High-temperature small-scale fracture mechanics and plasticity of a hardcoating system

Forging and cutting tools for high-temperature applications are often protected using hard nanostructured ceramic coatings. While a moderate amount of knowledge exists for material properties at room temperatures, significantly less is known about the system constituents at the elevated temperatures generated during service. For rational engineering design of such systems, it is therefore important to have methodologies for testing these materials to understand their properties under such conditions. Additionally, small-scale mechanical testing is of inherent importance for thin-films systems and materials subject to surface modification or treatment as for plasma nitrided steels. In this work, we present results on both the hard ceramic coating and the nitrided steel substrate using in situ micro-mechanical measurements at temperatures to 500 °C. The fracture and plastic yield behavior of FIB milled micro-pillars of plasma-nitrided tool steel was first investigated using in situ compression experiments. It was found that the yield strength of nitrided steel is particularly sensitive to temperatures within the service range. Elevated temperature led to significant softening of the nitrided steel and transition from slip-based to more ductile plastic flow. A 70% reduction in yield stress was observed when transitioning from room-temperature to 500 °C, which was then recovered upon cooling back to RT indicating a mechanistic activation at high temperature. The fracture toughness behavior of a hard CrN coating was then investigated using various micro-geometries and notching parameters. Toughness measurements at high temperatures highlighted the profound effect of the notching ion during small-scale fracture measurements. It was found that gallium ion implantation led to significant toughening of CrN, based on gallium dosage experiments and alternative notching using both xenon and helium sources. The effect of different notching ions was additionally understood through Monte Carlo simulations of energetic ion interactions in a dense ceramic matrix

Plasma-CVD-coated glass beads as photocatalyst for water decontamination

AmorphousTiO2 films were deposited on glass microbeads using a specially designed circulating fluidized bed plasma-CVD reactor. The film thickness was varied between 7 and 120 nm. While only little carbon impurity was found, XPS analysis revealed the presence of silicon, sodium and alkaline earth elements in the titania coating. Reduced amounts of these substrate-originating impurities were observed in the thicker films. By ToF-SIMS imaging, cross-sectional TEM andtime-resolved dissolution, the titania coatings were proven to be uniform, both per particle and in terms of the film thickness distribution.The photocatalytic performance of the composite particles was evaluated in a fully irradiated fluidized-bed photoreactor. The thinnest films had some photocatalytic activity in the as-deposited state, possibly induced by the high specific power of the microwave plasma or silicon doping. The thicker films needed a post-deposition calcination at 723K to achieve catalytic activity. Both the degree of anatase crystallization and the activity were improved by applying thicker films and after UV irradiation-plus-calcining. All films showed good adhesion and abrasion resistance during the photocatalytic tests. The best plasma-CVD films were about 70% as efficient (per unit reactor volume) as the reference material, P-25 immobilized on quartz sand.Fil: Karches, Martin. Eidgenossische Technische Hochschule Zurich;Fil: Morstein, Marcus. Eidgenossische Technische Hochschule Zurich;Fil: Rudolf von Rohr, Philipp. Eidgenossische Technische Hochschule Zurich;Fil: Pozzo, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Giombi, Jose Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Baltanas, Miguel Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentin

Ni nanocluster composites for enhanced impact resistance of multilayered arc-PVD ceramic coatings

Optimally, hard protective coatings should effectively absorb impact energy to reduce the likelihood of failure events. In this work, an arc-PVD approach was utilised for the deposition of thick ceramic multilayer AlCrTiN/CrN-based coatings containing a distribution of metallic nickel inclusions throughout sequential CrN-based interlayers. The aim of such coatings is to provide resistance to impact loading in intensive application environments, such as drop forging of steel and turbine blades exposed to abrasive particles. The structure and micro-structural development of the ceramic was first investigated using transmission electron microscopy, where discrete Ni inclusions were observed as both larger discs (d ~ 600–800 nm, h ~ 200 nm) and smaller (d ~ 50 nm) nanoclusters. Confirmation of the distribution and nanocluster chemistry was achieved using atom probe tomography. For mono-block coatings, XRD data showed drastically reduced internal stresses as a result of the Ni inclusions, enabling the creation of thicker protective coatings which minimise substrate stress concentration upon loading. Nickel inclusion additionally provides a softening of the containing hard ceramic layer, allowing for tuning of mechanical properties. Supporting this idea, in situ micropillar compression measurements of the multi-layered coating systems showed that Ni clusters hindered crack propagation through the coating during failure, while the fracture strength could be increased by incorporating both Ti and Ni in the softer CrN-based layer. High-load impact testing highlighted the influence of Ni ‘shock absorbers’ in reducing circumferential cracking, which was further confirmed by an industrial die forging test that demonstrated a 15–22% life-time increase compared to a much thicker hard-chrome plated reference. The results of this study demonstrate an exciting way to produce Ni nanocluster integrated hard ceramic multi-layers using arc-PVD in a single processing step. Such tuneable thin-film composite systems show great promise in minimising damage from impact loading, even under severe working conditions such as in hot forging