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Bolelli G.,University of Modena and Reggio Emilia | Berger L.-M.,Fraunhofer Institute for Material and Beam Technology | Bonetti M.,University of Modena and Reggio Emilia | Lusvarghi L.,University of Modena and Reggio Emilia
Wear | Year: 2014

Recent studies of HVOF-sprayed WC-(W,Cr)2C-Ni coatings indicated an applicability at temperatures above 600°C. Therefore, in the present paper the dry sliding wear behaviour of coatings prepared from one selected feedstock powder was studied up to 750°C and compared to a WC-10%Co-4%Cr coating as reference.Coatings were prepared onto stainless steel substrates by a liquid-fueled high velocity oxygen-fuel (HVOF) spray process. A Cr-W-Ni-C mixed matrix phase and chromium-rich large (W,Cr)2C grains were formed as a result of metallurgical reactions between the components WC, Cr3C2 and Ni during spraying; therefore, the resulting coating can be better designated as WC-(W,Cr)2C-Ni.The dry unidirectional sliding wear behaviour of the coatings, studied according to ASTM G99 against Al2O3 counterbodies at four different temperatures (25°C, 400°C, 600°C, 750°C) and at two sliding speeds, was interpreted on the basis of their microstructure, micromechanical properties and oxidation behaviour. The morphology and composition inside the wear scars were also studied by SEM and micro-Raman spectroscopy. The results show that the WC-(W,Cr)2C-Ni coating exhibits good wear resistance up to 750°C, whereas WC-CoCr, in spite of its very mild wear at low temperature, suffers from catastrophic oxidation at 750°C. © 2013 Elsevier B.V.

Perez-Hernandez H.,Fraunhofer Institute for Material and Beam Technology
Biointerphases | Year: 2012

Micropatterned surfaces with cell adhesive areas, delimited by protein repellent microstructures, are in high demand for its potential use as relevant biological assays. This is not only because such surfaces allow directing cell growth in a spatially localized and restricted manner, but also because they can be used to elucidate basic cell growth and orientation mechanisms. Here, it is presented a laser-assisted micropatterning technique to fabricate large area microstructures of poly (ethylene glycol) hydrogel onto a cell adhesive surface: a biofunctional maleic anhydride copolymer. By varying photoinitiator, laser intensity, copolymer as well as the hydrogel layer thickness, the optimum conditions to produce high quality features were found. The suitability of these micropatterned substrates for bioassay applications was proved by cell adhesion studies. The introduced procedure could be used to prepare a broad range of microarrays for certain bioanalytical approaches and to create different types of biofunctional surfaces.

Berger L.-M.,Fraunhofer Institute for Material and Beam Technology
International Journal of Refractory Metals and Hard Materials | Year: 2015

Thermally sprayed hardmetal coatings have a typical thickness within the range 100-500 μm. Thus, thermal spray enables the functionality of hardmetals to be realized on the surface of large parts, which cannot be produced by powder metallurgy for technical and economical reasons. This article reviews the different types of thermal spray processes, with particular focus on the high velocity HVOF and HVAF deposition techniques which are ofmost relevance to the application of hardmetal coatings. Feedstock powder preparation technologies are presented. The majority of hardmetal thermal spray coatings are based either onWC or Cr3C2 or hard phases appearing as a result of their interaction. As an alternative, TiC-based compositions aremost intensively studied. Thermal spraying generates significant changes in the hardmetal chemical and phase compositions between the feedstock powder to the sprayed coating. Coating formation and microstructures as well as selected properties, such as hardness, the effect of heat treatments and the oxidation in service, as well as corrosion resistance are discussed. As an example for wear protection applications, abrasion wear resistance is shortly discussed. This paper is a partly updated and condensed version of the chapter: "Coatings by thermal spray" in the book "Comprehensive Hard Materials", V.K. Sarin (Editor-in-Chief) & D. Mari & L. Llanes (Vol. Ed.), Vol. 1 (pp. 471-506), Elsevier, 2014. © 2014 Elsevier Ltd.

Mabuchi Y.,Nissan Motor Co. | Higuchi T.,Nissan Motor Co. | Weihnacht V.,Fraunhofer Institute for Material and Beam Technology
Tribology International | Year: 2013

Hydrogen-free diamond-like carbon (DLC) coatings display markedly lower friction coefficients under a lubricated condition owing to the adsorption of oiliness agents and their decomposed components. This study examined the effect on friction properties of the sp2/sp3 bonding ratio of DLC coatings and the addition of nitrogen for improving surface wettability. Evaluations of friction properties under a lubricated condition showed that DLC coatings with a lower ratio of sp3 bonds and nitrogenated DLC coatings displayed higher friction coefficients. Results obtained by electron spin resonance revealed that adding nitrogen decreased the number of dangling bonds of DLC coatings in proportion to the sp3 bond ratio. Assuming that such dangling bonds are oiliness agent adsorption sites, it is concluded that adding nitrogen is not conducive to reducing friction coefficients. © 2013 Elsevier Ltd.

Schultrich B.,Fraunhofer Institute for Material and Beam Technology
Diamond and Related Materials | Year: 2011

The structure of tetrahedral bonded amorphous carbon (ta-C) is determined by the energy of the impinging carbon ions, the angle of incidence, the substrate temperature and the deposition rate. Up to now modeling of the ta-C growth was concentrated on the energy effects by investigating the early stages of the ion impact and the short-time relaxation in the thermal spike. A new analytical model is presented, which emphasizes the role of long-time diffusion, driven by the stress induced drift towards the surface. In accordance with the experimentally found trends, the model allows the estimation of the sp 3 content in dependence on the technological relevant deposition parameters ion energy, angle, temperature, rate and their distributions. This opens the way for the realistic simulation of ta-C growth in industrial coaters. © 2011 Elsevier B.V. All rights reserved.

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