Conventional Plasma Spraying Research Articles

Fabrication of nanoporous multilayer graphene nanoplatelets membrane for water desalination

In recent years, graphene-based membranes are extensively explored for desalination process to fulfil the pure water scarcity. Still, there is a noticeable gap in the research on a technique that simultaneously deposits a membrane and also generates pores in the flake of membrane. In this work, we demonstrated a one-step and scalable protocol i.e. a conventional atmospheric plasma spraying (APS) to fabricate a graphene nanoplatelets (GNP) membrane for water desalination application. Various characterization tools: FE-SEM, Raman, and TEM etc. utilized to evaluate the deposition of GNPs flakes and induced sub-nanometer pores in the large area GNP membrane. Plasma-sprayed GNP membrane showed impressive desalination performance in terms of water flux, salt rejection, and permeability around ~67 Lm−2 h−1, ~99 %, and ~ 115 Lm−2h−1bar−1 respectively at 0.6 bar of transmembrane pressure (TMP). This performance of the GNP membrane was attributed to the critical role of graphene in terms of its structures, including induced pores within graphene sheets and its functions. Our deposited GNP membrane is cost-effective and showed the applicability of conventional plasma spraying in the design of nanomaterials-based membranes for various water purification protocols.

Role of bond coat deposition method in the hot corrosion behavior of CSZ thermal barrier coatings

In this research, a NiCrAlY bond coat was applied using two methods – namely, conventional air plasma spraying (APS) and new spark plasma sintering (SPS) technique – on an Inconel-738 substrate. The ceria-stabilized zirconia (CSZ) topcoat was similar in both samples and deposited using the APS method. The hot corrosion performance of the fabricated thermal barrier coatings (TBCs) was studied at 950°C in a sodium sulfate (Na2SO4)–55 wt.% vanadium (V) oxide (V2O5) corrosive salt environment. The results showed that the hot corrosion resistance of the coating system with an SPS-processed NiCrAlY bond coat and a CSZ topcoat was much better than that of the conventional TBC sample because the adhesion of the bond coat to the substrate was very good in this sample, and the interface was free of any defects such as cracks and porosities. According to these results, no spallation was observed in the SPS-processed coating up to 38 h, while for the CSZ topcoat with an APS-processed bond coat, spallation occurred after 24 h.

Effects of the TiO2 content on the mechanical properties and galvanic corrosion resistance of Al2O3 coatings

In this study, the conventional air plasma spraying (APS) technology was employed to deposit Al2O3 coatings with varying TiO2 contents (AT coatings) on a Ti80 substrate. The coating microstructure was characterized using scanning electron microscopy (SEM), as well as the mechanical properties including fracture toughness and Vickers hardness by indentation method. Besides, the galvanic corrosion behavior of a 921A/AT coating system couples was thoroughly investigated in comparison to a 921A/Ti80 couple. The influence of TiO2 content on the resistance of Al2O3 coatings to galvanic corrosion were discussed experimentally and theoretically. Microstructural observation indicated that the introduction of TiO2 into Al2O3 coating was conducive to improving coating quality, exhibiting a decreasing value in surface roughness and porosity. Besides, improved fracture toughness and decreasing hardness were observed for samples with higher contents of TiO2. Both experimental and theorical results demonstrated that the AT coatings could potentially inhibit the galvanic corrosion which occurred on 921A/Ti80 couples, but its resistance to galvanic corrosion weakened conversely with the increase of TiO2 content. Thereinto, the volume resistivity played a decisive role.

Investigation of sintering mechanism for novel composite structural thermal barrier coating

AbstractA micro‐agglomerated particle embedded–thermal barrier coating (TBC) structure was prepared by an improved plasma spray process to withstand the sintering‐induced degradation of TBCs during service. In this study, the sintering resistance and thermophysical and mechanical properties of conventional and novel‐structured TBCs were systematically characterized. The results suggested that the thermal conductivity and sintering shrinkage of the novel‐structured TBCs were approximately 30% lower than those of conventional air plasma spraying TBCs. The elastic modulus of the novel‐structured coating is only 32% of that of the conventional structure after thermal exposure at 1300°C for 100 h. The distinct structure of the coating is the main factor that influences its performance. The relationship between the structural evolution and residual strain of the coating was analyzed using electron backscatter diffraction and transmission electron microscopy. Significant differences were observed in the sintering behavior of the dense matrix and embedded particle regions in the coating. Some columnar grains near the intersplat pores in the dense matrix have similar lattice orientations, and they tend to connect and consequently heal the intersplat pores. The large pores between the agglomerated particles and non‐oriented submicron‐sized grains that constitute these particles are responsible for the sintering resistance of the coating.

The influence of HV-APS process parameters on microstructure and erosion resistance of metalloceramic WC-CrC-Ni coatings

This article presents an analysis of the effect of high velocity plasma spraying (HV-APS) parameters of WC-CrC-Ni powder. Plasma spraying processes were carried out using a three-electrode Axial III type torch with central powder injection, in which a supersonic nozzle with a diameter of 5/16 inch was installed. The following were taken as variable parameters: power current per electrode (140, 160, 180A), hydrogen flow rate (10, 20, 30 NLPM) and powder feed rate (15, 20, 25 g/min). The obtained coatings were characterized by thicknesses exceeding 200 μm and their porosity was very low (<1%), similar to those obtained by the HVOF method most commonly used for spraying carbide coatings. The obtained coatings were characterized by hardness in the range of 800–950 HV0.5 - depending on spraying parameters. The solid particle erosion test showed that the WC-CrC-Ni coating - regardless of the spraying parameters - has good erosion resistance. The analysis of the obtained test results indicates that the use of the HV-APS process allows producing carbide coatings with properties similar to those obtained in the HVOF spraying method and significantly better compared to conventional plasma spraying (APS).

Thermal Barrier Coatings: An Insight into Conventional Plasma Spray and Water-Stabilized Plasma Spray

Thermal barrier coating (TBC) systems have presented an ongoing design issue in bids to improve the lifespan of coatings. A TBC can support an extended lifespan by repairing cracks between interfacial layers during high thermal exposure while at the same time increasing coating thickness. Two deposition techniques, atmospheric plasma spray and water-stabilized plasma spray (WSP), have been distinguished to understand mechanical and thermal performance based on their contrasting torch systems and microstructural characterization. This insight paper will underline the superiority of WSP coating and the need to leverage existing technology by optimizing better deposition parameters for future fatigue-resistant TBC production.

Preparation and CMAS Wettability Investigation of CMAS Corrosion Resistant Protective Layer with Micro-Nano Double Scale Structure

Solution precursor plasma spray (SPPS) can prepare thermal barrier coatings (TBCs) with nanostructures, which can modify the adhesion and wettability of molten silicate environmental deposits (CMAS) on the surface of TBCs, thereby improving the resistance of TBCs to CMAS corrosion. In this study, SPPS layers with micro-nano double scale structures were prepared on the surface of conventional atmospheric plasma spraying (APS) coatings. The effect of process parameters on the micro-nano double scale structures and the wetting and infiltration behavior of molten CMAS on the surface of coatings were investigated. The results show that micron structure is more sensitive to process parameters. Lower precursor viscosity, closer spraying distance, and smoother APS layer are favorable to form more typical and dense micron structures. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% and the steady-state contact angle increased up to three times. The reason is that the micro-nano double scale structures can effectively trap air and form an air layer between the coating surface and the molten CMAS. In addition, nano-particles play a more important role in the formation of the air layer, which in turn determines the steady-state wettability properties. While micron structures can influence the time needed to reach the steady state. However, the SPPS layers composed of nano-particles have a very loose structure and weak cohesion, and they degrade and fail rapidly after the infiltration of molten CMAS. Therefore, maintaining the excellent CMAS wetting resistance of the SPPS layers while taking into account their lifetime and reliability has become the focus of further research.

Synthesis of low-cost Ti4O7 membrane electrode for electrooxidation of tetracycline under flow-through conditions: Performance, kinetics and mechanism

The high cost of anode material and the limitation of liquid-mass transfer on anode surface in electrochemical advanced oxidation processes always restrict the removal efficiency of organics. To overcome the limitations, this study synthesized a low-cost Ti4O7 membrane electrode by simple sol-gel method, and developed a flow-through electrooxidation system based on Ti4O7 membrane electrode. The membrane electrode presented high crystallinity, high specific surface area (10.18 m2/g), concentrated pore sizes distribution (0.1–1 µm), and high oxygen evolution potential (2.2 V vs. SHE). Furthermore, compared with stirring conditions, the flow-through conditions could enhance the liquid-mass transfer on anode surface, resulting in a high tetracycline degradation efficiency (97.24%) and a low energy consumption (0.18 kWh/gDOC). High degradation rates of tetracycline (more than 95%) were both observed with the initial tetracycline concentration ranged from 10 to 50 mg/L. The increase of pipeline pressure and current density had positive influence on tetracycline degradation. Hydroxyl radicals and sulfate radicals produced on the electrode surface are the main oxidants for tetracycline degradation. The degradation pathway of tetracycline included oxygenation, hydroxylation, demethylation, decarbonylation, ring-open, and C-N bond cleavage. Compared with materials prepared by conventional hot pressing and plasma spraying method, this simply synthesized Ti4O7 membrane electrode exhibited efficient oxidation capability and competitive electric cost for contaminants degradation, demonstrating its practicability, feasibility, and facilitation for the potential application.

Microstructure and properties of Ni-Al coatings obtained by conventional and high-velocity atmospheric plasma spraying

In this paper, we report on the microstructure and properties study of the Ni-Al system (75/25 at. %) coatings obtained by atmospheric plasma spraying (APS) and high-velocity plasma spraying (HV-APS) methods which differ in the speed of the sprayed particles. The structure and phase composition were investigated using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray phase analysis (XRD). The paper presents the measurements results of porosity and microhardness, as well as the resistance of coatings to high-temperature oxidation. Additionally, we propose a phase transformations model that explains the difference in structure formation. For HV-APS and APS coatings, the difference in structure is shown to be due to different residence conditions of the sprayed particles in the plasma jet. Particles in the HV-APS coatings were completely melted, while a significant proportion of unmelted particles was observed in the APS coatings. HV-APS coatings have better high-temperature oxidation resistance compared to APS coatings, which is explained by a decrease in their porosity to ∼2 %.

Effect of microstructure on thermal cycle fatigue resistance in a SPS-TBC

The turbine blades of gas turbine engines used in thermal power plants are coated with a thermal barrier coating (TBC) made of ceramics to provide heat resistance. In recent years, gas turbines for power generation have been utilized as a regulating power source for engines, which are subject to frequent start-up and shutdown and output fluctuations. In this context, the suspension plasma spraying (SPS) method, which has been developed recently, has been studied intensively because it has a porous columnar structure and superior thermal cycle properties compared with the conventional atmospheric plasma spraying method. In this study, we evaluated the thermal cycle properties of TBCs with different microstructures prepared by varying the deposition conditions of the SPS method, and investigated the effect of microstructures on the thermal cycle properties. The experimental results indicated that the thermal cycle fatigue life of SPS-TBC increased with decreasing column diameter. The effect of microstructure on the thermal cycle fatigue life was discussed based on the thermal stress in top-coating evaluated by the FE analysis.

Wear Behavior Analysis of Al2O3 Coatings Manufactured by APS and HVOF Spraying Processes Using Powder and Suspension Feedstocks

Thermally sprayed ceramic coatings are applied for the protection of surfaces that are exposed mainly to wear, high temperatures, and corrosion. In recent years, great interest has been garnered by spray processes with submicrometric and nanometric feedstock materials, due to the refinement of the structure and improved coating properties. This paper compares the microstructure and tribological properties of alumina coatings sprayed using conventional atmospheric plasma spraying (APS), and various methods that use finely grained suspension feedstocks, namely, suspension plasma spraying (SPS) and suspension high-velocity oxy-fuel spraying (S-HVOF). Furthermore, the suspension plasma-sprayed Al2O3 coatings have been deposited with radial (SPS) and axial (A-SPS) feedstock injection. The results showed that all suspension-based coatings demonstrated much better wear resistance than the powder-sprayed ones. S-HVOF and axial suspension plasma spraying (A-SPS) allowed for the deposition of the most dense and homogeneous coatings. Dense-structured coatings with low porosity (4 vol.%) and good cohesion to the metallic substrate, containing a high content of α–Al2O3 phase (56 vol.%) and a very low wear rate (0.2 ± 0.04 mm3 × 10−6/(N∙m)), were produced with the S-HVOF method. The wear mechanism of ceramic coatings included the adhesive wear mode supported by the fatigue-induced material delamination. Moreover, the presence of wear debris and tribofilm was confirmed. Finally, the coefficient of friction for the coatings was in the range between 0.44 and 0.68, with the highest values being recorded for APS sprayed coatings.

Corrosion and mechanical performance of HVOF WC-based coatings with alloyed nickel binder for use in marine hydraulic applications

Hydraulic components used in the maritime environment suffer damage due to the effects of corrosion and marine biofouling accumulation. The application of engineered coatings can overcome these problems. This study investigated the corrosion and mechanical performance of novel high velocity oxygen fuel (HVOF) sprayed ceramic-metal composite coatings; i.e., WC-18 wt% Hastelloy C and WC-10 wt% Ni-5 wt% Cr, designed for the protection of marine hydraulic components. A conventional atmospheric plasma sprayed (APS) ceramic coating (i.e., Al2O3-40 wt% TiO2) and uncoated Monel K500 substrate were tested for benchmarking purposes. The corrosion performance of the samples was assessed using a combination of laboratory-based tests (i.e., electrochemical polarization, neutral salt spray, hot water immersion) and field exposure tests by immersion in seawater. The mechanical properties of the samples were assessed via a drop-weight impact test and the tensile adhesion test. The results showed that the HVOF coatings exhibited better corrosion resistance and mechanical performance compared to the baseline APS ceramic coating and uncoated Monel K500 substrate.

A Study on YSZ Abradable Seal Coatings Prepared by Atmospheric Plasma Spray and Mixed Solution Precursor Plasma Spray

The yttria-stabilized zirconia (YSZ) ceramic has been proved to be a promising material for abradable seal coatings. In this research, abradable seal coatings named C-YSZ and M-YSZ were prepared by applying the conventional plasma spraying method with improved nanostructured powders and by the mixed solution precursor plasma spraying technique, respectively. A study on the microstructure, mechanical properties, oxidation resistance and hot corrosion resistance was conducted on both C-YSZ and M-YSZ coatings by using scanning electron microscopy, transmission electron microscopy, muffle furnace and x-ray diffractometer. The results show that the C-YSZ coating presents larger-size pores with denser YSZ structures and grains showing size larger than 100 nm distributed around the pores compared with M-YSZ, in which the pores are much smaller and the grain size is 20-30 nm. As a result, the M-YSZ coating displays lower hardness and bond strength compared with C-YSZ. The oxidation tests show that the M-YSZ coating possesses better oxidation resistance due to the nanostructure topcoat compared with C-YSZ. However, the hot corrosion tests indicate that both the C-YSZ coating and the M-YSZ coating fail to present ideal hot corrosion resistance.

Microstructures and corrosion properties of novel Fe46.8-Mo30.6-Cr16.6-C4.3-B1.7 metallic glass coatings manufactured by vacuum plasma spray process

Metallic glass coating layers were fabricated using new Fe46.8-Mo30.6-Cr16.6-C4.3-B1.7 metallic glass powders via vacuum plasma spray process (VPS). The microstructure and corrosion properties of the VPS Fe-based metallic glass coating layer were examined and compared with coating layer of same composition prepared through the conventional atmospheric plasma spray process (APS). Broad halo peaks were found in the metallic glass powder feedstock and two coating layers. Splat particles and minor un-melted powders were observed in the two metallic glass coating layers. For APS coating layer deposited under aerobic condition, Fe–Cr based oxides were detected at the particle boundaries and Cr element was found depleted in regions near the oxide. Meanwhile, all the alloying elements in the VPS material were evenly distributed throughout the coating layer. Corrosion tests showed that the corrosion properties of the APS coating layers were Ecorr: −0.347 V and Icorr: 1.38 × 10−5A/cm2, whereas for the VPS coating layer, Ecorr: −0.342 V and Icorr: 3.21 × 10−6A/cm2, signifying superior corrosion resistance in the VPS material to the APS material. The corrosion started from the pores and particle boundaries in both the coating layers and then advanced along the particle boundaries. It was apparent that the Cr depletion area inside the APS coating layer was susceptible to corrosion. Based on the above results, the corrosion mechanisms of the Fe-based metallic glass coating layers fabricated by the VPS and the APS processes were also discussed.

Changes in the Coating Composition Due to APS Process Conditions for Al2O3-Cr2O3-TiO2 Ternary Powder Blends

Thermally sprayed coatings from the single oxides and binary compositions of the Al2O3-Cr2O3-TiO2 system show multifunctional properties. Ternary compositions are promising for further improvement in their performance. The stability of the composition during coating formation is an important issue for blended feedstock powders in order to obtain the desired properties. This work focuses on the compositional changes of a ternary blend of Al2O3, Cr2O3 and TiOx powders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H2 plasma gas mixture. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H2 ratio were varied. For the highest argon flow rate, this resulted in an average particle velocity of 140% and an average particle temperature of 90% of the initial values, respectively. Coating composition and microstructure were studied by optical microscopy, SEM, including EDS analyses, and XRD. In addition, the coating hardness and electrical impedance were also measured. Differences in the “difficulty of melting factor” (DMF) and the thermal diffusivity of the three oxides appear to be responsible for the dramatic change of the coating composition with an increasing argon flow rate. For the highest argon flow rate applied, besides TiO2, the coating contains only 8 wt.% Al2O3, while the Cr2O3 content remained almost constant. At the same time, the change of the Ar/H2 ratio resulted in the formation of stoichiometric TiO2 in the coating by oxidation of TiOx in the feedstock powder. Moreover, a small content of titanium was found in the Cr2O3 splats, showing that there are only limited interactions between the large oxide powder particles. Thus, the study has shown that stability of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions.

Microstructure evolution and mechanical properties of Al2O3-40%TiO2 coating by Hybrid-Low Velocity OxyFuel process

This study presents a new thermal spray coating manufacturing process to deposit high performance ceramic coatings named ‘Hybrid-LVOF’ (Low Velocity OxyFuel). By this process, both thin and thick ceramic coatings can be deposited on metallic or ceramic substrates. In this work, the microstructural and mechanical properties of H-LVOF sprayed Al2O3-TiO2 coatings were compared with plasma sprayed (PS) Al2O3-TiO2 coatings; with the additional condition that typical bond coat was not applied. The α-Al2O3 and γ-Al2O3 contents have been quantified in the as-sprayed coatings. As-sprayed Hybrid-LVOF coatings show better properties than conventional plasma-sprayed coatings including better fracture toughness. Noise and radiation levels are very low in the hybrid-LVOF process. Furthermore, a 50% reduction in overall production cost was achieved. H-LVOF could be therefore a possible alternative to a conventional plasma spray process.

Fabrication of NiAl intermetallic alloy integrated materials chips with continuous one-dimensional composition gradients by plasma spray deposition and laser remelting

Integrated materials chips (IMCs) contain a multitude of discrete materials with varying compositions on a single substrate to enable the rapid screening of material properties, and therefore offer the potential for accelerating the speed of materials research and development. The present work proposes a combinatorial technology based on conventional plasma spray (PS) deposition and laser remelting (LR), and employs the proposed technology to develop Ni-Al intermetallic alloy IMCs with a continuous composition gradient yielding various homogeneous compositions with unique microstructures and microhardness values along the length. The experimental results demonstrate that the proposed technology can effectively promote the discovery of relationships between the composition, microstructure, and the inherent properties of materials.

Study on Sol–Gel Synthesized IN800 Thermal Barrier Coatings Subjected to Thermal Cyclic Loading: Effect of Metallic Substrates

In this work, the effect of sol–gel deposited top-coat on thermal fatigue resistance of thermal barrier coatings (TBCs) subjected to thermal fatigue loading is evaluated experimentally. To obtain non-conventional sol–gel thermal barrier coatings (SGTBC), coated samples underwent thermal fatigue loading at 1100 °C for 10 min heating and cooling. The tested sol–gel thermal barrier coatings were then compared to conventional air plasma sprayed (APS) thermal barrier coatings as well. The life of samples was investigated as a function of number of sustaining thermal cycles to times. Furthermore, the performed experiment was analyzed using scanning electron microscope, energy dispersive spectroscopy and X-ray diffractometer. The obtained results indicated that the non-conventional sol–gel thermal barrier coatings exhibited 1.46 times better thermal fatigue life in IN800SGTBC against 1.31 times thermal fatigue life of IN718SGTBC but overall thermal fatigue life was found to be better in IN718 SGTBC, signified effects of metallic substrates in thermal fatigue life determination. However, the nanostructured SGTBC had higher thermal cyclic resistance than conventional APS TBC, resulted in improved lifetime indicating the increased adherence at the substrate interface. Results also showed that the dominant failure mechanism of TBCs was destabilization of top-coat (YSZ), resulting composition of (Al, Cr)2O3 and spinel as reaction products for depleting Y2O3 producing from yttria stabilized zirconia (YSZ). Furthermore, the results showed that the amount of the porosity percent in the sol–gel TBCs was 2.3% higher than the conventional TBCs. Figure: Material Processing, and thereafter testing analysis for present work.

High-Performance Al2O3 Coating by Hybrid-LVOF (Low-Velocity Oxyfuel) Process

Plasma spraying is now a standard coating process to deposit ceramic coatings for applications in automotive, aviation, medical and health care and others. This study deals with a hybrid thermal spray technique to deposit ceramic coatings. This technique named “hybrid-LVOF” (low-velocity oxyfuel) makes it possible to deposit uniform thin or thick ceramic coatings on metallic or ceramic substrates. In this work, the microstructural and mechanical properties of Al2O3 coating deposited by hybrid-LVOF were compared with those of Al2O3 coating deposited by the conventional plasma spray technique. The Rietveld refinement of XRD patterns was used to quantify the α-Al2O3 and γ-Al2O3 coating content. Hybrid-LVOF as-sprayed coatings have a smoother surface, lower porosity (2–3%), higher hardness and better adhesion strength than conventional plasma-sprayed coatings. The hybrid-LVOF coatings also exhibit better fracture toughness probably because of a high inter-splat bonding and low porosity.

Effect of hydrogen and argon shrouding gas flow rate on high-temperature oxidation behavior of NiCrAlY coating by solid shielding shrouded plasma spray (SSPS)

This paper investigates the effect of hydrogen and argon shrouding gas flow rates in solid shielding shrouded plasma spray (SSPS) method on in-flight oxidation, formation and growth of the TGO layer and high-temperature oxidation behavior of NiCrAlY coating. With this in mind, NiCrAlY coating was applied on the Hastelloy X substrate by solid shielding shrouded plasma spray (SSPS) and compared with the conventional atmospheric plasma spray (APS). High-temperature oxidation was carried out at 1000 °C for 200 h. Microstructure of the coatings before and after oxidation was evaluated by scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). To study the effect of solid shielding shrouded plasma spray on the properties of metallic coatings, the variable parameters such as the shroud gas nature (Ar, H2), the gas injection method (internal, external or both) and their flow rates, were examined. The results indicated the better operation of the coatings deposited under the protection of internal argon shroud gas with a flow rate of 75SLPM, which could be due to better protection of the metallic particles against in-flight oxidation. This sample represented the lowest thickness of the TGO layer (2.25 μm). The formation and growth of the TGO layer were higher in APS coating (3.82 μm) than that of SSPS. Consequently, this can prove by the preservation of mono-layered TGO of SSPS coating even after 200 h oxidation without the formation of any CSN clusters.