continue the previous variation trend. Here, the diffraction peaks of m-ZrO2 get sharper further, while the diffraction peaks of t-ZrO2 appearing lower. Furthermore, a new phase of ZrO appears, and the primary initial phases, ZrB2 can be observed again, although it exhibits minor intensity. ZrB2-SiC-Graphite may be oxidized in dissociated oxygen by the reactions by consulting the oxidation of ZrB2-SiC-Graphite in molecule oxygen as follows [8]: ZrB2+5O(g)→ZrO2(s)+B2O3(l)
(1)
SiC+3O(g)→SiO2(l)+CO(g)
(2)
C+O(g)→CO(g)
(3)
SiC+2O(g)→SiO (g)+CO(g) 2370°C
1170°C m-ZrO2
950°C
(4)
t-ZrO2
2370°C
c-ZrO2
(5)
Formation of ZrO2 is attributed to the oxidation of ZrB2 (Reaction 1), and all ZrO2 phases refer to t-ZrO2 when the specimens are heated to 1200 °C, 1400 °C, and 1600 °C in accordance with Reaction 5. With the temperature reduced to room temperature, t-ZrO2 is transformed to m-ZrO2 during cooling. However, high t-ZrO2 content, due to severe oxidation, results in a certain amount of t-ZrO2 still being reserved to the room temperature, such as the microstructure after the oxidation at 1400 °C and 1600 °C. When the temperature gets between 1400 °C and 1600 °C in atomic oxygen environment, there are no glass phases observed in Fig.1 to cover the oxidation surface. This is caused by volatilization of B2O3 and active oxidation of SiC (Reaction 4), which is similar in reference [17]. Thus, more atomic oxygen reacts with SiC preferentially, which results in that a few
7
initial ZrB2 still is being present. Similarly, the existence of ZrO is thought to be due to the lack of atomic oxygen content as well. Therefore, ZrO2 mainly covers the surface of the specimens after oxidized. ZrO2-induced crack-healing mechanism of the ZSG composite in atomic oxygen environment is affirmed by combining the SEM of the crack-healing specimens. The flexural strength of the original specimens and the pre-cracked specimens before and after oxidation can characterize the ZrO2-induced crack-healing ability properly. The test results are shown in Fig.3. The strength of as-sintered and polished specimens is about 480 MPa. The indentation cracks can reduce the flexural strength to 194.47 MPa, which corresponds to ~60% reduction. This indicates that the ZSG composite is indeed very sensitive to the cracks. After oxidized in 50 Pa atomic oxygen at different temperatures for 30 minutes, the flexural strength changes greatly, which is mainly due to the temperature. When the pre-cracked specimen is oxidized at 1200 °C, the flexural strength has little variation compared to the unoxidized pre-cracked specimen, which corresponds to the crack morphology at 1200 °C. Then, as the healed temperature increases to 1400 °C and 1600 °C, the average flexural strength of crack-healing specimens increase to 218 MPa and 270.27 MPa, which means 12.15% and 39.09% improvement, respectively. Unfortunately the bending strengths of all the treated specimens are still lower than the strength of as-sintered
and
polished
specimens
(480
MPa).
These
results
suggest
that
ZrB2-SiC-Graphite composite presents limited crack-healing ability from 1200 °C to 1600 °C in 50 Pa atomic oxygen environment. Moreover, the fracture pattern of pre-cracked specimens is analyzed by SEM (Fig.4). As shown in Fig.4a-d, all the oxidized pre-cracked specimens still retain distinct
8
indentation. The difference reflects that pre-cracked specimens and 1200 °C-oxidized pre-cracked specimens fractured along the diagonal line of Vickers indentation. It indicates that the pre-cracks have not been healed within these two conditions. While the fracture of the pre-cracked specimens oxidized at 1400 °C and 1600 °C occurs at the locations of the maximum sectional area, rather than the along the diagonal line of Vickers indentation exactly, which results from random distribution of graphite soft phase leading to partial collapse. As a result, the blurry creak-healing trace can still be seen in Fig.4c and Fig.4d. These are just able to coincide with the increase of flexural strength and the crack-healing observed in SEM. In consequence, the ability of the crack-healing was verified further. Based on the observations above, the mechanism of creak-healing is analyzed briefly and ZrO2-induced crack-healing schematic of the ZSG composite is shown in Fig.5. The crack-healing is induced mainly by the ZrO2 in high temperature atomic oxygen environment. The important mechanisms governing crack healing in ZSG composite ceramics are summarized into three aspects as follows. A. Local particle rearrangement which is induced by expansion from the chemical reaction (ZrB2→ZrO2), the phase transition (t-ZrO2→m-ZrO2) and the thermal expansion. As described above, oxidation under higher temperatures atomic oxygen environment will result in evaporation of SiO2 and B2O3, leaving behind a single outer layer of porous ZrO2 in the surface. It is known that the oxidation reaction from ZrB2 to t-ZrO2, the phase transition from t-ZrO2 to m-ZrO2, and the thermal expansion [25-26] can lead to the volume expansion. In fact, the volume expansion can rearrange local porous ZrO2 particle, and then finally blunt the track-tip, and fill the pores, as is represented in Fig.5. Additionally, it also contributes to narrowing of the crack greatly. The volume expansion degrees can be
9
calculated by the density “ρ”. The densities of ZrB2, t-ZrO2 and m-ZrO2 are 6.104 g·cm-3, 6.134 g·cm-3 and 5.817 g·cm-3, respectively, which are taken from the XRD. Now, all the phases are assumed to be number of moles “n”, owing to the same Zr atomic quantities, then their mass “m” can be acquired by the product of molar mass “M” and “n”. As a result, the volume ratio of ZrB2, t-ZrO2 and m-ZrO2 is calculated to be 18.486 : 20.088 : 21.183 by m/ρ. In other words, the conversion from ZrB2 to t-ZrO2 leads to a 8.67% volume expansion, and the conversion to m-ZrO2 further increase with a 14.59% volume expansion, which is a considerable value to promote the track-tip blunting and narrowing of the crack. Considering the inverted cone shape of the crack, the crack healing derived from volume expansion of ZrO2 initiates at the bottom of the crack and then continues up the whole crack gradually. The expansion degrees of the ZrO2 on the either side of the crack specimens also become different, and thus, the width of the healing crack do not keep a regular change. This is due to the uneven distribution of initial phases which means the uneven distribution of pores after the oxidation of SiC and graphite. B. The crack volume filling by ZrO2. Oxide evaporation (B2O3 and SiO2) and formation of gas (CO and SiO) also play a positive role in improving ZrO2-induced crack-healing behavior, and it is very different compared to the borosilicate glass phase-induced crack-healing mechanism, in which the evolved gas is thought to be detrimental. The gas produced by the oxidation of the internal initial phases of the crack can separate these ZrO2 particles easily, because of the smaller binding force among the newly generated ZrO2 particles. Then, the separated particles “flow” to the inward of the crack naturally, which is depicted by red dots in Fig.5. Thus, the crack opening space is filled gradually. The crack-healing derived from the ZrO2 filling the crack opening space
10
starts from the bottom of the crack as well. In addition, when the pre-cracked specimens are oxidized at 1600 °C, a small number of ZrO is produced with a larger density (7.22 g·cm-3), which can lead to a smaller expansion. But more gas which is caused by active oxidation of SiC makes up for the deficiencies. Finally, the pre-cracked specimens oxidized at 1600 °C possesses the best crack-healing ability. C. Diffusion controlled sintering. According to the reaction 1, ZrO2 as the crack-healing phase is derived from the oxidation of ZrB2. At the initial stage of the oxidation, so-called “oxide nuclei” is formed at the surface of each phases [27] and the density of nuclei depends on the oxidation temperature and oxygen pressure. Then, with the temperature rising and oxidation product ZrO2 content increasing, these nuclei may grow together to form a continuous surface film of oxide. Particularly when the temperature is close to the ZrO2 sintering temperature (~1500°C), the crack regression by diffusional controlled sintering becomes an effective crack-healing mechanism. 4 Conclusions Crack-healing of ZrB2-SiC-Graphite composite in high temperature atomic oxygen environment was carried out. The crack can heal gradually with the increasing temperature in high temperature atomic oxygen environment. The oxidation product to induce the crack-healing is inferred to be ZrO2 by the SEM and XRD analysis. The ZrO2-induced crack-healing mechanism is summarized into three aspects: A. Local particle rearrangement which is induced by expansion from the chemical reaction (ZrB2→ZrO2), the phase transition (t-ZrO2→m-ZrO2) and the thermal expansion. B. The crack volume filling by ZrO2 driven by the liberation of the reaction gas. C. Diffusional controlled sintering around ZrO2 sintering temperature. In the end, the residual strength of the crack-healing specimens
11
verifies ZrO2-induced crack-healing ability further. The maximum strength of ZrO2-induced crack-healing specimens (oxidized at 1600 °C) increases by 39.09% when compared to the pre-cracked specimens, although it is still lower than the initial strength. Therefore, the results presented in this paper indicate that the ZSG composite has limited crack-healing ability in high temperature atomic oxygen atmosphere, which is induced by ZrO2. The good crack-healing ability, in the high temperature atomic oxygen atmosphere, could improve the reliability of ZSG composite components in service, as it could heal defects and flaws autonomously. Our findings may also provide a new insight into the design of materials used in high temperature. Acknowledgements This work is supported by National Natural Science Foundation of China (11502058) and China Postdoctoral Science Foundation Funded Project (2015M571403). References [1] M. Khoeini, A. Nemati, M. Zakeri, M. Tamizifard, H. Samadie, Comprehensive study on the effect of SiC and carbon additives on the pressureless sintering and microstructural and mechanical characteristics of new ultra-high temperature ZrB2 ceramics, Ceram. Int. 41 (2015) 11456-11463. [2] M.S. Asl, M.G. Kakroudi, R.A. Kondolaji, H. Nasiriet, Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2-SiC composite, Ceram. Int. 41 (2015) 5843-5851. [3] H. Jin, M.S. Meng, Q. Yang, Y.W. Zhu, Thermal shock resistance of a ZrB2-SiC-graphite composite in low oxygen partial pressure environment, Ceram. Int. 39 (2013) 5591-5596.
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[4] X.H. Zhang, Z. Wang, P. Hu, W.B. Han, C.Q. Hong, Mechanical properties and thermal shock resistance of ZrB2-SiC ceramic toughened with graphite flake and SiC whiskers, Scr. Mater. 61 (2009) 809-812. [5] S.R. Levine, E.J. Opila, M.C. Halbig, J.D. Kiser, M. Singh, J.A. Salem, Evaluation of ultra-high temperature ceramics for aero propulsion use, J. Eur. Ceram. Soc. 22 (2002) 2757-2767. [6] H. Jin, M.S. Meng, Y.W. Zhu, Y.J. Zhou, Effect of environment atmosphere on thermal shock resistance of the ZrB2-SiC-graphite composite, Mater. Des. 50 (2013) 509-514. [7] S. Zhou, Z. Wang, W. Zhang, Effect of graphite flake orientation on microstructure and mechanical properties of ZrB2-SiC-graphite composite, J. Alloys Compd. 485 (2009) 181-185. [8] Z. Wang, Q. Qu, Z.J. Wu, G.D. Shi, Effect of oxidation at 1100 °C on the strength of ZrB2-SiC-graphite ceramics, J. Alloys Compd. 509 (2011) 6871-6875. [9] Z. Zhao, L. Zhang, J. Zheng, H. Bai, S. Zhang, B. Xu, Microstructures and mechanical properties of Al2O3/ZrO2 composite produced by combustion synthesis, Scr. Mater. 53 (2005) 995-1000. [10] F. Tavangarian, G. Li, Crack healing and strength recovery in SiC/spinel nanocomposite, Ceram. Int. 41 (2015) 8702-8709. [11] J. Lin, Y. Huang, H. Zhang, Crack healing and strengthening of SiC whisker and ZrO 2 fiber reinforced ZrB2 ceramics, Ceram. Int. 40 (2014) 16811-16815. [12] T.K. Gupta, Crack healing in thermally shocked MgO, J. Am. Ceram. Soc. 58 (1975) 143-143. [13] F.F. Lange, K.C. Radford, Healing of surface cracks in polycrystalline Al2O3, J. Am.
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[23] J. Marschall, D.A. Pejakovic, W.G. Fahrenholtz, G.E. Hilmas, F.Panerai, O. Chazot, Temperature
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Figure Captions: Fig.1. SEM micrographs of healed specimen surface: (a) 1200 °C, 30min; (b) 1400 °C, 30min; (c) 1600 °C, 30min, and their high-magnification SEM images. Fig.2. Patterns of XRD of ZrB2-SiC-Graphite at different temperatures under atomic oxygen. Fig.3. Effect of crack-healing environment on the strength recovery behavior. Fig.4. Fracture morphologies of (a) pre-cracked specimen and specimen after oxidized at
15
(b)1200 °C; (c) 1400 °C and (d) 1600 °C in atomic oxygen. Fig.5. ZrO2-induced crack-healing mechanism schematic of the ZSG composite.
25μm
5μm
25μm
5μm
25μm
5μm
Fig.1. SEM micrographs of healed specimen surface: (a) 1200 °C, 30min; (b) 1400 °C, 30min; (c) 1600 °C, 30min, and their high-magnification SEM images.
16
Fig.2. Patterns of XRD of ZrB2-SiC-Graphite at different temperatures under atomic oxygen.
17
Fig.3. Effect of crack-healing environment on the strength recovery behavior.
18
100μm
100μm
100μm
100μm
Fig.4. Fracture morphologies of (a) pre-cracked specimen and specimen after oxidized at (b)1200 °C; (c) 1400 °C and (d) 1600 °C in atomic oxygen.
Local particle rearrangement
Oxidized
Oxidation product filling the crack
Track-tip blunting
Fig.5. ZrO2-induced crack-healing mechanism schematic of the ZSG composite.
19