High Velocity Oxy-fuel Thermal Spraying Research Articles

Hard Quasicrystalline Coatings Deposited by HVOF Thermal Spray to Reduce Ice Accretion in Aero-Structures Components

Weather hazards, in particular icing conditions, are an important contributing factor in aviation accidents and incidents worldwide. Many different anti-icing strategies are currently being explored to find suitable long-lasting solutions, such as surface engineering, which can contribute to reduce ice accumulation. Quasicrystals (QCs) are metallic materials, but with similar properties to those of ceramic materials, such as low thermal and electrical conductivities, and high hardness. In particular, QCs that have low surface energy are commercially used as coatings to replace polytetrafluoroethylene (PTFE), also known as Teflon, on frying pans, as they do not scratch easily. PTFE exhibits excellent anti-wetting and anti-icing properties and therefore QCs appear as good candidates to be employed as ice-phobic coatings. Al-based QCs have been applied by High Velocity Oxyfuel (HVOF) thermal spray on typically used aeronautic materials, such as Ti and Al alloys, as well as steels. The coatings have been characterized and evaluated, including the measurement of hardness, roughness, wetting properties, ice accretion behavior in an icing wind tunnel (IWT), and ice adhesion by a double lap shear test. The coatings were studied, both as-deposited, as well as after grinding, in order to study the effect of the surface roughness and morphology on the ice accretion and adhesion properties. The QC coating was compared with PTFE and two polyurethane (PU)-based commercial paints, one of them known to have anti-icing properties, and the results indicate an ice accretion reduction relative to these two materials, and ice adhesion lower than bare AA6061-T6, or the PU paint in the ground version of one of the two QCs. Since the QC coatings are hard (GPa Vickers hardness > 5), a durable behavior is expected.

Modelling Knudsen number effects in suspension high velocity oxy fuel thermal spray

Suspension high velocity oxy fuel thermal spray is a system characterized by supersonic velocities and length scales of particles of the order of nm – µm. As the effects of rarefication become significant the assumptions within the continuum models begin to collapse, the effects of rarefication can be evaluated through the flow Knudsen number. Modifications to the numerical modelling must be made to incorporate the effects of rarefaction. This study looks to include the effects of rarefication into the computational fluid dynamics (CFD) models for the suspension high velocity oxy-fuel (SHVOF) thermal spray process. A model for the heat transfer coefficient that take into account the Knudsen and Mach number effects is employed. Finally, the Ranz-Marshall correlation for the Nusselt number is compared to the Kavanau correlation and a compressible Nusselt number correlation. The model is validated through comparisons of particle temperatures which are obtained from two colour pyrometry measurements using a commercially available Accuraspray 4.0 diagnostic system. This study shows that there is a significant improvement in the prediction of inflight particle temperatures when accounting for the effects of compressibility and the effects of rarefication on the Nusselt number.

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.

Microstructure, Mechanical Properties and Wear Behavior of High-Velocity Oxygen-Fuel (HVOF) Sprayed (Cr3C2-NiCr+Al) Composite Coating on Ductile Cast Iron

In the present work Cr3C2-NiCr powder containing Al particles was deposited on ductile cast iron with high-velocity oxy-fuel (HVOF) thermal spray coating technique. An investigation was conducted to determine the role of Al particles in the Cr3C2-NiCr coating produced with HVOF technique on microstructure, mechanical and wear properties in a system Cr2C3-NiCr coating/ductile cast iron. The microstructure of the HVOF-sprayed Cr3C2-NiCr+Al coating was characterized by light microscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS). Microstructure analysis reveals the formation of coating with low porosity, good adhesion to the substrate and dense structure with irregularly shaped particles of Al arranged in strips and finely fragmented Cr3C2 particles embedded in a nanocrystalline Ni-Cr alloy matrix. In addition, the results were discussed in reference to examination of bending strength considering cracking and delamination in the system of (Cr3C2-NiCr+Al)/ductile cast iron as well as microhardness and wear resistance of the coating. It was found that the addition of Al particles significantly increased resistance to cracking and wear behaviour in the studied system.

The Effect of Deposition Rate on Microstructural Evolution in WC-Co-Cr Coatings Deposited by High-Velocity Oxy-Fuel Thermal Spray Process

Tungsten carbide-reinforced cermet composites are widely used as wear- and erosion-resistant coatings due to their combination of hardness, strength, toughness and thermal properties. In this study, WC-Co-Cr coatings were deposited using high-velocity oxy-fuel thermal spray technology under different kinematic spray parameters. The microstructure of coatings was investigated using scanning electron microscopy and transmission electron microscopy techniques, and elemental/phase analysis was completed using x-ray diffraction, electron energy loss spectroscopy and x-ray energy-dispersive spectroscopy. The findings indicated the presence of W2C and W6Co6C in addition to WC and chromium oxide (Cr2O3) in a nanocrystalline cobalt-rich matrix. The newly developed phases (W2C and W6Co6C) were formed as a result of decarburization/oxidation of tungsten carbide (WC). They were found as individual particles (0.2-0.4 µm) embedded in the metallic binder phase as well as surrounding existing WC particles. Nonetheless, increasing the coating deposition rate, i.e., reduction of coating build-up per pass, increased the coating hardness from 1516 ± 124 to 1713 ± 106 HV0.3 and led to a higher degree of phase transformation reactions in the coating.

Dry sliding wear behaviour of HVOF thermal sprayed WC-Co-Cr and WC-CrxCy-Ni coatings

High velocity oxy-fuel (HVOF) thermal spray process has shown obvious advantages over other surface hardening techniques when depositing WC-based layers, such as the laser cladding, electrodeposition and chemical/physical vapour deposition (CVD/PVD) methods, due to its versatility, survivability of hardening phase and low cost. HVOF thermal sprayed WC-based coatings are widely used in components that operate in harsh environments needing excellent sliding, fretting, abrasion and erosion resistance. WC-CrxCy-Ni coating shows better wear performance than the WC-Co-Cr coating at high temperature but inferior wear performance at room temperature at lower loads according to literature; however, the wear performance and relevant mechanisms of these two coatings under higher loads has not been reported. To fill this knowledge gap, wear testing of HVOF thermal sprayed WC-CrxCy-Ni and WC-Co-Cr coatings under high loads (96, 240 and 318 N) against a sintered WC-Co (6 mm diameter ball) counter-body was studied in this paper. For WC-CrxCy-Ni coating, decarburization of CrxCy rather than WC, took place during spraying. While the decarburization of WC to W2C occurred in the WC-Co-Cr coating. The major hardening phase (WC) dominated the wear performance of the coatings given its high hardness and small size, and Co also appeared to be a superior binder phase than Ni. At the maximum load, the specific wear rate of WC-CrxCy-Ni coating against WC-Co counter body was 17.92 × 10-7 mm3 N-1m-1, which is two times that of WC-Co-Cr coating (9.81 × 10-7 mm3 N-1m-1). The wear mechanisms for WC-CrxCy-Ni coatings included abrasion of the matrix, cracking of the secondary carbide phase and pulling out of WC particles. For WC-Co-Cr coatings, abrasion of the matrix was marginal, and cracking of the secondary carbide was not observed. The presence of CrxCy of lower hardness than the WC decreased the wear resistance of entire WC-based coating at room temperature, and improved oxidation resistance of WC at high temperatures due to the higher affinity of Cr to O. Hence, the secondary carbide hardening phase may be detrimental when considering the wear applications of HVOF thermal sprayed WC-based coatings.

Ni20Cr coating on T24 steel pipes by HVOF thermal spray for high temperature protection

This investigation deals with the deposition of Ni20Cr coatings on the surface of T24 steel pipes using High Velocity Oxy-Fuel (HVOF) technique to improve the high temperature oxidation performance. The main parameters of the thermal spraying process as tube rotational speed, spraying distance, number of layers, gun speed over the tube surface or H2/O2 relation have been studied and optimized. The best parameters have been selected through the analysis of coating properties, employing six spraying conditions. The optimized parameters provided a homogeneous and fully dense coating with very low oxide content (4 ± 1 vol%), thickness of at least 260 ± 20 μm, micro-hardness around 460 ± 30 HV0.1, and adhesion strength higher than 29 ± 7 MPa. Thermal tests have been used to analyze the oxidation protection effectiveness of the coating at high temperatures. The optimized coating provided useful high temperature protection to T24 steel pipe up to 750 °C, while the uncoated steel only withstands up to 600 °C. After the exposure of the uncoated sample to 650 °C for 360 h, the oxide layer formed on the surface had more than 120 μm of thickness. However, in the coated samples exposed at 650 °C and 750 °C, no degradation of T24 substrate has been found. Thin layers of 1 μm (in the coating exposed at 650 °C) and 5 μm (in the coating exposed at 750 °C) of protective Cr2O3 and NiCr2O4 oxides have been observed on the external surface of the coatings.

Splat formation and microstructure of solution precursor thermal sprayed Nb-doped titanium oxide coatings

Solution precursor thermal spray can become a breakthrough technology for the deposition of coatings with novel chemistries; however, the understanding of the process that the feedstock material undergoes is still lacking when compared to more traditional presentations (i.e. powder and suspension). In this paper, niobium-doped TiO2 coatings were deposited by solution precursor high velocity oxy-fuel thermal spraying, studying its microstructure and phase. It was reported that a lower flame temperature produced a highly porous coating, while the porosity was reduced at higher flame temperature. Investigation of the phase content showed that, contrary to our current understanding, a higher flame power implied an increase of the anatase phase content for solution precursor spray. Three methods were used: Rietveld refinement, peak height and peak area of the x-ray diffraction patterns. Additionally, single splats were analysed, showing that as the precursor travels through the flame, pyrolysis and sintering takes place to form the solid material. These results were used to derive a model of the physico-chemical transformation of the solution precursor. This work proves that solution precursor thermal spray is a promising technique for the deposition of doped ceramic coatings, being the microstructure and phase content controllable through the spraying parameters.

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.

Parametric optimisation of high-velocity oxy-fuel nickel-chromium-silicon-boron and aluminium-oxide coating to improve erosion wear resistance

Nickel (Ni) based alloy coatings are gaining momentum due to its superior mechanical properties contributed by the dispersion of hard carbides and borides, which is substantially influenced by the processing technique. Accordingly, this work investigates High-Velocity Oxy-Fuel (HVOF) thermal spraying of Nickel-Chromium-Silicon-Boron (NiCrSiB) and Aluminium Oxide (Al2O3) at a 60:40 (wt%) ratio on AISI304 stainless steel substrate. The influence of HVOF spray parameters such as oxygen, fuel, and powder feed rate in addition to standoff distance on the erosion resistance was studied. The parametric model identified the rate of powder feed and standoff distance as the two most significant parameters affecting the erosion behaviour. The optimum parametric values for oxygen, fuel and powder feed rate was identified as 260 lpm, 65 lpm and 28 g min−1 respectively at a standoff distance of 250 mm for the highest wear resistance. The results of this study show that that NiCrSiB-Al2O3 HVOF coating features a ductile erosion behaviour and offers 1.6 times more wear resistance at a 90° impact angle in comparison to 30°.

A Computational and Experimental Investigation into Radial Injection for Suspension High Velocity Oxy-Fuel (SHVOF) Thermal Spray

Suspension high velocity oxy-fuel thermal spray typically utilizes axial injections of suspension into the combustion chamber. There are certain cases where the oxygen-sensitive nanoparticles benefit from a reduction in the time and temperature spent in the gas flow. Therefore, a radial injection outside of the nozzle can enable deposition of oxygen-sensitive nanomaterials. This study investigated the effect of the suspension flow rate, angle of injection and the injector diameter on the in-flight particle conditions. The combustion reaction is modeled using the eddy dissipation concept model with a robust reaction mechanism and compared to the current approach within the literature. This approach has not been employed within SHVOF thermal spray and provides a robust treatment of the reaction mechanisms. The suspension was modeled using a two-way coupled discrete particle model. Experimental observations were obtained using high-speed imaging, and observations of the liquid jet were compared to the numerical values.

Application of HVOF WC-Co-Cr coatings on the internal surface of small cylinders: Effect of internal diameter on the wear resistance

Due to the restrictions and mediocre performance of current methods of coating complex shaped parts in which line of sight processes currently struggle, the development of new coating methods is essential, with High Velocity Oxy-Fuel (HVOF) thermal spray coatings being a good candidate. In this study, a new compact High Velocity Oxy-Air Fuel (HVOAF) thermal spray torch designed to coat internal surfaces was traversed within cylindrical pipes of internal diameters (IDs) of 70 mm, 90 mm and 110 mm and a WC-10Co-4Cr coating was applied with a commercially available powder feedstock. Powder and coating microstructures were analysed using SEM/EDX and XRD. Fracture toughness and microhardness of the coatings were measured, and dry sliding wear performance was investigated at two loads: 96 and 240 N. It was found that the coating sprayed at 90 mm (medium ID) had a lower specific wear rates at both test loads due to the highest fracture toughness and microhardness; whereas, the coating sprayed at 110 mm (high ID) showed the highest specific wear rates at both low and high conditions due to poor fracture toughness.

Are Cr3C2-25(80Ni20Cr) coatings deposited by HVOF suitable for reciprocating sliding contacts under aqueous media?

To evaluate the wear performance of a Cr3C2-25(80Ni20Cr) in several corrosive media, coatings were deposited by High Velocity Oxy-Fuel (HVOF) thermal spraying on 316L stainless steel (SS) substrates. Ball-on-plate reciprocating wear tests were performed on both coating and substrate in air, double-distilled de-aerated water, 3.5 wt% NaCl, 0.2 M NaOH and 0.2 M H2SO4 solutions. Tribocorrosion tests were performed under open circuit potential, potentiodynamic and cathodic protection conditions. The results showed that the coating has a wear resistance comparable to 316L in distilled water, NaOH and NaCl solutions. For H2SO4, the coating showed significantly improved wear resistance relatively to SS.

Investigation of operating parameters on high-velocity oxyfuel thermal spray coating quality for aerospace applications

The coating quality by the high-velocity oxyfuel (HVOF) thermal spray process is greatly dependent on the operating parameters chosen during the operation. Depending on the application, the operating conditions are manipulated to enhance the desired coating property. Enhancement of multiple properties necessitates optimization of the operating conditions. This entails a comprehensive analysis of several coatings deposited at different operating conditions. The design of experiments is an effective method that helps in identifying optimum operating conditions with a reduced number of experiments. In the present study, a 24 factorial design approach is used to establish the relationships between four operating parameters (coating thickness, fuel/oxygen ratio, spraying distance, and powder injection rate) and three coating properties (roughness, microhardness, and contact angle). The results obtained are utilized in identifying the optimum parameters that produce the best coating quality. Furthermore, the study also compares the coating quality obtained by the HVOF thermal spray process with that by chrome plating. The results show that the HVOF thermal spray process produces superior coating quality and it can be a promising environmentally friendly candidate for replacing chrome plating.

Improving Impact Resistance of High-Velocity Oxygen Fuel-Sprayed WC-17Co Coating Using Taguchi Experimental Design

The impact resistance of the WC-17Co coating as a function of high-velocity oxy-fuel thermal spraying process parameters was studied and presented in this paper. Design of experiments using Taguchi method and ANOVA were used for optimizing process parameters including grit type, spray distance, oxygen flow rate, carrier gas flow rate, powder feed rate, substrate preheat temperature and coating thickness to attain the maximum impact resistance in the coating. A falling mass impact tester apparatus was designed and fabricated for measuring the impact energy that was absorbed by each coating before failure. After each test, the cracks of each coating were observed under an optical microscope. According to the results, grit type was the most influential factor on increasing the impact resistance of the coatings and the effects of carrier gas flow rate, powder feed rate and substrate preheat temperature on impact resistance of the coatings were found to be negligible. The result of confirmation test showed that Taguchi method was a useful approach in predicting optimum parameters.

High temperature oxidation and erosion behaviour of HVOF sprayed bi-layer Alloy-718/NiCrAlY coating

In the present work, bi-layer Alloy-718/NiCrAlY was deposited on grey cast iron substrate by using high velocity oxy-fuel (HVOF) thermal spray coating process. The deposited coating was subjected to microstructure studies, microhardness, high-temperature oxidation and erosion test. The average microhardness of the Alloy-718 coating was found to be 563 ± 15 HV0.2, which is about 2.5 times higher than that of the substrate. The oxidation test was carried out at 900 °C for 50 cycles in a laboratory tube furnace. The one oxidation cycle was comprised of 1 h heating at 900 °C followed by cooling of 20 min in ambient air. The erosion behaviour of the coated and uncoated specimens was investigated at 800 °C using a high temperature air-jet erosion test rig, at impact angles of 30° and 90°, respectively. The mechanism of material removal in the eroded samples was determined by using scanning electron microscopy images of the eroded surfaces. The test results revealed that the Alloy-718 coating offered superior protection to the grey cast iron substrate from oxidation and erosive wear at high temperature. The increased oxidation resistance can be attributed to the formation of favourable oxides such as Cr2O3, Al2O3, and TiO2 in the coating. The microcutting and the formation of craters caused by the repetitive impacts of erodent particles were identified as the mechanisms of material loss from the surface of coating during the erosion testing conducted at 30° and 90° impact angles. The higher hardness along with the formation of Cr2O3 and Al2O3 as stable oxides have contributed in the enhanced erosion performance of the bi-layer Alloy-718/NiCrAlY coating as compared to the substrate.

Microstructure and properties of WC-17Co cermets prepared using different processing routes

WC-17Co cermets were prepared using three different processing routes. An as-ball milled powder mixture (BMPM) and pre-solid solution treatment powder (PSTP) were used as feedstock. Sinter-hot isostatic pressing (SHIP) and high-velocity oxy-fuel (HVOF) thermal spraying were used as consolidation methods. The microstructure and properties of the WC-17Co cermets prepared using the BMPM-SHIP, PSTP-SHIP, and PSTP-HVOF routes were characterized and systematically compared. Solid-solution strengthening of the Co binder phase accounted for the higher hardness and lower fracture toughness (KIC) of the cermets produced using the PSTP-SHIP and PSTP-HVOF routes compared with those of the BMPM-SHIP-produced cermet. Moreover, the SHIP method yielded WC grains featuring an equilibrium truncated triangular prism morphology, which were faceted compared to the round-edged WC grains prepared using HVOF. Owing to the formation of a dense protective tribofilm on its surface, the cermet prepared using the BMPM-SHIP route exhibited the lowest wear rate of 0.13 × 10−7 mm3/(Nm), while the wear rates of the cermets prepared using PSTP-SHIP and PSPT-HVOF were 0.24 × 10−7 and 0.96 × 10−7 mm3/(Nm), respectively.

A low-friction graphene nanoplatelets film from suspension high velocity oxy-fuel thermal spray

The addition of graphene-based nanomaterials is known to improve the tribology properties of materials by lowering the coefficient of friction and reducing wear. The covering of small areas with thin graphene-based films is routinely carried out; however, a fast and efficient way of covering large areas represents an outstanding challenge. Here we present a method for the deposition of graphene nanoplatelets (GNPs) on stainless steel substrates based on suspension high-velocity oxy fuel thermal spray. GNPs were radially injected into the combustion jet, providing sufficient momentum and moderate heat transfer to facilitate effective bonding with the substrate. Upon unlubricated ball-on-disc wear testing against an alumina counterbody, GNPs undergo gradual exfoliation, covering the substrate and thus lowering the friction coefficient (<0.1). We have reported the formation of a thin layer, composed of GNPs having different amounts of disorder, which protects the underlying substrate from wear. GNP structural ordering is studied throughout deposition and wear tests, showing an increase of inter- and intralayer disorder at the nanoscale, whilst largely preserving the GNP microstructure.

Effects of Combustion Model and Chemical Kinetics in Numerical Modeling of Hydrogen-Fueled Dual-Stage HVOF System

The present work examines the effect of utilizing different combustion models and chemical kinetics in predicting the properties of gas and particle phases in a hydrogen-fueled, dual-stage high-velocity oxy-fuel (HVOF) thermal spray system. For this purpose, effects of two combustion models, eddy dissipation concept (EDC) and eddy dissipation model (EDM), on the temperature and velocity fields in the system are studied. The computations using EDC model are performed for detailed and reduced chemical kinetics and for a range of mixture from lean to rich. It is found that EDC with multi-step reaction mechanism predicts higher temperatures for the flow and particle in the warm spray system. In contrast to EDC, the EDM with one-step global reaction shows extra heat release outside the HVOF barrel for rich mixtures which leads to unphysical higher prediction of particle temperature. The simulations using EDC model with detailed and reduced chemical kinetics show some exothermic reactions in converging-divergent nozzle of the system. The heat release from these reactions has profound impacts on the flow and particle temperatures and affects the gas dynamic behavior of flow considerably. Finally, it is discussed that moving toward rich mixtures is more reliable way to control the particles temperature.