Impact of interfaces on mechanical properties of hard coating materials

Thin films are used in a wide range of applications spanning from microelectronics, to thermal barrier coatings, applied to gas turbines or aero-engine parts, and hard protective coatings. The latter are used to protect engineering components, e.g. cutting tools, from severe external loads and harsh environments. Ideally, protective coatings should be both hard and tough. As has been reported in the literature, the hardness of the mostly ceramic films can be increased by some hundred percent when two materials are synthesized in a multilayer architecture with the layer thickness of a few atomic layers, known as superlattices. Recently, the authors brought an experimental evidence that superlattices not only significantly enhance hardness but also increase fracture toughness, i.e. the ability of a material containing a crack to resist fracture. As the layer thickness of the superlattice structures are in the nanometer range, the interfaces between the layers become the crucial building blocks that determine the coatings properties, as their volume fraction inversely scales with the layer thickness. In our project, the influence of the superlattice architecture (e.g. layer thicknesses, volume ratio of the different materials), interface constitution characteristics (interface extension, composition, and structure) and impurities on the mechanical properties shall be studied. State-of-the-art micromechanical experiments will be used to determine mechanical properties which are not easily accessible using classical experimental techniques. There, micrometer-sized specimens (pre-notched single cantilevers, micropillars, micro tensile specimens) will be prepared from the superlattice film material using focused ion beam (FIB) milling. The specimens will be tested until fracture inside a scanning electron microscope (SEM) using a pico-indenter. Simultaneous recording of the load-displacement information as well as SEM images acquired during loading will allow to quantify mechanical properties (fracture toughness, shear and tensile strength) and to study crack initiation and propagation events in multilayer films. To gain fundamental understanding of the mechanical properties of superlattices, the experimental studies will be complemented with quantum mechanical simulations and continuum theory of dislocations. The findings of the project shall help to understand and to substantially improve the performance of novel materials with advanced architectures. 1