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Innovative galvanic coatings for high temperature applications
Full text linkThe research regards the production and the characterization of two different types of Ni based galvanic coatings, which should guarantee higher performances at high temperature applications in comparison to the pure Ni coatings. The work is divided in two parts, the first one on the production and characterization of Ni matrix composite coatings containing either micro- or nano-particles of Al and the second one on the production and characterization of Ni-B alloy coatings with low B content produced with dimethylamine borane (DMAB) as boron source in the plating bath. In both cases, after an initial optimization of the process parameters, different types of coatings have been produced and tested. The coatings microstructure, microhardness and chemical composition both prior and after heat-treatments at different temperatures have been studied and compared to pure Ni deposits. The coatings performances such as protective properties, wear and tribocorrosion resistance have been also evaluated and the results have been correlated to the microstructural and chemical modifications. As plated pure Ni coatings present a columnar structure, a hardness HV0.05 of about 170 and good corrosion resistance. After heat treatment at temperatures starting from 400°C the Ni deposits undergo recrystallization which lead to a noticeable decrease of the mechanical properties. Ni/Al micro-composite deposits heat treated at 600°C consists of a biphasic γ-Ni and γ′-Ni3Al system, while after heat treatment at 800°C of a solid substitutional solution of Al in the γNi. The diffusion of Al in the metal matrix hinders the recrystallization of the Ni matrix and changes the preferential orientation. The more uniform distribution of the Al nano-particles and the smaller dimensions lead to an advance of the diffusion at lower temperature (400°C). The heat treated Ni/Al nano-composite deposits consist of γNi and present a columnar structure with narrow and short columns as the fast diffusion of Al that blocks the recrystallization of the Ni matrix. Both micro- and nano-composite coatings present higher hardness in comparison to pure Ni deposits after heat treatments at high temperatures. The introduction of Al micro-particles decreases the protective properties while the use of nano-particles lead to a marked increase of the microhardness without penalizing the corrosion resistance of the matrix. Ni-B alloy coatings present a marked grain refinement and a linear increase of the hardness by increasing the B content. The B content must be kept below 0.12% wt. in order to avoid the formation of cracks. The presence of B hinders the recrystallization of the coatings up to 600°C. Crack-free Ni-B coatings present similar protective properties, higher wear and tibocorrosion resistance in comparison to pure Ni deposits
Production and characterization of ni matrix composite electrodeposits containing either micro- or nano- aluminum particles
No full textCorrosion resistance of Ni-Co electrodeposits obtained using plating baths with different composition
No full textSulla microstruttura e proprietà anticorrosive di rivestimenti galvanici compositi a matrice Ni contenenti micro- o nano- particelle di Al
No full textOn the microstructure and corrosion properties of Ni matrix composite electrodeposits containing either micro- or nano- aluminium particles
No full textResistenza alla corrosione di rivestimenti compositi in Ni contenenti micro- o nano- particelle di Al
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Production, microstructural characterization and corrosion properties of Ni/B electrodeposits with low B content
No full textDepositi in Ni-Co: resistere alla corrosione con la giusta microstruttura
No full textLa deposizione galvanica di leghe Ni/Co con bagni a base di solfati è ampiamente affermata grazie alle proprietà protettive e magnetiche di tali leghe. La necessità di rivestimenti spessi di leghe di Ni/Co per applicazioni ad elevata resistenza all’usura ha portato allo sviluppo di bagni galvanici a base di nichel solfammato.
Lo scopo di questo lavoro è stato la produzione e la caratterizzazione di rivestimenti di Ni/Co utilizzando un bagno di nichel solfammato dove sono stati aggiunti ioni Co2+ come CoSO4 oppure come Co(NH2SO3)2 a differenti concentrazioni.
12 differenti tipi di depositi sono stati prodotti e caratterizzati mediante analisi della composizione chimica, della microstruttura, della microdurezza e della resistenza ad usura e a corrosione. L’elettrodeposizione è stata effettuata in condizioni potenziostatiche utilizzando una configurazione a piastre parallele. La microstruttura dei depositi ottenuti è stata osservata al SEM sia in pianta sia in sezione dopo attacco metallografico ed analizzata mediante diffrattometria ai raggi X. La composizione chimica lungo lo spessore dei rivestimenti è stata valutata mediante analisi EDXS in sezione. Sono state inoltre eseguite misure di microdurezza Vickers Hv0.3 in sezione in maniera da escludere l’influenza del substrato. La resistenza a corrosione dei differenti depositi è stata valutata mediante prove di polarizzazione potenziodinamica in due ambienti di prova: in ambiente acido e in ambiente contenente cloruri.
L’incremento del contenuto di Co nel bagno galvanico porta ad un incremento pressoché lineare del contenuto di Co nel deposito e a marcate modifiche microstrutturali. All’aumentare del contenuto di Co la microstruttura del deposito si modifica da colonnare a lamellare. Tale effetto è più marcato con l’utilizzo di CoSO4 piuttosto che con l’utilizzo di Co(NH2SO3)2.
Le modifiche composizionali e microstrutturali portano ad un rilevante incremento della microdurezza e della resistenza a corrosione dei depositi e le proprietà dei depositi dipendono dalla quantità di cobalto in lega e non dal tipo di sale di cobalto utilizzato nel bagno galvanico
Production and microstructural characterization of Ni/B electrodeposits with low B content
No full textNanocrystalline or amorphous Ni-B coatings can be prepared by electroless or electrochemical deposition. Electroless deposition is performed using strong reducing agents, high temperature and high pH values. The plating process and the maintenance of the solution is much easier when electrodeposition is used as production method. In this work, Ni/B coatings with low amounts of B have been produced using a Ni sulfamate plating bath with the addition of dimethylaminborane. The influence of the B content on the various properties of the Ni-B alloys produced by electrodeposition has been investigated. The microstructure of the obtained coatings has been observed by SEM both on top surface and cross section after metallographic etching and by X-ray diffraction. The B content on the produced deposits has been evaluated using GDOES. Selected specimens have been heat-treated at different temperatures and the effect of the heat treatment on the microstructure has been evaluated. Both as plated and heat treated deposits have been characterized regarding their microstructure and microhardness
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