Advances in test and measurement of the interface adhesion and bond strengths in coating-substrate systems, emphasising blister and bulk techniques

In this paper, recent advances in the minimum-destructive testing of the adhesion of coating-substrate systems are reviewed, focusing on key techniques such as micro- and nano-scale levels of indentation, scratching, laser-induced wave shock, as well as the blister and buckle approach. Along with adhesion failure tests, the latest and most extensive applications of the adhesion test methods in nano-, micro- and bulk-coating technology and the associated techniques to determine the minimum damage defects left on the coatings are discussed and their use reviewed

Hydrogen Sensor Application of Anodic Titanium Oxide Nanostructures

Hydrogen (H2) fuel cells have been considered a promising renewable energy source. The recent growth of H2 economy has required highly sensitive, micro-sized and cost-effective H2 sensor for monitoring concentrations and alerting to leakages due to the flammability and explosiveness of H2 Titanium dioxide (TiO2) made by electrochemical anodic oxidation has shown great potential as a H2 sensing material. The aim of this thesis is to develop highly sensitive H2 sensor using anodized TiO2. The sensor enables mass production and integration with microelectronics by preparing the oxide layer on suitable substrate. Morphology, elemental composition, crystal phase, electrical properties and H2 sensing properties of TiO2 nanostructures prepared on Ti foil, Si and SiO2/Si substrates were characterized. Initially, vertically oriented TiO2 nanotubes as the sensing material were obtained by anodizing Ti foil. The morphological properties of tubes could be tailored by varying the applied voltages of the anodization. The transparent oxide layer creates an interference color phenomena with white light illumination on the oxide surface. This coloration effect can be used to predict the morphological properties of the TiO2 nanostructures. The crystal phase transition from amorphous to anatase or rutile, or the mixture of anatase and rutile was observed with varying heat treatment temperatures. However, the H2 sensing properties of TiO2 nanotubes at room temperature were insufficient. H2 sensors using TiO2 nanostructures formed on Si and SiO2/Si substrates were demonstrated. In both cases, a Ti layer deposited on the substrates by a DC magnetron sputtering method was successfully anodized. A mesoporous TiO2 layer obtained on Si by anodization in an aqueous electrolyte at 5°C showed diode behavior, which was influenced by the work function difference of Pt metal electrodes and the oxide layer. The sensor enabled the detection of H2 (20-1000 ppm) at low operating temperatures (50–140°C) in ambient air. A Pd decorated tubular TiO2 layer was prepared on metal electrodes patterned SiO2/Si wafer by anodization in an organic electrolyte at 5°C. The sensor showed significantly enhanced H2 sensing properties, and detected hydrogen in the range of a few ppm with fast response/recovery time. The metal electrodes placed under the oxide layer also enhanced the mechanical tolerance of the sensor. The concept of TiO2 nanostructures on alternative substrates could be a prospect for microelectronic applications and mass production of gas sensors. The gas sensor properties can be further improved by modifying material morphologies and decorating it with catalytic materials.Siirretty Doriast

Fabrication and nanoroughness characterization of specific nanostructures and nanodevice

Nanoroughness is becoming a very important specification for many nanostructures and nanodevices, and its metrology impacts not only the nanodevice properties of interest, but also its material selection and process development. This Ph.D. thesis presents an investigation into fabrication and nanoroughness characterization of nanoscale specimens and MIS (metal-insulator-semiconductor) capacitors with 2 HfO as a high k dielectric. Self-affine curves and Gaussian, non-Gaussian, self-affine as well as complicated rough surfaces were characterized and simulated. The effects of characteristic parameters on the CD (critical dimension) variation and the properties of these rough surfaces were visualized. Compared with experimental investigations, these simulations are flexible, low cost and highly efficient. Relevant conclusions were frequently employed in subsequent investigations. A proposal regarding the thicknesses of the deposited films represented by nominal linewidths and pitch was put forward. The MBE (Molecular Beam Epitaxy) process was introduced and AlGaAs and GaAs were selected to fabricate nanolinewidth and nanopitch specimens on GaAs substrate with nominal linewidths of 2nm, 4nm, 6nm and 8nm, and a nominal pitch of 5nm. HRTEM (High Resolution Transmission Electron Microscopy) image-based characterization of LER/LWR (Line Edge Roughness/Line Width Roughness) in real space and frequency domains demonstrated that the MBE-based process was capable of fabricating the desired nanolinewidth and nanopitch specimens and could be regulated accordingly. MIS capacitors with 2 HfO film as high k dielectric were fabricated, and SEM (Scanning Electron Microscope) image-based nanoroughness characterization, along with measurement of the MIS capacitor electrical properties were performed. It was concluded that the annealing temperature of the deposited 2 HfO film was an important process parameter and 700℃ was an optimal temperature to improve the properties of the MIS capacitor. Also, by quantitative characterization of the relevant nanoroughness, the fabrication process can be further regulated. The uncertainty propagation model of SEM based nanoroughness measurement was presented according to specific requirements of the relevant standards, ISO GPS (Geometric Product Specifications and Verification) and GUM (Guide to the Expression of Uncertainty in Measurement), and the method for implementating uncertainties was evaluated. The case study demonstrated that the total standard uncertainty of the nanoroughness measurement was 0.13nm, while its expanded uncertainty with the coverage factor k as 3 was 0.39nm. They are indispensable parts of LER/LWR measurement results

In Situ Observations of Phase Transitions in Metastable Nickel (Carbide)/Carbon Nanocomposites.

Nanocomposite thin films comprised of metastable metal carbides in a carbon matrix have a wide variety of applications ranging from hard coatings to magnetics and energy storage and conversion. While their deposition using nonequilibrium techniques is established, the understanding of the dynamic evolution of such metastable nanocomposites under thermal equilibrium conditions at elevated temperatures during processing and during device operation remains limited. Here, we investigate sputter-deposited nanocomposites of metastable nickel carbide (Ni3C) nanocrystals in an amorphous carbon (a-C) matrix during thermal postdeposition processing via complementary in situ X-ray diffractometry, in situ Raman spectroscopy, and in situ X-ray photoelectron spectroscopy. At low annealing temperatures (300 °C) we observe isothermal Ni3C decomposition into face-centered-cubic Ni and amorphous carbon, however, without changes to the initial finely structured nanocomposite morphology. Only for higher temperatures (400-800 °C) Ni-catalyzed isothermal graphitization of the amorphous carbon matrix sets in, which we link to bulk-diffusion-mediated phase separation of the nanocomposite into coarser Ni and graphite grains. Upon natural cooling, only minimal precipitation of additional carbon from the Ni is observed, showing that even for highly carbon saturated systems precipitation upon cooling can be kinetically quenched. Our findings demonstrate that phase transformations of the filler and morphology modifications of the nanocomposite can be decoupled, which is advantageous from a manufacturing perspective. Our in situ study also identifies the high carbon content of the Ni filler crystallites at all stages of processing as the key hallmark feature of such metal-carbon nanocomposites that governs their entire thermal evolution. In a wider context, we also discuss our findings with regard to the much debated potential role of metastable Ni3C as a catalyst phase in graphene and carbon nanotube growth

Microstructure, Mechanical and Tribological Properties of Ternary Transition Metal Nitride Coatings

The evolution of transition metal nitride (TMN) coatings with excellent mechanical and tribological properties has been shown to be a successful strategy in protecting tool components. In this thesis, the microstructure, mechanical properties and tribological performances of a range of TMN-based coatings, synthesised by physical vapor deposition techniques, were explored. First, the influence of substrate bias, on the structure and properties of magnetron sputtered TiSiN coatings was investigated. Enhanced scratch resistance i.e., higher critical loads (Lc1 and Lc2) was found in the coating deposited at the lower bias voltage (-40 V), which was ascribed to higher H/Er and H3/Er 2 ratios arising from fine nanocomposite structure and the presence of a higher compressive residual stress. An approximately 21% increase in wear rate was obtained for the coating prepared at higher bias (-50 V), which was attributed to slightly higher Si concentrations (~9.4 at.%) and, in turn, lower hardness. Further, a notable increase in Lc1 (~54%) and Lc2 (~27%) values, was obtained for a thick TiSiN coating (magnetron sputtered at a different condition) in comparison to the binary TiN coating that were underlain by its superior mechanical properties and graded structure, promoting the capacity to resist crack formation and delamination. Furthermore, the influence of Ni content, regulated by cathode composition, on the structure and properties of cathodic arc evaporated TiNiN coatings was examined. A transition from a fine columnar structure, at low Ni contents (~2 at.%), to a much finer equiaxed structure at higher Ni concentrations (≥ 4 at.%) was noted. In addition, the density of macroparticles generated during arcing was shown to be inversely related to the melting temperature of the target material. Finally, the effect of Ni content, controlled by the NiCr target current (INiCr) on the structure mechanical properties and scratch and wear behaviour of magnetron sputtered CrNiN coatings was studied. Significant damage-tolerance, coupled with good hardness values (greater than ~12 GPa), was found in the CrNiN coatings deposited at INiCr ≥2 A. The presence of a metallic nickel-rich phase, together with nanoscale porosity, may contribute to stress dissipation and help maintain structural integrity

Engineering aperiodic spiral order for photonic-plasmonic device applications

Thesis (Ph.D.)--Boston UniversityDeterministic arrays of metal (i.e., Au) nanoparticles and dielectric nanopillars (i.e., Si and SiN) arranged in aperiodic spiral geometries (Vogel's spirals) are proposed as a novel platform for engineering enhanced photonic-plasmonic coupling and increased light-matter interaction over broad frequency and angular spectra for planar optical devices. Vogel's spirals lack both translational and orientational symmetry in real space, while displaying continuous circular symmetry (i.e., rotational symmetry of infinite order) in reciprocal Fourier space. The novel regime of "circular multiple light scattering" in finite-size deterministic structures will be investigated. The distinctive geometrical structure of Vogel spirals will be studied by a multifractal analysis, Fourier-Bessel decomposition, and Delaunay tessellation methods, leading to spiral structure optimization for novel localized optical states with broadband fluctuations in their photonic mode density. Experimentally, a number of designed passive and active spiral structures will be fabricated and characterized using dark-field optical spectroscopy, ellipsometry, and Fourier space imaging. Polarization-insensitive planar omnidirectional diffraction will be demonstrated and engineered over a large and controllable range of frequencies. Device applications to enhanced LEDs, novel lasers, and thin-film solar cells with enhanced absorption will be specifically targeted. Additionally, using Vogel spirals we investigate the direct (i.e. free space) generation of optical vortices, with well-defined and controllable values of orbital angular momentum, paving the way to the engineering and control of novel types of phase discontinuities (i.e., phase dislocation loops) in compact, chip-scale optical devices. Finally, we report on the design, modeling, and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing, nanoantennas for resonant near-field optical concentration of radiation to individual nanowires, and aperiodic double resonance surface enhanced Raman scattering substrates

Metal Oxide Thin Films: Synthesis, Characterization and Applications

This Special Issue will compile recent developments in the field of metal oxide thin film deposition. The articles presented in this Special Issue will cover various topics, ranging from, but not limited to, the optimization of deposition methods, thin film preparations, the functionalization of surfaces with targeted applications, nanosensors, catalysis, electronic devices, biocidal coating, and the synthesis of nanostructures via the accurate control of thin film deposition methods, among others. Topics are open to metal oxide thin film deposition and characterization for the development of applications

Corrosion control of magnesium for stent applications

Biomaterials used for implants may be metallic, ceramic, polymeric or composite. Currently, metals that are gradually broken down in the body have been attracting much attention, as a new generation of biodegradable implants. Magnesium (Mg) and related alloys are promising candidates for degradable biomaterials, comprising temporary mechanical properties with biological acceptance to the human body. However, the target periods set clinically, with respect to the practical uses of Mg for biodegradable stents, have yet to be achieved. Hence, improved understanding of the corrosion behaviour of Mg in the biological environment is needed. Novel Mg narrow walled minitubes, for degradable stent applications, have been produced using radio frequency magnetron sputtering (RF-MS) physical vapour deposition (PVD). The microstructural development of the as-deposited minitubes have been investigated, as a function of annealing temperature, using the combined complementary analytical techniques of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD) and microhardness indentation. The as-deposited minitubes exhibited columnar grain structures with high levels of porosity, but were very brittle. Slight alteration to the crystal structure, from columnar to more isotropic grain growth, was demonstrated at elevated temperature, along with increasing material densification, hardness and corrosion resistance. It is suggested that stabilisation of the columnar grains and the formation of oxide layers during the sequential Mg-layer deposition process, acted as a barrier, preventing the development of a fully dense, equiaxed structures. The onset and development of Mg corrosion may be addressed by the use of coatings or near surface modification processes. Accordingly, the corrosion resistance of ~ 1-2 µm thick Al coatings, deposited by RF-MS on polished Mg surfaces, within Ar and Ar/H2 environments, were appraised. The coatings were heat-treated at 300°C and 450°C, with the aim of inducing the formation of bioinert Al2O3, and samples were corroded within phosphate buffered saline (PBS) solution at 37°C to mimic the biological environment. Both as-deposited and heat-treated coatings were found to delay the onset of corrosion, but showed higher initial corrosion rates, once established, as compared to the polished Mg surfaces. Slight improvement in coating performance was achieved through the addition of H2 to the system, which acted to inhibit Al-Mg alloying and enhance Al2O3 formation. However, localized accelerated corrosion associated with substrate polishing damage emphasised the need for improved process control and coating uniformity. Si-H coatings deposited on Mg surfaces within Ar/H2 ambient using a PVD technique was also investigated. The as-deposited coatings comprised dense, crack-free amorphous a-Si-H layers with thickness of ~ 1 µm. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses provided evidence for the presence of SiH2 as well as SiOx. The corrosion resistance of a-Si-H coated Mg increased significantly in contact with PBS, in both electrical and immersion tests, due to improved coverage of the substrate. The effect of rapid thermal processing techniques on the corrosion resistance of Mg surfaces was also investigated. Mg surfaces treated by large area electron beam (LAEB) irradiation showed refinement of the surface grain structure, with increased grain boundary delineation, although localised ablation, roughness and crater formation increased with increasing cathode voltage and number of pulses. The corrosion potential and corrosion rate of LAEB modified surfaces generally increased with increase the energy imparted to the surface. The extended corrosion performance of low energy EB processed surfaces, under immersion testing was consistent with the trend of improved corrosion resistance during the early stages of immersion in PBS. However, surfaces over-processed at high energies experienced higher corrosion rates in both potentiodynamic and immersion testing, due to the development of inclusions, craters and cracks on the modified surface. Further, Mg surfaces, modified by laser surface melting (LSM) under conditions of low energy laser irradiation, experienced rapid melting, causing surface smoothening and grain refinement centred along the laser beam tracks, whilst coarser grains decorated the overlapping regions, due to the Gaussian shape of the laser beam profile. More uniform surface processing was achieved by increasing the laser beam spot size, which acted to improve the corrosion resistance of Mg. Under high energy LSM processing conditions, Mg surfaces showed conventional laser melting rippled patterns, along with craters and cracks, and the redeposition of MgO particles, causing an increase in surface roughness and corrosion rate. The corrosion performance under immersion testing showed the corrosion rate similar to that of the original polished Mg samples, due to non-uniform surface modification and the mixed development of fine and coarser grains. However, observation revealed that refined grain regions along the centre of the laser tracks were able to resist corrosion for longer times. Generally, annealed Mg-minitubes produced by PVD, and the near surface modification of Mg by EB and LSM, showed that fine grained Mg can affect the electrochemical response of Mg within the physiological environment, due to the rapid, enhanced development of the passivation layer, promoted by improvements in surface homogeneity and an increase in grain boundary density

Corrosion control of magnesium for stent applications

Biomaterials used for implants may be metallic, ceramic, polymeric or composite. Currently, metals that are gradually broken down in the body have been attracting much attention, as a new generation of biodegradable implants. Magnesium (Mg) and related alloys are promising candidates for degradable biomaterials, comprising temporary mechanical properties with biological acceptance to the human body. However, the target periods set clinically, with respect to the practical uses of Mg for biodegradable stents, have yet to be achieved. Hence, improved understanding of the corrosion behaviour of Mg in the biological environment is needed. Novel Mg narrow walled minitubes, for degradable stent applications, have been produced using radio frequency magnetron sputtering (RF-MS) physical vapour deposition (PVD). The microstructural development of the as-deposited minitubes have been investigated, as a function of annealing temperature, using the combined complementary analytical techniques of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD) and microhardness indentation. The as-deposited minitubes exhibited columnar grain structures with high levels of porosity, but were very brittle. Slight alteration to the crystal structure, from columnar to more isotropic grain growth, was demonstrated at elevated temperature, along with increasing material densification, hardness and corrosion resistance. It is suggested that stabilisation of the columnar grains and the formation of oxide layers during the sequential Mg-layer deposition process, acted as a barrier, preventing the development of a fully dense, equiaxed structures. The onset and development of Mg corrosion may be addressed by the use of coatings or near surface modification processes. Accordingly, the corrosion resistance of ~ 1-2 µm thick Al coatings, deposited by RF-MS on polished Mg surfaces, within Ar and Ar/H2 environments, were appraised. The coatings were heat-treated at 300°C and 450°C, with the aim of inducing the formation of bioinert Al2O3, and samples were corroded within phosphate buffered saline (PBS) solution at 37°C to mimic the biological environment. Both as-deposited and heat-treated coatings were found to delay the onset of corrosion, but showed higher initial corrosion rates, once established, as compared to the polished Mg surfaces. Slight improvement in coating performance was achieved through the addition of H2 to the system, which acted to inhibit Al-Mg alloying and enhance Al2O3 formation. However, localized accelerated corrosion associated with substrate polishing damage emphasised the need for improved process control and coating uniformity. Si-H coatings deposited on Mg surfaces within Ar/H2 ambient using a PVD technique was also investigated. The as-deposited coatings comprised dense, crack-free amorphous a-Si-H layers with thickness of ~ 1 µm. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses provided evidence for the presence of SiH2 as well as SiOx. The corrosion resistance of a-Si-H coated Mg increased significantly in contact with PBS, in both electrical and immersion tests, due to improved coverage of the substrate. The effect of rapid thermal processing techniques on the corrosion resistance of Mg surfaces was also investigated. Mg surfaces treated by large area electron beam (LAEB) irradiation showed refinement of the surface grain structure, with increased grain boundary delineation, although localised ablation, roughness and crater formation increased with increasing cathode voltage and number of pulses. The corrosion potential and corrosion rate of LAEB modified surfaces generally increased with increase the energy imparted to the surface. The extended corrosion performance of low energy EB processed surfaces, under immersion testing was consistent with the trend of improved corrosion resistance during the early stages of immersion in PBS. However, surfaces over-processed at high energies experienced higher corrosion rates in both potentiodynamic and immersion testing, due to the development of inclusions, craters and cracks on the modified surface. Further, Mg surfaces, modified by laser surface melting (LSM) under conditions of low energy laser irradiation, experienced rapid melting, causing surface smoothening and grain refinement centred along the laser beam tracks, whilst coarser grains decorated the overlapping regions, due to the Gaussian shape of the laser beam profile. More uniform surface processing was achieved by increasing the laser beam spot size, which acted to improve the corrosion resistance of Mg. Under high energy LSM processing conditions, Mg surfaces showed conventional laser melting rippled patterns, along with craters and cracks, and the redeposition of MgO particles, causing an increase in surface roughness and corrosion rate. The corrosion performance under immersion testing showed the corrosion rate similar to that of the original polished Mg samples, due to non-uniform surface modification and the mixed development of fine and coarser grains. However, observation revealed that refined grain regions along the centre of the laser tracks were able to resist corrosion for longer times. Generally, annealed Mg-minitubes produced by PVD, and the near surface modification of Mg by EB and LSM, showed that fine grained Mg can affect the electrochemical response of Mg within the physiological environment, due to the rapid, enhanced development of the passivation layer, promoted by improvements in surface homogeneity and an increase in grain boundary density