Philip J. Martin

Practical measurement of the residual stress in coatings

Abstract The method adopted to measure residual stress depends on the degree of detailed information needed, the size of the part, the coating thickness and the costs which can be incurred. Three of the main approaches used at the present time are discussed here. These are: first, the hole drilling method which returns a value of the macrostress and an indication of any anisotropy in coatings of thickness greater than about 0.3 mm; second, the cantilever beam methods applicable to thinner coatings on thin substrates; third, the X-ray diffraction methods applicable to crystalline coatings of a thickness which can be penetrated by the X-ray beam. This last method can be sub-divided into two groups where, in the first, the standard Bragg-Brentano diffractometer available in most laboratories is used, and, second, the more recent glancing incidence methods in which the stress in thin coatings or the surface of thicker coatings can be studied at depths of as little as 1 μm.

Composite yarns of multiwalled carbon nanotubes with metallic electrical conductivity.

Unique macrostructures known as spun carbon-nanotube fibers (CNT yarns) can be manufactured from vertically aligned forests of multiwalled carbon nanotubes (MWCNTs). These yarns behave as semiconductors with room-temperature conductivities of about 5 x 10(2) S cm(-1). Their potential use as, for example, microelectrodes in medical implants, wires in microelectronics, or lightweight conductors in the aviation industry has hitherto been hampered by their insufficient electrical conductivity. In this Full Paper, the synthesis of metal-CNT composite yarns, which combine the unique properties of CNT yarns and nanocrystalline metals to obtain a new class of materials with enhanced electrical conductivity, is presented. The synthesis is achieved using a new technique, self-fuelled electrodeposition (SFED), which combines a metal reducing agent and an external circuit for transfer of electrons to the CNT surface, where the deposition of metal nanoparticles takes place. In particular, the Cu-CNT and Au-CNT composite yarns prepared by this method have metal-like electrical conductivities (2-3 x 10(5) S cm(-1)) and are mechanically robust against stringent tape tests. However, the tensile strengths of the composite yarns are 30-50% smaller than that of the unmodified CNT yarn. The SFED technique described here can also be used as a convenient means for the deposition of metal nanoparticles on solid electrode supports, such as conducting glass or carbon black, for catalytic applications.

Ion‐assisted deposition of mixed TiO2‐SiO2 films

Mixed thin films of TiO2 and SiO2 were produced by coevaporation from separate electron‐beam sources and simultaneous bombardment of the growing film with oxygen ions. The optical properties of the films were determined during growth by in situ ellipsometry and the surface composition of the deposited films studied by in situ ion scattering spectroscopy, ex situ x‐ray photoelectron spectroscopy, and energy filtered electron diffraction. The correlation between the optical and surface characterization is presented. There is evidence of local variations in the relative concentrations of TiO2 and SiO2. The position of the Si 2p binding energy depends on the TiO2 content in the film, indicating the possible formation of an intimate mixture.

Protective dielectric coatings produced by ion-assisted deposition

Optically transparent dielectric films, when prepared by thermal evaporation or sputtering, have had limited use for many applications due to their permeability and lack of stability. We report on protective dielectric films produced by ion-beam-assisted deposition which demonstrate significant improvements over films produced by conventional deposition techniques. This result can be explained in terms of the increased packing density of ion-assisted films over the porous columnar microstructure usually associated with evaporated films. In addition, ion bombardment of the metal films produced the most stable structures and also substantially improved the adhesion of the films. The endurance under chemical attack of these films was found to be limited by the surface finish of the substrates.

The deposition of TiN thin films by filtered cathodic arc techniques

A filtered cathodic arc source has been used to deposit thin films of titanium nitride. The properties of the films are influenced by the nature of the condensation process. TiN films may be deposited directly onto heated substrates in a nitrogen atmosphere or onto unbiased substrates at ambient temperature by condensing the Ti/sup +/ ion beam under 500 eV N/sub 2//sup +/ nitrogen ion bombardment. In the latter case, the film stoichiometry was varied from an N:Ti ratio of 0.8 to 1.2 by controlling the relative arrival rates of Ti and nitrogen ions. Simple models are used to describe the evolution of compressive stress as a function of arrival ratio and the composition of the N/sub 2//sup +/ ion-assisted TiN films.

Influence of ion assistance on the optical properties of MgF(2).

The optical properties of MgF(2) films prepared by evaporation and ion-assisted deposition have been determined from transmittance and near-normal incidence reflectance measurements and also from electron-energy loss spectroscopy (EELS). The results show that oxygen-ion assistance leads to higher extinction coefficients for wavelengths <180 nm. Transmission electron microscopy studies show that the crystal grain size of MgF(2) films is not strongly affected by oxygen or argon-ion bombardment. The presence of MgO in the films is inferred from RBS measurements and proposed to be the major factor influencing VUV losses. EELS is also demonstrated to be a valuable technique for determination of optical properties from the near-infrared to x-ray regions of the spectrum.

Ion‐assisted deposition of bulklike ZrO2 films

Stable thin films of zirconium dioxide are produced by ion‐assisted electron beam deposition at room temperature and 300 °C. The method yields films with substantially increased packing density resulting in refractive indices close to the bulk value. The improvement in optical properties is accompanied by an amorphous to cubic crystalline transition of the film structure which is mixed with a monoclinic phase for heated films.

Effect of substrate bias on AlN thin film preparation in shielded reactive vacuum arc deposition

Abstract Aluminum nitride (AlN) thin films were prepared using reactive cathodic vacuum arc deposition in conjunction with a macrodroplet shield plate. Various bias conditions, such as no bias (floating), 0-V bias (same potential as anode), DC bias of −10 to −30 V, and RF power of 25–200 W, were applied to the substrate table. For floating bias, a -axis-oriented film was obtained. For 0-V bias, the films prepared on molybdenum substrate showed no preferential orientation, although the film prepared on silicon and borosilicate glass showed a -axis-orientation. For RF bias, the orientation changed from the a - to the c -axis as the RF power increased. The hardest (27 GPa) film was obtained for 0-V bias, and the hardness of the other films ranged from 19 to 24 GPa. The refractive index of the film prepared on quartz substrate was approximately 2.0 over the visual and infrared regions for all films. The extinction coefficient was less than 0.01 over the visual and infrared regions, with the exception of the film prepared under the 0-V bias condition, which showed a higher value.

Harnessing the influence of reactive edges and defects of graphene substrates for achieving complete cycle of room-temperature molecular sensing.

Molecular doping and detection are at the forefront of graphene research, a topic of great interest in physical and materials science. Molecules adsorb strongly on graphene, leading to a change in electrical conductivity at room temperature. However, a common impediment for practical applications reported by all studies to date is the excessively slow rate of desorption of important reactive gases such as ammonia and nitrogen dioxide. Annealing at high temperatures, or exposure to strong ultraviolet light under vacuum, is employed to facilitate desorption of these gases. In this article, the molecules adsorbed on graphene nanoflakes and on chemically derived graphene-nanomesh flakes are displaced rapidly at room temperature in air by the use of gaseous polar molecules such as water and ethanol. The mechanism for desorption is proposed to arise from the electrostatic forces exerted by the polar molecules, which decouples the overlap between substrate defect states, molecule states, and graphene states near the Fermi level. Using chemiresistors prepared from water-based dispersions of single-layer graphene on mesoporous alumina membranes, the study further shows that the edges of the graphene flakes (showing p-type responses to NO₂ and NH₃) and the edges of graphene nanomesh structures (showing n-type responses to NO₂ and NH₃) have enhanced sensitivity. The measured responses towards gases are comparable to or better than those which have been obtained using devices that are more sophisticated. The higher sensitivity and rapid regeneration of the sensor at room temperature provides a clear advancement towards practical molecule detection using graphene-based materials.

Electron optical techniques for microstructural andcompositional analysis of thin films

Abstract Electron optical methods of determining the atomic level microstructure ofthin films are discussed with reference to examples. The use of energy filtered electron diffraction and electron energy loss spectroscopy (EELS) are illustrated by using them to distinguish between amorphous carbons prepared by vacuum evaporation and the material prepared by condensing the carbon plasma stream from a vacuum arc. The latter is shown to consist almost entirely of tetrahedrally bonded carbon. These electron optical techniques are also applied to hexagonal and cubic boron nitride and to silicon and germanium carbides. Other techniques discussed are high resolution electron imaging and Kramers-Kronig analysis of EELS spectra.