Effect of Silicon Addition on Mechanical Properties of Mullite-Sic Composite Refractories

$Effect of Silicon Addition on Mechanical Properties of Mullite-Sic Composite Refractories$

Journal of the Ceramic Society of Japan 120 [12] 574-578 2012 Paper Effect of silicon addition on mechanical properties of mullite-SiC composite refractories

Xiaochao LI, Zhaohui HUANG, Yangai LIU,³ Minghao FANG, Juntong HUANG, Jingyu ZHANG and Shusen CHEN

School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China

Mullite-SiC composite refractories were prepared using mullite, ¡-SiC, Si and Al2O3 as starting materials via carbonization reaction sintering by buried in coarse coal particles. The phase composition and mechanical properties of the composite refractories were studied. Results showed that main phases in the composite refractories consisted of mullite, ¡-SiC, ¢-SiC and a small amount of cristobalite. Both bulk density and flexural strength of the samples increased with the increase of silicon addition. The samples prepared at 1400°C, with 12 mass % of silicon content added in the initial raw materials, had better properties compared with the other samples, with bulk density of 2.28 g/cm3, flexural strength of 32.63 MPa, and retained flexural strength of 35.92 MPa after thermal shock by 5 times water quenching from 1200°C to room temperature. The slight increasing of flexural strength after 5 times thermal shock trials is arose from the protective oxidation of ¡/¢-SiC and the second-sintering of samples. ©2012 The Ceramic Society of Japan. All rights reserved.

Key-words : Mullite, SiC, Composite refractories, Carbonization reaction, Thermal shock

[Received May 17, 2012; Accepted September 9, 2012]

cles and sintered for carbonization reaction.16) This method may 1. Introduction direct firing in the furnace of oxygen atmosphere using nature gas High-temperature thermal shock and solid particle wear, which as the firing fuel. It is not required to use the nitriding furnace can result in major failure and damage of many industrial furnaces which can reduce production costs and improve energy efficiency. and equipments, is a serious issue in circulating fluidized bed Therefore, the preparation of mullite-SiC composite refractories boiler, garbage incineration boiler and coke dry quenching.1) At may provide new technical ideas and theoretical basis for high present, commonly used refractories mainly include Si3N4-Sialon thermal shock resistant as wear-resisting refractories. bonded SiC bricks, high aluminum bricks and castables, corundum bricks and castables, silicon carbide bricks and castables, and 2. Experimental procedures other complex phases refractories which includes silicon carbide, The starting materials are as follows: silicon powder (Si ² corundum and zirconia, etc.2)-5) Conventional wear resistant 98 wt %, grain size < 45 ¯m, Luoyang Refractory Research and materials cannot fully meet the requirement in higher-temperature Industry Trade Co., Ltd., China), mullite particles [Al2O3 + industries for either their short life expectancy or high cost. There SiO2 ² 98 wt %, grain size (10.5 mm, 0.50 mm),] and mullite is a growing demand for thermal shock resistant and wear- powder [Al2O3 + SiO2 ² 98 wt %, grain size <45 ¯m, Shanxi resisting of refractories. It is necessary to improve the mechanical Xixiaoping Refractories, Ltd., China], ¡-SiC particle [SiC ² 98 properties and thermal shock resistant of refractories. Therefore, it wt %, grain size (0.20 mm)] and ¡-SiC Powder (SiC ² 97 wt %, is essential to search for a new technology and method to prepare grain size <45 ¯m, Luoyang Refractory Research and Industry refractories with high performance and low cost. Trade Co., Ltd., China), ¡-Al2O3 powder (Al2O3 ² 99.9 wt %, Mullite and SiC are potential candidates as thermal shock grain size <0.5 ¯m, Aluminum Corporation of China, Ltd.), resistant and wear-resisting refractories due to their excellent nitrogen (N2 ² 99.99 wt %) and sintering additives. Table 1 was mechanical properties at high temperatures, high thermal shock tabulated to show different silicon powder addition in different resistant,6)-8) chemical corrosion and high wear resistance.9)-11) samples. Previous literatures mainly focus on the fabrication and proper- The starting materials were fully mixed together according to ties of mullitle-SiC composites for ceramic research field,12)-15) Table 1. The binder and water was then introduced additionally However, rarely have any research reported on mullitle-SiC into the mixture. After being mixed uniformity, the mixture was composites for refractories. Thus, the development of high ther- pressed to rectangular cylinders of 6 mm © 6mm© 40 mm under mal shock resistant of mullite-SiC composite refractories is 40 MPa holding for 30 s. After that, the green samples were critical for wear-resistant refractories. further compacted by cold isostatic pressing at 120 MPa for 90 s In this paper, mullite-SiC composition refractories were pre- and subsequently dried at 110°C for 12 h in a drying oven. pared by carbonization reaction at 1400°C and 1500°C for 3 h. Finally, the samples were buried in coarse coal particles with Effects of silicon powder addition on sintering properties, size of 510 mm and sintered at 1400°C or 1500°C for 3 h before mechanical properties and thermal shock behavior of final pre- cooling down naturally to room temperature. pared composites by in-situ carbonization treatment were investi- Archimedes method was employed in the determination of bulk gated. Specifically, samples were initially buried in carbon parti- density and apparent porosity. The crystalline phase was exam- ined via X-ray diffractometer (XRD; XD-3, Purkinje General ³ Corresponding auther: Y. G. Liu; E-mail: [email protected] Instrument Co., Ltd.) with CuKa radiation. The morphology was

Table 1. Compositions of the samples mass/% Mullite with different particle size SiC with different particle size Sample Si Al2O3 Binder 10.5 mm 0.50mm ¯45 ¯m 0.20mm ¯45 ¯m T1 23 14 14 23 23 0 3 3 T2 23 14 10 23 23 4 3 3 T3 23 14 6 23 23 8 3 3 T4 23 14 2 23 23 12 3 3 observed by scanning electron microscopy (SEM; JEM-6460LV, Japan Electron Optics Laboratory Co., Ltd.) and the chemical composition was determined by energy dispersive spectrometer (EDS). According to the ASTM standard [C1421-01b (R2007)], flexural strength of the composites was determined via conventional three-point bending method with a span of 20 mm and a crosshead of 0.05 mm/min at room temperature. Thermal shock testing was carried out on water quenched bars from 1200°C to room temperature, namely, a specimen was uniformly heated at 1200°C for 30 min in the furnace and then rapidly cooled in water to ambient temperature. The retained flexural strength values of samples were measured using three-point bending fixture.17) 3. Results and discussion 3.1 XRD analysis Figure 1 shows the XRD patterns of samples. As shown in Fig. 1(a), mullite, ¡-SiC, ¢-SiC and cristobalite can be founded when silicon powder was added. But in the sample T1 (no added silicon powder), which only mullite and ¡-SiC can be detected. As Fig. 1(b) shows, when silicon powder was added, the resulting phases consisted of mullite, ¡-SiC and ¢-SiC, while a amount of cristobalite was obviously detected as the addition of silicon powder increased to 12 mass %. Similarly, only mullite and ¡-SiC can be founded in the sample T1 (no added silicon powder). The appearance of cristobalite might due to the oxidation of silicon powder. The appropriate size of coarse coal particle of 510 mm, which existed a certain amount of air in clearance between particles resulted in the oxidation of silicon powder. To further clarified the oxidation and carbonization of silicon, as a validation the phase behavior of ¢-SiC (Fig. 2), powder mixtures T (with mullite powder 70 mass % and silicon powder 30 mass %) as the matrix of this mullite-SiC composite refractories were sintered Fig. 1. XRD patterns of samples with different silicon powder addition: at 1400 and 1500°C for 3 h, respectively. It can be seen from (a) sintered at 1400°C for 3 h; (b) sintered at 1500°C for 3 h. Fig. 2 that silicon powder completely reacted to form ¢-SiC and cristobalite at 1400°C, while only ¢-SiC accrued via carbonization reaction at 1500°C. The reason due to cristobalite may convert into ¢-SiC by firing at 1500°C in carbonization reducing atmospheres.18),19) This reaction consists of four steps:20),21)

CðSÞ þ SiO2ðsÞ ! SiOðgÞ þ COðgÞ ð1Þ

SiO2ðSÞ þ COðgÞ ! SiOðgÞ þ CO2ðgÞ ð2Þ

CðSÞ þ CO2ðsÞ ! 2COðgÞ ð3Þ

CðSÞ þ SiOðsÞ ! SiCðgÞ þ COðgÞ ð4Þ The partial pressure of CO gas and temperature play an impor- tant role on proceeding of reaction rightward in severe reducing atmosphere.18) The formation of SiC by carbothermal reduction reaction starts at temperatures greater than 1300°C and is more rapid as the sintering temperature increases.22)

3.2 Effect of silicon powder addition on bulk density and apparent porosity Fig. 2. XRD patterns of powder mixtures T (with mullite powder The relative bulk density and apparent porosity of samples 70 mass % and silicon powder 30 mass %) sintered at 1400 and 1500°C are shown in Fig. 3. As Fig. 3 reveals, under the condition of for 3 h.

575 JCS-Japan Li et al.: Effect of silicon addition on mechanical properties of mullite-SiC composite refractories

Fig. 3. Effect of silicon powder addition on bulk density and apparent porosity of samples sintered at 1400 and 1500°C for 3 h. Fig. 4. Effect of silicon powder addition on ﬂexural strength and retained ﬂexural strength after 5 times thermal shock trials of samples. the increasing additions of silicon powder, the bulk density of samples increased, associated with the decreasing of apparent porosity. The bulk density of sample T4 (12 mass % Si added) reached the highest of 2.28 g/cm3 at 1400°C and 2.27 g/cm3 at 1500°C. The apparent porosity of samples T4 (12 mass % Si added) reached the lowest of 17.8% at 1400°C and 18.73% at 1500°C. It was considered that the increasing quantity of samples was resulted from the carbonization of silicon powder, as well as a liquid-phase sintering promoted positively by silicon with low melting point 1410°C. As further revealed in Fig. 3, without silicon powder as addition, the bulk density of samples sintered at 1400°C was lower than that at 1500°C. To the contrary, as content of silicon powder was increased from 4 to 12 mass %, the bulk densities of such samples were higher at 1400°C than those at 1500°C. The results can be ascribed that the sintering property at 1500°C without adding silicon powder was better than that at 1400°C. At 1500°C, as the increment of silicon powder, more and more silicon was volatilized than at 1400°C, which lead to increasing of apparent porosity.

3.3 Effect of silicon powder addition on flexural strength and retained flexural strength after 5 times thermal shock trials The flexural strength and retained strength after 5 times thermal shock trials of samples are shown in Fig. 4. It can be seen that both the flexural strength and the retained strength after 5 times thermal shock trials of samples increased with the increase of silicon powder. The reason revealed that with the increasing of fl silicon powder, the apparent porosity decreased. The exural Fig. 5. SEM photographs and EDS spectrum on the surface of T4 % strength of samples T4 (12 mass Si added) reached the highest sample sintered at 1400°C: (a) white attachments; (b) EDS spectrum of of 32.63 MPa at 1400°C and 30.72 MPa at 1500°C. The retained point A in Fig. 6(a). flexural strength of samples T4 (12 mass % Si added) after 5 times thermal shock trials also reached the highest of 35.92 MPa at 1400°C and 33.26 MPa at 1500°C. The flexural strength of the Figure 6 displayed SEM photographs of surface of sample T4 samples increased slightly after thermal shock by 5 times water at different temperatures, the structure of which was loosened and quenching from 1200°C to room temperature. contained some ¢-SiC whisker [Figs. 6(a) and 6(c)]. While after In order to further investigate the flexural strength of samples 5 times thermal shock trials, its surface microstructure [Figs. 6(b) increased slightly after thermal shock trials by 5 times water and 6(d)] was denser and form a vitreous film. It was considered quenching from 1200°C to room temperature, microstructure that ¡/¢-SiC on the surface of samples were oxidized by 5 times of samples were observed. The sample with Si powder added thermal shock trials at 1200°C to form a thin glassy film which had a layer of white substance on the surface. Figure 5 shows can blunt surface cracks.5) Furthermore, after 5 times thermal surface SEM photographs and EDS spectrum of T4 sample. The shock trials, the fracture surface of sample T4 in Figs. 7(b) chemical composition of the fiber-like structure is Si, Al, O and C and 7(d) showed that there were a significant transgranular [Fig. 5(b)]. Based on XRD, SEM and EDS analysis, it is believed fracture. However, the fracture surface of sample T4 showed that the network-shaped product in Fig. 5(a) consisted mainly of mostly intergranular fracture [Figs. 7(a) and 7(c)]. From the mullite and cristobalite of ¢-SiC whiskers. above observation, the improvement of the flexural strength of

576 Journal of the Ceramic Society of Japan 120 [12] 574-578 2012 JCS-Japan

Fig. 6. SEM photographs of the surface of T4 samples: (a) surface microstructure of T4 sample at 1400°C; (b) surface microstructure of T4 sample at 1400°C after thermal shock by 5 times water quenching from 1200°C to room temperature; (c) surface microstructure of T4 sample at 1500°C; (d) surface microstructure of T4 sample at 1500°C after thermal shock by 5 times water quenching from 1200°C to room temperature.

Fig. 7. SEM photographs of surface fracture of sample T4: (a) surface fracture microstructure of T4 sample at 1400°C; (b) surface fracture microstructure of T4 sample at 1400°C after thermal shock by 5 times water quenching from 1200°C to room temperature; (c) surface fracture microstructure of T4 sample at 1500°C; (d) surface fracture microstructure of T4 sample at 1500°C after thermal shock by 5 times water quenching from 1200°C to room temperature. the samples after 5 times thermal shock trials can be contributed (see Fig. 5) interweaved to form a network structure, which to the second-sintering. The as-metioned second-sintering can be helped impeding crack propagations that initially intrigued by explained as that, for one thing, there is a certain amount of liquid thermal stress.5) Thus the ﬂexural strength of samples increased generated by originally existing cristobalite in samples during the slightly after 5 time’s thermal shock trials. thermal shock resistant experiment; for another thing, glass-like substance was formed by the oxidation of ¡/¢-SiC after 5 times 4. Conclusions of thermal shock resistant testing in the open air. Meanwhile, the (1) Mullite-SiC composite refractories were prepared using microstructure of samples is featured as ﬁber-like ¢-SiC crystals mullite, ¡-SiC, Si and Al2O3 as starting materials by carbon-

577 JCS-Japan Li et al.: Effect of silicon addition on mechanical properties of mullite-SiC composite refractories

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