May 2005
Sol–Gel Synthesis and Characterization of Alumina–Calcium Hexaaluminate Composites
1179
2
A. R. Boccaccini, ‘‘The Relationship between Wear Behaviour and Brittleness
Index in Engineering Ceramics and Dispersion Reinforced Ceramic Composites,’’
Interceram., 48 [3] 32–4 (1999).
Y. F. Li, C. D. Qin, and D. H. L. Ng, ‘‘Morphology and Grain Growth
Mechanism of Alumina Whiskers in Aluminium Based Metal Matrix Compos-
addition of calcia. During sintering, grain growth and densifi-
cation occur simultaneously as intergranular pores are removed
through grain-boundary diffusion until a few grains start to
grow extremely fast and become excessively large by consuming
small matrix grains. Then, further densification becomes difficult
because the majority of the pores are trapped within or between
the abnormally grown large grains. In the densification of alu-
mina with the addition of calcia, the AGG occurs. Since the
pores are trapped within or between the abnormally grown large
grains, further densification becomes difficult.
3
ites,’’ J. Mater. Res., 14 [7] 14–9 (1999).
M. M. Suabaugh, I. H. Kersent, and G. L. Messing, ‘‘Texture Development
4
by Temperature Control Grain Growth in Liquid Phase Sintered Alumina,’’
J. Am. Ceram. Soc., 80 [5] 1181–8 (1997).
5
S.-H. Hong and D. Y. Kim, ‘‘Effect of Liquid Content on the Abnormal Grain
Growth of Alumina,’’ J. Am. Ceram. Soc., 84 [7] 1597–600 (2001).
6
2 3 2 3
F. P. Glasser, ‘‘CaO–Al O System’’; pp. 149–51 in The CaO–Al O
System’ Phase Diagrams Material Science and Technology Refractory Materials:
A Series of Monographs, Vol. 6. II, Edited by A. M. Alper. Academic Press,
New York, 1970.
(
5) Microstructure
Figure 6(a)–(c) shows the optical micrographs of alumina–
5 vol% Calcia composites under varying magnifications. Fig-
7
2 3
B. Hallstedt, ‘‘Assessment of the CaO–Al O System,’’ J. Am. Ceram. Soc., 73,
1
5–23 (1990).
C. Dominguez, J. Chevalier, R. Torrecillas, and L. Gremillard, ‘‘Microstruc-
1
8
ure 7(a)–(c) shows the fracture surface of alumina–15 vol% cal-
cia composites viewed under SEM. This shows the dispersed
state of calcium hexaaluminate grains in the alumina matrix.
ture Development of Calcium Hexaluminate,’’ J. Eur. Ceram. Soc., 21, 381–7
2001).
(
9
L. An, H. M. Chan, and K. K. Soni, ‘‘Control of Calcium Hexaluminate Grain
Morphology in In-Situ Toughened Ceramic Composites,’’ J. Mater. Sci., 31,
223–9 (1996).
6
The CA grains are elongated platelike morphology while the
3
10
alumina grains are small and equiaxed. A typical bimodal
microstructure of AGG with large grains dispersed in a fine-
grained matrix appears.
D. Asmi and I. M. Low, ‘‘Physical and Mechanical Characteristics Insitu
Alumina/Calcium Hexa-Aluminate Composites,’’ J. Mater. Sci. Lett., 17, 1735–8
1998).
(
11
V. K. Singh and M. M. Ali, ‘‘Formation Kinetics of High Alumina Cement
Phases: I Monocalcium Aluminate,’’ Br. Ceram. Trans., 79, 112–4 (1980).
AGG in alumina and the density is affected by the amount of
liquid forming impurities. The higher the liquid content, a larger
number of grains grow abnormally. As a result, a typical bimo-
dal microstructure of AGG with large grains dispersed in a fine-
12
H. Ishida, K. Mabuchi, K. Sasaki, and I. Mitsuda, ‘‘Low Temperature Syn-
thesis of Ca SiO from Hillebrandite,’’ J. Am. Ceram. Soc., 75 [9] 2427–32 (1998).
L. An, Hyoung-Chan, and H. M. Chan, ‘‘High Strength Alumina/Alumina
Calcium Hexaluminate Layer Composites,’’ J. Am. Ceram. Soc., 81, 3321–4
1998).
2
4
13
2
3
grained matrix appears. Song and Coble have reported that
the number of abnormal platelike grains increases with increas-
ing doping concentration (CaO1SiO ) from 0.3 to 3 wt%. It is
(
14
V. K. Singh and K. K. Sharma, ‘‘Low Temperature Synthesis of Calcium
Hexa-Aluminate,’’ J. Am. Ceram. Soc., 84 [9] 769–72 (2002).
2
15
C. Dominguez, J. Chevalier, R. Torrecillas, L. Gremillard, and G. Fantozzi,
‘Thermomechanical Properties and Fracture Mechanisms of Calcium Hexa-Alu-
A
also because of the variation of abnormally grown grains (N )
‘
minate,’’ J. Eur. Ceram. Soc., 21, 907–17 (2001).
with liquid content. Materials containing calcium hexaalumin-
ate develop some intermediate liquid phase during sintering and
the matrix grains are therefore larger.
16
F. M. Lea, The Chemistry of Cement and Concrete, Vol. 48; 502pp. Edward
Arnold Ltd., London, UK, 1970.
24
17
O. Sbaizero, S. Maschio, G. Pezzotti, and I. J. Davies, ‘‘Microprobe Flour-
escence Spectroscopy Evaluation of Stress Fields Developed Along a Propogating
Crack in Al Ceramic Composites,’’ J. Mater. Res., 16 [10] 2798–
/CaO ꢂ 6Al
02 (2001).
V. K. Singh, ‘‘Sintering of Calcium Aluminate Mixes,’’ Br. Ceram. Trans., 98
4] 1213–6 (1999).
IV. Conclusions
O
2 3
2 3
O
8
18
(
a) Alumina–calcium hexaaluminate composites were syn-
[
thesized by the sol–gel technique.
b) The powders were characterized using TEM and EDS
19
B. D. Cullity, Elements of X-Ray Diffraction, 2nd edition, Addison-Wesley,
Reading, MA, 1978.
(
and the presence of alumina and calcium hexaaluminate were
confirmed.
20
V. K. Singh, M. M. Ali, and U. K. Mandal, ‘‘Formation Kinetics of Calcium
Aluminates,’’ J. Am. Ceram. Soc., 73 [4] 872–6 (1990).
C. A. Handwerker, P. A. Morris, and R. L. Coble, ‘‘Effect of Chemical
21
(
c) X-ray diffraction analysis confirmed the presence of
Inhomogeneities on the Grain Growth and Microstructure of Alumina,’’ J. Am.
Ceram. Soc., 73 [1] 130–6 (1987).
A. P. Goswami and S. Roy, ‘‘Impurity Dependent Morphology and Grain
the calcium hexaaluminate, calcium dialuminate, and alumina
phases.
22
(
d) The microstructural analysis shows the presence of
Growth in Liquid Phase Sintered Alumina,’’ J. Am. Ceram. Soc., 84 [7] 1620–6
(2001).
bimodal microstructure with equiaxed alumina grains and
elongated platelike calcium hexaaluminate grains.
23
H. Song and R. L. Coble, ‘‘Origin and Growth Kinetics in Plate Like Ab-
normal Grain Growth in Liquid Phase Sintered Alumina,’’ J. Am. Ceram. Soc., 73
7] 2077–85 (1990).
[
24
S. Mascio and G. Pezzotti, ‘‘Microstructure Development and Mechanical
Properties of Alumina–Hexaluminate Composites as Sintered and After
References
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&