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Influence of SPS temperature on the properties of TiC–SiCw composites
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Influence of SPS temperature on the properties of TiC–SiCw composites Mehdi Fattahia, Ahad Mohammadzadehb, Yaghoub Pazhouhanfarc, Shahrzad Shaddeld, Mehdi Shahedi Asle,∗, Abbas Sabahi Naminif,g,∗∗ a
Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam Department of Materials Engineering, Faculty of Engineering, University of Maragheh, P.O. Box 55136-553, Maragheh, Iran Aliyazh Sanat Sahand Ipak Company, P.O. Box: 51576-13536, Tabriz, Iran d Department of Materials Engineering, Sahand University of Technology, Tabriz, Iran e Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran f Department of Engineering Sciences, Faculty of Advanced Technologies, University of Mohaghegh Ardabili, Namin, Iran g Department of Engineering Sciences, Faculty of Advanced Technologies, Sabalan University of Advanced Technologies (SUAT), Namin, Iran b c
A R T I C LE I N FO
A B S T R A C T
Keywords: TiC-SiCw composite Densification Microstructural features Mechanical properties Thermal conductivity
Titanium carbide (TiC) composites containing 10 vol% silicon carbide whisker (SiCw) were spark plasma sintered at different temperatures of 1800, 1900, and 2000 °C under a pressure of 40 MPa and a holding time of 7 min. At the sintering temperature of 1900 °C, the relative density, Vickers hardness, and flexural strength of the sintered samples hit their maximum values of 98.7%, 24.4 GPa, and 511 MPa, respectively. The microstructural characteristics of the sintered samples were assessed by optical and field emission scanning electron microscopy (FESEM) and XRD. The results revealed that at 1900 °C, the dispersion of SiCw in the TiC matrix was homogenous, no chemical reaction took place between the reinforcement and the matrix, and produced a fine-grained microstructure. It was found that the thermal conductivity of SPSed samples did not have the same trend with relative density and mechanical properties. A maximum value of 32.3 W/mK was measured for the thermal conductivity of the composite sintered at 2000 °C.
1. Introduction Titanium carbide has been extensively used in the high-performance cutting tools, machining materials, and ultra-high temperature applications such as rocket nozzle throat liners and jet engine parts since they possess a variety of technological properties including high melting point, high chemical stability, low density, high hardness, and excellent thermal stability [1–9]. However, the weak sinterability and poor toughness of TiC materials restrict their applications under service conditions [10–19]. To improve the mechanical characteristics and densification of titanium carbide-based materials, incorporating reinforcements, such as SiC, WC, Al2O3, TiB2, and ZrC with different morphologies (particles or whiskers), into TiC matrix has been developed in the past few years [20–27]. It is known that the physicomechanical characteristics of ceramic materials can be improved through reinforcing with different additives [28–38]. Silicon carbide has been extensively used as reinforcement since it enhances the mechanical properties, promotes densification, increases oxidation resistance, and improves the thermal conductivity of other ceramic-based
∗
materials [39–54]. Spark plasma sintering (SPS) is one of the suitable methods for processing metal and ceramic matrix composites compared to other conventional techniques such as pressure-less sintering [25,55–66]. SPS can be employed for manufacturing near net-shaped compacts with a higher relative density and less grain growth at relatively lower sintering times and temperatures [67–78]. In recent years, numerous researches have evaluated the properties of SPSed TiC matrix composites, indicating that SPS is a powerful technique to consolidate these grades of materials. Babapoor et al. [55] fabricated monolithic TiC by SPS at various sintering temperatures in the range of 1800–2000 °C. They showed that 1900 °C was an optimal sintering temperature and could maximize the values of relative density (99.4%), thermal conductivity (17.9 W m−1K−1), and Vickers hardness (25.7 GPa). Cheng et al. [79] produced SPSed TiC containing SiC submicron particles with various volume fractions (14.6, 27.7, 39.7, and 50.6 vol%) and investigated the fracture toughness (KIC) and microstructure of prepared composites. They showed that the maximum KIC of 5.2 MPa m1/2 was obtained at 14.6 vol% SiC and the SiC particle addition caused grain refinement compared to the monolithic TiC.
Corresponding author. Corresponding author. Department of Engineering Sciences, Faculty of Advanced Technologies, University of Mohaghegh Ardabili, Namin, Iran. E-mail addresses: [email protected] (M. Shahedi Asl), [email protected] (A. Sabahi Namini).
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https://doi.org/10.1016/j.ceramint.2020.01.206 Received 24 December 2019; Received in revised form 21 January 2020; Accepted 21 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mehdi Fattahi, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.206
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displacement and displacement rate of the specimen sintered at 1900 and 2000 °C (Fig. 1a and b), higher sintered density values are expected compared to the sample sintered at 1800 °C. Here, the maximum displacement (4.5 mm) occurred in the material processed at 2000 °C. Babapoor et al. reported maximum amount of shrinkage (1.5 mm) in the monolithic TiC processed by SPS at 2000 °C [55]. It seems that the observed increase of shrinkage during SPS of TiC – 10 vol% SiCw is attributable to the presence of SiCw reinforcement in the TiC matrix. As can be seen in Fig. 1b, the consolidation process contains IV main sintering zones. In the zone I (0–1400 °C), the highest level of the applied energy is utilized as a heating source of the graphite die and the powders, and to remove the residual gasses between the components; as a result, the displacement rate is low in this zone [82]. The highest shrinkage during the sintering process happened in zone II (1400–1900 °C) due to the rearrangement of the particles leading to increased contact areas between particles, passing of intense spark discharges, and localized deformation as a consequence of surface softening of the particles. This zone is the most critical stage of the sintering process, and the uppermost porosity reduction happened in this zone. Additionally, the most significant displacement rate occurred in zone II, which could be attributed to the plastic deformation of the particles. By reaching a holding temperature of 1900 °C, zone III begins and lasts for 7 min. A gradual displacement can be seen at the beginning of this zone; however, the process progressively hits the stable condition [82]. Zone IV is considered the cooling stage of the process. The effect of the sintering temperature on relative density is presented in Fig. 2. The composite SPSed at 1800 °C showed a low relative density of 85.91%, while the relative density hits its maximum value of 98.73% by increasing the temperature to 1900 °C. As shown in Fig. 2, the rise of SPS temperature up to 2000 °C results in the reduction of relative density down to 95.17% compared to the sample processed at 1900 °C. The lower relative density of the specimen treated at 1800 °C indicates that this temperature is not enough to complete the consolidation process for obtaining a fully dense composite. At 2000 °C, on the other hand, grain growth causes an increase in the porosity percentage, so that lower densification and higher contraction can be observed in this body. The same observation was reported for spark plasma sintered TiC and B4C ceramics at higher sintering temperatures and prolonged holding times [55,83,84].
Shahedi Asl et al. [80] produced titanium carbide-based composites containing different amounts of SiC whiskers using SPS method and achieved maximum relative density, Vickers hardness, and thermal conductivity at 30 vol% SiCw. To the best of our knowledge, there are few researches discussing the effects of SiCw addition on TiC matrix composites SPSed at different temperatures. This work, therefore, describes the influences of SPS temperatures on the fabrication of SiCw reinforced TiC matrix composite. Also, the densification behavior, microstructural evolution, and physicomechanical characteristics of TiCSiCw composites were investigated in detail. 2. Materials and methods The TiC powder with a particle size of < 12 μm and a purity of 99% was used as the matrix material. The SiC whisker with a purity of 99%, a length of 5–30 μm, and a diameter of 0.1–1 μm was used as the reinforcement material. The characteristics and morphologies of the starting materials are reported elsewhere [80]. The TiC powder with 10 vol% SiCw was homogeneously wet mixed employing a magnetic stirrer for 2 h in ethanol, and then the slurries were dried in an oven at 80 °C for 12 h. Thereafter, the mixed powder was poured into an SPS cylindrical graphite mold with a diameter of 30 mm, and separated from the graphite mold with thick graphite sheets surrounded with boron nitride powder. SPS process (Nanozint 10i, Khala Poushan Felez Co., Iran) was accomplished under a vacuum of less than 2 Pa at 1800, 1900, and 2000 °C for 7 min by applying an external pressure of 40 MPa. After sintering, the specimen was cooled down in the furnace, and the adhered graphite foils were removed entirely from the surfaces of SPSed specimens by grinding and polishing procedures. The bulk density of consolidated TiC-SiCw composite was measured using the Archimedes principle followed by calculating the relative density of SPSed samples (ratio of bulk density to theoretical density). The theoretical density of samples was determined using the mixtures rule [81]. The phase analysis of the processed specimens was done by XRD (model: Siemens D5000). The microstructural features were evaluated using an optical microscope (model: PMG3, OLYMPUS) and FESEM (model: Mira3 Tescan) equipped with the EDS system. The hardness of the polished sintered bodies was determined using a Vickers hardness tester (Eseway, UK) by applying 5 Kg load for 20 s. All the hardness tests were repeated six times to report an average hardness value. Also, the bending strength was assessed on the samples with dimensions of 3 × 4 × 28 mm3 and a span length of 15 mm by a universal testing machine (model: STM-250). The fracture surfaces of the flexural strength specimens were inspected in a CAM SCAN 2300 scanning electron microscope (SEM). Finally, the thermal conductivity of SPSed samples was measured using a digital thermal conductivity meter (model: Sahand Co., Iran).
3.2. Microstructural assessments Fig. 3 depicts the XRD pattern of TiC-10 vol % SiCw SPSed at 2000 °C. Only SiC and TiC peaks are detected in the diffraction pattern, representing that no apparent chemical reaction happened between matrix and reinforcement in the SPS process. Shahedi Asl et al. observed the Ti3SiC2 peak during SPS of TiC-30 vol% SiCw composite at 1900 °C and in the same SPS condition as the current research [80]. Although the SPS temperature increased from 1900 °C to 2000 °C herein, no reaction occurred between TiC and SiC. It can, therefore, be concluded that occurring chemical reaction between TiC and SiC during a short time SPS pronouncedly depends on the SiC volume fraction. Fig. 4 shows optical images of the composite sintered at different temperatures. The micrographs display three distinct colors that can be identified as TiC (white), SiCw (gray), and pores (black). According to Fig. 4, SiC whiskers distributed homogeneously in the TiC matrix. There are also several dark regions in the microstructure of sintered samples that are mainly related to the grain pullout generating during the grinding and polishing processes. In the case of the sample processed at 1800 °C, it can be inferred that the sintering process was not complete, along with the occurrence of inadequate densification. The sintering process completed by raising the sintering temperature to 1900 °C, and a fine microstructure and well-distributed SiC whiskers were observed in the TiC matrix. Accordingly, this microstructure is good evidence for occurring maximum densification at 1900 °C. Moreover, fracture surfaces of sintered samples (Fig. 7, shown in the next sections) reveal that
3. Results and discussion 3.1. Densification Firstly, densification of the spark plasma sintered TiC–10 vol% SiCw composite was studied according to the displacement of graphite punches during the SPS process. Fig. 1a demonstrates the displacement and sintering temperature gradient versus sintering time for three samples. The heating rate for all holding temperatures is approximately the same, and the process is completed in about 60 min. This figure shows that the displacement of the punch at all three holding temperatures starts at 1200–1500 °C. The sharpest shrinkage of the composite during sintering began at approximately 1400 °C (after ~ 40 min), and the samples gradually start the consolidation procedure as the temperature rose from around 1400 °C to the holding temperatures. To further investigate the consolidation behavior of the sample processed at 1900 °C, the displacement, displacement rate, and temperature gradient vs. the sintering time are illustrated in Fig. 1b. By considering the 2
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Fig. 1. Effect of sintering temperatures on SPS parameters of the TiC – 10 vol% SiCw composite: (a) shrinkage (punch displacement) for samples sintered at different temperatures, (b) displacement and displacement rate for specimens sintered at 1900 °C.
TiC matrix of composite containing 10 vol% of the reinforcement are presented in Fig. 5. It is clear that some changes occurred in the size and morphology of pores and SiC whiskers in the TiC matrix with varying sintering temperature. In Fig. 5 (also see Fig. 4), the white regions are related to the TiC matrix, and the gray phase dispersed in the matrix belongs to the SiC whiskers. BSE images (Fig. 5) illustrate that increasing sintering temperature to 2000 °C caused grain growth. Generally, raising the sintering temperature and time accelerates grain growth [85,86]. The EDS analysis results (Fig. 6) confirms that the bright phase and the dark phase (gray phase) are related to the TiC matrix and SiC whisker, respectively. The fracture surfaces of the sintered samples obtained from the flexural test are displayed in Fig. 7. It is seen that the sintering temperature (1800 °C) is not enough, and the consolidation process is in its initial stage. Incomplete bonding between particles, remained pores in the microstructure (especially irregular in shape), and inter-granular fracture mode are the features of lower sintering temperatures. With rising the sintering temperature to 1900 °C, a homogeneous microstructure appeared with equiaxed and fine grains. In the TiC–SiC composite, the presence of SiCw retards the growth of TiC grains by pinning the grain-boundaries and inhibiting their movement. The occurrence of pore coalescence is evident, and the pores morphology altered from irregular to near-spherical shape. Here, the pores emerged in the microstructure prevented the formation of a full dense bulk material. At 1900 °C, it can be realized that the fracture mode is mostly transgranular (Fig. 7). Increasing the sintering temperature to 2000 °C led to grain growth and pore coarsening.
Fig. 2. Effect of SPS temperatures on relative densities of the TiC–10 vol% SiCw composites.
3.3. Mechanical properties The effect of the SPS temperature on Vickers hardness is depicted in Fig. 8. The hardness increases from 11.3 ± 0.5 GPa to 24.54 ± 0.4 GPa when sintering temperature rises from 1800 °C to 1900 °C, respectively. This hardness enhancement is attributed to decreased porosity, the homogenization of the microstructure, and the increase of relative density (from 85.91% to 98.73%). Then hardness decreases to 22.61 ± 0.6 GPa as due to the formation of large grains and generation of extra pores with increasing the SPS temperature to 2000 °C. Babapoor et al. reported lower hardness values of 8.1 GPa and 22.1 GPa for monolithic TiC obtained by SPS at 1800 °C and 2000 °C, respectively. This difference can be attributed to the SiCw dispersoids in the TiC matrix. Also, Babapoor et al. reported a value of 25.7 GPa for the hardness of monolithic TiC sintered at 1900 °C [55], which is about 1.16 GPa higher than that obtained for the specimen sintered at the same temperature in this work. This difference is assumedly owing to the lower hardness of SiC compared to TiC. Several researchers have
Fig. 3. XRD pattern of SPSed TiC- 10 vol% SiCw sintered at 2000 °C under an external pressure of 40 MPa and a holding time of 7 min.
grain growth occurred due to the sintering temperature enhancement to 2000 °C, which led to an increase in pores and prevented full densification. The proper distribution and different morphology of SiCw in the 3
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Fig. 4. Optical microscopy images of the polished surfaces of the sintered TiC–10 vol% SiCw at various SPS temperatures: (a) 1800 °C, (b) 1900 °C, and (c) 2000 °C.
Fig. 5. FESEM images of the polished surfaces of the sintered TiC–10 vol% SiCw at various SPS temperatures: (a) 1800 °C, (b) 1900 °C, and (c) 2000 °C.
Fig. 6. FESEM micrograph of the polished surface of TiC–10 vol% SiCw composite sintered at 1900 °C and equivalent EDS mapping analysis. 4
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Fig. 7. SEM micrograph of the fracture surfaces of TiC–10 vol% SiCw composite at various SPS temperatures: (a) 1800 °C, (b) 1900 °C, and (c) 2000 °C.
reported that full densification and consequently improved mechanical properties could be achieved in TiC based composites reinforced with 20–30 vol% of SiC [40,80]. Fig. 9 represents the variation of flexural strength versus sintering temperature. Increases in sintering temperature from 1800 to 1900 °C and from 1900 to 2000 °C resulted in an increase and a decrease in the flexural strength from 289 to 511 MPa and from 511 to 470 MPa, respectively. These data reveal that the flexural strength, hardness, and densification follow a similar variation trend vs. SPS temperature. In other words, the mechanical characteristics, such as flexural strength and hardness, decrease when the amount of porosity increases, and the grain growth occurs according to the Hall–Petch relationship [87].
3.4. Thermal conductivity The sintering temperature dependence of thermal conductivity for TiC – 10 vol% SiCw composite is plotted in Fig. 10. It is evident that the room temperature thermal conductivity of the SPSed specimens increases along with rising sintering temperature, with values of 11.4, 19.8 and 32.3 W/mK for the samples sintered at 1800, 1900 and 2000 °C, respectively. Here, the obtained thermal conductivity of SPSed composites is between that of pure SiC and TiC. It is clear that thermal conductivity does not have the same trend with relative density and mechanical characteristics (bending strength and hardness) of sintered
Fig. 8. Hardness of the SPSed composites as a function of sintering temperatures.
Fig. 9. Flexural strength of the SPSed composites as a function of sintering temperatures.
Fig. 10. Thermal conductivity of the SPSed composites as a function of sintering temperatures. 5
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References
compacts. This behavior is not in agreement with that of Babapoor et al. [55], who presented a decreasing trend in thermal conductivity of SPSed monolithic TiC by augmenting the processing temperature from 1900 °C to 2000 °C. In this work, it seems that porosity does not have a dominant effect on thermal conductivity with rising sintering temperature. The porosity, impurity, micro-cracks, composition, grain size, and grain-boundary thickness in a ceramic material can influence thermal conductivity. In a polyphase material, microstructure and composition can significantly affect the thermal conductivity. Cabrero et al. [39] concluded that in a TiC–SiC composite produced via SPS, the composition of the material (matrix and reinforcement) had a significant effect on the thermal conductivity rather than the microstructure. Also, this claim was supported in Ref. [80] for TiC-based composite reinforced with SiC whisker. Here, the occurrence of grain growth in the TiC matrix (fewer grain boundaries), the presence of homogeneously distributed SiC whiskers in the matrix, and likely relative dissolution of two phases in each other might have led to hitting higher thermal conductivity in the sample sintered at 2000 °C. At lower sintering temperatures, especially at 1900 °C, a finer microstructure with higher grain boundaries acts as a heat transfer barrier, yielding a lower thermal conductivity compared to the specimen processed at a higher temperature (2000 °C).
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4. Conclusions In the current work, the spark plasma sintering technique is successfully employed in the fabrication of TiC based composites containing 10 vol% SiCw. According to the obtained results, the effects of the SPS temperature on densification, microstructural evolution, mechanical properties, and thermal conductivity of the TiC- 10 vol% SiCw composite are summarized as follows: 1. The displacement, displacement rates and relative density values of the sintered composites reveal that maximum densification occurred at 1900 °C. 2. Phase identification results indicate that there is no chemical reaction between TiC and SiCw phases. Based on the micrographs obtained from polished and fracture surfaces, SiC whiskers are distributed homogeneously in the TiC matrix at 1900 °C and 2000 °C. Sintering at 1900 °C resulted in a finer microstructure. The growth of grains occurred by raising the processing temperature to 2000 °C, which prevented full densification. 3. Effects of SPS temperature on hardness and flexural strength show that maximum value of mechanical properties is related to the composite sintered at 1900 °C. 4. The result of thermal conductivity analysis reveals that there is a linear relationship between sintering temperature and thermal conductivity. A maximum value of 32.3 W/mK is obtained for the composite sintered at 2000 °C. It can, therefore, be concluded that the composition of the material due to the existence of SiCw reinforcement in TiC matrix has a more significant effect on the thermal conductivity than the porosity percentage. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research work was financially supported by Aliyazh Sanat Sahand Ipak Company, Tabriz, Iran (Grant No. 971210-4). The authors gratefully appreciate for practical support of Eng. Hamid Sharifiyan and Saeid Sharifiyan (members of the board of directors in Aliyazh Sanat Sahand Ipak Company). 6
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Influence of SPS temperature on the properties of TiC–SiCw composites Mehdi Fattahia, Ahad Mohammadzadehb, Yaghoub Pazhouhanfarc, Shahrzad Shaddeld, Mehdi Shahedi Asle,∗, Abbas Sabahi Naminif,g,∗∗ a
Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam Department of Materials Engineering, Faculty of Engineering, University of Maragheh, P.O. Box 55136-553, Maragheh, Iran Aliyazh Sanat Sahand Ipak Company, P.O. Box: 51576-13536, Tabriz, Iran d Department of Materials Engineering, Sahand University of Technology, Tabriz, Iran e Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran f Department of Engineering Sciences, Faculty of Advanced Technologies, University of Mohaghegh Ardabili, Namin, Iran g Department of Engineering Sciences, Faculty of Advanced Technologies, Sabalan University of Advanced Technologies (SUAT), Namin, Iran b c
A R T I C LE I N FO
A B S T R A C T
Keywords: TiC-SiCw composite Densification Microstructural features Mechanical properties Thermal conductivity
Titanium carbide (TiC) composites containing 10 vol% silicon carbide whisker (SiCw) were spark plasma sintered at different temperatures of 1800, 1900, and 2000 °C under a pressure of 40 MPa and a holding time of 7 min. At the sintering temperature of 1900 °C, the relative density, Vickers hardness, and flexural strength of the sintered samples hit their maximum values of 98.7%, 24.4 GPa, and 511 MPa, respectively. The microstructural characteristics of the sintered samples were assessed by optical and field emission scanning electron microscopy (FESEM) and XRD. The results revealed that at 1900 °C, the dispersion of SiCw in the TiC matrix was homogenous, no chemical reaction took place between the reinforcement and the matrix, and produced a fine-grained microstructure. It was found that the thermal conductivity of SPSed samples did not have the same trend with relative density and mechanical properties. A maximum value of 32.3 W/mK was measured for the thermal conductivity of the composite sintered at 2000 °C.
1. Introduction Titanium carbide has been extensively used in the high-performance cutting tools, machining materials, and ultra-high temperature applications such as rocket nozzle throat liners and jet engine parts since they possess a variety of technological properties including high melting point, high chemical stability, low density, high hardness, and excellent thermal stability [1–9]. However, the weak sinterability and poor toughness of TiC materials restrict their applications under service conditions [10–19]. To improve the mechanical characteristics and densification of titanium carbide-based materials, incorporating reinforcements, such as SiC, WC, Al2O3, TiB2, and ZrC with different morphologies (particles or whiskers), into TiC matrix has been developed in the past few years [20–27]. It is known that the physicomechanical characteristics of ceramic materials can be improved through reinforcing with different additives [28–38]. Silicon carbide has been extensively used as reinforcement since it enhances the mechanical properties, promotes densification, increases oxidation resistance, and improves the thermal conductivity of other ceramic-based
∗
materials [39–54]. Spark plasma sintering (SPS) is one of the suitable methods for processing metal and ceramic matrix composites compared to other conventional techniques such as pressure-less sintering [25,55–66]. SPS can be employed for manufacturing near net-shaped compacts with a higher relative density and less grain growth at relatively lower sintering times and temperatures [67–78]. In recent years, numerous researches have evaluated the properties of SPSed TiC matrix composites, indicating that SPS is a powerful technique to consolidate these grades of materials. Babapoor et al. [55] fabricated monolithic TiC by SPS at various sintering temperatures in the range of 1800–2000 °C. They showed that 1900 °C was an optimal sintering temperature and could maximize the values of relative density (99.4%), thermal conductivity (17.9 W m−1K−1), and Vickers hardness (25.7 GPa). Cheng et al. [79] produced SPSed TiC containing SiC submicron particles with various volume fractions (14.6, 27.7, 39.7, and 50.6 vol%) and investigated the fracture toughness (KIC) and microstructure of prepared composites. They showed that the maximum KIC of 5.2 MPa m1/2 was obtained at 14.6 vol% SiC and the SiC particle addition caused grain refinement compared to the monolithic TiC.
Corresponding author. Corresponding author. Department of Engineering Sciences, Faculty of Advanced Technologies, University of Mohaghegh Ardabili, Namin, Iran. E-mail addresses: [email protected] (M. Shahedi Asl), [email protected] (A. Sabahi Namini).
∗∗
https://doi.org/10.1016/j.ceramint.2020.01.206 Received 24 December 2019; Received in revised form 21 January 2020; Accepted 21 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mehdi Fattahi, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.206
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displacement and displacement rate of the specimen sintered at 1900 and 2000 °C (Fig. 1a and b), higher sintered density values are expected compared to the sample sintered at 1800 °C. Here, the maximum displacement (4.5 mm) occurred in the material processed at 2000 °C. Babapoor et al. reported maximum amount of shrinkage (1.5 mm) in the monolithic TiC processed by SPS at 2000 °C [55]. It seems that the observed increase of shrinkage during SPS of TiC – 10 vol% SiCw is attributable to the presence of SiCw reinforcement in the TiC matrix. As can be seen in Fig. 1b, the consolidation process contains IV main sintering zones. In the zone I (0–1400 °C), the highest level of the applied energy is utilized as a heating source of the graphite die and the powders, and to remove the residual gasses between the components; as a result, the displacement rate is low in this zone [82]. The highest shrinkage during the sintering process happened in zone II (1400–1900 °C) due to the rearrangement of the particles leading to increased contact areas between particles, passing of intense spark discharges, and localized deformation as a consequence of surface softening of the particles. This zone is the most critical stage of the sintering process, and the uppermost porosity reduction happened in this zone. Additionally, the most significant displacement rate occurred in zone II, which could be attributed to the plastic deformation of the particles. By reaching a holding temperature of 1900 °C, zone III begins and lasts for 7 min. A gradual displacement can be seen at the beginning of this zone; however, the process progressively hits the stable condition [82]. Zone IV is considered the cooling stage of the process. The effect of the sintering temperature on relative density is presented in Fig. 2. The composite SPSed at 1800 °C showed a low relative density of 85.91%, while the relative density hits its maximum value of 98.73% by increasing the temperature to 1900 °C. As shown in Fig. 2, the rise of SPS temperature up to 2000 °C results in the reduction of relative density down to 95.17% compared to the sample processed at 1900 °C. The lower relative density of the specimen treated at 1800 °C indicates that this temperature is not enough to complete the consolidation process for obtaining a fully dense composite. At 2000 °C, on the other hand, grain growth causes an increase in the porosity percentage, so that lower densification and higher contraction can be observed in this body. The same observation was reported for spark plasma sintered TiC and B4C ceramics at higher sintering temperatures and prolonged holding times [55,83,84].
Shahedi Asl et al. [80] produced titanium carbide-based composites containing different amounts of SiC whiskers using SPS method and achieved maximum relative density, Vickers hardness, and thermal conductivity at 30 vol% SiCw. To the best of our knowledge, there are few researches discussing the effects of SiCw addition on TiC matrix composites SPSed at different temperatures. This work, therefore, describes the influences of SPS temperatures on the fabrication of SiCw reinforced TiC matrix composite. Also, the densification behavior, microstructural evolution, and physicomechanical characteristics of TiCSiCw composites were investigated in detail. 2. Materials and methods The TiC powder with a particle size of < 12 μm and a purity of 99% was used as the matrix material. The SiC whisker with a purity of 99%, a length of 5–30 μm, and a diameter of 0.1–1 μm was used as the reinforcement material. The characteristics and morphologies of the starting materials are reported elsewhere [80]. The TiC powder with 10 vol% SiCw was homogeneously wet mixed employing a magnetic stirrer for 2 h in ethanol, and then the slurries were dried in an oven at 80 °C for 12 h. Thereafter, the mixed powder was poured into an SPS cylindrical graphite mold with a diameter of 30 mm, and separated from the graphite mold with thick graphite sheets surrounded with boron nitride powder. SPS process (Nanozint 10i, Khala Poushan Felez Co., Iran) was accomplished under a vacuum of less than 2 Pa at 1800, 1900, and 2000 °C for 7 min by applying an external pressure of 40 MPa. After sintering, the specimen was cooled down in the furnace, and the adhered graphite foils were removed entirely from the surfaces of SPSed specimens by grinding and polishing procedures. The bulk density of consolidated TiC-SiCw composite was measured using the Archimedes principle followed by calculating the relative density of SPSed samples (ratio of bulk density to theoretical density). The theoretical density of samples was determined using the mixtures rule [81]. The phase analysis of the processed specimens was done by XRD (model: Siemens D5000). The microstructural features were evaluated using an optical microscope (model: PMG3, OLYMPUS) and FESEM (model: Mira3 Tescan) equipped with the EDS system. The hardness of the polished sintered bodies was determined using a Vickers hardness tester (Eseway, UK) by applying 5 Kg load for 20 s. All the hardness tests were repeated six times to report an average hardness value. Also, the bending strength was assessed on the samples with dimensions of 3 × 4 × 28 mm3 and a span length of 15 mm by a universal testing machine (model: STM-250). The fracture surfaces of the flexural strength specimens were inspected in a CAM SCAN 2300 scanning electron microscope (SEM). Finally, the thermal conductivity of SPSed samples was measured using a digital thermal conductivity meter (model: Sahand Co., Iran).
3.2. Microstructural assessments Fig. 3 depicts the XRD pattern of TiC-10 vol % SiCw SPSed at 2000 °C. Only SiC and TiC peaks are detected in the diffraction pattern, representing that no apparent chemical reaction happened between matrix and reinforcement in the SPS process. Shahedi Asl et al. observed the Ti3SiC2 peak during SPS of TiC-30 vol% SiCw composite at 1900 °C and in the same SPS condition as the current research [80]. Although the SPS temperature increased from 1900 °C to 2000 °C herein, no reaction occurred between TiC and SiC. It can, therefore, be concluded that occurring chemical reaction between TiC and SiC during a short time SPS pronouncedly depends on the SiC volume fraction. Fig. 4 shows optical images of the composite sintered at different temperatures. The micrographs display three distinct colors that can be identified as TiC (white), SiCw (gray), and pores (black). According to Fig. 4, SiC whiskers distributed homogeneously in the TiC matrix. There are also several dark regions in the microstructure of sintered samples that are mainly related to the grain pullout generating during the grinding and polishing processes. In the case of the sample processed at 1800 °C, it can be inferred that the sintering process was not complete, along with the occurrence of inadequate densification. The sintering process completed by raising the sintering temperature to 1900 °C, and a fine microstructure and well-distributed SiC whiskers were observed in the TiC matrix. Accordingly, this microstructure is good evidence for occurring maximum densification at 1900 °C. Moreover, fracture surfaces of sintered samples (Fig. 7, shown in the next sections) reveal that
3. Results and discussion 3.1. Densification Firstly, densification of the spark plasma sintered TiC–10 vol% SiCw composite was studied according to the displacement of graphite punches during the SPS process. Fig. 1a demonstrates the displacement and sintering temperature gradient versus sintering time for three samples. The heating rate for all holding temperatures is approximately the same, and the process is completed in about 60 min. This figure shows that the displacement of the punch at all three holding temperatures starts at 1200–1500 °C. The sharpest shrinkage of the composite during sintering began at approximately 1400 °C (after ~ 40 min), and the samples gradually start the consolidation procedure as the temperature rose from around 1400 °C to the holding temperatures. To further investigate the consolidation behavior of the sample processed at 1900 °C, the displacement, displacement rate, and temperature gradient vs. the sintering time are illustrated in Fig. 1b. By considering the 2
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Fig. 1. Effect of sintering temperatures on SPS parameters of the TiC – 10 vol% SiCw composite: (a) shrinkage (punch displacement) for samples sintered at different temperatures, (b) displacement and displacement rate for specimens sintered at 1900 °C.
TiC matrix of composite containing 10 vol% of the reinforcement are presented in Fig. 5. It is clear that some changes occurred in the size and morphology of pores and SiC whiskers in the TiC matrix with varying sintering temperature. In Fig. 5 (also see Fig. 4), the white regions are related to the TiC matrix, and the gray phase dispersed in the matrix belongs to the SiC whiskers. BSE images (Fig. 5) illustrate that increasing sintering temperature to 2000 °C caused grain growth. Generally, raising the sintering temperature and time accelerates grain growth [85,86]. The EDS analysis results (Fig. 6) confirms that the bright phase and the dark phase (gray phase) are related to the TiC matrix and SiC whisker, respectively. The fracture surfaces of the sintered samples obtained from the flexural test are displayed in Fig. 7. It is seen that the sintering temperature (1800 °C) is not enough, and the consolidation process is in its initial stage. Incomplete bonding between particles, remained pores in the microstructure (especially irregular in shape), and inter-granular fracture mode are the features of lower sintering temperatures. With rising the sintering temperature to 1900 °C, a homogeneous microstructure appeared with equiaxed and fine grains. In the TiC–SiC composite, the presence of SiCw retards the growth of TiC grains by pinning the grain-boundaries and inhibiting their movement. The occurrence of pore coalescence is evident, and the pores morphology altered from irregular to near-spherical shape. Here, the pores emerged in the microstructure prevented the formation of a full dense bulk material. At 1900 °C, it can be realized that the fracture mode is mostly transgranular (Fig. 7). Increasing the sintering temperature to 2000 °C led to grain growth and pore coarsening.
Fig. 2. Effect of SPS temperatures on relative densities of the TiC–10 vol% SiCw composites.
3.3. Mechanical properties The effect of the SPS temperature on Vickers hardness is depicted in Fig. 8. The hardness increases from 11.3 ± 0.5 GPa to 24.54 ± 0.4 GPa when sintering temperature rises from 1800 °C to 1900 °C, respectively. This hardness enhancement is attributed to decreased porosity, the homogenization of the microstructure, and the increase of relative density (from 85.91% to 98.73%). Then hardness decreases to 22.61 ± 0.6 GPa as due to the formation of large grains and generation of extra pores with increasing the SPS temperature to 2000 °C. Babapoor et al. reported lower hardness values of 8.1 GPa and 22.1 GPa for monolithic TiC obtained by SPS at 1800 °C and 2000 °C, respectively. This difference can be attributed to the SiCw dispersoids in the TiC matrix. Also, Babapoor et al. reported a value of 25.7 GPa for the hardness of monolithic TiC sintered at 1900 °C [55], which is about 1.16 GPa higher than that obtained for the specimen sintered at the same temperature in this work. This difference is assumedly owing to the lower hardness of SiC compared to TiC. Several researchers have
Fig. 3. XRD pattern of SPSed TiC- 10 vol% SiCw sintered at 2000 °C under an external pressure of 40 MPa and a holding time of 7 min.
grain growth occurred due to the sintering temperature enhancement to 2000 °C, which led to an increase in pores and prevented full densification. The proper distribution and different morphology of SiCw in the 3
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Fig. 4. Optical microscopy images of the polished surfaces of the sintered TiC–10 vol% SiCw at various SPS temperatures: (a) 1800 °C, (b) 1900 °C, and (c) 2000 °C.
Fig. 5. FESEM images of the polished surfaces of the sintered TiC–10 vol% SiCw at various SPS temperatures: (a) 1800 °C, (b) 1900 °C, and (c) 2000 °C.
Fig. 6. FESEM micrograph of the polished surface of TiC–10 vol% SiCw composite sintered at 1900 °C and equivalent EDS mapping analysis. 4
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Fig. 7. SEM micrograph of the fracture surfaces of TiC–10 vol% SiCw composite at various SPS temperatures: (a) 1800 °C, (b) 1900 °C, and (c) 2000 °C.
reported that full densification and consequently improved mechanical properties could be achieved in TiC based composites reinforced with 20–30 vol% of SiC [40,80]. Fig. 9 represents the variation of flexural strength versus sintering temperature. Increases in sintering temperature from 1800 to 1900 °C and from 1900 to 2000 °C resulted in an increase and a decrease in the flexural strength from 289 to 511 MPa and from 511 to 470 MPa, respectively. These data reveal that the flexural strength, hardness, and densification follow a similar variation trend vs. SPS temperature. In other words, the mechanical characteristics, such as flexural strength and hardness, decrease when the amount of porosity increases, and the grain growth occurs according to the Hall–Petch relationship [87].
3.4. Thermal conductivity The sintering temperature dependence of thermal conductivity for TiC – 10 vol% SiCw composite is plotted in Fig. 10. It is evident that the room temperature thermal conductivity of the SPSed specimens increases along with rising sintering temperature, with values of 11.4, 19.8 and 32.3 W/mK for the samples sintered at 1800, 1900 and 2000 °C, respectively. Here, the obtained thermal conductivity of SPSed composites is between that of pure SiC and TiC. It is clear that thermal conductivity does not have the same trend with relative density and mechanical characteristics (bending strength and hardness) of sintered
Fig. 8. Hardness of the SPSed composites as a function of sintering temperatures.
Fig. 9. Flexural strength of the SPSed composites as a function of sintering temperatures.
Fig. 10. Thermal conductivity of the SPSed composites as a function of sintering temperatures. 5
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References
compacts. This behavior is not in agreement with that of Babapoor et al. [55], who presented a decreasing trend in thermal conductivity of SPSed monolithic TiC by augmenting the processing temperature from 1900 °C to 2000 °C. In this work, it seems that porosity does not have a dominant effect on thermal conductivity with rising sintering temperature. The porosity, impurity, micro-cracks, composition, grain size, and grain-boundary thickness in a ceramic material can influence thermal conductivity. In a polyphase material, microstructure and composition can significantly affect the thermal conductivity. Cabrero et al. [39] concluded that in a TiC–SiC composite produced via SPS, the composition of the material (matrix and reinforcement) had a significant effect on the thermal conductivity rather than the microstructure. Also, this claim was supported in Ref. [80] for TiC-based composite reinforced with SiC whisker. Here, the occurrence of grain growth in the TiC matrix (fewer grain boundaries), the presence of homogeneously distributed SiC whiskers in the matrix, and likely relative dissolution of two phases in each other might have led to hitting higher thermal conductivity in the sample sintered at 2000 °C. At lower sintering temperatures, especially at 1900 °C, a finer microstructure with higher grain boundaries acts as a heat transfer barrier, yielding a lower thermal conductivity compared to the specimen processed at a higher temperature (2000 °C).
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4. Conclusions In the current work, the spark plasma sintering technique is successfully employed in the fabrication of TiC based composites containing 10 vol% SiCw. According to the obtained results, the effects of the SPS temperature on densification, microstructural evolution, mechanical properties, and thermal conductivity of the TiC- 10 vol% SiCw composite are summarized as follows: 1. The displacement, displacement rates and relative density values of the sintered composites reveal that maximum densification occurred at 1900 °C. 2. Phase identification results indicate that there is no chemical reaction between TiC and SiCw phases. Based on the micrographs obtained from polished and fracture surfaces, SiC whiskers are distributed homogeneously in the TiC matrix at 1900 °C and 2000 °C. Sintering at 1900 °C resulted in a finer microstructure. The growth of grains occurred by raising the processing temperature to 2000 °C, which prevented full densification. 3. Effects of SPS temperature on hardness and flexural strength show that maximum value of mechanical properties is related to the composite sintered at 1900 °C. 4. The result of thermal conductivity analysis reveals that there is a linear relationship between sintering temperature and thermal conductivity. A maximum value of 32.3 W/mK is obtained for the composite sintered at 2000 °C. It can, therefore, be concluded that the composition of the material due to the existence of SiCw reinforcement in TiC matrix has a more significant effect on the thermal conductivity than the porosity percentage. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research work was financially supported by Aliyazh Sanat Sahand Ipak Company, Tabriz, Iran (Grant No. 971210-4). The authors gratefully appreciate for practical support of Eng. Hamid Sharifiyan and Saeid Sharifiyan (members of the board of directors in Aliyazh Sanat Sahand Ipak Company). 6
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