Plasma Spray Technique Research Articles

Performance of thermal-sprayed WC-Cr3C2-Ni coatings in slurry erosion for hydrodynamic turbines

ABSTRACT Hydrodynamic turbine and slurry pump components are frequently influenced by severe wear due to the impact of solid particles present in the water. A WC-based coatings deposited by thermal spray techniques is known for the protection against slurry erosion. In the present work, WC-Cr3C2-Ni powder was sprayed on a turbine steel substrate using high-velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS) techniques. Microstructural and phase constitutions of as-sprayed coatings were examined. The effect of speed, slurry concentration and particle size on slurry erosion resistances of both coatings and substrate materials was comparatively studied in a slurry pot erosion tester. The results exhibit that the HVOF coating has better properties in terms of density, micro-hardness and slurry erosion resistance than those of APS coating. The improvement in slurry erosion resistance of bare turbine steel by using APS and HVOF coatings is approximately 37% and 62%, respectively.

Design and fabrication of elastic two-component polymer-metal disks using a 3D printer under different loads for the lumbar spine

One of the significant issues in the biomechanics of the intervertebral discs is the origin of the forces on the spine in the lumbar region. This issue can be gravity and the effects of inertia, muscles, intra-abdominal pressure, fascia, and ligaments. Recently, the 3D printing technique with biocompatible metals is used to strengthen the intervertebral discs. In this study, the three-dimensional printing (3DP) technique was used to fabricate the body of the disc. The prepared disc was multi-component and coated with Hydroxyapatite (HA) bioceramic nanoparticles using the plasma spray technique. To investigate the mechanical properties, the compressive strength test, hardness test, the stress-strain diagram, and the toughness were obtained from mechanical testing. The biological behavior of the disc was investigated in the Simulated Body Fluid (SBF) after 21 days. To characterize HA and coated disc the morphology and phase structure of the surface were evaluated using Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) techniques. The obtained mechanical results of three samples with a metal layer, without metal layer, and metal layer coated with HA nanoparticles were used in the mechanical simulation of the component. The artificial disk is an implant with the ability to maintain movement on the surgical surface, which reduces the side effects of vertebral connection in the destruction of adjacent components. Since the success of using an artificial disk depends on maintaining the mechanical functions of the intervertebral disc, it is useful to study the behavior of the artificial disk under different loads using Finite Element Method (FEM). In this research, the models are evaluated by comparing their behavior with experimental data and FEM data. Then, the elastic modulus and the optimal Poisson ratio were used for the two parts of the nucleus (the central part of the disk) and the annulus (the layers around the nucleus).

Surface modification of plasma spraying Al2O3–13 wt% TiO2 coating by laser remelting technique

An Al2O3–13 wt% TiO2 composite ceramic coating was prepared on the TiAl alloy surface by plasma spraying and laser remelting combined technique. The morphology, microstructure, and phase composition of the prepared coating were analyzed by scanning electron microscopy, energy disperse spectroscopy, and x-ray diffraction. The bonding strength, microhardness, wear resistance, erosion resistance, and thermal shock resistance of the coating were also tested. Results demonstrated that after processing by laser remelting, the particles on the ceramic coating surface were refined, lamellar structure disappeared, and density increased. A remelting layer basically without crack and other defects was gained. Due to laser remelting, the metastable-phase γ-Al2O3 was converted into stable-phase α-Al2O3. Influenced by the low thermal conductivity of ceramic materials, remelting of the whole ceramic layer is impossible to realize during laser remelting. The remelted ceramic coating formed the isometric crystal remelting zone with small grain size, sintering zone, and lamellar residual plasma spraying zone. The bonding strength and microhardness of the coating improved significantly after laser remelting, and the wear resistance, erosion resistance, and thermal shock resistance were significantly superior to those of the original plasma spraying layer. Laser remelting specimens still represented typical brittle erosion characteristics. Cracks initiated and expanded on near surface, finally leading to breakage of the remelting layer, mainly manifested by grain peeling. With respect to thermal shock failure mode, the corner peeling is the major failure mode of the ceramic coating after plasma spraying. Differently, corner peeling and considerable local peelings were found at the center of the ceramic coating after laser remelting. The influences of laser remelting on the thermal shock performances of the coating are mainly manifested as the decreased initial failure resistance, decelerated crack expansion, and changes in failure modes of the coating.

Structure-property correlation and high-temperature erosion performance of Inconel625-Al2O3 plasma-sprayed bimodal composite coatings

In the present experimental work, Inconel-625 (IN625) was reinforced with Al2O3 (30 wt%) to develop composite coatings with plasma spraying technique on ASTM SA210 GrA1 boiler steel. Three composite coatings were developed by varying Al2O3 particle sizes in micrometric, nano, and bimodal forms. The Inconel625 + 30 wt% micrometricAl2O3, Inconel625 + 30 wt% nano-Al2O3 and Inconel625 + 15 wt% micrometric Al2O3 + 15%nano-Al2O3 combinations were considered. The developed composite coatings were analyzed for the detailed microstructure studies, microhardness, fracture toughness, and elevated temperature erosion test. The elevated temperature erosion tests for bare substrate and coatings were conducted at 900 °C by using an erosion test rig (air jet) at two impingement angles 30° and 90°. By studying the eroded surfaces through scanning electron microscopy (SEM) micrographs, the mechanism of material removal was predicted. The existence of grooves and lips at a 30° and 90° impact angle on surfaces indicate the erosion mechanism consists of ploughing and micro-cutting action in the substrate. At 30° and 90° impact angles, all composite coatings exhibited a brittle erosion mode as erosion characteristics. The micrographs of eroded surfaces indicated splat removal, cracks, and fracture as the main erosion mechanism. The outcomes of the tests revealed that the bimodal composite coatings successfully protect the underlying substrate owing to their hardness and fracture toughness which is higher than the other two coatings. The better outcome of bimodal coatings was related to refined microstructures and good interaction among nano and micrometric Al2O3 reinforcement.

Understanding on the hydrogen detection of plasma sprayed tin oxide/tungsten oxide (SnO2/WO3) sensor

The plasma spray technique was well proven in producing metal oxide based gas sensors in the last two decades using different powder feedstocks. However, limited research was made to fabricate hydrogen gas sensor from tin oxide layer coated over tungsten oxide layer. This paper attempts to interpret the hydrogen gas sensing performances of plasma sprayed coating derived by depositing tin oxide layer over tungsten oxide (SnO2/WO3) layer. Plasma sprayed SnO2/WO3 sensor showed maximum response of 90% at 150 °C in contrast to stand-alone WO3 (89% at 350 °C) and stand-alone SnO2 (89% at 250 °C). The lower operating temperature of SnO2/WO3 sensor without compromising gas response was attributed to the WO3–SnO2 hetero-junction. SnO2/WO3 sensor showed selective sensing towards hydrogen with respect to carbon monoxide and methane gases. This sensor also possessed repeatable characteristics after 39 days from the initial measurement. In a nut-shell, plasma spayed SnO2/WO3 sensor showed stability of base resistance, repeatability after successive response and recovery cycles, selective sensing towards 500 ppm H2 with significant magnitude of gas response of 90%, response time of 35 s and recovery time of 269 s at a temperature of 150 °C.

Exploration of tribocorrosion behavior of Fe-based amorphous coating in simulated seawater

FeCrNiMoCBSi amorphous coating was prepared on marine steel F22 using plasma spraying technique for marine protection. The tribocorrosion behavior of Fe-based amorphous coating was explored in simulated seawater. The material loss of the coating in tribocorrosion behavior was attributed to the combination of wear and corrosion. To identify the interaction between wear and corrosion, the friction behavior of the coating was studied at different applied potentials using an electrochemical workstation. These results revealed that wear was a major factor in the material loss of the coating under the present tribology condition, whereas corrosion was retarded due to the good anti-corrosion properties of the Fe-based amorphous coating. The interaction between corrosion and wear in seawater was positive, indicating that corrosion promoted wear and corrosion promoted wear. The findings provide new insights into the wear-corrosion synergy effect on Fe-based amorphous coating under load-chemistry coupled conditions, and offer available supports for the design of marine protective 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.

Plasma spraying Ti–Al–C based composite coatings from Ti/Al/graphite agglomerates: Synthesis, characterization and reaction mechanism

This study explored the feasibility of fabricating Ti–Al–C based composite coatings using 2Ti/xAl/0.5C (x = 0.2–1.5) agglomerated powders by plasma spraying technique. The effect of Al content on microstructure and mechanical properties of spray coatings was studied, and the reaction mechanism was revealed. Results showed that the typical structure of agglomerates developed from C@Ti to C@Ti–Al and isolated Al as increasing Al additions. All the as-sprayed coatings consist of TiC, (Ti,Al)Cx (metastable solid solution formed by Al replacing Ti site in TiC), Ti2AlC and some TixAly phases. Ti2AlC phase grew tightly surrounded by TiC and (Ti,Al)Cx nanoparticles embedded in the TixAly regions. With elevated Al additions, the Ti2AlC phase content increases first and then decreases at x = 1.0. Correspondingly, the hardness of the composite coatings first increased and then decreased. The study proved the possibility of reaction synthesis of Ti2AlC phase by plasma spraying element powder mixtures and its formation mechanism was proposed.

Cyclic Oxidation Properties of the Nanocrystalline AlCrFeCoNi High-Entropy Alloy Coatings Applied by the Atmospheric Plasma Spraying Technique

Microcrystalline and nanocrystalline AlCrFeCoNi high-entropy alloy (HEA) coatings were applied on Inconel 718 superalloy using the atmospheric plasma spraying (APS) process. The high-temperature oxidation behavior of the microcrystalline and nanocrystalline AlCrFeCoNi HEA-coated superalloy was examined at 1100 °C under the air atmosphere for 50 cycles under cyclic heating and cooling (1 h for each cycle). The oxidation kinetics of both nanocrystalline- and microcrystalline-coated superalloys were accordingly analyzed by weight change measurements. We noted that the uncoated and coated samples followed the parabolic rate law of the oxidation. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDS), elemental mapping and X-ray photoelectron spectroscopy (XPS) were used to analyze the oxidized coated and uncoated samples. In the HEA-coated superalloy, Fe, Ni, Co and Al were oxidized in the inter-splat region, whereas the splats, which consisted mainly of Ni and Cr, remained unoxidized. Due to the formation of compact and adhesive thin NiO, CoO oxides and spinels together with the Al2O3 oxide scale on the surface of the coating during oxidation, the developed nanocrystalline HEA coating showed better oxidation resistance compared with the microcrystalline HEA coating.

Sliding Wear Behavior of the Nanoceria-Doped AlCrFeCoNi High-Entropy Alloy Coatings Deposited by Air Plasma Spraying Technique

Sliding Wear Behavior of the Nanoceria-Doped AlCrFeCoNi High-Entropy Alloy Coatings Deposited by Air Plasma Spraying Technique

Microstructure and Corrosion Behavior of Atmospheric Plasma Sprayed NiCoCrAlFe High Entropy Alloy Coating.

High entropy alloys (HEAs) are multi-elemental alloy systems that exhibit a combination of exceptional mechanical and physical properties, and nowadays are validating their potential in the form of thermal sprayed coatings. In the present study, a novel synthesis method is presented to form high entropy alloy coatings. For this purpose, thermal sprayed coatings were deposited on Stainless Steel 316L substrates using atmospheric plasma spraying technique with subsequent annealing, at 1000 °C for 4 h, to assist alloy formation by thermal diffusion. The coatings in as-coated samples as well as in annealed forms were extensively studied by SEM for microstructure and cross-sectional analysis. Phase identification was performed by X-ray diffraction studies. The annealed coatings revealed a mixed BCC and FCC based HEA structure. Potentiodynamic corrosion behavior of SS316L sprayed as well as annealed coatings were also carried out in 3.5% NaCl solution and it was found that the HEA-based annealed coatings displayed the best corrosion resistance 0.83 (mpy), as compared to coated/non-annealed and SS 316 L that showed corrosion resistance of 7.60 (mpy) and 3.04 (mpy), respectively.

Strain‐Activated Copper Catalyst for pH‐Universal Hydrogen Evolution Reaction

AbstractDeveloping low‐cost and high‐activity pH‐universal hydrogen evolution reaction (HER) catalysts is very crucial to the industrialization of water electrolysis. However, the high price, low yield, and poor stability of current HER catalysts make them difficult to meet practical requirements. Herein, a plasma spraying technique is employed to prepare self‐supported Cu catalysts with tensile strain for the first time. The tensile strain upshifts the d‐band of Cu, improves the water dissociation and H* adsorption, eventually improves the intrinsic HER catalytic activity. As such, Cu electrode achieves overpotentials of 182 mV in 0.5 m H2SO4, 261 mV in 1 M PBS, and 121 mV in 1 M KOH at 10 mA cm–2. In addition, Cu electrode also performs well at high current densities, the overpotentials at 1 A cm–2 are much lower than those of Pt foil in acid, neutral, and alkaline solutions.

Hot corrosion behavior of dense CYSZ/YSZ bilayer coatings deposited by atmospheric plasma spray in Na2SO4 + V2O5 molten salts

In this work, the hot corrosion (HC) resistance of bilayer thermal barrier coatings (TBC) architectures composed of dense Ceria-Yttria Stabilized Zirconia (CYSZ) and Yttria-Stabilized Zirconia (YSZ) deposited using the Atmospheric Plasma Spray (APS) technique was evaluated. Two kinds of HC tests were performed using different salt concentrations and thermal cycling (adding fresh salt between cycles) with a constant salt mixture (Na 2 SO 4 -V 2 O 5 ) and constant testing temperature (900 °C). The results showed that the HC mechanism changes with the testing procedure, and the outer dense CYSZ layer acts as a barrier to protect the inner layers against HC degradation. However, the formation of vertical cracks during the coating deposition process affected the corrosion resistance of the TBC systems. • Dense CYSZ layer overlay increases the hot corrosion resistance of TBC system. • Mechanisms of coating's degradation were identified. • Vertical cracks reduce the hot corrosion resistance of the TBC system.

Zinc coating on steel by atmosphere plasma spray and their anti-corrosion behavior

In this paper, we report a simple process for Zn coatings based on atmosphere plasma spray (APS) techniques. The Zn coating, prepared at the optimal spraying current of 120A, shows excellent structure characteristics with low porosity and high adhesion strength of 10.42 MPa. After 3000 h of NaCl solution immersion test, no obvious corrosion damage occurred on the surface. The corrosion evolution of Zn-coated steel was mainly transformed from spontaneous passivation of the oxide film to pitting damage by corrosion medium. However, due to the isolation effect and lower self-corrosion potential, the Zn coating could protect steel from corrosion damages, which demonstrates the excellent anti-corrosion property.

Simulation of residual stress in thick thermal barrier coating (TTBC) during thermal shock: A response surface-finite element modeling

The current study demonstrates a well-designed response surface methodology (RSM), based on the generated dataset of finite element method (FEM) to establish an integrated model for simulation of residual stress distribution in a thick thermal barrier coating (TTBC). In this study, typical TTBCs were applied on Hastelloy X Nickel-based superalloy using air plasma spray technique followed by thermal cycling. The recorded stress data of Raman spectroscopy was employed to verify the proposed FEM model. A relatively good agreement was obtained between predicted residual stresses and measured ones. Verified FEM model was used to carry out the parametric studies to evaluate the effects of such various parameters as interface amplitude, wavelength, thermally grown oxide thickness and preheating temperature on the stress distribution in the TTBC during the thermal cycling. The computed data were subsequently used for the development of RSM model. In conclusion, experimentally verified numerical data was used to construct a statistical model based on RSM and successfully used to predict the residual stress distribution field in TTBC during thermal cycling. The obtained results of hybrid FEM- RSM model were in acceptable conformity with Raman spectroscopy measurements.

Investigate the corrosion properties of stellite coated on AZ91D alloy by plasma spray technique

Stellite alloys are primarily preferred in various component manufacturing industries due to its magnificent mechanical and electrochemical properties. The stellite powder is deposited over the materials by various thermal spray methods in industries. This research proposed investigating the microstructure and corrosion properties of stellite powder that is deposited on an AZ91D magnesium alloy substrate by atmospheric plasma spraying process. The SEM, optical microscopy, and atomic force microscopy are typical of the coatings. Immersion experiments in sodium chloride conditions are conducted at varying pH levels, chloride ion concentration, and exposure time. In addition, XRD examined the phase change properties of the powder and sprayed condition of coatings along with corrosion testing. The corrosion experiments were conducted to establish several regression models based on the central composite design and the response sur-face process. The accuracy of the regression models was sufficient to allow correlations between input parameters and responses through variance analysis. It was found that the pH value significantly influences the corrosion behaviour of stellite coatings followed by other parameters such as chloride ion concentration and exposure time, even in corrosion tests, plasma-sprayed coatings performed better.

Evaluation of Corrosion Resistance of Bi-layered Plasma-sprayed Coating on Titanium Implants

There are several attempts to improve surface characteristics of biomaterials with thick film coatings. In this research, a class of protective coating layers of bi-layered Hydroxyapatite (HA)/Al2O3-SiO2 (with 10, 20, 30 %wt SiO2) were deposited on titanium (Ti) surfaces by plasma spray technique. The surface features of the applied coating layers were evaluated in detail to confirm the effectiveness of the technique for further biomedical applications. The results demonstrated that uniform and bi-layered plasma sprayed coatings can be successfully prepared through the optimization of engineering parameters. Also, it was found that the roughness of the bi-layered coatings increases with increasing the number of coating layers. The corrosion behavior of the coated and uncoated samples was comparatively investigated using electrochemical tests. The measured current densities (icorr) for HA, (HA)/Al2O3-SiO2 (with 10, 20, 30 %wt SiO2) were 0.27μA/cm2, 0.28 μA/cm2, 0.23 μA/cm2, 0.79 μA/cm2, respectively. According to the results, corrosion resistance of samples with 20% SiO2 is significantly improved compared to the single-layer HA and bare Ti. The outcomes of FESEM results revealed that porosity and cavities related to evaporation of adhesive PVA and it is confirmed by increasing the percentage of silica to more than 20%, porosity has increased. In conclusion, the proposed coating system showed promising abilities for future biomedical applications. It could be optimized and improved by changing the structural characteristics of the substrate, chemical composition and porosity of the coating layers.

Erosion behavior of Al2O3 + Sm2SrAl2O7 composite thermal barrier coatings

Thermal barrier coatings (TBC) are applied in gas turbines to protect the components from critical operating conditions at high temperatures. Foreign object damage is a major failure cause of turbine components due to erosion of the topcoat ceramic layer of the TBC system. In the present study, Samarium Strontium Aluminate (Sm2SrAl2O7) powders have been synthesized in the laboratory through molten salt synthesis, and thermal barrier coatings were developed using Atmospheric Plasma Spray (APS) technique on an Inconel substrate with NiCrAlY as a bond coat. The erosion behavior of the composite TBC system at temperatures of 200 °C and 800 °C at 30° and 90° impingement angles was analyzed using alumina erodent with a grit size of 50 µm. The erosion weight loss was measured in regular intervals and plotted against erodent mass. The samples exhibited a higher volumetric erosion at 90° and 800 °C. The morphological analysis of developed 70% Al2O3 + 30% Sm2SrAl2O7 composite coatings and eroded areas were performed using XRD, SEM, and surface profilometer. Erosion scars and surrounding deformed areas have been identified in the samples. The microstructure of the eroded areas in the samples showed micro-cutting and plowing as the major material removal mechanisms. Detailed phase analysis of the eroded locations in samples has been reported.

Residual stress analysis on functionally graded 8% Y2O3-ZrO2 and NiCrAlY thermal barrier coatings

Thermal Barrier Coatings (TBCs) protect metallic components that operate in high temperature environments and enhance their service life. The conventional two-layered TBC system consists of a duplex ceramic top coat (TC) fabricated from 8 wt% yttria stabilized zirconia (8-YSZ) and an underlying bond coat (BC) comprised of intermetallic layers such as NiAl or MCrAlY (M = Co, Ni) etc. In the present study, functionally graded material (FGM) TBCs were fabricated by introducing a third blend layer of 8-YSZ and NiCrAlY, in between the BC and TC in order to enhance the thermal fatigue life of the TBC. The blend layer in FGM TBCs provides a smoother transition in thermal expansion properties between the metallic substrate and the top ceramic coat (8YSZ) which have widely different thermal expansion characteristics compared with each other. In service, thermal fatigue introduces severe tensile stresses between the coated layers and the substrates leading to ultimate detachment of the coatings from the substrates. In this work, residual stress analysis by Cos α method was carried out as a non-destructive assessment tool to foresee the likelihood of onset of failure in the TBCs, well before the damage was visible. The two-layered (conventional) and three-layered (FGM) TBCs were synthesized on Inconel 718 substrates by atmospheric plasma spray (APS) technique. The TBCs were subjected to thermal fatigue tests between 1200℃ (by using gas flame) and ambient temperature and evaluated for residual stress analysis at different stages of thermal fatigue testing. The goal was to assess if residual stress analysis could be used to determine if the TBC was about to fail well before the delamination occurred and the catastrophic failure could be avoided. The tests conducted and results obtained are presented.

Effects of Transient Thermal Shock on the Microstructure and Mechanical Properties of CoCrFeNiMn High-Entropy Alloy Coatings

CoCrFeNiMn high-entropy alloy (HEA) has great potential for engineering application due to its good ductility and high fracture toughness at low temperature. It can be deposited on components as coatings to take advantage of its excellent properties and reduce the cost. In this study, CoCrFeNiMn HEA coatings were deposited on 316L stainless steel substrates by atmospheric plasma spraying (APS) technique, and a series of transient thermal shock tests were performed. It was found that the coatings contained two main phases: a face-centered cubic (FCC) solid solution phase and a flocculent oxides phase. The elemental contents of Co, Cr, Fe, and Ni were close to equal atomic percentage in the coating, while Mn was reduced significantly. The oxygen was mainly distributed in the dark flocculent oxides phase. After transient thermal shock tests, these two phases remained stable, but some tiny cracks appeared on the surface. Meanwhile, the microhardness of the coating after transient thermal shock tests also showed stable, ∼ 420 HV. Weibull statistics were used to analyze the reliability of the microhardness, and the Weibull modulus m was distributed from 9 to 15. The CoCrFeNiMn HEA coating exhibited high phase stability and excellent properties under transient thermal shock, making it have service advantages in extreme environments, especially in the fields of the development of future nuclear and aerospace structural materials.