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

The central result of this project is the decoding of the mechanisms that effectively hinder crack propagation within artificially nanostructured materials at the atomic level. During the industrial processing of metals, cutting tools are subjected to extreme mechanical stress and high operating temperatures. To protect these components from rapid wear, ultra-thin ceramic coatings are commonly applied. While these ceramic materials exhibit exceptional hardness, they are inherently brittle and prone to cracking under severe operational stress. Within the scope of this project, it was investigated how nanotechnology can be utilized to overcome this limitation. The research focused on superlattice coatings, which are artificially engineered structures consisting of alternating layers of two different materials, each only a few atomic layers thick. Although the high hardness of these materials is already well established in science, this project specifically addressed the mechanisms that hinder crack propagation within these coatings. By combining advanced laboratory experiments with atomistic computer simulations, the underlying mechanical effects were successfully uncovered at the atomic level. The findings demonstrate that the crystalline architecture of the coatings directly dictates their mechanical performance. When synthesized as defect-free single crystals, the superlattice coatings exhibit significantly higher toughness and strength than polycrystalline microstructures. Furthermore, the specific choice of material pairings is crucial: the interplay of stiff and compliant layers effectively hinders crack propagation. For these enhanced properties to manifest, the interfaces between the individual nanolayers must remain atomically sharp. If these boundaries blur due to atomic intermixing, the mechanical crack resistance decreases significantly. These discoveries have direct and important implications for industrial applications. Manufacturing tools coated with these optimized nanostructures benefit from a significantly extended service life. This allows for higher machining speeds, ultimately increasing efficiency and productivity in production processes. The project also contributes to global sustainability goals. Extending the operational lifetime of engineering components conserves valuable resources and reduces energy consumption. Since the outstanding mechanical properties are achieved purely through the geometric design at the atomic level, there is no need for additional chemical alloying elements.