2012-High temperature oxidation behaviour of Ti2AlC-Kinetics for grain growth with EBSD
High temperature oxidation behaviour of Ti2AlC
ceramic at1200 C
G.M.Song a,V.Schnabel b,C.Kwakernaak a,S.van der Zwaag c,J.M.Schneider b
and W.G.Sloof a*
a Department of Materials Science and Engineering,Delft University of Technology,
Mekelweg2,2628CD Delft,The Netherlands
b Department of Materials Chemistry,RWTH Aachen University,
Mies-van-der-Rohe-Strasse10,D-52074Aachen,Germany
c Novel Aerospace Materials Group,Faculty of Aerospace Engineering,
Delft University of Technology,Kluyverweg1,2629HS Delft,The Netherlands
*E-mail:w.g.sloof@tudelft.nl
ABSTRACT
The oxidation process of Ti2AlC ceramics in dry synthetic air at1200 C was monitored with
thermogravimetry.The microstructural evolution of the oxide scale with time was characterised by X-
ray diffractometry,scanning electron microscopy,X-ray microanalysis and electron back scattering
diffraction.The oxide scale is comprised of a continuous a-Al2O3inner layer with isolated coarse
TiO2particles on top.At the early oxidation stage(roughly less than0.5h),a-Al2O3and TiO2are the
main reaction products,while at longer reaction times the oxidation only leads to the formation of
a-Al2O3.The a-Al2O3grains in the oxide scale grow in size upon high-temperature oxidation with
the grain size being uniform throughout the thickness of the scale.As diffusion of oxygen along the
grain boundaries dominates the oxide scale growth,the change in grain size affects the oxide scale
growth kinetics.A simple oxide scale growth model,that takes into account this change in fast
diffusion paths,describes the experimentally observed oxide scale growth kinetics perfectly.
Keywords:Ti2AlC,oxidation,grain size,growth kinetics
1.INTRODUCTION
Layered ternary compounds,so-called M nt1AX n
(n?1à3)phase ceramics,have unusual properties
combining metallic merits and ceramic attributes[1–3].
Recently,it has been discovered that cracks developed in
these compounds can be healed via oxidation at high service-
temperatures in ambient air[4,5].For instance,the Young’s
modulus and hardness of the crack-healed zones in damaged
Ti3AlC2were even slightly higher than those of the base
material[4].Recently,we discovered full strength recovery
of the Ti2AlC after the cracks were healed via high
temperature oxidation of the ceramic itself.Such a crack
healing ability undoubtedly expands the lifetime of this type
of ceramics in high temperature applications.Insight into the
nature of the oxides formed in a crack gap and their growth
kinetics at high temperatures is required for predicting or
even controlling the self-repair of components made of these
ceramics.
Previous thermogravimetric analysis by Wang et al.[6]on
the oxidation of Ti2AlC at high temperatures showed that the
oxidation kinetics of Ti2AlC obeyed a cubic law in the
1000–1300 C temperature range.It was thought that the
oxygen grain-boundary transport governed the growth of the
oxide https://www.wendangku.net/doc/ea11126432.html,ter Sundberg et al.[7]found that the oxide
growth of Ti2AlC at1100,1200and1300 C followed a
parabolic law.It is well known that for reaction products
staying the same throughout the oxidation process,parabolic
kinetics indicates that diffusion of reactants(such as O)
occurs through the oxide scale(i.e.through the grain
interior),whereas cubic growth behaviour implies that
diffusion takes place primarily along the grain boundaries
of the oxide scale[8].
However,during the entire oxidation process there is a
competition between the kinetics of the oxidation of the Al
and Ti atoms in the Ti2AlC matrix.The growth rates of the
Al2O3and TiO2probably obey different laws.It has been
shown that the formation of a fully dense Al2O3layer
strongly in?uences the growth rate of the TiO2[7–9].
Hence,the formation and thickening of the Al2O3layer
plays a key role in the oxidation rate and the composition
of the layer formed[10].
In this work,the isothermal oxidation behaviour of Ti2AlC
at1200 C in a dry synthetic air?ow is investigated with
thermogravimetry(TG),and subsequently the microstruc-
tural evolution of the oxide scale with time was observed
with X-ray diffractometry(XRD),scanning electron micro-
scopy(SEM),X-ray microanalysis(XMA)and electron back
scattering diffraction(EBSD).The oxidation mechanisms at
the early and later oxidation stage are discussed and the
205
doi:10.3184/096034012X13348496462140
MATERIALS AT HIGH TEMPERATURES29(3)
kinetics is captured in a model also taking into account the
microstructural changes during the oxidation.
2.EXPERIMENTAL PROCEDURES
Ti2AlC bulk material was prepared via an in situ solid–
liquid reaction of Ti,Al and graphite powders under hot-
pressing conditions[11].From this material,square samples
with a size of10mm and a thickness of1.5mm were cut
using a thin diamond blade.Subsequently,the surfaces of the
samples were ground,starting with400grit and?nished with
2400grit SiC emery paper.Next,the sample surfaces were
polished,starting with3m m and?nished with0.25m m
diamond grains.
The isothermal oxidation experiments were conducted in a
thermal gravimetric analyser(TGA,Setsys Evolution1750,
Setaram).After pre-heating the furnace of the TGA with an
inner diameter of25mm to1200 C,a sample suspended on a
sapphire hook was inserted into the furnace to impose an
isothermal oxidation in a mixture of pure(5N)and dry gases:
N2with20vol.%O2with a?ow rate of80cm3minà1.
During the oxidation treatment,the weight change of the
sample was measured every60s.The accuracy of the
electromagnetic balance of the TGA was+0.1m g.The
samples were also weighed before oxidation with a balance
having a weight resolution of10m g(Mettler Instrument AG,
Switzerland).The oxidation experiments were carried out
with different samples for0.5,1,2,4,8,16,32and80h,
respectively.
The morphology and composition of the surface oxide
layers were investigated with scanning electron microscopy
(SEM,using a JEOL JSM6500F)equipped with an energy
dispersive spectrometer[EDS,ThermoFisher UltraDry
detector(30mm2)operated with Noran System Seven soft-
ware]for X-ray microanalysis.X-ray diffractometery(XRD,
using a Bruker AXS D5005equipped with a Vantec position
sensitive detector and graphite monochromator)was
employed for phase identi?cation.
To study the oxide grain structure across the oxide layer,
cross-sections of the oxidised samples were prepared using a
cross-section ion-polisher(JEOL SM-09010),which mini-
mises polishing induced artefacts.To this end,the samples
were?rst cut in half using a thin diamond blade.Next,
electron backscatter diffraction(EBSD,with a Nordlys II
detector operated with Channel5software,HKL
Technology)was employed in the SEM to record phase
and grain orientation maps of the oxide layers with a pixel
size of100nm.A20keV focused electron beam with beam
current of approximately1nA was applied.The normal of
the sample surface at a working distance of25mm was tilted
to70 with respect to the electron beam towards the EBSD
detector.
3.RESULTS AND DISCUSSION
3.1Oxide scale composition and microstructure
The oxide scale formed after isothermal oxidation at1200 C
in dry N2with20vol.%O2(cf.Section2),consists mainly
of a-Al2O3with some patches of rutile TiO2on top,see
Figure1.The TiO2is formed at the early stages of oxidation,
i.e.within the?rst0.5h,see Figures1(a)and(c).When
comparing the amount of TiO2after0.5and80h oxidation,it
remains virtually the same.This observation suggests that
the formation TiO2is terminated once a dense and contin-
uous a-Al2O3layer is formed.Thus,the oxidation kinetics of
Ti2AlC is dominated by the growth of a pure a-Al2O3layer
after a short initial stage when also TiO2is formed.
Analysis of the a-Al2O3grain size as observed in the
SEM-EBSD images of cross-sections of all oxide scales(see
Figure2),shows that size of these grains is the same
throughout the thickness of the oxide scale.However,the Oxidation behaviour of Ti2AlC ceramic:W.G.Sloof et al.
206MATERIALS AT HIGH TEMPERATURES
https://www.wendangku.net/doc/ea11126432.html, Figure1SEM backscattered electron images of the surface and cross-section of Ti2AlC oxidised in dry synthetic air at1200 C.(a)and(b) surface morphology after0.5and80h oxidation,respectively.(c)and(d)cross-section after0.5h and80h oxidation,respectively.
a-Al2O3grain size increases with oxidation time,see
Figure3.Apparently,during high temperature oxidation,
the mobility of the atoms is suf?cient for movement and
subsequent annihilation of grain boundaries in the a-Al2O3
layer.This grain growth reduces the number of fast diffusion
paths for continuous growth of the a-Al2O3layer and
consequently affects the oxide scale growth kinetics(see
Section3.3.).
3.2Oxidation mechanisms
Upon oxidation of Ti2AlC there is a competition between the
kinetics of the oxidation of Al and Ti atoms in Ti2AlC
matrix.The hexagonal crystal lattice structure of Ti2AlC can
be conceived as a stack of Al and Ti2C layers[1].The
diffusion of Al atoms along the(0001)basal plane will be
much faster than that of Ti atoms,because the bond between
the Ti2C layer and Al layer is a metallic bond whereas the
movement of Ti atoms are strongly limited by the Ti22C
covalent bond.This ensures suf?cient supply of Al atoms
towards the oxide y Ti2AlC matrix interface for continuous
reaction of Al with O.
Moreover,from a thermodynamic point of view the
reaction between Al and O is preferred rather than between
Ti and O.The Gibbs free energy for the formation of Al2O3
from Al and O2,and for the formation of TiO2(or TiO)from
Ti in TiC x and O2,are both negative[12–14].However,the
Gibbs free energy for Al2O3formation upon the reaction of
Al atoms with O at1200 C isà408kJ per mole O,which is
much more negative than that for TiO2formation from TiC,
viz.à295kJ per mole O[13].
The chemical reaction of the selective oxidation of Al in
Ti2AlC can be written as:
4Ti2AlCt3x O2?4Ti2Al1àx Ct2x Al2O3e1T
Thus,Ti2AlC can become non-stoichiometric Ti2Al1àx C
(05x50:5),but its crystal structure does not change[15].
Only in the early stages of oxidation does some TiO2also
form which can be described with:
Ti2AlCt2y O2?Ti2e1àyTAlCt2y TiO2e2T
or:
Ti2Al1àx Ct2y O2?Ti2e1àyTAle1àxTCt2y TiO2e3T
However,diffusion of both Al and Ti within the Ti2AlC
matrix towards the oxide y Ti2AlC matrix interface will
restore its composition.
Oxidation behaviour of Ti2AlC ceramic:W.G.Sloof et al. https://www.wendangku.net/doc/ea11126432.html, MATERIALS AT HIGH TEMPERATURES
207 Figure2Phase maps recorded with EBSD of cross-sections of Ti2AlC oxidised in dry synthetic air at1200 C for(a)1h and(b)16h, respectively.
Figure3Average lateral grain size of Al2O3in the oxide scale of
Ti2AlC oxidised in dry synthetic air at1200 C as a function of
oxidation time(line to guide the eye).
3.3Oxidation kinetics
The oxidation kinetics of Ti2AlC upon isothermal oxidation
at1200 C in dry N2with20vol.%O2is observed with
TGA,i.e.the weight change of the sample is recorded as a
function of time.In Figure4,this weight change per unit
surface area is displayed.
It reveals that the weight gain(due to uptake of oxygen)is
very fast during the early stages of oxidation and slows down
dramatically at the later stages.In previous studies on the
oxidation kinetics of MAX phases,like Ti2AlC[7,8],
Ti3AlC2[16],Ti3SiC2[17]and Cr2AlC[18],the oxidation
kinetics was formulated as a power law:
D m
A
n?k m te4T
where D m is the weight change,A is the surface area of the
sample and t the isothermal oxidation time.k m denotes the
oxidation rate constant and n is the exponent.The exponent n
versus time t can be derived from the mass change data with
[19]:
n?qelog tTy qelogeD m y ATT:e5T
The result is plotted in Figure5,which shows that n starts at
about7and drops to about5at the early stages of oxidation.
Thereafter,the exponent n gradually decreases with an on
average value of4in the later stages of oxidation.Clearly,
for the oxidation times and temperature considered here,the
development of the oxide scale does neither obey a parabolic
(n equals2)nor a cubic(n equals3)growth rate law.The
high value for the exponent n at the very early stage of
oxidation may be related to the formation of TiO2and the
fast nucleation of a-Al2O3forming a closed layer.The oxide
scale growth rate behaviour at the later stages of oxidation
may be explained with the decrease of fast diffusion paths
due to grain growth of a-Al2O3,cf.Section3.1.
In order to analyse the oxide scale growth in more detail,
the weight change data are converted into oxide scale
thicknesses.Here,it is considered that the oxide scale
growth is dominated by the formation of a-Al2O3(with a
density of3.98g cmà3)according to Eqn[1].The evolution
of the oxide scale thickness with time thus obtained is
presented in Figure6.The oxide scale thickness derived
from the weight change data are in good agreement with the
oxide scale thicknesses as observed in cross sections(see
Figure1).This implies that the oxide scale growth indeed
proceed according to the reaction given in Eqn[1].
For the growth of a compact scale controlled by diffusion
of some species through the oxide scale,with thickness X,
itself is generally written[8]:
d X
d t?
k p
Xe6T
The growth rate‘constant’k p can be expressed as:
k p?O D eff D Ce7T
where O is a constant,D C the concentration gradient across
the oxide scale and D eff the effective diffusion coef?cient,
which comprises lattice(L)and grain boundary(GB)diffu-
sion and equals[20]:
D eff?D Lt3d d D GBe8T
where D L and D GB denote the lattice and grain boundary
diffusion coef?cient,respectively.d and d are the grain
boundary width and lateral grain size,respectively.The
fast initial and subsequent slow oxide scale growth is due
to the change in fast diffusion paths for oxygen.It has been
observed that the oxide scale grain size reduces with
oxidation time,see Figure3.
The experimental data on the evolution of the oxide grain
size with oxidation time do not allow an accurate quantitative
determination of this relation.However,grain growth due to
diffusional process in a?nite solid can often be described
with a parabolic law[8],such as d2tàd20?kt,where d t and
d0are the grain size at time t,d0a virtual grain size for t?0,
and k is a constant.In the case of layer growth,which start Oxidation behaviour of Ti2AlC ceramic:W.G.Sloof et al.
208MATERIALS AT HIGH TEMPERATURES
https://www.wendangku.net/doc/ea11126432.html, Figure4Oxidation kinetics of Ti2AlC in dry synthetic air at
1200 C in terms of weight gain per unit area versus oxidation
time.The insert shows the weight gain during the?rst0.5h.
Figure5Change of the exponent n with time in a power law[cf.
Eqn(4)]describing the oxidation kinetics of Ti2AlC in dry synthetic
air at1200 C.This exponent is derived from the weight gain data in
Figure4using Eqn(5).
with the nucleation of a large number of small grains and
eventually successively grow on the expense of other grains
the relation can be well represented by:d t?d0??t p.
Assuming that the growth of the oxide scale is dominated
by oxygen diffusion along the oxide grain boundaries,then it
holds that:
d X
d t
?O D GB3d
0??t p D C X)
d X
d t
?1??t p k n Xe9T
which after integration leads to:
1
2
X2?2k n??t p)X?2?????k n p?t1=4e10T
Fitting this equation,which corresponds with a value for the
exponent n equal to4[cf.Eqn(1)],to the experimental data
results in a perfect match,see Figure6.Thus,the anomaly in
the oxide scale growth kinetics is indeed due to the change in
fast diffusion paths due to oxide grain growth.
4.CONCLUSIONS
The oxidation of Ti2AlC in dry synthetic air at1200 C
leads initially to an a-Al2O3layer with isolated TiO2
particles on top.At longer oxidation times,the oxida-
tion leads to a further thickening of the a-Al2O3oxide
layer,which prevents Ti atoms diffusing outward and
reacting with oxygen.The continuous growth of the
oxide scale is governed by the following reaction:
4Ti2AlCt3x O2?4Ti2Al1àx Ct2x Al2O3(05x50:5),
while maintaining the MAX phase crystal lattice structure.
Upon high-temperature oxidation of Ti2AlC,uniform
grain growth in the a-Al2O3layer occurs which reduces
the number of fast diffusion paths.The oxide scale growth
kinetics is very fast during the early stages of oxidation,but
decreases drastically once a continuous layer of a-Al2O3is
formed.The growth of the a-Al2O3layer follows
a power law with an exponent equal to4,which
is in agreement with a simple model that accounts
for the reduction in lateral grain size with time at
constant temperature.
ACKNOWLEDGEMENTS
The?nancial support from the Research Program
on Self Healing Materials of the Delft Centre for
Materials research(DCMat)and the Netherlands
Innovation Orientated Program(IOP)on Self
Healing Materials under grant no.IOP-SHM
0871is gratefully acknowledged.
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Oxidation behaviour of Ti2AlC ceramic:W.G.Sloof et al. https://www.wendangku.net/doc/ea11126432.html, MATERIALS AT HIGH TEMPERATURES209
Figure6Oxidation kinetics of Ti2AlC in dry synthetic air at1200 C in terms of
a-Al2O3layer thickness versus oxidation time.The a-Al2O3layer thickness is
derived from the weight gain data(cf.Figure4)adopting the oxidation reaction
Eqn(1)(black curve).The a-Al2O3layer thickness as observed with SEM
analysis of cross sections is also plotted(blue dots).A power law with a value of
4for its exponent is?tted to the experimental data(red curve).This function[cf.
Eqn(10)]takes into account the reduction of fast diffusion paths due to grain
growth in the a-Al2O3layer(cf.Figure3).