Low Pressure Plasma Spraying Research Articles

Enhanced high temperature ablation resistance of dense ZrB2–Al2TiO5 composite coating by very low-pressure plasma spraying

ZrB2-based composite coatings with addition of Al2TiO5 were prepared via very low-pressure plasma spraying using mechanically mixed powder of ZrB2 and Al2O3-40 wt%TiO2 (AT40). The ablation behavior of the composite coating was investigated using a high temperature plasma jet at a temperature of ∼2500oC. Results demonstrate the addition of AT40 improved the ablation resistance of the ZrB2 coatings at high temperatures. The coating with 17 vol%AT40 addition exhibited the lowest ablation rate, approximately 1.33 μm/s, which is only one-third of that of the pure ZrB2 coating. The added AT40 filled the pores between ZrO2 crystals during ablation, reducing oxygen penetration and lowering the ablation rate. However, excessive addition of AT40 hindered gaseous B2O3 release and thus led to large pore formation, impairing the protective effect. The ZrB2-AT40 coating is a promising candidate for high-temperature applications.

Effect of Heat Treatment on Microstructure of Silicon-containing CrFeCoNi High Entropy Alloys Produced by Low-Pressure Plasma Spraying

Effect of Heat Treatment on Microstructure of Silicon-containing CrFeCoNi High Entropy Alloys Produced by Low-Pressure Plasma Spraying

Improvement of the durability of thermal barrier coating by pre-oxidation

To suppress thermally grown oxides (TGO) in thermal barrier coatings (TBCs), the effect of pre-oxidation treatment was investigated for different thermal spray processes, such as high-velocity oxy-fuel spraying (HVOF) and low-pressure plasma spray (LPPS). A pre-oxidation treatment was applied before or after the top coat processing. When the pre-oxidation treatment was applied before top coat processing, TGO growth was suppressed during isothermal oxidation compared to when pre-oxidation was applied after the top coat process. Thermal spray processing did not affect the TGO growth behavior.

Low-pressure plasma sprayed dense scandia-stabilized zirconia electrolyte and its effect on SOFC performance

The plasma spraying process has been proven capable of fabricating high-performance metal-supported solid oxide fuel cells (MS-SOFCs), and the critical technology is to prepare dense MS-SOFC electrolytes. Usually, very low-pressure plasma spraying (<1000 Pa) is adopted as it can significantly improve the electrolyte density and air tightness. However, at the same time, it also has the disadvantages of low deposition efficiency and high cost. This paper explores whether it is possible to prepare dense MS-SOFC electrolytes under low-pressure conditions (1000–10000 Pa) with ideal deposition efficiency and cost. Specifically, 4000 Pa and 7000 Pa were chosen to prepare scandia-stabilized zirconia (ScSZ) electrolytes. The effects of spraying pressure and distance on the electrolyte microstructure and cell electrochemical performance were investigated. It was found that the electrolyte prepared at a pressure of 4000 Pa and a spraying distance of 325 mm had the lowest porosity of 3.28%. Meanwhile, the cell exhibited the best electrochemical performance with an open circuit voltage of 1.008 V and a maximum power density of 970 mW/cm2 at 800 °C.

Fabrication of CrFeCoNiSi High Entropy Alloys Dispersed with Silicon Compounds by Low-Pressure Plasma Spraying

Fabrication of CrFeCoNiSi High Entropy Alloys Dispersed with Silicon Compounds by Low-Pressure Plasma Spraying

Correlation between microstructure, phase composition and oxidation behavior of CoNiCrAlY/nano-Al2O3 coatings using satellited feedstock deposited by LPPS process

In this study, satellited CoNiCrAlY/2, 4 wt% nano-Al2O3, and conventional CoNiCrAlY feedstocks were deposited through the low-pressure plasma spraying (LPPS) process on Inconel738 substrate. The oxidation test was done at 1050 °C for 10 min and 5, 50, 100, 150, and 200 h. The microstructure and phase composition of the powders and coatings were characterized by the FESEM and XRD, respectively. The results showed that the CoNiCrAlY and Al2O3 powders comprised γ-CoNiCr and β-(Co,Ni)Al, and α-Al2O3, respectively. In contrast, the as-sprayed coatings included supersaturated γ-CoNiCr and meta-stable γ-Al2O3 phases. With the increase in the percentage of nanoparticles (from 2 to 4 wt%), the amount of porosity (from 2 to 3.76 vol%), and the roughness of the coating (from 6.1 to 7.5 μm) increased. With oxidation time, dissolved β-(Co,Ni)Al was recovered and then gradually consumed to create a thermally grown oxide (TGO) layer. In the composite coatings, the transformation of γ-Al2O3 to stable α-Al2O3 occurred, creating defects across the TGO layer. Therefore, with the addition of Al2O3 nanoparticles, the oxidation rate was continuously increased. The oxidation rate constant (Kp × 10−13) of conventional and composite coatings (2, 4 %) was measured as 3.89, 4.35 and 5.08 cm2.s−1, respectively.

Innovative Tungsten Coatings for an Application in Modern and Future Fusion Devices

Tungsten is foreseen presently as the plasma-facing material for divertors in fusion power plants. In order to achieve durable operation of divertors of current fusion reactors, an efficient way of maintaining the divertor functionality is needed. A system capable of in situ tungsten coating of the divertor via low-pressure plasma spraying was proposed to maintain the divertor integrity. In this work, tungsten was deposited on NB31 carbon fibre composite substrates using the low-pressure plasma spraying technology to evaluate the feasibility of this technique. The thickness, porosity, composition, adhesion, and microstructure of the coatings were investigated by scanning electron microscopy image analysis and energy dispersive spectroscopy. Based on the initial results, the spray parameters were iteratively improved in a campaign-based study. The coatings exhibited improving properties through an adjusting of the carrier gas flow, the scanning speed, and the spray distance. By lowering the carrier gas flow, the porosity of the coatings was reduced, resulting in coatings of 98% bulk density. Adjusting the carrier gas flow reduced the amount of semi-molten particles in the coatings significantly. A decrease in both scanning speed and spray distance increased the substrate’s temperature, which led to better adhesion and porosity.

Interfacial bonding of low-pressure plasma-sprayed Fe-based amorphous coating on 8090 Al–Li alloy

ABSTRACT Fe-based amorphous coatings (AMC) are deposited on 8090 Al–Li alloy using low-pressure plasma spraying. Coating’s microstructure and interfacial characteristics are investigated. The coating is mainly amorphous in structure. Some crystalline phases were observed between the two splats. The corrosion current density of this coating is two orders of magnitude less compared to 8090. The coating has high density as the average porosity is less than 0.5%. Because of partial melting and quick cooling of the 8090 alloy in the process of molten droplets deposition, there is an amorphous transition zone formed at the interface of AMC/8090 Al–Li alloy, which indicates localized metallurgical bonding. In electrochemical testing, the coating shows an obvious passivation tendency, and crystallization between splats is the main cause of corrosion. Owing to the low oxygen content, the coating exhibits excellent wear resistance.

Study on the mechanism of ultra-high temperature ablation of ZrB2–SiC–TaSi2 coatings by low-pressure plasma spraying on the C/C composites

To improve the high-temperature oxidation resistance of C/C composites, ZrB2–SiC (ZS) and ZrB2–SiC–TaSi2 (ZST) ultra-high temperature ceramics coatings were prepared on their surfaces using induction plasma spheroidization and low-pressure plasma spraying (LPPS). Ablation behaviors of the ZS and ZST coatings were evaluated at 1800°C, 2000°C, and 2200°C using the oxyacetylene flame, we found that the ZS coatings had good ablation resistance at 1800°C, but rapidly failed at 2000°C. Whereas, the ZST coating always performed good oxidation and ablation resistance at even 2000°C with a mass ablation rate of 4.68 × 10-5 g/s and a linear ablation rate of -1.63 × 10-5 mm s-1. The enhancement of the high temperature ablation resistance property was attributed to the introduction of the TaSi2, which improved the density of the coatings prepared by LPPS, reduced the volatilization of the glassy phase at high temperature, and enabled ZrO2 to form a dense oxide layer.

MoSi2 addition for high ablation resistance of dense ZrB2-based composite coating prepared by very low-pressure plasma spraying

Dense ZrB2-MoSi2 coatings were deposited on SiC-coated C/C composites for ablation protection by very low-pressure plasma spraying with mechanically mixed ZrB2 with 10–20 vol% MoSi2. The 100 Pa low chamber pressure ensures a dense coating with porosity down to 2.5%. During the ablation, the added MoSi2 produces a solid oxygen-blocking MoB layer and liquid self-sealing SiO2. Both could effectively block the oxygen penetration and results in 10 times improved ablation resistance as compared with the pure ZrB2 coating.

Development of Plasma Parameters for the Manufacture of MCrAlY Bond Coats by Low‐Pressure Plasma Spraying Using a Cascaded Torch

MCrAlY bond coats (MNi, Co) for thermal barrier coating systems are often manufactured by low‐pressure plasma spraying (LPPS) to achieve dense coatings with low oxygen uptake at high deposition efficiencies. Herein, the novel SinplexPro 03C plasma spray torch (Oerlikon Metco) is characterized regarding this application, and appropriate process parameters are developed. The mass‐specific plasma enthalpy and the hydrogen–argon plasma gas ratio prove to be substantial factors. The best deposition efficiency and lowest porosity are achieved at a mass‐specific plasma enthalpy between 17 and 19 MJ kg−1. The oxygen content increases degressively with the enthalpy. Already at small hydrogen fractions in the plasma gas, a fast route is established for the recombination of argon ions. Consequently, the thermal treatment of the feedstock is improved. Moreover, the porosity decreases at increasing hydrogen content of the plasma gas, as the gun is operated in constant current mode and thus the input power increases. However, the deposition efficiency decreases slightly. On the other hand, the oxygen content decreases due to the reducing effect of hydrogen. The deposition efficiency, porosity, and oxygen content are also reasonable if no hydrogen is admixed at all, as the fluctuations of the plasma are reduced.

A New Method of Preparing In situ Synthesized Titanium Oxide Coatings by Very Low-pressure Reactive Plasma Spraying

A New Method of Preparing In situ Synthesized Titanium Oxide Coatings by Very Low-pressure Reactive Plasma Spraying

Thermal Spraying of MCrAlY Overlay Coating Using New Ethanol-Fueled HVOF Gun

The MCrAlY overlay coatings are widely used for high-temperature protection of hot section part of gas turbines and jet engines. This type of coatings are usually thermally sprayed using APS (Atmospheric Plasma Spraying), LPPS (Low Pressure Plasma Spraying) as well as HVOF (High Velocity Oxygen Fuel) methods. In present article the newly developed ethanol based HVOF gun was used for production of this type of coatings. The stainless steel 18-8 type was used as a base material. The AMDRY 386 (Oerlikon-Metco) NiCrAlY powder was used for coatings production. In the research different oxygen (400, 500, 600 NLPM) and ethanol (16.5, 18.3, 21.3, 23.6 and 26.6 dm3/h) flow ratio were selected for experimental processes. The powder feed ratio was also changed during process. After deposition the microstructural assessment using Scanning Electron Microscopy and chemical composition analysis using EDS method were conducted. The obtained results showed that coating was above 100 μm thick depending on the process parameters. The low concentration of pores and oxides was also observed on coatings cross-section. Using of ethanol HVOF gun enables to form good quality MCrAlY coatings with 50% reduction of oxygen consumption in comparison with conventional HP/HVOF torch using kerosene such as JP 5000. The other benefit of its using is lower CO2 emission and lower concentration of carbon in coating in comparison with classic JP 5000 HVOF gun. The ethanol HVOF is a promising technology and might be considered as an replacement of LPPS and HVOF process for production of MCrAlY type of coatings.

A novel reverse-polarity plasma torch with extended hot-wall nozzle for atmospheric plasma spraying of dense yttria-stabilized zirconia coatings

Due to the high heating temperature and low cost, atmospheric plasma spraying (APS) has been widely used in the preparation of ceramic coatings, but its coatings are normally formed by the stacked splats from fully or partially melted feedstock particles, which inevitably leads to some pores, unbonded interfaces or cracks. Thus, in this paper, a novel atmospheric plasma spraying technique using a reverse-polarity plasma torch (RPT) with extended hot-wall nozzle was proposed to fully melt and accelerate feedstock particles for the preparation of dense YSZ ceramic coatings. Firstly, the RPT spray system was designed in detail, and its main electro-thermal characteristics were experimentally studied. The results showed that the RPT exhibited higher arc voltage, thermal efficiency and lower arc voltage fluctuation than that of traditional rod-cathode/normal-polarity plasma torches (NPT) at the similar output power. Secondly, a numerical simulate was carried out to reveal the effect of the extended hot-wall nozzle on the temperature and velocity distributions of the plasma jet. The results showed that the extended hot-wall nozzle remarkably extended the high-temperature and high-velocity region of the plasma jet from the RPT exit to the downstream. Finally, the dense and uniform YSZ coatings with a porosity of about 5.3% were successfully prepared by the RPT with extended hot-wall nozzle. The results showed that the YSZ coating exhibited denser microstructure and fewer defects than that of the coating prepared by traditional APS process. Meanwhile, the hardness and Young's modulus of the YSZ coating were 11.9 ± 1.4 GPa and 197.9 ± 29.4 GPa, respectively, which were relatively higher than that of the coating prepared by traditional APS process, and were very close to that of the coating prepared by very low-pressure plasma spraying (VLPPS) process.

Microstructure and Ablation Behavior of Low-Pressure Plasma Sprayed ZrB2 Coatings Down to 100 Pa.

The effect of chamber pressure on the microstructure and ablation behavior of ZrB2 coatings deposited by low-pressure plasma spraying was investigated. The results showed that as the spray chamber pressure further was reduced to less than 50 kPa, the porosity of the coating deposited at the same distance decreased with the chamber pressure, and the coating prepared under 100 Pa presented the lowest porosity of about 0.89%. The ablation performance test subjected to high-temperature plasma jet revealed that the linear ablation rate of ZrB2 coating increased with the porosity of the coating. As a result, among the ZrB2 coatings deposited at chamber pressures of 100 Pa, 5 kPa, 10 kPa and 50 kPa, the dense coating deposited at 100 Pa showed the lowest ablation rate of 0.33 µm/s. The dense ZrB2 coating with a thickness of about 100 μm was able to withstand 300 s ablation by a plasma flame with a net power of 25 kW resulting in an ablating coating surface temperature of about 2000 °C. The ablation mechanism of the coating was also examined.

Study of nanocomposite hafnia and zirconia based layers produced by plasma spraying

Nanostructured and nanocomposite layers NiCoCrAlY+ ZrO2-7%Y2O3, ZrO2-7% Y2O3+HfO2-9%Y2O3, HfO2-9%Y2O3 with thickness of ∽ 20 μm were formed by low pressure plasma spraying. The structure and composition of the layers have been studied using a scanning electron microscopy, X-ray microanalysis, and XRD analysis. Thermal stability of the coatings has been analyzed using synchronous thermal analysis at temperatures up to 1600 °C. The results of structure and composition analysis of ZrO2-7%Y2O3+HfO2-9%Y2O3 layer suggest the formation ofnanocomposite co-doped regions of the ZrO2-HfO2-Y2O3 solid solution. The layer has greater thermal stability at temperatures up to 1600 ° C and a smaller subgrain size (∽ 33 nm) than for the ZrO2-7% Y2O3 and HfO2-9%Y2O3 layers.

Enhancing the Oxidation Resistance of NiCrAlY Bond Coat by High-Current Pulsed Electron Beam Irradiation

The bond coat of a NiCrAlY thermal barrier coating plays an important role in solving the thermal expansion mismatch between a metal matrix and a ceramic layer and in improving the oxidation resistance of the whole thermal barrier coating. However, the NiCrAlY bond coat prepared by low-pressure plasma spraying is not conducive to its oxidation resistance because its lamellar structure is loose, porous and the surface is rough. To improve the oxidation resistance of the bond coat, the NiCrAlY bond coat prepared by plasma spraying was modified by high-current pulsed electron beam with different energy densities. Under the electron beam irradiation, the surface of the coating became smooth, and there was a 3–5 μm thick remelting layer on the surface. Under the irradiation, the thickness of the thermal growth oxide layer decreased, and the oxidation resistance was significantly improved, the oxidation product being mainly Al2O3.

Thermal Cycling Behavior of Air Plasma-Sprayed and Low-Pressure Plasma-Sprayed Environmental Barrier Coatings

Yb2Si2O7/Si environmental barrier coatings (EBCs) were produced by air plasma spray (APS) and low-pressure plasma spray (LPPS) processes. The phase composition, microstructure, and bonding strength of APS and LPPS EBCs were investigated. Thermal cycling tests were performed in air and in steam atmosphere respectively at 1316 °C for both APS and LPPS EBCs. There is no coating failure in air atmosphere for both APS and LPPS EBCs after 900 cycles. In contrast, APS EBCs have an average life of 576 cycles in a steam cycling test in 90% H2O + 10% air at 1316 °C while LPPS EBCs survived 1000 cycles without failure. The superior durability of the LPPS EBCs compared to APS EBCs in the same steam cycling environment is attributed to the significantly reduced thermally grown oxide (TGO) growth rate because of the denser and crack-free microstructure, higher bonding strength, and reduced coefficient of thermal expansion (CTE) mismatch (less Yb2SiO5 phase) in the LPPS Yb2Si2O7/Si EBCs.

Nitrogen species in a thermal plasma under very low pressure (150 Pa): Application to reactive plasma spraying

Very Low-Pressure Plasma Spraying (VLPPS) is an emerging process allowing the manufacturing of metallic or ceramic coatings under reduced pressure conditions (i.e., in the 150 Pa range). It aims at vaporizing primary feedstock precursors in the warm core of the plasma flow and at carrying them, through the expanded plasma column, towards the surface to be coated, where they condense to form a deposit. This process also can be operated in a reactive mode. In such a case, a secondary reactive gaseous precursor is injected in the plasma flow where it can react with the primary vaporized precursors. In this work, nitrogen is considered as the secondary gaseous precursor. Thereby, nitriding reactions occur between this secondary precursor and the primary one, titanium in the considered case. Optical emission spectroscopy is implemented to identify the reactive nitrogen species in an Ar-H2-N2 plasma gas ternary mixture together with the reaction mechanisms giving rise to the nitriding processes. Two mechanisms have been identified. At first, a gas-gas reactions occur between titanium vapors and N2, N2+ and N species leading to the formation of TiN nitrides; At second, a solid-gas reactions occur between flattened titanium lamellae and N2 species, leading to the formation of Ti2N nitrides, which as evidenced by XRD analyses. Nevertheless, the number of nitrides is rather low, very likely due to the limited fraction of titanium vapors in the system: this is a consequence of a rather limited plasma energy (i.e., net power of about 26 kW, corresponding to an average plasma mass enthalpy of 15.21 kJ kg−1).

Low-pressure plasma spraying of ZrB2-SiC coatings on C/C substrate by adding TaSi2

To improve the density and ablation resistance of ZrB 2 -SiC coatings, TaSi 2 modified ZrB 2 -SiC coatings was prepared on C/C composites by low-pressure plasma spraying . Systematical investigations about microstructures of two kinds of ceramic coatings were performed by XRD, SEM, EDS , TEM and X-ray computed tomography . The presence of Ta element in the ZrB 2 -SiC-TaSi 2 coating is mainly in the form of fine particles with nanoscale , (Zr 0.8 Ta 0.2 )B 2 , visible fine particles and densification areas. The addition of TaSi 2 is conducive to reducing the porosity of the coatings and improving their mechanical and ablation resistance properties. • The IPS and low-pressure plasma spraying technology were both introduced to prepare the coating layers. • Systematical investigations about microstructures of ZS and ZST coatings were performed by methods of characterization. • The TaSi 2 addition in the ZST coatings prepared by LPPS have different microstructures is firstly found and reported. • The ZrB 2 -SiC-TaSi 2 coatings prepared by IPS and low-pressure plasma spraying have lower porosity and better performance.