A thermodynamic database for simulation of CMAS and TBC interactions

Transcription

1 Engineering Conferences International ECI Digital Archives Thermal Barrier Coatings IV Proceedings Summer A thermodynamic database for simulation of CMAS and TBC interactions Lina Kjellqvist Thermo-Calc Software AB, lina@thermocalc.se Johan Brattberg Thermo-Calc Software AB Ake Jansson Thermo-Calc Software AB Huahai Mao Thermo-Calc Software AB Follow this and additional works at: Part of the Materials Science and Engineering Commons Recommended Citation Lina Kjellqvist, Johan Brattberg, Ake Jansson, and Huahai Mao, "A thermodynamic database for simulation of CMAS and TBC interactions" in "Thermal Barrier Coatings IV", U. Schulz, German Aerospace Center; M. Maloney, Pratt & Whitney; R. Darolia, GE Aviation (retired) Eds, ECI Symposium Series, (2015). This Conference Proceeding is brought to you for free and open access by the Proceedings at ECI Digital Archives. It has been accepted for inclusion in Thermal Barrier Coatings IV by an authorized administrator of ECI Digital Archives. For more information, please contact franco@bepress.com.

2 A Thermodynamic Database for Simulation of CMAS and TBC Interactions Lina Kjellqvist (1), Huahai Mao (1,2) 1, Thermo-Calc software AB, Sweden 2, KTH Royal Institute of Technology, Sweden

3 Acknowledgement This work was supported by a US Office of Naval Research (ONR) phase I and II small business technology transfer grant to QuesTek and Prof. Carlos Levi of UCSB entitled Affordable CMAS-Resistant Thermal Barrier Coatings (contract #s: N M-0340 and N C-0339, program manager: Dr. David Shifler).

4 Outline Introduction CALPHAD Thermodynamic models Compound Energy Formalism (CEF) Ionic two-sublattice liquid model (I2SL) Assessments and Results Preliminary calculations on CMAS-TBC interaction

5 Outline Introduction CALPHAD Thermodynamic models Compound Energy formalism (CEF) Ionic two-sublattice liquid model Assessments and Results Preliminary calculations on CMAS-TBC interaction

6 Introduction Thermodynamic database developed through CALPHAD approach Phase diagram and thermodynamic calculations e.g. thermochemical interaction between the thermal barrier coatings (TBC), namely 7YSZ (yttria stabilized zirconia) and CMAS (CaO-MgO-Al 2 O 3 -SiO 2 ) deposits. CaO-MgO-Al 2 O 3 -SiO 2 -Y 2 O 3 -ZrO 2 is thus the core system for understanding and modelling of processes occurring between CMAS and TBC. In this work Y 2 O 3 -ZrO 2 was incorporated into an existing internalconsistent description of the CaO-MgO-Al 2 O 3 -SiO 2 system. Many pseudo-binaries and ternaries are assessed.

7 CALPHAD Methodology CALPHAD databases need good experimental data as well as theoretical estimates by e.g. DFT calculations. The language of CALPHAD gives the possibility to extract a much more precise description (materials genome) based on both experimental data and theoretical estimates.

8 Thermo-Calc Software AB Thermo-Calc Software AB (TCSAB) was founded in 1997 and originates in research that had been made at the Royal Institute of Technology (KTH), Stockholm, Sweden, and that had resulted in the CALPHAD based softwares Thermo-Calc and DICTRA. Several 1000 users in more than 70 countries Swedish steel producer Sandvik pioneers the use of CALPHAD based computational tools and databases for industrial purposes in 1983 when developing a new generation duplex stainless steels. f α = 0.5 f α = 0.5 Both TCSAB and KTH are SGTE members. A classic example from 1983 SAF 2507

9 CALPHAD (CALculation of PHAse Diagrams) Computer Coupling of Phase Diagrams and Thermochemistry Self consistent framework to relate thermodynamics and phase equilibria through the Gibbs energy Thermochemical measurements: enthalpy, heat capacity, activity etc Phase equilibria: Phase diagram Gibbs Energy of Individual Phases G m f ( x, T, P) Applications

10 CALPHAD Gibbs energy is used as the modeled thermodynamic property. A function of temperature, pressure and composition. Other thermodynamic properties can easily be derived. CALPHAD databases are built from: Pure elements Phases Unary, binary, ternary and sometimes higher order systems. CALPHAD constitutes an efficient tool to estimate the equilibrium state of high order systems.

11 Outline Introduction CALPHAD Thermodynamic models Compound Energy formalism (CEF) Ionic two-sublattice liquid model Assessments and Results Preliminary calculations on CMAS-TBC interaction

12 Compound Energy Formalism (CEF) A general formalism widely used in CALPHAD assessments is the Compound Energy Formalism (CEF). CEF was constructed in order to describe models of the thermodynamic properties of solution phases with two or more sublattices, e.g. (A,B ) m (C,D ) n A,B,C,D... are various species e.g. atoms, ions, vacancy... m and n are site numbers of 1st and 2nd sublattice respectively.

13 G m (at 1 atm) by CEF (A,B) m (C,D) n reference physical G m = y i y j o G i:j + phys G m + i j RT(m y i i ln y i + n y j j ln y j ideal mixing ) + E G m excess The site fraction y i S is the mole fraction of species i in the sublattice S.

14 Ionic two-sublattice liquid model (I2SL) Within the framework of the CEF, the I2SL model was developed when there is a tendency for ionization in the liquid. The same model can be used both for metallic and ionized melts. At low levels of ionization, the model becomes equivalent to a substitutional solution model between metallic atoms.

15 I2SL model in CEF Two sublattices are assumed v (C i v i ) P (A j j, Va Q, B 0 k ) Q C represents cations A anions Va vacancies B neutrals i, j, k denote specific constituents Charged vacancies are introduced to keep electroneutrality when the composition approaches metallic liquid.

16 Liquid modeling of the CMAS-YZ system P and Q on the sublattice vary so that electroneutrality is maintained: P = y Aj ( v j ) j + Qy Va Examples: Pure Al: (Al +3 ) 3 (Va -3 ) 3 Al 2 O 3 : (Al +3 ) 1 (AlO 1 2 ) 3 Model in this work: Q = i y Ci v i (Al +3, Ca +2, Mg +2, Y +3, Zr +4 ) P (AlO 2 1, O 2, SiO 4 4, SiO 2 ) Q

17 Outline Introduction CALPHAD Thermodynamic models Compound Energy formalism (CEF) Ionic two-sublattice liquid model Assessment and Results Preliminary calculations on CMAS-TBC interaction

18 Assessments Al 2 O 3 -Y 2 O 3 -ZrO 2 Al 2 O 3 -Y 2 O 3 /ZrO 2 CaO-Y 2 O 3 /ZrO 2 MgO-Y 2 O 3 /ZrO 2 SiO 2 -Y 2 O 3 /ZrO 2 Y 2 O 3 -ZrO 2 Al 2 O 3 -Y 2 O 3 -SiO 2 Al 2 O 3 -CaO-ZrO 2 CaO-Y 2 O 3 -ZrO 2 MgO-Y 2 O 3 -ZrO 2 CaO-MgO-ZrO 2 Al 2 O 3 -MgO-ZrO 2 Al 2 O 3 -CaO-MgO-SiO 2 Assessed systems Al 2 O 3 -SiO 2 -ZrO 2 CaO-SiO 2 -ZrO 2 MgO-SiO 2 -ZrO 2 SiO 2 -Y 2 O 3 -ZrO 2 Al 2 O 3 -CaO-Y 2 O 3 Al 2 O 3 -MgO-Y 2 O 3 CaO-SiO 2 -Y 2 O 3 MgO-CaO-Y 2 O 3 MgO-SiO 2 -Y 2 O 3 To be assessed/ongoing

19 Al2O3-ZrO2 and Y2O3-ZrO2

20 SiO 2 activity in CaO-Al 2 O 3 -SiO 2 Assessed by Mao et al., J. Am. Ceram. Soc., (2006),

21 Al 2 O 3 -Y 2 O 3 -ZrO 2 Isothermal section at 1600 K Liquidus projection

22 the join forsterite - anorthite i.e. Mg 2 SiO 4 - CaAl 2 Si 2 O 8 of CMAS Experiments (Osborn and Tait, 1952) Calculations (This work)

23 The pseudo ternary diopside-forsterite-anorthite i.e. CaMgSi 2 O 6 -Mg 2 SiO 4 - CaAl 2 Si 2 O 8 of CMAS Experiments (Osborn and Tait, 1952) Calculations (This work)

24 Preliminary calculations for CMAS+TBC CMAS: 33% CaO - 9% MgO 13% AlO 3/2 45% SiO 2 (mole) ZrO % ZrO % YO 3/2 (mole) This is an equilibrium calculation, i.e. no kinetic is taken into account. In order to be able to describe the reality more closely a kinetic description of the system is necessary in addition to the present thermodynamic data.

25 CMAS-ZrO 2 L=Liquid T=t-ZrO 2 W=Wollastonite PW=Pseudo-wollastonite CP=Clino-pyroxene A=Anorthite M=Melilite Z=Zircon CMAS = 33% CaO - 9% MgO 13% AlO 3/2 45% SiO 2 (mole)

26 CMAS-YSZ L=Liquid T=t-ZrO 2 C=c-ZrO 2 W=Wollastonite PW=Pseudo-wollastonite CP=Clino-pyroxene A=Anorthite M=Melilite Z=Zircon YA= -Y 2 Si 2 O 7 B=b-Y 2 Si 2 O 7 CMAS = 33% CaO - 9% MgO 13% AlO 3/2 45% SiO 2 (mole)

27 Summary This work will aim at describing the CMAS interaction with the TBC. A complete analysis requires detailed knowledge of thermodynamics and phase equilibria in the system Al 2 O 3 -CaO-MgO-SiO 2 -Y 2 O 3 -ZrO 2. This is an ongoing project at QuesTek, where experimental work is performed at UCSB by Prof. Carlos Levi and the thermodynamic assessments at Thermo-Calc Software AB.