Long-term aging of Ag/a-C:H:O nanocomposite coatings in air and in aqueous environment

Abstract

Nanocomposite coatings of silver particles embedded in a plasma polymer matrix possess interesting properties depending on their microstructure. The film microstructure is affected among others also by the RF power supplied during the deposition, as shown by transmission electron microscopy. The optical properties are characterized by UV–vis–NIR spectroscopy. An anomalous optical absorption peak from the Ag nanoparticles is observed and related to the microstructure of the nanocomposite films. Furthermore, a long-term aging of the coatings is studied in-depth in ambient air and in aqueous environments. It is shown that the studied films are not entirely stable. The deposition conditions and the microstructure of the films affect the processes taking place during their aging in both environments.     

Residual stress characterization of Al/SiC nanoscale multilayers using X-ray synchrotron radiation

Nanolayered composites are used in a variety of applications such as wear resistant coatings, thermal barrier coatings, optical and magnetic thin films, and biological coatings. Residual stresses produced in these materials during processing play an important role in controlling their microstructure and properties. In this paper, we have studied the residual stresses in model metal-ceramic Al/SiC nanoscale multilayers produced by physical vapor deposition (magnetron sputtering). X-ray synchrotron radiation was used to measure stresses in the multilayers using the sin²Ψ technique. The stresses were evaluated as a function of layer thicknesses of Al and SiC and also as a function of the number of layers. The stress state of Al in the multilayer was largely compressive, compared to single layer Al stresses. This is attributed to a peening mechanism due to bombardment of the Al layers by SiC and Ar neutrals during deposition. The stress evolution was numerically modeled by a simplified peening process to qualitatively explain the Al thickness-dependent residual stresses.     

Preparation and properties of RF sputtered indium-tin oxide thin films for applications as heat mirrors in photothermal solar energy conversion

In the present research, in order to deposit indium-tin oxide (ITO) thin films the method of RF reactive sputtering was used. Sputtering of two types indium-tin targets in the presence of oxygen as reactive gas was made. The technological parameters were optimized to obtain films with good quality on different substrates. The films' microstructure was studied by TEM and SAED. To identify the optical properties of the films the methods of infrared spectrometry and laser ellipsometry were used. UV-VIS spectrophotometry showed the high visible transmittance of the RF sputtered ITO films. Heating of the substrates during the films sputtering and their post deposition thermal treatment also were studied. The ultimate goal of the present research activities was to develop new technological processes leading to low-cost, highly effective optical coatings for application in photo thermal solar energy conversion and utilization.     

Growth and optical properties of nanostructured ZnS:Mn films

Nanostructured ZnS:Mn films have been grown and their structure, optical properties, and photoluminescence have been studied. The nanostructured ZnS:Mn films have been grown on silicon and glass substrates via hydrochemical deposition from solution. The crystal structure and microstructure of the films have been studied by X-ray diffraction and atomic force microscopy. The band gap of the nanostructured ZnS:Mn films has been determined. The intensity of their photoluminescence bands has been shown to increase with decreasing nanoparticle size.     

Recent approaches to reduce aging phenomena in oxygen- and nitrogen-containing plasma polymer films: An overview

Plasma polymer films (PPFs) are well-known for their enhanced stability compared to conventional polymer coatings. However, PPFs tend to undergo aging in air or in aqueous environments due to oxidation, hydrophobic recovery, hydrolysis and dissolution of oligomeric fragments. Such aging mechanisms cause modifications of the PPFs that entail a change in surface properties. For example, PPF surfaces which are probed for protein adsorption or cell adhesion might therefore be substantially different from the initial PPF. It becomes thus necessary to understand the chemical reactions involved in the chemical modification (and/or degradation) of PPFs. Here, a summary of the most important aging mechanisms occurring in PPFs is given. More precisely, chemical reactions that can potentially occur in oxygen- and nitrogen-containing plasma polymer films when stored in air and in water were highlighted. On the basis of this understanding, recent strategies to reduce or delay aging mechanisms and/or to provide time-controlled degradable PPFs are discussed: the enhancement of the degree of cross-linking, the formation of a gradient structure in the PPF during plasma deposition, and the chemical post-plasma treatment to reduce the number of reactive sites. Finally, potential applications of such coatings will be considered.     

Microstructural design of hard coatings

Microstructural design has attracted increasing interest in modern development of hard coatings for wear-resistant applications. In plasma-assisted vapor deposited thin films, the material’s microstructure can be designed during growth or post-deposition annealing treatments. In this review, we demonstrate the correlation between microstructure and mechanical as well as tribological properties of hard ceramic coatings. This is done for single-phase coatings and composition or phase modulated layers. In the latter case, the microstructure can be designed by choice of the deposition technique chosen, understanding the growth processes taking place on a film surface, either by sequential deposition of layers or by taking advantage of newly discovered self-organization processes including segregation effects of the elements. Consequently, the effects of individual microstructural features like grain size, defect density (and hence residual stress), phase arrangements in a one-, two- or three-dimensional manner on the mechanical properties are treated. Here, especially TiN–TiB₂ is used as a model system to describe the development of two- and three-dimensional coating nanostructures. Due to their particular structures, such coatings can exhibit superhardness (H  40 GPa). The microstructural changes of hard ceramic coatings during a post-deposition annealing treatment are discussed in detail. Although the significance of heat treatments to optimize properties (by a well-designed microstructure) for specific applications is recognized in bulk material science, only a few elements have been applied for hard ceramic coatings so far. Due to limited atomic assembly kinetics during the deposition process (e.g., by using a low substrate temperature), defects (point-, line-, and area-defects), supersaturated, and metastable phases can easily be obtained. For example, growth of (Ti,Al)N and Ti(B,N) films can result in the formation of a supersaturated TiN based phase. Such films undergo age hardening processes during post-deposition annealing due to the decomposition of the supersaturated phases into their stable constituents. The review clearly shows that nanostructure dependent hardness increase (compared to hardness of the bulk counterparts) sustains higher annealing temperatures than hardness increase due to an increased density of point- and/or line-defects. Tribological properties of hard thin films can be engineered by adding phases with lubricious properties at operation temperature (either room or elevated temperatures) and prevailing environment. Especially in high speed and dry cutting applications, low-friction and lubricating mechanisms of the thin film itself are required in addition to excellent mechanical properties.     

Hartstoffschichten für das Pressen und Prägen von Gläsern bei hohen Temperaturen

Hard and wear resistant coatings for the moulding and embossing of glasses at elevated temperatures Hard and wear resistant coatings of Titanium Aluminium Nitride TiAlN were deposited on various substrates by the application of different reactive deposition processes: RF-magnetron-sputtering, ion beam-sputtering and by the energetic cluster impact (ECI) process. The deposition of the coatings was performed under variation of biasing conditions and of process parameters such as pressures and flow rates of the process gases argon and nitrogen as well as of energies of species hitting the substrate surfaces. The microstructure particularly the growth morphology of several films was investigated by pictures of film cross sections recorded by transmission electron microscopy. Residual intrinsic film stresses were analyzed by measuring deflections of substrates in an interference optical microscope before and after the deposition of the coatings. By heating coated substrates and in-situ observation of deflections at elevated temperatures dependencies of thermally induced stresses on temperatures and variations of intrinsic stresses due to changes within the films could be analyzed and related to microstructure and growth conditions. In the paper specific characteristics of the deposition processes occuring on the scale both of atoms and of clusters which may contain several thousand of atoms are described and related to microstructure, residual stress states and damaging conditions. Different contributions to residual film stresses are analyzed on the base of theoretical considerations taking into account deposition kinetics and thermomecanical properties. The significance of achieved film properties for application, i. e. for the coating of tools for the manufacturing of optical components by moulding and embossing of glasses is discussed.     

Optical and structural study of EB-PVD ZrO2 thin films

The behaviour of the crystalline properties of ZrO₂ films prepared by electron beam physical vapour deposition (EB-PVD) is investigated as a function of their deposition rates. In this paper, the conditions for the preparation of tetragonal zirconia from yttria stabilized zirconia and from monoclinic powder as starting materials are reported. The variation of the crystallite size as a function of the deposition rate is studied and, additionally, the optical characterization that permits to determine the refractive index is presented. The obtained values are in agreement with the bulk ones showing that EB-PVD prepared samples have good performance for optical and protective coatings.     

Mechanical characteristics of optical coatings prepared by various techniques: a comparative study

Good performance of optical coatings depends on the appropriate combination of optical and mechanical properties. Therefore, successful applications require good understanding of the relationship between optical microstructural and mechanical characteristics and film stability. In addition, there is a lack of standard mechanical tests that allow one to compare film properties measured in different laboratories. We give an overview of the methodology of mechanical measurements suitable for optical coatings; this includes depth-sensing indentation, scratch resistance, friction, abrasion and wear testing, and stress and adhesion evaluation. We used the techniques mentioned above in the same laboratory to systematically compare the mechanical behavior of frequently used high- and low-index materials, namely, TiO2, Ta2O5, and SiO2, prepared by different complementary techniques. They include ion-beam-assisted deposition by electron-beam evaporation, magnetron sputtering, dual-ion-beam sputtering, plasma-enhanced chemical-vapor deposition, and filtered cathodic arc deposition. The mechanical properties are correlated with the film microstructure that is inherently related to energetic conditions during film growth.     

The deposition and optical properties of Ge1−xCx thin film and infrared multilayer antireflection coatings

The effects of deposition parameters on the deposition rate, microstructure, and composition of Ge_1−xC_x thin films prepared by plasma enhanced chemical vapor deposition were studied and the films' infrared optical properties were investigated. The results show that the carbon content of these films increases as the precursor gas flow ratio of CH₄:GeH₄ increases, while the infrared refractive index of these films decreases from 4 to 2. The deposition rate increases with the radio-frequency power and reaches a constant value when the power goes above 60 W. Ge_1−xC_x/diamond-like carbon infrared antireflection coatings were prepared, and the transmittance of the coatings in the band of 8 to 14 μm was 88%, which is superior to that of Zinc Sulfide substrate by 14%.     

Superhard nanocrystalline titanium nitride films formed by inductively coupled plasma-assisted sputtering

Nano fabrication technology of superhard TiN films with sub-nanometered crystallites was developed using an Inductively coupled plasma (ICP) during deposition. Nanocrystalline TiN coatings were fabricated by ICP assisted sputtering and the properies of the coatings were investigated. The ICP assisted TiN coatings showed a much higher nano-hardness (>43 GPa) compared to coatings produced by the conventional DC sputtering process. The ICP assisited TiN coatings also showed superior properties in dense microstructure and surface roughness compared to the DC sputtered TiN coatings. The superior mechanical properties of ICP assistted TiN coatings were attributed to the fine and dense microstructure and high compressive residual stress.     

Influence of nitrogen on the optical and mechanical properties of diamond-like carbon films

Optical and mechanical properties of diamond-like carbon films grown by chemical vapor deposition were investigated as a function of the nitrogen content in the gas mixture. The nonmonotonic variation of the optical band gap and the microhardness of the films are interpreted using a model which allows for the influence of nitrogen on their structure. It is shown that nitrogen-containing diamond-like carbon films hold out promise as protective and antireflection coatings for silicon solar cells. Pis’ma Zh. Tekh. Fiz. 24, 87–92 (May 26, 1998)     

DEPOSITION OF OPTICAL COATINGS

A review of the preparation of optical coatings is presented. After considering the practical problems owing to the microstructure of thin films, some specific techniques for preparation of optical coatings, including electric-field assisted deposition, ion-beam-assisted deposition, magnetron sputtering, and ion plating are discussed, which represent the more significant advances of the last decade.     

Comparison study of microstructure, chemical composition and optical properties between resistive heating and electron beam evaporated LaF3 thin films

LaF₃ thin films were deposited by electron beam (EB) and resistive heating (RH) evaporation, respectively. Properties such as microstructure, chemical composition, surface morphology and optical constants of the LaF₃ thin films were characterized by measurements of X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and spectrophotometer, then comparison was made between this two deposition methods. It's found that the microstructure properties of the LaF₃ films deposited by these two methods were different, and slight content of oxyfluoride films was formed during deposition according to the result of chemical composition analysis. The microstructure of LaF₃ bulk materials after interaction with electron beam and resistive heating was also characterized to analyze how the two deposition processes affect the formation of LaF₃ thin films and their microstructure properties. When it was for the laser resistance of the films, although the EB evaporated LaF₃ thin films occupied lower absorption and optical loss than those of the RH films, they showed slightly smaller laser induced damage thresholds at 355 nm, which was thought to be related to their much more rougher surface and higher tensile stress.     

Wachstum von TiN-Schichten bei periodisch wechselnden Einfallswinkelverhältnissen der schichtbildenden Teilchen

Preferably, the magnetron sputtering process is used for the deposition of hard coatings in the low temperature range. The bombardement of energetical particles as well as the angular distribution of particle incidence onto the substrate are well known to influence the growth of the film substantielly. Especielly, layers with open columnar microstructure grow if the film-forming particles arrive the substrate at a large angle against the normal of the substrate. These films are unsuitable for tribological protection. For thickness homogenization and for effective utilization of the deposition chamber, a motion of the substrates is nessesary. This motion causes time dependent variations of the angle of the particles incidence. The microstructure of the films will be shown to deteriorate due to the amount of particles arriving the substrate at a large angle against the normal of the substrate. The possibility to suppress the formation of the open columnar microstructure during deposition of hard coatings on substrates (e.g. tools) by suitable procedures is discussed.     

Gasochromic switching of reactively sputtered molybdenumoxide films: A correlation between film properties and deposition pressure

Molybdenumoxide (MoO_x) thin films can change their optical properties upon exposure to hydrogen. Since the film properties strongly depend on process parameters we have studied how the films are affected by the total pressure during deposition. Stoichiometric and sub-stoichiometric MoO_x films were prepared by reactive direct current magnetron sputtering in an atmosphere of argon and oxygen. Substoichiometric films were coated with platinum as a catalyst and were colored in diluted hydrogen atmosphere and bleached in air. Optical spectroscopy, X-ray reectometry, spectroscopic ellipsometry and simulations of the measured spectra were used to characterize the films ex situ. In situ switching characteristics as revealed by optical spectroscopy and changes in stress were measured as well. We find that the total pressure during sputter deposition has a strong influence on the optical constants, the film density, and the sputter rate. The mechanical stresses and switching Preprint submitted to Elsevier Science 10 March 2006 cycles during the film coloration and bleaching also strongly depend on the total pressure. The influence of the sputter pressure on film properties is explained by the kinetics during the sputter process.     

Gadolinium and lutetium hafnates: Materials for the preparation of high-quality optical coatings

We have studied the spectroscopic properties and service performance of gadolinium hafnate and lutetium hafnate optical coatings. The coatings have been shown to possess improved optical parameters in comparison with binary oxide films and enhanced optical damage threshold     

Control of morphology and optical properties of PbS nanostructured thin films by deposition parameters: study of mechanism

PbS thin films were grown on glass substrates by chemical bath deposition (CBD) using lead nitrate, thiourea and sodium hydroxide in aqueous solutions at three different temperatures (22, 36 and 50?°C). The microstructure and morphology evolution of the films were investigated using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Optical properties were studied using UV–Vis–IR spectroscopy. The results indicate that temperature plays an important role in controlling the morphology and optical properties of nanostructured PbS thin films through changing deposition mechanism. The active deposition mechanism changed from cluster to ion-by-ion mechanism with an increase in deposition temperature from 22 to 50?°C, and consequently, film properties such as morphology, optical absorption and preferred orientation changed completely.     

准晶薄膜与涂层的制备、性能和应用

本文综述了近年来准晶薄膜的制备方法、性能及其应用等方面的研究工作。准晶薄膜的制备方法主要有物理汽相沉积法和热喷涂法两大类。准晶薄膜具有优异的力学、热学与光学等性能,已经应用于不粘锅涂层、热障涂层和太阳能吸收器等。     

Characterization of mesoporous ZnO:SiO2 films obtained by the sol-gel method

ZnO:SiO₂ films are intensively investigated for optical and electronic applications. Additionally, porous ZnO:SiO₂ films are of great interest as catalyst and gas-sensing materials. The sol-gel method is an efficient and low-cost process for the deposition of meso- and microporous silica-based films. The present paper studies the effect of the withdrawal speed on the microstructure and optical properties of mesoporous ZnO:SiO₂ films obtained by the sol-gel method. The morphology of the films was investigated by atomic force microscopy and the overall structure was studied by X-ray diffraction. The structure and size of the zinc oxide nanoparticles embedded in the silica matrix were investigated in more detail by transmission electron microscopy. These techniques showed ZnO:SiO₂ films with crack-free mesoporous morphology and highly efficient embedding of ZnO nanoparticles with (100) preferred orientation. Furthermore, the optical transmittance (in the visible and near infrared regions) and the optical band gap value were observed to vary with withdrawal speed. It is shown that ZnO:SiO₂ nanocomposites films which possess ZnO particles exhibiting a (100) orientation, with possible special applications in non-linear optics, could be prepared by the low-temperature crystallization sol-gel method.