This study assesses the potential of scandia-stabilized zirconia (ScSZ) produced by very low-pressure plasma spraying (VLPPS) for metal-supported solid oxide fuel cell (MS-SOFC) applications. To investigate the microstructure of ScSZ, coating samples were deposited at spraying distances of 150, 250, 350 mm. The fragile nature of coating cross-sections suggests that the typical lamellar structure of zirconia is replaced by a transgranular structure. Nonetheless, apparent porosity, ionic conductivity, open circuit voltage, and ohmic resistance measurements indicate that VLPPS is a viable method for producing MS-SOFCs.

This study demonstrates a method of plasma spraying in which the plasma is maintained in a laminar (rather than turbulent) state, achieving a much greater jet length with less ambient air engulfment. In the experiments, NiCr coatings were produced at spraying distances between 250 mm and 500 mm, showing that specific structures can be realized by changing stand-off distance. Structures with high porosity, for example, are generated at relatively short distances; dense structures, on the other hand, are obtained at longer stand-off distances that allow feedstock powder to reach a fully melted state. XRD analysis shows that the spraying process does not change the chemical composition of the material, and EDS results indicate that chemical and metallurgical bonding are achieved.

Rare-earth complex oxide ceramics are promising candidate materials for next-generation thermal barrier coatings due to their low thermal conductivity and phase stability. During plasma spraying, the component with a higher vapor pressure may experience severe preferential vaporization, resulting in significant composition change from the starting powder. In this study, Gd 2 Zr 2 O 7 (GZO) powder with a hollow spherical structure is used as the feedstock material to assess the vaporization behavior of G d2 O 3 during atmospheric plasma spraying (APS). Isolated Gd 2 Zr 2 O 7 splats with regular disc shapes in different sizes were deposited on stainless steel substrates at a deposition temperature of 300 °C to study the effects of particle size on vaporization loss. The elemental composition of each splat was analyzed by EDS, and the ratio of Gd to Zr in different splats with different diameters was obtained. The results show that the vaporization loss of Gd increases markedly with decreasing particle size due to the preferential vaporization of G d2 O 3 . Using Gd 2 Zr 2 O 7 powders of a certain size can reduce Gd loss, although the effect is determined by molten droplet size rather than apparent powder particle size.

In this study, the mechanisms responsible for enhancing the adhesion strength of thermally sprayed metallic coatings subjected to vacuum heat treatment were investigated using atmospheric plasma sprayed (APS) CoNiCrAlY coatings as an example. The formation of metallurgical bonding between the coating and the substrate, which determined the increase in the adhesion strength of the coatings, was studied by analyzing the effect of morphological changes of the oxide film in the coating. The results showed that during the vacuum heat treatment process, the oxide film formed during the coating deposition gradually broke down and subsequently shrank into round oxides. After vacuum heat treatment, the adhesion strength of the coating improved significantly, and there was a positive nonlinear relationship between the treatment time and the adhesion strength. The increase in the adhesion strength was caused by the formation of metallurgical bonding between the coating and the substrate. However, the prerequisite for the formation of metallurgical bonding was that the oxide film had to break during the vacuum heat treatment process. A thermodynamic 2D model based on the thermal grooving theory was proposed to explore the essential conditions for the breaking and shrinking of the oxide film.

It is usually difficult to deposit dense ceramic coatings with splats well bonded by plasma spraying at 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 materials melting point. Thus, it was proposed to control the lamellae bonding through the deposition temperature. In this study, to examine the feasibility of the bonding theory, a novel approach for the development of coating microstructure through materials design is proposed. Accordingly typical ceramic materials were selected of relative low melting point for plasma spraying of dense coating with well bonded splats. The experiment was conducted by using K 2 Ti 6 O 13 for splat deposition at ~110°C cooling down from a higher temperature to avoid substrate adsorbates and coating was deposited at room temperature in ambient temperature without substrate preheating. Results show that the splat is fully bonded with a ceramic substrate, while the coatings present a dense microstructure with a similar fracture morphology to sintered bulk ceramics. Moreover, the erosion test at 90° further confirmed the formation of a ceramic coating with lamellae fully bonded.

In the Plasma Spray-Physical Vapor Deposition (PS-PVD) process, the vapor atom of feedstock material is one deposition unit of the columnar structure coating. It is reported that the gas phase may be transformed into cluster when the powder feeding rate increases from small to large or the sedimentation distance increases from a certain distance to another distance. In order to understanding the variation of vaporized coating material in free plasma jet, the gaseous material capacity of plasma jet must be fundamentally understood. In this work, the thermal characteristics of plasma were firstly measured by optical emission spectrometry (OES). The results show that the free plasma jet is in the local thermal equilibrium due to a typical electron number density from 2.1×1015 to 3.1×1015 cm -3 . In this condition, the temperature of gaseous zirconia can be equal to the plasma temperature. A model was developed to obtain the vapor pressure of gaseous ZrO 2 molecules as a two dimensional map of jet axis and radial position corresponding to different average plasma temperatures. The overall gaseous material capacity of free plasma jet was further established. At a position of plasma jet, clusters may form when the gaseous material exceeds local maximum gaseous material capacity.

High manufacturing costs and long-term degradation are the main problems that have become a “bottleneck” and impeded SOFC’s further development. It is well known that a high operating temperature is the major cause that leads to these problems. As such, reducing the operating temperature becomes a hotspot of research. It has been reported that a uniform and dense coating can be prepared by using very low pressure plasma spraying (VLPPS) technology. The current study focuses on VLPPS for application in large-area (~100 × 100 mm) porous metal supported solid oxide fuel cell (MSSOFC), especially for the preparation of the electrolyte. It was found that the densification of the electrolyte was very good, as confirmed by the open-circuit voltage (OCV) of the cell. In the temperature range of 550~750°C, the OCV of the cell stabilized between 1.05 V and 1.1 V. The power density of the cell was also measured.

Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) with lamellar structure exhibit low thermal conductivity and low stiffness. However, high temperature exposure for certain long duration causes the sintering which heals two-dimensional (2D) inter-lamellar pores and intrasplat pores. Such sintering effect increases the stiffness and thermal conductivity of the coatings and consequently reduces the stability and durability of TBCs. It can be expected that large 2D pores with a wide opening is difficult to be eliminated. In this study, inter-lamellar 2D pores with large opening width were fabricated in the La 2 Zr 2 O 7 (LZO) coatings through spraying LZO+SrO coatings and removing the SrO splats in the water. Then, the conventional LZO coating and the porous LZO coating were subjected to high temperature exposure in the air at 1300 °C for different durations. The microstructure evolution especially in terms of the shape and density of inter-lamellar 2D pores was examined. In addition, the change of thermo-physic properties and the mechanical properties of the coatings with increasing exposure duration were studied. Results show that the 2D pores in LZO coating created by those SrO splats inherit primarily large opening width from 200nm to about 1 µm which endows the LZO coating with high sustainability at high temperature environment. Under thermal exposure at 1300°C, it was found that 2D pores resulting from SrO splats are free from healing while conventional 2D inter-lamellar pores with small opening width formed during splat cooling became healed rapidly. Thus, thermal conductivity and Young's modulus of the conventional LZO coating increased rapidly, while unhealed 2D pores in the highly porous LZO coatings contributed to the low Young's modulus and low thermal conductivity of LZO coating with remarkably high stability. With addition of 30% SrO in spray powder, a LZO coating with a thermal conductivity of about 0.39 W.m -1 .K -1 in the as-prepared state was obtained. The coating maintained a thermal conductivity of 0.57 W.m -1 .K -1 even after 100 hours exposure at 1300°C. The present results indicated that high sintering-resistant thermal barrier coating can be fabricated though designing inter-lamellar 2D pores with large opening width in the coating by the present novel approach.

The corrosion resistance of thermal barrier coatings against CMAS deposit at high temperature is significantly affected by the microstructure of the coatings. Enhancing the bonding ratio between splats can reduce the inter-connected pores and then obstructs the penetration of the molten CMAS into the coatings. In this study, atmospheric plasma sprayed ZrO 2 contains 8 wt. % Y 2 O 3 (8YSZ) coating with improved lamellar bonding ratios was deposited with full-molten droplets at an enhanced deposition temperature. The microstructure of the dense 8YSZ coating and conventional 8YSZ coating before and after thermal exposure with CMAS were characterized. It was clearly revealed that by adjusting the microstructure and designing a ceramic layer with high bonding ratio, the corrosion resistance of the thermal barrier coating could be enhanced. Moreover, by designing double-ceramic-layer (DCL) TBCs composed of a porous ceramic layer and well-bonded ceramic layer, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved.

The thermally grown oxide (TGO), formed on the bond coat (BC) surface of thermal barrier coatings (TBCs), has a significant impact on the TBCs life time. In order to reduce growth kinetics of TGO, vacuum pre-treatment was taken for some bond coats before pre-oxidation and isothermal oxidation in air. The other bond coats were directly subjected to pre-oxidation and isothermal oxidation. The grain size and grain boundary distribution of the TGO were analyzed by electron backscattered diffraction (EBSD). Element diffusion was taken into account to analyze growth kinetics of the TGO. Results indicate that the change of Al 2 O 3 TGO thickness with isothermal oxidation duration doesn’t fit well to conventional oxidation theory, i.e. exponential function. It is attributed to the change of the grain structure during the pre-treatment. This mechanism would benefit the development of long-life and highly reliable TBCs.

High efficiency gas turbine needs high temperature sealing by abradable porous ceramic coatings. In this study, porous Al 2 O 3 coatings were deposited by flame spraying through controlling melting of spray powder particle in a semi-molten state. The effect of melting degree of spray particles changed via spray conditions on coating microstructure and porosity was investigated. The melting degree of spray particles was characterized by using 3D confocal laser microscopy. The porosity of the coating was estimated by image analysis. The results showed that the melting degree of alumina particles can be changed from 80 down to 30% and thus the coating porosity can be increased from 30% up to about 60%. The standard hardness test yielded no effective data for the porous coatings deposited by spray particles of a melting degree less than 60%, and hardness of 32-75 HR15Y for Al 2 O 3 coatings deposited by spray particles with a melting degree higher than 60%. The pin-on-disk abrasion test of Inconel 738 nickel-based superalloy spherical pin of 5 mm in diameter at room temperature against porous alumina coating was conducted to evaluate abradability of porous Al 2 O 3 coatings. It was found that for the coatings of hardness less than 32HR15Y and porosity over 40% the wear weight loss of the IN738 pin became negligible despite high wear rate of the coating. It is evident that the flame-sprayed porous alumina coatings of high porosity by the present approach are promising abradable coatings applicable to gas turbine operating at high temperature.

The stiffness and thermal conductivity of thermal barrier coatings (TBCs) are inevitably changed by healing up of intersplat pores and intra-splat cracks during high temperature exposure, which results in less compliance and thermal insulating performance. However most publications on sintering of plasma sprayed TBCs are based on free-standing coatings, which ignore residual stress and the stress produced by the mismatch of thermal expansion coefficient between substrate and ceramic top coatings. In this paper, individual splat of YSZ and YSZ coatings have been sprayed on substrate of YSZ and Ni-based superalloy. Evolution of healing and morphology of 2D cracks and some properties, such as hardness and thermal conductivity, have been revealed during thermal exposure. Results showed that, during heating stage, the shear stress coming from substrate caused some tearing of bonding area tips and narrowing of inter-splat pores. Some recoverable and unrecoverable widening on intra-splat cracks occurred also due to shear stress. During annealing stage, compared with free-standing coating, the surface hardness of the coating deposited onto the Ni-based superalloy showed enhanced increasing due to the faster healing of inter-splat pores by narrowing down, and the hardness in cross-section presented retarding increasing due to the widening of cracks in out-plane direction leading to slower healing. The case of YSZ substrate fell between free-standing case and Ni-based superalloy. This would benefit the further in-depth understanding of the thermal cycling failure mechanism of plasma sprayed TBCs.

In this study, in-situ shot-peening approach was introduced by mixing large stainless steel (SS) shots with diameters over 150 μm with IN718 spray powders to aim at developing a novel approach to achieve dense coating. Effect of mixed stainless steel shot content on the microstructure and mechanical properties of the IN718 coating was examined. Results show that IN718 coating can be deposited without any shot peening particle inclusions for their relatively low velocity. It was revealed that the deposition efficiency of the IN 718 powders is improved by the in-situ shot peening effect. With increasing shot fraction in the powder mixture from 0% to 50 vol.%, the coating porosity decreased from 5.6% to 0.2% only by using N 2 accelerating gas. Remarkable work hardening induced by impact of the shot peening particles was detected.

The effect of substrate template effect on the crystalline structure of plasma sprayed 8YSZ (8mol%Y 2 O 3 ) splats was investigated by high resolution transmission electron microscopy (HR-TEM) examination of FIB-processed splat samples. 8YSZ splats were deposited by the atmospheric plasma spraying (APS) on the polished sintered tetragonal structure substrate (3YSZ) and cubic structure substrate (8YSZ) at different preheating temperatures. The focused ion beam (FIB) was utilized to prepared TEM cross-sectional sample of splats. The crystalline structures of both the splat and the underlying substrate were examined by HRTEM. Results showed that the 8YSZ splats deposited on the polished sintered cubic structure 8YSZ substrate at a substrate surface temperature of 900°C exhibited cubic structure and the epitaxial grain growth was confirmed between the crystalline of splat grain and immediately underlying cubic crystalline substrate grain. Moreover, epitaxial grain growth was confirmed between the crystalline of splat grain and the tetragonal structure substrate when substrate surface temperature was increased to 1200°C. The present results suggest that the crystalline structure formation of 8YSZ splats produced by plasma spraying was affected by the substrate template effect.

The bonding between flattened particles in plasma-sprayed metal coatings dominates their corrosion behavior by influencing the porosity in coatings, especially the porosity connected to the substrate for coatings used in a corrosive environment. Therefore, how to efficiently enhance the lamellar interface bonding in metallic coatings has been an important issue which has not been settled effectively. In this study, a shell-core structured powder particle designing with cladding spherical Ni20Cr powders with refractory molybdenum as alloying element is proposed to limit the evaporation of low melting point elements and subsequently raise particle temperature significantly high enough to cause impact melting. Results show that a dense coating with much low porosity was obtained due to the improved lamellar interface bonding by gas shrouded plasma spraying of the composite NiCr -Mo particles. Electrochemical method was employed to evaluate the polarization behavior of the NiCr - Mo coating to estimate its connected porosity. It was revealed that NiCr-Mo coating of excellent corrosion resistance with low connected porosity can be obtained by designing the shell-core-structured powder.

Thermally sprayed coatings exhibit a lamellar structure with a bonding ratio less than 32% when a coating is deposited at ambient temperature. The lamellar bonding is one of the most important factors controlling the mechanical, thermal and electrical properties of the coatings. However, it is not clear why only limited lamellar bonding exists in a thermal spray coating even though many studies have been focused on the formation of bonding. In our previous study, it was found that there exists a critical deposition temperature for depositing ceramic splats to form the bonding with the underlying identical substrate, i.e., critical bonding temperature. Moreover, the critical bonding temperature is related to the interface temperature prior splat solidification which is determined by the glass transition temperature of splat material. In the present study, the critical bonding temperature and its relationship with interface temperature are used to understand the limited lamellar bonding ratio in a coating. A numerical simulation model involving heat transfer among depositing splat was proposed to establish the sufficient condition for liquid splat to form the bonding with the underlaying splats. The non-uniform splat thickness model was established to calculate theoretically the interface bonding formation. The calculation based on the model yielded a bonding ratio of 38.5% which agrees reasonably with the observed maximum interface bonding ratio.

It is only performed light of sight deposition in atmospheric plasma spray (APS) and electron beam physical vapor deposition (EB-PVD). The PS-PVD processing is realized no light of sight deposition which bridge the gap between APS and EB-PVD. In this study, shielding deposition process was used to produce YSZ coatings and the microstructure and deposition behavior were also researched. A shrouded plasma torch was used to proceed on PS-PVD working. Multiple samples were placed on the same straight line to realize multistage shielding. The surface morphology and cross section morphology of coatings were observed by SEM. It is found that the surface is shown an island structure and the nano particles consist of island are grown smaller with the increased shielding stage. The cross section morphology of fracture coatings is shown columnar structure. This confirmed that the YSZ coatings are deposited via vapor phase at the condition of shielding. The deposition behavior of multi-stage shielding is also researched.

The evaporation of both CeO 2 and La 2 O 3 in high temperature plasma arc leads to deviation of plasma sprayed La 2 Ce 2 O 7 coating composition from the starting powder particle. Such change results in significantly inhomogeneity of compositions within a coating which influences the performance of calcium-magnesium-alumina-silicate (CMAS) corrosion. In this study, the pellets with different Ce / La ratio were interacted with molten CMAS glass. The penetration of molten CMAS on different pellets was characterized by scanning electron microscopy and energy dispersive spectrum (EDS) and the phase of different powder mixtures treated at 1250 °C was characterized by X-ray diffraction ( XRD ). The effects of pellets with different Ce / La ratio on the CMAS corrosion after thermal exposure at 1250 °C for 50 h were investigated. The result demonstrated that pellets with ratio of Ce / La greater than or equal to 1.0 were completely dissolved into the molten CMAS, at the same time the others were also damaged, forming diffusion layer and reacted layer, respectively. The difference of the ratio of Ce / La indeed affected their performance against CMAS attack.

La 2 Ce 2 O 7 (LCO) is one of the promising candidates of high temperature thermal barrier coating materials. However, during plasma spraying, the evaporation of both CeO 2 and La 2 O 3 in high temperature plasma jet leads to deviation of coating composition from the starting powder particle. Such change results in significant inhomogeneity of compositions within a coating which influences the performance of resultant coating. In this study, an agglomerated La 2 Ce 2 O 7 powder with Ce / La ratio of 1.5 and in a size range of 5-60 μm was used to deposit the splats and coatings under different plasma spray conditions. The compositions of La 2 Ce 2 O 7 splats in different sizes were characterized by scanning electron microscopy and energy dispersive spectrum (EDS), and splat dimensions were characterized by 3D laser microscopy. The relationship between droplet size and splat composition was examined. Results showed that the Ce / La ratio in splats changes significantly with the splats diameter in a range from 1.5 to 0.7. There is a big difference in the composition of APS coatings deposited by coarse and fine powders. The mechanism causing splat composition change was examined based on the selective elemental loss during plasma heating of LCO particles in APS, aiming to control LCO coating composition.

Pre-diffusion treatment (high temperature vacuum treatment) plays an important role in the growth of the thermally grown oxide (TGO) on the low pressure plasma spray (LPPS) bond coat surface. Results show that the initial thin oxide scale, formed during deposition process, on the as-sprayed bond coating surface has broken and shrunk to discontinuous oxide particles through the elements diffusion during the pre-diffusion. Two kinds of pre-diffused bond coats deposited in different oxygen partial pressure atmosphere show different results of the average distance between the individual oxide particles on the pre-diffused bond coating surface. In this study, a three dimensional model with thermal grooving theory was developed to explore the essential condition for the scale breaking and explain the differences of these results. This research can provide reference for the preparation optimization and pre-treatment optimization of bond coat towards high performance TBCs.