Sintering and microstructure of silicon carbide ceramic with Y₃Al₅O₁₂ added by sol-gel method

Sintering and microstructure of silicon carbide ceramic with Y 3 Al 5 O 12 added by sol-gel method first-author †E-mail:gxzh_zju@163.com †E-mail:gxzh_zju@163.com gxzh_zju@163.com Center of Nano-Science & Nano-Technology, Zhejiang University, Hangzhou 310027, China Center of Nano-Science & Nano-Technology, Zhejiang University, Hangzhou 310027, China Silicon carbide (SiC) ceramic with YAG (Y 3 Al 5 O 12 ) additive added by sol-gel method was liquid-phase sintered at different sintering temperatures, and the sintering mechanism and microstructural characteristics of resulting silicon carbide ceramics were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and elemental distribution of surface (EDS). YAG (yttrium aluminum garnet) phase formed before the sintering and its uniform distribution in the SiC/YAG composite powder decreased the sintering temperature and improved the densification of SiC ceramic. The suitable sintering temperature was 1860 °C with the specimen sintered at this temperature having superior sintering and mechanical properties, smaller crystal size and fewer microstructure defects. Three characteristics of improved toughness of SiC ceramic with YAG added by sol-gel method were microstructural densification, main-crack deflection and crystal ‘bridging’. Silicon carbide Ceramic Yttrium aluminum garnet (YAG) Sol-gel INTRODUCTION 1 Sintering and properties of SiC materials with incorporated oxides had been widely studied (Lee and Kim, 2003 2003 ; Izhevsikyi and Genova, 2001 2001 ; Jensen and Luecke, 2000 2000 ; Liden and Carlstrom, 1995 1995 ; Ye and Pujar, 1999 1999 ; Kim and Kim, 2000 2000 ; Biswas and Rixecker, 2001 2001 ; van Dijen and Mayer, 1996 1996 ). These oxides are added to improve the sintering and fracture toughness of SiC materials. Study of silicon carbide ceramics with additives of yttria/alumina mixture (Samanta and Dhargupta, 2001 2001 ; Zhang and Ru, 2002 2002 ; Zhou and Hirao, 2003 2003 ; She and Ueno, 1999a 1999a ; 1999b 1999b ; Huang and Jia, 2003 2003 ) has become a hotspot of SiC materials research. The oxide phase greatly decreased the sintering temperature and improved the fracture toughness of the materials due to the formation of an in situ YAG (yttrium aluminum garnet) phase. At present yttria/alumina mixture is added into SiC powder by mechanical mixing method. It is difficult to achieve the homogeneity of their distribution, which affects the sintering, properties and microstructure of SiC materials. Wet chemical method is an effective approach for adding the oxides into the SiC powder, and can achieve uniformity of YAG phase in the SiC composite. Liden and Carlstrom ( 1995 1995 ) studied the homogeneous distribution of Al 2 O 3 (2 wt%) and Y 2 O 3 (1 wt%) sols in liquid-phase sintered silicon carbide by an electron probe microanalyzer, with the addition of sols yielding a more homogeneous microstructure for green compacts. Zhang and Ru ( 2002 2002 ) studied the YAG phase added by chemical co-precipitation to prepare the SiC/YAG composite ceramics, but which did not enhance the sintering properties of SiC materials. We adopted sol-gel method to add the YAG additive into SiC powder to obtain SiC sealing material with superior properties and manipulable microstructure. The preparation and characteristics of SiC/YAG composite powder will be presented elsewhere. The aim of the present work was to study the effects of sintering temperature on the sintering and microstructure of SiC ceramic with YAG additive added by sol-gel method, to study the sintering behavior and mechanism of reinforcing SiC materials. EXPERIMENTAL DETAILS 1 The starting materials used in this experiment were α-SiC powder (with average particle size of 0.75 μm), Al(NO 3 ) 3 ·9H 2 O, Y(NO 3 ) 3 ·6H 2 O and (CH 2 ) 6 N 4 . Al(NO 3 ) 3 ·9H 2 O and Y(NO 3 ) 3 ·6H 2 O were mixed homogeneously according to the stoichiometry of 10 wt% YAG (Y:Al=3:5). (CH 2 ) 6 N 4 as catalytic agent was added into the above two aqueous solutions, and the mixed solution was stirred for a short time at the room temperature and yielded some sol which became transparent when heated to 50 °C in water-bath box. Ultra-fine SiC powder was added into the sol, and with the continuous stirring and gelating for some time and adjustment of pH value, the SiC/sol transformed into SiC/gel, which was then dried in air at 80 °C for 24 h and calcined at 900 °C for 2 h respectively to obtain the SiC/YAG composite powder after grinding and sieving through a 200-mesh screen. The composite powders were uniaxially pressed at 100 MPa in a carbon steel die into rectangular specimens. Subsequently, these green bodies were compacted at 250 MPa in a cold isostatic press, and then sintered in a vacuum pressureless-sintering furnace. 1860 °C, 1950 °C and 2000 °C were chosen as the sintering temperatures, and the holding time was 45 min. After finishing by machining, the specimens were prepared. Phase identification was performed by the X-ray diffraction (XRD) method on an XJ10-60 X-ray diffractometer using nickel filtered Cu K α radiation in the range of 2 θ =10°~80° at scanning rate of 2° per minute. The morphology and element content of the sintered samples were observed by scanning electron microscopy (SEM, Model: FEI SIRION) and Genesis 4000 EDAX spectral analysis. Densities of the sintered specimens were measured by a conventional water displacement method. The theoretical density was calculated according to the rule of mixtures. Flexural strength was measured by three-point bending tests with a 30 mm span at a cross-head speed of 0.5 mm/min. Fracture toughness was determined by using the single edge notch beam (SENB) method with a notch width of 0.25 mm and a support span of 20 mm. RESULTS AND ANALYSIS 1 The morphology of SiC/YAG composite powder by sol-gel method is shown in Fig. 1 1 . Note that the gray larger particle (around 1 μm) is considered to be SiC. There is a white fine particle (0.1~0.5 μm) on the surface of the SiC particle. The white fine particle is attributed to be YAG according to EDS analysis (Fig. 2 2 ). The distribution of elemental yttrium, aluminum and magnesium in the composite powders is more homogeneous, which is caused by the gel network structure on the surface of the SiC particle. The uniform distribution of YAG in the SiC powder is conductive to the decrease of the sintering temperature and sintering time and enhances the sintering properties of the SiC composite. The sintering and mechanical properties of the final SiC sintered body are given in Table 1 1 . Note that all the sintering and mechanical properties of the SiC sample sintered at 1860 °C are higher than those of the samples sintered at 1950 °C and 2000 °C. Compared with those of the sintered SiC with YAG additives added by mechanical mixture method (Jensen and Luecke, 2000 2000 ; She and Ueno, 1999b 1999b ; van Dijen and Mayer, 1996 1996 ), the sintering temperature of the sintered SiC with additives added by sol-gel method is lower than 100~200 °C and the properties are much superior, thus indicating that YAG additives added by sol-gel method can decrease the sintering temperature of the SiC composite and enhance the sintering and mechanical properties of the final SiC ceramics. 2 Samples 2 Bulk density (g/cm 3 ) 2 Relative density (%) 2 Fracture toughness (MPa·m 1/2 ) 2 Bending strength (MPa) 2 Crystalline phases Major Secondary Sintered at 1860 °C 3.19 96.1 5.7 486 6H-SiC YAG Sintered at 1950 °C 3.11 93.7 4.9 437 6H-SiC YAG Sintered at 2000 °C 3.01 89.2 3.3 356 6H-SiC YAG Crystalline analysis (Fig. 3 3 ) on the sintered samples by XRD revealed that the major phase of the sintered specimens was α-SiC (6H) and the secondary phase was YAG, with no existence of Al 2 O 3 or Y 2 O 3 phases. This means that the YAG phase was formed during the preparation of SiC/YAG powder by sol-gel method. The YAG phase decreased with increase of sintring temperature, which indicates that YAG reacts with SiC and volatilizes at a higher temperature. Fig. 4 4 shows the fracture structure of SiC ceramic sintered at 1860 °C, 1950 °C and 2000 °C. Note that the crystal size of the ceramics is much larger than that of the powder (0.83 μm of the average size), which is 0.5~5 μm, and that the crystals show columnar shape with complete crystallization. As also seen in Fig. 4 4 , compared with those sintered at 1950 °C and 2000 °C (Fig. 4b 4b ), the fracture surface of the SiC ceramics sintered at 1860 °C (Fig. 4a 4a ) has higher densification, smaller crystal size (0.5–2.5 μm) and fewer microstructure defects. The fracture modes of the ceramics sintered at 1860 °C is mostly intergranular, and the fracture modes at 1950 °C and 2000 °C are predominantly trans-granular. The microstructure of the specimen sintered at 1860 °C exhibits obvious ‘drawing’ characteristic of crystal particles, with the films (50 nm thickness) remaining on the grain boundary. EDS (Fig. 5 5 ) showed that grains (column particles) consist of Si-Y-Al-O, which indicates that SiC grains have core/rim structure where the rim contains smaller amounts of dissolved Y, Al and O; and the secondary phase consists of Y-Al-O-Si, which indicates that YAG phase exists in the intergranular boundary and contains SiC smaller grains. This implies that YAG phase has a ‘weak’ wetting on the surface of SiC grains and that SiC grains with smaller and/or rounded sections will be under dissolution effects in the liquid-phase sintering. The YAG phase becomes thin or messy points and disappears in some triple pockets at 1950 °C and 2000 °C, as shown from Figs. 4b and 4c 4b and 4c . DISCUSSIONS 1 According to the sintering mechanism of liquid-phase sintering, the sintering of ceramics is related to the particle size of the raw materials, additive and its distribution, and sintering procedure. The effect of additive on the sintering of ceramic relates to its content and uniformity. The distribution of additive with the same content is very important for the sintering. Some researchers maintained that the uniform liquid phase is beneficial for decreasing the sintering temperature and improving the densification during the sintering procedure (Liden and Carlstrom, 1995 1995 ; She and Ueno, 1999a 1999a ; Biswas and Rixecker, 2001 2001 ; Huang and Jia, 2003 2003 ). In this study, the SiC/YAG composite powder was prepared by sol-gel method. After hydrolysis and polymerization, the inorganic salts formed the sol, which after a short time was transformed into the gel with network structure and enwrapped on the SiC particles. After drying and heat treatment, the YAG phase (nanometer size) was uniformly distributed in the SiC/YAG composite powder, thus ensuring the homogeneity of the YAG liquid phase during sintering period and improving liquid-phase sintering of SiC ceramic. Compared with the mechanical mixing method, YAG added by sol-gel method formed in advance and resolved the problem of inhomogeneity of phase distribution during the sintering period. The sintering temperature affects the sintering properties and microstructure of SiC ceramic. At a high temperature, the content of liquid phase increases to improve the densification, while possibly leading to the over-growth of crystal, the volatilization of YAG phase and the reaction of SiC with other oxides. At a low temperature, the content of liquid phase is not enough for the required densification of SiC ceramics. The lowest eutectic temperature in the Al 2 O 3 -Y 2 O 3 system is 1760 °C (YAG), the presence of surface silica on the SiC starting powder particles clearly reduces the eutectic temperatures. In this study, with YAG phase formed before the sintering procedure, the sintering temperatures of 1950 °C and 2000 °C were higher by 200 °C and 240 °C than the liquid phase temperature of YAG (1760 °C) respectively, which attains rapid densification rate. Results in the reaction of SiC with other oxides and rapid growth of SiC grains are shown in Figs. 3 3 and 4 4 . This confirmed that SiC ceramic with YAG added by sol-gel method had over-sintering phenomenon above 1950 °C. The specimen sintered at 1860 °C possessed high densification, small crystal size and ideal microstructure, indicating that the liquid phase content met the requirement of densification and that the crystal did not grow too much at the temperature. Therefore, 1860 °C was more suitable sintering temperature for SiC/YAG composite powder. One of the approaches to enhance the strength and improve the toughness of the ceramic is obtaining fine, dense, uniform and pure crystal structure. Since YAG liquid phase existed from the beginning, it provided easy paths for fine SiC grains to slide and grow, which was enhance by the constant interdiffusion between films and grains. Once YAG crystal formed and the interdiffusion was complete, they slowed and blocked further grain sliding and over-growing, centralized in the intergranular boundary and triple points and led to the eventual microstructure. Three characteristics of improved toughness of SiC ceramic with YAG added by sol-gel method were as follows: 1. Microstructural densification: Combined with its crystal size and porosity, the strength of ceramics can be expressed as (Brook, 1999 1999 ): σ =( σ 0 + k 1 d −1/2 )e − np where σ is true strength; σ 0 is strength with no micropore; k 1 and n are constants; d is crystal size and p is porosity. As seen in Eq.( 1 1 ), the strength increases with the decrease of crystal size and porosity. In this work, the specimen sintered at 1860 °C had ideal microstructural densification with a fine crystal size and few pores, which improved the strength and fracture toughness. According to the fracture mechanism, the finer the crystal particle, the larger the proportion of grain boundary and the longer the expanding route of crack; furthermore, the finer the crystal particle, the smaller the initial crack size and the higher the critical stress. They increased the fracture toughness of materials, which was why the mechanical properties of the specimen sintered at 1860 °C were better than those sintered above 1950 °C. 2. Main-crack deflection: The thermal expansion and elasticity of YAG crystal were different from those of SiC crystal, which led to the microcracks (grey arrow pointing) during the cooling, as shown in Fig. 6 6 . The production of microcracks dissipates the forcing energy of main-cracks expansion, inducing the passivation of cusp of main-crack; on the other hand, there occurs volume expansion with microcracks forming, which results in the remaining strain after the disappearance of stress. They compel the main-crack to deflect and prolong its route expansion, thus increasing the fracture toughness of the SiC materials. In the intergranular mode of fracture, the main-cracks deflect along the crystal. This also prolongs the route of main-crack expansion, thus increasing the fracture toughness. 3. Crystal ‘bridging’: Grain boundary and triple pockets of SiC crystals contain the YAG crystal (phase) and played a role of ‘bridging’ of the YAG crystals and SiC crystals and enhanced the fracture toughness of SiC ceramic. CONCLUSION 1 1. Silicon carbide ceramic with YAG additive added by sol-gel method was liquid-phase sintered, and YAG phase forming ahead of the sintering and uniform distribution of YAG in the SiC/YAG composite powder decreased the sintering temperature and improved the densification of SiC ceramic. 2. Silicon carbide ceramic exhibited over-sintering phenomenon above 1950 °C, and the sintering temperature of 1860 °C is recommended; specimen sintered at 1860 °C had higher sintering and mechanical properties, smaller crystal size and fewer microstructure defects than those sintered above 1950 °C; and the main crystalline phase was α-SiC(6H) with secondary phase of YAG. 3. Three characteristics of improved toughness of SiC ceramic with YAG added by sol-gel method were microstructural densification with fine grains, main-crack deflection by microcracks and intergranular fracture, and ‘bridging’ of YAG crystals. References K Biswas , G Rixecker . Liquid phase sintering and microstructure-property relationships of silicon carbide ceramics with oxynitride additives . Materials Chemistry and Physics , 2001 . 67 180 - 191 . . Liquid phase sintering and microstructure-property relationships of silicon carbide ceramics with oxynitride additives JR Brook . Processing of Ceramics Part II , 1999 . : Beijing Science Press of China , 88 - 105 . (in Chinese) . Processing of Ceramics Part II ZH Huang , DC Jia . A new sintering additive for silicon carbide ceramic . Ceramics International , 2003 . 29 13 - 17 . . A new sintering additive for silicon carbide ceramic VA Izhevsikyi , LA Genova . Microstructure and properties tailoring of liquid-phase sintered SiC . International Journal of Refractory Metal & Hard Materials , 2001 . 19 407 - 409 . . Microstructure and properties tailoring of liquid-phase sintered SiC RP Jensen , EW Luecke . High-temperature properties of liquid-phase-sintered α-SiC . Materials Science and Engineering A , 2000 . 282 109 - 114 . . High-temperature properties of liquid-phase-sintered α-SiC YW Kim , JY Kim . Effect of initerial particle size on microstructure of liquid-phase sintered α-silicon carbide . Journal of the European Ceramic Society , 2000 . 20 945 - 949 . . Effect of initerial particle size on microstructure of liquid-phase sintered α-silicon carbide Y Lee , YW Kim . Fabrication of dense nanostructured silicon carbide ceramics through two-step sintering . Journal of American Ceramic Society , 2003 . 86 ( 10 ): 1803 - 1805 . . Fabrication of dense nanostructured silicon carbide ceramics through two-step sintering E Liden , E Carlstrom . Homogeneous distribution of sintering additives in liquid-phase sintered silicon carbide . Journal of the American Ceramic Society , 1995 . 78 ( 7 ): 1761 - 1768 . . Homogeneous distribution of sintering additives in liquid-phase sintered silicon carbide AK Samanta , KK Dhargupta . Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering . Ceramics International , 2001 . 27 123 - 133 . . Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering JH She , K Ueno . Effect of additive content on liquid-phase sintering on silicon carbide ceramics . Materials Research Bulletin , 1999 . 34 ( 10-11 ): 1629 - 1636 . . Effect of additive content on liquid-phase sintering on silicon carbide ceramics JH She , K Ueno . Densification behavior and mechanical properties of pressureless-sintered silicon carbide ceramics with alumina and yttria additions . Materials Chemistry and Physics , 1999 . 59 139 - 142 . . Densification behavior and mechanical properties of pressureless-sintered silicon carbide ceramics with alumina and yttria additions FK van Dijen , E Mayer . Liquid phase sintering of silicon carbide . Journal of the European Ceramic Society , 1996 . 16 413 - 420 . . Liquid phase sintering of silicon carbide H Ye , VV Pujar . Coasening in liquid-phase-sintered α-SiC . Acta mater , 1999 . 47 ( 2 ): 481 - 487 . . Coasening in liquid-phase-sintered α-SiC L Zhang , HQ Ru . Sintering behavior and mechanical properties of the powder mixtures of SiC/YAG ceramics . Journal of Northeastern University (Natural Science) , 2002 . 23 ( 7 ): 667 - 670 . (in Chinese) . Sintering behavior and mechanical properties of the powder mixtures of SiC/YAG ceramics Y Zhou , K Hirao . Fracture-mode change in alumina-silicon carbide composites doped with rare-earth impurities . Journal of the European Ceramic Society , 2003 . 24 265 - 270 . . Fracture-mode change in alumina-silicon carbide composites doped with rare-earth impurities 10.1631/jzus.2005.B0213 Project (No. 2004C31044) supported by the Science & Technique Project of Zhejiang Province, China 2004-10-26 2004-12-20 2005-02-20 2022-01-05 Copyright © 2005, Journal of Zhejiang University Science 2005 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 03 6B