The fracture behavior of nanometer scale brittle/ductile multilayers

Composites with alternating layers of stiff, brittle ceramics or intermetallics and ductile metals are found in many applications, including electronic devices, electronic packaging, and structural coatings. Recent interest has centered on multilayers with thicknesses in the nanometer range, in which mechanisms for the strengthening of both brittle and ductile materials arise. The present research focuses on the fracture behavior of a model nanoscale brittle/ductile multilayered system.; The first part of this study dealt with the formation of cracks within brittle layers. Specimens comprised of Si and Cu were produced by physical vapor deposition. The layers were evaporated onto ductile substrates consisting of stainless steel with a thin polyimide surface coating. Straining of the substrate induced cracking of the Si. Cu/Si/Cu trilayers of various thickness combinations were produced and tested. Cracking strains {dollar}ge{dollar}1% were observed. The minimum strain needed to induce fracture was found to be in agreement with the tunnel cracking mechanism, which predicts a lower-bound critical strain that increases with decreasing Si layer thickness. Si cracking was also found to be influenced by the elastic/plastic properties of the adjacent ductile layers.; Determination of the conditions under which an initial brittle layer crack propagates through the multilayer comprised the other part of this research. Two factors which influence crack progression, the thickness of the intervening metal layer and interfacial debonding, were investigated. To examine the role of the metal layer, Si/Cu/Si trilayers were produced and tested. Cracking in both Si layers, with no apparent offset of the crack plane, was observed, suggesting that brittle phase crack renucleation occurred readily. Additionally, in dramatic contrast with thick-layered laminates having similar metal volume fractions, rupture of the nanoscale Cu layers was always observed. This difference was attributed to the deterministic nature of the tunnel cracking phenomenon and the high strains necessary for its occurrence. Finite-element modeling indicated that because of the strain concentration associated with the crack tip, fracture in one Si layer readily induced crack renucleation in the next Si layer. Since the maximum deformation in a metal layer is on the order of its thickness, nanoscale Cu layers are particularly susceptible to rupture at the high critical strains required for tunnel cracking in the Si. Conversely, cracking in thick brittle layers occurs at relatively low strains because fracture is dependent upon the flaw distribution within the layers, instead of a lower-bound critical strain. Cracking in one layer does not necessarily induce renucleation in the next. This leads to stochastic, damage tolerant behavior if a sufficient volume fraction of metal is present.; Finally, weak interfaces were introduced in Cu/Si/Cu/Si multilayers through the deposition of thin Au interlayers. Interfacial debonding was found to be an effective means of preventing a crack in one Si layer from renucleating in the next. However, the fracture energy of the interface must be sufficiently low, so that the debond crack does not preferentially kink into the next Si layer.; Observations based on this model system suggest that the attainment of high critical cracking strains in the brittle phase is the principal advantage associated with nanoscale brittle/ductile multilayers. Damage tolerance is not expected when the metal layers are thin, even at high metal volume fractions. Control of the interfacial properties is crucial if damage tolerant behavior is required.