High Velocity Oxy-fuel Research Articles

Advanced Coatings by Thermal Spray Processes

Coatings are pivotal in combating problems of premature component degradation in aggressive industrial environments and constitute a strategic area for continued development. Thermal spray (TS) coatings offer distinct advantages by combining versatility, cost-effectiveness, and the ability to coat complex geometries without constraints of other in-chamber processes. Consequently, TS techniques like high-velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS) are industrially well-accepted. However, they have reached limits of their capabilities while expectations from coatings progressively increase in pursuit of enhanced efficiency and productivity. Two emerging TS variants, namely high-velocity air-fuel (HVAF) and liquid feedstock thermal spraying, offer attractive pathways to realize high-performance surfaces superior to those hitherto achievable. Supersonic HVAF spraying provides highly adherent coatings with negligible porosity and its low processing temperature also ensures insignificant thermal ‘damage’ (oxidation, decarburization, etc.) to the starting material. On the other hand, liquid feedstock derived TS coatings, deposited using suspensions of fine particles (100 nm–5 µm) or solution precursors, permits the production of coatings with novel microstructures and diverse application-specific architectures. The possibility of hybrid processing, combining liquid and powder feedstock, provides further opportunities to fine tune the properties of functional surfaces. These new approaches are discussed along with some illustrative examples.

Wear on Thermal &Plasma Spray Coated Al-2014 Alloy under Dry Sliding Conditions

In this paper, wear are common problems encountered in the piston. Thermal spray coatings re one of numerous methods for alteration of part's surface properties. The innovation relies upon the standard of dissolving and speeding of fine particles and their quick cementing delayed consequence on the substrate. Among every one of the strategies of thermal spray coatings, High-Velocity Oxy-Fuel and Plasma spray procedure is broadly utilized in different applications. In this research, chemical powder with explicit properties against wear was chosen, and it was showered with high velocity oxygen fuel (HVOF) and plasma spray strategies onto Aluminium 2014 alloy substrates. Both the coatings have same binder with equivalent rate. The coatings were portrayals were done with the help of Scanning Electron Microscope, X-ray Diffractometer, and Pin-on-disc wear testing machine. A pin on disc machine was used to analyse the wear behaviour by applying 10N and 20N loads.

Synergistic effect of CeO2 and Al2O3 nanoparticle dispersion on the oxidation behavior of MCrAlY coatings deposited by HVOF

In this study, the as-received and nano-scaled oxide dispersion strengthened (ODS) MCrAlY coatings were deposited using high-velocity oxy-fuel (HVOF) spraying process. The high-energy planetary ball-milling process was utilized to prepare CeO2 and Al2O3 nanoparticles. ODS-NiCoCrAlY feedstock powders were also developed using the ball-milling process. The various formulations of Al2O3 and CeO2 nanoparticles (0.5 and 1.0 wt%) were chosen to apply different types of ODS-NiCoCrAlY coatings. The microstructure of the as-received and ODS coatings were evaluated by field emission scanning electron microscope (FESEM) as well as the commercial and ODS powders. Furthermore, the microhardness of different compositions of ODS coatings was accordingly investigated and the obtained results were compared with as-received coating. On account of the measurement of oxidation kinetics, the freestanding as-received and ODS coatings were exposed to air at 1000 °C up to 500 h and the thickness growth rate of the α-Al2O3 oxide layer was simultaneously examined. The results exemplified that NiCoCrAlY+1.0 wt% nano-CeO2+0.5 wt% nano-Al2O3 coating had a better oxidation resistance and lower oxide scale growth rate under the synergistic effects of both CeO2 and Al2O3 nanoparticles.

In-vitro comparison of hydroxyapatite coatings obtained by cold spray and conventional thermal spray technologies.

Hydroxyapatite (HA) coatings onto Ti6Al4V alloy substrates were obtained by several thermal spray technologies: atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF), together with the cold spray (CS) technique. A characterization study has been performed by means of surface and microstructure analyses, as well as biological performance. In-vitro tests were performed with primary human osteoblasts at 1, 7 and 14 days of cell culture on substrates. Cell viability was tested by MTS and LIVE/DEAD assays, cell differentiation by alkaline phosphatase (ALP) quantification, and cell morphology was analyzed by scanning electron microscopy. The HA coatings showed an increase of HA crystallinity from 62,4% to 89%, but also an increase of hydrophilicity from ∼32° to 0°, with the decrease of the operating temperature of the thermal spray techniques (APS > HVOF > CS). Additionally, APS HA coatings showed more surface micro-features than HVOF and CS HA coatings; cells onto APS HA coatings showed faster attachment by acquiring osteoblastic morphology in comparison with the rounded cell morphology observed onto CS HA coatings at 1 day of cell culture. HVOF HA coatings also showed proper cell adherence but did not show extended filopodia as cells onto APS HA coatings. However, at 14 days of cell culture, higher cell proliferation and differentiation was detected on HA coatings with higher crystallinity (HVOF and CS techniques). Cell attachment is suggested to be favoured by surface micro-features but also moderate surface wettability whereas cell proliferation and differentiation is suggested to be highly influenced by HA crystallinity and crystal size.

Investigations of hot corrosion resistance of HVOF coated Fe based superalloy A-286 in simulated gas turbine environment

The main aim of the present work is to investigate the high-temperature corrosion behavior of bare, high-velocity oxy-fuel coated on iron-based superalloy A-286. Ni-20%Cr and Cr3C2-25%NiCr powders are deposited on the Fe based superalloy A-286 material by high-velocity oxy-fuel (HVOF) process. Surface roughness and coating thickness of the coated specimen is evaluated using surface roughness tester and optical microscope (OM). The hot corrosion study was performed on bare, and HVOF coated Fe-based superalloy A-286 in Na2SO4-50%V2O5-10%NaCl environment at 700 °C under cyclic conditions. Kinetics of corrosion has been determined by thermo-gravimetric analysis using mass change measurement; Cr3C2-25%NiCr coating shows the better corrosion resistance due to the formation of stable Cr2O3 layer. The uncoated superalloy is severely suffered by spallation and sputtering of oxide scales during hot corrosion study. The surface microstructure, chemical composition and phase identification of coatings before and after hot corrosion are analyzed using scanning electron microscope/ energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis. Depth of corrosion attack and oxide scales are characterized using EDS compositional analysis and elemental mapping technique. The result of the cross-sectional image shows that uncoated material was severely suffered by hot corrosion to a depth of 200 µm.

Synthesis, characterisation and erosion behaviour of AlCoCrMoNi high entropy alloy coating

AlCoCrMoNi equiatomic high entropy alloy (HEA) powders were prepared by mechanical alloying. High energy planetary ball-mill was employed to mill the powders till single solid solution formation. X-Ray Diffraction (XRD) analysis of as prepared powder was done at 5, 10, 15 and 20 hours of milling. A single BCC phase was formed after 20 h. Field emission scanning electron microscopy (FESEM) was used to study the microstructure of the prepared powder. After 20 hours of milling, a homogenous mixture formed, which was further confirmed by EDAX mapping of as prepared powders. The alloy was coated onto a 316 steel substrate using High-velocity Oxy-fuel (HVOF) coating. Cross-section of the coating was analysed using scanning electron microscope (SEM). The slurry erosion resistance exhibited by the coating was tested using Water-jet Erosion tester with 20 m s−1, 25 m s−1 and 30 m s−1 jet velocity at 30°, 60° and 90° angular impingement. The coating exhibited high ductility along with superior erosion resistance compared to conventional wear resistant coatings.

Estimation of residual stress and deformation of laser deposited tracks of Ni-5Mo-5Al powder using thermo-mechanical finite element simulation

Exhaust fans and manifolds, valve and pump seats, connecting rods etc. are coated with Ni-5Mo-5Al powder for service temperature up to 650 °C. Till date, it has been coated on structural steels etc. substrate mostly using plasma spray and High-Velocity Oxy-Fuel (HVOF) spraying. However, a few pioneering works, reported cladding by melt and deposition of the same powder using laser. Laser being much more precise heat source has the potential to achieve cladding without causing much thermally induced damage. In this work, laser cladding of Ni-5Mo-5Al powder was simulated probably for the first time. Layer thickness and width were input from the experiments. A significant amount of tensile residual stress (up to 500 MPa normal stress component along the scan track) and deformation (up to 32 μm) of the deposited layer was found in the simulation results. These were more at higher laser power. Experiments consisted of depositing overlapping tracks, on AISI 1020 steel cylindrical substrates having 39 mm diameter and 22.4 mm thickness, by blown powder method, with the help of a 2 KW Yb fibre laser having 1.07 μmwavelength.

Impact of high‐velocity oxy‐fuel sprayed TiAlN surface coating on mechanical and slurry erosion performance of aluminium alloys

AbstractTiAlN coatings were deposited on AA1050 and AA5083 aluminium alloys by high‐velocity oxy‐fuel (HVOF) spray process and evaluated for their mechanical and slurry erosion performance. In comparison to base alloys, the mechanical properties were found to enhance upon coating. The effect of working parameters namely impingement angle, impact velocity, erodent size and erodent feed rate on the slurry erosion wear rate has been investigated. The slurry erosion wear rate of the uncoated samples was found to decrease with an increase in the impingement angle whereas for coated samples the slurry erosion wear rate first increased, reached to a maximum value at 60° and then decreased with further increase in the impingement angle. For uncoated and coated samples the slurry erosion wear rate was found to increase with an increase in impact velocity, erodent size and erodent feed rate. Finally, the morphology of the eroded surfaces was analyzed using scanning electron microscopy and the possible erosion mechanisms have been studied.

Experimental Investigation of the Effect of Brake Disconnished by Hvof Method

In this study; brake discs used in vehicles have investigated. The behavior of these discs during braking, wear and braking distance performance of the coated and uncoated surfaces during the braking process were evaluated. The surface of the commercially produced discs was coated with a thickness of 1 mm using the HVOF (High Velocity Oxy-Fuel) coating method. In the coating process CoNiCrAlY, WC-Co (88/12), NiCrBSi and MoNiCrBSiFe (Mo 25%) 4 (four) different ceramic powders were used respectively. Wear and brake distance tests of coated and uncoated discs were performed by braking in a total of 20 brakes in 15 seconds on the brake-suspension tester. After the brake tests, the surface roughness changes of the discs, surface hardness change, mass loss and the abrasion values on the disc surfaces were determined due to the brake force applied to the discs during the experiment. The most wear on the discs was on the CoNiCrAlY ceramic powder coated disc at 0,4 g, while the least wear occurred on the disc coated with NiCrBSi ceramic powder at 0,2 g.

Sliding wear performance of thermally sprayed Ni-Cr based alloy deposited on SAE 1045 steel

ABSTRACTThe present work has been carried out in order to investigate the sliding wear performance of a Ni-Cr based alloy, containing boron, carbon, silicon and iron, deposited onto a SAE 1045 steel substrate. The coatings were thermally sprayed by means of a High Velocity Oxy-Fuel system and post heat-treated for one hour at 760°C in an argon atmosphere. The relative sliding performance was evaluated under unlubricated dry conditions at different applied normal loads and sliding velocities, in order to analyze the influence of these parameters on the wear response. Test results are discussed and interpreted on the basis of both microstructural evaluations and mechanical properties measurements. The surface morphology and topography of the wear scars have been studied through of a Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques. The results have shown an improved wear resistance of the coated steel samples as compared to the uncoated ones.

Prediction and analysis of high velocity oxy fuel (HVOF) sprayed coating using artificial neural network

Thermal spray comprises a group of coating processes for coating manufacturing in which metallic or nonmetallic materials are deposited in a molten or semi-molten condition. Most often, the coating properties are significantly influenced by the operating parameters. However, obtaining a comprehensive modeling or analytical analysis of the thermal spray process is too difficult to be practical due to the complex chemical and thermodynamic reactions. Accordingly, the present study aims at applying an artificial neural network (ANN) model to predict the HVOF sprayed Cr3C2−25NiCr coatings and analyze the influence of operating parameters regardless of the intermediate process. The process parameters, which were automatically recorded by the homemade HVOF spray system during the spray process, were served as the inputs for the ANN model. Then, the porosity, microhardness and wear rate of coatings were measured and considered as targets for the ANN model. After configuring and training procedure of the model, the predicted results were compared to the results of experimental data. The good consistency found between these results permits to verify the reliability and accuracy of the trained ANN model. Additionally, the mean impact value (MIV) analysis was conducted to quantitatively explore the relative significance of each input variable for improving the effective prediction.

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.

Synthesis and properties of high velocity oxy-fuel sprayed FeCoCrNi2Al high entropy alloy coating

The present study aims to develop dense FeCoCrNi2Al high entropy alloy (HEA) coating of thickness ≅ 200 μm by high velocity oxy-fuel (HVOF) process. Studies on the microstructure and mechanical properties like microhardness, erosion resistance of the HVOF coated FeCoCrNi2Al HEA has not been reported by any investigator. An adherent coating has been deposited displaying a lamellar microstructure. The X-ray diffractogram of the coating revealed the formation of a major FCC and a minor BCC phase with small amount of oxide. The oxide phase was confirmed to be Al2O3 using Raman spectroscopy. The microhardness of the coating is found to be 600 ± 30 VHN with no crack formation upto a load of 1 kgf. The erosion resistance of the HEA coated steel was evaluated upto a temperature of 800 °C. The coating displays good erosion resistance and the depth of penetration of the erodent was well within the coating thickness with no delamination of the coating, thereby making it suitable for high temperature applications.

Effect of APS flash bond coatings and curvature on TBC performance on rod specimens

The addition of an air plasma sprayed (APS) “flash” bond coating layer on top of a high velocity oxy-fuel (HVOF) bond coating significantly extended the lifetime of APS yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on rod specimens of superalloy 247 tested using 100-h cycles in air+10%H2O at 1100 °C. The flash bond coatings of both NiCoCrAlY and NiCoCrAlYHfSi powder were compared to an HVOF-only and a vacuum plasma sprayed (VPS) NiCoCrAlYHfSi bond coating. The flash coatings appear to form a mixed oxide-metal zone, which likely inhibits crack formation and therefore extends lifetime compared to conventional bond coatings. The underlying dense HVOF layer appeared to act as a source of Al for this intermixed zone and prevented the oxide from penetrating deeper into the bond coating. Residual stress in the thermally-grown Al2O3 scale was measured using photo-luminescence piezospectroscopy (PLPS) as a function of time for each coating variation, including a comparison of concave and convex surfaces on a specially designed rod.

In-situ SEM observations of ultrasonic cavitation erosion behavior of HVOF-sprayed coatings.

Several typical high-velocity oxy-fuel (HVOF)-sprayed coatings, including WC-10Co4Cr coatings, Co-based coatings, WC-10Co4Cr/Co-based composite coatings, and Fe-based amorphous/nanocrystalline coatings were fabricated, and their cavitation behavior was evaluated in deionized water. Further, in-situ SEM surface observations were used to understand the microstructure of tested coatings. The results show that cavitation erosion initially occurred at pre-existing defects in the coatings. Meanwhile, it was found that cavitation erosion damage of the WC-10Co4Cr/Co-based composite coating, which contained a hard reinforcing phase (WC-10Co4Cr phase) and a soft matrix phase (Co-based phase), preferentially occurred at or around pores and microcracks in the reinforcement, rather than in the defect free matrix. This suggested that defects were a critical contributing factor to cavitation damage of the composite coatings. Furthermore, a mechanism was suggested to explicate the cavitation behavior of composite coatings. The approach of using in-situ SEM surface observations proved to be useful for the analysis of the cavitation mechanism of engineering materials and protective coatings.

Tribological behaviors of CrN/Cr3C2-NiCr duplex coating at elevated temperatures

Duplex coating, composing of a thick Cr3C2-NiCr interlayer and a top CrN film, were deposited by high velocity oxy-fuel (HVOF) and arc ion plating technologies, and the microstructure, mechanical properties and thermal shock resistance of the duplex coating were investigated compared to those of the single CrN film, as well as tribological behaviors at elevated temperatures. Results showed that the duplex coating exhibited high surface hardness, H/E and H3/E2 ratios, load-bearing capacity and adhesive strength due to the supporting of the Cr3C2-NiCr interlayer, which contributed to improve the wear properties of duplex coating sliding against Al2O3 ball. At 25 °C, the duplex coating had a higher friction coefficient and a relatively low wear loss, and the main wear mechanism was abrasive wear. As the temperature increased to 350 °C, the top CrN film of duplex coating was worn out totally. The severe mismatch of coefficient of thermal expansion (CTE) between Cr3C2-NiCr interlayer and substrate that might have a negative impact on wear resistance of duplex coating at high temperature. In addition, a large number of debris produced due to oxidation peeling of the coatings were compacted to form a thick transfer film during the sliding process, resulting in badly adhesive wear. The wear mechanisms were adhesive and oxidation wear accompanied with abrasive wear at high temperature (from 150 °C to 550 °C).

A Review on Suspension Thermal Spray Patented Technology Evolution

Since the introduction of suspension thermal spray (STS) in 1997, the rapid evolution of this technology has resulted in a critical number of inventions justifying a comprehensive review. Today STS is one of the most important research and development topics in thermal spray. Due to the efforts of different industries, universities, and governmental institutions, STS has reached a certain maturity for implementation in different applications. STS includes any thermal spray process that is based on injection of a suspension of solid particles in a liquid carrier into the gas jet, such as suspension plasma spray, suspension high-velocity oxy-fuel, and high-velocity suspension flame spray. This review is on the state of the art of the STS technology and includes an overview of the current patent situation including the number of patents published in English language, patenting institutions and invention domains. Apparatus and methods, feedstock, and new material systems and applications were identified as the three main domains of focus for the STS inventions. The presented patents show a general perspective of the current situation and present the technology advancements in each domain. It also shows the potential of implementing this technology in new applications based on the needs of thermal spray market through further technology development.

Tribological properties of HVOF sprayed WC-Cr3C2-Ni coating

The deposition of WC-Cr3C2-Ni coating was done by High velocity Oxy Fuel (HVOF) technique. Microstructural and mechanical properties of coating were measured. Friction and abrasive wear response was evaluated with varying loads and sliding velocities. An increase of 1.6–2 times in friction coefficient (COF) was noted with rise in load and COF reduced by around 15%–20% for rise in sliding velocity. Abrasion wear rate reduced by around 2 times with rise in sliding velocity. The decrease in wear rate and COF at higher sliding velocity was due to lubricating properties of tribo oxides formed due to frictional heating. The composition of tribo oxides on worn surface as indicated by XRD analysis revealed different oxides of tungsten, chromium and nickel such as WO3, W3O8, NiO2, NiWO4, NiCr2O4 and CrO2. With increasing load, carbide pull out, delamination cracking, carbide fracture and inter splat fracture were the dominating mechanisms of material removal, whereas with increasing velocity tribo oxidation leading to oxidative wear was dominant wear mechanism.

Slurry Erosion Behavior of HVOF-Sprayed WC-10Co-4Cr Coated SS 316 Steel with and Without PTFE Modification

Several hydro-machinery components such as impellers of the submersible pump and turbine blades generally suffer from slurry erosion during their operation. High-velocity oxy-fuel (HVOF)-sprayed coatings are promising materials to prevent such components from slurry erosion. Moreover, polytetrafluoroethylene (PTFE) is a well-accepted hydrophobic material, which has several attractive properties such as chemical inertness and considerable thermal stability. Therefore, PTFE coating has been explored as a topcoat for a HVOF-sprayed WC-10Co-4Cr coating, with an aim to enhance the performance of the latter and analyzed with different slurry erosion conditions. A slurry erosion test rig was used for experimentation, and the effect of four different erosion parameters, namely average particle size, slurry concentration, impact angle, and velocity of jet, was investigated. A comparative study between PTFE-modified HVOF-sprayed coating and the standalone HVOF-sprayed coating was also done with respect to slurry erosion behavior and hydrophobicity. The overall results indicate that the slurry erosion of the HVOF-sprayed coated SS 316 steel got enhanced after modification by PTFE topcoat. Furthermore, the topcoat of PTFE on HVOF-sprayed coating resulted in a remarkable improvement in hydrophobicity, showing a superhydrophobic behavior, which might have also provided a better slurry erosion resistance to the HVOF-sprayed steel.