Journal of Thermal Spray Technology最新文献

Thirty-five (35) years ago, efforts to incorporate high-temperature ceramics as hot section components of gas turbine engines were initiated by the High-Temperature Engine Materials Program (HITEMP) and followed by the High Speed Research–Enabling Propulsion Materials (HSR-EPM) and the Ultra-Efficient Engine Technology (UEET) programs. HITEMP and UEET were government programs, while the HSR-EPM program was a collaboration between the private sector and government research. Under these efforts, silicon carbide (SiC_f/SiC) ceramic matrix composites (CMCs) were identified as the most promising material for ultra-high-temperature conditions. However, these materials react with water vapor at high temperatures to form volatile hydroxides that can subsequently cause recession of the base SiC substrate. To combat the loss of material, a durable, protective layer referred to as an environmental barrier coating (EBC) was deemed necessary early on for the success of SiC-based CMCs. Thermal spray processing has been the ideal method for applying the candidate EBCs, and this field has grown considerably over the past 30 years. This retrospective takes a literature-driven approach to review the initial development of EBCs and the progression to current state-of-the-art coatings in flight. It also provides a perspective on the future of EBC materials and processing methods that may pave a path forward toward increased turbine inlet temperatures and reduced emissions.

In high-temperature, adverse environments, the premature failure of environmental barrier coatings (EBCs) is a critical phenomenon that can significantly impact their applications in both aircraft engines and land-based gas turbines. The delamination failure of EBCs typically occurs at the topcoat/TGO or TGO/bond–coat interfaces, primarily due to thermal and shrinkage strains, as well as water vapor corrosion, resulting in crack propagation and coating spallation. This paper presents a systematic study of the degradation of bi-layer disilicate Yb₂Si₂O₇ (YbDS)/Si EBCs using COMSOL Multiphysics methodologies. Thermal-cycle-induced temperature fields were implemented into the EBC model, aiming to simulate the system’s in-service operation. The high-temperature creep models of topcoat YbDS, monosilicate Yb₂SiO₅ (YbMS), silica TGO, Si bond coat, and SiC substrate are included for the built-up undulated coating geometries. Based on the local stress evolution and distribution in the EBC system during thermal cycles in water vapor environments, and on the transformation from YbDS to YbMS and TGO growth kinetics observed at elevated temperatures, the experimentally observed EBC degradation mode was explained in terms of the simulated results. The J-integral, virtual crack extension, and phase-field damage model were implemented to investigate crack nucleation and propagation under thermal cycles, considering the cristobalite TGO, which undergoes a displaced β-α phase transformation between 220 °C and 270 °C with an associated large volume shrinkage upon cooling. The significant phase-field value of TGO, due to its high tensile stress, leads to the automatic nucleation of cracks in TGO and their subsequent bifurcation and propagation along both YbDS/TGO and Si/TGO interfaces. The linking of these bifurcation-induced cracks during cooling cycles could be the primary mechanism leading to the EBC’s spallation and failure. The simulated TGO crack growth pattern was compared with that experimentally observed in the literature.

In surface treatment methods, thermal spray is one of the promising technologies to enhance material functionality and life. This study presents a numerical investigation of molten droplet impact on engineered substrates using the smoothed particle hydrodynamics (SPH) method on splat morphology. Aluminum substrates incorporating two cubic extensions and machine lines were examined. Experimental observations of alumina droplet impact were used to validate the SPH model, demonstrating the model’s capability to reproduce spreading, flow of molten material along or across the grooves, pore formation, and solidification. Simulations were extended to yttria-stabilized zirconia (YSZ) droplets to assess the role of thermophysical properties, including low thermal conductivity and high melting point, on the splat morphology. Results from the simulations show that YSZ droplets exhibit longer molten lifetime, deeper penetration into the valleys, and a complex lateral flow pattern that induces interfacial pores, particularly in denser configurations. Comparative analysis between alumina and YSZ highlights the coupled effects of surface patterns and material thermal behavior on spreading dynamics. These findings provide new insights into tailoring the substrate design and material selection to control splat morphology in additive manufacturing technologies based on different application conditions.

Nickel coatings are widely employed in industrial applications due to their excellent corrosion resistance and high-temperature stability. Among thermal spray techniques, cold spraying is increasingly preferred for its high deposition rate and ability to produce thick, dense coatings with low porosity while avoiding phase transformations. The corrosion performance of cold-sprayed coatings is primarily governed by inter-splat bonding, which plays a more critical role than the intrinsic properties of the feedstock material. Insufficient inter-splat bonding permits the ingress of corrosive media, leading to localized attack at splat boundaries. Additionally, the microstructure of the coating influences the formation and protective nature of the native oxide layer. This study investigates the effect of initial feedstock condition, specifically grain size variations produced via different atomization techniques, on cold-sprayed nickel coatings' deposition characteristics and corrosion behavior. Microstructural evolution during spraying and subsequent heat treatment was characterized using electron backscatter diffraction (EBSD), and corrosion performance was evaluated by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution. Coatings exhibiting severe plastic deformation and improved inter-splat bonding demonstrated enhanced corrosion resistance in the as-sprayed condition. Meanwhile, in heat-treated coatings, improved corrosion resistance is achieved by promoting a uniform grain size distribution, increasing the fraction of low-angle grain boundaries, and enhancing twin density.

The velocity at which particles impact the substrate during cold spray processes is a critical factor influencing both the quality of the deposition and the efficiency of metallic particle adherence. The nozzle serves as the primary source of aerodynamic force in the cold spray system and significantly influences the impact velocity of particles. This study proposes an optimized design method for cold spray nozzles based on nozzle-flow dynamics, aiming to enhance aerodynamic acceleration and thereby improving the deposition efficiency. Initially, a nozzle-flow model is developed based on the nozzle’s geometric configuration and subsequently modified to incorporate the specific characteristics of cold spray processes. Following this, an optimized nozzle design model is established to identify the maximum outlet flow velocity under conditions of zero outlet pressure difference, integrating considerations of practical manufacturing constraints and aerodynamic performance criteria. Ultimately, an optimized solution is obtained. The results indicate that the theoretical maximum outlet flow velocity can reach 944.6 m/s, while simultaneously enhancing aerodynamic acceleration effects and maintaining an optimal outlet flow pattern. Furthermore, simulation analyses and particle acceleration experiments are performed on the optimized nozzle. The simulation results demonstrate that the optimized nozzle exhibits superior acceleration performance across various process conditions. Experimental findings further reveal that the average particle velocity at the nozzle outlet is 531 m/s, confirming that particles undergo significant entrainment acceleration within the nozzle-flow.

Hastelloy X exhibits excellent high-temperature strength and oxidation resistance, making it suitable for applications like including nuclear reactors, jet engines, and gas turbine engines. Conventional thermal spray techniques can induce thermal stresses, phase changes, and oxidation in Hastelloy X deposits. Cold spray is a solid-state technique that enables the deposition of dense, oxide-free coatings while maintaining the alloy’s original microstructure. This numerical study investigates the impact of various powder injection angles (0°, 30°, 60°, and 90°) on the cold spray process for Hastelloy X superalloy powder. Using computational fluid dynamics (CFD), we analyze particle velocity and flow characteristics to identify the optimal injection angle. The selected turbulence model, k-ω SST, effectively characterizes turbulent flows, making it suitable for elucidating the dynamics of gas-particle interactions. Complex flow structures, including Mach diamonds and shock waves, were observed, creating alternating pressure and velocity fields that significantly affect particle momentum transfer. Among all cases, the 30° injection angle achieved the highest particle velocity and the most focused particle stream, enabling more efficient entrainment and mixing. It also showed the most focused particle stream. Experimental validation using Accuraspray CS recorded an in-flight particle velocity of 710 ± 20 m/s 30° powder injection angle, in good agreement with the CFD-predicted 765 m/s. Microstructural analysis of deposited coatings revealed dense Hastelloy X layers with low porosity (0.70% ± 0.02%) and high deposition efficiency (92%). These results demonstrate that injection-angle optimization is critical for achieving superior particle acceleration and coating quality in cold spray of Ni-based superalloys.

In this study, the aim was to enhance the CMAS resistance and thermal cycling lifetime of the environmental barrier coating produced by the atmospheric plasma spray method (APS) through laser glazing. In plasma-sprayed environmental barrier coatings, the transition from an amorphous to a crystalline phase under operational conditions leads to deterioration, reducing their longevity. The surface of the YbSi coating, which was applied to the SiC substrate using APS, was modified using a CO₂ laser that operated at 1.5 kW power, had a wavelength of 10.6 µm, and moved at a speed of 170 mm/s. Thermal cycle tests were conducted to examine the influence of laser surface modification on the thermal cycling performance of the coatings in corrosive conditions. Microstructures before and after laser surface alteration were analysed using SEM with EDS capability. XRD investigations were conducted to examine phase changes prior to and following thermal cycle tests. Laser glazing converted the amorphous phases in the YbSi layer into crystalline structures. XRD investigations were conducted to examine phase changes before and following thermal cycle tests. After laser glazing, the surface crystallinity of the extremely amorphous EBC coating increased by 39%. After surface modification, the surface roughness value of the coating was improved by roughly 40%, and the thermal cycle life was enhanced by up to 10 times. After the thermal cycling test in corrosive environment, cross-sectional EDS results showed that the development of corrosive salts such as V, Na, and Ca in the coated region was improved by at least 50%. Moreover, surface EDS data showed that after laser modification, corrosive salts accumulated more than in the unglazed sample.

Graphical Abstract

Cold spray has been used extensively to repair critical military and aerospace components, particularly aluminum and magnesium parts. To achieve high strength and excellent ductility, many repairs have been developed using helium process gas. Unfortunately, the high cost and limited supply of helium require alternative processing methodologies. This study evaluated the influence of in situ peening via ~ 50 mm ZrO₂-based particles on the deposition characteristics, microstructure, and mechanical properties of nitrogen-deposited Al6061 cold spray coatings. Three powder mixtures comprising Al6061 and 25, 50, and 75 wt% ZrO₂ powder were deposited using nitrogen gas on Al6061 and ZE41 magnesium substrates. Results were compared to 100% Al6061 coatings produced using nitrogen and helium. In situ peening resulted in signification plastic deformation of the Al6061 deposit, which scaled with increasing ZrO₂ content, resulting in increasing levels of dynamic recrystallization that exceeded helium deposited coatings. Al6061-ZrO₂ coatings exhibited up to 11-13% higher microhardness, 10-21% higher adhesion strength, and 30-33% lower mass loss, but with 12-28% lower tensile strength compared to helium deposited coatings. The latter was attributed to reduced particle-particle cohesive strength. All coatings exhibited compressive residual stress, which decreased with increasing ZrO₂ content due to widespread dynamic recrystallization. The findings from this study show the potential to achieve “helium-like” properties for Al6061 coatings produced with nitrogen gas.