Effects of Alumina on Cristobalite Crystallization and Properties of Silica-Based Ceramic Cores

Abstract

Abstract:

In this work, the influences of alumina addition on cristobalite crystallization and properties of injection molded silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the samples, and the XRD result indicated that the addition of alumina promoted crystallization of fused silica during sintering at 1180-1220?°C and thus increases the amount of cristobalite. The increased amount of cristobalite as well as alumina addition led to much more thermal dilation due to their higher coefficients of thermal expansion than that of fused silica. The flexural strengths at room temperature and 1500?°C were tested, and it was shown that alumina addition could not affect room temperature strength, but decreased the flexural strength at 1500?°C. In addition, deflection resistance during heating to high temperatures was investigated, and the result indicated that alumina addition speeded up high temperature softening of the samples. XRD and scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) analysis suggested that this softening behavior was related with viscous flow sintering which could be accelerated by the reaction of alumina and silica with a product of mullite.

Key words: Silica-based ceramic cores, Alumina, Cristobalite crystallization, Properties

Liang J.J,Lin Q.H,Zhang X,Jin T,Zhou Y.Z,Sun X.F,Choi B.G,Kim I.S,Do J.H,Jo C.Y. Effects of Alumina on Cristobalite Crystallization and Properties of Silica-Based Ceramic Cores[J]. J. Mater. Sci. Technol., 2017, 33(2): 204-209.

Figures/Tables 11

Table 1 Weight percentages of compounds in the raw materials (wt%)

	SiO₂	Al₂O₃	Fe₂O₃	TiO₂	MgO	Na₂O	K₂O
Fused silica	99.87	0.017	0.004	0.002	0.001	0.015	0.004
Alumina	0.028	99.66	0.0096	0.0023	0.0008	0.081	0.0002

Fig. 1 Particle size distribution of the materials used in this work

Fig.2 Schematic diagram of sample position in deflection test: (a) original position and (b) drooping distance of the hanged end

Fig.3 Microstructures of the samples after being sintered at different temperatures: CKB1 sintered at (a) 1180?°C/4?h, (b) 1200?°C/4?h, (c) 1220?°C/4?h. CKB2 sintered at (d) 1180?°C/4?h, (e) 1200?°C/4?h, (f) 1220?°C/4?h. CKB3 sintered at (g) 1180?°C/4?h, (h) 1200?°C/4?h, (i) 1220?°C/4?h

Fig.4 XRD patterns of the core materials after sintering

Fig.5 Effect of alumina addition on crystallization of fused silica after sintering

Fig.6 Linear thermal expansion curves during heating up to 1550?°C of the core samples sintered at 1220?°C

Fig.7 Flexural strengths tested at room temperature and 1500?°C

Fig.8 XRD patterns of the samples after flexural strength test at 1500?°C

Fig.9 (a) SEM microstructure of CKB1 after flexural strength test at 1500?°C, and (b) EDS analysis result of the area marked by a red arrow in (a)

Fig.10 Deflection of the samples tested at a heating rate of 10?°C/min

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