Role of ion beam assisted deposition in the synthesis and fracture of metal-ceramic multilayers

Abstract

Multilayer metal-ceramic films have the potential to serve as strong, tough and environmentally resistant films and coatings for a wide variety of applications. They derive their properties from the multilayer structure (architecture), the microstructure and, hence, mechanical properties of the individual layers and the stress state of the film. However, in order to realize the potential of microlaminates, these features (architecture, microstructure and stress) must be controlled. Ion beam assisted deposition (IBAD) holds the promise to provide this control. In an effort to understand how IBAD can control mechanical properties of films, single trilayer and five-bilayer metal-ceramic, Al-Al2O3 films were fabricated on ductile metal substrates using IBAD over a range of thicknesses and normalized energies. Results of bending and tension experiments revealed that the stress state is critical in determining the fracture strain (ductility) of the film. A residual compressive stress is beneficial and can be formed in the oxide phase by bombardment of the film with Ar during deposition. The behavior of film stress correlates well with Ar gas incorporation and the film consists of a high density of small cavities. Gas incorporation into the cavities or the surrounding matrix may be responsible for the observed residual compressive stress. The density of surface cracks at high strains is a function of the film architecture, film strength and the interfacial shear strength. Use of a multilayer structure reduces the crack density over a monolithic oxide film by increasing the strain needed to form through-thickness cracks and by increasing the intrinsic strength of the brittle layers by decreasing their thickness. Ion bombardment of the metal layers resulted in radiation damage and grain size refinement, both of which result in a stronger film and a lower crack density. It was shown that architecture, microstructure and stress are the key ingredients in microlaminate properties and are uniquely controllable by IBAD.