Crack Healing in Ti
2
Al
0.5
Sn
0.5
C–Al
2
O
3
Composites
Guo Ping Bei,
‡,†
Birgit Joana Pedimonte,
‡
Marc Pezoldt,
‡
Johannes Ast,
§
Tobias Fey,
‡
Mathias Goeken,
§
and Peter Greil
‡
‡
Department of Materials Science (Glass and Ceramics), University of Erlangen-Nuernberg, Martensstr. 5,
Erlangen 91058, Germany
§
Department of Materials Science (General Material Properties), University of Erlangen-Nuernberg, Martensstr. 5,
Erlangen 91058, Germany
Oxidation induced crack healing of Al
2
O
3
composites loaded
with a MAX phase based repair filler (Ti
2
Al
0.5
Sn
0.5
C) was
examined. The fracture strength of 20 vol% repair filler loaded
composites containing artificial indent cracks recovered fully to
the level of the virgin material upon isothermal annealing in air
atmosphere after 48 h at 700°C and 0.5 h at 900°C. SEM-
EBSD analysis of crack microstructure indicates two different
oxidation reaction regimes to govern the crack filling: near the
surface SnO
2
, TiO
2
, and Al
2
O
3
were formed whereas deeply
inside the cracks Al
2
O
3
and TiO
2
and metallic Sn were
detected. The presence of elemental Sn was attributed to par-
tial oxidation of aluminum and titanium which lowered the
local oxygen concentration below a threshold value required
for Sn oxidation to SnO
2
. Thus, Ti
2
Al
0.5
Sn
0.5
C may represent
an efficient repair filler system to trigger oxidation induced
crack healing in ceramic composites at temperatures below
1000°C.
I. Introduction
S
INTERED Al
2
O
3
is one of the most important engineering
ceramics which is widely being used for numerous wear,
chemical, electrical, medical, and other applications. Depend-
ing on the residual porosity and purity it may offer high
hardness and wear resistance, excellent chemical inertness,
high strength, and moderate thermal conductivity as well as
good nuclear stability.
1
However, its relatively low fracture
toughness <6 MPam
1/2
may give rise for failure of a mechan-
ically loaded component primarily in the presence of surface
cracks. Improving the flaw tolerance envisages toughening of
Al
2
O
3
by embedding particle or fibers to trigger process zone
or crack bridging energy-transfer mechanisms.
2
Another
approach is to induce crack healing to recover strength after
damage induced crack formation or growth.
3
Research work on crack healing in Al
2
O
3
ceramic can be
traced back to 1970s.
4
Gupta et al. reported on the crack heal-
ing and strength recovery behavior of thermally shocked
Al
2
O
3
upon annealing above 1400°C.
5
Crack closure was
attributed to grain growth and sintering as the dominating
mechanisms. Enhanced healing ability was observed when the
monolithic Al
2
O
3
ceramics were loaded with repair fillers such
as SiC particles or whiskers which trigger oxidation crack heal-
ing at temperatures below 1400°C.
6,7
Different parameters
affecting the healing ability such as annealing temperature and
time,
6
crack dimensions,
7
content and characteristics of repair
filler,
8,9
healing environment
10
as well as oxygen pressure
11
were investigated. A volume fraction of 15%–20% of SiC
repair filler was found to give rise for complete strength recov-
ery of the Al
2
O
3
/SiC composite.
6–9
Surface cracks of 100–
250 lm in length were filled with silica oxidation product after
annealing in air at 1300°C for 1 h. Prolonged annealing peri-
ods of 10 and 300 h were required when the temperature was
reduced to 1200°C and 1000°C,
6
respectively .
More recently, a group of MAX phases M
n+1
AX
n
(n = 1
to 3) where M is a transition metal, A is an A group ele-
ment, and X is either carbon or nitrogen
12
where shown to
exhibit interesting crack healing abilities.
13
For example, in
Ti
3
AlC
2
extended cracks with a length up to 7 mm and a
width of 5 lm could be fully healed after heat treatment at
1100°C for 2 h in air. Superior healing capability observed
on Ti
2
AlC and Cr
2
AlC ceramics was attributed to the forma-
tion of adhesive Al
2
O
3
filling the space between the disrupted
crack surfaces.
14,15
Furthermore, repeatable crack healing
was demonstrated on Ti
2
AlC.
15
We have reported on the oxi-
dation behavior of Ti
2
Al
(1x)
Sn
x
C MAX phases solid solu-
tion.
16
Substitution of Al by Sn was demonstrated to reduce
the onset temperature of oxidation from 900°C (Ti
2
AlC)
down to 700°C (Ti
2
SnC). Loading Al
2
O
3
with Ti
2
SnC repair
filler was able to achieve crack healing of the composite at
temperatures below 1000°C.
17
The mechanism of crack space
filling reaction, however, still remained an open question.
Thus, the scope of this work is to investigate the crack filling
and strength recovery of Al
2
O
3
composite loaded with
Ti
2
Al
0.5
Sn
0.5
C solid solution repair filler. Analyses of the ele-
mental and the phase distribution of the crack filling material
were correlated with the recovery kinetics to identify opti-
mized healing conditions.
II. Experimental Procedure
A high purity (> 99.99%) submicrometer alumina powder
(AKP-53; Sumitomo Chemical Co., Ltd, Tokyo, Japan) with
a mean particle size d
(0.5)
0.1–0.3 lm served as the matrix
material for the preparation of the Al
2
O
3
–MAX phase com-
posites. Ti
2
Al
0.5
Sn
0.5
C solid solution repair filler with
d
(0.5)
19 lm was synthesized from reactant powder mix-
tures consisting of Ti (4.5 lm, 99.4% purity), Al (<45 lm,
99.5% purity), Sn (2 lm, 99.4% purity) and TiC (2 lm,
99% purity) with a molar composition corresponding to Ti–
0.5Sn–0.5Al–0.9TiC and annealed under vacuum at 1400°C
for 1 h.
Al
2
O
3
–Ti
2
Al
0.5
Sn
0.5
C composites with repair filler fractions
of 5, 10, and 20 vol% were sintered at 1350°C for 4 h in Ar
atmosphere (Heraeus Holding GmbH, Hanau, Germany)
applying a heating rate of 5 K/min. Samples dedicated for
mechanical investigation were polished to 1 lm surface finish
with diamond suspension and cut into bar specimens with
dimensions of 2.5 mm
3
9 2.0 mm
3
9 27 mm
3
. Surface
cracks were generated by means of Vickers’ indentation
F. Wakai—contributing editor
Manuscript No. 35832. Received October 27, 2014; revised January 8, 2015;
approved January 8, 2015.
†
Author to whom correspondence should be addressed. e-mail: guoping.bei@ww.
uni-erlangen.de
1604
J. Am. Ceram. Soc., 98 [5] 1604–1610 (2015)
DOI: 10.1111/jace.13496
© 2015 The American Ceramic Society
J
ournal