1,721,121 research outputs found
Il Tungsteno (Parte 2)
No full textNella Parte 1 abbiamo parlato della formazione del tungsteno nell’Universo e del perché abbia due nomi, per poi approfondire le
proprietà tecnologiche del suo composto più utilizzato, il carburo di formula WC. In questa seconda parte viene sottolineata
l’importanza , per i Paesi manifatturieri, del carburo di tungsteno, che rappresenta circa il 60% dellʼimpiego globale dellʼelemento.
Infine, viene presentato, in linea con gli obiettivi dellʼAgenda 2030 dell'ONU, un metodo alternativo, energeticamente più efficiente e a
minore impatto ambientale, per la produzione del WC dal minerale.In the previous Part 1 we discussed the formation in the Universe of the element with Z = 74, where its two names (tungsten and wolfram) come from, and the technological importance of
tungsten monocarbide, WC. In Part 2 we show that tungsten is of utmost strategic importance for manufacturing countries, and WC represents more than half of the global use of the element. Finally, an alternative process with lower environmental impact for the direct synthesis of WC powders from the minerals is proposed
On the use of CrN/Cr and CrN interlayers in hot filament chemical vapour deposition (HF-CVD) of diamond films onto WC-Co substrates
No full textCrN/Cr-based films were deposited using PVD-arc technique onto Co-cemented tungsten carbide (WC-Co) substrates and, then, seeded with diamond powder suspension or mechanically treated by Fluidized Bed Peening (FBP) of brittle diamond powders. Multilayered coatings were obtained from the superimposition of 4 mu m-thick diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The effectiveness of fluidized bed peened CrN/Cr interlayers on the adhesion enhancement of diamond on WC-Co substrates was studied and compared to diamond coated WC-Co substrates with unpeened CrN/Cr or CrN interlayers, or pre-treated with two-step chemical etching (Murakatni's reagent and Caro's acid, MC-treatment). In particular, growth, morphology, wear endurance and adhesion of the CVD deposited diamond films onto peened CrN/Cr interlayer were looked into. Diamond coatings on peened CrN/Cr interlayers exhibited a rougher surface morphology than as-prepared CrN/Cr films as a result of the surface roughening of the ductile Cr layer produced by the repeated impacts on it of the diamond powders during FBP. FBP was found to be a necessary step in improving the scarce adhesion of CVD diamond onto CrN/Cr-interlayer. However, the use of FB peened CrN/Cr interlayer did not represent the best way to pre-treat WC-Co substrates, as the unpeened single-layer CrN, or the use of MC pretreatment, was found to ensure better adhesion and wear endurance. (C) 2008 Published by Elsevier B.V
Impact of seed density on continuous ultrathin nanodiamond film formation
No full textAn analytical mean field approach for describing the time evolution of film growth by seeding has been developed. The modeling deals with the generic case of anisotropic growth with different growth rates, respectively on – and normal to – the substrate plane. The finite size of the seeds is considered by including spatial correlation effects among seeds through hard-core interactions. The approach, based on probability theory, provides solution in closed form for mean film thickness as a function of substrate coverage, seed density and initial size of the seeds. For negligible values of the initial coverage of the substrate by seeds, manageable analytical expressions are attained. The model has been validated by comparison with experimental data available in the literature. This study is significant in connection to the possibility of determining optimal growth conditions for ultrathin nanocrystalline diamond (NCD) film. In fact, the knowledge of the seeding/nucleation density that allows a given minimum average thickness of continuous film is of utmost importance for the development of technologically advanced applications
Diamond nucleation on cleaved Si(111)
Full text linkDiamond crystallites have been nucleated and grown by hot filament chemical vapor deposition at 600°C on the untreated fracture surface of a cleaved Si(111) sample. The flat surface of the cleaved crystal was inactive towards diamond nucleation while, on the terraced surface formed by the propagating crack, a high density of nuclei was found. The crystallites were nucleated in correspondence of edges between (111) terrace planes and step planes. The occurrence of edges, as determined by scanning electron microscopy (SEM) observation, is a necessary but not sufficient condition for the nucleation and this fact suggests that particular atomic arrangements are required for the diamond nucleus formation
Al2O3 thin coating of AA 6082 T6 components using a fast regime fluidized bed
No full textFluidized bed processing is a relatively novel method for coating metal substrates. A detailed study was carried out into surface property and in micro-structure changes induced by fluidized bed processing. In particular, fluidized bed processing of AA 6082 T6 aluminum alloy components using alumina Al2O3 powder was investigated. Firstly, the build up of Al2O3 films was studied and characterized in terms of coating thickness and adhesion. It was found that trends of deposited Al2O3 were consistent with fluidized bed processing time. Secondly, the effect of fluidized bed treatment on surface properties of processed components was examined. Surface morphology was significantly affected and its evolution according to processing time was accounted for. Both compressive residual stresses and increased dislocation density were induced by treatment of external layers of samples, and significant hardening was also detected. Lastly, the ability of fluidized bed processing was tested on 'ad hoc' fatigue samples. Rupture of fluidized bed treated samples as well as untreated samples was also discussed. At any rate, the fatigue behavior of processed components significantly improved. This quite new and unprecedented result is ascribed to the compressive residual stresses and work hardening induced by FB treatment in the outermost surface layers of the aluminum alloy. (c) 2005 Elsevier B.V. All rights reserved
Procedimento per la produzione di un substrato in metallo duro rivestito con pellicola aderente di diamante
Full text linkProcedimento per la realizzazione di un rivestimento protettivo antiusura di diamante su manufatti in metallo duro mediante deposizione chimica da fase vapore (CVD), in cui la fase metallica che agisce da legante nel materiale in metallo duro, usualmente responsabile della scarsa adesione della pellicola di diamante, viene eliminata dalla superficie del substrato durante le prime fasi dello stesso processo di deposizione.
Nel corso del processo, la temperatura interna del substrato viene dapprima mantenuta a 600-800 °C per consentire la nucleazione del diamante e la segregazione superficiale di una fase contenente la fase metallica legante, poi innalzata a 900-1100 °C per ottenere la rimozione della fase metallica legante dalla superficie del substrato e quindi mantenuta a 600-900 °C per un tempo sufficiente ad ottenere lo spessore desiderato di pellicola di diamante
Wear resistance of nano- and micro-crystalline diamond coatings onto WC-Co with Cr/CrN interlayers
No full textCr/CrN bi-layers have been used recently to promote the growth of high quality Hot Filament Chemical Vapour Deposition (HFCVD) diamond coatings onto Co-cemented tungsten carbide (WC-6 wt.%Co) substrates. In the present investigation, the influence of the crystalline size of the diamond coatings on their wear endurance is looked into. Nano- (NDC) and micro-crystalline Diamond Coatings (MDC) were deposited by HFCVD onto untreated and Fluidized Bed (FB) treated Cr/CrN interlayers. NDCs, characterized by a cauliflower-like morphology, showed improved wear resistance. However, the superimposition of NDCs onto Cr/CrN interlayers micro-corrugated by FB treatment was found to be the most promising choice, leading to the formation of highly adherent and wear resistant coatings. © 2010 Elsevier B.V. All rights reserved
Diamond as a wear-resistant coating for cutting tools, part 2
No full textSince the discovery of diamond film synthesis by chemical vapour deposition (CVD), many technologically important applications have been developed. In the machining industry, Co-cemented tungsten carbide (WC-Co) tool inserts can benefit from CVD diamond technology insofar as their wear life is concerned. However, premature failure of diamond-coated WC-Co tools caused by thermal and mechanical stresses still remains a challenging difficulty. In this paper, the authors discuss the state-of-the-art of CVD diamond films adhesion, since the most important failure mechanisms of diamond-coated hard metal tools are usually related to film delamination during machining. The main issues related to the development of CVD diamond-coated wear re sistant parts are addressed. The literature analysis shows that substrate surface pretreatments play an important role in determining sufficiently large interface toughness values. Therefore, a survey of advancements in techniques to enhance adhesion of CVD diamond coatings is presented. In addition, the combined effect of substrate pretreatment and microstructure on the cutting performance of CVD diamond-coated WC-Co inserts is discussed on the basis of metal matrix composites (MMCs) machining results
HF-CVD of diamond coatings onto fluidized bed (FB) treated CrN interlayers
No full textIn the present investigation, Fluidized Bed (FB) treatment is applied to pre-treat CrN interlayers onto WC-Co substrates to promote the growth on them of highly adherent diamond coatings. During FB treatment, the CrN interlayers are submitted to high speed impacts of loose abrasives. The action of their cutting edges is able to deeply change the starting morphology of the as-deposited Physical Vapour Deposition (PVD) CrN interlayers, thus promoting the establishment of a highly corrugated surface on which to grow Hot Filament-Chemical Vapour Deposition (HF-CVD) diamond coatings. Growth, morphology, adhesion and wear resistance of the CVD deposited diamond coatings onto the FB treated and just seeded CrN interlayers were looked into and compared to diamond coated WC-Co substrates with the untreated CrN interlayers or pre-treated with a two-step chemical etching (Murakami's reagent and Caro's acid, MC-treatment) or with FB. FB treatment proved to be an effective technique to tailor the surface morphology and roughness of CrN films deposited by PVD-arc technique, and was found to be very useful in improving the adhesion and wear resistance of CVD diamond onto the CrN interlayers. © 2010 Elsevier B.V. All rights reserved
Chemical Vapor Deposition of Highly Adherent Diamond Coatings onto Co-Cemented Tungsten Carbides Irradiated by High Power Diode Laser
No full textThe present investigation deals with the definition of a new eco-friendly alternative to pretreat Co-cemented tungsten carbide (WC−Co) substrates before diamond deposition by hot filament chemical vapor deposition (HFCVD). In particular, WC-5.8 wt %Co substrates were submitted to a thermal treatment by a continuous wave-high power diode laser to reduce surface Co concentration and promote the reconstruction of the WC grains. Laser pretreatments were performed both in N2 and Ar atmosphere to prevent substrate oxidation. Diamond coatings were deposited onto the laser pretreated substrates by HFCVD. For comparative purpose, diamond coatings were also deposited on WC-5.8 wt %Co
substrates chemically etched by the well-known two-step pretreatment
employing Murakami’s reagent and Caro’s acid. Surface morphology, microstructure, and chemical composition of the WC-5.8 wt %Co substrates after the different pretreatments and the deposition of diamond coatings were assessed by surface profiler, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses. Wear performance of the diamond coatings was checked by dry sliding linear reciprocating tribological tests. The worn volume of the diamond coatings deposited on the laser pretreated substrates was always found lower than the one measured on the chemically etched substrates, with the N2 atmosphere being particularly promising
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