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Multicomponent synergistically affected mechanical properties, microstructure, and oxidation resistance of Zr–Al(Si)–C based composites
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Multicomponent synergistically affected mechanical properties, microstructure, and oxidation resistance of Zr–Al(Si)–C based composites Xuhong Wanga,b,c, Wangjin Jia,b, Jian Hub, Hui Liua,b, Jianhao Zhangb, Zhefei Wangb,c, Lei Yua,b,c,∗, Shilong Yinb,c a
School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, China School of Chemistry and Materials Engineering, Changshu Institute of Technology, Changshu, 215500, China c Suzhou Key Laboratory of Functional Ceramic Materials, Changshu Institute of Technology, Changshu, 215500, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Zr3[Al(Si)]4C6–ZrB2–SiC Mechanical properties Oxidation behavior Ultra high-temperature ceramics (UHTCs) Synergistic mechanisms
Herein, in-situ Zr3[Al(Si)]4C6-based composites with 10–40 vol% ZrB2–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB2–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB2–SiC, while the hardness increases near linearly. Zr3[Al (Si)]4C6–ZrB2–SiC shows high strength (623 MPa), toughness (7.59 MPa m1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB2–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10−5 kg2 m−4 s−1 when oxidized at 1400 °C.
1. Introduction Owing to the combination of partial properties of both ceramics and metals, a new kind of Zr–Al(Si)–C layered quaternary compound beyond the MAX phases have attracted much attention since they were synthesized in 2007 [1,2]. As a typical representative, Zr3[Al(Si)]4C6 ceramic exhibits superior oxidation resistance, fracture toughness, and bending strength to the corresponding binary carbide ZrC [3–10]. Also, it shows better high-temperature mechanical properties than those of some MAX phases [4,5]. For instance, the strength of Zr3[Al(Si)]4C6 ceramic can maintain up to 1400 °C, and the residual ratio of its Young's modulus at 1600 °C can remain about 80% [4–7]. The excellent roomand high-temperature properties render Zr3[Al(Si)]4C6 as a potential material applicated in high or ultra-high temperature environments. And, the relatively low density (4.85 g/cm3) makes Zr3[Al(Si)]4C6 advantageous in aerospace applications with lightweight requirements. Nevertheless, it is very limited to be used in high-temperature environments in terms of its current performances. The fracture toughness (KIC = 4.62 MPa m1/2) of Zr3[Al(Si)]4C6 ceramic is obviously lower compared with some ternary layered carbides (e.g., Ti–Al/Si–C) [11], its hardness (HV = 12.4 GPa) is only about 50% of ZrC, and its strength (σc = 312 MPa) remains to be improved compared with other structural ∗
ceramics [12]. Furthermore, it shows unsatisfactory oxidation resistance during oxidation in the temperature range of 900 °C–1300 °C. Its oxidation kinetics almost follows a line law [3], and its oxidation rates increase very rapidly with the increasing temperature. To further improve the properties and applicability of Zr3[Al(Si)]4C6, effective approaches need to be designed. As one of the most studied UHTCs, ZrB2–SiC–ZrC composites exhibit much better mechanical properties, oxidation and ablation resistance, and thermal shock resistance than monolithic ZrB2, and also superior to ZrB2–SiC composites [13–15], which can be ascribed to the synergistic effect to the ZrB2 matrix produced by SiC–ZrC binary reinforcements. The achievements in ZrB2–SiC–ZrC composites imply that incorporation of appropriate phases can improve the comprehensive performance of materials effectively, and the synergistic mechanism between two phases in ceramic-matrix shows significantly toughening and strengthening effects [16,17]. Thus, various UHTCs systems with good performance are designed and fabricated, which is one of the main research focus in advanced ceramics. In our previous works, based on the ideals of multicomponent strengthening and toughening, a hotpressed Zr3[Al(Si)]4C6–ZrB2–ZrC ternary composite ceramic was prepared using in situ reaction method. The composite ceramic has attractive room-temperature mechanical properties (HV = 16.4 GPa,
Corresponding author. School of Chemistry and Materials Engineering, Changshu Institute of Technology, Changshu, 215500, China. E-mail address: [email protected] (L. Yu).
https://doi.org/10.1016/j.ceramint.2019.09.001 Received 12 June 2019; Received in revised form 19 August 2019; Accepted 1 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xuhong Wang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.001
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σc = 621 MPa, KIC = 7.37 MPa m1/2, etc.) [18]. Moreover, its hightemperature (1300 °C) strength in air is around 369 MPa, showing a 35% increase compared with Zr3[Al(Si)]4C6 (about 274 MPa) ceramic. However, its oxidation resistance is reduced because of a lack of effective oxidation protective layer. It is well known that SiC can be a good reinforcement to improve both the oxidation resistance and mechanical properties of materials [19–21]. Therefore, a novel Zr3[Al (Si)]4C6-based composite incorporated with binary ZrB2–SiC were prepared in our previous works [22]. The composite shows slightly better mechanical properties (σc = 648 MPa, KIC = 7.69 MPa m1/2, HV = 17.5 GPa, etc.) than Zr3[Al(Si)]4C6–ZrB2–ZrC [18,22]. Moreover, it can maintain a higher high-temperature bending strength (around 439 MPa) than Zr3[Al(Si)]4C6–ZrB2–ZrC under the same test conditions [18,22]. By comprehensive comparison, Zr3[Al(Si)]4C6–ZrB2–SiC composites possess much improved properties and exhibit a potential prospect as UHTCs. However, the previous works did not show the influence of ZrB2–SiC with a fixed ratio on the mechanical properties of materials prepared at a lower temperature. And also, the oxidation behavior of the corresponding composites has not been investigated, especially above 1300 °C. In this work, therefore, in-situ Zr3[Al(Si)]4C6-based composites incorporated by controlled ZrB2–SiC content (10–40 vol%) with a 2-to-1 molar ratio were prepared by at 1850 °C. And, the effects of ZrB2–SiC on the microstructure, mechanical properties, and the oxidation resistance of composites were discussed and investigated.
2.2. Characterization The Archimedes method was used to characterize the sintering properties of materials, including the porosity and density. And, according to the rule of mixtures, the theoretical densities of sintered specimens were calculated. The phase analysis of materials was identified by X-ray diffraction (XRD) (Rigaku Dmax-2200PC, Tokyo, Japan). A scanning electron microscopy (SEM, ZEISS SIGMA-500, Oberkochen, Germany) and a transmission electron microscope (TEM, JEOL JEM2010, Tokyo, Japan) were used to observe the microstructures of materials, including the polished and fracture surfaces, grains, and grain boundaries. Both of the SEM and TEM can be used to analyze the chemical composition via EDS within the instruments. 2.3. Mechanical properties test A universal mechanical instrument (SUNS UTM6203, Shenzhen, China) and an automatic hardness tester (TIME 6614AT, Beijing, China) were used to test the mechanical properties including the strength, the toughness, and the hardness of specimens, respectively. For the bending strength test, according to the three point bending (TPB) method, the specimens with the dimension of 36 mm × 4 mm × 3 mm were tested, and the loading speed and span length and were set to 0.5 mm/min and 30 mm, respectively. For the fracture toughness test, the specimens with the dimension of 22 mm × 2 mm × 4 mm were tested according to the single edge notched beams (SENB) method. The depth and width of the notch for specimens were 2 mm and 0.25 mm, respectively. Meanwhile, the span length and loading speed were set to 16 mm and 0.05 mm/min, respectively. While in hardness testing, the load was set to 9.8 N maintained for 10s.
2. Experimental procedure 2.1. Material preparation In-situ Zr3[Al(Si)]4C6–ZrB2–SiC (abbreviated as ZAS) composites were prepared by hot-pressing sintering at 1850 °C in flowing Ar atmosphere. Commercial powders of zirconium hydride (ZrH2, 99.9%, ≤37 μm), aluminum (Al, 99.9%, ≤48 μm), silicon (Si, 99.9%, ≤48 μm), graphite (C, 99%, ≤12 μm), and boron carbide (B4C, 99.9%, ≤10 μm) were used as starting materials. The detailed preparation process has been reported elsewhere [22], and the in-situ reaction is as follows:
2.4. Oxidation test The oxidation resistance of sintered specimens with a dimension of 8 mm × 4 mm × 3 mm was tested in a tube furnace (Kejing GSL-1500X50, Hefei, China) at 1300 °C, and 1400 °C, respectively. While testing, both ends of the tube furnace are connected with air and were inserted insulating bricks to maintaining the temperature in the constant temperature region. The specimens were put into the furnace immediately when the temperature rose to the set value. After oxidation for a period of time, the specimens were taken out directly. An electronic microbalance with the precision within 10−4 (Mettler Toledo ME204E, Shanghai, China) was used to weigh and measure the weight change of sintered samples before and after oxidation. Also, above SEM and XRD were used to analyze the microstructure and phase composition of the oxide products, respectively.
(2x+3)Zr + xB4C + xSi + 4Al(Si) + 6C = x(2ZrB2 + SiC) + Zr3[Al (Si)]4C6 (1) Here, according to the above reaction, ZAS composites with different ZrB2–SiC content of 10, 20, 30, and 40 vol% (x = 0.21, 0.47, 0.81, and 1.26, respectively) and given molar ratio of 2:1 for ZrB2 to SiC were designed and prepared. And, for the convenience in description, the composites were donated as ZAS1, ZAS2, ZAS3, and ZAS4, corresponding to the composites with 10, 20, 30, and 40 vol% ZrB2–SiC, respectively. The corresponding chemical composition of specimens was listed in Table 1. In addition, when the raw powders of B4C and Si are not used, namely x = 0, Zr3[Al(Si)]4C6 ceramic can be obtained by the same process.
3. Results and discussion 3.1. Phase composition The XRD patterns of ZAS composites and Zr3[Al(Si)]4C6 ceramic are shown in Fig. 1. The pattern of the Zr3[Al(Si)]4C6 sample consists with the reports [1,6,7] and our previous works [18,22], indicating that a
Table 1 Chemical composition and sintering properties of prepared samples in the ZAS system. Samples
Zr3[Al(Si)]4C6 ZAS1 ZAS2 ZAS3 ZAS4
x values in reaction (1)
0 0.21 0.47 0.81 1.26
Chemical composition/vol% SiC
ZrB2
Zr3[Al(Si)]4C6
– 2.52 5.04 7.56 10.08
– 7.48 14.96 22.44 29.92
100 90 80 70 60
Measured density/g/cm3
Theoretical density/g/cm3
Relative density/%
Apparent porosity/%
4.76 4.87 4.91 4.97 5.02
4.85 4.90 4.95 5.00 5.05
98.4 ≥99
0.18 ≤0.12
2
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content, both the toughness and strength of the materials increase greatly. And, when incorporated ZrB2–SiC content is 30 vol%, the ZAS3 composite exhibits the highest toughness and strength of 7.59 ± 0.36 MPa m1/2 and 623 ± 30 MPa, respectively, which is about 79% and 86% higher than monolithic Zr3[Al(Si)]4C6 (4.24 ± 0.24 MPa m1/2, and 334 ± 26 MPa) ceramic, respectively. With further increasing ZrB2–SiC content to 40 vol%, an obvious decrease in bending strength can be observed, while the fracture toughness decreases slightly. The corresponding value of ZAS4 is 7.28 ± 0.26 MPa m1/2 and 522 ± 24 MPa, respectively, which is still 72% and 56% higher than Zr3[Al(Si)]4C6. And, as shown in Fig. 2(b), because of the high Vickers hardness of ZrB2 (~23 GPa) [24] and SiC (~25 GPa) [8], with the increasing content of ZrB2–SiC, the Vickers hardness of materials also show an obvious increase. It increases near linearly from 12.0 ± 0.3 GPa for Zr3[Al(Si)]4C6 to 18.6 ± 0.3 GPa for ZAS4. It is indicated that there is a linear relationship between the changes of Vickers hardness of the samples and the content of ZrB2–SiC.
HV = 12.64 + 15.8 Vr
(2)
Fig. 1. XRD patterns of ZAS composites and Zr3[Al(Si)]4C6 ceramic sintered at 1850 °C.
Where HV is the Vickers hardness of samples, Vr is the volume content of reinforcement. In addition, for the composites, the linear relationship is better, and the fitting formula is as follows:
phase pure Zr3[Al(Si)]4C6 was prepared at 1850 °C. According to the XRD patterns of the composites, only the peaks of Zr3[Al(Si)]4C6, ZrB2, and SiC can be identified. In addition, with increasing the content of ZrB2–SiC, the corresponding peak intensity of the two phases increases, while obvious decrease in intensity is observed for the peaks of Zr3[Al (Si)]4C6 at about 16.2°, 27.2°, 34.4°, and 39.3° et al. 2θ. The results of phase analysis preliminarily mean that the designed reactions can proceed completely and in-situ hot-pressed ZAS composites can be successfully prepared at 1850 °C. The results of sintering properties are listed in Table 1. It shows that all the specimens have a low porosity of ≤0.2% and a high relative density of ≥98%, indicating the sintered materials are dense. The incorporation of ZrB2–SiC is benefit for the sintering of the composites, and the composites are more dense than the Zr3[Al(Si)]4C6 ceramic, showing lower apparent porosity and higher relative density than Zr3[Al(Si)]4C6. In addition, the theoretical density of Zr3[Al(Si)]4C6 is 4.85 g/cm3 [7], and according to the rule of mixtures, the calculated theoretical density of ZrB2–SiC with a 2-to-1 molar ratio is about 5.37 g/cm3 [23]. So, with the increasing content of ZrB2–SiC, the measured density of materials increases from 4.78 g/cm3 to 5.04 g/cm3. The prepared composites all have a lower density than the most studied UHTC ZrB2-20 vol%SiC composite (5.72 g/cm3) [20]. The relatively low density makes ZAS more attractive for aerospace and other ultra-high temperature applications.
HV = 13.60 + 12.6 Vr
(3)
The above results show that the introduced binary reinforcements of ZrB2–SiC induce a significant toughening and strengthening effect, and the ZAS composite has improved mechanical properties. In addition, to obtain a Zr3[Al(Si)]4C6-matrix composite with optimum mechanical properties, the content of ZrB2–SiC reinforcements should not exceed 40 vol%, an optimum content of ZrB2–SiC may be around 30 vol% in the present work. In the previous work [18], a Zr3[Al(Si)]4C6-based composite incorporated with 30 vol% ZrB2–ZrC with a molar ratio of 2:1 (denoted as ZAC3) was successfully prepared at 1900 °C. Thus, two novel Zr3[Al (Si)]4C6-based composites were obtained in our previous and present works [18,22]. Some properties of the two Zr3[Al(Si)]4C6-based composites are compared in Table 2. It shows that the mechanical properties of ZAS3 prepared at 1850 °C in this work are almost the same as those of the composite prepared at 1900 °C. So, ZAS with excellent mechanical properties can be obtained at a lower temperature. In addition, ZAC also show excellent mechanical properties, however, given the poorer oxidation resistance of ZrC than that of SiC, the high temperature bending strength in air is only 84% of that of ZAS. These results render Zr3[Al(Si)]4C6–ZrB2–SiC composites as more promising UTHCs. 3.3. Microstructure
3.2. Mechanical properties TEM and EDS results of ZAS3 are shown in Fig. 3. According to the results of EDS area analysis of the grains shown in Fig. 3(e–g) and combining the XRD results above, the successful synthesis of Zr3[Al
Fig. 2 shows the mechanical properties of the samples. As can be seen from Fig. 2(a), with the incorporation and increase of ZrB2–SiC
Fig. 2. Mechanical properties of specimens incorporated with changed ZrB2–SiC content: (a) fracture toughness and bending strength, and (b) Vickers hardness. 3
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SEM images of crack propagation on the polished surfaces of the specimens are shown in Fig. 5. From Fig. 5(b), three phases with distinct brightness can be identified for the ZAS composite. According to the EDS analysis in our previous work [26], the black grains are SiC, the offwhite grains are ZrB2, and the gray grains are the Zr3[Al(Si)]4C6 matrix. As can be seen, both SiC grains and ZrB2 grains distribute homogenously in the matrix and have a fine grain size of about 1–3 μm. In addition, the growth of the Zr3[Al(Si)]4C6 grains can be effectively inhibited by the addition of ZrB2–SiC, and the composite exhibits a finegrained microstructure, showing a reduced grain size and increased aspect ratio of Zr3[Al(Si)]4C6 grains from 5-13 μm and 5 in Zr3[Al (Si)]4C6 to 2–6 μm and 7 in the composite, respectively. According to the Hall-Petch equation [16], apparently, a fine-grained strengthening effect will be generated.
Table 2 Comparison of partial properties of as-prepared ZAS composite and ZAC composite. Samples Properties
ZAC3 [18]
ZAS3 [22]
ZAS3
Sintering temperature/°C Measured density/g/cm3 Theoretical density/g/cm3 Relative density/% Bending strength/MPa Bending strength at 1300 °C in air/ MPa Fracture toughness/MPa·m1/2 Vickers hardness/GPa Young's modulus/GPa
1900 5.24 5.27 99.4 621 ± 23 369 ± 18
1900 4.98 5.00 99.6 648 ± 32 439 ± 12
1850 4.97 99.4 623 ± 30 /
7.37 ± 0.18 16.4 ± 0.2 415
7.69 ± 0.28 17.5 ± 0.2 418
7.59 ± 0.36 17.5 ± 0.2 /
σf = σ0 + (Si)]4C6–ZrB2–SiC composite is furthermore confirmed. From Fig. 3(a), it can be seen that the reinforcing phase ZrB2 presents a columnar morphology, and the length and aspect ratio of ZrB2 grain are about 1 μm and 2.5, respectively. In addition, as a result of the in-situ preparation process, it is notable that the reinforcing phases ZrB2 and SiC are embedded in the matrix grains, constituting an intragranular structure. It is benefit for inhibiting the growth of the Zr3[Al(Si)]4C6 grains and results in a toughening and strengthening effect. From Fig. 3(b), (c), and (d), it can be seen that the grain boundaries have no crystalline phases or other amorphous and are tightly connected, which can be ascribed to the advantages of in-situ preparation [25]. The SEM results of the fracture surfaces of the sintered materials are shown in Fig. 4. It can be seen that there are no obvious pores existed in the materials. Both the composites and Zr3[Al(Si)]4C6 are dense, which is consistent with the results of sintering properties. Similar to the predominantly transgranular fracture of MAX phases, a relatively flat morphology of the fracture surface of Zr3[Al(Si)]4C6 (Fig. 4(a)) can be observed. While the fracture surfaces of the composites (Fig. 4(b–e)) exhibit a rough morphology with the incorporation of ZrB2–SiC, showing mixed modes of intergranular and transgranular fracture. In addition, some obvious pits and outcrops, clearly shown in Fig. 4(f), can be observed due to grains’ pull out, which is more and more obvious with the increasing content of ZrB2–SiC. These morphological characteristics are benefit for the energy dissipation during fracture.
k d
(4)
Where σf is the yield strength of materials, σ0 is equivalent to the yield strength for a single crystal, k is a constant related to the structure of grain boundary in materials, and d is the average size of the grains in materials. Comparing the illustrations of (a) and (b) in Fig. 5, it can be seen that the cracks propagate in monolithic Zr3[Al(Si)]4C6 ceramic straightly without cark bridging. For the composite, the cracks propagate tortuously, and the carks deflect markedly when meeting the reinforcing particles. In addition, crack bridging can be observed in the composite. Such cark propagation in the composite can effectively dissipate energy during fracture. Due to the high modulus and hardness of both ZrB2 (E = 500 GPa, HV = 23 GPa) [24] and SiC (E = 400 GPa, HV = 25 GPa) [8], a residual stress toughening and high deformation resistance will be induced by the incorporation of ZrB2–SiC. In addition, as is known that after the as-sintered material cooled down, the mismatch of coefficient of thermal expansion (CTE) between different phases will induce the residual thermal stress in the particles and at the interfaces [16]. The residual stress at the reinforcement/matrix interfaces can be evaluated as follows [22]:
σr =
(α p − α m ) ΔT 1 + νm 2Em
+
1 − 2ν p Ep
(5)
Fig. 3. (a) TEM images of the composite with 30 vol% ZrB2–SiC, (b–d) HRTEM images of the interfaces, and EDS area results of the grains of (e) Zr3[Al(Si)]4C6, (f) ZrB2, and (g) SiC. 4
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Fig. 4. SEM images of the fracture surfaces of (a) Zr3[Al(Si)]4C6 and ZAS composites with (b) 10 vol%, (c) 20 vol%, (d) 30 vol%, and (e) 40 vol% ZrB2–SiC as well as (f) the zoomed-in micrograph.
Fig. 5. SEM images of the cracks on the polished surfaces of (a) Zr3[Al(Si)]4C6 ceramic, and ZAS composites with (b) 30 vol%, and (c) 40 vol% ZrB2–SiC, respectively.
σt = −
σr 2
be the reasons that the toughness and strength of the composite with 40 vol% ZrB2–SiC decrease. Nevertheless, ZAS4 still exhibits excellent mechanical properties and superior to Zr3[Al(Si)]4C6. To summary, the binary reinforcements of ZrB2–SiC produce remarkable toughening and strengthening effects to the composite by the synergistic mechanisms including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc.
(6)
Where σr and σt are the radial and tangential matrix stress, respectively, ΔT is the temperature range, αm, vm, and Em is the CTE, Poisson's ratio, and Young's modulus of the matrix material, respectively, and αp, vp, and Ep are the corresponding parameters of the particle phases. The CTEs of both ZrB2 (6.8 × 10−6 K−1) [24] and SiC (5.1 × 10−6 K−1) [8] are relatively low compared with Zr3[Al(Si)]4C6 (about 7.7 × 10−6 K−1) [7], and a negative σr can be estimated. It indicates that the residual thermal stresses at the interfaces of SiC–Zr3[Al(Si)]4C6 and ZrB2–Zr3[Al(Si)]4C6 are compressive radial stress and tensile tangential stress [16,27]. Therefore, the tensile tangential stress makes the cracks propagate readily in Zr3[Al(Si)]4C6, while the compressive radial stress induces obvious crack deflection and makes the carks propagate along the grain boundaries as the carks meet the SiC and ZrB2 grains, showing an inhibitory effect on the crack propagation. Also, the grain refinement effect is benefit for increasing the propagation paths of cracks. Ascribed to the above results, the cracks exhibit tortuous and prolonged propagation paths, and lots of energy are consumed during fracture [16,18,22], which is of significance for improving the toughness of composites. Though the introduction of binary ZrB2–SiC shows benefits for improving the mechanical properties, excess reinforcements definitely have opposite effects, which is a universal law and similar with other particle reinforced ceramic-matrix composites [21,28]. The incorporation of excess ZrB2–SiC is disadvantageous to the distribution uniformity of particles, resulting in the increased probability of particle agglomerations. The crack propagates directly through the agglomerated particles (circled in Fig. 5(c)) due to the weak interfacial stress in these particles, resulting in low energy dissipation. In addition, these agglomerated particles can be considered as second phases with larger grain sizes, which does not profit the mechanical properties. These may
3.4. Oxidation behavior As a representative, the oxidation resistance of ZAS3 composite and Zr3[Al(Si)]4C6 ceramic were tested and compared. Fig. 6 shows the weight gain per unit surface area of the two sintered ceramics (Δw/s) after oxidation at 1300 °C and 1400 °C, respectively. A rapidly increased Δw/s of Zr3[Al(Si)]4C6 and ZAS3 are observed at the initial oxidation stage under each condition. It can be inferred that the surfaces of materials are oxidized quickly and the oxide scales form immediately, which is benefit for inhibiting the further oxidation. With the increasing oxidation time, the Δw/s of the two materials show a slow increase at every temperature. It indicates that the oxidation process of materials is gradually into the oxidation passivation stage. When oxidized at the same conditions, ZAS3 shows a lower Δw/s than Zr3[Al(Si)]4C6. Moreover, with the increasing oxidation time, the difference is more obvious, especially at a higher temperature. After oxidation at 1400 °C for 5 h, the Δw/s of ZAS3 is about 0.394 kg m−2, which is reduced by about 36.6% compared with Zr3[Al(Si)]4C6 (about 0.621 kg m−2). So, the incorporated binary ZrB2–SiC show beneficial effects in the improvement of the oxidation resistance. The Δw/s squared of the two specimens, i.e. (Δw/s)2, for different oxidation time are plotted in Fig. 7. Based on the data, quite good linear fitting curves with coefficients of determination (R2) ≥ 0.99 are displayed. It indicates that the oxidation kinetics of both the samples accord with a parabolic law [26] in this 5
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Fig. 6. Weight gain per unit surface area of the samples (Δw/s) after oxidation at (a) 1300 °C and (b) 1400 °C, respectively.
Fig. 7. Square of Δw/s of the materials, i.e. (Δw/s)2, after oxidation at (a) 1300 °C and (b) 1400 °C, respectively.
substrate. Meanwhile, the kc of ZAS3 composite decreases from about 84% to about 42% of the corresponding values of Zr3[Al(Si)]4C6 ceramic. The above results show that the incorporated binary ZrB2–SiC can improve the oxidation resistance of materials, and the increasing temperature makes the improvement more significant. The XRD results of the oxide layers on the specimens after oxidation in different conditions are shown in Fig. 8. As can be seen from Fig. 8(a) and (c), both of Zr3[Al(Si)]4C6 and ZAS3 have been oxidized completely after oxidation at 1300 °C for 1 h, and the oxide scales of both the specimens consist of ZrO2, Al2O3, mullite, and ZrSiO4. The difference is that more obvious peaks of mullite and ZrSiO4 can be observed for the oxide scale of ZAS3 compared with Zr3[Al(Si)]4C6. With the further oxidation, the product composition of the oxide scale of ZAS3 has no significant change, while the peaks of Al2O3, mullite, and ZrSiO4 almost disappear, and the oxide scale of Zr3[Al(Si)]4C6 consist of mainly ZrO2. The typical appearances of the specimens after oxidation are presented in Fig. 8(b). It can be seen that Zr3[Al(Si)]4C6 suffered a more serious oxidation process, resulting in lots of oxide products formed on the surface of Zr3[Al(Si)]4C6. Meanwhile, Zr3[Al(Si)]4C6 presents a weaker combination between the unoxidized substrate and the oxide layer, and the exfoliation of the oxide layers can be observed after oxidation. By contrast, ZAS3 shows relatively light oxidation, and it retains a good apparent shape after oxidation. In addition, it is important to note that
Table 3 Calculated oxidation rate constant (kc) of Zr3[Al(Si)]4C6 and ZAS3. Samples
Oxidation temperature (°C)
kc ( × 10−5 kg2 m−4 s−1)
Zr3[Al(Si)]4C6
1300 1400 1300 1400
1.70 2.09 1.43 0.87
ZAS3
experiment condition:
(
Δw 2 ) = kc t s
(7)
Where kc is the oxidation constant, and t is the time for oxidation. Fig. 7 shows the fitting lines according to the above equation, and the obtained slopes represent kc, which can be seen in Table 3. The calculated kc of ZAS3 is obviously lower than Zr3[Al(Si)]4C6 when oxidized at the same temperature, which is consistent with the analysis results of Fig. 6. Generally, with the increasing oxidation temperature, the kc of materials will become greater, and the kc of Zr3[Al(Si)]4C6 increases from 1.70 × 10−5 (1300 °C) to 2.09 × 10−5 kg2 m−4 s−1 (1400 °C). But in the case of ZAS3, the kc decreases from 1.43 × 10−5 to 0.87 × 10−5 kg2 m−4 s−1 with the increasing temperature. This can be ascribed to the more effective formation of protective products on the
Fig. 8. XRD patterns of the oxide scales of (a) Zr3[Al(Si)]4C6 and (c) ZAS3, and (b) the typical macrographs of the specimens after oxidation. 6
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Fig. 9. Surface morphologies of (a, b) Zr3[Al(Si)]4C6 and (c, d) ZAS3 oxidized at (a, c) 1300 °C for 3 h and (b, d) 1400 °C for 3 h, respectively.
Fig. 10. Cross-sectional morphologies of (a, b) Zr3[Al(Si)]4C6 and (c, d) ZAS3 oxidized at (a, c) 1300 °C for 3 h and (b, d) 1400 °C for 3 h, respectively.
at 1300 °C and 1400 °C for 3 h, respectively. For Zr3[Al(Si)]4C6, there are many cracks and holes on the oxide surfaces, and the morphologies of the surface are relatively single and show a porous structure consisting of mainly ZrO2. For ZAS3, complicated morphologies can be
ZAS3 seems to show lighter oxidation when oxidized at 1400 °C than that at 1300 °C for the same oxidation time, which is consistent with the results of the oxidation kinetics. Fig. 9 shows the surface morphologies of specimens after oxidation 7
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References
Table 4 Oxidized layer thickness of Zr3[Al(Si)]4C6 and ZAS3. Samples
Oxidation temperature and time (°C, h)
Oxidized layer thickness (μm)
Zr3[Al(Si)]4C6
1300, 1400, 1300, 1400,
763 899 704 539
ZAS3
3 3 3 3
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observed after oxidation at 1300 °C for 3 h. The cracks and holes also can be observed, but the needle-like oxides, presumably mullite, show suture effects, which is benefit for improving the oxidation resistance. When oxidized at 1400 °C for 3 h, the oxidized surface of ZAS3 shows a more dense structure without cracks and pores. Owing to the higher content of Si in the composite, the formation of SiO2-rich glass phases can be obviously observed. The abundant glass phases can produce a filling and healing effect on the cracks and pores formed during oxidation, which will prevent further oxidation effectively [26]. The corresponding cross-sectional morphologies of samples after oxidation are shown in Fig. 10. Though cracks at the interfaces between the oxide scales and substrates can be observed for both the specimens, there are some adhesions (marked with arrows) at the interface for ZAS3, showing a relatively better combination between the substrate and the oxide scale than Zr3[Al(Si)]4C6. In addition, with the further oxidation, it is notable that the thickness of the oxide layer of ZAS3 decreases from about 704 μm to about 539 μm (listed in Table 4), while that of Zr3[Al (Si)]4C6 increases form about 763 μm to about 899 μm. Thus, the obtained results suggest that ZAS3 composite shows significantly superior oxidation resistance to Zr3[Al(Si)]4C6 ceramic, especially at a higher temperature. 4. Conclusions Dense in-situ Zr3[Al(Si)]4C6-based composites reinforced by 10–40 vol% ZrB2–SiC (2-to-1 molar ratio) have been prepared using hot pressing sintering at 1850 °C. In the composite, the reinforcement grains embed in the matrix grains, constituting an intragranular structure. The in-situ incorporated binary ZrB2–SiC produces remarkable toughening and strengthening effects to the composites by the synergistic mechanisms including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, after ZrB2–SiC incorporated, the oxidation kinetics of the composite follows the parabolic law, and its oxidation resistance is improved significantly when oxidized at 1300–1400 °C. The superior oxidation resistance of the composite can be owing to the formed protective oxides having more mullite and ZrSiO4, and SiO2-rich glass phases. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51902031), the Natural Science Foundation of the Jiangsu Higher Education Institute of China (No. 18KJB430002), and the Scientific Research Foundation of Changshu Institute of Technology (No. XZ1639).
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Multicomponent synergistically affected mechanical properties, microstructure, and oxidation resistance of Zr–Al(Si)–C based composites Xuhong Wanga,b,c, Wangjin Jia,b, Jian Hub, Hui Liua,b, Jianhao Zhangb, Zhefei Wangb,c, Lei Yua,b,c,∗, Shilong Yinb,c a
School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, China School of Chemistry and Materials Engineering, Changshu Institute of Technology, Changshu, 215500, China c Suzhou Key Laboratory of Functional Ceramic Materials, Changshu Institute of Technology, Changshu, 215500, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Zr3[Al(Si)]4C6–ZrB2–SiC Mechanical properties Oxidation behavior Ultra high-temperature ceramics (UHTCs) Synergistic mechanisms
Herein, in-situ Zr3[Al(Si)]4C6-based composites with 10–40 vol% ZrB2–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB2–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB2–SiC, while the hardness increases near linearly. Zr3[Al (Si)]4C6–ZrB2–SiC shows high strength (623 MPa), toughness (7.59 MPa m1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB2–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10−5 kg2 m−4 s−1 when oxidized at 1400 °C.
1. Introduction Owing to the combination of partial properties of both ceramics and metals, a new kind of Zr–Al(Si)–C layered quaternary compound beyond the MAX phases have attracted much attention since they were synthesized in 2007 [1,2]. As a typical representative, Zr3[Al(Si)]4C6 ceramic exhibits superior oxidation resistance, fracture toughness, and bending strength to the corresponding binary carbide ZrC [3–10]. Also, it shows better high-temperature mechanical properties than those of some MAX phases [4,5]. For instance, the strength of Zr3[Al(Si)]4C6 ceramic can maintain up to 1400 °C, and the residual ratio of its Young's modulus at 1600 °C can remain about 80% [4–7]. The excellent roomand high-temperature properties render Zr3[Al(Si)]4C6 as a potential material applicated in high or ultra-high temperature environments. And, the relatively low density (4.85 g/cm3) makes Zr3[Al(Si)]4C6 advantageous in aerospace applications with lightweight requirements. Nevertheless, it is very limited to be used in high-temperature environments in terms of its current performances. The fracture toughness (KIC = 4.62 MPa m1/2) of Zr3[Al(Si)]4C6 ceramic is obviously lower compared with some ternary layered carbides (e.g., Ti–Al/Si–C) [11], its hardness (HV = 12.4 GPa) is only about 50% of ZrC, and its strength (σc = 312 MPa) remains to be improved compared with other structural ∗
ceramics [12]. Furthermore, it shows unsatisfactory oxidation resistance during oxidation in the temperature range of 900 °C–1300 °C. Its oxidation kinetics almost follows a line law [3], and its oxidation rates increase very rapidly with the increasing temperature. To further improve the properties and applicability of Zr3[Al(Si)]4C6, effective approaches need to be designed. As one of the most studied UHTCs, ZrB2–SiC–ZrC composites exhibit much better mechanical properties, oxidation and ablation resistance, and thermal shock resistance than monolithic ZrB2, and also superior to ZrB2–SiC composites [13–15], which can be ascribed to the synergistic effect to the ZrB2 matrix produced by SiC–ZrC binary reinforcements. The achievements in ZrB2–SiC–ZrC composites imply that incorporation of appropriate phases can improve the comprehensive performance of materials effectively, and the synergistic mechanism between two phases in ceramic-matrix shows significantly toughening and strengthening effects [16,17]. Thus, various UHTCs systems with good performance are designed and fabricated, which is one of the main research focus in advanced ceramics. In our previous works, based on the ideals of multicomponent strengthening and toughening, a hotpressed Zr3[Al(Si)]4C6–ZrB2–ZrC ternary composite ceramic was prepared using in situ reaction method. The composite ceramic has attractive room-temperature mechanical properties (HV = 16.4 GPa,
Corresponding author. School of Chemistry and Materials Engineering, Changshu Institute of Technology, Changshu, 215500, China. E-mail address: [email protected] (L. Yu).
https://doi.org/10.1016/j.ceramint.2019.09.001 Received 12 June 2019; Received in revised form 19 August 2019; Accepted 1 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xuhong Wang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.001
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σc = 621 MPa, KIC = 7.37 MPa m1/2, etc.) [18]. Moreover, its hightemperature (1300 °C) strength in air is around 369 MPa, showing a 35% increase compared with Zr3[Al(Si)]4C6 (about 274 MPa) ceramic. However, its oxidation resistance is reduced because of a lack of effective oxidation protective layer. It is well known that SiC can be a good reinforcement to improve both the oxidation resistance and mechanical properties of materials [19–21]. Therefore, a novel Zr3[Al (Si)]4C6-based composite incorporated with binary ZrB2–SiC were prepared in our previous works [22]. The composite shows slightly better mechanical properties (σc = 648 MPa, KIC = 7.69 MPa m1/2, HV = 17.5 GPa, etc.) than Zr3[Al(Si)]4C6–ZrB2–ZrC [18,22]. Moreover, it can maintain a higher high-temperature bending strength (around 439 MPa) than Zr3[Al(Si)]4C6–ZrB2–ZrC under the same test conditions [18,22]. By comprehensive comparison, Zr3[Al(Si)]4C6–ZrB2–SiC composites possess much improved properties and exhibit a potential prospect as UHTCs. However, the previous works did not show the influence of ZrB2–SiC with a fixed ratio on the mechanical properties of materials prepared at a lower temperature. And also, the oxidation behavior of the corresponding composites has not been investigated, especially above 1300 °C. In this work, therefore, in-situ Zr3[Al(Si)]4C6-based composites incorporated by controlled ZrB2–SiC content (10–40 vol%) with a 2-to-1 molar ratio were prepared by at 1850 °C. And, the effects of ZrB2–SiC on the microstructure, mechanical properties, and the oxidation resistance of composites were discussed and investigated.
2.2. Characterization The Archimedes method was used to characterize the sintering properties of materials, including the porosity and density. And, according to the rule of mixtures, the theoretical densities of sintered specimens were calculated. The phase analysis of materials was identified by X-ray diffraction (XRD) (Rigaku Dmax-2200PC, Tokyo, Japan). A scanning electron microscopy (SEM, ZEISS SIGMA-500, Oberkochen, Germany) and a transmission electron microscope (TEM, JEOL JEM2010, Tokyo, Japan) were used to observe the microstructures of materials, including the polished and fracture surfaces, grains, and grain boundaries. Both of the SEM and TEM can be used to analyze the chemical composition via EDS within the instruments. 2.3. Mechanical properties test A universal mechanical instrument (SUNS UTM6203, Shenzhen, China) and an automatic hardness tester (TIME 6614AT, Beijing, China) were used to test the mechanical properties including the strength, the toughness, and the hardness of specimens, respectively. For the bending strength test, according to the three point bending (TPB) method, the specimens with the dimension of 36 mm × 4 mm × 3 mm were tested, and the loading speed and span length and were set to 0.5 mm/min and 30 mm, respectively. For the fracture toughness test, the specimens with the dimension of 22 mm × 2 mm × 4 mm were tested according to the single edge notched beams (SENB) method. The depth and width of the notch for specimens were 2 mm and 0.25 mm, respectively. Meanwhile, the span length and loading speed were set to 16 mm and 0.05 mm/min, respectively. While in hardness testing, the load was set to 9.8 N maintained for 10s.
2. Experimental procedure 2.1. Material preparation In-situ Zr3[Al(Si)]4C6–ZrB2–SiC (abbreviated as ZAS) composites were prepared by hot-pressing sintering at 1850 °C in flowing Ar atmosphere. Commercial powders of zirconium hydride (ZrH2, 99.9%, ≤37 μm), aluminum (Al, 99.9%, ≤48 μm), silicon (Si, 99.9%, ≤48 μm), graphite (C, 99%, ≤12 μm), and boron carbide (B4C, 99.9%, ≤10 μm) were used as starting materials. The detailed preparation process has been reported elsewhere [22], and the in-situ reaction is as follows:
2.4. Oxidation test The oxidation resistance of sintered specimens with a dimension of 8 mm × 4 mm × 3 mm was tested in a tube furnace (Kejing GSL-1500X50, Hefei, China) at 1300 °C, and 1400 °C, respectively. While testing, both ends of the tube furnace are connected with air and were inserted insulating bricks to maintaining the temperature in the constant temperature region. The specimens were put into the furnace immediately when the temperature rose to the set value. After oxidation for a period of time, the specimens were taken out directly. An electronic microbalance with the precision within 10−4 (Mettler Toledo ME204E, Shanghai, China) was used to weigh and measure the weight change of sintered samples before and after oxidation. Also, above SEM and XRD were used to analyze the microstructure and phase composition of the oxide products, respectively.
(2x+3)Zr + xB4C + xSi + 4Al(Si) + 6C = x(2ZrB2 + SiC) + Zr3[Al (Si)]4C6 (1) Here, according to the above reaction, ZAS composites with different ZrB2–SiC content of 10, 20, 30, and 40 vol% (x = 0.21, 0.47, 0.81, and 1.26, respectively) and given molar ratio of 2:1 for ZrB2 to SiC were designed and prepared. And, for the convenience in description, the composites were donated as ZAS1, ZAS2, ZAS3, and ZAS4, corresponding to the composites with 10, 20, 30, and 40 vol% ZrB2–SiC, respectively. The corresponding chemical composition of specimens was listed in Table 1. In addition, when the raw powders of B4C and Si are not used, namely x = 0, Zr3[Al(Si)]4C6 ceramic can be obtained by the same process.
3. Results and discussion 3.1. Phase composition The XRD patterns of ZAS composites and Zr3[Al(Si)]4C6 ceramic are shown in Fig. 1. The pattern of the Zr3[Al(Si)]4C6 sample consists with the reports [1,6,7] and our previous works [18,22], indicating that a
Table 1 Chemical composition and sintering properties of prepared samples in the ZAS system. Samples
Zr3[Al(Si)]4C6 ZAS1 ZAS2 ZAS3 ZAS4
x values in reaction (1)
0 0.21 0.47 0.81 1.26
Chemical composition/vol% SiC
ZrB2
Zr3[Al(Si)]4C6
– 2.52 5.04 7.56 10.08
– 7.48 14.96 22.44 29.92
100 90 80 70 60
Measured density/g/cm3
Theoretical density/g/cm3
Relative density/%
Apparent porosity/%
4.76 4.87 4.91 4.97 5.02
4.85 4.90 4.95 5.00 5.05
98.4 ≥99
0.18 ≤0.12
2
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content, both the toughness and strength of the materials increase greatly. And, when incorporated ZrB2–SiC content is 30 vol%, the ZAS3 composite exhibits the highest toughness and strength of 7.59 ± 0.36 MPa m1/2 and 623 ± 30 MPa, respectively, which is about 79% and 86% higher than monolithic Zr3[Al(Si)]4C6 (4.24 ± 0.24 MPa m1/2, and 334 ± 26 MPa) ceramic, respectively. With further increasing ZrB2–SiC content to 40 vol%, an obvious decrease in bending strength can be observed, while the fracture toughness decreases slightly. The corresponding value of ZAS4 is 7.28 ± 0.26 MPa m1/2 and 522 ± 24 MPa, respectively, which is still 72% and 56% higher than Zr3[Al(Si)]4C6. And, as shown in Fig. 2(b), because of the high Vickers hardness of ZrB2 (~23 GPa) [24] and SiC (~25 GPa) [8], with the increasing content of ZrB2–SiC, the Vickers hardness of materials also show an obvious increase. It increases near linearly from 12.0 ± 0.3 GPa for Zr3[Al(Si)]4C6 to 18.6 ± 0.3 GPa for ZAS4. It is indicated that there is a linear relationship between the changes of Vickers hardness of the samples and the content of ZrB2–SiC.
HV = 12.64 + 15.8 Vr
(2)
Fig. 1. XRD patterns of ZAS composites and Zr3[Al(Si)]4C6 ceramic sintered at 1850 °C.
Where HV is the Vickers hardness of samples, Vr is the volume content of reinforcement. In addition, for the composites, the linear relationship is better, and the fitting formula is as follows:
phase pure Zr3[Al(Si)]4C6 was prepared at 1850 °C. According to the XRD patterns of the composites, only the peaks of Zr3[Al(Si)]4C6, ZrB2, and SiC can be identified. In addition, with increasing the content of ZrB2–SiC, the corresponding peak intensity of the two phases increases, while obvious decrease in intensity is observed for the peaks of Zr3[Al (Si)]4C6 at about 16.2°, 27.2°, 34.4°, and 39.3° et al. 2θ. The results of phase analysis preliminarily mean that the designed reactions can proceed completely and in-situ hot-pressed ZAS composites can be successfully prepared at 1850 °C. The results of sintering properties are listed in Table 1. It shows that all the specimens have a low porosity of ≤0.2% and a high relative density of ≥98%, indicating the sintered materials are dense. The incorporation of ZrB2–SiC is benefit for the sintering of the composites, and the composites are more dense than the Zr3[Al(Si)]4C6 ceramic, showing lower apparent porosity and higher relative density than Zr3[Al(Si)]4C6. In addition, the theoretical density of Zr3[Al(Si)]4C6 is 4.85 g/cm3 [7], and according to the rule of mixtures, the calculated theoretical density of ZrB2–SiC with a 2-to-1 molar ratio is about 5.37 g/cm3 [23]. So, with the increasing content of ZrB2–SiC, the measured density of materials increases from 4.78 g/cm3 to 5.04 g/cm3. The prepared composites all have a lower density than the most studied UHTC ZrB2-20 vol%SiC composite (5.72 g/cm3) [20]. The relatively low density makes ZAS more attractive for aerospace and other ultra-high temperature applications.
HV = 13.60 + 12.6 Vr
(3)
The above results show that the introduced binary reinforcements of ZrB2–SiC induce a significant toughening and strengthening effect, and the ZAS composite has improved mechanical properties. In addition, to obtain a Zr3[Al(Si)]4C6-matrix composite with optimum mechanical properties, the content of ZrB2–SiC reinforcements should not exceed 40 vol%, an optimum content of ZrB2–SiC may be around 30 vol% in the present work. In the previous work [18], a Zr3[Al(Si)]4C6-based composite incorporated with 30 vol% ZrB2–ZrC with a molar ratio of 2:1 (denoted as ZAC3) was successfully prepared at 1900 °C. Thus, two novel Zr3[Al (Si)]4C6-based composites were obtained in our previous and present works [18,22]. Some properties of the two Zr3[Al(Si)]4C6-based composites are compared in Table 2. It shows that the mechanical properties of ZAS3 prepared at 1850 °C in this work are almost the same as those of the composite prepared at 1900 °C. So, ZAS with excellent mechanical properties can be obtained at a lower temperature. In addition, ZAC also show excellent mechanical properties, however, given the poorer oxidation resistance of ZrC than that of SiC, the high temperature bending strength in air is only 84% of that of ZAS. These results render Zr3[Al(Si)]4C6–ZrB2–SiC composites as more promising UTHCs. 3.3. Microstructure
3.2. Mechanical properties TEM and EDS results of ZAS3 are shown in Fig. 3. According to the results of EDS area analysis of the grains shown in Fig. 3(e–g) and combining the XRD results above, the successful synthesis of Zr3[Al
Fig. 2 shows the mechanical properties of the samples. As can be seen from Fig. 2(a), with the incorporation and increase of ZrB2–SiC
Fig. 2. Mechanical properties of specimens incorporated with changed ZrB2–SiC content: (a) fracture toughness and bending strength, and (b) Vickers hardness. 3
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SEM images of crack propagation on the polished surfaces of the specimens are shown in Fig. 5. From Fig. 5(b), three phases with distinct brightness can be identified for the ZAS composite. According to the EDS analysis in our previous work [26], the black grains are SiC, the offwhite grains are ZrB2, and the gray grains are the Zr3[Al(Si)]4C6 matrix. As can be seen, both SiC grains and ZrB2 grains distribute homogenously in the matrix and have a fine grain size of about 1–3 μm. In addition, the growth of the Zr3[Al(Si)]4C6 grains can be effectively inhibited by the addition of ZrB2–SiC, and the composite exhibits a finegrained microstructure, showing a reduced grain size and increased aspect ratio of Zr3[Al(Si)]4C6 grains from 5-13 μm and 5 in Zr3[Al (Si)]4C6 to 2–6 μm and 7 in the composite, respectively. According to the Hall-Petch equation [16], apparently, a fine-grained strengthening effect will be generated.
Table 2 Comparison of partial properties of as-prepared ZAS composite and ZAC composite. Samples Properties
ZAC3 [18]
ZAS3 [22]
ZAS3
Sintering temperature/°C Measured density/g/cm3 Theoretical density/g/cm3 Relative density/% Bending strength/MPa Bending strength at 1300 °C in air/ MPa Fracture toughness/MPa·m1/2 Vickers hardness/GPa Young's modulus/GPa
1900 5.24 5.27 99.4 621 ± 23 369 ± 18
1900 4.98 5.00 99.6 648 ± 32 439 ± 12
1850 4.97 99.4 623 ± 30 /
7.37 ± 0.18 16.4 ± 0.2 415
7.69 ± 0.28 17.5 ± 0.2 418
7.59 ± 0.36 17.5 ± 0.2 /
σf = σ0 + (Si)]4C6–ZrB2–SiC composite is furthermore confirmed. From Fig. 3(a), it can be seen that the reinforcing phase ZrB2 presents a columnar morphology, and the length and aspect ratio of ZrB2 grain are about 1 μm and 2.5, respectively. In addition, as a result of the in-situ preparation process, it is notable that the reinforcing phases ZrB2 and SiC are embedded in the matrix grains, constituting an intragranular structure. It is benefit for inhibiting the growth of the Zr3[Al(Si)]4C6 grains and results in a toughening and strengthening effect. From Fig. 3(b), (c), and (d), it can be seen that the grain boundaries have no crystalline phases or other amorphous and are tightly connected, which can be ascribed to the advantages of in-situ preparation [25]. The SEM results of the fracture surfaces of the sintered materials are shown in Fig. 4. It can be seen that there are no obvious pores existed in the materials. Both the composites and Zr3[Al(Si)]4C6 are dense, which is consistent with the results of sintering properties. Similar to the predominantly transgranular fracture of MAX phases, a relatively flat morphology of the fracture surface of Zr3[Al(Si)]4C6 (Fig. 4(a)) can be observed. While the fracture surfaces of the composites (Fig. 4(b–e)) exhibit a rough morphology with the incorporation of ZrB2–SiC, showing mixed modes of intergranular and transgranular fracture. In addition, some obvious pits and outcrops, clearly shown in Fig. 4(f), can be observed due to grains’ pull out, which is more and more obvious with the increasing content of ZrB2–SiC. These morphological characteristics are benefit for the energy dissipation during fracture.
k d
(4)
Where σf is the yield strength of materials, σ0 is equivalent to the yield strength for a single crystal, k is a constant related to the structure of grain boundary in materials, and d is the average size of the grains in materials. Comparing the illustrations of (a) and (b) in Fig. 5, it can be seen that the cracks propagate in monolithic Zr3[Al(Si)]4C6 ceramic straightly without cark bridging. For the composite, the cracks propagate tortuously, and the carks deflect markedly when meeting the reinforcing particles. In addition, crack bridging can be observed in the composite. Such cark propagation in the composite can effectively dissipate energy during fracture. Due to the high modulus and hardness of both ZrB2 (E = 500 GPa, HV = 23 GPa) [24] and SiC (E = 400 GPa, HV = 25 GPa) [8], a residual stress toughening and high deformation resistance will be induced by the incorporation of ZrB2–SiC. In addition, as is known that after the as-sintered material cooled down, the mismatch of coefficient of thermal expansion (CTE) between different phases will induce the residual thermal stress in the particles and at the interfaces [16]. The residual stress at the reinforcement/matrix interfaces can be evaluated as follows [22]:
σr =
(α p − α m ) ΔT 1 + νm 2Em
+
1 − 2ν p Ep
(5)
Fig. 3. (a) TEM images of the composite with 30 vol% ZrB2–SiC, (b–d) HRTEM images of the interfaces, and EDS area results of the grains of (e) Zr3[Al(Si)]4C6, (f) ZrB2, and (g) SiC. 4
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Fig. 4. SEM images of the fracture surfaces of (a) Zr3[Al(Si)]4C6 and ZAS composites with (b) 10 vol%, (c) 20 vol%, (d) 30 vol%, and (e) 40 vol% ZrB2–SiC as well as (f) the zoomed-in micrograph.
Fig. 5. SEM images of the cracks on the polished surfaces of (a) Zr3[Al(Si)]4C6 ceramic, and ZAS composites with (b) 30 vol%, and (c) 40 vol% ZrB2–SiC, respectively.
σt = −
σr 2
be the reasons that the toughness and strength of the composite with 40 vol% ZrB2–SiC decrease. Nevertheless, ZAS4 still exhibits excellent mechanical properties and superior to Zr3[Al(Si)]4C6. To summary, the binary reinforcements of ZrB2–SiC produce remarkable toughening and strengthening effects to the composite by the synergistic mechanisms including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc.
(6)
Where σr and σt are the radial and tangential matrix stress, respectively, ΔT is the temperature range, αm, vm, and Em is the CTE, Poisson's ratio, and Young's modulus of the matrix material, respectively, and αp, vp, and Ep are the corresponding parameters of the particle phases. The CTEs of both ZrB2 (6.8 × 10−6 K−1) [24] and SiC (5.1 × 10−6 K−1) [8] are relatively low compared with Zr3[Al(Si)]4C6 (about 7.7 × 10−6 K−1) [7], and a negative σr can be estimated. It indicates that the residual thermal stresses at the interfaces of SiC–Zr3[Al(Si)]4C6 and ZrB2–Zr3[Al(Si)]4C6 are compressive radial stress and tensile tangential stress [16,27]. Therefore, the tensile tangential stress makes the cracks propagate readily in Zr3[Al(Si)]4C6, while the compressive radial stress induces obvious crack deflection and makes the carks propagate along the grain boundaries as the carks meet the SiC and ZrB2 grains, showing an inhibitory effect on the crack propagation. Also, the grain refinement effect is benefit for increasing the propagation paths of cracks. Ascribed to the above results, the cracks exhibit tortuous and prolonged propagation paths, and lots of energy are consumed during fracture [16,18,22], which is of significance for improving the toughness of composites. Though the introduction of binary ZrB2–SiC shows benefits for improving the mechanical properties, excess reinforcements definitely have opposite effects, which is a universal law and similar with other particle reinforced ceramic-matrix composites [21,28]. The incorporation of excess ZrB2–SiC is disadvantageous to the distribution uniformity of particles, resulting in the increased probability of particle agglomerations. The crack propagates directly through the agglomerated particles (circled in Fig. 5(c)) due to the weak interfacial stress in these particles, resulting in low energy dissipation. In addition, these agglomerated particles can be considered as second phases with larger grain sizes, which does not profit the mechanical properties. These may
3.4. Oxidation behavior As a representative, the oxidation resistance of ZAS3 composite and Zr3[Al(Si)]4C6 ceramic were tested and compared. Fig. 6 shows the weight gain per unit surface area of the two sintered ceramics (Δw/s) after oxidation at 1300 °C and 1400 °C, respectively. A rapidly increased Δw/s of Zr3[Al(Si)]4C6 and ZAS3 are observed at the initial oxidation stage under each condition. It can be inferred that the surfaces of materials are oxidized quickly and the oxide scales form immediately, which is benefit for inhibiting the further oxidation. With the increasing oxidation time, the Δw/s of the two materials show a slow increase at every temperature. It indicates that the oxidation process of materials is gradually into the oxidation passivation stage. When oxidized at the same conditions, ZAS3 shows a lower Δw/s than Zr3[Al(Si)]4C6. Moreover, with the increasing oxidation time, the difference is more obvious, especially at a higher temperature. After oxidation at 1400 °C for 5 h, the Δw/s of ZAS3 is about 0.394 kg m−2, which is reduced by about 36.6% compared with Zr3[Al(Si)]4C6 (about 0.621 kg m−2). So, the incorporated binary ZrB2–SiC show beneficial effects in the improvement of the oxidation resistance. The Δw/s squared of the two specimens, i.e. (Δw/s)2, for different oxidation time are plotted in Fig. 7. Based on the data, quite good linear fitting curves with coefficients of determination (R2) ≥ 0.99 are displayed. It indicates that the oxidation kinetics of both the samples accord with a parabolic law [26] in this 5
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Fig. 6. Weight gain per unit surface area of the samples (Δw/s) after oxidation at (a) 1300 °C and (b) 1400 °C, respectively.
Fig. 7. Square of Δw/s of the materials, i.e. (Δw/s)2, after oxidation at (a) 1300 °C and (b) 1400 °C, respectively.
substrate. Meanwhile, the kc of ZAS3 composite decreases from about 84% to about 42% of the corresponding values of Zr3[Al(Si)]4C6 ceramic. The above results show that the incorporated binary ZrB2–SiC can improve the oxidation resistance of materials, and the increasing temperature makes the improvement more significant. The XRD results of the oxide layers on the specimens after oxidation in different conditions are shown in Fig. 8. As can be seen from Fig. 8(a) and (c), both of Zr3[Al(Si)]4C6 and ZAS3 have been oxidized completely after oxidation at 1300 °C for 1 h, and the oxide scales of both the specimens consist of ZrO2, Al2O3, mullite, and ZrSiO4. The difference is that more obvious peaks of mullite and ZrSiO4 can be observed for the oxide scale of ZAS3 compared with Zr3[Al(Si)]4C6. With the further oxidation, the product composition of the oxide scale of ZAS3 has no significant change, while the peaks of Al2O3, mullite, and ZrSiO4 almost disappear, and the oxide scale of Zr3[Al(Si)]4C6 consist of mainly ZrO2. The typical appearances of the specimens after oxidation are presented in Fig. 8(b). It can be seen that Zr3[Al(Si)]4C6 suffered a more serious oxidation process, resulting in lots of oxide products formed on the surface of Zr3[Al(Si)]4C6. Meanwhile, Zr3[Al(Si)]4C6 presents a weaker combination between the unoxidized substrate and the oxide layer, and the exfoliation of the oxide layers can be observed after oxidation. By contrast, ZAS3 shows relatively light oxidation, and it retains a good apparent shape after oxidation. In addition, it is important to note that
Table 3 Calculated oxidation rate constant (kc) of Zr3[Al(Si)]4C6 and ZAS3. Samples
Oxidation temperature (°C)
kc ( × 10−5 kg2 m−4 s−1)
Zr3[Al(Si)]4C6
1300 1400 1300 1400
1.70 2.09 1.43 0.87
ZAS3
experiment condition:
(
Δw 2 ) = kc t s
(7)
Where kc is the oxidation constant, and t is the time for oxidation. Fig. 7 shows the fitting lines according to the above equation, and the obtained slopes represent kc, which can be seen in Table 3. The calculated kc of ZAS3 is obviously lower than Zr3[Al(Si)]4C6 when oxidized at the same temperature, which is consistent with the analysis results of Fig. 6. Generally, with the increasing oxidation temperature, the kc of materials will become greater, and the kc of Zr3[Al(Si)]4C6 increases from 1.70 × 10−5 (1300 °C) to 2.09 × 10−5 kg2 m−4 s−1 (1400 °C). But in the case of ZAS3, the kc decreases from 1.43 × 10−5 to 0.87 × 10−5 kg2 m−4 s−1 with the increasing temperature. This can be ascribed to the more effective formation of protective products on the
Fig. 8. XRD patterns of the oxide scales of (a) Zr3[Al(Si)]4C6 and (c) ZAS3, and (b) the typical macrographs of the specimens after oxidation. 6
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Fig. 9. Surface morphologies of (a, b) Zr3[Al(Si)]4C6 and (c, d) ZAS3 oxidized at (a, c) 1300 °C for 3 h and (b, d) 1400 °C for 3 h, respectively.
Fig. 10. Cross-sectional morphologies of (a, b) Zr3[Al(Si)]4C6 and (c, d) ZAS3 oxidized at (a, c) 1300 °C for 3 h and (b, d) 1400 °C for 3 h, respectively.
at 1300 °C and 1400 °C for 3 h, respectively. For Zr3[Al(Si)]4C6, there are many cracks and holes on the oxide surfaces, and the morphologies of the surface are relatively single and show a porous structure consisting of mainly ZrO2. For ZAS3, complicated morphologies can be
ZAS3 seems to show lighter oxidation when oxidized at 1400 °C than that at 1300 °C for the same oxidation time, which is consistent with the results of the oxidation kinetics. Fig. 9 shows the surface morphologies of specimens after oxidation 7
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References
Table 4 Oxidized layer thickness of Zr3[Al(Si)]4C6 and ZAS3. Samples
Oxidation temperature and time (°C, h)
Oxidized layer thickness (μm)
Zr3[Al(Si)]4C6
1300, 1400, 1300, 1400,
763 899 704 539
ZAS3
3 3 3 3
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observed after oxidation at 1300 °C for 3 h. The cracks and holes also can be observed, but the needle-like oxides, presumably mullite, show suture effects, which is benefit for improving the oxidation resistance. When oxidized at 1400 °C for 3 h, the oxidized surface of ZAS3 shows a more dense structure without cracks and pores. Owing to the higher content of Si in the composite, the formation of SiO2-rich glass phases can be obviously observed. The abundant glass phases can produce a filling and healing effect on the cracks and pores formed during oxidation, which will prevent further oxidation effectively [26]. The corresponding cross-sectional morphologies of samples after oxidation are shown in Fig. 10. Though cracks at the interfaces between the oxide scales and substrates can be observed for both the specimens, there are some adhesions (marked with arrows) at the interface for ZAS3, showing a relatively better combination between the substrate and the oxide scale than Zr3[Al(Si)]4C6. In addition, with the further oxidation, it is notable that the thickness of the oxide layer of ZAS3 decreases from about 704 μm to about 539 μm (listed in Table 4), while that of Zr3[Al (Si)]4C6 increases form about 763 μm to about 899 μm. Thus, the obtained results suggest that ZAS3 composite shows significantly superior oxidation resistance to Zr3[Al(Si)]4C6 ceramic, especially at a higher temperature. 4. Conclusions Dense in-situ Zr3[Al(Si)]4C6-based composites reinforced by 10–40 vol% ZrB2–SiC (2-to-1 molar ratio) have been prepared using hot pressing sintering at 1850 °C. In the composite, the reinforcement grains embed in the matrix grains, constituting an intragranular structure. The in-situ incorporated binary ZrB2–SiC produces remarkable toughening and strengthening effects to the composites by the synergistic mechanisms including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, after ZrB2–SiC incorporated, the oxidation kinetics of the composite follows the parabolic law, and its oxidation resistance is improved significantly when oxidized at 1300–1400 °C. The superior oxidation resistance of the composite can be owing to the formed protective oxides having more mullite and ZrSiO4, and SiO2-rich glass phases. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51902031), the Natural Science Foundation of the Jiangsu Higher Education Institute of China (No. 18KJB430002), and the Scientific Research Foundation of Changshu Institute of Technology (No. XZ1639).
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