Plasma Spray Process Research Articles

Numerical techniques to find optimal input parameters for achieving mean particles\u2019 temperature and axial velocity in atmospheric plasma spray process

We numerically find values of four process input parameters, namely, the argon flow rate, the hydrogen flow rate, the powder feed rate, and the current, that yield the desired mean particles’ temperature and the mean particle velocity (collectively called mean particles’ characteristics, or MPCs) in an atmospheric plasma spray process just before the particles arrive at the substrate to be coated. Previous studies have shown that the coating quality depends upon the MPCs. The process is simulated by using the software, LAVA-P-3D, that provides MPCs close to their experimental values. Thus, numerical rather than physical experiments are conducted. We first use the design of experiments to characterize the sensitivity of the MPCs to process parameters. We then identify relationships between the significant input parameters and the MPCs by using two methods, namely, the least squares regression and the response surface methodology (RSM). Finally, we employ an optimization algorithm in conjunction with the weighted sum method to find optimum values of the process input variables to achieve desired values of the MPCs. The effects of weights assigned to the objective functions for the temperature and the velocity, and the difference in using the regression and the RSM model have been studied. It is found that these values of the process parameters provide MPCs within 5% of their desired values. This methodology is applicable to other coating processes and fabrication technologies such as hot forging, machining and casting.

La influencia del tamaño inicial del polvo de Óxido de Circonio en fase tetragonal estabilizado con Óxido de Itrio en la formación de una Barrera de protección térmica

En este trabajo se estudian los efectos en las propiedades de porosidad y microdureza al usar distintos diámetros de polvos de ZrO2 +8% Y2O3 (menores a 40 µm, entre 40 a 60 µm y mayores a 80 µm) a depositar en la capa superior de un sistema TBC (barrera de protección térmica) de doble capa conformado por NiCrCoAlY y ZrO2 +8% Y2O3 usando el proceso de pulverización en plasma, con el propósito de encontrar el polvo adecuado y factible de usar como capa superior. Para ello se utilizaron instrumentos como el Microscopio electrónico de barrido TESCAN Vega SB con microanalizador dispersivo INCA Energy ng electronic microscope con el cual se pudo detectar la porosidad en distintas zonas de evaluación de la capa superior y el durómetro de Vickers para medir la microdureza. Comprobándose que con diámetros de partículas menores a 40 µm las propiedades anteriormente mencionadas son las adecuadas para la conformación de la capa superior de ZrO2 +8% Y2O3.

Numerical Study of the Effects of Twin-Fluid Atomization on the Suspension Plasma Spraying Process

Suspension plasma spraying (SPS) is an effective technique to enhance the quality of the thermal barrier, wear-resistant, corrosion-resistant, and superhydrophobic coatings. To create the suspension in the SPS technique, nano and sub-micron solid particles are added to a base liquid (typically water or ethanol). Subsequently, by using either a mechanical injection system with a plain orifice or a twin-fluid atomizer (e.g., air-blast or effervescent), the suspension is injected into the high-velocity high-temperature plasma flow. In the present work, we simulate the interactions between the air-blast suspension spray and the plasma crossflow by using a three-dimensional two-way coupled Eulerian–Lagrangian model. Here, the suspension consists of ethanol (85 wt.%) and nickel (15 wt.%). Furthermore, at the standoff distance of 40 mm, a flat substrate is placed. To model the turbulence and the droplet breakup, Reynolds Stress Model (RSM) and Kelvin-Helmholtz Rayleigh-Taylor breakup model are used, respectively. Tracking of the fine particles is continued after suspension’s fragmentation and evaporation, until their deposition on the substrate. In addition, the effects of several parameters such as suspension mass flow rate, spray angle, and injector location on the in-flight behavior of droplets/particles as well as the particle velocity and temperature upon impact are investigated. It is shown that the injector location and the spray angle have a significant influence on the droplet/particle in-flight behavior. If the injector is far from the plasma or the spray angle is too wide, the particle temperature and velocity upon impact decrease considerably.

Failure Mechanisms of APS-YSZ-CoNiCrAlY Thermal Barrier Coating Under Isothermal Oxidation and Solid Particle Erosion

The high-temperature oxidation and solid particle erosion of thermal barrier coating (TBC) system which consists of a 8 wt.% yttria-partially stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited on Inconel 718 substrate via air plasma spraying (APS) process are studied experimentally. Isothermal oxidation tests of the APS-TBCs are conducted at 1050, 1100 and 1150 °C in air for up to 1970 h. Solid particle erosion tests are also performed on both as-deposited and heat-treated APS-TBC specimens at selected particle impingement angles and velocities in room temperature. The scanning electron microscopy (SEM) analyses of the cross sections of the APS-TBC specimens after the oxidation tests show the formation of thermally grown oxide (TGO) scale due to the oxidation of CoNiCrAlY bond coat, and the oxidation kinetics of TGO growth is described by the parabolic rate equation. The failure of the APS-TBC system under isothermal oxidation is associated with the spallation of the top coat through propagation and coalescence of cracks along the coating interface, which is affected by the TGO growth. The solid particle erosion rate of the top coat is found to increase with impingement angle and reach the maximum erosion rate at normal impingement of particles. The erosion resistance of the APS-TBC is observed to increase after the APS-TBC specimen has been exposed at 1100 and 1150 °C for 72 h, probably due to the sintering effect on the top coat, which results in reduction of top coat porosity.

Fabrication and Property Evaluation of the Al2O3-TiO2 Composite Coatings Prepared by Plasma Spray

The Al2O3-13 wt.% TiO2 (AT13) composite coatings were prepared on Q235 steel by plasma spray technique. The spray parameters were designed by the orthogonal experiments, and the properties of the coating were evaluated. Results showed that with respect to the bond strength of the coating, the optimized spraying parameters were the plasma current of 530 A, Ar flow of 41 L/min, H2 flow of 10 L/min, and spray standoff distance of 100 mm. The plasma spray process led to the transition of α-Al2O3 to γ-Al2O3, resulting in the increase in the porosity of AT13 coating prepared at nonoptimized parameters. Meanwhile, the porosity and cracks were also increased due to the decrease in the Ar flow and the increase in spray standoff distance. The low porosity, a few cracks, and the uniformly dispersed TiO2 particles contributed the enhanced properties including mechanical and corrosion behaviors of the AT13 coating prepared at optimized parameters. The bond strength, microhardness, and thermal shock resistance of the AT13 coating could reach 25.01 MPa, 1000.6 HV0.5, and 40 times when the coating was prepared at optimized parameters, respectively. Especially, the static Icorr of the AT13 coating prepared at optimized parameters was two order of magnitude less than that of Q235 steel. In addition, the erosion weight loss of Q235 steel could be decreased about 30 times by the protection of the AT13 coating.

Design of advanced one-step hydroxyapatite coatings for biomedical applications using the electrostatic spray deposition

Calcium phosphate ceramics (CaP), particularly hydroxyapatite Ca10(PO4)6(OH)2 (HAP), are widely used as surface coatings on top of titanium and its alloys in biomedical applications thanks to their ability to form strong bonds with natural bone tissues. In this paper, the electrostatic spray deposition technique (ESD) by using an organic phosphorus (V) precursor is proposed as an alternative to the widely used plasma-spraying process. This bottom-up approach enables the one-step fabrication of single-phase crystalline HAP coatings with unique morphologies. The microstructure and composition of the deposited CaP layers result from a delicate compromise between the physicochemical properties of the precursor solution and the ESD deposition conditions. An in-depth study of their structural, microstructural, compositional properties and their reactivity with Ti6Al4V is undertaken. It shows that the Ca/P molar ratio and the nature of phases are strongly dependent on hydrolysis and evaporation of the P(V) precursor in the ESD process. Optimized deposition conditions are found by using solvents of low-boiling point, for which the coatings deposited above 325 °C correspond to a well crystalline, nanostructured, and single-phase HAP. Infrared heat treatment of a coating prepared from the one optimized precursor solution promotes nanostructured HAP.

Surface resistivity and bonding strength of atmosphere plasma sprayed copper‐coated alumina substrate

AbstractCeramic substrate metallization is widely used in electronic device packaging and ceramic circuit board. Currently used methods of ceramic metallization are thick film technology (TFC), activity metal blazing (AMB), direct bonded copper (DBC), and direct plated copper (DPC) etc Here we report a facile approach for the metallization of alumina substrate using an atmosphere plasma spray (APS) process. The APS‐coated copper layer is dense, high purity, and attached very firmly with alumina substrate. The maximum bonding strength is 9.02 MPa, which is higher than that obtained by conventional methods. With EDS mapping, it is found that element interdiffusion occurred at the interface region during coating, which may be one of the reason of high bonding strength. The high dense and purity Cu layer demonstrates quite lower surface resistivity as 8.7 × 10−5Ω⋅mm. The influence of APS current on the quality of Cu coating has been systematically investigated, and the results proved that the method is efficient and economical.

High temperature tribological performance of nickel-based composite coatings by incorporating multiple oxides (TiO2–ZnO–MoO3)

The potential of lower friction and desired wear for nickel-based composite coating system at high temperatures was explored, where multiple oxides were incorporated into nickel matrix during the atmospheric plasma spraying process. The phase composition and microstructure of wear track were characterized by XRD, Raman, SEM/EDS, and FIB/STEM. The further addition of MoO3 allows for an effective reduction of friction coefficient at 800 °C. The surficial and interfacial analysis well confirmed that a protective tribo-layer presenting on the contacting surface, in which binary oxides (A-TiO2, R–TiO2, ZnO, NiO), molybdates and titanates (ZnMoO4, NiMoO4, and Zn2TiO4) were identifiable. The association of friction and wear properties with interface chemistry was discussed for the nickel-based coatings with the addition of multiple oxides (TiO2–ZnO–MoO3).

Structure and Wear Resistance of Plasma-Sprayed NiCrBSiC–TiCrC Composite Powder Coatings

The NTC20 and NTC40 composite powders were produced by conglomeration from the NiCrBSiC self-fluxing alloy with additions of 20 and 40 wt.% TiCrC, respectively, using an organic binder. Since the organic binder burnt out in the plasma spray process, components of the composite powders segregated and a certain amount (up to 15–20 vol.%) of the TiCrC particles was lost. The plasma-sprayed NTC20 and NTC40 coatings showed a heterophase lamellar structure, consisting of a nickel-base matrix in which fine chromium boride and carboboride grains (1–2 μm) and TiCrC particles (5–8 μm) were evenly distributed. Electron microprobe analysis revealed a higher content of oxides in the NTC20 and NTC40 coatings compared to the NiCrBSiC coating, which was associated with the oxidation of TiCrC particles in the plasma spray process. The introduction of TiCrC additions into the NiCrBSiC self-fluxing alloy increased porosity of the plasma-sprayed NTC20 and NTC40 coatings (up to 8%) versus the NiCrBSiC coating (5%). Wear tests of the plasma-sprayed NTC20 and NTC40 coatings were performed in dry sliding friction conditions using 65G steel as a counterface. For comparison, a plasma-sprayed coating consisting of the serial NiCrBSiC self-fluxing alloy was tested. Additions of 20 and 40 wt.% TiCrC particles to the NiCrBSiC self-fluxing alloy increased the wear resistance of the plasma-sprayed coatings by 2 to 2.3 times. As the sliding speed increased from 4 to 12 m/sec, the NiCrBSiC coating underwent catastrophic wear (I ≈ 60 μm/km), while the wear rate of the NTC20 and NTC40 coatings remained constant (I ≈ 12–22 μm/km).

Microstructure Characteristics, Tribology and Nano-Hardness of Plasma Sprayed NiCrRe Coating

This paper presents the results of the investigation of NiCrRe coating deposited by plasma spray process at the atmospheric pressure on boiler steel substrate. These coatings were characterized by means of a scanning electron microscopy, and Energy-dispersive X-ray spectroscopy. The wear resistance of plasma sprayed NiCrRe coatings has been investigated under dry sliding conditions at applied load of 10 N in air. The continuous stiffness measurement (CSM) method was used for the investigation of nanohardness using Agilent G200 Nano-indenter in order to determine the mechanical properties of the coatings. Microstructural observations pointed out that the NiCr layer with white isles of rhenium possessed porosity, oxidized, un-melted and semi-melted particles, and inclusions. According to the results the thickness of the layer is 450 µm, the indentation modulus 158 ± 24.4 GPa, hardness 3.74 ± 0.76 GPa and the coefficient of friction is 0.45.

Experimental and Numerical Investigation of Residual Stress in Coatings on Steel

Abstract In the present work, an alumina–titania (13 %) coating has been deposited by the air plasma spray process on a mild steel substrate by preheating the substrate to different temperatures. Coating thickness was measured by a scanning electron microscope, the hardness of coatings was measured, and morphology was also checked. The crack compliance technique has been used to determine the residual stress, as it is a very simple method, and also, it is compatible with conventional machining equipment and wire electric discharge machining equipment. Specimens were cut by the diamond cutter up to certain depth in microns, and the strain relieved was recorded. For finding out the residual stress, these recorded values of strain have been used by doing a finite element analysis (FEA) using the ANSYS 15.0 (Ansys, Inc., Canonsburg, PA) software. For validation of the model, uniaxial tension was applied on a sample by using the INSTRON universal testing machine (Instron Inc., Norwood, MA), and strain values at both the coating side and the substrate side were recorded and matched with the model under the same value of the load. After verifying the model, an equation has been developed using the Minitab software (Minitab Inc, Bengaluru, India), and the strains previously recorded were used as an input to the equation obtained, and the value of residual stress was calculated. The maximum value of residual stress was found at the coating–substrate interface, and the values of residual stress for sample preheated at 250°C, 200°C, and 150°C were found to be compressive, with the magnitude of −176.28 MPa, −243.54 MPa, and −255.42 MPa, respectively.

Preparation of dense ScSZ thin films by plasma spraying with densified ScSZ powders

Low-pressure plasma spray (LPPS) technology was applied to make scania-stabilized zirconia (ScSZ) thin films for solid oxide fuel cell (SOFC). Agglomerated ScSZ powders and densified ScSZ powders sintered at high temperature were synthesized and sprayed to make dense ScSZ thin films in the LPPS processes. SEM results indicate that the densified ScSZ powders can be easier molten in plasma stream to form more uniform and denser thin films. The influence of spraying parameters on the microstructure of the films was investigated. It is found that longer spraying distance was beneficial to the melting of the flying powders to make high-quality thin films. The ionic conductivity of the thin film (about 20 μm thick) was about 0.025 S/cm measured at 700 °C. A half-cell sample was made via air plasma spray process by using the densified ScSZ powder. The result indicated that a denser and thinner ScSZ should be made for higher electrochemical property.

Thermal Stability of YSZ Thick Thermal Barrier Coatings Deposited by Suspension and Atmospheric Plasma Spraying

Two types of segmentation-crack structured yttria-stabilized zirconia (YSZ) thick thermal barrier coatings (>500 μm, TTBCs) were deposited onto the stainless steel substrates using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS) process, respectively. In this work, thermal aging behaviors, such as the microstructures, phase compositions, grain growth, and mechanical properties of APS TTBCs and SPS TTBCs, were systematically investigated. Results showed that both as-sprayed TTBCs exhibited a typical segmentation-crack structure in the through-thickness direction. APS coatings mainly comprised of larger columnar crystals, while a large number of smaller equiaxed grains existed in SPS coatings. Both of the coatings underwent tetragonal-monoclinic phase transformation after 155 °C/40 h heat treatment. The poorer phase stability of SPS TTBCs may have a connection with smaller grain size. Thermal-aged APS and SPS coatings exhibited a significant increase in H and E values with the rising of thermal aging temperature, and for the samples that thermal aged at 1550 °C, the H and E values increased sharply during initial stage then decreased after 80 h due to the phase decomposition. The segmented APS coatings had weak bonding between the lamellaes during thermal exposure, which caused the mean Vickers hardness value of APS TTBCs to be much lower than that of SPS TTBCs.

Plasma sprayed alumina-yttria composite ceramic coating for electrical insulation applications

A composite ceramic coating consisting of 50:50 weight fraction of alumina (Al2O3)-yttria (Y2O3) blend was deposited as an electrically insulating topcoat on Oxygen-Free Electronic Copper (OFEC) substrates with a cermet interlayer of 30 wt% NiAl-70 wt% Al2O3 blend, both deposited by Atmospheric Plasma Spray (APS) process, and with a NiCrAlY bond coat deposited by High-Velocity Oxy-Fuel (HVOF) process for application in Direct Current (DC) conduction pump of the sodium-cooled fast reactor. The cermet interlayer in between topcoat and bond coat is beneficial for a reduction in the thermal expansion mismatch stresses and further contribute to the improvement in the thermal fatigue life of the multilayer coating system. The coated samples were subjected to a thermal fatigue cycle between 600 °C and 200 °C, simulating sodium-cooled fast reactor temperature patterns in regular operation and transient conditions. The multilayer composite ceramic coating with cermet interlayer successfully endured up to 1000 thermal cycles with no cracking, spallation, or delamination of the topcoat. Surface cross-sectional micrographs before and after the thermal cycle revealed neither significant changes in the splat and lamellae morphology nor distribution of phases, except for the densification effects. X-ray Diffraction (XRD) analysis revealed neither phase change nor the formation of any new phase upon thermal cycling. The adhesion test as per ASTM-C633 revealed that the bonding strength of the coatings lower by ~20% after thermal cycling. The insulation resistance of thermal cycled samples showed a two order reduction in the magnitude, compared to that of the as-coated samples.

High temperature solid particle erosion behaviour of SS 316L and thermal sprayed WC-Cr3C2–Ni coatings

The boiler tubes, steam and gas turbines are commonly affected by severe high-temperature erosion wear due to impingement of solid particles entrained in the stream of fluid. The investigation focuses on the applicability of thermal sprayed WC based coatings for turbines and boiler tubes by improving the erosion resistance of the base material. The present work includes high-temperature solid particle erosion behaviour of WC-Cr3C2–Ni coatings deposited by atmospheric plasma spray (APS) and high-velocity oxy-fuel (HVOF) processes. The effects of temperature and impact angle on erosion performance of uncoated and coated specimens were comparatively studied using air-jet erosion tester (ASTM G76). The eroded surface morphology was analysed using a scanning electron microscope (SEM). Uncoated steel exhibit ductile mechanisms; however, both coated specimens follow the mixed mode of material removal under the same operating conditions. Erosion resistance of APS and HVOF coatings is approximately two and three times higher than uncoated specimens at higher temperatures under 30° and 90° impact angles. The HVOF coating offers higher erosion resistance than APS coating due to lower porosity, greater splat adhesion, a lower degree of decarburisation and higher inter-splat sintering at elevated temperatures.

Microstructure and Cyclic Oxidation of Yttria-Stabilized Zirconia/Nanostructured ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3 Thermal Barrier Coating at 1373 K

This study is intended to improve the high-temperature oxidation of nano-ZGYbY: ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3 in order to apply it in the new generation of defect cluster thermal barrier coatings (TBCs) through the employment of an intermediate conventional yttria-stabilized zirconia (micro-YSZ) layer between the bond coat (CoNiCrAlY) and top coat. The specimens were deposited with an atmospheric plasma spray (APS) process on IN738LC superalloy. The cyclic oxidation test was performed in air at 1373 K with 4 h in each cycle. The microstructure of the nano-ZGYbY was studied by field emission scanning electron microscopy, revealing the formation of a bimodal microstructure consisted of nanosized particles retained from the initial APS-processed nanopowder and columnar grains, whereas the microstructure of intermediate micro-YSZ layer consisted of columnar grain splats only. X-ray diffraction of TBCs confirmed the formation of non-transformable (t′) ZrO2 phase ( $$ \frac{c}{a\sqrt 2 } $$ < 1.01) as well as the stability of this phase after oxidation. Also, applying an intermediate conventional YSZ layer with a higher CTE and KIC than that of nano-ZGYbY between the bond and top coats improved mechanical properties in new TBCs and it increased the oxidation life.

Fabrication and characteristics of a HfC/TiC multilayer coating by a vacuum plasma spray process to protect C/C composites against oxidation

A multilayer coating of hafnium carbide and titanium carbide was fabricated on SiC-coated carbon/carbon(C/C) composite using a vacuum plasma spraying (VPS) system. The microstructure, composition, adhesion strength and ablation performance of the HfC/TiC multilayer coating were analyzed. Cracks and delamination were not observed at the interfaces. Moreover, oxidized layers or impurities were not observed in the coated layer. The adhesive strength was superior to the HfC single layer coating. After the ablation experiment, based on the mass removal rate, surface temperature, microstructure, and compositional analysis, it was confirmed that the HfC/TiC multilayer coating can effectively protect the carbon composite material.

Multifunctional plasma-sprayed nanocomposite coating based on FA-ZnO-GO with improved bioactivity and wear behaviour

The purpose of this study was to fabricate and characterize the FA-ZnO-GO nanocomposite coating on Ti6Al4V (Ti64) using plasma spray process and investigate its tribological and biological properties. Our results confirmed that the presence of GO in the nanocomposite coating increased the microhardness and the wettability from 450 ± 16 Hv to 761 ± 41 Hv, and from 81 ± 3.3° to 58 ± 5°, respectively. FA-ZnO-GO coating induced bone-like apatite formation on Ti64, after soaking in simulated body fluid, demonstrating its bioactivity. In addition, FA-ZnO-GO coating on Ti64 significantly improved MG63 cell proliferation and attachment. Finally, GO nanosheets considerably decreased the friction coefficient of FA-ZnO coated Ti64, from 0.6 to 0.4 and wear rate, from 58.02 to 24.10 mg. It could be concluded that the synergic roles of FA, ZnO and GO could provide a multifunctional coating for orthopedic and dental implants.

Process and properties of dense and porous vertically-cracked yttria stabilized zirconia thermal barrier coatings

Highly durable thermal barrier coatings (TBCs) are desired for various applications in hot sections of gas turbine engines. In this work, dense vertically-cracked (DVC) and porous vertically-cracked (PVC) yttria stabilized zirconia (YSZ) TBCs were produced using the air plasma spray process with a specially engineered powder. The effects of plasma enthalpy on the in-flight particle behavior and coating microstructures were investigated. Thermal conductivity, thermal cycling behavior, as well as, failure mechanisms of both DVC TBCs and PVC TBCs were evaluated. The thermal conductivity of PVC TBCs was found to be approximately 40%–50% lower than that of DVC TBCs due to their higher porosity. In addition, the thermal cycling life of PVC TBCs was demonstrated to be more than 1.8 times that of DVC TBCs with the same substrates and bond coats. PVC TBCs also showed excellent sintering resistance after 743 one-hour thermal cycles at 1125 °C. This work demonstrates that highly durable 8YSZ PVC TBCs with simultaneously high strain tolerance, low thermal conductivity, and high sintering resistance can be produced by the application of an engineered powder feedstock.

Effect of CeO2 on the Microstructure and Properties of Plasma-Sprayed Al2O3-ZrO2 Ceramic Coatings

Al2O3-ZrO2 eutectic ceramic composite coatings with and without CeO2 addition were prepared by atmospheric plasma spraying. The microstructure and properties of the plasma-sprayed Al2O3-ZrO2-CeO2 coating were investigated. The results show that the Al2O3-ZrO2-CeO2 coating contained t-ZrO2, α-Al2O3 and γ-Al2O3 phases. Amorphous phase was found in the Al2O3-ZrO2 and Al2O3-ZrO2-CeO2 coatings, which was determined by the characteristics of the plasma spraying process and the eutectic composition of the composite powder. Compared the Al2O3-ZrO2-CeO2 coating with the Al2O3-ZrO2 coating, it was found that the Al2O3-ZrO2 coating with 4 wt.% CeO2 had lower porosity, more uniform and denser microstructure. The addition of 4 wt.% CeO2 was beneficial to improving the hardness, toughness, bond strength and wear resistance of the Al2O3-ZrO2 coating.