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Contribution of SiC particle size and spark plasma sintering conditions on grain growth and hardness of TiB2 composites
Ceramics International 43 (2017) 13924–13931
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Contribution of SiC particle size and spark plasma sintering conditions on grain growth and hardness of TiB2 composites
MARK
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Mehdi Shahedi Asla, , Zohre Ahmadib, Soroush Parvizic, Zohre Balakd, Iman Farahbakhshe a
Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran Young Researchers and Elite Club, Miyaneh Branch, Islamic Azad University, Miyaneh, Iran c Department of Materials Engineering, Faculty of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran d Department of Materials Science and Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran e Department of Mechanical Engineering, Quchan Branch, Islamic Azad University, Quchan, Iran b
A R T I C L E I N F O
A BS T RAC T
Keywords: TiB2–SiC composites Spark plasma sintering Taguchi method Hardness Grain growth
In this paper, the significance and contribution of SiC particle size and spark plasma sintering (SPS) parameters on the mean TiB2 grain size and Vickers hardness of TiB2–20 vol% SiC ceramic composites were studied. An L9 orthogonal array was used to design of experiments by the Taguchi method to investigate the effects of four processing parameters including the SPS temperature, SPS soaking time, SPS pressure and SiC particle size. Three levels were selected for each parameter and the analysis of variance (ANOVA) was employed for optimization of processing parameters. The SPS temperature was identified as the most effective parameter on the as-sintered grain size and hardness of TiB2–SiC ceramics, but, the SPS soaking time does not have an obvious influence on these characteristics. The presence of SiC together with the in-situ formed nano-sized TiC phases acted as grain growth inhibitors which was helpful to obtain a fine-grained composite microstructure. The ANOVA disclosed that the sintering temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa and the SiC particle size of 200 nm are the optimal processing conditions. A Vickers hardness of 28.5 GPa was predicted at such optimized conditions which is near to the experimentally measured value of 27.4 GPa as the confirmation test.
1. Introduction
affect mechanical properties and densification. Removing the oxide impurity layers (TiO2 and B2O3) that exist on the surface of the TiB2 starting powders is the initial role of these additive to improve the sinterability [9–13]. However, using the metallic additives and their continuity at the grain boundaries leads to the unfavorable properties after densification for high-temperature applications. Therefore, nonmetallic secondary phases are usually required to improve the sinterability and high-temperature performance of TiB2 [14]. Various non-metallic additive such as SiC, AlN, Si3N4, ZrO2, MoSi2 and WC were added to reach higher densities and good mechanical properties for TiB2-based ceramics [15–28]. SiC addition results in inhibiting the grain growth, enhancing the mechanical properties, and improving the sinterability of TiB2 by removing the oxide impurity layer as well as forming a glassy SiO2 phase during the sintering process [22]. Also, pervious research works displayed that adding SiC to TiB2 leads to the formation of in-situ TiC phase as a result of a chemical reaction between TiO2 and SiC components. Such a phenomenon enhances the densification according to high density of TiC phase [15–17]. Similar to TiB2–SiC composites, such phenomena are observed in ZrB2–SiC ceramics doped with other additives, i.e. the formation of ZrC
Titanium diboride is one of the ultra-high temperature ceramics (UHTCs) that has unique mechanical and physical properties such as high melting point, high hardness, good abrasion resistance, high elastic modulus, and excellent electrical and thermal properties. This material is a potential candidate to use in different industries owing to such characteristics including cutting tools, wear-resistant parts, armor materials, wear-resistance parts, and cathode materials for aluminum refining [1– 6]. However, some limitations such as low sinterability, poor fracture toughness, weak flexural strength, and low oxidation resistance have restricted its applications. Low self-diffusion coefficient and strong covalent bonding can also lead to the difficulty in mass transport phenomenon which limits the densification process. Hence, an elevated temperature, upper than 2000 °C, is necessary for sintering of TiB2. However, exaggerated grain growth and resultant microcracks are serious challenges against the mechanical performance during the cooling step [7,8]. Some metallic and non-metallic additives are introduced to TiB2 as sintering aids to improve its sinterability. Additive materials can directly
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Corresponding author. E-mail address: [email protected] (M. Shahedi Asl).
http://dx.doi.org/10.1016/j.ceramint.2017.07.121 Received 8 June 2017; Received in revised form 15 July 2017; Accepted 15 July 2017 Available online 16 July 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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during the sintering. In-situ formed ZrC phase was detected when short carbon fiber [29], graphene [30], nano-graphite [31] or nano-ZrO2 [32,33] were added to ZrB2–SiC composites. However, it was not seen in the case of other carbon or carbide sintering aids like B4C [34,35], carbon nanotube [36], carbon black [37], and nitrogen sources such as SiAlON [38], AlN [39], and Si3N4 [40] or in ZrB2–SiC composites [41–46]. Grain growth has industrial importance because of the impressionability of microstructure and mechanical properties from final grain size. For example, using fine-grained materials is usually needed in structural application at low temperature to reach high toughness and strength. While, a large-grained microstructure is required to improve the creep resistant at high temperature applications. Therefore, the optimization of grain growth has specific importance for controlling both mechanical properties and microstructure of fabricated materials during the manufacturing process. Some factors affect the final grain size during the densification process are: initial powder size, presence of impurities, addition of secondary phases, and processing condition such as soaking time, temperature, and applied pressure [47,48]. Several papers have been published on the effect of SiC addition on mechanical properties of TiB2-based ceramics. It was shown that the addition of SiC to TiB2 is an effective way to enhance hardness, toughness, and sinterability [15,16]. Different densification routes such as hot pressing, pressureless sintering, and spark plasma sintering are broadly employed for sintering of TiB2-based materials. Hot pressing and spark plasma sintering are known as the effective processes to reduce the sintering temperature, in order to obtain fully dense as well as fine-grained materials. In both methods, an external pressure is applied during the sintering, but the soaking time of spark plasma sintering is very shorter than that of hot pressing [49,50]. The influences of processing parameters on the characteristics of HfB2 [51] and ZrB2-based ceramic materials [29,32,33,41–43,49,52– 54] were comprehensively discussed in literature. To the best of our knowledge, there is no published paper on the effect of processing parameters and SiC particle size on grain growth and hardness of spark plasma sintered TiB2–SiC composites. Hence, in this research work, the effects of four processing parameters including the spark plasma sintering temperature, the soaking time, the applied external pressure, and the SiC particle size on the TiB2 grain size, and hardness of ZrB2– SiC composites were investigated using the Taguchi method. 2. Experimental procedure 2.1. Design of experiment The statistical design of experiments, compared to the classic methods, is an interesting methodology to escape performing a large number of tests, save time, and optimize the processing conditions [55–57]. Hence, in this research, Taguchi methodology was employed to design the experiments as well as determining the influence of processing parameters and specifying the relationship between input and output data. A property can be analyzed in three classifications: lower is better, higher is better or nominal is better. Obtaining both higher hardness and finer microstructure is demanded in the current study; therefore, the statistical estimations are performed with the “higher is better” and the “lower is better” classes for evaluation of hardness and TiB2 grain size, respectively. In addition, the signal-tonoise ratios (S/N) are employed to investigate the influences of processing parameters on the TiB2 grain size and hardness. The analysis of variance (ANOVA) is used as a statistical evaluation technique to calculate the significance and contribution of processing parameters on the grain size and hardness of the samples, as the output results. Moreover, the optimal processing condition is predicted by signal to noise ratios and analysis of variance. The statistical analyses and mathematical calculations are completed using Qualitek-4 (Nutek Inc., USA) which is a user-friendly software developed for the design of
Table 1 Processing conditions established upon four parameters in three levels. Parameter
Level 1
Level 2
Level 3
SPS temperature (°C) SPS soaking time (min) SPS pressure (MPa) SiC particle size (nm)
1600 5 10 2000
1700 10 20 200
1800 15 30 20
experiments by Taguchi method. The influences of four processing parameters, with three levels, are investigated in this research work. The mentioned parameters are as follows: SPS temperature (1600 °C, 1700 °C and 1800 °C), SPS soaking time (5 min, 10 min and 15 min), SPS pressure (10 MPa, 20 MPa and 30 MPa), and SiC particle size (2 µm, 200 nm and 20 nm). The processing parameters and selected levels are summarized in Table 1. For studying the mentioned conditions, a number of 81 experimental tests is needed in the classical factorial method, but the Taguchi's design of experiments only requires 9 tests, as described in Table 2 based on an L9 orthogonal array. 2.2. Materials and process Commercially available TiB2 powder (particle size of ∼ 2 µm, purity > 99%, Xuzhou Hongwu Nanometer Material Co., China), α-SiC powder (particle size of ∼ 2 µm, purity > 99%, Carborundum Universal Limited, India), β-SiC powder (particle size of ~ 200 nm, purity > 98%, PlasmaChem GmbH, Germany), and β-SiC powder (particle size of ~ 20 nm, purity > 99.5%, PlasmaChem GmbH, Germany) were used as the raw materials. SEM images of the asreceived materials are shown in Fig. 1 which confirms the justness of particle size of initial powders. At the beginning, SiC powder was dispersed in ethanol (96%) for 60 min using an ultrasonic stirrer (Mercury UC4, Turkey). Then, by adding TiB2 powder to the slurry, mixtures of TiB2–20 vol% SiC was prepared and ball-mixed in a polyethylene cup by zirconia balls for 60 min at 90 rpm. Three powder mixtures, with same composition but containing different SiC particle size, were obtained. The slurries were dried on a rotary heater at 90 °C and then sieved. The powder mixtures were loaded into the graphite dies, lined with flexible graphite foils, to fabricate disk-shaped samples with a diameter of ~ 50 mm and a thickness of ~ 7 mm. Sintering processes were completed in a SPS furnace (SPS-20T-10, China). Spark plasma sintering was performed at temperature of 1600, 1700 or 1800 °C for soaking time of 5, 10 or 15 min under pressure of 10, 20 or 30 MPa, as described in Table 2. At the end of the SPS process, the furnace was naturally cooled to the room temperature and the samples were removed from the die. 2.3. Characterization The surfaces of spark plasma sintered samples were pulverized to detach the graphite layers. The bulk densities of spark plasma sintered Table 2 Processing conditions of nine experiments designed by Taguchi methodology.
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Sample
Temperature (°C)
Soaking time (min)
Pressure (MPa)
SiC particle size (nm)
1 2 3 4 5 6 7 8 9
1600 1600 1600 1700 1700 1700 1800 1800 1800
5 10 15 5 10 15 5 10 15
10 20 30 20 30 10 30 10 20
2000 200 20 20 2000 200 200 20 2000
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Fig. 1. SEM micro/nanographs of starting materials: (a) TiB2, (b) 2-µm SiC, (c) 200-nm SiC and (d) 20-nm SiC.
samples were calculated by the Archimedes technique, using the distilled water as immersion medium. The polishing process was performed using diamond slurries before the microstructure evaluation as well as hardness testing. The microstructural investigation was done by means of a field emission scanning electron microscope (FESEM: Sigma/VP, Zeiss). The mean TiB2 grain size in the as-sintered samples was estimated from the SEM fractographs by an image processing software (Image J: 1.44p, Wayne Rasband, National Institute of Health, USA). The phase analysis was performed using an X-ray diffractomerer (XRD: Cu lamp, λ = 1.54 Å, Philips PW1800). The hardness was measured by a Vickers indenter (ZHV10, Zwick Roell, Germany) on the polished surface of spark plasma sintered samples with 49 N load. The average hardness value for each sample was achieved after five indentations. Thermodynamical calculation was done using a software (HSC Chemistry, Ver. 5.11, Outokumpu Research Oy, Pori, Finland).
Table 3 Experimental results and corresponding S/N ratios for relative density of composites and TiB2 grain size. Sample
Relative density (g/cm3)
S/N ratio
Grain size (µm)
S/N ratio
1 2 3 4 5 6 7 8 9 Grand average
76.8 ± 0.3 81.7 ± 0.5 87.0 ± 2.5 85.2 ± 0.2 87.6 ± 0.3 90.2 ± 0.3 97.1 ± 0.5 88.3 ± 0.3 87.7 ± 0.5 86.9
37.71 38.24 38.75 38.61 38.85 39.11 39.74 38.92 38.86 38.75
2.8 ± 0.1 2.2 ± 0.1 2.5 ± 0.1 2.6 ± 0.2 3.6 ± 0.2 3.3 ± 0.4 3.5 ± 0.4 5.1 ± 0.5 5.5 ± 0.5 3.5
−9.0 −6.9 −8.0 −8.4 −11.2 −10.6 −11.1 −14.3 −15.0 −10.5
As previously expressed in Section 2.1, the statistical analysis of TiB2 grain size is performed using the “lower is better” class. Therefore, the S/N ratios for this class are calculated by the following equation [56]:
3. Results and discussion The results of relative density and mean TiB2 grain size in the spark plasma sintered TiB2–SiC composites as well as their corresponding S/ N ratios are reported in Table 3. Relative density of a sintered material not only shows how perfect the sample is consolidated, but also controls its characteristics such as hardness and toughness. The density analysis and densification behavior of TiB2–SiC composites will be published elsewhere.
⎛1 S = − 10 log⎜⎜ N ⎝n
n
⎞
i=1
⎠
∑ y2i ⎟⎟
(1)
where yi (i = 1, 2 …n) shows the response values with “n” repetitions. Fig. 2 shows the main effects plots of S/N ratios for mean TiB2 grain size in the TiB2–SiC composites versus the processing parameters. Based on these plots, obtaining a fine-grained microstructure is conceivable by setting the
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Fig. 2. Main effect plots of S/N ratios for TiB2 grain size in TiB2–SiC composites.
SPS temperature and SPS soaking time at their inferior levels. Such an outcome expresses that the TiB2 grains grow by increasing the sintering temperature and/or dwell time of SPS process. In addition, it seems that the SiC particle size of 200 nm can prevent the coarsening of TiB2 grains in a beneficial manner, compared to the other sizes. Fig. 3 shows a SEM nanograph of the polished surface of sample 6 which was spark plasma sintered at 1700 °C for 15 min under 10 MPa
and reinforced with 200-nm SiC particles (as shown in Table 2). Microstructural observation along with EDS spectera verified the presence of three distinctive phases in the as-sintered sample. Regarding the XRD results (not shown here), the EDS analysis disclosed that the gray (Point A), the dimgray (Point 2), and the lightgray (Point 3) phases corresponded to TiB2, SiC and TiC, respectively. As it can be clearly seen in Fig. 3, the in-situ formation
Fig. 3. SEM nanograph of the polished surface and corresponding EDS spectra from points A, B and C of sample 6: spark plasma sintered at 1700 °C for 15 min under 10 MPa and reinforced with 200-nm SiC particles.
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of nano-sized interfacial TiC phases was due to the chemical reaction between the TiO2 oxide layer on the surface of TiB2 and SiC, in accord with the following equation [15]:
2TiO2 + 3SiC = 2TiC + 3SiO (g) + CO (g)
ΔG° = 1113.30–0.68 T (KJ) (2)
This reaction is thermodynamically favorable above 1640 °C; hence, it occurs at the vacuum condition of the SPS process even at lower temperatures. It was reported that the presence of oxide impurities on the surface of diborides (e.g. ZrB2, TiB2 and HfB2) [15,52] not only disturbs the densification process, but also prompts the grain growth. In this research, on one hand, by the addition of SiC to the TiB2, the oxide impurities were reacted and eliminated from the surface of TiB2 particles (Eq. (2)) which led to the prevention of fanatic grain growth. On the other hand, the unreacted SiC grains as well as the in-situ formed interfacial TiC phases acted as the colonized grain growth inhibitors by pinning the grain boundaries. ANOVA results of significance of processing parameters on the S/N ratio of TiB2 grain size are reported in Table 4. The significances of SPS temperature, SPS soaking time, SPS pressure and SiC particle size are estimated 75%, 8%, 5% and 12%, respectively. The significance pie chart of the mentioned processing parameters on the TiB2 grain size is also presented in Fig. 4. Hence, it can be demonstrated that the SPS temperature is the most striking factor on the TiB2 grain size in TiB2– SiC composites. Hardness is an explanation of how material resists against the plastic deformation while an external compressive force is applied [53]. The results of hardness measurements, by Vickers indentation method, and their corresponding S/N ratios are reported in Table 5. The hardness values showed a relatively wide range and varied from ~ 16 GPa to ~ 28 GPa. Sample 1 and sample 7 have the lowest and the highest hardness values, respectively (Table 5). It should be reminded that the former has the minimum and the latter has the maximum relative density values, as reported in Table 3. Comparing the density and hardness measurements in Tables 3 and 5, it seems that the relative density is an important property which controls the hardness value. In other words, as the pores in a sample show no resistance against the indentation load of Vickers machine, more porous materials have lower hardness values. The statistical analysis of hardness is fulfilled by the “higher is better” class, as already declared in Section 2.1. Hence, the S/N ratios for this class are calculated by the subsequent formula [56,59]:
⎛1 S = − 10 log⎜⎜ N ⎝n
n
∑ i=1
1 ⎞⎟ ⎟ y 2i ⎠
(3)
The main effects plots of S/N ratios versus the processing parameters for the hardness of TiB2–SiC composites are shown in Fig. 5. A sharp increase in the S/N ratio is clearly seen in this figure with raising the SPS temperature. A slight decrease, and then, a higher increase in the S/N ratios is monitored with increasing the SPS pressure or SPS soaking time. Contrarily, a sharp increase, and then, an obvious drop in the S/N ratio is happened with decreasing SiC particle size. In general, the hardness of TiB2–SiC ceramics boosts with the increase in the SPS conditions (temperature, time and pressure). In addition, from the standpoint of SiC particle size, the highest value of S/N ratio for the hardness is
Fig. 4. Significance pie chart of processing parameters on TiB2 grain size in TiB2–SiC composites. Table 5 Experimental results and corresponding S/N ratios for Vickers hardness (GPa) of TiB2– SiC composites. Sample
Trial 1
1 14.1 2 16.7 3 19.5 4 17.1 5 23.3 6 23.4 7 27.2 8 19.8 9 18.3 Grand average
Trial 2
Trial 3
Trial 4
Trial 5
Average
S/N ratio
16.1 21.8 22.2 21.6 23.1 20.9 30.6 23.4 22.2
17.4 17.1 19.2 22.2 19.1 25.1 26.1 22.4 23.6
19.1 19.2 23.3 20.1 20.7 23.9 27.2 21.6 22.8
13.7 17.6 22.4 20.3 21.9 22.4 28.5 23.1 21.2
16.1 18.5 21.3 20.3 21.6 23.1 27.9 22.1 21.6 21.4
23.92 25.21 26.49 26.02 26.63 27.24 28.88 26.82 26.59 26.42
corresponded to the size of 200 nm. Therefore, the optimal SPS conditions for the TiB2-based ceramic composites are 1800 °C, 15 min, and 30 MPa which have been reinforced with 200-nm SiC particles. The ANOVA results of significance of processing parameters on the Vickers hardness of TiB2–SiC composites are reported in Table 6. Based on this analysis, the SPS temperature is the main controlling parameter which has an important effect on the hardness. After the temperature, the SPS pressure and SiC particle size have also effective roles on the hardness of composites. On the basis of ANOVA, the SPS soaking time does not have a significant influence on the hardness. The summary of AVONA results is illustrated in Fig. 6, in the form of significance pie chart of processing parameters on the hardness. The significances of SPS temperature, SPS soaking time, SPS pressure and SiC particle size are estimated 51%, 4%, 25%, and 20%, respectively. Table 7 describes the performance and contribution of processing conditions for Vickers hardness at optimal state. The highest hardness value is obtainable at such optimal conditions i.e. the sintering
Table 4 ANOVA results explaining the significance of processing parameters on the S/N ratio of TiB2 grain size. Processing parameters
Degrees of freedom
Sum of squares
Variance
Pure sum
Significance (%)
SPS temperature SPS soaking time SPS pressure SiC particle size Total
2 2 2 2 8
46.606 4.753 2.981 7.524 61.866
23.303 2.376 1.490 3.762 –
46.606 4.753 2.981 7.524 –
75.333 7.683 4.819 12.163 100.000
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Fig. 5. Main effect plots of S/N ratios for Vickers hardness of TiB2–SiC composites.
Table 6 ANOVA results explaining the significance of processing parameters on Vickers hardness. Processing parameters
Degrees of freedom
Sum of squares
Variance
Pure sum
Significance (%)
SPS temperature SPS soaking time SPS pressure SiC particle size Total
2 2 2 2 8
7.591 0.558 3.727 2.929 14.807
3.795 0.279 1.863 1.464 –
7.591 0.558 3.727 2.929 –
51.265 3.769 25.173 19.786 100.000
Fig. 6. Significance pie chart of processing parameters on Vickers hardness of TiB2–SiC composites.
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Table 7 Optimum conditions and performance for Vickers hardness of TiB2–SiC composites. Parameter
Level
Level description
SPS temperature (°C) 3 1800 SPS soaking time 3 15 (min) SPS pressure (MPa) 3 30 SiC particle size (nm) 2 200 Total contributions of all parameters Current grand average of hardness Expected result at optimal processing conditions Experimented result at optimal processing conditions
Contribution (GPa) 2.5 0.6 2.2 1.8 7.1 21.4 28.5 27.4 ± 3.1
temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa, and the SiC particle size of 200 nm. On one hand, the grand average of Vickers hardness values of nine samples was calculated as 21.4 GPa (Table 5). On the other hand, the contributions of SPS temperature, SPS soaking time, SPS pressure, and SiC particle size on the hardness improvement at the determined optimal conditions are 2.5, 0.6, 2.2, and 1.8 GPa, respectively. Hence, the total contributions of all processing parameters is 7.1 GPa. Therefore, a hardness of 28.5 GPa is expected at optimal processing conditions. The experimentally achieved result at the optimal conditions for checking the reliability and validity of ANOVA predictions is called the confirmation test. This means that the confirmation test is performed to compare the predicted outcomes with the experimental results. A Vickers hardness of 27.4 GPa was experimentally measured as the result of confirmation test at the optimal conditions. This outcome is relatively near to the predicted value of 28.5 GPa. Hence, the usefulness of the design of experiments by Taguchi's method and ANOVA optimization for the processing parameters is verified. 4. Conclusions The effects of four processing parameters including the SPS temperature, SPS soaking time, SPS pressure, and SiC particle size on the TiB2 grain size, and hardness of TiB2–20 vol% SiC composites were studied by the Taguchi method. The significances of sintering temperature, time, pressure and SiC particle size on the grain growth were estimated 75%, 8%, 5% and 12%, respectively. Therefore, the SPS temperature was recognized as the most significant parameter which controls the assintered TiB2 grain size. The microstructural investigations verified that the SiC and in-situ formed nano-sized TiC phases had effective roles in achieving a fine-grained TiB2–SiC composite due to their functions as grain growth inhibitors. The significances of SPS temperature, time, pressure, and SiC particle size on the hardness of composites were obtained 51%, 4%, 25% and 20%, respectively. Hence, the soaking time did not show a drastic influence on the grain size and hardness of spark plasma sintered ceramics. The confirmation test was performed at the optimal processing conditions suggested by AVOVA as follows: the SPS temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa, and the SiC particle size of 200 nm. The predicted result for hardness at the optimal conditions (28.5 GPa) was close to the experimentally measured outcome of the confirmation test (27.4 GPa). Acknowledgments The authors would like to thank the Arta Research Group (www. artaresearch.com) for its financial support through Grant ARG-2500-001. References [1] S.K. Bhaumik, C. Divakar, A.K. Singh, G.S. Upadhyaya, Synthesis and sintering of TiB2 and TiB2–TiC composite under high pressure, Mater. Sci. Eng. A 279 (2000) 275–281. [2] A. Li, Y. Zhen, Q. Yin, L. Ma, Y. Yin, Microstructure and properties of (SiC, TiB2)/ B4C composites by reaction hot pressing, Ceram. Int. 32 (2006) 849–856.
[3] H. Zhao, Y.B. Cheng, Formation of TiB2-TiC composites by reactive sintering, Ceram. Int. 25 (1999) 353–358. [4] J. Matsushita, T. Suzuki, A. Sano, Wire electrical discharge machining of TiB2 composite, J. Ceram. Soc. Jpn. 100 (1992) 219–222. [5] M. Shibuya, Y. Yamamoto, M. Ohyanagi, Simultaneous densification and phase decomposition of TiB2–WB2 solid solutions activated by cobalt boride addition, J. Eur. Ceram. Soc. 27 (2007) 307–312. [6] G.J. Zhang, Z.Z. Jin, X.M. Yue, Effects of Ni addition on mechanical properties of TiB2/SiC composites prepared by reactive hot pressing (RHP)), J. Mater. Sci. 32 (1997) 2093–2097. [7] A. Sabahi Namini, M. Azadbeh, M. Shahedi Asl, Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites, Adv. Powder Technol. 28 (2017) 1564–1572. [8] T.S. Srivatsan, G. Guruprasad, D. Black, R. Radhakrishnan, T.S. Sudarshan, Influence of TiB2 content on microstructure and hardness of TiB2–B4C composite, Powder Technol. 159 (2005) 161–167. [9] M.A. Janney, Mechanical properties and oxidation behaviour of a hot-pressed SiC15 vol%-TiB2 composite, Am. Ceram. Soc. Bull. 66 (1987) 322–324. [10] G.J. Zhang, Z.Z. Jin, X.M. Yue, Reaction synthesis of TiB2-SiC composites from TiH2-Si-B4C, Mater. Lett. 25 (1995) 97–100. [11] M.L. Gu, C.Z. Huang, J. Wang, Improvements in mechanical properties of TiB2 ceramics tool materials by the dispersion of Al2O3 particles, Key Eng. Mater. 315/ 316 (2006) 123. [12] Y.S. Kang, S.H. Kang, D.J. Kim, Effect of addition of Cr on the sintering of TiB2 ceramics, J. Mater. Sci. 40 (15) (2005) 4153–4155. [13] J. Jaroszewicz, A. Michalski, Preparation of a TiB2 composite with a nickel matrix by pulse plasma sintering with combustion synthesis, J. Eur. Ceram. Soc. 26 (2006) 2427–2430. [14] S. Baik, P.F. Becher, Effect of oxygen contamination of densification of TiB2, J. Am. Ceram. Soc., vol. 70, 1987, pp. 527–530. [15] 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, Int. J. Refract. Met. Hard Mater., vol. 51, 2015, pp. 169–179. [16] K. Farhadi, A.S. Namini, M. Shahedi Asl, A. Mohammadzadeh, M. Ghassemi Kakroudi, Characterization of hot pressed SiC whisker reinforced TiB2 based composites, Int. J. Refract. Met. Hard Mater. 61 (2016) 84–90. [17] M. Shahedi Asl, A. Sabahi Namini, M. Ghassemi Kakroudi, Influence of silicon carbide addition on the microstructural development of hot pressed zirconium and titanium diborides, Ceram. Int. 42 (2016) 5375–5381. [18] T.S.R.C. Murthy, B. Basu, R. Balasubramaniam, A.K. Suri, C. Subramonian, R.K. Fotedar, Processing and properties of TiB2 with MoSi2 sinter-additive: a first report, J. Am. Ceram. Soc. 89 (1) (2006) 131–138. [19] L.H. Li, H.E. Kim, E.S. Kang, Sintering and mechanical properties of titanium diboride with aluminum nitride as a sintering aid, J. Eur. Ceram. Soc. 22 (2002) 973–977. [20] S. Torizuka, T. Kishi, Effect of SiC and ZrO2 on sinterability and mechanical properties of titanium nitride, titanium carbonitride and titanium diboride, Mater. Trans. JIM 37 (4) (1996) (7872-787). [21] S. Torizuka, J. Harada, H. Nishio, High strength TiB2, Ceram. Eng. Sci. Proc., vol. 11, no. 9-10, 1990, pp. 1454–1460. [22] S. Torizuka, K. Sato, H. Nishio, T. Kishi, Effect of SiC on interfacial reaction and sintering mechanism of TiB2, J. Am. Ceram. Soc., vol. 78, no. 6, 1995, pp. 1606– 1610. [23] T. Graziani, A. Bellosi, Sintering and characterization of TiB2–B4C–ZrO2 composites, Mater. Manuf. Process. 9 (4) (1994) 767–780. [24] E.S. Kang, C.H. Kim, Improvements in mechanical properties of TiB2 by the dispersion of B4C particles, J. Mater. Sci. 25 (1990) 580–584. [25] Y. Murata, H.P. Julien, E.D. Whitney, Densification and wear resistance of ceramic systems: I. titanium diboride, Ceram. Bull. 46 (7) (1967) 643–648. [26] J.H. Park, Y. Koh, H. Kim, C. Hwang, E. Kong, Densification and mechanical properties of titanium diboride with silicon nitride as sintering aid, J. Am. Ceram. Soc. 82 (11) (1999) 3037–3042. [27] R. Telle, S. Meyer, G. Petzow, E.D. Franz, Sintering behavior and phase reactions of TiB2 with ZrO2 additives, Mater. Sci. Eng. A 105/106 (1988) 125–129. [28] Y. Muraoka, M. Yoshinaka, K. Hirota, O. Yamaguchi, Hot isostatic pressing of TiB2–ZrO2 (2 mol% Y2O3) composite powders, Mater. Res. Bull. 31 (7) (1996) 787–792. [29] 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, Ceram. Int. 42 (2016) 4498–4506. [30] M. Shahedi Asl, M. Ghassemi Kakroudi, Characterization of hot-pressed graphene reinforced ZrB2–SiC composite, Mater. Sci. Eng. A 625 (2015) 385–392. [31] 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, Ceram. Int. 41 (2015) 5843–5851. [32] 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, Ceram. Int. 41 (2015) 8388–8396. [33] N. Pourmohammadie Vafa, B. Nayebi, 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 II: mechanical behavior, Ceram. Int. 42 (2016) 2724–2733. [34] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, A fractographical approach to the sintering process in porous ZrB2–B4C binary composites, Ceram. Int. 41 (2015)
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M. Shahedi Asl et al. 379–387. [35] 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, Ceram. Int. 42 (2016) 17009–17015. [36] M. Shahedi Asl, I. Farahbakhsh, B. Nayebi, Characteristics of multi-walled carbon nanotube toughened ZrB2–SiC ceramic composite prepared by hot pressing, Ceram. Int. 42 (2016) 1950–1958. [37] I. Farahbakhsh, Z. Ahmadi, M. Shahedi Asl, Densification, microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black, Ceram. Int. 43 (2017) 8411–8417. [38] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, P. Pirmohammadi, M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered SiAlONdoped ZrB2–SiC composites, Mater. Charact. 102 (2015) 137–145. [39] Z. Ahmadi, B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered AlN-doped ZrB2–SiC composites, Mater. Charact. 110 (2015) 77–85. [40] Z. Ahmadi, B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, I. Farahbakhsh, Sintering behavior of ZrB2–SiC composites doped with Si3N4: a fractographical approach, Ceram. Int. 43 (2017) 9699–9708. [41] 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, Ceram. Int. 41 (2015) 6439–6447. [42] 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, Int. J. Refract. Met. Hard Mater. 51 (2015) 81–90. [43] 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, Ceram. Int. 41 (2015) 9628–9636. [44] M. Shahedi Asl, M. Ghassemi Kakroudi, Fractographical assessment of densification mechanisms in hot pressed ZrB2–SiC composites, Ceram. Int. 40 (2014) 15273–15281. [45] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, M. Shokouhimehr, Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol% SiC composites, Int. J. Refract. Met. Hard Mater. 54 (2016) 7–13.
[46] M. Shahedi Asl, M. Ghassemi Kakroudi, A processing–microstructure correlation in ZrB2–SiC composites hot-pressed under a load of 10 MPa, Universal J. Mater. Sci. 3 (2015) 14–21. [47] F. Guanghai, Y. Yanqing, Z. Guangming, Z. Wei, L. Xian, H. Bin, Effect of hot isostatic pressing parameters on the microstructure sand grain growth behavior of the matrix of SiCf/Ti-6Al-4V composites, Rare Met. Mater. Eng. 43 (2014) 1839–1845. [48] M. Shahedi Asl, M. Ghassemi Kakroudi, S. Noori, Hardness and toughness of hot pressed ZrB2–SiC composites consolidated under relatively low pressure, J. Alloy. Compd. 679 (2015) 481–487. [49] M. Shahedi Asl, M. Ghassemi Kakroudi, I. Farahbakhsh, B. Mazinani, Z. Balak, Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: grain growth, Ceram. Int. 42 (2016) 18612–18619. [50] A.S. Namini, M. Azadbeh, M. Shahedi Asl, Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites, Adv. Powder Technol. 28 (2017) 1564–1572. [51] R.K. Enneti, C. Carney, S.J. Park, Sundar V. Atre, Taguchi analysis on the effect of process parameters on densification during spark plasma sintering of HfB2-20 SiC, Int. J. Refract. Met. Hard Mater. 31 (2012) 293–296. [52] 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. J. Refract. Met. Hard Mater. 50 (2015) 140–145. [53] 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, Int. J. Refract. Met. Hard Mater. 50 (2015) 313–320. [54] Z. Balak, M. Shahedi Asl, M. Azizieh, H. Kafashan, R. Hayati, Effect of different additives and open porosity on fracture toughness of ZrB2–SiC-based composites prepared by SPS, Ceram. Int. 43 (2) (2017) 2209–2220. [55] S. Naghibi, M.A. Faghihi Sani, H.R. Madaah Hosseini, Application of the statistical Taguchi method to optimize TiO2 nanoparticles synthesis by the hydrothermal assisted sol–gel technique, Ceram. Int. 40 (2014) 4193–4201. [56] P.J. Ross, Taguchi Techniques for Quality Engineering, McGraw- Hill, New York, 1996. [57] Z. Balak, M. Azizieh, H. Kafashan, M. Shahedi Asl, Z. Ahmadi, Optimization of effective parameters on thermal shock resistance of ZrB2-SiC-based composites prepared by SPS: using Taguchi design, Mater. Chem. Phys. 196 (2017) 333–340.
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Contribution of SiC particle size and spark plasma sintering conditions on grain growth and hardness of TiB2 composites
MARK
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Mehdi Shahedi Asla, , Zohre Ahmadib, Soroush Parvizic, Zohre Balakd, Iman Farahbakhshe a
Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran Young Researchers and Elite Club, Miyaneh Branch, Islamic Azad University, Miyaneh, Iran c Department of Materials Engineering, Faculty of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran d Department of Materials Science and Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran e Department of Mechanical Engineering, Quchan Branch, Islamic Azad University, Quchan, Iran b
A R T I C L E I N F O
A BS T RAC T
Keywords: TiB2–SiC composites Spark plasma sintering Taguchi method Hardness Grain growth
In this paper, the significance and contribution of SiC particle size and spark plasma sintering (SPS) parameters on the mean TiB2 grain size and Vickers hardness of TiB2–20 vol% SiC ceramic composites were studied. An L9 orthogonal array was used to design of experiments by the Taguchi method to investigate the effects of four processing parameters including the SPS temperature, SPS soaking time, SPS pressure and SiC particle size. Three levels were selected for each parameter and the analysis of variance (ANOVA) was employed for optimization of processing parameters. The SPS temperature was identified as the most effective parameter on the as-sintered grain size and hardness of TiB2–SiC ceramics, but, the SPS soaking time does not have an obvious influence on these characteristics. The presence of SiC together with the in-situ formed nano-sized TiC phases acted as grain growth inhibitors which was helpful to obtain a fine-grained composite microstructure. The ANOVA disclosed that the sintering temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa and the SiC particle size of 200 nm are the optimal processing conditions. A Vickers hardness of 28.5 GPa was predicted at such optimized conditions which is near to the experimentally measured value of 27.4 GPa as the confirmation test.
1. Introduction
affect mechanical properties and densification. Removing the oxide impurity layers (TiO2 and B2O3) that exist on the surface of the TiB2 starting powders is the initial role of these additive to improve the sinterability [9–13]. However, using the metallic additives and their continuity at the grain boundaries leads to the unfavorable properties after densification for high-temperature applications. Therefore, nonmetallic secondary phases are usually required to improve the sinterability and high-temperature performance of TiB2 [14]. Various non-metallic additive such as SiC, AlN, Si3N4, ZrO2, MoSi2 and WC were added to reach higher densities and good mechanical properties for TiB2-based ceramics [15–28]. SiC addition results in inhibiting the grain growth, enhancing the mechanical properties, and improving the sinterability of TiB2 by removing the oxide impurity layer as well as forming a glassy SiO2 phase during the sintering process [22]. Also, pervious research works displayed that adding SiC to TiB2 leads to the formation of in-situ TiC phase as a result of a chemical reaction between TiO2 and SiC components. Such a phenomenon enhances the densification according to high density of TiC phase [15–17]. Similar to TiB2–SiC composites, such phenomena are observed in ZrB2–SiC ceramics doped with other additives, i.e. the formation of ZrC
Titanium diboride is one of the ultra-high temperature ceramics (UHTCs) that has unique mechanical and physical properties such as high melting point, high hardness, good abrasion resistance, high elastic modulus, and excellent electrical and thermal properties. This material is a potential candidate to use in different industries owing to such characteristics including cutting tools, wear-resistant parts, armor materials, wear-resistance parts, and cathode materials for aluminum refining [1– 6]. However, some limitations such as low sinterability, poor fracture toughness, weak flexural strength, and low oxidation resistance have restricted its applications. Low self-diffusion coefficient and strong covalent bonding can also lead to the difficulty in mass transport phenomenon which limits the densification process. Hence, an elevated temperature, upper than 2000 °C, is necessary for sintering of TiB2. However, exaggerated grain growth and resultant microcracks are serious challenges against the mechanical performance during the cooling step [7,8]. Some metallic and non-metallic additives are introduced to TiB2 as sintering aids to improve its sinterability. Additive materials can directly
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Corresponding author. E-mail address: [email protected] (M. Shahedi Asl).
http://dx.doi.org/10.1016/j.ceramint.2017.07.121 Received 8 June 2017; Received in revised form 15 July 2017; Accepted 15 July 2017 Available online 16 July 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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during the sintering. In-situ formed ZrC phase was detected when short carbon fiber [29], graphene [30], nano-graphite [31] or nano-ZrO2 [32,33] were added to ZrB2–SiC composites. However, it was not seen in the case of other carbon or carbide sintering aids like B4C [34,35], carbon nanotube [36], carbon black [37], and nitrogen sources such as SiAlON [38], AlN [39], and Si3N4 [40] or in ZrB2–SiC composites [41–46]. Grain growth has industrial importance because of the impressionability of microstructure and mechanical properties from final grain size. For example, using fine-grained materials is usually needed in structural application at low temperature to reach high toughness and strength. While, a large-grained microstructure is required to improve the creep resistant at high temperature applications. Therefore, the optimization of grain growth has specific importance for controlling both mechanical properties and microstructure of fabricated materials during the manufacturing process. Some factors affect the final grain size during the densification process are: initial powder size, presence of impurities, addition of secondary phases, and processing condition such as soaking time, temperature, and applied pressure [47,48]. Several papers have been published on the effect of SiC addition on mechanical properties of TiB2-based ceramics. It was shown that the addition of SiC to TiB2 is an effective way to enhance hardness, toughness, and sinterability [15,16]. Different densification routes such as hot pressing, pressureless sintering, and spark plasma sintering are broadly employed for sintering of TiB2-based materials. Hot pressing and spark plasma sintering are known as the effective processes to reduce the sintering temperature, in order to obtain fully dense as well as fine-grained materials. In both methods, an external pressure is applied during the sintering, but the soaking time of spark plasma sintering is very shorter than that of hot pressing [49,50]. The influences of processing parameters on the characteristics of HfB2 [51] and ZrB2-based ceramic materials [29,32,33,41–43,49,52– 54] were comprehensively discussed in literature. To the best of our knowledge, there is no published paper on the effect of processing parameters and SiC particle size on grain growth and hardness of spark plasma sintered TiB2–SiC composites. Hence, in this research work, the effects of four processing parameters including the spark plasma sintering temperature, the soaking time, the applied external pressure, and the SiC particle size on the TiB2 grain size, and hardness of ZrB2– SiC composites were investigated using the Taguchi method. 2. Experimental procedure 2.1. Design of experiment The statistical design of experiments, compared to the classic methods, is an interesting methodology to escape performing a large number of tests, save time, and optimize the processing conditions [55–57]. Hence, in this research, Taguchi methodology was employed to design the experiments as well as determining the influence of processing parameters and specifying the relationship between input and output data. A property can be analyzed in three classifications: lower is better, higher is better or nominal is better. Obtaining both higher hardness and finer microstructure is demanded in the current study; therefore, the statistical estimations are performed with the “higher is better” and the “lower is better” classes for evaluation of hardness and TiB2 grain size, respectively. In addition, the signal-tonoise ratios (S/N) are employed to investigate the influences of processing parameters on the TiB2 grain size and hardness. The analysis of variance (ANOVA) is used as a statistical evaluation technique to calculate the significance and contribution of processing parameters on the grain size and hardness of the samples, as the output results. Moreover, the optimal processing condition is predicted by signal to noise ratios and analysis of variance. The statistical analyses and mathematical calculations are completed using Qualitek-4 (Nutek Inc., USA) which is a user-friendly software developed for the design of
Table 1 Processing conditions established upon four parameters in three levels. Parameter
Level 1
Level 2
Level 3
SPS temperature (°C) SPS soaking time (min) SPS pressure (MPa) SiC particle size (nm)
1600 5 10 2000
1700 10 20 200
1800 15 30 20
experiments by Taguchi method. The influences of four processing parameters, with three levels, are investigated in this research work. The mentioned parameters are as follows: SPS temperature (1600 °C, 1700 °C and 1800 °C), SPS soaking time (5 min, 10 min and 15 min), SPS pressure (10 MPa, 20 MPa and 30 MPa), and SiC particle size (2 µm, 200 nm and 20 nm). The processing parameters and selected levels are summarized in Table 1. For studying the mentioned conditions, a number of 81 experimental tests is needed in the classical factorial method, but the Taguchi's design of experiments only requires 9 tests, as described in Table 2 based on an L9 orthogonal array. 2.2. Materials and process Commercially available TiB2 powder (particle size of ∼ 2 µm, purity > 99%, Xuzhou Hongwu Nanometer Material Co., China), α-SiC powder (particle size of ∼ 2 µm, purity > 99%, Carborundum Universal Limited, India), β-SiC powder (particle size of ~ 200 nm, purity > 98%, PlasmaChem GmbH, Germany), and β-SiC powder (particle size of ~ 20 nm, purity > 99.5%, PlasmaChem GmbH, Germany) were used as the raw materials. SEM images of the asreceived materials are shown in Fig. 1 which confirms the justness of particle size of initial powders. At the beginning, SiC powder was dispersed in ethanol (96%) for 60 min using an ultrasonic stirrer (Mercury UC4, Turkey). Then, by adding TiB2 powder to the slurry, mixtures of TiB2–20 vol% SiC was prepared and ball-mixed in a polyethylene cup by zirconia balls for 60 min at 90 rpm. Three powder mixtures, with same composition but containing different SiC particle size, were obtained. The slurries were dried on a rotary heater at 90 °C and then sieved. The powder mixtures were loaded into the graphite dies, lined with flexible graphite foils, to fabricate disk-shaped samples with a diameter of ~ 50 mm and a thickness of ~ 7 mm. Sintering processes were completed in a SPS furnace (SPS-20T-10, China). Spark plasma sintering was performed at temperature of 1600, 1700 or 1800 °C for soaking time of 5, 10 or 15 min under pressure of 10, 20 or 30 MPa, as described in Table 2. At the end of the SPS process, the furnace was naturally cooled to the room temperature and the samples were removed from the die. 2.3. Characterization The surfaces of spark plasma sintered samples were pulverized to detach the graphite layers. The bulk densities of spark plasma sintered Table 2 Processing conditions of nine experiments designed by Taguchi methodology.
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Sample
Temperature (°C)
Soaking time (min)
Pressure (MPa)
SiC particle size (nm)
1 2 3 4 5 6 7 8 9
1600 1600 1600 1700 1700 1700 1800 1800 1800
5 10 15 5 10 15 5 10 15
10 20 30 20 30 10 30 10 20
2000 200 20 20 2000 200 200 20 2000
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Fig. 1. SEM micro/nanographs of starting materials: (a) TiB2, (b) 2-µm SiC, (c) 200-nm SiC and (d) 20-nm SiC.
samples were calculated by the Archimedes technique, using the distilled water as immersion medium. The polishing process was performed using diamond slurries before the microstructure evaluation as well as hardness testing. The microstructural investigation was done by means of a field emission scanning electron microscope (FESEM: Sigma/VP, Zeiss). The mean TiB2 grain size in the as-sintered samples was estimated from the SEM fractographs by an image processing software (Image J: 1.44p, Wayne Rasband, National Institute of Health, USA). The phase analysis was performed using an X-ray diffractomerer (XRD: Cu lamp, λ = 1.54 Å, Philips PW1800). The hardness was measured by a Vickers indenter (ZHV10, Zwick Roell, Germany) on the polished surface of spark plasma sintered samples with 49 N load. The average hardness value for each sample was achieved after five indentations. Thermodynamical calculation was done using a software (HSC Chemistry, Ver. 5.11, Outokumpu Research Oy, Pori, Finland).
Table 3 Experimental results and corresponding S/N ratios for relative density of composites and TiB2 grain size. Sample
Relative density (g/cm3)
S/N ratio
Grain size (µm)
S/N ratio
1 2 3 4 5 6 7 8 9 Grand average
76.8 ± 0.3 81.7 ± 0.5 87.0 ± 2.5 85.2 ± 0.2 87.6 ± 0.3 90.2 ± 0.3 97.1 ± 0.5 88.3 ± 0.3 87.7 ± 0.5 86.9
37.71 38.24 38.75 38.61 38.85 39.11 39.74 38.92 38.86 38.75
2.8 ± 0.1 2.2 ± 0.1 2.5 ± 0.1 2.6 ± 0.2 3.6 ± 0.2 3.3 ± 0.4 3.5 ± 0.4 5.1 ± 0.5 5.5 ± 0.5 3.5
−9.0 −6.9 −8.0 −8.4 −11.2 −10.6 −11.1 −14.3 −15.0 −10.5
As previously expressed in Section 2.1, the statistical analysis of TiB2 grain size is performed using the “lower is better” class. Therefore, the S/N ratios for this class are calculated by the following equation [56]:
3. Results and discussion The results of relative density and mean TiB2 grain size in the spark plasma sintered TiB2–SiC composites as well as their corresponding S/ N ratios are reported in Table 3. Relative density of a sintered material not only shows how perfect the sample is consolidated, but also controls its characteristics such as hardness and toughness. The density analysis and densification behavior of TiB2–SiC composites will be published elsewhere.
⎛1 S = − 10 log⎜⎜ N ⎝n
n
⎞
i=1
⎠
∑ y2i ⎟⎟
(1)
where yi (i = 1, 2 …n) shows the response values with “n” repetitions. Fig. 2 shows the main effects plots of S/N ratios for mean TiB2 grain size in the TiB2–SiC composites versus the processing parameters. Based on these plots, obtaining a fine-grained microstructure is conceivable by setting the
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Fig. 2. Main effect plots of S/N ratios for TiB2 grain size in TiB2–SiC composites.
SPS temperature and SPS soaking time at their inferior levels. Such an outcome expresses that the TiB2 grains grow by increasing the sintering temperature and/or dwell time of SPS process. In addition, it seems that the SiC particle size of 200 nm can prevent the coarsening of TiB2 grains in a beneficial manner, compared to the other sizes. Fig. 3 shows a SEM nanograph of the polished surface of sample 6 which was spark plasma sintered at 1700 °C for 15 min under 10 MPa
and reinforced with 200-nm SiC particles (as shown in Table 2). Microstructural observation along with EDS spectera verified the presence of three distinctive phases in the as-sintered sample. Regarding the XRD results (not shown here), the EDS analysis disclosed that the gray (Point A), the dimgray (Point 2), and the lightgray (Point 3) phases corresponded to TiB2, SiC and TiC, respectively. As it can be clearly seen in Fig. 3, the in-situ formation
Fig. 3. SEM nanograph of the polished surface and corresponding EDS spectra from points A, B and C of sample 6: spark plasma sintered at 1700 °C for 15 min under 10 MPa and reinforced with 200-nm SiC particles.
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of nano-sized interfacial TiC phases was due to the chemical reaction between the TiO2 oxide layer on the surface of TiB2 and SiC, in accord with the following equation [15]:
2TiO2 + 3SiC = 2TiC + 3SiO (g) + CO (g)
ΔG° = 1113.30–0.68 T (KJ) (2)
This reaction is thermodynamically favorable above 1640 °C; hence, it occurs at the vacuum condition of the SPS process even at lower temperatures. It was reported that the presence of oxide impurities on the surface of diborides (e.g. ZrB2, TiB2 and HfB2) [15,52] not only disturbs the densification process, but also prompts the grain growth. In this research, on one hand, by the addition of SiC to the TiB2, the oxide impurities were reacted and eliminated from the surface of TiB2 particles (Eq. (2)) which led to the prevention of fanatic grain growth. On the other hand, the unreacted SiC grains as well as the in-situ formed interfacial TiC phases acted as the colonized grain growth inhibitors by pinning the grain boundaries. ANOVA results of significance of processing parameters on the S/N ratio of TiB2 grain size are reported in Table 4. The significances of SPS temperature, SPS soaking time, SPS pressure and SiC particle size are estimated 75%, 8%, 5% and 12%, respectively. The significance pie chart of the mentioned processing parameters on the TiB2 grain size is also presented in Fig. 4. Hence, it can be demonstrated that the SPS temperature is the most striking factor on the TiB2 grain size in TiB2– SiC composites. Hardness is an explanation of how material resists against the plastic deformation while an external compressive force is applied [53]. The results of hardness measurements, by Vickers indentation method, and their corresponding S/N ratios are reported in Table 5. The hardness values showed a relatively wide range and varied from ~ 16 GPa to ~ 28 GPa. Sample 1 and sample 7 have the lowest and the highest hardness values, respectively (Table 5). It should be reminded that the former has the minimum and the latter has the maximum relative density values, as reported in Table 3. Comparing the density and hardness measurements in Tables 3 and 5, it seems that the relative density is an important property which controls the hardness value. In other words, as the pores in a sample show no resistance against the indentation load of Vickers machine, more porous materials have lower hardness values. The statistical analysis of hardness is fulfilled by the “higher is better” class, as already declared in Section 2.1. Hence, the S/N ratios for this class are calculated by the subsequent formula [56,59]:
⎛1 S = − 10 log⎜⎜ N ⎝n
n
∑ i=1
1 ⎞⎟ ⎟ y 2i ⎠
(3)
The main effects plots of S/N ratios versus the processing parameters for the hardness of TiB2–SiC composites are shown in Fig. 5. A sharp increase in the S/N ratio is clearly seen in this figure with raising the SPS temperature. A slight decrease, and then, a higher increase in the S/N ratios is monitored with increasing the SPS pressure or SPS soaking time. Contrarily, a sharp increase, and then, an obvious drop in the S/N ratio is happened with decreasing SiC particle size. In general, the hardness of TiB2–SiC ceramics boosts with the increase in the SPS conditions (temperature, time and pressure). In addition, from the standpoint of SiC particle size, the highest value of S/N ratio for the hardness is
Fig. 4. Significance pie chart of processing parameters on TiB2 grain size in TiB2–SiC composites. Table 5 Experimental results and corresponding S/N ratios for Vickers hardness (GPa) of TiB2– SiC composites. Sample
Trial 1
1 14.1 2 16.7 3 19.5 4 17.1 5 23.3 6 23.4 7 27.2 8 19.8 9 18.3 Grand average
Trial 2
Trial 3
Trial 4
Trial 5
Average
S/N ratio
16.1 21.8 22.2 21.6 23.1 20.9 30.6 23.4 22.2
17.4 17.1 19.2 22.2 19.1 25.1 26.1 22.4 23.6
19.1 19.2 23.3 20.1 20.7 23.9 27.2 21.6 22.8
13.7 17.6 22.4 20.3 21.9 22.4 28.5 23.1 21.2
16.1 18.5 21.3 20.3 21.6 23.1 27.9 22.1 21.6 21.4
23.92 25.21 26.49 26.02 26.63 27.24 28.88 26.82 26.59 26.42
corresponded to the size of 200 nm. Therefore, the optimal SPS conditions for the TiB2-based ceramic composites are 1800 °C, 15 min, and 30 MPa which have been reinforced with 200-nm SiC particles. The ANOVA results of significance of processing parameters on the Vickers hardness of TiB2–SiC composites are reported in Table 6. Based on this analysis, the SPS temperature is the main controlling parameter which has an important effect on the hardness. After the temperature, the SPS pressure and SiC particle size have also effective roles on the hardness of composites. On the basis of ANOVA, the SPS soaking time does not have a significant influence on the hardness. The summary of AVONA results is illustrated in Fig. 6, in the form of significance pie chart of processing parameters on the hardness. The significances of SPS temperature, SPS soaking time, SPS pressure and SiC particle size are estimated 51%, 4%, 25%, and 20%, respectively. Table 7 describes the performance and contribution of processing conditions for Vickers hardness at optimal state. The highest hardness value is obtainable at such optimal conditions i.e. the sintering
Table 4 ANOVA results explaining the significance of processing parameters on the S/N ratio of TiB2 grain size. Processing parameters
Degrees of freedom
Sum of squares
Variance
Pure sum
Significance (%)
SPS temperature SPS soaking time SPS pressure SiC particle size Total
2 2 2 2 8
46.606 4.753 2.981 7.524 61.866
23.303 2.376 1.490 3.762 –
46.606 4.753 2.981 7.524 –
75.333 7.683 4.819 12.163 100.000
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Fig. 5. Main effect plots of S/N ratios for Vickers hardness of TiB2–SiC composites.
Table 6 ANOVA results explaining the significance of processing parameters on Vickers hardness. Processing parameters
Degrees of freedom
Sum of squares
Variance
Pure sum
Significance (%)
SPS temperature SPS soaking time SPS pressure SiC particle size Total
2 2 2 2 8
7.591 0.558 3.727 2.929 14.807
3.795 0.279 1.863 1.464 –
7.591 0.558 3.727 2.929 –
51.265 3.769 25.173 19.786 100.000
Fig. 6. Significance pie chart of processing parameters on Vickers hardness of TiB2–SiC composites.
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Table 7 Optimum conditions and performance for Vickers hardness of TiB2–SiC composites. Parameter
Level
Level description
SPS temperature (°C) 3 1800 SPS soaking time 3 15 (min) SPS pressure (MPa) 3 30 SiC particle size (nm) 2 200 Total contributions of all parameters Current grand average of hardness Expected result at optimal processing conditions Experimented result at optimal processing conditions
Contribution (GPa) 2.5 0.6 2.2 1.8 7.1 21.4 28.5 27.4 ± 3.1
temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa, and the SiC particle size of 200 nm. On one hand, the grand average of Vickers hardness values of nine samples was calculated as 21.4 GPa (Table 5). On the other hand, the contributions of SPS temperature, SPS soaking time, SPS pressure, and SiC particle size on the hardness improvement at the determined optimal conditions are 2.5, 0.6, 2.2, and 1.8 GPa, respectively. Hence, the total contributions of all processing parameters is 7.1 GPa. Therefore, a hardness of 28.5 GPa is expected at optimal processing conditions. The experimentally achieved result at the optimal conditions for checking the reliability and validity of ANOVA predictions is called the confirmation test. This means that the confirmation test is performed to compare the predicted outcomes with the experimental results. A Vickers hardness of 27.4 GPa was experimentally measured as the result of confirmation test at the optimal conditions. This outcome is relatively near to the predicted value of 28.5 GPa. Hence, the usefulness of the design of experiments by Taguchi's method and ANOVA optimization for the processing parameters is verified. 4. Conclusions The effects of four processing parameters including the SPS temperature, SPS soaking time, SPS pressure, and SiC particle size on the TiB2 grain size, and hardness of TiB2–20 vol% SiC composites were studied by the Taguchi method. The significances of sintering temperature, time, pressure and SiC particle size on the grain growth were estimated 75%, 8%, 5% and 12%, respectively. Therefore, the SPS temperature was recognized as the most significant parameter which controls the assintered TiB2 grain size. The microstructural investigations verified that the SiC and in-situ formed nano-sized TiC phases had effective roles in achieving a fine-grained TiB2–SiC composite due to their functions as grain growth inhibitors. The significances of SPS temperature, time, pressure, and SiC particle size on the hardness of composites were obtained 51%, 4%, 25% and 20%, respectively. Hence, the soaking time did not show a drastic influence on the grain size and hardness of spark plasma sintered ceramics. The confirmation test was performed at the optimal processing conditions suggested by AVOVA as follows: the SPS temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa, and the SiC particle size of 200 nm. The predicted result for hardness at the optimal conditions (28.5 GPa) was close to the experimentally measured outcome of the confirmation test (27.4 GPa). Acknowledgments The authors would like to thank the Arta Research Group (www. artaresearch.com) for its financial support through Grant ARG-2500-001. References [1] S.K. Bhaumik, C. Divakar, A.K. Singh, G.S. Upadhyaya, Synthesis and sintering of TiB2 and TiB2–TiC composite under high pressure, Mater. Sci. Eng. A 279 (2000) 275–281. [2] A. Li, Y. Zhen, Q. Yin, L. Ma, Y. Yin, Microstructure and properties of (SiC, TiB2)/ B4C composites by reaction hot pressing, Ceram. Int. 32 (2006) 849–856.
[3] H. Zhao, Y.B. Cheng, Formation of TiB2-TiC composites by reactive sintering, Ceram. Int. 25 (1999) 353–358. [4] J. Matsushita, T. Suzuki, A. Sano, Wire electrical discharge machining of TiB2 composite, J. Ceram. Soc. Jpn. 100 (1992) 219–222. [5] M. Shibuya, Y. Yamamoto, M. Ohyanagi, Simultaneous densification and phase decomposition of TiB2–WB2 solid solutions activated by cobalt boride addition, J. Eur. Ceram. Soc. 27 (2007) 307–312. [6] G.J. Zhang, Z.Z. Jin, X.M. Yue, Effects of Ni addition on mechanical properties of TiB2/SiC composites prepared by reactive hot pressing (RHP)), J. Mater. Sci. 32 (1997) 2093–2097. [7] A. Sabahi Namini, M. Azadbeh, M. Shahedi Asl, Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites, Adv. Powder Technol. 28 (2017) 1564–1572. [8] T.S. Srivatsan, G. Guruprasad, D. Black, R. Radhakrishnan, T.S. Sudarshan, Influence of TiB2 content on microstructure and hardness of TiB2–B4C composite, Powder Technol. 159 (2005) 161–167. [9] M.A. Janney, Mechanical properties and oxidation behaviour of a hot-pressed SiC15 vol%-TiB2 composite, Am. Ceram. Soc. Bull. 66 (1987) 322–324. [10] G.J. Zhang, Z.Z. Jin, X.M. Yue, Reaction synthesis of TiB2-SiC composites from TiH2-Si-B4C, Mater. Lett. 25 (1995) 97–100. [11] M.L. Gu, C.Z. Huang, J. Wang, Improvements in mechanical properties of TiB2 ceramics tool materials by the dispersion of Al2O3 particles, Key Eng. Mater. 315/ 316 (2006) 123. [12] Y.S. Kang, S.H. Kang, D.J. Kim, Effect of addition of Cr on the sintering of TiB2 ceramics, J. Mater. Sci. 40 (15) (2005) 4153–4155. [13] J. Jaroszewicz, A. Michalski, Preparation of a TiB2 composite with a nickel matrix by pulse plasma sintering with combustion synthesis, J. Eur. Ceram. Soc. 26 (2006) 2427–2430. [14] S. Baik, P.F. Becher, Effect of oxygen contamination of densification of TiB2, J. Am. Ceram. Soc., vol. 70, 1987, pp. 527–530. [15] 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, Int. J. Refract. Met. Hard Mater., vol. 51, 2015, pp. 169–179. [16] K. Farhadi, A.S. Namini, M. Shahedi Asl, A. Mohammadzadeh, M. Ghassemi Kakroudi, Characterization of hot pressed SiC whisker reinforced TiB2 based composites, Int. J. Refract. Met. Hard Mater. 61 (2016) 84–90. [17] M. Shahedi Asl, A. Sabahi Namini, M. Ghassemi Kakroudi, Influence of silicon carbide addition on the microstructural development of hot pressed zirconium and titanium diborides, Ceram. Int. 42 (2016) 5375–5381. [18] T.S.R.C. Murthy, B. Basu, R. Balasubramaniam, A.K. Suri, C. Subramonian, R.K. Fotedar, Processing and properties of TiB2 with MoSi2 sinter-additive: a first report, J. Am. Ceram. Soc. 89 (1) (2006) 131–138. [19] L.H. Li, H.E. Kim, E.S. Kang, Sintering and mechanical properties of titanium diboride with aluminum nitride as a sintering aid, J. Eur. Ceram. Soc. 22 (2002) 973–977. [20] S. Torizuka, T. Kishi, Effect of SiC and ZrO2 on sinterability and mechanical properties of titanium nitride, titanium carbonitride and titanium diboride, Mater. Trans. JIM 37 (4) (1996) (7872-787). [21] S. Torizuka, J. Harada, H. Nishio, High strength TiB2, Ceram. Eng. Sci. Proc., vol. 11, no. 9-10, 1990, pp. 1454–1460. [22] S. Torizuka, K. Sato, H. Nishio, T. Kishi, Effect of SiC on interfacial reaction and sintering mechanism of TiB2, J. Am. Ceram. Soc., vol. 78, no. 6, 1995, pp. 1606– 1610. [23] T. Graziani, A. Bellosi, Sintering and characterization of TiB2–B4C–ZrO2 composites, Mater. Manuf. Process. 9 (4) (1994) 767–780. [24] E.S. Kang, C.H. Kim, Improvements in mechanical properties of TiB2 by the dispersion of B4C particles, J. Mater. Sci. 25 (1990) 580–584. [25] Y. Murata, H.P. Julien, E.D. Whitney, Densification and wear resistance of ceramic systems: I. titanium diboride, Ceram. Bull. 46 (7) (1967) 643–648. [26] J.H. Park, Y. Koh, H. Kim, C. Hwang, E. Kong, Densification and mechanical properties of titanium diboride with silicon nitride as sintering aid, J. Am. Ceram. Soc. 82 (11) (1999) 3037–3042. [27] R. Telle, S. Meyer, G. Petzow, E.D. Franz, Sintering behavior and phase reactions of TiB2 with ZrO2 additives, Mater. Sci. Eng. A 105/106 (1988) 125–129. [28] Y. Muraoka, M. Yoshinaka, K. Hirota, O. Yamaguchi, Hot isostatic pressing of TiB2–ZrO2 (2 mol% Y2O3) composite powders, Mater. Res. Bull. 31 (7) (1996) 787–792. [29] 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, Ceram. Int. 42 (2016) 4498–4506. [30] M. Shahedi Asl, M. Ghassemi Kakroudi, Characterization of hot-pressed graphene reinforced ZrB2–SiC composite, Mater. Sci. Eng. A 625 (2015) 385–392. [31] 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, Ceram. Int. 41 (2015) 5843–5851. [32] 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, Ceram. Int. 41 (2015) 8388–8396. [33] N. Pourmohammadie Vafa, B. Nayebi, 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 II: mechanical behavior, Ceram. Int. 42 (2016) 2724–2733. [34] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, A fractographical approach to the sintering process in porous ZrB2–B4C binary composites, Ceram. Int. 41 (2015)
13930
Ceramics International 43 (2017) 13924–13931
M. Shahedi Asl et al. 379–387. [35] 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, Ceram. Int. 42 (2016) 17009–17015. [36] M. Shahedi Asl, I. Farahbakhsh, B. Nayebi, Characteristics of multi-walled carbon nanotube toughened ZrB2–SiC ceramic composite prepared by hot pressing, Ceram. Int. 42 (2016) 1950–1958. [37] I. Farahbakhsh, Z. Ahmadi, M. Shahedi Asl, Densification, microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black, Ceram. Int. 43 (2017) 8411–8417. [38] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, P. Pirmohammadi, M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered SiAlONdoped ZrB2–SiC composites, Mater. Charact. 102 (2015) 137–145. [39] Z. Ahmadi, B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, Fractographical characterization of hot pressed and pressureless sintered AlN-doped ZrB2–SiC composites, Mater. Charact. 110 (2015) 77–85. [40] Z. Ahmadi, B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, I. Farahbakhsh, Sintering behavior of ZrB2–SiC composites doped with Si3N4: a fractographical approach, Ceram. Int. 43 (2017) 9699–9708. [41] 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, Ceram. Int. 41 (2015) 6439–6447. [42] 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, Int. J. Refract. Met. Hard Mater. 51 (2015) 81–90. [43] 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, Ceram. Int. 41 (2015) 9628–9636. [44] M. Shahedi Asl, M. Ghassemi Kakroudi, Fractographical assessment of densification mechanisms in hot pressed ZrB2–SiC composites, Ceram. Int. 40 (2014) 15273–15281. [45] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, M. Shokouhimehr, Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol% SiC composites, Int. J. Refract. Met. Hard Mater. 54 (2016) 7–13.
[46] M. Shahedi Asl, M. Ghassemi Kakroudi, A processing–microstructure correlation in ZrB2–SiC composites hot-pressed under a load of 10 MPa, Universal J. Mater. Sci. 3 (2015) 14–21. [47] F. Guanghai, Y. Yanqing, Z. Guangming, Z. Wei, L. Xian, H. Bin, Effect of hot isostatic pressing parameters on the microstructure sand grain growth behavior of the matrix of SiCf/Ti-6Al-4V composites, Rare Met. Mater. Eng. 43 (2014) 1839–1845. [48] M. Shahedi Asl, M. Ghassemi Kakroudi, S. Noori, Hardness and toughness of hot pressed ZrB2–SiC composites consolidated under relatively low pressure, J. Alloy. Compd. 679 (2015) 481–487. [49] M. Shahedi Asl, M. Ghassemi Kakroudi, I. Farahbakhsh, B. Mazinani, Z. Balak, Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: grain growth, Ceram. Int. 42 (2016) 18612–18619. [50] A.S. Namini, M. Azadbeh, M. Shahedi Asl, Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites, Adv. Powder Technol. 28 (2017) 1564–1572. [51] R.K. Enneti, C. Carney, S.J. Park, Sundar V. Atre, Taguchi analysis on the effect of process parameters on densification during spark plasma sintering of HfB2-20 SiC, Int. J. Refract. Met. Hard Mater. 31 (2012) 293–296. [52] 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. J. Refract. Met. Hard Mater. 50 (2015) 140–145. [53] 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, Int. J. Refract. Met. Hard Mater. 50 (2015) 313–320. [54] Z. Balak, M. Shahedi Asl, M. Azizieh, H. Kafashan, R. Hayati, Effect of different additives and open porosity on fracture toughness of ZrB2–SiC-based composites prepared by SPS, Ceram. Int. 43 (2) (2017) 2209–2220. [55] S. Naghibi, M.A. Faghihi Sani, H.R. Madaah Hosseini, Application of the statistical Taguchi method to optimize TiO2 nanoparticles synthesis by the hydrothermal assisted sol–gel technique, Ceram. Int. 40 (2014) 4193–4201. [56] P.J. Ross, Taguchi Techniques for Quality Engineering, McGraw- Hill, New York, 1996. [57] Z. Balak, M. Azizieh, H. Kafashan, M. Shahedi Asl, Z. Ahmadi, Optimization of effective parameters on thermal shock resistance of ZrB2-SiC-based composites prepared by SPS: using Taguchi design, Mater. Chem. Phys. 196 (2017) 333–340.
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