High Velocity Oxy-fuel Thermal Spraying Research Articles

An analytical approach to the β-phase coarsening behaviour in a thermally sprayed CoNiCrAlY bond coat alloy

This paper investigates the β-phase coarsening behaviour during isothermal heat treatment of free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) coatings prepared by high velocity oxy-fuel (HVOF) thermal spraying. The microstructure of the coatings was characterised using scanning electron microscopy with energy dispersive X-ray (EDX) analysis and electron backscatter diffraction (EBSD). It comprises a two phase structure of fcc γ-Ni matrix and bcc β-NiAl precipitates. The volume fraction of the γ-Ni and the β-NiAl phases were measured to be around 70% and 30% respectively, with grain sizes varying largely from 0.5 to 2 μm for both phases. Isothermal heat treatments of the free-standing coatings were carried out at 1100 C for times up to 250 h. The β-phase coarsening behaviour during isothermal heat treatments was analysed by quantitative metallography. It is shown that the coarsening behaviour of β phase in the CoNiCrAlY alloy followed the classical Lifshitz-Slyozov-Wagner (LSW) theory of Ostwald ripening. By incorporating a dimensionless factor which correlates with volume fraction of the β phase, a modified LSW model coupled with formulaic interfacial energy and effective diffusion coefficient of the CoNiCrAlY alloy was utilised to interpret the coarsening behaviour of the β phase. The coarsening rate coefficient obtained from the modified LSW model shows good agreement with the corresponding experimental result.

Role of Oxides and Porosity on High-Temperature Oxidation of Liquid-Fueled HVOF Thermal-Sprayed Ni50Cr Coatings

High chromium content in Ni50Cr thermally sprayed coatings can generate a dense and protective scale at the surface of coating. Thus, the Ni50Cr coating is widely used in high-temperature oxidation and corrosion applications. A commercially available gas atomized Ni50Cr powder was sprayed onto a power plant steel (ASME P92) using a liquid-fueled high velocity oxy-fuel thermal spray with three processing parameters in this study. Microstructure of as-sprayed coatings was examined using oxygen content analysis, mercury intrusion porosimetry, scanning electron microscope (SEM), energy-dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD). Short-term air oxidation tests (4 h) of freestanding coatings (without boiler steel substrate) in a thermogravimetric analyzer at 700 °C were performed to obtain the kinetics of oxidation of the as-sprayed coating. Long-term air oxidation tests (100 h) of the coated substrates were performed at same temperature to obtain the oxidation products for further characterization in detail using SEM/EDX and XRD. In all samples, oxides of various morphologies developed on top of the Ni50Cr coatings. Cr2O3 was the main oxidation product on the surface of all three coatings. The coating with medium porosity and medium oxygen content has the best high-temperature oxidation performance in this study.

High Temperature Oxidation Behaviour of HVOF Thermally Sprayed NiCrAlY Coating on T-91 Boiler Tube Steel

In this work, NiCrAlY coating was deposited on T-91 boiler tube steel by high velocity oxy fuel (HVOF)thermal spraying system. The comparative cyclic high temperature oxidation behavior of NiCrAlY coating with uncoated substrate has been investigated experimentally. The cyclic oxidation studies were carried out at 900 °C for 50 cycles. Each cycle consists of 1 hour heating in silicon carbide tube furnace followed by 20 minutes cooling in air. The kinetics of oxidation of coated and uncoated boiler steel has been established with weight change measurements after each cycle. The corrosion products were characterized by X-ray diffractometry, Scanning Electron Microscopy (SEM) and Electron Dispersive Spectroscopy (EDS). The NiCrAlY coating showed better resistance to high temperature oxidation and corrosion due to the formation of Al2O3, Cr2O3, NiO and CrO protective oxides which lead to a reduced rate of corrosion. The uncoated steel suffered high rate of corrosion in form of spallation and peeling of unprotected Fe2O3 oxide scale.

Numerical investigation of dual-stage high velocity oxy-fuel (HVOF) thermal spray process: A study on nozzle geometrical parameters

The present study takes the advantage of computational fluid dynamics (CFD) methods to model steady-state, two-dimensional, axisymmetric, turbulent, compressible and combusting flow in a dual-stage high velocity oxy-fuel (HVOF) thermal spray system. The Eulerian method is used to solve the continuum gas phase and the Lagrangian method is utilized for tracking the particles. The effects of particle loads on the continuous gas phase are included in the simulation. Thus, compared to the previous studies, we investigate the influence of coupling between the particle and gas phases in modeling of the dual-stage HVOF process. It is found that decouple modeling of the particle and the continuous phase causes a significant error in velocity of particle at the impact moment, even for low powder particle loading. We further investigate the effects of four geometrical parameters on the behavior of gas phase and consequently the particle phase. Results also show that the turbulent intensity of flow at different sections of the warm spray process is the most important factor determining the radial distribution of nitrogen and temperature in the barrel. It also determines the radial distribution of oxygen in the free jet outside of the barrel. It is further found that reduction of the first nozzle diameter and increasing the length of the divergent section (for a fixed divergent angle) of the convergent-divergent nozzle reduce the particle temperature while these changes do not affect the particle velocity. In other words, changing these geometrical parameters has a desirable effect on the particle temperature without causing an undesirable change on the particle velocity.

Experimental Investigations on Slurry Erosion Behaviour of HVOF and HVOLF Sprayed Coatings on Hydraulic Turbine Steel

Slurry erosion behaviour of high velocity oxy fuel (HVOF) and high velocity oxy liquid fuel (HVOLF) sprayed coatings on hydraulic turbine material (i.e. CA6NM steel) was investigated at different levels of various parameters. The 50 % (WC–Co–Cr) and 50 % (Ni–Cr–B–Si) coating powder was deposited on CA6NM steel samples by HVOF and HVOLF thermal spraying techniques. Erosion tests were conducted on self-made erosion test rig with various factors as explained in the "experimentation" section. Coated and uncoated samples of CA6NM steel were investigated by following a design of experiments based on the L9 Taguchi technique, which was used to obtain the data of erosion test in a controlled way. Four parameters used in L9 experiment were velocity, impact angle, slurry concentration and average particle size. The study revealed that the velocity, impact angle and slurry concentration were most significant among various parameters, influencing the wear rate of the coatings. The average particle size did not show any significant effect on both the coatings. In comparison, coated samples showed approximately two times better results in erosion resistance than uncoated samples. Scanning electron microscopy of eroded surface showed different mechanisms of erosion on different samples under various conditions.

Evaluation of the ductile-to-brittle transition temperature in a thermally sprayed CoNiCrAlY coating by small punch multi-step loading tests

High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY (Co–31.7%Ni–20.8%Cr–8.1%Al–0.5%Y, all in wt%) coatings of an approximate thickness of 0.5 mm. Small punch tests under multi-step loading conditions were performed between room temperature and 600 ℃ on these samples to evaluate the ductile-to-brittle transition temperature. The microstructure of the coatings was characterised using a scanning electron microscope with energy-dispersive X-ray analysis. A two-phase structure consisting of fcc γ-Ni and bcc β-NiAl was found to exist. The displacements obtained from small punch multi-step loading tests at each load increment were relatively small and similar at temperatures below 500 ℃ but a significant increase in displacement was noted at 600 ℃. Fractographic investigation showed that the main fracture mode was dominated by extensive γ matrix tearing at elevated temperatures. A distinct stress and strain behaviour was found at 600 ℃, indicating that the ductile-to-brittle transition temperature of this CoNiCrAlY coating occurred between 500 ℃ and 600 ℃.

Gas and liquid-fuelled HVOF spraying of Ni50Cr coating: Microstructure and high temperature oxidation

Ni50Cr thermally sprayed coatings are widely used for high temperature oxidation and corrosion in thermal power plants. In this study, a commercially available gas atomised Ni50Cr powder was sprayed onto a power plant alloy (ASME P92) using both gas and liquid fuelled high velocity oxy-fuel (HVOF) thermal spray. Microstructures of the two coatings were examined using SEM-EDX, XRD, oxygen content analysis and mercury intrusion porosimeter. The gas fuelled coating had higher levels of oxygen content and porosity. Shorter term air oxidation tests (4h) of the free-standing deposits in a thermogravimetric analyser (TGA) and longer term air oxidation tests (100h) of the coated substrates were performed at 700°C. The kinetics of oxidation and the oxidation products were characterized in detail in SEM/EDX and XRD. In both samples, oxides of various morphologies developed on top of the Ni50Cr coatings. Cr2O3 was the main oxidation product on the surface of the coatings along with a small amount of NiO and NiCr2O4. Rietveld analysis was performed on the XRD data to quantify the phase composition of the oxides on both Ni50Cr coatings and their evolution with exposure time.

Erosion behaviour of WC–10Co–4Cr coating on 23-8-N nitronic steel by HVOF thermal spraying

WC–10Co–4Cr coating was deposited by high-velocity oxy-fuel (HVOF) process on cast and solution treated (1220°C/150min) 23-8-N nitronic steel substrates. Solution treated substrate has shown higher toughness, ductility and impact energy with a marginal reduction in hardness as compared to cast substrate. This influence the coating deposition efficiency and erosion behaviour. Erosion resistance of coatings was evaluated by air jet erosion tester on two different impact angles (30° and 90°). Phases, microstructure and eroded surface of the coating were characterised by X-ray diffraction (XRD) analysis and field-emission scanning electron microscopy (FESEM) respectively. It is observed that coating on solution treated substrate was superior to cast substrate. The reason being that coating has dense and well-bonded structure with low porosity, less decarburization and inter-splat oxidation. Higher erosion resistance of coated solution treated 23-8-N nitronic steel was attributed to combination of high fracture toughness and hardness of coating including optimum mechanical properties of the substrate. FESEM analysis reveals that erosion response of WC–10Co–4Cr coating also dependent on the relative size of the impact crater with respect to the WC grain size. Coating is removed by combined mode of ductile and brittle erosion.

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

This paper investigates the β-phase depletion behaviour during oxidation of free-standing CoNiCrA1Y (Co-31.7%Ni-20.8%Cr-8.1%A1-0.5%Y, all in wt%) bond coats prepared by high velocity oxy-fuel (HVOF) thermal spraying. The microstructure of the coatings was characterised using scanning electron microscopy with energy dispersive X-ray (EDX) analysis and electron backscatter diffraction (EBSD). It comprises a two phase structure of fcc γ-Ni and bcc β-NiA1, with grain sizes varying largely from 0.5 to 2 μm for both phases. Isothermal oxidation tests of the free-standing coatings were carried out at 1100 °C for times up to 250 h. The β phase depletion behaviour at the surface was measured and was also simulated using Thermo-Calc and DICTRA software. An A1 flux function derived from an oxide growth model was employed as the boundary condition in the diffusion model. The diffusion calculations were performed using the TTNi7 thermodynamic database together with the MOB2 mobility database. Reasonable agreement was achieved between the measured and the predicted element concentration and phase fraction profiles after various time periods. Grain boundary diffusion is likely to be important to element diffusion in this HVOF sprayed CoNiCrA1Y coating due to the sub-micron grains.

Optimization and erosion wear response of NiCrSiB/WC–Co HVOF coating using Taguchi method

In this work, high velocity oxy-fuel (HVOF) thermal spraying was used to develop 65wt% NiCrSiB–35wt% WC–Co coatings on AISI 304 stainless steel substrate. Taguchi method was taken to optimize the spray parameters like oxygen flow rate, fuel flow rate, powder feed rate and standoff distance to obtain better erosion resistance at 90° impact angle. Standoff distance and powder feed rate were identified as the highly contributing parameters on the erosion wear loss. The important sequence of the spray parameter is standoff distance>powder feed rate>fuel flow rate>oxygen flow rate. The combination of 220lpm of oxygen flow rate, 65lpm of fuel flow rate, 28g/min of powder feed rate and 300mm of standoff distance was identified as the optimum condition for minimum erosion wear loss. The microstructure of the optimized coating was characterized using Scanning Electron Microscope (SEM) equipped with Energy dispersive spectroscopy (EDS), Optical microscope (OM), and X-Ray Diffraction (XRD). The microhardness, porosity and surface roughness of the coating was also measured. Finally, erosion wear testing of the optimized coating was conducted at 30°, 60° and 90° impact angle using hot air jet erosion testing machine. The SEM images of the erodent samples were taken to analyse the erosion mechanism.

The effect of heat treatment and thermal spray processes on the grain growth of nanostructured composite CoNiCrAlY/YSZ powders

This research investigated thermal stability of mechanically milled MCrAlY/YSZ composites during heat treatment and high velocity oxy-fuel (HVOF) thermal spray processes. MCrAlY powder was mixed with 5%, 10% and 15% YSZ and then milled for 12 and 24 h. A powder without YSZ reinforcement ball was milled for 12 and 24 h too. The composite powders were annealed for 10 h at 1273 K to investigate thermal stability. Nanocrystalline and commercial powders were deposited on Inconel-617 substrate using the HVOF process. The morphology and thermal stability of mechanically milled and heat treated powders and coatings were investigated using X-ray diffractometry, scanning electron microscopy (SEM) and optical microscopy (OM). It was found that the increase in milling time resulted in the uniform distribution of reinforcements in the γ phase matrix. The uniform distribution of reinforcements caused reduction in grain growth during heat treatment process. On the other hand, increasing YSZ percentage decreased grain growth, but when the YSZ amount exceeded 10%, the ceramic reinforcements could not prevent the grain growth of nanostructure powders. The heat absorbed by nanostructured powders during thermal spraying process resulted in the grain growth of the γ-phase, but due to the presence of YSZ reinforcement, the grain growth was very lower than that of unreinforced coatings. It could be suggested that nanostructure coating formed due to thermo mechanical phenomena occurred in commercial powder particles during thermal spray process.

Fabrication of nanostructured WC–Co coating with low decarburization

Three kinds of thermal spray powders with different densities and WC particle sizes were prepared using in-situ synthesized nanoscale WC–Co composite powder as raw material. The WC–Co coatings were then fabricated by high velocity oxy-fuel thermal spraying. The phase constitution of the as-sprayed coatings and microstructures of the feedstock particles and the coatings were analyzed. The results show that the coating prepared using powder with a relatively higher density and smaller WC particle size exhibits simultaneously lower decarburization and higher density, and further enhanced hardness. The present study suggests an effective method to fabricate nanostructured WC-based cermet coatings with high properties.

Microstructural, Hyperfine, and Magnetic Properties of FeSiBCuNb Deposits

Mechanically alloyed Fe73.5Si13.5B9Nb3Cu1 powders were used as feedstock materials for depositing nano/amorphous coatings by high-velocity oxy-fuel thermal spraying process. Morphology, structure, microstructure, hyperfine, and magnetic properties have been investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Mossbauer spectrometry, and vibrating sample magnetometer. The coating structure consists of bcc Fe(B)-type, bcc Fe(Si)-type, and amorphous phases. The crystallite sizes of Fe(Si) and Fe(B) solid solutions increase after spraying to about (72–82) and (48–81) nm, respectively. The Mossbauer spectrometry results reveal the presence of Fe2B boride. The squareness ratio Mr/ Ms and the coercivity values of the coatings decrease with increasing the number of passes.

Characterisation and corrosion performance of HVOF sprayed Ni–10 wt-%Ti coatings

Mechanically alloyed Ni–10 wt-%Ti powders were deposited on 1045 steel substrate using high velocity oxy-fuel (HVOF) thermal spraying process. The microstructure and phase composition of coatings were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The results showed that the phase composition of feedstock powders was preserved during spraying and only supersaturated Ni(Ti) solid solution was present in the coatings. The hardness measurements showed that the hardness of coatings was about 510 HV (50 g). Tafel polarisation tests performed after 1 h immersion revealed that the corrosion behaviour of HVOF Ni–Ti coatings was not affected by substrate. However, delamination of air plasma sprayed (APS) Ni–10 wt-%Ti coatings and significant corrosion of substrate was observed in similar conditions. For HVOF Ni–Ti coating and after 24 h of immersion time, the corrosion of underlying substrate was observed after polarisation measurements but no delamination of coating occurred.

Development of a semi automated dual feed unit to produce FGM coatings using the HVOF thermal spray process

The application of functionally graded materials (FGMs) is quite difficult, but thermal spray processes like Plasma spray have demonstrated their unique potential in producing graded deposits, where researchers have used twin powder feed systems to mix different proportions of powders. FGMs vary in composition and/or microstructure from one boundary (substrate) to another (top service surface), and innovative characteristics result from the gradient from metals to ceramics or non-metallic to metals. The present study investigates an innovative modification of a high velocity oxy-fuel (HVOF) thermal spray process to produce functionally graded thick coatings. In order to deposit thick coatings, certain problems have to be overcome. Graded coatings enable gradual variation of the coating composition and/or microstructure, which offers the possibility of reducing residual stress build-up with in coatings. In order to spray such a coating, modification to a commercial powder feed hopper was required to enable it to deposit two powders simultaneously which allows deposition of different layers of coating with changing chemical compositions, without interruption to the spraying process. Various concepts for this modification were identified and one design was selected, having been validated through use of a process model, developed using ANSYS Flotran finite element analysis. In the current research the mixing of different proportions of powders were controlled by a computer using LabVIEW software and hardware, which allowed the control and repeatability of the microstructure when producing functionally graded coatings.

Corrosion resistance of Fe-based amorphous metallic matrix coating fabricated by HVOF thermal spraying

The Fe-based amorphous metallic matrix coating (Fe-AMMC) was fabricated with the powder mixtures of Fe-based metallic glass synthesized with industrial raw materials, NiCr alloy and WC particle by high velocity oxy-fuel (HVOF) spraying. The corrosion resistance of Fe-AMMC was investigated by potentiodynamic polarization tests in 1 mol/L HCl, NaCl, H2SO4 and NaOH solutions, respectively. The surface morphologies corroded were observed by SEM. The results indicate that Fe-AMMC exhibits excellent corrosion resistance, higher corrosion resistance than 304L stainless steel in the chloride solutions. The low corrosion current density and passive current density of Fe-AMMC with a wide spontaneous passivation region are about 132.0 μA/cm2 and 9.0 mA/cm2 in HCl solution, and about 2.5 μA/cm2 and 2.3 mA/cm2 in NaCl solution. The excellent corrosion resistance demonstrates that Fe-based amorphous metallic matrix powder is a viable engineering material in practical anti-corrosion and anti-wear coating applications.

Micro-Scale Abrasion of WC-Based Coatings with Different Abrasive Type

Various research programmes have been conducted examining cermet coatings regarding wear, corrosion and the combination of both (erosion-corrosion and abrasion-corrosion). Several methods have been used to deposit cermet coatings, the most common being thermal spraying or hard facing (weld overlaying). In the current work, the micro-scale abrasion of coatings deposited using both high velocity oxy-fuel (HVOF) thermal spraying and weld overlay techniques are compared. The weld-overlayed WC-nickel alloy systems have the carbide particles are typically two orders of magnitude larger than in the sprayed coatings. Micro-scale abrasion tests were performed using silicon carbide, alumina and silica particle slurries with abrasive particle sizes in the range of 2-10 μm in all cases. Wear rates were determined and the wear scars were examined using SEM to elucidate the dominant wear mechanisms. The wear rate is generally observed to decrease with decreasing abrasive hardness from silicon carbide, followed by alumina and silica.

Hot Corrosion Behaviour of HVOF Sprayed Stellite-6 Coatings on Gas Turbine Alloys

The coal burned natural gas in contact with gas turbine can contain impurities of sodium, sulfur, vanadium, silicon and possibly lead and phosphorous, induce accelerated hot corrosion during long term operation. Coatings are frequently applied on gas turbine components in order to restrict surface degradation and to obtain accurate lifetime expectancies. High velocity oxy-fuel thermal spraying has been used to deposit Stellite-6 alloy coatings on turbine alloys. Hot corrosion behavior of the coatings were investigated for 50 cycles of 1 h heating at 800 °C followed by 20 min cooling in presence of Na2SO4 + 50 % V2O5 measuring weight gain (or loss). X-ray diffraction and SEM/EDAX techniques were used to characterize the oxide scale formed. The superior performance of Stellite-6 coating can be attributed to continuous and protective thin oxide scale of CoO, Cr2O3 and SiO2 formed on the surface. The coating region beneath this thin oxide scale was partially oxidized. Uncoated SuperCo-605 and MDN-121 showed less weight gain than Stellite-6 coated samples, but they showed spalling or sputtering during cyclic oxidation. Stellite-6 coating was dense and pore free even after 50 cycles, indicating that it can resist the hot corrosion cycle.

Application of small punch creep testing to a thermally sprayed CoNiCrAlY bond coat

High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY (Co–31.7% Ni–20.8% Cr–8.1% Al–0.5% Y (wt%)) bond coat alloy samples approximately 0.5mm thick. Creep tests were conducted at 750°C on these samples using a small punch (SP) creep test method. The samples were characterised before and after creep testing using scanning electron microscopy with electron backscatter diffraction (EBSD). EBSD revealed a two phase fcc γ-Ni and bcc B2 β-NiAl microstructure with grain sizes ~1–2μm for both phases, which did not change significantly following testing. The constant temperature SP test data were characterised by a minimum creep strain rate, ε̇min, and a total time to failure, tf, at different applied stresses. The data are fitted to conventional power law equations with a stress exponent for creep close to 8 in the Norton power law and between 7 and 10 in the Monkman–Grant creep rupture law. Creep rupture was predominantly due to creep cavitation voids nucleating at both the γ–β interphase boundaries and the γ–γ grain boundaries leading to final failure by void linkage. However, rupture life was influenced by the quantity of oxide entrained in the coating during the spray deposition process.

A study on erosive wear behavior of HVOF sprayed nanostructured WC-CoCr coatings

WC-CoCr cermet coatings were deposited on stainless steel substrate using high-velocity oxy-fuel (HVOF) thermal spray process. The coatings were developed with two different thermal spray powders: one has WC grains of conventional micron size and the other is composed of nanosized (near-nanostructured) grains. HVOF spraying was assisted with in-flight particle temperature and velocity measurement system to control the process parameters that have resulted in quality coatings. Cavitation erosion testing was performed using a vibratory test apparatus based on ASTM standard G32-98. Surface morphology of powders and coatings was examined using the FESEM images, and phase identification was performed by XRD analysis. The erosion behavior of coatings and mechanism of material removal was discussed by examining the microstructure images of worn-out surfaces. WC-CoCr cermet coating deposited with nanosized WC grains exhibited higher cavitation erosion resistance as compared to conventional coating.