Biomimetic hard and tough nanoceramic Ti-Al-N film with self-assembled six-level hierarchy.

Nature uses self-assembly of a fairly limited selection of components to build hard and tough protective tissues like nacre and enamel. The resulting hierarchical micro/nanostructures provide decisive toughening mechanisms while preserving strength. However, to mimic microstructural and mechanical characteristics of natural materials in application-relevant synthetic nanostructures has proven to be difficult. Here, we demonstrate a biomimetic synthesis strategy, based on chemical vapour deposition technology, employed to fabricate a protective high-temperature resistant nanostructured ceramic TiAlN thin film with six levels of hierarchy. By using just two variants of gaseous precursors and through bottom-up self-assembly, an irregularly arranged hard and tough multilayer stack was formed, consisting of hard sublayers with herringbone micrograins, separated by tough interlayers with spherical nanograins, respectively composed of lamellar nanostructures of alternating coherent/incoherent, hard/tough, single-/poly-crystalline platelets. Micro- and nanomechanical testing, performed in situ in scanning and transmission electron microscopes, manifests intrinsic toughening mechanisms mediated by five types of interfaces resulting in intergranular, transgranular and cleavage fracture modes with zigzag-like crack patterns at multiple length-scales. The hierarchical 2.7 µm thick film self-assembled during ∼15 minutes of deposition time shows hardness, fracture stress and toughness of ∼31 GPa, ∼7.9 GPa and ∼4.7 MPa m0.5, respectively, as well as phase/microstructural thermal stability up to ∼950/900 °C. The film's microstructural and mechanical characteristics represent a milestone in the production of protective and wear-resistant thin films.

In-situ Observation of Cross-Sectional Microstructural Changes and Stress Distributions in Fracturing TiN Thin Film during Nanoindentation.

Load-displacement curves measured during indentation experiments on thin films depend on non-homogeneous intrinsic film microstructure and residual stress gradients as well as on their changes during indenter penetration into the material. To date, microstructural changes and local stress concentrations resulting in plastic deformation and fracture were quantified exclusively using numerical models which suffer from poor knowledge of size dependent material properties and the unknown intrinsic gradients. Here, we report the first in-situ characterization of microstructural changes and multi-axial stress distributions in a wedge-indented 9 µm thick nanocrystalline TiN film volume performed using synchrotron cross-sectional X-ray nanodiffraction. During the indentation, needle-like TiN crystallites are tilted up to 15 degrees away from the indenter axis in the imprint area and strongly anisotropic diffraction peak broadening indicates strain variation within the X-ray nanoprobe caused by gradients of giant compressive stresses. The morphology of the multiaxial stress distributions with local concentrations up to -16.5 GPa correlate well with the observed fracture modes. The crack growth is influenced decisively by the film microstructure, especially by the micro- and nano-scopic interfaces. This novel experimental approach offers the capability to interpret indentation response and indenter imprint morphology of small graded nanostructured features.