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Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites
Ceramics International 46 (2020) 156–164
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Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites a
Farhad Adibpur , Seyed Ali Tayebifard a b c
a,∗∗
b
, Mohammad Zakeri , Mehdi Shahedi Asl
T
c,∗
Semiconductors Department, Materials and Energy Research Center (MERC), Karaj, Iran Ceramic Department, Materials and Energy Research Center (MERC), Karaj, Iran Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Spark plasma sintering ZrB2–SiC composites ZrC Carbon fiber Mechanical properties Microstructure
Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB2–SiC–ZrC–Cf ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB2:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB2–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (Cf: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:Cf components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% Cf had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.
1. Introduction ZrB2, with a hexagonal crystal structure and powerful covalent bonds, has a high melting temperature about 3245 °C, high chemical stability and strength. Hence, these features of ZrB2 make it an attractive ultrahigh-temperature ceramic to be used as high-temperature electrodes, microelectronics, cutting tools, and refractory linings [1–7]. In addition to low density (6.119 g/cm3), ZrB2 has a relatively good electrical and thermal conductivity (1.0 × 107 S/m and 60 W/m.K, respectively). These properties makes it suitable for specific environments associated with the atmospheric re-entry and hypersonic flights [8–13]. Among other UHTC materials, these characteristics make it one of the best appropriate materials against the thermal shock resistance [14]. Making parts by electrical discharge machining processes (EDM) is another distinguishing feature of this material [15–17]. However, it is not sufficient for the ZrB2 to be used in raw form in industrial parts alone; therefore, getting it reinforced with SiC particulates is proved to be effective in intensification of both the oxidation resistance and room temperature strength [18–20]. Besides enjoying good ablative and oxidation resistance properties for ZrB2–SiC UHTCs, studies have proved that presence of SiC acts as a grain growth inhibitor [21,22]. Introducing 10, 20 and 30 vol% of SiC to ZrB2 ceramics, will not only improve the oxidation behavior up to 1600 °C through formation of
∗
less-volatile borosilicate glasses, but will also promote the mechanical properties [23,24]. It is established that adding 20 vol% SiC to ZrB2 will result in achievement of optimal high-temperature thermo-mechanical application for ZrB2–SiC composites [25]. Overally, excellent densification behavior, oxidation and thermal shock resistance, because of the smaller coefficient of thermal expansion compared to ZrB2 and ZrC, make silicon carbide one of the most important materials for making efficient composites with ZrB2 [26,27]. In ceramic composites, fabricated with the ZrB2 base, ZrC is a proper choice for reinforcement of materials. As for the ternary composite systems, consisting of the already mentioned ZrB2, SiC and ZrC, it has been revealed that under similar condition [26,28] the systems yield more excellent combination properties compared to those done by the ZrB2–SiC composites. It is proved that adding ZrC to boride and carbidebased composites will inhibit abnormal grain growth. Moreover, sinterability and mechanical properties may be promoted because of good compatibility of the ZrC (110) and ZrB2 (0001) crystallographic planes [28,29]. Carbon fiber reinforced ZrB2–ZrC–SiC ternary ceramics matrix has a better ablation and mechanical erosion resistance compared to the binary carbon reinforced SiC ceramic matrix. In addition, non-brittle fracture behavior and good mechanical properties achieved by carbon
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S.A. Tayebifard), [email protected] (M. Shahedi Asl).
∗∗
https://doi.org/10.1016/j.ceramint.2019.08.243 Received 18 July 2019; Received in revised form 25 August 2019; Accepted 26 August 2019 Available online 27 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Table 1 Mechanical properties of several ZrB2-based composites reported in the literature. Composite
Sintering route
ZrB2–SiC ZrB2–SiC ZrB2–ZrC ZrB2–SiC–Cf ZrB2–SiC–Cf ZrB2–SiC ZrB2–SiC ZrB2–SiC–ZrC ZrB2–SiC–Cf ZrB2–SiC ZrB2–SiC ZrB2–SiC–Cf ZrB2–SiC ZrB2–SiC Cf/ZrB2-SiC-ZrC ZrB2–SiC
HP HP SPS HP PS HP PS MASPS SPS SPS HP PS HP HP SPS
Mechanical Properties
Ref.
Flexural strength (MPa)
Fracture toughness (MPa.m
500 605 – 445 269 486 537 620 – 108.6 592 335 260 – 100 –
6.4 3.65 2.9–5.1 6.56 – 5.61 4.2 5.7 6.8 1.7 5.34 6.05 – 4.2 – 4.8
1/2
)
Elastic modulus (GPa)
Hardness (GPa)
– – – – – – 499 – – – – – – 494 – –
– 15.9 13.6–17.8 – – 15.94 22.4 19.3 14.8 – – – 17.5 – – 24.5
[60] [61] [58] [62] [63] [64] [24] [28] [43] [65] [66] [67] [68] [69] [70]
Table 2 Starting materials and their specifications. Material
Purity (%)
Particle size
Supplier
ZrB2 SiC ZrC Cf
> 99 > 99 > 99 > 99
< 5 μm < 5 μm ~ 10 μm d ~5 μm, L < 2 mm
Xuzhou Hongwu Xuzhou Hongwu Xuzhou Hongwu Toray Carbon fibers America Inc.
fiber reinforced ZrB2–ZrC–SiC composite [30–32]. Poor thermal shock tolerance is still a source of concern because of low fracture toughness of such ceramics. Introducing carbon fiber phase as a toughening and strengthening agent is a practical trick for enhancing mechanical properties because of its high-temperature endurance and ultrahigh specific strength and stiffness [33–35]. Carbon fiber has newly been used as a filler in the ZrB2 ceramics, resulting in nearly complete densification, while increasing the degree of fracture toughness by about 15% (the hardness and thermal conductivity did not vary, however.) [25]. Microstructurally investigating, it is revealed that carbon fiber acts as a sintering aid and as eliminator of the oxide impurities (including ZrO2, B2O3 and SiO2) from the raw materials’ surfaces [36–39]. Constructing ceramic matrix composites using ZrB2, SiC and ZrC is proved in many processing routes such as reactive hot pressing (RHP), hot pressing (HP), pressureless sintering (PS) and spark plasma sintering (SPS). Among these sintering processes, spending less sintering time, and achieving higher mechanical characteristics are significant signs of SPS [28,40,41]. Since borides and carbides have strong covalent bonding, their sintering characteristics are poor, so their densification is difficult through conventional sintering processes. Therefore, SPS technique is extremely favored to reduce the sintering time and temperature, while enhancing the mechanical properties due to the better densification [29,42]. Shahedi Asl et al. [43] showed that the most effective process parameter for sintering ZrB2–20 vol% SiC–10 vol % Cf composite is temperature (1850 °C) followed by soaking time (6 min) under 30 MPa punch pressure by applying SPS process. Loaded sample powder in a graphite die with punching it from top and bottom and passing direct current through it, generate spark discharge and/or plasma between the particles, causes rust removing from particle's surface, so diffusion process from grain boundaries will be enhanced for better densification [44–56]. Moreover, low sintering temperature and short process time will prevent abnormal grain growth [57,58]. Also there are many researches on ZrB2 and its composites, but among them, the ones that have mostly focused on mechanical properties have been studied [59]. Loehman et al. [60] built ZrB2–SiC composites with 0, 2, 5 and
Fig. 1. SEM images of size and morphology of raw materials: (a) ZrB2 (b) SiC (c) ZrC and (d) Cf.
20 vol% SiC, being processed by hot pressing under 2000 °C and 34.5 MPa process condition. The best results were obtained for ZrB2–20 vol% SiC with 4–6 MPa m1/2 for toughness and 450–500 MPa for strength. Zhang et al. [61] compared ZrB2 and ZrB2–20 vol% SiC from microstructure and mechanical points of view. They were made of from 1 to 5 μm for ~0.5 μm particle size in order to be applied in ZrB2 and αSiC, respectively, through the hot pressing process. Adding 20 vol% SiC to ZrB2 resulted in grain refinement and grain growth prevention, while changing the fracture mode from transgranular to transgranular/intergranular fracture mode. Detected strength mechanism was grain refinement and for toughness, crack deflection, crack branching and grain refinement. The best mechanical properties were observed for ZrB2–20 vol% SiC with 605 MPa and 3.65 MPa m1/2 in terms of strength and toughness respectively. Yang et al. [62] compared ZrB2–20 vol% SiC and ZrB2–20 vol% Cf–20 vol% SiC from mechanical points of view. The hot pressing process was employed for producing samples at 2000 °C, 30 MPa and 1 h sintering time conditions. Adding carbon fiber to ZrB2–20 vol% SiC, 157
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Fig. 2. Selection path to determine the amounts of ZrC and Cf additives for five samples with a constant ZrC:Cf ratio of 2:1.
564.7 MPa flexural strength and 5.52 MPa m1/2 fracture toughness. Zhang et al. [24] selected pressureless sintering process for producing ZrB2–SiC composites, which for them nominated SiC volume percentage included 10, 20 and 30%. Sintering occurred at 2000 °C for 3 h. This research revealed that SiC content prevents grain growth, while changing such mechanical properties as flexural strength and toughness to 492 MPa from 404 and to 3.5 MPa m1/2 from 3, respectively. Emami et al. [28] produced ZrB2–SiC–ZrC composites through mechanical activation spark plasma synthesis (MASPSyn) process. For this purpose, selected SiC content was about 31.6 vol% and ZrC content varied from 7.5 to 19.5 vol% and what remained was balanced by ZrB2 as the matrix. The best results for flexural strength and toughness were obtained for 15 vol% of ZrC content by measuring 620 MPa and 5.7 MPa m1/2, respectively. Tsuchida et al. [58] used MA-SHS process to obtain ZrB2 and ZrC from elemental raw materials to produce ZrB2–ZrC composites through the SPS process. The mechanical characteristics of the composites, estimated by Vickers indentation technique, showed the fracture toughness of 2.9–5.1 MPa m1/2 and Vickers hardness of 13.6–17.8 GPa, depending on the molar ratio of ZrB2/ZrC. Shahedi Asl et al. [43] investigated the hardness and fracture toughness of ZrB2–20 vol% SiC–10 vol% Cf composite prepared by the SPS route. They revealed that the optimal attainable properties
Table 3 Chemical composition and theoretical density of composite samples. Sample No.
ZrB2 (vol%)
SiC (vol%)
ZrC (vol%)
Cf (vol%)
ρth (g/cm3)
1 2 3 4 5
80 77 74 71 68
20 19.25 18.5 17.75 17
0 2.5 5 7.5 10
0 1.25 2.5 3.75 5
5.506 5.490 5.475 5.459 5.443
increased the fracture toughness from 4.25 to 6.56 MPa m1/2, while decreasing the flexural strength 445 MPa from 502 and that of density to 4.63 g/cm3 from 5.5. Detected mechanisms for toughness increase are detachment and pull out carbon fiber from the matrix and crack deflection. Graphitization in grain boundaries caused decrease of flexural strength. Padmavathi et al. [63] selected SiC as matrix and ZrB2 contents were changed by 5, 10, 15 and 20 vol% by impregnation process and sintering at 1600 °C and 1700 °C which resulted in sintering at 1600 °C, times better than that for samples at 1700 °C. Li et al. [64] employed the hot pressing process at 1750 °C for ZrB2–15 vol% SiC plus 10, 15 and 20 vol% of MoSi2 as sintering aid. The best results achieved for ZrB2–15 vol% SiC–15 vol% MoSi2 with
Fig. 3. Displacement-time-temperature curve for sample No. 3. 158
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Fig. 4. Relative density of samples No. 1–5 after the sintering process.
Fig. 5. XRD patterns of as-sintered samples No. 1–5.
carbon fibers were purchased as starting materials. The specifications of these materials, including purity, particle size and suppliers, are reported in Table 2 and their morphologies are displayed in Fig. 1. Five composite samples, with different compositions, were prepared using mixtures of the above-mentioned starting materials. Based on the literature on ZrB2–SiC composites, in order to get an acceptable combination of mechanical properties, the volume ratio of ZrB2:SiC powders were chosen 4:1 in all samples. The composite mixtures were arranged by the addition of 0–10 vol% ZrC and 0–5 vol% Cf as reinforcements to the ZrB2–SiC baseline with a constant volume ratio of 2:1 for ZrC:Cf. The details of composition of samples are presented in Fig. 2 and Table 3. The dry-mixing process was carried out using a low-energy ballmilling (200 rpm) with a ball to powder ratio of 5:1 in an argon atmosphere for 2 h. It's worth noting that the soft carbon fibers were not added to the cup during the milling process. They were initially dispersed in acetone-ethanol solution by ultrasonication and then the dry mixture was added to the slurry. Beaker of mixed starting materials slurry was located on heating magnetic stirrer under a laboratory fume
(14.8 GPa and 6.8 MPa m1/2 for hardness and fracture toughness, respectively) were achieved at the 1850 °C temperature, at the dwelling time of 6 min and the pressure level of 30 MPa. The mechanical characteristics reported by the researchers are summarized in Table 1. This research is distinctive for and follows the following purposes to report improvement in relative density, flexural strength and hardness in the considered composites simultaneously. Concurrent increase of both Cf and ZrC additives in the ZrB2–SiC composites and investigation of their impact on mechanical properties of the base composite (ZrB2–20 vol% SiC) are the major objectives of this research. Furthermore, the method of sampling multiple components composites, genuinely used by the authors, marks one more instance of novelty initiated by the authors within the scope of this research.
2. Methodology 2.1. Materials and process Commercially attainable ZrB2, SiC and ZrC powders as well as short 159
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Fig. 6. High-magnification SEM images of the polished surfaces of (a) sample No. 1 and (b) sample No. 5 and corresponding EDS spectra of (c) zone A (d) zone B and (e) zone C.
3. Results and discussion
hood for drying the slurry. Each of the prepared mixtures was loaded into a graphite die with an inner diameter of 30 mm, while being and covered by flexible graphite foils. The composite samples were fabricated by vacuum spark plasma sintering at 1800 °C temperature for 5 min dwelling time under 30 MPa pressure using a DC current rate of 400 A/min up to a final current of 2700 A. The thickness of such sintered disk-shaped samples was ~6 mm.
Fig. 3 displays the displacement-time-temperature (DTT) curve for sample No. 3 which can be extended to other samples due to their similar trends in temperature and compaction variations. After 5.5 min from the beginning of the SPS process, a sharp increase occurred in the temperature up to the level of 1100 °C. This can be due to growing electrical conductivity of the sample during the sintering. It should be noted that the electrical conductivity is low at the lower temperatures due to the significant amount of porosity between the powder particles. It is shown that the electrical conductivity increases due to the connection of the powders together [71]. Increased electrical conductivity will encourage, more electrical current leading through the sample, prompting faster sample heat up. Such a remarkable increase in the temperature may be due to this warming phenomenon. No significant progress in displacement is seen up to 8 min because the densification condition is not appropriate enough for all particles to be rearranged that can fill the voids between themselves. First remarkable displacement with a gentle slope up to 2 mm occurred from the 8th to the 12th minute when the sample had reached the final sintering temperature. In the 12th minute, the final pressure of 30 MPa was applied on the powder compact for 5 min at about 1800 °C. The main displacement happened in this stage from 2 mm up to more than 6 mm as a result of good progression of the sintering process. Finally, the sample was cooled down by disconnecting the electricity and removing the applied pressure from the graphite die. Relative density values of the composite samples after the SPS process are presented in Fig. 4. With an average value of 99.58%, the highest relative density belongs to sample No. 3. The results of water absorption and open porosity measurements (not reported here), with the lowest values for sample No. 3 compared to other ones, are in good
2.2. Characterization The Archimedes principles were used to measure the bulk density of sintered composites. Samples relative density was calculated through ASTM C373-88: B = D/(M-S)
(1)
where B, D, M and S are bulk density, dry mass, saturated mass and impregnated specimen mass while suspended in the distilled water, respectively. X-ray diffraction (XRD) analysis was made for the phase characterization of the fabricated samples using a Philips-PW3710 diffractometer. A field emission scanning electron microscope (FESEM), equipped with an energy dispersive spectroscope (EDS), was employed for microstructural and chemical analyses. The hardness of samples was evaluated by a Koopa Vickers hardness testing machine with 5 kg load for 10 s. Flexural strength was measured based on ASTM C1161-02c standard by Santam test machine STM-20 model. Test of the samples with dimensions of 1.5 × 2 × 25 mm3 (prepared via electro-discharge machining) is done under 0.2 mm/min pressing rate with 20 mm span length.
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Fig. 7. Low-magnification SEM images of the polished surfaces of (a) sample No. 2 (b) sample No. 3 (c) sample No. 4 and (d) sample No. 5 and (e) EDS spectrum of carbon fiber (marked by D arrows).
Fig. 8. Flexural strength of composite samples No. 1–5.
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Fig. 9. SEM fractography of samples (a) No. 1 (b) No. 2 (c) No. 3 (d) No. 4 and (e) No. 5.
Fig. 10. Hardness of composite samples No. 1–5.
Obviously, because of the detection of only starting materials in the sintered bulks, it can be claimed that no new in-situ phase was formed during the SPS process (Figs. 6–7). Hence, the sintering of quadruplet ZrB2–SiC–ZrC–Cf composites seems to be non-reactive. Flexural strength values of the composite samples are presented in Fig. 8. A close look into the data shows that, the baseline composite (sample No. 1) has the lowest flexural strength (393 MPa) compared to the ZrC and Cf co-reinforced ones. The highest flexural strength with a value 565 MPa belongs to sample No. 3. However, by adding higher amounts of such reinforcements (> 5 vol% ZrC and > 2.5 vol% Cf), the flexural strength will decrease and get dropped to the point of 401 MPa in the sample No. 5. Fig. 9 shows the SEM micrographs of the fracture surfaces of the thus sintered composites. As the figure shows, the dominant fracture mode of ZrB2 grains in the sample No. 1 is transgranular, which has resulted in weak flexural strength for the baseline ZrB2–SiC composite.
harmony with this outcome. Sample No. 1, as the baseline which is free of ZrC and Cf additives, reached a relative density of 93.66%. The low relative density of this sample can not only be related to the insufficient sintering temperature, but can also to presence of oxide impurities on the surface of ZrB2 and SiC particles in the absence of carbon additive as a reductant/sintering aid. This is while; the relative density enhances (samples No. 2&3) by the addition of ZrC and Cf up to 5 vol% and 2.5 vol%, respectively. Such positive roles of ZrC and Cf additions on densification behavior of ZrB2–SiC composites were also reported in the literature [28,36]. Anyway, by adding higher amounts of these reinforcements, the negative role of extra Cf on densification can be expected since the relative densities of samples No. 4 and No. 5 were dropped to 95.38% and 90.67%, respectively. Such a low relative density of sample No. 5 can be related to the agglomeration of carbon fibers in some areas. Fig. 5 shows the XRD patterns of the sintered composite samples. 162
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Introducing the appropriate amounts of ZrC and Cf additives (samples No. 2 and 3) changes the fracture mode dramatically to intergranular from transgranular. In other words, the cracks tended to spread in ZrB2/ ZrC interfaces rather than inside the ZrB2 grains, which in turn strengthened those samples. Sharp drop in the flexural strength values of sample No. 4 can be related to overwhelming impact of the Cf addition on flexural strength compared to that of ZrC addition (Table 1). As for the Sample No. 5, the Cf addition will not only undermine the in flexural strength, but will also cause unbalanced distribution of the carbon fibers as their agglomeration in the sintered bulks will intensify this reduction. The presence of such problems in the structure of composite ceramics will result in weakening and wide deviations of mechanical characteristics (Fig. 8). Generally speaking, the variations in flexural strength of investigated composites can be due to three factors. The first factor is the grain size and the strength of materials increases at the cost of decrease in the grain size. The relative density of the sintered sample is the second factor as growth in the relative density, the better progression of the sintering process, will enhance the flexural strength. Since sample No. 3 is a fully dense composite (Fig. 4) and its relative density has the highest value compared to that of other samples of this research, it can be claimed that it should have the highest flexural strength. The third factor is presence of the ZrC additive, which intensifies the strength by inhibiting the growth of ZrB2 grains via locating as a barrier at the grain boundaries [72,73]. Based on the mentioned factors, which certainly correlate with each other, achieving the highest value of 565 MPa for sample No. 3 seems to be logical. The hardness values of composite samples after the SPS process are presented in Fig. 10. The curve shows that with increase in the amounts of ZrC and Cf additives in the composition of ZrB2–SiC-based UHTCs, the hardness starts degrading from 19.7 GPa (sample No. 1) to 14.8 GPa (sample No. 5).
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4. Conclusions This study investigated the impact of the amount of ZrC (0–10 vol%) and carbon fiber (0–5 vol%) on the densification and mechanical properties of quadruplet ZrB2–SiC–ZrC–Cf composite within the ratios of ZrB2/SiC = 4 and ZrC/Cf = 2 for all samples. Sintering process was carried out at the temperature of 1800 °C for the dwelling time of 5 min under the punch press of 30 MPa in the vacuum atmosphere. The highest performance, i.e. with a relative density of 99.6%, a hardness of 18 GPa, and a flexural strength of 565 MPa was observed in the ZrB2–18.5 vol% SiC composite being co-reinforced with 5 vol% ZrC and 2.5 vol% Cf. References [1] E. Ghasali, M. Shahedi Asl, Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite, Ceram. Int. 44 (2018) 18078–18083, https://doi. org/10.1016/j.ceramint.2018.07.011. [2] M. Shahedi Asl, A statistical approach towards processing optimization of ZrB2–SiC–graphite nanocomposites. Part I: relative density, Ceram. Int. 44 (2018) 6935–6939, https://doi.org/10.1016/j.ceramint.2018.01.122. [3] M. Shahedi Asl, B. Nayebi, M.G. Kakroudi, M. Shokouhimehr, Investigation of hot pressed ZrB2–SiC–carbon black nanocomposite by scanning and transmission electron microscopy, Ceram. Int. (2019), https://doi.org/10.1016/j.ceramint.2019. 05.211. [4] Z. Ahmadi, M. Zakeri, A. Habibi-Yangjeh, M. Shahedi Asl, A novel ZrB2–C3N4 composite with improved mechanical properties, Ceram. Int. (2019), https://doi. org/10.1016/j.ceramint.2019.07.144. [5] M. Shahedi Asl, B. Nayebi, M. Shokouhimehr, TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite, Ceram. Int. 44 (2018) 15269–15273, https://doi.org/10.1016/j.ceramint.2018.05.170. [6] M. Shahedi Asl, B. Nayebi, A. Motallebzadeh, M. Shokouhimehr, Nanoindentation and nanostructural characterization of ZrB2–SiC composite doped with graphite nano-flakes, Compos. B Eng. 175 (2019) 107153, https://doi.org/10.1016/j. compositesb.2019.107153. [7] M. Khoeini, A. Nemati, M. Zakeri, M. Shahedi Asl, Pressureless sintering of ZrB2 ceramics codoped with TiC and graphite, Int. J. Refract. Metals Hard Mater. 81 (2019) 189–195, https://doi.org/10.1016/j.ijrmhm.2019.02.026.
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Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites a
Farhad Adibpur , Seyed Ali Tayebifard a b c
a,∗∗
b
, Mohammad Zakeri , Mehdi Shahedi Asl
T
c,∗
Semiconductors Department, Materials and Energy Research Center (MERC), Karaj, Iran Ceramic Department, Materials and Energy Research Center (MERC), Karaj, Iran Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Spark plasma sintering ZrB2–SiC composites ZrC Carbon fiber Mechanical properties Microstructure
Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB2–SiC–ZrC–Cf ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB2:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB2–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (Cf: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:Cf components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% Cf had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.
1. Introduction ZrB2, with a hexagonal crystal structure and powerful covalent bonds, has a high melting temperature about 3245 °C, high chemical stability and strength. Hence, these features of ZrB2 make it an attractive ultrahigh-temperature ceramic to be used as high-temperature electrodes, microelectronics, cutting tools, and refractory linings [1–7]. In addition to low density (6.119 g/cm3), ZrB2 has a relatively good electrical and thermal conductivity (1.0 × 107 S/m and 60 W/m.K, respectively). These properties makes it suitable for specific environments associated with the atmospheric re-entry and hypersonic flights [8–13]. Among other UHTC materials, these characteristics make it one of the best appropriate materials against the thermal shock resistance [14]. Making parts by electrical discharge machining processes (EDM) is another distinguishing feature of this material [15–17]. However, it is not sufficient for the ZrB2 to be used in raw form in industrial parts alone; therefore, getting it reinforced with SiC particulates is proved to be effective in intensification of both the oxidation resistance and room temperature strength [18–20]. Besides enjoying good ablative and oxidation resistance properties for ZrB2–SiC UHTCs, studies have proved that presence of SiC acts as a grain growth inhibitor [21,22]. Introducing 10, 20 and 30 vol% of SiC to ZrB2 ceramics, will not only improve the oxidation behavior up to 1600 °C through formation of
∗
less-volatile borosilicate glasses, but will also promote the mechanical properties [23,24]. It is established that adding 20 vol% SiC to ZrB2 will result in achievement of optimal high-temperature thermo-mechanical application for ZrB2–SiC composites [25]. Overally, excellent densification behavior, oxidation and thermal shock resistance, because of the smaller coefficient of thermal expansion compared to ZrB2 and ZrC, make silicon carbide one of the most important materials for making efficient composites with ZrB2 [26,27]. In ceramic composites, fabricated with the ZrB2 base, ZrC is a proper choice for reinforcement of materials. As for the ternary composite systems, consisting of the already mentioned ZrB2, SiC and ZrC, it has been revealed that under similar condition [26,28] the systems yield more excellent combination properties compared to those done by the ZrB2–SiC composites. It is proved that adding ZrC to boride and carbidebased composites will inhibit abnormal grain growth. Moreover, sinterability and mechanical properties may be promoted because of good compatibility of the ZrC (110) and ZrB2 (0001) crystallographic planes [28,29]. Carbon fiber reinforced ZrB2–ZrC–SiC ternary ceramics matrix has a better ablation and mechanical erosion resistance compared to the binary carbon reinforced SiC ceramic matrix. In addition, non-brittle fracture behavior and good mechanical properties achieved by carbon
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S.A. Tayebifard), [email protected] (M. Shahedi Asl).
∗∗
https://doi.org/10.1016/j.ceramint.2019.08.243 Received 18 July 2019; Received in revised form 25 August 2019; Accepted 26 August 2019 Available online 27 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Table 1 Mechanical properties of several ZrB2-based composites reported in the literature. Composite
Sintering route
ZrB2–SiC ZrB2–SiC ZrB2–ZrC ZrB2–SiC–Cf ZrB2–SiC–Cf ZrB2–SiC ZrB2–SiC ZrB2–SiC–ZrC ZrB2–SiC–Cf ZrB2–SiC ZrB2–SiC ZrB2–SiC–Cf ZrB2–SiC ZrB2–SiC Cf/ZrB2-SiC-ZrC ZrB2–SiC
HP HP SPS HP PS HP PS MASPS SPS SPS HP PS HP HP SPS
Mechanical Properties
Ref.
Flexural strength (MPa)
Fracture toughness (MPa.m
500 605 – 445 269 486 537 620 – 108.6 592 335 260 – 100 –
6.4 3.65 2.9–5.1 6.56 – 5.61 4.2 5.7 6.8 1.7 5.34 6.05 – 4.2 – 4.8
1/2
)
Elastic modulus (GPa)
Hardness (GPa)
– – – – – – 499 – – – – – – 494 – –
– 15.9 13.6–17.8 – – 15.94 22.4 19.3 14.8 – – – 17.5 – – 24.5
[60] [61] [58] [62] [63] [64] [24] [28] [43] [65] [66] [67] [68] [69] [70]
Table 2 Starting materials and their specifications. Material
Purity (%)
Particle size
Supplier
ZrB2 SiC ZrC Cf
> 99 > 99 > 99 > 99
< 5 μm < 5 μm ~ 10 μm d ~5 μm, L < 2 mm
Xuzhou Hongwu Xuzhou Hongwu Xuzhou Hongwu Toray Carbon fibers America Inc.
fiber reinforced ZrB2–ZrC–SiC composite [30–32]. Poor thermal shock tolerance is still a source of concern because of low fracture toughness of such ceramics. Introducing carbon fiber phase as a toughening and strengthening agent is a practical trick for enhancing mechanical properties because of its high-temperature endurance and ultrahigh specific strength and stiffness [33–35]. Carbon fiber has newly been used as a filler in the ZrB2 ceramics, resulting in nearly complete densification, while increasing the degree of fracture toughness by about 15% (the hardness and thermal conductivity did not vary, however.) [25]. Microstructurally investigating, it is revealed that carbon fiber acts as a sintering aid and as eliminator of the oxide impurities (including ZrO2, B2O3 and SiO2) from the raw materials’ surfaces [36–39]. Constructing ceramic matrix composites using ZrB2, SiC and ZrC is proved in many processing routes such as reactive hot pressing (RHP), hot pressing (HP), pressureless sintering (PS) and spark plasma sintering (SPS). Among these sintering processes, spending less sintering time, and achieving higher mechanical characteristics are significant signs of SPS [28,40,41]. Since borides and carbides have strong covalent bonding, their sintering characteristics are poor, so their densification is difficult through conventional sintering processes. Therefore, SPS technique is extremely favored to reduce the sintering time and temperature, while enhancing the mechanical properties due to the better densification [29,42]. Shahedi Asl et al. [43] showed that the most effective process parameter for sintering ZrB2–20 vol% SiC–10 vol % Cf composite is temperature (1850 °C) followed by soaking time (6 min) under 30 MPa punch pressure by applying SPS process. Loaded sample powder in a graphite die with punching it from top and bottom and passing direct current through it, generate spark discharge and/or plasma between the particles, causes rust removing from particle's surface, so diffusion process from grain boundaries will be enhanced for better densification [44–56]. Moreover, low sintering temperature and short process time will prevent abnormal grain growth [57,58]. Also there are many researches on ZrB2 and its composites, but among them, the ones that have mostly focused on mechanical properties have been studied [59]. Loehman et al. [60] built ZrB2–SiC composites with 0, 2, 5 and
Fig. 1. SEM images of size and morphology of raw materials: (a) ZrB2 (b) SiC (c) ZrC and (d) Cf.
20 vol% SiC, being processed by hot pressing under 2000 °C and 34.5 MPa process condition. The best results were obtained for ZrB2–20 vol% SiC with 4–6 MPa m1/2 for toughness and 450–500 MPa for strength. Zhang et al. [61] compared ZrB2 and ZrB2–20 vol% SiC from microstructure and mechanical points of view. They were made of from 1 to 5 μm for ~0.5 μm particle size in order to be applied in ZrB2 and αSiC, respectively, through the hot pressing process. Adding 20 vol% SiC to ZrB2 resulted in grain refinement and grain growth prevention, while changing the fracture mode from transgranular to transgranular/intergranular fracture mode. Detected strength mechanism was grain refinement and for toughness, crack deflection, crack branching and grain refinement. The best mechanical properties were observed for ZrB2–20 vol% SiC with 605 MPa and 3.65 MPa m1/2 in terms of strength and toughness respectively. Yang et al. [62] compared ZrB2–20 vol% SiC and ZrB2–20 vol% Cf–20 vol% SiC from mechanical points of view. The hot pressing process was employed for producing samples at 2000 °C, 30 MPa and 1 h sintering time conditions. Adding carbon fiber to ZrB2–20 vol% SiC, 157
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Fig. 2. Selection path to determine the amounts of ZrC and Cf additives for five samples with a constant ZrC:Cf ratio of 2:1.
564.7 MPa flexural strength and 5.52 MPa m1/2 fracture toughness. Zhang et al. [24] selected pressureless sintering process for producing ZrB2–SiC composites, which for them nominated SiC volume percentage included 10, 20 and 30%. Sintering occurred at 2000 °C for 3 h. This research revealed that SiC content prevents grain growth, while changing such mechanical properties as flexural strength and toughness to 492 MPa from 404 and to 3.5 MPa m1/2 from 3, respectively. Emami et al. [28] produced ZrB2–SiC–ZrC composites through mechanical activation spark plasma synthesis (MASPSyn) process. For this purpose, selected SiC content was about 31.6 vol% and ZrC content varied from 7.5 to 19.5 vol% and what remained was balanced by ZrB2 as the matrix. The best results for flexural strength and toughness were obtained for 15 vol% of ZrC content by measuring 620 MPa and 5.7 MPa m1/2, respectively. Tsuchida et al. [58] used MA-SHS process to obtain ZrB2 and ZrC from elemental raw materials to produce ZrB2–ZrC composites through the SPS process. The mechanical characteristics of the composites, estimated by Vickers indentation technique, showed the fracture toughness of 2.9–5.1 MPa m1/2 and Vickers hardness of 13.6–17.8 GPa, depending on the molar ratio of ZrB2/ZrC. Shahedi Asl et al. [43] investigated the hardness and fracture toughness of ZrB2–20 vol% SiC–10 vol% Cf composite prepared by the SPS route. They revealed that the optimal attainable properties
Table 3 Chemical composition and theoretical density of composite samples. Sample No.
ZrB2 (vol%)
SiC (vol%)
ZrC (vol%)
Cf (vol%)
ρth (g/cm3)
1 2 3 4 5
80 77 74 71 68
20 19.25 18.5 17.75 17
0 2.5 5 7.5 10
0 1.25 2.5 3.75 5
5.506 5.490 5.475 5.459 5.443
increased the fracture toughness from 4.25 to 6.56 MPa m1/2, while decreasing the flexural strength 445 MPa from 502 and that of density to 4.63 g/cm3 from 5.5. Detected mechanisms for toughness increase are detachment and pull out carbon fiber from the matrix and crack deflection. Graphitization in grain boundaries caused decrease of flexural strength. Padmavathi et al. [63] selected SiC as matrix and ZrB2 contents were changed by 5, 10, 15 and 20 vol% by impregnation process and sintering at 1600 °C and 1700 °C which resulted in sintering at 1600 °C, times better than that for samples at 1700 °C. Li et al. [64] employed the hot pressing process at 1750 °C for ZrB2–15 vol% SiC plus 10, 15 and 20 vol% of MoSi2 as sintering aid. The best results achieved for ZrB2–15 vol% SiC–15 vol% MoSi2 with
Fig. 3. Displacement-time-temperature curve for sample No. 3. 158
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Fig. 4. Relative density of samples No. 1–5 after the sintering process.
Fig. 5. XRD patterns of as-sintered samples No. 1–5.
carbon fibers were purchased as starting materials. The specifications of these materials, including purity, particle size and suppliers, are reported in Table 2 and their morphologies are displayed in Fig. 1. Five composite samples, with different compositions, were prepared using mixtures of the above-mentioned starting materials. Based on the literature on ZrB2–SiC composites, in order to get an acceptable combination of mechanical properties, the volume ratio of ZrB2:SiC powders were chosen 4:1 in all samples. The composite mixtures were arranged by the addition of 0–10 vol% ZrC and 0–5 vol% Cf as reinforcements to the ZrB2–SiC baseline with a constant volume ratio of 2:1 for ZrC:Cf. The details of composition of samples are presented in Fig. 2 and Table 3. The dry-mixing process was carried out using a low-energy ballmilling (200 rpm) with a ball to powder ratio of 5:1 in an argon atmosphere for 2 h. It's worth noting that the soft carbon fibers were not added to the cup during the milling process. They were initially dispersed in acetone-ethanol solution by ultrasonication and then the dry mixture was added to the slurry. Beaker of mixed starting materials slurry was located on heating magnetic stirrer under a laboratory fume
(14.8 GPa and 6.8 MPa m1/2 for hardness and fracture toughness, respectively) were achieved at the 1850 °C temperature, at the dwelling time of 6 min and the pressure level of 30 MPa. The mechanical characteristics reported by the researchers are summarized in Table 1. This research is distinctive for and follows the following purposes to report improvement in relative density, flexural strength and hardness in the considered composites simultaneously. Concurrent increase of both Cf and ZrC additives in the ZrB2–SiC composites and investigation of their impact on mechanical properties of the base composite (ZrB2–20 vol% SiC) are the major objectives of this research. Furthermore, the method of sampling multiple components composites, genuinely used by the authors, marks one more instance of novelty initiated by the authors within the scope of this research.
2. Methodology 2.1. Materials and process Commercially attainable ZrB2, SiC and ZrC powders as well as short 159
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Fig. 6. High-magnification SEM images of the polished surfaces of (a) sample No. 1 and (b) sample No. 5 and corresponding EDS spectra of (c) zone A (d) zone B and (e) zone C.
3. Results and discussion
hood for drying the slurry. Each of the prepared mixtures was loaded into a graphite die with an inner diameter of 30 mm, while being and covered by flexible graphite foils. The composite samples were fabricated by vacuum spark plasma sintering at 1800 °C temperature for 5 min dwelling time under 30 MPa pressure using a DC current rate of 400 A/min up to a final current of 2700 A. The thickness of such sintered disk-shaped samples was ~6 mm.
Fig. 3 displays the displacement-time-temperature (DTT) curve for sample No. 3 which can be extended to other samples due to their similar trends in temperature and compaction variations. After 5.5 min from the beginning of the SPS process, a sharp increase occurred in the temperature up to the level of 1100 °C. This can be due to growing electrical conductivity of the sample during the sintering. It should be noted that the electrical conductivity is low at the lower temperatures due to the significant amount of porosity between the powder particles. It is shown that the electrical conductivity increases due to the connection of the powders together [71]. Increased electrical conductivity will encourage, more electrical current leading through the sample, prompting faster sample heat up. Such a remarkable increase in the temperature may be due to this warming phenomenon. No significant progress in displacement is seen up to 8 min because the densification condition is not appropriate enough for all particles to be rearranged that can fill the voids between themselves. First remarkable displacement with a gentle slope up to 2 mm occurred from the 8th to the 12th minute when the sample had reached the final sintering temperature. In the 12th minute, the final pressure of 30 MPa was applied on the powder compact for 5 min at about 1800 °C. The main displacement happened in this stage from 2 mm up to more than 6 mm as a result of good progression of the sintering process. Finally, the sample was cooled down by disconnecting the electricity and removing the applied pressure from the graphite die. Relative density values of the composite samples after the SPS process are presented in Fig. 4. With an average value of 99.58%, the highest relative density belongs to sample No. 3. The results of water absorption and open porosity measurements (not reported here), with the lowest values for sample No. 3 compared to other ones, are in good
2.2. Characterization The Archimedes principles were used to measure the bulk density of sintered composites. Samples relative density was calculated through ASTM C373-88: B = D/(M-S)
(1)
where B, D, M and S are bulk density, dry mass, saturated mass and impregnated specimen mass while suspended in the distilled water, respectively. X-ray diffraction (XRD) analysis was made for the phase characterization of the fabricated samples using a Philips-PW3710 diffractometer. A field emission scanning electron microscope (FESEM), equipped with an energy dispersive spectroscope (EDS), was employed for microstructural and chemical analyses. The hardness of samples was evaluated by a Koopa Vickers hardness testing machine with 5 kg load for 10 s. Flexural strength was measured based on ASTM C1161-02c standard by Santam test machine STM-20 model. Test of the samples with dimensions of 1.5 × 2 × 25 mm3 (prepared via electro-discharge machining) is done under 0.2 mm/min pressing rate with 20 mm span length.
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Fig. 7. Low-magnification SEM images of the polished surfaces of (a) sample No. 2 (b) sample No. 3 (c) sample No. 4 and (d) sample No. 5 and (e) EDS spectrum of carbon fiber (marked by D arrows).
Fig. 8. Flexural strength of composite samples No. 1–5.
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Fig. 9. SEM fractography of samples (a) No. 1 (b) No. 2 (c) No. 3 (d) No. 4 and (e) No. 5.
Fig. 10. Hardness of composite samples No. 1–5.
Obviously, because of the detection of only starting materials in the sintered bulks, it can be claimed that no new in-situ phase was formed during the SPS process (Figs. 6–7). Hence, the sintering of quadruplet ZrB2–SiC–ZrC–Cf composites seems to be non-reactive. Flexural strength values of the composite samples are presented in Fig. 8. A close look into the data shows that, the baseline composite (sample No. 1) has the lowest flexural strength (393 MPa) compared to the ZrC and Cf co-reinforced ones. The highest flexural strength with a value 565 MPa belongs to sample No. 3. However, by adding higher amounts of such reinforcements (> 5 vol% ZrC and > 2.5 vol% Cf), the flexural strength will decrease and get dropped to the point of 401 MPa in the sample No. 5. Fig. 9 shows the SEM micrographs of the fracture surfaces of the thus sintered composites. As the figure shows, the dominant fracture mode of ZrB2 grains in the sample No. 1 is transgranular, which has resulted in weak flexural strength for the baseline ZrB2–SiC composite.
harmony with this outcome. Sample No. 1, as the baseline which is free of ZrC and Cf additives, reached a relative density of 93.66%. The low relative density of this sample can not only be related to the insufficient sintering temperature, but can also to presence of oxide impurities on the surface of ZrB2 and SiC particles in the absence of carbon additive as a reductant/sintering aid. This is while; the relative density enhances (samples No. 2&3) by the addition of ZrC and Cf up to 5 vol% and 2.5 vol%, respectively. Such positive roles of ZrC and Cf additions on densification behavior of ZrB2–SiC composites were also reported in the literature [28,36]. Anyway, by adding higher amounts of these reinforcements, the negative role of extra Cf on densification can be expected since the relative densities of samples No. 4 and No. 5 were dropped to 95.38% and 90.67%, respectively. Such a low relative density of sample No. 5 can be related to the agglomeration of carbon fibers in some areas. Fig. 5 shows the XRD patterns of the sintered composite samples. 162
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Introducing the appropriate amounts of ZrC and Cf additives (samples No. 2 and 3) changes the fracture mode dramatically to intergranular from transgranular. In other words, the cracks tended to spread in ZrB2/ ZrC interfaces rather than inside the ZrB2 grains, which in turn strengthened those samples. Sharp drop in the flexural strength values of sample No. 4 can be related to overwhelming impact of the Cf addition on flexural strength compared to that of ZrC addition (Table 1). As for the Sample No. 5, the Cf addition will not only undermine the in flexural strength, but will also cause unbalanced distribution of the carbon fibers as their agglomeration in the sintered bulks will intensify this reduction. The presence of such problems in the structure of composite ceramics will result in weakening and wide deviations of mechanical characteristics (Fig. 8). Generally speaking, the variations in flexural strength of investigated composites can be due to three factors. The first factor is the grain size and the strength of materials increases at the cost of decrease in the grain size. The relative density of the sintered sample is the second factor as growth in the relative density, the better progression of the sintering process, will enhance the flexural strength. Since sample No. 3 is a fully dense composite (Fig. 4) and its relative density has the highest value compared to that of other samples of this research, it can be claimed that it should have the highest flexural strength. The third factor is presence of the ZrC additive, which intensifies the strength by inhibiting the growth of ZrB2 grains via locating as a barrier at the grain boundaries [72,73]. Based on the mentioned factors, which certainly correlate with each other, achieving the highest value of 565 MPa for sample No. 3 seems to be logical. The hardness values of composite samples after the SPS process are presented in Fig. 10. The curve shows that with increase in the amounts of ZrC and Cf additives in the composition of ZrB2–SiC-based UHTCs, the hardness starts degrading from 19.7 GPa (sample No. 1) to 14.8 GPa (sample No. 5).
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4. Conclusions This study investigated the impact of the amount of ZrC (0–10 vol%) and carbon fiber (0–5 vol%) on the densification and mechanical properties of quadruplet ZrB2–SiC–ZrC–Cf composite within the ratios of ZrB2/SiC = 4 and ZrC/Cf = 2 for all samples. Sintering process was carried out at the temperature of 1800 °C for the dwelling time of 5 min under the punch press of 30 MPa in the vacuum atmosphere. The highest performance, i.e. with a relative density of 99.6%, a hardness of 18 GPa, and a flexural strength of 565 MPa was observed in the ZrB2–18.5 vol% SiC composite being co-reinforced with 5 vol% ZrC and 2.5 vol% Cf. References [1] E. Ghasali, M. Shahedi Asl, Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite, Ceram. Int. 44 (2018) 18078–18083, https://doi. org/10.1016/j.ceramint.2018.07.011. [2] M. Shahedi Asl, A statistical approach towards processing optimization of ZrB2–SiC–graphite nanocomposites. Part I: relative density, Ceram. Int. 44 (2018) 6935–6939, https://doi.org/10.1016/j.ceramint.2018.01.122. [3] M. Shahedi Asl, B. Nayebi, M.G. Kakroudi, M. Shokouhimehr, Investigation of hot pressed ZrB2–SiC–carbon black nanocomposite by scanning and transmission electron microscopy, Ceram. Int. (2019), https://doi.org/10.1016/j.ceramint.2019. 05.211. [4] Z. Ahmadi, M. Zakeri, A. Habibi-Yangjeh, M. Shahedi Asl, A novel ZrB2–C3N4 composite with improved mechanical properties, Ceram. Int. (2019), https://doi. org/10.1016/j.ceramint.2019.07.144. [5] M. Shahedi Asl, B. Nayebi, M. Shokouhimehr, TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite, Ceram. Int. 44 (2018) 15269–15273, https://doi.org/10.1016/j.ceramint.2018.05.170. [6] M. Shahedi Asl, B. Nayebi, A. Motallebzadeh, M. Shokouhimehr, Nanoindentation and nanostructural characterization of ZrB2–SiC composite doped with graphite nano-flakes, Compos. B Eng. 175 (2019) 107153, https://doi.org/10.1016/j. compositesb.2019.107153. [7] M. Khoeini, A. Nemati, M. Zakeri, M. Shahedi Asl, Pressureless sintering of ZrB2 ceramics codoped with TiC and graphite, Int. J. Refract. Metals Hard Mater. 81 (2019) 189–195, https://doi.org/10.1016/j.ijrmhm.2019.02.026.
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