Plasma Spray-physical Vapor Deposition Research Articles

Molten salt corrosion behavior of laser remelted PS-PVD YSZ thermal barrier coatings

Molten salt from the service environment leaches Y2O3 from yttria-stabilized zirconia (YSZ), threatening the security of thermal barrier coatings (TBCs). In this study, we compared the molten salt corrosion resistances of YSZ coatings prepared via plasma-spraying physical vapor deposition (PS-PVD) before and after laser remelting. At 1000 °C, the molten salt reacted with YSZ particulates in the PS-PVD YSZ coating and infiltrated the gaps between quasi-columnar crystals, resulting in phase transition of the coating and column breakage. A dense glazed layer was formed on the YSZ coating surface after remelting by the low-energy-density laser, which prohibited the penetration of molten salt and improved the hot corrosion resistance of the YSZ coating. By contrast, the failure of the high-energy-density laser-treated YSZ coating was accelerated due to the significant interfacial stress from the thick glazed layer and volume expansion caused by the corrosion products. Our investigation revealed the structural evolution of the PS-PVD YSZ coating under hot corrosion conditions, emphasized the role of laser remelting, and could serve as a guideline for TBCs modification.

Collapse characteristics and deposition behaviour of PS–PVD powders with different crushing strengths

Plasma spray–physical vapour deposition uses finely agglomerated spherical powders as raw materials; these powders undergo a complex process of dispersing–melting–evaporation–condensation in high-temperature and high-speed plasma jets to attain the desired physical phase and high-performance columnar-like structure thermal barrier coatings. Currently, the interactions between the powders and the jets have not been fully disclosed. The article examines the crushing strength of powders with various particle sizes and properties. To further elaborate the crushing process, discrete element simulation was used. Additionally, powder spray deposition experiments were conducted to disclose the association between powder properties and the efficiency of coating deposition. The findings indicate that the stiffness of the powder and the size of the internal particles have an impact on the crushing strength of the powder. The stronger the bond strength caused by greater powder stiffness, the more effective the dispersion effect becomes. Furthermore, the smaller the particle size, the higher the compressive and collapse strength. Therefore, the greater the crushing strength of the powder, the higher the rate of powder deposition.

Reactive deposition of CYSZ coatings using PS-PVD technology

CYSZ (CeO2-Y2O3-ZrO2) coatings were prepared via PS-PVD (plasma spray physical vapor deposition) using untreated YSZ and CeO2 blend powders instead of commercially CYSZ powders. 8YSZ was mixed with different proportions of CeO2 (10, 20and 30 wt%). CYSZ agglomerated composite powders were then produced via spray drying and CYSZ coatings were deposited through PS-PVD spraying. Microstructures of composite powders and coatings were observed using SEM and EDS, and characterization of powders and coatings were evaluated via XRD, laser particle size measurement and XPS. Results showed agglomerated composite powders possessed high percentages of particles with optimum size and were suitable for subsequent PS-PVD process. The coatings prepared with reactive deposition had a typical columnar structure as current coating method. The coatings exhibited uniform distribution of all elements and consisted mainly of the t-ZrO2 phase. Crystal structure and properties of coatings were identical to coating deposited via solid phase reactive powder.

Influence of structural characteristics of a Si nanoparticulate anode on all-solid-state Li-ion batteries

Si nanoparticles with independently controlled size and oxygen content have been produced by plasma spraying physical vapor deposition followed by the retarded oxidation. These nanoparticles are used as the negative electrode of all-solid-state batteries with sulfide solid electrolyte, and the influence of size and oxygen content on battery performance has been analyzed. The cells containing Si nanoparticles smaller than 150 nm with the oxygen content x in SiO x smaller than 0.1 have attained relatively high capacity and a good stable cyclability simultaneously after 50 cycles. This could be due to the formation of unique and uniform synaptic-like Si network with small Si nanoparticles within the electrode maintaining a firm contact with the Cu foil, which contrasts to large lateral crack formation for the cell with large Si particles.

Layer structure formation induced by in-situ deposit surface temperature under different scanning methods during thermal spraying

Plasma spraying-physical vapor deposition (PS-PVD) and atmospheric plasma spraying (APS), as new thermal spraying process, could prepare advanced thermal barrier coatings by successive scanning deposition layer by layer. To investigate the layer structure of the coating, the transient in-situ deposit surface temperature was investigated by varying the scanning methods. Results show that, compared to unidirectional scanning method, alternating scanning method results in doubling heating duration, higher in-situ deposit temperature rising and thereby thicker well-bonded layer structure during each spray cycle for the coating deposited at the edge position of the scanning path. The erosion resistance of the coatings deposited by unidirectional scanning method is independent on deposition positions including edge position and center position. However, the erosion resistance of the coatings deposited at edge position by alternating scanning method is one time higher than those deposited by unidirectional scanning method. Finally, the erosion mechanism of layer spallation is revealed to explain the difference between different scanning methods.

Mechanical properties and simulated thermal conductivity of biomimetic structured PS-PVD (Gd0.9Yb0.1)2Zr2O7 thermal barrier coatings

Plasma spray physical vapor deposition (PS-PVD) (Gd0.9Yb0.1)2Zr2O7 (GYbZ) thermal barrier coatings (TBCs) exhibited better silicate-phobicity than coatings produced by electron beam physical vapor deposition. In combination with PS-PVD and ultrafast laser direct writing technology, biomimetic structured GYbZ TBCs, with a triple-scale micro/nano surface microstructure, were obtained. Laser ablating on the PS-PVD GYbZ coating enhanced the surface roughness, improving its wear resistance without increasing the surface hardness. Furthermore, during the laser ablation processing, numerous nanoparticles were deposited in-situ in the gaps between columns of the coating, reducing the coating Young's modulus. The simulated temperature field and heat flux field demonstrated that the presence of numerous interfaces between small columns of the PS-PVD coatings is beneficial to thermal insulation. However, laser ablation decreased the coating thickness, reducing the thermal insulation by around 20%–30% as compared to its PS-PVD counterpart, suggesting that a moderate increase in the coating thickness should be considered when designing an efficient TBC system.

Microstructure and mechanical properties evolution in tri-layer Si-HfO2/Yb2Si2O7/Yb2SiO5 environmental barrier coating by PS-PVD during post-annealing

In recent years, the plasma spray-physical vapor deposition (PS-PVD) process has garnered significant attention as an advanced method for producing environmental barrier coatings (EBCs), and the tri-layer Si-HfO2/Yb2Si2O7/Yb2SiO5 coating system as the EBCs of SiCf/SiC has received widespread attention for potential applications in hot sections of aero-engines. Unfortunately, the peeling phenomenon of these EBCs in the process of high-temperature gas erosion is serious and reduces the service life of the coating system. In this research, tri-layer Si-HfO2/Yb2Si2O7/Yb2SiO5 coatings were deposited on SiCf/SiC with PS-PVD. After spraying, the samples were annealed at 1200 °C, 1300 °C and 1400 °C for 10 h, 30 h and 50 h, respectively. The effects of annealing parameters on the microstructure and mechanical properties of the coating were investigated. Annealing resulted in an increase in grain size, healing of microcracks and pores, accompanied by a decrease in surface roughness and porosity. The change in coating microstructure led to variations in the mechanical properties of the coating, including the residual stress, hardness and elastic modulus of the coating. In addition, the diffusion behavior of bonding layer elements was investigated. Furthermore, the bonding strengths of as-sprayed and annealed samples were obtained by tensile testing. High-temperature annealing enhanced the bonding strength between coating and substrate by promoting the diffusion of silicon elements into the substrate to form strong intergranular bonding. The prerequisite for the diffusion of silicon was that it was not oxidized during annealing. However, long-term high-temperature annealing leads to the formation of pores in the bonding layer, which reduce the bonding strength.

Insights into the primitives and sequence deposition mechanism of 7YSZ TBCs respectively by EB-PVD and PS-PVD

The deposition mechanism of 7YSZ (7 wt% Y2O3-ZrO2) thermal barrier coatings (TBCs) prepared by plasma spray–physical vapor deposition (PS-PVD) and electron beam–physical vapor deposition (EB-PVD) were investigated in detail. These coating primitives (grains) were deposited using atomic vapor. The PS-PVD primitives were short rod–shaped, while the EB-PVD primitives were needle-shaped. The PS-PVD coating primitives were ordered in the interior but disordered on the surface due to quenching under the influence of the plasma flow. However, the EB-PVD coating primitives were ordered from the interior to the surface. The feather-like columnar structure in PS-PVD was sequenced due to the vapor-liquid-solid mixed-phases deposition. In contrast, the needle-like columnar structure in EB-PVD was sequenced because of the single-vapor phase deposition. In addition, we investigated the effect of non-line sight deposition of PS-PVD on the coating, which can reveal the coating pattern on complex blade shapes. The results show that the deposition mechanism of 7YSZ TBCs can be divided into two stages, including primitives and sequences.

The failure behavior analysis based on finite element simulation of PS-PVD (Gd0.9Yb0.1)2Zr2O7/YSZ coatings during burner rig tests

A double-ceramic-layer (DCL) thermal barrier coating consisting of quasi-columnar structured (Gd0.9Yb0.1)2Zr2O7 (GYbZ) on top and yttria-stabilized zirconia (YSZ) beneath was deposited by plasma spray-physical vapor deposition (PS-PVD). The as-deposited GYbZ/YSZ coating is composed of a dense continuous layer at the bottom and a columnar layer growing on it. The columnar layer comprises taller coarse columns, shorter tapered columns, and particles filling in the gaps between the columns. The thermal shock behavior of the coating is evaluated in a burner rig at 1300 °C, during which the coating mainly fails in the form of delamination. The coating initially delaminates in the regions where the particles are embedded beneath the taller GYbZ columns tips. Subsequently, delamination occurs in the lower region of GYbZ coating and eventually fails at the TGO/YSZ interface. A finite element model tailored to the microstructural characteristics of the DCL coating was established. Stress distributions of the coating indicate that the discontinuities of the strain tolerance caused by abrupt microstructural or componential changes lead to multiple out-of-plane stress (σ22) concentration spots. The position and procedure of the delamination of GYbZ coating obtained based on the energy release rate of each σ22 concentration spot are consistent with experimental results.

Enhancing CMAS corrosion resistance of PS-PVD Si/Yb2Si2O7 EBCs via Al-modification

Enhancing the resistance to molten silicate corrosion is crucial for the long service life of environmental barrier coatings (EBCs). In this study, we used the Al-modification technique to enhance the CMAS corrosion resistance of Si/Yb2Si2O7 coatings prepared by plasma spray-physical vapor deposition. The results show that the Al-modified Yb2Si2O7 coating had higher resistance to CMAS corrosion than the Yb2Si2O7 coating annealed at 1300 ℃ for 100 h, which is related to the refractory mullite and Yb2Mg(AlO2)2O3 generated during the CMAS exposure of Al-modified Yb2Si2O7 coating. The Al-modified Yb2Si2O7 coating also exhibited excellent resistance to oxygen penetration. The Al-modification technology provides the direction for the corrosion resistance of Yb2Si2O7 system to CMAS.

Research on reducing infrared emissivity of 8YSZ coating by regulating microstructure

Yttria stabilized zirconia (YSZ) has low thermal conductivity and high electrical conductivity in high temperature environment, and has great application potential in the field of infrared stealth. Herein, we used plasma spraying physical vapor deposition (PS-PVD) and air plasma spraying (APS) to prepare 8YSZ coatings with different microstructures by regulating the spraying distance and surface treatment, and explored the influence of the microstructures on the emissivity of the 8YSZ coating. The results show that the 8YSZ coating prepared by APS has a distinctive lamellar porous structure, and its emissivity is lower than that of the 8YSZ feather columnar prepared by PS-PVD. In addition, the plasma surface treatment further reduces the emissivity of the coating prepared by APS. The emissivity significantly decreased from 0.54 to 0.43 at 3–5 μm waveband along with decreased surface roughness.

Molten-Volcanic-Ash-Phobic Thermal Barrier Coating based on Biomimetic Structure.

Volcanic ash is a major threat to aviation safety. The softening/melting temperatures of volcanic ash lie far below typical aero-engine operating temperatures. Thus, molten ash can accelerate the failure of thermal barrier coatings (TBCs). Here, inspired by natural superhydrophobic surfaces (e.g., the lotus leaf), a molten-volcanic-ash-phobic TBC, which provides a large possibility to eliminate molten ash issues of TBCs, is developed. A hierarchically structured surface is first prepared on a (Gd0.9 Yb0.1 )2 Zr2 O7 (GYbZ) pellet by ultrafast laser direct writing technology, aiming to confirm the feasibility of the biomimetic microstructure to repel molten volcanic ash wetting. Then biomimetic-structured GYbZ TBCs are successfully fabricated using plasma spray physical vapor deposition, which reveals "silicate" phobicity at high temperatures. The exciting molten-volcanic-ash-phobic attribute of the designed surfaces is attributed to the lotus-leaf-like dual-scale microstructure, emulating in particular the existence of nanoparticles. These findings may be an important step toward the development of next-generation aviation engines with greatly reduced vulnerability to environmental siliceous debris.

Heterogeneous layered structure in thermal barrier coatings by plasma spray-physical vapor deposition

The unique columnar structure endows thermal barrier coatings (TBCs) prepared by plasma spray-physical vapor deposition (PS-PVD) with high thermal insulation and long lifetime. However, the coating delamination failure resulting from an intra-column fracture (within a column rather than between columns) is a bottleneck in the solid dust particle impact environment for aero-engine. To clarify the intra-column fracture mechanism, a basic layer deposition model is developed to explore a heterogeneous weak-to-strong layered structure formed by a local transient <i>in-situ</i> deposit temperature. During the PS-PVD, an <i>in-situ</i> deposit surface is continuously updated due to constantly being covered by vapor condensation, showing a transient temperature, which means that the <i>in-situ</i> deposit surface temperature rises sharply in short period of 0.2 s of depositing a thin layer during a single pass. Meanwhile, the increasing temperature of the <i>in-situ</i> deposit surface results in an experimentally observed heterogeneous weak-to-strong structure, showing a continuous transition from a porous weak structure at the bottom region to a dense strong structure at the top region. This structure easily makes the intra-column fracture at the porous weak region. The results shed light on improving TBC lifetime by restraining the intra-column fracture.

Calcia-magnesia-alumino-silicate-induced degradation of (Gd0.9Yb0.1)2Zr2O7 thermal barrier coatings prepared by plasma spray-physical vapor deposition (PS-PVD)

(Gd0.9Yb0.1)2Zr2O7 (GYbZ) coatings with quasi-columnar structure prepared by PS-PVD were isothermally exposed to Calcia-magnesia-alumino-silicate (CMAS) at T = 1300 °C for t0 = 0.1 h, t1 = 1 h, t2 = 10 h, and t3 = 100 h. The resistance of GYbZ coatings against the CMAS melt penetration was investigated by wetting test and the reaction products were analyzed by energy-dispersive X-ray spectroscopy (EDS). The results show that a dense reaction layer was formed after isothermal exposure for t1 = 1 h, and the thickness of the reaction layers was almost the same from t1 = 1 h to t3 = 100 h, which indicated that the penetration of CMAS was basically suppressed. The particles between columns played a significant role in the rapid generation of a large amount of CMAS barrier apatite layer which prevented the penetration of the CMAS melt in the axial direction. The main reaction products were Gd-apatite, fluorite and spinel of which the Gd-apatite slowly grew from “free-island” to “net-frame”.

Dual-phase nanostructured coating with high hydrophobicity

It has been extensively studied how lotus leaf self-cleans, but most biomimetic research materials are polymers, where the surface structure is vulnerable to damage. In this research, we proposed a dual-phase nanostructured coating with high hydrophobic structure, aiming to obtain the bionic lotus leaf surface structure. We created the surface structure of the bionic lotus leaf using plasma spray-physical vapor deposition technology. It was demonstrated that the contact angle was greater than 130°, which has great application potential in the field of waterproofing and hydrophobicity.

Corrosion behavior of PS-PVD spray Yb2Si2O7 environmental barrier coatings during continuous water vapor exposure

The corrosion behavior of Si/Yb2Si2O7 environmental barrier coatings prepared by plasma spray-physical vapor deposition during continuous water vapor exposure were investigated at 1350–1450 ℃ over 500 h. The results showed that the temperature had a major impact on the corrosion morphology of the EBCs. The various corrosion morphologies of Yb2Si2O7 affected the oxidation kinetics of the TGO, and therefore the TGO had a porous structure, as was attributed to the Si-SiO2 phase transition and H2 placeholder aggregation. Consequently, the Si/Yb2Si2O7 EBCs need to be added a protecting layer or film that is more resistant to steam corrosion.

Al2O3-modified 7YSZ thermal barrier coatings for protection against volcanic ash corrosion

To prevent volcanic ash corrosion using thermal barrier coatings (TBCs), a novel method using Al2O3-modification was proposed to reduce molten CMAS wettability on TBCs surface through a lotus effect. Plasma spray-physical vapor deposition (PS-PVD), a third-generation method for TBCs fabrication, was adopted to deposit 7YSZ TBCs with a feather-like microstructure. Then, Al2O3 modification was introduced to fabricate a dense Al2O3 overlay with nano/micro-sized grains on the TBCs surface. The wetting ability of CMAS on APS/EB-PVD/PS-PVD 7YSZ TBCs was comparatively in situ observed at 1250 °C for 3600 s. The results indicated that the dense Al2O3 overlay inhibited penetration of molten CMAS. Additionally, the micro/nano dual-sized structure, which is similar to the papillary structure of a lotus leaf, reduced molten CMAS wettability on the TBCs surface. The results demonstrated that the Al2O3-modified TBCs had better CMAS corrosion resistance than the as-deposited APS/EB-PVD/PS-PVD TBCs.

Thermally Grown Oxide Stress in PS-PVD and EB-PVD Thermal Barrier Coatings Observed at Various Lifetimes Via Synchrotron X-ray Diffraction

Abstract The current standard application method for thermal barrier coatings (TBCs) on turbine blades for jet engines is electron-beam physical vapor deposition (EB-PVD) due to its high strain tolerance and low thermal conductivity. An emerging deposition method, plasma-spray physical vapor deposition (PS-PVD), presents an opportunity for a tailorable microstructure, and non-line-of-sight deposition that is faster and less expensive. To compare the lifetime behavior of both PS-PVD and EB-PVD coatings, samples subjected to 300 and 600 thermal cycles were measured during a 1 h thermal cycle to determine the strains, which were converted to stress, in the thermally grown oxide (TGO) layer of the TBCs using synchrotron X-ray diffraction (XRD). Room temperature XRD measurements indicated among samples that PS-PVD coatings experienced greater variation in in-plane room temperature strain in the TGO after cycling than the EB-PVD coatings. In-situ XRD measurements indicated similar high-temperature strain and no spallation after 600 thermal cycles for both coatings. Microscopy imaging after cycling showed greater rumpling in PS-PVD coatings that led to different failure modes between the two coatings’ TGO layers. The tailorability of PS-PVD coatings allows for adjustments in the processing parameters to improve their overall performance after aging and bridge the differences between the two deposition methods.

Effects of different nano-agglomerated powders on the microstructures of PS-PVD YSZ coatings

Plasma spray physical vapor deposition (PS-PVD) is a technology that combines the advantages of traditional atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). As the feedstock of the PS-PVD, nano-agglomerated powder is critical on determining the microstructure of the obtained coating. In this study, a method to characterize the cohesion of nano-agglomerated powders was investigated. The nano-agglomerated powders fractured into smaller particles under ultrasonic waves. Their particle size distributions were measured to quantitatively compare their cohesiveness. The change rate in the percentage of powders with particle size less than 5 μm was selected as the value for the cohesion comparison. A high change rate corresponded to a faster fracture and lower powder cohesion. Furthermore, the fracture behavior and heat and mass transfer process of nano-agglomerated powders in the plasma torch were studied by combining 3-D simulation and observation of the microstructures of PS-PVD coatings sprayed with different powders. To obtain a quasi-columnar coating, the nano-agglomerated powder required high cohesion. Finally, a suitable powder was selected and quasi-columnar structure coatings were obtained by optimizing the PS-PVD parameters.

Influence of air plasma spraying process parameters on the thermal barrier coating deposited with micro- and nanopowders

Abstract This study investigates the optimal conditions to deposit a thermal barrier coating using micro- and nanopowders in the air plasma spraying (APS) process. The influence of the APS process parameters on the thickness, porosity and hardness of the yttria-stabilized zirconia (YSZ; ZrO2 × 8Y2O3) coatings deposited with a single-electrode plasma gun was determined. The temperature and velocity of melted particles were determined by the DPV diagnostic system to decrease the number of experimental processes. The current and H2 flow rate were changed in this research. Metco-6700 YSZ micropowder has already been used in plasma spray physical vapor deposition. The results of this study suggest the possibility of using it for APS. The particles of this powder are characterized by high temperature (2,700°C–2,900°C) and high speed (&gt;380 m/s). The highest thickness of the coating was obtained with 6 NLPM (normal liter per minute) H2 flow and 800 A current. Difficulties were observed with the feeding of the powder particles at higher H2 flow. The results showed that using APS, deposition of Metco-6609, a nanopowder normally used in suspension plasma spraying, is possible. In this research, this powder was fed using a carrier gas. The coatings were around 40 μm thick and had high porosity. The lowest porosity of the coating was obtained at a current of 600 A and H2 flow rate of 12 NLPM. In the coatings, unmelted spherical particles were also visible.