Conference PaperPDF Available

Abstract

Plasma spraying of large-scale components (such as paper mill rolls, rocket exit nozzles, parts of the land-based turbines for power generation, thermally sprayed free-standing tubes, etc.) and also powder spheroidization) are just several examples of industrial applications where the processing time and costs are critical. To ensure productivity and cost-effectivity of the process, it is beneficial to use plasma torches which can process high powder throughputs, i.e. with feed rates in tens of kilograms per hour. Such torches must have sufficiently high enthalpy and plasma temperature, to ensure homogeneous particle treatment and high deposition efficiency. Hybrid water-stabilized plasma (WSP-H) system with its enthalpy of more than 140 MJ/kg and plasma temperature of 25 000 K is able to process up to 40 kg of powders per hour. Since the WSP-H torch consumes typically only 15 slm of Ar at full power of 180 kW, it is a very powerful and also economical tool that meets all the prerequisites for large-scale plasma spraying applications. Spraying of three representative ceramics: alumina, zircon, and yttria-stabilized-zirconia is presented in this study. It reveals only limited influence of increasing powder throughput on particle velocity, temperature and coating microstructure. Deposition efficiency of the processes is discussed and the deposited coatings are analyzed by SEM and XRD.
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Powder
Spraying
distance
(mm)
Feeding
distance
(mm)
Feed
rate
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T
(°C)
vmean
(m/s)
Porosity
(%)
Number of
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per pass
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1
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30
10
137 ± 27
6.3
28
45.2
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Al2O3
380
45
10
113 ± 22
N/A
N/A
N/A
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Al2O3
380
60
10
103 ± 17
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28
50.4
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4
Al2O3
380
30
20
135 ± 29
7.7
20
99.6
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5
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300
50
10
122 ± 24
7.1
24
47.2
65.4 ± 0.2
6
ZrSiO4
300
50
20
103 ± 20
6.3
16
92.6
67.8 ± 0.3
7
YSZ
300
50
11
113 ± 18
8.9
28
27.2
52.2 ± 0.4
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Run #
Powder
Spraying
distance
(mm)
Feeding
distance
(mm)
Feed
rate
(kg/h)
Nozzle
diam.
(mm)
T
mean
(°C)
vmean
(m/s)
Deposition
efficiency
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8
Al2O3
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45
10
8
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78 ± 14
82.9 ± 1.2
9
Al2O3
380
45
20
8
2415 ± 79
73 ± 13
N/A
10
Al2O3
380
45
40
8
2385 ± 64
72 ± 12
N/A
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... Hybrid water stabilized plasma spraying (WSP-H) is a new deposition method providing high plasma enthalpy allowing very high feed rates (tens of kg/h). The WSP-H plasma torch can operate at power up to 180 kW consuming about 15 slpm of argon making it ideal for cost-effective spraying of large scale components [4]. WSP-H spraying is typ-ically carried out in open-air atmosphere. ...
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Full-text available
NiCrAlY deposited by different thermal spraying methods is commonly used as the bond coat material in thermal barrier coatings (TBCs). In the present study, two experimental coatings were deposited by hybrid water stabilized plasma (WSP-H) and radio frequency inductively coupled plasma (RF-ICP) using the same feedstock powder. Spark plasma sintering (SPS) was used to manufacture a compact NiCrAlY from the same feedstock powder as a reference material. Microstructure, internal oxidation, phase characterization and quantification of the mechanical behaviour in terms of microhardness were studied. The investigations clearly showed microstructural and mechanical differences between the NiCrAlY samples manufactured by different plasma technologies. The results confirmed that SPS and RF-ICP provide dense structures with no oxides due to the fabrication under protective atmosphere and similar mechanical properties. Thus, RF-ICP may be used for deposition of very dense coatings with microstructure and hardness comparable to compacted materials prepared by SPS.
... Principal deposition parameters are listed in Table 1. For details, see [10][11][12]. ...
Article
Full-text available
Mechanical characterization of plasma-sprayed coatings at microscopic level represents a major challenge due to the presence of numerous inherent microstructural features such as cracks, pores, or splat boundaries, which complicate coatings characterization by conventional testing methods. Need for reliable testing of structural integrity of newly developed multiphase plasma-sprayed coatings introduced even more complexity to the testing. In this study, applicability of indirect vibratory cavitation test (adapted from ASTM G32 standard) for such testing was evaluated. Three plasmasprayed coatings having distinctive microstructures were tested: i) conventional alumina coating deposited from coarse powder, ii) hybrid coating deposited by co-spraying of coarse alumina powder and fine yttria-stabilized zirconia (YSZ) suspension, and iii) compact alumina coating deposited from fine ethanol-based suspension. Differences in the coatings internal cohesion were reflected in different failure mechanisms observed within the cavitation crater by scanning electron microscopy and mean erosion rates being i) 280 μm/hour, ii) 97 μm/hour and iii) 14 μm/hour, respectively.
... Economical deposition of thick coatings on large areas can be provided by hybrid water/argon-stabilized plasma (WSP-H) torch technology ( Ref 35,36 In this study, complete TBCs consisting of NiCrAlY bond-coat and four different top-coats were deposited. Powder, suspension, and solution spraying routes were used in order to deposit top-coats having i) rather conventional lamellar, ii) porous feathery/cauliflower-like, or iii) dense vertically cracked microstructures, respectively. ...
Article
Hybrid water-stabilized plasma (WSP-H) torches provide high-enthalpy plasma which may be utilized for high-throughput and yet economical spraying of coatings from powders, suspensions, and solutions. It was previously demonstrated that microstructures and functional properties of the WSP-H coatings may be tailored to a wide extent for new applications, namely those requiring high coating thickness and/or coating of large components. In this study, applicability potential of WSP-H technology for spraying of novel thermal barrier coatings (TBCs) is demonstrated. WSP-H technology was used for spraying of yttria-stabilized zirconia (YSZ) top-coats from powder, suspension, and solution. Yttria content in the top-coat feedstock was 7-8 wt.%. In addition, gadolinium zirconate (Gd2Zr2O7-GZO) was sprayed from suspension for comparison. NiCrAlY bond-coat was also deposited by WSP-H, and Hastelloy-X alloy was used as substrate material. Microstructure, phase composition, and endurance of the deposited coatings in thermal cycling fatigue (TCF) test and during high-temperature short-term annealing were evaluated. All coatings showed excellent high-temperature stability and TCF resistance withstanding more than 650 cycles, surpassing some of the currently commercially used TBCs. Lifetime of the TBC with columnar top-coat deposited from YSZ suspension exceeded even more than 900 cycles.
Conference Paper
In suspension spraying, the two most frequently used solvents are water and ethanol. In this study, we test a potential alternative, a high-molecular weight solvent. Two organic solvents are compared: ethanol (serving as a benchmark, suspension formulated at 10 wt.% solid load) and di-propylene glycol methyl ether (two suspensions at 10 wt.% and 20 wt.%). Submicron alpha-alumina powder is used as a model material to formulate the suspensions. It is shown that ethanol- and ether-based-feedstock coatings are fully comparable in terms of their microstructure, porosity content, surface roughness, and hardness. However, the ether-based coatings exhibit slightly higher levels of α-Al2O3 phase than their ethanol-based counterpart (17 wt.% vs. 6 wt.%). The use of 20 wt.% solid load in the ether solvent leads to a twofold increase in the deposition rate while, as opposed to ethanol, successfully retaining a dense microstructure. Ether also costs less than ethanol and is safer to handle.
Article
In suspension spraying, two most frequently used solvents are water and ethanol. In this study, we test the potential of using alternative, high molecular weight solvent and demonstrate the associated advantages. For that, two organic solvents are directly compared: ethanol (serving as a benchmark, suspension formulated at 10 wt.% solid load) and di-propylene glycol methyl ether (two suspensions at 10 wt.% and 20 wt.% solid load). As a model material, \(\hbox {Al}_{2}\hbox {O}_3\) is selected, a frequently sprayed ceramics employed in many industrial sectors. Sub-micron 100% alpha-alumina powder is used to formulate the suspensions. Identical spray conditions are then used to deposit the coatings using hybrid water-stabilized plasma torch. Shadowgraphy monitoring of the suspension fragmentation as well as in situ measurement of the particle in-flight properties is employed, showing no significant differences between the three series. Further, it is shown that the ethanol- and ether-based-feedstock coatings are fully comparable in terms of their microstructure, porosity content, surface roughness as well as hardness and adhesion to the substrates. Importantly, the ether-based coatings exhibit slightly higher levels of \(\alpha\)-\(\hbox {Al}_{2}\hbox {O}_3\) phase when compared to their ethanol-based counterpart (17 wt.% vs. 6 wt.%). The use of 20 wt.% solid load in the ether solvent leads to twofold increase in the deposition rate while (as opposed to ethanol) successfully retaining a dense microstructure. Lastly, the ether is significantly cheaper and safer to handle than ethanol.
Article
High-enthalpy hybrid water/argon-stabilized plasma (WSP-H) torch may be used for efficient deposition of coatings from dry powders, suspensions, and solutions. WSP-H torch was used to deposit two complete thermal barrier coatings (TBCs) with multilayered top-coat. NiCrAlY was used as bond-coat and deposited on nickel-based superalloy substrates. Top-coat consisted of up to three sublayers: i) yttria-stabilized zirconia (ZrO2-8wt%Y2O3 - YSZ) deposited from solution, ii) gadolinium zirconate (Gd2Zr2O7 - GZO) deposited from suspension, and iii) optional yttrium aluminum garnet (Y3Al5O12 - YAG) overlayer deposited from suspension. Each of the sublayers was intended to provide different functionality, namely improved fracture toughness, low thermal conductivity, and high erosion resistance, respectively. High temperature performance and thermal shock resistance of the deposited coatings was tested by thermal cycling fatigue “TCF” test (maximum temperature 1100 °C, 1 hour dwell per cycle) and “laser-rig” test (maximum temperature ~1530 °C, 5 minutes dwell per cycle) exposing samples to isothermal and gradient thermal conditions, respectively. In both tests, coatings endured around 800 test cycles which shows great potential for further development of these layers and their application in demanding thermal conditions. Analysis of the samples after the test showed microstructural changes and identified reason of ultimate coating failure.
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The novel method of hybrid suspension plasma spraying of dry coarse aluminum oxide powder with chromium oxide suspension using hybrid water/argon-stabilized (WSP-H 500) plasma torch was utilized for the deposition of coatings with very high α-phase content reaching up to 90%. The deposition mechanism and phase composition were compared with those of coatings deposited from i) intermixed alumina-chromia suspension and ii) alumina suspension doped with chromium nitrate nonahydrate solution. All deposition routes showed alternative ways of preparation of novel multimaterial coatings. It was demonstrated that the chromia addition and the deposition route play the crucial role in the pronounced formation of the thermodynamically stable α-phase.
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