Plasma Spray-physical Vapor Deposition Research Articles

Fabrication of Dense Gadolinia-Doped Ceria Coatings via Very-Low-Pressure Plasma Spray and Plasma Spray–Physical Vapor Deposition Process

Gadolinia-doped ceria (GDC) is a promising electrolyte material for low-temperature solid oxide fuel cells (LT-SOFCs). Many works used ceramic sintering methods to prepare the GDC electrolyte, which was mature and reliable but presented difficulties in rapidly preparing a large area of GDC electrolyte without cracks. The low-pressure plasma spray (LPPS) process has the potential to solve this problem, but few studies have been conducted to date. In this work, submicron GDC powder was agglomerated by a spray drying method to achieve the proper granularity with D50 about 10 μm, and then two dense GDC coatings were fabricated with this agglomerated GDC powder using very-low-pressure plasma spray (VLPPS) and plasma spray–physical vapor deposition (PS-PVD), respectively. The results indicate that the two GDC coatings exhibited similar microstructure but with different densification mechanisms. The VLPPS coating was mainly built up in the form of liquid splats, which had lower mechanical properties due to the lower density and crystallinity, while the PS-PVD coating was co-deposited with the vapor clusters and liquid splats, which had higher density, crystallinity, and mechanical properties. It can therefore be concluded that the GDC coating prepared by PS-PVD is more appropriate for the LT-SOFC application.

Investigation of TGO stress in thermally cycled plasma-spray physical vapor deposition and electron-beam physical vapor deposition thermal barrier coatings via photoluminescence spectroscopy

Gas turbine engines for aircraft propulsion require thermal barrier coatings (TBCs) to protect metallic components in the hot section from the extreme temperatures of operation, which may exceed 1200 ∘C. One standard method of coating deposition is electron-beam physical vapor deposition (EB-PVD), which produces a high quality and strain tolerant coating with a smooth surface, which does not impair aerodynamical flow. Plasma-spray physical vapor deposition (PS-PVD) is a promising technique that offers several advantages over EB-PVD, including lower cost, shorter coating time, customization of microstructure by varying processing parameters, and the possibility of non-line-of-sight coating. However, the properties and microstructure of PS-PVD coatings and their effect on performance are not fully understood. This work investigates the residual stress in the thermally grown oxide (TGO) layer of samples made by EB-PVD and PS-PVD, thermally cycled at multiple times (uncycled, 300 cycles, and 600 cycles), to evaluate the effects of these two deposition methods on coating lifetime. Area stress maps of the samples were made with photoluminescence piezospectroscopy (PLPS). The PS-PVD samples exhibited stress relaxation with cycling time, suggesting stress relief from damage, and the PS-PVD coatings exhibited greater stress relaxation between 300 and 600 cycles; however, no samples exhibited spallation. SEM imaging revealed different damage modes for the two coating types. These results reveal PS-PVD's potential as a candidate deposition method for coatings in aero-engine applications.

Thermal Stability of YSZ Coatings Deposited by Plasma Spray–Physical Vapor Deposition

The plasma spray–physical vapor deposition (PS–PVD) process has received considerable attention due to its non-line of sight deposition ability, high deposition rates, and cost efficiency. Compared with electron beam–physical vapor deposition (EB–PVD), PS–PVD can also prepare thermal barrier coatings (TBCs) with columnar microstructures. In this paper, yttria-stabilized zirconia (YSZ) coatings were fabricated by PS–PVD. Results showed that the as-deposited coating presented a typical columnar structure and was mainly composed of metastable tetragonal (t′-ZrO2) phase. With thermal exposure, the initial t′ phase of YSZ evolved gradually into monoclinic (m-ZrO2) phase. Significant increase in hardness (H) and the Young’s modulus (E) of the coating was attributed to the sintering effect of the coating during the thermal exposure, dependent on exposure temperature and time. However, the values of H and E decreased in the coatings thermally treated at 1300–1500 °C for 24 h, which is mainly affected by the formation of m-ZrO2 phase.

Transport and deposition behaviors of vapor coating materials in plasma spray-physical vapor deposition

The plasma spray-physical vapor deposition (PS-PVD) technology offers great potential to support the development of non-line-of-sight deposition. However, many researches indicated that the shadowing effect is a significant mechanism of PS-PVD for the coating growth and formation of the micro-structure. There seems to be an irreconcilable contradiction between the two theories. In this study, both the transport and deposition behavior of PS-PVD processes are researched from a macroscopic and microscopic perspective, respectively. The deposition processes of vapor coating materials are separated by the last collision position. The process before reaching the last collision position is defined as transport behavior, and the process after the last collision position is defined as deposition behavior. The results of simulation and experiment indicated that the flow behavior of plasma jet and the frequent collisions between plasma gas and vapor coating material provide opportunities for the vapor coating material to deposit in shadowed surfaces or non-line-of-sight positions. The shadowing effect under micro line-of-sight deposition is a major factor in the growth of columnar structures, and it accompanies the entire process of coating growth. This study divides the deposition process of vapor coating materials into the macro non-line-of-sight transport and micro line-of-sight deposition, illuminates the macro-non-line-of-sight transport behavior of vapor-phase materials, and sheds light upon the micro-line-of-sight deposition mechanism of the PS-PVD process.

Investigation on growth mechanisms of columnar structured YSZ coatings in Plasma Spray-Physical Vapor Deposition (PS-PVD)

By Plasma Spray-Physical Vapor Deposition (PS-PVD), major fractions of the powder feedstock can be evaporated so that the coating builds up mainly from vapor phase. In this work, the deposition mechanisms at different PS-PVD conditions were investigated. Depending on the plasma flow conditions and the substrate temperature, the columns in the coatings possess successively pyramidal, cauliflower, and lamellar shaped tops. In addition, the different microstructures show characteristic crystallographic textures, in which different in-plane and out-of-plane orientations were observed by pole figures. Based on investigations by electron back-scatter diffraction (EBSD), the overall coating growth process can be roughly divided into three subsequent stages: equiaxed growth, competitive growth, and preferential growth. Influences of diffusion and shadowing on final coating microstructure and orientation were discussed. The formation of equiaxed grains was proposed to be caused by high nucleation rates, which are probably induced by large undercooling and super-saturation at the beginning of deposition. The preferential growth orientation was preliminarily explained based on an evolutionary selection mechanism.

Lanthanum tungstate membranes for H2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition

In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28−xW4+xO54+δ, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H2 flux of ca. 0.4 ml/min cm2 measured at 825 °C in 50 vol% H2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H2 separation membranes and it is well comparable with the H2 flux reported for other newly developed dual phase cer-cer and cer-met membranes.

Microstructure and Properties of YSZ Coatings Prepared by Plasma Spray Physical Vapor Deposition for Biomedical Application

Microstructure and Properties of YSZ Coatings Prepared by Plasma Spray Physical Vapor Deposition for Biomedical Application

Novel thermal barrier coatings repel and resist molten silicate deposits

Optimization of the resistance of thermal barrier coatings (TBCs) to environmental particles is an area of considerable current research. Here, we present a novel methods, incorporating plasma spray physical vapor deposition (PS-PVD), in order to fabricate a novel yttria-stabilized zirconia (YSZ) TBC which is highly phobic (i.e. non-wettable) with respect to CaO-MgO-Al2O3-SiO2 (CMAS) particles. At 1250 °C, CMAS particles form liquid droplets which are readily repelled by the TBC surface, diminishing adherence to and penetration into the TBC. This effectiveness of the TBC in resisting wetting by molten CMAS deposits is interpreted to result from its lotus-leaf-like dual-scale microstructure.

Structural evolution of Al-modified PS-PVD 7YSZ TBCs in thermal cycling

Feather-like columnar 7YSZ thermal barrier coatings (TBCs) were prepared by plasma spray-physical vapor deposition (PS-PVD). In order to improve the thermal cycle performance of TBCs, Al-modification was carried out with the as-sprayed TBCs. Al film was deposited on the surface of TBCs. Subsequently, the Al-deposited TBCs were conducted with vacuum heat treatment. After that, a dense overlay consisting of α-Al2O3, Al3Zr phases was generated on the TBCs surface due to the reactions of Al and O2, Al and ZrO2. The Al-modified TBCs show a better thermal cycle performance than the as-sprayed TBCs because of the overlay generation. Moreover, the structural evolution of overlay was investigated emphatically before and after thermal cycle testing, showing good phase stability. No crack and spallation were observed in the Al-modified TBCs.

Thermodynamic conditions for cluster formation in supersaturated boundary layer during plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition (PS-PVD) is used to produce columnar microstructure coatings for vapor materials under particular operating parameters. However, due to the particular conditions of PS-PVD, the state of vapor material transported in plasma jet is difficult to predict by experimental method. Moreover, the properties of plasma jet changes sharply when the hot plasma jet approaches a relatively low temperature substrate. It is more difficult to examine the behavior variation of vapor material by experiment. In this work, the thermodynamic condition for state variation of the vapor material along with methods to change this condition were studied. The distribution of plasma temperature near the substrate surface was obtained through simulation and the vapor pressure of vapor materials was calculated by the obtained temperature distribution. The supersaturated boundary layer was obtained from the distribution of the vapor pressure and partial pressure of the vapor atoms. The environment of this supersaturated boundary layer agreed with the thermodynamic conditions of the state variation of vapor materials. Further, this layer could be varied by changing the spraying distance and powder feed rate, thus allowing further control of the state variation of vapor materials. For PS-PVD process, especially for the deposition of columnar coating, it is very important to understand the properties of boundary layer and condensation conditions of vapor materials; this study provides the basis for future research on the transport of materials in the boundary layer.

Numerical Study on Particle–Gas Interaction Close to the Substrates in Thermal Spray Processes with High-Kinetic and Low-Pressure Conditions

In thermal spray processes, the interaction between the gas jet and the particulate feedstock can affect the coating build-up mechanisms considerably. In particular under high-kinetic and low-pressure conditions, small particles are subjected to rapid deflection and velocity changes close to the substrate. In this work, numerical studies were carried out to investigate the interaction between gas and particles in the substrate boundary layers (BL). Typical conditions for suspension plasma spraying (SPS), plasma spray-physical vapor deposition (PS-PVD), and aerosol deposition (AD) were taken as a basis. Particular importance was attached to the consideration of rarefaction and compressibility effects on the drag force. Typical Stokes numbers for the different thermal spray processes were calculated and compared. Possible effects on the resulting coating build-up mechanisms and microstructure formation are discussed. The results show that just for larger particles in the SPS process the laminar flow attached to the particles begins to separate so that the drag coefficients have to be corrected. Furthermore, slip effects occur in all the investigated processes and must be considered. The comparison of calculated Stokes numbers with critical values shows that there is a disposition to form columnar microstructures or stacking effects depending on the particle size for PS-PVD and SPS, but not for AD.

Microstructural evolution, mechanical properties and degradation mechanism of PS-PVD quasi-columnar thermal barrier coatings exposed to glassy CMAS deposits

Thermal barrier coatings (TBCs) applied in aero-engines tend to be attacked by molten calcia-magnesia-alumino-silicate (CMAS) at high operating temperatures. Yttria-stabilized zirconia (YSZ) coatings with quasi-columnar microstructure were fabricated by plasma spray physical vapor deposition (PS-PVD) technique. The chemical changes, microstructural transformation, mechanical properties and degradation mechanisms of the CMAS-interacted TBCs in the thermal cycling tests were investigated. Feathered YSZ grains were dissolved in the CMAS melts, and then the ZrO2 grains were reprecipitated with spherical shape, accompanying with phase transformation from tetragonal (t) to monoclinic (m). The thermal cycling tests reveal that the YSZ coating fails at the early stage due to the attack of CMAS. The fractures in intra-columns lead to partial spallation of the coatings. The failure of the coating occurs at the interfaces between thermally grown oxides (TGO) layer and YSZ topcoat; especially, the hardness and Young’s modulus of the YSZ coatings increase intensively, as the coatings were infiltrated by the CMAS for a long time.

Evolution of Residual Stresses in PS-PVD Thermal Barrier Coatings on Thermal Cycling

Plasma spray-physical vapor deposition (PS-PVD) is an advanced technique to fabricate quasi-columnar structured thermal barrier coatings (TBCs) with excellent thermal cyclic lifetime. In this study, PS-PVD TBCs were investigated via burner rig test. The residual stresses in both of the topcoat layer and the thermally grown oxide (TGO) scale were measured non-destructively using Raman spectroscopy and Cr3+ photoluminescence piezo-spectroscopy, respectively. Evolution of the microstructures and distribution of residual stresses in such kind structured TBCs before and after thermal cycling test were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly and then essentially maintained at a relatively stable state with further testing. Moreover, the pre-heating of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat.

Influence of microstructure on thermal cycling lifetime and thermal insulation properties of yttria-stabilized zirconia thermal barrier coatings for diesel engine applications

Thermal barrier coatings (TBCs) may improve the fuel efficiency of heavy-duty diesel engines by reducing heat losses. A combination of durability, low thermal conductivity, and high directional hemispherical reflectance is required for a TBC in the combustion chamber. These properties are evaluated for yttria-stabilized zirconia coatings, produced using atmospheric plasma spraying (APS) and plasma spray–physical vapour deposition (PS-PVD). The influences of different types of microstructure and metallic coatings on the surface are studied. APS coatings with segmentation cracks and PS-PVD coatings with columnar microstructure have the best thermal cycling lifetime, while nanostructured and conventional APS coatings have the lowest thermal conductivities. The nanostructured APS coating has the highest reflectance at low temperatures, while the columnar PS-PVD coating has the highest reflectance at elevated temperatures. It is further demonstrated that a thin silver layer improves the reflectance of a dense, segmented APS YSZ coating.

Conditions for nucleation and growth in the substrate boundary layer at plasma spray-physical vapor deposition (PS-PVD)

Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition can come off not only from liquid splats but also from vapor phase. Moreover, there is the suggestion that also nano-sized clusters can be formed by homogeneous nucleation and contribute to deposition. In this work, the conditions for nucleation and growth of such nano-sized particles in the plasma flow around the substrate under PS-PVD conditions were investigated. A boundary layer kinetics model was coupled to an approach for homogeneous nucleation from supersaturated vapors and primary particle growth by condensation as well as secondary particle formation by coagulation. The results confirm the importance of the boundary layer on the substrate. However, since these particles are relatively small, their contribution to coating deposition is limited. Furthermore, microstructure or crystallographic orientations are unlikely to be affected by this cluster deposition.

Regional characteristic of 7YSZ coatings prepared by plasma spray-physical vapor deposition technique

7YSZ coating was prepared by plasma spray-physical vapor deposition (PS-PVD) technique based on a specific experimental design. The microstructure and deposition properties of 7YSZ coating along the radius of plasma jet were investigated in detail. Results show that the coating presents regional characteristic in the radial direction, which could be divided into three typical zones: In Zone I, the coating is all composed of columnar structures with cauliflower structure, and the coating properties including the surface roughness and deposition efficiency (DE) are almost stable; in Zone III, the coating is made up of solid particles, droplet and gas phase mixed without columnar structures; Zone II is between Zone I and Zone III, in which there are columns with domed top and small particles. Based on experiment results, a model on the state and distribution of particles in plasma jet was proposed to clarify the regional characteristic. This study is helpful to comprehend and control coatings deposition by PS-PVD technique.

CMAS corrosion and thermal cycle of Al-modified PS-PVD environmental barrier coating

Environmental barrier coatings (EBCs) with a tri-layer structure consisting of silicon, mulltite and Yb2SiO5 were deposited by plasma spray-physical vapor deposition (PS-PVD) on SiC/SiC ceramic matrix composite (CMC). To improve the calcium-aluminum-magnesium-silicates (CMAS) corrosion resistance of EBCs, a novel method called “Al-modification of EBCs surface” was proposed, wherein an Al film was first deposited on surface of Yb2SiO5 top coating by magnetron sputtering, and afterwards the vacuum heat treatment was carried out on the Al-deposited EBCs samples. The thermal cycle performance of both as-sprayed and Al-modified EBCs was characterized from 1300 °C to the temperature of environmental water. Moreover, the CMAS corrosion property of both types of EBCs at 1300 °C has been compared with each other. And the corrosion mechanism between CMAS and Al-modified EBCs has been investigated. The results show that the Al-modified EBCs have a better thermal cycle and CMAS corrosion performance than that of as-sprayed EBCs.

A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)

The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed.

Wetting, infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250 °C for short terms (1, 5, 30 min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (∼100 μm thick) was fully penetrated by CMAS melt within 1 min. After the CMAS attack for 30 min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO2 grains in the interaction regions.

Microstructural evolution of plasma spray physical vapor deposited thermal barrier coatings at 1150 °C studied by impedance spectroscopy

Plasma spray physical vapor deposition (PS-PVD) is a new promising technology for processing thermal barrier coatings (TBCs). In this study, the microstructural evolution of PS-PVD yttria-stabilized zirconia (YSZ) TBCs at 1150 °C was investigated by scanning electron microscopy, electron backscattered diffraction, and impedance spectroscopy. Bridging is observed between the secondary columns of the coatings during the first 8 h of heat treatment, while sintering necks are observed between the quasi-columns over the heat-treatment interval of 12–20 h. As the heat-treatment time increases to 100 h, the nanogaps between the secondary columns disappear and a large number of shrinkage pores are observed in the columns, leading to a typical cellular structure. Besides, the average grain size of the YSZ coatings increases from 0.20 (as-deposited) to 0.55 µm (after 50 h of heat treatment). The resistance of the YSZ grains increases while that of the YSZ grain boundaries decreases because of grain coarsening. During the first 4 h of heat treatment, the resistance of the thermally grown oxide (TGO) scale increases because of its thickening. However, its resistance decreases during the next 4 h of heat treatment mainly because of the change in its composition (from pure alumina to mixed oxides).