Sintering and Deformation Mechanisms in Plasma-Sprayed 7 wt.% Y 2 O 3 -ZrO 2 Thermal Barrier Coatings

Transcription

1 Sintering and Deformation Mechanisms in Plasma-Sprayed 7 wt.% Y 2 O 3 -ZrO 2 Graeme Dickinson John Levin* Christophe Deschaseaux Rod Trice, Ph.D. School of Materials Engineering Purdue University October 24, 2003 NSF CAREER Award (DMR ) Purdue Research Foundation *Undergraduate support partly by REU DMR

2 Introduction Hot Gas Flow Ceramic top-coat (~200 µm) Bondcoat (~100 µm) Superalloy Blade for a gas turbine engine Typical bondcoat materials: MCrAlY, PtAl Typical topcoat material: 7 wt% Y 2 O 3 -ZrO 2 (YSZ) focus of current research A 200 µm thick YSZ topcoat provide a 200 o C drop engine efficiency increases by ~ 6-12% R.A. Miller, Surf. Coat. Tech., 30, pp (1987) R. Stevens, An Introduction to Zirconia, Publication of Magnesium Elektron Inc.

3 Overview of Plasma Spray Process Introduction

4 Plasma-Sprayed YSZ Microstructure Introduction BF TEM/Cross-Sectional Orientation BF TEM/Plan View Orientation Lamellae are observed Interlamellar pores Intralamellar cracks Columnar grains Porosity of 15-20%

5 Focus of Our Research: Link the microstructure, before and after high temperature exposure, to the thermal and mechanical properties of the YSZ coating off-of-the-substrate Introduction Experimental Procedure Outline Results and Discussion -Room temperature mechanical behavior -Sintering behavior -Stress relaxation behavior Conclusions

6 Experimental Procedure Processing of Stand-Alone Coating Samples YSZ Powder Cool air Plasma plume Melted Powder Alumina Rod Seven axis robot Turntable Cut Etching Al in HCl To remove alumina core YSZ stand-alone coating

7 Experimental Procedure Transmission Electron Microscopy Procedures Bright-field TEM analysis Cross Section Cross-section and plan-view Tripod polishing technique Gatan ion-mill JEOL 2000FX Plan View

8 Overview of Stand-Alone Coating Overview of YSZ Stand-Alone Coatings Sample Characteristics Diameter = 13 mm Height = mm Thickness = µm Porosity = 13-18% Yttria rich, non-transformable t-zro 2 coating Cut-away view of tube

9 Room Temperature Mechanical Behavior In-Situ Observation of Crack Behavior in Plasma-Sprayed Coatings Compressive Stress Hysteresis Change in slope on loading Strain -Two distinct slopes in stress-strain data -Significant cracking heard with acoustic emission techniques during loading K.F. Wesling, D.F. Socie, and B. Beardsley, J. Am. Ceram. Soc., 77 [7] (1997). R.W. Trice, D.W. Prine, K.T. Faber, J. Am. Ceram. Soc., 83 [12] (2000).

10 Room Temperature Mechanical Behavior ESEM with Load Frame ESEM Load Frame -Electro Scan 2020 ESEM -Operated at 25 kev -Working distance of 27 mm

11 Room Temperature Mechanical Behavior Close-up of Load Frame YSZ Tube -Strain gages attached to sides of tube -View outer surface of the coating

12 Room Temperature Mechanical Behavior Procedures Followed for Compression Testing of Tubes in ESEM A region was located which exhibited cracks oriented in various directions with respect to the applied stress. The sample was then incrementally stressed, with strain data and micrographs taken at each stress increment (approximately 10 MPa per step)

13 Stress-Strain Results Room Temperature Mechanical Behavior 140 Stress Direction Strain gages Stress (MPa) Vertical Crack Horizontal Crack 62.1 GPa -Distinct change in modulus with stress GPa Strain, m/ m

14 Room Temperature Mechanical Behavior Effect of Stress on Intralamellar Cracks Micrographs of top surface 20 µm 20 µm 0 MPa MPa - Cracks formed during cooling

15 Room Temperature Mechanical Behavior Intralamellar Cracks Investigated in Present Study 20 µm Horizontal Crack 2 - Chose 5 cracks to investigate Vertical Crack 2 Horizontal Crack 3 - Measured change in crack width as a function of applied stress Vertical Crack 1 Horizontal Crack 1

16 Room Temperature Mechanical Behavior Results of Crack Measurements 1.6 Crack Width (µm) Vertical Crack 1 Vertical Crack 2 Horizontal Crack 3 Horizontal Crack 2 Horizontal Crack Applied Stress (MPa) -Vertically oriented cracks open, horizontally oriented cracks close -Seems plausible that the increase in E with stress is due to partial closure of horizontally oriented cracks

17 Room Temperature Mechanical Behavior Crack Nucleation and Propagation 33.3 MPa 58.3 MPa 83.3 MPa MPa These images show a crack nucleating and propagating from an existing angled crack Significant crack propagation prior to failure Consistent with acoustic emission results on PS alumina Trice, Prine, Faber, J. Am. Ceram. Soc., 83 [12] (2000)

18 Crack Initiation in Angled Micro-Cracks Ashby and Hallam Results* on PMMA Crack Close-up Original Crack *Ashby, M. F. and Hallam, S. D., Acta Metall., 34, pp (1986)

19 Sintering Behavior of PS YSZ Sintering Behavior 3.5 Thermal Conductivity, W/m/K o C/50-hr 1200 o C/50-hr 1000 o C/50-hr As-Sprayed Observed Microstructural Changes -Cont. sintering/some m-zro 2 -Partial closure of interlamellar pores -Full closure of intralamellar cracks -Closure of intralamellar cracks Measurement Temperature, C Trice, Su, Mawdsley, Faber, de Arellano-Lopez, Wang, Porter, J. Mat. Sci (2002)

20 Dilatometry Measurements Sintering Behavior Top View Linear resolution: 1 µm (~ 0.02% for a 15mm coating) Dilatometer data are deconvoluted in order to isolate the shrinkage behavior of the YSZ coating. Temperatures investigated: 800 C thru 1400 C, simple ramps and 10-hr isothermal tests

21 1500 Effect of Multiple Heating/Cooling Cycles 0.1 Sintering Behavior Temperature, o C th Run 3rd Run 2nd Run 1st Run Percent Linear Shrinkage, % Subtracted out the expansion of apparatus and coating Measurable shrinkage, detected at 950 o C Time, (minutes)

22 Sintering Behavior Effect of 10-hr Isotherms on Linear Shrinkage of YSZ Tubes o C, 900 o C 1000 o C Linear Shrinkage, % o C 1200 o C 1300 o C 1400 o C Time at Indicated Temperature, min

23 Effect of Isothermal Temperature on Shrinkage Rate Sintering Behavior o C 1000 o C Linear Shrinkage Rate, % / min o C 1300 o C 1200 o C Significant shrinkage still occurring after 10-hrs At 1300 o C, 1400 o C Time at Temperature, min

24 Link Shrinkage Behavior with Microstructural Changes After 800 o C Cross-sectional view After 900 o C Sintering Behavior Plan view Narrow intralamellar microcracks observed

25 Microstructural Changes with Temperature After 1000 o C Sintering Behavior Cross-sectional view Plan view No narrow intralamellar microcracks were observed. Pores and wide intralamellar cracks remain.

26 Cross-sectional view Microstructural Changes with Temperature After 1200 o C Sintering Behavior Cross-sectional view Interlamellar lenticular pores are breaking down into channels of isolated globular pores by pinching between adjacent lamellae.

27 Stress Relaxation Stress Relaxation Experiments on Stand-Alone YSZ Coatings Apply constant strain, monitor load Test temps: 1000 o C, 1050 o C, 1100 o C, 1200 o C

28 Stress State in the YSZ During Heat-up Effective biaxial compressive stress applied within the plane of the coating

29 Stress Relaxation Stress Relaxation Behavior as a Function of Temperature Stress (MPa) Original Stress Applied was 20 MPa Temp. o C Time to 0 MPa, min C C 1050 C C Time (min)

30 Stress Relaxation Stress Relaxation Behavior As a Function of Beginning Stress Level At 1000 o C Stress (MPa) MPa Initial Stress Modeling behavior of the material 5 20 MPa Initial Stress 10 MPa Initial Stress TEM experiments Time (min)

31 Summary The stress-strain curve for the YSZ tube showed an increasing modulus with two distinct regions of differing modulus linked to crack closure. Micrographs taken using an ESEM showed that cracks oriented parallel to the applied stress opened and cracks oriented perpendicularly to the applied stress closed.

32 Summary The shrinkage and shrinkage rate of PS YSZ increased with temperature and decreased with time. PS YSZ is still shrinking after 10 hours at 1300 o C and 1400 o C. Narrow intralamellar cracks are healing around 1000 o C. All cracks have healed after 10 hours at 1000 o C. Interlamellar pores were found to close at temperatures > 1000 o C. This process seems to be triggered by pinching between two adjacent grains as they assume an equilibrium shape.