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Synthesis and Heat Treatment of Sprayed High�Temperature NiAl–Ni3Al Coatings by In�Flight Combustion Synthesis (CAFSY)
Abstract
Synthesis and heat treatments of NiAl-Ni3Al high-temperature sprayed coatings by in-flight combustion synthesis (CAFSY)
Amalia Marinou1,2, Galina Xanthopoulou1, George Vekinis1, Angeliki Lekatou2, Michail Vardavoulias3
1 Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310, Athens, Greece
2 Department of Materials Science and Engineering, University of Ioannina, 45110, Greece
3 Pyrogenesis SA, Technological Park of Lavrion, 19500 Lavrion, Greece
E-mail: a.marinou@inn.demokritos.gr
Abstract
Combustion-Assisted Flame SpraYing (“CAFSY”) of intermetallic coatings is a new, cost-efficient and on-site-applicable thermal spraying process for applying Ni-Al intermetallic overlays or bond-coats on metallic substrates for protection at high temperature. The method is characterised by the synthesis of the desired intermetallic phases in-flight and in-situ on the substrates during oxy-acetylene thermal spraying, using only low cost base-metal powders. By adjusting the spraying conditions (initial composition, spraying distance, substrate temperature and flame temperature), excellent Ni-Al-based coatings have been produced on various substrates, including mild steel, stainless steel and aluminium alloys.
In some cases however, the intermetallic phases formed on the substrates during CAFSY have been found to be metastable or the nickel and aluminium powders have not reacted sufficiently. In such cases, post-spraying heat treatments of the coatings post-spraying allow the solid-state combustion reactions to proceed to completion in the coating. In many cases, it was found that increasing the temperature and the duration of the heat treatment increases the amount of intermetallic compounds (NiAl, Ni3Al, NiAl3 and Ni2Al3) in the coating up to as much as 90% by volume. In particular, all remaining aluminium reacts completely by forming Ni-Al intermetallics in the coatings. In all cases, porosity of the coatings remains below 3% while adhesion strength increases and reaches up to 57MPa.
The CAFSY method is a special manifestation of combustion synthesis, along the lines of the well known SHS method (Self-propagating High Temperature Synthesis). Without the need for expensive pre-alloyed intermetallic powders, optimisation of spraying conditions allows very fast, in-flight reactions between component base metal powders to produce the required coating alloys when they reach the surface of the substrate. The actual mixture of the intermetallic phases of the coatings and their properties can be optimized for any industrial use by control of the spraying conditions and composition of the powder mixtures.
Keywords: in-flight combustion synthesis (CAFSY); flame spraying; intermetallic phases; thermal treatment
... During CAFSY, the Ni and Al powder mixture reacted in an SHS synthesis regime, both in-flight and on the surface of coupons, producing various nickel aluminides. Table 1 presents the parameters employed in the present study, as previously determined [20,28]. Four different parameters were studied: (1) composition of the initial powder (COMPO), (2) distance of thermal spraying (DIST), (3) substrate temperature (SUBTEM) and (4) postheat treatment (COATR). ...
... However, this can be dangerous for the spray gun, as there is a risk of overheating due to the short distance from the substrate. Figure 4 (using data of [28]) also reveals that the porosity increases with increasing spray distance, while exhibiting a minimum value at the spraying distance of 2.5 inches. This can be attributed to the reduction of the fully melted particles when they impacted the substrate, resulting in fewer backscattered droplets, and hence, a smaller number of cavities. ...
... Semiquantitative analysis based on the peak ratios (Al, hkl: 111, Ni hkl: 200, NiAl, hkl: 220, Ni3Al, hkl: 311 and NiAl3, hkl: 112 [55]) in Figure 6 (using data of [28]) confirms that an increase in the substrate temperature caused an increase in the concentration of the intermetallic compounds. NiAl3 manifests the largest increase since, as mentioned above, heating of the substrate at temperatures over 550 °C increased the reactivity of Al, especially when the temperature is close to the melting point. ...
Combustion-assisted flame spraying (CAFSY) is a novel method that allows in-flight synthesis of alloys during flame spraying. The in-flight synthesis of alloys by the CAFSY method during flame spraying combines two different methods: the self-propagating high-temperature synthesis (SHS) and flame spraying (FS). The present work studies the corrosion performance (by cyclic polarization and chronoamperometry in aerated 3.5 wt.% NaCl) of NiAl coatings fabricated by the CAFSY technique in relation to main process parameters (composition of the initial feedstock, spraying distance, substrate temperature, postdeposition heat treatment) and their effect on the microstructure and porosity of the coatings. Most of the coatings exhibited limited susceptibility to localized corrosion. In all cases, the steel substrate remained intact despite corrosion. Interconnected porosity was the main parameter accelerating uniform corrosion. Localized corrosion had the form of pitting and/or crevice corrosion in the coating that propagated dissolving Al and Al-rich nickel aluminides along coating defects. Substrate preheating and postdeposition heat treatment negatively affected the corrosion resistance. A short spraying distance (1.5 inches) increased the corrosion resistance of the coatings.
... They are used as coatings in hightemperature applications such as energy conversion systems, internal combustion engines, voltage converters, gas turbines, and jet engines in order to protect the substrate against the oxidation. One of the requirements for these coatings is to minimize the thermal stress at the coating/substrate interface by promoting coating adhesion [10][11][12][13][14][15]. In this regard, Ni-Al alloy coatings can be applied by thermal spray process [6]. ...
... The flame spray is a general, available, and cost-effective method for producing coating from a wide range of materials with a low cost. As a post-treatment, annealing is required to improve coating performance because it can reduce porosity and increase adhesion [1,10]. Hence, with a convenient and low-cost heat treatment, a good coating like costly HVOF ones can be created. ...
In this study, the effects of time and temperature of heat treatment on Ni-20 wt.% Al flame spraying coatings were studied. The objectives are to find the optimal sample in terms of microstructure and compare its oxidation resistance with the as-sprayed coating. The coatings were deposited by flame spraying on low carbon (St 37) steel substrate. The specimens were heat-treated in vacuum at 850 °C and 950 °C for 1 h and at 1050 °C for 1, 3, and 6 h. Microstructural characterization and phase analysis were performed by scanning electron microscopy, energy-dispersive spectrometry, and x-ray diffraction, respectively. The results showed that the lowest porosity (0.43 %) and the highest NiAl content were achieved for the sample heat-treated at 1050 °C for 1 h. Optimized heat-treated, as-sprayed, and bare steel specimens were exposed to ambient air at 800 °C in order to evaluate their durability and oxidation resistance. The results showed that their oxidation rate constants were 24.66, 65.06, and 269.42, respectively. Most of the NiAl content in the coating was responsible for covering the coating surface by a protective γ-Al2O3 layer during the high-temperature oxidation process.
... Combustion-assisted flame spraying ( " CAFSY " ) combines conventional flame spraying and powder combustion synthesis into a single step [1,2]. Parameters including the temperature of the flame, the temperature of the substrate, the powder ratio and any pre-treatment of the metal powders, the gas speed and feed rate, and size and condition of starting powders can all be optimized for any particular coating needed. ...
... CAFSY is attractive for production catalysts because (1) it is a simple operation (requiring minimal operator training and can be used with hand-held sprayers); (2) it gives high spray rates and deposit efficiencies; (3) it works also with low-cost base metal powders of sizes ranging from 5 to 300 µm; (4) it is possible to change, at any time, the composition of the initial charge, and by adjusting the flame spray conditions, different properties of the coating can be achieved; and, finally; (5) different intermetallic phases can be deposited in one step. Our previous work [1,2] showed that the Ni–Al composite coatings applied by CAFSY were principally composed of three different intermetallic phases (i.e., NiAl, phases as well as traces of unreacted nickel and aluminium. In a recent paper [29], we confirmed the catalytic properties of those CAFSY coatings, and the objective of the present paper is to elaborate further on the catalytic properties of Ni–Al coated by CAFSY on spinel and refractory substrates for dry reforming of methane. ...
Combustion-assisted flame spraying (" CAFSY ") has been used to produce catalytically active nickel aluminide coatings on ceramic substrates. Their catalytic activity was studied in CO 2 (dry) reforming of methane, which is particularly significant for environmental protection as well as production of synthesis gas (CO + H 2). By varying the CAFSY processing parameters, it is possible to obtain a range of Ni–Al alloys with various ratios of catalytically active phases on the substrate. The influence of the number of coating layers and the type of substrate on the final catalyst composition and on the catalytic activity of the CAFSY coatings was studied and is presented here. The morphology and microstructure of the composite coatings were determined by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) elemental analysis, X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) specific area analysis. Catalytic tests for dry reforming of methane were carried out using crushed pellets from the coatings at temperatures of 750–900 • C, and gas chromatography showed that methane conversion approached 88% whereas that of carbon dioxide reached 100%. The H 2 /CO ratio in the synthesis gas produced by the reaction varied from about 0.7 to over 1.2, depending on the catalyst and substrate type and testing temperature.
... Method is attractive for industrial production: much lower energy consumption than traditional production methods, much lower energy costs, possibility for "just-in-time" manufacturing, high productivity, cheap catalysts, relatively simple process -easily adaptable to industrial scale, controlled physico-chemical properties of the products, large range of new materials which can be used in catalysis, it has wide diapason of structural forms of products -from granules of different size to blocks of honeycomb structure and different geometric forms. In addition, the environmental impact of SHS is very much lower than that of the traditional method, a fact which decreases even further the indirect cost of production (Marinou et al., 2015). For over the last few years, conversion of CO2, in particular by its reaction with methane to form CO and hydrogen (commonly known as dry reforming of methane), has gained a lot of attention. ...
Processing of natural gas into motor fuel has become one of the major problems of chemistry. Partial oxidation and CO2 reforming of CH4 attract a great attention in recent years. Resistant to temperature extremes and thermal shocks is one of the most important requirements for catalysts. Works on research of intermetallic compounds as a contact mass for conversion of methane were carried out. Method of self-propagating high temperature synthesis was used for synthesis of catalysts. Investigation of the activity of catalysts based on the initial mixture of metal oxides produced in the solution combustion synthesis process was carried out in the reaction of carbon dioxide conversion and partial oxidation of methane. 100 % methane conversion at 750 °C was carried out on the catalyst, whereas the conversion of CO2 reached 81.7 % at 900 o C. H2 yield reached 99.2 %, yield of CO-99.1 % in the ratio of H2/CO = 1.2. Effective catalysts for the production of synthesis gas from methane have been developed.
... The process has been subdivided into self-propagating hightemperature synthesis (SHS) and thermal explosion (TE) modes and has been used to synthesize various refractory compounds including ceramics and intermetallics [7][8][9]. The combustion synthesis of intermetallic compounds such as NiAl [10][11][12], TiAl [13,14], CoAl [15], and FeAl [16] has been successfully carried out during the past decade. However, the complex nature of the process, high temperature gradients, and high rates of reaction have made it difficult to study the kinetics of the reactions. ...
In this study, the reactions in Ti–50 at.%Al powder mixture upon heating at constant heating rates were studied using DSC analysis. Heating up the mixture resulted in melting of aluminum at temperatures close to its melting temperature, which was manifested as an endothermic peak in DSC curves. The melting of aluminum was the onset of an intense exothermic reaction, so-called combustion synthesis reaction which results in the formation of titanium aluminide. The shift in exothermic peak temperature in various linear heating rates was used to calculate the apparent activation energy according to Kissinger-type model-free methods. Heating rates of 10, 20, 30, 40, and 50 K min−1 were used to estimate the activation energy based on DSC data. In order to study the effect of ball-milling on reaction behavior, the starting powder mixture was ball-milled for different times, and a comparison was made between non-ball-milled and ball-milled DSC curves. The results showed that ball-milling tends to diminish the endothermic peak of melting of aluminum and shift the exothermic reaction temperature to lower temperatures, apparently altering the mechanism from a solid–liquid to an almost solid–solid one. The activation energy of the process in the non-ball-milled state was found to be close to the activation energy of diffusion of aluminum in TiAl (220 kJ mol−1). However, the application of same model-free methods to the ball-milled samples showed unexpectedly increased values for activation energy. It would be appropriate to check other methods as well as for calculation of activation energy in the ball-milled systems.
Atmospheric Plasma Spray (APS) is one of the most leading industrial techniques for protective coating, by improving the performance of parts in the thermal barrier, and wear resistance. Ni-Al alloys are very effective players in the field of design of protective coatings. Accordingly, mixed Al, Ni/Al, and Ni5Al powders were applied on 304stainless steel substrate to develop plasma sprayed coatings. The effect of different compositions on microstructure, microhardness, and porosity was measured. The microstructures of the as-deposited films were characterized utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), and microhardness measurements. The results showed the formation of two intermetallic compounds, namely: NiAl and Ni3Al. The existence of NiAl is inevitable in all samples, despite the amount of Ni-based alloys in mixtures, or even the atomic percentage of nickel, where the appearance of Ni3Al depends only on increasing the amount of the Ni-based alloy to 50 % percent in mixtures. As regards the steel substrate, the microhardness of the interdiffusion zone of the substrate has been significantly enhanced. Results have shown that the microhardness of the different tested coatings is increased directly with the increment of Ni-based percentage in the coating mixture. The average porosity of the plasma sprayed coatings has proven to be within the normal range.
Abstract: A new, cost-efficient and on-site-applicable thermal spraying process for depositing NiAl metallic overlay or bond-coat coatings for high temperature applications by synthesizing the desired intermetallic phases in-flight during oxy-acetylene flame spraying is presented. Base-metal powders were used for spraying and, by adjusting the spraying conditions, excellent NiAl-based coatings were achieved on various substrates, including mild steel, stainless steel and aluminium alloys. Expensive, pre-alloyed or agglomerated powders are avoided and the method is very promising for in-situ work and repairs. We call the new method “Combustion-Assisted Flame Spraying” (CAFSY) and its viability has been demonstrated at a pre-industrial level for coating metallic substrates. The NiAl-based coatings produced by CAFSY exhibit very high integrity with good adhesion, very low porosity, high surface hardness and high erosion resistance at a substantially lower cost than equivalent coatings using pre-prepared alloy powders.
Preface
My contact with thermal spraying started in autumn 1973 as I was asked to choose the subject of MSc thesis.As most of my fellow - students of Electronic Technology Institute of Technical University of Wroclaw (Poland),I was attracted rather by the technology of semiconductors.Then,my tutor Prof.Licznerski changed my mind,by showing very thick coatings and explaining their deposition technique:plasma spraying.
This deposition method is one from the family of thermal spraying techniques.Thermal spraying is today present in the research laboratories of universities and in the industry.The laboratories are about to complete the understanding of the physical and chemical phenomena at spraying.The industry explores new applications of the coatings.The expansion of the thermal spraying in the laboratories and industry created the need for the book.
This book is therefore addressed to the professionals of different educational level.The highly trained researchers and engineers can find a physical background of the thermal spraying processes,the description of coatings characterization techniques and up-to-dated review of the coatings properties.
The sales officers and technicians would hopefully appreciate the basic informations concerning the consumables using in thermal spraying and the methods of pre - and post - spraying treatment.
Finally,the entrepreneurs,who wish to invest into thermal spray would find the description of the thermal spraying techniques and the remarks concerning organization of the thermal spray workshop.
The book gives a complete description of the technology of thermal spraying,starting with the discussions of the powder manufacturing and testing techniques and the methods of pre-spray treatment.The most important techniques of thermal spraying being in present use as well as those in the research and development stage are discussed.Finally,the techniques of post spraying treatment such as mechanical finishing,high pressure and high temperature treatments and laser treatment are outlined.
The book explains also the physics of thermal spraying and the phenomena as acceleration or heating the solid particles in the flames.The problems related to the coatings buildup,that starts with a splash of an individual particle until the generation of thermal stresses in the coating,are also discussed.The special attention is paid to the methods of the coatings characterizations including microstructure investigation as well as the testing of mechanical and physical properties and the non-destructive methods enabling control of their quality.The coatings properties determined with the discussed methods,their relation to the coatings microstructure and processing parameters are systematically reviewed.Similarly,the coatings applications in
such important branches of modern industry as aeronautic,printing and electronic and others are presented.
A chapter related to the creation and organization of modern thermal spray shop finishes the book.
Many persons contributed in the more or less direct way in the book creation.I wish to acknowledge the friendly support of Prof.P.Fauchais - Head of Plasma Spray Laboratory at the University of Limoges (France).Ing.F.Kilp of Valco,Düsseldorf (Germany) made available the papers about the powders production methods and Prof.A.Vardelle of University of Limoges - about impact of sprayed particles.Dr.S.Sturlese of C.S.M. Rome (Italy) made a lot of useful comments concerning the thermophysical and electrical properties of coatings and Dr.R.C.Tucker,Jr.of Praxair,Indianapolis,Indiana (USA) revised a section concerning D-gunTM technique.Dr.J.Takeuchi of Tocalo,Kobe (Japan) made available the photographs of the coated elements applied in paper and steel industries.
I want to express my gratitude to the following colleagues or the organizations who send me the requested photographs and permitted me to use them as the figures in the book:Mr.J.Andresen,Dr.Ing.Z.Babiak,Batelle Columbus Labs.,Dipl.Ing.P.Bork,Dr.T.Cosack,Mr.F.J.Driller,Prof.H.Herman,
Mr.B.Kushner,Mrs. and Mr.Lemmens,Dr.E.H.Lutz,Dr.M.Marchese,Dr.A.Mascanzoni,Matrasur,Dr.Monerie-Moulin,Ms.B.Ottesen,Dipl.Ing.E.Prinz,Mr.G.Slaughter,Sherritt Inc.
Finally,I appreciate an efficient and supportive collaboration with the editor John Wiley & Sons and in particular with Mrs. V.Lutman.Many thanks to Muryel Wehr for drawing a lot of figures and her support during the last months of the book writing.
Nozay
14 april 1994
A description of the important physical metallurgy aspects of N13Al and NiAl encompassing structure, crystallographic defects, slip systems and phase stability has been presented in this
article. The microstructures generated in the two alloys by conventional as well as novel processing techniques have been
discussed. The effect of alloying additions on the microstructure has been enumerated. Besides description of the aforementioned
physical metallurgy aspects, an important purpose of this review is to focus on the reasons of brittleness in the two alloys
and means of alleviating this problem primarily by alloying. The effect of alloying on the slip behaviour has also been described.
Ni/Al alloy powders were synthesized by ball milling of nickel-aluminum powder mixture with a Ni/Al atomic ratio of 1:1. Ni/Al
alloy coating was deposited by cold spraying using N2 as accelerating gas. NiAl intermetallic compound was evolved in situ through postspray annealing treatment of cold-sprayed
Ni/Al alloy coating. The effect of annealing temperature on the phase transformation behavior from Ni/Al mechanical alloy
to intermetallics was investigated. The microstructure of the mechanically alloying Ni/Al powder and NiAl coatings was characterized
by scanning electron microscopy and x-ray diffraction analysis. The results show that a dense Ni/Al alloy coating can be successfully
deposited by cold spraying using the mechanically alloyed powder as feedstocks. The as-sprayed alloy coating exhibited a laminated
microstructure retained from the mechanically alloying powder. The annealing of the subsequent Ni/Al alloy coating at a temperature
higher than 850°C leads to complete transformation from Ni/Al alloy to NiAl intermetallic compound.
Advanced NiAl-based high temperature materials are developed and characterized for structural applications in energy conversion systems. The intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high temperature mechanical properties, and excellent oxidation resistance. Disadvantages of polycrystalline pure NiAl are the lack in plasticity and fracture toughness at room temperature and insufficient high temperature strength at temperatures above 800 °C. The refractory metals Cr, Mo, and Re form with NiAl quasi-binary eutectic systems which enable to produce metal fibres reinforced NiAl-based alloys in the as-cast condition and by performing directional solidification. These in-situ composites show fine-grained and thermally stable microstructures possessing high temperature strength, superior creep resistance and sufficient room temperature ductility.
The possibilities of the oxidation reduction of nickel aluminide plasma sprayed layers were studied. The NiO, Al203 and NiAl204. oxides occure in the Ni-Al coatings. The introduction of the deoxidiser of Ni e.g. P leads to the decrease in the oxidation numbers and allows the removal of NiO and NiAl204 from the layers. Coatings without NiO and NiAl204 oxides indicated better adhesion to the base than conventional Ni-Al type layers.
Ni-Al intermetallic compounds from Ni and Al powder were obtained by thermal explosion. Effects of molar ratio of Ni to Al in raw materials on the phases, microstructures and microhardness of the final products were studied. The results show that a single phase of NiAl is obtained with the composition corresponding to n(Ni):n(Al)=1:1. However, when the molar ratio of Ni to Al increases to 2:1, the product is composed of Ni3Al and NiAl, in which NiAl phase is in the majority with an irregular morphology and Ni3Al is in the minority mainly along the grain boundary of the NiAl. As the molar ratio of Ni to Al continues to increase to 3:1, the microstructures of the product are diversified. Lots of M-NiAl with needle-like or long lathing shape and many γ-Ni3Al with irregular and pointed strip-like morphology even appear besides β-NiAl with dendritic structure, γ-Ni3Al with irregular or net-like morphology because of the non-equilibrium cooling and large difference in composition of β-NiAl.
The adiabatic reaction temperature of stoichiometric Ni+Al and 3Ni+Al elemental mixtures and non-stoichiometric Ni/Al reaction systems under various initial conditions are calculated and compared with experiments. The experiments are based on the measurement of temperature–time profiles of the combustion reactions carried out in a nearly adiabatic condition. The adiabatic reaction temperature changes with the fraction of Ni with a maximum at about equal atomic ratio. The experimental results are in good agreement with the calculations.
In the present research, mechanically alloyed Ni-AI powder was utilized to develop plasma sprayed coatings, and the effect of the spray distance and heat treatment on the phases, microstructure, and hardness of the coatings were examined. Coatings were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and through microhardness measurements. Although mechanically alloyed Ni-AI powder showed no intermetallic phases, the coatings did. Different spray distances from 5 to 19 cm were employed for plasma spray and the specimens were heat treated at different temperatures, then the amount of oxides, porosity and hardness of the coatings were changed according to the spray condition. The thermal energy of the plasma spray caused the formation of NiAl phases while particles flew to the substrate or after that. Extreme increase in heat treatment temperature and spray distance resulted in oxidation and reduction in the quality of the coating. Furthermore, the best spray distance and heat treatment temperature to gain the NiAl intermetallic coating were established.
Effects of heating rate on the temperature profile of the thermal explosion mode of combustion synthesis and on the homogeneity of the reaction product are investigated. Analysis of the temperature profile using a simple mathematical model indicated that a significant fraction of reactants is consumed during the precombustion duration, forming intermetallic compounds at the reactant interface, which was confirmed experimentally. The relationship between the parameters measured from the temperature profile and the degree of conversion of reactants to products is discussed.
Alloys and coatings designed to resist oxidizing environments at high temperatures should be able to develop a surface oxide layer, which is thermodynamically stable, slowly growing and adherent. During oxidation in the temperature range 850–1300 °C, MCrAlY coatings should form Al2O3-rich scales, which are reasonably effective for long-term applications under isothermal or thermal cyclic oxidation conditions.This study is concerned with the isothermal oxidation behaviour of high-velocity oxygen fuel (HVOF)-sprayed CoNiCrAlY coatings containing 12 wt.% aluminium. Specimens treated in different ways (ground, polished and EB-remelted) were oxidized at 950 °C in synthetic air for various periods of time. Oxidation studies include thermogravimetric weight gain measurements under isothermal conditions in order to quantify the parabolic oxidation rate constant. The microstructure and morphology of as-sprayed and of oxidized coatings were characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD).The ground and polished samples show internal oxidation as well as relatively high values of the oxidation rate constants. In the case of EB-remelted coatings, the oxide-scale growth rate is substantially smaller and no internal oxidation was observed.
A new high velocity oxygen fuel (HVOF) spraying process to produce MCrAlY coatings was developed and optimized. The HVOF sprayed MCrAlY coatings were isothermally oxidized at 950 °C and 1050 °C in air as well as in different oxidizing atmospheres (synthetic air, He-10% O2 and He-10% synthetic air). The oxidation behaviour of HVOF-sprayed coatings is compared with that of VPS coatings. Under the chosen oxidation conditions, which assure a high oxygen partial pressure, the oxidation kinetics of the two coatings are very different, i.e. the oxidation rate of the HVOF sprayed coating is considerably lower than that of the VPS coating. In the present paper, this observation is explained by the presence of finely divided α-Al2O3 particles in the HVOF-sprayed coating, which are formed during spraying. The Al2O3 probably hinders the grain boundary diffusion of the elements. As a consequence, the oxide scale growth is very low. This effect is more evident at high temperature. The as-sprayed coating as well as the oxidized coatings were examined by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy to study the appearance and the composition of the oxide scales and the phase transformations in the MCrAlY coating.
Turbine blades are protected against high temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO2/Y2O3) and a metal bond coating (MCrAlY, M Ni, Co). At high temperatures and under oxidative conditions, between the MCrAlY and the ceramic top coating an oxide scale is formed, which protects the metal against further oxidation. The oxidation behaviour of the thermally sprayed MCrAlY is influenced by the coating process and the composition of the metal alloys.This work is concerned with the isothermal oxidation behaviour of vacuum plasma sprayed (VPS) MCrAlY coatings. The MCrAlY powders used have different aluminium contents: 8 and 12 wt.%. The MCrAlY specimens are oxidized at 1050 °C in air as well as in helium with 1% O2 and the oxidation kinetics are determined thermogravimetrically. The microstructure, morphology and thickness of the oxide scales formed are characterized by metallography, SEM, TEM and XRD.After short time oxidation (6 h) θ-Al2O3 is the main constituent of the oxide scale. Exposure times of 500 h and more lead to oxide scales consisting of α-Al2O3. Moreover, after a long time oxidation, Cr2O3 and CoO (CoO on the coatings with 8 wt.% Al) are formed. The oxidation rates of both MCrAlY coatings are the same. Beneath the oxide scale an Al-depleted zone is formed and this zone is considerably thicker with the coating with 8 wt.% Al, because the amount of β-NiAl phase in this coating is lower than that in the coating with 12 wt.% Al. The oxide scale formed in He—1% O2 consists of α-Al2O3 and Cr2O3 on both MCrAlY coatings.
The self-propagating high-temperature synthesis (SHS) of 0–40wt.% ZrB2 or 40wt.% TiB2 reinforced Ni–Al matrix composite powders has been investigated. The SHS reactions were conducted starting from elemental powders of Zr, Ti, Ni, Al, and amorphous B. For all initial compositions, the product contained only the NiAl and ZrB2 or TiB2 compounds without any trace of cross-reaction phases. The powder, obtained by grinding the reacted pellet, contained ZrB2 or TiB2 homogeneously dispersed in the NiAl matrix. The dimension and distribution of the reinforcement phase make the powder suitable as precursor for thermal spray applications. After sintering of the composite powder up to 85% of theoretical density, microhardness was measured. The microhardness of pure NiAl was 394±37HV0.2. It increased considerably by the addition of the boride phases. Samples containing 40wt.% ZrB2 and TiB2 attained values of 812±94 and 967±104HV0.2, respectively.
Intermetallic NiAl has the potential to be used for elevated temperature applications. To date different ignition techniques have been utilized to synthesize NiAl and produce coatings. Self-propagating high-temperature synthesis (SHS) has been developed as a relatively simple route to obtain intermetallics. This paper considers using induction heating to preheat and ignite the synthesis directly and investigates the effect of processing parameters on the phase transformation, microstructures and properties of Ni/Al compacts synthesized by SHS. The results show that single phase NiAl can be produced by induction heating whilst processing parameters such as heating rates and green densities have a significant effect on the properties and structures of sintered products.
The aim of this work was to develop a new process for the synthesis of TiC and NiAl/TiC composite in which the combustion reaction was ignited using a high frequency induction heater. High density, two-layer TiC–NiAl composites were also produced using this process. Temperature profiles during synthesis were measured with an IR thermometer and a high resolution thermal image camera was used to monitor the reaction process. Phase transformation was investigated using XRD and SEM was used to characterize the microstructure of the synthesized composites. The mechanical properties of the products were evaluated by measuring hardness. The results show that the reaction was complete and that stoichiometric products of NiAl and TiC were produced. The properties of NiAl/TiC composites were found to be functions of composition and processing parameters. The reaction mechanism was analyzed using temperature monitoring, thermodynamic analysis and microstructure investigation.
Advanced NiAl-based high temperature materials are developed and characterized for structural applications in energy conversion systems. The intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high temperature mechanical properties, and excellent oxidation resistance. Disadvantages of polycrystalline pure NiAl are the lack in plasticity and fracture toughness at room temperature and insufficient high temperature strength at temperatures above 800 °C. The refractory metals Cr, Mo, and Re form with NiAl quasi-binary eutectic systems which enable to produce metal fibres reinforced NiAl-based alloys in the as-cast condition and by performing directional solidification. These in-situ composites show fine-grained and thermally stable microstructures possessing high temperature strength, superior creep resistance and sufficient room temperature ductility.
This paper describes our efforts at combining vacuum plasma spraying and exothermic in-situ reaction processing techniques to obtain a cost-effective method of producing dense intermetallic materials for high temperature structural applications. It has been demonstrated that it is possible to produce, using this technique, a layered interpenetrating Ni3Al/NiAl composite with less than 5 vol.% residual Ni and less than 3% residual porosity. Exothermic reaction control through powder mixture composition and deposit thermal management proved to be critical steps in synthesizing this composite. Mechanical testing of the Ni3Al/NiAl composite produced in this study revealed that the presence of Ni3Al improved appreciably the room temperature properties of the composite (compared to NiAl) and imparted an anomalous yield behavior, where the yield strength initially increased with temperature. Furthermore, the layered structure of the Ni3Al/NiAl composite was observed to yield a marked anisotropy in its fracture toughness. The mechanical properties of the Ni3Al/NiAl composite produced in this study were observed to be in good agreement with properties reported in literature for similar materials produced by other techniques, indicating that with further process optimization the technique explored here can become a viable alternative to currently employed techniques.
The production of intermetallic compound was carried out in an electrical resistance furnace in open air under 150 MPa uniaxial pressure at 1050 °C for 1 hour using aluminum powder with 15 μm size and Carbonyl-nickel powder with 4–7 μm size having 99% and 99.8% purity, respectively. The formation temperature of intermetallic compound was determined by Differential Scanning Calorimeter analysis, and exothermic reaction of powder mixture was determined to occur at 655 °C. Optical microscope, scanning electron microscopy and X-ray diffraction analysis were used to characterize produced samples. These samples consist of single phase NiAl with very low porosity. Based on the Archimedes' principle, the relative density of the samples was 99.6%. The microhardness of the samples was approximately 367 ± 17 HV1.0. It was observed that NiAl intermetallic exhibited good oxidation resistance at high temperatures in open atmosphere. The distribution of alloying elements within intermetallic compound was determined by energy-dispersive X-ray spectroscopy.
Intermetallic compounds in the Al–Ni system were prepared by Self-propagating high-temperature synthesis (SHS). The front high speed in the range of 1–100 mm/s – typical of these processes made necessary the use a very intense synchrotron source (ESRF) coupled to a very fast acquisition system. Finely divided powders of Ni and Al were intimately mixed in a 1:1 mole ratio, and then compressed to form compact cylindrical of the right density to assure a steady combustion front. Diffraction patterns were acquired every 100 ms. As the reaction proceeded, patterns from the preheated, reaction front, post-heated and cooling zones were sampled. The crystal phases appearing were identified. Similarly, reaction mechanisms and nucleation kinetics were established. Also, high speed pyrometers were used to obtain combustion front temperature profiles and its propagation was video recorded. Finally, SEM images were taken to characterize the microstructure of end products.
Reaction synthesis principles have been extended to plasma spraying to obtain coatings consisting of mixed oxide phases and iron aluminides. Elemental powders of iron and aluminium were fed through a d.c. plasma torch to deposit intermetallic coatings on carbon steel substrates. Carbon steel substrates were also pre-heated with a plasma flame to create an iron oxide surface on the substrate such that an exothermic thermite reaction takes place when molten splats of aluminium impinge the pre-heated substrate at sub- or supersonic velocities. A thermite reaction between iron oxide and aluminium allowed the formation of alumina, FeAl2O4, iron, and iron aluminide phases. The presence of FeAl2O4 and Al2O3 increased the surface hardnesses of the coating, and the hardnesses of the coatings are significantly higher than the hardnesses of steel substrate, and aluminium particles. X-ray analysis of the coatings, microstructural observations, and microhardness measurements suggest that plasma spraying conditions can be tailored to obtain coatings with high hardness values with in situ synthesized reinforcements (spinel and alumina) or iron aluminide phases. Aluminium-rich phases were observed in the as-deposited coatings when a mixture of aluminium and iron or aluminium and nickel were fed through the plasma gun in ratios equivalent to Fe3Al, FeAl, Ni3Al, and NiAl. In some cases, annealing allowed the formation of iron-rich or nickel-rich aluminide phases. High solidification rates of molten splats allowed very limited diffusional reactions between the splats of aluminium and iron, or aluminium and nickel because the available diffusional time for exothermic interfacial reactions is limited to a fraction of a second at best. Oxidation of part of the aluminium led to the formation of alumina in the as-deposited coatings, and therefore, a vacuum plasma spraying technique is desirable to obtain intermetallic phases. The results suggest that reactive spraying will allow deposition of coatings by utilizing the heats of reaction between the constituents, and reactive spraying will broaden the engineering applications of reaction synthesis techniques.
Reactive spraying of nickel aluminides was accomplished via reaction synthesis techniques in which nickel and aluminum powders
were fed through a direct- current plasma torch onto carbon steel substrates. The as- sprayed coatings obtained by reactive
spraying were characterized by x- ray diffraction and microscopic techniques. Reactive spraying of nickel and aluminum resulted
in coatings consisting of Ni, Al, Ni
3Al, NiAl3, Ni5Al3, NiAl, and Al2O3, depending on the experimental conditions. Nickel aluminide phases observed in plasma spray depositions were compared with
the phases obtained by combustion synthesis techniques, and the formation of phases in reactive spraying was attributed to
the exothermic reaction between splats of aluminum and nickel. Primary and secondary reactions leading to the formation of
nickel aluminides were also examined. The splat thickness and the reaction layer suppressed the formation of desired equilibrium
phases such as Ni3Al and NiAl. As- sprayed coatings were annealed to enhance the diffusional reactions between the product phases and aluminum
and nickel. Coatings obtained by reactive spraying of elemental powders were compared with as- sprayed and annealed coatings
obtained with a bond coat material in which nickel was deposited onto aluminum particles.
Microstructures arrested at different stages of completion in case of both the modes of self-propagating high-temperature synthesis (SHS) of NiAl were studied in detail to compare the process evolution. In case of the thermal explosion mode where combustion cannot be arrested directly once the ignition point is reached, it was quenched after different extents of post-ignition combustion by varying the heating rate and using coarse Ni powder as in situ heat sink. This indirect method successfully simulates the wave-quenching operation, which is applicable only to the plane wave propagation (PWP) mode. The sequence of events accompanying the combustion was reconstructed on the basis of the incompletely converted microstructures in both the cases. It was shown for the first time that one unified reaction mechanism, very close to the one proposed by Aleksandrov and Korchagin [Comb. Expl. Shock Waves 23 (1988) 557], was operative in both the modes of SHS. This is irrespective of the fact that the pre-combustion diffusive reactions do not occur in the PWP mode.
Processing and characterization of Ni–Al coating on metal substrates, Master’s Thesis, National Institute of Technology
- M Chaithanya
Advanced Plasma Spray Applications
- H Jazi
- H. Jazi
Precipitation hardening in nickel copper alloy monel K500
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Handbook of Thermal Spray Technology
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Processing and characterization of Ni-Al coating on metal substrates
- M Chaithanya
- M. Chaithanya