Cheng-Xin Li

Relationships between the microstructure and properties of thermally sprayed deposits

Thermally sprayed deposits have layered structure composed of individual splats. The individual splats have quenching microstructure of quasi-stable preferred fine grains. However, this fine-grained microstructure of the deposits is usually not reflected by improved performance of the deposits because a layered structure with two-dimensional voids occurs between lamellar interfaces. The microstructure of the thermal spray deposits with the emphasis on the layer structural parameters is reviewed. Conventionally, one of the most common quantitative parameters used to characterize the microstructure of the thermally sprayed deposits is the porosity, measured by different methods. However, it is illustrated that the relationships between properties and porosity for bulk porous materials processed by conventional processes cannot be applied to thermally sprayed deposits owing to the two-dimensional characteristics of voids. The total porosity in the deposits is not meaningful from the viewpoint of prediction of the deposit properties. An idealized structural model and related parameters, instead of porosity, are proposed to characterize quantitatively the microstructure of the thermally sprayed deposit. The relationships between the properties and the structural parameters are presented for the plasma-sprayed ceramic deposits based on the proposed microstructure model. The properties include the Young’s modulus, fracture toughness, erosion resistance, and thermal conductivity of the plasma sprayed ceramic deposits. The correlations of theoretical relationships with reported experimental data are discussed. An agreement of theoretical with observed values suggests that the lamellar structure of the deposit with limited interface bonding is the dominant factor controlling the performance of the deposit.

Development of Particle Interface Bonding in Thermal Spray Coatings: A Review

Thermal spray ceramic coatings deposited following the conventional routine exhibit a typical lamellar structure with a limited interface bonding ratio. The bonding between particles in the coating dominates coating properties and performance. In this review paper, the bonding formation at the interface between thin lamellae in the coating is examined. The effect of spray parameters on the bonding ratio is presented to reveal the main droplet parameters controlling bonding formation, which reveals that the temperature of the spray particle rather than its velocity dominates the bonding formation. The limitation to increase significantly the ceramic particle temperature inherent to the thermal spray process leads to the observation of a maximum bonding ratio of about 32%, while through controlling the surface temperature of the coating prior to molten droplet impact, the bonding at the lamellar interface can be significantly increased. Consequently, it is shown that with the proper selection of deposition conditions and control of the deposition temperature, the bonding ratio of ceramic deposits can be altered from a maximum of 32% for a conventional deposit to a maximum of 100%. Such wide adjustability of the lamellar bonding opens new possibilities for using thermal spray coatings in various applications requiring different microstructures and properties. The examination of recent studies shows that the bonding control makes it possible to fabricate porous deposits through surface-molten particles. Such an approach could be applied for the fabrication of porous materials, the deposition of high temperature abradable ceramic coatings, and for forming functional structured surfaces, such as a surface with super-hydrophobicity or a solid oxide fuel cell cathode interface with high specific surface area and high catalytic performance. Furthermore, complete interface bonding leads to crystalline structure control of individual splats through epitaxial grain growth.

Effect of solid carbide particle size on deposition behaviour, microstructure and wear performance of HVOF cermet coatings

Abstract The deposition process of WC – Co and Cr3 C2 – NiCr cermet coatings through a high velocity oxyfuel (HVOF) spray process was examined using powders of different carbide particle sizes that ranged from 1.5 m m to over 15 m m. The effect of carbide particle size on the thickness of flattened particles was investigated. The carbide size in the coating was compared to that in the original feedstock. Results showed that most of the carbide particles remained in a solid state after passing through the flame, and the deposition of the cermet coating involves lateral flattening of a solid – liquid two phase droplet rather than a single liquid droplet, as in plasma spraying of metallic and ceramic materials under optimised conditions. It was found that the size of carbide particles in a sprayed coating depends greatly on the corresponding carbide size in the starting powder. The large solid carbide particles in a two phase droplet tend to rebound easily during HVOF spraying when the droplet impacts on a substrate surface, while small carbide particles in the droplet may follow the flattening of the liquid binder phase and be easily retained in the deposit. Using a powder consisting of small carbide particles leads to substantial retention of carbide particles in the coating and consequently an improved wear performance. A model is proposed to explain the effect of carbide particle size on the splat formation by a solid – liquid two phase droplet.

Effect of types of ceramic materials in aggregated powder on the adhesive strength of high velocity oxy-fuel sprayed cermet coatings

Abstract The effects of the types and compositions of the binder phase in WC cermet powders, types of solid materials in solid–liquid two-phase particle, and high velocity oxy-fuel spray conditions on the adhesive strength of high velocity oxy fuel (HVOF) sprayed composite coatings were studied using three aggregated WC cermet powders, Cr 3 C 2 –25NiCr, Al 2 O 3 –Ni powders, and W–Ni powder. The examination of the coating microstructure using optical and scanning electron microscopy revealed that all the coatings were deposited through solid–liquid two-phase particles with ceramic particles or W in the powder being in a solid state while the binder in a melted liquid state. It was found that an adhesive strength higher than the strength of the test adhesives of approximately 65 MPa can be reached for all HVOF coatings deposited by WC cermet powders and W–Ni powder. The adhesive strength of HVOF WC cermet coatings was not significantly influenced by the composition of the binder, the powder structure, or the spray parameters. However, for Cr 3 C 2 –25NiCr coating the adhesive strength ranged from 35 MPa to over 92 MPa and was significantly influenced by the spray parameters. Finally, Al 2 O 3 –Ni coatings had a very limited adhesive strength. The results suggested that the formation of a solid–liquid two-phase particle in the HVOF process is a necessary condition to deposit HVOF coatings of high adhesive strength, while the high density of WC and W particles in the powders is a necessary condition to achieve high adhesive strength for HVOF WC cermet and W–Ni coatings. The adhesive strength of HVOF cermet coatings will be increased with an increase in the density of the solid material component in the two-phase particle.

Facile and Scalable Fabrication of Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells in Air Using Gas Pump Method

Control of the perovskite film formation process to produce high-quality organic-inorganic metal halide perovskite thin films with uniform morphology, high surface coverage, and minimum pinholes is of great importance to highly efficient solar cells. Herein, we report on large-area light-absorbing perovskite films fabrication with a new facile and scalable gas pump method. By decreasing the total pressure in the evaporation environment, the gas pump method can significantly enhance the solvent evaporation rate by 8 times faster and thereby produce an extremely dense, uniform, and full-coverage perovskite thin film. The resulting planar perovskite solar cells can achieve an impressive power conversion efficiency up to 19.00% with an average efficiency of 17.38 ± 0.70% for 32 devices with an area of 5 × 2 mm, 13.91% for devices with a large area up to 1.13 cm(2). The perovskite films can be easily fabricated in air conditions with a relative humidity of 45-55%, which definitely has a promising prospect in industrial application of large-area perovskite solar panels.

Characterization of YSZ solid oxide fuel cells electrolyte deposited by atmospheric plasma spraying and low pressure plasma spraying

Yttria doped zirconia has been widely used as electrolyte materials for solid oxide fuel cells (SOFC). Plasma spraying is a cost-effective process to deposit YSZ electrolyte. In this study, the 8 mol% Y2O3 stabilized ZrO2 (YSZ) layer was deposited by low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with fused-crushed and agglomerated powders to examine the effect of spray method and particle size on the electrical conductivity and gas permeability of YSZ coating. The microstructure of YSZ coating was characterized by scanning electron microscopy and x-ray diffraction analysis. The results showed that the gas permeability was significantly influenced by powder structure. The gas permeability of YSZ coating deposited by fused-crushed powder is one order lower in magnitude than that by agglomerated powder. Moreover, the gas permeability of YSZ deposited by LPPS is lower than that of APS YSZ. The electrical conductivity of the deposits through thickness direction was measured by potentiostat/galvanostat based on three-electrode assembly approach. The electrical conductivity of YSZ coating deposited by low pressure plasma spraying with fused-crushed powder of small particle size was 0.043 S cm−1 at 100 °C, which is about 20% higher than that of atmospheric plasma spraying YSZ with the same powder.

Relationship Between Lamellar Structure and Elastic Modulus of Thermally Sprayed Thermal Barrier Coatings with Intra-splat Cracks

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.

Measurement and numerical simulation of particle velocity in cold spraying

The velocity of cold spray particles was measured by a diagnostic system designed for thermal spray particles that is based on thermal radiation. A laser beam was used to illuminate the cold spray particles in cold spraying to obtain a sufficient radiant energy intensity for detection. The measurement was carried out for copper particles of different mean particle sizes. The particle velocity was also estimated using a two-dimensional axisymmetric model developed previously. The simulated velocity agreed well with the measured result. This fact indicates that particle velocity in cold spraying can be predicted reasonably by simulation. Therefore, it is possible to optimize the cold spray process with the aid of the simulation results.

Material nucleation/growth competition tuning towards highly reproducible planar perovskite solar cells with efficiency exceeding 20%

Since there are still challenges in fabricating high-quality large-area perovskite films, perovskite solar cells have limitations for industrial application. Here, we develop a material nucleation/growth competition theory to guide us to tune the nucleation and grain growth process of perovskite. Subsequently, we introduce a gas-flow-induced gas pump approach for the large-area deposition of dense, uniform and full-coverage perovskite films, and this process is simple with high manufacturing efficiency and a wide process window. This enabled us to fabricate uniform perovskite films on substrates with the largest area of up to 144 cm2. Normal planar perovskite solar cells were fabricated at pressures of 100 Pa, 500 Pa and 1500 Pa, achieving average efficiencies of 19.25 ± 0.50%, 19.17 ± 0.46% and 18.98 ± 0.51% respectively for 0.1 cm2 devices (84 devices in total) with ultrahigh reproducibility. A high fill factor of up to 80% was obtained at different pressures. A champion cell with an efficiency of 20.44% was obtained which is one of the highest efficiencies for normal planar perovskite solar cells. Furthermore, we achieved an efficiency of 17.03%, the highest efficiency for normal perovskite solar cells with the device area exceeding 1 cm2 and an average efficiency of 15.63 ± 0.80% with an area of 1.1275 cm2 (for 30 devices).

Preparation of flexible perovskite solar cells by a gas pump drying method on a plastic substrate

A uniform and full coverage perovskite film is of significant importance for flexible perovskite solar cells. In this study, highly efficient flexible perovskite solar cells were assembled using a flexible conductive plastic substrate by a one-step gas pump drying method to prepare high performance perovskite films under air conditions. The SEM results show that the perovskite film deposited on the flexible conductive plastic substrate was uniform, compact, and pinhole-free. The AFM results show that the film presented an extremely smooth surface morphology with a root mean square roughness of 15.8 nm in a large and representative scan area of 18 × 18 μm2. The flexible planar perovskite solar cell was fabricated, and all devices were prepared at 100 °C or below under air conditions. The highest efficiency on flexible substrates had reached 11.34% with an average efficiency of 8.93% for 14 solar cell devices.