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Microstructure and High-Temperature Oxidation Behavior of Cold Gas-sprayed Ni-Al2O3 Coatings
Abstract
Cermet coatings are widely used for high-temperature industrial applications. This study investigates the effect of high-temperature oxidation on cold gas dynamic-sprayed Ni-Al2O3 coatings. For this purpose, high-temperature oxidation tests were performed at 520 and 640 °C. The selected exposure times were 24, 48, 72, 168, and 336 h. The microstructural evolution during exposure at high temperature was analyzed by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), and x-ray diffraction (XRD). The oxidation kinetics was estimated by thickness measurements. The results show that the coatings protect the substrates against oxidation. In order to study possible changes in the mechanical properties of the system, Vickers microhardness experiments on the coatings and on the 10CrMo9-10 steel substrates were conducted. It was observed that hardness decreased by exposing the specimens to high temperature.
TZM is an important Mo based alloy for high temperature applications, however, oxide structures formed in an air atmosphere at high temperatures restrict its commercial applications. In this study, TZM alloys were coated with Ni-Zn-Al2O3 by low pressure cold spray (LPCS) in order to improve the high temperature oxidation behaviour. The effects on the microstructure, oxidation, and tribological properties of the MMC protective coating were examined. According to microstructural examinations, the LPCS solid-state coating had a wave-like lamellar structure due to an interlocking mechanism and consisted of ceramic alumina particles embedded in a Ni and Zn metal matrix. The EDS and XRD results showed that phase changes or oxidation were not formed in the LPCS coating due to the solid state deposition mechanism and low process temperature. The oxidation test results showed that the specific mass losses of the TZM alloy were sharply increased at high temperature due to the deformation of the oxygen in the molybdenum matrix. The Ni-Zn-Al2O3 MMC coating prevented oxidation on the TZM alloy at 600–1000 °C due to the reaction of the oxygen with the Ni and Zn elements generating a NiO and ZnO protective film layer on the coating surface. Dry sliding tests of the MMC coating were performed at different test loads for determination of the tribological performance. Although MMC coatings showed similar friction behaviour at different test loads, the wear rate in the 4 N test load was about 1.2 times higher than the 2 N loading.
Oxidation of TiAl alloys has been recognized as an obstacle for high-temperature applications such as aero engine and gas turbine. Substantial efforts have been made to improve oxidation resistance of TiAl alloys at elevated temperatures. In this study, multilayered thermal barrier coatings are prepared to protect a high-Nb-containing TiAl alloy from oxidation by air plasma spraying. The combination of Al2O3-13wt.%TiO2 ceramic coatings and NiCoCrAlY metallic coatings can improve thermal stability and increase the service lifetime of coatings. The fully melted TiO2 particles distribute in ceramic coatings uniformly and act as sealing pores and microcracks, which decrease porosity of the ceramic coatings and reduce diffusion channels of oxygen atoms. The porosity of surface and cross-section morphology are 5.5 ± 0.8 and 5.1 ± 0.8%, respectively. The results of oxidation experiment carried out at 800 and 900 °C for 100 h indicate that the coatings can effectively protect a high-Nb-containing TiAl alloy from oxidation. The mass gain of the high-Nb-containing TiAl alloys with coatings is lower than that of the one without coatings. The ceramic coatings retard diffusion of large amount of oxygen atoms, and bond coatings avoid to be excessively oxidized. Thus, the multilayered thermal barrier coatings exhibit an excellent long-term stability.
The composite coatings containing of nanocrystalline Ni matrix and hard nano-sized gamma-Al2O3 were electrodeposited in a system with a rotating disk electrode. The bath composition (nickel salts and buffer concentration, presence of surface-active agents and inert particles) influence on kinetics of Ni electrodeposition as well as on structural properties (morphology, phase composition, texture, residual stresses) and microhardness of Ni/Al2O3 coatings has been investigated. SEM and TEM studies show more uniform arrangement of Al2O3 particles in the matrix, however a tendency to agglomeration is observed. Surfactant application to a saccharine containing bath effectively improves the dispersion of nanoparticles into the nickel matrix. The addition of Al2O3 particles results in decrease of the average Ni crystallite size. The introduction of additive into electrolyte solution resulted in change of stress character of composite coatings (from tensile to compressive). The microhardness of Ni matrix was enhanced (about 40%) due to incorporation of ceramic particles.
Cold gas dynamic spray or simply cold spray (CS) is a process in which solid powders are accelerated in a de Laval nozzle toward a substrate. If the impact velocity exceeds a threshold value, particles endure plastic deformation and adhere to the surface. Different materials such as metals, ceramics, composites and polymers can be deposited using CS, creating a wealth of interesting opportunities towards harvesting particular properties. CS is a novel and promising technology to obtain surface coating, offering several technological advantages over thermal spray since it utilizes kinetic rather than thermal energy for deposition. As a result, tensile residual stresses, oxidation and undesired chemical reactions can be avoided. Development of new material systems with enhanced properties covering a wide range of required functionalities of surfaces and interfaces, from internal combustion engines to biotechnology, brought forth new opportunities to the cold spraying with a rich variety of material combinations. As applications multiply, the total number of studies on this subject is expanding rapidly and it is worth summarizing the current state of knowledge. This review covers different material systems that have been studied up to now with an emphasis on potential innovative applications. This includes metallic, ceramic and metal matrix composite (MMC) coatings and their applications. Polymer (both as substrate and coating) and metal embedment in the polymer are also covered. CS has emerged as a promising process to deposit nanostructured materials without significantly altering their microstructure whereas many traditional consolidation processes do. Relevant material systems containing nanostructured powders are also considered. A critical discussion on the future of this technology is provided at the final part of the paper focusing on the microstructural bonding mechanisms for those relatively less explored material systems. These include MMCs involving more than one constituent, ceramics, polymers and nanostructured powders. Future investigations are suggested especially to quantitatively link the process parameters and the behaviour of the material systems of interest during impact.
This paper presents the results of an oxidation behavior study for a thermal barrier coating (TBC) with air plasma sprayed yttria-stabilized zirconia top coat and CoNiCrAlY bond coat deposited using low pressure plasma spray (LPPS) and cold spray (CS). The TBC is subjected to isothermal oxidation and creep tests at 900 °C and evaluated using scanning electron microscopy, energy dispersive x-ray spectrometry transmission electron microscopy and electron backscatter diffraction. The thermally grown oxide (TGO) developed in the TBC with the LPPS bond coat was composed of only α-Al2O3 and the TGO developed in the TBC with a CS bond coat is composed of α-Al2O3 and γ-Al2O3. Despite the presence of this metastable γ phase, the TGO in the CS specimens exhibits a dense microstructure and lower amounts of mixed oxides. The correlation between γ-Al2O3 and the formation of mixed oxides was investigated through the measurement of γ-Al2O3 thickness ratio and mixed oxides coverage ratio. It was found that the mixed oxides coverage ratio is inversely proportional to the γ-Al2O3 thickness ratio.
In the present paper, aluminum bronze–alumina composite coatings have been applied on mild steel substrate using conventional plasma spray and a new route based on cold spray techniques. The microstructure, phase distribution, microhardness, adhesion strength and tribological behavior of the composite coatings, consisting of alumina reinforcing phase distributed in a bronze matrix, were studied. Ball-on-disk dry sliding wear tests, Rubber Wheel tests and Erosion tests were conducted to examine the tribological behavior of the composite coatings. The tribological properties of the bronze coatings were improved by the addition of alumina. Friction coefficient of coatings depends strongly on the reinforcing particles content and spraying process. Wear mechanisms of the composite coatings, such as ploughing and particle delamination, were considered. In the case of abrasive wear test, the wear rate was greatly reduced due to the reinforcing ceramic particles. Relationships between size and volume fraction of the ceramic reinforcement Al2O3 and the wear rate are discussed. On the other hand, erosion wear behavior of coatings with higher bronze content showed the best results.
Composite powder obtained from mechanically crushed titanium carbide—metal binder cermet compacts deserves special mention
for plasma spraying of wear-resistant coatings. However, cermet coatings sprayed using this powder have comparatively high
porosity. The porosity causes the mechanical strength of the coating to largely deteriorate, and it also lowers the strength
of the bond between the coating and the substrate. Computational and physical experiments were performed in this area to reveal
the possibilities offered by pulsed electron beam irradiation for structural modification of 70 vol.%TiC-(Ni-Cr) powder coatings.
The authors evaluated optimal values of process parameters for suitability in implementing a controlled thermal treatment
of coatings under conditions of solid-liquid interaction of components in the cermet composition with each other and with
the steel substrate. Evolution of the structure and physical properties of the cermet coatings under rapid heating and following
cooling in a wide range of temperatures typical of pulsed irradiation conditions have been examined.
Coating build-up mechanisms and properties of cold-sprayed aluminum-alumina cermets were investigated using two spherical
aluminum powders having average diameters of 36 and 81μm. Those powders were blended with alumina at several concentrations.
Coatings were produced using a commercial low-pressure cold spray system. Powders and coatings were characterized by electronic
microscopy and microhardness measurements. In-flight particle velocities were monitored for all powders. The deposition efficiency
was measured for all experimental conditions. Coating performance and properties were investigated by performing bond strength
test, abrasion test, and corrosion tests, namely, salt spray and alternated immersion in saltwater tests. These coating properties
were correlated to the alumina fraction either in the starting powder or in the coating.
The oxidation kinetics of high-purity nickel were studied between 500 and 1200C, in pure oxygen at atmospheric pressure, for average oxide-scale thicknesses of 1, 5, 10, and 30 m. In the overall temperature range studied, a decrease in the parabolic rate constant kp with increasing scale thickness was observed. Depending on temperature and oxide-scale thickness, growth kinetics can be interpreted as a mixture of parabolic- and cubic-growth kinetics. Possible correlations between growth kinetics and microstructures of the oxide scales were investigated. From this set of experimental data, oxidation-kinetics models were tested. In particular, the effect of grain-boundary diffusion on NiO-growth kinetics was discussed. The correlations between growth kinetics and oxide microstructures appear to be more complex than usually reported.
This research delivers a generic understanding of the design and integrated performance of the coating-substrate systems under
impact loading, and comprehends the understanding of underpinning failure mechanisms. Repeated severe impacts to the coatings
often result in poor performance by cracking and delamination from the coating-substrate interface. The durability of coatings
thus depends on the choice of coating and substrate materials, coating deposition process, and service conditions. The design
of thermal spray coatings thus requires an optimization of these parameters. This investigation provides insight into the
role of coating and substrate properties on the impact resistance of coated materials, and maps the relationship between the
impact resistance of WC cermet coatings on a variety of substrates. Results indicate that the delamination resistance of the
coating during impact loading not only depends upon the hardness and roughness of the substrate material, but, more importantly,
substrates with a higher work-hardening coefficient indicate a higher delamination resistance.
This paper summarizes on some of the extensive experimental data and corresponding models suggested to account for the oxidation mechanism of Ni in the temperature range 500–1400 °C. In addition it reports on in-house experimental data from investigations related to the oxidation of high-purity Ni from 500 to 1300 °C in the oxygen pressure range 1×10−4–1 atm based on TG, measurements of surface kinetics, two-stage oxidation, scanning electron microscopy, atomic force microscopy, secondary ion mass spectroscopy etc. The main part of this paper focuses on the more complex models suggested to account for experimental observations of the oxidation kinetics and the oxide morphology below 1000 °C.
The electrospark deposition (ESD) technique has been successfully applied to deposit at room temperature a cermet layer of Cr7C3 carbide bound with 20 wt.% Cr on a AISI 304 stainless steel substrate. It has been possible to obtain a fully dense, uniform and strong adherent coating layer of 10–30-μm thickness. The corrosion properties of this coating system have been evaluated in HCl electrolyte by conventional anodic polarization tests and impedance measurements. The results indicate an exceptional resistance of the coating to both general and localized corrosion attack. Some surface stress relief microcracks formed during the ESD deposit cycles do not impair the corrosion resistance of the coating even after long time immersion in HCl.
A straightforward treatment describing the oxidation processes of metals and alloys at elevated temperatures. This 2006 second edition retains the fundamental theory but incorporates advances made in understanding degradation phenomena. The first half provides an authoritative introduction to the basic principles, covering thermodynamics and mechanisms of high temperature corrosion of metals and alloys. The latter half extends the discussion to oxidation processes in complex systems, from reactions in mixed environments to protective techniques, including coatings and atmosphere control. The authors provide a logical and expert treatment of the subject, producing a revised edition that will be a comprehensive guide to material scientists and engineers requiring an understanding of this elementary process.
A new solar selective absorbing coating of AlNi-Al2O3 cermet maltilayer is designed and prepared on Cu or stainless steel (SS) substrate by magnetron sputtering. The coating consists of an IR-reflective metallic layer, an absorption bilayer and an anti-reflection layer from substrate to top. The IR-reflective metallic layer is Cu or Mo. The absorption bilayer consists of a high metal volume fraction (HMVF) AlNi-Al2O3 cermet sublayer and a low metal volume fraction (LMVF) AlNi Al2O3 cermet sublayer. The anti-reflection layer is Al2O3. The optical performance (alpha/epsilon) of the coating is 0.94/0.07 on Cu substrate and 0.95/0.078 on SS substrate by optimizing the metal volume fraction and thickness of each layer. The samples deposited on SS substrate were heated at 500 degrees C for 138 h and 284 h in vacuum to evaluate the optical stability. Experimental results showed that the high temperature stable AlNi intermetallic compound, instead of Mo in the traditional Mo-Al2O3 coatings, displayed a good thermal stability at 500 degrees C. There is no significant change appearing in the optical properties of the coating after the heat treatment. It means that the new coating is a good potential candidate as a high temperature solar selective absorbing coating for parabolic trough concentrated solar power (CSP). (c) 2013 Elsevier Ltd. All rights reserved.
High velocity oxy fuel (HVOF) and cold spray techniques have been used to form oxidation resistant hard coatings for bond coat application for gas turbine blades. Though both techniques accelerate powder particles with high kinetic energy, the coatings formed differ considerably in their microstructures. In the former, high pressure is created by burning gases such as acetylene, propane or kerosene at high pressure and generating high temperature (3500 to 4500 °C) in the gun. This gives high acceleration to powder particles which melt and deposit on substrate layer by layer with splat cool mechanism. On the other hand, large kinetic energy (KE) is generated in cold spray by passing carrier gases such as He or N2 through a converging-diverging nozzle, with a lower gun temperature of around 500 °C. Here the particles are not liquid droplets because of lower temperature and the deposition mechanism is not a splat cooling, but a high impact of solid particles, which results in plastic deformation, making a very adherent coating.
Hard protective nitride coatings are often applied to cutting tools operating at high temperature. To further develop and optimize their performance, in-situ investigation of structure, oxidation, mechanical and tribological properties at elevated temperature is required. In this study we focus on the high temperature behavior of a nanolayered CrAlTiN coating deposited on WC substrates by cathodic arc evaporation. The coating's chemical composition and the bonding state were evaluated by electron probe microanalysis and by X-ray photoelectron spectroscopy (XPS). The structure of as-deposited and annealed samples was analyzed using X-ray diffraction. The adhesion was investigated by scratch test and the mechanical properties were studied by depth sensing nanoindentation. The main objective of this work was to have a detailed analysis of friction and wear properties tested by high temperature tribometer (pin-on-disc) with alumina balls as counterparts in the temperature range of 20–800 °C. Selected wear track cross-sections were prepared by focused ion beam and analyzed by transmission electron microscopy; the wear track was investigated as well by XPS (chemical depth profile) and by Raman spectroscopy. The coating showed an excellent thermal stability and wear resistance. The friction reached a maximum at 500 °C and then decreased, whereas the wear rate was negligible up to 600 °C and then increased significantly for higher temperatures. Oxidation of the worn surfaces was surprisingly low even at the highest temperature corroborating results of oxidation tests. The main identified wear mechanism was polishing combined with a nanoscale delamination of thin coating layers; nanoscale multilayer proved to be a vital factor blocking vertical crack propagation.
A series of Fe–xCr cored wires (x = 15, 20, 25, 30, 35 and 40% in weight) were used to produce protective coatings for components used in high-temperature environments by wire-arc spraying. The microstructure, morphology and high-temperature oxidation behavior of the Fe–xCr coatings were investigated. The Fe–xCr coatings presented a homogeneous structure with thin oxide inclusions and low porosity. Thermogravimetric analysis was used to evaluate the high-temperature oxidation behavior of the coatings at a temperature of 650 °C under cyclic oxidation conditions. The results showed that all the Fe–xCr coatings exhibited significantly lower weight gain than the uncoated substrate. As Cr content of the coatings increased, the thickness of oxide scales decreased and oxidation resistance improved.
Background
A meta-analysis of published data was conducted to investigate the overall risks of hypertension and QTc prolongation in patients with advanced non-small cell lung cancer (NSCLC) who were receiving vandetanib.
Methods
A computerized search through electronic databases, including PubMed and Embase (until Dec 2014), was performed to obtain eligible randomized controlled trials (RCTs) that compared hypertension and/or QTc prolongation profile of vandetanib alone or plus chemotherapy with control groups (placebo, single targeted therapy, chemotherapy, or a combination of them) in patients with advanced NSCLC. The outcome measures were the overall risks of hypertension and QTc prolongation. Relative risk (RR) and 95 % confidence interval (CI) were calculated and pooled using a random effects model.
Results
A total of nine RCTs, which involved 4813 patients, were enrolled in the present study. A significant increase in risk was observed for all-grade hypertension (RR 5.58; 95 % CI 4.16 to 7.48; P < 0.00001) and grade ≥3 hypertension (RR 4.79; 95 % CI 2.31 to 9.93; P < 0.0001) in advanced NSCLC patients who were receiving vandetanib compared with the controls. Moreover, vandetanib significantly prolonged all-grade QTc interval (RR 7.90; 95 % CI 4.03 to 15.50; P < 0.00001) and grade ≥3 QTc interval (RR 3.12; 95 % CI 1.01 to 9.63; P = 0.05).
Conclusions
Current evidence showed that significant risks in developing hypertension and QTc prolongation exist in advanced NSCLC patients who were receiving vandetanib. Thus, appropriate monitoring and management of these events are recommended.
Cold spraying shows a great potential in the fabrication of metal matrix composites. In this study, a nickel-coated Al2O3 powder produced with the hydrothermal hydrogen reduction method was employed aiming to increase the volume fraction of ceramic particles in the cold sprayed composites. The effect of oxidation temperatures on the microstructure and microhardness of as-cold-sprayed and vacuum heat-treated Ni/Al2O3 composite coatings was characterized. In addition, the high temperature oxidation behaviors of as-sprayed and heat-treated coatings were compared and discussed. Eventually, the tribological performance of as-sprayed coating was examined at 25 °C and 400 °C. Results show that the oxidized coatings are denser compared with the as-sprayed coating. The oxide films morphologies of as-sprayed coatings are obviously different under different oxidizing temperatures. Moreover, the as-sprayed coating has a similar oxide film feature to the heat-treated coating when oxidized at 850 °C, which may suggest the direct use of coating after deposition without any heat-treatment. The microhardness values of the oxidized coatings are comparable with the as-sprayed coating and even the heat-treated coating. Finally, friction tests indicate that the as-sprayed coating shows a good tribological performance at elevated working temperatures.
The aim of this work is modify the surface composition of a Gr22 ferritic steel component with Ni(Al) using a high-power diode laser (HPDL). Two different combinations of parameters were used to obtain a superficial alloy modification macroscopically homogenous, with low porosity and low presence of cracks. From a microscopically point of view the surface presents a band structure due to the rapid cooling process. Isothermal oxidation tests at 640 °C were performed to evaluate the performance of the solution at high temperature. The introduction of Ni(Al) with this technique has allowed to reduce the degradation of the components, without modifying the ferritic structure of the steel.
In the current work, Ni–Al2O3 based composite coatings were fabricated using High Velocity Flame Spray (HVFS) system. The effect of alumina content on the microstructure and various mechanical properties viz. microhardness, fracture toughness, density, residual stress and scratch resistance was studied. Microstructure of the coating was characterized with the help of Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), Optical Microscope (OM) and X-ray Diffraction (XRD). It was observed that microstructural properties such as splat size, porosity, un-melted particles and surface roughness are highly affected by the alumina content in the coatings. Microhardness improved with the rise in alumina fraction and a maximum of 1151 HV could be achieved with 60 wt.% alumina blending. Cracking and spallation were found to be the mechanisms responsible for damage of coatings during scratch testing. A model has been proposed to explain the variation in surface roughness and porosity of the Ni–Al2O3 coatings based on coating build-up mechanism.
Most erosion–oxidation research has been directed at high erodent flux, low impact velocity conditions typical of fluidised bed conditions. Few works have investigated erosion–oxidation under high temperature, high impact velocity, and low erodent flux conditions found within turbines. This work addresses the gap in knowledge of the high temperature erosion–oxidation mechanism of Cr3C2–NiCr coatings under long-term simulated turbine conditions. The same generic mechanism was observed under erosion-affected-oxidation and oxidation-affected-erosion conditions. The carbide and Cr3C2–NiCr phase interface was preferentially oxidised. An internal oxidation front progressed into the coating in advance of the slower erosive mass loss front at the surface.
A composite coating was deposited on Inconel 600 substrate by cold spray method using pure Ni powder (60wt.%) blended with α-Al2O3 (40wt.%) as feedstock. It is expected to be applied to repair the bellows eroded by the liquid droplet erosion (LDE). Microstructure of the coating was observed using optical microscope (OM) and scanning electron microscopy (SEM). Microhardness of the coating was determined by Vickers hardness tester. Cavitation erosion (CE) experiments were carried out in the distilled water. Jet impingement erosion (JIE) experiments were performed in slurry containing 1wt.% quartz particle with the flow velocity of 15m/s at impingement angles of 30°, 60° and 90°, respectively. Cumulative mass loss vs. testing time was used to evaluate the erosion rate of the coating. The erosion mechanism was analyzed by OM, SEM, X-ray diffraction (XRD) and the microhardness measurement. The results show that the composite coating has compact microstructure and relatively high hardness. The resistance to CE of the coating is not as good as that of Inconel 600 substrate due to the weak bonds of the Al2O3 particles. However, the results of the JIE test indicate that the slurry erosion resistance of the coating is better than that of Inconel 600 at the impact angles of 30° and 60°, but not at the normal impact angle.
The effect of impurities on the oxidation mechanism of nickel and on the mechanical characteristics of NiO films was studied on two industrial nickel grades compared to a pure nickel. In this part, influence of impurities on the oxidation mechanism and on the NiO film microstructure will be detailed. The oxidation mechanism, especially studied at 800°C in air, was clarified using complementary techniques, SEM and STEM microstructural and analytical investigations, XPS analyses, profilometry, oxygen isotopic exchange and SIMS. Depending on the Ni purity, the morphology of the oxide scale and the porosity amount differs. Duplex oxide layers and internal oxidation are observed on industrial grades, while a simple NiO film grows on pure nickel without internal oxidation. An oxidation mechanism, which differs according to the presence or not of impurities in the nickel substrate, is proposed. The mechanical characteristics will be characterised and discussed in the following paper (part II) in relation with the microstructural modifications induced by changing the nickel purity.
Solar energy is the most abundant source of renewable energy. The direct method of harnessing solar energy is the solar thermal conversion method using solar absorbers. The absorbers are coated with solar selective coatings with high absorptance and low thermal emittance. Spectrally selective coatings which are stable up to temperatures ≤300°C (in air and vacuum) have been developed in the past. These coatings are mainly deposited from wet chemical routes (e.g., electrodeposition) and have been reviewed widely in the literature. Because of the environmental issues as well as low thermal stability of these wet chemical deposited coatings, researchers all over the world started looking for other alternative routes such as physical vapor deposited (PVD) coatings. A great deal of research has been carried out since 1990s to develop PVD coatings for both mid- and high-temperature applications. The mid-temperature coatings are used mainly for solar hot water and industrial process heat applications, whereas, the high-temperature absorber coatings are used in concentrating solar power systems for solar thermal power generation. It is well recognized that in order to increase the efficiency of solar thermal power plants, solar selective coatings with high thermal stability are required. In recent years, significant efforts have been made in the field of solar selective coatings to achieve high solar absorptance and low thermal emittance at higher temperatures (T≥400°C). Transition metal based cermets have emerged as novel high temperature solar selective coatings, which are currently being used for solar thermal power plants for electricity generation. Solar selective coatings based on transition metal nitrides, oxides and oxynitrides also hold great potential for high-temperature applications because of their excellent mechanical and optical properties, which are yet to be commercialized. In this review, we present the state-of-the-art of the physical vapor deposited solar selective coatings used for solar thermal applications with an emphasis on sputter deposited coatings for high-temperature applications. A detailed survey, covering the period 1970-present, has been made for the PVD deposited solar selective coatings with high absorptance and low emittance. This review article also describes in detail about the commercially available PVD coatings for flat-plate/evacuated tube collectors and solar thermal power generation applications.
Cermets coatings are extensively used in energy applications both because of their high wear resistance as required, for example, in components like gas turbine sealants, and because of their specific functionality as required in solar absorbers. So far, high-temperature thermal spraying and physical vapor deposition have traditionally been used to deposit this kind of coatings. In this study, Ni-Al2O3 coatings have been deposited using a Kinetic®3000 cold-spray system starting from Ni and Al2O3 powders blend; five blends have been prepared setting the alumina content in the feedstock to 10, 25, 50, 75, and 90 wt.%. The embedded alumina ranges between a few percent weight up to 16 and 31 wt.%, while the microhardness shows a deep increase from 175 Vickers in the case of pure Ni coatings up to 338 Vickers. The spray and coating growth mechanism have been discussed, with special attention to the fragmentation of the ceramic particles during the impact. Finally, the coating behavior at high temperature was analyzed by oxidation tests performed in air at 520 °C emphasizing a good oxidation resistance that could represent a very promising basis for application in power generation systems.
This monograph reviews available information on the production, properties, and uses of high-purity and commercial forms of nickel, a d on the properties and applications of its important alloys, both ferrous and nonferrous. It is a revision of National Bureau of Standards Circular 592, issued in 1958.
The oxidation of cold‐worked and annealed pure Ni in 1 atm O2 was investigated from 700° to 1270°C in continuous weighing experiments for periods from 1 min to 20 hr and the oxidized
specimens examined by diffraction, electron‐optical techniques, and metallographic cross sections. Cold‐worked Ni was found
to oxidize faster than annealed Ni and form finer‐grained oxide. Plots of the apparent parabolic rate constant, Kp, for cold‐worked Ni show an initial high value that decreases rapidly as the oxide coarsens with time. For annealed Ni, Kp is lower, grain size larger, and both change little with time. Accordingly, a range of Kp values is obtained at each temperature and, on an Arrhenius plot of Ni oxidation, there is a corresponding variation in apparent
activation energy, EA
. These results can be plausibly interpreted on the basis of oxide grain boundaries acting as easy diffusion paths for Ni
through the
layer: for fine‐grained oxide, Kp is higher and EA
lower. The oxide is thinnest (and the derived EA
highest) on Ni grains which form a single crystal overgrowth. A break in the Arrhenius plot around 1000°C is the result of
transport below 1000° being largely by leakage paths while at higher temperatures volume transport is more important. The
estimated activation energy for growth of the oxide layer is
for transport by lattice diffusion and appreciably less for transport by grain boundary diffusion.
This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr2O3 and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.
The use of coatings to improve the tribological properties of components such as tools for metal cutting and forming, and machine elements e.g. sliding bearings, seals and valves is constantly increasing. This paper presents tribological design considerations of coating composites and recommends methods and techniques for their evaluation. It is aimed both for those involved in scientific development of coatings and for the practising tribology engineer.Current design concepts of coatings for tools and machine elements are exemplified together with successfully coated components. Techniques for evaluation of some of the more important fundamental and tribological properties will be presented and several examples are given mainly with thin physical vapour deposition (PVD) coatings. The evaluation techniques can be utilised both in coatings selection and in development of new coatings. Some recent trends and speculations about the future end the paper.
Previous studies have shown that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al2O3, were used as reinforcements, it was diffcuclt to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al2O3 powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni–Al2O3 composite coating could be deposited with the Ni-coated Al2O3 powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (nominal: 40–45 vol.%) due to the rebound of some Al2O3 particulates upon kinetic impacting. The microhardness of the composite coating was about 173 ± 33Hv0.2.
TiCN–Ni-based cermets are attractive cutting tools because of the combination of high hardness and wear resistance with improved toughness and thermal shock resistance. The present work reports the effect of WC addition (0–15 wt.%) on the machining performance of TiCN–20 wt.% Ni cermets against boiler steel. The cutting force was measured on-line using dynamometer, with respect to varying cutting speed and feed rate, in dry and orthogonal cutting conditions. The principal aim of the present investigation is to evaluate the dominant mechanisms, responsible for material removal on the rake face of cermets, using SEM–EDS. The cutting performance of TiCN–Ni cermet is observed to improve with the addition of WC content upto 10 wt.%. While, the adhesion of tribochemical layer is dominant with limited WC content, the presence of abrasive grooves and pull-outs are observed for TiCN–20Ni cermets containing higher amount of WC (>10 wt.%).
TiCNT-Ni-based cermets are attractive cutting tools because of the combination of high hardness and wear resistance with improved toughness and thermal shock resistance. The present work reports the effect of WC addition (0-15 wt.%) on the machining performance of TiCN-20 wt.% Ni cermets against boiler steel. The cutting force was measured on-line using dynamometer, with respect to varying cutting speed and feed rate, in dry and orthogonal cutting conditions. The principal aim of the present investigation is to evaluate the dominant mechanisms, responsible for material removal on the rake face of cermets, using SEM-EDS. The cutting performance of TiCN-Ni cermet is observed to improve with the addition of WC content upto 10 wt.%. While, the adhesion of tribochemical layer is dominant with limited WC content, the presence of abrasive grooves and pull-outs are observed for TiCN-20Ni cermets containing higher amount of WC (>10 wt.%).
Material degradation at high temperatures takes place due to loss in mechanical properties with increase in temperature as well as due to the chemical interaction of metal with the environment. This chemical interaction is further sub-divided into oxidation, sulfidation, and hot corrosion. While oxidation leads to the formation of oxide, which can be deleterious if the oxide is fast growing and spalls extensively, however, if the scale formed is adherent, thin and slow growing, it provides protection to the base metal or alloy. Sulfidation is a much severe degradation process and several times faster than the oxidation. In many industrial environments, it is a mixed gas environment, leading to oxidation and sulfidation simultaneously. Hot corrosion is another degradation mechanism which is even more severe than the oxidation and the sulfidation. Here, oxidation/sulfidation occurs in the presence of a molten salt on the surface of the substrate. Related issues, such as role of defect structure, active element effect and stress generation, during oxide growth process, have also been discussed. Finally, a guide to material selection for high temperature application is presented.
Al-Al(2)O(3) composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al(2)O(3) particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al(2)O(3) particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al(2)O(3) content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al-Al(2)O(3) bonds compared with stronger Al-Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted. Crown Copyright