Low Power Plasma Spraying Research Articles

Study of the La1−xSrxMnO3 Cathode Film Prepared by a Low Power Plasma Spray Method with Liquid Solution Precursor for a Solid Oxide Fuel Cell

A perovskite La1−xSrxMnO3 cathode thin film for an oxygen ion conducting solid oxide fuel cell was prepared using a low power (8.8 kW) liquid solution plasma spray method. Usually, a 30–50 kW Ar plasma torch with temperature higher than all the melting points of solid precursors is essential to synthesis oxides thin film. However, using the liquid precursors as the feeding materials, the required power can be reduced and save a lot of thermal budget. The precursors are water solutions of lanthanum nitrate hexahydrate, manganese(II) nitrate tetrahydrate, and strontium nitrate. The atomic percentage of La in the plasma sprayed La1−xSrxMnO3 cathode film is lower than that of La in the feeding precursor into the torch, which is due to the low boiling temperature of La(NO3)3 precursor. The oxygen stoichiometry of La1−xSrxMnO3−δ deduced from the valence state of Mn measured by X-ray absorption spectroscopy shows an oxygen deficit structure. The measured low resistivity of 0.07–0.09 Ωcm at room temperature for this La1−xSrxMnO3−δ is essential for oxygen ion transport in the cathode thin film of a solid-state fuel cell.

Alternative low-power plasma-sprayed inconel 625 coatings for thermal solar receivers: Effects of high temperature exposure on adhesion and solar absorptivity

Over recent years, renewable energy technologies have focused on increasing performance and efficiency, and on the reduction of maintenance costs. In this work, thermal-sprayed Inconel 625 coatings have been studied as an alternative for concentrated solar power plants receivers. A low-power compact plasma spray system was used to deposit coatings onto two substrates: grade 22 ferritic steel and AISI 316 L austenitic steel. This system may be used for in-situ maintenance or repair purposes. The coatings were heat-treated at two temperatures: 520 °C and 800 °C, at different exposure times. The aim of this work was to evaluate the effect of this treatment on the adherence and solar absorptivity of the Inconel 625 coatings. The results showed that, at higher temperatures and longer exposure times, better adherence and absorptivity are achieved. Adherence values above 60 MPa were obtained due to diffusion in the coating-substrate interface. Additionally, absorptivity values above 93% were measured due to oxide formation on the coating surface during heat treatment. Furthermore, the highest temperature of the oxidized treatment reported the highest values of absorptivity. These results show that the developed Inconel 625 coatings could be considered as a possible alternative to improve the performance of concentrated solar power plants. • Inconel 625 coatings can be deposited by compact low-power plasma spray. • The adherence is increased at high temperature exposure by interface diffusion. • Inconel 625 coatings reach solar absorptivity values above 93%. • The formation of oxide phases is responsible of the incremental solar absorptivity. • Inconel 625 coatings could be an interesting alternative for solar receivers.

Low power plasma spray assisted thermal barrier coating repair without the plugging of cooling holes

Abstract Currently operating new generation aircraft engines are equipped with multi-hole combustion chambers, coated with a thick ZrO2-4 mol% Y2O3 (YpSZ) thermal barrier coating (TBC). The local repair of such multi-hole combustors requires the development of specific deposition methods. This paper describes a modified version of a low power plasma reactor (LPPR), upgraded with two opposite gas flows to prevent cooling channel plugging. The influence of some experimental parameters (plasma power density, spray distance and precursor concentration) on the repaired TBC microstructure is examined. A short spray distance, associated with a low power density and a low concentration of nitrate precursors, led to a suitable microstructure of the deposited YpSZ. Moreover, FTIR spectroscopy and SEM-EDS analysis revealed the efficient conversion of the nitrate precursors into oxide and the good infiltration of the deposited YpSZ into the damaged area. Finally, it is shown that, once the deposition parameters have been optimized, a dual flow system effectively allows local repair to be carried out without plugging the cooling system.

Influence of NiCrAlY content on dielectric and microwave absorption properties of NiCrAlY/Al2O3 composite coatings

NiCrAlY/Al2O3 coatings with varying NiCrAlY content were deposited by low-power plasma spraying technology. The influence of NiCrAlY content on dielectric and microwave absorption properties have been investigated in the X-band (8.2–12.4 GHz). The results indicate that metallic NiCrAlY exhibits typically string or splat-like structure in the coatings, and the elevated NiCrAlY filling content is beneficial to the formation of conductive metallic clusters. The real and imaginary parts of complex permittivity are enhanced with increase in NiCrAlY content, which results from interfacial polarization, relaxation loss and conductance loss. Owing to the excellent impedance matching and the preferable attenuation coefficient, the coating with 20 wt.% NiCrAlY and 2.0 mm thickness possesses effective bandwidth (<-10 dB) 1.3 GHz in 8.2–9.5 GHz and minimum reflection loss −15.7 dB at 8.9 GHz, exhibiting the most favorable microwave absorption properties.

Optimization of atmospheric low-power plasma spraying process parameters of Al2O3-50wt%Cr2O3 coatings

The work is focused on the effect of the processing parameters of Al2O3-50wt%Cr2O3 coatings manufactured by low-power plasma spraying systems on the microstructure, mechanical properties, and tribological behaviour. Design of experiments followed by an optimization process using desirability functions has been applied for the selection of the processing parameters. The results indicate that power is the crucial parameter that controls the microstructure and consequently, the mechanical response of the coatings. At high powers, the highest volume of hard phases is attained, which leads to the highest hardness values and the best wear resistance at high applied loads. On the contrary, at low powers, the highest volume of spinels (tough phases) is observed, resulting in the highest indentation fracture toughness values and the best wear response at low applied loads. Therefore, the mechanical properties are directly related to the contents of the tough and hard phases.

Influence of Cr content and initial Cr particle size on the dielectric properties of plasma-sprayed Cr/Al2O3 coatings

Cr/Al2O3 coatings were deposited by low-power plasma spray process with dry-crushed powders. The coatings exhibited a microstructure formed by fully molten splat-like metallic lamellae and partially molten or un-molten submicron Al2O3 particles, surround by a fully molten alumina matrix. The content and size of metallic Cr in the coatings played a dominant role in the formation of metallic clusters. Complex permittivity measurement showed that the coatings with higher Cr content and larger Cr particle size exhibited higher ε′ and ε″ values due to the enhanced interfacial polarization and conductance loss. Furthermore, strong frequency dependency was observed for the coatings with 15wt% Cr addition and 50–74μm Cr particle size.

Thin-thickness FeSiAl/flake graphite-filled Al2O3 ceramics with enhanced microwave absorption

Thin and enhanced microwave absorption FeSiAl/ flake graphite (FG) filled Al2O3 ceramics were fabricated by low power plasma spraying. The characteristics of the sprayed ceramics, such as phase composition, microstructure, electromagnetic (EM) properties and reflection loss (RL) were investigated. Suitable values and frequency dependence of the complex permittivity of the FeSiAl/ FG filled Al2O3 ceramics can be obtained in the Ku-band due to the larger internal boundary layer capacitance and the combination of the advantages of the FeSiAl and FG. RL values less than −7dB (larger than 80% absorption) can be obtained in the whole Ku-band for the FeSiAl/FG filled Al2O3 ceramics with a sample thickness of 0.8mm. These results suggest a possible use of these ceramics as effective microwave absorbing materials for EM applications.

Effect of initial particle size on microstructure and dielectric properties of Cr/Al2O3 coatings deposited by plasma spraying

Cr/Al2O3 coatings were deposited by low-power plasma spray technology using spray-dried feedstock powders with different sized initial Cr particles. XRD results revealed that γ-Al2O3 phase formed in the process of plasma spraying. The effect of initial Cr particle size on the microstructure was investigated. It was found that the coatings deposited with larger initial Cr particles possessed denser structure and lower porosity, and the Cr clusters in these coatings were more prone to form due to the connections of adjacent metal lamellae. Moreover, the initial Cr particle size had a significant effect on the dielectric properties. The complex permittivity of as-sprayed Cr/Al2O3 coatings was evidently enhanced with increasing initial Cr particle size, which was attributed to the interfacial polarization and the formation of metallic conductive clusters.

High performance electromagnetic interference shielding of lamellar MoSi2/glass composite coatings by plasma spraying

MoSi2/glass composite coatings with different MoSi2 content were fabricated by low power plasma spraying. The microstructure, electrical conductivity, dielectric properties and electromagnetic interference (EMI) shielding effectiveness (SE) of MoSi2/glass composite coatings were investigated in this study. The EMI SE is found to be related with the electrical conductivity and dielectric property, which are attributed to the high aspect ratio of lamellar structure and well dispersion state of MoSi2. The average value of EMI SE is 24.2 dB in the frequency range of 8.2–12.4 GHz for the MoSi2/glass composite coating with 30wt% MoSi2 in 1.5 mm. The results indicate that the plasma sprayed MoSi2/glass composite coatings can be applied to effective EMI shielding materials.

Dielectric and microwave absorption properties of plasma sprayed short carbon fibers/glass composite coatings

Short carbon fibers (SCFs)/glass composite coatings were fabricated by low power plasma spraying system. The morphology and microstructure of the coatings were observed. The dielectric and microwave absorption properties of the coatings with homogeneously dispersed SCFs were investigated in the frequency range of 8.2–12.4 GHz. The results show that both e′ and e″ monotonically increased with the increase of SCFs content, which could be ascribed to the enhanced electronic relaxation polarization, interfacial polarization and conductivity. And the Cole–Cole plots indicate that the interfacial polarization plays a dominant role on polarization mechanism with the increase of SCFs content. The effects of SCFs content and thickness on the microwave absorption properties were investigated. The composite coating containing 6.04 wt% SCFs shows that reflection loss below −10 dB (over 90 % microwave absorption) is in the frequency range of 8.9–11.4 GHz and the minimum value was −27.9 dB at 10.1 GHz with the thickness of 2.2 mm. The results indicate that SCFs/glass composite coating could be a potential microwave absorbing material used in 8.2–12.4 GHz.

Enhanced dielectric and electromagnetic interference shielding properties of FeSiAl/Al2O3 ceramics by plasma spraying

FeSiAl/Al2O3 ceramics with flaky metallic FeSiAl were fabricated by low power plasma spraying. High complex permittivity of the flaky FeSiAl/Al2O3 ceramics can be obtained in the X-band due to the larger internal boundary layer capacitance and conductive network. The reflection loss (RL) values less than −7 dB (larger than 80% absorption) can be obtained in the whole X-band for the FeSiAl/Al2O3 ceramics with 20 wt% FeSiAl and a thickness of 1.18 mm. The total electromagnetic interference (EMI) shielding effectiveness is over 36 dB in the whole X-band for the FeSiAl/Al2O3 ceramics with 40 wt% FeSiAl and a thickness of 2 mm. These results indicated that such ceramics with conductive networks and high complex permittivity can be used to produce effective shielding materials for EM application.

Effect of feedstock characteristics on the dielectric and microwave absorption properties of plasma sprayed NiCrAlY/Al2O3 coatings

NiCrAlY/Al2O3 coatings were fabricated by low power plasma spraying with two different feedstock powders. The microstructure, phase composition and dielectric properties of the coatings were investigated. The result showed that the particle melting degree and the connection of NiCrAlY in the coatings were closely related to the feedstock characteristics, such as particle size and morphology, which played an important role in the microstructure and dielectric properties. The enhanced complex permittivity of the coating deposited from dried and crushed powder is attributed to interfacial polarization and conductance loss. Compared with the coating prepared from milled and agglomerated powder, the reflection loss of the coating fabricated with dried and crushed powder reaches −20.5 dB at 9.9 GHz and the eligible microwave absorption (below −10 dB) can be obtained in the frequency range 8.7–10.8 GHz.

Deposition of YSZ Coatings in a Chamber at Pressures below 100 Pa Using Low-Power Plasma Spraying with an Internal Injection Powder Feeding

By decreasing the chamber pressure, the effective mean free path length in a plasma jet increases, resulting in a distinct decrease in the heat transfer from the plasma jet to the powder feedstock. Thus, instantaneous, complete melting or partial evaporation of the powders in the plasma jet becomes difficult. To create yttria partially stabilized zirconia coatings in a relatively low-power plasma jet and a low-pressure environment of 100 Pa, a new plasma torch with a specialized nozzle with an internal injection powder system was designed. Optical emission spectroscopy indicated that the feedstock powder could be partially vaporized in a 700-A plasma arc in either 40Ar–10H2 or 20Ar–20He–10H2 standard liters per minute (SLPM) plasma gas. Denser coatings were deposited at arc current of 700 A by changing the plasma gas mixture from argon–hydrogen to argon–helium–hydrogen. A compact coating was obtained as the amount of vaporized powder increased, but the microstructure of the coating lost its columnar morphology due to the limited amount of vapor phase. In addition, the porosity and microhardness of the coatings were measured.

The Preparation of Al–Cu–Fe Quasicrystalline Coating by Low Power Plasma Spraying

Al-Cu-Fe with an atomic ratio of 70:20:10 was selected for preparing the quasicrystalline coating by low power plasma spraying. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for analyzing the phase component and microstructure of the quasicrystalline coating. It was found that the best quality of quasicrystalline coating was obtained under a current of 400A and a voltage of 40V. XRD results show that, using these parameters, a relatively higher content of quasicrystalline phase can be acquired in the coating. SEM results show that the sprayed coating has a low level of porosity.

Thin Yttria-Stabilized Zirconia Coatings Deposited by Low-Energy Plasma Spraying Under Very Low Pressure Condition

In recent decades, very low pressure plasma spraying (VLPPS) technology (less than 10 mbar), as a next-generation coating process, has been extensively studied, because it can fully evaporate the materials to deposit dense, thin, and columnar grain coatings. This research aims at applying VLPPS with low-energy plasma source to melt or evaporate ceramic materials to develop high-quality thermal barrier coatings. Thin and homogeneous yttria-stabilized zirconia coatings were deposited successfully on a stainless steel substrate using low-power plasma spraying torch F100 (23 kW maximal) under very low pressure (1 mbar). The optical emission spectroscopy was used to analyze the properties of the plasma jet. The phase composition and the microstructure of the coatings were characterized by x-ray diffraction and scanning electron microscopy. The results showed that the YSZ powder was fully melted and partially evaporated, and the coatings had a hybrid microstructure that was combined with the condensation of the YSZ vapor and the melted particles. In addition, the porosity and microhardness of the coatings were evaluated.

Study on the Tribological Properties of Low-Power Plasma-Sprayed Al-Cu-Cr Quasicrystalline Coating

Al-Cu-Cr quasicrystalline coatings were prepared by low power plasma spraying with axially-fed powder systerm. The Al65Cu20Cr15 powders were deposited on AISI 1045 steel substrate at the power ranged from 4.0 to 5.5 kW. The effects of H2/Ar flow ratio on the phase composition, microstructure and tribological properties of the as-sprayed coatings were investigated. The XRD results showed that the feedstock powders and as-sprayed coatings contained a predominant icosahedral quasicrystalline phase I-Al65Cu24Cr11 and three minor crystalline phases, including a body-centered cubic α-Al69Cu18Cr13, a monoclinic θ-Al13Cr2 (i.e. Al83Cu4Cr13) and a hexagonal ε-Al2Cu3. A qualitative analysis on the XRD patterns indicated that the volume fraction of any crystalline phase (α, ε or θ) in the coatings increased, while the quasicrystalline I-phase decreased with the rise of hydrogen (H2) content in the working gas. However, as increasing the plasma power, the friction and wear resistance of the coating were improved under the same dry sliding wear test conditions. The improvementincrease of the wear resistance may be attributed to the high hardness of quasicrystal.

Al–Cu–Cr quasicrystalline coatings prepared by low power plasma spraying

Al–Cu–Cr quasicrystalline coatings were prepared by low power plasma spraying with axially-fed powder systerm. The Al 65Cu 20Cr 15 powders were deposited on AISI 1045 steel substrate at the power ranged from 4.0 to 6.0 kW. The effects of H 2/Ar flow ratio on the phase composition, microstructure and microhardness properties of the as-sprayed coatings were investigated. The XRD results showed that the original powders and as-sprayed coatings contained a predominant icosahedral quasicrystalline phase I-Al 65Cu 24Cr 11 and three minor crystalline phases, including a body-centered cubic α-Al 69Cu 18Cr 13, a monoclinic θ-Al 13Cr 2 (i.e. Al 83Cu 4Cr 13) and a hexagonal ε-Al 2Cu 3. A qualitative analysis on the XRD patterns indicated that the volume fraction of any crystalline phase ( α, ε or θ) in the coatings increased, while the quasicrystalline I-phase decreased with increasing hydrogen content in the plasma gas. As H 2/Ar flow ratio increased from 4.8% to 18.8%, the coating hardness increased whilst its porosity decreased, and they reached the maximum (4.98 Gpa) and the minimum (8%) respectively. However, with increasing H 2 content further, the hardness of the coating slightly decreased and its porosity increased because of the excessive vaporization of aluminum at higher plasma energy.

Properties of Al2O3-TiO2 Coating Prepared by Plasma Spraying with an Internally-Fed Powder System

Alumina-titania (60: 40 by weight) coatings were prepared by low power plasma thermal spraying with two types of plasma spray systems: internally-fed and externally-fed powder system. The coatings were investigated by many techniques, respectively X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-hardness tester. It was found that the thickness of coatings by low power plasma spray with an internally-fed powder system is higher than that of the coatings with an externally-fed powder system. The internally-fed powder system not only decreases the loss of powder but also increases the deposition efficiency for more than 80%. To better qualify the characteristics of these coatings prepared from spray-dried powders, microhardness was also studied.

Microstructure and hardness properties of cermet coating sprayed by low power plasma

Conventional (micron size) WC–12Co coatings were sprayed on stainless steel substrate with low power plasma spraying process. The plasma torch was designed such that the powder can be sprayed at low power plasma (3.9–9.1 kW) and internally injected by carrier gas in the region between the anode and cathode. The cermet powders were sprayed via this new technique at different plasma power (3.9–9.1 kW). The results show that, at low power (3.9 kW), the main structure of the coatings is composed of WC grains. However, beside WC, W 2C phase has been detected in coatings sprayed at high plasma power (5–9 kW). On the other hand, intensity of WC peak decreased and that of W 2C increased as the plasma power was increased. The influence of plasma power on the coatings hardness has been investigated. The results indicated that the coatings hardness increased and reached about 1500 HV at plasma power of 6.5 kW. Over this power value, the hardness of the coatings slightly decreased mainly due to the appearance of brittle α-W 2C grains.

Influence of binder materials on the properties of low power plasma sprayed cermet coatings

Thermal spraying of cermet coatings is widely used for protection of machined parts against wear and corrosion. These coatings consist of WC particles in metal binders such as Co, Cr, and Ni. In this study, three kinds of WC powders with different metal binders (Co, NiCr and CoCr) were sprayed by low power plasma spray system on Al–Si–Cu alloy substrate. Fundamental aspects of sprayed cermet coatings, including (i) the effects of binder type on the coating structure, (ii) the microhardness properties and (iii) the microstructure were investigated. All the cermet coatings had the same phase structure such as WC and W 2C. However, the phase intensities of these phases are different in each coating, mainly due to the different in solidification rate in each case. Moreover, the microhardness measurements were found to be different in each coating. The results show that the binder type has a significance effect of the physical and mechanical properties of the sprayed coatings.