Application of phase diagrams in development of transparent polycrystalline oxides and thermal barrier coating Phase diagrams for production of transparent polycrystalline materials Polycrystalline oxides are used for optical applications The Al ID: 554196
Download Presentation The PPT/PDF document "Phase diagrams for thermal barrier coati..." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Phase diagrams for thermal barrier coating
Application of phase diagrams in development of transparent polycrystalline oxides and thermal barrier coating
Phase diagrams for production of transparent polycrystalline materialsSlide2
Polycrystalline oxides are used for optical applications. The Al
2O3 is a key element in high pressure sodium lamp. Number of oxides have been developed for optical applications. They are Al2O3, MgAl
2O4, AlON, Y2O3. The aim is to obtain transparent polycrystalline single phase without porosity.Y2O3High 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 Y
2O3 among rare earths is restricted to La and Gd, because these ions have no electronic transitions in the visible or infrared frequencies.
Phase chemistry in the development of transparent polycrystalline oxidesSlide3
Phase diagram was
calculated using assessed thermodynamic data and reproduced 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.Y2O
3 with La2O3 additive
Calculated phase diagram of the Y2O3-La2
O3 systemSingle phase microstructure achieved by sintering at 2150°C in two-phase region and annealed at 1900°C in single phase region , (A) as polished and (B) as etched [1981Rhodes].Slide4
S
cintillators 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 Eu
2O3, 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.
The Y
2
O
3-Gd
2O3 solid solutions for optical applications
XRD shows that at
10 mol.%
Y
2
O
3
the B
phase was produced. The measurements showed that light
output was gradually increased with Y
2
O
3
decrease down to 20 mol.% Y
2O3 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]Slide5
Al
2O3-MgOPresence of
MgO enhanced sintering by 1 to 2 %. Without MgO grain growth kinetic was unaffected prior to the onset of 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 pore-boundary 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.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.
Al2O
3
Sintered microstructure of Al2
O3: Low porosity high-transmittance resulting from MgO sintering aid (a); pores entrapment without MgO (b)Slide6
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 MgAl2
O4 possibly precipitate during cooling.Using of the Y2O
3 in combination with MgO as sintering aid for translucent Al2O3 allows 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.
Al
2
O
3
-MgO
Solubility limit (mass.%
MgO
) in corundumSlide7
Spinel MgAl
2O4Transparent MgAl
2O4 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 have no optical anisotropy. Phase diagram shows 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 MgAl
2O4 and not related to solid solution limits (the same at 1600 and 1850°C). In case of deviation in Al2O3 rich side, grain size is maximal at the border
Sp/Sp+Cor. The highest density is achieved in two-phase field. Grain growth control is difficult and it is a key to the achievement of transparent spinel. One of possibilities to improve microstructure of MgAl
2
O
4
is sintering with the aid of
LiF
.
Calculated phase diagram of the MgO-Al
2
O
3
system
Average grain size vs. Al
2
O
3 content in 1640-1685°C
Transmittance of spinel vs. MgO contentSlide8
Thermal barrier coating
Application of thermal barrier coating (TBC)
enables to increase operating temperature of gas turbines and therefore increases 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 Al
2O3 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 7YSZSlide9
Phase diagram of the ZrO
2
-Y2O3 system
7YSZ = 7 mass.% Y2O3 = 4
mol.% Y2O3 = 7.6 mol.% YO1.5
T‘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).
T
e
F
e
Fluorite structure
(F)Slide10
Stabilization mechanism of metastable phases t‘ and t‘‘
Metastable-stable phase diagram in the ZrO
2
-YO
1.5 system [1996Yashima]
Phase diagram of the ZrO2-Y2O3 system: stable and metastable phase boundaries
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.Slide11
New materials for TBC
Co-doping of YSZ by rare earth oxidesRare earth zirconates
with pyrochlore structureOther materials
Gd
Sm
Nd
La
r(RE
+3
)
Stability of
pyrochlores
increases with radius of RE
+3
: La
2
Zr
2
O
7
melts congruently, while other
pyroclores
transform to fluorite structure below
liquidus
. Temperature of transformation of
pyrochlore
to fluorite decreases with decrease of ionic radius.
Experimental studies show that YSZ co-doped with other rare earth and
pyrochlores
RE
2
Zr
2
O
7
demonstrate lower thermal conductivity than 7YSZ.Slide12
Gd
Sm
Nd
La
r(RE
+3
)
Composition of phase should correspond to non-transformable range, i.e. it should not intersect T
0
(T/M) curve at ambient temperatures. With the ionic radius increase T
0
(F/T) are shifting to higher RE content. For La position of T
0
(T/M) substantially shifted to higher RE concentration thus making tetragonal phase transformable to T
M disruptive transformation. For the other considered RE (
Gd
, Sm,
Nd
) position
of T
0
(T/M)
is not influenced by RE nature and RESZ are more stable. 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.
Calculated ZrO
2
-rich part of ZrO
2
-REO
1.5
phase diagrams along with T
0
-lines.Slide13
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 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.Driving force for precipitation to stable phase equilibrium assemblage T+F: a RESZ RE=
Gd, Sm, Nd, La; b- ZrO2-(Y1-xGdx
)O1.5.Stability of RE stabilized ZrO2
.
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. Slide14
Compatibility of TBC top coat with TGO
In the ZrO
2-RE2O3 systems RE2Zr2O7 with pyrochlore
structure forms for RE=La-Gd and d-RE
4Al3O12 for RE=Y, Er-Lu. In the RE2O
3-Al2O3 systems REAl11O18 with
b-alumina structure forms for RE=La, Nd (b-alumina for Nd at high T) , REAlO
3 perovskite for RE=Nd, Sm-Gd and RE
3
Al
5
O
12
garnet for RE=Y,
Er
-Lu. Phase diagrams show that RE
2
Zr
2
O
7
should react with Al
2O3 (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 Al2O
3. 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 Al
2
O
3
.
Experimental results for diffusion couple between Gd
2
Zr
2
O
7
and Al
2
O
3
heat treated at 1200°C [2005Leckie].
7YSZ is chemically compatible with Al
2
O
3
. However thermal expansion misfit between Al
2
O
3
and 7YSZ
decreases
durability. For new candidate materials chemical compatibility with Al
2
O
3
should be checked
.Slide15
Compatibility of TBC with TGO and phase diagrams of the ZrO
2-RE2O3-Al2O
3 systemsSlide16
The TiO
2
additions to YSZ enhances the toughness of YSZ and durability of coating. The TiO2 is one of small number of dopants that increases tetragonality of zirconia solid solutions without compromising its phase stability. Co-doping 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-Y2
O3-Ta2O5 system presents several properties attractive for TBC application. The solubility of Ta+5 in ZrO2
is quite modest, but co-doping with Y+3 shows synergism and solubility of YTaO4
is much higher than Y
+3
and Ta
+5
alone. The YTaO
4
rich
compositions
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
exhibit
toughness slightly above than 7YSZ that improves their durability as TBC.
Other additives of YSZ for TBC applicationsSlide17Slide18
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 co-doped 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-alumo-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.
Concluding remarks about novel materials for TBC