Plasma Spray and Pack Cementation Process Optimization and Oxidation Behaviour of Novel Multilayered Coatings By Feng Gao B. Eng., M.A.Sc. Materials A thesis submitted to the Faculty of Graduate and Postdoctoral Affairs in partial fulfilment of the degree requirements of Doctor of Philosophy Ottawa-Carleton Institute for Mechanical and Aerospace Engineering Department of Mechanical and Aerospace Engineering Carleton University Ottawa, Ontario, Canada December, 2012 Copyright © 2012 Feng Gao Library and Archives Bibliotheque et Canada Archives Canada Published Heritage Direction du 1+1 Branch Patrimoine de I'edition 395 Wellington Street 395, rue Wellington Ottawa ON K1A0N4 Ottawa ON K1A 0N4 Canada Canada Your file Votre reference ISBN: 978-0-494-94216-1 Our file Notre reference ISBN: 978-0-494-94216-1 NOTICE: AVIS: The author has granted a non­ L'auteur a accorde une licence non exclusive exclusive license allowing Library and permettant a la Bibliotheque et Archives Archives Canada to reproduce, Canada de reproduire, publier, archiver, publish, archive, preserve, conserve, sauvegarder, conserver, transmettre au public communicate to the public by par telecommunication ou par I'lnternet, preter, telecommunication or on the Internet, distribuer et vendre des theses partout dans le loan, distrbute and sell theses monde, a des fins commerciales ou autres, sur worldwide, for commercial or non­ support microforme, papier, electronique et/ou commercial purposes, in microform, autres formats. paper, electronic and/or any other formats. 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Canada ABSTRACT The hot section components in gas turbines are subjected to a harsh environment with the temperature being increased continuously. The higher temperature has directly resulted in severe oxidation of these components. Monolithic coatings such as MCrAlY and aluminide have been traditionally used to protect the components from oxidation; however, increased operating temperature quickly deteriorates the coatings due to accelerated diffusion of aluminum in the coatings. To improve the oxidation resistance a group of multilayered coatings are developed in this study. The multilayered coatings consist of a Cr-Si co-deposited layer as the diffusion barrier, a plasma sprayed NiCrAlY coating as the middle layer and an aluminized top layer. The Cr-Si and aluminized layers are fabricated using pack cementation processes and the NiCrAlY coatings are produced using the Mettech Axial III™ System. All of the coating processes are optimized using the methodology of Design of Experiments (DOE) and the results are analyzed using statistical method. The optimal processes are adopted to fabricate the multilayered coatings for oxidation tests. The coatings are exposed in air at 1050°C and 1150°C for 1000 hr. The results indicate that a Cr layer and a silicon-rich barrier layer have formed on the interface between the Cr-Si coating and the NiCrAlY coating. This barrier layer not only prevents aluminum and chromium from diffusing into the substrate, but also impedes the diffusion of other elements from the substrate into the coating. The results also reveal that, for optimal oxidation resistance at 1050°C, the top layer in a multilayered coating should have at least Al/Ni ratio of one; whereas the multilayered coating with the Al/ Ni ratio of two in the top layer exhibits the best oxidation resistance at 1150°C. The DOE methodology provides an excellent means for process optimization and the selection of oxidation test matrix, and also offers a more thorough understanding of the effects of process parameters on the coating microstructure, and the effects of layers and their interactions on the oxidation behavior of the multilayered coatings. ACKNOWLEDGEMENTS I owe my sincere gratitude to my supervisors, Prof. Xiao Huang, Prof. Rong Liu (Carleton University, Canada), and Dr. Qi Yang (National Research Council, Canada), for their constant support and valuable suggestions. Thank you for being there for me when I needed your advice or recommendations. I would like to express my great appreciation to my family for their continuous support and patience. I am grateful to Mr. Fred Barrett (Carleton University, Canada) for spraying all specimens and to Mr. Yunfen Qian for his help in the preparation of the metallographic specimens. I thank the staff of the Mechanical and Aerospace Engineering at Carleton University and Institute of Aerospace Research at National Research Council Canada (NRC) for all the help that they have provided me with throughout my studies at Carleton University. TABLE OF CONTENTS ABSTRACT................................................................................................................................ ii ACKNOWLEDGEMENTS......................................................................................................iv TABLE OF CONTENTS........................................................................................................... v LIST OF TABLES......................................................................................................................x LIST OF FIGURES.................................................................................................................xiv NOMENCLATURE.................................................................................................................xx LIST OF ACRONYMS........................................................................................................xxiii Chapter 1: Introduction ...............................................................................................................1 1.1 Background and Significance .................................................................................... 1 1.1.1 Temperature Environments of Hot Section in Gas Turbines ............................. 1 1.1.2 Oxidation of Coatings ............................................................................................2 1.1.3 Development of Coatings with Oxidation Resistance ........................................2 1.2 Research Objectives and Methodologies ................................................................. 3 1.2.1 Designing Multilayered Coatings .........................................................................4 1.2.2 Optimizing Coating Processes ..............................................................................4 1.2.3 Fabricating Multilayered Coatings.......................................................................5 1.2.4 Investigating the Oxidation Behavior of Multilayered Coatings ...................... 5 1.3 Thesis Structure.......................................................................................................... 5 Chapter 2: Literature Review..................................................................................................... 8 2.1 Operating Conditions of the Hot Section Components in Gas Turbines ................8 v 2.2 Coating Degradation Mechanisms ..............................................................................9 2.2.1 Oxidation of Metals ..............................................................................................10 2.2.2 Oxidation of Coatings ...........................................................................................12 2.2.3 Requirements for Oxidation Resistant Coatings ................................................15 2.3 Coating Processes .......................................................................................................16 2.3.1 Diffusion Coatings ................................................................................................17 2.3.2 Overlay Coatings ..................................................................................................29 2.3.3 Diffusion between Coating and Substrate ......................................................... 36 2.3.4 Duplex Layer and Multilayered Coatings .......................................................... 40 2.4 Design of Experiments Methodology ...................................................................... 42 2.4.1 Two-Level Full Factorial Design ........................................................................43 2.4.2 Two-level Fractional Factorial Design .............................................................. 44 2.4.3 Response Surface Methodology (RSM) ............................................................ 45 2.4.4 Taguchi Method ....................................................................................................47 2.4.5 Analysis of Variance (ANOVA) Table ............................................................. 52 2.5 Summary of Literature Review