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The effect of nano-TiO2 additions on the densification and mechanical properties of SiC-matrix composite
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The effect of nano-TiO2 additions on the densification and mechanical properties of SiC-matrix composite Mahdi Khodaeia,∗, Omid Yaghobizadehb, Hamid Reza Baharvandia, Alireza Alipour Shahrakia, Hesam mohammadic a
Composite Materials & Technology Center, Malek Ashtar University of Technology, Tehran, Iran Department of Materials Engineering, Imam Khomeini International University (IKIU), Qazvin, Iran c Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC-TiC composite Nano-TiO2 Crack deflection Crack branching In-situ reaction
This research studied the effect of adding TiO2 nanoparticles along with Al2O3 and Y2O3 additives on the physical and mechanical properties of a SiC-matrix composite. The samples were fabricated through a pressureless process at 1900 °C. The results showed that the addition of TiO2 nanoparticles up to 4.5 wt% inhibited the excessive growth of SiC grains. According to the investigations, the microstructure and final properties of composites were affected by density, synthesized phases as well as their distribution in the matrix, and grain size. The highest density, Young's modulus, hardness, indentation fracture resistance, and flexural strength, alongside with the lowest brittleness index were respectively, 98.7%, 401.2 GPa, 27.1 GPa, 6.1 MPa m1/2, 522.7 MPa and 292.19 × 10−6 m−1, attributed to sample containing 4.5 wt% TiO2. Microstructural observations showed that the stresses caused by the difference in the thermal expansion coefficients of SiC and TiC, the formation of microcracks, the occurrence of crack deflection and crack branching mechanisms, as well as grain pull-out were the main reasons for the improvements in indentation fracture resistance.
1. Introduction Due to its low thermal expansion coefficient and the stability of its mechanical strength, silicon carbide (SiC) can withstand thermal cycles at high temperatures. Since SiC enjoys a considerable thermal shock resistance, its application at high temperatures is preferable to other ceramics, even Si3N4. The replacement of metallic super-alloys by SiC brings 10–50% energy saving [1–3]. Silicon carbide is a hard material (17–25 GPa), whose Young's modulus is 400–450 GPa. Its reactivity with other materials is low at room temperature [4]. Given the mentioned properties, SiC ceramics are used in industrial thermal converters, vapor turbines, glass as well as metallurgy industries, ceramics, thermal and nuclear power plants, and aerospace structures [5–8]. Despite all these beneficial properties, some undesired properties, such as low fracture toughness, have not only limited the application of SiC but also made researchers search for a solution [9,10]. According to researchers, the addition of a material as a reinforcement to SiC ceramic structures is the best way to overcome the mentioned weakness—low fracture toughness [9–11]. SiC-matrix composites are fabricated via different methods—such as hot pressing, SPS, pressureless sintering, and reactive sintering ∗
processes [11–13]. Among these methods, pressureless sintering, which makes the fabrication of complex-shaped or large pieces possible, is considered as a well-established method. The pressureless method is applied via the sintering of a solid and a liquid phase for achieving over 95% of the theoretical density [9,11]. The main advantage of liquidstate sintering is achieving high and uniform densification in the presence of a liquid phase at low temperatures. The quality of liquid-phase sintering depends on the composition of original materials and oxide additives [9,11,14]. Given the previous projects, Al2O3 and Y2O3 additives can react with the SiO2 phase present on the surface of SiC powders, which develops eutectic composition and improves sinterability [15–17]. Researches have indicated that SiC sintering in the presence of Al2O3–Y2O3 additives is initiated at 1600 °C and that with increasing the temperature up to 2000 °C, density rises [18]. Because another limitation of SiC ceramics is their weak sinterability, caused by strong covalent bonds in the structure, in order to improve sinterability and mechanical properties, many reinforcements—such as TiN, B4C, CaO, TiO2, and TiC—have been added to SiC composites so far [9,11,19–23]. Previous projects have demonstrated that the application of TiO2 particles and the in-situ formation of TiC particles can improve the sinterability and mechanical properties of
Corresponding author. E-mail address: [email protected] (M. Khodaei).
https://doi.org/10.1016/j.ceramint.2019.11.128 Received 29 October 2019; Received in revised form 14 November 2019; Accepted 15 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mahdi Khodaei, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.128
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The average of 5 measurements was reported as indentation fracture resistance. Young's modulus was measured according to ASTM C1198 standard. Brittleness index was calculated according to Equation (3) [29]:
SiC bodies. TiC particles limit the grain growth of SiC and affect both the strength and toughness of the sample by forming a fine-grained structure. In addition, the localization of TiC particles on the crack growth path increases fracture energy by crack deflection. Additionally, the residual stress caused by the mismatch between the thermal expansion coefficient of TiC and that of SiC boosts grain boundary strength and stimulates the formation of micro-cracks, each of which has a specific role in improving mechanical properties [14,19,24–26]. Apart from that, previous researches have shown that the application of nano-scale reinforcements results in superior properties, because the specific area of nanoparticles is higher than that of micro-reinforcements [27]. The basic properties of the ceramic-matrix composites reinforced by nanoparticles include low-temperature sintering, high density, and inhibition from the excessive growth of matrix grains. Thus, controlling particle size distribution and particle dispersion in the matrix phase of composites is necessary to improve their properties [9,27,28]. This research investigated the effect of different amounts of nano-TiO2 nanoparticles on the sinterability, mechanical properties, crack growth path, and fracture mode of SiC-TiC composites sintered via pressureless sintering method.
B=
MOR =
3FI 2bd 2
(4)
F, I, b, and d represent the fracture force (N), support distance (mm), the width of the sample (mm), and the thickness of the sample (mm), respectively. The phase analysis of samples was performed by an Inel EQUINOX 3000, equipped with a copper cathode; further, the microstructure of composites was studied by a scanning electron microscope (SEM, VEGA\TESCAN) equipped with EDS. The average grain size (G) was determined by the linear intercept method using the following equation:
2.1. Materials and methods
G = 1.56L/(M × N )
In this project, α-SiC powder whose mean particle size was 0.3 μm as well as sub-micron Al2O3 and Y2O3 powders (4.3 wt% and 5.7 wt%, respectively) were used for liquid-phase formation. Also, different amounts of TiO2 powders (0, 1.5, 3, 4.5, 6, 7.5, 9 and 12 wt %) with a mean particle size of 20 nm were used as reinforcement. In order to prepare the samples, raw materials were milled and mixed by a planetary mill, whose rotational speed was 180 rpm, for 3 h. Subsequently, the obtained compounds were dried at 100 °C for 4 h. The samples were pressed into cylindrical shaped pieces. Afterward, green samples were pyrolyzed at 600 °C with a heating rate of 2 °C/min; finally, they were sintered at 1900 °C for 1.5 h. Of note, before reaching the sintering temperature, all samples were heat-treated at 1800 °C for 0.5 h so that in-situ TiC forms from the reaction of SiC and TiO2.
(5)
where L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of lines intersected by grain boundaries. 3. Results and discussion 3.1. Phase analysis According to X-ray diffraction patterns, the TiC phase was formed by the reaction of TiO2 with SiC (reaction 1). Phase analysis results of the composites containing 4.5 wt% and 12 wt% TiO2 nanoparticles sintered at 1900 °C are shown in Fig. 1. SiC was the primary phase, and YAG (JCPDS card 01-079-1891) was the grain boundary phase. As is obvious, the increase in the amount of TiO2 raised the intensity of TiC peaks. According to the Al2O3-Y2O3 phase diagram (Fig. 2), as was expected, the formation of the YAG phase was the result of adding 43 wt% Al2O3 and 57 wt% Y2O3 [11]. In this research, non-stoichiometric products, as secondary phases, did not form, which was in accordance with other researches [14,25]. Investigations have indicated that the toughened phase, which is formed in situ, has better activity and thermodynamic stability than the TiC particles directly added to the composite [14]. In fact, TiO2 reacts with SiC and decreases the carbon content of SiC, diminishing carbon activity, thereby assuring that no detrimental carbide phase is produced during synthesis.
2.2. Characterization In this research, the relative density of samples was measured using Archimedes' method, according to the ASTM C373 standard. MicroHardness was measured via the Vickers method, according to ASTM C1327. Initially, the surfaces of samples were polished by diamond pastes of 30, 6, and 1 μm diamond, respectively. Hardness was measured according to Equation (1); the average of 5 measurements was reported: (1)
TiO2 + 3SiC → TiC + 3Si + 2CO(g)
where d is the indentation diagonal length in μm, H is Vickers hardness (MPa), and P is the applied force (N). In addition, the indentation fracture resistance of samples was determined according to Equation (2):
E 0.5 P KIFR = a ⎛ ⎞ × ⎛ 3/2 ⎞ H ⎝ ⎠ ⎝c ⎠
(3)
where HV is the Vickers hardness, E is Young's modulus, and KIFR is indentation fracture resistance. To measure the flexural strength, the composites were polished using 30, 6, and 1-μm diamond polishing pastes, and their flexural strengths were measured according to ASTM C1161 standard.
2. Experimental
P H = (1.854) × ⎛ 2 ⎞ ⎝d ⎠
Hv E 2 KIFR
(6)
3.2. The investigation of microstructure and grain size In Fig. 3, the microstructure of the composites containing 4.5 wt% and 12 wt% TiO2 nanoparticles is represented. As is observed, increasing the percentage of additives to 12 wt% led to a non-uniform distribution of the reinforcement phase and grain growth. It is worth mentioning that the localization of reinforcement particles on grain boundaries limits the mobility of grain boundaries, which is observed when reinforcement particles are added at the critical level. However, if the reinforcement content of the nanocomposite structure exceeds a specific limit, sample properties deteriorate. The agglomeration of reinforcement particles plays a part in this negative trend [27]. As can be seen in Fig. 3 (b), most TiC phases were agglomerated in
(2)
where: a: 0.016 ± 0.004; E: Young's modulus (GPa); H: Vickers hardness (GPa); P: Applied force (N); C: The half-length of radial crack (m).
2
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Fig. 1. Phase analysis of the samples containing (a) 4.5 wt% and (b) 12 wt% TiO2 nanoparticles, sintered at 1900 °C.
reduces grain size [30].
G=
4r 3f
(7)
where G, r, and f are the grain size of the matrix, the radius of reinforcement particles, and the volume fraction of reinforcement particles, respectively. Results indicated that even a meager amount of additive could be enough for inhibiting the growth of SiC grains. Thus, the TiC reinforcement phase would effectively control grain growth. In addition to halting grain growth, the in-situ synthesis of TiC particles led to SiC grain pull-out, which was in agreement with Liang and Ahmoye [14,19,25]. In fact, the precipitation of TiC particles on SiC grain boundaries could limit the equiaxial growth of SiC grains. 3.3. Density Different decisive factors play a role in the density of SiC-matrix composites. The results suggested that the amount of TiO2 nanoparticles affected the density of the composite. Fig. 4 shows relative density versus the variations of the weight percentages of TiO2 nanoparticles. At first, relative density increased with the increase in the content of TiO2 nanoparticles and reached its maximum, 98.7%, at 4.5 wt% TiO2 nanoparticles; afterward, it went down and reached 92.8%. The increase in the density of composites with the increase in the amount of TiO2 can be attributed to the in-situ reaction in grain boundaries and the limitation of excessive grain growth [24,26]. Moreover, the formation of in-situ TiC would reduce the reaction
Fig. 2. Phase diagram of Al2O3-Y2O3.
the composite containing 12 wt% TiO2 nanoparticles. Therefore, a nonuniform distribution of reinforcements could have reduced the density and other properties of the sample. The results of the mean grain size of different composites are presented in Table 1. Increasing the amount of TiO2 nanoparticles decreased grain size. This result was in agreement with the Zener pinning effect, according to which an increase in the amount of reinforcement
Fig. 3. SEM images of the samples containing (a) 4.5 wt% and (b) 12 wt% TiO2 nanoparticles. 3
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Table 1 The effects of TiO2 nanoparticles amount on the grain size and flexural strength of the composites. TiO2 (wt. %)
0
1.5
3
4.5
6
7.5
9
12
Grain size (μm( Flexural strength (MPa)
1.39 ± 0.04 454.2 ± 10
1.34 ± 0.02 471.4 ± 13
1.3 ± 0.3 493.6 ± 14
1.27 ± 0.6 522.7 ± 10
1.39 ± 0.09 503.6 ± 7
1.63 ± 0.0.7 487.4 ± 6.2
1.9 ± 0.1 470.1 ± 9
2.36 ± 0.13 456.2 ± 12
Fig. 4. Relative density and Young's modulus variations versus TiO2 content. Fig. 5. Hardness variations versus TiO2 content.
tendency for SiC and other components through reactions 2 and 3. Therefore, the amount of released CO and SiO gases would decrease, thereby raising the density [19,24,26].
SiC + 2Y2O3 → SiO + 4YO + CO (g)
(8)
SiC + Al2 O3 → SiO + Al2 O+ CO (g)
(9)
3.5. Hardness In Fig. 5, the effect of adding different amounts of TiO2 nanoparticles on the hardness of the samples is represented. As can be seen, at first, up to 4.5 wt%, hardness increased but then decreased. The composite containing 4.5 wt% TiO2 nanoparticles had the highest hardness (27.1 GPa). In general, given the difference between the hardness of TiC (20–32 GPa) and SiC (17–25 GPa), the formation of TiC particles among SiC grains would improve the hardness in the SiCTiC composite [37,38]. As is shown, the in-situ synthesis of TiC particles inhibited grain growth and improved sinterability as well as the density of the samples. All of the mentioned parameters was effective in increasing hardness. In this regard, previous researches have shown that the formation of TiC particles in the SiC matrix reduces the SiC lattice parameter, which enhances the hardness [11]. It is observed that the addition of more than 4.5 wt% TiO2 lowered the hardness. Different explanations can be represented for this adverse trend, including the decrease in relative density as well as the agglomeration of TiC particles and their non-uniform distribution on grain boundaries. Additionally, increasing the reinforcement phase increased the residual stress derived from the mismatch between the thermal expansion coefficients of SiC and TiC phases. This residual stress caused some micro-cracks to form (Fig. 6) and, consequently, lowered the hardness [24,26].
The decrease in relative density in the composites containing more than 4.5 wt% TiO2 can be related to the agglomeration of reinforcement particles and the exhaustion of CO gas as a result of the reaction between TiO2 and SiC (reaction 1) [19,24,26]. In fact, under such conditions, the formation rate of these gases would become higher than the elimination rate of the pores. During sintering, pore elimination occurred as a result of the capillary forces derived by increasing the amount of additive. Generally, since in-situ synthesized particles decrease stress concentration, the efficiency and properties of in-situ reinforced SiC composites improve, which can be explained by the increase in masstransformation processes in SiC [31–33]. 3.4. Elastic modulus In Fig. 4, variations in the elastic modulus of a SiC-TiC nano-composite versus the weight percentages of TiO2 nanoparticles are illustrated. Maximum Yang's modulus was 401.2 GPa and was obtained in the sample containing 4.5 wt% TiO2 nanoparticle. According to Equation (7), the elastic modulus is dependent on the porosity content of composites. In the following equation, E is the elastic modulus, and P is the amount of porosity in the sintered sample [34].
E = 66.82 − 192.58 P −
1551.21 P 2
3.6. Indentation fracture resistance In Fig. 7, the variations of the indentation fracture resistance of sintered composites are indicated. The best indentation fracture resistance was observed in the sample containing 4.5 wt% addition. Many parameters are decisive in the fracture toughness of composites. In-situ synthesized TiC particles form a fine-grained and uniform microstructure through prohibiting grain growth. Moreover, the in-situ formed TiC particles act as barriers to crack growth and positively influence their growth path. Crack deflection occurs when the reactions between the crack tip and secondary phase particles lead to non-planar crack growth–this kind of propagation alleviates the stress intensity on the crack tip [39].
(10)
Porosity amount and Young's modulus are reversely related [35]. Since adding up to 4.5 wt% TiO2 nanoparticles increased relative density, by adding up to 4.5 wt% TiO2, Young's modulus of the composite increased at first and reached its maximum but then decreased. Furthermore, Young's modulus of TiC (680 GPa) is higher than that of SiC (410 GPa) [36]; thus, according to the rule of mixtures, the simultaneous increase in Young's modulus and TiC amount would be predictable. 4
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respectively. In this research, EP and Em were 680 and 410 GPa, respectively. αm and αp were equal to 4.16 × 10−6/°C and 7.4 × 10−6/°C, and v p and v m were equal to 0.25 and 0.17, respectively [36]. Thus, the tensile radius stress around TiC was positive, and compressive stress was negative. The field of residual stress forms micro-cracks around TiC particles. Although stress-derived micro-cracks improve fracture toughness, excess micro-cracking can lead to the particle pull-out and lower fracture toughness [43]. According to Faber et al. [44], the main reason for the decrease in fracture toughness with the increase in reinforcement amount is the great number of formed micro-cracks. Furthermore, micro-cracking depends on grain size. Previous researches have shown that the rise in grain size leads to the propagation of micro-cracking [45,46]. Therefore, here, the decrease in the indentation fracture resistance of the composite containing 12 wt% TiO2 with an increase in TiO2 amount can be attributed to enhance micro-cracking and, also, the increase in SiC grain size. As is observed in Fig. 9, the fracture mode was a mixture of intragranular and intergranular fracture surfaces, and the share of intergranular fracture was higher than that of intragranular fracture. As is shown, SiC grain pull-out is another mechanism responsible for the increased fracture toughness.
Fig. 6. SEM image of the propagation of micro-cracks and micro voids in the sample containing 9 wt% TiO2.
3.7. Brittleness index Fig. 7 illustrates the brittleness index of composites versus different amounts of nano-TiO2. As is observed, increasing nano-TiO2 additions up to 4.5 wt% diminished brittleness; however, upon further increasing nano-TiO2, the brittleness index increased. These alterations were in harmony with Young's modulus, hardness, and indentation fracture resistance in different percentages of TiO2 nanoparticles. 3.8. Flexural strength The effects of different additions of TiO2 nanoparticles on flexural strength are indicated in Table 1. The results showed that the addition of reinforcement nanoparticles up to 4.5 wt% improved the strength of SiC-TiC nanocomposite; nevertheless, adding more than 4.5 wt% TiO2 reduced strength. The addition of TiO2 limited the mobility of grain boundaries, which not only lowered grain size but improved strength according to the Hall-Petch equation as well [24,26]. When the amount of additive was less than 4.5 wt%, density was directly proportional to the amount of additive, demonstrating an improvement in strength as well as a reduction in structural defects. Still, with the addition of more than 4.5 wt% of reinforcement particles, strength decreased, which can be attributed to the agglomeration of particles and the formation of intergranular pores, suitable places for crack nucleation. Excessive addition of reinforcement particles increased the residual stress in the structure. If the level of residual stress exceeds the tolerance of the matrix phase, the crack will nucleate and propagate in the matrix, and fracture can occur even by low external stress.
Fig. 7. Effect of different amounts of TiO2 nanoparticles additives on the indentation fracture resistance and brittleness index of a SiC-TiC composite.
Crack deflection absorbs a high amount of energy, resulting in toughness enhancement. As is observed in Fig. 8, crack deflection, crack branching, and crack bridging occurred in the obtained composite structure; each of these mechanisms contributed to toughness improvement. Another important mechanism that boosts fracture toughness in SiCmatrix composites is micro-cracking, which occurs in the regions with residual tensile stress [40,41]. Residual stress can be the result of a mismatch between linear expansion coefficients and Young's modulus of the SiC matrix and the TiC reinforcement particles, calculated by Equations (8) and (9) [42].
σmr = (αp − αm) ΔT /{[(1 + vm)/2Em] + [(1 − 2vp)/ Ep]}
(11)
σmt = −σmr /2
(12)
4. Conclusions In this research, the effect of adding different amounts of TiO2 nanoparticles on the sinterability and properties of a SiC-Al2O3-Y2O3 composite was investigated. The results of phase analysis showed that the whole amount of TiO2 transformed into TiC. The best density, elastic modulus, hardness, fracture toughness, flexural strength, and the lowest brittleness index were obtained in the composite containing 4.5 wt% TiO2. Investigations showed that the amount of reinforcement phase, relative density, grain size, the distribution of TiC particles among SiC grains, and residual stress were the deciding parameters in
where σmr and σmt are the radial matrix stress and the tangential matrix stress around the TiC particles, respectively. ν is Poisson's ratio, ΔT is the temperature difference over which stress is relieved by the diffusive process, and subscripts p and m refer to the particle and matrix, 5
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Fig. 8. SEM images of the crack path in the sample containing 4.5 wt% nano-TiO2.
Fig. 9. SEM images of the fracture surfaces of the composite containing 4.5 wt% TiO2 nanoparticles.
the activation of crack deflection and bridging mechanisms, as well as the formation of micro-cracks. It was also determined that an excessive increase in reinforcements led to grain growth, a decrease in density, and an increase in micro-cracks, thereby reducing the mechanical
the measured properties. It was also clarified that the addition of TiO2 up to 4.5 wt% inhibited grain growth. Microscopic observations showed that the dominant mode of fracture was intergranular, and decisive factors in the fracture toughness of samples included SiC grain pull-out, 6
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properties of SiC-TiC composites.
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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. References [1] J. Zhang, D. Jiang, Q. Lin, Z. Chen, Z. Huang, Properties of silicon carbide ceramics from gel casting and pressureless sintering, Mater. Des. 65 (2015) 12–16. [2] S.E. Saddow, A. Agarwal, Advances in Silicon Carbide Processing and Applications, Artech House, 2004 ,. [3] G. Magnani, A. Brentari, E. Burresi, G. Raiteri, Pressureless sintered silicon carbide with enhanced mechanical properties obtained by the two-step sintering method, Ceram. Int. 40 (2014) 1759–1763. [4] S. Zhu, M. Mizuno, Y. Kagawa, Y. Mutoh, Monotonic tension, fatigue and creep behavior of SiC-fiber-reinforced SiC-matrix composites: a review, Compos. Sci. Technol. 59 (1999) 833–851. [5] D.W. Richerson, Modern Ceramic Engineering: Properties, Processing, and Use in Design, CRC press, 2005. [6] T. Graziani, D.J. Baxter, C.A. Nannetti, Degradation of silicon carbide-based materials in a high temperature combustion environment, Key Eng. Mater. 113 (1996) 153–166. [7] D. Bucevac, Heat treatment for strengthening silicon carbide ceramic matrix composites, Advances in Ceramic Matrix Composites, 2014, pp. 141–163. [8] K. Yamada, M. Mohri, Properties and applications of silicon carbide Ceramics, Silicon Carbide Ceramics-1, Springer, Dordrecht, 1991, pp. 13–44. [9] M. Khodaei, O. Yaghobizadeh, A.A. Shahraki, S. Esmaeeli, Investigation of the effect of Al2O3–Y2O3–CaO (AYC) additives on sinterability, microstructure and mechanical properties of SiC matrix composites: a review, Int. J. Refract. Metals Hard Mater. 78 (2018) 9–26. [10] K. Raju, D.H. Yoon, Sintering additives for SiC based on the reactivity: a review, Ceram. Int. 42 (2016) 17947–17962. [11] M. Khodaei, O. Yaghobizadeh, H.R. Baharvandi, A. Dashti, Effects of different sintering methods on the properties of SiC-TiC, SiC-TiB2 composites, Int. J. Refract. Metals Hard Mater. 70 (2018) 19–31. [12] F. Guillard, A. Allemand, J.D. Lulewicz, J. Galy, Densification of SiC by SPS-effects of time, temperature and pressure, J. Eur. Ceram. Soc. 27 (2007) 2725–2728. [13] X. Qiang, Z. Shizhen, N. Chuanhao, C. Jianling, Z. Junfeng, F. Chao, W. Fuchi, Study on SPS sintering mechanism of SiC ceramic, Rare Metal Mater. Eng. 36 (2007) 341. [14] H. Liang, X. Yao, H. Zhang, X. Liu, Z. Huang, In situ toughening of pressureless liquid phase sintered α-SiC by using TiO2, Ceram. Int. 40 (2014) 10699–10704. [15] F.K. Van Dijen, E. Mayer, Liquid phase sintering of silicon carbide, J. Eur. Ceram. Soc. 16 (1996) 413–420. [16] J.K. Lee, H. Tanaka, H. Kim, Movement of liquid phase and the formation of surface reaction layer on the sintering of β-SiC with an additive of yttrium aluminium garnet, J. Mater. Sci. Lett. 15 (1996) 409–411. [17] H. Gu, T. Nagano, G.D. Zhan, M. Mitomo, F. Wakai, Dynamic evolution of grain‐boundary films in liquid phase sintered ultrafine silicon carbide material, J. Am. Ceram. Soc. 86 (2003) 1753–1760. [18] M. Omori, H. Takei, Pressureless sintering of SiC, J. Am. Ceram. Soc. 65 (1982). [19] D. Ahmoye, V.D. Krstic, Reaction sintering of SiC composites with in situ converted TiO2 to TiC, J. Mater. Sci. 50 (2015) 2806–2812. [20] Z.N. Wing, TiN modified SiC with enhanced strength and electrical properties, J. Eur. Ceram. Soc. 37 (2017) 1373–1378. [21] K.J. Kim, K.M. kim, Y.W. Kim, Highly conductive SiC ceramics containing Ti2CN, J.
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The effect of nano-TiO2 additions on the densification and mechanical properties of SiC-matrix composite Mahdi Khodaeia,∗, Omid Yaghobizadehb, Hamid Reza Baharvandia, Alireza Alipour Shahrakia, Hesam mohammadic a
Composite Materials & Technology Center, Malek Ashtar University of Technology, Tehran, Iran Department of Materials Engineering, Imam Khomeini International University (IKIU), Qazvin, Iran c Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC-TiC composite Nano-TiO2 Crack deflection Crack branching In-situ reaction
This research studied the effect of adding TiO2 nanoparticles along with Al2O3 and Y2O3 additives on the physical and mechanical properties of a SiC-matrix composite. The samples were fabricated through a pressureless process at 1900 °C. The results showed that the addition of TiO2 nanoparticles up to 4.5 wt% inhibited the excessive growth of SiC grains. According to the investigations, the microstructure and final properties of composites were affected by density, synthesized phases as well as their distribution in the matrix, and grain size. The highest density, Young's modulus, hardness, indentation fracture resistance, and flexural strength, alongside with the lowest brittleness index were respectively, 98.7%, 401.2 GPa, 27.1 GPa, 6.1 MPa m1/2, 522.7 MPa and 292.19 × 10−6 m−1, attributed to sample containing 4.5 wt% TiO2. Microstructural observations showed that the stresses caused by the difference in the thermal expansion coefficients of SiC and TiC, the formation of microcracks, the occurrence of crack deflection and crack branching mechanisms, as well as grain pull-out were the main reasons for the improvements in indentation fracture resistance.
1. Introduction Due to its low thermal expansion coefficient and the stability of its mechanical strength, silicon carbide (SiC) can withstand thermal cycles at high temperatures. Since SiC enjoys a considerable thermal shock resistance, its application at high temperatures is preferable to other ceramics, even Si3N4. The replacement of metallic super-alloys by SiC brings 10–50% energy saving [1–3]. Silicon carbide is a hard material (17–25 GPa), whose Young's modulus is 400–450 GPa. Its reactivity with other materials is low at room temperature [4]. Given the mentioned properties, SiC ceramics are used in industrial thermal converters, vapor turbines, glass as well as metallurgy industries, ceramics, thermal and nuclear power plants, and aerospace structures [5–8]. Despite all these beneficial properties, some undesired properties, such as low fracture toughness, have not only limited the application of SiC but also made researchers search for a solution [9,10]. According to researchers, the addition of a material as a reinforcement to SiC ceramic structures is the best way to overcome the mentioned weakness—low fracture toughness [9–11]. SiC-matrix composites are fabricated via different methods—such as hot pressing, SPS, pressureless sintering, and reactive sintering ∗
processes [11–13]. Among these methods, pressureless sintering, which makes the fabrication of complex-shaped or large pieces possible, is considered as a well-established method. The pressureless method is applied via the sintering of a solid and a liquid phase for achieving over 95% of the theoretical density [9,11]. The main advantage of liquidstate sintering is achieving high and uniform densification in the presence of a liquid phase at low temperatures. The quality of liquid-phase sintering depends on the composition of original materials and oxide additives [9,11,14]. Given the previous projects, Al2O3 and Y2O3 additives can react with the SiO2 phase present on the surface of SiC powders, which develops eutectic composition and improves sinterability [15–17]. Researches have indicated that SiC sintering in the presence of Al2O3–Y2O3 additives is initiated at 1600 °C and that with increasing the temperature up to 2000 °C, density rises [18]. Because another limitation of SiC ceramics is their weak sinterability, caused by strong covalent bonds in the structure, in order to improve sinterability and mechanical properties, many reinforcements—such as TiN, B4C, CaO, TiO2, and TiC—have been added to SiC composites so far [9,11,19–23]. Previous projects have demonstrated that the application of TiO2 particles and the in-situ formation of TiC particles can improve the sinterability and mechanical properties of
Corresponding author. E-mail address: [email protected] (M. Khodaei).
https://doi.org/10.1016/j.ceramint.2019.11.128 Received 29 October 2019; Received in revised form 14 November 2019; Accepted 15 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mahdi Khodaei, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.128
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The average of 5 measurements was reported as indentation fracture resistance. Young's modulus was measured according to ASTM C1198 standard. Brittleness index was calculated according to Equation (3) [29]:
SiC bodies. TiC particles limit the grain growth of SiC and affect both the strength and toughness of the sample by forming a fine-grained structure. In addition, the localization of TiC particles on the crack growth path increases fracture energy by crack deflection. Additionally, the residual stress caused by the mismatch between the thermal expansion coefficient of TiC and that of SiC boosts grain boundary strength and stimulates the formation of micro-cracks, each of which has a specific role in improving mechanical properties [14,19,24–26]. Apart from that, previous researches have shown that the application of nano-scale reinforcements results in superior properties, because the specific area of nanoparticles is higher than that of micro-reinforcements [27]. The basic properties of the ceramic-matrix composites reinforced by nanoparticles include low-temperature sintering, high density, and inhibition from the excessive growth of matrix grains. Thus, controlling particle size distribution and particle dispersion in the matrix phase of composites is necessary to improve their properties [9,27,28]. This research investigated the effect of different amounts of nano-TiO2 nanoparticles on the sinterability, mechanical properties, crack growth path, and fracture mode of SiC-TiC composites sintered via pressureless sintering method.
B=
MOR =
3FI 2bd 2
(4)
F, I, b, and d represent the fracture force (N), support distance (mm), the width of the sample (mm), and the thickness of the sample (mm), respectively. The phase analysis of samples was performed by an Inel EQUINOX 3000, equipped with a copper cathode; further, the microstructure of composites was studied by a scanning electron microscope (SEM, VEGA\TESCAN) equipped with EDS. The average grain size (G) was determined by the linear intercept method using the following equation:
2.1. Materials and methods
G = 1.56L/(M × N )
In this project, α-SiC powder whose mean particle size was 0.3 μm as well as sub-micron Al2O3 and Y2O3 powders (4.3 wt% and 5.7 wt%, respectively) were used for liquid-phase formation. Also, different amounts of TiO2 powders (0, 1.5, 3, 4.5, 6, 7.5, 9 and 12 wt %) with a mean particle size of 20 nm were used as reinforcement. In order to prepare the samples, raw materials were milled and mixed by a planetary mill, whose rotational speed was 180 rpm, for 3 h. Subsequently, the obtained compounds were dried at 100 °C for 4 h. The samples were pressed into cylindrical shaped pieces. Afterward, green samples were pyrolyzed at 600 °C with a heating rate of 2 °C/min; finally, they were sintered at 1900 °C for 1.5 h. Of note, before reaching the sintering temperature, all samples were heat-treated at 1800 °C for 0.5 h so that in-situ TiC forms from the reaction of SiC and TiO2.
(5)
where L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of lines intersected by grain boundaries. 3. Results and discussion 3.1. Phase analysis According to X-ray diffraction patterns, the TiC phase was formed by the reaction of TiO2 with SiC (reaction 1). Phase analysis results of the composites containing 4.5 wt% and 12 wt% TiO2 nanoparticles sintered at 1900 °C are shown in Fig. 1. SiC was the primary phase, and YAG (JCPDS card 01-079-1891) was the grain boundary phase. As is obvious, the increase in the amount of TiO2 raised the intensity of TiC peaks. According to the Al2O3-Y2O3 phase diagram (Fig. 2), as was expected, the formation of the YAG phase was the result of adding 43 wt% Al2O3 and 57 wt% Y2O3 [11]. In this research, non-stoichiometric products, as secondary phases, did not form, which was in accordance with other researches [14,25]. Investigations have indicated that the toughened phase, which is formed in situ, has better activity and thermodynamic stability than the TiC particles directly added to the composite [14]. In fact, TiO2 reacts with SiC and decreases the carbon content of SiC, diminishing carbon activity, thereby assuring that no detrimental carbide phase is produced during synthesis.
2.2. Characterization In this research, the relative density of samples was measured using Archimedes' method, according to the ASTM C373 standard. MicroHardness was measured via the Vickers method, according to ASTM C1327. Initially, the surfaces of samples were polished by diamond pastes of 30, 6, and 1 μm diamond, respectively. Hardness was measured according to Equation (1); the average of 5 measurements was reported: (1)
TiO2 + 3SiC → TiC + 3Si + 2CO(g)
where d is the indentation diagonal length in μm, H is Vickers hardness (MPa), and P is the applied force (N). In addition, the indentation fracture resistance of samples was determined according to Equation (2):
E 0.5 P KIFR = a ⎛ ⎞ × ⎛ 3/2 ⎞ H ⎝ ⎠ ⎝c ⎠
(3)
where HV is the Vickers hardness, E is Young's modulus, and KIFR is indentation fracture resistance. To measure the flexural strength, the composites were polished using 30, 6, and 1-μm diamond polishing pastes, and their flexural strengths were measured according to ASTM C1161 standard.
2. Experimental
P H = (1.854) × ⎛ 2 ⎞ ⎝d ⎠
Hv E 2 KIFR
(6)
3.2. The investigation of microstructure and grain size In Fig. 3, the microstructure of the composites containing 4.5 wt% and 12 wt% TiO2 nanoparticles is represented. As is observed, increasing the percentage of additives to 12 wt% led to a non-uniform distribution of the reinforcement phase and grain growth. It is worth mentioning that the localization of reinforcement particles on grain boundaries limits the mobility of grain boundaries, which is observed when reinforcement particles are added at the critical level. However, if the reinforcement content of the nanocomposite structure exceeds a specific limit, sample properties deteriorate. The agglomeration of reinforcement particles plays a part in this negative trend [27]. As can be seen in Fig. 3 (b), most TiC phases were agglomerated in
(2)
where: a: 0.016 ± 0.004; E: Young's modulus (GPa); H: Vickers hardness (GPa); P: Applied force (N); C: The half-length of radial crack (m).
2
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Fig. 1. Phase analysis of the samples containing (a) 4.5 wt% and (b) 12 wt% TiO2 nanoparticles, sintered at 1900 °C.
reduces grain size [30].
G=
4r 3f
(7)
where G, r, and f are the grain size of the matrix, the radius of reinforcement particles, and the volume fraction of reinforcement particles, respectively. Results indicated that even a meager amount of additive could be enough for inhibiting the growth of SiC grains. Thus, the TiC reinforcement phase would effectively control grain growth. In addition to halting grain growth, the in-situ synthesis of TiC particles led to SiC grain pull-out, which was in agreement with Liang and Ahmoye [14,19,25]. In fact, the precipitation of TiC particles on SiC grain boundaries could limit the equiaxial growth of SiC grains. 3.3. Density Different decisive factors play a role in the density of SiC-matrix composites. The results suggested that the amount of TiO2 nanoparticles affected the density of the composite. Fig. 4 shows relative density versus the variations of the weight percentages of TiO2 nanoparticles. At first, relative density increased with the increase in the content of TiO2 nanoparticles and reached its maximum, 98.7%, at 4.5 wt% TiO2 nanoparticles; afterward, it went down and reached 92.8%. The increase in the density of composites with the increase in the amount of TiO2 can be attributed to the in-situ reaction in grain boundaries and the limitation of excessive grain growth [24,26]. Moreover, the formation of in-situ TiC would reduce the reaction
Fig. 2. Phase diagram of Al2O3-Y2O3.
the composite containing 12 wt% TiO2 nanoparticles. Therefore, a nonuniform distribution of reinforcements could have reduced the density and other properties of the sample. The results of the mean grain size of different composites are presented in Table 1. Increasing the amount of TiO2 nanoparticles decreased grain size. This result was in agreement with the Zener pinning effect, according to which an increase in the amount of reinforcement
Fig. 3. SEM images of the samples containing (a) 4.5 wt% and (b) 12 wt% TiO2 nanoparticles. 3
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Table 1 The effects of TiO2 nanoparticles amount on the grain size and flexural strength of the composites. TiO2 (wt. %)
0
1.5
3
4.5
6
7.5
9
12
Grain size (μm( Flexural strength (MPa)
1.39 ± 0.04 454.2 ± 10
1.34 ± 0.02 471.4 ± 13
1.3 ± 0.3 493.6 ± 14
1.27 ± 0.6 522.7 ± 10
1.39 ± 0.09 503.6 ± 7
1.63 ± 0.0.7 487.4 ± 6.2
1.9 ± 0.1 470.1 ± 9
2.36 ± 0.13 456.2 ± 12
Fig. 4. Relative density and Young's modulus variations versus TiO2 content. Fig. 5. Hardness variations versus TiO2 content.
tendency for SiC and other components through reactions 2 and 3. Therefore, the amount of released CO and SiO gases would decrease, thereby raising the density [19,24,26].
SiC + 2Y2O3 → SiO + 4YO + CO (g)
(8)
SiC + Al2 O3 → SiO + Al2 O+ CO (g)
(9)
3.5. Hardness In Fig. 5, the effect of adding different amounts of TiO2 nanoparticles on the hardness of the samples is represented. As can be seen, at first, up to 4.5 wt%, hardness increased but then decreased. The composite containing 4.5 wt% TiO2 nanoparticles had the highest hardness (27.1 GPa). In general, given the difference between the hardness of TiC (20–32 GPa) and SiC (17–25 GPa), the formation of TiC particles among SiC grains would improve the hardness in the SiCTiC composite [37,38]. As is shown, the in-situ synthesis of TiC particles inhibited grain growth and improved sinterability as well as the density of the samples. All of the mentioned parameters was effective in increasing hardness. In this regard, previous researches have shown that the formation of TiC particles in the SiC matrix reduces the SiC lattice parameter, which enhances the hardness [11]. It is observed that the addition of more than 4.5 wt% TiO2 lowered the hardness. Different explanations can be represented for this adverse trend, including the decrease in relative density as well as the agglomeration of TiC particles and their non-uniform distribution on grain boundaries. Additionally, increasing the reinforcement phase increased the residual stress derived from the mismatch between the thermal expansion coefficients of SiC and TiC phases. This residual stress caused some micro-cracks to form (Fig. 6) and, consequently, lowered the hardness [24,26].
The decrease in relative density in the composites containing more than 4.5 wt% TiO2 can be related to the agglomeration of reinforcement particles and the exhaustion of CO gas as a result of the reaction between TiO2 and SiC (reaction 1) [19,24,26]. In fact, under such conditions, the formation rate of these gases would become higher than the elimination rate of the pores. During sintering, pore elimination occurred as a result of the capillary forces derived by increasing the amount of additive. Generally, since in-situ synthesized particles decrease stress concentration, the efficiency and properties of in-situ reinforced SiC composites improve, which can be explained by the increase in masstransformation processes in SiC [31–33]. 3.4. Elastic modulus In Fig. 4, variations in the elastic modulus of a SiC-TiC nano-composite versus the weight percentages of TiO2 nanoparticles are illustrated. Maximum Yang's modulus was 401.2 GPa and was obtained in the sample containing 4.5 wt% TiO2 nanoparticle. According to Equation (7), the elastic modulus is dependent on the porosity content of composites. In the following equation, E is the elastic modulus, and P is the amount of porosity in the sintered sample [34].
E = 66.82 − 192.58 P −
1551.21 P 2
3.6. Indentation fracture resistance In Fig. 7, the variations of the indentation fracture resistance of sintered composites are indicated. The best indentation fracture resistance was observed in the sample containing 4.5 wt% addition. Many parameters are decisive in the fracture toughness of composites. In-situ synthesized TiC particles form a fine-grained and uniform microstructure through prohibiting grain growth. Moreover, the in-situ formed TiC particles act as barriers to crack growth and positively influence their growth path. Crack deflection occurs when the reactions between the crack tip and secondary phase particles lead to non-planar crack growth–this kind of propagation alleviates the stress intensity on the crack tip [39].
(10)
Porosity amount and Young's modulus are reversely related [35]. Since adding up to 4.5 wt% TiO2 nanoparticles increased relative density, by adding up to 4.5 wt% TiO2, Young's modulus of the composite increased at first and reached its maximum but then decreased. Furthermore, Young's modulus of TiC (680 GPa) is higher than that of SiC (410 GPa) [36]; thus, according to the rule of mixtures, the simultaneous increase in Young's modulus and TiC amount would be predictable. 4
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respectively. In this research, EP and Em were 680 and 410 GPa, respectively. αm and αp were equal to 4.16 × 10−6/°C and 7.4 × 10−6/°C, and v p and v m were equal to 0.25 and 0.17, respectively [36]. Thus, the tensile radius stress around TiC was positive, and compressive stress was negative. The field of residual stress forms micro-cracks around TiC particles. Although stress-derived micro-cracks improve fracture toughness, excess micro-cracking can lead to the particle pull-out and lower fracture toughness [43]. According to Faber et al. [44], the main reason for the decrease in fracture toughness with the increase in reinforcement amount is the great number of formed micro-cracks. Furthermore, micro-cracking depends on grain size. Previous researches have shown that the rise in grain size leads to the propagation of micro-cracking [45,46]. Therefore, here, the decrease in the indentation fracture resistance of the composite containing 12 wt% TiO2 with an increase in TiO2 amount can be attributed to enhance micro-cracking and, also, the increase in SiC grain size. As is observed in Fig. 9, the fracture mode was a mixture of intragranular and intergranular fracture surfaces, and the share of intergranular fracture was higher than that of intragranular fracture. As is shown, SiC grain pull-out is another mechanism responsible for the increased fracture toughness.
Fig. 6. SEM image of the propagation of micro-cracks and micro voids in the sample containing 9 wt% TiO2.
3.7. Brittleness index Fig. 7 illustrates the brittleness index of composites versus different amounts of nano-TiO2. As is observed, increasing nano-TiO2 additions up to 4.5 wt% diminished brittleness; however, upon further increasing nano-TiO2, the brittleness index increased. These alterations were in harmony with Young's modulus, hardness, and indentation fracture resistance in different percentages of TiO2 nanoparticles. 3.8. Flexural strength The effects of different additions of TiO2 nanoparticles on flexural strength are indicated in Table 1. The results showed that the addition of reinforcement nanoparticles up to 4.5 wt% improved the strength of SiC-TiC nanocomposite; nevertheless, adding more than 4.5 wt% TiO2 reduced strength. The addition of TiO2 limited the mobility of grain boundaries, which not only lowered grain size but improved strength according to the Hall-Petch equation as well [24,26]. When the amount of additive was less than 4.5 wt%, density was directly proportional to the amount of additive, demonstrating an improvement in strength as well as a reduction in structural defects. Still, with the addition of more than 4.5 wt% of reinforcement particles, strength decreased, which can be attributed to the agglomeration of particles and the formation of intergranular pores, suitable places for crack nucleation. Excessive addition of reinforcement particles increased the residual stress in the structure. If the level of residual stress exceeds the tolerance of the matrix phase, the crack will nucleate and propagate in the matrix, and fracture can occur even by low external stress.
Fig. 7. Effect of different amounts of TiO2 nanoparticles additives on the indentation fracture resistance and brittleness index of a SiC-TiC composite.
Crack deflection absorbs a high amount of energy, resulting in toughness enhancement. As is observed in Fig. 8, crack deflection, crack branching, and crack bridging occurred in the obtained composite structure; each of these mechanisms contributed to toughness improvement. Another important mechanism that boosts fracture toughness in SiCmatrix composites is micro-cracking, which occurs in the regions with residual tensile stress [40,41]. Residual stress can be the result of a mismatch between linear expansion coefficients and Young's modulus of the SiC matrix and the TiC reinforcement particles, calculated by Equations (8) and (9) [42].
σmr = (αp − αm) ΔT /{[(1 + vm)/2Em] + [(1 − 2vp)/ Ep]}
(11)
σmt = −σmr /2
(12)
4. Conclusions In this research, the effect of adding different amounts of TiO2 nanoparticles on the sinterability and properties of a SiC-Al2O3-Y2O3 composite was investigated. The results of phase analysis showed that the whole amount of TiO2 transformed into TiC. The best density, elastic modulus, hardness, fracture toughness, flexural strength, and the lowest brittleness index were obtained in the composite containing 4.5 wt% TiO2. Investigations showed that the amount of reinforcement phase, relative density, grain size, the distribution of TiC particles among SiC grains, and residual stress were the deciding parameters in
where σmr and σmt are the radial matrix stress and the tangential matrix stress around the TiC particles, respectively. ν is Poisson's ratio, ΔT is the temperature difference over which stress is relieved by the diffusive process, and subscripts p and m refer to the particle and matrix, 5
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Fig. 8. SEM images of the crack path in the sample containing 4.5 wt% nano-TiO2.
Fig. 9. SEM images of the fracture surfaces of the composite containing 4.5 wt% TiO2 nanoparticles.
the activation of crack deflection and bridging mechanisms, as well as the formation of micro-cracks. It was also determined that an excessive increase in reinforcements led to grain growth, a decrease in density, and an increase in micro-cracks, thereby reducing the mechanical
the measured properties. It was also clarified that the addition of TiO2 up to 4.5 wt% inhibited grain growth. Microscopic observations showed that the dominant mode of fracture was intergranular, and decisive factors in the fracture toughness of samples included SiC grain pull-out, 6
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properties of SiC-TiC composites.
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