Electrical Properties of Atmospheric Plasma-Sprayed La10(SiO4)6O3 Electrolyte Coatings

In this present work, La₁₀(SiO₄)₆O₃ as a promising electrolyte candidate for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been synthesized and its electrical property was investigated as a function of temperature. In order to improve the density and oxide ion conductivity of La₁₀(SiO₄)₆O₃, the feedstock powder was prepared by sintering the oxide mixture powders at proper sintering temperatures and times. The hexagonal apatite-type ceramic coatings with a typical composition of La₁₀(SiO₄)₆O₃ were deposited by atmospheric plasma spraying (APS) with different hydrogen flow rates. With increasing hydrogen flow rate oxide ion conductivity successively decreases. The highest ionic conductivity of the dense composite electrolyte coatings reaches a value of 2.4 mS/cm at 900 °C in air, which is comparable to other apatite-type lanthanum silicate (ATLS) conductors.     

Preparation and Thermophysical Properties of La2Zr2O7 Coatings by Thermal Spraying of an Amorphous Precursor

Free-standing La₂Zr₂O₇ coatings were obtained by plasma spraying, using an amorphous La-O-Zr precursor as the feedstock. The La-O-Zr precursor powder was prepared by coprecipitation. During thermal spraying, the formation of coatings can be regarded as a joint process of melting-solidification, thermal decomposition, and crystallization. The time required for crystal growth was significantly shortened during spraying. Consequently, the average grain size of coatings was approximately 200 nm, with a narrow distribution (100-500 nm). Coatings prepared by this method show better thermophysical properties than those prepared with crystalline La₂Zr₂O₇ powder as the feedstock. The thermal conductivity of the as-sprayed coating was approximately 0.36-0.47 W/m K and the average coefficient of thermal expansion (CTE) is 11.1 × 10^?6/K.     

Phase Stability of Ce-Modified La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Coatings and Chemical Compatibility with YSZ

Ce-modified La₂Zr₂O₇ powders, i.e., La₂Zr₂O₇ (LZ), La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3), and La₂(Zr_0.3Ce_0.7)₂O₇ (LZ3C7), were used to produce thermal barrier coatings by atmospheric plasma spray process. The chemical compatibility of the CeO₂-doped La₂Zr₂O₇ with the traditional YSZ was investigated in LZ-YSZ powder mixtures and LZ-YSZ bilayer coatings by x-ray diffraction and scanning electron microscope. The powder mixtures and coatings were aged at 1200 and 1300 °C for 100 h. The results showed that LZ and LZ7C3 presented single pyrochlore structure after the heat treatments at both 1200 and 1300 °C. For LZ3C7, however, fluorite structure was observed at 1300 °C, indicating a poor phase stability of LZ3C7 at the elevated temperature. The results further showed that La₂(Zr_0.3Ce_0.7)₂O₇ reacted with YSZ in the bilayer ceramic coatings due to the diffusion of cerium, zirconium, and yttrium. While for La₂Zr₂O₇(LZ) and La₂(Zr_0.7Ce_0.3)₂O₇, a better chemical compatibility with YSZ was shown.     

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.     

Plasma spraying of lanthanum silicate electrolytes for intermediate temperature solid oxide fuel cells (ITSOFCs)

A new challenge in the field of solid oxide fuel cells (SOFCs) concerns reducing their operating temperature to 973 K. Apatite ceramics are interesting candidates for SOFC electrolytes due to their high ionic conductivity at this temperature. The present work reports on the fabrication and characterization of La₉SrSi₆O_26.5 coatings obtained by atmospheric plasma spraying with two different plasma spray powers. The microstructure and the composition of the as-sprayed and heat-treated coatings were investigated by several techniques including X-Ray Diffraction, Inductively Coupled Plasma-Atomic Emission Spectroscopy and Scanning Electron Microscopy. The open porosity of the coatings was evaluated by the Archimedean method. It was found that the as-sprayed apatite coatings were composed of an amorphous phase as well as of a crystalline apatite phase, and that they contained chemical heterogeneities resulting from Si volatilization in the high-temperature plasma. Furthermore, a heat treatment rendered it possible to obtain denser, fully crystallized apatite coatings. Ionic conductivity measurements carried out with impedance spectroscopy demonstrated that the conductivity of the apatite coatings - depending on the spraying conditions - increased with sintering.     

Study of plasma- and detonation gun-sprayed alumina coatings using taguchi experimental design

Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al₂O₃) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics. The present study is aimed at comparing the characteristics of Al₂O₃ coatings deposited using the above techniques by using Taguchi experimental design. Alumina coating experiments were conducted using a Taguchi fractional-factorial (L₈) design parametric study to optimize the spray process parameters for both APS and D-gun. The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes. The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear. The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed. Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis.     

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.     

Microstructure and Thermal Properties of Nanostructured 4?wt.%Al2O3-YSZ Coatings Produced by Atmospheric Plasma Spraying

Microstructure and phase composition of the nanostructured Al₂O₃ doped YSZ coatings by atmospheric plasma spraying method have been characterized with XRD, TEM and SEM. The nanostructured 4AlYSZ coatings consist mainly of t-ZrO₂, crystalline Al₂O₃ phase is absent in the coatings and the grain size of the 4AlYSZ coating is about 65 nm. The APS 4AlYSZ coating is characterized by nanozones, dense area and voids. After doping, the coefficient of thermal expansion of YSZ is decreased to 10.928 × 10⁻⁶/K. The addition of Al₂O₃ has a great influence on decreasing the thermal conductivity of nano-YSZ, which is mainly caused by the point defect scattering and grain-boundary scattering. The lifetime of nanostructured 4AlYSZ coating is about 1000 cycles at 1100 °C.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nano-Particles on Corrosion Performance of Plasma Electrolytic Oxidation Coatings Formed on 6061 Aluminum Alloy

Corrosion resistance improvement of plasma electrolyte oxidation coatings on 6061 aluminum alloy in silicate electrolyte containing Al₂O₃ nano-particles was studied, with particular emphasis on the microstructure, coating growth, and corrosion behavior in 3.5 wt.% NaCl solution. The microstructure of coatings, their thickness, and phase composition were characterized using scanning electron microscopy and x-ray diffraction. All characterization data showed that the maximum coating thickness and lowest amount of porosity were obtained in a low concentration of KOH, a high concentration of Na₂SiO₃, and moderate concentration of Al₂O₃ nano-particles in the electrolyte. This combination describes the optimum plasma electrolytic oxidation electrolyte, which has the best conductivity and oxidizing state, as well as the highest incorporation of electrolyte components in the coating growth process. On the other hand, incorporation and co-deposition of Al₂O₃ nano-particles were more pronounced than SiO₃ ^2? ions in some level of molar concentration, which is due to the higher impact of electron discharge force on the adsorption of Al₂O₃ nano-particles. The electrochemical results showed that the best protective behavior was obtained in the sample having a coat with the lowest porosity and highest thickness.     

Air Plasma-Sprayed Yttria and Yttria-Stabilized Zirconia Thermal Barrier Coatings Subjected to Calcium-Magnesium-Alumino-Silicate (CMAS)

Yttria (Y₂O₃) and zirconia (ZrO₂) stabilized by 8 and 20 wt.%Y₂O₃ thermal barrier coatings (TBCs) subjected to calcium-magnesium-alumino-silicate (CMAS) have been investigated. Free-standing Y₂O₃, 8 and 20 wt.%YSZ coatings covered with synthetic CMAS slurry were heated at 1300 °C in air for 24 h in order to assess the effect of Y₂O₃ on the corrosion resistance of the coatings subjected to CMAS. The microstructures and phase compositions of the coatings were characterized by SEM, EDS, XRD, RS, and TEM. TBCs with higher Y₂O₃ content exhibited better CMAS corrosion resistance. Phase transformation of ZrO₂ from tetragonal (t) to monoclinic (m) occurred during the interaction of 8YSZ TBCs and CMAS, due to the depletion of Y₂O₃ in the coating. Some amounts of original c-ZrO₂ still survived in 20YSZ TBCs along with a small amount of m-ZrO₂ that appeared after reaction with CMAS. Furthermore, Y₂O₃ coating was found to be particularly highly effective in resisting the penetration of molten CMAS glass at high temperature (1300 °C). This may be ascribed to the formation of sealing layers composed of Y-apatite phase [based on Ca₄Y₆ (SiO₄)₆O and Y_4.67(SiO₄)₃O] by the high-temperature chemical interactions of Y₂O₃ coating and CMAS glass.     

Reinvestigation of the Phase Equilibria in the La2O3-P2O5 System

The phase equilibria between the solid phases in the La₂O₃-P₂O₅ system were reinvestigated with solid state reaction method. The existence of La₃PO₇, La₇P₃O₁₈, LaPO₄, and LaP₃O₉ was confirmed. La₇P₃O₁₈ can form only at ~1200 °C or higher possibly due to kinetic reasons, but it is stable at lower temperatures. On the other hand, three intermediate compounds proposed to exist, La₅PO₁₀, La₄(P₂O₇)₃, and La₂P₄O₁₃ were not obtained in this study. Based on the literature, La₄(P₂O₇)₃ and La₂P₄O₁₃ are considered to be metastable phases, although thermodynamic evidence is still required.     

Properties of alumina-based coatings deposited by plasma spray and detonation gun spray processes

Alumina, Al₂O₃ + 3 to 40 wt% TiO₂, and Al₂O₃ + 40 wt% ZrO₂ coatings were deposited by atmospheric plasma spraying (APS) and detonation gun spraying (DGS). The coatings were evaluated by optical microscopy, microhardness measurements, and X- ray diffraction. Wear resistance of the coatings was evaluated by rubber wheel sand abrasion and particle erosion test methods. Detonation gun- sprayed coatings exhibited more homogeneous microstructures and somewhat higher microhardness than corresponding plasma- sprayed coatings. Small additions of TiO₂ (3 wt%) improved both the abrasion and erosion wear resistance, whereas 40 wt% TiO₂ significantly decreased the erosion wear resistance of both APS and DGS coatings. Alumina + 40% ZrO₂ coatings exhibited the best abrasion wear resistance of both APS and DGS coatings, but the erosion wear resistance of these coatings was lower than that of the Al₂O₃ and Al₂O₃ + 3 wt% TiO₂ coatings. The best abrasion wear resistance of the coatings studied was obtained with DGS Al₂O₃ + 40 wt% ZrO₂ and Al₂O₃ + 3 to 40 wt% TiCh coatings. These coatings exhibited lower wear rates than bulk Al₂O₃. The best erosion wear resistance was obtained with the DGS Al₂O₃ + 3 wt% TiO₂ coating; however, it exhibited a higher wear rate than bulk Al₂O₃. In general, detonation gun- sprayed coatings showed significantly enhanced abrasion and erosion wear resistance than the corresponding plasma- sprayed coatings.     

Improving Atmospheric Plasma Spraying of Zirconate Thermal Barrier Coatings Based on Particle Diagnostics

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of La₂Zr₂O₇ a considerable amount of La₂O₃ can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La₂O₃-ZrO₂ system, in the composition range of pyrochlore structure, the stoichiometric La₂Zr₂O₇ has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro?-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (Gd₂Zr₂O₇) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance.     

Mechanical Properties of Layered La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Thermal Barrier Coatings

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La₂Zr₂O₇ and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La₂Zr₂O₇ + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La₂Zr₂O₇ TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La₂Zr₂O₇. The erosion rate of La₂Zr₂O₇ TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La₂Zr₂O₇. DCL porous 8YSZ + La₂Zr₂O₇ had a lower erosion rate than other SCL and DCL La₂Zr₂O₇ coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.     

Atmospheric Plasma Spraying of High Melting Temperature Complex Perovskites for TBC Application

High melting materials have always been very attractive candidates for materials development in thermal barrier coating (TBC) applications. Among these materials, complex perovskites with Ba(Mg_1/3Ta_2/3)O₃ and La(Al_1/4Mg_1/2T_1/4)O₃ compositions have been developed and deposited in TBC systems by atmospheric plasma spraying. Spray parameters were optimized and in-flight particle temperatures were recorded using Accuraspray-g3 and DPV 2000. Plasma sprayed coatings were found to undergo non-stoichiometric decomposition of components which could have contributed to early failure of the coatings. Particle temperature diagnostics suggest that gun power of ~15 kW or lower where majority of the particles have already solidified upon impact to the substrate could probably prevent the decomposition of phases. Additionally, it has been found that the morphology of the powder feedstock plays a critical role during atmospheric plasma spraying of complex perovskites.     

Application of Plasma-Sprayed Complex Perovskites as Thermal Barrier Coatings

In an effort to improve the performance of heat engines at high temperatures, advanced surface coatings have been developed from complex perovskites. Materials of Ba(Mg_1/3Ta_2/3)O₃ and La(Al_1/4Mg_1/2Ta_1/4)O₃ composition were synthesized and applied as ceramic topcoats of thermal barrier coating (TBC) systems by atmospheric plasma spraying (APS) in single layer and in double-layer combination with conventional yttria stabilized zirconia (YSZ). Microstructural and phase analyses reveal that plasma spraying of complex perovskites is accompanied with the formation of vertical crack networks and secondary oxide phases which influence the failure mechanism of the TBCs. The low value of fracture toughness for the complex perovskites and the thermally grown oxide at the topcoat-bondcoat interface of the TBCs are, however, the major factors which lead to the coating failure on thermal cycling at about 1250 °C.     

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.     

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.     

Mechanical and Thermal Transport Properties of Suspension Thermal-Sprayed Alumina-Zirconia Composite Coatings

Micro-laminates and nanocomposites of Al₂O₃ and ZrO₂ can potentially exhibit higher hardness and fracture toughness and lower thermal conductivity than alumina or zirconia alone. The potential of these improvements for abrasion protection and thermal barrier coatings is generating considerable interest in developing techniques for producing these functional coatings with optimized microstructures. Al₂O₃-ZrO₂ composite coatings were deposited by suspension thermal spraying (APS and HVOF) of submicron feedstock powders. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to unique and novel fine-scaled microstructures. The suspensions were injected internally using a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to highly alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states upon their impact. Mechanical and thermal transport properties of the coatings were compared. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1125 VHN_{3 N}, as well as the highest abrasion wear resistance (following ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (following ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia-alloyed matrix. This microstructure also exhibited the lowest thermal diffusivity, which is explained by the amorphous phase content and limited particle bonding, generating local thermal resistances within the structure.     

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.