Scientific Papers in SCI


Title: Chemical and microstructural characterization of (Y or Zr)-doped CrAlN coatings
Author(s): Rojas, T. C.; El Mrabet, S.; Dominguez-Meister, S.; Brizuela, M.; Garcia-Luis, A.; Sanchez-Lopez, J. C.
Source: Surface and Coatings Technology, 211 (2012) 104-110

abstract | fulltext

Magnetron sputtered chromium aluminium nitride films are excellent candidates for advanced machining and protection for high temperature applications. In this work CrAlN-based coatings including Y or Zr as dopants (≈ 2 at.%) are deposited by d.c. reactive magnetron sputtering on silicon substrates using metallic targets and Ar/N2 mixtures. The hardness properties are found in the range of 22–33 GPa with H/E ratios close to 0.1. The influence of the dopant element in terms of oxidation resistance after heating in air at 1000 °C is studied by means of X-ray diffraction (XRD), cross-sectional scanning electron microscopy (X-SEM) and energy dispersive X-ray analysis (EDX). The microstructure and chemical bonding are investigated using a transmission electron microscope (TEM) and electron energy-loss spectroscopy (EELS) respectively. The improvement in oxidation resistance as compared to pure CrN coating is manifested in the formation of a Al-rich outer layer that protects the underneath coating from oxygen diffusion. The best performance obtained with the CrAlYN film is investigated by in situ annealing of this sample inside the TEM in order to gain knowledge about the structural and chemical transformations induced during heating.

October, 2012 | DOI: 10.1016/j.surfcoat.2011.07.071

Title: XPS and AES analyses of cerium conversion coatings generated on AA5083 by thermal activation
Author(s): Sanchez-Amaya, JM; Blanco, G; Garcia-Garcia, FJ; Bethencourt, M; Botana, FJ
Source: Surface and Coatings Technology, 213 (2012) 105-116

abstract | fulltext

This paper describes the deep analysis of cerium conversion coatings developed with thermal activation on AA5083 under optimum processing conditions. Scanning electron microscopy (SEM), electron dispersive spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were employed to study these layers. Ar+ sputtering was also employed to analyse the coatings' core. Although conversion coatings based on Ce salts have been widely characterised in the literature for different aluminium alloys, the coatings developed with thermal activation on Al–Mg alloys have not been previously investigated with these techniques. SEM/EDX studies have demonstrated the existence of a heterogeneous layer formed by a film of aluminium oxide/hydroxide on the matrix as well as a series of dispersed islands of cerium deposited on the cathodic intermetallics. These results have been further confirmed by means of XPS. The XPS and AES results revealed that the outer layer comprises a mixture coating of Ce3 + (70%) and Ce4 + (30%) compounds. Although only Ce3 + compounds were detected at the inner part of the coating, possible reduction of Ce(IV) to Ce(III) due to the Ar+ beam could not be discarded. Obtained results allowed authors to confirm that the cerium conversion coatings developed have a similar structure to those previously reported for other aluminium alloys.

December, 2012 | DOI:

Title: Bulk TiCxN1−x–15%Co cermets obtained by direct spark plasma sintering of mechanochemical synthesized powders
Author(s): Borrell, A; Salvador, MD; Rocha, VG; Fernandez, A; Aviles, MA; Gotor, FJ
Source: Materials Research Bulletin, 47 (2012) 4487-4490

abstract | fulltext

TiCxN1−x–15 wt.%Co cermets were obtained by a mechanically induced self-sustaining reaction (MSR) and sintered by spark plasma sintering (SPS) technique at different temperatures (1200–1400 °C) for 1 min in vacuum under a uniaxial load of 80 MPa. The evolution of microstructure and mechanical properties was investigated. SPS allowed high densification with limited grain growth at a relatively low temperature. Material sintered at 1300 °C showed a good combination of mechanical properties with Vickers hardness of 17.1 ± 0.5 GPa, fracture toughness of 5.51 ± 0.29 MPa m1/2 and bending strength of 904 ± 12 MPa. Lower sintering temperature resulted in a decrease in bending strength due to poor cohesion between the ceramic and binder phases. An increase in sintering temperature would allow tailoring the cermet microstructure and, therefore, adjusting the Vickers hardness/fracture toughness relation.

December, 2012 | DOI: 10.1016/j.materresbull.2012.09.066

Title: An international round-robin calibration protocol for nanoindentation measurements
Author(s): Cabibbo, M; Ricci, P; Cecchini, R; Rymuza, Z; Sullivan, J; Dub, S; Cohen, S
Source: Micron, 43 (2012) 215-222

abstract | fulltext

Nanoindentation has become a common technique for measuring the hardness and elastic–plastic properties of materials, including coatings and thin films. In recent years, different nanoindenter instruments have been commercialised and used for this purpose. Each instrument is equipped with its own analysis software for the derivation of the hardness and reduced Young's modulus from the raw data. These data are mostly analysed through the Oliver and Pharr method. In all cases, the calibration of compliance and area function is mandatory. The present work illustrates and describes a calibration procedure and an approach to raw data analysis carried out for six different nanoindentation instruments through several round-robin experiments. Three different indenters were used, Berkovich, cube corner, spherical, and three standardised reference samples were chosen, hard fused quartz, soft polycarbonate, and sapphire. It was clearly shown that the use of these common procedures consistently limited the hardness and reduced the Young's modulus data spread compared to the same measurements performed using instrument-specific procedures. The following recommendations for nanoindentation calibration must be followed: (a) use only sharp indenters, (b) set an upper cut-off value for the penetration depth below which measurements must be considered unreliable, (c) perform nanoindentation measurements with limited thermal drift, (d) ensure that the load–displacement curves are as smooth as possible, (e) perform stiffness measurements specific to each instrument/indenter couple, (f) use Fq and Sa as calibration reference samples for stiffness and area function determination, (g) use a function, rather than a single value, for the stiffness and (h) adopt a unique protocol and software for raw data analysis in order to limit the data spread related to the instruments (i.e. the level of drift or noise, defects of a given probe) and to make the H and Er data intercomparable.

February, 2012 | DOI: 10.1016/j.micron.2011.07.016

Title: Absence of the core-rim microstructure in TixTa1-xCyN1-y-based cermets developed from a pre-sintered carbonitride master alloy
Author(s): Chicardi, E; Cordoba, JM; Sayagues, MJ; Gotor, FJ
Source: International Journal of Refractory Metals & Hard Materials, 33 (2012) 38-43

abstract | fulltext

(Ti,Ta)(C,N) solid solution-based cermets with cobalt as the binder phase were synthesised by a two-step milling process. The titanium-tantalum carbonitride solid solution (the ceramic phase) was obtained via a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental Ti, Ta, and graphite powder blends in a nitrogen atmosphere. Elemental Co (the binder phase) was added to the ceramic phase, and the mixture was homogenised by mechanical milling (MM). The powdered cermet was then sintered in a tubular furnace at temperatures ranging from 1400°C to 1600°C in an inert atmosphere. The chemical composition and microstructure of the sintered cermets were characterised as ceramic particles grown via a coalescence process and embedded in a complex (Ti,Ta)-Co intermetallic matrix. The absence of the typical core-rim microstructure was confirmed.

July, 2012 | DOI: 10.1016/j.ijrmhm.2012.02.005

Title: Inverse core-rim microstructure in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced self-sustaining reaction
Author(s): Chicardi, E; Cordoba, JM; Sayagues, MJ; Gotor, FJ
Source: International Journal of Refractory Metals & Hard Materials, 31 (2012) 39-46

abstract | fulltext

Cermets with a nominal composition (Tia(0.8)Ta(0.2)C(0.5)N(0.5)-20 wt.% Co) were synthesised by a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental powder blends. The MSR allowed the production of a complex (Ti,Ta)(C,N) solid solution, which was the raw material used for the sintering process. The pressureless sintering process was performed at temperatures between 1400 degrees C and 1600 degrees C in an inert atmosphere. The microstructural characterisation showed a complex microstructure composed of a ceramic phase with an unusual inverse core-rim structure and a Ti-Ta-Co intermetallic phase that acted as the binder.

March, 2012 | DOI: 10.1016/j.ijrmhm.2011.09.003

Title: Attenuation lengths of high energy photoelectrons in compact and mesoporous SiO2 films
Author(s): Ferrer, FJ; Gil-Rostra, J; Gonzalez-Garcia, L; Rubio-Zuazo, J; Romero-Gomez, P; Lopez-Santos, MC; Yubero, F
Source: Surface Science, 606 (2012) 820-824

abstract | fulltext

We have experimentally evaluated attenuation lengths (AL) of photoelectrons traveling in compact and micro and mesoporous (∼ 45% voids) SiO 2 thin films with high (8.2-13.2 keV) kinetic energies. The films were grown on polished Si(100) wafers. ALs were deduced from the intensity ratio of the Si 1s signal from the SiO 2 film and Si substrate using the two-peaks overlayer method. We obtain ALs of 15-22 nm and 23-32 nm for the compact and porous SiO 2 films for the range of kinetic energies considered. The observed AL values follow a power law dependence on the kinetic energy of the electrons where the exponent takes the values 0.81 ± 0.13 and 0.72 ± 0.12 for compact and porous materials, respectively.

May, 2012 | DOI: 10.1016/j.susc.2012.01.017

Title: Influence of PVP in magnetic properties of NiSn nanoparticles prepared by polyol method
Author(s): Bobadilla, LF; Garcia, C; Delgado, JJ; Sanz, O; Romero-Sarria, F; Centeno, MA; Odriozola, JA
Source: Journal of Magnetism and Magnetic Materials, 324 (2012) 4011-4018

abstract | fulltext

The influence of PVP on the magnetic properties of NiSn nanoparticles prepared by polyol method has been studied. NiSn nanoparticles exhibit superparamagnetic behavior although there is a ferromagnetic contribution due to particles agglomerated below the blocking temperature. The particle size is controlled by the addiction of PVP in varying amounts. The addition of PVP also favours the particles isolation, narrow the particle size distribution and decrease the interparticle interaction strength increasing the superparamagnetic contribution.

November, 2012 | DOI: 10.1016/j.jmmm.2012.07.005

Title: Interfacial characterization of silicon nitride/silicon nitride joints brazed using Cu-base active metal interlayers
Author(s): Singh, M; Fernandez, JM; Asthana, R; Rico, JR
Source: Ceramics Intenational, 38 (2012) 2793-2802

abstract | fulltext

Silicon nitride/silicon nitride joints were vacuum brazed at 1317 K for 5 min and 30 min using ductile Cu-base active metal interlayers. The joints were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM). An inhomogeneous Ti-rich reaction layer (similar to 2-3 mu m thick) formed in 5 min at the Si3N4/braze interface. The inhomogeneity disappeared after brazing for 30 min and was replaced with a compact and featureless reaction zone. TEM studies revealed fine grains in the reaction layer, and larger grains in the inner part of the joint interfaces. The joints were crack-free and presented features associated with plastic deformation, which indicated accommodation of strain associated with CTE mismatch. Electron Backscatter diffraction (EBSD) revealed a highly textured braze alloy interlayer and its crystallographic orientation was determined. The formation of additional phases at the joint interface during brazing is discussed.

May, 2012 | DOI: 10.1016/j.ceramint.2011.11.050

Title: Characterisation of ternary TixV1-xNy nitride prepared by mechanosynthesis
Author(s): Roldan, MA; Alcala, MD; Real, C
Source: Ceramics Intenational, 38 (2012) 687-693

abstract | fulltext

In the present manuscript the authors have systematically investigated the composition and microstructure of a series of ternary nitrides (TixV1-xNy) (0.0 <= x <= 1.0) prepared by mechanosynthesis, using XRD, SEM, EELS, XAS and TGA. The ternary titanium-vanadium nitride (TixV1-xNy) has been obtained in all range of compositions by the mechanical treatment of the two metals under nitrogen pressure in a planetary mill with a maximum milling time of 3 h and without any post-heating treatment. The materials' microhardnesses were measured after sinterisation and compared to those reported in the literature for these types of materials. When compared with the previously reported data for bulk samples, these values are similar or higher for compositions within the range x = 0.5 to x = 0.77 (TixV1-xN).

January, 2012 | DOI: 10.1016/j.ceramint.2011.07.057

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