Abstract

Plasma sprayed alumina and zirconia coatings are widely used coatings for many industrial applications. One of the most important applications is the production of thermal barrier coatings (TBCs). As sprayed alumina-zirconia coatings have relatively high degree of porosity and the properties of these coatings, such as high temperature, corrosion resistance, toughness and abrasion resistance may thereby be reduced. Laser surface treatment is one novel method that has potential for eliminating porosity and producing a homogeneous surface layer. In this research work the effect of excimer laser annealing on the surface of alumina-zirconia coatings was investigated. Alumina-40% zirconia (AZ-40) coatings were sprayed with a water-stabilized plasma spray gun. The coated surface was treated by excimer laser having a wavelength of 248 nm and pulse duration of 24 ns. In the first phase of the work an analytical model was developed in COMSOL Multiphysics 4.2 in order to investigate the effect of the defects on the heat distribution at the surface of samples irradiated by KrF beam. The model revealed that much higher temperatures were localized at areas having defects than at continuous surfaces. A detailed parametric study was carried out to investigate the effects of different laser surface treatment parameters including laser energy density (fluence), pulse repetition rate (PRR), and number of pulses on the microstructure, surface morphology, and mechanical properties of the coatings. The surface structure of the treated coating was examined by field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Treating the surface with low laser energy of 200mJ/cm2 etched a very thin layer from the coating, which helped revealing the microstructures initially present but hidden on the surface of as sprayed coatings. High laser energy of 800mJ/cm2 resulted in significant changes in the coat surface morphology where eutectic colonies growing in a pool of zirconia matrix were identified on the surface. The surface of untreated coating was continuously alternating up and down; it had a zigzag nature. After irradiating the surface with high laser fluence of 800mJ/cm2 the zigzag nature of roughness profile of untreated coating disappeared. Also irradiating the surface with high pulse repetition rate exhibited "dome-like" structures on the surface, which were associated with an increase in surface hardness.

Department

Mechanical Engineering Department

Degree Name

MS in Mechanical Engineering

Graduation Date

6-1-2013

Submission Date

May 2013

First Advisor

Salem, Hanadi

Committee Member 1

Salem, Hadi

Committee Member 2

Sedky, Sherif

Extent

137 p.

Document Type

Master's Thesis

Library of Congress Subject Heading 1

Metal spraying.

Library of Congress Subject Heading 2

Plasma spraying.

Rights

The author retains all rights with regard to copyright. The author certifies that written permission from the owner(s) of third-party copyrighted matter included in the thesis, dissertation, paper, or record of study has been obtained. The author further certifies that IRB approval has been obtained for this thesis, or that IRB approval is not necessary for this thesis. Insofar as this thesis, dissertation, paper, or record of study is an educational record as defined in the Family Educational Rights and Privacy Act (FERPA) (20 USC 1232g), the author has granted consent to disclosure of it to anyone who requests a copy.

Institutional Review Board (IRB) Approval

Not necessary for this item

Comments

This dissertation would not have been possible without the guidance and help of many people. First, I am indebted to my supervisor Dr. Sherif Sedky, who gave me the opportunity to join his research team at Yousef Jameel Science and Technology Research Center. He gave me the chance to get hand on experience and get familiar with different equipment. His ability to select and to approach compelling research problems, his high scientific standards, and his hard work set an example. The results obtained would not have been possible without his guidance and support. I would also like to sincerely thank Dr. Hadi Salem my supervisor for her valuable contribution to this research work. She was always orienting and supporting me with promptness and care. Her experience and knowledge in the field were of great importance during different phases of this research. Last but not least, I would like to express my utmost appreciation and gratitude to Dr. Ahmed Ibrahim. I have been very privileged to get to know such a great Professor and to work with him over the last two years. He has always been patient and encouraging in times of new ideas and difficulties. He was always promptly replying to my frequent questions and ideas. The work would not have been completed without his effort and time. Sparing the time during his vacation to come to AUC to work with me was beyond expectation and is unforgettable and extremely valued. This work of thesis was supported by The Yousef Jameel Science and Technology Research Center (YJ-STRC). STRC has provided me with state-of-art equipment to conduct this work. I am also profoundly thankful to the entire center staff for offering me their generous support throughout my research work. I have been very privileged to get to know great people in the center who became friends. I would like to thank my colleagues and friends Eng. Rami Wasfi, Dr. Jouma El-Rifai, Eng. Ahmed El Ghazaly, and Eng. Mohamed Ibrahim, Eng. Ahmed Abdel Gawad for dedicating much of their time to help and support me. Special thanks to Eng. Mohamed Ashour for his help in the modeling part of this work. I would also like to thank with all my heart Eng. Ahmed Sayed Salem, my husband, for his support, encouragement and constant help with his expertise in the field of Material Science. Filly, I would like to thank my family for their constant support throughout everything. They were and always will be my everlasting source of love and inspiration.

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