Journal of Thermal Spray Technology最新文献

Interface microstructures of metallic coatings at end faces are critical for durability of metallic polypropylene (MPP) capacitors. However, the microstructures are difficult to be regulated efficiently due to the specificity of the polymer substrates. Therefore, ZnAl coatings were deposited at end faces of MPP capacitors by arc spraying. The temperature and velocity of in-flight droplets were regulated with different spray distance. A modified numerical simulation with a combustion model was employed to calculate the temperature and velocity of the droplets. The interface microstructures and equivalent series resistance of the capacitors were characterized. The results show that the temperature of the droplets continued to increase during the flight due to exothermic oxidation. The interface of the coatings with the spray distance of 150 mm presented a dendritic microstructure with deeper embedment depth and more bonding layers. The bonding layers reduced as the spray distance decreased to 120 mm because of the damage of the MPP layers. The embedment depth of the coatings decreased as the spray distance increased to 180 mm due to lower temperature of the droplets. The equivalent series resistance of the capacitors decreased to 7.84 mΩ with the dendritic interface microstructures. The research provides a new numerical model to optimize arc spraying and improve the quality of MPP capacitors.

Alumina nanoparticles were incorporated into CoNiCrAlY powders to fabricate an overlay coating of improved oxidation resistance for gas turbine blades via thermal spraying. In this regard, 6 wt.% alumina nanoparticles were agglomerated with CoNiCrAlY powders by modified suspension route and applied to samples of CMSX-4 nickel-based superalloy by high velocity oxygen fuel (HVOF) process. The coatings were characterized by X-ray diffraction, scanning electron microscopy and field emission scanning electron microscopy, EDS and elemental mapping, Vickers hardness and roughness measurement. Cyclic oxidation tests were performed to study the high-temperature oxidation behavior at 1100 °C. The results showed an increase in hardness, roughness and porosity with the addition of alumina nanoparticles to the coating. Furthermore, the oxidation resistance of CoNiCrAlY + 6 wt.% Al₂O₃ was improved as compared to conventional CoNiCrAlY after 100 cycles of oxidation; a reduction in the thickness of oxide layer and β depletion zone was observed. Formation of a dense and protective α-Al₂O₃ phase, instead of θ-Al₂O₃, was confirmed during the oxidation process in the coatings containing nanoparticles. It was concluded that nanoparticles prevent the penetration of elements to the surface and reduce the formation of non-protective oxide layer.

A computational framework is proposed for modelling particle bonding in cold spray. The model is based on the commonly-held view that bonding is a consequence of jetting, namely, the large plastic strains occurring at extreme rates upon particle impact. The model incorporates a bonding criterion at contacting boundaries by introducing a novel strain-like history variable referred to as the bonding parameter conjugate to a rate-dependent evolution law. In doing so, an analogy is made with classic damage mechanics where bonding is viewed as a similar but opposite process to fracture. Two new material constants are introduced, namely, the bonding toughness and the bonding toughness rate. Furthermore, a numerical implementation of the model in the Material Point Method (MPM) is presented which, thanks to a proposed regularization technique, is free of non-physical dependence on discretization parameters. The mesh-free nature of the MPM allows avoiding the numerical issues in conventional Lagrangian and Eulerian methods such as mesh distortion and artificial dissipation. The model is calibrated numerically for aluminum-aluminum material pair using an in-house computer program. Several numerical results are presented to demonstrate that the model can accurately capture material jetting and directly relate it to bonding within the simulation.

Thermal spray is an important surface treatment technique used in many industrial applications. Thermal spray processes involve molten droplets sprayed onto substrates. Heat transfer between the droplet and the substrate at different temperatures results in sharp temperature gradients and a phase change. Quenching stresses arise as a combined effect of phase change and the thermal mismatch between materials. It is important to characterize quenching stress for predicting material durability. However, such characterization is challenging due to the complex physics involved. In this study, the smoothed particle hydrodynamics method is used to predict the quenching stress in the thermal spray process for different droplet materials, including yttrium-stabilized zirconia (YSZ), stainless steel (SS), aluminum (Al), and alumina (Al₂O₃) impinging on various substrate materials. The present numerical model is validated against the experiments and previous numerical studies for splat behavior, time evolution of substrate temperature, and quenching stress. A parametric study investigates the main contributing factors to quench stress. The parametric study reveals that elevated substrate temperatures reduce thermal gradient, thus quenching stress. Compared to the differences in droplet material, the quenching stress shows increased sensitivity to the substrate material. Additionally, materials with high thermal diffusivity, such as SS, exhibit lower quenching stress due to their ability to dissipate heat quickly. Conversely, materials with lower thermal diffusivity, such as YSZ, show higher quenching stress because of slower heat dissipation. These findings provide critical insights into optimizing thermal spray processes to minimize quenching stress and enhance material durability.

Wear and corrosion of boiler tubes in coal-based boilers are one of the serious problems. Trying to solve this issue, FeCr alloy with 45%Cr-content coating and Fe-based coating with 13%Cr-content were arc sprayed onto carbon steel substrates to enhance both the wear and corrosion resistance of boiler heat exchanger pipelines. The microstructure, chemical compositions, and phases of the coatings were analyzed using a scanning electron microscopy, energy-dispersive spectrometer, and x-ray diffraction, respectively. The wear resistance of the coatings was assessed at 25 and 300 °C using a ball-on-disk wear tester. The corrosion resistance of the coatings was evaluated based on seawater immersion, electrochemical impedance, and polarization tests. The porosities of FeCr alloy and Fe-based coatings were 4.05 and 5.75%, respectively. The microhardness values of FeCr alloy and Fe-based coatings were 377.50 ± 46.88 HV_0.5 and 666.69 ± 57.64 HV_0.5, respectively. FeCr alloy coating with lamellar structure was mainly composed of FeCr solid solution phase and a small amount of Cr oxide and Fe₃O₄ phases, and Fe-based coating was composed of a mixture phase of amorphous and crystalline, and a small amount of Fe₃O₄ phase. FeCr alloy coating had better wear resistance than Fe-based coating at both 25 and 300 °C. The wear mechanisms of the coatings were also studied. The corrosion resistance of FeCr alloy coating was better than that of Fe-based coating in corrosive solutions. Therefore, FeCr alloy coating can provide better high-temperature wear resistance and anticorrosion performance for boiler heat exchanger piping, compared with Fe-based coating.

The effect of porosity on the wear behavior of YSZ abradable coating under simulated working conditions was studied using the high-temperature and ultra-high-speed abradability test rig. The results show that the porosity significantly influences the macroscopic morphology and abradability of the YSZ coating at the experimental temperatures of 1000 °C, with the blade tip velocity of 350 m/s, and the feed rate of 50 μm/s. The wear degree of the blade gradually decreases as porosity increases, and the incursion depth ratio (IDR) dramatically decreases. When the porosity reaches its maximum value, the wear scar of the coating is smoothest, and there is no discernible wear on the blade, the IDR value reaches its minimum, and the abradability of the coating reaches its maximum. Besides, brittle fracture in the YSZ coating with high porosity is concluded to be the reason for better abradability.

Wire thermal spraying, one of the significant coating preparation technologies in the field of thermal spraying, has the advantages of low cost, high material utilization rate and fast coating deposition. Powder-cored wires, with easily controllable compositions, are used as spraying materials to prepare functional coatings with special properties. Coatings prepared by traditional wire thermal spraying technologies, mainly including wire flame spraying (WFS), wire arc spraying (WAS) and plasma wire spraying, have some defects, such as weak bonding strength and high porosity. In this paper, the plasma transferred wire arc spraying (PTWAS) technology was innovatively proposed, by which Al/SiC powder-cored wires were successfully sprayed to deposit the aluminum (Al)/Nicalon (SiC) composite coating. Furthermore, the influences of spraying current (I), argon (Ar) flow rate (L_Ar), hydrogen (H₂) flow rate (L_H2) and other factors on Al/Sic powder-cored wires prepared by PTWAS and the optimization of the coating preparation process were mainly studied via the single factor method and the response surface methodology. After experimental exploration and analysis, the optimized process parameters were finally determined as follows: L_Ar was 120 L min⁻¹, I was 160 A, L_H2 was 5 L min⁻¹, the spraying distance was 100 mm, the wire feeding speed (V) was 0.18 m s⁻¹, and the distance between the wire and nozzle (d) was 10 mm. It was found in the test that the porosity of the optimized Al/SiC composite coating was only 1.6%, the average microhardness was 102 HV_0.1, and the average bonding strength was 36.5 MPa. The comprehensive properties of this coating were better than those of the Al/SiC composite coatings prepared by WFS and WAS.

This study presents a numerical additive manufacturing simulation aimed at simulating the shape recovery process of a steam turbine blade damaged by corrosion, using laser-directed energy deposition (LDED). The simulation integrates the finite element (FE) method with heat conduction and thermo-elastoplastic constitutive equations, incorporating phase transformation. The additive manufacturing process by LDED was modeled using the death-birth algorithm, wherein a deposition layer is defined as a virtual element. Its stiffness and thermal properties activated when the laser irradiation regions overlapped. In this study, the shape of the virtual element was determined based on the cross-sectional shape of the deposition layer manufactured under various laser conditions. To validate the numerical simulation results, additive manufacturing was conducted for one pass deposition in the width direction at the center of a cantilever-supported plate made of SUS304 steel, and the changes in displacement at the free edges with respect to the process time were compared. The obtained FE results are in good agreement with the experimental results. Finally, an FE simulation was performed for the shape recovery of a steam turbine blade thinned due to corrosion damage. The results revealed that the residual stress component becomes more compressive as the laser output decreases and scanning speed increases, which is advantageous for improving the fatigue strength of steam turbine blades.

Tungsten (W) has a high melting point, excellent thermal conductivity, and irradiation resistance, making it the most promising plasma facing material for divertors in fusion reactors, which are currently under development. However, since the divertor is exposed to an extremely harsh environment, it is considered necessary to develop suitable and cost-effective repair techniques. In this study, the applicability of the atmospheric plasma spraying (APS) method using a gas shroud as a repair technique for W components was investigated, in particular the possibility of strengthening the repaired part by applying friction stir processing (FSP) as a post-treatment. It was found that the application of a gas shroud can suppress in-flight oxidation to some extent, even when the W is deposited in air. In addition, the FSP treatment reduced grain size and porosity, resulting in an increase in microhardness of approximately 37.5% compared to the base material (W substrate) and 203.5% compared to the as-sprayed material. The gas shroud APS and FSP post-treatments have been shown to have potential as repair techniques for tungsten components in future fusion reactors.

The growth of a thermally grown oxide (TGO) layer has been identified as a major driving force for the failure of environmental barrier coatings (EBCs). It is always desirable to reduce the TGO growth rate in order to achieve a highly durable EBC system. In this study, an Al₂O₃-modified Si bond coat was developed for EBC applications. Both a Yb₂Si₂O₇/Si baseline EBC and a Yb₂Si₂O₇/(Si-Al₂O₃)-modified EBC were deposited using the air plasma spray process. The TGO growth behavior and cycling life of the EBCs were evaluated at 1316 °C in a 90% H₂O (g) + 10% air environment. The TGO growth rate in the baseline EBC is over four times faster than that of the modified EBC. The modified EBC survived 1000 cycles of steam testing without failure, while the baseline EBC has an average life of 576 cycles under identical conditions. The superior durability of the modified EBC can be attributed to the significantly reduced TGO growth rate.