Sam Zhang

Recent advances of superhard nanocomposite coatings: a review

In this paper, a review of the present status of the research and technological development in the field of superhard nanocomposite coatings is attempted. Various deposition techniques have been used to prepare nanocomposite coatings. Among them, reactive magnetron sputtering is most commonly used. Nanocomposite coating design methodology and synthesis are described with emphasis on the magnetron sputtering deposition technique. Also discussed are the hardness and fracture toughness measurements of the coatings and the size effect. Superhard nanocomposite thin films are obtainable through optimal design of microstructure. So far, much attention is paid to increasing hardness, but not enough to toughness. The development of next generation superhard coatings should base on appropriate material design to achieve high hardness and at the same time high toughness.

FUNCTIONALLY GRADED TIN/TINI SHAPE MEMORY ALLOY FILMS

The presence of an adherent and hard TiN layer (200 nm) on TiNi-based shape memory alloy (SMA) film (3.5 μm) formed a passivation layer, improved hardness and tribological properties, without sacrificing the phase transformation and shape memory effect of the TiNi film.

Deposition of Cr Species at ( La , Sr ) ( Co , Fe ) O3 Cathodes of Solid Oxide Fuel Cells

Deposition process of Cr species at the (La,Sr)(Co,Fe)O 3 (LSCF) electrode and Gd 0 . 2 Ce 0 . 8 O 2 (GDC) electrolyte system is investigated under the O 2 reduction conditions in the presence of a Fe-Cr alloy interconnect for solid oxide fuel cells. Deposition of Cr species preferentially occurs on the surface of the LSCF electrode with and without the cathodic polarization at 900°C, forming SrCrO 4 and Cr 2 O 3 phase. At the initial stage of the reaction, Cr deposition was not detected inside the LSCF electrode or at the LSCF electrode/GDC electrolyte interface. Deposition of Cr species on the LSCF electrode surface under the rib of Fe-Cr alloy interconnect is substantial in comparison to that under the channel of the interconnect. The results demonstrate clearly that the deposition of Cr species at the LSCF electrode is essentially a chemical reaction and is kinetically controlled by nucleation reaction between the gaseous Cr species and SrO-enriched/segregated on the LSCF electrode surface.

Ambient fabrication of large-area graphene films via a synchronous reduction and assembly strategy

A synchronous reduction and assembly strategy is designed to fabricate large-area graphene films and patterns with tunable transmittance and conductivity. Through an oxidation-reduction reaction between the metal substrate and graphene oxide, graphene oxide is reduced to chemically converted graphene and is organized into highly ordered films in situ. This work will form the precedent for industrial-scale production of graphene materials for future applications in electronics and optoelectronics.

Magnetron sputtering of nanocomposite (Ti,Cr)CN/DLC coatings

Superhard nanocrystalline (Ti, Cr)CN/DLC coatings were prepared through co-sputtering of Ti, Cr and graphite targets in an argon/nitrogen atmosphere. Results from both transmission electron microscopy (TEM) and grazing incident X-ray diffraction (GIXRD) indicated that the grain size of (TiCr)CxNy crystals was approximately 10–20 nm. X-ray photoelectron spectroscopic studies confirmed that an increase in the sputtering power at the Ti target not only increased the Ti composition in the film but also brought about an increase in sp3 bonding in DLC matrix, in agreement with the raising hardness with Ti sputtering power. Film hardness and elastic modulus were measured with a nano-indenter, and film hardness reached 40 GPa. Tribological behaviors of the films were evaluated using a ball-on-disk tribometer, and the films demonstrated properties of low-friction and good wear resistance.

Diffusion in mechanical alloying

Abstract This paper studies the roles of two key factors in the mechanical alloying process, these factors being: activation energy, which is related to the formation of defects during the collision of powder particles; and crystalline size, which is related to the formation of nanometer crystalline during mechanical alloying. According to thermodynamic theory, the decrease in activation energy can result in an increase in diffusivity at constant temperature. Therefore, a decrease in activation energy is equivalent to an increase in temperature. High diffusivity can be obtained by creating a large number of defects through mechanical alloying. In addition, by creating nanometer size crystalline particles through the repeated fracturing and cold-welding of the powder particles, diffusion can take place easily through the grain boundaries. Consequently, elements which are difficult to diffuse may be alloyed using this technique.

Early interaction between Fe-Cr alloy metallic interconnect and Sr-doped LaMnO3 cathodes of solid oxide fuel cells

The initial stages of the interaction between the Fe–Cr alloy metallic interconnect and Sr-doped LaMnO 3 (LSM) electrode of solid oxide fuel cells (SOFC) were investigated under cathodic polarization at the temperature range of 700–900 °C. Cr deposits on the Y 2 O 3 –ZrO 2 (YSZ) electrolyte surface increased with the polarization time. However ,i t was observed that at the early stages of the reaction, there is no preferential Cr deposition at the three-phase boundary areas at the LSM electrode/YSZ electrolyte interface region. With the decrease of the temperature the Cr deposition reduced significantly, probably due to the significant reduction in the partial pressure of the gaseous Cr species and the cationic diffusivities in the LSM electrode. The results clearly demonstrated that the deposition of Cr species at the LSM electrode/YSZ electrolyte is basically a chemical reaction and kinetically controlled by the nucleation reaction between the gaseous Cr species and the Mn 2+ species generated under cathodic polarization.

Magnetron sputtered hard a-C coatings of very high toughness

Hydrogen-free amorphous carbon coatings of high hardness (≈30 GPa) and toughness (plasticity from 50 to 60%) were deposited on 440C steel substrates by DC magnetron sputtering at target power density of 10.5 W/cm2 in the bias range from −20 to −150 V. The surface topography, hardness and tribological behavior of the coatings were investigated. With the increase of bias voltage, coating hardness and surface smoothness increased at expense of some adhesion strength and an increase of coefficient of friction. All coatings showed low friction in humid air and graphitization was observed after a high number of rotation cycles. The graphitization adds more benefit aside from reducing friction: the graphite layer can considerably reduce the adhesive wear since it prevents the asperities of the two surfaces to be adhered to each other.

Cyclic Voltammetry Studies of Sputtered Nitrogen Doped Diamond-Like Carbon Film Electrodes

The conductive nitrogen-doped diamond-like carbon film (N-DLC) of about 0.14 m in thickness was deposited on highly conductive silicon wafer (111) with DC magnetron sputtering system. The electrochemical characteristics of the film have been studied with cyclic voltammetry (CV). The N-DLC film electrodes compare very favorably with conventional carbon based electrodes such as glassy carbon. The N-DLC film electrode exhibits a low double-layer capacitance, a large electrochemical potential window, and a relatively high electrochemical activity toward ferricyanide reduction. In addition, the electrode exhibits catalytic activity for Cl2/Cl as well as durability to high anodic potential, and a high signal for the trace analysis of Pb 2 . These characteristics demonstrate great promise of the N-DLC film as a novel electrode material for electrochemical analysis.

Sol-gel preparation of bioactive apatite films

The surface of biomedical metallic implants covered by a bioactive apatite film can create bioactivity of the implant and shorten healing time. In this work, apatite films on Ti6Al4V were prepared by sol–gel route using Ca(NO ) , P O and HPF as 32 2 5 6 the precursors, in vitro evaluations of the resulting hydroxyapatite (HA) and fluorapatiteyhydroxyapatite solid solution (FHA) films were done in Kokubo’s simulated body fluid and citric acid modified phosphate buffer solution (CPBS). HPF showed a 6 good reagent for the incorporation of fluorine into apatite films. The FHA film demonstrated to have good bioactivity, and to have better stability in CPBS and higher adhesion strength than the HA film. When fluorine is incorporated into the film, an increase in crystallinity of the apatite film and a decrease in intrinsic solubility of the FHA film could make significant contributions to the improvement in the stability; the thermal expansion coefficient of the FHA film getting closer to that of Ti6Al4V could be responsible for the increase in adhesion strength. 2002 Elsevier Science B.V. All rights reserved.