Toughness measurement of Thermal Barrier Coatings (TBCs)

Transcription

1 Class: Master Project of Engineering Science Advisor # 1: Professor Ernesto Gutierrez-Miravete Advisor # 2: Professor Bose, Sudhangshu Name: Danh Tran Toughness measurement of Thermal Barrier Coatings (TBCs) The main topic of this project is about how to measure the toughness of Thermal Barrier Coating on Turbine Vanes. But before getting to that topic, the followings are also discussed: 1. What are the functions of the vane in a jet engine? 2. Why is vane chosen in this analysis? There are 2 sets of sample with TBCs by APS and EB-PVD. 3. What is a Thermal Barrier Coatings (TBCs) or ceramic coating? 4. Benefits of TBC are used in the jet engine. 5. What will happen to vane or other components without TBC? 6. Introduce Air Plasma Sprayed (APS) and Electron Beam Physical Vapor Deposited (EB-PVD) are 2 methods/processes for TBCs. 7. Microstructure of TBC in APS and in EB-PVD 8. Tensile test cannot be used to measure mechanical properties of ceramic coating because of its microstructure columnar grains. 9. Propose Fracture Toughness with Nanoindentation as a different method to measure coating hardness. 10. Discussion results of measurement with pictures. 11. Conclusions 12. References

2 A gas turbine engine divides into many stages. The number of stages depends on engine type. Each stage has a ring of vanes and blades. Size of stage is increasing so vanes size and blades size are also increased. Vanes and blades are the most important components in a turbine jet engine. They always see very high gas temperature because the turbine is right behind the combustor. Vanes are the very first components facing hot gas. Vanes are stationary components. Blades are rotational components. Mass flow from combustor is guided by vane airfoil onto blade airfoil to create rotation for shaft and next stages. Increasing or decreasing the angle of vane airfoil or blade airfoil in the first stage will affect entire performance. In this experiment, 2 nd stage vanes of a gas turbine engine are chosen because their airfoils are larger than blades. The critical information of these samples is briefly described because they are classified. There are 2 sets of samples with finished ceramic coatings (TBC) from 2 different processes; Air Plasma Spray (APS) and Electron Beam Physical Vapor Deposited (EB-PVD). The reason of choosing 2 coating methods on the same type of sample is because their TBC microstructure are difference as well as their toughness. TBCs are systems consisting of a ceramic coating, the TGO, and a metallic bond coat on the surface alloy (see figure 1). The approximate thickness range of each layer is as follows: ceramic 125 to 1000µm, bond coat 50 to 125µm, and TGO 0.5 to about 10µm. TBC has multiple layers of coatings, with each layer having a specific function and requirement. The topmost layer provides thermal insulation ad consists of a ceramic, with low thermal conductivity, typically ZrO 2, know as zirconia. The ceramic insulating layer is deposited on the substrate alloy with an intervening oxidation resistant metallic layer called bond coat. The general composition of this bond coat is NiCoAlY conforming to the susbtrate alloy composition. During the ceramic coating deposition, a thermally grown oxide (TGO), predominantly Al 2 O 3, forms on bond coat surface at the ceramic-bond coat interface. TGO binds the ceramics layer to the bond coat. Figure 1: Microstructure of 3 layers of TBC systems

3 TBC provides many benefits to aircrafts and land-based gas turbines. TBC was introduced in the early 1980s on the turbine blades in Pratt & Whitney JT9D engines powering jumbo jets. The systems temperature capability improved by as much as 200 C (360 F). TBC reduces oxidation and significantly reduces creep damage for rotating components as blades. High percentage of the room-temperature structural properties of the substrate is maintained because the components are now cooler. Compressor air used to cool components may be rerouted to produce thrust in order to increase engines efficiency. TBCs reduce local variations in temperature; therefore, distortion of components is reduced and thermal fatigue life is improved. Air Plasma Sprayed (APS) and Electron Beam Physical Vapor Deposited (EB-PVD) will be discussed later. See figure 2 and 3 for microstructure of TBC done by these 2 processes. Figure 2: EB-PVD columnar microstructure Figure 3: Microstructure of 7YSZ by APS process on a NiCoCrAlY bond coat deposited by LPPS

4 Blades and Vanes in a gas turbine engine are subjected to hot corrosion type 1 and type 2. See figure 4 and 5. Details of these corrosion types will be discussed later. Figure 4: Hot corrosion Type 1 and Type 2 in gas turbine engine. Figure 5: Example of Hot Corrosion on turbine blade.

5 TBC columnar grains Loading direction Loading direction Substrate Failure Figure 6: Failure when applying tensile load on TBCs columnar grains in the direction of the blue arrows. Because tensile stress cannot be applied to measure mechanical properties of TBCs (figure 6), another method called Fracture Toughness with Nanoindentation is introduced. Nanoindentation is widely used for measuring the modulus E and the hardness H of small volumes of material and thin films. Fracture toughness K IC is a critical parameter but much less frequently investigated with nanoindentation. K IC is a measure of materials resistance against crack propagation. This analysis will focus on this critical parameter; K IC as shown in equation (1) where 1 2 E P K IC = κ 3 2 (1) H c K IC is the fracture toughness of material κ is an empirical constant that depends on the geometry of the indenter E is the elastic modulus of material H is hardness of material c is crack length P is load applied for indenter This is a proposal for this project. The paper body has more pictures and discussions in details. Because of the sensitivity of the information, samples (vanes) and pictures of the samples will be briefly mentioned or described.

6 References 1. Sudhangshu, Bose. High Temperature Coatings. Oxford: Elsevier, W.C. Oliver and G. M. Pharr, J. Mater. Res (1992) Nanoindentaion techniques for assessing mechanical reliability at the nanoscale 3. B. R. Lawn, A. G. Evans, and D. B. Marshall, J. Am. Ceram. Soc 73, 574 (1990). 4. G. R. Anstis, P. Chantikul, B. R. Lawn, and D. B. Marshall, J. Am. Ceram. Soc. 64, 533 (1981). 5. D. S. Harding, W. C. Oliver, and F. M. Pharr, Mater. Res. Soc. Symp. Proc. 356, 663 (1995). 6. J. S. Field, M. V. Swain, and R. D. Dukino, J. Mater. Res. 18, 1412 (2003). 7. C. Tromas, J. Colin, C. Coupeau, J. C. Girard, J. Woirgard, and J. Grilhe, Eur. Phys. J.: Appl. Phys. 8, 123 (1999). 8. M. T. Laugier, J. Mater. Sci. Lett. 6, 355 (1987). 9. J. E. Bradby, J. S. Williams, and M. V. Swain, J. Mater. Res. (in press). 10. J. S. Field and M. V. Swain, J. Mater. Res (1993). 11. M. Iwasa, T. Ueno, and R. C. Bradt, J. Soc. Mater. Sci. Jpn. 30, 1001 (1981). 12. F. Meschke, O. Raddatz, A. Kolleck, and G. A. Schneider, J. Am. Ceram. Soc. 83, 353 (2000). 13. K. Fritscher, C. Leyens and U. Schulz. ; Why Do EB-PVD NiCoCrAlY Coatings Oxidize Faster than their LPPS Counterparts? ; Materials Science Forum Vols (2001) pp online at ; Trans Tech Publications, Switzerland. 14. Milton V. Kofskey and Kerry L McLallin. ; Effect of Stator Vane End Clearances on Performance. ; National Aeronautics and Space Administration Lewis Research Center, December Prepared for U.S. DEPARTMENT OF ENERGY, Office of Conservation and Solar Applications Division of Transportation Energy Conservation.