Thermal Spray Method Research Articles

TiB2-Coated Graphite As Wettable Cathode for Al Production

A significant reduction in greenhouse gas emissions from aluminum smelters will only be achieved with the emergence of new technologies. The most effective way to decrease significantly CO2 emissions from this industry is to replace the consumable carbon anodes that are used nowadays with so-called inert anodes that emit O2 instead of CO2 during Al electrolysis [1]. However, the implementation of inert anodes in Al smelters will induce an increase of the cell voltage, which could be compensated by reducing the cell ohmic drop by decreasing the anode-to-cathode distance. For that purpose, the thickness of the molten Al layer presents on the surface of the cathode must be minimized, which requires a high wettability of molten Al on the cathode. To date, the most promising cathode material is TiB2, which has an excellent Al wettability, good electrical conductivity and good chemical stability under Al electrolysis conditions. However, the manufacturing of bulk TiB2 electrodes at the industrial scale poses challenges. Sintering of TiB2 must be performed at high temperature (~2000°C), which induces significant crystal growth that has dramatic consequences on the mechanical properties of the electrode. Faced with these constraints, various consolidation methods operating at lower temperatures have been evaluated (sintering under pressure, spark plasma sintering...), but they are expensive and/or poorly adapted to the production of large electrodes of complex shape as required for Al electrolysis application.In the present study, new avenues based on the elaboration of TiB2 coated graphite cathodes by electrochemical and thermal spray methods are evaluated. The influence of the deposition conditions on the crystalline structure, morphology, mechanical resistance and Al wettability of TiB2 coatings will be presented in this communication.

Yttrium Aluminum Garnet Powder Feedstock for Atmospheric Plasma Spray

High demands for higher turbine efficiency bring attention to newer and more advanced insulating materials for the high temperature components in the turbine. Yttrium aluminum garnet (YAG) has shown good insulating properties in the previous published research, such as higher temperature limitation and better resistance to calcium–magnesium–alumina–silicate environmental contaminant penetration than the more conventional yttria-stabilized zirconia systems. Whereas in literature, coatings of YAG are typically prepared by solution deposition processes, in the present work YAG powder has been prepared for more conventional thermal spraying methods. The goal is to show the potential YAG powders have as a thermal barrier coating. Different approaches for obtaining a successful deposition and a good coating have been explored. Small-sized industrial-supplied powder and larger in-house-made powder have been compared, emphasizing the importance of energy used for deposition and crystallinity in the final coating. Highly crystalline material has successfully been produced with F4 atmospheric plasma spray system without post-treatment or substrate heating.

Дослідження механічних властивостей детонаційно-газових покриттів методом мікроіндентування

Розглянуто перспективи використання методу неруйнівних випробувань безперервним вдавленням індентора для визначення механічних властивостей газотермічних покриттів. Проведено випробування з оцінки мікротвердості й модуля пружності композиційних покриттів на основі карбідів вольфраму і хрому, отриманих методом детонаційно-газового напилення. Розглянуто діаграми проникнення для покриттів з порошків ВК8, ВК15, ВК18С, КХН15С. Показано, що при оптимальних режимах напилювання мікротвердість одержуваних покриттів близька до мікротвердості спечених твердих сплавів того ж складу. Визначено значення коефіцієнтів варіації значень мікротвердості зі збільшенням глибини відбитка для зразків зі спеченого твердого сплаву і напилених покриттів. При оптимальних режимах напилювання досягаються не тільки максимальні значення мікротвердості та модуля пружності покриттів, але і більш однорідні структура і механічні властивості покриттів. Більш чутливими до змін технологічних параметрів є значення модуля пружності напилених покриттів, що більш зручно при відпрацюванні технологічних процесів.

Tribological Behaviour of Plasma and HVOF-sprayed NiCrSiBFe Coatings

ABSTRACTInvestigation of tribological behaviour of NiCrSiBFe coating on SS 316L sliding against AISI 52100 bearing steel, alumina (Al2O3), and silicon nitride (Si3N4) under dry conditions carried out. Air Plasma Spraying (APS) and High Velocity Oxy Fuel (HVOF) thermal spraying methods used to deposit the coatings. The dry sliding wear tests were carried out with the ball on disc machine following the ASTM G99 standard and the normal loads used were 10N, 15N, and 20N along with the constant sliding velocity of 0.5 m s−1 at room temperature. The particle analysis, XRD, SEM, EDX, and nanoindentation were used for the characterisation of the powder, coatings, and worn surfaces. The spraying methods, counter materials, and normal loads were ranked based on the wear loss and coefficient of friction. The main wear mechanisms on the coating's wear track are abrasive and adhesive. The HVOF spraying method revealed enhanced wear resistance property as compared to the plasma-sprayed coating.

Electromagnetic Shielding Performance of Carbon Black Mixed Concrete with Zn-Al Metal Thermal Spray Coating.

The electromagnetic pulse (EMP) is a destructive phenomenon which harms the building, telecommunication, and IT based infrastructure. Thus, it is required to reduce the effect of EMP using shielding materials. In the present study, we have used different thickness of concrete walls by incorporating 1 and 5 wt% of carbon black, as well as 100 µm thick Zn–Al coating using the arc thermal metal spraying method (ATMSM). The EMP was evaluated using waveguide measurement fixture for shielding performance of the concrete wall in the range of 0.85 to 1 GHz frequency. The results reveal that the maximum value, i.e., 41.60 dB is shown by the 5-300-N specimen before application of Zn–Al coating where the thickness of concrete wall was 300 mm and 5% carbon black. However, once the 100 µm thick Zn–Al coating was applied on concrete specimen, this value was increased up to 89.75 dB. The increase in shielding values around 48 dB after using the Zn–Al coating is attributed to the reflection loss of the metal thermal spray coating. Thus, the Zn–Al coating can be used for EMP application instead of metallic plate.

Multi-scale mechanical properties of Fe-based amorphous/nanocrystalline composite coating synthesized by HVOF spraying

Fe-based (Fe–Cr–B–P–C) amorphous/nanocrystalline composite coatings were synthesized by high velocity oxy-fuel (HVOF) thermal spray method with varying powder feed rates and the multi-scale wear behaviour of the coatings’ is reported here. Microstructural characterization of the composite coating envisaged the presence of embedded nanocrystalline phases in the amorphous matrix. The porosity content decreased, whereas the amorphicity of the coating increased gradually with increment in the feed rate. The combined effect of (i) splat morphology and (ii) extent of devitrification on the mechanical and tribological properties of the various coatings was investigated. Increasing the powder feed rate resulted in higher hardness of the coating, which was attributed to better inter-splat bonding and reduction in α-Fe content in the amorphous matrix of the coating due to lower extent of devitrification. Nanotribology test on a single-splat revealed increment in wear resistance at elevated feed rate due to the reduction in volume fraction of softer nanocrystalline α-Fe phases. Besides, dry sliding wear investigated the coatings’ wear behaviour on a global basis and revealed decreasing trends for both wear rate and coefficient of friction with increasing feed rate. Most importantly, the Fe-based composite coatings exhibited low wear volume during nanotribology and lower friction coefficient, low wear rate of dry sliding wear study, compared to an SS316L coating, prepared using industrially optimized parameters. The enhanced wear resistance of the composite coating compared to that of the stainless steel coating makes it an effective method of surface protection for metallic substrates.

High Velocity Suspension Flame Spraying (HVSFS) of Metal Suspensions.

Thermal spraying of metal materials is one of the key applications of this technology in industry for over a hundred years. The variety of metal-based feedstocks (powders and wires) used for thermal spray is incredibly large and utilization covers abrasion and corrosion protection, as well as tribological and electrical applications. Spraying metals using suspension- or precursor-based thermal spray methods is a relatively new and unusual approach. This publication deals with three metal types, a NiCr 80/20, copper (Cu), and silver (Ag), sprayed as fine-grained powders dispersed in aqueous solvent. Suspensions were sprayed by means of high-velocity suspension spraying (HVSFS) employing a modified TopGun system. The aim was to prepare thin and dense metal coatings (10–70 µm) and to evaluate the process limits regarding the oxygen content of the coatings. In case of Cu and Ag, possible applications demand high purity with low oxidation of the coating to achieve for instance a high electrical conductivity or catalytic activity. For NiCr however, it was found that coatings with a fine dispersion of oxides can be usable for applications where a tunable resistivity is in demand. The paper describes the suspension preparation and presents results of spray experiments performed on metal substrates. Results are evaluated with respect to the phase composition and the achieved coating morphology. It turns out that the oxidation content and spray efficiency is strongly controlled by the oxygen fuel ratio and spray distance.

Optimizing high velocity oxy fuel spray coating process parameters for reducing emissions in ZrO2/Al2O3 coated internal combustion engines

It is a major issue in the world that only one third of the total fuel input gets con-verted to work, whereas the remaining two thirds of the fuel input goes unused through the exhaust and cooling systems. In the present years, enormous research has been done for reducing the undesirable emissions in internal combustion engines. In this paper, using high velocity oxy fuel thermal spray method, ZrO2 and Al2O3 were coated over the piston head by fluctuating the important thermal spraying process parameters. A central composite design model was developed for conducting twenty different experiments with different spray process parameters. The coated piston was used in internal combustion engine to evaluate the emissions, which were measured using AVL5 exhaust gas analyzer to evaluate the smoke density and NOx content. Empirical relations were created between input process parameters with the responses. Analysis of variance was used to evaluate the significance of developed model. Response surface method-ology was used to optimize, thereby predicting the parameters of oxygen flow rate at 229 L/m, fuel flow rate at 51.2 L/m, spray distance of 194 mm to predict minimum smoke density of 27.7 HSU and NOx of 220.8 ppm, which was validated to a high level of accuracy.

The Effect of Some Physical and Mechanical Properties of Cermet Coating on Petroleum Pipes Prepared by Thermal Spray Method

The cermet materials have wide industrial applications, one of which is the protection of damaged petroleum pipelines as a result of working and environmental conditions coated with those materials. The current research aims to treat damaged parts of petroleum pipes, turbine blades and other parts of the devices which are exposed to heat and damage. Using thermal spray method with different spraying distances (8, 10, 12, 14 and 16 cm), the coating material ensures the foundation (TiO2) and supportive material (Al–Ni) with supportive ratios (25, 35, 50 and 65%) for the formation of cermet coatings. Physical tests (porosity and adhesion force) as well as mechanical (hardness and wear rate), compositional tests (atomic force microscope) were conducted for only 2 h before and after heat treatment (1000 °C) The results showed that the best supportive ratio is 50% and the best spray distance is 12 cm at heat treatment (1000 °C) for 2 h, where the porosity gave a magnitude of 8.1% and adhesion force of 27.6 Mpa, the hardness is 42.4 Hv and the lowest wear rate at the sliding speed is 4.5 m/s. Meanwhile, atomic force microscope clarification to be the best topographical surface was at the same standard conditions mentioned above. Microscopic hardness was also calculated at different temperatures (450, 650, 850, 950, 1150 °C). It was observed that the best hardness value was with heat treatment at 950 °C.

Photocatalytic surfaces obtained through one-step thermal spraying of titanium

Advanced oxidation processes catalyzed by functional surfaces represent one of the most versatile methods for removing potentially toxic and persistent organic compounds from aqueous environments. This study is focused on obtaining two types of coatings with complex surface chemistry, involving titanium dioxide, nitrogen-doped titanium dioxide, titanium suboxides, and titanium nitride through a one-step thermal spraying method, starting from titanium feedstock powder. The differences in morphological and structural characteristics of coatings arise from the two different standoff distances between thermal spray nozzle and aluminum substrate (150 mm and 200 mm). The surface energies of the coatings range from 31 to 34 mN/m, which increase with up to 20% upon irradiation with UV light. The optical band gap values of the coatings are 2.01 eV and 1.55 eV respectively, indicating the possibility to harvest electromagnetic radiation from the visible domain in relation to photocatalysis applications. The photocatalytic activity of the surfaces has been tested against methylorange under UV (254 nm and 365 nm) and visible light irradiation. The photodegradation of methyl orange followed a pseudo-first order mechanism, with rate constants in the 0.0039–0.0871 min−1 interval. High photodegradation (91–99%) and dye mineralization (85–97%) efficiencies were registered for both types of coatings.

Tribological performance of AlCrN–MoS2/PTFE hard-lubricant composite coatings

In order to improve the tribological performance of the physical vapor-deposited AlCrN coatings, molybdenum disulfide (MoS2)/poly tetra fluoroethylene (PTFE) coatings were fabricated on the AlCrN coatings surface through the thermal spraying method. The microstructure, adhesive strength, hardness, and tribological properties were investigated. Reciprocating sliding tests against SiC ball were executed with a ball-on-plate tribometer. Results showed that the adhesive strength between the AlCrN–MoS2/PTFE composite coatings and substrate was increased by about 15% compared with single AlCrN coatings. Compared with the single MoS2/PTFE coatings, the hardness of the AlCrN–MoS2/PTFE composite coatings surface was increased by about 15%. The MoS2/PTFE layer can availably reduce the friction coefficient of single AlCrN layer, and the AlCrN–MoS2/PTFE composite coatings exhibited the lowest and the most stable friction coefficient. In addition, the MoS2/PTFE layer existed on the wear track and accumulated on both the sides, which was the main reason that the friction coefficient was still lower compared with the samples without MoS2/PTFE coatings.

Impact of gamma rays on physical properties of chromium oxide thin films

The effects of three different gamma (γ) irradiations (50, 100 and 200 Gy) are reported on thin films of chromium oxide (Cr2O3). The chemical thermal spray method was applied where the samples were deposited on a glass substrate at 300 °C temperature. XRD data indicated the crystallite size of the Cr2O3 thin film was 50.76 nm as-deposited, while after irradiation it was observed to be 57.03 nm, 68.21 nm and 77.21 nm for the (012) peak. AFM analyses indicated that the grain size and roughness increased with irradiation. The optical properties were measured in the wavelength range of 250–950 nm utilising a spectrophotometer before and after gamma irradiation. There was an observed reduction in the optical band gap and with an increased irradiation dose, the electrical I–V characteristics indicated a downward trend in resistance. The defects generated by γ-irradiation play a key part in the film's electrical and optical properties. This paper discusses the procedure for the induced irradiation, its physical effect, and possible applications such as dosimetry.

Improving hot corrosion resistance of Cr3C2–20NiCr coatings with CNT reinforcements

ABSTRACT Carbon Nanotubes (CNT) reinforced Cr3C2–20NiCr composite coatings were prepared and deposited on alloy steel using high-velocity oxyfuel (HVOF) thermal spraying method. The comparative effects of variation in CNT content (from 1 to 2 weight percentage) on mechanical properties and hot corrosion behaviour in super-heater zone of actual coal-fired boiler of a thermal power plant at 900°C have been investigated. After exposure to boiler environment, the corrosion products have been analysed by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, energy dispersive and cross-sectional analysis techniques. The results confirmed that variation in CNT content improved the mechanical and microstructural properties of surface coatings by modifying their surface characteristics. The variation in CNT content improved the corrosion protection property of composite coating at high temperature of exposure. Reduction in corrosion rate was observed with increase in CNT content from 1 to 2 weight percentage in the Cr3C2–20NiCr coating matrix.

A Study on the Corrosion Inhibition of Fe-Based Amorphous/Nanocrystalline Coating Synthesized by High-Velocity Oxy-Fuel Spraying in an Extreme Environment

In this work, Fe-based (Fe-Cr-B-P-C) amorphous/nanocrystalline composite coatings were deposited by high-velocity oxy-fuel thermal spray method with varying powder feed rates and their behavior in saline environment was investigated. An SS316L coating with optimized parameters was also prepared for comparison purpose. The microstructural characterization of the composite coatings revealed the presence of embedded nanocrystalline phases in the amorphous matrix. The amorphicity of the coating increased, whereas the porosity content decreased gradually with the increment in feed rate during the spraying process. The combined effect of extent of devitrification and porosity content on the corrosion behavior of the various coatings was analyzed. Potentiodynamic polarization and electrochemical impedance spectroscopy studies revealed that the Fe-based composite coatings exhibited significantly lower corrosion current density and higher polarization resistance than that of the mild steel substrate as well as SS316L coating. The enhanced corrosion resistance of the composite coatings is ascribed to the combined effect of lower porosity content and retained amorphous phase. In addition, the formation of chromium hydroxide along with some of the oxides and hydroxides of iron in the post-corroded coating samples aids in impeding the corrosive solution penetration, thereby increasing the corrosion inhibition efficiency of the composite coatings.

Comparison of cold sprayed and arc sprayed Zn15Al alloy coating on AZ91D magnesium substrate

Purpose The purpose of this paper is to examine the performance of cold-sprayed Zn15Al alloy coating whether it is capable of protecting magnesium alloy from corrosion, and to compare it with arc-sprayed Zn15Al alloy coating. Design/methodology/approach In this paper, Zn15Al alloy coating was prepared with CS-6000 cold spraying system and HDX-800 arc-sprayed system. Corrosion behaviors of the two kinds of coatings were examined with potentiodynamic polarization curves methods combined with SEM, EDS, XRD, etc. Findings Corrosion behavior of cold-sprayed Zn15Al alloy coating is superior to arc-sprayed Zn15Al alloy coating. The bonding strength and density of cold-sprayed Zn15Al alloy coating is much higher than that of arc-sprayed Zn15Al alloy coating. The cold-sprayed coating has a dense structure which separate magnesium from corrosion medium completely. The samples behave as Zn15Al instead of AZ91D alloy. The coating has a low probability of pitting corrosion comparing with cold sprayed Al coating through potentiodynamic polarization curve. Practical implications Cold-sprayed Zn15Al coating can be used to improve the anticorrosion performance of magnesium significantly and low down the risk of pitting corrosion of coating. Social implications Cold-sprayed Zn15Al coating is an environmentally friendly anticorrosion method for light alloy, which is also the most effective way among thermal spray, chemical vapor deposition, sol–gel, plating and anodizing or microarc oxidation. Originality/value The present paper used cold spray method to deposit Zn15Al coating, which has an overwhelming performance both in physical and anticorrosion to traditional thermal spray method.

Microstructure and corrosion resistance of Ni/Cr3C2-NiCr composite coating

PurposeIn this study, electrochemical deposition method which have cheaper equipment than thermal spraying methods and is available for the production of composite coatings were used.Design/methodology/approachComposite coatings were electrodeposited from a Watts's bath solution in which the suspended Cr3C2-NiCr particles were dispersed in the bath solution during deposition. Potentiodynamic polarization and electrochemical impedance spectroscopy techniques have been used to evaluate the corrosion resistance of the composite coating in the 3.5 Wt.% NaCl solution.FindingsIt was found that the submicron Cr3C2-NiCr particles distributed uniformly in the coating and depend on the current density of deposition, different amount of particles can be incorporated in the coating. The results showed that the corrosion resistance of the Ni/ Cr3C2-NiCr composite coatings is more comparable to the pure nickel coating.Originality/valueProduction of Ni-base composite coating from an electrolytic bath containing Cr3C2-NiCr particles is possible via electrodeposition.

Movable hood tube in LD convertor: Failure analysis and coating solution

An assembly of movable and fixed hood tubes provide safe passage to LD gas produced during primary steel making process. These water-cooled tubes, made up of ASTM A106: Grade B steel are exposed to most extreme environment involving severe thermal cycling, corrosion from hot LD gas as well as cooling water, wear and erosion from lime and slag splashing, etc. Current work analyzed the failure of movable tubes that led to generation of multiple cracks on the outer surface of the tube facing the LD gas.Analysis revealed cracks parallel to each other in circumferential direction of the tubes. Most of the cracks originated from outer wall of the tube and propagated inwards, however, some of the cracks emanated from inner wall which was in contact with cooling water as well. Elemental analysis revealed the presence of calcium, potassium and sulphur in the oxide scale along the cracks. The nature of scale was confirmed using Energy Dispersive Spectroscopy and Raman Spectroscopy. Presence of these elements are known to reduce the thermal conductivity of the scale and thus can make the cooling inefficient and promote greater thermal stresses. A number of circumferential and longitudinal cracks were observed in tube-to-tube weld zone as well and these were related to microstructural abnormality therein. Overall failure analysis suggested that “thermo-mechanical corrosion fatigue” resulted in premature failure of the hood tubes. Based on detailed laboratory study and field trial, Inconel 625 coating using twin wire thermal spray method has been proposed as a viable techno-economic solution for improved reliability of these tubes.

Ultrasonic transducers from thermal sprayed lead-free piezoelectric ceramic coatings for in-situ structural monitoring for pipelines

Direct-write ultrasonic transducers comprised of lead-free ceramic coatings were designed and in situ fabricated by thermal spray process onto stainless steel pipe structures. The experimental measurements and simulation analyses showed that both external and internal defects on the pipe structures could be effectively detected with the ultrasonic Lamb waves propagating along the axial direction, which were excited and detected using the direct-write ultrasonic transducers. The results suggested that the implementation of the direct-write ultrasonic transducers made of the lead-free piezoelectric ceramic coating by thermal spray method is promising for realizing structural health monitoring for pipeline structures with improved consistency and reliability. It is also an important step moving toward smart pipelines with real-time self-diagnostic function.

Development of Evaluation Method for Damage of Oxidation CoNiCrAlY Coating

The bond coat plays an important role in the failure of the thermal barrier coating (TBC) system used for gas turbines [1,2]. In this research, the CoNiCrAlY coated Ni-base superalloy specimens were used for developing evaluation method for interfacial damage in the coat. Samples were exposed at 1000°C and 1100°C for up to 1000 hours. The morphology and residual stress in the thermally grown oxide (TGO) layer on the CoNiCrAlY coating were characterized by microscopic observation and luminescence spectroscope, respectively. The microstructure and damage o\n both the coating surfaces and the cross sections were observed by optical microscope and scanning electron microscope. According to the results, the low pressure plasma sprayed CoNiCrAlY coating (LPPS) showed the thinnest TGO layer and lowest residual stress.Residual stress decreased with an increase in exposure time, depending on the morphology of TGO layer. The effects of thermal spraying methods on the oxidation of yttrium in TGO layer and BC layer and its influence on interfacial damage were discussed.

Physical and Electrochemical Performances of Cold Sprayed Pb Electrodes

Titanium-based PbO2 electrodes are widely used for chemical industries, such as electrodialysis, electrolysis, and electrodepositing, to improve the mechanical and life cycle properties of Pb metal electrodes. However, PbO2 electrodes are usually electrodeposited onto rigid metals due to its soft characteristic, which results in severe passivation problems requiring thin thickness and high porosity. It is of great importance to develop a rigid Pb metal electrode system since thermal spraying and welding methods fail to manufacture such a promising electrode. In the present work, the cold spraying method was used to deposit a pure Pb metal coating with thickness of above 500 μm on Q235 steel substrate. The coating has good physical performances, the porosity is less than 1%, and the bonding strength ranges from 6.25 to 7.75 MPa. The cross-sectional morphology suggests that no through-thickness pores exist in the coating. The oxygen evolution potential is larger than 1.5 V vs. SCE, which is similar to the potential of the titanium-based PbO2 electrode. Dynamic polarization curves and cyclic voltammetry curves of coated sample in sodium sulfate solution indicate that cold sprayed Pb coating is a good electrode for electrochemical reduction reactions. All our results mean that cold spraying is capable of manufacturing electrode materials for electrochemical industries.