Oxide surface films on metal crystals.

In their Report “Sample dimensions influence strength and crystal plasticity” (13 Aug., p. [986][1]), M. D. Uchic et al. demonstrate that the dimensions of micrometer-sized metal crystals have dramatic effects on the properties of the crystals. However, they do not mention a critical factor; namely, that oxide films were present on their specimens. All metals oxidize. Therefore, except for short times in ultra-high vacua, they have oxide (or hydroxide) surface films. These films have negligible effects on the behaviors of most macroscopic metals. As sizes decrease, however, they become increasingly important. The necessity for shearing through these films may well account for the increase in jaggedness of the stress-strain curves observed by the authors with decreasing size. Also, this interpretation is consistent with the lack of a “size effect” observed for the nickel-based superalloy, as well as the large glide offsets. # Response {#article-title-2} Gilman points out that a thin surface film (oxide or hydroxide) on a micrometer-sized crystal could affect the sample flow behavior, especially as the sample size decreases. He raises an important point. However, it is not entirely clear to us how a surface film could contribute significantly to the strength under progressively rising stresses, without also leading to severe strain localization. High-resolution scanning electron micrographs of deformed samples generally show slip traces at many points along the sample gage length. One might expect much more slip localization in these samples if oxide films were acting as strong barriers to deformation. The “jaggedness” of the stress-strain response that Gilman mentions may be simply the result of mechanical testing at the micrometer scale. A slip event in a micrometer-sized crystal will be more clearly resolved than if the same event occurred in a millimeter-sized sample. In addition, Gilman indicates that the lack of a “size effect” in nickel-based superalloys is consistent with the interpretation that surface films are affecting the observed mechanical behavior. Although this is open to interpretation, we have recently learned through additional testing that sample size effects of the type discussed in our Report do occur for a nickel-based superalloy at sample sizes that are 5 μm in diameter and smaller ([1][2]). Finally, significant sample size effects have recently been observed in testing of 1-μm-diameter single crystals of pure gold ([2][3]), using the same fabrication and testing methodology. The magnitude of the size-dependent strengthening is similar to the results for pure nickel. However, gold does not form a native oxide. Clearly, more research is needed to positively establish the connection between sample size effects to either changes in deformation mechanisms or extrinsic effects such as surface case hardening. 1. 1.[↵][4] 1. M. D. Uchic, 2. D. M. Dimiduk , in preparation. 2. 2.[↵][5] 1. J. R. Greer, 2. W. C. Oliver, 3. W. D. Nix , presentation at the 2004 Fall Minerals, Metal, and Materials Society Meeting, New Orleans, LA, 26 to 29 Sept. 2004. [1]: /lookup/doi/10.1126/science.1098993 [2]: #ref-1 [3]: #ref-2 [4]: #xref-ref-1-1 "View reference 1. in text" [5]: #xref-ref-2-1 "View reference 2. in text"