2,475 research outputs found
Raman spectroscopy characterization of diamond films on steel substrates with titanium carbide arc-plated interlayer
No full textDiamond chemical vapour deposition (CVD) on steel represents a difficult task. The major problem is represented by large diffusion of carbon into steel at CVD temperatures. This leads to very low diamond nucleation and degradation of steel microstructure and properties. Recent work [R. Polini, F. Pighetti Mantini, M. Braic, M. Amar, W Ahmed, H. Taylor, Thin Solid Films 494 (2006) 116] demonstrated that well-adherent diamond films can be grown on high-speed steels by using a TiC interlayer deposited by the PVD-arc technique. The resulting multilayer (TiC/ diamond) coating had a rough surface morphology due to the presence of droplets formed at the substrate surface during the reactive evaporation of TiC. In this work, we first present an extensive Raman investigation of 2 mu m, 4 mu m and 6 mu m thick diamond films deposited by hot filament CVD on TiC interlayers obtained by the PVD-arc technique. The stress state of the diamond was dependent on both the films thickness and the spatial position of the coating on the substrate. In fact, on the top of TiC droplets, the stress state of the diamond was much lower than that of diamond in flatter substrate areas. These results showed that diamond films deposited on rough TiC interlayers exhibited a wide distribution of stress values and that very large compressive stress exists in the diamond film grown on flat regions of steel substrates with a TiC interlayer. Diamond films could accommodate stresses as large as 10 GPa without delamination
Diamond nucleation on cleaved Si(111)
No full textDiamond 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
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
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
Kriging quick adaptive sampling for multivariate optimization
No full textThe paper describes a general adaptive sampling methodology useful in multivariate optimization involving finite element analysis that typically requires a long time. The proposed adaptive sampling method was obtained by starting from a model-based statistical methodology developed to design an adaptive inspection plan for the geometric control of mechanical parts. The novelty of the new kriging adaptive sampling procedure consists in overcoming some critical issues, such as the reduction of the estimation time to have an efficient tool for quick application. The others are related to how to set up a kriging-based adaptive sampling, how to choose the stopping rule, and how to remove the dependence on the statistical variability. These issues are the subject of this work to develop a useful tool for multivariate optimization. By improving the methodology in these aspects, it was possible to obtain a feasible and useful instrument for the application in a quick sampling procedure within the inspection process. After this enhancement process, the method is sufficiently general to be extended in the multivariate optimization field that requires a multi-factorial finite element analysis. To demonstrate this, an illustrative real case study from an industrial application is presented and it was found that the proposed method reduces significantly the time required to reach the optimal solutio
Analysis of size distribution functions of diamond crystallites formed in the early stages of chemical vapour deposition
No full textSize distribution functions of diamond particles formed in the early stages of chemical vapour deposition (CVD) can be used to distinguish between seeding and heterogeneous nucleation on the basis of their shape and of the d(max)/d(min) ratio, d(max) and d(min), being the maximum and the minimum diameters, respectively. A monomodal size distribution function with a d(max)/d(min) ratio much greater than 1.2-1.3 indicates diamond formation to occur via heterogeneous nucleation. In this case the nucleation kinetics can be calculated once the growth law of the crystallites has been established. The nucleation kinetics at copper substrates have been derived from distribution functions and described by a new kinetic model which includes the generation of nucleation sites
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
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
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
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