Influence of Gradient Residual Stress and Tip Shape on Stress Fields Inside Indented TiN Hard Coating

Nanoindentation of treated surfaces, thin films, and coatings is often used as a simple method to measure their hardness and stiffness. These quantities are technologically highly relevant and allow to qualitatively compare different material and surface treatments but fail to capture the entire extent of the highly complex mechanical interaction between indenter tip and the tested surface. Many studies have addressed this question by analytical or numerical modeling, but they must rely on verification by recalculating indentation curves or ex situ microscopy of surface deformation postexperiment. Herein, results from in situ measurements of the multiaxial stress distributions forming beneath an indenter tip while the tested sample is still under load are presented. A 9 μm-thick TiN hard coating is tested in 1) as-deposited state and 2) shot-peened by Al2O3 particles, using two diamond wedges as indenter tips, with 60° and 143° opening angle, respectively. The results reveal a strong influence of the tip shape on the deformation behavior and the main stress component developing inside the sample while under load. In addition, a crack-closing effect can be attributed to the exponentially declining near-surface compressive residual stress gradient that is present in the shot-peened sample

On the stability of the Higher Manganese Silicides

We investigated in this work the stability of the Higher Manganese Silicides (HMS). Several alloys in the composition range 62-66 at.% Si were prepared from their constitutive elements by arc-melting. The prepared alloys were then analysed by in situ X-ray diffraction measurements and Electron Probe Micro-Analyser (EPMA). The whole results allow us to suggest that whatever the composition is, only Mn(27)Si(47) is stable for the temperatures 500 degrees C and 800 degrees C. At higher temperatures, the studied samples undergo two phase transformations which consecutively lead to the formation of Mn(15)Si(26) and Mn(11)Si(19). Mn(4)Si(7) was never evidenced in the present work. It is shown for the first time in this work that Mn(27)Si(47) is the only HMS stable phase at room temperature. (C) 2011 Elsevier B.V. All rights reserved

On the synthesis of WS2 nanotubes: reaction mechanism revelation by in-situ scanning and ex-situ transmission electron microscopy

This study provides a comprehensive understanding of the WS2 nanotube synthesis mechanism by conducting in-situ SEM and ex-situ TEM analyses of the sulfidation reaction of W18O49 nanowhiskers. The formation of WS2 nanotubes initiates with the rapid passivation of the reactive tungsten suboxide surface, followed by the evaporation of the oxide core and pressure buildup inside the nascent nanotube. The compactness and defectiveness of the initial passivation layer, gas pressure differences, and the structure of the W18O49 nanowhisker play crucial roles in determining the morphology of the final WS2 nanotubes. Additionally, this work elucidates the cause of open or closed nanotube tips based on gas pressure conditions. The combination of in-situ SEM technology and ex-situ sequential TEM analysis emerges as a robust and reliable methodology for investigating high-temperature heterogeneous reactions

Lignin-based multiwall carbon nanotubes

| openaire: EC/H2020/| 681885/EU//SCATAPNUTCarbon particles were produced by carbonization of perfectly spherical, sub-micron beads obtained from Kraft lignin. While the bulk of the particles consisted of carbon with only moderate structural order, high-resolution transmission electron microscopy revealed the presence of highly ordered multilayered carbon structures adhering to the surface of the carbon spheres. Besides irregularly shaped multiwall carbon, well-defined multiwall carbon nanotubes were identified. Energy dispersive x-ray analysis hints at a catalytic action of metallic impurities present in lignin in the synthesis of these lignin-based carbon nanostructures. Lignin-based multiwall carbon nanotubes are considered of high interest with regard to providing bio-based alternatives for fossil-based high-performance materials.Peer reviewe

W18O49 Nanowhiskers Decorating SiO2 Nanofibers: Lessons from in-situ SEM/TEM Growth to Large Scale Synthesis and Fundamental Structural Understanding

Tungsten suboxide W18O49 nanowhiskers is a material of great interest due to its potential high-end applications in electronics, near-infrared light shielding, catalysis, and gas sensing. The present study introduces three main approaches for the fundamental understanding of W18O49 nanowhisker growth and structure. Firstly, W¬18O49 nanowhiskers were grown from WO3/a SiO2¬ nanofibers in-situ in a scanning electron microscope (SEM) utilizing a specially designed micro-reactor (uReactor). It was found that irradiation by the electron beam (e-beam) slows the growth kinetics of the W18O49 nanowhisker markedly. Following this, an in-situ¬ TEM study led to some new fundamental understanding of the growth mode of the crystal shear planes in the W¬18O49 nanowhisker and the formation of domain (bundle) structure. High-resolution scanning transmission electron microscopy (HRSTEM) analysis of a cross-sectioned W18O¬49 nanowhisker revealed the well-documented pentagonal Magnéli columns and hexagonal channels characteristics for this phase. Furthermore, a highly crystalline and oriented domain structure and previously unreported mixed structural arrangement of tungsten oxide polyhedrons were analyzed. The tungsten oxide phases found in the cross-section of the W18O49 nanowhisker were analyzed by nanodiffraction and electron energy loss spectroscopy (EELS), which was discussed and compared in the light of theoretical calculations based on the density functional theory (DFT) method. Finally, the knowledge gained from the in-situ SEM and TEM experiments was valorized in developing a multigram synthesis of W18O49/a-SiO2 urchin-like nanofibers in a flow reactor

Effect of Pressure and Temperature on Microstructure of Self-Assembled Gradient AlxTi1−xN Coatings

The correlation between structural properties of Al-rich self-assembled nano-lamellar AlxTi1−xN coatings and process parameters used during their chemical vapor deposition (CVD) remains unexplored. For this article, two gradient AlxTi1−xN coatings were prepared by a stepwise increase in temperature and pressure in the ranges of 750–860 °C and 1.56 to 4.5 kPa during the depositions at a constant composition of the process gas mixture. The cross-sectional properties of the coatings were analyzed using X-ray nanodiffraction (CSnanoXRD) and electron microscopy. Experimental results indicate that the variation of the process parameters results in changes in microstructure, grain morphology, elastic strain, nanolamellae’s chemistry and bi-layer period. At temperatures of ~750–800 °C and pressures of 2.5–4.5 kPa, preferably cubic nanolamellar grains are formed, whose microstructure correlates with the build-up of tensile stresses, which become relaxed in coating regions filled with nanocrystalline grains. CSnanoXRD superlattice satellite reflections indicate the period of the cubic Al(Ti)N-Ti(Al)N bilayers, which changes from 6.7 to 9 nm due to the temperature increase from 750 to ~810 °C, while it remains nearly unaffected by the pressure variation. In summary, our study documents that CVD process parameters can be used to tune microstructural properties of self-assembled AlxTi1−xN nanolamellae as well as the coatings’ grain morphology

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 m$^{0.5}$, 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

Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics

Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions