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Influence of NH4F additive on the combustion synthesis of β-SiAlON in air
Author’s Accepted Manuscript Influence of NH4F additive on the combustion synthesis of β-SiAlON in air O. Tavassoli, M. Bavand-vandchali
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S0272-8842(17)32929-2 https://doi.org/10.1016/j.ceramint.2017.12.220 CERI17100
To appear in: Ceramics International Received date: 18 August 2017 Revised date: 28 December 2017 Accepted date: 28 December 2017 Cite this article as: O. Tavassoli and M. Bavand-vandchali, Influence of NH 4F additive on the combustion synthesis of β-SiAlON in air, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.220 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of NH4F additive on the combustion synthesis of β-SiAlON in air O. Tavassolia, M. Bavand-vandchalia,* a.
Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
Abstract: The preparation of β-SiAlON, Si6–zAlzOzN8–z (z = 1–4), was investigated using the combustion synthesis (CS) technique with Si, Al, and CO(NH2)2 as the main raw materials in air. Different amounts of NH 4F were used as a low-melt additive to the raw materials to decrease the reaction temperature and promote β-SiAlON formation. Phase and microstructural analyses of the reaction products were performed by X-Ray diffraction (XRD) and FE-SEM/energy dispersive X-ray spectrometer techniques. FT-IR spectroscopy was also used to demonstrate the formation of β-SiAlON in the final product. The phase and microstructural analysis results showed that NH4F addition improved the exothermic reaction of β-SiAlON phase formation and increased its content in the final synthesized powders. Additional heat treatment of the sample at the optimum z-value (z = 4) and different percentages of NH4F revealed the positive effect of the low-melt additive on β-SiAlON phase formation under very low N2 pressure. Keywords: CS technique, β-SiAlON, NH4F, Z-values, low-melt additive.
1. Introduction SiAlONs are silicon nitride-based solid solutions in which equivalent Al–O groups replace Si–N groups in the Si3N4 structure. Different well-known polymorphs of SiAlON phases such as -, β-, and O-SiAlON are isostructural with -Si3N4, β-Si3N4, and Si2ON2, respectively [1, 2]. SiAlONs are widely considered promising high-temperature structural materials owing to their excellent mechanical properties (e.g., strength, fracture toughness, and wear resistance) and superior chemical stability in severe environments [3, 4]. Consequently, they are used extensively as high-temperature engineering ceramics in cutting tools, abrasive materials, location pins, welding nuts, heater tubes, and wear components in the chemical, oil, and gas industries [5, 6]. β-SiAlON, which presents a hexagonal lattice structure, is described by the formula β-Si6–zAlzOzN8–z (0 < z ≤ 4.2) and has a microstructure composed of elongated grains; thus, the compound is characterized with high fracture toughness [7]. Several different techniques have been reported to synthesize SiAlON ceramic powders, including (i) direct reaction of Al2O3, AlN, and Si3N4 in a high-pressure N2 atmosphere at high temperatures of usually >1600 C [2]; (ii) carbothermal reduction–nitridation of a mixture of carbon and aluminosilicate clays (e.g., kaolin) or a mixture of Si, Al, SiO2, and Al2O3 under a N2 or NH3 atmosphere [6–9]; (iii) pressure-less sintering of Si3N4 in the presence of sintering aids such as Al2O3 and Y2O3; (iv) reaction of Si3N4, AlN, and stabilizing alkali cations [1]; and (v) the self-propagating high-temperature synthesis (SHS) method [10– 13]. The SHS technique is a combustion synthesis (CS) mode that begins with an extremely exothermic chemical reaction that propagates through the sample. This process has attracted interest due to its low energy consumption, short reaction time, high-purity products, and simple equipment requirements [14, 15]. It has also been proven to be an efficient technique for synthesizing several engineering materials, such as nitrides [16, 17], hydrides [18], oxides [19], and intermetallic materials [20]. CS of β-SiAlON powders is usually performed in a high-pressure N2 atmosphere ranging from 0.5 to several tens of megapascals [21, 22]. βSiAlON powders have been successfully synthesized throughout combustion with the addition of diluents [23, 24] and mechanical activation at the relatively low nitrogen pressure of 1 MPa [25, 26]. Decreased combustion temperatures and slowed reaction kinetics are mainly why diluents such as β-SiAlON, Si3N4, and AlN are used in the reaction instead of Si and Al metal powders. The use of these diluents also improves the conversion rate of the reactants, prevents the agglomeration or conglomeration of molten particles, improves the 1
nitridation process, and, subsequently, increases the purity of the final synthesized products. Some researchers have reported the effect of different oxides as a raw material on the CS of β-SiAlON synthesized using the SHS technique [27], and these scholars have found that SiO2 is a better oxygen source than Al2O3 when synthesizing β-SiAlON via the CS method under low pressure. However, the purity of the β-SiAlON powders obtained using SiO2 is low due to the presence of remaining unreacted Si in the product, which was attributed to the melting and subsequent agglomeration of Si caused by the extremely high reaction temperature and rate of the CS process and prevention of N2 diffusion into the combustion front [28]. Addition of diluents has been reported to cause a number of problems, such a difficulty in obtaining βSiAlON powders with fine or submicron-size particles; the high cost of diluent materials must also be considered. SiAlONs exhibit superplastic deformation when their grain size is reduced to the nanometer scale [29]. Thus, pressing demands to develop an alternative diluent with low cost and high efficiency have grown to advance efforts to synthesize fine β-SiAlON particles. NaCl, NH4Cl, and NH4F are low-melt additives that have attracted considerable attention as diluents to produce ceramic and intermetallic powders through the CS technique; these diluents have an important influence in decreasing the adiabatic temperature and reducing the rate of agglomeration and grain growth of the resulting particles [30–33]. Previous reports emphasizing the synthesis of SiAlON ceramics in high-pressure N2 gas indicate that diluents use and some nitrogen source additive can be considered as accelerators to improve SiAlON ceramic synthesis via the CS process. The objective of this study is to investigate the effect of NH4F additive as a diluent and urea as a nitrogen source on β-SiAlON formation through the CS process in air. 2. Experimental Procedure The starting materials used were elemental silicon (commercial grade, >99%, D50 = 65 m), aluminum metal (commercial grade, >99.7%, D50 = 45 m), urea (≥99%, 108487, Merck), and NH4F (≥98%, 101164, Merck), which functioned as the diluent additive. The raw materials were mixed according to the stoichiometric ratio based on the reaction below to synthesize β-Si6–zAlzOzN8–z with z-values varying from 1 to 4 and different amounts of NH4F (i.e., 0, 15, and 30 wt%). The starting compositions of each sample are summarized in Table 1. (6 − z) Si + 2 Al + k (8 − z) CO(NH2)2 → β-Si6–zAlzOzN8–z
(1)
The obtained mixtures were uniaxially pressed at 100 MPa to form pellets (20 mm Ø 10 mm H), and hot alumina–mullite plates were used as an ignition source for the CS reaction. The plates were inserted into an elevator furnace and heated until 1600 °C, which was maintained for 30 min. The bottom door of the furnace was then lowered, and the sample pellets were immediately placed on the alumina–mullite plates. Figure 1 illustrates the experimental procedure with the corresponding times from heating of the alumina–mullite plates in the furnace to placing of the sample pellets on the hot plates. The ignition process was initiated at the interface between the samples and the hot plate and propagated immediately throughout the samples, as shown in Figure 2. The combustion process for all of the samples was completed after a few minutes, and the products were allowed to cool to room temperature and prepared for characterization. To investigate the effect of heat treatment on the phase composition and morphology of the products, powdered samples with the optimum z-value and different NH4F contents were uniaxially pressed and then sintered in an electric tube furnace at 1500 °C for 3 h in a N2 atmosphere flowing at a rate of 7 L·min−1. XRD (D-500, Siemens Corporation, Germany, Cu2
K, = 1.54056 nm) was used to assess the phase composition of the samples, and FE-SEM (TESCAN MIRA3, Czech Republic) was used to determine their morphology. In addition, an energy dispersive X-ray spectrometer (EDS) was used for elemental analysis of the products. β-SiAlON formation was confirmed by FT-IR spectrometry (PerkinElmer Spectrum 400) in the mid-IR range.
3. Results and Discussion 3.1. Changes in sample appearance The green and ignited samples are shown in Figure 3. Samples with 15 and 30 wt% NH4F had a lighter appearance than NH4F-free samples after combustion. All of the samples were ignited just after coming into contact with the ceramic hot plate that incorporated with a large flame due to urea decomposition and burning during the combustion reaction. Combustion was initiated just after the flame was extinguished and propagated through the NH4Fcontaining samples. Thus, the appearance of these samples differs from those of the NH4Ffree samples. The figure also shows that all of the combusted samples were porous and that samples with lower z-values and higher Si contents present some deformations, although the S430 sample was completely deformed after the process. High reaction temperatures and fast heating rates are the main characteristics of the CS process. In general, during the CS of β-SiAlON in air, N2 and O2 gases are important reactants because they play a key role in the related combustion reactions. At high reaction temperatures and fast heating rates, Al and Si particles in the raw materials melt and agglomerate quickly, thereby inhibiting reactions with N2 and nitride phase formation. In addition, the more-reactive O2 reacts with the metal particles first, resulting in the formation of oxide phases on the surface of the samples. This phenomenon can be observed in the freeNH4F samples, which show black powders formed on the surface of the pellets (Figure 3) due to metal droplet agglomeration and rapid oxidation. In comparison with these samples, the NH4F in additive-containing samples acts as a diluent that reduces the agglomeration of metal droplets and facilitates oxidation. The presence of an NH4F diluent additive also increases the pressure ratio of N2/O2 in the central portion of the samples after additive decomposition, leading to reduced oxidation of Si and Al particles and promotion of the formation of nitride phases. Liu and et al. [30] indicated that NH4F exerts a catalytic effect on the nitridation of Al and Si as follows: NH4F(s) = NH3(g) + HF(g) NH3(g) = 1/2 N2(g) + 3/2 H2(g) HF(g) = 1/2 H2(g) + 1/2 F2(g) Si(s, l) + HF(g) SiFx(g) + H2(g) SiFx(g) + N2(g) + NH3(g)/H2(g) Si3N4(s) + HF(g) Si(g) + N2(g) + NH3(g) [Six(NH)y]n(s) [Six(NH)y]n(s) Si3N4(s) + NH3(g)
(1) (2) (3) (4) (5) (6) (7)
In this research, NH4F was considered a low-melt additive that creates N2 gas in the reaction mixture after decomposition and enhances the nitridation reaction. Since NH4F is fully decomposed at temperatures above approximately 250 °C and N2 gas is released as a reaction product during the decomposition process in reactions 1 and 2, the N2 supply increases and the nitridation reaction becomes more extensive. Urea was also used as another main source of N2 gas in the samples to promote nitridation during the CS of SiAlON; a similar phenomenon has been observed by other authors [31, 32]. 3
As described in the experimental section, samples with the optimum z-value (z = 4) were heat-treated once more at 1500 °C for 3 h under low-pressure N2 to decrease amount of nonSiAlON phases formed during CS and increase the quantity of β-SiAlON phase. The appearances of the heat-treated samples, labeled P400, P415, and P430, are shown in Figure 4. The appearance of P400, which has no NH4F, is completely different from that of P415 and P430, which have NH4F. Whereas the surface of P400 was covered by molten metal, those of P415 and P430 were not. 3.2. Phase analysis Figure 5 shows the XRD patterns of the combusted products with different z-values and various amounts of NH4F. When NH4F was not used (i.e., samples S100, S200, and S400), a large amount of Al, Si, and SiO appeared and no β-SiAlON phase formed in the combusted products. A small amount of Si3N4 also formed due to the reaction of Si metal and N2 gas originating from the decomposition of urea during combustion. The presence of Al and Si can be attributed to the melting and subsequent coagulation of the metals caused by the extremely high reaction temperature and rate of the combustion reactions, which retards the easy access of the reactant N2 gas to these metals and interrupts the completion of their nitridation. The generation of the reducing gases H2 and CO due to urea decomposition also restricts the accessibility of the oxygen needed to oxidize Al and Si. Therefore, SiO and unreacted Al and Si were present in the NH4F-free samples, as clearly detected in the XRD patterns of S100, S300, and S400. Analysis of the XRD patterns of the NH4F-containing samples revealed that addition of NH4F decreased the quantity of Si, SiO, and Al phases formed, improved the conversion rate of the reaction, and increased the formation of β-SiAlON after combustion. For all z-values, higher NH4F contents increased the formation of β-SiAlON as the main phase; the contents of secondary phases, such as Al2O3, AlN, and Si3N4, in the final combusted products also increased (Figure 5). According to reaction 8, SiO reacts with Al under N2 gas and forms Al2O3 and Si3N4 phases. SiO(s) + Al(l) + N2(g)
Al2O3(s) + Si3N4(s)
(8)
This reaction is similar to the results obtained by other researchers who showed an increase in the amount of the β-SiAlON phase formed in the combusted product by the addition of higher contents of NH4F as an additive [30]. Higher contents of NH4F promote the formation of the Si3N4 phase, which then transforms into the β-SiAlON phase. Figure 5 shows that a higher z-value (z = 4) and higher NH4F content yield better results and form higher amounts of β-SiAlON. In fact, higher NH4F/Si ratios in z = 4 samples promoted the catalytic effect of NH4F after its decomposition during the combustion reaction and improved the nitridation process. The higher melting point and thermal capacity of Si in comparison with those of Al means most of the thermal energy conducted from the ceramic hot plate to the sample bottom was consumed in melting Si and Al. Thus, as the z-value increased, the Si content decreased and the Al content increased in the green pellets. Lower Si melt and higher amount of NH4F additive helped to decrease the adiabatic temperature (Tad) and significantly increase the nitridation reaction during the combustion process. However, higher Si contents in the samples (low z-value) caused Si particles to fuse, subsequently oxidize as SiO, and harden under the fast reaction rate, which caused it to remain in the final products. Therefore, at lower z-values, a larger amount of NH4F is needed to increase the combustion temperature and obtain higher contents of the β-SiAlON phase in the final products. Three main reasons support the concept of SiO instead of SiO2 formation in the final S-series samples: (1) urea 4
decomposition during the combustion process and H2/CO gas formation restrict the ability of oxygen to oxidize Si; (2) Al oxidation (−1582.3 KJ/mole) has a more negative free Gibbs energy (∆G°) than SiO2 formation (−856.4 KJ/mole) [33]; and (3) Al2O3 formation requires a lower molar Al/O ratio (1.5) SiO2 formation (2). The XRD patterns of the P-series samples are shown in Figure 6. Compared with the Sseries samples, all secondary phases, such as SiO, Al, AlN, and Si3N4, disappeared in the NH4F-containing samples, and the intensity of the peaks related to the β-SiAlON phase increased after heat treatment. Based on the XRD results, the increase in β-SiAlON peak intensity corresponded with the decrease of the intensities of peaks corresponding to Si3N4, AlN, and Al2O3. Therefore, formation of β-SiAlON could be concluded to occur by the reaction of Si3N4 with AlN and Al2O3 (reaction 9) in the sintered sample under a low-pressure flow of N2 gas in the furnace, which partially diffuses into the sample and promotes βSiAlON formation. The XRD results of the P-series samples and comparison of the relative intensities of the peaks indicated that the final composition of the powder products includes up to 70% β-SiAlON, approximately. This is very interest that high quantity of β-SiAlON phase has been formed with usage of nitrogen source additive in the air trough the CS process. Si3N4(s) + AlN(s) + Al2O3(s)
β-Si3Al3O3N5(s)
(9)
Low-melt NH4F plays a vital role in the formation of the β-SiAlON phase, and this additive was not detected during phase analysis of the P400 sample, in contrast to the results of the P415 and P430 samples. In contrast to P415 and P430, reactions among Al, Si, and N2 gases in P400 mainly resulted in the formation of composite phases, such as Al2O3, AlN, and Si3N4, and large amounts of unreacted molten Si were obtained from the sample (Figure 3). Recall that NH4F addition promoted β-SiAlON phase formation in P415 and P430 by decreasing Tad and increasing N2 availability to form the β-SiAlON phase. The lone oxygen source for the P-series samples during the sintering process was SiO, from which oxygen separates between 1000 and 1440 °C, leading to the formation of SiO2 and Si [34,35]. The formed SiO2 then contributed to react Al2O3 and Si3N4 incorporated with AlN that was generated from the decomposition of NH4AlF4 in the P430 sample. In P415, the low contents of SiO, NH4AlF4, Si3N4, and AlN and high contents of unreacted Si and Al formed mullite as the dominant phase and excess amounts of Si3N4 and Al2O3. The Si3N4 phase was generated by the reaction of Si and N2 gas originating from NH4AlF4 decomposition and the furnace atmosphere. The low content of Al2O3 in P400 confirms this theory. In fact, NH4F decomposes during the combustion reaction and increases the porosity of the sample, thereby favoring the N2 gas reaction. The NH3 and HF gases produced by the decomposition of NH4F can react with Si and promote its nitridation and further reaction with Al2O3 to produce β-SiAlON. In the CS of β-SiAlON, the quantity of unreacted Si was evidently reduced by the addition of NH4F, and Tad decreased so that more β-SiAlON could be produced. 3.3. FT-IR analysis The FT-IR spectra of combustion-synthesized sample (S-series) with the optimum z-value (z = 4) and heat-treated samples (P-series) are shown in Figures 7 and 8, respectively. In both series of samples, peaks with wave numbers between 400 and 1000 cm−1, which could be attributed to Al–O, Al–N, Si–O, and Si–N bond vibration modes, were observed. This finding demonstrates the presence of β-SiAlON, Al2O3, AlN, and Si3N4 phases in the samples. Peaks observed at about 607 and 670 cm−1 are due to Si–O–Si bending vibrations [36], and the peak 5
at about 3440 cm−1 can be attributed to the stretching vibrations of the Si–N group [37]. The absorption peak at 60 cm−1 found in the S- and P-series sample spectra could be attributed to the Al–O bond vibrations in [AlO6], which exists in Al2O3 and mullite [38], and the absorption peaks at 876 and 700 cm−1 found in the spectra of S430 and P430 could be attributed to Si–N and the stretching vibrations of Al–O–Si, respectively [39, 40]. The peaks at 879 and 839 cm−1 could be attributed to Si–N and Al–N bond vibrations, respectively. The peaks at 1121 cm−1 in the S400 spectrum, that at 1123 cm−1 in the P400 spectrum, and that at 1099 cm−1 could be the stretching vibrations of Si–O bonds as Al–O replaces Si–N in the SiAlON phases [41]. These findings agree with the XRD results and confirm the formation of Si3N4, AlN, and Al2O3 phases in the S400 and P400 samples and the formation of β-SiAlON in the S430 and P430 samples. The peaks at about 711 cm−1 could be attributed to the Al–N stretching vibration mode [40]. The lack of a specific absorption peak at 603 cm−1, the reduction in peak intensity at about 3400 cm−1, and the high intensity of the peak at about 711 cm−1 altogether confirmed the XRD results that are consequences of SiO decomposition and formation of AlN, Si 3N4 from S400 to P400 samples. 3.4. Microstructural analysis Figure 9 shows FE-SEM micrographs of combusted samples with z = 4 and no (S400), 15 wt% (S415), and 30 wt% NH4F (S430). Without addition of NH4F, the morphology of the particles obtained was inhomogeneous, and the particles presented different sizes. Fine agglomerates and tabular-shaped particles were observed in the NH4F-containing samples, including S430, which contained the highest content of NH4F among the samples studied. Details of the elemental mass analysis of marked points in S-series samples with z = 4 and different NH4F contents in Figure 8 are presented in Table 2. EDS analysis of different particles with various morphologies demonstrated unreacted Al particles (spot A) and some particles composed of Al, O, and C elements with a small amount of Si (spots B and C). According to the XRD results in Figure 5, these powders clearly showed the unreacted Al particles and some compositions that formed by the reaction of Al and CO gas originating from urea decomposition during the combustion process. EDS analysis of spots D, E, F, and G revealed agglomerates of well-crystallized particles conforming to the SiAlON phase, together with Al2O3, Si3N4, mullite, and AlN phases. The formation of these compounds can be attributed to the very high combustion temperature and fast reaction kinetics of the reaction system. The morphology of the S430 powder sample obtained after CS (Figure 9) showed dominant phases including thick various-sized rod-like grains with high aspect ratios and a few small agglomerated grains. EDS analysis of the point marked D in the S430 sample showed the formation of β-SiAlON grains. Investigation on the S-series samples, revealed some thick rod-like phases formed in the S230 sample after combustion (Figure 10). EDS analysis of these grains also showed that these phases were composed of SiAlON but neither points A nor B were identified as β-SiAlON. Samples with 30 wt% NH4F showed β-SiAlON formation, thus emphasizing the positive effect of the low-melt additive. 4. Conclusions In this study, the characteristics of β-SiAlON (z = 1–4) phase formation via the CS technique was investigated using NH4F as a low-melt additive in air, which contrasts previous studies done in a high-pressure N2 atmosphere. The purity of the β-SiAlON products 6
increased steadily with addition of the NH4F diluent, and highly pure β-SiAlON was obtained when 30% NH4F was added to the raw materials. This positive effect is due to the influence of the NH4F diluent in weakening the agglomeration of the Si and Al raw materials and accelerating the Si nitridation process. The phase compositions of the final products were also strongly affected by their NH4F content and z-values. While some non-SiAlON phases, such as AlN and Al2O3, formed during combustion in the final product, the presence of these phases is not very negative and they can play a significant role as sintering aids in the sintering process of nitride-based ceramics.
References [1] R. B. Heimann, Classic and advanced ceramics: from fundamentals to applications., Weinheim, Germany: John Wiley & Sons, 2010. [2] G. Petzow, M. Herrmann, Silicon Nitride Ceramics. Chapter 2. In High Performance NonOxide Ceramics II, (Vol. 102). [M. Jansen (Ed.)]. Berlin, Germany: Springer-Verlag, 2003. [3] Maznoy, A. Z., Prospects for Resource-Saving Synthesis of Advanced Ceramic Materials on the Basis of Tomsk Oblast Raw Materials. In Proceedings of the 16th International Scientific and Practical Conference of Students, Post-graduates and Young Scientists, Modern Technique and Technologies MTT’ 2010 (Section VI: Material science, pp. 58-60). Piscataway, NJ, United States: IEEE. [4] S. Hampshire, Silicon nitride ceramics–review of structure, processing and properties., Journal of Achievements in Materials and Manufacturing Engineering, 24(1), 2007, 43-50. [5] C. A. G. Pereira, Development of SiAlONs colloidally processed in aqueous medium., (Ph.D. Thesis), University of Aveiro, Portugal, 2009. [6] N. R. Gajum, Rare-earth doped (α'/β')-Sialon ceramics., (Ph.D. Thesis), University of Warwick, Coventry, UK, 2001. [7] Y. Tang, H. Yin, H. Yuan, H. Shuai, Y. Xin, Carbothermal reduction nitridation of slag, glass and minerals: Formation process of SiAlON powders with different morphology. Ceramics International, 42(6), 2016, 7499-7505. [8] V. G. Gilev, IR Spectra and Structure of Si–Al–O–N Phases Prepared by Carbothermal Reduction of Kaolin in Nitriding Atmosphere., Inorganic Materials, 37(10), 2001, 1041-1045. [9] C. B. Raju, S. Vermab, M. N. Sahua, P. K. Jaina, S. Choudary, Silicon nitride/SiAION ceramics-A review., Indian Journal of Engineering & Materials Sciences, 8, 2001, 36-45. [10] J. Niu, K. Harada, S. Suzuki,I. Nakatsugawa, N. Okinaka, T. Akiyama, Fabrication of mixed α/β-SiAlON powders via salt-assisted combustion synthesis., Journal of Alloys and Compounds, 604, 2014, 260-265. [11] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion Synthesis of β‐Si6− zAlzOzN8−z (z=1–4)., Journal of the American Ceramic Society, 90(2), 2007, 626-628. [12] X. Yi, K. Watanabe & T. Akiyama, Preparation of β-Si6−zAlzOz N8−z (z=1–3) by combustion synthesis., In IOP Conference Series: Materials Science and Engineering (Vol. 18, No. 7, p. 072004). Mare, Romania: IOP Publishing, 2011. [13] X. Yi, J. Niu,T. Nakamura, T. Akiyama, Reaction mechanism for combustion synthesis of βSiAlON by using Si, Al, and SiO2 as raw materials., Journal of Alloys and Compounds, 561, 2013, 1-4.
7
[14] P. Mossino, Some aspects in self-propagating high-temperature synthesis., Ceramics International, 30(3), 2004, 311-332. [15] A. Varma, A. S. Rogachev, A. S. Mukasyan, S. Hwang, Combustion synthesis of advanced materials: principles and applications., Advances in Chemical Engineering, 24, 1998, 79-226. [16] L. N. Chukhlomina,Y. M. Maksimov and L. N. Skvortsova, Synthesis of Nitrides by SHS of Ferroalloys in Nitrogen, in Nitride Ceramics: Combustion Synthesis, Properties, and Applications (eds A. A. Gromov and L. N. Chukhlomina), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2014. [17] V. Rosenband & A. Gany, Activated self-propagating high-temperature synthesis of aluminum and titanium nitrides. Experimental Thermal and Fluid Science, 31(5), 2007, 461467. [18] S. K. Dolukhanyan, A. G. Aleksanyan,V. S. Shekhtman,H. G. Hakobyan,D. G. Mayilyan,N. N. Aghadjanyan&O. P. Ter-Galstyan, O. P, Synthesis of transition metal hydrides and a new process for production of refractory metal alloys: An auto review., International Journal of Self-Propagating High-Temperature Synthesis, 19(2), 2010, 85-93. [19] N. Liu, G. Zhang, Y. Xiao & Z. Peng, A General Self-Propagating High-Temperature Synthesis Method for Fast and Easy Preparation of Metal Oxide Nanostructures from Low Melting Point Metals. NANO, 10(01), 2015, 1550015. [20] S. Gennari, F. Maglia,U. Anselmi-Tamburini & G. Spinolo, SHS (Self-sustained hightemperature synthesis) of intermetallic compounds: effect of process parameters by computer simulation., Intermetallics, 11(11), 2003, 1355-1359. [21] J. Niu, X. Yi, I. Nakatsugawa & T. Akiyama, T, Salt-assisted combustion synthesis of βSiAlON fine powders., Intermetallics, 35, 2013, 53-59. [22] J. Niu, K. Harada, I. Nakatsugawa & T. Akiyama, Morphology control of β-SiAlON via saltassisted combustion synthesis., Ceramics International, 40(1), 2014, 1815-1820. [23] J. Niu, T. Nakamura, I. Nakatsugawa & T. Akiyama, Reaction characteristics of combustion synthesis of β-SiAlON using different additives. Chemical Engineering Journal, 241, 2014, 235-242. [24] G. Liu, K. Chen, H. Zhou, X. Ning & J. M. F. Ferreira, Effect of diluents and NH4F additive on the combustion synthesis of Yb α-SiAlON., Journal of the European Ceramic Society, 25(14), 2005, 3361-3366. [25] K. Xu, G. He, G. Liu, Z. Han & J. Li, Combustion Synthesis of Eu2+ Doped β-SiAlON Phosphors by Salt Assistance., ECS Journal of Solid State Science and Technology, 2(11), 2013, 230-232. [26] K. Aoyagi, R. Sivakumar, X. Yi, T. Watanabe & T. Akiyama, Effect of diluents on high purity β-SiAlONs by mechanically activated combustion synthesis., Journal of the Ceramic Society of Japan, 117(1366), 2009, 777-779. [27] M. Sopicka-Lizer, M. Tańcula, T. Włodek, K. Rodak, M. Hüller,V. Kochnev & K. MacKenzie, The effect of mechanical activation on the properties of β-sialon precursors., Journal of the European Ceramic Society, 28(1), 2008, 279-288. [28] C. L. Yeh, F. S. Wu &Y. L. Chen, Effects of α-and β-Si3N4 as precursors on combustion synthesis of (α+β)-SiAlON composites., Journal of Alloys and Compounds, 509(9), 2011, 3985-3990. [29] D. G. Liu, K. Chen, J. Li, Combustion synthesis of SiAlON ceramic powders: A review., Materials and manufacturing processes., 28(2), 2013, pp: 113-125.
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[30] G. H. Liu, K. X. Chen, H. P. Zhou, X. S. Ning, J. M. F. Ferreira, Effects of diluents and NH 4F additive on the combustion synthesis of Yb α-SiAlON. Journal of the European Ceramic Society, 25, 2005, 3361-3366. [31] G. H. Liu, K. X. Chen, H. P. Zhou, X. S. Ning, J. M. F. Ferreira, Effects of technical parameters on the phase composition and microstructure of Yb α-SiAlON prepared by combustion synthesis., Key Engineering Materials, 280-283, 2005, 1237-1240. [32] K. Chen, H. Jin, M. Oliveira, H. Zhou, J. M. F. Ferreira, Microstructure and formation mechanism of combustion-synthesized rodlike Ca α-SiAlON crystals., Journal of Material Resource, 16 (7), 2001, 1928-1934. [33] V. N. Antsiferov, V. G. Gilev & V. I. Karmanov, Infrared spectra and structure of Si 3N4, Si2ON2, and SiAlON., Refractories and Industrial Ceramics, 44(2), 2003, 108-114. [34] K. Chen, H. Jin, M. Oliveira, H. Zhou, J. M. F. Ferreira, Microstructure and formation mechanism of combustion-synthesized rodlike Ca α-SiAlON crystals., Journal of Material Resource, 16 (7), 2001, 1928-1934. [35] A. D. John, Lange’s handbook of chemistry (15th ed.)., New York, USA: McGraw-Hill, (pp. 6.82, 6.112). 1999. [36] W. Hertl, W. W. Pultz, Disproportionation and vaporization of solid silicon monoxide., Journal of the American Ceramic Society, 50(7), 1967, 378-381. [37] K. AlKaabi, D. L. Prasad, P. Kroll, N. W. Ashcroft, R. Hoffmann, Silicon monoxide at 1 atm and elevated pressures: crystalline or amorphous?, Journal of the American Chemical Society, 136(9), 2014, 3410-3423. [38] V. N. Antsiferov, V. G. Gilyov, V. I. Karmanov, Infrared spectra and structure of Si3N4, Si2ON2, and sialons., Refractories and Industrial Ceramics, 44(2), 2003, 108-114. [39] V. N. Antsiferov, V. G. Gilyov, V. I. Karmanov, IR-spectra and phases structure of sialons., Vibrational Spectroscopy, 30(2), 2002, 169-173. [40] M. Panneerselvam, K. J. Rao, A microwave method for the preparation and sintering of βSiAlON., Materials Research Bulltein, 38(4), 2003, 663-674. [41] H. Liu, F. Meng, Q. Li, Z. Huang, S. Luo, L. Yin, etal, Growth mechanism and PL properties of β-sialon nanobelts/nanowires synthesized by a process of aluminothermic reduction nitridation of zircon., Cryst. Eng. Comm., 17(7), 2015, 1591-1596.
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Table 1 The composition of the starting raw materials at different Z values and NH4F additive. Composition [mass%] Sample code Z value Si Al Urea S100 1 23.89 4.59 71.52 S200 2 21.33 10.24 68.42 S300 3 18.10 17.39 64.51 S400 4 13.89 26.69 59.41 S115 1 20.78 3.98 62.19 S215 2 18.56 8.90 59.50 S315 3 15.74 15.11 56.09 S415 4 12.09 23.20 51.67 S130 1 18.38 3.53 55.01 S230 2 16.41 7.88 52.63 S330 3 13.92 13.38 49.62 S430 4 10.68 20.53 45.70
Fig. 1. Combustion process steps of samples in different time just after flame out.
10
NH4F 0
13.04
23.08
Fig. 2. Self-propagation and combustion process in S430 sample after putting on hot plate at different time.
11
Fig. 3. The appearance of green pellets (up) and combusted S-series samples (below).
Fig. 4. The appearance of P-series samples, (a): P400, (b): P415, (c): P430.
Figure 2 XRD patterns of combusted S100, S115, S130.
Figure 4 XRD patterns of combusted S300, S315, S330.
Fig. 3 shows XRD patterns of samples S200, S215, and S230. In contrast to previous samples, no NH4AlF4 phase was indentified. Although, the new phase of AlF3 was seen which its 100% peak was overlapped with peaks of β-SiAlON, Al2O3 at 2θ=28.3˚; also, two unidentified peaks was observed. These peaks may belong to one or two different phases that overlapped with Al2O3, Si peaks at 2θ=52.5˚ and 2θ=55˚, respectively. SiO peaks overlapped with Si peaks; Although, Si is the dominant phase. For samples with z=1, Al content increased with z increasing.
In addition, NH4F/Si ratio was increased by decreasing Si content of green pellets which led to higher catalytic effect of NH4F after its decomposition during the reaction; this improved the nitridation process. Despite the fact that the effect of NH4F on oxidation process had never been reported previously, the effect was highly considerable here. Due to higher Si melting point and thermal capacity comparing to Al (Table 1), for S430 most of th thermal energy conducted from ceramic hot-plate to the bottom of the sample consumed to partially melting of Si, Al. As z enhanced, melted Si content raised, contributed to higher content of SHS products; including β-SiAlON, Si3N4, Al2O3, AlN, NH4AlF4. All identified β-SiAlON phases in the S series samples had a peak shift of 1.8˚ toward higher 2θ angles which means lower d spacing. For all the S series samples except for S415, S430 samples, it was believed 1
5
7
Figure 3 XRD patterns of combusted S200, S215, S230.
Fig. 5. XRD patterns of combusted S-series samples include different percent of NH4F. : Alumina, β: β –SiAlON, SiO: silicon monoxide, Si: Silicon, SN: Si3N4, Al: Aluminium, M: mullite.
XRD patterns of S300, S315, S330 samples are shown in Fig. 4. The phases were identified are as same as those in Fig. 3. Fig. 5 depicts XRD patterns of S400, S415, S430 samples. Similar to Fig. 5, the same phases were identified. The only difference is the higher content of β-SiAlON, Si3N4, Al2O3, AlN phases in the pattern of S430. Comparing Fig. 3 to Fig. 6, it was found that for a specific z value, higher amounts of NH4F led to enhanced SiAlON, nitride, and oxide phases’ contents; the samples with 30wt% of NH4F additive had maximum content of β-SiAlON. In order to investigate effect of z value, in optimum 30%wt of NH4F, on synthesized phases, especially β-SiAlON, XRD pattern of S130, S230, S330, and S430 samples were compared in figure 7. All identified phases were β-SiAlON, SiO, Si3N4, Al2O3, AlN, NH4AlF4, AlF3, and mullite. Increasing z value, Al increased and Si decreased (Table 3); led to higher content of β-SiAlON, SiO, Si3N4, Al2O3, AlN, and AlF3 which was not observed in S130 and appeared in S230 to S430 samples.
6
12
Si3N4 and SiO2 were not occurred. Thus, only a little Al2O3 was formed instead of mullite. It is believed that the formation of Al2O3 is a result of partially reaction of Al, SiO, and N2 near the surfaces of the pellet under N2 atmosphere.
Figure 8 XRD patterns of combusted P400, P415, P430.
Fig. 6. XRD patterns of combusted P-series samples P400, P415, P430. : Alumina, β: β –SiAlON, SiO: silicon monoxide, Si: Silicon, SN: Si3N4, Al: Aluminium.M: mullite. 10
Fig. 7. FTIR spectra of S-series samples S400, S415, and S430.
13
Fig. 8. FTIR spectra of P-series samples P400, P415, and P430.
14
Fig. 9. FE-SEM images of combusted S-series samples, (a): S400, (b) and (c): S415, and (d): S430.
15
Table 2 Elemental mass% analysis of marked points of S400, S415, S430 samples Elemental [mass%] Sample code point Si Al O N F A 3.25 92.00 3.61 1.02 0.13 S400 B 4.99 23.19 19.06 3.63 0.47 C 7.39 33.27 20.99 5.04 0.61 S415 D 2.94 35.19 52.41 3.62 5.84 E 2.90 54.33 36.40 6.38 S430 F 1.87 46.43 46.15 5.55 ---G 8.71 53.13 28.43 9.73
phase C ---48.66 32.70 ----
Al Al-O-C composition Al-O-C composition Al2O3+SiAlON
----
Al2O3+SiAlON
Fig. 10. FE-SEM images and EDS spectra of S230 sample.
16
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PII: DOI: Reference:
S0272-8842(17)32929-2 https://doi.org/10.1016/j.ceramint.2017.12.220 CERI17100
To appear in: Ceramics International Received date: 18 August 2017 Revised date: 28 December 2017 Accepted date: 28 December 2017 Cite this article as: O. Tavassoli and M. Bavand-vandchali, Influence of NH 4F additive on the combustion synthesis of β-SiAlON in air, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.220 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of NH4F additive on the combustion synthesis of β-SiAlON in air O. Tavassolia, M. Bavand-vandchalia,* a.
Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
Abstract: The preparation of β-SiAlON, Si6–zAlzOzN8–z (z = 1–4), was investigated using the combustion synthesis (CS) technique with Si, Al, and CO(NH2)2 as the main raw materials in air. Different amounts of NH 4F were used as a low-melt additive to the raw materials to decrease the reaction temperature and promote β-SiAlON formation. Phase and microstructural analyses of the reaction products were performed by X-Ray diffraction (XRD) and FE-SEM/energy dispersive X-ray spectrometer techniques. FT-IR spectroscopy was also used to demonstrate the formation of β-SiAlON in the final product. The phase and microstructural analysis results showed that NH4F addition improved the exothermic reaction of β-SiAlON phase formation and increased its content in the final synthesized powders. Additional heat treatment of the sample at the optimum z-value (z = 4) and different percentages of NH4F revealed the positive effect of the low-melt additive on β-SiAlON phase formation under very low N2 pressure. Keywords: CS technique, β-SiAlON, NH4F, Z-values, low-melt additive.
1. Introduction SiAlONs are silicon nitride-based solid solutions in which equivalent Al–O groups replace Si–N groups in the Si3N4 structure. Different well-known polymorphs of SiAlON phases such as -, β-, and O-SiAlON are isostructural with -Si3N4, β-Si3N4, and Si2ON2, respectively [1, 2]. SiAlONs are widely considered promising high-temperature structural materials owing to their excellent mechanical properties (e.g., strength, fracture toughness, and wear resistance) and superior chemical stability in severe environments [3, 4]. Consequently, they are used extensively as high-temperature engineering ceramics in cutting tools, abrasive materials, location pins, welding nuts, heater tubes, and wear components in the chemical, oil, and gas industries [5, 6]. β-SiAlON, which presents a hexagonal lattice structure, is described by the formula β-Si6–zAlzOzN8–z (0 < z ≤ 4.2) and has a microstructure composed of elongated grains; thus, the compound is characterized with high fracture toughness [7]. Several different techniques have been reported to synthesize SiAlON ceramic powders, including (i) direct reaction of Al2O3, AlN, and Si3N4 in a high-pressure N2 atmosphere at high temperatures of usually >1600 C [2]; (ii) carbothermal reduction–nitridation of a mixture of carbon and aluminosilicate clays (e.g., kaolin) or a mixture of Si, Al, SiO2, and Al2O3 under a N2 or NH3 atmosphere [6–9]; (iii) pressure-less sintering of Si3N4 in the presence of sintering aids such as Al2O3 and Y2O3; (iv) reaction of Si3N4, AlN, and stabilizing alkali cations [1]; and (v) the self-propagating high-temperature synthesis (SHS) method [10– 13]. The SHS technique is a combustion synthesis (CS) mode that begins with an extremely exothermic chemical reaction that propagates through the sample. This process has attracted interest due to its low energy consumption, short reaction time, high-purity products, and simple equipment requirements [14, 15]. It has also been proven to be an efficient technique for synthesizing several engineering materials, such as nitrides [16, 17], hydrides [18], oxides [19], and intermetallic materials [20]. CS of β-SiAlON powders is usually performed in a high-pressure N2 atmosphere ranging from 0.5 to several tens of megapascals [21, 22]. βSiAlON powders have been successfully synthesized throughout combustion with the addition of diluents [23, 24] and mechanical activation at the relatively low nitrogen pressure of 1 MPa [25, 26]. Decreased combustion temperatures and slowed reaction kinetics are mainly why diluents such as β-SiAlON, Si3N4, and AlN are used in the reaction instead of Si and Al metal powders. The use of these diluents also improves the conversion rate of the reactants, prevents the agglomeration or conglomeration of molten particles, improves the 1
nitridation process, and, subsequently, increases the purity of the final synthesized products. Some researchers have reported the effect of different oxides as a raw material on the CS of β-SiAlON synthesized using the SHS technique [27], and these scholars have found that SiO2 is a better oxygen source than Al2O3 when synthesizing β-SiAlON via the CS method under low pressure. However, the purity of the β-SiAlON powders obtained using SiO2 is low due to the presence of remaining unreacted Si in the product, which was attributed to the melting and subsequent agglomeration of Si caused by the extremely high reaction temperature and rate of the CS process and prevention of N2 diffusion into the combustion front [28]. Addition of diluents has been reported to cause a number of problems, such a difficulty in obtaining βSiAlON powders with fine or submicron-size particles; the high cost of diluent materials must also be considered. SiAlONs exhibit superplastic deformation when their grain size is reduced to the nanometer scale [29]. Thus, pressing demands to develop an alternative diluent with low cost and high efficiency have grown to advance efforts to synthesize fine β-SiAlON particles. NaCl, NH4Cl, and NH4F are low-melt additives that have attracted considerable attention as diluents to produce ceramic and intermetallic powders through the CS technique; these diluents have an important influence in decreasing the adiabatic temperature and reducing the rate of agglomeration and grain growth of the resulting particles [30–33]. Previous reports emphasizing the synthesis of SiAlON ceramics in high-pressure N2 gas indicate that diluents use and some nitrogen source additive can be considered as accelerators to improve SiAlON ceramic synthesis via the CS process. The objective of this study is to investigate the effect of NH4F additive as a diluent and urea as a nitrogen source on β-SiAlON formation through the CS process in air. 2. Experimental Procedure The starting materials used were elemental silicon (commercial grade, >99%, D50 = 65 m), aluminum metal (commercial grade, >99.7%, D50 = 45 m), urea (≥99%, 108487, Merck), and NH4F (≥98%, 101164, Merck), which functioned as the diluent additive. The raw materials were mixed according to the stoichiometric ratio based on the reaction below to synthesize β-Si6–zAlzOzN8–z with z-values varying from 1 to 4 and different amounts of NH4F (i.e., 0, 15, and 30 wt%). The starting compositions of each sample are summarized in Table 1. (6 − z) Si + 2 Al + k (8 − z) CO(NH2)2 → β-Si6–zAlzOzN8–z
(1)
The obtained mixtures were uniaxially pressed at 100 MPa to form pellets (20 mm Ø 10 mm H), and hot alumina–mullite plates were used as an ignition source for the CS reaction. The plates were inserted into an elevator furnace and heated until 1600 °C, which was maintained for 30 min. The bottom door of the furnace was then lowered, and the sample pellets were immediately placed on the alumina–mullite plates. Figure 1 illustrates the experimental procedure with the corresponding times from heating of the alumina–mullite plates in the furnace to placing of the sample pellets on the hot plates. The ignition process was initiated at the interface between the samples and the hot plate and propagated immediately throughout the samples, as shown in Figure 2. The combustion process for all of the samples was completed after a few minutes, and the products were allowed to cool to room temperature and prepared for characterization. To investigate the effect of heat treatment on the phase composition and morphology of the products, powdered samples with the optimum z-value and different NH4F contents were uniaxially pressed and then sintered in an electric tube furnace at 1500 °C for 3 h in a N2 atmosphere flowing at a rate of 7 L·min−1. XRD (D-500, Siemens Corporation, Germany, Cu2
K, = 1.54056 nm) was used to assess the phase composition of the samples, and FE-SEM (TESCAN MIRA3, Czech Republic) was used to determine their morphology. In addition, an energy dispersive X-ray spectrometer (EDS) was used for elemental analysis of the products. β-SiAlON formation was confirmed by FT-IR spectrometry (PerkinElmer Spectrum 400) in the mid-IR range.
3. Results and Discussion 3.1. Changes in sample appearance The green and ignited samples are shown in Figure 3. Samples with 15 and 30 wt% NH4F had a lighter appearance than NH4F-free samples after combustion. All of the samples were ignited just after coming into contact with the ceramic hot plate that incorporated with a large flame due to urea decomposition and burning during the combustion reaction. Combustion was initiated just after the flame was extinguished and propagated through the NH4Fcontaining samples. Thus, the appearance of these samples differs from those of the NH4Ffree samples. The figure also shows that all of the combusted samples were porous and that samples with lower z-values and higher Si contents present some deformations, although the S430 sample was completely deformed after the process. High reaction temperatures and fast heating rates are the main characteristics of the CS process. In general, during the CS of β-SiAlON in air, N2 and O2 gases are important reactants because they play a key role in the related combustion reactions. At high reaction temperatures and fast heating rates, Al and Si particles in the raw materials melt and agglomerate quickly, thereby inhibiting reactions with N2 and nitride phase formation. In addition, the more-reactive O2 reacts with the metal particles first, resulting in the formation of oxide phases on the surface of the samples. This phenomenon can be observed in the freeNH4F samples, which show black powders formed on the surface of the pellets (Figure 3) due to metal droplet agglomeration and rapid oxidation. In comparison with these samples, the NH4F in additive-containing samples acts as a diluent that reduces the agglomeration of metal droplets and facilitates oxidation. The presence of an NH4F diluent additive also increases the pressure ratio of N2/O2 in the central portion of the samples after additive decomposition, leading to reduced oxidation of Si and Al particles and promotion of the formation of nitride phases. Liu and et al. [30] indicated that NH4F exerts a catalytic effect on the nitridation of Al and Si as follows: NH4F(s) = NH3(g) + HF(g) NH3(g) = 1/2 N2(g) + 3/2 H2(g) HF(g) = 1/2 H2(g) + 1/2 F2(g) Si(s, l) + HF(g) SiFx(g) + H2(g) SiFx(g) + N2(g) + NH3(g)/H2(g) Si3N4(s) + HF(g) Si(g) + N2(g) + NH3(g) [Six(NH)y]n(s) [Six(NH)y]n(s) Si3N4(s) + NH3(g)
(1) (2) (3) (4) (5) (6) (7)
In this research, NH4F was considered a low-melt additive that creates N2 gas in the reaction mixture after decomposition and enhances the nitridation reaction. Since NH4F is fully decomposed at temperatures above approximately 250 °C and N2 gas is released as a reaction product during the decomposition process in reactions 1 and 2, the N2 supply increases and the nitridation reaction becomes more extensive. Urea was also used as another main source of N2 gas in the samples to promote nitridation during the CS of SiAlON; a similar phenomenon has been observed by other authors [31, 32]. 3
As described in the experimental section, samples with the optimum z-value (z = 4) were heat-treated once more at 1500 °C for 3 h under low-pressure N2 to decrease amount of nonSiAlON phases formed during CS and increase the quantity of β-SiAlON phase. The appearances of the heat-treated samples, labeled P400, P415, and P430, are shown in Figure 4. The appearance of P400, which has no NH4F, is completely different from that of P415 and P430, which have NH4F. Whereas the surface of P400 was covered by molten metal, those of P415 and P430 were not. 3.2. Phase analysis Figure 5 shows the XRD patterns of the combusted products with different z-values and various amounts of NH4F. When NH4F was not used (i.e., samples S100, S200, and S400), a large amount of Al, Si, and SiO appeared and no β-SiAlON phase formed in the combusted products. A small amount of Si3N4 also formed due to the reaction of Si metal and N2 gas originating from the decomposition of urea during combustion. The presence of Al and Si can be attributed to the melting and subsequent coagulation of the metals caused by the extremely high reaction temperature and rate of the combustion reactions, which retards the easy access of the reactant N2 gas to these metals and interrupts the completion of their nitridation. The generation of the reducing gases H2 and CO due to urea decomposition also restricts the accessibility of the oxygen needed to oxidize Al and Si. Therefore, SiO and unreacted Al and Si were present in the NH4F-free samples, as clearly detected in the XRD patterns of S100, S300, and S400. Analysis of the XRD patterns of the NH4F-containing samples revealed that addition of NH4F decreased the quantity of Si, SiO, and Al phases formed, improved the conversion rate of the reaction, and increased the formation of β-SiAlON after combustion. For all z-values, higher NH4F contents increased the formation of β-SiAlON as the main phase; the contents of secondary phases, such as Al2O3, AlN, and Si3N4, in the final combusted products also increased (Figure 5). According to reaction 8, SiO reacts with Al under N2 gas and forms Al2O3 and Si3N4 phases. SiO(s) + Al(l) + N2(g)
Al2O3(s) + Si3N4(s)
(8)
This reaction is similar to the results obtained by other researchers who showed an increase in the amount of the β-SiAlON phase formed in the combusted product by the addition of higher contents of NH4F as an additive [30]. Higher contents of NH4F promote the formation of the Si3N4 phase, which then transforms into the β-SiAlON phase. Figure 5 shows that a higher z-value (z = 4) and higher NH4F content yield better results and form higher amounts of β-SiAlON. In fact, higher NH4F/Si ratios in z = 4 samples promoted the catalytic effect of NH4F after its decomposition during the combustion reaction and improved the nitridation process. The higher melting point and thermal capacity of Si in comparison with those of Al means most of the thermal energy conducted from the ceramic hot plate to the sample bottom was consumed in melting Si and Al. Thus, as the z-value increased, the Si content decreased and the Al content increased in the green pellets. Lower Si melt and higher amount of NH4F additive helped to decrease the adiabatic temperature (Tad) and significantly increase the nitridation reaction during the combustion process. However, higher Si contents in the samples (low z-value) caused Si particles to fuse, subsequently oxidize as SiO, and harden under the fast reaction rate, which caused it to remain in the final products. Therefore, at lower z-values, a larger amount of NH4F is needed to increase the combustion temperature and obtain higher contents of the β-SiAlON phase in the final products. Three main reasons support the concept of SiO instead of SiO2 formation in the final S-series samples: (1) urea 4
decomposition during the combustion process and H2/CO gas formation restrict the ability of oxygen to oxidize Si; (2) Al oxidation (−1582.3 KJ/mole) has a more negative free Gibbs energy (∆G°) than SiO2 formation (−856.4 KJ/mole) [33]; and (3) Al2O3 formation requires a lower molar Al/O ratio (1.5) SiO2 formation (2). The XRD patterns of the P-series samples are shown in Figure 6. Compared with the Sseries samples, all secondary phases, such as SiO, Al, AlN, and Si3N4, disappeared in the NH4F-containing samples, and the intensity of the peaks related to the β-SiAlON phase increased after heat treatment. Based on the XRD results, the increase in β-SiAlON peak intensity corresponded with the decrease of the intensities of peaks corresponding to Si3N4, AlN, and Al2O3. Therefore, formation of β-SiAlON could be concluded to occur by the reaction of Si3N4 with AlN and Al2O3 (reaction 9) in the sintered sample under a low-pressure flow of N2 gas in the furnace, which partially diffuses into the sample and promotes βSiAlON formation. The XRD results of the P-series samples and comparison of the relative intensities of the peaks indicated that the final composition of the powder products includes up to 70% β-SiAlON, approximately. This is very interest that high quantity of β-SiAlON phase has been formed with usage of nitrogen source additive in the air trough the CS process. Si3N4(s) + AlN(s) + Al2O3(s)
β-Si3Al3O3N5(s)
(9)
Low-melt NH4F plays a vital role in the formation of the β-SiAlON phase, and this additive was not detected during phase analysis of the P400 sample, in contrast to the results of the P415 and P430 samples. In contrast to P415 and P430, reactions among Al, Si, and N2 gases in P400 mainly resulted in the formation of composite phases, such as Al2O3, AlN, and Si3N4, and large amounts of unreacted molten Si were obtained from the sample (Figure 3). Recall that NH4F addition promoted β-SiAlON phase formation in P415 and P430 by decreasing Tad and increasing N2 availability to form the β-SiAlON phase. The lone oxygen source for the P-series samples during the sintering process was SiO, from which oxygen separates between 1000 and 1440 °C, leading to the formation of SiO2 and Si [34,35]. The formed SiO2 then contributed to react Al2O3 and Si3N4 incorporated with AlN that was generated from the decomposition of NH4AlF4 in the P430 sample. In P415, the low contents of SiO, NH4AlF4, Si3N4, and AlN and high contents of unreacted Si and Al formed mullite as the dominant phase and excess amounts of Si3N4 and Al2O3. The Si3N4 phase was generated by the reaction of Si and N2 gas originating from NH4AlF4 decomposition and the furnace atmosphere. The low content of Al2O3 in P400 confirms this theory. In fact, NH4F decomposes during the combustion reaction and increases the porosity of the sample, thereby favoring the N2 gas reaction. The NH3 and HF gases produced by the decomposition of NH4F can react with Si and promote its nitridation and further reaction with Al2O3 to produce β-SiAlON. In the CS of β-SiAlON, the quantity of unreacted Si was evidently reduced by the addition of NH4F, and Tad decreased so that more β-SiAlON could be produced. 3.3. FT-IR analysis The FT-IR spectra of combustion-synthesized sample (S-series) with the optimum z-value (z = 4) and heat-treated samples (P-series) are shown in Figures 7 and 8, respectively. In both series of samples, peaks with wave numbers between 400 and 1000 cm−1, which could be attributed to Al–O, Al–N, Si–O, and Si–N bond vibration modes, were observed. This finding demonstrates the presence of β-SiAlON, Al2O3, AlN, and Si3N4 phases in the samples. Peaks observed at about 607 and 670 cm−1 are due to Si–O–Si bending vibrations [36], and the peak 5
at about 3440 cm−1 can be attributed to the stretching vibrations of the Si–N group [37]. The absorption peak at 60 cm−1 found in the S- and P-series sample spectra could be attributed to the Al–O bond vibrations in [AlO6], which exists in Al2O3 and mullite [38], and the absorption peaks at 876 and 700 cm−1 found in the spectra of S430 and P430 could be attributed to Si–N and the stretching vibrations of Al–O–Si, respectively [39, 40]. The peaks at 879 and 839 cm−1 could be attributed to Si–N and Al–N bond vibrations, respectively. The peaks at 1121 cm−1 in the S400 spectrum, that at 1123 cm−1 in the P400 spectrum, and that at 1099 cm−1 could be the stretching vibrations of Si–O bonds as Al–O replaces Si–N in the SiAlON phases [41]. These findings agree with the XRD results and confirm the formation of Si3N4, AlN, and Al2O3 phases in the S400 and P400 samples and the formation of β-SiAlON in the S430 and P430 samples. The peaks at about 711 cm−1 could be attributed to the Al–N stretching vibration mode [40]. The lack of a specific absorption peak at 603 cm−1, the reduction in peak intensity at about 3400 cm−1, and the high intensity of the peak at about 711 cm−1 altogether confirmed the XRD results that are consequences of SiO decomposition and formation of AlN, Si 3N4 from S400 to P400 samples. 3.4. Microstructural analysis Figure 9 shows FE-SEM micrographs of combusted samples with z = 4 and no (S400), 15 wt% (S415), and 30 wt% NH4F (S430). Without addition of NH4F, the morphology of the particles obtained was inhomogeneous, and the particles presented different sizes. Fine agglomerates and tabular-shaped particles were observed in the NH4F-containing samples, including S430, which contained the highest content of NH4F among the samples studied. Details of the elemental mass analysis of marked points in S-series samples with z = 4 and different NH4F contents in Figure 8 are presented in Table 2. EDS analysis of different particles with various morphologies demonstrated unreacted Al particles (spot A) and some particles composed of Al, O, and C elements with a small amount of Si (spots B and C). According to the XRD results in Figure 5, these powders clearly showed the unreacted Al particles and some compositions that formed by the reaction of Al and CO gas originating from urea decomposition during the combustion process. EDS analysis of spots D, E, F, and G revealed agglomerates of well-crystallized particles conforming to the SiAlON phase, together with Al2O3, Si3N4, mullite, and AlN phases. The formation of these compounds can be attributed to the very high combustion temperature and fast reaction kinetics of the reaction system. The morphology of the S430 powder sample obtained after CS (Figure 9) showed dominant phases including thick various-sized rod-like grains with high aspect ratios and a few small agglomerated grains. EDS analysis of the point marked D in the S430 sample showed the formation of β-SiAlON grains. Investigation on the S-series samples, revealed some thick rod-like phases formed in the S230 sample after combustion (Figure 10). EDS analysis of these grains also showed that these phases were composed of SiAlON but neither points A nor B were identified as β-SiAlON. Samples with 30 wt% NH4F showed β-SiAlON formation, thus emphasizing the positive effect of the low-melt additive. 4. Conclusions In this study, the characteristics of β-SiAlON (z = 1–4) phase formation via the CS technique was investigated using NH4F as a low-melt additive in air, which contrasts previous studies done in a high-pressure N2 atmosphere. The purity of the β-SiAlON products 6
increased steadily with addition of the NH4F diluent, and highly pure β-SiAlON was obtained when 30% NH4F was added to the raw materials. This positive effect is due to the influence of the NH4F diluent in weakening the agglomeration of the Si and Al raw materials and accelerating the Si nitridation process. The phase compositions of the final products were also strongly affected by their NH4F content and z-values. While some non-SiAlON phases, such as AlN and Al2O3, formed during combustion in the final product, the presence of these phases is not very negative and they can play a significant role as sintering aids in the sintering process of nitride-based ceramics.
References [1] R. B. Heimann, Classic and advanced ceramics: from fundamentals to applications., Weinheim, Germany: John Wiley & Sons, 2010. [2] G. Petzow, M. Herrmann, Silicon Nitride Ceramics. Chapter 2. In High Performance NonOxide Ceramics II, (Vol. 102). [M. Jansen (Ed.)]. Berlin, Germany: Springer-Verlag, 2003. [3] Maznoy, A. Z., Prospects for Resource-Saving Synthesis of Advanced Ceramic Materials on the Basis of Tomsk Oblast Raw Materials. In Proceedings of the 16th International Scientific and Practical Conference of Students, Post-graduates and Young Scientists, Modern Technique and Technologies MTT’ 2010 (Section VI: Material science, pp. 58-60). Piscataway, NJ, United States: IEEE. [4] S. Hampshire, Silicon nitride ceramics–review of structure, processing and properties., Journal of Achievements in Materials and Manufacturing Engineering, 24(1), 2007, 43-50. [5] C. A. G. Pereira, Development of SiAlONs colloidally processed in aqueous medium., (Ph.D. Thesis), University of Aveiro, Portugal, 2009. [6] N. R. Gajum, Rare-earth doped (α'/β')-Sialon ceramics., (Ph.D. Thesis), University of Warwick, Coventry, UK, 2001. [7] Y. Tang, H. Yin, H. Yuan, H. Shuai, Y. Xin, Carbothermal reduction nitridation of slag, glass and minerals: Formation process of SiAlON powders with different morphology. Ceramics International, 42(6), 2016, 7499-7505. [8] V. G. Gilev, IR Spectra and Structure of Si–Al–O–N Phases Prepared by Carbothermal Reduction of Kaolin in Nitriding Atmosphere., Inorganic Materials, 37(10), 2001, 1041-1045. [9] C. B. Raju, S. Vermab, M. N. Sahua, P. K. Jaina, S. Choudary, Silicon nitride/SiAION ceramics-A review., Indian Journal of Engineering & Materials Sciences, 8, 2001, 36-45. [10] J. Niu, K. Harada, S. Suzuki,I. Nakatsugawa, N. Okinaka, T. Akiyama, Fabrication of mixed α/β-SiAlON powders via salt-assisted combustion synthesis., Journal of Alloys and Compounds, 604, 2014, 260-265. [11] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion Synthesis of β‐Si6− zAlzOzN8−z (z=1–4)., Journal of the American Ceramic Society, 90(2), 2007, 626-628. [12] X. Yi, K. Watanabe & T. Akiyama, Preparation of β-Si6−zAlzOz N8−z (z=1–3) by combustion synthesis., In IOP Conference Series: Materials Science and Engineering (Vol. 18, No. 7, p. 072004). Mare, Romania: IOP Publishing, 2011. [13] X. Yi, J. Niu,T. Nakamura, T. Akiyama, Reaction mechanism for combustion synthesis of βSiAlON by using Si, Al, and SiO2 as raw materials., Journal of Alloys and Compounds, 561, 2013, 1-4.
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[14] P. Mossino, Some aspects in self-propagating high-temperature synthesis., Ceramics International, 30(3), 2004, 311-332. [15] A. Varma, A. S. Rogachev, A. S. Mukasyan, S. Hwang, Combustion synthesis of advanced materials: principles and applications., Advances in Chemical Engineering, 24, 1998, 79-226. [16] L. N. Chukhlomina,Y. M. Maksimov and L. N. Skvortsova, Synthesis of Nitrides by SHS of Ferroalloys in Nitrogen, in Nitride Ceramics: Combustion Synthesis, Properties, and Applications (eds A. A. Gromov and L. N. Chukhlomina), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2014. [17] V. Rosenband & A. Gany, Activated self-propagating high-temperature synthesis of aluminum and titanium nitrides. Experimental Thermal and Fluid Science, 31(5), 2007, 461467. [18] S. K. Dolukhanyan, A. G. Aleksanyan,V. S. Shekhtman,H. G. Hakobyan,D. G. Mayilyan,N. N. Aghadjanyan&O. P. Ter-Galstyan, O. P, Synthesis of transition metal hydrides and a new process for production of refractory metal alloys: An auto review., International Journal of Self-Propagating High-Temperature Synthesis, 19(2), 2010, 85-93. [19] N. Liu, G. Zhang, Y. Xiao & Z. Peng, A General Self-Propagating High-Temperature Synthesis Method for Fast and Easy Preparation of Metal Oxide Nanostructures from Low Melting Point Metals. NANO, 10(01), 2015, 1550015. [20] S. Gennari, F. Maglia,U. Anselmi-Tamburini & G. Spinolo, SHS (Self-sustained hightemperature synthesis) of intermetallic compounds: effect of process parameters by computer simulation., Intermetallics, 11(11), 2003, 1355-1359. [21] J. Niu, X. Yi, I. Nakatsugawa & T. Akiyama, T, Salt-assisted combustion synthesis of βSiAlON fine powders., Intermetallics, 35, 2013, 53-59. [22] J. Niu, K. Harada, I. Nakatsugawa & T. Akiyama, Morphology control of β-SiAlON via saltassisted combustion synthesis., Ceramics International, 40(1), 2014, 1815-1820. [23] J. Niu, T. Nakamura, I. Nakatsugawa & T. Akiyama, Reaction characteristics of combustion synthesis of β-SiAlON using different additives. Chemical Engineering Journal, 241, 2014, 235-242. [24] G. Liu, K. Chen, H. Zhou, X. Ning & J. M. F. Ferreira, Effect of diluents and NH4F additive on the combustion synthesis of Yb α-SiAlON., Journal of the European Ceramic Society, 25(14), 2005, 3361-3366. [25] K. Xu, G. He, G. Liu, Z. Han & J. Li, Combustion Synthesis of Eu2+ Doped β-SiAlON Phosphors by Salt Assistance., ECS Journal of Solid State Science and Technology, 2(11), 2013, 230-232. [26] K. Aoyagi, R. Sivakumar, X. Yi, T. Watanabe & T. Akiyama, Effect of diluents on high purity β-SiAlONs by mechanically activated combustion synthesis., Journal of the Ceramic Society of Japan, 117(1366), 2009, 777-779. [27] M. Sopicka-Lizer, M. Tańcula, T. Włodek, K. Rodak, M. Hüller,V. Kochnev & K. MacKenzie, The effect of mechanical activation on the properties of β-sialon precursors., Journal of the European Ceramic Society, 28(1), 2008, 279-288. [28] C. L. Yeh, F. S. Wu &Y. L. Chen, Effects of α-and β-Si3N4 as precursors on combustion synthesis of (α+β)-SiAlON composites., Journal of Alloys and Compounds, 509(9), 2011, 3985-3990. [29] D. G. Liu, K. Chen, J. Li, Combustion synthesis of SiAlON ceramic powders: A review., Materials and manufacturing processes., 28(2), 2013, pp: 113-125.
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[30] G. H. Liu, K. X. Chen, H. P. Zhou, X. S. Ning, J. M. F. Ferreira, Effects of diluents and NH 4F additive on the combustion synthesis of Yb α-SiAlON. Journal of the European Ceramic Society, 25, 2005, 3361-3366. [31] G. H. Liu, K. X. Chen, H. P. Zhou, X. S. Ning, J. M. F. Ferreira, Effects of technical parameters on the phase composition and microstructure of Yb α-SiAlON prepared by combustion synthesis., Key Engineering Materials, 280-283, 2005, 1237-1240. [32] K. Chen, H. Jin, M. Oliveira, H. Zhou, J. M. F. Ferreira, Microstructure and formation mechanism of combustion-synthesized rodlike Ca α-SiAlON crystals., Journal of Material Resource, 16 (7), 2001, 1928-1934. [33] V. N. Antsiferov, V. G. Gilev & V. I. Karmanov, Infrared spectra and structure of Si 3N4, Si2ON2, and SiAlON., Refractories and Industrial Ceramics, 44(2), 2003, 108-114. [34] K. Chen, H. Jin, M. Oliveira, H. Zhou, J. M. F. Ferreira, Microstructure and formation mechanism of combustion-synthesized rodlike Ca α-SiAlON crystals., Journal of Material Resource, 16 (7), 2001, 1928-1934. [35] A. D. John, Lange’s handbook of chemistry (15th ed.)., New York, USA: McGraw-Hill, (pp. 6.82, 6.112). 1999. [36] W. Hertl, W. W. Pultz, Disproportionation and vaporization of solid silicon monoxide., Journal of the American Ceramic Society, 50(7), 1967, 378-381. [37] K. AlKaabi, D. L. Prasad, P. Kroll, N. W. Ashcroft, R. Hoffmann, Silicon monoxide at 1 atm and elevated pressures: crystalline or amorphous?, Journal of the American Chemical Society, 136(9), 2014, 3410-3423. [38] V. N. Antsiferov, V. G. Gilyov, V. I. Karmanov, Infrared spectra and structure of Si3N4, Si2ON2, and sialons., Refractories and Industrial Ceramics, 44(2), 2003, 108-114. [39] V. N. Antsiferov, V. G. Gilyov, V. I. Karmanov, IR-spectra and phases structure of sialons., Vibrational Spectroscopy, 30(2), 2002, 169-173. [40] M. Panneerselvam, K. J. Rao, A microwave method for the preparation and sintering of βSiAlON., Materials Research Bulltein, 38(4), 2003, 663-674. [41] H. Liu, F. Meng, Q. Li, Z. Huang, S. Luo, L. Yin, etal, Growth mechanism and PL properties of β-sialon nanobelts/nanowires synthesized by a process of aluminothermic reduction nitridation of zircon., Cryst. Eng. Comm., 17(7), 2015, 1591-1596.
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Table 1 The composition of the starting raw materials at different Z values and NH4F additive. Composition [mass%] Sample code Z value Si Al Urea S100 1 23.89 4.59 71.52 S200 2 21.33 10.24 68.42 S300 3 18.10 17.39 64.51 S400 4 13.89 26.69 59.41 S115 1 20.78 3.98 62.19 S215 2 18.56 8.90 59.50 S315 3 15.74 15.11 56.09 S415 4 12.09 23.20 51.67 S130 1 18.38 3.53 55.01 S230 2 16.41 7.88 52.63 S330 3 13.92 13.38 49.62 S430 4 10.68 20.53 45.70
Fig. 1. Combustion process steps of samples in different time just after flame out.
10
NH4F 0
13.04
23.08
Fig. 2. Self-propagation and combustion process in S430 sample after putting on hot plate at different time.
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Fig. 3. The appearance of green pellets (up) and combusted S-series samples (below).
Fig. 4. The appearance of P-series samples, (a): P400, (b): P415, (c): P430.
Figure 2 XRD patterns of combusted S100, S115, S130.
Figure 4 XRD patterns of combusted S300, S315, S330.
Fig. 3 shows XRD patterns of samples S200, S215, and S230. In contrast to previous samples, no NH4AlF4 phase was indentified. Although, the new phase of AlF3 was seen which its 100% peak was overlapped with peaks of β-SiAlON, Al2O3 at 2θ=28.3˚; also, two unidentified peaks was observed. These peaks may belong to one or two different phases that overlapped with Al2O3, Si peaks at 2θ=52.5˚ and 2θ=55˚, respectively. SiO peaks overlapped with Si peaks; Although, Si is the dominant phase. For samples with z=1, Al content increased with z increasing.
In addition, NH4F/Si ratio was increased by decreasing Si content of green pellets which led to higher catalytic effect of NH4F after its decomposition during the reaction; this improved the nitridation process. Despite the fact that the effect of NH4F on oxidation process had never been reported previously, the effect was highly considerable here. Due to higher Si melting point and thermal capacity comparing to Al (Table 1), for S430 most of th thermal energy conducted from ceramic hot-plate to the bottom of the sample consumed to partially melting of Si, Al. As z enhanced, melted Si content raised, contributed to higher content of SHS products; including β-SiAlON, Si3N4, Al2O3, AlN, NH4AlF4. All identified β-SiAlON phases in the S series samples had a peak shift of 1.8˚ toward higher 2θ angles which means lower d spacing. For all the S series samples except for S415, S430 samples, it was believed 1
5
7
Figure 3 XRD patterns of combusted S200, S215, S230.
Fig. 5. XRD patterns of combusted S-series samples include different percent of NH4F. : Alumina, β: β –SiAlON, SiO: silicon monoxide, Si: Silicon, SN: Si3N4, Al: Aluminium, M: mullite.
XRD patterns of S300, S315, S330 samples are shown in Fig. 4. The phases were identified are as same as those in Fig. 3. Fig. 5 depicts XRD patterns of S400, S415, S430 samples. Similar to Fig. 5, the same phases were identified. The only difference is the higher content of β-SiAlON, Si3N4, Al2O3, AlN phases in the pattern of S430. Comparing Fig. 3 to Fig. 6, it was found that for a specific z value, higher amounts of NH4F led to enhanced SiAlON, nitride, and oxide phases’ contents; the samples with 30wt% of NH4F additive had maximum content of β-SiAlON. In order to investigate effect of z value, in optimum 30%wt of NH4F, on synthesized phases, especially β-SiAlON, XRD pattern of S130, S230, S330, and S430 samples were compared in figure 7. All identified phases were β-SiAlON, SiO, Si3N4, Al2O3, AlN, NH4AlF4, AlF3, and mullite. Increasing z value, Al increased and Si decreased (Table 3); led to higher content of β-SiAlON, SiO, Si3N4, Al2O3, AlN, and AlF3 which was not observed in S130 and appeared in S230 to S430 samples.
6
12
Si3N4 and SiO2 were not occurred. Thus, only a little Al2O3 was formed instead of mullite. It is believed that the formation of Al2O3 is a result of partially reaction of Al, SiO, and N2 near the surfaces of the pellet under N2 atmosphere.
Figure 8 XRD patterns of combusted P400, P415, P430.
Fig. 6. XRD patterns of combusted P-series samples P400, P415, P430. : Alumina, β: β –SiAlON, SiO: silicon monoxide, Si: Silicon, SN: Si3N4, Al: Aluminium.M: mullite. 10
Fig. 7. FTIR spectra of S-series samples S400, S415, and S430.
13
Fig. 8. FTIR spectra of P-series samples P400, P415, and P430.
14
Fig. 9. FE-SEM images of combusted S-series samples, (a): S400, (b) and (c): S415, and (d): S430.
15
Table 2 Elemental mass% analysis of marked points of S400, S415, S430 samples Elemental [mass%] Sample code point Si Al O N F A 3.25 92.00 3.61 1.02 0.13 S400 B 4.99 23.19 19.06 3.63 0.47 C 7.39 33.27 20.99 5.04 0.61 S415 D 2.94 35.19 52.41 3.62 5.84 E 2.90 54.33 36.40 6.38 S430 F 1.87 46.43 46.15 5.55 ---G 8.71 53.13 28.43 9.73
phase C ---48.66 32.70 ----
Al Al-O-C composition Al-O-C composition Al2O3+SiAlON
----
Al2O3+SiAlON
Fig. 10. FE-SEM images and EDS spectra of S230 sample.
16