In the last ten years amorphous alumina coatings, deposited by Pulsed Laser Deposition, emerged as potential key enabling technology in the fields of heavy liquid metal fast reactors (lead and lead-bismuth) and fusion. In the former, as coating of the steel fuel cladding and in the latter as multifunctional coating providing a barrier against tritium permeation, steel corrosion and electrical insulation. Nevertheless, a detailed knowledge of the behavior of this thermodynamically metastable material at high temperatures and under neutron irradiation is still unknown. A knowledge gap that is mandatory to fill up for the deployment of this barrier technology. In the present work, we present a first step towards this goal, by the in-situ dynamic observation of the radiation induced crystallization processes of thin films of amorphous Al2O3, induced by ion-irradiation over an extensive range of temperatures (400-800 degrees C). The study was performed at the Intermediate Voltage Electron Microscope (IVEM)-Tandem Facility at Argonne National Laboratory. The experimental findings allow to elucidate the dependence of the grain growth on ion dose and temperature. A kinetic approach has been used to derive the process activation energies and other important parameters.

In this paper, a novel experimental-numerical methodology is outlined, which aims at estimating the homogenized elastic properties of thin nanostructured coatings, utilized in a variety of industrial contexts. Focus is posed on amorphous titanium dioxide (TiO2) films, a few micrometer thick, produced through Pulsed Laser Deposition in the form of columnar elements coating a Silicon flat substrate. The proposed inverse strategy rests on digital images acquired by a Scanning Electron Microscope on coated samples, with a nanometer resolution, and consists of the following steps: (i) within a representative volume element, cavities existing in between the nanostructured columns are identified in the raster images by an automatic edge detection procedure; (ii) information concerning the location of the pore borders are vectorized and passed through a Python script to a commercial finite element code; (iii) the homogenized stiffness matrix is reconstructed on the basis of finite element simulations of such a representative volume subjected to elementary loading conditions, each corresponding to a single component of the effective strain tensor. Diverse mechanical models with periodic boundary conditions have been comparatively assessed, fully three dimensional or under plane conditions, assuming for the TiO2 columns in an amorphous state (as corroborated by Raman spectroscopy) an isotropic behaviour, with a priori known Young’s modulus and Poisson’s ratio. The homogenized elastic moduli and coefficients of TiO2 thin films, at varying the deposition process parameter, are estimated by the novel procedure, resulting in an orthotropic, transversely-isotropic behaviour.

Materials at the nanoscale often have properties which differ from those they have in the bulk form. These properties significantly depend on the production process, and their measurement is not trivial. The elastic properties characterize the ability of materials to deform in a reversible way; they are of interest by themselves, and as indicators of the type of nanostructure. As for larger scale samples, the measurement of the elastic properties is more straightforward, and generally more precise, when it is performed by a deformation process which involves exclusively reversible strains. Vibrational and ultrasonic processes fulfill this requirement. Several measurement techniques have been developed, based on these processes. Some of them are suitable for an extension towards nanometric scales. Until truly supramolecular scales are reached, the elastic continuum paradigm remains appropriate for the description and the analysis of ultrasonic regimes. Some techniques are based on the oscillations of purpose-built testing structures, mechanically actuated. Other techniques are based on optical excitation and/or detection of ultrasonic waves, and operate either in the time domain or in the frequency domain. A comparative overview is given of these various techniques.

In this work, we investigate the correlation between morphology, composition, and the mechanical properties of metallic amorphous tungsten-oxygen and amorphous tungsten-oxide films deposited by Pulsed Laser Deposition. This correlation is investigated by the combined use of Brillouin Spectroscopy and the substrate curvature method. The stiffness of the films is strongly affected by both the oxygen content and the mass density. The elastic moduli show a decreasing trend as the mass density decreases and the oxygen-tungsten ratio increases. A plateaux region is detected in correspondence of the transition between metallic and oxide films. The compressive residual stresses, moderate stiffness and high local ductility that characterize compact amorphous tungsten-oxide films make them promising for applications involving thermal or mechanical loads. The coefficient of thermal expansion is quite high (i.e. 8.9 $\cdot$ 10$^{-6}$ K$^{-1}$), being strictly correlated to the amorphous structure and stoichiometry of the films. Under thermal treatments they show a quite low relaxation temperature (i.e. 450 K). They crystallize into the $\gamma$ monoclinic phase of WO$_3$ starting from 670 K, inducing an increase by about 70\% of material stiffness.

In this work, we exploit nanosecond laser irradiation as a compact solution for investigating the thermomechanical behavior of tungsten materials under extreme thermal loads at the laboratory scale. Heat flux factor thresholds for various thermal effects, such as melting, cracking and recrystallization, are determined under both single and multishot experiments. The use of nanosecond lasers for mimicking thermal effects induced on W by fusion-relevant thermal loads is thus validated by direct comparison of the thresholds obtained in this work and the ones reported in the literature for electron beams and millisecond laser irradiation. Numerical simulations of temperature and thermal stress performed on a 2D thermomechanical code are used to predict the heat flux factor thresholds of the different thermal effects. We also investigate the thermal effect thresholds of various nanostructured W coatings. These coatings are produced by pulsed laser deposition, mimicking W coatings in tokamaks and W redeposited layers. All the coatings show lower damage thresholds with respect to bulk W. In general, thresholds decrease as the porosity degree of the materials increases. We thus propose a model to predict these thresholds for coatings with various morphologies, simply based on their porosity degree, which can be directly estimated by measuring the variation of the coating mass density with respect to that of the bulk.

Oxide nanoceramics combine the enhanced radiation tolerance of nanocrystalline materials with the chemical inertness of oxides, and are promising materials for highly corrosive and intense radiation environments. In this work, nanocrystalline Al2O3 thin films are irradiated at 600 degrees C with either 12 MeV Au5++18 MeV W8+ or 4 MeV Ni2+ ions. The radiation damage exposure exceeds 450 displacements per atom. A comprehensive analysis of the irradiated samples is accomplished by X-Ray Diffractometry (XRD), Transmission Electron Microscopy (TEM) and Scanning-TEM (STEM). Results are compared in an effort to establish correlations between the irradiation spectrum and the response of this class of materials to radiation environments. The results show that grain growth is the main microstructural change induced by ion irradiation in the material, regardless of the ion utilized in this work. The phase evolution may be depth-dependent, and depends strongly on the ion utilized and on the irradiation spectrum. 12 MeV Au5++18 MeV W8+ irradiations favor the formation of gamma-A1203 and alpha-Al2O3, while 4 MeV Ni2+ irradiations yield mainly delta-Al2O3, accompanied by small alpha-Al2O3 centers. Molecular dynamics simulations of displacement cascades are used to support discussions on the mass effect brought about by the different ions. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The in plane coefficient of thermal expansion (CTE) and the residual stress of nanostructured W based coatings are extensively investigated. The CTE and the residual stresses are derived by means of an optimized ad-hoc developed experimental setup based on the detection of the substrate curvature by a laser system. The nanostructured coatings are deposited by Pulsed Laser Deposition. Thanks to its versatility, nanocrystalline W metallic coatings, ultra-nano-crystalline pure W and W-Tantalum coatings and amorphous-like W coatings are obtained. The correlation between the nanostructure, the residual stress and the CTE of the coatings are thus elucidated. We find that all the samples show a compressive state of stress that decreases as the structure goes from columnar nanocrystalline to amorphous-like. The CTE of all the coatings is higher than the one of the corresponding bulk W form. In particular, as the grain size shrinks, the CTE increases from 5.1 10$^{-6}$ K$^{-1}$ for nanocrystalline W to 6.6 10$^{-6}$ K$^{-1}$ in the ultra-nano-crystalline region. When dealing with amorphous W, the further increase of the CTE is attributed to a higher porosity degree of the samples. The CTE trend is also investigated as function of materials stiffness. In this case, as W coatings become softer, the easier they thermally expand.

Bare and Al2O3-coated austenitic steel samples are exposed to lead-fast-reactor\relevant corrosive conditions. Selective leaching of Ni, Mn and Cr is observed in bare samples exposed to high temperature stagnant lead (550 degrees C, 10(-8) wt.% oxygen, 1000 and 4000 h). By contrast, corrosion is not observed in either pristine (4000 h) or irradiated (1000 h) coated samples. Further characterization and testing methods include SEM, TEM, STEM, EDS, cyclic nanoimpact, microindentation, scratch, and thermal cycling. Overall, the results show that the coatings retain structural integrity under the conditions investigated, which is a crucial prerogative for corrosion protection with ceramic coatings.

The high operating temperatures and radiation damage levels foreseen in innovative nuclear systems are expected to accelerate corrosion kinetics in most of the reactor designs. Among several corrosion mitigation strategies, ceramic coatings stand as a promising option. Here, fully dense and compact Al2O3 coatings are grown at room temperature by Pulsed Laser Deposition (PLD). The utility of the coatings as corrosion resistant barriers is examined through short-term exposure of coated 1515Ti plates to stagnant lead at 550°C (500 hours, 10-10 wt.% O). Post-test analyses by SEM reveal no signs of corrosion. The effect of radiation damage on the coatings is investigated through heavy ion irradiation up to damage levels of 20, 40 and 150 displacements per atom at 600°C. TEM analyses reveal that the as-deposited coatings are mainly amorphous and contain a homogeneous dispersion of ultra-fine (6±4 nm) crystalline nano-domains. The structural evolution upon irradiation is characterized by crystallization of the amorphous phase, which is accompanied by a dramatic increase of the nanoindentation hardness. Increasing radiation damage levels induce grain growth, twinning and Hall-Petch softening. The performance of the coatings under impact loading is examined by nanoimpact tests. The impact energy is dissipated more efficiently in the irradiated samples –specifically, through localized crystalline-toamorphous transformations. In spite of the high radiation damage levels reached, no delamination is observed at the coating-substrate interface. Overall, PLD-grown Al2O3 is a promising coating material for innovative nuclear systems.

Micron-thick boron films have been deposited by Pulsed Laser Deposition in vacuum on several substrates at room temperature. The use of high energy pulses (> 700 mJ) results in the deposition of smooth coatings with low oxygen uptake even at base pressures of 10− 4–10− 3 Pa. A detailed structural analysis, by X-Ray Diffraction and Raman, allowed to assess the amorphous nature of the deposited films as well as to determine the base pressure that prevents boron oxide formation. In addition the crystallization dynamics has been characterized showing that film crystallinity already improves at relatively low temperatures (800 °C). Elastic properties of the boron films have been determined by Brillouin Spectroscopy. Finally, micro-hardness tests have been used to explore cohesion and hardness of B films deposited on aluminum, silicon and alumina. The reported deposition strategy allows the growth of reliable boron coatings paving the way for their use in many technology fields.

Pulsed Laser Deposition allows to obtain W and W-Ta alloy coatings with different nanostructures, monitored by X-ray diffraction. The correlation between such structures and the elastic properties is investigated for amorphous-like, ultra-nano- and nano-crystalline coatings obtained by tuning the gas pressure during deposition, annealing temperature and Ta concentration. The full elastic characterization is achieved by surface Brillouin spectroscopy, interpreted by isotropic and anisotropic film models. Amorphous like coatings are obtained with He pressures of tens of Pa. In comparison with bulk W, they have lower stiffness, by about 60%, closely correlated to the mass density (lower by about 40%). In the nanocrystalline regime the stiffness is more correlated to the average grain size, approaching the bulk values for increasing crystallite size. Vacuum annealing of amorphous like coatings leads to the nucleation of ultra-nano crystalline seeds, embedded in an amorphous matrix with intermediate values for mass density and stiffness. Here, the stiffness results from an interplay between the crystal size and the density. Alloying with Ta leads to properties which are consistent with the lever rule in the nanocrystalline regime, and deviate from it when the higher Ta concentration, interfering with crystal growth, induces an ultra-nano crystalline structure.

The lack of suitable materials solutions stands as a major challenge for the development of advanced nuclear systems. Most issues are related to the simultaneous action of high temperatures, corrosive environments and radiation damage. Oxide nanoceramics are a promising class of materials which may benefit from the radiation tolerance of nanomaterials and the chemical compatibility of ceramics with many highly corrosive environments. Here, using thin films as a model system, we provide new insights into the radiation tolerance of oxide nanoceramics exposed to increasing damage levels at 600 °C – namely 20, 40 and 150 displacements per atom. Specifically, we investigate the evolution of the structural features, the mechanical properties, and the response to impact loading of Al2O3 thin films. Initially, the thin films contain a homogeneous dispersion of nanocrystals in an amorphous matrix. Irradiation induces crystallization of the amorphous phase, followed by grain growth. Crystallization brings along an enhancement of hardness, while grain growth induces softening according to the Hall-Petch effect. During grain growth, the excess mechanical energy is dissipated by twinning. The main energy dissipation mechanisms available upon impact loading are lattice plasticity and localized amorphization. These mechanisms are available in the irradiated material, but not in the as-deposited films.

Viene descritta la progettazione di prove di irraggiamento con ioni pesanti di rivestimenti di allumina prodotti per ablazione laser da eseguire presso la piattaforma sperimentale JANNUS dei centri di ricerca CEA di Saclay (SRMP) e di Orsay (CSNSM). In confronto all’utilizzo di neutroni, gli ioni pesanti hanno il vantaggio di permettere il raggiungimento di un danno da irraggiamento elevato in tempi brevi ed a bassi costi, il tutto senza attivare i campioni irraggiati, il che facilita la loro successiva caratterizzazione. I campioni irraggiati verranno caratterizzati con prove di nanoindentazione, scratch e spettroscopia Brillouin, per valutarne le proprieta meccaniche. I campioni verranno inoltre caratterizzati dal punto di vista della microstruttura tramite analisi XRD, SEM, spettroscopia Raman e TEM ex-situ. Per seguire l’evoluzione della struttura durante l’irraggiamento verranno eseguite osservazioni TEM in situ durante gli esperimenti di irraggiamento.

A preliminary experimental comparison of the behaviour of aluminium and magnesium alloys subjected to Liquid Hot Isostatic Pressing (LHIP) is proposed. The two metals melt at approximately the same temperature. However, as a consequence of a larger deformability of magnesium at elevated temperatures, the choice of LHIP parameters – and especially the temperature at which the pressure is applied – in the present exploratory case was constrained to values far smaller than those one would like to select in order to improve the ultimate tensile stress and the elongation to fracture.

In this work, we propose a nanocrystalline Al2O3/amorphous Al2O3 composite coating for protecting steels operating in heavy liquid metals (HLMs) at high temperature. The coating is grown by Pulsed Laser Deposition (PLD). Previous work by Garcia Ferre et al. [27] has shown that PLD-grown Al2O3 attains an unusual ensemble of metal-like mechanical properties and strong interfacial bonding, all combined with the chemical stability and resistance to wear of ceramics. In a short term corrosion test (500 h), 9Cr1 Mo steel samples were coated and exposed to stagnant lead at 550 degrees C. Cross-sectional SEM and EDX analysis revealed no signs of corrosion. (C) 2013 Elsevier Ltd. All rights reserved.

© 2013 Beghi et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Combination of Acoustic Methods and the Indentation Technique for the Measurement of Film Properties