Samir Aouadi

Carbon Nanotube−MoS2 Composites as Solid Lubricants

Solid lubricants (SLs) characterized by low coefficients of friction (mu) and wear rates (w) drastically improve the life span of instruments that undergo extreme frictional wear. However, the performance of SLs such as sputtered or nanoparticulate molybdenum disulfide (MoS(2)), tungsten disulfide (WS(2)), or graphite deteriorates heavily under extreme operational conditions such as elevated temperatures and high humidity. Here, we present our preliminary results, which demonstrate that composites of carbon nanotubes (CNTs) and MoS(2) produced by electrodeposition of MoS(2) on vertically aligned CNT films have low mu ( approximately 0.03) and w (approximately 10(-13) mm(3)/N.mm) even at 300 degrees C, which are about 2 orders of magnitude better than those of nanoparticulate MoS(2)-based coatings. The high load-bearing capacity of CNTs provides a strong enduring support to MoS(2) nanoclusters and is responsible for their ultralow w. The incorporation of these composites in liquid lubricants reduces the friction coefficient of the liquid lubricants by approximately 15%. The technique described here to produce SL coatings with extremely appealing frictional properties will provide valuable solutions for a variety of tribological applications where the coatings encounter high temperature, reduced pressure, and/or low- and high-humidity conditions.

Growth and characterization of Cr2N/CrN multilayer coatings

A series of monolithic and multilayer coatings of chromium nitride with various compositions and architectures were deposited at low temperatures (<200°C) on silicon substrates using ion-assisted reactive magnetron sputtering. All coatings had a total thickness in the 1.5±0.3 μm range. The multilayer coatings were designed such that their period and CrN fraction varied in the range 30–150 nm and 0.50–0.93, respectively. Real-time in situ ellipsometry was used to monitor and control the deposition process. The deposited coatings were characterized post-deposition using X-ray diffraction (XRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The primary chromium nitride phases (Cr2N and CrN) in the films were identified using XRD. The chemical composition of selected samples was determined from RBS and XPS measurements. The phase composition of the deposited layers was deduced from the analysis of the SE data. The mechanical properties of the coatings were evaluated using a nanoindenter. The measured hardness values were in excess of 20 GPa. The results of the different characterization and testing techniques were correlated and follow-up work on this project suggested.

Structural and mechanical properties of TaZrN films: Experimental and ab initio studies

This paper reports on the growth and characterization of the structural and mechanical properties of tantalum zirconium nitride films and the subsequent simulation of these properties using an ab initio calculation based on density functional theory (DFT) within the generalized gradient approximation. The films were deposited by reactive unbalanced magnetron sputtering and their physical and chemical properties were studied by means of x-ray diffraction (XRD), Rutherford backscattering (RBS), and nanoindentation. XRD revealed that these films formed a solid solution and that the lattice constant decreased linearly with Ta content. RBS provided the elemental composition of the films. Nanoindentation was used to evaluate the hardness and the elastic modulus. The hardness was found to have high values for a Ta∕(Ta+Zr) of 30% and 100%. The elastic modulus was found to increase monotonically with Ta content. The intrinsic elastic constants were calculated using DFT and the results were compared to the experime...

Zirconium nitride/silver nanocomposite structures for biomedical applications

Silver zirconium nitride films deposited by unbalanced magnetron sputtering were studied by means of x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, spectroscopic ellipsometry, and nanoindentation. Coatings were deposited on silicon substrates at room temperatures with bias voltages in the −45 to −160 V range. The concentration of zirconium and silver was regulated by controlling the power to the sputtering guns. The nitrogen concentration was selected so that the nitrogen flow rate corresponded to the production of stoichiometric zirconium nitride. The films consisted of nanocrystals of zirconium nitride embedded in a silver matrix. The grain size was deduced from the width of the XRD peaks using the Scherrer formula and was found to decrease with the addition of silver. The chemical and phase composition was determined from XPS measurements. The optical constants were measured using spectroscopic ellipsometry. A correlation between film structure/composition and op...

Room temperature synthesis and high temperature frictional study of silver vanadate nanorods

We report the room temperature (RT) synthesis of silver vanadate nanorods (consisting of mainly beta-AgV O(3)) by a simple wet chemical route and their frictional study at high temperatures (HT). The sudden mixing of ammonium vanadate with silver nitrate solution under constant magnetic stirring resulted in a pale yellow coloured precipitate. Structural/microstructural characterization of the precipitate through x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the high yield and homogeneous formation of silver vanadate nanorods. The length of the nanorods was 20-40 microm and the thickness 100-600 nm. The pH variation with respect to time was thoroughly studied to understand the formation mechanism of the silver vanadate nanorods. This synthesis process neither demands HT, surfactants nor long reaction time. The silver vanadate nanomaterial showed good lubrication behaviour at HT (700 degrees C) and the friction coefficient was between 0.2 and 0.3. HT-XRD revealed that AgV O(3) completely transformed into silver vanadium oxide (Ag(2)V(4)O(11)) and silver with an increase in temperature from RT to 700 degrees C.

Characterization of TiBN films grown by ion beam assisted deposition

This paper presents one of the first attempts to measure and model the ellipsometric data for ternary nitride coatings in general and TiBN coatings in particular. TiBN coatings with a functionally graded underlayer of Ti/TiN have been deposited at low temperatures (<200 °C) on a silicon substrate using ion beam assisted deposition (IBAD). The coating selected for detailed analysis had a total thickness of 1.5±0.2 μm. The deposited structure was characterized post-deposition using X-ray diffraction (XRD), atomic force microscopy (AFM), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), infrared spectroscopic ellipsometry (IR-SE), and visible-light spectroscopic ellipsometry (VIS-SE). The primary phases (TiB2, TiN, and BN) in the film were identified using XRD. The surface morphology and nanocrystalline nature of the coating (grain size of 5–7 nm) were deduced using AFM. The chemical composition and phase composition of the sample was determined from RBS and XPS measurements and was subsequently deduced from the analysis of the VIS-SE data. The refractive indices for the constituent phases were deduced from the investigation of TiB2, TiN and BN single layers with SE. Good correlation was observed between RBS, XPS and VIS-SE for the data on the TiBN sample. XPS and IR-SE suggested that BN formed in the amorphous form. The chemical composition study using these various techniques shows that in-situ SE is a potential technique to control the growth of ternary nitride coatings. Finally, the mechanical properties of the coating were evaluated using a nanoindenter. The hardness and elastic modulus were measured to be 42 GPa and 325 GPa, respectively.

Characterization of Ti-based nanocrystalline ternary nitride films

Ternary nitride coatings of Ti–X–N, where X= Cr, Zr, or B, were deposited at low temperatures (<200 °C) on silicon substrates using ion beam assisted deposition (IBAD). The deposited films were characterized postdeposition using x-ray diffraction (XRD), atomic force microscopy, Rutherford backscattering (RBS), x-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and nanoindentation. The elemental and phase composition of the films were investigated using XRD, RBS, and XPS measurements. The TiZrN coating was found to consist of a single phase in contrast with TiCrN and TiBN which consisted of three phases, namely, Cr+TiN+Cr2N and TiB2+TiN+BN, respectively. Their optical constants were determined from SE measurements and the phase compositions were deduced from the analysis of these SE data. The elemental compositions deduced from the optically determined phase compositions correlated well with the results obtained from the more direct chemical analysis techniques, namely RBS and XPS. Th...

Characterization of CrBN films deposited by ion beam assisted deposition

This article reports on the growth and analysis of CrBN nanocrystalline materials using an ion beam assisted deposition process. In addition, this article addresses the utilization of spectroscopic ellipsometry for in situ analysis of ternary nitrides. Coatings, with a total thickness of 1.5±0.2 μm, were deposited at low temperatures (<200 °C) on silicon substrates using ion beam assisted deposition. These coatings were characterized postdeposition using x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), visible-light spectroscopic ellipsometry (VIS-SE), infrared spectroscopic ellipsometry (IR-SE), and nanoindentation. The primary phases in the films were investigated using XRD. The surface morphology and nanocrystalline nature of the coatings (grain size of 5–7 nm) were deduced using AFM. The elemental composition and phase composition of the samples were determined from XPS and AES measurements and were subsequently deduced ...

Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics

Cr/Sc and Ni/V multilayers, intended as normal incidence soft x-ray mirrors and Brewster angle polarizers, have been synthesized by employing a novel modulated low-energy and high-flux ion assistance as a means of engineering the interfaces between the subnanometer layers on an atomic scale during magnetron sputter deposition. To reduce both roughness and intermixing, the ion energy was modulated within each layer. The flat and abrupt interfaces yielded soft x-ray mirrors with near-normal incidence reflectances of R = 20.7% at the Sc 2p absorption edge and R = 2.7% at the V 2p absorption edge. Multilayers optimized for the Brewster angle showed a reflectance of R = 26.7% and an extinction ratio of R(s)/R(p)=5450 for Cr/Sc and R = 10% and R(s)/R(p)=4190 for Ni/V. Transmission electron microscopy investigations showed an amorphous Cr/Sc structure with an accumulating high spatial frequency roughness. For Ni/V the initial growth mode is amorphous and then turns crystalline after approximately 1/3 of the total thickness, with an accumulating low spatial frequency roughness as a consequence. Elastic recoil detection analyses showed that N was the major impurity in both Cr/Sc and Ni/V with concentrations of 15 at. % and 9 at. %, respectively, but also O (3 at. % and 1.3 at. %) and C (0.5 at. % and 1.9 at. %) were present. Simulations of the possible normal incidence reflective properties in the soft x-ray range of 100-600 eV are given, predicting that reflectivities of more than 31% for Cr/Sc and 5.8% for Ni/V can be achieved if better control of the impurities and the deposition process is employed. The simulations also show that Cr/Sc is a good candidate for mirrors for the photon energies between the absorption edges of B (E = 188 eV) and Sc (E = 398.8 eV).

Grain Boundary Sliding Mechanisms in ZrN-Ag, ZrN-Au, and ZrN-Pd Nanocomposite Films

Nanocomposite films of ZrN-Me (Me=Ag, Au, or Pd) were produced by reactive unbalanced magnetron sputtering and were found to form a dense and homogeneous microstructure whereby nanocrystals of Me are distributed evenly throughout the ZrN matrix. Interestingly, the Young’s modulus was found to decrease much more dramatically with the increase in metal content for the ZrN-Ag system. A systematic ab initio study was undertaken to understand the mechanism of grain boundary sliding in these nanostructures. The maximum energy variation during the sliding was found to be the largest and the smallest for ZrN-Pd and ZrN-Ag, respectively.