Erjia Liu

Calibration procedures for frictional measurements with a lateral force microscope

This paper outlines a procedure for the calibration of the lateral force applied to a triangular cantilever tip by means of a lateral force microscope. The force is directed perpendicular to the cantilever major axis. The lateral force results in a torsion of the cantilever. The torsion signal is monitored as a voltage on a four-sector photodiode. The conversion from a torsion signal to a lateral force depends on the optical geometry of the instrument and the cantilever shape, dimensions and material properties. The calibration procedure used requires a calculation of the elasticity of the cantilever and experimental data. In case tripping forces are ignored, the lateral force can be taken as the friction force.

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.

Properties of nitrogen doped tetrahedral amorphous carbon films prepared by filtered cathodic vacuum arc technique

The properties of nitrogen doped tetrahedral amorphous carbon films prepared by the filtered cathodic vacuum arc technique have been studied. The doping species, nitrogen ions, were produced by an ion beam source. The nitrogen flow rate was varied from 0.5 to 10 sccm while keeping other deposition conditions constant. The nitrogen content in deposited films was determined by Rutherford backscattering technique and ranged from 5 to 34 at.% depending on the nitrogen flow rate. The surface morphology, mechanical, optical, and electronic properties of the films were measured. The compressive stress, the hardness and the optical band gap all increased at low nitrogen content to a maximum at 5 at.% nitrogen and then decreased with increasing nitrogen content. The activation energy first increased and then decreased with increasing nitrogen content. We attribute these changes to the Fermi level moving up in the band gap, from below the midgap to near conduction band. We achieved continuously adjustable band gap and complex refractive index with nitrogen incorporation. Possible mechanisms of N ion in the ta-C:N films are discussed.

Transport of vacuum arc plasma through an off-plane double bend filtering duct

A new magnetic macroparticle filter design consisting of an off-plane double-bend (OPDB) filter is described. The transport of the vacuum arc plasma through this OPDB filter is investigated using Langmuir and deposition probes. Films of amorphous hard carbon have been deposited using a 90° single bend and the OPDB filter and the macroparticle contents of the films are compared. The experimental results were found to be in good agreement with the simulations results based on an improved drift approximation model. The results demonstrate that OPDB filter has a relatively better transmission efficiency than the 90° single bend filter, lower macroparticle counts and is suitable for preparation of diamond-like carbon coatings with high quality.

Comparative study between macrotribology and nanotribology

The behavior of different materials under macrotribological and nanotribological conditions has been compared. The materials, hydrogenated amorphous diamondlike carbon, highly ordered pyrolitic graphite, and mica, were submitted to a fretting test (macrotribological behavior) and a combined atomic force microscopy/lateral force microscopy (AFM/LFM) (nanotribological behavior). The coefficient of friction in the macroscopic regime under fixed experimental conditions considerably changed with the test duration, whereas the friction coefficient measured from the nanoscopic regime was relatively constant during the test. In the macroscopic regime, evident wear, elastoplastic deformation, and material transfer were noticed, while in the nanoscopic regime under the used testing conditions no wear phenomena were observed. The coefficient of friction showed some dependence on the amplitude and frequency of lateral movement of the sample relative to the counterbody in both the fretting and AFM/LFM tests as well as...

Stripping Voltammetric Analysis of Heavy Metals at Nitrogen Doped Diamond-Like Carbon Film Electrodes

Conductive nitrogen doped diamond-like carbon (N-DLC) film electrodes were used to investigate the possibility of detecting heavy metals such as lead, copper and cadmium by differential pulse anodic stripping voltammetry (DPASV) in the absence of mercury film. The preconcentration conditions (deposition potential, deposition time) and solution pH were optimized for the determination of lead in aqueous solution. A linear dependence of lead stripping current peak within the concentration (5 10 7 to 2 10 6 MP b 2 ) and deposition time (30 to 300 s at 1.00 V vs. SCE) was obtained. A multi-elemental analysis (Pb 2 ,C d 2 and Cu 2 ) illustrated that the N-DLC film electrode provided a significant stripping response for determination of multi-metals simultaneously. The present novel electrode showed great promise for the analysis of heavy metals.

Development of Sol–Gel Icephobic Coatings: Effect of Surface Roughness and Surface Energy

Sol-gel coatings with different roughness and surface energy were prepared on glass substrates. Methyl triethoxysilane (MTEOS), 3-Glycidyloxypropyl trimethoxysilane (GLYMO) and fluoroalkylsilane (FAS) were used to obtain a mechanically robust icephobic coating. Different amount of hydrophobic silica nano particles was added as fillers to introduce different roughness and surface energy to the coatings. The microstructure, roughness, and surface energy, together with elemental information and surface chemical state, were investigated at room temperature. The contact angle and sliding angle were measured at different temperatures to correlate the wetting behavior at low temperature with the anti-icing performance. The ice adhesion shear strength was measured inside an ice chamber using a self-designed tester. The factors influencing the ice adhesion were discussed, and the optimum anti-icing performance found in the series of coatings. It was found that lower surface energy leads to lower ice adhesion regardless of the roughness, while the roughness plays a more complicated role. The wetting behavior of the droplet on surface changes as temperature decreases. The anti-icing performance is closely related to the antiwetting property of the surfaces at subzero temperatures.

PHASE TRANSFORMATIONS IN PLASMA SPRAYED HYDROXYAPATITE COATINGS

Hydroxyapatite (HA: Ca 10(PO4)6(OH)2) is a bioactive material that has recently been receiving much attention mainly because of its ability to bond chemically with bone [1–3]. It is often applied clinically as a coating on an inert metallic implant such as Ti-6Al-4V. Plasma spraying is a widely used coating process to deposit HA powders onto the metallic implants. Amorphous calcium phosphate is normally produced in the coatings during the plasma spraying, not only due to the high cooling rate but also due to the relevant intrinsic properties of HA [4]. In addition, calcium phosphate phases other than crystalline HA and amorphous calcium phosphate are usually identified in the as-sprayed coatings. These phases include tri-calcium phosphate (TCP), tetra-calcium phosphate (TTCP) and/or CaO depending on the plasma conditions and the type of the HA material. A high crystallinity level is desirable in order for the materials to have good bioactive properties. Like other amorphous phases [5], the amorphous calcium phosphate in the as-sprayed coatings is thermodynamically metastable and an appropriate thermal treatment could induce a crystallization process to occur. That is why the as-sprayed coatings are usually subjected to a post heat-treating cycle. In this study, by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD), we aim to understand the phase transformations that could take place in an as-sprayed hydroxyapatite coating during a post heat-treatment process. We investigated the temperatures at which the phase transformations took place and elucidated fundamental understandings about the phase transformations (i.e. activation energy and enthalpy).

Ultraviolet and visible Raman studies of nitrogenated tetrahedral amorphous carbon films

Nitrogenated, tetrahedral amorphous carbon (ta-C) films prepared by the filtered cathodic vacuum arc (FCVA) technique have been studied using ultraviolet (UV, 244 nm) and visible (514 nm) micro-Raman scattering. The nitrogen ions were produced by a RF ion-beam source with a nitrogen flow-rate varying from 0 to 10.0 sccm, which results in a nitrogen content from 0 to 10.8 at.% in the deposited films. In the visible Raman spectra, only vibrational modes of sp 2 -bonded carbon (G and D peaks) are observed, while a new wide peak, called the T peak, located at 1090‐1320 cm 21 , associated with the vibrational mode of sp 3 -bonded carbon, appears in the UV-Raman spectra. In the visible Raman spectra, the G-peak width (100‐113 cm 21 ) and the intensity ratio ID/IG (0.34‐0.94) are both sensitive to the structural changes induced by N incorporation. In the UV-Raman spectra, the G-peak position almost linearly decreases from 1665 to 1610 cm 21 , and the Tpeak position increases tremendously from 1095 to 1314 cm 21 with increasing N content. The G-peak position and width, and the T-peak position, are all sensitive to the bonding structure of the films. q 2000 Elsevier Science S.A. All rights reserved.

Tribological behaviour of different diamond-like carbon materials

Abstract This paper comparatively studies the tribological behaviour of different types of diamond-like carbon (DLC) coatings in a reciprocating sliding wear test. The results are interpreted in terms of structure and surface morphological characteristics. At the beginning of each reciprocating sliding wear test, the higher coefficient of friction of the DLC coatings reflects the original surface conditions of contacting counterfaces, whereas the low coefficient of friction achieved under the steady regime is linked to the presence of wear debris and an enlarged real contact area. sp2 carbon bonding has been related to the occurrence of a higher coefficient of friction and surface damage of DLC materials compared to bulk diamond. The a-C coatings and ta-C films have a lower coefficient of friction at the steady regime than the a-C:H coatings, possibly due to a significant amount of hydrogen in the a-C:H coatings.