Application of phase diagrams in development of transparent
polycrystalline oxides and thermal barrier coating
Phase diagrams for production of
transparent polycrystalline materials
Phase diagrams for thermal barrier coating
Phase chemistry in the development of transparent
polycrystalline oxides
Polycrystalline oxides are used for optical applications. The Al2O3 is a key element in
high pressure sodium lamp. Number of oxides have been developed for optical
applications. They are Al2O3, MgAl2O4, AlON, Y2O3. The aim is to obtain transparent
polycrystalline single phase without porosity.
1. Y2O3
High melting point and capacity for wideband transmittance of Y2O3 presents interest
for many infrared applications. Transparent Y2O3-Gd2O3 are unique X-ray scintillators.
Pure un-doped Y2O3 can be fabricated to transparency by employing combination of
fine active powders and high – temperature press forging and hot isostatic pressing.
Success of these approaches relies not only on powder properties and application of
pressure to enhance densification kinetics, but also on high purity to prevent
precipitation of second phases or coloration from transitions or rare earth ions. Phase
relations become critical when pressure-less method of sintering is selected to attain
transparency. The options are liquid phase sintering, transient second phase sintering
and doped solid state sintering. Non coloring aliovalent sintering aids for Y2O3 among
rare earths is restricted to La and Gd, because these ions have no electronic transitions
in the visible or infrared frequencies.
Y2O3 with La2O3 additive
Phase diagram was calculated using assessed
thermodynamic data and reproduce available
experimental data very well.
Composition with 9 mol.% La2O3 was prepared
by co-precipitation from the oxalates
[1981Rhodes]. After calcination to get oxides,
milling and isostatic pressing to get compacts,
samples were heated up to 2150 °C in two
phase C+H region with 25 vol.% H phase. Grain
growth was retarded by the second phase
formation. Pores were thought to remain
attached to slow moving grain boundaries and
annihilate by classic solid-state diffusion
process. Temperature was lowered to 1900 °C
inside single phase C solid solution field.
Holding at this temperature dissolved H phase
by slow diffusion process. Change was not
displacive nor accompanied by volume change
generating porosity. Instead pore free
microstructure was achieved.
Calculated phase diagram of the Y2O3-La2O3 system
Single phase microstructure achieved by sintering at 2150 in twophase region and annealed at 1900°C in single phase region , (A)
as polished and (B) as etched [1981Rhodes].
The Y2O3-Gd2O3 solid solutions for optical applications
Scintillators are the materials which after being
exited by ionizing radiation re-emit the absorbed
energy in the form of light. Ceramic scintillators
contain Y2O3-Gd2O3 solid solutions and one or more
rare-earth activation oxides such as Eu2O3, Yb2O3 etc.
The Gd2O3 is essential in absorbing X-rays while
activators undergo electronic transitions that convert
the energy to visible wavelengths for photodiode
detection.
According to the phase diagram of Y2O3-Gd2O3
system cubic(C)/monoclinic (B) phase boundary is
~1500°C at ~20 mol.% Y2O3. Ceramic processing
temperatures such as powder synthesis (~1300°C)
and sintering (1400°C) are lower and cubic phase is
maintained.
XRD shows that at 10 mol.% Y2O3 the
B phase was produced. The
measurements showed that light
output was gradually increased with
Y2O3 decrease down to 20 mol.% Y2O3
and then abruptly reduced because of
lower scintillator conversion of B
phase. The optimal composition is 20
mol.% Y2O3 and 3 mol.% Eu2O3.
Phase diagram of Y2O3-Gd2O3 system [2007Zin]
X-ray data and relative light output from [2004Kim]
Al2O3
Translucent Al2O3 is a key element in high-pressure sodium lamps. High-pressure
discharge experiments in sapphire tubing show considerable resistance to chemical
attacks and improved color emission. Translucent Al2O3 envelope is used to prevent Na
from attacking SiO2 in any glass.
Al2O3-MgO
Presence of MgO enhanced sintering by 1 to 2 %.
Without MgO grain growth kinetic was unaffected prior
discontinuous grain growth, which occurred at 99%
density. MgO allowed normal grain growth up to
achievement of theoretical density. Role of MgO is to
decrease grain-boundary mobility and therefore poreboundary tendency for separation (pore entrapment
within grains). MgO increases densification rate and
grain-growth rate as well as surface diffusivity which
keeps the pores on the boundary until they are
annihilated by solid-state diffusion. MgO is
microstructure stabilizer against green density variation,
which might lead to porosity and other in-homogeneities.
Sintered microstructure
of Al2O3: Low porosity
high-transmittance
resulting from MgO
sintering aid
Al2O3-MgO
Phase diagram shows very small solubility of MgO
in Al2O3. Addition of TiOx and MgTiO3 increases
solubility more than MgO alone. The ZrO2+MgO
addition to Al2O3 also increases the solubility.
Solid solution limit is an issue in development of
translucent Al2O3 because MgAl2O4 precipitates
act as light scattering centers due to their index
of refraction difference with Al2O3. However pores
are much greater scattering centers. Spinel
MgAl2O4 precipitates in Al2O3 with 0.08 mass%
MgO sintered at 1900°C in H2. According to phase
diagram this composition should be within
solubility range, but MgAl2O4 possibly precipitate
during cooling.
Using of the Y2O3 in combination with MgO as
sintering aid for translucent Al2O3 allow to
decrease sintering temperature and time, but the
grain growth is more difficult to control. In case
of sintering above eutectic temperature the
porosity is reduced, but presence of liquid results
in bimodal grain size distribution.
Solubility limit (mass.% MgO) in corundum
Spinel MgAl2O4
Transparent MgAl2O4 spinel has been examined for
potential applications as lamp envelopes, watch lenses,
bullet-proof windows, pressure vessel windows, optical
components and infrared windows.
The hardness of spinel is 16.1 GPa which is slightly less
than sapphire. Spinel has cubic structure which means
that single crystals has no optical anisotropy. Phase
diagram show substantial deviation from stoichiometric
composition of MgAl2O4 at high temperature.
Maximum transmittance in case of
composition deviation in MgO-rich
side is close to stoichiometric
composition MgAl2O4 and not
related to solid solution limits. In
case of deviation in Al2O3 rich side
grain size is maximal at the border
Sp/Sp+Cor. Grain growth control is
difficult and it is a key to the
achievement of transparent spinel.
One of possibilities to improve
microstructure of MgAl2O4 is
sintering with the aid of LiF.
Calculated phase diagram of
the MgO-Al2O3 system
Transmittance of spinel vs.
MgO content
Average grain size vs. Al2O3 content
in 1640-1685°C
Thermal barrier coating
Application of thermal barrier coating (TBC) enable to increase
operating temperature of gas turbines and therefore increase their
efficiency. TBC is complex multi-layered system consisting from top
coat (ZrO2 partially stabilised by Y2O3 with tetragonal structure),
bond coat (BC) chemically modified surface of Ni super-alloy and
thermally grown oxide (TGO) which is dense layer of Al2O3 grown
during service by thermal oxidation.
Top coat is 125-250 mm layer of porous ZrO2 stabilised with 7 mass.% Y2O3 applied either by
air-plasma spray(APS) or electron-beam physical vapour deposition (EB-PVD). Both
processes lead to porous microstructure that benefit the thermal insulating efficiency and
allow the coating to tolerate the thermal stress associated with the cycling temperatures in
the engine. Bond coats are classified into two groups. (1) single phase b-(Ni,Pt)Al, applied by
electrodeposition of Pt and subsequent aluminising by chemical vapour pressure deposition
(CVD) and (2) overlay two-phase (g’+b/g) MCrAlY (M=Ni,Co), applied by low pressure plasma
spray or EB-PVD. TGO is thin layer (<10 mm) of Al2O3 which play environmental protection
role against oxidation and corrosion.
Current technology is based on one thermal
barrier material 7YSZ. The selection of this
composition is based on longest durability.
Higher Y content would improve insulating
property, but durability is more important.
Microstructure of EB-PVD coating 7YSZ
Phase diagram of the ZrO2-Y2O3 system
7YSZ = 7 mass.% Y2O3 = 4 mol.% Y2O3 = 7.6 mol.% YO1.5
Te
Fe
T‘
F‘
Fluorite structure
(F)
Phase t‘ is tetragonal phase containing more Y
than equilibrium t phase. Equilibrium tetragonal
phase will transform to monoclinic phase on
cooling, while t‘ is stable (non-transformable).
Phase diagram of the ZrO2-Y2O3 system: stable and
metastable phase boundaries
Stabilization mechanism of
metastable phases t‘ and t‘‘
Phase diagram of the ZrO2-Y2O3 system
according to experimental data and calculations.
At temperatures below 1200°C phase
equilibrium can not be attained. Therefore
experimental data (DTA, dilatometry, HT-XRD)
are referred to diffusion-less transformations at
temperatures below 1200°C.
Metastable-stable phase diagram in the ZrO2-YO1.5
system [1996Yashima]
New materials for TBC
Experimental studies show that YSZ co-doped with other rare earth and pyrochlores RE2Zr2O7
demonstrate lower thermal conductivity than 7YSZ.
1. Co-doping of YSZ by rare earth oxides
2. Rare earth zirconates with pyrochlore structure
3. Other materials
Stability of pyrochlores increases with radius of RE+3:
La2Zr2O7 melts congruently, while other pyroclores
transform to fluorite structure below liquidus.
Temperature of transformation of pyrochlore to fluorite
decreases with decrease of ionic radius.
r(RE+3)
Gd
Sm
Nd
La
Calculated ZrO2-rich part of ZrO2-REO1.5 phase diagrams along
with T0-lines.
Composition of phase should
correspond to non-transformable
range, i.e. it should not intersect
T0(T/M) curve at ambient
temperatures. With the ionic radius
increase T0(F/T) are shifting to higher
RE content. For La position of T0(T/M)
substantially shifted to higher RE
concentration thus making tetragonal
phase transformable to TM
disruptive transformation. The other
important issue is stability of T’ phase
against partitioning to equilibrium
assemblage at high temperatures,
because equilibrium tetragonal phase
transforms to monoclinic phase on
cooling.
r(RE+3)
Gd
Sm
Nd
La
Stability of RE stabilized ZrO2.
Driving force for precipitation to stable phase
equilibrium assemblage T+F: a RESZ RE=Gd, Sm,
Nd, La; b- ZrO2-(Y1-xGdx)O1.5.
Phase stability of singly doped zirconias
(circles), and co-doped ZrO2-(Y0.5M0.5)O1.5
(diamonds). The curves present the
maximum temperature before significant
monoclinic formation was observed.
The effect of different RE stabilizers on the resistance of TBC against partitioning to equilibrium
assemblage and thus ensuing monoclinic transformation is shown in the figures. The maximal driving
force is for La. The equilibrium tetragonal phase resulting from partitioning of T‘/F‘ is transformable and
jeopardize durability of coating. Co-doped materials do not show partitioning at temperature below
1500°C except for La co-doped ZrO2. Durability of 7YSZ is higher.
Compatibility of TBC top coat with TGO
7YSZ is chemically compatible with Al2O3. However thermal
expansion misfit between Al2O3 and 7YSZdecrease durability. For
new candidate materials chemical compatibility with Al2O3 should
be checked.
Experimental results for diffusion couple between Gd2Zr2O7 and Al2O3
heat treated at 1200°C [2005Leckie].
In the ZrO2-RE2O3 systems RE2Zr2O7 with pyrochlore structure
forms for RE=La-Gd and d-RE4Al3O12 for RE=Y, Er-Lu. In the RE2O3Al2O3 systems REAl11O18 with b-alumina structure forms for RE=La,
Nd (b-alumina for Nd at high T) , REAlO3 perovskite for RE=Nd,
Sm-Gd and RE3Al5O12 garnet for RE=Y, Er-Lu. Phase diagrams show
that RE2Zr2O7 should react with Al2O3 (TGO). Therefore
pyrochlores can be used as TBC only by incorporating diffusion
barrier (usually 7YSZ). 7YSZ is compatible with Al2O3 since no
compounds appear at the interface. Phase diagram shows that
YSZ compatibility limit (X*) at 1200°C is 20 mol. % YO1.5 when YAG
appears in equilibrium with fluorite and Al2O3. Compatibility limit
of GdSZ is 32 mol.% GdO1.5 when GAP (perovskite) appears in
equilibrium with fluorite and Al2O3. The lowest compatibility limit
for LaSZ is equal to ~5 mol.% LaO1.5, when b-alumina appears in
equilibrium with tetragonal phase and Al2O3.
Compatibility of TBC with TGO and phase diagrams of
the ZrO2-RE2O3-Al2O3 systems
Other additives of YSZ for TBC applications
The TiO2 additions to YSZ enhances the
toughness of YSZ and its durability of coating.
The TiO2 is one of small number of dopants that
increase tetragonality of zirconia solid solutions
without compromising its phase stability. Codoping with Y+3 and Ti+4 yields the compositions
that do not transform to monoclinic phase even
after decomposition of metastable solid solution
into T+F equilibrium assemblage.
F
F
The ZrO2-Y2O3-Ta2O5 system presents several
properties attractive for TBC application. The
solubility of Ta+5 in ZrO2 is quite modest, but codoping with Y+3 show synergism and solubility of
YTaO4 is much higher than Y+3 and Ta+5 alone. The
YTaO4 rich composition of tetragonal phase are
stable up to 1500°C and non-transformable to
monoclinic phase upon cooling. In addition Y+Ta
stabilized compositions within non-transformable
tetragonal phase exhibits toughness slightly above
than 7YSZ that improves their durability as TBC.
Concluding remarks about novel materials for TBC
7YSZ is presently used as TBC. The issue is ageing effect on phase stability at temperatures
above ~1200°C and necessity to decrease thermal conductivity.
The requirements to the new candidate materials are: (1) high melting point; (2) no phase
transformations in the range between room temperature and operation temperature; (3)
low thermal conductivity; (4) chemical inertness; (5) thermal expansion match with metallic
substrate and TGO; (6) good adherence to metallic substrate and TGO; (7) low sintering rate
of porous microstructure.
To reduce thermal conductivity two groups of candidate materials were considered: RE codoped YSZ and RE zirconates with pyrochlore structure. Multiple co-doping with one
smaller cation (Yb, Sc) and one larger (La, Nd, Sm, Gd). The defect nano-cluster system
arises which contribute to phonon scattering and reduction of thermal conductivity.
Pyrochlores have many advantages such as lower thermal conductivity, high microstructural
stability. The disadvantages are low thermal expansion coefficient and therefore larger
misfit with TGO and reaction with TGO forming aluminates. The double layer concept can
be a solution of these problems: pyrochlore can be used as top layer to reduce thermal
conductivity and 7YSZ as next layer for better compatibility with TGO. Testing of double
layer coating indicated better durability than 7YSZ during cycling at 1300°C [2004Vassen].
Additional issues arise when considering environmental effect on novel TBC. Many of
suggested materials exhibit lower erosion resistance. The other concerns are related to
attacks of molten silicates. Molten Ca-Mg-alumino-silicates (CMAS) appear to readily
penetrate all ZrO2 based compositions. Understanding of reactions between TBC and CMAS
is important to develop TBC resistance to CMAS attacks.