Mechanical behavior of nanostructured thin films and multilayers

It is well known that laminated composites with micrometer layer thickness can enhance the strength of these composites. The strengthening mechanism at micrometer length scale is basically governed by dislocation pile-up between layer interfaces. Consequently, the hardness of composites is proportional to 1/ (or h ^-0.5), where h is the layer thickness. This is evident from the linear dependence of hardness on 1/ as shown in the figure.

However, recent research has shown that the continuum dislocation pile up model will not work at a much smaller length scale, such as a few nanometers. At this length scale, the strength of nanolayered composites is determined by the strength of interface barriers. Therefore, the hardness typically reaches a plateau when h is only a few nanometers as shown in the figure.

Strengthening mechanisms at different length scale

As described before, the strengthening mechanisms at micrometer length scale is mainly originated from dislocation pile-up. At smaller length scale, h ~ tens of nanometers, dislocation will tend to bow within each layer. The yield strength, as a result is proportional to lnh/h. At even smaller length scale, the interface barrier strength will dominate. In this case, strengthening can be due to either Young’s modulus mismatch (Kohler stress) or lattice mismatch (coherency stress).

References

1. “Strengthening mechanisms in nanostructured copper/304 stainless steel multilayers”, Journal of Materials Research, 18 (2003) 1600. (PDF)

2. “Enhanced Hardening in Cu/330 Stainless Steel Multilayers by Nanoscale Twinning”, Acta Materialia, 52 (2004) 995. (PDF)

3. “Effects of Deposition Parameters on Residual Stresses, Hardness and Electrical Resistivity of Nanoscale Twinned 330 Stainless Steel Thin Films”, Journal of Applied Physics, in press.

4. “Work hardening in rolled nanolayered metallic composites”, A. Misra, X. Zhang, D. Hammon, R.G. Hoagland

Acta Materialia, 53 (2005) 221. (PDF)

5. “Residual Stresses in Sputter-deposited Copper / 330 Stainless Steel Multilayers”, Journal of Applied Physics, 96 (2004) 7173. (PDF)

6. “Effects of Deposition Parameters on Residual Stresses, Hardness and Electrical Resistivity of Nanoscale Twinned 330 Stainless Steel Thin Films”, Journal of Applied Physics, in press.