Low Reflectivity Coatings Research Articles

Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High‐Temperature Turbine Applications

High reflectance thermal barrier coatings consisting of 7% Yittria‐Stabilized Zirconia (7YSZ) and Al2O3 were deposited by co‐evaporation using electron beam physical vapor deposition (EB‐PVD). Multilayer 7YSZ and Al2O3 coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 μm wavelength. The variable 7YSZ and Al2O3 multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 μm. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m‐K for the 20‐layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations.

Gradual degradation in 980 nm InGaAs/AlGaAs pump lasers

High temperature (140 °C) and high current density (17 kA cm −2) aging tests have been applied to InGaAs/AlGaAs 980 nm single-mode pump lasers. The use of low reflectivity (≅1%, AR) coatings on both facets leads to a symmetrical cavity configuration with relatively low power per facet emission (≈50–60 mW per facet). In this way, a quasi-uniform strongly accelerated stress is applied to the laser cavity and the normally very low gradual degradation rate becomes observable after a few hundreds hours of operation as a strong threshold current shift, while the external efficiency of the laser is almost unaffected. The degradation of the laser structure has been studied by low level I( V) curves analysis, Low Temperature (80 K) Spectrally Resolved Cathodoluminescence (LT-SRCL) and Electrical/Optical Noise (EON) techniques; the degradation seems to be longitudinally uniform and localized in the vicinity of the QW. Some laser thus degraded has been subsequently aged in more standard conditions, after applying a high reflection coating (HR) on the back facet; preliminary results of these tests are discussed.

Investigation of low-reflective ZrCN–PVD-arc coatings for application on medical tools for minimally invasive surgery

Medical tools for minimally invasive surgery are often used under difficult environmental conditions. If the surgeons have to work with a microscope, the operating area must be brightly lit. Reflection and scattering of light from tool-surfaces disturb the visual field. To decrease this kind of disturbance, the tools can be covered by a dark PVD thin film. In the present study, the properties of low-reflective PVD-arc coatings of ZrCN were considered in comparison to TiCN. Different pure metal targets of zirconium and titanium were utilized in a reactive arc-coating process. The coating device was an industrial PVD engine of the HTC 1000/4 ABS type. Caused by the purpose of aimed investigations the engine merely operated in the arc-coating-mode. Amounts of carbon and nitrogen in the layers were steered by systematic changes of gas flow parameters from batch-to-batch. Substrate temperature and bias voltage were held at constant levels. Besides some mechanical and optical properties, aspects of biocompatibility of ZrCN on stainless steel AISI 304 were focused on. Summarizing the obtained results with regard to questions of biocompatibility, all of the investigated thin films have fulfilled the requirements for short-term applications to the human body.

Single shot laser damage in ultraviolet mirrors with a stepwise reflectivity profile

The photoacoustic and photothermal beam deflection techniques and microscopic inspection have been used to measure the laser damage threshold of mirrors with a stepwise reflectivity profile designed for KrF excimer lasers (248 nm). Laser damage resistance studies on the low and high reflectivity regions of the mirrors, as well as on the border between the high and low reflectivity regions, have been performed with a KrF laser delivering pulses of 25 ns duration. It is shown that the presence of a sharp step in the coating thickness, and consequently, in the mirror reflectivity profile does not significantly affect the damage threshold. Indeed, the damage threshold fluence of 1.8 J/cm2 in the high reflectivity area reduces to 1.1 J/cm2 in the low reflectivity region as it would for two independent high and low reflectivity coatings. The damage threshold in the border region is 1.4 J/cm2.

Nanoporous silica antireflective coatings for solar heat collectors

To increase the efficiency of solar energy collection, low reflective coatings in the solar spectrum must be used for solar heat collectors. Nanoporous silica films can form broadband antireflectance in the region. Based on a base/acid two-step catalysed tetraethoxysilane (TEOS) system and the sol-gel process, the nanoporous antireflective silica coatings were prepared by dipcoating and annealing in air or ammonia atmosphere. Abrasion tests of the porous silica coatings indicate that treated coatings have scratch-resistant properties. The films were characterised by ellipsometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). Reflectance spectra were recorded with a spectrophotometer. Based on optimised design, the average reflectance of the obtained coatings in the solar spectrum is less than 2%. Finally, a strengthening mechanism of the films is discussed.

Bio-compatible low reflective coatings for surgical tools using reactive d.c.-magnetron sputtering and arc evaporation — a comparison regarding steam sterilization resistance and nickel diffusion

Abstract The reflection and scattering of bright light from the surfaces of surgical tools disturb the visual operating field of the surgeon. To decrease this kind of disturbance, a hard black PVD thin film is used as a possible precaution. In the present paper, the two dark PVD finish layers TiAlN and TiCN on stainless-steel substrates were considered. For corrosion protection, a Nb intermediate layer as diffusion barrier was studied. One of the questions to be answered was the long-term steam oxidation resistance due to multiple sterilization cleansings of the surgical tools. The long-term oxidation stability of multilayer Nb/TiAlN and Nb/TiCN could be significantly affected by the PVD coating methods and their very different conditions in reactive modes. Here, the results of unbalanced d.c.-magnetron sputtering and arc-evaporation coatings are represented for an industrial PVD engine of the HTC 1000/4 ABS ™ type. To study the effect of the Nb diffusion barrier layer, GDO(E)S depth-profile analyses were carried out. Because of the so-called nickel allergy problem, which restricts surgical operations in some cases, a special emphasis was the study of nickel diffusion into the coating layers. Furthermore, the bio-compatibility of the multilayers has been estimated. Summarizing the obtained results, all of the investigated coatings have fulfilled the requirements for application to surgical instruments independent of the coating method used.

Interferometric fibre sensors for measurement of surface heat transfer rates on turbine blades

We describe a prototype sensor for surface heat transfer measurements based on a miniature fibre Fabry-Perot (FFP) interferometer. These sensors are being developed for a particular application where heat transfer data are currently obtained using conventional platinum thin film resistance thermometers. The specification that the sensors must exceed is: (i) temperature resolution of 25 mK over a 50 K range; (ii) temporal response of 10 μs; (iii) an ability to operate as a calorimetric heat transfer gauge. The sensor consists of a short length of single mode optical fibre (∼ 3 mm) to which low reflectivity coatings have been applied at each end. It is illuminated and interrogated by an arbitrary length of addressing fibre. A laser diode is used as the source and we have exploited the facility to frequency modulate the diode in a novel signal processing scheme. To determine the performance of the sensor, short duration heat pulses derived from a pulsed Nd: YAG laser were applied to one end of the FFP. The response time was found to be 8 μs and the sensor operation as a calorimeter was verified.

Silica low-reflection coatings for collector covers, by a dip-coating process

A procedure for the deposition of antireflection coatings onto collector cover plates, by means of a dip-coating process using an ethanol-based colloidal silical sol, is described. By use of this process the solar transmittance of such cover plates can be increased by an increment 0.05 thus eliminating over 60 per cent of the reflectance losses. This increased transmittance increases the daily heat output for a single-glazed flat plate collector by a factor of approximately 1.1, and for double-glazed units by a factor of approximately 1.2. These antireflection films can be deposited on glass and on some types of plastic substrate. The abrasion resistance of the deposits on glass can be improved greatly by baking to 500°C. Outdoor weathering tests have shown the films probably have adequate stability for use in solar collector applications.

Low-Reflectance Eyeglass Lenses: A Viewing Aid to Radiologists

Low-reflectance coatings can significantly improve the low contrast resolution (MTF) of eyeglass lenses. The reflectance per surface can be reduced from 4.25% for uncoated lenses to 0.5% for multi-layer coated lenses. The radiologist who wears glasses will note improved viewing comfort with coated lenses and possibly a reduced error rate.

Low Reflectance Coatings and the Sensitivity Curve of the Eye

Heterochromatic light is treated and the spectral sensitivity of the human eye is taken into account. In spite of certain simplifying assumptions, conclusions of a general character may be drawn, the most important being that the optical properties required for low reflectance coatings can be obtained by treating the incident light as monochromatic.

An Analysis of Low-Reflection Coatings as Applied to Glass

A brief summary of the development of the low-reflection coating is given. As instruments were improved, a point was soon reached where further improvement in optical design required elimination of surface reflections. An explanation of the color effect observed in a coating and a method of applying a film whose index varies from top to bottom which will eliminate the color effect is described. A comparison is drawn between the coating produced by natural aging, nitric acid, hydrofluoric acid, magnesium fluoride, and the American Optical Company low-reflection coatings. This shows that the magnesium fluoride is the most practical film to date when all factors are taken into consideration. The American Optical method of applying coatings without using vacuum is described. There are four main advantages in using a low-reflection coating: (1) elimination of reflections, (2) increased transmission, (3) increased contrast, and (4) increased chemical stability. Several tests are outlined which indicate that glass properly coated with magnesium fluoride will withstand more chemical abuse than uncoated glass. A new method of removing a high-temperature-baked low-reflection coating using melted crystals of potassium bisulfate is described which will materially speed the decoating process.