Effects of Silver Addition on Properties and Performance of Plasma Sprayed La0.6Sr0.4Co0.2Fe0.8O3−δ Interconnect Layer

Interconnect layers on stainless steel substrates (STS430) for solid oxide fuel cells (SOFC) were built up by atmospheric plasma spraying (APS) using spray dried La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) and blended LSCF/Ag composites. The microstructure and phase of each coating were analyzed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) studies, respectively. Furthermore, bond strength, microhardness, performance in a thermal cycle test and in an oxidation test, and electrical conductivity were measured and compared. The coatings prepared from spray dried LSCF have higher porosity and more cracks within the splats and at intersplat boundaries. In contrast, the coatings prepared from LSCF/Ag had fewer cracks and less porosity due to the relatively high ductility of silver. After oxidation testing at 800 °C for 200 h, the weight change of the STS430 substrate and the LSCF and LSCF/Ag-coated alloys were found to be 0.06833, 0.01950, and 0.01656 mg/cm², respectively. Also the electrical conductivity of LSCF and LSCF/Ag coatings were higher than that of STS430 by two orders.     

Tribological Behavior of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-20 wt.%TiO<Subscript>2</Subscript> Coating

Al₂O₃-20 wt.% TiO₂ ceramic coatings were deposited on the surface of Grade D steel by plasma spraying of commercially available powders. The phases and the microstructures of the coatings were investigated by x-ray diffraction and scanning electron microscopy, respectively. The Al₂O₃-20 wt.% TiO₂ composite coating exhibited a typical inter-lamellar structure consisting of the γ-Al₂O₃ and the Al₂TiO₅ phases. The dry sliding wear behavior of the coating was examined at 20 °C using a ball-on-disk wear tester. The plasma-sprayed coating showed a low wear rate (~4.5 × 10^?6 mm³ N^?1 m^?1), which was <2% of that of the matrix (~283.3 × 10^?6 mm³ N^?1 m^?1), under a load of 15 N. In addition, the tribological behavior of the plasma-sprayed coating was analyzed by examining the microstructure after the wear tests. It was found that delamination of the Al₂TiO₅ phase was the main cause of the wear during the sliding wear tests. A suitable model was used to simulate the wear mechanism of the coating.     

Effect of Spraying Powders Size on the Microstructure,Bonding Strength,and Microhardness of MoSi2 Coating Prepared by Air Plasma Spraying

Molybdenum disilicide (MoSi₂) coatings were deposited on carbon steel by air plasma spraying technology with different feedstock powder sizes (i.e., powder A: ?15 + 2.5 μm, powder B: ?30 + 15 μm, powder C: ?54 + 30 μm, powder D: ?74 + 54 μm and powder E: ?106 + 74 μm). Phase composition and microstructure of coatings were characterized by x-ray diffraction (XRD) and scanning electron microscope. The bonding strength and microhardness of coatings were also evaluated. The XRD results show that there exists mutual transformation between T-MoSi₂ and H-MoSi₂ phase and part of Mo-rich phases are formed because of oxidization during the spraying process. With the increase of spraying powders size, the content of Mo-rich phases (Mo or Mo₅Si₃) and molybdenum oxide (MoO₃) in coatings decreases, and that of disilicide-rich phase (MoSi₂) in coatings increases. The oxidation degree of MoSi₂ particle gradually decreases during the spraying process with the increase of spraying powders size. The MoSi₂ is the main phase of the as-sprayed coatings when the spraying powders size is beyond 30 μm. With the increase of spraying powders size, the porosity of the as-sprayed coating increases, and the bonding strength of the coating gradually decreases. The hardness of coatings first increases and then decreases with the increase of spraying powders size.     

Effect of Graphite Addition on Structure and Properties of Ti(CN) Coatings Deposited by Reactive Plasma Spraying

Ti(CN) coatings with graphite addition ranging from 0 to 50 wt.% were prepared using reactive plasma spraying technology and their microstructure, mechanical, and tribological properties were investigated using scanning and transmission electron microscopy, x-ray diffraction analysis, x-ray photoelectron spectroscopy, Vickers microhardness testing, and block-on-ring wear testing. The results showed that graphite addition resulted in crystallite size refinement and an increase in the amount of amorphous phase. The Ti(CN) coatings consisted of a mixture of Ti(CN), graphite, CN _x , and amorphous phases. The hardness first increased then decreased as the graphite content was increased, with a maximum of 1450 HV_0.2 for 30 wt.% graphite addition. The fracture toughness decreased from 4.38 MPa m^1/2 to 2.76 MPa m^1/2 with increasing graphite content. The friction coefficient decreased due to unreacted graphite embedded in the matrix. Also, the wear rate first decreased then increased, with a minimum value of 2.65 × 10^?6 mm³ N^?1 m^?1 for 30 wt.% graphite addition. The wear mechanisms of the Ti(CN) coatings included abrasive, adhesive, and oxidation wear.     

Synthesis and characterization of La0.8Sr0.2MO3−δ (M=Mn, Fe, or Co) cathode materials by induction plasma technology

In this work, nanopowders of perovskite cathode materials (La_0.8Sr_0.2MnO_3−δ, La_0.8Sr_0.2FeO_3−δ, and La_0.8Sr_0.2CoO_3−δ), for use in solid oxide fuel cells (SOFC), were successfully synthesized, using induction plasma techniques. Their compositions, structures, morphology, particle size distributions, and BET specific surface areas were determined for comparison with their counterparts prepared by the Pechini method and by the glycine-nitrate combustion (GNC) technique. The particle sizes of the plasma-synthesized powders are mostly around 63 nm. These plasma-synthesized powders are generally globular, their BET specific surface areas being about 26 m²g⁻¹, approximately twice those of powders prepared by the GNC and Pechini methods. These plasma-synthesized powders are readily reproducible and are not agglomerated. Their individual particle sizes and distributions are very independent of their composition.     

Microstructures and magnetic properties of carbon nanotube/Co-oxide nanocomposite powders

Carbon nanotube embedded cobalt oxide (CNT/Co-oxide) nanocomposite powders were synthesized by molecular-level mixing followed by calcination. The surface morphology of the fabricated nanocomposite powders shows Co-oxide coated CNTs homogeneously dispersed in the Co-oxide nanopowders. XRD patterns reveal that CoO–Co₃O₄ nanopowders were obtained as a complex phase. The nanocomposite powders exhibit ferromagnetism with a saturation magnetization of 75emu g^?1, while CoO–Co₃O₄ nanopowders without CNTs are paramagnetic. These results indicate that incorporating CNTs into Co-oxide nanopowders triggers a change of the magnetic properties towards ferromagnetism.     

Oxidation and Hot Corrosion Behavior of Plasma-Sprayed MCrAlY–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

The oxidation and hot corrosion behavior of two atmospheric plasma-sprayed NiCoCrAlY–Cr₂O₃ and CoNiCrAlY–Cr₂O₃ coatings, which are primarily designed for wear applications at high temperature, were investigated in this study. The two coatings were exposed to air and molten salt (75%Na₂SO₄–25%NaCl) environment at 800 °C under cyclic conditions. Oxidation and hot corrosion kinetic curves were obtained by thermogravimetric technique. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive x-ray spectrometry were employed to characterize the coatings’ microstructure, surface oxides, and composition. The results showed that both coatings provided the necessary oxidation resistance with oxidation rates of about 1.03 × 10^?2 and 1.36 × 10^?2 mg/cm² h, respectively. The excellent oxidation behavior of these two coatings is attributed to formation of protective (Ni,Co)Cr₂O₄ spinel on the surface, while as-deposited Cr₂O₃ in the coatings also acted as a barrier to diffusion of oxidative and corrosive substances. The greater presence of Co in the CoNiCrAlY–Cr₂O₃ coating restrained internal diffusion of sulfur and slowed down the coating’s degradation. Thus, the CoNiCrAlY–Cr₂O₃ coating was found to be more protective than the NiCoCrAlY–Cr₂O₃ coating under hot corrosion condition.     

Comparison of 10 μm and 20 nm Al-Al2O3 Metal Matrix Composite Coatings Fabricated by Low-Pressure Cold Gas Dynamic Spraying

Cold-gas dynamic spraying (“cold spraying”) was used to deposit aluminum-alumina (Al-Al₂O₃) metal-matrix composite (MMC) coatings onto 6061 Al alloy. The powders consisted of ?45 μm commercially pure Al that was admixed with either 10 μm or agglomerated 20 nm Al₂O₃ in weight fractions of 25, 50, 75, 90, and 95 wt.%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted to determine the microstructure, properties, and the volume fractions of reinforcing particles in the coatings, which was then converted to weight fractions. As the weight fraction of the Al₂O₃ in the coatings increased, the hardness values of the MMC coatings increased. A maximum hardness of 96 ± 10 HV_0.2 was observed for the MMC coating that contained the agglomerated 20 nm Al₂O₃ particles, while a maximum hardness of 85 ± 24 HV_0.2 was observed for the coatings with the 10 μm Al₂O₃ particles. The slight increase in hardness of the coating containing the agglomerated 20 nm Al₂O₃ particles occurred in a coating of Al₂O₃ content that was lower than that in the coating that contained the 10 μm reinforcing Al₂O₃ particles. The increased hardness of the MMC coatings that contained the agglomerated 20 nm Al₂O₃ particles and at lower reinforcing particle content was attributed to the increased spreading of the nanoagglomerated particles in the coating, which increased load-sharing and reinforcement capability of the particles. These results suggest that the use of nanoagglomerated, reinforcing hard-phase particles in cold-sprayed MMC coatings may be a more efficient alternative to the use of conventional micronsized reinforcing particles.     

Characterization and Synthesis of Co(ss)/WSi2-CoWSi Nanocomposite by Mechanical Alloying and Subsequent Sintering

In this study, Co_(ss)/WSi₂-CoWSi nanocomposite was synthesized via mechanical alloying and heat treatment. In order to fabricate bulk composite, 50-h-milled powders were cold pressed and subsequently sintered at 1150 °C for 4 h in Ar atmosphere. Phase development and structural changes were investigated by x-ray diffraction technique and scanning electron microscopy. After various milling times, the powders were investigated by differential thermal analysis and microhardness measurements. The starting powder mixture has two allotropic structures of Co (fcc and hcp). After 10-h milling, an allotropic transformation takes place in Co (fcc to hcp), and a composite microstructure consisting of cold-welded Co, W, and Si phases is formed. After 20 h, new peaks related to WSi₂ appeared in x-ray diffractograms. Increasing milling time to 50 h caused the formation of (Co, W, and Si) solid solution, WSi₂, and CoWSi phases. DTA analysis of 30- and 50-h-milled powders confirmed an increase in the degree of ordering. The 50-h-milled powders exhibited high microhardness value of about 1050 HV_0.1. XRD result of sintered material demonstrated that only ordered Co_(ss)/WSi₂-CoWSi nanostructured composite is present. Consolidated sample showed 12% porosity. Nanoindentation results showed that the sintered composite an exhibited a high hardness of 700 HV_0.1 with an elastic modulus of 107 GPa.     

Alloying Behavior and Properties of FeSiBAlNiCo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High Entropy Alloys Fabricated by Mechanical Alloying and Spark Plasma Sintering

In this paper, FeSiBAlNiCo_x (x = 0.2, 0.8) high-entropy alloy (HEA) powders were fabricated by mechanical alloying process, and the powders milled for 140 h were sintered by spark plasma sintering (SPS) technique. The microstructures and properties of as-milled powders and as-sintered samples were investigated. The results reveal that the final milling products (140 h) of both sample powders present the fully amorphous structure. The increased Co contents obviously enhance the glass forming ability and thermal stability of amorphous HEA powders, which are reflected by the shorter formation time of fully amorphous phase and the higher onset crystallization temperature, respectively. According to coercivity, the as-milled FeSiBAlNiCo_x (x = 0.2, 0.8) powders (140 h) are the semi-hard magnetic materials. FeSiBAlNiCo_0.8 HEA powders possess the highest saturation magnetization and largest remanence ratio. The SPS-ed products of both bulk HEAs are composed of body-centered cubic solid solution, and FeSi and FeB intermetallic phases. They possess the high relative density above 97% and excellent microhardness exceeding 1150 HV. The as-sintered bulks undergo the remarkable increase in saturation magnetization compared with the as-milled state. The SPS-ed FeSiBAlNiCo_0.8 HEA exhibits the soft magnetic properties. The electrochemical corrosion test is carried out in 3.5% NaCl solution. The SPS-ed FeSiBAlNiCo_0.2 HEA reveals the better passivity with low passive current density, and the higher pitting resistance with wide passive region.     

Sintering Behavior of Al/B4C-AlB12-Al2O3 Nanostructure Composite Synthesized by In Situ Processing Method

The aim of this article is to study the effect of time and temperature of sintering on the properties and microstructure of Al/B₄C-AlB₁₂-Al₂O₃ nanocomposite synthesized via in situ processing method. In this method, a mixture of Al-B₂O₃-C, as starting materials, was activated and reacted in a planetary ball mill under Argon atmosphere. The phase analysis, morphology, and particles size were studied by x-ray diffraction and scanning electron microscopy methods. The composite powders were compressed with uniaxial cold press and then were sintered at different temperatures (470-600 °C) for various times (30-90 min). The density, hardness, and compressive strength of sintered samples were investigated. The results revealed that by increasing the sintering temperature and sintering time, both hardness and compaction strength of nanocomposite reach to maximum at 560 °C. The results showed that the sample sintered at 560 °C for 90 min had highest sintered density (≈2590 kg/m³) and highest hardness value of ~88 BHN with compaction strength of ~313 MPa. Nanosize and homogeneous distribution of in situ formed ceramic particles were observed in the synthesized composite.     

Cobalt Ferrite in YSZ for Use as Reactive Material in Solar Thermochemical Water and Carbon Dioxide Splitting, Part I: Material Characterization

The synthesis, characterization, and evaluation of different weight loadings of cobalt ferrite (CoFe₂O₄) in 8 mol.% yttria-stabilized zirconia (8YSZ) via the co-precipitation method are reported. Prepared powders were calcined at 1350°C for 36 h and 1450°C for 4 h in air. These powders were then formed into a porous structure using sacrificial pore formation via oxidation of co-mixed graphite powder. These formed structures obtained were then characterized using thermogravimetric analysis (TGA), x-ray diffraction, high-temperature x-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Brunauer–Emmett–Teller surface area analysis was performed on the most promising of the structures before being subjected to 50 thermal reduction-CO₂ oxidation (redox) cycles using TGA. Together, these results indicate that CoFe₂O₄-8YSZ can provide a lower reduction temperature, maintain syngas production performance from cycle to cycle, and enhance utilization of the reactive material within the inert support in comparison to iron oxide only structures.     

Plasma Spray Physical Vapor Deposition of La1−x Sr x Co y Fe1−y O3−δ Thin-Film Oxygen Transport Membrane on Porous Metallic Supports

Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes.     

Corrosion Resistance of APS- and HVOF-Sprayed Coatings in the Al2O3-TiO2 System

In this article, the results of corrosion investigations performed on thermally sprayed ceramic coatings with different compositions in the Al₂O₃-TiO₂ system (Al₂O₃, Al₂O₃-3%TiO₂, Al₂O₃-40%TiO₂, and TiO _x ) are presented. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high-velocity oxy-fuel (HVOF) spraying processes and characterized by means of optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD). The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄, at room temperature, 60 °C, and 85 °C, as well as in hydrothermal conditions with deionized water at 100 °C and 200 °C. The corrosion stability of the coatings depended on coating characteristics (spraying method, microstructure, and crystalline phase composition) and the corrosive environment (media, test temperature, and duration). In contrast to expectations, APS-sprayed coatings were found to be more corrosion-resistant than the HVOF-sprayed coatings. Addition of TiO₂ to Al₂O₃ increased the corrosion stability, especially for the HVOF-sprayed coatings. In this work, TiO _x coatings were found to be more corrosion-resistant than the Al₂O₃-based coatings.     

Effect of Heat Treatment on the Microstructure and Microhardness of Nanostructural Al2O3 Coatings

Nanostructural Al₂O₃ coatings were formed on a steel substrate surface using a multichamber detonation sprayer. The Al₂O₃ coatings were characterized by a dense microstructure with porosity below 1% and hardness of 1300 ± 25 HV_0.3. The transition layer between the coating and substrate was up to 15 μm thick, containing Fe-Al-type intermetallic compounds (FeAl₃, Fe₂Al₅). Postdeposition heat treatment of the samples at 850 °C for 3 h was carried out in air and argon environments. The effect of heat treatment on the microstructure and microhardness of the Al₂O₃ coatings was investigated by optical microscopy, scanning and transmission electron microscopy, scanning probe microscopy, x-ray phase analysis, and Vickers hardness testing. A positive impact of postcoating heat treatment on the coating microstructure and microhardness was observed. Heat treatment resulted in an increase in the coating hardness from 1300, to 1350 ± 25 HV_0.3 and 1600 ± 25 HV_0.3 after annealing in air and argon, respectively. Heat treatment in argon led to a more significant increase in the α-Al₂O₃ phase from 47 to 81%.     

Effects of Plasma-spraying Powers on Microstructure and Microhardness of In-Situ Nanostructured FeAl2O4 Composite Coatings

In-situ nanostructured FeAl₂O₄ composite coatings were prepared using plasma spraying of Al/Fe₂O₃ composite powders applying different spraying powers. The effects of plasma-spraying powers on microstructure and property of FeAl₂O₄ composite coatings were investigated. The results indicated the composite coatings had the microstructure with thin lamellar splats rich in FeAl₂O₄ as matrix, and dispersed granules rich in Fe and thin lamellar splats rich in Al₂O₃ as second phases. The reaction degree of Al/Fe₂O₃ composite powders increased while applying spraying power of 25-30 kW and then decreased while applying spraying power of 30-40 kW, which first resulted in the increase and then in the decrease of the Al₂O₃ content. The coating prepared by applying spraying power of 30 kW had the maximum microhardness, which was attributed to the maximum Al₂O₃ content present in the coating and the most uniform microstructure of the coating.     

Deposition of Composite LSCF-SDC and SSC-SDC Cathodes by Axial-Injection Plasma Spraying

The performance of solid oxide fuel cell cathodes can be improved by increasing the number of electrochemical reaction sites, by controlling microstructures, or by using composite materials that consist of an ionic conductor and a mixed ionic and electronic conductor. LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3???) and SSC (Sm_0.5Sr_0.5CoO₃) cathodes were manufactured by axial-injection atmospheric plasma spraying, and composite cathodes were fabricated by mixing SDC (Ce_0.8Sm_0.2O_1.9) into the feedstock powders. The plasma power was varied by changing the proportion of nitrogen in the plasma gas. The microstructures of cathodes produced with different plasma powers were characterized by scanning electron microscopy and gas permeation measurements. The deposition efficiencies of these cathodes were calculated based on the mass of the sprayed cathode. Particle surface temperatures were measured in-flight to enhance understanding of the relationship between spray parameters, microstructure, and deposition efficiency.     

Microstructure and High-Temperature Oxidation Behavior of Cold Gas-sprayed Ni-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

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.     

Degradation of Thermal Barrier Coatings by Fuel Impurities and CMAS: Thermochemical Interactions and Mitigation Approaches

Degradation of free-standing yttria-stabilized zirconia (YSZ) and CoNiCrAlY coatings (300 μm) due to V₂O₅ and a laboratory-synthesized CMAS was investigated at temperatures up to 1400 °C. Reactions, phase transformations, and microstructural development in coatings were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The molten deposits destabilized the YSZ and reacted with the thermally grown oxide with various phase transformations and reaction product formation. A dense, continuous environmental barrier overlay, based on oxides, applied by electrophoretic deposition was effective in mitigating the molten deposit attack. Enriching CMAS composition with Al promoted the crystallization of anorthite platelets and MgAl₂O₄ spinel, and mitigated CMAS ingression. EPD MgO overlay was also effective in protection against V₂O₅ melt by formation of magnesium vanadates. EPD alumina overlay deposited on thermal barrier coatings with APS 8YSZ and bond-coated IN939 superalloy retained its adhesion and structural integrity after prolonged furnace thermal cycle test at 1100 °C.     

Lu2O3-SiO2-ZrO2 Coatings for Environmental Barrier Application by Solution Precursor Plasma Spraying and Influence of Precursor Chemistry

As environmental barrier coatings are subjected to thermal stress in gas turbine engines, the introduction of a secondary phase as zircon (ZrSiO₄) is likely to increase the stress resistance of Lu₂Si₂O₇ coatings generated by induction plasma spraying using liquid precursors. In a first step, precursor chemistry effect is investigated by the synthesis of ZrO₂-SiO₂ nanopowders by induction plasma nanopowder synthesis technique. Tetraethyl orthosilicate (TEOS) as silicon precursor and zirconium oxynitrate and zirconium ethoxide as zirconium precursors are mixed in ethanol and produce a mixture of tetragonal zirconia and amorphous silica nanoparticles. The use of zirconium ethoxide precursor results in zirconia particles with diameter below 50 nm because of exothermic thermal decomposition of the ethoxide and its high boiling point with respect to solvent, while larger particles are formed when zirconium oxynitrate is employed. The formation temperature of zircon from zirconia and silica oxides is found at 1425 °C. Second, coatings are synthesized in Lu₂O₃-ZrO₂-SiO₂ system. After heat treatment, the doping effect of lutetium on zirconia grains totally inhibits the zircon formation. Dense coatings are obtained with the use of zirconium ethoxide because denser particles with a homogeneous diameter distribution constitute the coating.