Niobium–niobium oxide multilayered coatings for corrosion protection of proton-irradiated liquid water targets for [18F] production

Abstract Chemically inert coatings on Havar ® entrance foils of the targets for [ 18 F] production via proton irradiation of enriched water at pressurized conditions are needed to decrease the amount of ionic contaminants released from Havar ® . During current investigation, magnetron sputtered niobium and niobium oxide were chosen as the candidates for protective coatings because of their superior chemical resistance. Aluminated quartz substrates allowed us to verify the protection efficiency of the desirable coatings as diffusion barriers. Two modeling corrosion tests based on the extreme susceptibility of aluminum to liquid gallium and acid corrosion were applied. As far as niobium coatings obtained by magnetron sputtering are columnar, the grain boundaries provide a fast diffusion path for active species of corrosive media to penetrate and to corrode the substrate. Amorphous niobium oxide films obtained by reactive magnetron sputtering showed superior barrier properties according to the corrosion tests performed. In order to prevent degrading of brittle niobium oxide at high pressures, multilayers combining high ductility of niobium with superior diffusion barrier efficiency of niobium oxide were proposed. The intercalation of niobium oxide interlayers was proved to interrupt the columnar grain growth of niobium during sputtering, resulting in improved diffusion barrier efficiency of obtained multilayers. The thin layer multilayer coating architecture with 70 nm bi-layer thickness was found preferential because of higher thermal stability

A method to improve the quality of 2.5 dimensional micro-and nano-structures produced by focused ion beam machining

The present work deals with a new technique to produce complex micro- and nano-scale patterns with high accuracy by FIB micro machining. The proposed method is related to the production of stream file, which is optimized through a software interface. A unique sampling approach is used to optimize the conversion from a 3D meshed CAD object to the focused ion beam (FIB) digital to analogue converter (DAC). The method uses a novel scan strategy, sensitive to the pattern local geometry and size, to define the optimal ion beam path, dwell time and the scanning pitch. This not only allows to minimize the redeposition but also to obtain accurate and scalable milling routines. In order to show the applicability of the method, a hemisphere and a pyramid shape are milled and compared to the shapes obtained using the conventional techniques. Results show that the method is very effective in producing complex shapes while overcoming the detrimental effect of conventional raster/serpentine FIB strategies, such as redeposition. Lastly, a fish-net structure with a pitch of ∼200 nm as well as a series of truncated cones with sub-micrometrical details are realized to show the potential impact of this new method. Results show that a spatial resolution of less than 100 nm is achievable with the help of this method

Metodo della Rosetta Micrometrica per la misura multidirezionale degli stress residui tramite tecnologia a fascio ionico focalizzato

In questo lavoro è presentato un metodo innovativo per il calcolo dello stato tensionale residuo di un materiale. Il metodo, denominato Rosetta Micrometrica (Micrometric Rosette, MR), applica i cui principi validati in precedenti pubblicazioni degli stessi autori [13, 14, 15], e consiste nell’utilizzo combinato del fascio ionico focalizzato (FIB), del microscopio elettronico a scansione (SEM) e della correlazione digitale di immagini (DIC). La tecnica consente la valutazione dello stress residuo di un materiale misurando il rilassamento dello stesso a seguito della rimozione del volume circostante che lo costringe e lo mantiene in tensione. L’implementazione del metodo, presentata in questo lavoro, consiste nella capacità di analizzare il rilassamento del materiale lungo qualsivoglia direzione nel piano della superficie del campione, e la capacità di individuarne le direzioni principali di tensione, consentendo così la valutazione completa dello stato tensionale residuo con un solo unico test di misura. La procedura è realizzata con una ripetizione ciclica di rimozione del materiale consentendo la valutazione del profilo dello stress residuo in funzione dell’affondamento. La risoluzione del gradiente è dell’ordine di 100 nm. La procedura sviluppata è semi automatizzata, grazie alla realizzazione, da parte degli stessi autori, di uno script per il continuo controllo e riallineamento degli strumenti coinvolti. Lo script permette di variare le dimensioni dell’area di materiale analizzato, il numero di cicli eseguiti, l’affondamento relativo di ogni ciclo, la numerosità e l’orientazione delle direzioni da analizzare, consentendo di adattare perfettamente la misura alle peculiarità del provino che deve essere studiato. Il metodo viene eseguito su un materiale caratterizzato da uno stato tensionale non equibiassiale, un laminato.A new method for the calculation of the residual stress state of materials is presented in this work. The method, named Micrometric Rosette (MR), applies the principles validated in previous publications by the same authors [13, 14, 15], and consists of the combined use of focused ion beam (FIB), the scanning electron microscope (SEM) and the digital correlation of images (DIC). The technique allows the evaluation of the residual stress of a material by measuring the relaxation of the same after the removal of the volume surrounding that forces him and keeps it in tension. The implementation of the method, presented in this work, consists in the ability of analyzing the relaxation of the material along any direction in the plane of the surface of the material, and the ability to identify the main directions of tension, thereby enabling the complete evaluation of the residual stress state with one and only test measurement. The measure is carried out with a cyclic repetition of removal of the material, as shown in fig 5, allowing the realization of a profile of residual stress as a function of the sinking. The resolution of the gradient is of the order of 100 nm. The developed procedure is semi-automated thanks to the implementation, by the same authors, of a script for the continuous control and realignment of the instruments. The script allows to vary the size of the area being analyzed, the number of cycles performed, the relative sinking of each cycle, the numerosity and the orientation of the directions to be analyzed, allowing to perfectly adapt the test to the peculiarities of the specimen which is to be studied. In this work the method was executed on a material characterized by a non equi biaxial stress state, a rolled sheet

Production and characterization of duplex coatings (HVOF and PVD) on TI6AL4V substrate. nanoSMat2005, International Conference on Surfaces, Coatings and Nanostructured Materials.

Mechanical resistance of non ferrous alloy components subject to severe wear conditions strongly depends on the thickness of the surface protective wear resistant layer. PVD coatings shows both very low values for wear rate and friction coefficient and high hardness. Nevertheless PVD techniques allow to obtain just thin coatings (< 10&#956;m), which do not warrant good mechanical resistance in presence of highly localized loads, during the whole lifetime of component. Such limit is currently overcame by utilizing duplex processes or multilayer coatings, which allow to reach high thickness with mechanical properties increasing gradually from the substrate-coating interface to the outer surface. Present paper deals with production and characterization of HVOF PVD duplex coatings deposited on Ti6Al4V for application in automotive industry.Mechanical resistance of non ferrous alloy components subject to severe wear conditions strongly depends on the thickness of the surface protective wear resistant layer. PVD coatings shows both very low values for wear rate and friction coefficient and high hardness. Nevertheless PVD techniques allow to obtain just thin coatings (< 10&#956;m), which do not warrant good mechanical resistance in presence of highly localized loads, during the whole lifetime of component. Such limit is currently overcame by utilizing duplex processes or multilayer coatings, which allow to reach high thickness with mechanical properties increasing gradually from the substrate-coating interface to the outer surface. Present paper deals with production and characterization of HVOF PVD duplex coatings deposited on Ti6Al4V for application in automotive industry

Wear mechanisms and in-service surface modifications of a Stellite 6B Co-Cr alloy

In the present paper, the in-service wear mechanisms and cross-sectional microstructural evolution of a Stellite 6B cobalt-based alloy subjected to sliding contact conditions were analysed by focused ion beam (FIB), transmission electron microscopy (TEM) and nanoindentation techniques. Samples under investigation consisted of a lip-seal adopted in a tunnel boring machine (TBM). In the working conditions, the lip-seal is in sliding contact with a rubber seal in a pressurised oil environment. In this specific case, an unexpected low wear resistance was observed during service, and a long machine downtime was necessary to remove the component. Results of FIB-SEM/EDS characterisation showed deep grooving due to sand (SiO2) particles embedded in the rubber seal inside the contact area, while three-body abrasive wear is observed immediately outside it. Microstructural evaluation of Stellite 6 B Co-based alloy of the cross-section also showed the presence of a nano-crystalline hardened layer with diffuse presence of stacking faults. The presence of a tribo-film with a complex structure was also clearly observed. It is concluded that wear resistance of such components could be significantly enhanced either by a proper control of abrasive contaminant particles in the lubricant oil or by introducing surface microstructural modifications of the component (e.g. a nanostructured thick coating). (C) 2012 Elsevier B.V. All rights reserved

Study on the Correlation between Microstructure Corrosion and Wear Resistance of Ag-Cu-Ge Alloys

In this work, a morphological and structural characterization of a ternary Ag-Cu-Ge alloy of known composition was performed with the aim of evaluating how the passivation parameters (time and temperature) influence the morphological features of the material surface. A nanomechanical characterization was performed in order to correlate the morphology and microstructure of the alloy with its tarnish, wear, and scratch resistance. It was found that the addition of germanium to the alloy not only provides the material with tarnish and fire-stain resistance, but it also improves the scratch and wear resistance owing to the formation of a dense and stable thin oxide layer

Focused ion beam and transmission electron microscopy as a powerful tool to understand localized corrosion phenomena

"The present study deals with focused ion beam (FIB) coupled with transmission electron microscopy (TEM) analysis of corrosion mechanisms in cathodic arc evaporation (CAE)-PVD CrN\/NbN multilayer coatings on aluminum alloy substrate. Samples were exposed to a 500-h ASTM B-117 salt spray test. Surface morphology of tested specimens was preliminary analyzed by SEM-EDS techniques. The corrosion rate in CAE-PVD coated components (ASTM B-117 standard) was significantly lower than uncoated substrates. Crevice corrosion in relation to surface defects (microdroplets) was observed after FIB and TEM cross-section analysis. Nevertheless, results of TEM analysis also showed a more complex crevice\/galvanic corrosion mechanism in relation to the bottom droplet\/coating voids, close to the coating\/substrate interface, which could be concurrent with usually proposed corrosion mechanisms. Moreover, modification of coating microstructure and composition is observed close to the peripheral area of droplets. These results give further and deeper insight into the corrosion mechanisms for PVD thin coatings, and could only be obtained by FIB sectioning and TEM analysis. Obtained results also allow the correct selection of the most suitable coating material and process parameters as a function of the nature of the substrate and the specific environmental conditions.

Effect of aging on microstructure and mechanical properties of ZnAl15Cu1 alloy for wrought applications

This study investigates the aging behavior of the recently developed ZnAl15Cu1 alloy and its change of mechanical properties with time. Samples obtained from drawn bars were artificially aged at 80 and 220°C for up to several weeks. The mechanical properties were monitored periodically by hardness testing, until a plateau was reached. The microstructural evolution was studied by optical and scanning electron microscope analyses. Bidimensional X-ray diffraction, transmission electron microscopy and selected area electron diffraction experiments were carried out to study the recrystallization phenomena. Nanoindentation measurements were also performed to determine the hardness of the single constituents. It was demonstrated that the aging causes a drop of about 30% in hardness with time. This reduction was related to the recrystallization of the Zn-rich phase network, occurring at both the studied temperatures