High Velocity Oxy-fuel Spray Process Research Articles

Identifying the optimal HVOF spray parameters to attain minimum porosity and maximum hardness in iron based amorphous metallic coatings

Flow based Erosion – corrosion problems are very common in fluid handling equipments such as propellers, impellers, pumps in warships, submarine. Though there are many coating materials available to combat erosion–corrosion damage in the above components, iron based amorphous coatings are considered to be more effective to combat erosion–corrosion problems. High velocity oxy-fuel (HVOF) spray process is considered to be a better process to coat the iron based amorphous powders. In this investigation, iron based amorphous metallic coating was developed on 316 stainless steel substrate using HVOF spray technique. Empirical relationships were developed to predict the porosity and micro hardness of iron based amorphous coating incorporating HVOF spray parameters such as oxygen flow rate, fuel flow rate, powder feed rate, carrier gas flow rate, and spray distance. Response surface methodology (RSM) was used to identify the optimal HVOF spray parameters to attain coating with minimum porosity and maximum hardness.

Termal Püskürtme Yöntemi ile Üretilmiş AISI 316L Paslanmaz Çelik Kaplamaların Karşılaştırmalı İncelenmesi

In this study, austenitic stainless steel (316L) coatings were formed using both Wire Flame Spray Forming (WSF) and High Velocity Oxy Fuel (HVOF) deposition processes after a NiCr bond layer was deposited on low carbon steel substrate. The coatings were characterized by light optical microscope (LOM) and microhardness measurements. Corrosion resistance of 316L austenitic stainless steel coatings applied onto low carbon steel via WSF and HVOF spray processes was analyzed by using the potentiodynamic polarization scanning (PDS) technique. The wear behaviour of the coatings against Al 2 O 3 ball was investigated with a reciprocating tester under dry friction conditions. It was found that the HVOF sprayed coating showed higher hardness and better wear resistance than the WSF coating. However, the overall corrosion resistance of the HVOF deposited coating was inferior to that of the WSF coating.

High-Temperature Exposure Studies of HVOF-Sprayed Cr3C2-25(NiCr)/(WC-Co) Coating

In this research, development of Cr3C2-25(NiCr) + 25%(WC-Co) composite coating was done and investigated. Cr3C2-25(NiCr) + 25%(WC-Co) composite powder [designated as HP2 powder] was prepared by mechanical mixing of [75Cr3C2-25(NiCr)] and [88WC-12Co] powders in the ratio of 75:25 by weight. The blended powders were used as feedstock to deposit composite coating on ASTM SA213-T22 substrate using High Velocity Oxy-Fuel (HVOF) spray process. High-temperature oxidation/corrosion behavior of the bare and coated boiler steels was investigated at 700 °C for 50 cycles in air, as well as, in Na2SO4-82%Fe2(SO4)3 molten salt environment in the laboratory. Erosion-corrosion behavior was investigated in the actual boiler environment at 700 ± 10 °C under cyclic conditions for 1500 h. The weight-change technique was used to establish the kinetics of oxidation/corrosion/erosion-corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), and EDS elemental mapping techniques were used to analyze the exposed samples. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in all the environments. The oxidation/corrosion/erosion-corrosion resistance of the HVOF-sprayed HP2 coating was found to be better in comparison with standalone Cr3C2-25(NiCr) coating. A simultaneous formation of protective phases might have contributed the best properties to the coating.

Numerical Modeling of Suspension HVOF Spray

A three-dimensional two-way coupled Eulerian-Lagrangian scheme is used to simulate suspension high-velocity oxy-fuel spraying process. The mass, momentum, energy, and species equations are solved together with the realizable k-e turbulence model to simulate the gas phase. Suspension is assumed to be a mixture of solid particles [mullite powder (3Al2O3·2SiO2)], ethanol, and ethylene glycol. The process involves premixed combustion of oxygen-propylene, and non-premixed combustion of oxygen-ethanol and oxygen-ethylene glycol. One-step global reaction is used for each mentioned reaction together with eddy dissipation model to compute the reaction rate. To simulate the droplet breakup, Taylor Analogy Breakup model is applied. After the completion of droplet breakup, and solvent evaporation/combustion, the solid suspended particles are tracked through the domain to determine the characteristics of the coating particles. Numerical simulations are validated against the experimental results in the literature for the same operating conditions. Seven or possibly eight shock diamonds are captured outside the nozzle. In addition, a good agreement between the predicted particle temperature, velocity, and diameter, and the experiment is obtained. It is shown that as the standoff distance increases, the particle temperature and velocity reduce. Furthermore, a correlation is proposed to determine the spray cross-sectional diameter and estimate the particle trajectories as a function of standoff distance.

Tribological behaviour of NiCrBSi–WC(Co) coatings

Parts working under extreme conditions are in need of surface coatings to achieve better performance and longer service life. NiCrBSi alloys have been a good choice against wear combined with corrosion at high temperature but need further improvements. In this work, a series of NiCrBSi–WC(Co) coatings with WC(Co) content from 0 wt-% to as high as 80 wt-% were prepared using a high velocity oxy-fuel spraying process. The influences of the WC(Co) content on the wear behaviour of the metal matrix composite coatings were studied using an SRV reciprocating sliding wear system. The relationship of the wear mechanism and the WC(Co) content was discussed, and an optimal WC(Co) content was proposed according to the study results.

벌크 비정질 용사코팅과 비정질 기지 복합재료의 건조 마찰특성

The friction behaviors of bulk amorphous thermal spray coating (BAC) and second phase-reinforced composite coatings using a high velocity oxy-fuel spraying process were investigated using a ball-on-disk test rig that slides against a ceramic ball in an atmospheric environment. The surface temperatures were measured using an infrared thermometer installed 50 mm from the contact surface. The crystallinities of the coating layers were determined using X-ray diffraction. The morphologies of the coating layers and worn surfaces were observed using a scanning electron microscope and energy-dispersive spectroscopy. The results show that the friction behavior of the monolithic amorphous coating was sensitive to the testing conditions. Under lower than normal loads, a low and stable friction coefficient of about 0.1 was observed, whereas under a higher relative load, a high and unstable friction coefficient of greater than 0.3 was obtained with an instant temperature increase. For the composite coatings, a sudden increase in friction coefficient did not occur, i.e., the transition region did not exist and during the friction test, a gradual increase occurred only after a significant delay. The BAC morphology observations indicate that viscous plastic flow was generated with low loads, but severe sur- face damage (i.e., tearing) occurred at high loads. For composite coatings, a relatively smooth surface was observed on the worn surface for all applied loads. Keywordsamorphous thermal spray coating (비정질 용사 코팅), amorphous metallic matrix composite (비정질 기지 복합재료), friction characteristic (마찰 특성), sliding interface temperature (미끄럼 계면 온도)

Optimizing HVOF spray process parameters to attain minimum porosity and maximum hardness in WC–10Co–4Cr coatings

Many thermal and thermo-chemical surface treatment techniques are being used to modify the surface of the material with desired properties for various applications in service. Nowadays, high velocity oxy-fuel (HVOF) spray is being widely used in industrial applications due to its ability to produce a high-quality coating with required hardness and low oxide content due to its high velocity impact inherent in the process. Porosity and hardness are two important coating properties for wear and corrosion related applications, and they play a significant role in the service life of the components. In HVOF spraying, the parameters such as oxygen flow rate, liquefied petroleum gas (LPG) flow rate, powder feed rate and spray distance have greater influence on the final properties of the coatings. The present work aims to select the optimum HVOF spray parameters to attain minimum porosity and maximum hardness in WC–10Co–4Cr coating sprayed on naval brass substrate material. Statistical tools such as design of experiments (DOE), analysis of variance and response surface methodology (RSM) were used to meet the stated objectives. From the results, it was observed that the oxygen flow rate has greater influence on coating porosity and hardness followed by LPG flow rate, powder feed rate and spray distance, respectively.

Production of Nanocrystalline Ni-20Cr Coatings for High-Temperature Applications

Presynthesized nanocrystalline Ni-20Cr powder was deposited on SA 516 and T91 boiler steels by a high-velocity oxy-fuel spraying process. Ni-20Cr powder was synthesized by the ball milling approach. The high-temperature oxidation behavior of bare and coated samples was then studied under cyclic isothermal conditions at 900 °C for 50 cycles. The kinetics of oxidation was established using weight change measurements for the bare and coated boiler steels. Uncoated and coated samples of T91 steel were exposed to the superheated zone of a power plant boiler at 750 °C under cyclic conditions for 15 cycles. Each cycle consisted of 100 h of heating followed by 1 h of cooling. Attempts were made to study the kinetics of erosion–corrosion using weight change and thickness loss data for the samples. Different characterization techniques were used to study the oxidized and eroded–corroded samples, including x-ray diffraction, scanning electron microscopy/energy-dispersive spectroscopy, and x-ray mapping analyses. The Ni-20Cr alloy powder coating was found to offer excellent oxidation resistance to the base steels and was successful in reducing the weight gain of SA 516 steel by 98.5 % and that of T91 steel by 65 %. The coating was observed to reduce the erosion–corrosion rate of T91 steel by 86 % in terms of thickness loss. This indicates that the investigated nanostructured coating can be a better choice over conventional coating for erosion–corrosion control of boiler tubes.

Effect of Feed Rate and Impact Angle on the Erosive Wear of High Velocity Oxy-fuel Sprayed Coating Under High Temperature Conditions

In the present investigation erosive wear behavior of EWAC 1006 Co-base alloy deposited by high velocity oxy-fuel (HVOF) spraying process was investigated. The microstructure, porosity and hardness of the coating was evaluated. The effect of impact angles of 30°, 60° and 90° and erodent feed rate of 1, 3, 5 g/min on the erosive wear were evaluated. All erosive wear tests were conducted at high temperature (450°C) against erodent velocity of 40 m/s. The erosive wear was found to decrease with the increase in impact angle of the erodent and increase with the increase in erodent feed rate. The erosive wear of the coating was 3-4 times lower than the substrate. Analysis of the scanning electron microscope images revealed cutting and lip formation as the material removal mechanisms in these coatings under erosive wear conditions used in this investigation.

Characterization of Ni-Based Alloy Submicron WS<sub>2</sub>/CaF<sub>2</sub> Composite Coatings Deposited by High Velocity Oxy-Fuel (HVOF) Spray Process

Ni-based alloy submicron WS2/CaF2 composite coatings were successfully produced through high velocity oxy-fuel (HVOF) spraying process. Microstructure characterization and morphology analysis on coatings were conducted using scanning electron microscopy and X-ray diffractometry. Tribological properties of coatings were carried out using a pin-on-ring tribometer at room temperature and 500°C, respectively. Results showed that WS2 in particles did not entirely decomposed during spraying process, submicron WS2 and CaF2 were uniformly distributed throughout the coating. Which lead a dense coating and a lower friction coefficient and better wear resistance. In ambient temperature, WS2 in coatings played a main role of lubrication, friction coefficient of coatings was in the rage of 0.25~0.35. But with the increasing of temperature, WS2 failed to provide solid lubricating films for pyrolysis, and CaF2 did not fully transfer from brittleness to plasticity, restrained solid lubricating film forming, friction coefficient reached to 0.45. Furthermore, average adhesive strength between coating/substrate was above 30MPa, which had positive affect on wear resistance of coatings.

Developing Empirical Relationship to Predict Hardness in WC- 10Co-4Cr HVOF Sprayed Coatings

Surface treatments of engineering materials have recently become important for serviceable engineering components. Many techniques such as thermal and thermo-chemical surface treatments have been used to develop surface characteristics of materials. Presently, high velocity oxy-fuel (HVOF) spray is considered as the best method of producing a high-quality hard metal coating due to its moderate process temperatures and high gas velocities. Hardness is the most important property, which influences considerably service life characteristics of coatings. HVOF spray parameters, namely oxygen flow rate, LPG flow rate, powder feed rate, spray distance playsa major role in determining the hardness of coating. In this study, WC-10Co-4Cr coatings were deposited on naval brass substrates by high velocity oxy-fuel (HVOF) spray process. The aim of this work is to develop an empirical relationship to predict the hardness of WC-10Co-4Cr coatings incorporating HVOF spray parameters using statistical tools such as design of experiments, analysis of variance and regression analysis. Statistical calculations showed that the effects of the four variables on the hardness were different. Moreover, the results of this study may help to control the spray parameters of HVOF process to achieve higher coating hardness.

HVOF Spraying Process Optimization for the Deposition of Submicron Structural WC-12Co Coatings

The submicron structural WC-12Co coatings were deposited by a high-velocity oxy-fuel (HVOF) system from agglomerated WC-12Co powders with carbide grain size of 0.7~0.9μm in this study. Effects of HVOF spraying parameters on the microstructure and properties of submicron structural WC-12Co coatings were examined. The results showed that both the fuel flow rate and spraying distance influenced significantly the microhardness and porosity of coatings, and cooling condition also had a few effect on the porosity of coatings. Through a series of experiments, the spraying parameters for the WC-12Co coatings preparation were optimized as follows: propane gas pressure was 0.65 MPa, propane flow rate of 16.7 L/min, oxygen pressure of 0.7 MPa, oxygen flow rate of 100 L/min, spraying distance of 270 mm and with compressed air cooling. The porosity of WC-12Co coating sprayed at the optimum parameters was lower than 1%, and its microhardness was HV0.11141±67.

Propylene flow, microstructure and performance of WC–12Co coatings using a gas-fuel HVOF spray process

Five WC–12Co coatings were deposited by a high velocity oxy-fuel (HVOF) system using constant oxygen flow and varying propylene flow. The phase composition, microstructure, as well as abrasive and sliding wear performance of the as-sprayed coatings were investigated. The degree of tungsten carbide (WC) decarburization in the as-sprayed coatings increases while the coating porosity decreases with the increase of the propylene flow. The coating hardness, fracture toughness, resistance to abrasive and sliding wear increases with the increase of the propylene flow, reaches maximum and then decreases. At the low flow of the propylene, relatively loose coating microstructure is formed, which leads to fracturing and pulling off the WC particles during abrasive and sliding wear process. Herewith, at the high flow of the propylene, the high degree of the WC decarburization and high brittleness of the coating leads to micro-cutting during abrasive wear as well as to cracking and delamination of the coating in the sliding wear process.

Deposition of WC-Co Coatings by a Novel High Pressure HVOF

WC-Co coatings are primarily deposited using the high velocity oxy-fuel (HVOF) spray process. However, the decomposition and decarburization of carbides during spraying affects the wear performance and fracture toughness of the coatings. In this paper, a novel high pressure HVOF was developed to achieve lower particle temperature and higher particle velocity. It enables combustion chamber pressures up to 3.0 MPa. The influence of combustion chamber pressure and oxygen/fuel ratio on WC-Co particle velocity and temperature levels were analyzed by numerical simulation. The experimental results show that the combustion chamber pressure and the oxygen/fuel ratio have a significant influence on particle velocity and melting degree, as well as on the microstructure and microhardness of the coating. High velocity WC-Co particles in different states, i.e., molten, semi-molten, and non-molten can be readily obtained by changing the spraying conditions. A comparison to the conventional JP-5000 was also performed.

Slurry erosion behaviour of HVOF sprayed WC–10Co–4Cr and Al2O3+13TiO2 coatings on a turbine steel

Hydroturbine components generally suffer from severe wear due to slurry erosion caused by solid particles entrained in the flowing water. High-velocity oxy-fuel (HVOF) spray process is known for depositing protective coatings on the materials. In this work, WC–10Co–4Cr and Al2O3+13TiO2 coatings were deposited on CF8M turbine steel by HVOF spray process and studied with regard to their performance under slurry erosion conditions. High speed erosion test rig was used for slurry erosion tests and effect of three parameters namely average particle size, speed (rpm) and slurry concentration on slurry erosion of these materials was investigated. The bare steel and Al2O3+13TiO2 coating followed ductile and brittle mechanisms respectively under slurry erosion, whereas the WC–10Co–4Cr coating exhibited mixed behaviour (mainly ductile). WC–10Co–4Cr coating was found to be useful to increase the slurry erosion resistance of steel remarkably.

Wear study of Ni–WC composite coating modified with CeO2

In the present investigation, Ni–WC composite powder was modified with the addition of CeO2 in order to form a new composition of Ni–WC–CeO2. The Ni–WC and Ni–WC–CeO2 compositions were used for coating deposition by high-velocity oxy-fuel (HVOF) spraying process so as to study the effect of CeO2 addition on microstructure, distribution of various elements, hardness, formation of new phases, and abrasive wear behavior. Further, the effect of load, abrasive size, sliding distance, and temperature on abrasive wear behavior of these HVOF-sprayed coatings was investigated by response surface methodology. To investigate the abrasive wear behavior of HVOF-sprayed coatings four factors such as load, abrasive size (size in micrometers), sliding distance (meters), and temperature (°C) with three levels of each factor were investigated. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the load, abrasive size, and sliding distance were the main significant factors while load and abrasive size, load and sliding distance, abrasive size and sliding distance were the main significant interactions. Thus an abrasive wear model was developed in terms of main factors and their significant interactions. The validity of the model was evaluated by conducting experiments under different wear conditions. A comparison of modeled and experimental results showed 4–9% error. The abrasive wear resistance of coatings increases with the addition of CeO2. This is due to increase in hardness with the addition of CeO2 in Ni–WC coatings.

A Comparison between the Corrosion Resistances of High Velocity Oxy Fuel (HVOF) Sprayed Coatings and Hard Chromium Coatings

HVOF-sprayed coatings (WC–17%Co) and hard chromium coatings corrosion resistances have been compared through electrochemical polarization test in 3.5% NaCl solution. WC–17%Co alloy coatings were deposited on mild steel substrates by High Velocity Oxy-Fuel (HVOF) spray process. The layers of standard and crack free hard chromium coatings were prepared by using Direct Current (DC) and Pulse Current (PC) electroplating process on the mild steel substrates. Hard chromium coatings was characterized as a reference material, to verify whether HVOF-sprayed coatings are suitable as a hard chromium coatings replacement. The microstructure of the coatings was examined by OM, SEM and XRD. Standard hard chromium coatings passivate in NaCl environment, but crack free hard chromium coatings were prepared by using Pulse Current (PC) electroplating do not passivate. The lowest corrosion current densities (Icorr) were recorded for crack free hard chromium coatings. Comparative electrochemical test results showed that, the Standard hard chromium coating has the highest Icorr and were significantly damaged after the electrochemical tests. It is seem to be that WC–17%Co alloy coatings can be substituted for standard hard chromium coatings but crack free chromium coatings bring new challenge for HVOF-sprayed coatings!

Evaluation of Thermocyclic Oxidation Behavior of HVOF-Sprayed NiCrFeSiB Coatings on Boiler Tube Steels

High velocity oxy-fuel (HVOF) spray process has grown into a well-accepted industrial technology for obtaining coatings resistant to significant surface degradation processes. In the present study, HVOF process was used to deposit Ni-based hardfacing NiCrFeSiB alloy powder on kinds of boiler tube steels designated as SA210 grade-A1, SA213-T11, and SA213-T22. The microstructures and several properties of the as-sprayed coatings have been investigated. Thermocyclic oxidation studies were performed in static air at 900 °C. NiCrFeSiB-coated steels showed slow oxidation kinetics and considerably lower weight gains than that of uncoated steels. The superior performance of NiCrFeSiB coatings can be attributed to continuous and protective thin oxide scale of amorphous SiO2 and Cr2O3 formed on the surface of the oxidized coatings. The combined technique of x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, and electron probe microanalysis are used to characterize reaction products of the oxidized surfaces.

Thick tool steel coatings using HVOF spraying for wear resistance applications

This paper deals with the processing of thick cold work tool steel coating using high velocity oxy-fuel (HVOF) spraying process. A full factorial experimental design was established to identify the influencing process parameters on the formation of dense coating with low oxidation. Microstructural analysis of the coating was carried out using optical, SEM and XRD techniques. Cold work tool steel coatings with a thickness up to 2mm were developed on bond coated low carbon steel substrates for wear resistance evaluation. A pin on disc test was performed to examine the wear resistance of thick cold work tool steel coatings on different types and sizes of abrasive papers. The wear results were compared with the wear resistance of a standard high speed steel pin. The abrasive wear resistance of cold work tool steel coated pins was found to be superior against soft and fine abrasive papers than the standard high speed steel. Besides, the performance of the coated pins against hard and coarser abrasive papers was found to be similar to standard high speed steel. The study showed the potential of HVOF spraying on the development of thick cold work tool steel coatings for wear resistance applications.

Characterisation of HVOF sprayed Cr3C2-50(Ni20Cr) coating and the influence of binder properties on solid particle erosion behaviour

Abstract The coating Cr 3 C 2 with 50 wt.% Ni20Cr deposited by high velocity oxy-fuel (HVOF) spray process was characterized in detail to investigate the effect of annealing on the solid particle erosion behaviour and understand the influence of the binder properties. Systematic characterization of the coating was carried out using electron microscopy (scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA)), X-ray diffraction (XRD), microindentation and nanoindentation techniques. The solid particle erosion tests were done on the as-sprayed coating and coatings annealed at 400 °C, 600 °C and 800 °C using silica erodent particles. The coefficient of restitution of the coated samples was also measured by WC ball impact tests to simulate dynamic impacts. The as-sprayed coating consisted of primary carbides and binder that was a mixture of amorphous and nanocrystalline phases. Annealing leads to recrystallisation of binder phase and precipitation of secondary carbides. The coating hardness and binder ductility change with annealing temperature. The erosion resistance improves with annealing up to 600 °C. In the as-sprayed coating, the amorphous phase, inter-splat boundaries and the elastic rebound characteristics affect the erosion response. While in the case of the coating annealed at 600 °C, the presence of ductile crystalline binder, fine carbide precipitates and embedment of erodent particles together improve solid particle erosion resistance.