The cracking resistance of nanoscale layers and films
- PDF / 895,134 Bytes
- 7 Pages / 576 x 792 pts Page_size
- 111 Downloads / 178 Views
Thin film cracking has been studied for a nanometer brittle/ductile layered system consisting of Si and Cu. Single Si films and Cu/Si/Cu trilayers were fabricated by physical vapor deposition. The films were deposited onto a ductile substrate consisting of stainless steel with a thin polyimide interlayer. Straining of the substrate induced cracking of the Si. Cracking strains > 1 % were observed, particularly in layers ^100 nm thick. The critical cracking strain was found to depend upon the Si layer thickness, as well as the thickness and elastic/plastic properties of the adjacent ductile layers. Si cracking was suppressed by the presence of adjacent Cu layers. The measured strains were compared with lower-bound critical strains for tunnel and channel cracking. Comparisons indicated that these mechanisms control the critical strain found in trilayers, because trilayer fabrication introduces edge flaws larger than the Si layer thickness. Conversely, for Si films, the measured critical strains exceed the channel cracking strain, because the flaws in these films are relatively small. I. INTRODUCTION Multilayers made from constituents with widely differing physical properties have been advocated as materials with exciting new thermomechanical properties.!~3 The concepts are founded on the unusual behavior of naturally occurring materials, such as shells.4 The approach has been to combine a hard, brittle material with a softer, ductile material in order to achieve an optimal combination of damage tolerance, stiffness, and strength. Generally, this has involved combinations of a ceramic with a polymer or metal and an intermetallic with a metal.1"8 Research on ceramic/metal and intermetallic/metal multilayers has provided some instructive principles.5^8 One of the critical performance requirements is the attainment of damage tolerance while maintaining high strength.8 The principal mode of damage consists of crack formation in the brittle layers.8 The variables that influence such cracking are addressed in this study. Particular emphasis is placed on the role of the layer thickness and the mechanical characteristics of the adjoining material, such as modulus and yield strength. Theoretical work on brittle layer damage has identified tunneling or channeling modes of crack extension9-10 (Fig. 1). These modes operate when flaws pre-exist and propagate in steady state along the layers. They provide a criterion for the lower bound critical strain, ec, with resulting implications for the fail-safe design of multilayers.9-11 The criterion is manifest as a cracking number Z given by9
Z = Tb/tbe2cEb
(1)
J. Mater. Res., Vol. 10, No. 7, Jul 1995 http://journals.cambridge.org
Downloaded: 02 Dec 2014
where tb is the layer thickness, Eb its modulus, and Yb its fracture energy. The cracking number is of order unity, but depends somewhat on the elastic properties and yield strengths of the materials comprising the layers. A central implication of Eq. (1) is that ec increases when the layer thickness becomes small. Thin layers in the
Data Loading...
