Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
CRACK HEALING OF ADVANCED MACHINABLE
HIGH TEMPERATURE Ti3AlC2 CERAMICS
G.M. Song a,*, W.G. Sloof b, S.B. Li c, S. van der Zwaag a
a
Fundamental of Advanced Materials, Faculty of Aerospace Engineering, Delft University of
Technology, Kluyverweg 1, 2629HS, Delft, the Netherlands
b
Department of Materials Science and Engineering, Delft University of Technology,
Mekelweg 2, 2628CD, Delft, the Netherlands,
c
Materials Engineering Center, Beijing Jiaotong University, Beijing 100044, China
* Tel: +31-15-2781607
Fax: +31-15-2784472
e-mail: g.song@tudelft.nl
Crack healing of advanced machinable ternary carbide Ti3AlC2 was investigated by oxidizing a cracked sample
at high temperatures to explore the potential application of the material at high temperatures. A crack with a
length of ~7 mm was introduced into the sample by tensile loading. After an oxidation treatment at 1100 ºC for 2
h in air, the whole crack was completely healed by being fully filled with the oxidation products consisting
primarily of α-Al2O3 as well as a small quantity of rutile TiO2, demonstrating an excellent crack healing ability.
The preferential oxidation of Al atoms in Ti3AlC2 grain at the crack surface resulted in the dominant α-Al2O3
particles inside the crack.
Keywords: Crack healing; Ti3AlC2; Healing mechanism; Preferential oxidation
1
Introduction
The investigation on crack healing of engineering ceramics over the past two decades focused
mainly on SiC, Si3N4 and their composites because these ceramics possess a relative good
crack-healing ability by filling the cracks with the oxidation products [1-3]. The good crack
healing ability undoubtedly increases the reliability and competitive potential of these
ceramics as high temperature structural components. Whereas the oxide ceramics such as
alumina and mullite show poor crack healing abilities because crack healing in these oxide
ceramics relies on thermal diffusion of mass [4], similar to the densification mechanism of
ceramics during sintering. The rate of crack healing by means of oxidation depends strongly
on the oxidation environment, temperature and time [1-3], i. e. high temperature, long
oxidation time and sufficient oxygen are favorable to crack healing.
Our interest in the advanced layered ternary carbide Ti3AlC2 is stimulated by its unusual
properties.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
Ti3AlC2 has an excellent combination of properties of both ceramics and metals [5-7]. It has a
low density (4.2 g/cm3), a low thermal expansion coefficient (9.0×10-6/K), a high Young’s
modulus (297 GPa), a good high-temperature strength and a good high-temperature oxidation
resistance, typical for ceramics. Meanwhile, it posses an excellent electrical conductivity
(2.9×106 Ω-1⋅m-1) and a good thermal shock resistance, has a quiet readily machinability and
is tolerant to damage, as most metals. The machinability can greatly decreases the cost of
Ti3AlC2 ceramics as precise structural and functional components. This combination of
properties makes Ti3AlC2 ceramics very promising candidates for a variety of hightemperature structural and functional applications, such as engines, hypersonic vehicles,
electrodes, furnace elements and core components in nuclear power station.
To date, Ti3AlC2 has not received extensive attention because it is a relatively new ductile
ceramic, and was first synthesized in 1994 by Pietzka et al [8]. The existed reports on Ti3AlC2
focus mainly on the synthesis techniques [5-7], oxidation behaviour [9, 10], bending, shear,
compressive and damage behaviour [5, 6, 11], and some physical properties such as
tribophysical, elastic and electrical properties [5-7]. Considering the potential application and
integrity of Ti3AlC2 ceramics at high temperatures, it is expected that the damage of Ti3AlC2
ceramics can heal spontaneously at their operating temperatures, i.e. the cracks are selfhealing. However, the crack-healing behaviour of Ti3AlC2 ceramics, to our knowledge, has
not been reported yet.
This present work is to explore the self-healing ability of Ti3AlC2 at high temperatures. First,
a long crack in a Ti3AlC2 sample was created by tensile loading. Next, the cracked sample
was exposed to high temperatures in an oxidizing environment to explore the crack healing
ability.
2
Experimental
2.1
Materials preparation
The Ti3AlC2 bulk sample was prepared by an in-situ solid-liquid reactive hot-pressing method
using titanium, aluminum and graphite powders as starting materials. Ti, Al and graphite
powders with the desired atomic stoichiometry of 3:1.1:2 were mixed by ball milling for 4 h
in an ethanol solution. The slurry was dried at 60 ºC and cold-pressed into lumps in a graphite
die under 8 MPa, and then hot-pressed at 1425 °C under 20 MPa for 30 min in flowing argon
gas. The final dimension of these Ti3AlC2 lumps were 5×25×35 mm.
2.2
Crack introduction and healing treatment
In order to introduce a long crack in Ti3AlC2 ceramics under controlled conditions, a Ti3AlC2
piece with a size of 8×2×0.3 mm was glued onto a steel tensile bar, as shown in Fig. 1. A
notch was made on one side of the Ti3AlC2 piece and the steel bar with a saw. The depth of
the notch in the Ti3AlC2 piece and the steel bar were 0.5 mm and 2.5 mm, respectively. At the
notch root a crack was initiated under tensile deformation. To this end, the single edge
notched tensile bar was amounted in a micro tensile stage (Deben Micro Tensile Device5KN), which was inserted into the vacuum chamber of a Scanning Electron Microscope
(SEM, JEOL JSM 6500F).
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
The tensile load was applied to the tensile bar with a 0.02mm/min extension rate. After a long
crack was formed in the Ti3AlC2 piece, the load was released to zero.
For the crack healing experiment, the cracked ceramic piece was removed from the steel bar
by dissolving the glue in acetone for 24 h. Next, the sample was thoroughly cleaned twice
with isopropanol. Finally, the sample was heated at 1100 °C for 2h in air.
Figure 1: A single edge notched Ti3AlC2 piece bonded on a single edge notched tensile bar
2.3
Microstructure analysis
Surfaces of the samples were ground and polished with 0.5 μm diamond paste in the final
step. To reveal the grains, some of the polished samples were slightly etched for 20 s in a
1:1:2 (by volume) solution of HNO3, HF and H2O. The microstructures of the sample were
studied using a JSM 6500F-SEM equipped with an Energy Dispersive Spectrometer (EDS).
The crystalline phases present in the sample before and after heat treatment were identified
with X-ray diffractometry (XRD) using a Bruker AXS D5005.
3
Results and discussion
3.1
Microstructure and crack profile before healing
The microstructure of the polycrystalline Ti3AlC2 is shown in Fig. 2: large lamellar grains
were homogenously distributed in the matrix consisting of small equiaxed grains. The average
size of the large lamellar grains is about 8 μm in length and 2 μm in thickness. SEM
observation of the un-etched sample showed that the material is fully dense. A small amount
of Al3O2 particles are present in the material as impurities, which were not detected by XRD.
Figure 2: Microstructure of the etched surface of Ti3AlC2
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
A crack with a length of ~7 mm in the Ti3AlC2 sample is shown in Fig. 3. Once the crack was
initiated, it propagated rapidly perpendicular to the applied tensile loading direction. This
indicated the brittle nature of the Ti3AlC2 sample. The average width of the crack is about 5
μm. Images at higher magnification (Fig. 4) show that the crack grew in a zigzag mode
mainly along the basal planes of the hexagonal Ti3AlC2 lamellar grains. The typical angle of
the crack deflection direction to the tensile load direction is close to 45°; see Fig. 4b. This
zigzag crack pattern in combination with local crack bridging is responsible for the high
toughness of the material. The Ti3AlC2 crystal is hexagonal (hcp) and consists of planar closepacked Al layers linked together by edge-sharing Ti3C2 octahedral layers [7], where the
bonding between the Al layer and the TiC layer is relatively weak compared the bonding
between Ti and C. The bonding between Al layer and TiC layer is governed by the metallic
Ti-Al bond, which allows for the motion of the dislocations and the subsequent shear failure
and delamination along the basal plane. The bonding between Ti atoms and C atoms is
governed by Ti-C covalent bond, which is mainly responsible for the brittle failure.
Figure 3: A crack with a length of about 7 mm in Ti3AlC2 sample
Figure 4: (a) SEM micrograph of zigzag crack of Ti3AlC2; (b) Crack propagated mainly along the basal planes of
the hexagonal grains. Most of the crack deflection angles to the tensile loading direction are close to 45°
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
3.2
G.M. Song et al.
Crack healing
The topology of the cracked sample after a heat treatment at 1100 ºC for 2 h is shown in Fig.
5. The thick outer oxide layer was removed to expose the microstructure of the crack. SEM
observation on the subsurface underneath the top oxide layer reveals that the crack gap was
filled by new particles (Fig 5b and c), identified as the oxidation products of Al and Ti with
EDS.
Figure 5: The surface of the crack healed Ti3AlC2. (a) Outer layer at the left part was removed; (b) The crack
with a width of about 10 μm was filled with oxides; (c) High magnification image of the crack, showing that the
crack gap was fully filled with oxides of Ti and Al
The XRD analysis of the exposed surface of the healed sampled after the outer layer was
removed indicated that the crack gap was filled with rutile TiO2 and α-Al2O3; see Fig. 6
Figure 6: XRD patterns of the (a) exposed surface and (b) outer layer surface of the oxidized Ti3AlC2 (hcp)
sample
Fig. 7a shows the whole crack path along the cross-sectional fracture surface of the healed
sample. High magnification images reveal that the crack is fully filled by fine oxides particles
through the thickness; see Fig. 7b. The elemental maps of Ti, Al and O on the cross section
show that a high Al content band together with a high O content band is present in the crack
gap, whereas the Ti content is lower.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
High Al content combining with high O content is also detected at some sporadic points at the
sides of the main crack. This suggests the existence of microcrack branches filled with Al2O3.
Figure 7: Cross-section of the healed Ti3AlC2 sample. (a)The healed crack crossed through the thickness of the
sample; (b) The crack was filled with fine particles. The chemical composition at point A is 62 at.% O, 31 at.%
Al, and 7 at.% Ti
Figure 8: Cross-section of the healed Ti3AlC2 sample. (a) SEM image; (b) Ti map; (c) Al map; (d) O map
Chemical composition analysis of the healed crack in combination with XRD demonstrates
that the crack gap is mainly filled with α-Al2O3 and some rutile TiO2. For example, the
composition in point A in Fig. 7b is 62 at.% O, 31 at.% Al, and 7 at.% Ti.
3.3
Crack healing mechanism
The crack healing by selective oxidation of ternary carbides may be different from the binary
carbides and nitrides. The oxidation of Ti3AlC2 strongly depends on the activities of Al and Ti
reacting with oxygen, and the diffusion of Al and Ti from the Ti3AlC2 matrix to the oxide
scale.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
The activity of Ti in Ti3AlC2 is lower than that of Al because the Ti-C bonding is strongly
covalent, whereas the Ti-Al bonding is weak [9], which is thermodynamically favorable for
the preferential oxidation of Al in the lamellar grains. Additionally, Al has higher affinity
with oxygen than Ti and C because the Gibbs free energy for the reaction of Al with oxygen
to form Al2O3 is more negative than Ti and C [10].
During the crack healing process of Ti3AlC2 at 1100 ºC in air, Al at the crack surface
preferentially reacted with oxygen to form α-Al2O3, probably as a continuous thin Al2O3 film
at the Ti3AlC2 fracture surface. Ti3AlC2 grains become depleted of Al atoms and subsequently
transform into Ti3C2Oy. Then, the oxidation of Al is preceded by the oxidation of Ti, which
results in the formation of rutile TiO2. Thus:
Ti3AlC2 + O2 → Ti3C2Oy + Al2O3,
y <1,
(1)
where Ti3C2Oy is a substoichiometric cubic Ti3C2 (or TiCx, x<1) in which some oxygen ions
are dissolved. This dissolution of oxygen ions in TiC cell only results in a small shift of TiC
cell lattice parameter [9]. Next, the oxidation of Ti3C2Oy occurs:
Ti3C2Oy + O2 → TiO2 + CO (or CO2),
y <1,
(2)
The TiO2 particles effectively mix with Al2O3 particles to form a (TiO2+Al2O3)-mixed layer
on the surface of the Al2O3. Further, after prolonged oxidation, a thick TiO2 layer develops as
a top oxide scale. Previous investigation [9, 10, 12] on the oxidation of Ti3AlC2 at 1000~1200
ºC in air showed that the formation of Al2O3 film on Ti3AlC2 sample preceded the formation
of TiO2 layer.
The oxygen partial pressure and limited space of the crack gap will influence the healing rate
and eventually the nature of oxidation products.
Oxygen can not be supplied sufficiently inside the crack due to narrow zigzag crack path
which is frequently bridged by the lamellar Ti3AlC2 grains. The rougher the crack surface, the
longer the time for crack healing. Fortunately, a low oxygen partial pressure is very favorable
for the preferential oxidation of Al atoms in Ti3AlC2 grains [9]. Therefore, more Al2O3 is
expected to form within the crack gap. The crack healing process is schematically shown in
Fig. 9. First, preferential oxidation of Al atoms occurs at the sample surface, crack mouth and
some areas inside the crack. Subsequently, the oxidation of Ti occurs on the surface of the
Al2O3 scale. Due to the low oxygen partial pressure inside the crack, the growth rate of the
oxide scale in the crack interior is much slower than that at the sample surface. If the crack
gap is very small (maybe less than 1 μm), the crack gap is expected to be fully filled by the
initial Al2O3 scale because the low oxygen is favorable for the preferential formation of
Al2O3. If the crack gap is relatively large (for example, is 10 μm in width), the content of
TiO2 will be higher; see Fig. 5b. The wider the crack gap, the higher the TiO2 content. In this
study, the width of the main part of the crack path is about 5 μm and the crack is mainly filled
with α-Al2O3.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
Figure 9: The crack healing process of Ti3AlC2 sample. (a) A crack in the sample; (b) Al2O3 scale first forms on
the crack surface and sample surface; (c) TiO2 scale forms on top of the Al2O3 scale, meanwhile Al2O3 particles
continuously forms and mix with TiO2 particles; (d) A thick TiO2 scale forms as a outer layer of the sample, and
the crack gap is fully filled with the Al2O3 particles and TiO2 particles
The oxidation product α-Al2O3 is stable at high temperatures and exhibits an excellent high
temperature strength, whereas TiO2 is weak. In addition, the adhesion between the rutile TiO2
outer layer and the Al2O3 inner scale is week. High-resolution transmission electron
microscopy investigation [14] of the interface between the Al2O3 scale and Ti3AlC2 showed
that no amorphous phase existed at the Al2O3/Ti3AlC2 interface. The orientation relationships
and
at
the
interface
are
identified
as
(0001)Al2O3lI(0001)Ti3AlC2
[ 1 2 1 0] Al2O3lI [1120] Ti3AlC2 or [ 1 1 00] Al2O3lI [112 0] Ti3AlC2, which means Al2O3 scale
can grow “epitaxially” on Ti3AlC2 during oxidation. The difference between the coefficients
of thermal expansion of Ti3AlC2 (9.0×10-6/K [5]) and α-Al2O3 (8.8×10-6/K parallel to the caxis and 7.9×10-6/K normal to the c-axis [14]) are 2% (normal to the c-axis of Al2O3 crystal)
and 12% (parallel to the c-axis of Al2O3 crystal) respectively, therefore the thermal misfit
stresses are small [10]. The adhesion between Al2O3 scale and Ti3AlC2 substrate is believed to
be good [10, 13] although no direct measurement of the interface adhesion has been done yet.
Therefore, it is highly desirable for a pure α-Al2O3 scale to fill the crack, because then a good
strength recovery of the healed sample is expected. Further research is needed to confirm the
hypothesis.
In general a high crack-healing rate and thus a high oxidation rate is desired. However,
oxidation occurs not only on the crack surface but also on the sample surface. The formation a
thick and less protective oxide TiO2 layer on the sample surface is unwanted, because this
may severely influence the properties of the components. Therefore, methods to control the
oxidation resistance of Ti3AlC2 outer surfaces needs to be developed, without affecting the
oxidation resistance of freshly formed crack surfaces.
4
Summary
The investigation on the crack healing of Ti3AlC2 ceramics shows that a crack with a length
up to ~ 7 mm was healed by oxidizing the cracked sample at 1100 ºC for 2 h in air,
demonstrating a good crack healing ability of the machiable high temperature Ti3AlC2
ceramics.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
G.M. Song et al.
The crack with an average width of ~5 μm was fully filled with the oxidation products
consisting primarily of α-Al2O3 with a small quantity of rutile TiO2. Preferential oxidation of
Al atoms in Ti3AlC2 grains at the crack surface is responsible for the dominant α-Al2O3
within the crack gap. A fair strength recovery of the healed Ti3AlC2 may be expected
considering the good adhesion between the α-Al2O3 scale and the Ti3AlC2 matrix.
ACKNOWLEDGEMENTS
The financial supported of this work comes from the Dutch Ministry of Economic Affairs via its Innovation
Oriented research Program on Self Healing Materials (IOP-SHM).
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