D. Martinez-Martinez

Mechanical behavior and oxidation resistance of Cr(Al)N coatings

Nanocrystalline chromium nitride and ternary chromium aluminium nitride thin films were deposited by reactive magnetron sputtering of Cr and Al targets in argon/nitrogen atmosphere varying the sputtering power and gas composition. The coatings were characterized in terms of crystal phase, chemical composition, microstructure, and mechanical properties by x-ray diffraction, x-ray photoelectron spectroscopy, including x-ray-induced Auger electron spectroscopy, transmission electron microscopy, selected-area electron diffraction, electron energy-loss spectroscopy, cross-sectional scanning electron microscopy, and ultramicrohardness tester. The incorporation of Al in the composition of the films produces an increase in the mechanical properties (hardness and reduced Young’s modulus) and an increased thermal resistance against oxidation in comparison to the pure CrN composition. The hardness behavior was attributed mainly to a reduction of the CrN crystallite size according to a Hall–Petch relationship. The ox...

Tailored synthesis of TiC∕a-C nanocomposite tribological coatings

Composite coatings made of nanocrystalline TiC (nc-TiC) particles and amorphous carbon (a-C) have been prepared in a double magnetron sputtering system using graphite and titanium targets under Ar bombardment. Chemical composition and microstructure of coatings were studied by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and x-ray diffraction (XRD) for a set of samples prepared varying the ratio and intensity of power applied to each magnetron. Changes in coatings microstructure, from a quasipolycrystalline TiC to a nanocomposite formed by nanocrystals of TiC embedded in an amorphous matrix of carbon (nc-TiC∕a-C), are observed depending on the synthesis conditions. Tribological and mechanical properties of coatings were tested using a pin-on-disk tribometer and an ultramicrohardness indenter, respectively. Coatings with moderate hardness (7–27GPa), low friction (0.1–0.2), and low wear rates (k∼10−7mm3∕Nm) were obtained. A percentage between 15% and 30% of TiC is found ...

On the determination of local residual stress gradients by the slit milling method

This paper concentrates on an extension of a rather new methodology to determine local residual gradients at an enhanced lateral resolution using the so-called slit milling technique. The method is based on stress relaxation by making a slit using a focused ion beam and the displacements are measured through digital image correlation so as to calculate the residual stress. Our novel approach consists of a multiple fitting procedure along the length of the slit instead of the commonly applied averaging method. The applicability of our approach is demonstrated when stress gradients exist. In accordance to the Saint-Venant principle in linear elasticity, longer slits are better than shorter slits because of end effects. The proof-of-principle is supported by measurements on steel under controlled bending and by finite element modeling.

Influence of Plasma Treatments on the Frictional Performance of Rubbers

The frictional performance of several rubbers after pulsed-DC plasma treatments has been examined. In all cases, the treated rubbers showed better performance than the corresponding untreated ones. Stronger treatments, in terms of longer process time and/or higher substrate bias voltage, led to larger reductions of coefficient of friction and wear. The addition of hydrogen to the argon plasma did not show any additional positive effect. Nevertheless, different degrees of improvement were observed for different rubbers. In fact, the energy consumed during the tribotest scales with the maximum working temperature of the rubbers, indicating that the plasma treatment is more effective in the case of more sensitive rubbers.

On the nature of the coefficient of friction of diamond-like carbon films deposited on rubber

In this paper, the nature of the coefficient of friction (CoF) of diamond-like carbon (DLC)-protected rubbers is studied. The relative importance of the viscoelastic and adhesive contributions to the overall friction is evaluated experimentally by modifying the contact load and the adhesive strength between the surface and the counterpart. The results indicate that the increase of CoF during the tribotests under non-lubricated conditions is caused by the increase of the adhesive contribution to friction motivated by the growth of the contact area during the test. In the case of oil lubricating condition, the adhesive force is minimized and the CoF is observed to decrease during the tribotest. This is caused by the reduction of the viscoelastic contribution due to the variation of the shape of the contact area. The role of the microstructure of the DLC film on the efficiency of the oil lubrication is also discussed.

Performance of diamond-like carbon-protected rubber under cyclic friction. II. Influence of substrate viscoelasticity on the friction evolution

In this paper, the influence of the mechanical properties of rubber substrate on frictional behavior of DLC-protected rubber is studied by numerical methods. The viscoelastic contribution to the friction during a tribotest was simulated according to a “mattress” approach composed by Voigt or standard linear solid units. The latter approach demonstrated more accurate predictions. In both cases, the results show an increase of the contact depth, which is in agreement with the experimental observations. The simulations also show a progressive reduction of the viscoelastic contribution of the coefficient of friction (CoF) with the number of laps due to the elongation of the front part of the contact area. This prediction does not agree with the experimentally observed increase of CoF, and suggests a dominant contribution of the adhesive component to the friction under real conditions.

Selective functionalization of patterned glass surfaces

Tailored writing and specific positioning of molecules on nanostructures is a key step for creating functional materials and nano-optical devices, or interfaces for synthetic machines in various applications. We present a novel approach for the selective functionalization of patterned glass surfaces with functional probes of any nature. The presented strategy is optimized for imaging fluorophore labeled nanostructures for (single-molecule) fluorescence microscopy. The first step in the protocol is coating a glass surface, here a microscope cover slide, with a 60 nm thick diamond-like carbon film. Subsequently, the pattern is defined by either writing silicon oxide on the coating with a focused electron beam, or by etching the coating with a focused ion beam to expose the glass surface. Finally, the pattern is silanized and functionalized. We demonstrate the selective binding of organic fluorophores and imaging with high contrast, especially in total-internal-reflection mode. The presented approach is flexible and combines bottom-up assembly with high-resolution lithography on glass cover slides to precisely position and image functional molecules of any type.

On the 3D reconstruction of diatom frustules: a novel method, applications, and limitations

Because of the importance of diatoms and the lack of information about their third dimension, a new method for the 3D reconstruction is explored, based on digital image correlation of several scanning electron microscope images. The accuracy of the method to reconstruct both centric and pennate (symmetrical and asymmetrical) diatoms was shown, independently of valve size and shape, and considering not only the general frustule morphology but also the intricate ornamentation. Several measurements were obtained, such as of the surface and projected areas and the valve volume. These results were validated by focused ion beam transverse cross section of one valve, and the quantitative results were compared with geometric models commonly applied. Furthermore, direct volume calculations based on 3D reconstructions have several advantages, such as it has higher taxonomic accuracy than the methods based on light microscopy; it solves the problems of the light halos; it allows the precise measurement of all linear dimensions, including the often neglected third dimension; natural samples can be measured directly; and it provides an exact estimate of cell volume, independently of its shape or alterations due to life cycle stage. This approach provides therefore a simple way to measure the morphological features of diatoms, even at a nanoscale, and can be applied to other microorganisms commonly illustrated by means of scanning electron microscopy.

Calibration-free quantitative surface topography reconstruction in scanning electron microscopy

This work presents a new approach to obtain reliable surface topography reconstructions from 2D Scanning Electron Microscopy (SEM) images. In this method a set of images taken at different tilt angles are compared by means of digital image correlation (DIC). It is argued that the strength of the method lies in the fact that precise knowledge about the nature of the rotation (vector and/or magnitude) is not needed. Therefore, the great advantage is that complex calibrations of the measuring equipment are avoided. The paper presents the necessary equations involved in the methods, including derivations and solutions. The method is illustrated with examples of 3D reconstructions followed by a discussion on the relevant experimental parameters.

Formation of nanoporous Si upon self-organized growth of Al and Si nanostructures.

Nanostructured materials offer unique electronic and optical properties compared to their bulk counterparts. The challenging part of the synthesis is to create a balance between the control of design, size limitations, up-scalability and contamination. In this work we show that self-organized Al nanowires in amorphous Si can be produced at room temperature by magnetron co-sputtering using two individual targets. Nanoporous Si, containing nanotunnels with dimensions within the quantum confinement regime, were then made by selective etching of Al. The material properties, film growth, and composition of the films were investigated for different compositions. In addition, the reflectance of the etched film has been measured.