Suspension Plasma Spraying Research Articles

A comparative study of the optical and microstructural properties of suspension and atmospheric plasma sprayed thermal barrier coatings

Thermal radiation is an increasingly important issue for thermal barrier coating (TBC) applications in aircraft engine components. As operating temperatures increase, the radiative part of the heat load increases significantly faster than the conductive portion; as such, TBCs may not be as efficient in protecting their underlying heat-sensitive metallic components. To address this issue, new coating morphologies must be investigated. In this study, we compare the optical performance of atmospheric plasma sprayed (APS) and suspension plasma sprayed (SPS) yttria-stabilized zirconia (YSZ) coatings. First, we extract their absorption and scattering coefficients from spectrophotometry measurements via the inverse adding-doubling (IAD) method. The microstructure of the coatings is then analyzed and compared using scanning electron microscopy (SEM) and mercury infiltration porosimetry (MIP). SPS samples are found to offer higher radiation blocking properties due to a more uniform distribution of pores with an average smaller pore diameter than the APS samples. This results in an overall higher pore density, with each pore interface acting as a scattering site, and therefore a higher propensity for scattering events.

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 (>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.

Investigation into microstructure and mechanical properties effects on sliding wear and cavitation erosion of Al2O3–TiO2 coatings sprayed by APS, SPS and S-HVOF

In this work, different Al2O3–TiO2 coatings, i.e., Al2O3, Al2O3-13 wt% TiO2 and Al2O3-40 wt% TiO2, produced by various thermal spray processes, namely: (i) atmospheric plasma spraying (APS), (ii) suspension plasma spraying (SPS), and (iii) suspension high-velocity oxy fuel spraying (S-HVOF), were investigated. For spray purposes, micrometer-sized powders and water-based suspensions of fine submicrometer-sized powders were used. The study aimed to investigate the influence of spray technology and spray feedstock characteristics, mainly chemical composition, i.e., TiO2 content, on the properties of the resulting coating.The sprayed coatings were characterised in terms of selected mechanical and wear properties, including fracture toughness, Vickers and instrumental hardness, Young's modulus, sliding wear resistance, and cavitation erosion resistance.The study showed that the sliding wear rate decreased with the increasing hardness, Young's modulus, and the content of the α-Al2O3 phase in the coating. The dominant wear mechanism was fatigue-induced brittle delamination, followed by the adhesive smearing of the wear debris. The dense microstructures characterised by a high hardness and an increased content of α-Al2O3 favored the resistance of coatings to cavitation erosion. Cavitation erosion was initiated at the microstructural discontinuities like pores or microcracks, resulting in damage by brittle failure with cracking and spallation, which ended up forming large cavitation craters.

Phase composition and thermophysical properties of Yb–Y Co-doped SrZrO3 coating prepared by suspension plasma spray

In this study, the Yb–Y co-doped SrZrO3 (SZ) coatings [SZYY-1 (Sr1.0(Zr0.9Yb0.05Y0.05)O2.95), SZYY-2 (Sr0.9(Zr0.9Yb0.05Y0.05)O2.85), and SZYY-3 (Sr0.8(Zr0.9Yb0.05Y0.05)O2.75)] were deposited by SPS (Suspension plasma spray) using the corresponding co-precipitation prepared precursors suspension. In the three as-sprayed SZYY coatings, there were some typical suspension plasma-sprayed coating structures, such as columnar crystals, vertical cracks, and micro/nanopores.The TCs (Thermal conductivities) of the three as-sprayed SZYY coatings were 1.23–1.88, 0.80–1.41, and 0.88–1.43 W/(m · K) at 100–1400 °C, respectively, which were at least 19% lower than that of the SZ coating (1.25 W/(m · K), 1000 °C). The TECs (Thermal expansion coefficients) of the three as-sprayed SZYY coatings were (7.8–8.7) × 10−6, (8.0–9.8) × 10−6, and (8.1–9.8) × 10−6/K, respectively (200–1360 °C). The sintering coefficients of the three as-sprayed SZYY coatings were 5.69 × 10−6, 4.29 × 10−6, and 3.84 × 10−6/s, respectively. The SZYY-3 coating showed the best SR (Sintering resistance), followed by the SZYY-2 coating and SZYY-1 coating. The SPS SZYY-2 coating with good phase stability, low TC (Thermal conductivity) and good SR is a promising new TBCs (Thermal barrier coatings) material. It is expected to be an alternative TBCs material for the next generation of high performance gas turbines.

Synthesizing AlN Coatings Using Suspension Plasma Spraying: Effect of Promotional Additives and Aluminum Powder Particle Size

Synthesizing AlN Coatings Using Suspension Plasma Spraying: Effect of Promotional Additives and Aluminum Powder Particle Size

Palladium Membrane with High Density of Large-Angle Grain Boundaries to Promote Hydrogen Diffusivity.

A higher density of large-angle grain boundaries in palladium membranes promotes hydrogen diffusion whereas small-angle grain boundaries suppress it. In this paper, the microstructure formation in 10 µm thick palladium membranes is tuned to achieve a submicronic grain size above 100 nm with a high density of large-angle grain boundaries. Moreover, changes in the grain boundaries’ structure is investigated after exposure to hydrogen at 300 and 500 °C. To attain large-angle grain boundaries in Pd, the coating was performed on yttria-stabilized zirconia/porous Crofer 22 APU substrates (intended for use later in an ultracompact membrane reactor). Two techniques of plasma sprayings were used: suspension plasma spraying using liquid nano-sized powder suspension and vacuum plasma spraying using microsized powder as feedstock. By controlling the process parameters in these two techniques, membranes with a comparable density of large-angle grain boundaries could be developed despite the differences in the fabrication methods and feedstocks. Analyses showed that a randomly oriented submicronic structure could be attained with a very similar grain sizes between 100 and 500 nm which could enhance hydrogen permeation. Exposure to hydrogen for 72 h at high temperatures revealed that the samples maintained their large-angle grain boundaries despite the increase in average grain size to around 536 and 720 nm for vacuum plasma spraying and suspension plasma spraying, respectively.

Magnetic properties and Tb diffusion of sintered Nd–Fe–B magnets by suspension plasma spraying

For the first time, a TbF3 coating as a diffusion source was prepared on the surface of sintered Nd–Fe–B magnets by suspension plasma spraying. A sintered Nd–Fe–B magnet with a high coercivity and low remanence loss was obtained. The uniform and dense coating favored grain-boundary diffusion of heavy rare-earth elements, which improved the coercivity of Nd–Fe–B magnets. The effect of grain-boundary diffusion and TbF3 coating amount on the microstructure and magnetic properties of the magnet were studied. Optimal magnetic properties were obtained for a heat treatment of 900 °C for 10 h, when the TbF3 coating amount was 0.2132 mg/mm2. The coercivity reached 23.72 kOe with an increase of 60.16% compared with the original magnet, whereas the remanence did not decrease significantly. Microstructure analysis showed that a Tb-layer core–shell structure formed around the Nd2Fe14B grains after diffusion. The high magnetocrystalline anisotropy of the Tb core–shell structure formation improved the Nd–Fe–B magnet coercivity significantly. Therefore, the method of plasma spray grain-boundary diffusion is efficient and feasible.

Oxidation behavior of double-ceramic-layer thermal barrier coatings deposited by atmospheric plasma spraying and suspension plasma spraying

This paper focuses on the oxidation behavior of novel double-ceramic-layer thermal barrier coatings (DCL TBCs) deposited by atmospheric plasma spraying (APS) and suspension plasma spraying (SPS). Four kinds of APS-SPS DCL TBCs with dense/porous columnar structured or vertically cracked microstructures were prepared. The oxidation behavior of the APS-SPS DCL TBCs were tested and the underlying mechanisms were further discussed. Results showed that the developed APS-SPS DCL TBCs have a better oxidation resistance than the single layer SPS TBC that was tested for comparison. In the long-term oxidation, the thermally grown oxide (TGO) can be divided into two layers, the outer mixed oxide and inner Al2O3 layer, in which the growth rate of mixed oxide in TGO changed during oxidation. In terms of the oxidation rate and oxidation lifetime, segmented APS-SPS TBCs has a slightly better performance than the columnar APS-SPS TBCs. Among the four different APS-SPS TBCs, the segmented dense APS-SPS TBCs with low vertical crack density appears to have more potential to be used for industrial application.

Li ion transport properties of amorphous/crystalline Li-La-Zr-Nb-O solid electrolyte thick films prepared by suspension plasma spraying

Amorphous Li-La-Zr-O films are considered very attractive as a solid-state electrolyte in future batteries due to their low processing temperatures and resistance to Li-dendrite formation. The knowledge of amorphous Li-La-Zr-O phases is limited and thick films and bulk samples of amorphous Li-La-Zr-O have yet to be explored. Only thin film forms (~0.5 μm) of these amorphous Li-La-Zr-O phases have been prepared. In this work, we investigated the Li-ion transport properties of amorphous/crystalline mixtures of Li-La-Zr-Nb-O thick films (30 to 55 μm) prepared by the suspension plasma spray process. The process was manipulated to alter the amorphous Li-La-Zr-Nb-O content in the as-deposited films. Properties of the films that contained mostly amorphous Li-La-Zr-Nb-O (~76 wt%) were compared vs. mostly cubic Li garnet-type oxide (~55 wt%). The latter showed Li-ion transport properties that were similar to previously reported properties of cubic Li-garnet type oxides whereas the amorphous Li-La-Zr-Nb-O films exhibited significantly different properties. Their AC conductivity spectra show a Jonscher power law (JPL) behavior with the dimensionless exponent of ~0.92 that is followed by superlinear power law (SLPL) behavior with the dimensionless exponent of ~1.65 in the high frequency regime. These behaviors have not been reported earlier for the amorphous Li-La-Zr-Nb-O phases. Both of the JPL and SLPL behaviors have thermally activated characteristic with the activation energies of 0.76 eV and 0.64 eV, respectively. A lower value of ~0.45 eV was evaluated for the activation energy by solid-state 7Li NMR spectroscopy. The effect of temperature on the Li-ion dynamic process was studied by scaling the conductivity spectra and it was found to be temperature independent.

Relationship between the mechanical properties and structure of a suspension plasma-sprayed thermal barrier coating with columnar microstructure

To investigate the mechanical properties of a suspension plasma-sprayed (SPS) thermal barrier coating (TBC) with a unique cauliflower-like columnar structure, shear and cantilever bending tests were conducted on its single submillimeter-sized columns. In the shear test, the fracture of the single columns occurred at the porous layer near the top-coating/bond-coating interface. Compared with conventional atmospheric plasma-sprayed (APS) TBCs having a lamellar microstructure, the cantilever bending test revealed a significantly low Young's modulus along the out-of-plane direction and strong anisotropy in the elastic modulus of the SPS TBC. The shear strength and out-of-plane Young's modulus of the single columns increased due to the sintering resulting from thermal aging; however, the increasing ratio of the shear strength was higher than that of the Young's modulus. Both experimental and finite element analysis results indicated that the mechanical properties of the SPS TBC, as well as their variation with thermal aging, are dominated by those of the porous layer within the single column undergoing sintering. Moreover, the experimental results suggested that SPS TBCs having a cauliflower-like columnar microstructure have superior durability to thermal cycles compared with APS TBCs.

Yttrium Oxyfluoride Coating Deposited with a Y5O4F7/YF3 Suspension by Suspension Plasma Spraying Under Atmospheric Pressure

Yttrium Oxyfluoride Coating Deposited with a Y5O4F7/YF3 Suspension by Suspension Plasma Spraying Under Atmospheric Pressure

Challenging zircon coatings by suspension plasma spraying

Zircon is a ceramic material that decomposes at high temperature, limiting its use by conventional thermal spraying. In this work, it is intended to use thermal spraying from concentrated aqueous suspensions to evaluate the possibility of obtaining coatings in which a significant proportion of zircon could be preserved. For this purpose, stable concentrated suspensions of zircon have been prepared, which have been subsequently sprayed at two different spraying distances. The coatings were characterised in terms of microstructural features and the amount of zircon present in the coatings was quantified. All the coatings obtained display the typical microstructure derived from the deposition of liquid feedstocks by plasma spraying. In all cases, the XRD analysis demonstrates the partial decomposition of zircon into zirconia and residual silica, but also that a significant percentage (about 20%) is preserved without decomposing, which marks a strong difference with respect to reported data for atmospheric plasma spraying.

In-Flight Mechanisms in Suspension Plasma Spraying: Issues and Perspectives

Suspension plasma spraying has aroused much research efforts over the two last decades to describe and understand the in-flight mechanisms of suspension plasma processing by developing advanced diagnostics and models. This research field has highlighted the intricate and prevailing dependence of the coating properties, and especially the rich variety of attainable microstructures, on the thermal and dynamic histories of the submicron particles in the plasma jet. This paper gives an overview of the major key points to control from the plasma torch operation up to the coating build-up. The main diagnostics methods are examined and the most advanced models as well. The perspectives focus on the key issues to improve the basic understanding of the in-flight phenomena. They also insist on a necessary multiscale approach to combine the diagnostics and the models in order to form relevant data flows for machine learning algorithms and better process prediction and reliability.

Sintering Characteristics of Thermal Barrier YSZ Coatings Made by Suspension Plasma Spraying

Sintering Characteristics of Thermal Barrier YSZ Coatings Made by Suspension Plasma Spraying

Effect of Low-CTE Oxide-Dispersion-Strengthened Bond Coats on Columnar-Structured YSZ Coatings

Thermal barrier coatings (TBCs) are commonly used to protect gas turbine components from high temperatures and oxidation. Such coatings consist of ceramic top coats and metallic bond coats. The mismatch in thermal expansion of the top coat, the bond coat and the component material is one main factor leading to the failure of the coating system. Columnar-structured top coats offer an enhanced tolerance to the strain during thermal cycling. On a flat bond coated surface, these TBCs reach higher thermal cycling performance. However, on rough surfaces, as used for thermal spray coatings, the performance of these thermal barrier coatings seems to be restricted or even stays below the performance of atmospheric-plasma-sprayed (APS) thermal barrier coatings. This low performance is linked to out-of-plane stresses at the interface between the top coat and the bond coat. In this study, a thin additional oxide-dispersion-strengthened (ODS) bond coat with high alumina content provides a reduced mismatch of the coefficient of thermal expansion (CTE) between the top coat and the bond coat. Columnar suspension plasma sprayed (SPS), yttria-stabilized zirconia (YSZ) TBCs were combined with low-CTE ODS bond coats. The behavior of these TBCs was characterized with respect to thermal cycling performance and degradation in a burner-rig facility. The comparison showed an up-to-four-fold increase in the performance of the new system.

Erosion performance of suspension plasma spray thermal barrier coatings — A comparison with state of art coatings

Suspension plasma spray (SPS) thermal barrier coatings are currently at an early stage of industrial adoption. There remain questions about the performance of SPS columnar coatings under different engine environmental conditions as it may influence which established engine coatings can be replaced by SPS coatings. One particular area of concern has been the erosion resistance of SPS coatings.In this study a columnar SPS coating has been evaluated against three types of state of art air plasma spray coatings: conventional porous coating, high porosity coating and dense vertically cracked coating. Air-jet erosion testing was performed on coatings at a glancing angle of 30 degrees and with direct impact at 90 degrees. Coatings have been ranked according to their mass loss per unit erodent mass. Coatings were also evaluated for their microstructure, porosity content, hardness, and fracture toughness. The erosion damage created during testing has also been investigated using electron microscopy to observe the damage mechanism. The results of this study demonstrate that SPS coatings can outperform porous APS coatings in erosion resistance and could be considered a match for dense vertically cracked coatings. The SPS columnar coatings have shown a decreasing erosion rate with exposure time that suggest the influence of surface roughness on initial erosion behaviour.

Columnar-structured thermal barrier coatings deposited via the water-based suspension plasma spray process

Suspension plasma spray (SPS) has been developed as a rapid, facile and cost-effective process to deposit columnar-structured thermal barrier coatings (TBCs). In contrast to the most commonly used ethanol-based suspensions, water-based suspensions have not been used in the SPS process to deposit columnar-structured TBCs due to their high surface tension, although they are much cheaper and safer. In this work, a new water-based SPS process was prepared by adding surfactant to lower the surface tension. The optimum content of dispersant and surfactant added to the suspension was determined via measurements of viscosity, particle size, surface tension, contact angles, and atomized droplet size. Coatings deposited using suspensions with and without surfactant showed typical columnar-structured microstructures and vertically cracked microstructures, respectively. The coatings deposited using suspensions with surfactant also showed evolution from columnar-structured microstructures to mixed microstructures of columns and cracks, and to homogeneous microstructures with the increase in standoff distance. The formation of different coating microstructures was correlated to the size of droplets after aerodynamic breakup and the Stokes number of in-flight particles. The new water-based suspension together with the water-based SPS process show great potential to be a cheap and effective alternative to the ethanol-based SPS process.

Role of phase content on the photocatalytic performance of TiO2 coatings deposited by suspension plasma spray

In this work, suspension plasma spraying (SPS) with different hydrogen (H2) flow rates was employed to produce TiO2 coatings with various phase contents, oxygen contents, and roughnesses. To eliminate the role of the morphology and oxygen content on the photocatalytic activity, all coatings were polished to reach the same roughness followed by heat-treatment at 550 °C in air for 48 h. Then coatings were analyzed by X-ray diffractometer (XRD), confocal laser microscope, scanning electron microscope (SEM), UV–visible spectrometer, Raman microscope, and thermogravimetric analyzer. The XRD data indicated that the percentage of anatase decreased as function of H2 flow rates, and almost 46% of anatase transformed to rutile during SPS process at the highest H2 flow rate. Moreover, the photocatalytic performance was evaluated by monitoring the degradation of methylene blue under visible light irradiation, and the results indicated that anatase phase positively enhances the photocatalytic activity of TiO2 coatings.

Attractive tribological properties of Al2O3 coating prepared by SPS in comparison with APS in different environment

Al 2 O 3 coatings were prepared by atmospheric plasma spraying (APS) and suspension plasma spraying (SPS). The microstructure, phase composition and nano-hardness of the two coatings were tested. Meanwhile, the friction and wear performance of SPS and APS in different environments were systematically studied. The results showed that both coatings exhibited a stable friction state and low wear rate under different environments. The special fine structure and high hardness value of SPS coating made its wear resistance much higher than that of APS coating. Additionally, the wear mechanisms of the two coatings in dry sliding friction were mainly abrasive wear, fatigue wear and adhesive wear. The APS coating had slight abrasion, while the SPS coating exhibited a negative abrasion state, in the seawater environment.

Coercivity Enhancement of Sintered Nd-Fe-B Magnets by Diffusing Tb4O7 Based on Suspension Plasma Spraying

Coercivity Enhancement of Sintered Nd-Fe-B Magnets by Diffusing Tb4O7 Based on Suspension Plasma Spraying