Mechanical Properties of Thin Films and Coatings: Understanding and Controlling the Residual Stresses

William D. Nix

Department of Materials Science and Engineering
Stanford University, Stanford CA

Abstract

The mechanical properties of polycrystalline thin films and coatings are important even if the films and coatings have no mechanical function.  Residual stresses created during deposition and processing can lead to deleterious effects, including film fracture and delamination from the substrate.  Managing these failure processes depends on controlling the stresses in the film, by controlling the stress generation or stress relaxation mechanisms.  It is shown that crystallite coalescence during the early stages of film formation leads to very large tensile stresses while the insertion of excess matter into the film in the later stages of growth can create equally large compressive stresses.  The stresses produced by these mechanisms are especially troublesome in the thinnest films because the thinnest films are also the strongest.  While grain size strengthening and the confinement of dislocations by the film thickness are well-accepted mechanisms for thin film strength, recent experiments suggest that very thin films may be strong because of the paucity of dislocations sources within the film.   When these mechanisms inhibit stress relaxation in the thinnest films, control of fracture and delamination requires control of the stress generation mechanisms and/or adhesion of the film to the substrate.  It is shown that the average stresses in multilayered films can be controlled precisely, by balancing tensile and compressive stresses in the alternating layers.  It is also shown that interfacial adhesion can be controlled by deposition conditions in some cases. 

Biography

Professor Nix obtained his B.S. degree in Metallurgical Engineering from San Jose State College, and his M.S. and Ph.D. degrees in Metallurgical Engineering and Materials Science, respectively, from Stanford University.  He joined the faculty at Stanford in 1963 and was appointed Professor in 1972.  He was named the Lee Otterson Professor of Engineering at Stanford University in 1989 and served as Chairman of the Department of Materials Science and Engineering from 1991 to 1996.  He became Professor Emeritus in 2003.  In 2001 he was awarded an Honorary Doctor of Engineering Degree by the Colorado School of Mines and in 2007 an honorary degree of Doctor of Engineering by the University of Illinois. 

In 1964 Professor Nix received the Western Electric Fund Award for Excellence in Engineering Instruction, and in 1970, the Bradley Stoughton Teaching Award of ASM.  He received the 1979 Champion Herbert Mathewson Award and in 1988 was the Institute of Metals Lecturer and recipient of the Robert Franklin Mehl Award of the Metallurgical Society (TMS).  In 1995 he received the Educator Award from TMS.  He was selected by ASM International to give the 1989 Edward DeMille Campbell Memorial Lecture and in 1998 received the ASM Gold Medal.  He gave the Alpha Sigma Mu Lecture to ASM in 2000 and received the Albert Easton White Distinguished Teacher Award in 2002 and the Albert Sauveur Achievement Award in 2003, both from ASM.  He also received a Distinguished Alumnus Award from San Jose State University in 1980.  In 1993 he received the Acta Metallurgical Gold Medal and in 2001 he received the Nadai Medal from the American Society of Mechanical Engineers. He was elected Fellow of the American Society for Metals in 1978 and elected Fellow of the Metallurgical Society of AIME in 1988. He received the von Hippel Award from the Materials Research Society in 2007.  In 1987 he was elected to the National Academy of Engineering and in 2002 was elected as a Fellow of the American Academy of Arts and Sciences.  Prof. Nix was elected to the National Academy of Sciences in 2003. 

In 1966 he participated in Ford Foundation's "Residence in Engineering Practice " program as Assistant to the Director of Technology at the Satellite Division of Union Carbide Corporation.  From 1968 to 1970 Professor Nix was Director of Stanford's Center for Materials Research.  Professor Nix is engaged in research on the mechanical properties of solids.  He is principally concerned with the relation between structure and mechanical properties of materials in both thin film and bulk form.  He is co-author of over 425 publications in these and related fields and he has trained 75 Ph.D. students in these subjects in his years at Stanford.  Professor Nix teaches courses on dislocation theory and mechanical properties of materials.  He is co-author of "The Principles of Engineering Materials", published in 1973 by Prentice-Hall, Incorporated.