by Professor Jim Low
Centre for Materials Research, Department of Imaging and Applied
Physics, Curtin University of Technology, GPO Box U1987, Perth, WA, 6845,
MAX phases exhibit a unique combination of characteristics of both ceramics
and metals with unusual mechanical, electrical and thermal properties1-3. These materials are nano-layered ceramics with the
general formula Mn+1AXn (n = 1 - 3), where
M is an early transition metal, A is a group A element, and
X is either carbon and/or nitrogen. The unique combination of these
interesting properties enables these ceramics to be promising candidate
materials for use in diverse fields which include automobile engine components,
heating elements, rocket engine nozzles, aircraft brakes, racing car brake pads
and low-density armour .
However, the high-temperature stability in MAX phases has hitherto generated
much controversy among researchers. For instance, several researchers have
reported that Ti3SiC2 became unstable at
temperatures greater than 1400ºC in an inert atmosphere by dissociating into Si,
Ti5Si3Cx4-8. A similar phenomenon has also been observed for
Ti3AlC2 whereby it decomposes in vacuum to
form TiC and Ti2AlC9-12.
In other studies, Zhang et al.13 reported
Ti3SiC2 to be thermally stable up to 1300ºC
in nitrogen, but above this temperature drastic degradation and damage occurred
due to surface decomposition. Feng et al.14
annealed the Ti3SiC2-based bulk samples at
1600ºC for 2h and 2000ºC for 0.5 h in vacuum (10-2 Pa) and found that
TiCx was the only phase remaining on the surface. According to
Gao et al.15 the propensity of decomposition
of Ti3SiC2 to TiCx was
related to the vapour pressure of Si, i.e., the atmosphere where the
Ti3SiC2 exits. They believed that the
partial pressure of Si plays an important role in maintaining the stability of
Ti3SiC2 whereby it has a high propensity to
decompose in N2, O2 or CO atmospheres at
temperatures above 1400ºC. This process of surface-initiated phase decomposition
was even observed to commence as low as 1000 - 1200ºC in
Ti3SiC2 thin films during vacuum annealing16. The large difference in observed decomposition
temperatures between bulk and thin-film
Ti3SiC2 has been attributed to the
difference in diffusion length scales involved and measurement sensitivity
employed in the respective studies. In addition,
Ti3SiC2 has also been observed to react
readily with molten Al, Cu, Ni and cryolite
(Na3AlF6) at high temperatures.
In contrast, Barsoum and co-workers17 have shown
that Ti3SiC2 was thermodynamically stable up
to at least 1600ºC in vacuum for 24h and in argon atmosphere for 4h. They
further argued that the reduced temperature at which
Ti3SiC2 decomposed as observed by others was
due to the presence of impurity phases (e.g. Fe or V) in the starting powders
which interfered with the reaction synthesis of
Ti3SiC2, and thus destabilized it following
prolonged annealing in an inert environment18.
However, mixed results have been reported by Radhakrishnan et al.19. In their investigation,
Ti3SiC2 was shown to be stable in a
tungsten-heated furnace for 10h at 1600ºC and 1800ºC in an argon atmosphere, but
dissociated to TiC x under the same conditions when using a graphite heater.
These conflicting results suggest that the thermochemical stability of MAX
phases is still poorly understood although its susceptibility to thermal
decomposition is strongly influenced by factors such as:
- Purity of powders and sintered materials
- Vapour pressure
- Atmosphere, and
- The type of heating elements used.
In addition, the nature of the microstructure of the decomposed surface layer
formed during annealing remains controversial, especially in relation to the
role of pore sizes in the decomposition kinetics at the near surface.
In this article, the use of in-situ neutron diffraction to investigate the
effect of vacuum annealing on the kinetics of thermal stability of several MAX
phases in the temperature range 1000-1800°C is described. The role of pore size
on the kinetics of phase decomposition is discussed.
Thermal Stability and Phase Transitions of MAX Phases
The phase transitions in several MAX phases and their relative phase
abundances at various temperatures as revealed by in-situ neutron diffraction is
shown in Figure 1. A weight loss of ~ 4% was observed for decomposed
Ti3SiC2 which may be attributed to the
release of gaseous Ti and Si by sublimation during the decomposition process.
For Ti3AlC2, its decomposition into TiC and
Ti2AlC as lower order or intermediate phase was observed at
≥1400°C. However, at higher temperatures, when compared to TiC, a smaller growth
rate for Ti2AlC may indicate that Ti2AlC
experienced further decomposition into TiC via the sublimation of Al, similar to
decomposition of Ti3SiC2. In contrast to
Ti3AlC2, no intermediate or lower order
phase was observed for the decomposition of
Ti3SiC2. This difference can be attributed
to the fact that Ti3SiC2 is the only stable
ternary phase in Ti-Si-C diagram.
Research conducted in our laboratories showed that a weight loss of up to 20%
was observed as a result of decomposition for all the MAX phases can be
attributed to the release of gaseous Al by sublimation during the decomposition
process because the vapour pressures of the A elements exceed the ambient
pressure of the furnace (i.e. ≤5x10-5 torr) at ≥ 1500°C. Since the
vapor pressure of a substance increases non-linearly with temperature according
to the Clausius-Clapeyron relation, the volatility of A elements will increase
with any incremental increase in temperature .
Figure 1. Phase abundance as a function of temperature
for the decomposition of (a) Ti3SiC2, (b)
Ti3AlC2, (c) Ti2AlC, and
(d) Ti2AlN in vacuum.
It is well known that A elements such as Si and Al have high vapour pressures
and become volatile at elevated temperatures. Thus, at temperatures of well over
1500°C used in this study, both Al and Si should become volatile and sublime
readily and continuously in a dynamic environment of high vacuum. When the vapor
pressure becomes sufficient to overcome the ambient pressure in the vacuum
furnace, bubbles will form inside the bulk of the substance which eventually
appear as voids on the surface of decomposed MAX phase. The evidence of surface
voids formation can be clearly discerned from the scanning electron micrographs
of decomposed MAX phases shown in Figure 2 . Since Si has a lower vapour
pressure than Al, it helps to explain why
Ti3SiC2 is more resistant to decomposition
than Ti3AlC2 or Ti2AlN. In
all cases, the kinetics of decomposition process are driven mainly by a highly
restricted out-diffusion and sublimation of high vapour pressure A
element (e.g. Al, Si) from the bulk to the surface of the sample and into the
Mn+1Xn + A
Figure 2. Scanning electron micrographs of the surface
microstructures of vacuum-decomposed MAX phases; (a) Ti2AlN,
(b) Ti4AlN3, (c)
Ti3SiC2 and (d)
Role of Pore Size on Decomposition Kinetics
Research conducted in our laboratories show that all the calculated
activation energies are positive except for bulk
Ti3AlC2. However, when
Ti3AlC2 powder was used a positive
activation energy was obtained which implies the importance of pore
microstructures in the decomposition kinetics. Table 1 shows that the activation
energies calculated from the Arrhenius equation for several MAX phases and the
proposed reactions. A negative activation energy indicates that the rate of
decomposition in Ti3AlC2 decreased with
increasing temperature due to the formation of a dense TiC surface layer with
very fine pores (<1.0 µm) which exert an increasing resistance to the
sublimation process as the temperature increases (Fig. 2d). In contrast, a more
porous decomposed layer with coarser pores (>2.0 µm) formed in other MAX
phases and in powdered Ti3AlC2 which enabled
the sublimation of Al or Si to progress with minimum resistance, thus resulting
in an increasing rate of decomposition with temperature. In short, the pore
sizes play a critical role in determining the value of activation energy and the
rate of decomposition. Thus, the ability to manipulate the pore microstructure
either through densification to reduce pore-size or engineering of pore-free
microstructures will allow the process of decomposition in MAX phases to be
minimized or arrested.
Table 1. Comparison of the kinetics of thermal
decomposition in MAX-phase samples.
1.0 - 3.0
Ti3SiC2 --> 3TiC0.67
0.5 - 0.8
2.0 – 10.0
Ti2AlC --> 2TiC0.5
2.0 – 8.0
1.8 – 3.0
All the calculated activation energies are positive except for
Ti3AlC2. A negative activation energy indicates that the
rate of decomposition in Ti3AlC2 decreased with increasing
temperature due to the formation of a TiC surface layer with very fine pores
(<1.0 µm) which exert an increasing resistance to the sublimation process as
the temperature increases. In contrast, coarser pores (>5.0µm) formed in
other MAX phases which enable the sublimation of Al or Si to progress with
minimum resistance and thus an increasing rate of decomposition with
temperature. In short, the pore sizes play a critical role in determining the
value of activation energy and the rate of decomposition.
The kinetics of isothermal phase decomposition as modelled using the Avrami
equation and the Avrami exponents (n) of isothermal decomposition of the
MAX phases are shown in Table 2. The low values (i.e. <1) of the exponent
indicate that in all cases the decomposition is a highly restricted diffusion
process, presumably of Al or Si from the bulk of the sample to its surface.
Table 2. Comparison of the Avrami decomposition kinetics
in MAX phases.
Avrami exponent (n)
Avrami constant (k) mol% (min)
Before MAX phases can be widely used in extreme environments, the issues
pertaining to their susceptibility to thermal decomposition need to be fully
addressed. In particular, there remain several unresolved issues relating to the
phase and thermal stability that require further study:
- The vapour pressure of element A is critical to the phase stability
of MAX phases. The higher the vapour pressure of element A, the
more susceptible the MAX phase is to phase dissociation at elevated
- The Avrami kinetics of phase dissociation is dependent on the rate of
removal of vapourised element A. A dynamic atmosphere with a flowing gas
or in high vacuum will facilitate the continual removal of the vapourised
A and thus the continuous dissociation of the MAX phase. In contrast, a
static atmosphere is expected to be most conducive for a MAX phase to resist
- The role of microstructural modification due to phase dissociation on the
mechanical performance of MAX phases. It remains unknown how microstructural
changes will affect the mechanical properties. New stabilizers will be
formulated to arrest the susceptibility of MAX phases to thermal dissociation at
elevated temperature. TiSi2 is an effective stabilizer for
- Development of improved models to describe the chemical processes and
kinetics of phase dissociation. No such models exist currently that can
adequately describe and predict the property modification, especially for the
Fundings from the Australian Research Council (
ISIS and AINSE (P329, P606, P1431 & MI1488 ) are acknowledged. Also thanked
are contributions from Drs W.K. Pang, R. Smith, V. Peterson, S. Kennedy and
E/Prof. B. O’Connor.
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