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Effect of annealing environment on the crack healing and mechanical properties of (Mo0.97Nb0.03)(Si0.97Al0.03)2
Journal of Alloys and Compounds 634 (2015) 109–114
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of annealing environment on the crack healing and mechanical properties of (Mo0.97Nb0.03)(Si0.97Al0.03)2 Gaoming Zhu a, Xiaohong Wang a,⇑, Zhi Sun a, Peizhong Feng a, Farid Akhtar b a b
School of Material Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China Division of Materials Science, Luleå University of Technology, Luleå 97187, Sweden
a r t i c l e
i n f o
Article history: Received 3 November 2014 Received in revised form 7 February 2015 Accepted 10 February 2015 Available online 16 February 2015 Keywords: Silicides Molybdenum disilicide Crack healing Strength recovery
a b s t r a c t Crack healing of Nb and Al alloyed MoSi2 notched ceramics had been investigated during thermal treatment from 900 to 1500 °C in air, vacuum, argon and nitrogen environments. Notched (Mo0.97Nb0.03) (Si0.97Al0.03)2 ceramics showed significant recovery of bending strength after heat treatment in air. Bending strength recovery of 250% was found after heat treatment in air at 1200 °C. Oxide layer formation healed the cracks during annealing in air. Re-sintering was found dominant mechanism of crack healing during annealing in vacuum, argon and nitrogen atmosphere. Bending strength recovery of 208% was found after heat treatment in vacuum at 1200 °C. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum disilicide (MoSi2) has received significant attention for high-temperature structural applications [1–4]. MoSi2 has high melting point (2030 °C), low density (6.24 g cm 3) and excellent high temperature resistance to oxidation [4]. The low fracture toughness at room temperature (2–3 MPa m1/2) and poor creep resistance at high temperature (>Brittle–Ductile Transition Temperature) have limited its application in load bearing applications [5,6]. Several approaches have been investigated to improve the fracture toughness at room temperature and creep resistance of MoSi2 at high temperature [7,8]. Most promising routes reported to improve fracture toughness are (I) developing composites and (II) addition of alloying elements [9]. Alloying of MoSi2 has been widely adopted to improve fracture toughness. Alloying of MoSi2 with metals results in formation of metallic–covalent bonds and improves mechanical properties of MoSi2 by promoting the dislocation plasticity at low temperature. Commonly used alloying elements to structural silicides are Nb, V, Cr, Zr, Ta, Re and Al [9–11]. Al addition to MoSi2 has shown significant improvement in oxidation resistance at moderate temperatures [10]. Nb addition improves low temperature deformability and high temperature strength [12]. Nb-alloyed MoSi2 has shown anomalous strengthening behavior with a maximum strength at 1600 °C [13,14]. The co-substitution of Nb and ⇑ Corresponding author. Tel.: +86 516 83591879; fax: 86 516 83591870. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.02.072 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
Al in MoSi2 leads to improvement in the oxidation resistance at pest oxidation temperatures, reduction in the hardness and stiffness values and increase in the fracture toughness [11]. Structural ceramics, including MoSi2, are brittle and sensitive to superficial defects such as cracks and pores. The presence of these defects greatly reduces the component reliability. The component reliability can be improved by improving the fracture toughness and/or promoting the defect healing in ceramics. Early researches show that structural ceramics such as SiC [15], Si3N4 [16], Al2O3 [17], Ti3AlC2 [18], UO2 [19] and MoSi2 [20] exhibit crack healing during carefully devised heat treatment cycle in air. The crackhealing in these structural ceramics is mainly driven by oxidation process at crack surface during heat treatment in air. Korouš et al. studied the crack healing behavior of pre-cracked SiC ceramics in air from 600 to 1500 °C. They identified formation of SiO2 in crack healing regions by X-ray diffraction and related it to the observed crack healing phenomenon [21]. Zhang et al. heat treated ZrB2–SiC composites at 800 °C for 180 min in air. They found that a B2O3–SiO2 glass layer was formed with a 15% increase in strength [22]. Our preliminary work on MoSi2-montmorillonite composite heating elements showed that excellent self-healing occurred with a bending strength recovery from 133 MPa to 345 MPa after heat treatment at 1500 °C for 1 h in air [20]. In contrast to several studies addressing the issue of crack healing in SiC and Si3N4 based ceramics and composites, this is the first study reporting the defect and crack healing of MoSi2 based bulk ceramics by oxidation and re-sintering mechanisms at high temperatures. Moreover, we show that contrary to SiC and Si3N4 based
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ceramics, MoSi2 ceramics exhibit defect healing properties in non-oxidizing conditions. In order to study the crack healing properties, (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramics were prepared by self-propagation high-temperature synthesis (SHS) and vacuum hot-pressing (HP). The sintered (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramics were pre-cracked by Vickers indentation method and investigated for crack healing in air, vacuum, argon and nitrogen atmospheres from 900 to 1500 °C. Bending strength, crack-healing mechanisms and fracture mode of (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramics are reported.
Pre-cracked specimens were annealed in air at 900, 1200 and 1500 °C. The specimens annealed at 1200 °C in air exhibited the highest bending strength. To make a comparison, the pre-cracked specimens were annealed at 1200 °C in vacuum (<2 10 2 Pa), argon (99.99%, 0.1 MPa) and nitrogen (99.99%, 0.1 MPa). The bending strength of polished, pre-cracked and heat treated (Mo0.97Nb0.03)(Si0.97Al0.03)2 specimens was tested by a three-point loading system, using a 30 mm span and a crosshead speed of 0.5 mm min 1, and at least five specimens were tested for each condition. Surfaces and fracture patterns of (Mo0.97Nb0.03)(Si0.97Al0.03)2 specimens were analyzed by scanning electron microscopy (SEM, FEI Quanta TM 250) equipped with energy-dispersive spectroscopy (EDS, Quantax 400-10). The phase compositions were identified by Bruker D8 Advance X-ray diffractometer (XRD) using Cu Ka (k = 0.15406 nm) radiation.
2. Experimental procedure
3. Results and discussions
Molybdenum (2.0–2.5 lm, 99.9% purity, Zhuzhou cemented carbide group Co. Ltd., China), silicon ( 300 mesh, 99.9% purity, WODETAI (Beijing) science and technology development Co. Ltd., China), niobium ( 300 mesh, 99.9% purity, Zhuzhou cemented carbide group Co. Ltd., China) and aluminum (100–200 mesh, 99.0% purity, Chinasun Specialty Products Co. Ltd., China) powders with atomic ratio of 0.97:1.94:0.03:0.06 were ball-milled for 240 min at 450 rounds per minute. Absolute ethanol was used as milling media. After milling, the slurry was dried. The powder mixture was cold-pressed into cylindrical compacts of 16 mm diameter and 15 mm height at 200 MPa pressure. The relative density of the compacts was 65% of the theoretical. Combustion synthesis of the compacts was conducted in a steel combustion chamber (in-house manufactured), under pure argon (99.99%, 0.1 MPa) atmosphere [23]. The details of the combustion synthesis process are summarized in Supplementary Information (see SI, Part A). The synthesized (Mo0.97Nb0.03)(Si0.97Al0.03)2 porous compacts were ground to powder ( 100 mesh). The powder was ball-milled for 240 min at 450 rounds per minute as before. The ball-milled (Mo0.97Nb0.03)(Si0.97Al0.03)2 powder was dried and hot-pressed at 1500 °C at 27.5 MPa for 2 h in vacuum (<6.0 10 2 Pa). The sintered (Mo0.97Nb0.03) (Si0.97Al0.03)2 has the following properties: fracture toughness (KIC) of 2.5 ± 0.2 MPa m1/2, Vickers hardness (Hv) of 10.6 ± 0.1 GPa. The sintered disks (u50 mm 5 mm) were used to machine test specimens of 36 mm 3 mm 4 mm by wire-electrode cutting. The surface of test specimens was polished to mirror finish. The edges of all the specimens were chamfered to minimize the effect of stress concentration due to machining flaws. Cracks induced by Vickers indentation are used as model cracks for crack healing studies [24]. For this reason, a crack was introduced at the center of the test specimen using a Vickers indenter, at a load of 98 N for 20 s.
The surface SEM micrographs of the indentation are shown in Fig. 1. After indentation, microcracks appear around the indent (Fig. 1a). The semi-elliptical radial surface cracks are 0.5–0.8 lm wide at half-length of the crack and 100 lm long (the half crack length, c, was measured from the center of the indentation to the crack tip). The effect of crack length on the strength of brittle ceramics is discussed in Supplementary Information (see SI, Part B). After heat treatment at 900 °C in air (Fig. 1b) SiO2 particles (white) of 5–10 lm appear on the surface, determined by energy dispersive spectroscopy (see SI, Fig. S7). After annealing at 1200 °C (Fig. 1c) in air, the indentation edges passivate and the specimen surface is covered with a discontinuous glass layer. Annealing at 1500 °C (Fig. 1d) show that the cracks and diagonals of the indentation disappear and indent edges passivate. X-ray diffraction (XRD) data (see SI, Fig. S10) confirms that SiO2 diffraction peaks appear after annealing at 1200 and 1500 °C in air. At 1500 °C, the oxidation happens at a faster rate with accelerated volatilization of MoO3 and results in reconstitution of SiO2 [25]. A dense SiO2 layer forms on the surface at 1500 °C (Fig. 1d). The dense SiO2 layer inhibits the diffusion of O2 , and Mo5Si3 is produced because of the selective oxidation of MoSi2 (see SI,
Fig. 1. (a) SEM micrographs of the untreated surface shape; samples at different temperatures for 1 h in air: (b) 900 °C; (c) 1200 °C; and (d) 1500 °C. The inset of (d) is a higher-magnification image of the box.
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Table 1 Effect of crack-healing environments and temperatures on the bending strength. Specimens
Healing environment
Healing temperature (°C)
Bending strength (MPa)
Fracture location (outside the indent/all)
Smooth
– Air – Air Air Air Vacuum Argon Nitrogen
– 1200 – 900 1200 1500 1200 1200 1200
287 ± 59 380 ± 34 103 ± 10 187 ± 24 361 ± 39 331 ± 22 317 ± 35 256 ± 45 262 ± 32
– – 0/5 1/6 4/6 5/6 2/5 1/5 1/5
Cracked (series a)
Cracked (series b)
Fig. S10). We observed formation of smoke by naked eye during annealing at 1500 °C and attributed it to the volatilization of MoO3. The smoke did not appear during annealing at 900 °C and 1200 °C. The bending strength of as-sintered, pre-cracked and crackhealed (Mo0.97Nb0.03)(Si0.97Al0.03)2 materials in air is summarized in Table 1. It can be seen that the bending strength decreases from 287 MPa to 103 MPa, when the cracks are introduced. After heat treatment in air, the bending strength increases to 187 MPa (900 °C), 361 MPa (1200 °C) and 331 MPa (1500 °C). It is evident that the bending strength shows a significant recovery after heat treatment at 1200 °C for 1 h and exceeds the bending strength of as-sintered materials. The increase in strength has been reported for Al2O3/SiC composite after heat treated at 1573 K for 1 h in air, from 660 MPa to 890 MPa [26]. In our case, the bending strength of heat treated materials (1200 °C for 1 h) is 32% higher (380 MPa) than as-sintered materials (287 MPa). It can be concluded that microcracks and residual porosity exist in as synthesized materials and reduce the mechanical properties of (Mo0.97Nb0.03) (Si0.97Al0.03)2 materials. Besides, residual stress and microscopic cracks may form during the mechanical preparation processes such as wire cutting and polishing [27]. Increase in strength compared to as sintered materials supports the notion that residual stress is relieved [28] and defects in as sintered materials are healed due to the formation of SiO2 at the high energy defect sites. The strength show slight decrease after heat treatment at 1500 °C compared to 1200 °C (Table 1). This decrease in the strength is attributable to the formation of defects in the thick oxide layer. The microcracks appear in the oxide layer and scales with the thickness of the oxide layer due to the thermal expansion mismatch between the MoSi2 substrate and the oxide layer (8.1 10 6 K 1 for MoSi2 versus 0.5 10 6 K 1 for SiO2 [5,29]) and volume change due to phase transformations [30,31]. The cracks are also observed in the oxide layer in Maruyama’s report [31]. Two cases of fracture locations (from/outside the indent, see SI, Fig. S11) in the bending strength test were analytically analyzed (Table 1). Because of the stress concentration at the crack tips [26], all of the untreated specimens fractured from the indentation zone. After annealing, 67% (1200 °C) and 83% (1500 °C) of the specimens fractured outside the indentation zone. These analyses indicate that the indentation cracks are not critical after healing and passivation of crack tips. The cross-section of indented surface in Fig. 2a shows that semielliptical cracks appear at the center of the tension surface. Intergranular fracture appears around the indent and transcrystalline fracture appears far from the indent. It is well know that five independent deformation modes are necessary for polycrystalline ductility, while only four independent slip systems ({0 1 3) < 1 0 0], {1 1 0)1/2 < 1 1 1], {0 1 1) < 1 0 0] and {0 1 3)1/2 < 3 3 1]) are observed in MoSi2 [32,33]. Therefore, grains in MoSi2 crystal cannot deform coordinately with adjacent grains during the indentation
Fig. 2. Cross-sections of the indentation: (a) overview of untreated specimen with indentation pattern and microcracks; (b) overview of specimen treated under 1500 °C for 1 h in air; and (c) enlarged view of the box in (b).
experiment, which causes the destruction of the continuity between grains and results in the generation of micro-cracks along grain boundaries, significantly around the center of indent. Microcracks greatly weaken the grain boundary and contribute to the intergranular fracture around the indent. The cross-sectional view of (Mo0.97Nb0.03)(Si0.97Al0.03)2 annealed at 1500 °C for 1 h show that the semi-elliptical boundary between intergranular fracture and transcrystalline fracture remains after heat treatment (Fig. 2b). We suspect that crack surfaces do not heal with oxide completely during heat treatment at 1500 °C for 1 h. This might be because of formation of oxide layer forms rapidly on the surface in the beginning of heat treatment at 1500 °C. This layer reduces the diffusion of oxygen and reduces the oxide formation rate. On a closer look at the semi-elliptical area (Fig. 2c), it can be observed that grains are rounded than the untreated material (Fig. 2a). The higher dislocation density inside the grains provides driving force for re-sintering of grains [34].
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Fig. 3. SEM micrographs of treated specimens at 1200 °C for 1 h under different environments: (a) air; (b) vacuum; (c) argon; and (d) nitrogen. The insets are highermagnification images of the cracks, scale bars, 10 lm.
Fig. 4. (a) Schematic diagram of indentation locations; (b) cross-section overview of indentation; (c and d) enlarged views of the boxes in (b).
G. Zhu et al. / Journal of Alloys and Compounds 634 (2015) 109–114
In addition to the influence of crack-healing temperature in air, pre-cracked specimens are treated at 1200 °C for 1 h in vacuum, argon and nitrogen. The bending strength of heat treated specimens in Table 1 (series b) show increase in strength. Early researches on Si3N4 [35], ZrB2 [36] and Al2O3/SiC [37] show that specimens heat treated in vacuum, argon and nitrogen show insignificant strength recovery. In our work, the bending strength recovery is significant after heat treatment in vacuum (317 MPa), argon (256 MPa) and nitrogen (262 MPa). However, the strength is lower than materials annealed in air (Table 1). This indicates that in addition to the oxide layer formation, there can be other crack healing mechanisms operative at high temperature in (Mo0.97Nb0.03)(Si0.97Al0.03)2 materials. Fig. 3 compares the surfaces of pre-cracked materials treated at 1200 °C for 1 h in air, vacuum, argon and nitrogen. After treated in air at 1200 °C (Fig. 3a), the surface is covered with a thin oxide layer and cracks are filled with oxide (inset in Fig. 3a). However, after heat treatment in vacuum, argon and nitrogen (Fig. 3b–d), surface views and cracks are remained as close to as-cracked materials. In order to investigate the mechanism of crack healing under vacuum, an experimental scheme is designed as shown in Fig. 4a. After the treatment of pre-cracked specimens at 1200 °C for 1 h in vacuum, two indentations (Y1 and Y2) are made beside the original indentation (O). The cross-section of the specimen is observed at the position of indentations (Fig. 4). In Fig. 4b–d, re-sintering of grains is visible. The grains around the indent are rounded (Fig. 4c) compared to as-sintered specimen (Fig. 2a) and cracks inside are not detectable. It is interesting that the new indents give the similar morphology of the surface with Fig. 2a supporting the notion that the heat treatment in vacuum facilitates the crack healing process by re-sintering. Over the past 40 years, crack healing behavior of structural ceramics has been widely studied. The mechanisms proposed are: re-sintering [38], relaxation of tensile residual stress at the indentation site [39] and cracks bonding by oxidation [35]. Our current work on Nb and Al alloyed MoSi2 shows that it has excellent self-healing properties at high temperature in air, vacuum, argon and nitrogen at 1200 °C for 1 h. The bending strength recovery is 250%, 208%, 149% and 153% in air, vacuum, argon and nitrogen, respectively. Chou et al. indicated that the bending strength of Al2O3/5 vol% SiC increased by up to 50% after annealing at 1300 °C for 2 h in argon [37]. Zhang et al. investigated the crack healing behavior of ZrB2/20 vol% SiC in air and vacuum. They showed that all composite healed in air recovered bending strength (889 MPa) compared with pre-cracked (279 MPa). However, in vacuum the bending strength increased to 351 MPa, 20% higher than precracked samples [36]. In our case, the bending strength has exceeded the as-sintered bending strength after heat treatment at 1200 °C for 1 h in air and vacuum and considerably improved in argon (149%) and nitrogen (153%). The crack healing behavior with the addition of Nb and Al to MoSi2 is very important to improve the component reliability.
4. Conclusions After annealing in air from 900 to 1500 °C, crack healing behavior was observed. The bending strength of treated (Mo0.97Nb0.03) (Si0.97Al0.03)2 ceramics showed complete recovery and exceed the strength of as-sintered ceramics at 1200 °C. We concluded that the formation of a silica-rich glassy phase facilitated crack rebinding. MoSi2 based ceramics showed good recovery of bending strength in non-oxidizing conditions. After annealing in vacuum (317 MPa), argon (256 MPa) and nitrogen (262 MPa), no oxide was formed and bending strength increased due to re-sintering and residual stress relaxation.
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Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2012QNB04) and National Nature Science Foundation of China (51202289).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.02. 072.
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Effect of annealing environment on the crack healing and mechanical properties of (Mo0.97Nb0.03)(Si0.97Al0.03)2 Gaoming Zhu a, Xiaohong Wang a,⇑, Zhi Sun a, Peizhong Feng a, Farid Akhtar b a b
School of Material Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China Division of Materials Science, Luleå University of Technology, Luleå 97187, Sweden
a r t i c l e
i n f o
Article history: Received 3 November 2014 Received in revised form 7 February 2015 Accepted 10 February 2015 Available online 16 February 2015 Keywords: Silicides Molybdenum disilicide Crack healing Strength recovery
a b s t r a c t Crack healing of Nb and Al alloyed MoSi2 notched ceramics had been investigated during thermal treatment from 900 to 1500 °C in air, vacuum, argon and nitrogen environments. Notched (Mo0.97Nb0.03) (Si0.97Al0.03)2 ceramics showed significant recovery of bending strength after heat treatment in air. Bending strength recovery of 250% was found after heat treatment in air at 1200 °C. Oxide layer formation healed the cracks during annealing in air. Re-sintering was found dominant mechanism of crack healing during annealing in vacuum, argon and nitrogen atmosphere. Bending strength recovery of 208% was found after heat treatment in vacuum at 1200 °C. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum disilicide (MoSi2) has received significant attention for high-temperature structural applications [1–4]. MoSi2 has high melting point (2030 °C), low density (6.24 g cm 3) and excellent high temperature resistance to oxidation [4]. The low fracture toughness at room temperature (2–3 MPa m1/2) and poor creep resistance at high temperature (>Brittle–Ductile Transition Temperature) have limited its application in load bearing applications [5,6]. Several approaches have been investigated to improve the fracture toughness at room temperature and creep resistance of MoSi2 at high temperature [7,8]. Most promising routes reported to improve fracture toughness are (I) developing composites and (II) addition of alloying elements [9]. Alloying of MoSi2 has been widely adopted to improve fracture toughness. Alloying of MoSi2 with metals results in formation of metallic–covalent bonds and improves mechanical properties of MoSi2 by promoting the dislocation plasticity at low temperature. Commonly used alloying elements to structural silicides are Nb, V, Cr, Zr, Ta, Re and Al [9–11]. Al addition to MoSi2 has shown significant improvement in oxidation resistance at moderate temperatures [10]. Nb addition improves low temperature deformability and high temperature strength [12]. Nb-alloyed MoSi2 has shown anomalous strengthening behavior with a maximum strength at 1600 °C [13,14]. The co-substitution of Nb and ⇑ Corresponding author. Tel.: +86 516 83591879; fax: 86 516 83591870. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.02.072 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
Al in MoSi2 leads to improvement in the oxidation resistance at pest oxidation temperatures, reduction in the hardness and stiffness values and increase in the fracture toughness [11]. Structural ceramics, including MoSi2, are brittle and sensitive to superficial defects such as cracks and pores. The presence of these defects greatly reduces the component reliability. The component reliability can be improved by improving the fracture toughness and/or promoting the defect healing in ceramics. Early researches show that structural ceramics such as SiC [15], Si3N4 [16], Al2O3 [17], Ti3AlC2 [18], UO2 [19] and MoSi2 [20] exhibit crack healing during carefully devised heat treatment cycle in air. The crackhealing in these structural ceramics is mainly driven by oxidation process at crack surface during heat treatment in air. Korouš et al. studied the crack healing behavior of pre-cracked SiC ceramics in air from 600 to 1500 °C. They identified formation of SiO2 in crack healing regions by X-ray diffraction and related it to the observed crack healing phenomenon [21]. Zhang et al. heat treated ZrB2–SiC composites at 800 °C for 180 min in air. They found that a B2O3–SiO2 glass layer was formed with a 15% increase in strength [22]. Our preliminary work on MoSi2-montmorillonite composite heating elements showed that excellent self-healing occurred with a bending strength recovery from 133 MPa to 345 MPa after heat treatment at 1500 °C for 1 h in air [20]. In contrast to several studies addressing the issue of crack healing in SiC and Si3N4 based ceramics and composites, this is the first study reporting the defect and crack healing of MoSi2 based bulk ceramics by oxidation and re-sintering mechanisms at high temperatures. Moreover, we show that contrary to SiC and Si3N4 based
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ceramics, MoSi2 ceramics exhibit defect healing properties in non-oxidizing conditions. In order to study the crack healing properties, (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramics were prepared by self-propagation high-temperature synthesis (SHS) and vacuum hot-pressing (HP). The sintered (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramics were pre-cracked by Vickers indentation method and investigated for crack healing in air, vacuum, argon and nitrogen atmospheres from 900 to 1500 °C. Bending strength, crack-healing mechanisms and fracture mode of (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramics are reported.
Pre-cracked specimens were annealed in air at 900, 1200 and 1500 °C. The specimens annealed at 1200 °C in air exhibited the highest bending strength. To make a comparison, the pre-cracked specimens were annealed at 1200 °C in vacuum (<2 10 2 Pa), argon (99.99%, 0.1 MPa) and nitrogen (99.99%, 0.1 MPa). The bending strength of polished, pre-cracked and heat treated (Mo0.97Nb0.03)(Si0.97Al0.03)2 specimens was tested by a three-point loading system, using a 30 mm span and a crosshead speed of 0.5 mm min 1, and at least five specimens were tested for each condition. Surfaces and fracture patterns of (Mo0.97Nb0.03)(Si0.97Al0.03)2 specimens were analyzed by scanning electron microscopy (SEM, FEI Quanta TM 250) equipped with energy-dispersive spectroscopy (EDS, Quantax 400-10). The phase compositions were identified by Bruker D8 Advance X-ray diffractometer (XRD) using Cu Ka (k = 0.15406 nm) radiation.
2. Experimental procedure
3. Results and discussions
Molybdenum (2.0–2.5 lm, 99.9% purity, Zhuzhou cemented carbide group Co. Ltd., China), silicon ( 300 mesh, 99.9% purity, WODETAI (Beijing) science and technology development Co. Ltd., China), niobium ( 300 mesh, 99.9% purity, Zhuzhou cemented carbide group Co. Ltd., China) and aluminum (100–200 mesh, 99.0% purity, Chinasun Specialty Products Co. Ltd., China) powders with atomic ratio of 0.97:1.94:0.03:0.06 were ball-milled for 240 min at 450 rounds per minute. Absolute ethanol was used as milling media. After milling, the slurry was dried. The powder mixture was cold-pressed into cylindrical compacts of 16 mm diameter and 15 mm height at 200 MPa pressure. The relative density of the compacts was 65% of the theoretical. Combustion synthesis of the compacts was conducted in a steel combustion chamber (in-house manufactured), under pure argon (99.99%, 0.1 MPa) atmosphere [23]. The details of the combustion synthesis process are summarized in Supplementary Information (see SI, Part A). The synthesized (Mo0.97Nb0.03)(Si0.97Al0.03)2 porous compacts were ground to powder ( 100 mesh). The powder was ball-milled for 240 min at 450 rounds per minute as before. The ball-milled (Mo0.97Nb0.03)(Si0.97Al0.03)2 powder was dried and hot-pressed at 1500 °C at 27.5 MPa for 2 h in vacuum (<6.0 10 2 Pa). The sintered (Mo0.97Nb0.03) (Si0.97Al0.03)2 has the following properties: fracture toughness (KIC) of 2.5 ± 0.2 MPa m1/2, Vickers hardness (Hv) of 10.6 ± 0.1 GPa. The sintered disks (u50 mm 5 mm) were used to machine test specimens of 36 mm 3 mm 4 mm by wire-electrode cutting. The surface of test specimens was polished to mirror finish. The edges of all the specimens were chamfered to minimize the effect of stress concentration due to machining flaws. Cracks induced by Vickers indentation are used as model cracks for crack healing studies [24]. For this reason, a crack was introduced at the center of the test specimen using a Vickers indenter, at a load of 98 N for 20 s.
The surface SEM micrographs of the indentation are shown in Fig. 1. After indentation, microcracks appear around the indent (Fig. 1a). The semi-elliptical radial surface cracks are 0.5–0.8 lm wide at half-length of the crack and 100 lm long (the half crack length, c, was measured from the center of the indentation to the crack tip). The effect of crack length on the strength of brittle ceramics is discussed in Supplementary Information (see SI, Part B). After heat treatment at 900 °C in air (Fig. 1b) SiO2 particles (white) of 5–10 lm appear on the surface, determined by energy dispersive spectroscopy (see SI, Fig. S7). After annealing at 1200 °C (Fig. 1c) in air, the indentation edges passivate and the specimen surface is covered with a discontinuous glass layer. Annealing at 1500 °C (Fig. 1d) show that the cracks and diagonals of the indentation disappear and indent edges passivate. X-ray diffraction (XRD) data (see SI, Fig. S10) confirms that SiO2 diffraction peaks appear after annealing at 1200 and 1500 °C in air. At 1500 °C, the oxidation happens at a faster rate with accelerated volatilization of MoO3 and results in reconstitution of SiO2 [25]. A dense SiO2 layer forms on the surface at 1500 °C (Fig. 1d). The dense SiO2 layer inhibits the diffusion of O2 , and Mo5Si3 is produced because of the selective oxidation of MoSi2 (see SI,
Fig. 1. (a) SEM micrographs of the untreated surface shape; samples at different temperatures for 1 h in air: (b) 900 °C; (c) 1200 °C; and (d) 1500 °C. The inset of (d) is a higher-magnification image of the box.
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Table 1 Effect of crack-healing environments and temperatures on the bending strength. Specimens
Healing environment
Healing temperature (°C)
Bending strength (MPa)
Fracture location (outside the indent/all)
Smooth
– Air – Air Air Air Vacuum Argon Nitrogen
– 1200 – 900 1200 1500 1200 1200 1200
287 ± 59 380 ± 34 103 ± 10 187 ± 24 361 ± 39 331 ± 22 317 ± 35 256 ± 45 262 ± 32
– – 0/5 1/6 4/6 5/6 2/5 1/5 1/5
Cracked (series a)
Cracked (series b)
Fig. S10). We observed formation of smoke by naked eye during annealing at 1500 °C and attributed it to the volatilization of MoO3. The smoke did not appear during annealing at 900 °C and 1200 °C. The bending strength of as-sintered, pre-cracked and crackhealed (Mo0.97Nb0.03)(Si0.97Al0.03)2 materials in air is summarized in Table 1. It can be seen that the bending strength decreases from 287 MPa to 103 MPa, when the cracks are introduced. After heat treatment in air, the bending strength increases to 187 MPa (900 °C), 361 MPa (1200 °C) and 331 MPa (1500 °C). It is evident that the bending strength shows a significant recovery after heat treatment at 1200 °C for 1 h and exceeds the bending strength of as-sintered materials. The increase in strength has been reported for Al2O3/SiC composite after heat treated at 1573 K for 1 h in air, from 660 MPa to 890 MPa [26]. In our case, the bending strength of heat treated materials (1200 °C for 1 h) is 32% higher (380 MPa) than as-sintered materials (287 MPa). It can be concluded that microcracks and residual porosity exist in as synthesized materials and reduce the mechanical properties of (Mo0.97Nb0.03) (Si0.97Al0.03)2 materials. Besides, residual stress and microscopic cracks may form during the mechanical preparation processes such as wire cutting and polishing [27]. Increase in strength compared to as sintered materials supports the notion that residual stress is relieved [28] and defects in as sintered materials are healed due to the formation of SiO2 at the high energy defect sites. The strength show slight decrease after heat treatment at 1500 °C compared to 1200 °C (Table 1). This decrease in the strength is attributable to the formation of defects in the thick oxide layer. The microcracks appear in the oxide layer and scales with the thickness of the oxide layer due to the thermal expansion mismatch between the MoSi2 substrate and the oxide layer (8.1 10 6 K 1 for MoSi2 versus 0.5 10 6 K 1 for SiO2 [5,29]) and volume change due to phase transformations [30,31]. The cracks are also observed in the oxide layer in Maruyama’s report [31]. Two cases of fracture locations (from/outside the indent, see SI, Fig. S11) in the bending strength test were analytically analyzed (Table 1). Because of the stress concentration at the crack tips [26], all of the untreated specimens fractured from the indentation zone. After annealing, 67% (1200 °C) and 83% (1500 °C) of the specimens fractured outside the indentation zone. These analyses indicate that the indentation cracks are not critical after healing and passivation of crack tips. The cross-section of indented surface in Fig. 2a shows that semielliptical cracks appear at the center of the tension surface. Intergranular fracture appears around the indent and transcrystalline fracture appears far from the indent. It is well know that five independent deformation modes are necessary for polycrystalline ductility, while only four independent slip systems ({0 1 3) < 1 0 0], {1 1 0)1/2 < 1 1 1], {0 1 1) < 1 0 0] and {0 1 3)1/2 < 3 3 1]) are observed in MoSi2 [32,33]. Therefore, grains in MoSi2 crystal cannot deform coordinately with adjacent grains during the indentation
Fig. 2. Cross-sections of the indentation: (a) overview of untreated specimen with indentation pattern and microcracks; (b) overview of specimen treated under 1500 °C for 1 h in air; and (c) enlarged view of the box in (b).
experiment, which causes the destruction of the continuity between grains and results in the generation of micro-cracks along grain boundaries, significantly around the center of indent. Microcracks greatly weaken the grain boundary and contribute to the intergranular fracture around the indent. The cross-sectional view of (Mo0.97Nb0.03)(Si0.97Al0.03)2 annealed at 1500 °C for 1 h show that the semi-elliptical boundary between intergranular fracture and transcrystalline fracture remains after heat treatment (Fig. 2b). We suspect that crack surfaces do not heal with oxide completely during heat treatment at 1500 °C for 1 h. This might be because of formation of oxide layer forms rapidly on the surface in the beginning of heat treatment at 1500 °C. This layer reduces the diffusion of oxygen and reduces the oxide formation rate. On a closer look at the semi-elliptical area (Fig. 2c), it can be observed that grains are rounded than the untreated material (Fig. 2a). The higher dislocation density inside the grains provides driving force for re-sintering of grains [34].
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Fig. 3. SEM micrographs of treated specimens at 1200 °C for 1 h under different environments: (a) air; (b) vacuum; (c) argon; and (d) nitrogen. The insets are highermagnification images of the cracks, scale bars, 10 lm.
Fig. 4. (a) Schematic diagram of indentation locations; (b) cross-section overview of indentation; (c and d) enlarged views of the boxes in (b).
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In addition to the influence of crack-healing temperature in air, pre-cracked specimens are treated at 1200 °C for 1 h in vacuum, argon and nitrogen. The bending strength of heat treated specimens in Table 1 (series b) show increase in strength. Early researches on Si3N4 [35], ZrB2 [36] and Al2O3/SiC [37] show that specimens heat treated in vacuum, argon and nitrogen show insignificant strength recovery. In our work, the bending strength recovery is significant after heat treatment in vacuum (317 MPa), argon (256 MPa) and nitrogen (262 MPa). However, the strength is lower than materials annealed in air (Table 1). This indicates that in addition to the oxide layer formation, there can be other crack healing mechanisms operative at high temperature in (Mo0.97Nb0.03)(Si0.97Al0.03)2 materials. Fig. 3 compares the surfaces of pre-cracked materials treated at 1200 °C for 1 h in air, vacuum, argon and nitrogen. After treated in air at 1200 °C (Fig. 3a), the surface is covered with a thin oxide layer and cracks are filled with oxide (inset in Fig. 3a). However, after heat treatment in vacuum, argon and nitrogen (Fig. 3b–d), surface views and cracks are remained as close to as-cracked materials. In order to investigate the mechanism of crack healing under vacuum, an experimental scheme is designed as shown in Fig. 4a. After the treatment of pre-cracked specimens at 1200 °C for 1 h in vacuum, two indentations (Y1 and Y2) are made beside the original indentation (O). The cross-section of the specimen is observed at the position of indentations (Fig. 4). In Fig. 4b–d, re-sintering of grains is visible. The grains around the indent are rounded (Fig. 4c) compared to as-sintered specimen (Fig. 2a) and cracks inside are not detectable. It is interesting that the new indents give the similar morphology of the surface with Fig. 2a supporting the notion that the heat treatment in vacuum facilitates the crack healing process by re-sintering. Over the past 40 years, crack healing behavior of structural ceramics has been widely studied. The mechanisms proposed are: re-sintering [38], relaxation of tensile residual stress at the indentation site [39] and cracks bonding by oxidation [35]. Our current work on Nb and Al alloyed MoSi2 shows that it has excellent self-healing properties at high temperature in air, vacuum, argon and nitrogen at 1200 °C for 1 h. The bending strength recovery is 250%, 208%, 149% and 153% in air, vacuum, argon and nitrogen, respectively. Chou et al. indicated that the bending strength of Al2O3/5 vol% SiC increased by up to 50% after annealing at 1300 °C for 2 h in argon [37]. Zhang et al. investigated the crack healing behavior of ZrB2/20 vol% SiC in air and vacuum. They showed that all composite healed in air recovered bending strength (889 MPa) compared with pre-cracked (279 MPa). However, in vacuum the bending strength increased to 351 MPa, 20% higher than precracked samples [36]. In our case, the bending strength has exceeded the as-sintered bending strength after heat treatment at 1200 °C for 1 h in air and vacuum and considerably improved in argon (149%) and nitrogen (153%). The crack healing behavior with the addition of Nb and Al to MoSi2 is very important to improve the component reliability.
4. Conclusions After annealing in air from 900 to 1500 °C, crack healing behavior was observed. The bending strength of treated (Mo0.97Nb0.03) (Si0.97Al0.03)2 ceramics showed complete recovery and exceed the strength of as-sintered ceramics at 1200 °C. We concluded that the formation of a silica-rich glassy phase facilitated crack rebinding. MoSi2 based ceramics showed good recovery of bending strength in non-oxidizing conditions. After annealing in vacuum (317 MPa), argon (256 MPa) and nitrogen (262 MPa), no oxide was formed and bending strength increased due to re-sintering and residual stress relaxation.
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Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2012QNB04) and National Nature Science Foundation of China (51202289).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.02. 072.
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