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Functionality of optical systems requires optical coatings of
high quality which have to be adapted to the application in
terms of the performance of optical parameters such as
reflectivity (AR, low, defined, high), the substrate material of
interest (polymers, ceramics, and glass), the environmental
conditions (such as temperature or humidity), and the service
conditions (such spectral range and power density). Moreover,
in many cases, additional non-optical requirements have
to be met, including mechanical features such as scratch
resistance, hardness and modulus.
Consequently, thickness and overall uniformity requirements
are steadily increasing. In general, these demands can usually
not be achieved by a single film or coating. Instead, films with
specific design features (such as gradients), layer stacks and
multilayers are a prerequisite for high quality layers and their
reliability. For the development of such layer systems, a
versatile cluster deposition system consisting of three clusters
and a cluster-to-cluster handler has been designed. Besides a
PECVD cluster for DLC coatings, two PVD clusters implement
ion assisted electron beam evaporation and plasmabased
sputter technology. In addition to metallic films, preparation
of oxide and nitride optical coatings (SiO2, TiO2, Ta2O5,
ITO, Al2O3, TiN, AlN) in various process modifications (nonreactive,
partially reactive, full-reactive; rf-, dc-, mf- mode;
PEM control) has been carried out.
Within this paper, the effect of several process parameters on
layer quality (optical constants, density, and uniformity) is
discussed for selected applications examples (AR- and HR
coatings, applications at elevated temperatures and applications
of enhanced mechanical performance).
It has been shown that specific features of ion and plasma
assisted PVD processes, including ion-beam and plasma preand
post-treatment of substrates and layer systems, may
widen the application range and improve functionality features
of given coating/substrate systems.
Public, hidden, and forensic features either encoded or directly legible are used for authentication. Fabry-Perot layer stacks as information carriers in combination with imaging ellipsometry as optical read-out system provide all-in-one anti-counterfeiting capability that may establish the new security level 'encoded forensic'. Different layer designs are described with regard to all three security levels: public features (e.g. color and tilt effect) perceptible by the human eye, hidden features (e.g. spectroscopic response in the UV or IR), and forensic features (ellipsometric quantities Ψ and Δ as a function of wavelength λ and angle of incidence AOI). Physically uncloneable functions (PUF) could be realized as a result of a multi-material and a multi-parameter deposition approach as well as by means of specific design features of the Fabry-Perot layer stack. Hence, they are not subject to any reverse engineering strategies. Examples of stratified, micro-structured, and laser-modified Fabry-Perot layer systems are considered that may be used at all perception levels (e.g. human eye, bar code reader, and imaging ellipsometry) for authentication against product counterfeiting and related areas. --------------------------------------------------------------------------------------------------------------------
Zur Authentifizierung werden offene, versteckte und forensische Merkmale entweder codiert oder direkt auslesbar verwendet. Mit Fabry-Perot Schichtsystemen als Informationsträger in Kombination mit der abbildenden Ellipsometrie als optischem Auslesesystem steht ein 'all-in-one' Fälschungsschutz-System zur Verfügung, das ein neues Sicherheitsniveau 'kodiert-forensisch' darstellt. Es werden unterschiedliche Schichtdesigns beschrieben, mit denen alle drei Sicherheitsniveaus realisiert werden können: offene Merkmale (z.B. Farbe und Kippeffekt), die mit dem menschlichen Auge wahrnehmbar sind, versteckte Merkmale (z.B. die spektrale Antwort im Sichtbaren oder im IR) sowie forensische Merkmale (ellipsometrische Größen Ψ und Δ als Funktion der Wellenlänge λ und des Einfallswinkels AOI).
Physikalisch nicht klonbare Funktionen (PUF) können im Ergebnis einer Multi-Material- und Multi-Parameter Beschichtungsstrategie sowie durch spezifische Design-Merkmale des Fabry-Perot Schichtsystems realisiert werden. Folglich sind solche Schichtsysteme dem 'reverse engineering' nicht zugänglich. Es werden Beispiele für lateral ausgedehnte, mikrostrukturierte und Laser modifizierte Schichtsysteme auf verschiedenen Ausleseniveaus (menschliches Auge, Barcode-Lesegerät, abbildendes Ellipsometer) vorgestellt, die zur Authentifizierung von Produkten und verwandte Gebiete geeignet sind.
The irradiation of ~0.9-µm-thick hydrogenated amorphous carbon (a-C:H) layers deposited on silicon substrates with single femtosecond (fs) laser pulses (35 fs pulse duration, 790 nm centre wavelength) in air is studied experimentally. Irradiation spots have been generated with different peak fluences and subsequently investigated by optical topometry, micro Raman spectroscopy and microscale mechanical indentation in order to evaluate their microscopic, topographical, structural and mechanical properties (e.g. elastic modulus). By this multi-method approach, a clear separation of different effects (delamination and graphitisation) becomes possible. The joint application of mechanical and spectroscopic techniques provides unique insights into the effects of the fs-laser radiation on the carbon layer.
Fluid-assisted mass transport reactions by dissolution-precipitation, where a precursor mineral reacts with a fluid, play an important role in metamorphism and metasomatism. We investigated titanite growth on rutile in time series experiments between one and 107 days at constant P-T conditions of 600 °C and 400 MPa in the system TiO2-CaO-SiO2-Na2O-HCl-H2O. A two-capsule assemblage allows for transport of Ca and Si from dissolving wollastonite to dissolving rutile, the Ti-source, in a NaCl-bearing aqueous fluid, according to the general reaction CaSiO3 + TiO2 = CaTiSiO5. Complete overgrowth of rutile by titanite occurred after just one day of experiment. Fine-grained lozenge-shaped titanite crystals of short-time runs (up to 14 days) reorganize to larger predominantly prismatic crystals after >14 days.
After investigation by scanning electron microscopy, the titanite overgrowth was removed from the rutile by hydrofluoric acid, to provide a three-dimensional view of the dissolution-reaction front on the rutile surface. The morphology of the rutile surface is dominated by humps or ridges beneath the central region of a titanite crystal and valleys at the grain boundaries between adjacent titanite crystals. The dissolution pattern on the rutile surface mimics the titanite overgrowth and changes with changing grain size and shape of the titanite with longer run times. The preferred dissolution of rutile in the valleys is clearly linked to the position of the titanite grain boundaries, which served as pathways for fluid-assisted element transport. Rutile-titanite and titanite-titanite boundaries show a significant porosity in transmission electron microscopy images of foils prepared by focused ion beam milling. The large-scale dissolution pattern on the rutile surface is independent of the crystallographic orientation of the rutile and entirely dominated by the arrangement of titanite crystals in the overgrowth. Dissolution features on a scale smaller than ~1 µm are dominated by stepwise dissolution and etch-pits following the crystallographic orientation of the rutile. Similar observations were made in experiments with an additional Al-source, although these experiments result in a different overgrowth pattern; i.e., an exposed rutile surface is always present and solitary titanite crystals are accompanied by partial overgrowths. Quantitative characterization of the surface morphology by white-light interference microscopy demonstrates that, with increasing grain size of titanite, dissolution of rutile is strongly enhanced at the titanite grain boundaries.
Natural examples of titanite overgrowths on rutile show the same relations between element pathways, arrangement of titanite crystals and 3D dissolution pattern on rutile as in the experimental systems. We conclude that the transport of Ti away from the rutile and of Ca + Si into the reaction rim occurred in a grain boundary fluid, the composition of which must have been strongly different from the composition of the bulk fluid in the experiment, as well as in the natural system. The reaction progress depends on the availability of a fluid, and relicts of rutile in titanite indicate restricted availability of fluid in the natural system (e.g., a fluid pulse that was consumed by the reactions). The reaction examined here can serve as a proxy for other reactions of the conversion of oxide minerals (e.g., spinel or corundum) into silicates.
The new promoted ignition combustion test apparatus at BAM matches the requirements of ASTM test method G 124 [ASTM G 124, Standard Test Method for Determining the Combustion Behavior of Metallic Materials in Oxygen-Enriched Atmospheres, 2003] and of EIGA publication, IGC Doc 13/02/E [EIGA Gas Association Document IGC Doc 13/02/E, Oxygen Pipeline Systems, 2003]. The test apparatus is most likely the only one in Europe. The test chamber allows investigations with pure oxygen or oxygen mixtures at pressures up to 500 bar and at temperatures up to 400°C at static or flowing gas conditions. The ignition device consists of an electrical ignition wire and a promoter. IR-sensors, distributed over the whole length of the rod, measure and record the burning temperature and allow measurement of the burn rate. A video system records the experiment for visual characterization of the burning behavior. BAM test results are compared with published data. Results on investigations with coated rods are presented. Ceramic coatings seem to be a solution for the use of certain less compatible metallic materials at higher oxygen pressures.
Two different test apparatuses are used at BAM to evaluate the safety of oxygen components: First, the promoted ignition combustion test according to ASTM G 124 [1] and to EIGA IGC Doc 13/02/E [2], and second, the new metallic disk ignition test that has not been standardized yet. The standard test method ASTM G 124 allows a ranking of metallic materials for use at high pressure oxygen. However, the transfer of those test results to valves and fittings in real-life situations is rather complex. The metallic disk ignition test, developed at BAM, is more practice oriented and uses metallic disks of different thicknesses of the investigated material. A commonly used nonmetallic sealing ring is placed in front of this disk and serves as an igniter. This test array is then exposed to high pressure gaseous oxygen impacts in a standardized pneumatic impact tester according to DIN EN 1797 [3].
Hydrogen-Containing Amorphous Carbon Layers as Optical Materials in the Near-IR Spectral Range
(2007)
Hydrogenated amorphous carbon layers were deposited on various substrates by means of a plasma CVD process with a RF substrate bias as well as an ECR plasma source. The optical properties of the a-C:H layers were obtained via spectroscopic ellipsometry and correlated with their mechanical and chemical properties. The layers from pure RF plasma exhibit a higher absorption constant in the visible spectral range and a higher refractive index. All layers are nearly transparent in the NIR spectral range making them candidates for optical thin layer systems. The laser damage behaviour of the a-C:H layers was investigated with ultrashort pulses. The damage thresholds were consistent with the absorption constants of the layers. Interesting damage morphologies were observed indicating a sensitivity of this experiment to sub-structures in the layer.
Most of the polymeric materials used are easy to ignite and show extensive flame spread along their surfaces. Apart from extensive heat release rates, their short time to ignition (tig), in particular, is a key fire hazard. Preventing ignition eliminates fire hazards completely. Protection layers that shift tig by more than an order of magnitude are powerful flame retardancy approaches presenting an alternative to the usual flame retardancy concepts.
Coatings are proposed that consist of a three-layer system to ensure adhesion to the substrate, acting as an infrared (IR) mirror and protecting against oxidation. The IR-mirror layer stack is realised by physical vapour deposition in the sub-micrometre (<1 µm) range, reducing heat absorption by up to an order of magnitude. Not only is the ease of ignition diminished (tig is increased by several minutes), the flame spread and fire growth indices are also remarkably reduced to as little as 1/10 of the values of the uncoated polymers open for further optimization. Sub-micrometre thin IR-mirror coatings yielding surface absorptivity <0.1 are proposed as a novel and innovative flame retardancy approach.