Diffusional Creep: Stresses and Strain Rates in Thin Films and Multilayers
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Diffusional Creep:
Stresses and Strain Rates in Thin Films and Multilayers D. Josell, T.P. Weihs, and H. Gao
Abstract In this article, we discuss creep deformation as it relates to thin films and multilayer foils. We begin by reviewing experimental techniques for studying creep deformation in thin-film geometries, listing the pros and cons of each; then we discuss the use of deformation-mechanism maps for recording and understanding observed creep behavior. We include a number of cautionary remarks regarding the impact of microstructural stability, zero-creep stresses, and transient-creep strains on stress–strain rate relationships, and we finish by reviewing the current state of knowledge for creep deformation in thin films. This includes both thin films that are heated on substrates as well as multilayer films that are tested as freestanding foils. Keywords: diffusional creep, mechanical properties, thin films.
Introduction In studying the mechanical properties of materials, most researchers emphasize yield or fracture properties and test for these quantities using simple tension tests and bend tests. However, while yield strength and fracture toughness are useful properties for designing materials and structures, they are rarely experienced in applications involving thin films and multilayers. On the other hand, many materials do exhibit time-dependent deformation, or creep, under stresses well below those needed to cause yielding or fracture. This time-dependent deformation occurs by means of a variety of deformation processes. In thin-film applications, creep deformation is often driven by stresses that arise from mismatched thermal expansions of a film and the substrate on which it is deposited. Stress relaxation by means of creep deformation is ubiquitous in the processing of multilevel, thin-film devices that require multiple processing steps at numerous temperatures. Because rates for several creep mechanisms increase rapidly as grains get smaller, they cannot be ignored in thin-film structures, even at room temperature. We therefore discuss creep
MRS BULLETIN/JANUARY 2002
deformation as it relates to homogeneous films on substrates and stacked thin films, or so-called multilayers. As noted in other articles in this issue, atomistic simulations have become an important resource for modeling a large number of deformation processes. However, even with recent increases in computational speed, simulations are limited to extreme cases of total strain or strain rate that do not yet yield useful predictions in the case of creep deformation. To predict plastic strains of a realistic magnitude, one is restricted to strain rates of 107 s1 or higher1 that are many orders of magnitude greater than experimental strain rates. Alternatively, if realistic strain rates are used (105–109 s1), creep strains are limited to magnitudes (1012) that are far too small to be informative. Therefore, this article focuses on experimental techniques and analytical models for characterizing creep deformation in thin films. In
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