Crystalline Coatings Research Articles

An Investigation on the Mechanical Properties of Ni-Nano-Mg Composite Coatings on ASTM A284 Grade-A by Pulsed Electro-Deposition using Taguchi Method

The main objective of this research paper focuses on investigating the enhancement of the mechanical properties of nano Magnesium coatings on ASTM A284 Grade A specimen. Taguchi's L9 orthogonal array was used to optimize the micro hardness and analyzing the corrosive resistivity of Ni-nano crystalline Mg composite coatings over the ASTM A284 Grade A specimen by pulsed electro deposition method. Pulse frequency 80 Hz, pulse duty cycle 25% and pulse peak current density 1.2 amps/cm2 are found to be optimal values. However, the influences of the optimized pulse parameters over the microstructure, corrosion resistance, and X-ray diffraction (XRD) of Ni-nano-Mg coatings were also analyzed. The micro hardness, microstructure of the composite coatings was significantly influenced by pulse frequency. The EDX results confirmed that the composites were fairly spreaded over the specimen. The hardness was improved nearly 12.8% and also the corrosion resistance of the specimen was greatly improved.

Direct fabrication of crystalline hydroxyapatite coating on zirconium by single-step plasma electrolytic oxidation process

Abstract This is the first report on the growth of a crystalline hydroxyapatite (HA) coating on zirconium substrate using a facile plasma electrolytic oxidation (PEO) method. The crystalline HA layers were fabricated in a Ca(CH3COO)2 × 5H2O, C3H7Na2O6P × 5H2O, and KOH containing electrolyte. The prepared coatings were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), contact angle goniometry (CAG) and energy dispersive spectroscopy (EDS). In addition, Fourier transform infrared spectroscopy (FTIR) was used to examine the molecular bond structure of the coating. The results revealed that the composition of the oxide layer formed on zirconium surface includes hydroxyapatite, calcium zirconium oxide, and monoclinic zirconia. The FTIR and XRD results confirmed the formation of crystalline HA layers on the surface of Zr substrate. It was concluded that PEO of Zr substrate is a valid method to prepare crystalline HA coatings with a low contact angle value for biomedical applications due to its easy and fast nature.

Technology for the next gravitational wave detectors

This paper reviews some of the key enabling technologies for advanced and future laser interferometer gravitational wave detectors, which must combine test masses with the lowest possible optical and acoustic losses, with high stability lasers and various techniques for suppressing noise. Sect. 1 of this paper presents a review of the acoustic properties of test masses. Sect. 2 reviews the technology of the amorphous dielectric coatings which are currently universally used for the mirrors in advanced laser interferometers, but for which lower acoustic loss would be very advantageous. In sect. 3 a new generation of crystalline optical coatings that offer a substantial reduction in thermal noise is reviewed. The optical properties of test masses are reviewed in sect. 4, with special focus on the properties of silicon, an important candidate material for future detectors. Sect. 5 of this paper presents the very low noise, high stability laser technology that underpins all advanced and next generation laser interferometers.

Plasma-sprayed amorphous–crystalline coatings based on TiO2, SiO2, Al2O3, and ZrO2 oxides

The results of studies of composites based on TiO2, SiO2, Al2O3, and ZrO2 oxides used as conglomerated powders (microcomposites) and plasma-sprayed coatings made therefrom have been summarized. It has been shown that, for the first time in the practice of thermal spraying, high-density gradientamorphous–crystalline ceramic coatings reinforced with ultradispersed stishovite phases were obtained. Inclusions with amorphous shells were detected. The effect of the particle size and composition of the initial microcomposites on the phase composition of the sprayed layers was investigated.

Control of hydroxyapatite coating by self-assembled monolayers on titanium and improvement of osteoblast adhesion.

Self-assembly technique was applied to introduce functional groups and form hydroxyl-, amine-, and carboxyl-terminal self-assembled monolayers (SAMs). The SAMs were grafted onto titanium substrates to obtain a molecularly smooth functional surface. Subsequent hydrothermal crystal growth formed homogeneous and crack-free crystalline hydroxyapatite (HA) coatings on these substrates. AFM and XPS were used to characterize the SAM surfaces, and XRD, SEM, and TEM were used to characterize the HA coatings. Results show that highly crystalline, dense, and oriented HA coatings can be formed on the OH-, NH2 -, and COOH-SAM surfaces. The SAM surface with -COOH exhibited stronger nucleating ability than that with -OH and -NH2 . The nucleation and growth processes of HA coatings were effectively controlled by varying reaction time, pH, and temperature. By using this method, highly crystalline, dense, and adherent HA coatings were obtained. In addition, in vitro cell evaluation demonstrated that HA coatings improved cell adhesion as compared with pristine titanium substrate. The proposed method is considerably effective in introducing the HA coatings on titanium surfaces for various biomedical applications and further usage in other industries. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 124-135, 2017.

Absorption-induced scattering and surface plasmon out-coupling from absorber-coated plasmonic metasurfaces.

Interactions between absorbers and plasmonic metasurfaces can give rise to unique optical properties not present for either of the individual materials and can influence the performance of a host of optical sensing and thin-film optoelectronic applications. Here we identify three distinct mode types of absorber-coated plasmonic metasurfaces: localized and propagating surface plasmons and a previously unidentified optical mode type called absorption-induced scattering. The extinction of the latter mode type can be tuned by controlling the morphology of the absorber coating and the spectral overlap of the absorber with the plasmonic modes. Furthermore, we show that surface plasmons are backscattered when the crystallinity of the absorber is low but are absorbed for more crystalline absorber coatings. This work furthers our understanding of light–matter interactions between absorbers and surface plasmons to enable practical optoelectronic applications of metasurfaces.

Structure and corrosion behavior of sputter deposited cerium oxide based coatings with various thickness on Al 2024-T3 alloy substrates

Cerium oxide based coatings from ∼100 to ∼1400nm in thickness were deposited onto Al 2024-T3 alloy substrates by magnetron sputtering of a 99.99% pure CeO2 target. The crystallite size of CeO2 coatings increased from 15nm to 46nm as the coating thickness increased from ∼100nm to ∼1400nm. The inhomogeneous lattice strain increased from 0.36% to 0.91% for the ∼100nm to ∼900nm thick coatings and slightly decreased to 0.89% for the ∼1400nm thick coating. The highest adhesion strength to Al alloy substrates was for the ∼210nm thick coating, due to a continuous film coverage and low internal stress. Electrochemical measurements indicated that sputter deposited crystalline CeO2 coatings acted as physical barriers that provide good cathodic inhibition for Al alloys in saline solution. The ∼900nm thick CeO2 coated sample had the best corrosion performance that increased the corrosion resistance by two orders magnitude and lowered the cathodic current density 30 times compared to bare Al 2024-T3 substrates. The reduced defects and exposed surface, along with suppressed charge mobility, likely accounts for the improved corrosion performance as coating thickness increased from ∼100nm to ∼900nm. The corrosion performance decreased for ∼1400nm thick coatings due in part to an increase in coating defects and porosity along with a decrease in adhesion strength.

Bacterial adhesion on amorphous and crystalline metal oxide coatings.

Several studies have demonstrated the influence of surface properties (surface energy, composition and topography) of biocompatible materials on the adhesion of cells/bacteria on solid substrates; however, few have provided information about the effect of the atomic arrangement or crystallinity. Using magnetron sputtering deposition, we produced amorphous and crystalline TiO2 and ZrO2 coatings with controlled micro and nanoscale morphology. The effect of the structure on the physical-chemical surface properties was carefully analyzed. Then, we studied how these parameters affect the adhesion of Escherichia coli and Staphylococcus aureus. Our findings demonstrated that the nano-topography and the surface energy were significantly influenced by the coating structure. Bacterial adhesion at micro-rough (2.6 μm) surfaces was independent of the surface composition and structure, contrary to the observation in sub-micron (0.5 μm) rough surfaces, where the crystalline oxides (TiO2>ZrO2) surfaces exhibited higher numbers of attached bacteria. Particularly, crystalline TiO2, which presented a predominant acidic nature, was more attractive for the adhesion of the negatively charged bacteria. The information provided by this study, where surface modifications are introduced by means of the deposition of amorphous or crystalline oxide coatings, offers a route for the rational design of implant surfaces to control or inhibit bacterial adhesion.

Effect of silicate doping on the structure and mechanical properties of thin nanostructured RF magnetron sputter-deposited hydroxyapatite films

Silicon-doped hydroxyapatite-based (Si-HA) coatings were deposited via radio frequency (RF) magnetron sputtering on the surface of titanium that was treated with a pulsed electron beam. This study aimed to evaluate the effect of Si doping on the structure and mechanical properties of thin HA films. The content of the silicon was 1.2 and 4.6at.% for the coatings prepared using the Si-HA precursor powders with a chemical formula Ca10(PO4)6−x(SiO4)x(OH)2−x where, x=0.5 and 1.72. Pure HA (Ca10(PO4)6(OH)2) coatings were deposited for comparison. The as-deposited films were analysed with respect to their composition, state of chemical binding and microstructure using XPS, FTIR, XRD, and SEM. We hypothesized that the addition of Si would affect the mechanical features of the coatings due to microstructure changes. The effect of the introduction of Si on the nanohardness and the Young's modulus as well as the adhesion strength and scratch resistance of the HA coating was investigated using nanohardness testing and a scratch test, respectively. Examination of the coating microstructure using SEM and AFM revealed that Si doping influenced the surface morphology and led to a smaller grain size. The tendency to form an amorphous structure also increased with an increase in the Si content. A monotonous decrease in both the nanohardness and the elastic modulus was observed with an increase in the Si content. A maximum nanohardness of ~7GPa was obtained for the Si-free HA coating, whereas the hardness decreased to ~4.3GPa for the films with a Si content of 1.2at.%. The addition of 4.6at.% Si to the HA coating resulted in a reduction in the elastic modulus, whereas the nanohardness was very similar to that of the uncoated substrate. The adhesion behaviour of the coatings demonstrated different responses. In the case of pure HA coatings, failure occurred due to the low cohesion of the coating, whereas the crystalline Si-HA coatings with a Si content of 1.2at.% deformed plastically without crack formation and without detaching from the titanium substrate, which resulted in a greater coating stability.

Chemical Alkaline Etching of Silicon Mie Particles

Chemical etching via alkaline solutions is associated with electronic litho­graphy to structure at a nanometer scale silicon Mie resonators. Two different alkaline solutions are employed and their influences on the shape of the resonators are discussed. The method is applied on both amorphous and crystalline silicon coatings and silicon resonators are characterized by electron microscopy and optical spectroscopy, and then the different modes are identified thanks to numerical simulations. This method avoids the use of reactive ion etching and appears to be very well adapted to design silicon particles at a nanometer scale for applications in nanophotonics.

Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 μm

The sensitivity of second and third generations of interferometric gravitational wave (GW) detectors will be limited by the thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating that is optimized for high reflectivity for a wavelength of 1064 nm. The absorption was measured at 1530 nm, where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than ppm, which is equivalent to ppm for a coating stack that is highly reflective at 1530 nm. While this is a very promising low absorption result for alternative low-loss coating materials, further work will be necessary to reach the requirements of ppm for future GW detectors.

Amorphous and crystalline TiO2 coatings synthesized with the RF PECVD technique from metalorganic precursor

Thin films of titanium dioxide (TiO2) were synthesized with the help of plasma enhanced chemical vapour deposition (RF PECVD) technique, using titanium (IV) ethoxide and gaseous oxygen as precursor compounds. The glow discharge power and the flow rate of oxygen were used as process operational parameters. Phase composition of the films was studied with the X-ray diffraction (XRD) technique, their morphology was followed with the scanning electron microscopy (SEM), elemental composition was investigated with the energy dispersive spectroscopy (EDS) and their thickness and optical properties were assessed with the help of variable angle spectroscopic ellipsometry (VASE) and UV-VIS spectroscopy.The coatings obtained at the glow discharge power of 300 W and the oxygen flow rate of 400 sccm were characterized by the best titanium dioxide stoichiometry and the highest refractive index of 2.2. As deposited, the films had strictly amorphous character. They crystallized after thermal annealing at 500 °C, with an increase of their refractive index to 2.35. Thermal treatment also modified water wettability of the coatings: annealing produced a decrease of water contact angle by more than 30°. An illumination of the films with UV radiation brought about further drop of water contact angle which eventually resulted in a strongly hydrophilic surface.

Sensing earth's rotation with a helium-neon ring laser operating at 1.15 μm.

We report on the operation of a 2.56 m2 helium-neon based ring laser interferometer at a wavelength of 1.152276μm using crystalline coated intracavity supermirrors. This work represents the first implementation of crystalline coatings in an active laser system and expands the core application area of these low-thermal-noise cavity end mirrors to inertial sensing systems. Stable gyroscopic behavior can only be obtained with the addition of helium to the gain medium as this quenches the 1.152502μm (2s4→2p7) transition of the neon doublet which otherwise gives rise to mode competition. For the first time at this wavelength, the ring laser is observed to readily unlock on the bias provided by the earth's rotation alone, yielding a Sagnac frequency of approximately 59Hz.

Phase stability and biological property evaluation of plasma sprayed hydroxyapatite coatings for orthopedic and dental applications

In this work we have investigated the effects of strontium (Sr) dopant on in vitro protein release kinetics and in vivo osteogenic properties of plasma sprayed hydroxyapatite (HA) coatings, along with their dissolution behavior. Plasma sprayed HA coatings are widely used in load-bearing implants. Apart from osseointegration, the new generation of HA coating is expected to deliver biomolecules and/or drugs that can induce osteoinduction. This paper reports the preparation of crystalline and amorphous HA coatings on commercially pure titanium (Cp-Ti) using inductively coupled radio frequency (RF) plasma spray, and their stability at different solution pH. Coatings prepared at 110mm working distance from the nozzle showed an average Ca ion release of 18 and 90ppm in neutral and acidic environments, respectively. Decreasing the working distance to 90mm resulted in the formation of a coating with less crystalline HA and phases with higher solubility products, and consequently higher dissolution over 32days. A 92% release of a model protein bovine serum albumin (BSA) in phosphate buffer with pH of 7.4 was measured for Sr-doped HA (Sr-HA) coating, while only a 72% release could be measured for pure HA coating. Distortion of BSA during adsorption on coatings revealed a strong interaction between the protein and the coating, with an increase in α-helix content. Osteoid formation was found on Sr-HA implants as early as 7weeks post implantation compared to HA coated and uncoated Ti implants. After 12weeks post implantation, osteoid new bone was formed on HA implants; whereas, bone mineralization started on Sr-HA samples. While no osteoid was formed on bare Ti surfaces, bone was completely mineralized on HA and Sr-HA coatings after 16weeks post implantation. Our results show that both phase stability and chemistry can have a significant influence toward in vitro and in vivo response of HA coatings on Ti implants.

Enhanced osteogenesis on titanium implants by UVB photofunctionalization of hydrothermally grown TiO₂ coatings.

Even though Ti-based implants are the most used materials for hard tissue replacement, they may present lack of osseointegration on the long term, due to their inertness. Hydrothermal treatment (HT) is a useful technique for the synthesis of firmly attached, highly crystalline coatings made of anatase titanium dioxide (TiO2), providing favorable nanoroughness and higher exposed surface area, as well as greater hydrophilicity, compared to the native amorphous oxide on pristine titanium. The hydrophilicity drops even more by photofunctionalization of the nanostructured TiO2-anatase coatings under UV light. Human mesenchymal stem cells exhibited a good response to the combination of the positive surface characteristics, especially in respect to the UVB pre-irradiation. The results showed that the cells were not harmed in terms of viability; even more, they were encouraged to differentiate in osteoblasts and to become osteogenically active, as confirmed by the calcium ion uptake and the formation of well-mineralized, bone-like nodule structures. In addition, the enrichment of hydroxyl groups on the HT-surfaces by UVB photofunctionalization accelerated the cell differentiation process and greatly improved the osteogenesis in comparison with the nonirradiated samples. The optimal surface characteristics of the HT-anatase coatings as well as the high potentiality of the photo-induced hydrophilicity, which was reached during a relatively short pre-irradiation time (5 h) with UVB light, can be correlated with better osseointegration ability in vivo; among the samples, the superior biological behavior of the roughest and most hydrophilic HT coating makes it a good candidate for further studies and applications.

Measurement of the mechanical loss of prototype GaP/AlGaP crystalline coatings for future gravitational wave detectors

Thermal noise associated with the dielectric optical coatings used to form the mirrors of interferometric gravitational wave detectors is expected to be an important limit to the sensitivity of future detectors. Improvements in detector performance are likely to require coating materials of lower mechanical dissipation. Typically, current coatings use multiple alternating layers of ion-beam-sputtered amorphous silica and tantalum pentoxide (doped with titania). We present here measurements of the mechanical dissipation of promising alternative crystalline coatings that use multi-layers of single crystal gallium phosphide (GaP) and aluminium gallium phosphide (AlGaP) that are epitaxially grown and lattice matched to a silicon substrate. Analysis shows that the dissipation of the crystalline coating materials appears to be significantly lower than that of the currently used amorphous coatings, potentially enabling a reduction of coating thermal noise in future gravitational wave detectors.

Fabrication of TiO2 Crystalline Coatings by Combining Ti-6Al-4V Anodic Oxidation and Heat Treatments.

The bio- and hemocompatibility of titanium alloys are due to the formation of a TiO2 layer. This natural oxide may have fissures which are detrimental to its properties. Anodic oxidation is used to obtain thicker films. By means of this technique, at low voltages oxidation, amorphous and low roughness coatings are obtained, while, above a certain voltage, crystalline and porous coatings are obtained. According to the literature, the crystalline phases of TiO2, anatase, and rutile would present greater biocompatibility than the amorphous phase. On the other hand, for hemocompatible applications, smooth and homogeneous surfaces are required. One way to obtain crystalline and homogeneous coatings is by heat treatments after anodic oxidation. The aim of this study is to evaluate the influence of heat treatments on the thickness, morphology, and crystalline structure of the TiO2 anodic coatings. The characterization was performed by optical and scanning electron microscopy, X-ray diffraction, and X-ray reflectometry. Coatings with different colors of interference were obtained. There were no significant changes in the surface morphology and roughness after heat treatment of 500°C. Heat treated coatings have different proportions of the crystalline phases, depending on the voltage of anodic oxidation and the temperature of the heat treatment.

Nano zinc phosphate coatings for enhanced corrosion resistance of mild steel

Nano crystalline zinc phosphate coatings were developed on mild steel surface using nano zinc oxide particles. The chemical composition and morphology of the coatings were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The particles size of the nano zinc phosphate coating developed was also characterized by TEM analysis. Potentiodynamic polarization and electrochemical impedance studies were carried out in 3.5% NaCl solution. Significant variations in the coating weight, morphology and corrosion resistance were observed as nano ZnO concentrations were varied from 0.25 to 2g/L in the phosphating baths. The results showed that nano ZnO particles in the phosphating solution yielded phosphate coatings of higher coating weight, greater surface coverage and enhanced corrosion resistance than the normal zinc phosphate coatings (developed using normal ZnO particles in the phosphating baths). Better corrosion resistance was observed for coatings derived from phosphating bath containing 1.5g/L nano ZnO. The activation effect brought about by the nano ZnO reduces the amount of accelerator (NaNO2) required for phosphating.

Facile synthesis of novel one-dimensional hierarchical SiC@SiO2@c-C nanostructures and their field emission properties

One-dimensional hierarchical nanostructures composed of SiC cores, SiO2 interlayers and crystalline carbon coatings (SiC@SiO2@c-C) were successfully synthesized by Ni-assisted chemical vapor deposition method. The nanostructures delivered a low turn-on field of 0.8 V μm−1 and threshold field of 2.6 V μm−1 when evaluated as a field emission cathode material. With the advantages of synergy between different layers, the attractive performance was adequately illustrated by the energy band diagram structures of the interface area. The coaxial nanostructures with a semiconductor–insulator–semiconductor geometry have promising applications in nanoelectronics display devices.

Structural formation and photocatalytic activity of magnetron sputtered titania and doped-titania coatings.

Titania and doped-titania coatings can be deposited by a wide range of techniques; this paper will concentrate on magnetron sputtering techniques, including “conventional” reactive co-sputtering from multiple metal targets and the recently introduced high power impulse magnetron sputtering (HiPIMS). The latter has been shown to deliver a relatively low thermal flux to the substrate, whilst still allowing the direct deposition of crystalline titania coatings and, therefore, offers the potential to deposit photocatalytically active titania coatings directly onto thermally sensitive substrates. The deposition of coatings via these techniques will be discussed, as will the characterisation of the coatings by XRD, SEM, EDX, optical spectroscopy, etc. The assessment of photocatalytic activity and photoactivity through the decomposition of an organic dye (methylene blue), the inactivation of E. coli microorganisms and the measurement of water contact angles will be described. The impact of different deposition technologies, doping and co-doping strategies on coating structure and activity will be also considered.