High-velocity Oxygen-fuel Thermal Spray Research Articles

HVOF THERMAL SPRAYING METHOD-PROPERTIES, CHALLENGES, AND OPPORTUNITIES IN BOILER/TURBINE APPLICATIONS: A REVIEW

The technique of thermal spray proves invaluable in helping to enhance the surface of solid materials. This specially and versatile process finds application in coating various materials and components, endowing them with increased resistance against wear, erosion, corrosion, abrasion, or heat. Highvelocity oxygen fuel (HVOF) coating is included in the category of most rapidly utilized coating techniques and is very good for making dense coatings at moderate temperatures, boasting reduced porosity (typically < 1%). Industries worldwide have broadly implemented HVOF because of its numerous benefits. This research article offers a comprehensive overview of numerous thermal spray coating techniques, and characteristics are outlined, showcasing the purpose of HVOF deposition in safeguarding materials from deterioration at elevated heat levels in extremely diverse environments. The article also focuses on previous research efforts in the surrounding context of HVOF-sprayed coatings techniques, challenges, advancements, and opportunities in boiler/turbine applications.

SPRAYING PROCESS FOR Cr<sub>3</sub>C<sub>2</sub>-NiCr COATING

In this paper, the theoretical research of Cr3C2-NiCr coating for zirconia substrate is discussed. To obtain this coating, one of the multi-efficiency methods high velocity oxygen-fuel thermal spraying (HVOF) was investigated. The coatings were processed by different thicknesses of zirconia substrate sample of 3–5 mm also with length of 20 mm and width of 30 mm, at spraying speed of 600–700 m/s. The temperature during HVOF spraying is about 3000 ºC and the cooling temperature is 27 ºC. Investigating the theoretical data of Cr3C2-NiCr coatings, the development and thermal stresses after HVOF treatment were determined using Stone's method and Brenner-Senderoff equations with a coating thickness not exceeding 0.6 mm for carbide coatings. According to the results of theoretical research, the deposition efficiency values were found by the method proposed by Kosaku Shinoda. According to the theoretical and mathematical calculations, the deposition efficiency for Cr3C2-NiCr coating with zirconia substrate is in the range of 59.5%–69.4%. Thus, it was found that the deposition efficiency for the coatings depends on the thickness of the substrate, the powder feed rate, and the mass of the applied material and consequently the number of spraying passes.

Wear behavior of bare and coated 18Cr8Ni turbine steel exposed to sediment erosion: A comparative analysis

Hydropower plant components are exposed to sediment erosion due to the impact of hard particles flowing along with the water. As a consequence, significant material loss and substantial reduction in efficiency are encountered during plant operation. This work investigates the sediment erosion problem in hydraulic turbines using a slurry erosion tester. Experiments are performed on bare and coated 18Cr8Ni turbine steel at three impact velocities i.e. 6, 8, and 12 m/s, and two impact angles 30° and 90°. The size of impacting particles varies between 50 to 350 µm with a slurry concentration of 2500 ppm. The erosion rate is found to be maximum at 30° impact angle and minimum at 90° impact angle. With an aim to minimize the erosion of turbine steel, the test samples were coated with in-house formulated ceramic-based coating material using the high-velocity oxygen-fuel thermal spray technique. Vickers hardness test was performed to determine the hardness of the coatings. For metallurgical characterization, scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy techniques were used. A comparison of both bare and coated samples is carried out to observe the erosion-resistant aspects. A higher erosion resistance has been detected for the coated material as compared to the bare turbine material. The reduction in the erosion rate for coatings is 13.4% at 30° and 17.62% at 90° impingement angles. The obtained results are related to the tested materials’ microstructural, mechanical, and metallurgical aspects.

Fabrication, microstructure, and wear properties of novel Fe–Cr–B–Nb–Mo metamorphic alloy coatings manufactured by the HVOF thermal spray process

Novel Fe–Cr–B based metamorphic alloy coating layers were manufactured using newly designed Fe–Cr–B–C–Mo–Nb metamorphic alloy powders (M1 (Nb:3.50 wt%), M2 (Nb: 1.75 wt%)) and an HVOF (high-velocity oxygen fuel) thermal spray process. The microstructures and wear properties of the coating layers were investigated and compared with those of a conventional Fe–Cr–B based metamorphic alloy (M) coating layer. The investigation of the microstructure revealed that the (Cr, Fe)2B phase is densely distributed in the α-Fe matrix and metallic glass phases in the M and M2 coating layers. The M1 coating layer was mainly composed of the α-Fe phase, some metallic glass, and nano-sized Nb boride particles. The results of measuring the oxide fraction indicated that the M2 layer showed 10 times better in-flight oxidation resistance than the M layers. A wear test resulted in the wear rates of M, M1, and M2 being 1.68 × 10−6 mm3/Nm, 7.65 × 10−6 mm3/Nm, and 8.16 × 10−7 mm3/Nm respectively, indicating that the wear resistance of the M2 coating layer was 2.06 times higher than that of the M coating layer. Based on the above results, the dependence of the wear mechanism of the HVOF Fe–Cr–B based metamorphic alloy coating layers on the additive element content is also discussed.

Mechanism study on the influence of combustion models and spray gun geometric parameters on high-velocity oxygen-fuel (HVOF) thermal spraying

HVOF thermal spraying technology has unique advantages for spraying WC-12Co ceramic powder. The spraying process is complex, involving the gas-liquid-solid multiphase flow, chemistry, heat transfer and intelligent control. It is vital to quantitatively reveal the reaction mechanism of combustion components, which optimizes the thermal spraying process and improves the coating quality. A numerical model of HVOF thermal spraying was established, including the gas dynamics model, chemical thermodynamics model and discrete phase model for spraying particle. The difference between the eddy dissipation model (EDM) and EDC model in the traditional calculation was compared. The results show that the EDC model reveal the chemical reaction mechanism of the combustion in detail step by step, and quantitatively display the exothermic phenomenon during whole spraying. The exothermic reaction predicted by the EDC model continues into the air domain with the flame flow. The geometric structure of spray gun was analyzed based on the EDM model. The results show that the diameter of the combustion chamber is 30.2 mm, the diameter of the parallel nozzle is 10.5 mm, and the length from the particle inlet to the nozzle starting point is 13.3 mm, the flight temperature and velocity for particles are relatively high.

Time-varying evolutionary mechanism analysis of the multiphase flow during high-velocity oxygen-fuel (HVOF) thermal spraying WC-12Co particle

HVOF thermal spraying with the fuel of aviation kerosene has unique advantages for spraying WC-12Co ceramic particles. The time-varying evolution mechanism of the multiphase flow formed by the particles wrapping with the combustion flame flow is quantitatively revealed, which is essential for preparing of high-quality coatings. In this study, the breakup and gasification process of kerosene droplet was considered. The fuel droplet, flame flow and spraying particles were directly coupled. Based on the computational fluid dynamics (CFD) method, a transient evolution model of HVOF thermal spraying multiphase flow with JP5000 spray gun was established. The Euler-Lagrange method of bidirectional coupling was used to track the continuous and discrete phases. The spraying combustion flame characteristics were expressed by the realizable k-ε turbulence model and eddy dissipation model (EDM). The results show that the multiphase coupling transient evolution model quantitatively captures the nonlinear changes of the breaking and gasification for the kerosene droplet, combustion reaction, particle flight behavior and others during spraying. The kerosene droplets in the combustion chamber aggregate with each other to form large droplets, which are rapidly broken and gasified in the peripheral areas. Due to the violent combustion, a gas-phase circulation was formed by the heat and cold air. As the combustion progresses, the flame flow becomes stable and orderly. When particles with a size of 20 to 30 μm impact the substrate, the particle temperature is close to the solid phase temperature, and the velocity is fast. The particle temperature and velocity decrease with the increase of the particle size.

Development of rGO doped alumina-based wear and corrosion resistant ceramic coatings on steel using HVOF thermal spray

This study deals with the development of hard, wear-resistant ceramic-based coatings containing Alumina (Al2O3) doped with 0.8% Ceria (CeO2) and reduced graphene oxide (rGO, 0–0.2%) on 17-4 PH steel using High-Velocity Oxygen Fuel (HVOF) thermal spray process. The tribological response of the coatings were recorded using Universal Tribo Tester. The corrosion response of the coatings was studied using potentiostat galvanostat corrosion setup. The introduction of rGO to the Al2O3–CeO2 helps in reducing the coefficient of friction and better corrosion response in the coatings. The elastic modulus and nano hardness of Al2O3–CeO2-rGO coating are ≈101% and ≈ 87% higher than that of Al2O3–CeO2 coating, respectively. The specific wear rate of Al2O3–CeO2-rGO coating is reduced by ten times compared to that of 17-4 PH steel. The improved properties can be attributed to the dense microstructure of the coatings with the introduction of rGO and participation of rGO in the tribological process.

Fretting corrosion behavior of WC-10Co-4Cr coating on Inconel 690 alloy by HVOF thermal spraying

WC-10Co-4Cr coating was deposited through supersonic flame of high-velocity oxygen fuel (HVOF) process on the outside surface of Inconel 690 alloy tube. The fretting corrosion behavior of substrate- and coating- treated alloy tubes under dry state and seawater are compared. Results show remarkable differences between substrate- and coating-treated samples in electrochemical corrosion response and wear behavior. The fretting wear resistance of the WC-10Co-4Cr coating-sprayed alloy tube considerably improved because its surface mechanical properties are better than that of the substrate. Though the initial static corrosion performance of the coating is poorer than the substrate alloy tube, its excellent wear resistance helps decrease corrosion rate. In addition, the predominant damage mechanism of fretting corrosion was abrasive wear, fatigue delamination, and corrosion.

The dry sliding wear rate of a Fe-based amorphous coating prepared on mild steel by HVOF thermal spraying

Wear-resistant amorphous coatings Fe49.7Cr18Mn1.9 Mo7.4W1.6B15.2C3.8Si2.4 of about 300 μm thickness were deposited on mild steel substrate using a high-velocity oxygen fuel (HVOF) thermal spray method. The median Vickers hardness of the as-sprayed coating is 751.9 HV300 g, which further increases by 127% and by 112% when the coating is vacuum heat treated at 650 °C and 800 °C, respectively. The longer the amorphous phase is, the higher the Vickers hardness is. The relatively low porosity indicates excellent wear resistance of these amorphous coating, which were measured using a pin-on-disc method. The wear mechanism of the current Fe-based amorphous coating varies from oxidative wear to delamination wear when the sliding speed changes from low (0.4 cm/s) to high (4.0 cm/s). It is believed that the upsurge in hardness and the wear resistance of the applied coating is due to the optimized selection of the heat treatment process (i.e., at 650 °C), which has a significant impact on the microstructure, as well as the low porosity, with better coating adherence and fewer cracks. Under dry sliding wear circumstances, the coating demonstrated good wear resistance, with wear rates of (2.81–16.30) x 10−6 mm3/N m. Due to the properties, low porosity, and high hardness especially in case vacuum heat treatment 650 °C, these coatings produced by (HVOF) exhibit a high ability to resist better friction and wear performance that indicates to ability application it in facilities steel.

Numerical Analysis of High-Velocity Oxygen Fuel Thermal-Spray Process for Fe-Based Amorphous Coatings

High-velocity oxygen fuel (HVOF)-sprayed amorphous alloy coatings usually have advantages of a dense structure that improve their resistance to corrosion, wear, and fatigue in the substrate. The flame flow characteristics and particle behaviors during the spray process have a significant influence on the amorphous coating structure and properties. In this study, a computational fluid dynamics model is enforced to analyze the flame flow and Fe-based amorphous alloy particle behavior in an HVOF spray process. The flame flow temperature, velocity characteristics, and the Fe48Cr15Mo14C15B6Y2 Fe-based amorphous alloy particles’ velocities, temperatures, flight trajectories, and mass concentration distribution characteristics are simulated. Moreover, the effects of the oxygen/fuel ratio, particle morphology parameter, particle-injection rate, and angle on the particle behavior are also investigated. Judging from the simulation results, the optimum amorphous alloy particle size varies between 20 and 30 μm, the shape factor is within the range of 0.9–1, the optimum O/F ratio is 3.4, the optimum injection angle is 45°, and the optimum injection rate is 10 m/s. With these conditions, most of the particles settled toward the centerline of the spray gun and are in a semisolid or solid state before affecting the substrate, giving the materials optimal coating structure and performance.

Effects of different HVOF thermal sprayed cermet coatings on tensile and fatigue properties of AISI 1045 steel

The objective of the present work was to investigate the effect of different cermet coatings on tensile and high-cycle fatigue properties of AISI 1045 steel using High-Velocity Oxygen Fuel (HVOF) thermal spray process. In this study, the WC-12Co, WC-10Co-4Cr, and Cr3C2–25NiCr cermet powders were coated onto the surface of annealed AISI 1045 steel specimens via the HVOF thermal spray process. The corresponding HVOF thermal spraying results were compared with those obtained from hard chrome (Cr) electroplating process. The thermal spray coated specimens showed load drop tendency beyond the yield point with significant work hardening in the tensile results, but low fatigue life than the base material in high-cycle fatigue results due to defects and poor adhesion at the interface. However, those cermet coating materials showed higher fatigue strengths than the hard chrome plated material. Fractography tests showed that fatigue strength was not only dependent on residual stress, but also influenced by the modes of fatigue failure related to the coated layer's cohesion and adhesion strength. Overall, the Cr3C2–25NiCr cermet coating is preferable due to its less porosity, high fatigue property, and better adhesion than the other coated materials.

Effects of internal oxide contents on the oxidation and β-phase depletion behaviour in HOVF CoNiCrAlY coatings

This paper studied the effects of internal oxide contents on the high temperature oxidation and β-phase depletion behaviour of high velocity oxygen fuel (HVOF) thermally sprayed CoNiCrAlY coatings. Two oxygen-to-kerosene ratios were employed in the HVOF thermal spraying process to generate different internal oxide contents in the as-deposited CoNiCrAlY coatings. The isothermal oxidation behaviour of the as-deposited CoNiCrAlY coatings were examined in air at 1100 °C. The as-deposited HVOF CoNiCrAlY coatings before and after oxidation were characterised by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was shown that the high oxygen-to-kerosene ratio caused significant oxidation of in-flight particles, resulting in a large volume fraction of the internal oxides in the as-deposited CoNiCrAlY coatings. Furthermore, it was found that the CoNiCrAlY coating with a higher content of internal oxides exhibited larger mass gain and significant β-phase depletion during oxidation. This is likely due to that the Al in the coating was initially consumed during the spraying process and the discrete internal oxide stringers were not effective diffusion barriers. • Two CoNiCrAlY coatings were prepared at different oxygen-to-kerosene ratios. • The internal oxides were found to be Al-rich oxides by EDS analysis. • High mass gain was found in the coating with high internal oxide content. • Rapid β-phase depletion occurred in the coating with high internal oxide content.

Mechanical and Tribological behaviour of velocity oxygen fuel thermal spray coating: A Review

For over decades thermal spraying has been present and optimized over the time and now it has become an effective method to thick coating deposition. Thermal spray coating especially high velocity oxygen fuel (HVOF) have served various industrial sector such as aerospace, defence and thermal electricity generation. To overcome disadvantages of traditional coating there was a need of smart coating process which could integrate new functionalities and coherent responses. The aim of this paper is to present a detailed review of current technological changes in HVOF thermal spray coatings. This paper gives insight view of different type of coatings on substrates, their mechanical and tribological properties. HVOF coating categorised according to various application sector has also been discussed in this paper.

Effect of Coating Thickness on Fatigue Behavior of AISI 1045 Steel with HVOF Thermal Spray and Hard Chrome Electroplating

The effects of coating thickness on the fatigue lives of AISI 1045 steel shaft-bending specimens with WC-10%Co-4%Cr high-velocity oxygen fuel (HVOF) coatings and hard chrome electroplated coatings under cyclic loading conditions are investigated. Residual stress distributions of the HVOF-coated layer and the hard chrome-plated layer were measured. During cyclic fatigue testing, cracks are initiated near the interface between the HVOF coating and the steel substrate and propagate through the substrate to cause final fracture based on experimental results. For the specimens with hard chrome-plated coatings, the fatigue crack is initiated near the top of the coating, propagates through the coating, and then grows in the thickness direction of the steel substrate to cause final fracture. As the coating thickness of the HVOF-coated specimens increases, the fatigue strength of the specimens increases, which can be attributed to the fact that the magnitude of the compressive residual stress near the interface between the substrates and the HVOF coatings is higher when the coating thickness gets higher. As for the hard chrome-plated specimens, the fatigue strength is deteriorated when the coating thickness is increased. The microcrack density in the hard chrome coatings increases with the coating thickness and, therefore, may result in the decreased fatigue strength.

Influence of high-velocity oxygen fuel spraying and plasma nitriding on microstructure properties of iron-nickel-chromium alloy using hybrid surface heat treatment

In this investigation, an attempt has been made to develop the surface hardness of iron-nickel-chromium alloy (alloy20) and the analysis on the micro structure and mechanical properties were performed. The surface modification of alloy20 was performed using HVOF (High-Velocity Oxygen Fuel) coating, plasma nitriding (PN) processes and hybrid coating (HVOF combined with PN) on 2 mm thickness plate. The HVOF thermal spray coating was performed at 450 °C using Molybdenum and the PN process was done at 500 °C for 12 h to strengthen the surface material. The microstructure and mechanical properties were investigated by x-ray diffractometry (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) techniques, tensile and bending. The result showed that the hybrid surface treatment enhanced the surface hardness of base material by 18%, tensile strength reached to 826 MPa which is 21% greater than that of base material, residual stress and corrosion rate decreased by 5 and 6% respectively compared to base material. Further, a transformation/diffusion model has been used and correlated with experimental hardness value and concluded that both the experimental and theoretical values displayed similar trend. The present investigation concluded that the formation of expanded austenite reduced the development of Cr2N and this led to the better results of hardness, tensile and bending strength.

Multi-response optimization of 67Ni18Cr5Si4B coating by HVOF spray using Taguchi-OEC technique

High Velocity Oxygen Fuel (HVOF) spray is being widely used in many industries to deposit erosion-resistant coatings or restore the dimension for components. The porosity, adhesion strength, and hardness are the essential properties commonly used to access the quality of coatings and thus the effectiveness of a HVOF spray process. To achieve desired coating properties and reduce production cost, optimization of process parameters in HVOF spray has developed immensely during last decades. Among the spray parameters, particle velocity, powder feed rate and standoff distance contribute a significant influence on the final properties of the coatings. In the present study, experiments of HVOF thermal spray were prepared for the 67Ni18Cr5Si4B alloy powder. The process parameters including spray velocity, powder feed rate and spray distance were optimized for the objectives of adhesion strength, porosity and microhardness of 67Ni18Cr5Si4B coating. Based on the Taguchi method, signal-to-noise (S/N) ratio and overall evaluation criteria (OEC) are determined to solve the optimization problem considering multiple performance characteristics. Moreover, analysis of variance (ANOVA) was conducted to analyse the contribution percentage of each process parameter. The optimal process parameters in HVOF spray of 67Ni18Cr5Si4B were found for the multi-response optimization as velocity of 1000 m/s, powder feed rate of 30 g/min, and spray distance of 0.2 m. The results of ANOVA indicated that the spray distance was the most significant parameter affecting the output spray quality among the investigated process parameters, subsequently to the powder flow rate and velocity of spray. Finally, a confirmatory experiment was carried out to support the accuracy of optimal process parameters.

Tribological and microstructural behaviour in coatings applied using GTAW and HVOF (thermal spraying process) and used to recover AISI/SAE D2 grade tool steels

ABSTRACTFe-Cr-Mo alloys are used in coatings for protecting components that are subjected to conditions involving wear and corrosion. The high velocity oxygen fuel (HVOF) method used for tool steel alloys for cold working with the use of Fe-Cr-Mo substrates has yielded good results in the field of spray coatings. HOVF is a great method for applying coatings and recovering diameters that become worn down during the operating process. New surfaces and materials can be provided without the distortion caused by conventional welding processes. It is a great method for materials that are susceptible to cracking due to the formation of hard and off-balance phases with compositions with high contents of carbon and chromium, due to the conditions of the process, which combine a relatively low flame temperature and a time of low exposure. The microstructure and characteristics of the coating are determined by the physical and chemical properties of the particles impregnated in the substrate which in turn depend on a lot of parameters such as the design of the spray gun, oxygen/fuel ratio, injection method, particle size and shape, among others. This study investigates the influence of the processing parameters on the mechanical and microstructural properties of the Diamalloy 1008 coating (mixed Fe-Cr-Mo alloy), which is applied using high velocity oxygen fuel (HVOF) and five different types of electrode applied using TIG (GTAW) welding in a tool steel for cold working. These coatings are usually used as a protection against corrosion and wear, but also have the ability to recover damaged areas as is the case with stamping dies in the automotive industry. The samples were characterised using optical microscopy and scanning electron microscopy in order to evaluate the quality of the material deposited on the substrate; Vickers microhardness and Pin-Ball Disc were used to quantify the mechanical properties of the coatings.

Tribology of FeVCrC coatings deposited by HVOF and HVAF thermal spray processes

This work studies FeVCrC-based coatings as potential alternatives to conventional Ni- and Co-based alloys for wear protection. Specifically, the microstructure and tribological properties of the coatings are characterized as a function of the particle size distribution of the feedstock powder, of the deposition technique – High Velocity Oxygen-Fuel (HVOF) or High Velocity Air-Fuel (HVAF) spraying – and of specific processing parameters.HVOF-sprayed coatings obtained from fine feedstock powder exhibit numerous oxide inclusions, which provide high hardness (≈ 900 HV0.3) but do not excessively impair fracture toughness, as determined through scratch testing techniques. HVAF-sprayed coatings obtained from the same feedstock powder contain much fewer oxide inclusions, and some of them possess simultaneously high hardness and high toughness. Defects (e.g. speckles) are instead formed in case unsuitable HVAF torch hardware is employed. A coarse feedstock powder always results in unmelted inclusions, which impair the cohesion of the coatings, particularly of the HVAF-sprayed ones.Most coatings anyway exhibit very low sliding wear rates < 3 × 10−6mm3/(Nm); abrasive grooving and surface fatigue-induced pitting are the main wear mechanisms. Oxide inclusions do not affect negatively the response of HVOF coatings, whereas too many unmolten particles increase pitting under severe test conditions. Rubber-wheel abrasion testing produces comparatively more severe grooving.

Experimental study on the erosion behavior of WC-based high-velocity oxygen-fuel spray coating

In oil and gas industries, erosion wear presents a major problem in the hydraulic fracturing process and severely reduces the erosion life of tools. Tungsten carbide (WC)-based high-velocity oxygen-fuel (HVOF) thermal spray coating is widely applied to improve surface properties. However, the erosion wear mechanism of this coating in liquid–solid two-phase flows is not clearly understood. Therefore, in this study, the HVOF thermal spray WC-based coating was characterized, and the erosion behavior of the coating was studied in a wet erosion test rig. Results showed that the experimental parameters, namely, attack angle, impact speed, particle size, and erosion time, exerted significant effects on the erosion rates of the specimens. The coated specimens exhibited higher hardness and better erosion wear resistance than the uncoated substrate due to the hard phase particles of the coating. The surfaces of the coated specimens showed microcutting, craters, and brittle chipping, as a visible manifestation of material removal. Furthermore, the erosion wear mechanism of the deposited coating was established by analyzing the eroded morphologies. The findings of this study can help enrich the understanding of the erosion wear mechanism of WC coatings in liquid–solid two-phase flows.

Braking performance and noise in excessive worn brake discs coated with HVOF thermal spray process

In this paper, two excessive worn brake discs of a light commercial vehicle were coated with High velocity oxygen fuel (HVOF) thermal spray process for reuse. The coated discs were compared with uncoated Original equipment manufacturer (OEM) D1 disc in terms of braking performance and noise. The D2 disc was coated with tungsten-carbide-cobalt (88 %WC-12 %Co) powder and NiCr (80/20) binder, and 500 Gm thick coating was obtained. The D3 disc was coated with Colmonoy-88 (Ni-W-Cr-B-Si) powder and NiCr (80/20) binder and 600 Gm thick coating was obtained. Several test procedures (e.g. bedding, vibration, and structural-strength tests) was applied to three discs using inertia brake dynamometer. Results showed that coated D2 disc with tungsten carbide cobalt provided lower brake noise and higher brake performance compared with OEM disc. Considering the coefficient of friction and temperature, coated D3 disc has approximately equal braking performance with OEM D1 disc despite the high braking noise value.