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The effect of TiO2 additive on the electrical resistivity and mechanical properties of pressureless sintered SiC ceramics with Al2O3-Y2O3
International Journal of Refractory Metals & Hard Materials 76 (2018) 141–148
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The effect of TiO2 additive on the electrical resistivity and mechanical properties of pressureless sintered SiC ceramics with Al2O3-Y2O3
T
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Mahdi Khodaeia, Omid Yaghobizadehb, , Naser Ehsania, Hamid Reza Baharvandia a b
Composite Materials & Technology Center, Malek Ashtar University of Technology, Tehran, Iran Department of Materials Engineering, Imam Khomeini International University (IKIU), Qazvin, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC Electrical resistivity Insulation Mechanical properties Grain boundary phase TiO2 additive Toughening mechanism
In this study, SiC-TiC composite was fabricated by the reaction between TiO2 and SiC and in addition to the effect of TiO2 additive, influence of sintering temperatures on the electrical resistivity, the relationship between electrical resistivity and microstructure, density, indentation fracture resistance and hardness were investigated. The main goal of this study was to improve electrical resistivity while preserving mechanical properties of SiC body. The results showed that with 10 wt% Al2O3-Y2O3, the electrical resistivity reached to 9 × 108 Ω·m. Increasing the amount of TiO2 particles from 2.5 to 10 wt% and changing the sintering temperature from 1850 °C to 1950°C made the electrical resistivity to be variable in the range of 2.2×105 Ω·m to 9 × 108 Ω·m. In this report, the highest density, hardness and indentation fracture resistance for the samples containing 5 wt% additive which were sintered at 1900 °C were 96.2%, 24.4 GPa and 5.8 MPa.m1/2, respectively. Microscopic images showed that if the grain boundary phase is located in the triple points or multiple points of grain boundary, the electrical resistivity will decrease and if it is located in full circumference of the SiC particles, due to failure of conducting pathways through SiC grains, the resistance of ceramics will increase instead.
1. Introduction In recent years, studying the variation of electrical conductivity in structured ceramics is of interest for the applied industries. For insulating ceramics to gain electrical conductivity, the distribution of conducting secondary phase particles is one of the solutions. In these composites, using 12 to 30 vol% of the conducting secondary phase is necessary. But the researches have shown that increasing the amount of secondary phase up to 70 vol% is essential for increment of resistivity in conductive ceramics [1–9]. It is worth to note that addition of the large amount of secondary phase is undesirable because it can severely affect the mechanical properties of final ceramic. Thus changing the electrical resistivity of ceramics by adding a little of secondary phase is important. It is wellknown that in ceramics which are sintered in the liquid phase, the grain boundary phase in the grain boundary pockets such as the triple points or multiple points is sporadically distributed in the whole body. Although it is also possible for the grain boundary phase to remain in the interface of two boundaries [10–12]. It can be concluded that if the grain boundary phase is insulator and located not only in the triple points of grain boundary but also in the surface of grain boundary, therefore addition of only 3 vol% of it will be sufficient to change the
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Corresponding author. E-mail address: [email protected] (O. Yaghobizadeh).
https://doi.org/10.1016/j.ijrmhm.2018.06.005 Received 16 April 2018; Received in revised form 5 June 2018; Accepted 11 June 2018 0263-4368/ © 2018 Elsevier Ltd. All rights reserved.
electrical properties of the final ceramic. In this condition, if the grain boundary is conductive, it will act as a path for electrical current and if it is insulator, it will weaken the connections of the matrix grains and the electrical resistivity will increase. Non-oxide ceramics such as nitrides, carbides and borides have very good mechanical properties no only at room temperatures but also at high temperature, which is due to covalent bond between its atoms. Moreover, non-oxide ceramics, except AlN and Si3N4 are electrically conductive showing either metallic conductivity or semi-conductivity properties. Among the non-oxide ceramics, SiC is one of the best candidates for industrial applications such as gas turbines, piston engines and heat exchangers, due to its good strength and creep resistance at high temperatures [13, 14]. The most important characteristics of this material include high thermal and electrical conductivity, high dielectric loss (in some cases), high melting point and low sinter-ability [15]. Recently, the SiC ceramics have been used as a useful material for manufacturing semi-conductors. In the case of wafer-boats and susceptors, high thermal conductivity and low electrical resistivity are needed, because the mentioned industry needs to accelerate the speed of warming and cooling of these parts in the production process [16–18]. In fact, electronic equipment and semiconductor sectors need to have different
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electrical resistivity, for example, the electrostatic chucks require a resistivity of 1 × 108 Ω·m-1 × 1014 Ω·m to be able to provide sufficient adsorption force [19], also, the vacuum chucks needs 1 × 106 Ω·m resistance to prevent the accumulation and discharge of static electricity. Due to the inherent resistance of SiC ceramics, variable electrical resistivity along with unchangeable mechanical properties is highly desirable for this category of applications. But one of the present problems occurring within the fabrication process of the SiC bodies is its low sinterability. About 88% of chemical bonds between carbon and silicon atoms in SiC are covalent. This fact reduces atomic diffusion so makes it difficult to achieve higher density [20]. In order to solve this problem and to obtain a denser silicon carbide body, a sintering aid is included in the sintering process. Sintering process of SiC body was done by Prochaska for the first time, using boron and carbon as additives. It should be noted that sintering temperature of SiC body with boron and carbon additives is higher than 2000 °C and mechanical properties of the final sample are relatively poor [17, 21]. Therefore, in order to improve the mechanical properties and also to reduce the temperature of pressure-less sintering process, extensive researches have been conducted on various additives. So far, various combinations including Al-C, Al2O3-C, Al2O3-Y2O3 have been studied to improve and reduce growth rate of the grains during the sintering process of SiC body [22–29]. Recent studies have shown that the use of Al2O3-Y2O3 additives at the temperature range of 1850 °C to 2000°C can lead to the formation of a denser body. The molten phase formation and dissolution-deposition process increase the density of silicon carbide bodies. Insulating grain boundary phase forms a eutectic phase, through additive reactions with SiO2 on the surface of SiC particles during the sintering process [30–36]. It is believed that the including oxides form the eutectic liquid and also react with SiO2 forming on the surface of SiC particles during the sintering process; this eutectic phase in addition to the recovery of condensation, improves some properties such as flexural strength as well. Also according to the insulating grain boundary phase, this phase will cause a change in the electrical conductivity of the sample [37]. The reason of increased strength and toughness could be attributed to stress concentration reduction and also reduction in porosities of the body. Additionally variation of fracture modes are one of the advantages of oxide additive [38]. Electrical resistivity can also change with matrix grain size, the amount of additives and their distribution in the sample. Given the above example, changing the electrical resistivity of SiC ceramics in such a way that their mechanical properties remain fixed, is invaluable. So far, few reports about the electrical resistivity of SiC and SiC-TiC bodies containing Al2O3-Y2O3 additives have been published [37, 39] and in these researches, reinforcement particles are added directly to raw materials. Among the reports published about reaction sintering of SiC composites with in situ converted TiO2 to TiC [40, 41], the effect of sintering temperature on the synthesized phases, mechanical properties and the electrical resistivity has not been investigated yet. In this research then the effect of different amounts of TiO2 additive and sintering temperature on the density, electrical resistivity, microstructure and the relationship between microstructure and electrical resistivity, indentation fracture resistance and hardness of the SiC and SiC-TiC bodies with Al2O3-Y2O3 additives have been investigated.
Table 1 Raw material compositions of each sample. wt. % TiO2 Raw materials
0
1
2.5
5
7.5
10
SiC (wt. %) TiO2 (wt. %) Al2O3 (wt. %) Y2O3 (wt. %)
90 0 4.3 5.7
89 1 4.3 5.7
87.5 2.5 4.3 5.7
85 5 4.3 5.7
82.5 7.5 4.3 5.7
80 10 4.3 5.7
order to remove the binders, all the samples were pyrolyzed at temperatures up to 600 °C with a heating rate of 2 °C/min. Then all the samples were sintered under argon atmosphere at 1850 °C, 1900 °C and 1950 °C, for 1.5 h. Before reaching to the sintering temperature, samples were heated at 1800 °C for 0.5 h to provide the suitable conditions for the reaction between SiC and TiO2. The densities of the samples were measured according to the ASTMC373–88 standard method. The hardness of the samples was calculated according to the standard Vickers method ASTM-C-1327; for this purpose, the surface of the samples was polished with diamond paste of 30 μm, 6 μm and 1 μm. Each sample went under load of 5 Kg (49.03 N) for 10 s. Five measurements were carried out on each sample and they were reported according to the following relationship:
H = (1.854) × (P/d2)
(1)
Where (d) is the average diameter of the indentation diagonals, (H) is Vickers hardness in MPa and P is force. Indentation fracture resistance was calculated according to an expression contrived by Anstis et al. [42].
E 0.5 P KIFR = a ⎛ ⎞ × ⎛ 3/2 ⎞ × 10−6 ⎝H⎠ ⎝c ⎠
(2)
Where in: a: 0.0160.004 ± E: Modulus of elasticity (GPa). H: Vickers hardness (GPa). P: Applied force (N). c: The half-length of radial crack (m). KIFR: Indentation fracture resistance (MPa.m1/2). The average amount of five measurements was reported as the indentation fracture resistance. Phase analysis of samples were conducted by Inel EQUINOX 3000 equipped with copper cathode and microstructure was investigated using scanning electron microscopy (SEM, VEGA\\TESCAN) equipped with EDS. The average grain size (G) was determined by the linear intercept method by using the following equation:
G = 1.56 L/(M × N)
(3)
Where L is the random line length on the micrograph, M the magnification of the micro-graph and N the number of line intersected by grain boundaries. The electrical resistivity of the SiC samples was measured using a 2point probe method using a high resistance meter (model 4339B, Agilent Technologies, Santa Clara, CA, USA).
2. Experimental procedures 3. Results and discussion In this study, α-SiC powders with an average size of 0.3 μm, Al2O3, Y2O3 and TiO2 powders with a particle size below 1 μm were utilized. In order to prepare samples, each raw material composition according to Table 1 was milled in planetary mill for 3 h at 180 rpm. Then the resulting slurries were dried at 100 °Cfor 4 h. The samples were pressed by uniaxial hydraulic press (90 MPa) in a cylindrical shape with a diameter and height of 1.2 cm and 0.5 cm, respectively. In
3.1. Relative density and phase analysis of samples In Fig. 1, the effects of various additives and sintering temperature on the density of composites are shown. As it can be seen, density of the samples increased at first then decreased by increasing the amount of additives in each three temperature. It should be noted that the effect of 142
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density of the samples increased from 93.3% to 96.2% and from 90% to 94.12%, respectively. An increase in the density of samples was observed due to the presence of very fine TiC particles which were the result of the reaction between TiO2 particles and SiC according to the reaction (4) and these fine particles were located on SiC boundaries. These particles reduced the migration rate of the boundaries, thus the extreme growth of the grains was minimized and the density increased.
TiO2 + 3SiC → TiC + 3Si + 2CO(g)
(4)
On the other hand, increased surface permeability and porosity mobility led to the reduction of the cavities in the sample therefore the density increased during the sintering process [45]. Also, with the formation of TiC, the reaction between SiC and Al2O3 was partially reduced leading to the decrease of the amount of released gases such as CO, Al2O, and SiO, i.e. increase of the density [40]. As it could be seen, at all three temperatures, the density decreased after a certain amount of the additive. The reason of this behavior is that by increase of the amount of additives, the rate of gas phase formation from the reaction between the oxide additives and SiC (reactions (5), (6)) is faster than the porosity removal rate by the capillary forces, therefore no significant increase in the density of the samples is observed.
Fig. 1. Relative density of samples with different additive content, sintered at 1850 °C, 1900 °C and 1950 °C.
SiC + 2Y2O3 → SiO + 4YO(g) + CO(g)
(5)
SiC + Al2 O3 → SiO(g) + AlO(g) + CO(g)
(6)
As it is obvious, increasing the temperature from 1850 °C to 1900 °C, the density of the specimens was increased. The decrease in the viscosity of the molten phase which was produced during the heat treatment process also reduced the amount of porosities and led to the increase of density. On the other hand, the density of specimens decreased slightly by increasing the temperature up to 1950 °C. The reason for this behavior is that when the temperature is higher than 1900 °C, the reaction between SiC and oxide phases intensifies. Although at this temperature the viscosity of the liquid phase is less than that in 1900 °C, but since the liquid phase reacts with SiC and the oxide phases are generated, resulting porosities of exhaust of gases reduce the density of the samples. The X-ray diffraction patterns of the samples containing 0 wt% and 10 wt% of the TiO2 additive which were sintered at 1900 °C to 1950 °C are shown in Fig. 3. As it can be seen, at 1900°C, the samples contain the 3Y2O3.5Al2O3 phase in addition to the dominant SiC phase. This phase is known as the YAG phase. In samples containing TiO2 additive, the derived X-ray diffraction patterns show no trace of TiO2 phase. This phenomenon represents the complete conversion of TiO2 to TiC in these compounds (according to the reaction 4). In Fig. 4(a), the backscattered image of the sample with 5 wt% of TiO2 is shown. The line scan analysis confirms the presence of very fine TiC particles along the grain boundary phase of the YAG.
Fig. 2. Al2O3-Y2O3 phase diagram.
Al2O3-Y2O3 is so much important, because the presence of additives in pressure-less sintering method leads to the formation of molten phase. To achieve the YAG phase at the lowest temperature possible, an Al2O3:Y2O3 ratio of 4.3:5.7 wt% was considered according to the Al2O3Y2O3 phase diagram (Fig. 2) and Ahmoye et al. [40] and Bucevac et al. [43] reports. Therefore, the amount of sintering aids which directly affect the volume fraction of the molten phase during the sintering process is a key factor in controlling the density of ceramics. If the amount of molten phase is not adequate, the SiC particles will not become wet and large amounts of porosity along the boundaries will form [44]. Therefore, inclusion of Al2O3 and Y2O3 additives with values of 4.3 and 5.7 wt%, respectively, leads to the condensation of the samples through the formation of the liquid phase during sintering. Regarding the effect of TiO2 at 1850 °C, the relative density increased from 70.1% to 71.24% by increasing amount of TiO2 from 0 to 7.5 wt% but by further addition of TiO2 up to 10 wt%, the relative density decreased. Also at 1900 °C and 1950 °C, by increasing TiO2 content from 0 to 5 wt%, the relative
3.2. Electrical resistivity and microstructure of SiC bodies The use of Al2O3-Y2O3 additives, increases electrical resistivity of SiC bodies; due to the grain boundary phase insulation and also the presence of Al2O3 particles. In fact during the heat treatment some Al impurities substitute in Si sites and act as deep acceptors for trapping carrier. Fig. 5 shows the electrical resistivity of samples with different amounts of TiO2 which are sintered at three different temperatures. As it is known, electrical resistivity of samples gradually dropped by increasing the amount of TiO2. The slope of this reduction decreased slightly at 1850 °C with 7 wt% of the additive and also decreased at 1900 °C and 1950 °C with 5 wt% of the additive. It is also clear that the specimens sintered at 1850 °C and 1950 °C have the highest and lowest resistance, respectively. 143
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Fig. 3. Phase analysis of the samples containing 0 wt% (a, c) and 10 wt% (b, d) of the TiO2 additive, sintered 1900 °C and 1950 °C, respectively.
Fig. 4. Microstructure of etched, polished surface of samples containing 5 wt% TiO2 additive sintered at 1900 °C and 1950 °C (a, b) and line scan analysis of sintered sample at 1900 °C (c). 144
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Fig. 5. Electrical resistivity of SiC samples with different additive content. Fig. 6. Microstructure of etched, polished surface of sample containing 7.5 wt% TiO2 additive sintered at 1850 °C.
As it is shown in Table 2, the average length and width of SiC grains steadily diminish with addition of TiO2. These results confirm that the in-situ formation of TiC reduces the grain boundaries migration and thus the grain size of SiC. As it is known, reducing the grain size, increases the number of grain boundaries and subsequently enhances the electrical resistivity. However, since the electrical resistivity of TiC is about 5.3 × 10−5 Ω·m, electrical resistivity of samples decreases by insitu formation of this phase. These two factors affect the electrical resistivity, according to the results, it could be suggested that the effect of synthesized TiC particles outweighs the effect of matrix grains size. The microstructure of the samples containing 7.5 wt% TiO2 additive is shown in Fig. 6. As it was mentioned earlier, the density of the sintered samples was low in 1850 °C and the conductive pathways through SiC grains were disconnected by air. The air electrical resistivity is in the range of 1.3 × 1016 Ω·m - 3.3 × 1016 Ω·m, therefore existence of pores increase the electrical resistivity of these series of samples. Microstructure of specimens with 5 wt% additive sintered at temperatures of 1900 °C and 1950 °C are shown in Fig. 4. As it is clear in the case of sintering at 1900 °C, the grain boundary phase filled the interstices between the SiC grains. The existence of the insulating grain boundary phases results in discontinuity of the conductance path, leads to the reduction of samples conductance. At temperature of 1950 °C, most of grain boundary phases migrate simultaneously with the growth of grains into triple or multiple points between SiC particles, so the electrical resistivity decreases due to increased contact between SiC grains. A higher sintering temperature introduces more impurities into SiC structure and leads to further drop in the electrical resistivity of the samples.
Fig. 7. Change of indentation fracture resistance with different additive content.
sintered samples at 1850 °C, it was not possible to measure the indentation fracture resistance of them. An increase of the indentation fracture resistance was observed for the sample containing up to 5 wt% TiO2 additive while by further addition of TiO2 up to 10 wt%, indentation fracture resistance decreased again. Meanwhile, not only the indentation fracture resistance of SiC-10 wt% TiO2 sample was suitable but also indentation fracture resistance of this sample was higher than ceramic samples without TiO2 additive at the same temperature. This
3.3. Effect of TiO2 additive and sintering temperature on indentation fracture resistance and hardness Changes in indentation fracture resistance of the samples as a function of the additive at two different temperatures, 1900 °C and 1950 °C, are shown in Fig. 7. Due to high levels of porosity in the
Table 2 The effect of the amount of TiO2 and sintering temperature on SiC grain size and hardness. % TiO2
0 5 10
Sintered at 1900 °C
Sintered at 1950 °C
Maximum diameter (μm)
Minimum diameter (μm)
Hardness (GPa)
Maximum diameter (μm)
Minimum diameter (μm)
Hardness (GPa)
1.61 ± 0.06 1.36 ± 0.05 1.29 ± 0.09
1.18 ± 0.1 0.85 ± 0.07 0.83 ± 0.04
22.64 ± 0.1 24.4 ± 0.3 23.5 ± 0.13
1.7 ± 0.12 1.48 ± 0.16 1.39 ± 0.1
1.35 ± 0.08 1.25 ± 0.02 1.2 ± 0.03
22.23 ± 0.1 23.3 ± 0.2 22.48 ± 0.1
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Fig. 8. Microstructure of etched, polished surface of sintered sample without TiO2 (a) and sample containing 5 wt% TiO2 additive (b).
shown in Fig. 10 can improve the indentation fracture resistance of the samples. As it can be seen, crack led to the fracture of some parts of SiC grains in the sample without additive. Therefore, it was expected that the indentation fracture resistance of this sample was lower than the other samples which were sintered at 1900 °C while in the samples containing 5 and 10 wt% additive, there are some observable mechanisms that each of them contributes to the energy absorption of the crack tip. One of these mechanisms is crack deflection due to the weak oxide phase that forms along the SiC boundaries. Also, this mechanism may be due to difference between thermal expansion coefficients of SiC and TiC. Other effective mechanisms for improvement of KIFR include the grains pull-out (as shown in Fig. 11) and crack bridging. These mechanisms together with elongated grains with high aspect ratio were so effective. In structures containing equiaxed grains, the contribution of these mechanisms in KIFR improvement becomes minimum. Here, the contribution of crack bridging mechanism is less because very small amounts of SiC grains are elongated, so the dominant mechanism in SiC-TiC composite is crack deflection (as shown in Fig. 11). As can be seen in Table 2, the highest hardness was achieved for the sample containing 5 wt% TiO2 at both 1900 °C and 1950 °C. This behavior is perfectly consistent with the density variation and the highest hardness was obtained for the sample with the maximum density. So it can be said that density is the most influential factor on hardness. According to the results, by increasing the TiO2 up to 5 wt%, hardness of the sintered samples at 1900 °C and 1950 °C increased by 7.7% and 4.8%, respectively. As it is shown, enhancing the amount of TiO2 from 2.5 to 10 wt% and sintering temperatures from 1850 °C to
increase of indentation fracture resistance has occurred despite the decrease in relative density in the samples containing 10 wt% TiO2. Consequently, it can be said that density is not the only factor that controls the KIFR of the specimens. It has also been shown by Bucevac et al. [43] that the density, the amount and type of reinforcement phase and its morphology are factors which can affect indentation fracture resistance. The microstructures of the sintered sample without TiO2 additive and the sample containing 5 wt% TiO2 additive are presented in Fig. 8. In the sample containing 5 wt% TiO2, formation of TiC phase in the SiC boundaries leads to the reduction of grains sizes and increases the barriers against the crack motion. In fact, the crack is blunted by grain boundaries and it requires a lot of energy for nucleation and regrowth, because it has to change its path so that it can continue to move in the appropriate planes in the adjacent grains. Another reason of indentation fracture resistance increment can be attributed to the longer crack propagation. Additionally, the in situ formation of TiC prevents grain growth and increases elongation of SiC grains (Fig. 9). According to, Evan's and Faber's equation is cited by Wei et al. [46], elongated SiC grains can improve the fracture toughness:
Km = K eq (1 + 0∙28V )
l d
(7)
Where Km is the fracture toughness of the SiC matrix, Keq is the fracture toughness of equiaxed SiC grains, V is the volume fraction of elongated SiC grains in the SiC matrix and l is the aspect ratio of the d elongated SiC. In addition to the stated expressions, the mechanisms which are
Fig. 9. The back scatter image of polished surface of sample without TiO2 (a) and sample containing 5 wt% TiO2 additive (b) sintered at 1900 °C. 146
International Journal of Refractory Metals & Hard Materials 76 (2018) 141–148
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Fig. 10. Crack propagation through SiC samples without additive (a), containing 5 wt% (b) and 10 wt% (c) TiO2 additive sintered at 1900 °C (The surfaces were polished and etched before observation).
4. Conclusion In this study, SiC-TiC composites were fabricated through a reaction between TiO2 and SiC the following results were obtained: - At temperature of 1900 °C, by increasing additive up to 5 wt%, density, hardness and indentation fracture resistance were equaled with 96.2%, 24.4 GPa and 5.8 MPa.m1/2 and by further addition up to 10 wt%, they were reduced to 91.9%, 23.5 GPa and 4.95 MPa.m1/ 2 , respectively. - By increasing the temperature to 1950 °C, density, hardness and indentation fracture resistance decreased slightly due to grain growth and further released gas from internal reactions and the maximum values for samples containing 5 wt% TiO2 were equaled with 94.2%, 23.3 GPa and 4.7 MPa.m1/2, respectively. - By changing the amount of TiO2, the electrical resistivity of sintered samples at 1900 °C is variable in the range of 5 × 107 Ω·m − 7.6 × 105 Ω·m and by increase of the temperature up to 1950 °C, electrical resistivity drops slightly. Depending on the amount of TiO2, electrical resistivity varies from 1.1 × 107 Ω·m to 2.2 × 105 Ω·m. - It is determined that the style of the grain boundary phase (YAG) settlement in the structure has a large effect on the electrical resistivity of the samples. For example, if it occupies the surroundings of particles, resistance increases because of blockade in conducting pathways through SiC grains and if it is located at triple or multiple points, the resistance decreases. - In the case of changing sintering temperature and amount of additive, the electrical resistivity of SiC bodies changed without deteriorating its mechanical properties. - Grain size reduction, crack bridging and crack deflection are the mechanisms contributing to the increase the indentation fracture resistance in the SiC-TiC composite.
Fig. 11. Grain pull-out in sample containing 5 wt% TiO2 additive, sintered at 1900 °C (simply fractured surface).
1950 °C, the electrical resistivity of the sample is variable from 2.2 × 105 Ω·m to 9 × 108 Ω·m. Among the samples sintered at 1900 °C and 1950 °C, hardness, indentation fracture resistance and density of the sample with the maximum resistance is 22.23 GPa, 3.7 MPa.m1/2 and 90%, respectively. Hardness, indentation fracture resistance and density of the sample with minimum resistance is 23.5 GPa, 4.95 MPa.m1/2 and 91.9%, respectively. The results indicate that by addition of 10 wt% of Al2O3-Y2O3 and altering TiO2 amount, the electric resistance of SiC bodies can be changed, while not only there is no reduction in their mechanical properties but also improvements have been observed in the SiC bodies without additives. 147
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References
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
The effect of TiO2 additive on the electrical resistivity and mechanical properties of pressureless sintered SiC ceramics with Al2O3-Y2O3
T
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Mahdi Khodaeia, Omid Yaghobizadehb, , Naser Ehsania, Hamid Reza Baharvandia a b
Composite Materials & Technology Center, Malek Ashtar University of Technology, Tehran, Iran Department of Materials Engineering, Imam Khomeini International University (IKIU), Qazvin, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC Electrical resistivity Insulation Mechanical properties Grain boundary phase TiO2 additive Toughening mechanism
In this study, SiC-TiC composite was fabricated by the reaction between TiO2 and SiC and in addition to the effect of TiO2 additive, influence of sintering temperatures on the electrical resistivity, the relationship between electrical resistivity and microstructure, density, indentation fracture resistance and hardness were investigated. The main goal of this study was to improve electrical resistivity while preserving mechanical properties of SiC body. The results showed that with 10 wt% Al2O3-Y2O3, the electrical resistivity reached to 9 × 108 Ω·m. Increasing the amount of TiO2 particles from 2.5 to 10 wt% and changing the sintering temperature from 1850 °C to 1950°C made the electrical resistivity to be variable in the range of 2.2×105 Ω·m to 9 × 108 Ω·m. In this report, the highest density, hardness and indentation fracture resistance for the samples containing 5 wt% additive which were sintered at 1900 °C were 96.2%, 24.4 GPa and 5.8 MPa.m1/2, respectively. Microscopic images showed that if the grain boundary phase is located in the triple points or multiple points of grain boundary, the electrical resistivity will decrease and if it is located in full circumference of the SiC particles, due to failure of conducting pathways through SiC grains, the resistance of ceramics will increase instead.
1. Introduction In recent years, studying the variation of electrical conductivity in structured ceramics is of interest for the applied industries. For insulating ceramics to gain electrical conductivity, the distribution of conducting secondary phase particles is one of the solutions. In these composites, using 12 to 30 vol% of the conducting secondary phase is necessary. But the researches have shown that increasing the amount of secondary phase up to 70 vol% is essential for increment of resistivity in conductive ceramics [1–9]. It is worth to note that addition of the large amount of secondary phase is undesirable because it can severely affect the mechanical properties of final ceramic. Thus changing the electrical resistivity of ceramics by adding a little of secondary phase is important. It is wellknown that in ceramics which are sintered in the liquid phase, the grain boundary phase in the grain boundary pockets such as the triple points or multiple points is sporadically distributed in the whole body. Although it is also possible for the grain boundary phase to remain in the interface of two boundaries [10–12]. It can be concluded that if the grain boundary phase is insulator and located not only in the triple points of grain boundary but also in the surface of grain boundary, therefore addition of only 3 vol% of it will be sufficient to change the
⁎
Corresponding author. E-mail address: [email protected] (O. Yaghobizadeh).
https://doi.org/10.1016/j.ijrmhm.2018.06.005 Received 16 April 2018; Received in revised form 5 June 2018; Accepted 11 June 2018 0263-4368/ © 2018 Elsevier Ltd. All rights reserved.
electrical properties of the final ceramic. In this condition, if the grain boundary is conductive, it will act as a path for electrical current and if it is insulator, it will weaken the connections of the matrix grains and the electrical resistivity will increase. Non-oxide ceramics such as nitrides, carbides and borides have very good mechanical properties no only at room temperatures but also at high temperature, which is due to covalent bond between its atoms. Moreover, non-oxide ceramics, except AlN and Si3N4 are electrically conductive showing either metallic conductivity or semi-conductivity properties. Among the non-oxide ceramics, SiC is one of the best candidates for industrial applications such as gas turbines, piston engines and heat exchangers, due to its good strength and creep resistance at high temperatures [13, 14]. The most important characteristics of this material include high thermal and electrical conductivity, high dielectric loss (in some cases), high melting point and low sinter-ability [15]. Recently, the SiC ceramics have been used as a useful material for manufacturing semi-conductors. In the case of wafer-boats and susceptors, high thermal conductivity and low electrical resistivity are needed, because the mentioned industry needs to accelerate the speed of warming and cooling of these parts in the production process [16–18]. In fact, electronic equipment and semiconductor sectors need to have different
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electrical resistivity, for example, the electrostatic chucks require a resistivity of 1 × 108 Ω·m-1 × 1014 Ω·m to be able to provide sufficient adsorption force [19], also, the vacuum chucks needs 1 × 106 Ω·m resistance to prevent the accumulation and discharge of static electricity. Due to the inherent resistance of SiC ceramics, variable electrical resistivity along with unchangeable mechanical properties is highly desirable for this category of applications. But one of the present problems occurring within the fabrication process of the SiC bodies is its low sinterability. About 88% of chemical bonds between carbon and silicon atoms in SiC are covalent. This fact reduces atomic diffusion so makes it difficult to achieve higher density [20]. In order to solve this problem and to obtain a denser silicon carbide body, a sintering aid is included in the sintering process. Sintering process of SiC body was done by Prochaska for the first time, using boron and carbon as additives. It should be noted that sintering temperature of SiC body with boron and carbon additives is higher than 2000 °C and mechanical properties of the final sample are relatively poor [17, 21]. Therefore, in order to improve the mechanical properties and also to reduce the temperature of pressure-less sintering process, extensive researches have been conducted on various additives. So far, various combinations including Al-C, Al2O3-C, Al2O3-Y2O3 have been studied to improve and reduce growth rate of the grains during the sintering process of SiC body [22–29]. Recent studies have shown that the use of Al2O3-Y2O3 additives at the temperature range of 1850 °C to 2000°C can lead to the formation of a denser body. The molten phase formation and dissolution-deposition process increase the density of silicon carbide bodies. Insulating grain boundary phase forms a eutectic phase, through additive reactions with SiO2 on the surface of SiC particles during the sintering process [30–36]. It is believed that the including oxides form the eutectic liquid and also react with SiO2 forming on the surface of SiC particles during the sintering process; this eutectic phase in addition to the recovery of condensation, improves some properties such as flexural strength as well. Also according to the insulating grain boundary phase, this phase will cause a change in the electrical conductivity of the sample [37]. The reason of increased strength and toughness could be attributed to stress concentration reduction and also reduction in porosities of the body. Additionally variation of fracture modes are one of the advantages of oxide additive [38]. Electrical resistivity can also change with matrix grain size, the amount of additives and their distribution in the sample. Given the above example, changing the electrical resistivity of SiC ceramics in such a way that their mechanical properties remain fixed, is invaluable. So far, few reports about the electrical resistivity of SiC and SiC-TiC bodies containing Al2O3-Y2O3 additives have been published [37, 39] and in these researches, reinforcement particles are added directly to raw materials. Among the reports published about reaction sintering of SiC composites with in situ converted TiO2 to TiC [40, 41], the effect of sintering temperature on the synthesized phases, mechanical properties and the electrical resistivity has not been investigated yet. In this research then the effect of different amounts of TiO2 additive and sintering temperature on the density, electrical resistivity, microstructure and the relationship between microstructure and electrical resistivity, indentation fracture resistance and hardness of the SiC and SiC-TiC bodies with Al2O3-Y2O3 additives have been investigated.
Table 1 Raw material compositions of each sample. wt. % TiO2 Raw materials
0
1
2.5
5
7.5
10
SiC (wt. %) TiO2 (wt. %) Al2O3 (wt. %) Y2O3 (wt. %)
90 0 4.3 5.7
89 1 4.3 5.7
87.5 2.5 4.3 5.7
85 5 4.3 5.7
82.5 7.5 4.3 5.7
80 10 4.3 5.7
order to remove the binders, all the samples were pyrolyzed at temperatures up to 600 °C with a heating rate of 2 °C/min. Then all the samples were sintered under argon atmosphere at 1850 °C, 1900 °C and 1950 °C, for 1.5 h. Before reaching to the sintering temperature, samples were heated at 1800 °C for 0.5 h to provide the suitable conditions for the reaction between SiC and TiO2. The densities of the samples were measured according to the ASTMC373–88 standard method. The hardness of the samples was calculated according to the standard Vickers method ASTM-C-1327; for this purpose, the surface of the samples was polished with diamond paste of 30 μm, 6 μm and 1 μm. Each sample went under load of 5 Kg (49.03 N) for 10 s. Five measurements were carried out on each sample and they were reported according to the following relationship:
H = (1.854) × (P/d2)
(1)
Where (d) is the average diameter of the indentation diagonals, (H) is Vickers hardness in MPa and P is force. Indentation fracture resistance was calculated according to an expression contrived by Anstis et al. [42].
E 0.5 P KIFR = a ⎛ ⎞ × ⎛ 3/2 ⎞ × 10−6 ⎝H⎠ ⎝c ⎠
(2)
Where in: a: 0.0160.004 ± E: Modulus of elasticity (GPa). H: Vickers hardness (GPa). P: Applied force (N). c: The half-length of radial crack (m). KIFR: Indentation fracture resistance (MPa.m1/2). The average amount of five measurements was reported as the indentation fracture resistance. Phase analysis of samples were conducted by Inel EQUINOX 3000 equipped with copper cathode and microstructure was investigated using scanning electron microscopy (SEM, VEGA\\TESCAN) equipped with EDS. The average grain size (G) was determined by the linear intercept method by using the following equation:
G = 1.56 L/(M × N)
(3)
Where L is the random line length on the micrograph, M the magnification of the micro-graph and N the number of line intersected by grain boundaries. The electrical resistivity of the SiC samples was measured using a 2point probe method using a high resistance meter (model 4339B, Agilent Technologies, Santa Clara, CA, USA).
2. Experimental procedures 3. Results and discussion In this study, α-SiC powders with an average size of 0.3 μm, Al2O3, Y2O3 and TiO2 powders with a particle size below 1 μm were utilized. In order to prepare samples, each raw material composition according to Table 1 was milled in planetary mill for 3 h at 180 rpm. Then the resulting slurries were dried at 100 °Cfor 4 h. The samples were pressed by uniaxial hydraulic press (90 MPa) in a cylindrical shape with a diameter and height of 1.2 cm and 0.5 cm, respectively. In
3.1. Relative density and phase analysis of samples In Fig. 1, the effects of various additives and sintering temperature on the density of composites are shown. As it can be seen, density of the samples increased at first then decreased by increasing the amount of additives in each three temperature. It should be noted that the effect of 142
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density of the samples increased from 93.3% to 96.2% and from 90% to 94.12%, respectively. An increase in the density of samples was observed due to the presence of very fine TiC particles which were the result of the reaction between TiO2 particles and SiC according to the reaction (4) and these fine particles were located on SiC boundaries. These particles reduced the migration rate of the boundaries, thus the extreme growth of the grains was minimized and the density increased.
TiO2 + 3SiC → TiC + 3Si + 2CO(g)
(4)
On the other hand, increased surface permeability and porosity mobility led to the reduction of the cavities in the sample therefore the density increased during the sintering process [45]. Also, with the formation of TiC, the reaction between SiC and Al2O3 was partially reduced leading to the decrease of the amount of released gases such as CO, Al2O, and SiO, i.e. increase of the density [40]. As it could be seen, at all three temperatures, the density decreased after a certain amount of the additive. The reason of this behavior is that by increase of the amount of additives, the rate of gas phase formation from the reaction between the oxide additives and SiC (reactions (5), (6)) is faster than the porosity removal rate by the capillary forces, therefore no significant increase in the density of the samples is observed.
Fig. 1. Relative density of samples with different additive content, sintered at 1850 °C, 1900 °C and 1950 °C.
SiC + 2Y2O3 → SiO + 4YO(g) + CO(g)
(5)
SiC + Al2 O3 → SiO(g) + AlO(g) + CO(g)
(6)
As it is obvious, increasing the temperature from 1850 °C to 1900 °C, the density of the specimens was increased. The decrease in the viscosity of the molten phase which was produced during the heat treatment process also reduced the amount of porosities and led to the increase of density. On the other hand, the density of specimens decreased slightly by increasing the temperature up to 1950 °C. The reason for this behavior is that when the temperature is higher than 1900 °C, the reaction between SiC and oxide phases intensifies. Although at this temperature the viscosity of the liquid phase is less than that in 1900 °C, but since the liquid phase reacts with SiC and the oxide phases are generated, resulting porosities of exhaust of gases reduce the density of the samples. The X-ray diffraction patterns of the samples containing 0 wt% and 10 wt% of the TiO2 additive which were sintered at 1900 °C to 1950 °C are shown in Fig. 3. As it can be seen, at 1900°C, the samples contain the 3Y2O3.5Al2O3 phase in addition to the dominant SiC phase. This phase is known as the YAG phase. In samples containing TiO2 additive, the derived X-ray diffraction patterns show no trace of TiO2 phase. This phenomenon represents the complete conversion of TiO2 to TiC in these compounds (according to the reaction 4). In Fig. 4(a), the backscattered image of the sample with 5 wt% of TiO2 is shown. The line scan analysis confirms the presence of very fine TiC particles along the grain boundary phase of the YAG.
Fig. 2. Al2O3-Y2O3 phase diagram.
Al2O3-Y2O3 is so much important, because the presence of additives in pressure-less sintering method leads to the formation of molten phase. To achieve the YAG phase at the lowest temperature possible, an Al2O3:Y2O3 ratio of 4.3:5.7 wt% was considered according to the Al2O3Y2O3 phase diagram (Fig. 2) and Ahmoye et al. [40] and Bucevac et al. [43] reports. Therefore, the amount of sintering aids which directly affect the volume fraction of the molten phase during the sintering process is a key factor in controlling the density of ceramics. If the amount of molten phase is not adequate, the SiC particles will not become wet and large amounts of porosity along the boundaries will form [44]. Therefore, inclusion of Al2O3 and Y2O3 additives with values of 4.3 and 5.7 wt%, respectively, leads to the condensation of the samples through the formation of the liquid phase during sintering. Regarding the effect of TiO2 at 1850 °C, the relative density increased from 70.1% to 71.24% by increasing amount of TiO2 from 0 to 7.5 wt% but by further addition of TiO2 up to 10 wt%, the relative density decreased. Also at 1900 °C and 1950 °C, by increasing TiO2 content from 0 to 5 wt%, the relative
3.2. Electrical resistivity and microstructure of SiC bodies The use of Al2O3-Y2O3 additives, increases electrical resistivity of SiC bodies; due to the grain boundary phase insulation and also the presence of Al2O3 particles. In fact during the heat treatment some Al impurities substitute in Si sites and act as deep acceptors for trapping carrier. Fig. 5 shows the electrical resistivity of samples with different amounts of TiO2 which are sintered at three different temperatures. As it is known, electrical resistivity of samples gradually dropped by increasing the amount of TiO2. The slope of this reduction decreased slightly at 1850 °C with 7 wt% of the additive and also decreased at 1900 °C and 1950 °C with 5 wt% of the additive. It is also clear that the specimens sintered at 1850 °C and 1950 °C have the highest and lowest resistance, respectively. 143
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Fig. 3. Phase analysis of the samples containing 0 wt% (a, c) and 10 wt% (b, d) of the TiO2 additive, sintered 1900 °C and 1950 °C, respectively.
Fig. 4. Microstructure of etched, polished surface of samples containing 5 wt% TiO2 additive sintered at 1900 °C and 1950 °C (a, b) and line scan analysis of sintered sample at 1900 °C (c). 144
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Fig. 5. Electrical resistivity of SiC samples with different additive content. Fig. 6. Microstructure of etched, polished surface of sample containing 7.5 wt% TiO2 additive sintered at 1850 °C.
As it is shown in Table 2, the average length and width of SiC grains steadily diminish with addition of TiO2. These results confirm that the in-situ formation of TiC reduces the grain boundaries migration and thus the grain size of SiC. As it is known, reducing the grain size, increases the number of grain boundaries and subsequently enhances the electrical resistivity. However, since the electrical resistivity of TiC is about 5.3 × 10−5 Ω·m, electrical resistivity of samples decreases by insitu formation of this phase. These two factors affect the electrical resistivity, according to the results, it could be suggested that the effect of synthesized TiC particles outweighs the effect of matrix grains size. The microstructure of the samples containing 7.5 wt% TiO2 additive is shown in Fig. 6. As it was mentioned earlier, the density of the sintered samples was low in 1850 °C and the conductive pathways through SiC grains were disconnected by air. The air electrical resistivity is in the range of 1.3 × 1016 Ω·m - 3.3 × 1016 Ω·m, therefore existence of pores increase the electrical resistivity of these series of samples. Microstructure of specimens with 5 wt% additive sintered at temperatures of 1900 °C and 1950 °C are shown in Fig. 4. As it is clear in the case of sintering at 1900 °C, the grain boundary phase filled the interstices between the SiC grains. The existence of the insulating grain boundary phases results in discontinuity of the conductance path, leads to the reduction of samples conductance. At temperature of 1950 °C, most of grain boundary phases migrate simultaneously with the growth of grains into triple or multiple points between SiC particles, so the electrical resistivity decreases due to increased contact between SiC grains. A higher sintering temperature introduces more impurities into SiC structure and leads to further drop in the electrical resistivity of the samples.
Fig. 7. Change of indentation fracture resistance with different additive content.
sintered samples at 1850 °C, it was not possible to measure the indentation fracture resistance of them. An increase of the indentation fracture resistance was observed for the sample containing up to 5 wt% TiO2 additive while by further addition of TiO2 up to 10 wt%, indentation fracture resistance decreased again. Meanwhile, not only the indentation fracture resistance of SiC-10 wt% TiO2 sample was suitable but also indentation fracture resistance of this sample was higher than ceramic samples without TiO2 additive at the same temperature. This
3.3. Effect of TiO2 additive and sintering temperature on indentation fracture resistance and hardness Changes in indentation fracture resistance of the samples as a function of the additive at two different temperatures, 1900 °C and 1950 °C, are shown in Fig. 7. Due to high levels of porosity in the
Table 2 The effect of the amount of TiO2 and sintering temperature on SiC grain size and hardness. % TiO2
0 5 10
Sintered at 1900 °C
Sintered at 1950 °C
Maximum diameter (μm)
Minimum diameter (μm)
Hardness (GPa)
Maximum diameter (μm)
Minimum diameter (μm)
Hardness (GPa)
1.61 ± 0.06 1.36 ± 0.05 1.29 ± 0.09
1.18 ± 0.1 0.85 ± 0.07 0.83 ± 0.04
22.64 ± 0.1 24.4 ± 0.3 23.5 ± 0.13
1.7 ± 0.12 1.48 ± 0.16 1.39 ± 0.1
1.35 ± 0.08 1.25 ± 0.02 1.2 ± 0.03
22.23 ± 0.1 23.3 ± 0.2 22.48 ± 0.1
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Fig. 8. Microstructure of etched, polished surface of sintered sample without TiO2 (a) and sample containing 5 wt% TiO2 additive (b).
shown in Fig. 10 can improve the indentation fracture resistance of the samples. As it can be seen, crack led to the fracture of some parts of SiC grains in the sample without additive. Therefore, it was expected that the indentation fracture resistance of this sample was lower than the other samples which were sintered at 1900 °C while in the samples containing 5 and 10 wt% additive, there are some observable mechanisms that each of them contributes to the energy absorption of the crack tip. One of these mechanisms is crack deflection due to the weak oxide phase that forms along the SiC boundaries. Also, this mechanism may be due to difference between thermal expansion coefficients of SiC and TiC. Other effective mechanisms for improvement of KIFR include the grains pull-out (as shown in Fig. 11) and crack bridging. These mechanisms together with elongated grains with high aspect ratio were so effective. In structures containing equiaxed grains, the contribution of these mechanisms in KIFR improvement becomes minimum. Here, the contribution of crack bridging mechanism is less because very small amounts of SiC grains are elongated, so the dominant mechanism in SiC-TiC composite is crack deflection (as shown in Fig. 11). As can be seen in Table 2, the highest hardness was achieved for the sample containing 5 wt% TiO2 at both 1900 °C and 1950 °C. This behavior is perfectly consistent with the density variation and the highest hardness was obtained for the sample with the maximum density. So it can be said that density is the most influential factor on hardness. According to the results, by increasing the TiO2 up to 5 wt%, hardness of the sintered samples at 1900 °C and 1950 °C increased by 7.7% and 4.8%, respectively. As it is shown, enhancing the amount of TiO2 from 2.5 to 10 wt% and sintering temperatures from 1850 °C to
increase of indentation fracture resistance has occurred despite the decrease in relative density in the samples containing 10 wt% TiO2. Consequently, it can be said that density is not the only factor that controls the KIFR of the specimens. It has also been shown by Bucevac et al. [43] that the density, the amount and type of reinforcement phase and its morphology are factors which can affect indentation fracture resistance. The microstructures of the sintered sample without TiO2 additive and the sample containing 5 wt% TiO2 additive are presented in Fig. 8. In the sample containing 5 wt% TiO2, formation of TiC phase in the SiC boundaries leads to the reduction of grains sizes and increases the barriers against the crack motion. In fact, the crack is blunted by grain boundaries and it requires a lot of energy for nucleation and regrowth, because it has to change its path so that it can continue to move in the appropriate planes in the adjacent grains. Another reason of indentation fracture resistance increment can be attributed to the longer crack propagation. Additionally, the in situ formation of TiC prevents grain growth and increases elongation of SiC grains (Fig. 9). According to, Evan's and Faber's equation is cited by Wei et al. [46], elongated SiC grains can improve the fracture toughness:
Km = K eq (1 + 0∙28V )
l d
(7)
Where Km is the fracture toughness of the SiC matrix, Keq is the fracture toughness of equiaxed SiC grains, V is the volume fraction of elongated SiC grains in the SiC matrix and l is the aspect ratio of the d elongated SiC. In addition to the stated expressions, the mechanisms which are
Fig. 9. The back scatter image of polished surface of sample without TiO2 (a) and sample containing 5 wt% TiO2 additive (b) sintered at 1900 °C. 146
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Fig. 10. Crack propagation through SiC samples without additive (a), containing 5 wt% (b) and 10 wt% (c) TiO2 additive sintered at 1900 °C (The surfaces were polished and etched before observation).
4. Conclusion In this study, SiC-TiC composites were fabricated through a reaction between TiO2 and SiC the following results were obtained: - At temperature of 1900 °C, by increasing additive up to 5 wt%, density, hardness and indentation fracture resistance were equaled with 96.2%, 24.4 GPa and 5.8 MPa.m1/2 and by further addition up to 10 wt%, they were reduced to 91.9%, 23.5 GPa and 4.95 MPa.m1/ 2 , respectively. - By increasing the temperature to 1950 °C, density, hardness and indentation fracture resistance decreased slightly due to grain growth and further released gas from internal reactions and the maximum values for samples containing 5 wt% TiO2 were equaled with 94.2%, 23.3 GPa and 4.7 MPa.m1/2, respectively. - By changing the amount of TiO2, the electrical resistivity of sintered samples at 1900 °C is variable in the range of 5 × 107 Ω·m − 7.6 × 105 Ω·m and by increase of the temperature up to 1950 °C, electrical resistivity drops slightly. Depending on the amount of TiO2, electrical resistivity varies from 1.1 × 107 Ω·m to 2.2 × 105 Ω·m. - It is determined that the style of the grain boundary phase (YAG) settlement in the structure has a large effect on the electrical resistivity of the samples. For example, if it occupies the surroundings of particles, resistance increases because of blockade in conducting pathways through SiC grains and if it is located at triple or multiple points, the resistance decreases. - In the case of changing sintering temperature and amount of additive, the electrical resistivity of SiC bodies changed without deteriorating its mechanical properties. - Grain size reduction, crack bridging and crack deflection are the mechanisms contributing to the increase the indentation fracture resistance in the SiC-TiC composite.
Fig. 11. Grain pull-out in sample containing 5 wt% TiO2 additive, sintered at 1900 °C (simply fractured surface).
1950 °C, the electrical resistivity of the sample is variable from 2.2 × 105 Ω·m to 9 × 108 Ω·m. Among the samples sintered at 1900 °C and 1950 °C, hardness, indentation fracture resistance and density of the sample with the maximum resistance is 22.23 GPa, 3.7 MPa.m1/2 and 90%, respectively. Hardness, indentation fracture resistance and density of the sample with minimum resistance is 23.5 GPa, 4.95 MPa.m1/2 and 91.9%, respectively. The results indicate that by addition of 10 wt% of Al2O3-Y2O3 and altering TiO2 amount, the electric resistance of SiC bodies can be changed, while not only there is no reduction in their mechanical properties but also improvements have been observed in the SiC bodies without additives. 147
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