Available online at www.sciencedirect.com
Journal of the European Ceramic Society 29 (2009) 3205–3209
High-temperature mechanical behavior of Al
2
O
3
/graphite composites
Eugenio Zapata-Solvas
a
, Rosalía Poyato
a
, Diego Gómez-García
a
,
Arturo Domínguez-Rodríguez
a,∗
, Nitin P. Padture
b
a
Departamento Física de la Materia Condensada, Universidad de Sevilla, 41080 Sevilla, Spain
b
Department of Materials Science & Engineering, Center for Emergent Materials, The Ohio State University, Columbus, OH 43210, USA
Received 31 March 2009; received in revised form 28 May 2009; accepted 4 June 2009
Available online 7 July 2009
Abstract
Uniaxial compressive creep behaviour of spark-plasma-sintered Al
2
O
3
/graphite particulate composites has been studied at temperature between
1250 and 1350
◦
C. Values of stress exponent, n, ranging from 1 to 1.4 and, activation energy, Q, of 600 ± 40 kJ/mol have been determined. With
10 vol% graphite in the composite, the creep deformation of the composite is controlled by the fine-grained Al
2
O
3
matrix, where Coble creep has
been identified as the dominant creep mechanism.
© 2009 Elsevier Ltd. All rights reserved.
Keywords: Composites; Ceramics; Graphite; Creep
1. Introduction
There have been extensive studies on the high-temperature
deformation of pure Al
2
O
3
polycrystalline ceramics.
1–3
Research in high-temperature deformation of Al
2
O
3
is driven
by opposing goals, while suppressing cavitation in both cases
3
:
(i) decrease creep resistance for improved superplastic forming
and (ii) increase creep resistance for improved high-temperature
mechanical properties.
There have been efforts to increase steady-state creep rate
in Al
2
O
3
by suppressing grain growth, through MgO doping
4
or TiO
2
/MgO codoping
5
or CuO/MgO codoping.
6
However,
ductility-limiting concurrent grain growth was still found to
occur. Grain-boundary pinning by second-phase particle disper-
sion has been found to be more effective in suppressing grain
growth, resulting in improved ductility of Al
2
O
3
with disper-
sions of Mg
2
Al
2
O
4
,
7
ZrO
2
,
8
and combined Mg
2
Al
2
O
4
/ZrO
2
.
9
In contrast, significant decreases in steady-state creep rates,
between 1 and 2 orders of magnitude, have been reported in
Al
2
O
3
singly doped with Zr, Y, La, or Nd
10
and codoped with
Nd/Zr.
11,12
The origin of grain growth suppression and creep
resistance enhancement appears to be related with the decrease
of grain-boundary diffusivity.
13
However, we did not observe C
∗
Corresponding author. Tel.: +34 954 557849.
E-mail address: adorod@us.es (A. Domínguez-Rodríguez).
in the HRTEM we performed. In any case it is difficult to ascribe
the same behaviour to C, than Zr, Y, Nb, Ca, etc. because the
radius of C is much smaller. For instance, in Y
2
O
3
-stabilised
tetragonal ZrO
2
polycrystals (YTZP) it has been shown that
cations with smaller ionic sizes decrease the flow stress, whereas
those with larger ionic sizes increase the flow stress.
14,15
More recently Al
2
O
3
ceramics (0.5 m grain size) containing
a 10 vol% dispersion of single-wall carbon nanotubes (SWNTs)
fabricated using spark-plasma sintering (SPS)
16,17
has been
found to be about 2 orders of magnitude more creep-resistant
compared to pure Al
2
O
3
of the same grain size.
18,19
The high-
temperature deformation mechanisms in these Al
2
O
3
/SWNTs
composites are intimately related to their unique grain-boundary
structures,
20,21
where high-temperature stretching of SWNTs
appears to impede grain-boundary sliding.
18
In a related study, nanocomposites of Al
2
O
3
with a disper-
sion of 10 vol% graphite particles were also fabricated using the
SPS method.
16
These Al
2
O
3
/graphite composites were found to
be resistant to contact damage under indentation.
16
The objec-
tive of this research is to study and model the high-temperature
deformation of these interesting composites.
2. Experimental
The Al
2
O
3
/graphite particulate composites with 10 vol%
graphite used here are from a previous study.
16
The fabricated
0955-2219/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jeurceramsoc.2009.06.002