Papers

ICMCTF2002 Session A1-1: Coatings to Resist High Temperature Corrosion

Monday, April 22, 2002 10:30 AM in Room Esquire/Towne

Monday Morning

Start	Invited?	Item
10:30 AM		A1-1-1 Deformation Behavior of Alumina and the Effect on Growth Stresses R.M. Cannon (Lawrence Berkeley National Laboratory) Studies of the deformation mechanisms of Al₂O₃ are critically reviewed in order to synthesize a comprehensive view of high temperature deformation behavior. The slip is highly anisotropic with order of magnitude differences in critical resolved yield stresses for different systems. Consequently, diffusional creep provides the deformation over a wide range of stress-grain size conditions. However, complications in interpretation and prediction arise as the rates are often controlled by grain boundary processes (sliding or point defect accommodation) rather than diffusion, as indicated by nonlinear stress dependencies. Also, there are no adequate tracer diffusion studies to indicate the levels of Al lattice diffusion or of grain boundary diffusion of either ion, and the associated impurity dependencies. These issues are important to understanding the relaxation of stresses that can arise during the formation of protective oxide scales or the deposition of alumina films, plus those from thermal expansion mismatch. Mechanisms of stress generation
11:10 AM		A1-1-3 Correlation of Short-term to Long-term Oxidation Testing for Alumina Forming Alloys and Coatings M.J. Stiger, F.S. Pettit, G.H. Meier (University of Pittsburgh) Materials used in high temperature applications, such as gas turbine engines, are protected from environmental attack by formation of an oxide scale. Life of components constructed from these materials is limited to the ability to maintain and reform this oxide scale. In modern alumina scale formers, this life could be thousands of hours. The focus of this investigation is to gain insight on the materials' performances by short-term testing. The materials studied are low sulfur N5, nickel base superalloy, and diffusion aluminide coatings on N5. Each material is exposed to a range of temperatures from 1050°C to 1175°C in dry air for 24 hours. Acoustic emissions, scale cracking and spalling, are recorded during these high temperature exposures and during the cooling to room temperature. XRD was also used to measure the stresses in the alumina scale. Behavior of the materials from these short-term tests are then compared to long-term cyclic oxidation experiments at 1100°C to attempt to develop accelerated testing failure criterion.
11:30 AM		A1-1-4 Martensitic Transformation in CVD NiAl and (Ni,Pt)Al Bond Coatings Y. Zhang (Tennessee Technological University); B.A. Pint, J.A. Haynes, I.G. Wright (Oak Ridge National Laboratory); W.Y. Lee (Steven Institute of Technology) The martensitic phase transformation in single-phase β-NiAl and (Ni,Pt)Al coatings on Ni-based superalloys synthesized by chemical vapor deposition (CVD) was investigated in both isothermal and cyclic oxidation tests. After isothermal exposure to 1150°C for 100 hours, the β phase in the coating was transformed to a martensitic phase during cooling to room temperature. A martensitic transformation also was observed in the (Ni,Pt)Al bond coat with Y₂O₃-stabilized-ZrO₂ thermal barrier coating (TBC) on top after thermal cycling at 1150°C for 700 1-h cycles. Interdiffusion and Al₂O₃ scale spallation/re-formation lead to a decrease of Al content in β-NiAl coating; once the Al content is below a certain critical level, the β-phase undergoes martensitic transformation. Further Al depletion in the β-phase also facilitates the β to γ ^' phase transformation. High-temperature X-ray diffraction studies on cast NiAl and (Ni,Pt)Al alloys indicate that the β to γ ^' phase transformation was repeated during each thermal cycle when the alloy composition was close to the two-phase β + γ ^' region with 35-37at% Al (20-22wt% Al), which might be responsible for macroscopic deformation of the cast specimens. The contributions of the martensite reaction to the early stages of rumpling of (Ni,Pt)Al bond coats during thermal cycling are discussed, based on the consideration of the associated volume change and the formation of surface relief.
11:50 AM		A1-1-5 CVD Aluminide Coatings for High Temperature Gas Turbine Applications W.M. O'Neill, F. O'Shea (SIFCO, Ireland); M.A. Morris (University College Cork, Ireland); A. Kennedy, M. Chunggaze (SIFCO, Ireland) Modern turbine components operate in highly oxidising and corrosive environments. The lifetime of these components is often determined by the durability of the protective aluminium coatings applied. Switching from older coating technologies to a dynamic CVD aluminising process can offer improvements in coating lifetime. This is explained through comparison of the cyclic and isothermal oxidation properties of CVD aluminide and traditional pack coatings. The CVD process is found to generate cleaner coatings with superior oxidative resistance and improved thermo-mechanical properties. Microstructure, mechanical properties and chemical composition are investigated to further probe these differences. It is shown that factors other than the aluminising step can also have a significant effect. Grit blasting, platinum plating and heat treatments can also have important implications for coating structure and performance.