Analisi del processo di “pack chromising” su superleghe di Ni per turbine a gas

I rivestimenti diffusivi sono utilizzati per la realizzazione di strati protettivi necessari a migliorare la resistenza dicomponenti soggetti a fenomeni degradativi ad alta temperatura, come ossidazione e corrosione a caldo (es.: componentiper impianti turbogas, bruciatori, etc.). In particolare, l’arricchimento in cromo della superficie di leghe metalliche, adesempio attraverso processi tipo “pack-cementation”, garantisce una buona protezione dalla corrosione a caldo di tipoII. Tuttavia, in letteratura, non esistono studi sistematici ed approfonditi sui meccanismi di formazione e sull’effetto deiparametri di processo sulla microstruttura di questi rivestimenti.Lo scopo del presente lavoro, pertanto, è stato lo studio dell’effetto della composizione della miscela di polveri (packmix)e della durata del trattamento termico (12 e 24 h) sulla microstruttura e sullo spessore dei rivestimenti ottenutiutilizzando il processo di pack-chromizing. Come substrato è stata impiegata la superlega Inconel 738, mentre il packmixutilizzato è composto da polvere di cromo (10 e 25 wt.%), NH4Cl (1, 2 e 5 wt.%) e Al2O3 (bal.) come inerte.Inoltre, il campione con il rivestimento più promettente è stato sottoposto ad un successivo trattamento di packaluminizing,per ottenere uno strato superficiale arricchito sia in Cr, sia Al, adeguato a conferire simultaneamenteresistenza a corrosione a caldo a 700 – 900 °C e ad ossidazione a temperature intorno ai 1000 °C.Le caratteristiche strutturali e microstrutturali dei riporti ottenuti sono state studiate tramite microscopiaelettronica a scansione (SEM) e diffrazione di raggi X. Le analisi eseguite hanno evidenziato che la durata deltrattamento termico favorisce i fenomeni interdiffusivi delle specie chimiche più mobili, Cr e Ni, portando adun generale aumento di spessore dei rivestimenti. La quantità di polvere di cromo nel pack mix influenza laconcentrazionedi questo elemento all’interno dei coating solamente se la composizione del pack-mix prevedeanche una quantità di attivatore sufficiente a garantirne un trasporto ottimale sulla superficie e successivamenteall’interno del componente. La presenza di particelle ricche di alluminio e ossigeno nello strato diffusivo dimostrache durante il ciclo termico avvengono anche reazioni secondarie che interessano l’inerte, portando allo sviluppodi ossigeno che reagisce con alcuni elementi del substrato

Comportamento ad alta temperatura di rivestimenti NiCrAlY prodotti mediante tecnica Laser Cladding

La tecnica Laser Cladding è un metodo alternativo alla spruzzatura termica per la produzione di rivestimentiresistenti alle alte temperature con una porosità molto bassa o addirittura nulla.Il presente studio ha riguardato il comportamento ad ossidazione di rivestimenti NiCrAlY ottenuti medianteLaser Cladding su substrati di Hastelloy X. Il comportamento ad ossidazione dei campioni rivestiti è statovalutato mediante prove in forno a circolazione d’aria alla temperatura di 1100°C con durate fino a 450 ore.I rivestimenti sono stati analizzati, prima e dopo le prove di ossidazione, mediante tecniche di microscopiaottica ed elettronica, microanalisi e diffrazione di raggi X. Dopo la deposizione il rivestimento ha mostratouna struttura dendritica con grani colonnari e porosità pressoché nulla. Dopo le prove di ossidazione l’analisimicroscopica ha mostrato la presenza di due distinti strati di ossidi, uno più esterno costituitoprevalentemente da ossidi di tipo spinello e uno strato più interno, continuo, costituito principalmenteda allumina. I risultati ottenuti suggeriscono che il rivestimento di NiCrAlY preparato per Laser Claddingè in grado di formare, ad alte temperature ed in atmosfera ossidativa, uno strato continuo di alluminache può proteggere il substrato dall’ossidazione

Studio per lo sviluppo di nuovi rivestimenti ceramico compositi impiegabili in sistemi abradibili

Una strategia per l’incremento delle prestazioni delle turbine a gas consiste nella riduzione del trafilamentodei gas nella zona ad alta temperatura della turbomacchina grazie alla riduzione della distanza tra lasommità delle palette e la copertura protettiva della cassa della turbina. A tal fine sono stati studiati sistemiabradibili realizzati mediante diverse tecniche. Sono stati realizzati sistemi compositi compositi a baseCoNiCrAlY/Al2O3 mediante tecnica Air Plasma Spray (APS), a base NiCrAlY/graphite mediante tecnica LaserCladding e barriere termiche spesse e porose. Le resistenze all’ossidazione e alla fatica termica dei rivestimentisono state valutate per mezzo di prove di ossidazione isoterma e ciclica. I rivestimenti compositiCoNiCrAlY/Al2O3 e NiCrAlY/graphite dopo 1000 ore a 1100°C non hanno mostrato rilevanti modificazionimicrostrutturali. La resistenza all’ossidazione dei nuovi rivestimenti compositi soddisfa le specifiche deiproduttori, gli “Original Equipment Manufacturer” (OEM). Le barriere termiche spesse e porose superano i testdi fatica termica imposti dalle procedure degli OEM. La durezza di tali rivestimenti suggerisce il loro uso incombinazione con palette dotate di rivestimento abrasivo sull’estremità. Le barriere termiche hanno mostratobuone caratteristiche di abradibilità

DEVELOPMENT AND CHARACTERIZATION OF TIO2 COATINGS PREPARED BY ELECTRIC ARC-PHYSICAL VAPOUR DEPOSITION SYSTEM

TiO2 thin coatings were prepared, on various substrates, through evaporation of metallic titanium in an oxidizing atmosphere by modified electric arc physical vapor deposition (EA-PVD). The coatings were characterized chemically (by means of XPS and SIMS) and from the structural point of view (by means of XRD and Raman spectroscopy), in order to understand the factors which lead to homoge-neous coatings with high anatase content. The type of substrate is the main parameter that influence the crystal structure of the coatings: when stainless steel is used as substrate the coatings consist es-sentially of rutile, while on glass substrates coatings containing mainly anatase are obtained. The photocatalytic activity of the samples upon UVA irradiation was tested by using phenol as the target molecule. Phenol in the solution can be photocatalytically and rapidly degraded through the EA-PVD anatase TiO2 coatings

Visuomotor Cerebellum in Human and Nonhuman Primates

In this paper, we will review the anatomical components of the visuomotor cerebellum in human and, where possible, in non-human primates and discuss their function in relation to those of extracerebellar visuomotor regions with which they are connected. The floccular lobe, the dorsal paraflocculus, the oculomotor vermis, the uvula–nodulus, and the ansiform lobule are more or less independent components of the visuomotor cerebellum that are involved in different corticocerebellar and/or brain stem olivocerebellar loops. The floccular lobe and the oculomotor vermis share different mossy fiber inputs from the brain stem; the dorsal paraflocculus and the ansiform lobule receive corticopontine mossy fibers from postrolandic visual areas and the frontal eye fields, respectively. Of the visuomotor functions of the cerebellum, the vestibulo-ocular reflex is controlled by the floccular lobe; saccadic eye movements are controlled by the oculomotor vermis and ansiform lobule, while control of smooth pursuit involves all these cerebellar visuomotor regions. Functional imaging studies in humans further emphasize cerebellar involvement in visual reflexive eye movements and are discussed

Molecular identification of a retinal cell type that responds to upward motion

The retina contains complex circuits of neurons that extract salient information from visual inputs. Signals from photoreceptors are processed by retinal interneurons, integrated by retinal ganglion cells (RGCs) and sent to the brain by RGC axons. Distinct types of RGC respond to different visual features, such as increases or decreases in light intensity (ON and OFF cells, respectively), colour or moving objects1, 2, 3, 4, 5. Thus, RGCs comprise a set of parallel pathways from the eye to the brain. The identification of molecular markers for RGC subsets will facilitate attempts to correlate their structure with their function, assess their synaptic inputs and targets, and study their diversification. Here we show, by means of a transgenic marking method, that junctional adhesion molecule B (JAM-B) marks a previously unrecognized class of OFF RGCs in mice. These cells have asymmetric dendritic arbors aligned in a dorsal-to-ventral direction across the retina. Their receptive fields are also asymmetric and respond selectively to stimuli moving in a soma-to-dendrite direction; because the lens reverses the image of the world on the retina, these cells detect upward motion in the visual field. Thus, JAM-B identifies a unique population of RGCs in which structure corresponds remarkably to function

Recycling in ceramic glazes of zirconia overspray from thermal barrier coatings manufacturing

Glazed ceramic tiles are the most common building material for floor and wall covering. Glazes are produced from frits. The aim of this work is to make a total or partial replacement of a raw material, zircon, widely used in ceramic tiles manufacturing, with a waste material, in order to prepare ceramic frits. The waste material used in this work, is the overspray zirconia, which is produced during the deposition process by atmospheric plasma spraying (APS) of thermal barrier coatings (TBC) on turbine blades. In particular, a replacement of 100. wt%, 1. wt% and 0.2. wt% of zirconium silicate with zirconia has been studied. Ceramic glazes prepared mixing frits and other raw materials are applied on a single-fired tile. The glazes obtained were characterized with different analytical techniques. This study has revealed that the substitution of zircon with waste zirconia is possible in small percentages due to the presence of small amount of chromophore ions in the overspray zirconia, which tend to colour the glaze

Analysis of pack chromizing of Nickel superalloys for gas turbines

The pack-cementation process is a widespread technique used to obtain protective coatings formed by diffusion mechanisms. The protection of the base component is achieved by the enrichment of the outer layer with scale formers, such as Al, Cr and Si which react with the substrate material to form new phases and intermetallics. During the working life these chemical species react with the pollutant elements to form stable oxides and compounds, increasing the oxidation and hot corrosion resistance of gas-turbine components. The component is embedded inside a mixture of powder and then exposed at high temperature. The pack-mix is based on a scale former source, an activator (usually halide salts) and an inert filler (Al2O3). The pack-cementation process consists of a heating step, an isothermal stage and a cooling step.During the thermal treatment the activator reacts with the metal source, forming a vapor phase that transports the metal source to the surface of the component, where chemical reactions lead to the inward diffusion of the selected element into the substrate, forming new phases and intermetallics.The present investigation deals with the study of formation mechanisms of diffusion coatings using chromium as scale former, in order to obtain hot corrosion resistant coating, for gas turbine applications. According to the pertinent literature aluminide coatings provide the protection of the component against oxidation, whereas chromium rich coating are more suitable against hot corrosion type II, that usually occurs at about 700 degrees C due to pollutant agentsThe main phases of these coatings depend on many parameters, such as temperature, duration of the isothermal stage and pack-mix composition. However a systematic investigation about the effect of the main process parameters on the formation mechanisms is not available in the pertinent literature.Therefore this work aims to realize an accurate investigation that leads to a better comprehension of the influence of time and pack-mix composition on the microstructure and thickness of these coatings.A Ni-base superalloy, Inconel 738, was used as substrate material in this work. The samples (approx. size 15x15x3mm) were polished with a 1000 SiC paper and the cleaned in acetone.In order to investigate the effect of the pack-mix composition on the final microstructure and on the coating thickness different formulations were selected, varying the amount of the chromium source, based on pure Cr powder (10, 25 wt.%) and the activator (NH4Cl, 1, 2 and 5 wt.%). Before each pack-chromizing cycle two samples were embedded inside a steel retort with the chosen pack-mix.Three different isothermal stage were used to evaluate the effect of the thermal cycle duration on the diffusion phenomena: 0, 12 and 24 hours at 1100 degrees C. A summary of the process parameters and the pack-mix formulations is listed in tab. 1.At the end of the pack-chromizing one sample for each deposition condition was heat treated in vacuum to evaluate the effect of the interdiffusion phenomena that occur during the vacuum cycle.The surface of the specimens were characterized by X-Ray diffraction (XRD), the cross-section of the coatings were embedded in an epoxy resin and analyzed by Scanning Electron Microscope (SEM) and Energy-Dispersion X-Ray Spectroscopy (EDS).The analysis on the coating C10A1t0 shows that the diffusion mechanism takes place even before the selected process temperature (1100 degrees C), leading to the formation of a thin Cr-rich layer (fig. 2). The dark region is based on Al2O3 rich particles, formed due to secondary reactions occurred between the salt activator and the inert filler that caused the oxidation of the aluminum originally present in the Inconel 738 composition.The comparison between the coating C10A1t0 and C10A1t12 reveals that during the isothermal stage the Cr atoms diffused inward and Ni outward. The coating is based on 3 different layer: a Cr-rich outer region, an alumina rich zone and an inner layer, mostly formed by Cr and Ti nitrides and carbides dispersed in a -Ni matrix.The XRD analysis (fig. 4) revealed that the external region in based on a Cr-rich -Ni matrix formed mainly due to the Cr inward diffusion. The Ti, Cr carbides and nitrides were formed due to the precipitation of these two elements caused by the depletion of Ni and their reaction with carbon and nitrogen. The former is an alloy element, the latter comes from the activator (NH4Cl).Increasing the amount of activator (from 1 wt.% to 2 wt.%) within the pack-mix made the transport of Cr to the sample surface easier: the concentration of this element in the outer layer is higher (fig. 3) whereas the microstructure and the coating thickness remain almost the same.The duration of the isothermal stage (from 12 to 24 h) affected the interdiffusion movements of Cr and Ni, bringing to a decrease of the Cr content in the outer layer, due to an improvement of the Ni outward diffusion (fig. 6).Increasing the Cr content from 10 to 25 wt.% made the Cr diffusion in the outer layer more relevant. Despite this it was found that the amount of activator (2 wt.%) was not enough to provide a good transport of Cr inside the coatings. Therefore, compared to the previous samples, no important changes in thickness were observed (fig. 8).To obtain thicker coatings a higher amount of activator is required (5 wt.%), in order to transport the Cr atoms not only to the substrate surface, but also inside the component (fig. 10).In order to realize a coating with a good resistance against both oxidation and hot corrosion a Cr,Al-rich diffusion coating was prepared, with a 2 step pack-chromizing and pack-aluminzing process. The coating C10A2t24 was embedded inside a pack-mix based on CoAl as aluminum source, AIF3 as activator and alumina as inert filler and then heat treated for 2.5 hours. Compared to the pack-chromized coating, the new resulting coating show a higher thickness, due to the inward diffusion of the new element. The microstructure is also different: SEM investigation shows the presence of an interdiffusion layer, that demonstrates also the outward diffusion of Ni (Fig. 12)