Arc Plasma Spraying Research Articles

Influence of coating microstructure on the corrosion behavior of Inconel 625 coatings fabricated by different thermal spraying processes

Inconel 625 coatings with dense microstructures (porosity <2 %) were fabricated on AISI 304L stainless steel substrates by flame spraying (F), twin-wire arc spraying (A), plasma spraying (P), and high-velocity oxygen fuel spraying (H). The quantity of oxide within the coating layers followed the descending order of F > A > P > H, with Cr2O3 identified as the predominant oxide in the Inconel 625 coating layers. The corrosion resistance of the Inconel 625 coating specimens decreased in the order of H > A > P > F. The outstanding corrosion resistance of the Inconel 625 coating prepared by high-velocity oxygen fuel spraying was attributed to the compact coating layer with a very low oxide content. It is suggested that the localized galvanic corrosion around the oxide/matrix interface may deteriorate the corrosion resistance of the coating layer, which is the primary factor contributing to the low corrosion resistance of the F specimen. Although the P specimen exhibited a lower oxide content than the A specimen, it demonstrated inferior corrosion resistance. This phenomenon can be attributed to the significant formation of microcracks within the P specimen, creating penetration tunnels for the corrosive solution and consequently reducing its corrosion resistance.

Structure and electrochemical behavior of coatings from complex concentrated alloys deposited by Metco Triplex Pro-210 gun

Complex concentrated alloys (CCAs) have gained a significant attention in the engineering and research community due to their unique phase structure and exceptional properties. CCAs are usually comprised of five or more principal alloying elements mixed in near-equimolar or equimolar ratio, which can derive their qualities from multiple principal elements owing to cocktail effect. In this study, atmospheric plasma spraying system equipped with a Metco Triplex Pro-210 gun was used to deposit CCAs designed coating compositions onto SS-316L steel substrate. The microstructure, mechanical and electrochemical behavior of the developed (CoCrFeNiMnx) CCAs were analyzed and compared to commercially used WC-17Co (Diamalloy 2005NS) and NiCr-75Cr2C3 (Metco 81NS) coatings deposited by APS Metco Triplex Pro-210 cascading arc plasma spray gun. The experimental results confirm that using Metco Triplex Pro-210 it is possible to manufacture a CCA coating layers with lower porosity, excellent phase stability in compare to conventional thermal spray processes. Coating characterization also reveals that the CCA coating’ grains experienced a significant amount of grain refinement as compared to those in as-received CCA feedstock powder materials due to the occurrence of shot peening effect and dynamic recrystallization caused by highly deformed splat boundaries during spraying process. The electrochemical results indicates that the CCA coatings have a lower corrosion rate in compare to its plasma sprayed counterparts.

Thermal Spray Coating Technology – A Review

Thermal spray coating involves heating of coating materials (ceramic, carbide, and metal alloys) to a semi-molten or molten state and propels to substrate. The flame temperature is in the range of 3,000 to 16,000OC but the surface temperature of the substrate rarely exceeds 500 OC depending on the thermal spray processes being used. The coating materials are feed into the spray gun in the form of powders, rods or wires. Thermal spray coating is use for the following purposes; (i) increase corrosion and wear resistance, (ii) protection against electromagnetic, or electrostatic, (iii) protection against radio frequency interference, (iv) metal buildup and, (iv) cosmetic. Thermal spray coating can be categorized into five most common processes; (i) Flame arc spraying, (ii) Electric Arc spraying, (iii) Plasma arc spraying, (iv) High-velocity Oxy/Fuel (HVOF) and (v) Detonation Gun. In this paper, thermal spraying processes will be explained, along with the advantages and disadvantages of one another.

Hot corrosion behavior of intermixed layered LTA/YSZ thermal barrier coatings

In present work a unique combination of hybridized intermixed five layered coatings structure is developed by using Arc plasma spray method. A Lanthanum-Titanium-Aluminum Oxide (LaTi2Al9O19) that is LTA powder is used as a ceramic top coat material along with YSZ. A bond coat of nickel dominant NiCrAlY having thickness of 90 µm (µm) is applied on the substrate Inconel 718 post shot blasting. A preceding layer of YSZ with 50% and remaining 50% of 50 µm bond coat is sprayed. After that a layer of YSZ having thickness of 100 µm is applied. Then a layer of 50 µm thickness having 50% YSZ and 50% LTA is applied. LTA layer of thickness 100 µm is applied at the end. Cyclic and isothermal hot corrosion is performed using salts Na2SO4 & NaCl in 3:1 mass proportion at 900 °C. The mass increment by as-sprayed intermixed coatings is less than 0.05 mg/cm2 and it is 0.02 mg/cm2 for annealed coating. An annealed intermixed coating shows negligible mass increment and has survived upto 100 h in case of isothermal hot corrosion. In case of cyclic hot corrosion annealed intermixed coatings shown no mass increment and is survived till 70 h. Mass increment of as-sprayed intermixed coatings is less than 0.05 mg/cm2 but after 50 h severe spallation of coating occurred. The coating surface morphology is evaluated through field emission scanning electron microscopy (FESEM) and x-ray (XRD) diffraction.

Effect of the technological parameters of plasma-arc spraying of flux-cored wire on the structure and properties of intermetallide coatings based on Fe3Al

Existing techniques for applying intermetallide layers are characterized by low productivity, difficulties associated with the maintenance and operation of technological equipment, as well as significant costs for the purchase of materials for spraying. Therefore, modern science shows considerable interest in the development of new, highly effective technologies to form intermetallide coatings on the surface of articles. Such promising techniques include the technology of plasma-arc spraying (PAS) of flux-cored wires. This technique has a number of significant advantages, namely high performance, relative simplicity, as well as the affordability of equipment and materials for coating. This paper reports a study into the structure and properties of coatings obtained by flux-cored wire PAS, in which the steel sheath and aluminum powder filler interact when heated with the exothermic effect of Fe3Al synthesis. The influence of technological parameters of PAS process on the structure and properties of Fe-Al coatings was investigated by means of mathematical planning of the experiment. It was found that in all samples the main phase is an intermetallide of the Fe3Al type. Tests for gas-abrasive wear resistance at room temperature showed that the wear resistance of coatings exceeds the stability of steel S235 by an average of 2 times. As a result of studying the electrochemical properties in a 3-% aqueous solution of NaCl and in a 0.5-% solution of H2SO4, the score of corrosion resistance for these media was determined, which was, respectively, 4 and 5 (coatings belong to the group of "resistant"). In this regard, the practical use of coatings based on the Fe3Al intermetallide is recommended for protection against oxidation, corrosion, and gas-abrasive wear of components and assemblies in the heat power industry (heat exchanger pipes, catalytic converters, steam turbine blades, shut-off valves, etc.)

Effect of Kariba Weed Biodiesel Blended with n-Pentane on the Chosen Parameters of a Ceramic-Coated Thermal Barrier Direct Injection Diesel Engine.

In this experimental investigation, Kariba weed biodiesel (KSB) blended with n-pentane has been tested in conventional and ceramic-coated thermal barrier engines, and the results have been compiled and presented. A single-cylinder, four-stroke, direct injection diesel engine has been used as the test engine with eddy current dynamometer loading as used in the experimental setup. The tests were repeated in various ambient conditions to get an optimal value. Ceramic coating has been done with partially stabilized zirconia by the plasma arc spraying process. Among the quantum of tests conducted, 90% KSB blended with 10% n-pentane showed appreciable results when it was compared with the test fuel (neat diesel). The brake thermal efficiency and brake-specific fuel consumption were found to be better when compared with neat diesel. At increasing load, unburnt hydrocarbon, carbon monoxide, and smoke opacity emissions were appreciably reduced.

THERMAL SPRAYING AND RELATED TECHNOLOGIES FOR THE SURFACE MODIFICATION OF AL ALLOYS: REVIEW

Aluminum (Al) and its alloys are attractive for a variety of applications due to its advantages like light weight, ease of processing, and high thermal/electrical conductivities. However, it suffers short comings in terms of strength, wear resistance, and corrosion resistance. Thermal spray processes are widely used techniques for the surface property improvement like thermal, wear, and corrosion resistances of aluminum and other light metals. This paper summarizes the recent developments in thermal spraying and related technologies. Plasma spraying, flame spraying, and arc spraying are the chief thermal spraying techniques, whereas the HVOF and D-gun processes are the derivatives of flame spraying process. The cold spraying technique is an emerging coating technique where the particles are heated to a temperature lower than their melting point travel at high velocity and coat on impact with the target material by plastic deformation. Different parameters like heat source characteristics, coating material properties, substrate properties, and spray torch operating circumstances affect the coating microstructure and properties.

Influence of accompanying compressing air flow on the coating structure and properties in plasma-arc spraying by consumable current-conducting wire

The Paton Welding Journal, 2022, №02. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

Role of Coating Processes on the Corrosion Kinetics and Mechanism of Zinc in Artificial Seawater.

Zinc (Zn) coating is being used to protect steel structures from corrosion. There are different processes to deposit the coating onto a steel substrate. Therefore, in the present study, a 100 µm thick Zn coating was deposited by arc and plasma arc thermal spray coating processes, and the corrosion resistance performance was evaluated in artificial seawater. Scanning electron microscopy (SEM) results showed that the arc thermal spray coating exhibited splats and inflight particles, whereas plasma arc spraying showed a uniform and dense morphology. When the exposure periods were extended up to 23 d, the corrosion resistance of the arc as well as the plasma arc thermal spray coating increased considerably. This is attributed to the blocking characteristics of the defects by the stable hydrozincite (Zn5(OH)6(CO3)2).

Plasma Arc Coatings Produced from Powder-Cored Wires with Steel Sheaths

Plasma arc coatings produced from powder-cored wires developed at the Paton Electric Welding Institute that were deposited onto a low-carbon steel substrate were characterized. Interaction between the steel sheath (constituting at least 80 wt.% of the wire) and powder cores, such as B4C, B4C + ZrO2(nano), B4C + (Cr, Fe)7C3, and B4C + (Cr, Fe)7C3 + Al, in the arc plasma spraying process was analyzed. The resultant coatings were free of defects and had low porosity (to 2.5%) and lamellar structure. The core carbide components were found to dope the coating ferritic matrix and strengthen it with carbide, carboboride, and boride precipitates. An addition of 0.5% ZrO2 nanopowder refined the coating structure and participated in the formation of Fe2B and Fe3B precipitates. An aluminum addition within 10 wt.% did not led to iron aluminides but, being easily melted, activated the interaction between components, reduced porosity, and improved adhesion strength of the coatings. The coatings reached a microhardness of 6.25–8.59 GPa, being 4 to 5.5 times higher than the microhardness of the steel wire sheath. The development and use of such powder-cored wires expanded the application of plasma arc spraying, particularly for the protection of equipment against abrasive gas wear in chemical engineering, in the production of parts for pumps, compressors, and other components, and for the recovery of worn parts.

Effect of WC particles with different shapes during plasma spray on the properties of Fe-based composites coatings

The WC particles reinforced Fe-based spray welding layer was fabricated on the surface of 45 steel by plasma arc spray welding technology. The WC particles were irregular polyhedral and spherical, which were fed into the tail of the molten pool. The microstructure, phase composition, microhardness, and dry -sliding wear resistance of the coating were evaluated. The results show that the microstructure of the composite coating exhibits a hypoeutectic structure in addition to the WC, and the spherical WC particles are better preserved than the irregular polyhedral WC particles. The coating added with spherical WC particles has the best wear resistance, which is 1.28 times that of the coating added with irregular polyhedral WC particles.

Structurization of Coatings in the Plasma Arc Spraying Process Using B4C + (Cr, Fe)7C3-Cored Wires

The structure and phase composition acquired by coatings deposited in optimum mode by the plasma arc spraying process were examined. The spraying materials were wires with a steel sheath and nB4C + 100 – n(Cr, Fe)7C3 cores. The coatings have a ferrite matrix doped with Cr, B, and C and hardened with Fe3(B, C), Cr2B, FeCr, and boron carbide particles. When the (Cr, Fe)7C3 amount in the wire increases from 30 to 50 wt.%, the matrix phase (α-Fe-based solid solution) content decreases from 65.8 to 37.6 wt.% and the weight content of the hardening phases increases from 28.2 to 59.3%. These processes improve hardness of the coatings to 6.25 and 8.6 GPa, respectively. All coatings are characterized by a fine lamellar structure and high adhesion to the substrate. They exhibit high density and low porosity (<2.5%) and their hardness exceeds that of the wire steel sheath by 4.0 to 5.5 times. The plasma arc coatings can be used as wear-resistant materials for protection of chemical engineering equipment against abrasive wear, in the production of parts for pumps, compressors, and other machines, and for the recovery of worn parts.

Microstructure and mechanical properties of Co-based alloy coatings fabricated by laser cladding and plasma arc spray welding

The presented paper concerned a comparison of microstructural characters and mechanical performances between the cobalt based alloy (Co-based) coatings of the same chemical composition and yet fabricated by laser cladding and plasma arc spray welding (PASW), respectively. The phase constituent and microstructure of the coatings was characterized by optical metallography, scanning electron microscopy (SEM), X-ray diffraction spectrum (XRD), transmission electron microscopy (TEM) and energy dispersive spectrum (EDS), while the mechanical properties of the coatings obtained by different procedures were comparatively evaluated according to the microhardness and high-temperature wear resistance tests. As the novelties of the presented research, it was interesting that the amorphous Co-based coating was directly fabricated by laser cladding rather than by cladding-and-remelting method with commercial self-flux alloy powder for the first time. The formation mechanism of the amorphous phase was explained by experiments and finite element simulation. The advantages of the amorphous coatings in mechanical performances were quantitatively revealed.

Developing an approach to increasing cohesive strength durability for details of a compressor for a gas turbine engine

Formulation of the problem. Optimization of plasma-arc spraying technology is complicated by its multi-criteria and multi-parameter nature. The search for ways to increase the performance characteristics of the running-in sealing coating for the compressor parts of a gas turbine engine is due to the fact that these coatings operate at a temperature not exceeding usually 650 0C, which limits the scope of their use. Therefore, the development of an approach to increase the cohesive strength of the running-in coating will increase the life of gas turbine engines. Materials and methods. In this paper, it is proposed to apply the method of planning extreme experiments to increase the cohesive strength of a sealing run-in coating. The application of this method involves conducting active experiments in a certain working area of the process given by the numerical values of the controlled variables of the thermal spraying process. It is assumed that in this part of the workspace of controlled variables, the indicators of the goal function have suboptimal values. An experiment planning matrix 25 is defined. Results and discussion. The range of values of eleven variables that affect cohesive strength is determined. To test the reproducibility of experiments, four parallel experiments were carried out at each point of the factor space (in each row of the matrix). Based on the analysis of the coefficients of the obtained multi-parameter equation from 11 given variables, the degree of influence of each variable on the target function is determined. This approach made it possible to establish a pair of variables X5 (power) and X7 (nitrogen consumption), which increase the cohesion strength indicators most strongly compared with the variables under consideration. X5 power was determined by current strength indicators of 280 − 400 A and voltage of 40 − 75 V. A model for predicting cohesive strength depending on the selected pair of variables is obtained. Conclusions. An approach is proposed to increase the cohesive strength of the coating for compressor parts of a gas turbine engine using the method of planning extreme experiments. This made it possible to determine for the selected criterion the most significant controlled variables that ensure its extremum in a given work area.

Non-transferred Arc Torch Simulation by a Non-equilibrium Plasma Laminar-to-Turbulent Flow Model

Non-transferred arc torches are at the core of diverse industrial applications, particularly plasma spray. The flow in these torches transitions from laminar inside the torch to turbulent in the emerging jet. The interaction of the plasma with the processing gas leads to significant deviations from local thermodynamic equilibrium (LTE) far from the arc core. The flow from a non-transferred arc plasma spray torch is simulated using a non-LTE (NLTE) plasma flow model solved by variational multiscale (VMS) and nonlinear VMS (VMSn) methods, which are suitable for unified laminar and turbulent flow simulations. Non-plasma turbulent jet simulations indicate that the VMSn method produces results comparable to those by the dynamic Smagorinsky method, often considered the workhorse for turbulent incompressible flow simulations. VMS and VMSn approaches are applied to the simulation of incompressible, compressible, and NLTE plasma flows in non-transferred arc torch operating at representative conditions found in plasma spray processes. The NLTE plasma flow simulations reproduce the dynamics of the arc inside the torch together with the evolution of turbulence in the produced plasma jet in a cohesive manner. However, the similarity of results by both methods indicates the need for numerical resolution significantly higher than what is commonly afforded in arc torch simulations.

High-Temperature Oxidation Resistance of NiAl Intermetallic Formed In Situ by Thermal Spraying

An Al/Ni composite coating was deposited on the surface of a pure Ti substrate by arc spray technology and plasma spray technology. In order to enable the in-situ reaction between the Al/Ni composite coating and the specimen, they were heated under different conditions. In addition, oxidation testing was conducted to test the oxidation-resistant property of the coating. The phase transition regulation of the coating after heating, the influence of heating at different temperatures and time on the reaction depth, and the correlated theory of the in-situ formation of the NiAl intermetallic compounds were studied and analyzed. The results showed that after the heat treatment, a ragged wave-like morphology was exhibited in the diffusion front of Al, and a small amount of the Ni in the diffusion region did not participate in the reaction. The growth of the NiAl intermetallic layer in the diffusion region of the Al/Ni/Ti specimen was obviously slower compared with the Al/Ni specimen.

Tribological Behavior of Surface-Modified Titanium and Polytetrafluoroethylene-based Composites in Seawater

In order to improve the wear resistance of Ti6Al4V titanium alloy contacted with polytetrafluoroethylene-based composites in seawater, plasma nitriding (PN), micro arc oxidation (MAO) and plasma spray tungsten carbide (PSTC) were used to modify the surface of Ti6Al4V alloy. The tribological properties of Ti6Al4V alloy matrix and its modified layers sliding with polytetrafluoroethylene-based (PTFE) composites in seawater were comparatively investigated. The PTFE composites were made with glass fiber, graphite and bronze separately by compression molding. The results indicate that low friction coefficients were shown in modified layers of the titanium alloy sliding with PTFE composites in the seawater environment. The Ti6Al4V alloy exhibits poor wear resistance and leads to the serious wear of PTFE composites. The above three kinds of surface treatments improve effectively the wear resistance of Ti6Al4V alloy. The PN treatment with reducing the wear of PTFE composites shows better wear resistance than the MAO treatment with accelerating the wear of PTFE composites. The glass fiber reinforced PTFE shows better wear resistance than the bronze reinforced PTFE in the same experimental condition. It has obvious advantages that the titanium alloy with PN treatment contacted with glass fiber reinforced PTFE were chosen as the materials of the tribological components used in hydraulic transmission in the marine environment application.

High-Performance Molybdenum Coating by Wire–HVOF Thermal Spray Process

Coating deposition on many industrial components with good microstructural, mechanical properties, and better wear resistance is always a challenge for the thermal spray community. A number of thermal spray methods are used to develop such promising coatings for many industrial applications, viz. arc spray, flame spray, plasma, and HVOF. All these processes have their own limitations to achieve porous free, very dense, high-performance wear-resistant coatings. In this work, an attempt has been made to overcome this limitation. Molybdenum coatings were deposited on low-carbon steel substrates using wire–high-velocity oxy-fuel (W-HVOF; WH) thermal spray system (trade name HIJET 9610®). For a comparison, Mo coatings were also fabricated by arc spray, flame spray, plasma spray, and powder-HVOF processes. As-sprayed coatings were analyzed using x-ray diffraction, scanning electron microscopy for phase, and microstructural analysis, respectively. Coating microhardness, surface roughness, and porosity were also measured. Adhesion strength and wear tests were conducted to determine the mechanical and wear properties of the as-sprayed coatings. Results show that the coatings deposited by W-HVOF have better performance in terms of microstructural, mechanical, and wear resistance properties, in comparison with available thermal spray process (flame spray and plasma spray).

Influence of Time-Modulation of Applied Current on Arc Stability in DC Pulsed Plasma Spray Torch

Conventional direct current arc plasma spray torches generate highly unstable plasma jets. This leads to some difficulties with control of coating properties especially when injecting liquid feedstock for the elaboration of finely structured coatings. At constant applied current, a properly designed arc plasma torch can work in self-sustained pulsed mode that produces large amplitude oscillations of the arc, which are interrupted by very repeatable restrikes. This paper investigates the influence of the modulation of the applied current on the arc stability. Electrical and acoustic diagnostics permit to show that amplification of arc oscillation and a stabilizing mechanism occur when the arc current frequency approaches the torch resonance frequency. Time-resolved optical emission spectroscopy is used to determine temperature and enthalpy profiles close to the resonance and the enthalpy modulation.

Pulsed arc plasma jet synchronized with drop-on-demand dispenser

This work concerns with the liquid injection in arc plasma spraying for the development of finely structured ceramics coatings. Nanostructured coatings can be now achieved with nanopowders dispersed in a liquid (SPS: Suspension Plasma Spraying) or with a salt dissolved into a liquid (SPPS: Solution Precursor Plasma Spraying) injected into the plasma jet. Controlling electric arc instabilities confined in non-transferred arc plasma torch is therefore a key issue to get reproducible coating properties. Adjustment of parameters with a mono-cathode arc plasma allows a new resonance mode called “Mosquito”. A pulsed arc plasma producing a periodic regular voltage signal with modulation of enthalpy is obtained. The basic idea is to synchronize the injection system with the arc to introduce the liquid material in each plasma oscillation in the same conditions, in order to control the plasma treatment of the material in-fly. A custom-developed pulsed arc plasma torch is used with a drop-on-demand dispenser triggered by the arc voltage. A delay is added to adjust the droplets emission time and their penetration into the plasma gusts. Indeed, the treatment of droplets is also shown to be dependent on this injection delay. A TiO2 suspension and an aqueous solution of aluminium nitrate were optimized to get ejectable inks forming individual droplets. The feasibility of the process was demonstrated for SPS and SPPS techniques. Coatings from the suspension and the solution were achieved. First synchronized sprayings show a good penetration of the droplets into the plasma. Coatings show a fine structure of cauliflowers shapes. The synchronization of the ejection allows a control of morphology and a better deposition efficiency. Further investigations will find the optimal operating parameters to show the full potential of this original liquid injection technique.