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Plasma spraying, an important coating and film preparation technology, provides a crucial method for depositing ceramic-based interconnects on tubular solid oxide fuel cells. However, traditional plasma-sprayed ceramic coatings exhibit typical lamellar porous structures with numerous unbound interfaces and gas-permeable channels, which makes it difficult to meet the microstructural requirements to prevent the leakage of fuel gas and oxidizing gas. This leads to the lower conductivity of the coating compared to the sintered bulk, and results in increased ohmic resistance and reduced output performance. To improve the interface bonding and conductivity of plasma-sprayed La0.2Sr0.8MnO3-La0.3Sr0.7TiO3 (LSM-LST) bilayer interconnects for tubular cells, a Co3O4 healing additive was added to the gap interfaces within the LST and LSM coatings. Results showed that metallurgical healing occurred at unbound interfaces and microcracks due to the liquid-phase sintering mechanism, and a bulk-like dense microstructure was obtained at a lower temperature (1200 °C) compared to the dense sintering temperature of the bulk. Moreover, the gas leakage rates of the stable coatings after interface healing were > 1 order of magnitude lower than that of the as-sprayed coatings. Additionally, their electrical conductivity was more than twice that of the as-sprayed coatings, which meets the microstructural and performance requirements of tubular cell interconnects. [ABSTRACT FROM AUTHOR]

Thermal spray metallic coatings usually present a lamellar structure with limited lamellar interface bonding due to inevitable oxidation involved. Such microstructure not only degrades the mechanical performances of the coatings significantly but also cannot provide effective corrosion protection to the substrate. In the present paper, the recent progresses to generate oxide-free molten metallic droplets by air plasma spraying will be summarized toward to deposition of highly dense metal coatings. It is revealed that by designing the metallic spray powders containing deoxidizers such as boron or carbon based on thermodynamics theory of oxidation, the oxide-free in-flight molten metal droplets can be created in open atmosphere during air plasma spraying. The thermodynamics and kinetics for the in-flight deoxidization are presented for the deoxidizers of boron and carbon. The tests with Ni-based and Cu-based alloy coatings using boron as deoxidizer showed that the oxygen content in the coatings can be reduced to less than 0.6 wt.%. It was demonstrated with NiCrAlY, NiAl and FeAl alloys that contain element Al the coatings with low oxide contents can be deposited by APS using carbon as deoxidizer. The post-impact oxidation is mainly responsible for the introduction of oxide inclusions in these coatings. Besides deoxidizer addition, ultra-high temperature is the other necessary condition for the generation of oxide-free molten droplets. It was revealed that a minimum deoxidizer content is necessary to maintain continuous oxidation protection of alloying elements along with rapid mass transfer mechanism within molten metal droplets during the whole spray distance. [ABSTRACT FROM AUTHOR]

The grain structure evolution and nanomechanical behavior of cold-sprayed Al coating on Ti substrate with friction stir spot processing (FSSP) were studied by the electron backscatter diffraction and nanoindentation methods. The low-angle boundaries (LAGBs) fraction and the density of the geometrically necessary dislocations (GNDs) decreased from the base zone (BZ) to the stir zone (SZ). The average grain size, the LAGBs fraction and the density of the GNDs were various in different locations of the SZ, which can be attributed to the variety of local shear strain and temperature gradient during FSSP. The B/ B - component, the C component, and the A*1/ A*2 component were mainly developed in the SZ. The highest intensity of the B/ B - component appeared in the 3/8D of the SZ, indicating that the plasticized materials flowed downward experienced the highest shear strain. The materials in the heat affected zone (HAZ) underwent static recrystallization, while the continuous dynamic recrystallization (CDRX) and the geometric dynamic recrystallization (GDRX) occurred in the thermo-mechanically affected zone (TMAZ) and SZ. The nano-hardness and elastic modulus of the cold-sprayed Al coating after FSSP were comparable to those of pure Al bulk. The grain size and dislocation density were the main factors affecting the nano-hardness in the SZ. [ABSTRACT FROM AUTHOR]

304 stainless steel (304SS) powder and novel Mo-cladded 304SS (304SS-Mo) powder were used as feedstocks to prepare metallic coatings with different inter-splat bonding qualities. Wear test was conducted to examine the dependence of wear behavior on the inter-splat bonding quality. The results showed that poor inter-splat bonding leads to much lower wear resistance in conventional 304SS coating relative to that of 304SS bulk, especially at high loading conditions (>75 N), where the wear rate increased to 5.53 × 10−3 mm3/(m⋅N) by 3.5 times higher than that at low-load range. However, the novel 304SS-Mo coating with metallurgical inter-splat bonding and minimized oxide inclusions exhibited a low wear rate comparable to that of 304SS bulk. Failure analysis of worn samples suggests that splat delamination contributes to low wear resistance of the 304SS coating; conversely, the absence of splat delamination results in higher wear resistance of the 304SS-Mo coating. Using Mo-cladded powders, significantly enhanced inter-splat bonding enables the use of plasma-sprayed metallic coating under high-load wear conditions. The strong dependence of wear resistance on the load in conventional coating implies that evaluation of wear performance of thermally sprayed metallic coating should consider both the wear rate and critical load. [ABSTRACT FROM AUTHOR]

A dense and continuous Cr coating with a thickness of approximately 140 μm was successfully deposited on the surface of a thin-walled Zr alloy cladding tube using high-speed laser cladding technology in this study. The microstructure, phase composition, microhardness, and resistance to high-temperature oxidation of the coating were investigated. The experimental results showed that the Cr coating exhibited high-strength metallurgical bond with the Zr alloy substrate, forming a narrow heat-affected zone with a thickness of 25 μm, and the coating consists of ZrCr2 and α-Zr phase. The average microhardness of the coating was 589 HV0.05, about 2.3 times that of the substrate. After oxidation at 1200 °C for 1200 s in air, with the formation of complete and dense protective Cr2O3 scale, the Cr-coated Zr alloy cladding tube showed better high-temperature oxidation resistance than uncoated tube. [ABSTRACT FROM AUTHOR]

In this study, cold spraying was introduced to produce an Al deposit on a Cu substrate, which was then modified through a solid-state method named friction-stir-processing (FSP). The pores and inter-particle interfaces within the Al deposit completely disappeared after FSP at traverse speeds less than 18 mm/min. Equiaxed grains containing a low density of dislocations occurred in the Al deposit after FSP. The grain size increased, while the dislocation density decreased with decreasing traverse speed. Meanwhile, three intermetallic compound (IMC) layers including Al2Cu, AlCu, and Al4Cu9 were developed at the Al/Cu interface after FSP. The thickness of the entire IMC layers and the continuity of the AlCu layer increased as the traverse speed was reduced. Due to the generation of the IMC layers, the adhesion strength of the deposit dramatically improved after FSP. The variation of the adhesion strength after FSP at different traverse speeds was associated with the continuity of the AlCu layer. The electrical conductivity of the Al deposit was remarkably enhanced after FSP due to the disappearance of pores and inter-particle interfaces. Furthermore, the changes in the electrical conductivity of the Al deposit and Al-Cu bimetallic structure after FSP at different traverse speeds were related to the number of grain boundaries and the thickness of IMC layers, respectively. [ABSTRACT FROM AUTHOR]

Stabilized bismuth oxide material with fluorite structure (δ-Bi2O3) has been studied as a promising electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high oxygen ion conductivity in mediate temperature. Especially, the ternary system Bi2O3-Er2O3-WO3 is widely concerned for its high ionic conductivity and thermal stability. In this study, regarding its low melting point, the possibility to deposit dense Bi2O3-Er2O3-WO3 ((Bi2O3)0.705 (Er2O3)0.245 (WO3)0.05, EWSB) electrolyte by plasma spraying was examined. It was confirmed that the sintered EWSB bulk presents a high ion conductivity of 0.34 S cm−1 at 750 °C and excellent stability that indicates no structure transformation and conductivity degradation after annealing at 600 °C for 1000 h. The phase structure and cross-sectional microstructure of plasma-sprayed EWSB were characterized by XRD and SEM. Results showed that the as-plasma-sprayed EWSB presents a dense microstructure with well bonded lamellae. The XRD showed the formation of EWSB with δ-phase and a trace of β-phase, while the β-phase disappeared after annealing at 750 °C for 10 h. The deposited EWSB electrolyte presented the excellent ionic conductivity of 0.26 S cm−1 at 750 °C which can be directly applied to SOFC at intermediate temperature. [ABSTRACT FROM AUTHOR]

Magnesium-based alloys are appropriate materials for transportation applications owing to their high strength-to-weight ratio. However, the poor corrosion performance of Mg alloy limits its lifetime. In this study, the pure Al coating was deposited onto an AZ91D substrate by cold spraying, and then modified by friction stir-spot-processing. The interfacial microstructure and phase were studied in detail, and the effect of the intermetallic compounds on the microhardness and corrosion properties of the coatings was investigated. Results show that the sound and dense coatings were obtained after friction stir-spot-processing at 2400 and 2700 rpm. Al12Mg17 was found in the upper part of the 1/2 radius zone in the processed coating. The thin and thick IMCs were identified at the interface in the center and 1/2 radius areas, respectively. In the thick IMCs, mainly two types of multi-phase coexistence structures were observed. The four-phase coexistence structure dominated by Al3Mg2 revealed a higher microhardness than the three-phase coexistence structure dominated by Al12Mg17. In the processed coating, the upper part of the 1/2 radius zone containing Al12Mg17 exhibited a higher microhardness than the center zone. Moreover, the cold-sprayed Al coating provided more effective corrosion protection for the AZ91D substrate after FSSP. [ABSTRACT FROM AUTHOR]

This study adopts a novel structure design of large-area planar metal-supported solid oxide fuel cell (MS-SOFC), which exhibits the characteristics of self-sealing and high thermal cycling resistance. The self-sealing structure of MS-SOFCs is achieved by brazing technology. Plasma spraying is performed to prepare all functional layers of cells. The size of cells is 10 × 10 cm2. The coefficients of thermal expansion of relevant parts (i.e., porous metal support, interconnector and compound structure of both) and the electrical conductivity of brazing solder were measured. The microstructure of brazing area was observed in situ at 800 °C for 500 h, and the distribution of fuel gas in the interconnector was simulated. The performance of cells was characterized and found that the maximum power density can reach up to 716 mW cm−2 at 700 °C. Moreover, the long-term stability of the cell was investigated for about 500 h with 6 times of thermal cycling implemented during this period. The open circuit voltage and the maximum power density of the cell showed a good stability. [ABSTRACT FROM AUTHOR]

Magnesium is a suitable material for structural and aerospace applications owing to its high strength to weight ratio, yet the wide usage of Mg alloys is restricted due to their poor corrosion and mechanical properties. In this study, cold gas spraying method was used to produce pure Al, Al-Al2O3 and Al-Al2O3-CNTs coatings with feedstock powder containing Al admixed with 5 wt.% Al2O3 and 1 wt.% CNTs to improve electrochemical and tribological properties of Mg-LA43M alloy. X-ray diffraction spectroscopy and scanning electron microscopy were performed for compositional and microstructural analysis of feedstock powders as well as coatings. Furthermore, corrosion properties of coatings were examined by photodynamic polarization, electrochemical spectroscopy and long-term immersion test in 3.5% NaCl solution. The obtained results were correlated with microstructural analysis and we found that reinforcement of hard particles, especially CNTs, resulted in better corrosion resistance of MMC coatings as compared to Mg substrate and pure Al coatings. Reinforcements of Al2O3 and CNTs also improved the sliding wear resistance of the coatings. Al-Al2O3-CNTs exhibited lower coefficient of friction and wear rate than the Mg substrate, pure Al, and Al-Al2O3 coatings due to the denser microstructure. [ABSTRACT FROM AUTHOR]

In this study, a composite coating with outstanding corrosion and wear resistance was successfully produced on LA43M substrates. The composite coating was composed of cold-sprayed aluminum, anodized Al2O3 and a SiO2 sealing coating, which was defined as the MAC composite coating. The properties of the MAC composite coating were characterized by water contact angle, potentiodynamic polarization, salt spray corrosion test, electrochemical impedance spectroscopy and ball-on-disk test, while the microstructure was investigated by scanning electron microscope. The wear results show that the double ceramic composite coating improves the wear resistance compared to the LA43M substrate. After sealing with polysilazane, the surface of the MAC composite coating became hydrophobic. Thus, the corrosion current density dropped to 5.4 × 10−9 A cm−2, which is by four orders of magnitude lower compared to that of the LA43M alloy. Furthermore, there was no significant change in surface roughness and profile after long-term corrosion test, demonstrating the outstanding corrosion resistance of the MAC composite coating. In addition, the MAC composite coatings presented the highest impedance loop, impedance moduli and the phase angles, hence confirming that the MAC composite coatings provided a high degree of corrosion resistance for LA43M alloy. [ABSTRACT FROM AUTHOR]

Compared with ceramic materials, the fabrication of dense metal films requires higher impact velocity in vacuum cold spraying (VCS), also known as aerosol deposition method. In this study, a supersonic nozzle for the fabrication of dense, thin and narrow copper lines was designed. The acceleration behavior of gas and copper particles was investigated through CFD numerical simulation. And the impact behavior of copper particles was studied through finite element analysis. The copper lines with the width of about 200 μm and the height of about 4 μm were directly fabricated on silicon wafers at room temperature without masking. The results show that there is an optimum particle diameter for the impact velocity in particle collision deposition systems. In order to obtain a higher particle impact velocity in VCS, the substrate should be placed behind the high-pressure region of gas shock wave, so that the position of the high-pressure region of the free jet and the position of the bow shock with the substrate coincide as much as possible. Copper particles undergo plastic deformation and particle flattening upon impact and subsequent particle compaction. The width of copper line increased with increasing standoff distance, and maximum height decreased with increasing standoff distance. [ABSTRACT FROM AUTHOR]

Rare earth element-doped bismuth oxides with the fluorite structure (δ-Bi2O3) exhibit high oxygen ion conductivity at low temperature, which is promising electrolyte materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, traditional co-sintering process is not applicable to the manufacturing of IT-SOFCs using low melting point Bi2O3-based electrolyte, while further high-temperature processing is not required for deposition Bi2O3-based electrolytes. In this study, plasma spraying was used to examine the possibility to deposit high-performance Bi2O3-based electrolytes without the following high-temperature process. (Bi2O3)0.75 (Y2O3)0.25 (YSB) spray powders were prepared by the sinter-crushing method. The YSB electrolytes were fabricated by plasma spraying at different deposition temperatures. The effects of deposition temperature on the coating microstructure, crystalline stability, and ion conductivity were investigated. Results showed that the as-sprayed YSB electrolytes present a dense microstructure with well-bonded lamellar interfaces. The pure δ-phase YSB electrolyte was deposited with 37.5-75 μm powders at a deposition temperature of 350 °C. The deposited YSB electrolyte presented the excellent ionic conductivity of 0.19 S cm−1 at 700 °C in comparison with 0.21 S cm−1 for sintered bulk. [ABSTRACT FROM AUTHOR]

A novel self-sealing structure for metal-supported solid oxide fuel cells (MS-SOFCs) is designed by applying brazing technology between the metal support and interconnector to solve the sealing problem on the anode side of planar SOFCs. A high-reliability self-sealing effect is thus realized at the anode side of the MS-SOFC. Plasma spraying technology is used to prepare cell functional layers including the anode, cathode, and electrolyte. A single cell is assembled with a 50–60-μm plasma-sprayed Sc2O3-stabilized ZrO2 (ScSZ) electrolyte layer. The gas permeability of the self-sealed MS-SOFC without a cathode layer is 0.42 × 10−17 m2. The open-circuit voltage of the cell is ~ 1.1 V in the operating temperature range from 550 to 750 °C. The power density of the cell reaches 1109 mW cm−2 at 750 °C under standard atmosphere. In addition, the use of a fuel gas pressure that is 20 kPa higher than the cathodic gas pressure results in a significant increase in the power density to 1782 mW cm−2 at 750 °C. The novel cell structure and gas tightness of the ScSZ electrolyte prepared by plasma spraying indicates that it can meet the requirements of SOFC applications. [ABSTRACT FROM AUTHOR]

This study deals with the improvement of the wear resistance of aluminum alloys by metal matrix composites (MMCs). The latter were fabricated by implanting high-speed solid particles into the metal surfaces. For that, stainless steel and Fe-based amorphous alloy particles were accelerated to the substrates using high-pressure nitrogen. The effect of multi-particle implantation, particle material properties and kinetic energy at impact, and pre-heating treatment of the substrate on particle implantation was investigated using numerical simulation. In addition, the effect of particle size on the MMCs microstructure, wear resistance, strengthening mechanism, and relative hardness was studied. The results showed that the method simultaneously achieved shot peening and metal matrix composite strengthening, that is, resulted in a double-strengthening effect. Furthermore, high-speed particle implantation effectively improved the wear resistance of the substrate: The wear volume of Fe-based amorphous alloy/Al MMCs was 5% of the untreated aluminum substrate and that of stainless steel/Al MMCs 14–44%. It is believed that laser-assisted particle implantation can be used to efficiently increase the thickness and surface properties of MMCs. [ABSTRACT FROM AUTHOR]

A particular YSZ feedstock for very low-pressure plasma spraying (VLPPS) has been designed which can be automatically divided into less than 5 μm YSZ molten particles in the long plasma jet. Fully molten particles were deposited on the substrate at different deposition distances of 250, 350 and 450 mm, respectively. The deposition behavior of less than 5 μm YSZ molten particles were studied aiming to obtain a thin gastight YSZ coating. The flattening ratio of particles at different deposition distances and the gas permeability of YSZ coating prepared by VLPPS were investigated. The results revealed that the thickness of flattened particles was about 0.10-0.35 μm and the flattening ratio of molten particles was about 4.7. The flattened particles were bonded well with substrates and the width of vertical cracks appeared in flattened particles was 0.01-0.02 μm. The gas permeability of coatings prepared at 350 mm was 1.5 × 10−7 cm4 (gf)−1 s−1. [ABSTRACT FROM AUTHOR]

Plasma spray–physical vapor deposition (PS–PVD) is a unique technology that enables highly tailorable functional films and coatings with various rare metal elements to be processed. This technology bridges the gap between conventional thermal spray and vapor deposition and provides a variety of coating microstructures composed of vapor, liquid, and solid deposition units. The PS–PVD technique serves a broad range of applications in the fields of thermal barrier coatings (TBCs), environmental barrier coatings (EBCs), oxygen permeable films, and electrode films. It also represents the development direction of high-performance TBC/EBC preparation technologies. With the PS–PVD technique, the composition of the deposition unit determines the microstructure of the coating and its performance. When coating materials are injected into a nozzle and transported into the plasma jet, the deposition unit generated by a coating material is affected by the plasma jet characteristics. However, there is no direct in situ measurement method of material transfer and deposition processes in the PS–PVD plasma jet, because of the extreme conditions of PS–PVD such as a low operating pressure of ~ 100 Pa, temperatures of thousands of degrees, and a thin and high-velocity jet. Despite the difficulties, the transport and transformation behaviors of the deposition units were also researched by optical emission spectroscopy, observation of the coating microstructure and other methods. This paper reviews the progress of PS–PVD technologies considering the preparation of advanced thermal barrier coatings from the perspective of the transport and transformation behaviors of the deposition units. The development prospects of new high-performance TBCs using the PS–PVD technique are also discussed. [ABSTRACT FROM AUTHOR]

Complex rare-earth oxide ceramics with pyrochlore structure (A2B2O7) are considered as promising candidate materials for next generation thermal barrier coatings (TBCs) due to low thermal conductivity and high phase stability. During plasma spraying, the component with a higher vapor pressure would experience preferential vaporization. In this study, Gd2Zr2O7 (GZO) powders with a hollow spherical structure in a wide range of particle size distribution were used to examine the preferential vaporization behavior of Gd2O3 during plasma spraying. Individual GZO splats were deposited to study the effect of particle size on Gd vaporization loss. The results show that almost all particles experienced Gd preferential vaporization loss that increased remarkably with the decrease in particle size. The Gd vaporization loss was significantly suppressed only when the particle size of molten droplet was larger than about 35 μm. The relationship between the Gd/Zr ratio and the droplet size reveals the significant effects of particle size and powder structure on Gd vaporization loss. It is clear that the vaporization behavior of in-flight GZO particles is dominated by molten droplet size rather than the size of feedstock powders with a porous hollow structure. The particle size effect on Gd vaporization resulted in wide distributions of Gd/Zr ratio in plasma-sprayed GZO coatings. Furthermore, it is found that after annealing at 1300 °C, the vaporization loss of Gd led to the formation of monoclinic ZrO2 from metastable pyrochlore phase in GZO coatings with inhomogeneous chemical composition. [ABSTRACT FROM AUTHOR]

Very low pressure plasma spraying (VLPPS) is an attractive method for metal-supported solid oxide fuel cells, wherein it can significantly avoid the oxidation of metal substrate. In this study, scandia-stabilized zirconia electrolyte was fabricated by VLPPS. To investigate the microstructure of coatings, the spraying distances were set to 150, 250 and 350 mm. The fractured morphology suggests that, at a relatively low deposition temperature (< 350 °C), typical lamellar structure was replaced by trans-granular structure. The apparent porosity of coatings ranges from 4.75 to 6.83%, as per the image analysis method. The ionic conductivity of coating at 250 mm was 0.068 S cm−1, which was ~ 68% of bulk. The ratio of surface area to projected area of coatings surface could reach 6.53-8.30 measured by a 3D confocal laser microscope. Finally, testing cells showed a maximum output power density of 1112 mW cm−2 and an OCV of 1.07 V at 750 °C. The ohmic resistance of whole cell was 0.1 Ω cm2, and the total resistance was 0.15 Ω cm2. The results suggest that very low pressure plasma spraying is a promising approach for the manufacturing of high-performance MS-SOFCs. [ABSTRACT FROM AUTHOR]

For tubular solid oxide fuel cells (SOFCs), individual cells are connected in series by the interconnector to obtain desired voltage and power output. As the most widely used ceramic interconnector for high-temperature SOFCs, lanthanum chromite-based material is quite difficult to sinter to high density due to the high vapor pressure of the chromium component. In this paper, dense strontium-doped lanthanum chromite (LSC) membranes with enhanced lamellar interface bonding were achieved at an elevated deposition temperature over the critical bonding temperature by atmospheric plasma spraying (APS). The effects of molten droplet particle size on the chemical composition and phase structure of LSC deposits were studied. The preferential vaporization of Cr during plasma spraying was of the greatest interest here. When the particle size of molten droplet was smaller than 30 μm, the vaporization loss of Cr increased remarkably with the decrease in particle size, resulting in impurity phases. When the particle size was larger than 30 μm, the vaporization loss of Cr was greatly reduced. The electrical conductivity of plasma-sprayed LSC deposits reached 8.7 and 2.1 S/cm at 850 °C in oxidizing and reducing atmospheres, respectively. The optimized LSC deposits exhibited good stabilities of phase structure and properties under operation conditions, which is adequate for SOFC interconnector application. [ABSTRACT FROM AUTHOR]

Magnesium (Mg) alloys have remarkable physical and mechanical properties for various aerospace applications. However, the high corrosion susceptibility and low wear resistance of Mg alloys restrict their wider use. To overcome these problems, 6061 Al alloy coatings were fabricated via the cold spraying process on the AZ91D Mg alloy substrate and a detailed comparative study on the wear and corrosion properties was carried out. The microstructure analysis of the compact coatings revealed low porosity with no phase change during the deposition process. The coatings showed improved corrosion resistance up to seven times as confirmed by potentiodynamic polarization and electrochemical impedance spectroscopy measurements in 3.5 wt.% NaCl aqueous solution. The as-deposited coatings were also compared with commercial bulk alloys of similar composition for their suitability to industrial applications. The tribological behavior of the coatings and substrate was investigated using tribometer. The wear resistance of the coatings improved more than two orders of magnitude as compared to the substrate. Abrasive and oxidative wear mechanisms were dominant in the coatings and substrate. The results showed that cold spraying of 6061 Al alloy on AZ91D Mg alloy could be an auspicious process to overcome the poor corrosion and wear resistance of this Mg alloy for practical industrial applications. [ABSTRACT FROM AUTHOR]

A novel direct current non-transferred arc plasma torch that can generate silent, stable and super-long laminar plasma jets in atmospheric air is investigated. The results showed that laminar plasma jets of length ranging from 100 to 720 mm in length can be generated by controlling the gas input rate ranging from 8.5 to 15 L min−1 and the output power from 8.5 to 28 kW. The length of the plasma jets generally increased with the output power and gas flow rate. Observations of temporal evolution of the plasma jet appearance and the voltage demonstrated that the jet is highly stable in the atmospheric environment. The fluid dynamic properties of the laminar plasma jet were studied using a numerical simulation incorporating a laminar flow model and an RNG turbulent flow model. Simulation results show the expansion of a high temperature region close to the torch nozzle exit, corresponding to a bright region observed in experiments. [ABSTRACT FROM AUTHOR]

It is usually difficult to deposit a dense ceramic coating with fully bonded splats by plasma spraying at a room temperature. Following the recent research progress on the splat interface bonding formation, it was found that there is a well-defined relationship between the critical bonding temperature and the melting point of spray material. Thus, it can be proposed to control the lamellar bonding through the deposition temperature. In this study, to examine the feasibility of the bonding formation theory, a novel approach to the development of ceramic coating with dense microstructure by plasma spraying through materials design with a low melting point is proposed. Potassium titanate K2Ti6O13 was selected as a typical ceramic material of a relatively low melting point for plasma spraying deposition of dense coating with well-bonded splats. Experiment was conducted by using K2Ti6O13 for both splat and coating deposition. Results show that the splat is fully bonded with a ceramic substrate at room temperature, and the K2Ti6O13 coating presents a dense microstructure and a fracture surface morphology similar to sintered bulk ceramic, revealing excellent interlamellar bonding formation. Moreover, both the hardness test and erosion test at 90° further confirmed the formation of the isotropic ceramic coating with fully bonded lamellae. [ABSTRACT FROM AUTHOR]

Cold spray bonding (CSB) has been proposed as a new method for joining aluminum and copper. At high speeds, solid Al particles impacted the groove between the two substrates to form a bond between Al and Cu. Compared to traditional welding technologies, CSB does not form distinct intermetallic compounds. Large stainless steel particles were introduced into the spray powders as in situ shot peen particles to create a dense Al deposit and to improve the bond strength of joints. It was discovered that introducing shot peen particles significantly improved the flattening ratio of the deposited Al particles. Increasing the proportion of shot peen particles from 0 to 70 vol.% decreased the porosity of the deposits from 12.4 to 0.2%, while the shear strength of joints significantly increased. The tensile test results of the Al-Cu joints demonstrated that cracks were initiated at the interface between the Al and the deposit. The average tensile strength was 71.4 MPa and could reach 81% of the tensile strength of pure Al. [ABSTRACT FROM AUTHOR]

In this study, vacuum cold spraying was used as a simple and fast way to prepare transparent super-hydrophobic coatings. Submicrometer-sized Al2O3 powder modified by 1,1,2,2-tetrahydroperfluorodecyltriethoxysilane and mixed with hydrophobic SiO2 aerogel was employed for the coating deposition. The deposition mechanisms of pure Al2O3 powder and Al2O3-SiO2 mixed powder were examined, and the effects of powder structure on the hydrophobicity and light transmittance of the coatings were evaluated. The results showed that appropriate contents of SiO2 aerogel in the mixed powder could provide sufficient cushioning to the deposition of submicrometer Al2O3 powder during spraying. The prepared composite coating surface showed rough structures with a large number of submicrometer convex deposited particles, characterized by being super-hydrophobic. Also, the transmittance of the obtained coating was higher than 80% in the range of visible light. [ABSTRACT FROM AUTHOR]

Corrosion of metal plays a detrimental role in service lifetime of parts or systems. Therefore, coating a protective film which is fully dense and defects free on the base metal is an effective approach to protect the base metal from corrosion. In this study, a dense NiCr-20Mo coating with excellent lamellar interface bonding was deposited by plasma spraying of the novel shell-core-structured Mo-clad-NiCr powders, and then post-spray shot peening treatment by cold spraying of steel shots was applied to the plasma-sprayed NiCr-20Mo coating to obtain a fully dense coating through eliminating possibly existed pores and un-bonded interfaces within the NiCr-20Mo coating. Corrosion behaviors of the NiCr-20Mo coatings before and after shot peening were tested to investigate the effect of the post-spray shot peening on the corrosion behavior of the NiCr-20Mo coating. Results showed that a much dense and uniform plasma-sprayed NiCr-20Mo coating with perfect lamellar bonding at most of interfaces was deposited. However, the electrochemical tests revealed the existence of through-thickness pores in the as-plasma-sprayed NiCr-20Mo coating. Through the post-spray shot peening treatment, a completely dense top layer in the coating was formed, and with the increase in the shot peening intensity from one pass to three passes, the dense top layer became thicker from 100 μm to reach 300 μm of the whole coating thickness. Thus, a fully dense bulk-like coating was obtained. Corrosion test results showed that the dense coating layer resulting from densification of shot peening can act as an effective barrier coating to prevent the penetration of the corrosive medium and consequently protect the substrate from corrosion effectively. Therefore, a fully dense bulk-like NiCr-20Mo coating with excellent corrosion resistance can be achieved through the plasma spraying of Mo-clad-NiCr powders followed by appropriate post-spray shot peening treatment. [ABSTRACT FROM AUTHOR]

Interlamellar bonding within plasma-sprayed coatings is one of the most important factors dominating the properties and performance of coatings. The interface bonding between lamellae significantly influences the erosion behavior of plasma-sprayed ceramic coatings. In this study, TiO and AlO coatings with different microstructures were deposited at different deposition temperatures based on the critical bonding temperature concept. The erosion behavior of ceramic coatings was investigated. It was revealed that the coatings prepared at room temperature exhibit a typical lamellar structure with numerous unbonded interfaces, whereas the coatings deposited at the temperature above the critical bonding temperature present a dense structure with well-bonded interfaces. The erosion rate decreases sharply with the improvement of interlamellar bonding when the deposition temperature increases to the critical bonding temperature. In addition, the erosion mechanisms of ceramic coatings were examined. The unbonded interfaces in the conventional coatings act as pre-cracks accelerating the erosion of coatings. Thus, controlling interlamellar bonding formation based on the critical bonding temperature is an effective approach to improve the erosion resistance of plasma-sprayed ceramic coatings. [ABSTRACT FROM AUTHOR]

Etanercept has been shown to be effective for the treatment of moderate-to-severe plaque psoriasis. Since most clinical trials examined etanercept in combination with other drugs, the efficacy and safety of etanercept monotherapy for moderate-to-severe plaque psoriasis have not been well established. This prospective study enrolled 61 Chinese patients with moderate-to-severe plaque psoriasis to explore the efficacy and safety of etanercept monotherapy. These patients were treated with etanercept at a subcutaneous dose of 25 mg, twice a week, for 12 weeks. All the 61 patients completed the treatment and showed significant improvement in psoriasis area and severity index (PASI) scores. At 4, 8, and 12 weeks after treatment, the response rates (PASI75) were 0%, 21.31%, and 40.98%, respectively. It was concluded that etanercept monotherapy is efficacious and safe for patients with moderate- to-severe plaque psoriasis. [ABSTRACT FROM AUTHOR]

In the plasma spray-physical vapor deposition process (PS-PVD), there is no obvious heating to the feedstock powders due to the free molecular flow condition of the open plasma jet. However, this is in contrast to recent experiments in which the molten droplets are transformed into vapor atoms in the open plasma jet. In this work, to better understand the heating process of feedstock powders in the open plasma jet of PS-PVD, an evaporation model of molten ZrO is established by examining the heat and mass transfer process of molten ZrO. The results reveal that the heat flux in PS-PVD open plasma jet (about 10 W/m) is smaller than that in the plasma torch nozzle (about 10 W/m). However, the flying distance of molten ZrO in the open plasma jet is much longer than that in the plasma torch nozzle, so the heating in the open plasma jet cannot be ignored. The results of the evaporation model show that the molten ZrO can be partly evaporated by self-cooling, whereas the molten ZrO with a diameter <0.28 μm and an initial temperature of 3247 K can be completely evaporated within the axial distance of 450 mm by heat transfer. [ABSTRACT FROM AUTHOR]

Super-hydrophobic surface has received widespread attention in recent years. Both the surface morphology and chemical composition have significant impact on hydrophobic performance. A novel super-hydrophobic surface based on plasma spray-vapor deposition was introduced in the present paper. Samaria-doped ceria, which has been proved as an intrinsic hydrophobic material, was used as feedstock material. Additionally, in order to investigate the influence of surface free energy on the hydrophobicity, chemical modification by low surface free energy materials including stearic acid and 1,1,2,2-tetrahydroperfluorodecyltrimethoxysilane (FAS) was used on coating surface. Scanning electron microscopy and Fourier transform infrared spectroscopy were employed to characterize the coating surface. The results show that the obtained surface has a hierarchical structure composed by island-like structures agglomerated with angular-like sub-micrometer-sized particles. Moreover, with the surface free energy decreases, the hydrophobic property of the surface improves gradually. The water contact angle of the as-sprayed coating surface increases from 110° to 148° after modification by stearic acid and up to 154° by FAS. Furthermore, the resultant surface with super-hydrophobicity exhibits an excellent stability. [ABSTRACT FROM AUTHOR]

In this study, we develop a large tubular solid oxide fuel cells design with several cells in series on a porous cermet support, which has many characteristics such as self-sealing, low Ohmic loss, high strength, and good thermal expansion coefficient matching. Here, we investigate aspects of the cell design, manufacture, performance, and application. Firstly, the cell length and number of cells in series are optimized by theoretical analysis. Then, thermal spraying is applied as a cost-effective method to prepare all the cell components. Finally, the performance of different types of cells and two types of stacks is characterized. The maximum output power of one tube, which had 20 cells in series, reaches 31 and 40.5 W at 800 and 900 °C, respectively. Moreover, the output power of a stack assembled with 56 tubes, each with ten cells in series, reaches 800 W at 830 °C. The excellent single tube and cell stack performance suggest that thermally sprayed tubular SOFCs have significant potential for commercialized application. [ABSTRACT FROM AUTHOR]

In this study, suspension plasma spraying (SPS) was applied to deposit double perovskite SrFeMoO (SFM) which can be used as both cathode and anode for solid oxide fuel cells. The effects of SFM concentration on the electrode phase composition, microstructure, and catalytic performance were investigated. The electrodes showed a dense structure when it was deposited at a concentration of 0.05 mol/L. The cathode performance was limited by the limited three-phase boundaries and poor gas diffusion. At 750 °C, cathode polarization ( R ) was 0.19 Ω cm. When the SFM concentration increased to 0.075 mol/L, the deposits revealed a porous microstructure with well-bonded fine particles. As a result, the R decreased significantly to 0.078 Ω cm at 750 °C. However, when the SFM concentration was further increased to 0.1 mol/L, the R increased owing to the limited interface bonding between the non-molten particles. As a result, it was found that the SFM suspension concentration should be optimized to achieve a highly active SFM by SPS process. Moreover, when the optimized deposit was employed as an anode and tested in a hydrogen atmosphere, it showed anode polarization resistance (R) of 1.5 Ω cm at 750 °C. [ABSTRACT FROM AUTHOR]

To explore the edge effect on intrasplat cracking of thermally sprayed ceramic splats, crack patterns of splats were experimentally observed and investigated through mechanical analysis. Both the polycrystalline splats and single-crystal splats showed obvious edge effects, i.e., preferential cracking orientation and differences in domain size between center fragments and edge fragments. In addition, substrate/interface delamination on the periphery was clearly observed for single-crystal splats. Mechanical analysis of edge effect was also carried out, and it was found that both singular normal stress in the substrate and huge peeling stress and shear stress at the interface were induced. Moreover, effective relief of tensile stress in splats is discussed. The good correspondence between experimental observations and mechanical analysis is elaborated. The edge effect can be used to tailor the pattern morphology and shed further light on coating structure design and optimization. [ABSTRACT FROM AUTHOR]

To comprehensively understand the formation mechanism of lamellar pores in splats, the delamination morphologies and crack patterns of yttria-stabilized zirconia (YSZ) and lanthanum zirconia splats were examined. Results showed that both types of splats grew epitaxially on well-polished YSZ substrates, evidently confirming the formation of chemical bonding between splats and substrate. However, the interfacial or sub-interfacial delamination was observed in all kinds of splats in this study. Residual vertical cracks passing through delaminated domains (on bare substrate) were also observed, which clearly indicated that transverse delamination followed vertical cracking. Mechanical analysis about delamination was addressed, and the results were consistent with the experimental data. [ABSTRACT FROM AUTHOR]

Two types of typical thermal cycling tests are used for the evaluation of thermal cycling lifetime of thermal barrier coatings. Those are the burner cycling test with a thermal gradient and the isothermal furnace cycling test. There are diverse explanations to test results up to now. Although certain correlations should exist between the results obtained by two types of the tests, no evident parameters in two tests were directly related, possibly due to large range of difference test conditions. In this investigation, a series of TBC samples with carefully prepared AlO-based TGO of different thicknesses were used for both the burner cycling and the furnace cycling tests. The relationships between thermal cycling lifetime and TGO thickness were obtained for two types of the tests. It was found that TGO thickness presents the same influence tendency despite of different types of thermal cycling test. The results reveal the existence of the critical TGO thickness by which the transition of failure mode takes place. Moreover, the values of the critical TGO thickness for two tests are comparable. The results evidently suggest that the lifetimes during different thermal cycling tests can be correlated by TGO effects on failure behavior. However, it is clear that the apparent dominant driving factors to TBC failure are different in two types of tests. Accordingly, the burner cycling test could be used for optimizing the durability of ceramic top coat by separating the effect of individual factors through test condition design, while the furnace cycling test results represent the integrated TBC durable performance of the bond coat and top ceramic coating. [ABSTRACT FROM AUTHOR]

Stainless steel splats were deposited on 304 stainless substrates with different thicknesses of oxide layer to examine the effect of substrate oxidation on splat morphology and splat-substrate interface bonding by inert low-pressure plasma spraying. The cross sections of splats showing the splat-substrate interface were prepared by focus ion beam (FIB). The splat morphology and splat-substrate interface bonding state were characterized by scanning electron microscopy. The interface bonding was also examined by an electrolytic etching process. Results showed that with increasing oxide layer thickness and surface roughness, the morphology of splat changed from disk shape to splashed finger-like shape. The examination into the interface bonding by using FIB-prepared cross-sectional samples revealed that the splat interface bonding depended on the oxide roughness and composition. The interface bonding with a ratio of 44% was formed at the inner part of a splat on the pre-oxidized substrate when iron oxide presented on the surface, and the roughness of oxide scale was <5 nm. When the pre-oxidizing temperature exceeded 800 °C, the surface roughness increased to 14 nm and chromium oxide covered the pre-oxidized surface, resulting in no effective bonding forming at the whole interface. Thus, surface roughness and oxide composition have a significant influence on the splat interface bonding formation. [ABSTRACT FROM AUTHOR]

Magnesium-based alloys have excellent physical and mechanical properties for a lot of applications. However, due to high chemical reactivity, magnesium and its alloys are highly susceptible to corrosion. In this study, Al6061 coating was deposited on AZ31B magnesium by cold spray with a commercial Al6061 powder blended with large-sized stainless steel particles (in-situ shot-peening particles) using nitrogen gas. Microstructure and corrosion behavior of the sprayed coating was investigated as a function of shot-peening particle content in the feedstock. It is found that by introducing the in-situ tamping effect using shot-peening (SP) particles, the plastic deformation of deposited particles is significantly enhanced, thereby resulting in a fully dense Al6061 coating. SEM observations reveal that no SP particle is deposited into Al6061 coating at the optimization spraying parameters. Porosity of the coating significantly decreases from 10.7 to 0.4% as the SP particle content increases from 20 to 60 vol.%. The electrochemical corrosion experiments reveal that this novel in-situ SP-assisted cold spraying is effective to deposit fully dense Al6061 coating through which aqueous solution is not permeable and thus can provide exceptional protection of the magnesium-based materials from corrosion. [ABSTRACT FROM AUTHOR]

Interlamellar bonding is an important factor controlling the mechanical, thermal and electrical properties of plasma sprayed ceramic coatings. In order to understand the formation of limited interlamellar bonding, a theoretical model is proposed based on the concept of the intrinsic bonding temperature. The numerical simulation of the interface temperature between a molten splat and underlying splats was performed for splats with uniform and non-uniform thickness, in order to reveal the conditions for the interlamellar bonding formation. The interlamellar bonding ratio was theoretically estimated based on the bonding forming conditions. The features of interlamellar bonding revealed by the simulation agree well with the experimental observations. The bonding ratio of plasma sprayed coatings is significantly influenced by the distribution of splat thickness. According to the distribution of AlO splat thickness in the coating, the theoretical estimation of bonding ratio yielded a value of 0.41 for the plasma sprayed AlO coating at the ambient atmosphere conditions, which is reasonably consistent with the observation value. Therefore, the limited interlamellar bonding can be reasonably explained based on the sufficient condition that the maximum interface temperature between a molten splat and underlying splats is larger than the intrinsic bonding temperature. [ABSTRACT FROM AUTHOR]

Active three-phase boundaries (TPBs) significantly influence cathode performance in solid oxide fuel cells, but obtaining long TPBs and understanding the mechanism underlying the improved cathode performance when the electrolyte is prepared with a smooth surface by a high-temperature sintering process remain essential challenges. In this work, we used flame spraying to deposit single-layer semimolten particles on a smooth electrolyte to build a three-dimensional surface with enlarged active surface area and thus increased TPBs. Meanwhile, LaSrMnO (LSM) cathodes with fine microstructure were deposited by solution-precursor plasma spraying (SPPS) on the designed electrolyte to establish a three-dimensional cathode-electrolyte interface. The deposition behavior of the semimolten particles on the smooth electrolyte and LSM cathodes on the three-dimensional electrolyte surface was studied. The effects of the area enlargement factor ( α ) on the polarization resistance of the SPPS LSM cathodes were investigated, using three-dimensional electrolytes with α from 1.29 to 2.48. The results indicated that convex particles with different molten states bonded well with the electrolytes. SPPS LSM cathodes also showed good interfacial bonding with convex particles. Finally, the cathode polarization ( R ) decreased linearly with increase of α . At 800 °C, R decreased from 0.98 to 0.32 Ω cm when α was increased from 1.29 to 2.48. [ABSTRACT FROM AUTHOR]

Intrasplat cracks, an essential feature of thermally sprayed ceramic coatings, play important roles in determining coating properties. However, final intrasplat crack patterns are always considered to be disordered and irregular, resulting from random cracking during splat cooling, since the detailed formation process of intrasplat cracks has scarcely been considered. In the present study, the primary formation mechanism for intrasplat cracking was explored based on both experimental observations and mechanical analysis. The results show that the intrasplat crack pattern in thermally sprayed ceramic splats presents a hierarchical structure with four sides and six neighbors, indicating that intrasplat crack patterns arise from successive domain divisions due to sequential cracking during splat cooling. The driving forces for intrasplat cracking are discussed, and the experimental data quantitatively agree well with theoretical results. This will provide insight for further coating structure designs and tailoring by tuning of intrasplat cracks. [ABSTRACT FROM AUTHOR]

A novel approach to prepare a coating system containing an in situ grown CrO diffusion barrier between a nickel top layer and 310SS was reported. Cold spraying was employed to deposit Ni(O) interlayer and top nickel coating on the Cr-contained stainless steel substrate. Ni(O) feedstock was prepared by mechanical alloying of pure nickel powders in ambient atmosphere, acting as an oxygen provider. The post-spray annealing was adopted to grow in situ CrO layer between the substrate and nickel coating. The results revealed that the diffusible oxygen can be introduced into nickel powders by mechanical alloying. The oxygen content increases to 3.25 wt.% with the increase of the ball milling duration to 8 h, while Ni(O) powders maintain a single phase of Ni. By annealing the sample in Ar atmosphere at 900 °C, a continuous CrO layer of 1-2 μm thick at the interface between 310SS and cold-sprayed Ni coating is formed. The diffusion barrier effect evaluation by thermal exposure at 750 °C shows that the CrO oxide layer effectively suppresses the outward diffusion of Fe and Cr in the substrate effectively. [ABSTRACT FROM AUTHOR]

Based on the specific structure of tubular solid oxide fuel cells, good chemical, microstructural, and phase stabilities for the protective coating are required in both the oxidizing and reducing environments. In this work, MnCoO coatings were deposited onto porous Ni50Cr50-AlO substrate by atmospheric plasma spray. The coated samples were tested at 800 °C with the coating exposed in air environment and the substrate in H environment. Reducing and pre-oxidizing treatments were performed prior to the stability test. The microstructural stability, phase composition, and electrical properties of the tested coatings were investigated. The surface morphology exhibited an excellent surface stability, and no obvious crystal coarsening was observed. With enhancement of the testing duration, the area-specific resistance presented a decreasing trend attributed to increase in the contact interface and densification of the upper layer. The cross-section views presented a dense upper layer and a relatively porous bottom layer. The x-ray diffraction results also indicated a single MnCoO phase in the upper layer exposed to air environment and a reduced phase structure in the bottom layer from the substrate side. The evolution mechanism between the oxidation frontier and the reduction interface was then discussed. [ABSTRACT FROM AUTHOR]

Atmospheric plasma-sprayed ceramic coatings with a lamellar structure exhibit low thermal conductivity. However, high-temperature exposure causes sintering, which heals inter-lamellar two-dimensional (2D) pores and intra-splat pores. Such sintering effect increases the thermal conductivity of the coatings and consequently reduces the thermal insulation ability of TBCs. In this study, inter-lamellar 2D pores with a large opening width were introduced into the LaZrO (LZO) coating through the spraying of a LZO-SrO coating and the removal of the SrO splats in water. Then, the conventional LZO coating and the porous LZO coating were subjected to high-temperature exposure at 1300 °C, for different durations. It was found that the 2D pores resulting from SrO splats present little healing during high-temperature exposure, while the conventional 2D inter-lamellar pores with a small opening width heal rapidly. Thus, the thermal conductivity of the conventional LZO coating increased rapidly, while the unhealed 2D pores in the highly porous LZO coating contributed to the coating low thermal conductivity. The present results indicated that a high heat insulating thermal barrier coating with high stability can be fabricated though the introduction of inter-lamellar 2D pores with large opening width. [ABSTRACT FROM AUTHOR]

The infiltration of molten CMAS in thermal barrier coatings (TBCs) at high temperature is significantly affected by the microstructure of the ceramic coating. Enhancing the bonding ratio between splats can reduce the interconnected pores and suppress the infiltration of the molten CMAS into the coating. In this study, a dual-layered (DL) TBC with the dense 8YSZ on the top of the conventional porous 8YSZ was proposed to enhance CMAS corrosion of atmospheric plasma-sprayed YSZ. The dense YSZ coating with improved lamellar bonding was deposited at a higher deposition temperature. The microstructure of the coatings before and after CMAS attack test was characterized by scanning electron microscopy. It was clearly revealed that by adjusting the microstructure and applying a dense ceramic layer with the improved interface bonding on the top of porous TBC, the infiltration of CMAS into porous YSZ coating can be effectively suppressed. Moreover, by designing DL TBCs, the thermal conductivity of the TBC system exhibits a limited increase. Thus with the design of DL structure, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved. [ABSTRACT FROM AUTHOR]

Perovskite-structured (LaSrCoFeO) LSCF has been widely studied as a cathode material for intermediate-temperature solid oxide fuel cells. However, the application of LSCF cathode is likely to be limited by its sluggish surface catalytic properties and long-term stability issues. Oxygen hyper-stoichiometric LaNiO with KNiF structure exhibits higher catalytic properties, ionic conductivity, and stability in comparison with LSCF cathode. Due to the good chemical compatibility of these two cathode materials, it is possible to prepare a composite cathode by the infiltration of LaNiO in the porous LSCF. This composite structure fully utilizes the advantages of the two cathodes and enhances the LSCF cathode performance. In this study, the LSCF cathode was deposited by using an atmospheric plasma spray technique, and the porous LSCF cathode was then infiltrated by LaNiO. The atmospheric plasma spray technique was used to reduce the SOFC manufacturing cost. The microstructure of coatings was characterized by SEM and EDS. The cathode polarization resistance was found to decrease by ~40% after the LaNiO infiltration. Also, the activation energy decreased from 1.53 to 1.40 eV. [ABSTRACT FROM AUTHOR]

The elastic modulus of plasma-sprayed top coating plays an important role in thermal cyclic lifetime of thermally sprayed thermal barrier coatings (TBCs), since the thermal stress is determined by the substrate/coating thermal mismatch and the elastic modulus of top coating. Consequently, much attention had been paid to understanding the relationship between elastic modulus and lamellar structure of top coating. However, neglecting the intra-splat cracks connected with inter-splat pores often leads to poor prediction in in-plane modulus. In this study, a modified model taking account of intra-splat cracks and other main structural characteristics of plasma-sprayed yttria-stabilized zirconia coating was proposed. Based on establishing the relationship between elastic modulus and structural parameters of basic unit, effects of structural parameters on the elastic modulus of coatings were discussed. The predicted results are well consistent with experimental data on coating elastic modulus in both out-plane direction and in-plane direction. This study would benefit the further comprehensive understanding of failure mechanism of TBCs in thermal cyclic condition. [ABSTRACT FROM AUTHOR]

The splat interface bonding state which changes heat transfer conditions and thus the cooling rate during splat cooling may influence the interface microstructure. In this paper, YSZ coating was deposited by atmospheric plasma spraying with substrate cooling during deposition. Subsequent characterization was implemented using high resolution transmission electron microscopy to examine the local microstructures near the interfaces at the bonded and unbonded zones. Selected area diffraction analyses of the splats across both the bonded interface and unbonded interface revealed that all bulk splats present a metastable tetragonal structure. Results showed that the size of columnar grains within a splat was significantly influenced by the interface bonding. At the unbonded region in the splat, large columnar grains form which can be attributed to poor thermal contact of melt to the underlying splat surface before its solidification. At the bonded zones, the splat presents a much fine columnar grain structure, which is attributed to good thermal contact of the melt to the underlying splat before solidification. Moreover, it is evident that the bonded interface region presents a distinct microstructure feature from the fine columnar grains suggesting the crystal defect of high density of dislocations at the interface. [ABSTRACT FROM AUTHOR]

During plasma spray-physical vapor deposition (PS-PVD) of yttria-stabilized zirconia (YSZ) coatings, evaporation of the YSZ powder is essential, but quite difficult when using a commercial <80 kW plasma torch. In this study, a shrouded plasma torch was examined to improve the YSZ evaporation. The plasma characteristics were diagnosed using optical emission spectroscopy. Results showed that the electron number density in the plasma jet was maintained at an order of magnitude of 10 cm, indicating local thermal equilibrium of the plasma jet. Compared with a conventional torch, the shrouded torch resulted in much higher plasma temperature and much lower electron number density. With the shrouded torch, more energy of the plasma was transferred to the YSZ material, leading to more evaporation of the YSZ powder and thereby a much higher deposition rate of the YSZ coating. These results show that use of a shrouded torch is a simple and effective approach to improve the evaporation of feedstock material during PS-PVD. [ABSTRACT FROM AUTHOR]

LaZrO (LZO) is widely expected to be one of the promising thermal barrier coating materials for application in high-temperature conditions (1200 °C). However, high-temperature exposure causes sintering which heals interlamellar two-dimensional (2D) pores and intrasplat pores. This sintering effect increases the stiffness and thermal conductivity of thermal barrier coatings, consequently reducing their durability. In this study, to reveal the possible critical opening of 2D pores above which they are free from sintering, LZO coating and splat were deposited by atmospheric plasma spraying and were exposed to 1300 °C for different durations. Thereafter, the evolution of the parameters of residual 2D pores in the coating and the surface morphology of LZO splat were characterized. It was found that there is a critical opening width for 2D pores above which grain bridging does not occur across the gaps. Accordingly, pores with an opening larger than this critical width are free from sintering across the 2D pores despite surface roughening of splats, whereas pores with an opening less than the critical width sinter rapidly at the early stage of thermal exposure through the formation of grain bridges. [ABSTRACT FROM AUTHOR]