Received 31 March 2015; accepted 24 June 2015; published 1 July 2015

ABSTRACT

The present work investigated the phase relations in SiC-Al₂O₃-Y₂O₃-SiO₂ (Si-Al-Y-O-C) system. As a continuation of our previous works, the purpose of this study is to understand the high temperature reaction behaviors of SiO₂ in the system and its effect on the phase relations of the valuable system of SiC-Al₂O₃-Y₂O₃. The phase compositions of six solid-state reacted samples with different components of Y₂O₃:Al₂O₃:SiC:SiO₂ were analyzed by XRD. The phase relations of the systems were determined. The subsolidus phase diagrams of ternary Al₂O₃-SiC-SiO₂ system and the tentative phase diagram of an extended quaternary Y₂O₃-Al₂O₃-SiC-SiO₂ system were presented latter involving several coexisting regions of four phases. The high temperature reaction behavior of SiO₂ in the system and its effect on the phase relations of system were discussed.

Keywords:

Phase Relation, Phase Diagram, SiC, SiO₂, YAG, YAM, YAP, Al₆Si₂O₁₃

1. Introduction

SiC ceramic is one of advanced structural ceramics with excellent mechanical properties, mainly in high temperature properties, high hardness, wear resistance, as well as good chemical resistance. It is widely applied in the industry. But its low strength and poor toughness restrict its more extensively applications [1] [2]. Y₂O₃- Al₂O₃ as sintering additives of SiC ceramic have been used for many years [3] [4]. The phase relationship of SiC-Y₂O₃-Al₂O₃ system has also been studied by calculation [5] and experiment [6]. However a few of SiO₂ on SiC surface caused by oxidation of SiC has the non-ignorable effect on the sintering of SiC-Y₂O₃-Al₂O₃ system at high temperature. Thus the effect of SiO₂ on phase relations of SiC-Y₂O₃-Al₂O₃ ternary systems is being concerned. For made it clear this effect, understanding the reaction of SiO₂ with other members in the system at high temperature is required. The phase relations of SiC-Si₃N₄-R₂O₃ (Si-C-N-O-R; R = La, Gd, Y) [7] and SiC- AlN-R₂O₃ (R = Nd, Gd, Yb, Y [8] systems have been reported by the authors. Within systems the effect of SiO₂ impurity on the formation of serial rare-earth nitrogen-containing silicates and silicon-aluminates had been revealed. As the continuation work of them, the present work investigates the high temperature reaction behavior of SiO₂ and the phase relations in the Y₂O₃-Al₂O₃-SiC-SiO₂ system. In this quaternary system the phase diagram of Y₂O₃-SiO₂ subsystem has been identified to form two Y₂Si₂O₇ and Y₂SiO₅ compounds [9] [10]. The phase relations of binary Y₂O₃-Al₂O₃ subsystem have also been reported, in which three compounds Y₄Al₂O₉ (YAM), YAlO₃ (YAP) and Y₃Al₅O₁₂ (YAG) were confirmed [11]-[13]. The phase diagram of Al₂O₃-SiO₂ subsystem has also been reported to form Al₆Si₂O₁₃ (Mullite), and is used in ceramic manufacture [14]. SiC is difficult to react with others oxides. SiC-SiO₂ has a simple binary phase diagram [15], but the SiC-Al₂O₃ phase relation remains unclear [16]. The phase relations of binary Y₂O₃-SiC subsystem has been identified that no any compound was formed [7] [8] [17]. Knowing the phase relations of Y₂O₃-Al₂O₃-SiC-SiO₂ quaternary system will provide help to design and manufacture of SiC ceramic. It will also provide strong evidence to know the factors of controlling the equilibrium of Y₂O₃-Al₂O₃-SiC-SiO₂system.

2. Experimental

2.1. Materials

The starting powders used for the experiments were β-SiC with 0.5% O₂ (mass ratio, the same below) (BF 12-A, H.C. Starck), Al₂O₃ purity ≥ 99.99%, (Xuan Cheng Jing Rui New Material Co., Ltd., China), Y₂O₃ > 99.99% purity (Baotou Research Institute of Rare Earth), SiO₂ (Tianjing Fuchen Chemical Reagents factory, China) respectively. Y₂O₃ and Al₂O₃ powders were calcined at 1100˚C in air for 2 hr to remove their hydrates before being used.

2.2. Experimental Procedure

Selected compositions were marked as YASS in order of Y₂O₃/Al₂O₃/SiC/SiO₂. For example, YASS 1422 represents as the sample with composition of Y₂O₃/Al₂O₃/SiC/SiO₂ = 1/4/2/2 (molar ratio). The details are shown in Table 1. The 20 g powder mixture was mixed and ground in an agate mortar for 1.5 - 2 hrs by adding alcohol (analytical reagent, 99.9% purity). After dried up, the prepared powders were cold isostatic pressed under 250 MPa. The conditions used for solid-state reaction were: in Ar atmosphere, at temperature of 1450˚C - 1600˚C, holding 2 hrs, and then cooling down to room temperature freely. In order to achieve the reaction equilibrium, the holding time prolonged. None of noticeable phase composition change is observed could be the judgment of whether the system equilibrium is achieved. The phase compositions of the sintered samples were analyzed by X-ray diffraction (XRD) using equipment SHIMADZU XRD-6000 with CuKα radiation in 0.2˚ scan step, 2˚∙min⁻¹. The experimental and the analysis method of phase compositions for samples are similar with our previous papers [6]-[8].

3. Results and Discussion

XRD analysis results of sintered samples are shown in Table 1. The effect of SiO₂ impurity on the phase relations in SiC ceramic system is paid close attention. However in our previous work of the phase relations of SiC- Al₂O₃-Y₂O₃ system [6], seemingly no such effect was found (see Figure 1). In order to identify SiO₂ caused by oxidation of SiC at high temperature, the SiC powder was heated to 1450˚C, hold 2 hours in Ar. Figure 2 is XRD pattern of β-SiC starting powder after heating at 1450˚C, showing no trace of SiO₂ could be found. The experimental results also indicated that SiO₂ impurity on the surface of SiC did not participated in the reaction with other oxides in system [6]. There might be too little amount of SiO₂ to detect or the SiO₂ and other components might form a few of liquid at the grain-boundaries, because the eutectic temperature of Y₂O₃-Al₂O₃-SiO₂

system is very low (1370˚C) [18]. Even so, understanding the high temperature react behavior of SiO₂ and its effect on the phase relations in the present system, as well as, further on the manufacture of SiC ceramic is required. For this reason, SiO₂ was chosen as a member of the quaternary Y₂O₃-Al₂O₃-SiC-SiO₂ system. Samples were sintered at 1500˚C - 1600˚C/2 hrs in Ar. Figure 3 is XRD patterns of YASS0133 and YASS0421 sintered samples. From Figure 3, it could be found that both samples have Al₆Si₂O₁₃ (mullite) and SiC formed. The different is YASS0133 contains SiO_2, butYASS0421 contains Al₂O₃. It can be confirmed that SiC can form a tie- line with mullite. Three phases could be identified in YASS0133 as SiC, SiO₂ and mullite. While in YASS0421 three phase coexistence of SiC, Al₂O₃ andAl₆Si₂O₁₃ could be found. Therefore the subsolidus phase diagram of Al₂O₃-SiC-SiO₂ system can be presented as Figure 4. Figure 5 is XRD patterns of specimens of YASS 1422 and YASS 3211. In Figure 5 it was found that the two samples both have SiC and yttrium silicates phases. The different is YASS1422 have Al₂O₃ and YAG (Y₃Al₅O₁₂), while YASS3211 have YAM (Y₄Al₂O₉) and YAP (YAlO₃). It indicates two points, one is mulite can not has a tie-line with SiC. The other is SiO₂ was almost used up to form yttrium silicates. Four phases could be identified as YAG, Al₂O₃, SiC and Y₂Si₂O₇ in YASS1422. And four phases of YAM, YAP, SiC and Y₂SiO₅ were found in YASS3211 specimen. Figure 6 is XRD patterns

of YASS1315 and YASS1414 samples. Phases identified by XRD analysis in YASS1315 are SiC, Al₆Si₂O₁₃, SiO₂ and Y₂Si₂O₇, for YASS1414 they are SiC, Al₂O₃, Al₆Si₂O₁₃ and Y₂Si₂O₇. In Figure 6 we found that the two samples both have Y₂Si₂O₇, Al₆Si₂O₁₃ and SiC. The different is, YASS1315 has SiO₂, YASS1414 has Al₂O₃. It is indicated that SiO₂ plays the role of forming mullite and yttrium silicates and leading to establish the phase equilibria with yttrium aluminates and SiC. The formation of above several four phase coexistence involving both yttrium silicates and mullite extends the ternary system SiC-Al₂O₃-Y₂O₃ into the quaternary system included SiO₂. Combing the phase diagrams of Al₂O₃-Y₂O₃-SiC system with Al₂O₃-Y₂O₃-SiO₂ system [18] the tentative phase diagram of quaternary SiC-Al₂O₃-Y₂O₃-SiO₂ (Si-Al-Y-O-C) system can be presented as Figure 7 which shows that the triangle equilibrium relations of SiC with three yttrium aluminates extend to the tetrahedral equilibrium relations with three yttrium silicates Y₂Si₂O₇, Y₂SiO₅ and mullite, respectively. Besides, it should be indicated that the tie-line YAP-Y₂SiO₅ in present work is better stead of YAG-YAMss tie-line in the phase diagram of Y₂O₃-Al₂O₃-SiO₂ system [18]. The possible YAMss solid-solution is not measured in present work.

4. Conclusion

The subsolidus phase diagram of the Al₂O₃-SiC-SiO₂ system was given. The phase relations in the SiC-Al₂O₃-

Y₂O₃-SiO₂ system were established. The tentative phase diagram of this quaternary system was presented in which SiO₂ plays the role of forming mullite and yttrium silicates and leading to establish the phase equilibria of yttrium silicates with yttrium aluminates and SiC.

Acknowledgements

The present work was financially supported by National Natural Science Foundation of China, NSFC51362001.

Cite this paper

Kan Wu,Laner Wu,Zhenkun Huang,Yong Jiang,Yun Ma, (2015) Phase Relations in Si-Al-Y-O-C Systems. Journal of Materials Science and Chemical Engineering,03,90-96. doi: 10.4236/msce.2015.37011

References

NOTES

^*Corresponding author.