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Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth
Author’s Accepted Manuscript Synergetic effects of SiC and C sf in ZrB2-based ceramic composites. Part II: Grain growth Mehdi Shahedi Asl, Mahdi Ghassemi Kakroudi, Iman Farahbakhsh, Babak Mazinani, Zohre Balak www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)31527-9 http://dx.doi.org/10.1016/j.ceramint.2016.08.205 CERI13657
To appear in: Ceramics International Received date: 1 August 2016 Revised date: 30 August 2016 Accepted date: 31 August 2016 Cite this article as: Mehdi Shahedi Asl, Mahdi Ghassemi Kakroudi, Iman Farahbakhsh, Babak Mazinani and Zohre Balak, Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.205 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth Mehdi Shahedi Asla*, Mahdi Ghassemi Kakroudib, Iman Farahbakhshc, Babak Mazinani d, Zohre Balake a
Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran
b c
Department of Mechanical Engineering, Quchan Branch, Islamic Azad University, Quchan, Iran
d e
Department of Materials Science and Engineering, University of Tabriz, Tabriz, Iran
Department of Materials Engineering, Malayer University, Malayer, Iran
Department of Materials Science and Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
*
Corresponding author. Tel.: +98 912 3277186, [email protected] (M. Shahedi Asl)
Abstract The synergetic effects SiC particles and short carbon fibers (Csf) as well as hot pressing parameters (sintering temperature, dwell time and applied pressure) on the grain growth of ZrB2-based composites were investigated. Taguchi methodology was employed for the design of experiments to study the microstructure and grain growth of ZrB2–SiC–Csf ceramic composites. Three hot pressing parameters and SiC/Csf ratio were selected as the scrutinized variables. The sintering temperature and SiC/Csf ratio were identified by ANOVA as the most effective variables on the gain growth of ZrB2-based samples. Removal of oxide impurities from the surface of starting particles by the reactant Csf, not only hindered the extraordinary grain growth of ZrB2 matrix, but also improved the sinterability of the ceramics. A fully dense ceramic with an average grain size of 8.3 µm was obtained by hot pressing at 1850 °C for 30 min under 16 MPa through adding 20 vol% SiC and 10 vol% Csf to the ZrB2 matrix. SEM observations and EDS analysis verified the in-situ formation of ZrC which can restrain the growth of ZrB2 particles, similar to the role of SiC, by the pinning of grain boundaries as another stationary secondary phase.
Keywords Hot pressing; ZrB2; SiC; Carbon fiber; Taguchi method; Grain growth. 1. Introduction Ultra high temperature ceramics (UHTCs) include carbides, borides and nitrides of transition metals which have melting point over 3000 °C. Zirconium diboride (ZrB2), as a member of UHTCs, has strong covalent bonding together with metallic characteristic, high hardness, high Young’s modulus, and high thermal and electrical conductivities. This coalition of properties, made ZrB2 as a proper candidate for several applications such as molten metal crucibles, cutting tools, and thermal protection systems (TPS) in aerospace industry. Nevertheless, achieving a fully dense ZrB2 is impossible even by hot pressing at a high temperature of 2000 °C, because the densification and grain growth occur simultaneously [1-9]. The industrial significance of grain growth comes from the dependence of densification behavior and mechanical properties (e.g. strength and fracture toughness) on the final grain size of the sintered microstructure. Hence, an appropriate comprehension of the grain growth phenomenon is an important requirement for controlling the microstructure and characteristics of the material during the manufacturing process [10-11]. Several parameters such as the size of starting materials, the presence of impurities (for example, oxide impurities on the surface of non-oxide ceramics), and the sintering conditions (temperature, dwell time and applied external pressure) affect the grain growth during the consolidation process [12]. Hot pressing is recognized as an efficient processing method to decrease the sintering temperature, in order to obtain fully dense and synchronously finegrained ceramics [13]. The addition of SiC was recognized as a key reinforcement phase which decelerates the rate of grain growth in ZrB2-based and other non-oxide ceramics [14-15]. In other words, SiC acts as an inhibitor of movement of ZrB2 grain boundaries at elevated sintering temperatures [16-117]. A kinetical analysis verified that the coarsening rate of ZrB2 matrix was controlled by the grain boundary immobilizing effect of SiC particles. However, SiC phase with higher proportion effectively prevented ZrB2 grain growth during the sintering
process [18]. With decreasing the size of SiC additive, the number of particles which can restrain the grain growth of ZrB2 increases; therefore, the densification behavior meaningfully improves [19]. Additionally, the oxide impurities of ZrO2 and B2O3 on the surface of starting powder not only hinder the densification but also motivate the grain coarsening [20-21]. Because B2O3 exists as a liquid or vapor phase during the sintering process which provides a rapid diffusion path [22]. A similar behavior has been reported for other diborides such as TiB2 and HfB2 [13-14, 23]. Carbon additives, for example in the form of carbon black [24-26], fiber [27-30], nanotube [31-33], graphite [34-39] or graphene [40-41], can react with and eliminate the surface oxide impurities which promote the densification process by minimizing the grain growth of ZrB2. In this research, SiC and Csf with different ratios were chosen as additives and several sintering temperature, times and pressures were selected as hot pressing variables. Although, the investigation of all mentioned variables seems not to be economically possible, the Taguchi method was used to design of experiments as an optimization plan. The goal of this paper, as the second part of a series of papers, is to study the influences of processing variables on the microstructure development of ZrB2–SiC–Csf ceramics. Additionally, the statistical investigations based on the signal to noise (S/N) ratios and the analysis of variance (ANOVA) were employed to determine the significances of processing variables on the grain growth of zirconium diboride. The effects of processing variables on the densification behavior and mechanical properties of ZrB2-based composites were reported in Part I [42] and will be reported in Part III, respectively.
2. Experimental procedure 2.1. Design of experiment The experiments were designed established upon the Taguchi methodology to conclude the influences of four processing variables on the microstructure development and grain growth of ZrB2-based composites. This technique is more economical than the classical methods, because minimum numbers of experiments are needed to supply a notable insight about the effects of variables on the investigated output. For example, if four processing variables
with three levels are mentioned for investigation, only a series of arrays with nine incorporations of input parameters (L9 orthogonal array) can be employed. The processing variables and selected levels in this research are displayed in Table 1 and the designed L9 arrays by the Taguchi method are presented in Table 2. Generally, the output characteristics can be analyzed in three exclusive classifications; smaller is better, higher is better or nominal is better. In this paper, as achieving a finegrained microstructure was desirable, the statistical analysis of ZrB2 grain size was fulfilled applying the smaller is better option. Hence, the S/N ratios for this option were evaluated according to the subsequent equation [43-44]:
(1)
S 1 n 10 log yi2 N n i 1
where yi (i=1, 2 …n) are the response values and n is the number of repetitions. Furthermore, the analysis of variance (ANOVA) was accomplished to determine the significance of processing variables on the mean ZrB2 grain size in the sintered microstructure. The data analyses were carried out using Qualitek-4 software (Automatic design and analysis of Taguchi experiments, Nutek Inc., Michigan, USA).
2.2. Materials and process In the present work, zirconium diboride (ZrB2: particle size ∼2 µm) and silicon carbide (SiC: particle size ~5 μm) powders as well as short carbon fibers (Csf: length ~1 mm, diameter ~8.5 µm) were purchased as raw materials from Leung Hi–tech Co. (China), Carborundum Universal Ltd. (India) and Torayca Sigmatex Ltd. (UK), respectively. As per technical data sheets provided by the suppliers, the purity of starting materials are higher than 99%. Three ZrB2-based mixed powders containing 25 vol% SiC–5 vol% Csf, 20 vol% SiC–10 vol% Csf and 15 vol% SiC–15 vol% Csf were prepared. At the beginning, the short carbon fibers were dispersed in ethanol for 0.5 h using an ultrasonic agitator (265-watt, Sonicator Q500, USA). Afterwards, the stoichiometric amounts of ZrB2 and SiC powders in each
batch were added to the slurry and mixed for 0.5 h by zirconia balls in a polyethylene cup and then dried at 110 °C for 24 h in a rotary evaporator (Tebazma HMS 14, Iran). Mixed batch powders were hot pressed at 1700, 1775 or 1850 °C in vacuum atmosphere (0.05 Pa) in a graphite element hot press furnace (Shenyang Weitai Science & Technology Development Co., Ltd., China) under a pressure of 8, 12 or 16 MPa and a dwell time of 30, 60 or 90 min. To minimize reactions between graphite die and powder mixture, inner wall of the die was coated with BN and lined with flexible graphite foil before the precursors were loaded. Diameter and thickness of the hot pressed pellets were around 25 and 5 mm, respectively.
2.3. Characterization The relative density of sintered samples was calculated using the Archimedes’ principle and the rule of mixtures. The microstructural and elemental analyses of the hot pressed composites were performed by field emission scanning electron microscopy (FESEM: Mira3 Tescan, Czech Republic) and energy dispersive spectroscopy (DXP-X10P Digital Xray Processor), respectively. The average ZrB2 grain size in the sintered microstructures was determined by ImageJ, (Image analysis software: Wayne Rasband, National Institute of Health, USA).
3. Results and discussion Table 3 presents the relative density and the mean ZrB2 grain size in the microstructure of composite samples which were hot pressed at different processing conditions. This table shows that the relative density and grain size values ranged from ~80% to ~100% and 4.6 µm to 9.2 µm, respectively, throughout the processing conditions of this research. Fig. 1 shows the microstructures of fracture surfaces of the composites sintered at 1700 °C. The porous microstructure of sample 1 (Fig. 1(a)), which was sintered at the weakest conditions of current research, is in consistent with its low relative density (~80%). A white colored cauliflower-like phase is clearly seen in this micrograph (marked by the arrow). A closer view of morphology of such a phase is shown in Fig. 2(a). EDS spectrum of this figure is displayed in Fig. 2(b), which verifies the presence of oxygen, zirconium and boron
because of the expected presence of oxide impurities such as ZrO2 and B2O3. Such oxides hinder the ceramics to be fully densified at inferior sintering conditions of sample 1. Moreover, it seems that adding only 5 vol% Csf was not enough to react with and eliminate such harmful impurities. However, with increasing Csf/SiC in the initial powder mixtures, the finer and denser microstructures were obtained at 1700 °C (Fig. 1 and Table 3). The microstructures of fracture surfaces of the composite samples densified at 1775 °C are shown in Fig. 3. A relative density of ~83% with an average grain size of 7.1 µm was achieved for sample 5, as the weakest result for the samples sintered at the temperature of 1775 °C. As it can be clearly seen in Fig. 3(b), the presence of large-size pores in sample 5 can be related to the not influential amount of Csf in the powder mixture. Compared to sample 1 which also contains 5 vol% Csf, with simultaneously increasing in the hot pressing variables (temperature, time and pressure), less than 3% enhancement in the relative density was occurred with no significant changes in the final grain size. On the other hand, a closer view of Fig. 3(a) which is related to sample 4 and contains high amount of Csf (15 vol%), is shown in Fig. 4. EDS spectrum of the moss-like phase, marked by arrow in Fig. 4(a), is presented in Fig. 4(b). This analysis detects the high amount of zirconium and carbon in the mentioned phase which may be a sign of in-situ formation of ZrC on the surfaces of ZrB2 grains via reacting with Csf. The details and related chemical reactions have been previously published in Part I [42]. Fig. 5 exhibits the microstructures of fracture surfaces of the samples hot pressed at 1850 °C. Based on these figures, it seems that samples 7 and 8 have reached near full density. The results of relative density measurements in Table 3 are in consistent with this observation. However, some pores are detectable in the microstructure of sample 9 (Fig. 5(c)), because the relative density of this sample was measured ~92%, may be due to the lack of carbon additives even at the highest sintering temperature of 1850 °C in this research. In addition, the largest ZrB2 grains in the final microstructure (9.2 ± 1.6 µm) are also belonged to sample 9. Therefore, the vital effect of Csf content in progress of the sintering process and also hindering of the grain growth in ZrB2-based composites can be verified. Nevertheless, the in-situ formation of some ZrC phases (marked by arrow in Fig.
6) in the interfaces of ZrB2 and Csf phases at 1850 °C was confirmed by EDS analysis (Point D). Fig. 7 displays the main effects plots showing the changes in S/N ratios with processing variables. As it can be seen in this figure, a fine-grained microstructure is obtainable at the lowest level of the sintering temperature (1700 °C) and the highest levels of dwell time (90 min) and applied pressure (16 MPa). Khoeini et al. [45] also reported the occurrence of significant grain growth in pressureless sintered ZrB2–SiC–C composites with the increase in sintering temperature. In addition, the grain size of ZrB2 in the final microstructure decreases with increasing Csf/SiC ratio. The higher proportion of Csf in the initial powder mixture results in the formation of further ZrC in the as-sintered ceramics, based on the chemical reactions discussed in Part I [42]. Liu et al. [46] reported the synergistic effect of SiC and ZrC on hindering the grain growth of ZrB2-based composites. In this research, therefore, with increasing Csf/SiC ratio and consequent formation of further ZrC, a finer microstructure can be achievable (Fig. 7), since simultaneous presence of SiC and ZrC is beneficial to inhibiting the grain growth synergistically. However, Nasiri et al. [47] observed the agglomeration of reinforcement raw materials (nano-sized SiC and Csf) due to the high-energy ball-milling, which boosts the grain coarsening of ZrB2-based composites. They reported that due to the agglomeration of such reinforcements at ZrB2 grain boundaries, the diffusion rate in the grains decreases which lead to the further diffusion and consequent grain growth in ZrB2 grains. In this research, such a problem was not occurred, because of using the micron-sized SiC as well as ultrasonication and ball-mixing in ethanol which effectively prevents the Csf from agglomeration. According to Fig. 7, the optimal hot pressing conditions, which lead to a finer microstructure, are predicted as temperature of 1700 °C, dwell time of 90 min and pressure of 16 MPa when the amount of SiC is equal to Csf (15 vol%). The analysis of variance reveals the effects of processing variables on the mean ZrB2 grain size with rank. The ANOVA results of the grain size are presented in Table 4, which recognize the sintering temperature and dwell time as the most and the least effective
variables on the grain size of ZrB2, respectively. This calculation verifies that the composition (SiC/Csf ratio) can also affect the grain size of the composites. The significances of temperature, dwell time, pressure, and composition on the grain size are approximately 68, 2, 9 and 21%, respectively. In monolithic ZrB2 ceramics, the sintering temperature was identified as the most critical variable having influence on the grain growth. The significances of temperature, dwell time and applied pressure were estimated around 56%, 33% and 1%, respectively, which showed that the external pressure cannot affect the mean ZrB2 grain size in the sintered microstructure. Fine-grained ceramics were obtained by hot pressing at the lowest sintering temperature / the shortest dwell time [48]. In ZrB2–(20-30 vol%) SiC composites, a fine-grained microstructure was also achieved at the lowest levels of the hot pressing variables (temperature, dwell time and pressure) with adding 25 vol% SiC. However, the fabrication of a fully dense sample was impossible at such weak hot pressing conditions. In these composites, both the sintering temperature and dwell time were recognized as the most affecting variables on the grain growth. The significances of temperature, dwell time, applied pressure and SiC content on the mean ZrB2 grain size were approximate 33, 47, 10 and 10%, respectively [49]. In ZrB2–25 vol% SiC composites (with three different SiC particle sizes), significances of temperature, dwell time, applied pressure and SiC particle size were almost 19, 31, 16 and 34%, respectively. The addition of nano-sized SiC particles (200 nm) had the efficient role in minimizing the grain growth during the hot pressing process. Choosing the lowest levels of variables as the hot pressing conditions, similar to the above-mentioned reports, was found to assist obtaining a fine-grained microstructure [50]. In the composites hot pressed at 1700 °C (samples 1-3), with increasing Csf content, the relative density increased and the ZrB2 grain size decreased. As the sintering dwell time and applied external pressure did not have significant influences on the relative density (discussed in Part I [42]) and mean ZrB2 grain size, the positive role of Csf in sintering process can be declared. Several research works have explained that the presence of oxide impurities (ZrO2 and B2O3) on the surface of ZrB2 particles causes grain growth, which
restrains the densification process [21, 48-49, 51]. Therefore, the amount of such detrimental oxides was reduced by adding Csf to the ZrB2-based composites. On one hand, due to its self-lubricating attribute, Csf helps the rearrangement of ZrB2 particles in the initial stage of densification process. On the other hand, Csf acts as a sintering aid in the final stage of the sintering process, based on some interfacial reactions (discussed in Part I [42]) which removes the oxide impurities from the surfaces of ZrB2 particles. It seems that the grain growth was not directly contributed by the external pressure. The influence of applied pressure becomes clearer when the rate of grain growth is relatively higher than that of densification. Since the applied pressure can increase the rate of densification, a lower sintering temperature is needed which indirectly hinders the exaggerated grain growth [52].
4. Conclusion The effects of hot pressing parameters (sintering temperature, holding time and applied external pressure) and composition (SiC/Csf ratio) on the grain growth of ZrB2-based composites were investigated using the Taguchi methodology. Analysis of variance showed that the hot pressing temperature and the composition of the ceramic are the most important parameters regarding the final grain size of ZrB2 matrix. Elimination of oxide impurities (e.g. B2O3, ZrO2 and SiO2) from the surface of starting materials, due to the addition of Csf, restrained the grain growth. Simultaneous presence of SiC and in-situ formed ZrC as the stationary secondary phases inhibited the grain growth synergistically by the grain boundary pinning effect.
Acknowledgment This work was supported by the Advanced Ceramic Research Group (ACRG), University of Tabriz, Iran.
References [1] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas, Pressureless sintering of zirconium diboride, Journal of the American Ceramic Society, vol. 89, pp. 450–456, 2006. [2] N. Pourmohammadie Vafa, B. Nayebi, M. Shahedi Asl, M. Jaberi Zamharir and M. Ghassemi Kakroudi, Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part II: Mechanical behavior, Ceramics International, vol. 42, pp. 2724–2733, 2016. [3] Z. Ahmadi, B. Nayebi, M. Shahedi Asl and M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered AlN-doped ZrB2–SiC composites, Materials Characterization, vol. 110, pp. 77–85, 2015. [4] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, M. Shokouhimehr, Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol% SiC composites, International Journal of Refractory Metals and Hard Materials, vol. 54, pp. 7–13, 2016. [5] M. Shahedi Asl, M. Ghassemi Kakroudi, A. Farzaneh, B. Nayebi, Influence of nano-SiC participation on densification and mechanical properties of ZrB2, in 10th Nanoscience and Nanotechnology Conference of Turkey (NanoTR10), Istanbul, 2014. [6] M. Shahedi Asl, M. Ghassemi Kakroudi, A processing–microstructure correlation in ZrB2–SiC composites hot-pressed under a load of 10 MPa, Universal Journal of Materials Science, vol. 3, pp. 14–21, 2015. [7] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas, Low-temperature densification of zirconium diboride ceramics by reactive hot pressing, Journal of the American Ceramic Society, vol. 89, pp. 3638–3645, 2006. [8] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, A fractographical approach to the sintering process in porous ZrB2–B4C binary composites, Ceramics International, vol. 41, pp. 379– 387, 2015. [9] N. Pourmohammadie Vafa, M. Shahedi Asl, M. Jaberi Zamharir, M. Ghassemi Kakroudi, Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Densification behavior, Ceramics International, vol. 41, pp. 8388–8396, 2015. [10] F. Guanghai, Y. Yanqing, Z. Guangming, Z. Wei, L. Xian, H. Bin, Effect of hot isostatic pressing parameters on the microstructures and grain growth behavior of the matrix of SiCf/Ti-6Al4V composites, Rare Metal Materials and Engineering, vol. 43, pp. 1839–1845, 2014. [11] M. Shahedi Asl, M. Ghassemi Kakroudi, S. Noori, Hardness and toughness of hot pressed ZrB2–SiC composites consolidated under relatively low pressure, Journal of Alloys and Compounds, vol. 619, pp. 481–487, 2015. [12] A. Kishimoto, M. Hanao, H. Hayashi, Anomalous grain growth during hot isostatic pressing of magnesia ceramics made from starting powders with different coarse/fine mixing ratios, Scripta Materialia, vol. 57, pp. 321–324, 2007. [13] W. G. Fahrenholtz, G. E. Hilmas, I. G. Talmy, J. A. Zaykoski, Refractory Diborides of Zirconium and Hafnium, Journal of the American Ceramic Society, vol. 90, pp. 1347–1364, 2007. [14] A. Sabahi Namini, S. N. Seyed Gogani, M. Shahedi Asl, K. Farhadi, M. Ghassemi Kakroudi, A. Mohammadzadeh, Microstructural development and mechanical properties of hot pressed SiC
reinforced TiB2 based composite, International Journal of Refractory Metals and Hard Materials, vol. 51, pp. 169–179, 2015. [15] F. Monteverde, Beneficial effects of an ultrafine α-SiC incorporation on the sinterability and mechanical properties of ZrB2, Applied Physics A, vol. 82, pp. 329–337, 2006. [16] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas, D. T. Ellerby, High-strength zirconium diboride-based ceramics, Journal of the American Ceramic Society, vol. 87, no. 6, pp. 1170–1172, 2004. [17] Q. Liu, W. Han, X. Zhang, S. Wang, J. Han, Microstructure and mechanical properties of ZrB2–SiC composites, Materials Letters, vol. 63, pp. 1323–1325, 2009. [18] Microstructural evolution and grain growth kinetics in ZrB2–SiC composites during heat treatment, Journal of the American Ceramic Society, vol. 92, pp. 2780–2783, 2009. [19] M. Jaberi Zamharir, M. Shahedi Asl, N. Pourmohammadie Vafa, M. Ghassemi Kakroudi, Significance of hot pressing parameters and reinforcement size on densification behavior of ZrB2– 25 vol% SiC UHTCs, Ceramics International, vol. 41, pp. 6439–6447, 2015. [20] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, H. Nasiri, Taguchi analysis on the effect of hot pressing parameters on density and hardness of zirconium diboride, International Journal of Refractory Metals and Hard Materials, vol. 50, pp. 313–320, 2015. [21] M. Shahedi Asl, M. Ghassemi Kakroudi, Fractographical Assessment of Densification Mechanisms in Hot Pressed ZrB2–SiC Composites, Ceramics International, vol. 40, pp. 15273– 15281, 2014. [22] S. Q. Guo, Densification of ZrB2-based composites and their mechanical and physical properties: A review, Journal of the European Ceramic Society, vol. 29, pp. 995–1011, 2009. [23] M. Shahedi Asl, A. Sabahi Namini, M. Ghassemi Kakroudi, Influence of silicon carbide reinforcement on the microstructural development of hot pressed zirconium and titanium diborides, Ceramics International, vol. 42, pp. 5375-5381, 2016. [24] S. Zhu, W. G. Fahrenholtz, G. E. Hilmas, S. C. Zhang, Pressureless sintering of carbon-coated zirconium diboride powders, Materials Science and Engineering A, vol. 459, pp. 167–171, 2007. [25] S. Zhou, Z. Wang, X. Sun, J. Han, Microstructure, mechanical properties and thermal shock resistance of zirconium diboride containing silicon carbide ceramic toughened by carbon black, Materials Chemistry and Physics, vol. 122, pp. 470–473, 2010. [26] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, P. Pirmohammadi, M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered SiAlON-doped ZrB2–SiC composites, Materials Characterization, vol. 102, pp. 137–145, 2015. [27] Z. Balak, M. Zakeri, M. Rahimipour, E. Salahi, Taguchi design and hardness optimization of ZrB2-based composites reinforced with chopped carbon fiber and different additives and prepared by SPS, Journal of Alloys and Compounds, vol. 639, pp. 617–625, 2015. [28] F. Yang, X. Zhang, J. Han, S. Du, Mechanical properties of short carbon fiber reinforced ZrB2–SiC ceramic matrix composites, Materials Letters, vol. 62, pp. 2925–2927, 2008. [29] F. Yang, X. Zhang, J. Han, S. Du, Processing and mechanical properties of short carbon fibers toughened zirconium diboride-based ceramics, Materials and Design, vol. 29, pp. 1817–1820, 2008.
[30] F. Yang, X. Zhang, J. Han, S. Du, Characterization of hot-pressed short carbon fiber reinforced ZrB2–SiC ultra-high temperature ceramic composites, Journal of Alloys and Compounds, vol. 472, pp. 395–399, 2009. [31] G. B. Yadhukulakrishnan, A. Rahman, S. Karumuri, M. M. Stackpoole, A. K. Kalkan, R. P. Singh, S. P. Harimkar, Spark plasma sintering of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ceramic composite, Materials Science and Engineering A, vol. 552, pp. 125–133, 2012. [32] W. B. Tian, Y. M. Kan, G. J. Zhang, P. L. Wang, Effect of carbon nanotubes on the properties of ZrB2–SiC ceramics, Materials Science and Engineering A, vol. 487, pp. 568–573, 2008. [33] M. Shahedi Asl, I. Farahbakhsh, B. Nayebi, Characteristics of multi-walled carbon nanotube toughened ZrB2–SiC ceramic composite prepared by hot pressing, Ceramics International, vol. 42, pp. 1950–1958, 2016. [34] Z. Wang, S. Wang, X. Zhang, P. Hu, W. Han, C. Hong, Effect of graphite flake on microstructure as well as mechanical properties and thermal shock resistance of ZrB2–SiC matrix ultrahigh temperature ceramics, Journal of Alloys and Compounds, vol. 484, pp. 390–394, 2009. [35] X. Zhang, Z. Wang, X. Sun, W. Han, C. Hong, Effect of graphite flake on the mechanical properties of hot pressed ZrB2–SiC ceramics, Materials Letters, vol. 62, pp. 4360–4362, 2008. [36] S. Zhou, Z. Wang, W. Zhang, Effect of graphite flake orientation on microstructure and mechanical properties of ZrB2–SiC–graphite composite, Journal of Alloys and Compounds, vol. 485, pp. 181–185, 2009. [37] W. Zhi, W. Zhanjun, S. Guodong, Fabrication, mechanical properties and thermal shock resistance of a ZrB2-graphite ceramic, Int. Journal of Refractory Metals and Hard Materials, vol. 29, pp. 351–355, 2011. [38] Z. Wang, C. Hong, X. Zhang, X. Sun, J. Han, Microstructure and thermal shock behavior of ZrB2–SiC–graphite composite, Materials Chemistry and Physics, vol. 113, pp. 338–341, 2009. [39] M. Shahedi Asl, M. Ghassemi Kakroudi, R. Abedi Kondolaji, H. Nasiri, Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite, Ceramics International, vol. 41, pp. 5843–5851, 2015. [40] G. B. Yadhukulakrishnan, S. Karumuri, A. Rahman, R. P. Singh, A. K. Kalkan, S. P. Harimkar, Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites, Ceramics International, vol. 39, pp. 6637–6646, 2013. [41] M. Shahedi Asl, M. Ghassemi Kakroudi, Characterization of hot-pressed graphene reinforced ZrB2–SiC composite, Materials Science & Engineering A, vol. 625, pp. 385–392, 2015. [42] M. Shahedi Asl, F. Golmohammadi, M. Ghassemi Kakroudi, M. Shokouhimehr, Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part I: Densification behavior, Ceramics International, vol. 42, pp. 4498–4506, 2016. [43] R. Roy, A primer on the Taguchi method. New York: Van Nostrand Rheinhold, 1990. [44] D. C. Montgomery, Introduction to Statistical Quality Control, Sixth Edition, Sixth Edition ed. United States of America: John Wiley & Sons, Inc., 2009. [45] M. Khoeini, A. Nemati, M. Zakeri, M. Tamizifar, H. Samadi, Comprehensive study on the effect of SiC and carbon additives on the pressureless sintering and microstructural and mechanical
characteristics of new ultra-high temperature ZrB2 ceramics, Ceramics International, vol. 41, pp. 11456–11463, 2015. [46] H. L. Liu, G. J. Zhang, J. X. Liu, H. Wu, Synergetic roles of ZrC and SiC in ternary ZrB2– SiC–ZrC ceramics, Journal of the European Ceramic Society, vol. 35, pp. 4389–4397, 2015. [47] Z. Nasiri, M. Mashhadi, A. Abdollahi, Effect of short carbon fiber addition on pressureless densification and mechanical properties of ZrB2–SiC–Csf nanocomposite, Int. Journal of Refractory Metals and Hard Materials, vol. 51, pp. 216–223, 2015. [48] M. Shahedi Asl, M. Ghassemi Kakroudi, M. Rezvani, F. Golestani-Fard, Significance of hot pressing parameters on the microstructure and densification behavior of zirconium diboride, Int. Journal of Refractory Metals and Hard Materials, vol. 50, pp. 140–145, 2015. [49] M. Shahedi Asl, M. Ghassemi Kakroudi, F. Golestani-Fard, H. Nasiri, A Taguchi approach to the influence of hot pressing parameters and SiC content on the sinterability of ZrB2-based composites, International Journal of Refractory Metals and Hard Materials, vol. 51, pp. 81–90, 2015. [50] M. Jaberi Zamharir, M. Shahedi Asl, M. Ghassemi Kakroudi, N. Pourmohammadie Vafa, M. Jaberi Zamharir, Significance of hot pressing parameters and reinforcement size on sinterability and mechanical properties of ZrB2–25 vol% SiC UHTCs, Ceramics International, vol. 41, pp. 9628– 9636, 2015. [51] S. Zhu, W. G. Fahrenholtz, G. E. Hilmas, Enhanced densification and mechanical properties of ZrB2–SiC processed by a preceramic polymer coating route, Scripta Materialia, vol. 59, pp. 123– 126, 2008. [52] M. Mazaheri, S. A. Hassanzadeh-Tabrizi, S. K. Sadrnezhaad, Hot pressing of nanocrystalline zinc oxide compacts: densification and grain growth during sintering, Ceramics International, vol. 35, pp. 991–995, 2009. [53] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, F. Farahbakhsh, M. Shokouhimehr, Interfacial phenomena and formation of nano-particles in porous ZrB2–40 vol% B4C UHTC, Ceramics International, 2016. [54] K. Farhadi, A. Sabahi Namini, M. Shahedi Asl, A. Mohammadzadeh, M. Ghassemi Kakroudi, Characterization of hot pressed SiC whisker reinforced TiB2 based composites, International Journal of Refractory Metals and Hard Materials, vol. 61, pp. 84–90, 2016. [55] I. Farahbakhsh, M. Shahedi Asl, M. Ghassemi Kakroudi, Sinterability improvement of zirconium diboride ceramics fabricated by hot pressing, 13th International Conference on Nanosciences & Nanotechnologies (NN16), Thessaloniki, Greece, 2016. [56] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Mazinani, B. Nayebi, The morphology-dependent properties for nano-carbon-reinforced ZrB2–SiC composited, 12th International Nanoscience and Nanotechnology Conference (NanoTR12), Darica-Kocaeli, Turkey, 2016. [57] I. Farahbakhsh, M. Shahedi Asl, M. Ghassemi Kakroudi, Influence of nano-carbon morphology on densification and fracture toughness of hot-pressed ZrB2–SiC composites, 13th International Conference on Nanosciences & Nanotechnologies (NN16), Thessaloniki, Greece, 2016.
[58] M. Shahedi Asl, Z. Ahmadi, B. Nayebi, M. Ghassemi Kakroudi, Effect of nano-AlN on densification behavior of hot-pressed ZrB2-based ceramics, 12th International Nanoscience and Nanotechnology Conference (NanoTR12), Darica-Kocaeli, Turkey, 2016.
Figures captions: Fig. 1. SEM micrographs of fracture surfaces of ZrB2–SiC–Csf composites hot pressed at 1700 °C: (a) sample 1, (b) sample 2 and (c) sample 3. Fig. 2. (a) SEM nanograph of the marked area in Fig. 1(a), shows the morphology of cauliflower-like nano-oxide impurities in sample 1 and (b) corresponding EDS spectrum. Fig. 3. SEM micrographs of fracture surfaces of ZrB2–SiC–Csf composites hot pressed at 1775 °C: (a) sample 4, (b) sample 5 and (c) sample 6. Fig. 4. SEM micrograph of the fracture surface of sample 4: ZrB2–15 vol% SiC–15 vol% Csf composite hot pressed at 1700 °C for 30 min under 12 MPa. Fig. 5. SEM micrographs of fracture surfaces of ZrB2–SiC–Csf composites hot pressed at 1850 °C: (a) sample 7, (b) sample 8 and (c) sample 9. Fig. 6. SEM micrograph of the fracture surface and corresponding EDS spectra from points A, B, C and D of sample 9: ZrB2–25 vol% SiC–5 vol% Csf composite hot pressed at 1850 °C for 90 min under 12 MPa. Fig. 7. S/N ratios of processing variables for the grain size of ZrB2. Table 1. Processing variables and selected levels. Processing variables
Range
Level 1
Level 2
Level 3
Temperature (°C)
1700–1850
1700
1775
1850
Dwell time (min)
30–90
30
60
90
Pressure (MPa)
8–16
8
12
16
vol% SiC / vol% Csf
1–5
5
2
1
Table 2. Conditions for processing of nine experiments designed by the Taguchi approach. Processing variables Sample 1
Temperature (°C) 1700
Dwell time (min) 30
Pressure (MPa) 8
2
1700
60
12
2
3
1700
90
16
1
4
1775
30
12
1
5
1775
60
16
5
vol% SiC / vol% Csf 5
6
1775
90
8
2
7
1850
30
16
2
8
1850
60
8
1
9
1850
90
12
5
Table 3. Relative density and mean ZrB2 grain size of the hot pressed composites.
Sample
Relative density (g/cm3)
Grain size (µm)
1
80.07 ± 0.08
6.8 ± 2.1
2
86.29 ± 0.08
5.5 ± 1.2
3
89.99 ± 0.10
4.6 ± 0.9
4
86.56 ± 0.06
5.9 ± 1.2
5
82.52 ± 0.08
7.1 ± 1.6
6
91.42 ± 0.12
6.9 ± 1.2
7
99.95 ± 0.24
8.3 ± 1.1
8
97.64 ± 0.02
8.4 ± 2.0
9
92.30 ± 0.20
9.2 ± 1.6
Table 4. ANOVA results describing the significance of each sintering factor on the S/N ratio of ZrB 2 grain size. Hot pressing parameters
Degrees of freedom (f)
Sum of squares (S)
Variance (V)
F-ratio (F)
Pure sum (Sʹ)
Significance P (%)
Temperature
2
19.929
9.964
-
19.929
67.815
Dwell time
2
0.562
0.281
-
0.562
1.913
Pressure
2
2.753
1.376
-
2.753
9.368
vol% SiC / vol% Csf
2
6.142
3.071
-
6.142
20.902
Other/Error
0
-
-
-
-
-
Total
8
29.387
-
-
-
100.000
PII: DOI: Reference:
S0272-8842(16)31527-9 http://dx.doi.org/10.1016/j.ceramint.2016.08.205 CERI13657
To appear in: Ceramics International Received date: 1 August 2016 Revised date: 30 August 2016 Accepted date: 31 August 2016 Cite this article as: Mehdi Shahedi Asl, Mahdi Ghassemi Kakroudi, Iman Farahbakhsh, Babak Mazinani and Zohre Balak, Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.205 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth Mehdi Shahedi Asla*, Mahdi Ghassemi Kakroudib, Iman Farahbakhshc, Babak Mazinani d, Zohre Balake a
Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran
b c
Department of Mechanical Engineering, Quchan Branch, Islamic Azad University, Quchan, Iran
d e
Department of Materials Science and Engineering, University of Tabriz, Tabriz, Iran
Department of Materials Engineering, Malayer University, Malayer, Iran
Department of Materials Science and Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
*
Corresponding author. Tel.: +98 912 3277186, [email protected] (M. Shahedi Asl)
Abstract The synergetic effects SiC particles and short carbon fibers (Csf) as well as hot pressing parameters (sintering temperature, dwell time and applied pressure) on the grain growth of ZrB2-based composites were investigated. Taguchi methodology was employed for the design of experiments to study the microstructure and grain growth of ZrB2–SiC–Csf ceramic composites. Three hot pressing parameters and SiC/Csf ratio were selected as the scrutinized variables. The sintering temperature and SiC/Csf ratio were identified by ANOVA as the most effective variables on the gain growth of ZrB2-based samples. Removal of oxide impurities from the surface of starting particles by the reactant Csf, not only hindered the extraordinary grain growth of ZrB2 matrix, but also improved the sinterability of the ceramics. A fully dense ceramic with an average grain size of 8.3 µm was obtained by hot pressing at 1850 °C for 30 min under 16 MPa through adding 20 vol% SiC and 10 vol% Csf to the ZrB2 matrix. SEM observations and EDS analysis verified the in-situ formation of ZrC which can restrain the growth of ZrB2 particles, similar to the role of SiC, by the pinning of grain boundaries as another stationary secondary phase.
Keywords Hot pressing; ZrB2; SiC; Carbon fiber; Taguchi method; Grain growth. 1. Introduction Ultra high temperature ceramics (UHTCs) include carbides, borides and nitrides of transition metals which have melting point over 3000 °C. Zirconium diboride (ZrB2), as a member of UHTCs, has strong covalent bonding together with metallic characteristic, high hardness, high Young’s modulus, and high thermal and electrical conductivities. This coalition of properties, made ZrB2 as a proper candidate for several applications such as molten metal crucibles, cutting tools, and thermal protection systems (TPS) in aerospace industry. Nevertheless, achieving a fully dense ZrB2 is impossible even by hot pressing at a high temperature of 2000 °C, because the densification and grain growth occur simultaneously [1-9]. The industrial significance of grain growth comes from the dependence of densification behavior and mechanical properties (e.g. strength and fracture toughness) on the final grain size of the sintered microstructure. Hence, an appropriate comprehension of the grain growth phenomenon is an important requirement for controlling the microstructure and characteristics of the material during the manufacturing process [10-11]. Several parameters such as the size of starting materials, the presence of impurities (for example, oxide impurities on the surface of non-oxide ceramics), and the sintering conditions (temperature, dwell time and applied external pressure) affect the grain growth during the consolidation process [12]. Hot pressing is recognized as an efficient processing method to decrease the sintering temperature, in order to obtain fully dense and synchronously finegrained ceramics [13]. The addition of SiC was recognized as a key reinforcement phase which decelerates the rate of grain growth in ZrB2-based and other non-oxide ceramics [14-15]. In other words, SiC acts as an inhibitor of movement of ZrB2 grain boundaries at elevated sintering temperatures [16-117]. A kinetical analysis verified that the coarsening rate of ZrB2 matrix was controlled by the grain boundary immobilizing effect of SiC particles. However, SiC phase with higher proportion effectively prevented ZrB2 grain growth during the sintering
process [18]. With decreasing the size of SiC additive, the number of particles which can restrain the grain growth of ZrB2 increases; therefore, the densification behavior meaningfully improves [19]. Additionally, the oxide impurities of ZrO2 and B2O3 on the surface of starting powder not only hinder the densification but also motivate the grain coarsening [20-21]. Because B2O3 exists as a liquid or vapor phase during the sintering process which provides a rapid diffusion path [22]. A similar behavior has been reported for other diborides such as TiB2 and HfB2 [13-14, 23]. Carbon additives, for example in the form of carbon black [24-26], fiber [27-30], nanotube [31-33], graphite [34-39] or graphene [40-41], can react with and eliminate the surface oxide impurities which promote the densification process by minimizing the grain growth of ZrB2. In this research, SiC and Csf with different ratios were chosen as additives and several sintering temperature, times and pressures were selected as hot pressing variables. Although, the investigation of all mentioned variables seems not to be economically possible, the Taguchi method was used to design of experiments as an optimization plan. The goal of this paper, as the second part of a series of papers, is to study the influences of processing variables on the microstructure development of ZrB2–SiC–Csf ceramics. Additionally, the statistical investigations based on the signal to noise (S/N) ratios and the analysis of variance (ANOVA) were employed to determine the significances of processing variables on the grain growth of zirconium diboride. The effects of processing variables on the densification behavior and mechanical properties of ZrB2-based composites were reported in Part I [42] and will be reported in Part III, respectively.
2. Experimental procedure 2.1. Design of experiment The experiments were designed established upon the Taguchi methodology to conclude the influences of four processing variables on the microstructure development and grain growth of ZrB2-based composites. This technique is more economical than the classical methods, because minimum numbers of experiments are needed to supply a notable insight about the effects of variables on the investigated output. For example, if four processing variables
with three levels are mentioned for investigation, only a series of arrays with nine incorporations of input parameters (L9 orthogonal array) can be employed. The processing variables and selected levels in this research are displayed in Table 1 and the designed L9 arrays by the Taguchi method are presented in Table 2. Generally, the output characteristics can be analyzed in three exclusive classifications; smaller is better, higher is better or nominal is better. In this paper, as achieving a finegrained microstructure was desirable, the statistical analysis of ZrB2 grain size was fulfilled applying the smaller is better option. Hence, the S/N ratios for this option were evaluated according to the subsequent equation [43-44]:
(1)
S 1 n 10 log yi2 N n i 1
where yi (i=1, 2 …n) are the response values and n is the number of repetitions. Furthermore, the analysis of variance (ANOVA) was accomplished to determine the significance of processing variables on the mean ZrB2 grain size in the sintered microstructure. The data analyses were carried out using Qualitek-4 software (Automatic design and analysis of Taguchi experiments, Nutek Inc., Michigan, USA).
2.2. Materials and process In the present work, zirconium diboride (ZrB2: particle size ∼2 µm) and silicon carbide (SiC: particle size ~5 μm) powders as well as short carbon fibers (Csf: length ~1 mm, diameter ~8.5 µm) were purchased as raw materials from Leung Hi–tech Co. (China), Carborundum Universal Ltd. (India) and Torayca Sigmatex Ltd. (UK), respectively. As per technical data sheets provided by the suppliers, the purity of starting materials are higher than 99%. Three ZrB2-based mixed powders containing 25 vol% SiC–5 vol% Csf, 20 vol% SiC–10 vol% Csf and 15 vol% SiC–15 vol% Csf were prepared. At the beginning, the short carbon fibers were dispersed in ethanol for 0.5 h using an ultrasonic agitator (265-watt, Sonicator Q500, USA). Afterwards, the stoichiometric amounts of ZrB2 and SiC powders in each
batch were added to the slurry and mixed for 0.5 h by zirconia balls in a polyethylene cup and then dried at 110 °C for 24 h in a rotary evaporator (Tebazma HMS 14, Iran). Mixed batch powders were hot pressed at 1700, 1775 or 1850 °C in vacuum atmosphere (0.05 Pa) in a graphite element hot press furnace (Shenyang Weitai Science & Technology Development Co., Ltd., China) under a pressure of 8, 12 or 16 MPa and a dwell time of 30, 60 or 90 min. To minimize reactions between graphite die and powder mixture, inner wall of the die was coated with BN and lined with flexible graphite foil before the precursors were loaded. Diameter and thickness of the hot pressed pellets were around 25 and 5 mm, respectively.
2.3. Characterization The relative density of sintered samples was calculated using the Archimedes’ principle and the rule of mixtures. The microstructural and elemental analyses of the hot pressed composites were performed by field emission scanning electron microscopy (FESEM: Mira3 Tescan, Czech Republic) and energy dispersive spectroscopy (DXP-X10P Digital Xray Processor), respectively. The average ZrB2 grain size in the sintered microstructures was determined by ImageJ, (Image analysis software: Wayne Rasband, National Institute of Health, USA).
3. Results and discussion Table 3 presents the relative density and the mean ZrB2 grain size in the microstructure of composite samples which were hot pressed at different processing conditions. This table shows that the relative density and grain size values ranged from ~80% to ~100% and 4.6 µm to 9.2 µm, respectively, throughout the processing conditions of this research. Fig. 1 shows the microstructures of fracture surfaces of the composites sintered at 1700 °C. The porous microstructure of sample 1 (Fig. 1(a)), which was sintered at the weakest conditions of current research, is in consistent with its low relative density (~80%). A white colored cauliflower-like phase is clearly seen in this micrograph (marked by the arrow). A closer view of morphology of such a phase is shown in Fig. 2(a). EDS spectrum of this figure is displayed in Fig. 2(b), which verifies the presence of oxygen, zirconium and boron
because of the expected presence of oxide impurities such as ZrO2 and B2O3. Such oxides hinder the ceramics to be fully densified at inferior sintering conditions of sample 1. Moreover, it seems that adding only 5 vol% Csf was not enough to react with and eliminate such harmful impurities. However, with increasing Csf/SiC in the initial powder mixtures, the finer and denser microstructures were obtained at 1700 °C (Fig. 1 and Table 3). The microstructures of fracture surfaces of the composite samples densified at 1775 °C are shown in Fig. 3. A relative density of ~83% with an average grain size of 7.1 µm was achieved for sample 5, as the weakest result for the samples sintered at the temperature of 1775 °C. As it can be clearly seen in Fig. 3(b), the presence of large-size pores in sample 5 can be related to the not influential amount of Csf in the powder mixture. Compared to sample 1 which also contains 5 vol% Csf, with simultaneously increasing in the hot pressing variables (temperature, time and pressure), less than 3% enhancement in the relative density was occurred with no significant changes in the final grain size. On the other hand, a closer view of Fig. 3(a) which is related to sample 4 and contains high amount of Csf (15 vol%), is shown in Fig. 4. EDS spectrum of the moss-like phase, marked by arrow in Fig. 4(a), is presented in Fig. 4(b). This analysis detects the high amount of zirconium and carbon in the mentioned phase which may be a sign of in-situ formation of ZrC on the surfaces of ZrB2 grains via reacting with Csf. The details and related chemical reactions have been previously published in Part I [42]. Fig. 5 exhibits the microstructures of fracture surfaces of the samples hot pressed at 1850 °C. Based on these figures, it seems that samples 7 and 8 have reached near full density. The results of relative density measurements in Table 3 are in consistent with this observation. However, some pores are detectable in the microstructure of sample 9 (Fig. 5(c)), because the relative density of this sample was measured ~92%, may be due to the lack of carbon additives even at the highest sintering temperature of 1850 °C in this research. In addition, the largest ZrB2 grains in the final microstructure (9.2 ± 1.6 µm) are also belonged to sample 9. Therefore, the vital effect of Csf content in progress of the sintering process and also hindering of the grain growth in ZrB2-based composites can be verified. Nevertheless, the in-situ formation of some ZrC phases (marked by arrow in Fig.
6) in the interfaces of ZrB2 and Csf phases at 1850 °C was confirmed by EDS analysis (Point D). Fig. 7 displays the main effects plots showing the changes in S/N ratios with processing variables. As it can be seen in this figure, a fine-grained microstructure is obtainable at the lowest level of the sintering temperature (1700 °C) and the highest levels of dwell time (90 min) and applied pressure (16 MPa). Khoeini et al. [45] also reported the occurrence of significant grain growth in pressureless sintered ZrB2–SiC–C composites with the increase in sintering temperature. In addition, the grain size of ZrB2 in the final microstructure decreases with increasing Csf/SiC ratio. The higher proportion of Csf in the initial powder mixture results in the formation of further ZrC in the as-sintered ceramics, based on the chemical reactions discussed in Part I [42]. Liu et al. [46] reported the synergistic effect of SiC and ZrC on hindering the grain growth of ZrB2-based composites. In this research, therefore, with increasing Csf/SiC ratio and consequent formation of further ZrC, a finer microstructure can be achievable (Fig. 7), since simultaneous presence of SiC and ZrC is beneficial to inhibiting the grain growth synergistically. However, Nasiri et al. [47] observed the agglomeration of reinforcement raw materials (nano-sized SiC and Csf) due to the high-energy ball-milling, which boosts the grain coarsening of ZrB2-based composites. They reported that due to the agglomeration of such reinforcements at ZrB2 grain boundaries, the diffusion rate in the grains decreases which lead to the further diffusion and consequent grain growth in ZrB2 grains. In this research, such a problem was not occurred, because of using the micron-sized SiC as well as ultrasonication and ball-mixing in ethanol which effectively prevents the Csf from agglomeration. According to Fig. 7, the optimal hot pressing conditions, which lead to a finer microstructure, are predicted as temperature of 1700 °C, dwell time of 90 min and pressure of 16 MPa when the amount of SiC is equal to Csf (15 vol%). The analysis of variance reveals the effects of processing variables on the mean ZrB2 grain size with rank. The ANOVA results of the grain size are presented in Table 4, which recognize the sintering temperature and dwell time as the most and the least effective
variables on the grain size of ZrB2, respectively. This calculation verifies that the composition (SiC/Csf ratio) can also affect the grain size of the composites. The significances of temperature, dwell time, pressure, and composition on the grain size are approximately 68, 2, 9 and 21%, respectively. In monolithic ZrB2 ceramics, the sintering temperature was identified as the most critical variable having influence on the grain growth. The significances of temperature, dwell time and applied pressure were estimated around 56%, 33% and 1%, respectively, which showed that the external pressure cannot affect the mean ZrB2 grain size in the sintered microstructure. Fine-grained ceramics were obtained by hot pressing at the lowest sintering temperature / the shortest dwell time [48]. In ZrB2–(20-30 vol%) SiC composites, a fine-grained microstructure was also achieved at the lowest levels of the hot pressing variables (temperature, dwell time and pressure) with adding 25 vol% SiC. However, the fabrication of a fully dense sample was impossible at such weak hot pressing conditions. In these composites, both the sintering temperature and dwell time were recognized as the most affecting variables on the grain growth. The significances of temperature, dwell time, applied pressure and SiC content on the mean ZrB2 grain size were approximate 33, 47, 10 and 10%, respectively [49]. In ZrB2–25 vol% SiC composites (with three different SiC particle sizes), significances of temperature, dwell time, applied pressure and SiC particle size were almost 19, 31, 16 and 34%, respectively. The addition of nano-sized SiC particles (200 nm) had the efficient role in minimizing the grain growth during the hot pressing process. Choosing the lowest levels of variables as the hot pressing conditions, similar to the above-mentioned reports, was found to assist obtaining a fine-grained microstructure [50]. In the composites hot pressed at 1700 °C (samples 1-3), with increasing Csf content, the relative density increased and the ZrB2 grain size decreased. As the sintering dwell time and applied external pressure did not have significant influences on the relative density (discussed in Part I [42]) and mean ZrB2 grain size, the positive role of Csf in sintering process can be declared. Several research works have explained that the presence of oxide impurities (ZrO2 and B2O3) on the surface of ZrB2 particles causes grain growth, which
restrains the densification process [21, 48-49, 51]. Therefore, the amount of such detrimental oxides was reduced by adding Csf to the ZrB2-based composites. On one hand, due to its self-lubricating attribute, Csf helps the rearrangement of ZrB2 particles in the initial stage of densification process. On the other hand, Csf acts as a sintering aid in the final stage of the sintering process, based on some interfacial reactions (discussed in Part I [42]) which removes the oxide impurities from the surfaces of ZrB2 particles. It seems that the grain growth was not directly contributed by the external pressure. The influence of applied pressure becomes clearer when the rate of grain growth is relatively higher than that of densification. Since the applied pressure can increase the rate of densification, a lower sintering temperature is needed which indirectly hinders the exaggerated grain growth [52].
4. Conclusion The effects of hot pressing parameters (sintering temperature, holding time and applied external pressure) and composition (SiC/Csf ratio) on the grain growth of ZrB2-based composites were investigated using the Taguchi methodology. Analysis of variance showed that the hot pressing temperature and the composition of the ceramic are the most important parameters regarding the final grain size of ZrB2 matrix. Elimination of oxide impurities (e.g. B2O3, ZrO2 and SiO2) from the surface of starting materials, due to the addition of Csf, restrained the grain growth. Simultaneous presence of SiC and in-situ formed ZrC as the stationary secondary phases inhibited the grain growth synergistically by the grain boundary pinning effect.
Acknowledgment This work was supported by the Advanced Ceramic Research Group (ACRG), University of Tabriz, Iran.
References [1] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas, Pressureless sintering of zirconium diboride, Journal of the American Ceramic Society, vol. 89, pp. 450–456, 2006. [2] N. Pourmohammadie Vafa, B. Nayebi, M. Shahedi Asl, M. Jaberi Zamharir and M. Ghassemi Kakroudi, Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part II: Mechanical behavior, Ceramics International, vol. 42, pp. 2724–2733, 2016. [3] Z. Ahmadi, B. Nayebi, M. Shahedi Asl and M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered AlN-doped ZrB2–SiC composites, Materials Characterization, vol. 110, pp. 77–85, 2015. [4] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, M. Shokouhimehr, Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol% SiC composites, International Journal of Refractory Metals and Hard Materials, vol. 54, pp. 7–13, 2016. [5] M. Shahedi Asl, M. Ghassemi Kakroudi, A. Farzaneh, B. Nayebi, Influence of nano-SiC participation on densification and mechanical properties of ZrB2, in 10th Nanoscience and Nanotechnology Conference of Turkey (NanoTR10), Istanbul, 2014. [6] M. Shahedi Asl, M. Ghassemi Kakroudi, A processing–microstructure correlation in ZrB2–SiC composites hot-pressed under a load of 10 MPa, Universal Journal of Materials Science, vol. 3, pp. 14–21, 2015. [7] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas, Low-temperature densification of zirconium diboride ceramics by reactive hot pressing, Journal of the American Ceramic Society, vol. 89, pp. 3638–3645, 2006. [8] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, A fractographical approach to the sintering process in porous ZrB2–B4C binary composites, Ceramics International, vol. 41, pp. 379– 387, 2015. [9] N. Pourmohammadie Vafa, M. Shahedi Asl, M. Jaberi Zamharir, M. Ghassemi Kakroudi, Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Densification behavior, Ceramics International, vol. 41, pp. 8388–8396, 2015. [10] F. Guanghai, Y. Yanqing, Z. Guangming, Z. Wei, L. Xian, H. Bin, Effect of hot isostatic pressing parameters on the microstructures and grain growth behavior of the matrix of SiCf/Ti-6Al4V composites, Rare Metal Materials and Engineering, vol. 43, pp. 1839–1845, 2014. [11] M. Shahedi Asl, M. Ghassemi Kakroudi, S. Noori, Hardness and toughness of hot pressed ZrB2–SiC composites consolidated under relatively low pressure, Journal of Alloys and Compounds, vol. 619, pp. 481–487, 2015. [12] A. Kishimoto, M. Hanao, H. Hayashi, Anomalous grain growth during hot isostatic pressing of magnesia ceramics made from starting powders with different coarse/fine mixing ratios, Scripta Materialia, vol. 57, pp. 321–324, 2007. [13] W. G. Fahrenholtz, G. E. Hilmas, I. G. Talmy, J. A. Zaykoski, Refractory Diborides of Zirconium and Hafnium, Journal of the American Ceramic Society, vol. 90, pp. 1347–1364, 2007. [14] A. Sabahi Namini, S. N. Seyed Gogani, M. Shahedi Asl, K. Farhadi, M. Ghassemi Kakroudi, A. Mohammadzadeh, Microstructural development and mechanical properties of hot pressed SiC
reinforced TiB2 based composite, International Journal of Refractory Metals and Hard Materials, vol. 51, pp. 169–179, 2015. [15] F. Monteverde, Beneficial effects of an ultrafine α-SiC incorporation on the sinterability and mechanical properties of ZrB2, Applied Physics A, vol. 82, pp. 329–337, 2006. [16] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas, D. T. Ellerby, High-strength zirconium diboride-based ceramics, Journal of the American Ceramic Society, vol. 87, no. 6, pp. 1170–1172, 2004. [17] Q. Liu, W. Han, X. Zhang, S. Wang, J. Han, Microstructure and mechanical properties of ZrB2–SiC composites, Materials Letters, vol. 63, pp. 1323–1325, 2009. [18] Microstructural evolution and grain growth kinetics in ZrB2–SiC composites during heat treatment, Journal of the American Ceramic Society, vol. 92, pp. 2780–2783, 2009. [19] M. Jaberi Zamharir, M. Shahedi Asl, N. Pourmohammadie Vafa, M. Ghassemi Kakroudi, Significance of hot pressing parameters and reinforcement size on densification behavior of ZrB2– 25 vol% SiC UHTCs, Ceramics International, vol. 41, pp. 6439–6447, 2015. [20] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, H. Nasiri, Taguchi analysis on the effect of hot pressing parameters on density and hardness of zirconium diboride, International Journal of Refractory Metals and Hard Materials, vol. 50, pp. 313–320, 2015. [21] M. Shahedi Asl, M. Ghassemi Kakroudi, Fractographical Assessment of Densification Mechanisms in Hot Pressed ZrB2–SiC Composites, Ceramics International, vol. 40, pp. 15273– 15281, 2014. [22] S. Q. Guo, Densification of ZrB2-based composites and their mechanical and physical properties: A review, Journal of the European Ceramic Society, vol. 29, pp. 995–1011, 2009. [23] M. Shahedi Asl, A. Sabahi Namini, M. Ghassemi Kakroudi, Influence of silicon carbide reinforcement on the microstructural development of hot pressed zirconium and titanium diborides, Ceramics International, vol. 42, pp. 5375-5381, 2016. [24] S. Zhu, W. G. Fahrenholtz, G. E. Hilmas, S. C. Zhang, Pressureless sintering of carbon-coated zirconium diboride powders, Materials Science and Engineering A, vol. 459, pp. 167–171, 2007. [25] S. Zhou, Z. Wang, X. Sun, J. Han, Microstructure, mechanical properties and thermal shock resistance of zirconium diboride containing silicon carbide ceramic toughened by carbon black, Materials Chemistry and Physics, vol. 122, pp. 470–473, 2010. [26] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, P. Pirmohammadi, M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered SiAlON-doped ZrB2–SiC composites, Materials Characterization, vol. 102, pp. 137–145, 2015. [27] Z. Balak, M. Zakeri, M. Rahimipour, E. Salahi, Taguchi design and hardness optimization of ZrB2-based composites reinforced with chopped carbon fiber and different additives and prepared by SPS, Journal of Alloys and Compounds, vol. 639, pp. 617–625, 2015. [28] F. Yang, X. Zhang, J. Han, S. Du, Mechanical properties of short carbon fiber reinforced ZrB2–SiC ceramic matrix composites, Materials Letters, vol. 62, pp. 2925–2927, 2008. [29] F. Yang, X. Zhang, J. Han, S. Du, Processing and mechanical properties of short carbon fibers toughened zirconium diboride-based ceramics, Materials and Design, vol. 29, pp. 1817–1820, 2008.
[30] F. Yang, X. Zhang, J. Han, S. Du, Characterization of hot-pressed short carbon fiber reinforced ZrB2–SiC ultra-high temperature ceramic composites, Journal of Alloys and Compounds, vol. 472, pp. 395–399, 2009. [31] G. B. Yadhukulakrishnan, A. Rahman, S. Karumuri, M. M. Stackpoole, A. K. Kalkan, R. P. Singh, S. P. Harimkar, Spark plasma sintering of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ceramic composite, Materials Science and Engineering A, vol. 552, pp. 125–133, 2012. [32] W. B. Tian, Y. M. Kan, G. J. Zhang, P. L. Wang, Effect of carbon nanotubes on the properties of ZrB2–SiC ceramics, Materials Science and Engineering A, vol. 487, pp. 568–573, 2008. [33] M. Shahedi Asl, I. Farahbakhsh, B. Nayebi, Characteristics of multi-walled carbon nanotube toughened ZrB2–SiC ceramic composite prepared by hot pressing, Ceramics International, vol. 42, pp. 1950–1958, 2016. [34] Z. Wang, S. Wang, X. Zhang, P. Hu, W. Han, C. Hong, Effect of graphite flake on microstructure as well as mechanical properties and thermal shock resistance of ZrB2–SiC matrix ultrahigh temperature ceramics, Journal of Alloys and Compounds, vol. 484, pp. 390–394, 2009. [35] X. Zhang, Z. Wang, X. Sun, W. Han, C. Hong, Effect of graphite flake on the mechanical properties of hot pressed ZrB2–SiC ceramics, Materials Letters, vol. 62, pp. 4360–4362, 2008. [36] S. Zhou, Z. Wang, W. Zhang, Effect of graphite flake orientation on microstructure and mechanical properties of ZrB2–SiC–graphite composite, Journal of Alloys and Compounds, vol. 485, pp. 181–185, 2009. [37] W. Zhi, W. Zhanjun, S. Guodong, Fabrication, mechanical properties and thermal shock resistance of a ZrB2-graphite ceramic, Int. Journal of Refractory Metals and Hard Materials, vol. 29, pp. 351–355, 2011. [38] Z. Wang, C. Hong, X. Zhang, X. Sun, J. Han, Microstructure and thermal shock behavior of ZrB2–SiC–graphite composite, Materials Chemistry and Physics, vol. 113, pp. 338–341, 2009. [39] M. Shahedi Asl, M. Ghassemi Kakroudi, R. Abedi Kondolaji, H. Nasiri, Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite, Ceramics International, vol. 41, pp. 5843–5851, 2015. [40] G. B. Yadhukulakrishnan, S. Karumuri, A. Rahman, R. P. Singh, A. K. Kalkan, S. P. Harimkar, Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites, Ceramics International, vol. 39, pp. 6637–6646, 2013. [41] M. Shahedi Asl, M. Ghassemi Kakroudi, Characterization of hot-pressed graphene reinforced ZrB2–SiC composite, Materials Science & Engineering A, vol. 625, pp. 385–392, 2015. [42] M. Shahedi Asl, F. Golmohammadi, M. Ghassemi Kakroudi, M. Shokouhimehr, Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part I: Densification behavior, Ceramics International, vol. 42, pp. 4498–4506, 2016. [43] R. Roy, A primer on the Taguchi method. New York: Van Nostrand Rheinhold, 1990. [44] D. C. Montgomery, Introduction to Statistical Quality Control, Sixth Edition, Sixth Edition ed. United States of America: John Wiley & Sons, Inc., 2009. [45] M. Khoeini, A. Nemati, M. Zakeri, M. Tamizifar, H. Samadi, Comprehensive study on the effect of SiC and carbon additives on the pressureless sintering and microstructural and mechanical
characteristics of new ultra-high temperature ZrB2 ceramics, Ceramics International, vol. 41, pp. 11456–11463, 2015. [46] H. L. Liu, G. J. Zhang, J. X. Liu, H. Wu, Synergetic roles of ZrC and SiC in ternary ZrB2– SiC–ZrC ceramics, Journal of the European Ceramic Society, vol. 35, pp. 4389–4397, 2015. [47] Z. Nasiri, M. Mashhadi, A. Abdollahi, Effect of short carbon fiber addition on pressureless densification and mechanical properties of ZrB2–SiC–Csf nanocomposite, Int. Journal of Refractory Metals and Hard Materials, vol. 51, pp. 216–223, 2015. [48] M. Shahedi Asl, M. Ghassemi Kakroudi, M. Rezvani, F. Golestani-Fard, Significance of hot pressing parameters on the microstructure and densification behavior of zirconium diboride, Int. Journal of Refractory Metals and Hard Materials, vol. 50, pp. 140–145, 2015. [49] M. Shahedi Asl, M. Ghassemi Kakroudi, F. Golestani-Fard, H. Nasiri, A Taguchi approach to the influence of hot pressing parameters and SiC content on the sinterability of ZrB2-based composites, International Journal of Refractory Metals and Hard Materials, vol. 51, pp. 81–90, 2015. [50] M. Jaberi Zamharir, M. Shahedi Asl, M. Ghassemi Kakroudi, N. Pourmohammadie Vafa, M. Jaberi Zamharir, Significance of hot pressing parameters and reinforcement size on sinterability and mechanical properties of ZrB2–25 vol% SiC UHTCs, Ceramics International, vol. 41, pp. 9628– 9636, 2015. [51] S. Zhu, W. G. Fahrenholtz, G. E. Hilmas, Enhanced densification and mechanical properties of ZrB2–SiC processed by a preceramic polymer coating route, Scripta Materialia, vol. 59, pp. 123– 126, 2008. [52] M. Mazaheri, S. A. Hassanzadeh-Tabrizi, S. K. Sadrnezhaad, Hot pressing of nanocrystalline zinc oxide compacts: densification and grain growth during sintering, Ceramics International, vol. 35, pp. 991–995, 2009. [53] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, F. Farahbakhsh, M. Shokouhimehr, Interfacial phenomena and formation of nano-particles in porous ZrB2–40 vol% B4C UHTC, Ceramics International, 2016. [54] K. Farhadi, A. Sabahi Namini, M. Shahedi Asl, A. Mohammadzadeh, M. Ghassemi Kakroudi, Characterization of hot pressed SiC whisker reinforced TiB2 based composites, International Journal of Refractory Metals and Hard Materials, vol. 61, pp. 84–90, 2016. [55] I. Farahbakhsh, M. Shahedi Asl, M. Ghassemi Kakroudi, Sinterability improvement of zirconium diboride ceramics fabricated by hot pressing, 13th International Conference on Nanosciences & Nanotechnologies (NN16), Thessaloniki, Greece, 2016. [56] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Mazinani, B. Nayebi, The morphology-dependent properties for nano-carbon-reinforced ZrB2–SiC composited, 12th International Nanoscience and Nanotechnology Conference (NanoTR12), Darica-Kocaeli, Turkey, 2016. [57] I. Farahbakhsh, M. Shahedi Asl, M. Ghassemi Kakroudi, Influence of nano-carbon morphology on densification and fracture toughness of hot-pressed ZrB2–SiC composites, 13th International Conference on Nanosciences & Nanotechnologies (NN16), Thessaloniki, Greece, 2016.
[58] M. Shahedi Asl, Z. Ahmadi, B. Nayebi, M. Ghassemi Kakroudi, Effect of nano-AlN on densification behavior of hot-pressed ZrB2-based ceramics, 12th International Nanoscience and Nanotechnology Conference (NanoTR12), Darica-Kocaeli, Turkey, 2016.
Figures captions: Fig. 1. SEM micrographs of fracture surfaces of ZrB2–SiC–Csf composites hot pressed at 1700 °C: (a) sample 1, (b) sample 2 and (c) sample 3. Fig. 2. (a) SEM nanograph of the marked area in Fig. 1(a), shows the morphology of cauliflower-like nano-oxide impurities in sample 1 and (b) corresponding EDS spectrum. Fig. 3. SEM micrographs of fracture surfaces of ZrB2–SiC–Csf composites hot pressed at 1775 °C: (a) sample 4, (b) sample 5 and (c) sample 6. Fig. 4. SEM micrograph of the fracture surface of sample 4: ZrB2–15 vol% SiC–15 vol% Csf composite hot pressed at 1700 °C for 30 min under 12 MPa. Fig. 5. SEM micrographs of fracture surfaces of ZrB2–SiC–Csf composites hot pressed at 1850 °C: (a) sample 7, (b) sample 8 and (c) sample 9. Fig. 6. SEM micrograph of the fracture surface and corresponding EDS spectra from points A, B, C and D of sample 9: ZrB2–25 vol% SiC–5 vol% Csf composite hot pressed at 1850 °C for 90 min under 12 MPa. Fig. 7. S/N ratios of processing variables for the grain size of ZrB2. Table 1. Processing variables and selected levels. Processing variables
Range
Level 1
Level 2
Level 3
Temperature (°C)
1700–1850
1700
1775
1850
Dwell time (min)
30–90
30
60
90
Pressure (MPa)
8–16
8
12
16
vol% SiC / vol% Csf
1–5
5
2
1
Table 2. Conditions for processing of nine experiments designed by the Taguchi approach. Processing variables Sample 1
Temperature (°C) 1700
Dwell time (min) 30
Pressure (MPa) 8
2
1700
60
12
2
3
1700
90
16
1
4
1775
30
12
1
5
1775
60
16
5
vol% SiC / vol% Csf 5
6
1775
90
8
2
7
1850
30
16
2
8
1850
60
8
1
9
1850
90
12
5
Table 3. Relative density and mean ZrB2 grain size of the hot pressed composites.
Sample
Relative density (g/cm3)
Grain size (µm)
1
80.07 ± 0.08
6.8 ± 2.1
2
86.29 ± 0.08
5.5 ± 1.2
3
89.99 ± 0.10
4.6 ± 0.9
4
86.56 ± 0.06
5.9 ± 1.2
5
82.52 ± 0.08
7.1 ± 1.6
6
91.42 ± 0.12
6.9 ± 1.2
7
99.95 ± 0.24
8.3 ± 1.1
8
97.64 ± 0.02
8.4 ± 2.0
9
92.30 ± 0.20
9.2 ± 1.6
Table 4. ANOVA results describing the significance of each sintering factor on the S/N ratio of ZrB 2 grain size. Hot pressing parameters
Degrees of freedom (f)
Sum of squares (S)
Variance (V)
F-ratio (F)
Pure sum (Sʹ)
Significance P (%)
Temperature
2
19.929
9.964
-
19.929
67.815
Dwell time
2
0.562
0.281
-
0.562
1.913
Pressure
2
2.753
1.376
-
2.753
9.368
vol% SiC / vol% Csf
2
6.142
3.071
-
6.142
20.902
Other/Error
0
-
-
-
-
-
Total
8
29.387
-
-
-
100.000