Atomistic Simulation and Virtual Diffraction Characterization of Alumina Interfaces: Evaluating Structure and Stability for Predictive Physical Vapor Deposition Models

The objectives of this work are to investigate the structure and energetic stability of different alumina (Al2O3) phases using atomistic simulation and virtual dif fraction characterization. To meet these objectives, this research performs molec ular statics and molecular dynamics simulations employing the reactive force-field (Rea xFF) potential to model bulk, interface, and surface structures in the θ-, γ-, κ-, and α-Al 2O3 system. Simulations throughout this study are characterized using a new virtual diffraction algor ithm, developed and implemented for this work, that creates both selected area electron diff raction (SAED) and x-ray diffraction (XRD) line profiles without assuming prior knowledge of t he crystal system. First, the transferability of the ReaxFF potential is evaluated by modelling diff erent alumina bulk systems. ReaxFF is shown to correctly predict the energetic stability of α-Al 2O3 among the crystalline alumina phases, but incorrectly predicts an even lower ener gy amorphous phase. Virtual XRD patterns uniquely identify each phase and validate the minim um energy bulk structures through experimental comparison. Second, stable and metast able alumina surfaces are studied at 0, 300, 500, and 700 K. ReaxFF predicts minimum energy sur face structures and energies in good agreement with prior studies at 0 K; however, selec t surface models at 500 and 700 K undergo significant reconstructions caused by the unnatural bias for a lower-energy amorphous phase. Virtual SAED analysis performed on alumina surfaces allow advanced characterization and direct experimental validation of select models. T hird, ReaxFF is used to model homophase and heterophase alumina interfaces at 0 K. Predicted m inimum energy structures of α-Al 2O3 interfaces show good agreement with prior works, wh ich provides the foundation for the first atomistic study of metastable alumina grain boundar ies and heterophase alumina interfaces. Virtual SAED patterns characterize select alumina i nterfaces and help guide the construction of low-energy heterophase alumina interfaces by provid ing insight into crystallographic compatibilities. Combined, the energetic data ext racted from bulk, surface, and interface simulations as well as insights gained through virt ual diffraction will aid the development of mesoscale predictive models of polycrystalline alum ina formation during physical vapor deposition. ACKNOWLEDGEMENTS There are many people to thank and acknowledge for their guidance and support during my graduate studies. First and foremost, I would like to xtend my gratitude to my academic and research advisor, Dr. Douglas E. Spearot. The dire ction and financial support for the enclosed research was provided by Dr. Spearot's National Sci ence Foundation (NSF) Faculty Early Career Development Grant (#0954505) titled: CAREER: Computational Modeling of Microstructure Evolution during Vapor Deposition. I would like to hank Dr. Spearot for all the time, helpful advice, and encouragements he has provided me throu ghout my graduate studies. I would also like to thank all my colleagues in Dr. Spearot's re earch group: James Stewart, Khanh Dang, Mehrdad Sichani, and Wesley Barrows for their alway s fruitful and entertaining discussions. As well, I would like to thank Dr. Laurent Capolungo a t the Georgia Institute of Technology for his collaboration on our seminal virtual diffraction pu blication. During my graduate studies, I had the opportunity t o conduct two internships at Sandia National Laboratories (SNL) in Albuqurue, NM. Thes e internships were conducted under the direction of Dr. Stephen Foiles and Dr. Christopher Weinberger whom I would like to thank for their guidance, support, and helpful advice. I wou ld also like to thank Dr. Garritt Tucker and Dr. Eric Homer whom I met as an intern during their postdoctoral appointments at SNL for their helpful encouragements and collaborations. In addi tion, I would like to thank Dr. Aidan Thompson and Dr. Steve Plimpton from SNL for their extensive work developing and maintaining the LAMMPS molecular dynamics simulator used throughout my research. Simulations in this work were performed on resource s supported in part by the NSF under grants #0963249, #0959124, #0918970, managed by the Arkansas High Performance Computing Center (AHPCC) as well as resources provided by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported b y NSF grant #ACI-1053575. I would like to thank all the support staff at AHPCC whom have impacted my computing experience, including: Mr. Jeff Pummill, Dr. David Chaffin, Dr. Pawel Wolinski, and Dr. Rick McMullen. In addition, I would like to thank the m e bers of the XSEDE extended collaborative support services team assigned to this project whom provided assistance optimizing the visual code and enabled high-throughput studies. They inc lude: Dr. Mark Vanmoer, Dr. Sudhakar Pamidighantam, Dr. Yang Wang, Dr. Lars Koesterke, D r. Luis Cueva-Parra, and Ms. Paula Bermudez. Lastly, I would like to thank my family and friends who have walked with me throughout this journey. Their unconditional support and guid ance have allowed me to become the person I am today – Thank you.