Thermal barrier
coating materials
by David R. Clarke† and Simon R. Phillpot‡
Improved thermal barrier coatings (TBCs) will enable
future gas turbines to operate at higher gas
temperatures. Considerable effort is being invested,
therefore, in identifying new materials with even
better performance than the current industry
standard, yttria-stabilized zirconia (YSZ). We review
recent progress and suggest that an integrated
strategy of experiment, intuitive arguments based on
crystallography, and simulation may lead most
rapidly to the development of new TBC materials.
Turbines should operate at as high temperature as
possible to maximize their efficiency. Until about
15 years ago, relentless increases in operating
temperatures were achieved through improved alloy
design, the development of blades composed of
textured microstructures and subsequently single
crystals, and internal cooling by air flow through
internal channels cast into the component. More
recent increases in operating temperatures have been
enabled by deposition of TBCs on high-temperature
gas turbine components1,2. TBCs are complex,
multifunctional thick films (typically 100 µm to 2 mm
thick) of a refractory material that protect the metal
part from the extreme temperatures in the gas (see
Fig. 1). Indeed, in the hottest part of many gas turbine
engines, the coatings enable metallic materials to be
used at gas temperatures above their melting points.
Under such heat flux conditions, it is the thermal
conductivity of the coating that dictates the
temperature drop across the TBC.
†Materials Department,
University of California,
Santa Barbara CA 93106, USA
E-mail: clarke@engineering.ucsb.edu
‡Department of Materials Science and Engineering,
University of Florida,
Gainesville FL 32611, USA
E-mail: sphil@mse.ufl.edu
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June 2005
To illustrate the benefit of TBCs, it has been estimated3
that a 50% reduction in thermal conductivity will reduce the
alloy temperature by about 55°C. This may not seem large,
but it actually corresponds to the increase in hightemperature capability achieved over the last ~20 years by
developments in single-crystal Ni-based superalloys.
The current material of choice for TBCs is YSZ in its
metastable tetragonal-prime structure. Since it has proven to
be a highly durable TBC material, it is likely to remain the
ISSN:1369 7021 © Elsevier Ltd 2005
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material of choice for turbines with current operating
temperatures. However, in anticipation of still higher
operating temperatures, for instance as embodied in the US
Department of Energy’s Next Generation Turbine (NGT)
program, the search is underway for TBCs that will be capable
of operating at higher temperatures and for longer times
than YSZ.
While the primary function of TBCs is as a thermal barrier,
the extremely aggressive thermomechanical environment in
which they must function demands that they also meet other
severe performance constraints. In particular, to withstand
the thermal expansion stresses associated with heating and
cooling, either as a result of normal operation or as a
consequence of a ‘flame-out’, the coatings must be able to
undergo large strains without failure. This ‘strain compliance’
is typically conferred through the incorporation of porosity in
the microstructure by, for example, forming the coating by
electron-beam evaporation or plasma spraying. Another less
stringent but nevertheless rather practical requirement is that
the material must not undergo phase transformations on
cycling between room temperature and high temperatures.
Such phase transformations are usually accompanied by
volume changes, which detract from the strain compatibility
and reversibility of the coating and, hence, its ability to
withstand repeated thermal cycling. Practical TBC materials
must also be able to resist erosion, which calls for high
resistance to fracture and deformation. For air-breathing
engines, which are by far the majority, the coatings must be
able to withstand prolonged high temperatures in an
oxidizing atmosphere. To satisfy this requirement, refractory
oxides are the focus of the search for new and alternative
TBC materials. Another perhaps less obvious requirement is
that the coating material is thermodynamically compatible
with the oxide formed by oxidation of the bond-coat. Indeed,
the choice of Ni-based superalloys for turbine applications is
based largely on their ability to form a slow-growing Al2O3,
under oxidative conditions typical of operation (Al2O3 has
the lowest oxygen diffusivity of the common oxides). This
suggests that compatibility with Al2O3 is an additional
constraint on the choice of new TBC materials, although it is
possible to envisage a two-layer coating, an inner layer
compatible with alumina, and an outer layer capable of
prolonged higher temperature exposure that need only be
compatible with the inner layer. TBCs are outstanding
examples of multifunctional materials.
Fig. 1 Cross-sectional image of a YSZ thermal barrier coating deposited by electron-beam
evaporation on a superalloy. During use at high temperatures, a thermally grown oxide
(TGO) of Al2O3 forms on the metal beneath the TBC.
While failure to meet any of the above performance
criteria can make any potential material unusable as a TBC,
suitable thermal transport properties remain the first design
criterion that must be met. In the remainder of this article,
we focus purely on the thermal transport properties. For a
review of TBCs as complete thermomechanical systems and
for the materials research aspects, see4,5.
Thermal conductivity of
high-temperature materials
Somewhat surprisingly, the experimental investigation of
thermal conductivity at very high temperatures has been a
largely neglected field since the work of Kingery and
colleagues in the 1950s6. They measured the thermal
conductivity of many oxides as a function of temperature and
studied the effects of porosity and of mixing two different
oxides. They also demonstrated that, after correction for the
temperature dependence of thermal expansion, the thermal
conductivity of almost all oxides decreases as 1/T, in accord
with thermal conductivity being controlled by the Umklapp
inelastic phonon-phonon scattering process. The majority of
their measurements (Fig, 2a) do not extend to the
temperatures of interest for future TBCs, but they did find
that three fluorite oxides, YSZ, UO2-x, and Th0.7U0.3O2+x,
exhibit temperature-independent thermal conductivity at
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REVIEW FEATURE
(a)
(b)
Fig. 2 (a) Thermal conductivity versus temperature for several refractory compounds (after9). The upturn A at the highest temperatures is a result of radiative transport through the
material during measurement. (b) Materials usually exhibiting low thermal conductivity.
high temperatures, quite different from other crystalline
oxides but very similar to that of fused silica. (Interestingly,
monoclinic zirconia, which does not contain any stabilizers
and hence no associated structural point defects, exhibits the
classical 1/T dependence caused by Umklapp scattering.) The
absence of the characteristic 1/T dependence was ascribed to
the fact that both YSZ and UO2-x contain very high
concentrations of point defects that scatter phonons. More
recent measurements on materials are shown in Fig. 2b.
High-temperature thermal
conductivity
The thermal conductivity of a material is a measure of heat
flow in a temperature gradient. In the first successful model
for thermal conductivity, Debye used an analogy with the
kinetic theory of gases to derive an expression of the thermal
conductivity7:
(1)
κ = CVνmΛ/3
where CV is the specific heat, νm is the speed of sound, and Λ
is the phonon mean free path. Both Kittel in 19498 and
Kingery in 19559 suggested that the minimum value of the
thermal conductivity at high temperatures was that given by
eq 1 with the phonon mean free path equal to the
interatomic spacing. This simple approach works quite well
because, at temperatures in excess of the Debye temperature
T > ΘD, the specific heat is close to its asymptotic,
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temperature-independent value of CV → 3kB per atom, as
predicted by the Dulong-Petit equation7. Other, more
sophisticated approaches also assume that the major
contribution to thermal conductivity in the high-temperature
regime is caused by phonons whose mean free path is the
interatomic spacing. For instance, Cahill and Pohl10 have
suggested that the value computed from their analysis for
the minimum thermal conductivity differs by just ~20% from
the limit of eq 1, where the phonon mean free path is equal
to the lattice parameter. In a similar way, the low
temperature-independent thermal conductivity of fused silica
and other glasses has been attributed to their random
structure precluding any long-wavelength phonon modes,
with the dominant phonon contributions being limited by the
size of the tetrahedral unit of the glass8,9.
The minimum thermal conductivity for more complex,
multicomponent materials also has a similar form11 and can
be expressed12 as:
(2)
κmin = kBνmΛmin → 0.87kBΩa-2/3(E/ρ)1/2
where Λmin is the minimum phonon mean free path,
Ωa = M/(mρNA) is the average volume per atom, E is the
elastic modulus, and ρ is the density. (The different atoms in
a molecule are replaced with an equivalent atom having a
mean atomic mass given by M/m, where M is the molecular
mass and m is the number of atoms per molecule.) The data
for a variety of materials is plotted in Fig. 3, illustrating that
REVIEW FEATURE
Fig. 3 Minimum thermal conductivity of materials of interest as TBCs, together with other
materials for comparison, calculated using eq 2. (Redrawn from12.)
materials with low thermal conductivity tend to have large
volumes per atom and low specific elastic modulus E/ρ.
A particularly important feature of the minimum thermal
conductivity is that, in contrast to conductivity at lower
temperatures, it is independent of the presence of defects
such as dislocations, individual vacancies, and long-range
strain fields associated with inclusions and dislocations. This
is largely because these defects affect phonon transport over
length scales much larger than the interatomic spacing. This
also means that measurements at low and intermediate
temperatures can be a poor guide to the thermal conductivity
at high temperatures.
YSZ – the current material of choice
The initial choice of YSZ (4 mol% Y2O3) as a TBC material
was largely based on the simple fact that zirconia was one of
the few refractory oxides that could also be deposited as
thick films using the then-known technology of plasmaspraying. The identification of yttria as the optimum
stabilizer and composition then followed from a series of
more exacting testing, especially under thermal cycling
conditions13. Originally, the low and temperatureindependent thermal conductivity of YSZ was attributed to
the presence of a high point defect concentration associated
with the substitution of Zr4+ ions by Y3+ ions in the fluorite
structure, producing a small spacing between point defects.
As an oxygen vacancy is introduced into the zirconia
structure for every two Y3+ ions that substitute for a Zr4+
ion, in 4 mol% YSZ, for instance, the average distance
between oxygen vacancies is only ~1 nm and the average
distance between Y3+ ions is ~0.5 nm.
Recently, a detailed simulation analysis14 of how the
nature of the vibrational modes changes with yttria
concentration has revealed that the picture of the phonon
scattering length being determined by the point defect
spacing alone is too simple. In monoclinic ZrO2, all of the
vibrational modes have well-defined wavevectors and
polarizations, characteristic of normal phonons. However, as
yttria is added to the zirconia, the nature of the vibrational
excitations changes. In particular, most vibrational modes no
longer have a well-defined wavevector or polarization. For
example, there is a phonon-like transverse acoustic mode in
monoclinic ZrO2 at about 3 THz, which shows complete
polarization along the z-direction. In a plot of the normalized
displacement in the x-y and x-z planes, this mode would be
characterized by a single point at the origin in the former
and spots at z = +1 and z = -1 in the latter. The
corresponding mode in 4 mol% YSZ (Fig. 4) shows an almost
completely uniform distribution in the x-y and y-z planes,
indicating an isotropic distribution of vibrations. This
demonstrates that the concepts of polarization and
wavevector are no longer relevant for this vibrational mode,
i.e. it is not a phonon-like mode but is actually better
characterized as a diffusive vibrational wavepacket.
Furthermore, analysis shows that the vast majority of
vibrational modes are similar in that, though they are still
relatively spatially extended modes, they move more slowly
than the sound velocity of phonon-like modes. As a result,
the thermal conductivity is greatly diminished. A further
significant contribution to the thermal conductivity comes
from the few remaining lowest-energy modes, which remain
phonon-like. A few of the high-frequency modes become
spatially localized; these do not contribute to the thermal
conductivity at all. Together, these results are very similar to
those of an earlier analysis of the vibrational modes in
amorphous Si (α-Si)15, and confirm the close correspondence
between the thermal transport mechanisms in the chemically
disordered YSZ and the structurally disordered α-Si. The new
insight that a large concentration of defects changes the
vibrational modes from pure phonons in undoped, monoclinic
zirconia to a variety of other modes in 4 mol% YSZ, even
though the material remains crystalline, may well be
important in identifying new candidate TBC materials.
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REVIEW FEATURE
(a)
(b)
Fig. 4 Scatter plot showing the normalized magnitudes of vibration of each atom in the x-y plane (a) and the y-z plane (b) for the mode close to 3 THz in 4 mol% Y2O3. For a pure
transverse acoustic (TA) phonon mode, all the atoms would vibrate in the z-direction only; this would be indicated by a single point at the origin in (a) and points at z=-1 and +1 in (b).
The larger scatter for this mode indicates that the concepts of polarization and wavevector are no longer meaningful in this material. (Reprinted from14. © 2001 Blackwell Publishing.)
The search for alternative TBC
materials
In seeking potential new TBC materials, it makes sense to
explore other refractory materials. However, since there are
numerous crystal structures known to the mineralogical and
crystal-chemistry communities, and each can be formed from
several different elements, there are literally thousands of
possible compounds to search. Faced with this complexity,
initial attempts have focused on exploring oxides with
structures related to zirconia. More recently, the search has
been broadened by using insights from atomistic simulations
and crystal chemistry.
Fluorite oxides
A natural place to look for other TBC materials is among
fluorite-structured materials. The obvious candidates include
HfO2, CeO2, and ThO2; UO2 and transuranic fluoritestructured oxides are precluded for obvious reasons. Also,
although doped ceria exhibits comparable thermal
conductivity, it is not a practical choice because of
volatilization. Measurements on both HfO2 and ThO2 are
similar to those on monoclinic ZrO2.
However, recent research has shown that co-doping
zirconia and hafnia can result in reductions in thermal
conductivity. The most intriguing observations have been
made on co-doping YSZ with a mixture of one trivalent ion
larger than Y3+ and another trivalent ion smaller than Y3+,
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June 2005
while still preserving the metastable zirconia structure16.
Similarly, reductions in thermal conductivity have been
reported17 for compositions in which some of the Zr4+ is
replaced with Hf4+. While the measurements have been made
on porous coatings rather than dense materials, hence the
contribution to the low thermal conductivity from porosity is
unknown, the results indicate that these materials warrant
further investigation.
Pyrochlore oxides
Since the fluorites do not offer any other viable candidate
materials, attention has turned to the pyrochlores,
A23+B24+O7, because several zirconate pyrochlores have
lower thermal conductivity than YSZ3. This class of materials
is also of fundamental interest because of the close
relationship between the fluorite and pyrochlore structures
(Fig. 5). The pyrochlore unit cell may be viewed as eight
fluorite unit cells, each of which contains, on average, a single
oxygen vacancy. The close relationship between the fluorite
and pyrochlore structures is well illustrated by the yttriazirconia system. The pyrochlore Y2Zr2O7 is actually unstable
to the disordered fluorite material (ZrO2)2-Y2O3, i.e. heavily
doped YSZ. However, replacing the Y3+ ion with larger ions,
such as La3+ or Gd3+, results in a stable pyrochlore structure
up to at least 1500°C. Likewise, replacement of the Zr4+ ion
by a smaller ion, such as Ti4+ or Mo4+, also stabilizes the
pyrochlore structure.
REVIEW FEATURE
(a)
(b)
Unoccupied 8a
Fig. 5 (a) The unit cell of the high-temperature cubic phase of zirconia has the fluorite structure, with O ions shown in red and the smaller Zr ions shown in yellow. (b) One-eighth of the
unit cell of the pyrochlore, A2B2O7 structure, with the oxygen in red, the B4+ ions in yellow, and the A3+ ions in blue.
The pyrochlores are also attractive because many are
refractory up to temperatures well in excess of 1500°C and
thermally stable. Moreover, they can be formed from a wide
range of cations18, since the A site can have a notional charge
of 3+ or 2+ and the B site cation can have a valence of either
4+ or 5+. Consequentially, there can be extensive
intermixing of different ions on the same crystallographic
sites.
Thermal conductivities ranging from ~1.1 W/mK to
~1.7 W/mK at temperatures between 700°C and 1200°C
have been reported for zirconates of Gd, Eu, Sm, Nd, and
La19-22. Although other pyrochlore compositions have yet to
be measured, they have been explored extensively using
atomistic simulations23. The predicted thermal conductivities
are shown in Fig. 6. These studies suggest that the zirconates
may indeed have the lowest thermal conductivities of the
stable pyrochlores – the plumbate pyrochlores have lower
thermal conductivities, but they decompose easily and are
unsuitable for environmental reasons.
Co-doping of pyrochlores on both A and B sites has been
proposed to further reduce their conductivity and also to
modify their thermal expansion coefficients22,24. Some
success has been achieved, at least up to ~800°C, as
exemplified by studies in which La2Zr2O7 was doped with
30 at.% of Nd, Eu, or Gd and the thermal conductivity
reduced from ~1.55 W/mK to ~0.9 W/mK for Gd doping. For
comparison, the undoped, fully dense stoichiometric Nd, Sm,
and Gd zirconates25, and 97% dense La2Zr2O720, were all
found to have essentially the same conductivity
(~1.5-1.6 W/mK) at 700°C. Whether this temperature is
sufficient to evaluate if the minimum thermal conductivity
has been attained was not reported, but any further
decreases are unlikely to be significant. Interestingly, site
disorder alone does not appear to have much effect, at least
Fig. 6 Contour map of thermal conductivity κ as a function of the ionic radii of the A and B ions for pyrochlores A2B2O7, determined at T=1200°C by simulation. (Reprinted with permission
from23. © 2004 Taylor and Francis.)
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REVIEW FEATURE
in dense Gd2Zr2O7; suitably heat-treated, this compound can
be produced as either the pyrochlore or its fluorite allotrope,
and yet the thermal conductivity was the same within
experimental accuracy25. Moreover, thermal conductivity was
found to vary only slightly over a wide range of
(ZrO2)2-Gd2O3 compositions away from the pyrochlore
stoichiometry, especially at high temperature25. Together,
these observations raise the intriguing question as to what
factors determine whether co-doping can lead to significant
reductions in high-temperature thermal conductivity.
Other oxides
Apart from the fluorites and pyrochlores, many other oxide
compounds have been proposed as candidate lowconductivity materials. These include the garnets
(Y3AlxFe5-xO12)26, monazite (LaPO4)27, and the
magnetoplumbite lanthanum hexaaluminate (LaMgAl11O19)28.
While they all have rather low thermal conductivity
(<~3 W/mK), none offer the prospect of compositions with
lower conductivity than the pyrochlore zirconates.
In contrast to these other classes of oxide, the perovskites,
ABO3, comprise a class of crystal structures that can
accommodate a wide variety of different ions in solid solution,
including ions with large atomic mass. Many compositions
are stable to very high temperatures. Although some
members exhibit rather low thermal conductivity at high
temperatures, none has yet been found to have conductivity
as low as the zirconate pyrochlores. One explanation is that
the perovskite structure is more rigid, as the octahedra are
corner sharing. Several that appear promising on the basis of
their mean molecular weight unfortunately undergo phase
transitions at intermediate temperatures. For instance,
SrZrO3 transforms from orthorhombic to pseudo-tetragonal
at about 730°C, accompanied by a change in volume20.
However, one hopeful development in the area of perovskites
is a recent report of very low thermal conductivity in a
coating made of a layered perovskite with Ruddlesden-Popper
structure29. While fully dense materials have not been
studied, the very wide range of potential compositions in this
class of crystal structure and the possibility of forming a
variety of other layered structures, there is plenty of scope
for further work on perovskites.
Glasses and nanocrystalline materials
For many years, it was considered that the high-temperature
conductivity of silica glass represented the lower limit – the
so-called amorphous limit – to the thermal conductivity of
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materials at high temperatures. Although amorphous
materials can have significantly lower thermal conductivity at
room temperatures than their crystalline counterparts, the
data suggest that the difference is not significant at
temperatures well in excess of the Debye temperature.
Nevertheless, some have advocated using nanocrystalline
materials, as they offer the potential of limiting thermal
conductivity by incorporating grain boundary scattering as an
extrinsic phonon-scattering phenomenon. This may prove to
be an effective strategy for many materials, but there is not
yet any definitive evidence at elevated temperatures. A first
study of nanocrystalline-stabilized zirconia ceramics has
indicated that there is no grain size effect30, while a later
study on films has shown a significant effect for grains below
~25 nm31. Even so, the benefit conferred by being
nanocrystalline was less than that associated with the
disorder of Y ions and O vacancies in YSZ.
A major concern with amorphous and nanocrystalline
materials is their long-term stability at elevated temperature:
glassy materials tend to crystallize and nanocrystalline
materials tend to rapidly coarsen and lose any initial
advantages of being nanocrystalline. Nanocomposites may be
more resistant to coarsening, but early investigations of
zirconia/alumina laminates indicated that they became
spheroid and then coarsened. Nevertheless, the potential of
nanocomposites for thermal barrier applications has not been
explored systematically. Of particular interest would be
nanoporous materials that resist coarsening of the porosity
while retaining their fracture toughness.
Future directions
The search for low thermal conductivity materials for thermal
barriers is only just beginning. The vast range of chemical
compositions of all refractory oxides and minerals precludes a
purely Edisonian approach to identifying promising
compositions; it is simply too time consuming and costly. The
most rapid progress will probably only be made by using a
combination of intuition about crystal structures and
complementary atomic-level simulations to guide
experiment. Two recent examples serve to illustrate this.
The first is the systematic investigation of pyrochlore
compositions using simulations, resulting in the predicted
thermal conductivities mapped in Fig. 6. The capabilities of
such simulations are illustrated in Fig. 7, which compares the
calculated and experimental values of the thermal
REVIEW FEATURE
Fig. 7 There is a strong linear correlation between experimental and calculated values of
the thermal conductivity, with the calculated values being typically ~10% larger. The data
are for the five experimentally investigated pyrochlores (circles) and for YSZ (square).
conductivity for YSZ and pyrochlore materials. Although the
simulated values of the thermal conductivity are uniformly
somewhat higher than the experimental values, they are
highly correlated. While the higher values could be the result
of limitations in the simulation approach, they are most likely
to be the result of the absence of chemical disorder,
impurities, and, in particular, porosity in the simulations.
The other example is the recent identification of a
previously unconsidered composition, based on the following
heuristic argument. The search for prospective, low thermal
conductivity materials mirrors, in some regards, the search
for materials with high thermal conductivity. Based on the
observations that diamond, beryllium oxide, and aluminum
nitride all exhibit high thermal conductivities at and around
room temperature, it has been concluded that highconductivity materials will have low atomic weight, highly
directional, covalent bonding, isotopic purity, and high
specific elastic modulus. So, conversely, materials with low
thermal conductivity at high temperatures can be expected
to have the opposite characteristics: high average atomic
weight, loose bonding, and highly disordered and distorted
structures. YSZ and glasses have many of these attributes. It
also suggests crystal structures containing elements of high
atomic weight that, although crystalline, would exhibit
considerable vibrational freedom of some of their structural
subunits. One such material, W3Nb14O44, which has the
tungsten bronze structure, has recently been identified on
this basis32. It consists of blocks of corner-shared octahedra
with Nb and W distributed among the octahedral sites and W
atoms in the channels between blocks. As shown in Fig. 2b,
its thermal conductivity is intermediate between zirconia and
the pyrochlore zirconates. This suggests that this heuristic
argument can be used for the initial identification of
candidate materials for simulation and testing. MT
REFERENCES
1. National Research Council, Coatings for High Temperature Structural Materials;
Trends and Opportunities, National Academy of Sciences, Washington, DC,
(1996)
15. Allen, P. B., et al., Philos. Mag. B (1999) 79 (11-12), 1715
16. Zhu, D., and Miller, R. A., Ceram. Eng. Sci Proc. (2002) 23 (4), 457
2. Jones, R. L., In Metallurgical and Ceramic Protective Coatings, Chapman and
Hall, London, (1996)
17. Murphy, K. S., US patent 0 118 873 (2003) A1
3. Maloney, M. J., Thermal Barrier Coatings Systems and Materials, US Patent
6 117 560, (2000)
19. Maloney, M., US Patents 6 177 200 and 6 231 991 (2001)
4. Clarke, D. R., and Levi, C. G., Annu. Rev. Mater. Res. (2003) 33, 383
5. Evans, A. G., et al., Prog. Mater. Sci. (2001) 46 (5), 505
6. Many of these results are summarized in Kingery, W. D., et al., Introduction to
Ceramics, 2nd edn, Wiley Interscience, New York, (1976)
7. See, for example, Kittel C., Introduction to Solid State Physics, Wiley
Interscience, New York, (1995)
8. Kittel, C., Phys. Rev. (1949) 75, 972
18. Hyde, B. G., et al., Mater. Sci. Technol. (1994) 11, 339
20. Vassen, R., et al., J. Am. Ceram. Soc. (2000) 83 (8), 2023
21. Suresh, G., et al., J. Alloy Compounds (1998) 269 (1-2), L9
22. Lehmann, H., et al., J. Am. Ceram. Soc. (2003) 86 (8), 1338
23. Schelling, P. K., et al., Philos. Mag. Lett. (2004) 84 (2), 127
24. Vassen, R., et al., International Patent Application, WO 02081768 (2002) A2
25. Wu, J., et al., J. Am. Ceram. Soc. (2002) 85 (12), 3031
26. Padture, N., et al., J. Am. Ceram. Soc. (1997) 80 (4), 1018
9. Kingery, W. D., J. Am. Ceram. Soc. (1955) 38 (7), 251
27. Sudre, O., et al., Ceram. Eng. Sci. Proc. (2001) 22 (3), 367
10. Cahill, D., and Pohl, R., Solid State Commun. (1989) 70 (10), 927
28. Friedrich, C. J., et al., J. Therm Spray Tech. (2001) 10 (4), 592
11. Clarke, D. R., Surf. Coat. Technol. (2003) 163-164, 67
29. See http://atfinet.com
12. Levi, C. G., Curr. Opin. Solid State Mater. Sci. (2004) 8 (1), 77
30. Raghavan, S., et al., Scripta Mater. (1998) 39 (8), 1119
13. Stecura, S., NASA Report TM-86905, NASA, Cleveland, (1985)
31. Yang, H.-S., et al., Acta Mater. (2002) 50 (9), 2309
14. Schelling, P. K., and Phillpot, S. R., J. Amer. Ceram. Soc. (2001) 84 (12), 2997
32. Winter, M., PhD Thesis, University of California, Santa Barbara, in preparation
June 2005
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