Polyelectrolyte multilayer films containing silver as antibacterial coatings

A facile approach to fabrication of transparent antimicrobial coatings based on polyelectrolyte multilayers (PEMs) is presented. Counterions existing in PEMs were utilized via ion exchange and in situ reduction to incorporate into the films silver ions and nanoparticles, and the antibacterial efficacy of the films against E. coli was assessed by the Kirby-Bauer method. The PEMs containing silver in the ionic form exhibited high activities in short terms, and the antibacterial effects depended on the ionic strength in the polyelectrolyte solutions used for the PEM fabrication. The PEMs loaded with silver nanoparticles showed lower initial bactericidal effects, but remained active after long periods of time, and the antimicrobial performance can be improved by increasing the silver loading through repeating the ion-exchange/reduction cycle for multiple times. The films were transparent in the visible region. Coatings containing multiple antimicrobial agents for possible synergistic effects can be fabricated in a single process using this method.     

Open-cell aluminium foams with graded coatings as passively controllable energy absorbers

Compared to most bulk materials, open-cell aluminium (Al) foams (OCAFs) are light-weight and can absorb a significant amount of energy in compression, e.g. during impact. When coated with nickel (Ni), OCAFs can absorb even more energy, making them more appropriate for impacts at higher velocities than uncoated OCAFs. When Ni-coated OCAFs experience low-velocity impact however, the stopping distance during the impact is small compared to that of uncoated OCAFs and hence, deceleration occurs fast. This exposes devices (and possibly human beings) protected by OCAFs to large internal forces leading to internal damage. An OCAF that combines the properties of uncoated and coated OCAFs can absorb energy during both low-velocity and high-velocity impact scenarios. This contribution introduces two of such OCAFs which are created by partially and gradually coating OCAFs. The general mechanics of the two OCAFs are revealed using experimental and numerical observation methods.     

Pyrolysis-induced polymetallosiloxane coatings for aluminium substrates

Inorganic amorphous polymetallosiloxane (PMS) coating films on aluminium substrate were produced through the polycondensation-pyrolysis reaction mechanisms of a sol-precursor solution. The precursor solution was formed by HCl-catalysed hydrolysis of a mixture ofN-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole (TSPI) and M(OC₃H₇) _n , (M=Zr, Ti and Al,n=3 or 4). The TSPI/Zr(OC₃H₇)₄ or Ti(OC₃H₇)₄ precursor systems formed higher quality thin coating films, compared to the /Al(OC₃H₇) system. This was because of the critical formation of the polyorganosiloxane terminated by end groups containing zirconium and titanium oxides. These end groups were derived by a dechlorinating reaction between the Cl, bonded to the propyl C in organosilane, and the hydroxylated Zr or Ti compounds in the sintering stages of the film production. Good film-forming performance resulted from moderate degrees of cross-linking of metal oxides to polysiloxane chains and of the densification of metal -O-Si linkages in the pyrolysis-induced PMS network. In addition to these factors, the formation of an oxane bond at the interface between PMS and aluminium provided corrosion protection for the aluminium substrate.     

Ion-plated aluminium/aluminium oxide coatings using a pulsed oxygen process

Pulsed oxygen was utilized in a thermionically assisted triode ion plating process to produce dense, hard and highly adherent aluminium/aluminium oxide coatings on steel substrates. The coatings were deposited in a 0.1 Pa argon discharge while oxygen gas was injected at various pulse rates into the chamber. The influence of the oxygen flow rates, the pulse rates and the substrate bias on the structure and crystal phase of the coating were investigated using scanning and transmission electron microscopy, X-ray diffraction and Rutherford backscattering techniques. The analysis showed that the resultant coating structure consist of a mixed state of crystalline and non-crystalline phases depending on the oxygen flow rate and on the substrate bias at a constant oxygen pulse rate. Analysis showed that the non-crystalline phase approached stoichiometry for Al₂O₃ at oxygen flow rates of 7.5 Torr l s^-1 and a substrate bias of 2 kV and that there was a substantial increase in hardness.     

Evaporated aluminium nitride encapsulating films

AIN films were deposited onto GaAs and vitreous carbon substrates held at room temperature, by reactive evaporation of aluminium in the presence of nitrogen and/or NH₃ gas mixture. These films and their combination with very thin layers of Si₃N₄ were successfully used as encapsulants for GaAs and were found to withstand annealing temperatures of up to 1100°C. Films grown by this novel method were analysed by Rutherford backscattering spectrometry and reflection high energy electron diffraction. Oxygen, nitrogen and aluminium were the only elements detected in the encapsulants. However, the best encapsulants were found to have the lowest oxygen content. The deposition conditions were found to be very important in preventing the reaction of the films with the surface of GaAs during heat treatment.     

Purity and morphology of aluminium films

A continuous wire feed source of aluminium of 99.999% purity in conjunction with a composite ceramic boat has been used to deposit aluminium films of high purity in vacuum. The impurity content was analysed by proton-induced X-ray analysis with a Van de Graaff accelerator, supplemented by X-ray fluorescence, optical emission and Auger electron spectroscopy. The low impurity content achieved ( 50 atomic ppm) is attributed to the small quantity of molten aluminium maintained in the boat at one time.

The morphology of the Al films was studied by reflection electron diffraction, X-ray diffraction and scanning electron microscopy. Thick Al films grew with {111} and {311} texture orientations. The grain size variation with thickness generally followed the known variation with the deposition rate.     

RF magnetron sputtered aluminium oxide coatings on iridium

The effects of process parameters on the microstructural morphology of aluminium oxide (Al₂O₃) coatings on Ir have been studied. Al₂O₃ coatings were deposited on Ir-coated isotropic graphite (IG) substrates at substrate temperatures of room temperature (RT)-1073 K, RF power of 200–600 W in an Ar, or Ar+1–10% O₂, sputtering gas atmosphere by RF magnetron sputtering. Al₂O₃ coatings which were deposited at high substrate temperatures and high RF powers in an Ar, or an Ar+O₂, sputtering gas atmosphere were found to contain a dense, fine columnar structure with a -Al₂O₃ phase, low Ar content and a relatively high hardness value of ca. 1050 H _v. Furthermore, high resolution transmission electron microscopy (HRTEM) results revealed the epitaxial growth of Al₂O₃ coatings on Ir-coated IG substrate. It was found that the interface between Al₂O₃ and Ir coatings was sharp and Al₂O₃ coatings remained intact with the Ir-coated IG substrate.     

Flexible inorganic films and coatings

This paper describes a new family of materials derived from layer minerals such as phillosilicates and includes vermiculite, montmorillonite, etc. They are structurally analogous to the organic crystalline polyethylene in which the crystalline lamellae are replaced by inorganic lamellae of comparable dimensions. Flexible films can be produced with tensile strengths of 30 to 160 MN m^–2, elongation of up to 6% and moduli of 14 G N m^–2; they are stronger than organic films. Suspension of discrete lamellae in water can be used as coating for a variety of surfaces. The most interesting is the vermiculite-coated glass fibre which has unique properties. Finally, the suspension can be used to make a wholly inorganic foam which has similar properties to organic foams with good compressive strength.     

Electrical properties of thin oxidized aluminium films

Thin aluminium films of thickness 40 to 200 nm were deposited on to glass substrates at 573 K in a high vacuum. The deposition was carried out layer by layer and the interfaces between these layers were exposed to oxygen. The electrical resistivity was studied as a function of the film thickness, annealing time, annealing temperature and oxygen pressure. The temperature coefficient of resistivity and the activation energy for the conduction electrons were studied as a function of the film thickness and oxygen pressure. Fuchs-Sondheimer theory for electrical conduction was applied to the experimental results. The mean free path of the conduction electrons was calculated as a function of temperature and agreed well with the theoretical relation.     

Porous silica xerogel films as antireflective coatings – Fabrication and characterization

The paper presents a simple fabrication method of porous silica xerogel films. By adding a surface active agent Triton X-100™ to the starting solution, we can considerably reduce the surface tension, which, in turn, allows to fabricate silica films of high porosity. The paper presents the influence of surfactant content and the influence of heating temperature on the refractive index and thickness of the fabricated films. We fabricated silica films of the minimum refractive index below 1.3 and corresponding porosity ∼50%. Due to low refractive index, the elaborated porous silica xerogel films can be applied to reduce the light reflection coefficient in optical systems. In this work the spectral characteristics of the refractive index, extinction coefficients, the reflection and transmission coefficients and also depolarization factor are presented. The paper also provides results of surface morphology of produced layers, obtained using an atomic force microscope.     

Helium ion bombardment of thin aluminium films

A study is presented of the effect of 5 keV helium ion bombardment on thin (about 2000 Å) aluminium films using proton backscattering, scanning and transmission electron microscopy and α particle energy loss spectroscopy. Measurements of helium content after irradiation using proton backscattering indicate low below-saturation retention for both room temperature and low temperature implantations (19% and 24% respectively). Electron microscopy examination of the films reveals a severe deformation in the form of coarse and fine-scale wrinkling whose amplitude increases with increasing helium dose. This deformation does not appear to be the result of bubble swelling. An attempt has been made to quantify the wrinkling by measuring the energy loss spectrum of α particles transmitted through irradiated films and the combination of these measurements with a simple sinusoidal deformation model indicates an increase in film area of up to 20%.     

Optical properties of obliquely evaporated aluminium films

Vacuum-evaporated aluminium films can show substantially different optical properties depending on the angle at which the vapour stream impinges on the substrates. Films deposited at 85° oblique incidence developed diffuse reflecting surfaces and showed greater absorption than those condensed at normal incidence. Electron micrographs of such films reveal chains of crystallites whose long axes are inclined to the substrate. The orientation of these crystallites gives rise to optical anisotropy and the optical transmission of the films strongly depends on the direction of the vapour beam. The surface density at which the films start to show surface roughness can be increased if the films are condensed in an argon atmosphere. These films were shown to have superior reflectivity and to exhibit very low absorption or scattering. Optical anisotropy in these films could hardly be observed.     

Biological effects of aluminium diffusion from plasma-sprayed alumina coatings

Plasma-sprayed alumina (Al₂O₃) coatings on metal stems of hip prostheses are used to favour bone apposition on the stem without fibrous interposition. We tested both in vitro and in vivo in rabbits, alumina coatings in order to evaluate the biological effect of this material on bone. Mice fibroblasts were grown on Al₂O₃-coated discs and time course of aluminium concentration was recorded in two phosphate and citrate buffers (pH 4 and 7) bathing the alumina coated discs. Alumina-coated cylinders were implanted into femur condyles of ten rabbits for periods of time from 1–6 months. Then, they were histologically analysed using light and scanning electron microscopy, and X-ray microanalysis. Cell proliferation was not affected on alumina coatings compared to controls. In pH 4 buffer, aluminium was released from the coatings. From a period of implantation of 4–6 months an increasing demineralization process took place in the bone at the coating contact. Aurine staining showed the presence of aluminium at the interface between the non-mineralized and the mineralized bone. These results suggest that aluminium is released from alumina coatings and leads to bone demineralization at the coating contact.     

Reinforcement coatings and interfaces in aluminium metal matrix composites

The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites.The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of in situ hybridising and in situ alloying are described.     

Modelling of degradation of nickel-pigmented aluminium oxide photothermal collector coatings

Thermal degradation phenomena occurring in nickel-pigmented aluminium oxide coatings were investigated by combining detailed microstructural analysis with modelling of the optical properties. Scanning electron microscopy and Auger electron spectroscopy sputter profiling showed that the initial film consisted of a nickel-pigmented aluminium oxide layer close to the substrate. This layer supported a porous aluminium oxide layer that had a rough outer surface. While the solar absorptance degraded substantially (from 0.94 to 0.71) after heat treatment at 350°C for tens of hours, the aluminium oxide film morphology and thickness remained virtually unchanged and there was apparently no redistribution of nickel within the coating. Instead, the optical quality of the film degraded through oxidation of the nickel particles. These observations were supported by an optical model of the coating which produced the spectral reflectance properties measured both before and after the thermal ageing.