Journal of Thermal Spray Technology最新文献

Ceramic matrix composites (CMCs), particularly SiC/SiC, exhibit outstanding properties making them ideal for applications in the hot sections of aircraft engines. However, they require protection against high-temperature water vapor corrosion: To address this challenge, environmental barrier coatings (EBCs) are currently one of the most widely used solutions for protecting CMC components. This work proposes the deposition and characterization of rare earth (RE) silicates produced using the atmospheric plasma spray (APS) technique. Four different multilayer RE silicate APS coatings were deposited on SiC substrates and a bond layer of Si, obtained by APS: (i) yttrium/ytterbium disilicate (YbYSi₂O₇), (ii) scandium disilicate (Sc₂Si₂O₇), (iii) ytterbium disilicate (Yb₂Si₂O₇) and (iv) ytterbium disilicate with ytterbium monosilicate as a topcoat (Yb₂Si₂O₇/Yb₂SiO₅). Since few research works focus on thermal spray-deposited YbYSi₂O₇ and Sc₂Si₂O₇, it was necessary to test several sets of APS parameters in order to find an optimized deposition strategy. EBC samples were exposed to a mixture of 90 mol.% H₂O and 10 mol.% air at 1300 °C for up to 500 h. All samples were characterized using SEM/EDS, XRD and image analysis (crack density and porosity) to assess the quality of the as-sprayed coatings and to analyze the microstructural modifications induced by the high-temperature exposure tests. The results highlight the strong dependence of the protective capability on the microstructural features, particularly in terms of coating cohesion and crack density.

In this study, a CNT-reinforced nanocomposite powder containing 2 wt.% carbon nanotubes was synthesized via spray drying and deposited onto an AA7075 aluminum alloy substrate using the low-pressure cold spray (LPCS) technique. The nanocomposite powder was produced by ultrasonically dispersing CNTs onto lead particles in an aqueous solution, followed by spray drying at low temperatures to avoid damaging the CNTs. The resulting powders were spherical and exhibited homogeneous CNT distribution. Coatings obtained through LPCS demonstrated a dense microstructure with a low porosity level of 2.77%, despite the relatively high CNT content. SEM/EDS analyses confirmed CNT preservation, while EDS mapping revealed a uniform carbon distribution within the matrix, indicating effective CNT embedding and reducing the likelihood of localized agglomeration. Elemental analysis verified CNT incorporation, showing 7.30 wt.% carbon and negligible sulfur, which—together with microstructural observations—confirmed CNT retention and their reinforcing effect. Nanoindentation and microhardness tests revealed that the incorporation of 2 wt.% MWCNTs into the lead–alumina matrix increased hardness significantly (from 13.1 to 20.5 HV), while the elastic modulus showed only a small increase (35 to 36 GPa). The CNT-reinforced coating also exhibited more elastic and limited deformation, indicating improved resistance to plastic deformation without a notable change in stiffness. The results indicate that the assumed approach effectively addresses common challenges in CNT/metal powder production, such as agglomeration and poor bonding, and enables the production of dense composite coatings with enhanced mechanical properties.

AISI 4340 steel was deposited using cold spray (CS) and laser-assisted cold spray (LACS) techniques at different surface temperatures of 500, 738, and 950 °C. To study the microstructure evolution and phase transformation, scanning electron microscopy and x-ray diffraction were used on both the starting powders and the CS and LACS deposits. The dislocation density was determined by x-ray line profile analysis at the top and bottom of the CS and LACS deposits. It was found that the grain size, dislocation density, and hardness show significant differences between the top and bottom of the deposits at higher surface temperatures, such as 738 and 950 °C. Laser heating at 500 °C during the CS deposition reduced the dislocation density compared to the cold-sprayed steel sample without laser heating. Higher laser temperatures (738 and 950 °C) resulted in significantly higher dislocation densities, which can be attributed to the strains caused by the martensitic phase transformation. The hardness variation up to 738 °C reflects changes in dislocation density. At 950 °C, the hardness is lower than expected due to grain growth compensating for the high dislocation density.

This study systematically investigated the effects of plasma-assisted cold spraying process and gas pressure (3.5-5.0 MPa) on the microstructure and properties of 2219 aluminum coatings. The results showed that increasing the gas pressure and introducing a plasma heat source significantly enhanced the coating thickness and improved particle bonding strength. This mechanism is attributed to the synergistic effect of gas pressure-enhanced particle kinetic energy and plasma heat source-induced particle thermal softening. The high temperature (450 °C) induced by the plasma heat source causes the decomposition of the Al₂Cu phase, forming a Cu-depleted zone, as confirmed by the reduced intensity of the Al₂Cu diffraction peak in XRD analysis. EBSD revealed an increase in dynamic recrystallization rate in the PACS coating, with the average grain size decreasing to 2.391 μm. In terms of mechanical properties, the bonding strength of the PACS coating increased to 43.32 at 5.0 MPa pressure, and the fracture surface exhibited ductile indentation characteristics, confirming that the plasma heat source promoted local metallurgical bonding. Additionally, the coating hardness increased to 107.98 HV (reaching 80% of the substrate hardness), and the wear rate decreased to 0.59 × 10⁻³ mm³/(N m), representing an 80% reduction compared to the CS coating under 3.5 MPa air pressure. This process overcomes the limitations of traditional cold spraying interface bonding by adjusting gas pressure and introducing a plasma heat source to induce thermal activation and dynamic recrystallization.

Bond coats are used to protect the superalloy from oxidation and to act as a bond between the ceramic thermal barrier coating (TBC) layer and the superalloy. During high-temperature exposures, a thermally grown oxide (TGO) layer forms between the bond coat and the topcoat due to oxygen diffusion. Although being fundamental for the substrate oxidation protection, further growth of the TGO creates severe thermal stresses at the bondcoat - topcoat interface, leading to coating failure in the components. This study aimed to investigate the TGO and microstructure evolution of three TBCs with different cold-sprayed bond coat alloys after isothermal heat treatments. The TBCs were heat treated at 1100 °C for periods of 12, 25, and 50 hours to study the effects of temperature on the microstructure and phase distribution. The microstructure of heat-treated bond coat alloys was examined using scanning electron microscopy and x-ray diffraction. The TBC with NiCoCrAlTaReY bond coat demonstrated superior performance compared to the other two systems. All TBC systems failed after 50 hours of thermal exposure, as the ceramic topcoat spalled due to crack propagation assisted by thermal stresses.

This work presents a modified approach to Focused Ion Beam (FIB) milling, specifically tailored to preparing cross-sections of plasma sprayed resolidified ceramic splats, was developed at the Center for Thermal Spray Research at Stony Brook University and refined at the Forschungszentrum Jülich Thermal Spray Center. Two key advantages were gained from this modified approach: relatively large (100 µm long by 25-50 µm wide by 5-10 µm deep) ion-milled trenches were possible in as little as several hours of milling time and the traditional platinum overcoat was deemed not necessary. Images of single and double-splat cross-sections show the through-thickness crack propagation of mudflat cracks. Refinement of this method allowed for even shorter (up to or less than one hour) milling times for splat cross-section viewing, and concurrent Energy Dispersive Spectroscopy for chemical analysis of unique splat combinations of Thermal Barrier Coating (TBC) splats atop Environmental Barrier Coating (EBC) splats. Additionally, time-lapse imaging during the ion-milling process garnered additional insights about subsurface features within the splats. The results in this paper and the time-lapse animations online revealed subsurface ‘bubbles’ embedded in EBC splats—dependent on the process parameters. Finally, the TBC-on-EBC splat cross-section points toward the presence of viscous flow of solid amorphous EBC material during microsecond solidification of a molten TBC splat.

In recent years, the increase in gas turbine inlet temperatures has led to the problem of excessive temperatures in gas turbine blades. To address this issue, thermal barrier coatings (TBCs), internal cooling, and film cooling have been applied to the hot-side surfaces of the blades. However, it remains necessary to manage crack initiation and spalling damage of the TBC caused by thermal fatigue around the cooling holes. In this study, a thermomechanical fatigue analysis was performed on a TBC/IN738LC plate-type substrate with cooling holes using finite element analysis software. The analysis incorporated a damage-coupled inelastic constitutive equation for TBC developed by our research group. The damage and thermal stress fields around the cooling holes under cyclic loading were analyzed under thermomechanical fatigue conditions, considering both in-phase conditions—where the temperature and load patterns were synchronized—and out-of-phase conditions, where they were out of sync. Under temperature and cyclic loading conditions where the mechanical stress level around the cooling holes was lower than the thermal stress, thermal fatigue cracks were initiated at the edges of the cooling hole, notably at the 3 and 9 o’clock positions, corresponding to the loading direction, and the cracks propagated near the top coating (TC)/substrate interface. Conversely, under loading conditions where the mechanical stress exceeded the thermal stress, thermal fatigue cracks initiated at the edges of the cooling hole at 6 and 12 o’clock positions, and the cracks propagated rapidly near the TC surface. The stress level was higher in the out-of-phase condition than in the in-phase condition, and the fatigue crack initiation life was shorter. However, the differences in the temperature loading patterns had little effect on the crack initiation location.