High Velocity Oxy-fuel Spray Technique Research Articles

Hot corrosion and CMAS degradation of laser-glazed YSZ coating with optimum parameter

The purpose of this research is to look into the effect of laser surface modification on the hot corrosion and hot corrosion + calcium–magnesium aluminosilicate (CMAS) resistance of YSZ thermal barrier coating. Metco AMDRY 997 (Ni–22Cr–10Al–Y) powder was coated as a bond coat on the AISI 304L stainless steel substrate using a high-velocity oxy-fuel spray technique. As a ceramic top layer, 204 NS YSZ (ZrO2·8Y2O3) particles were plasma sprayed on the bond coat. Following that, the entire coating surface was treated with a 20-kHz pulsed CO2 laser to increase the coatings’ surface performance. A corrosion test was carried out in a box furnace for 20 h at 1050 °C. SEM images of laser-glazed coatings revealed denser surfaces than those of air-spraying coatings. The laser-modified coating is more resistant to hot corrosion than unmodified coatings. The X-ray diffraction analysis revealed that the reaction between yttria (Y2O3) and V2O5 resulted in the formation of YVO4 crystals as a corrosion product. According to energy dispersive spectroscopy results, laser-glazed coatings reduced molten salt diffusion from surface to bond coat. As a result, laser-glazed coatings demonstrated improved hot corrosion resistance and decreased corrosion products on the bond coat zone. After the hot corrosion test, the laser-treated composite had a 55% lower thermally grown oxide layer than untreated APS YSZ. The optimum laser parameter was used as 77.7 W laser power, 120 mm/s scanning speed, and 215 mm laser distance.

Electrochemical Corrosion Behavior and Microstructural Characterization of HVOF Sprayed Inconel-718 Coating on Gray Cast Iron

In the present work, Inconel-718 (IN718) coating was developed on a gray cast iron (C.I) substrate by utilizing a high-velocity oxy-fuel spray technique. The microstructural analysis was performed to know the topological characteristics of the deposited IN718 coating. Corrosion testing of bare and coated samples was carried out at an ambient temperature in a NaCl solution (3.5 wt.%) using potentiodynamic polarization technique. The kinetics of corrosion and surface characteristics of the as-sprayed and corroded coatings were studied by scanning electron microscopy/energy-dispersive spectroscopy techniques. The different phases in the feedstock powder and as-sprayed coating were determined through X-ray diffraction analysis. The reported average microhardness value of the developed IN718 coating was 563 ± 15 HV0.2. The increased hardness of the IN718 coating is credited to the existence of Mo and Ti elements in the IN718 coatings. Corrosion testing showed that the IN718 coating exhibited the maximum corrosion resistance compared to a C.I. substrate due to a stable passive layer in the IN718 specimen.

Wear behavior of HVOF sprayed WC-Co/NiCrAlYSi (35–65%) and WC-Co/NiCrAlYSi (80–20%) coatings on turbine SS316 steel

Abstract The high-Velocity Oxy-Fuel (HVOF) process belongs to the family of thermal spray techniques and are widely used in many industries to protect the components against wear and erosion. HVOF spraying technique is widely used due to its viability to produce low porosity and dense coatings. The wear test will be carried on pin-on-disk for different loading and constant speed conditions on substrate and coated materials. After a thorough review, commercially available powder WC-Co/NiCrAlYSi (80–20%) of nominal composition and blended powder WC-Co/NiCrAlYSi (35–65%), is selected to be coated on the SS316 substrate material for this study. The coefficient of friction and wear rate of both the coatings is lower than that of uncoated samples and the coatings exhibit high hardness with a high volume fraction of carbides being preserved during the HVOF spraying process and have lower sliding wear behavior when compared to uncoated material.

Experimental investigations of carbon nanotubes reinforcement on properties of ceramic-based composite coating

This paper deals with the preparation and development of carbon nanotubes (CNT)-reinforced chromium oxide coatings. The 8% by weight CNT was reinforced with 92% by weight chromium oxide. The composite powder was blended in a ball mill and deposited successfully with high velocity oxy-fuel spraying technique. The microhardness of CNT-reinforced coating was found to be 30% higher than conventional coatings. The CNT reinforcement was able to decrease porosity of composite by 20%. The XRD spectrum indicated that CNT were present in pure graphite form and were found to be chemically inert during the spraying process. CNT did not react with chromium oxide to form carbides. The metallurgical analysis of surface and cross-section of composite coating revealed that CNT were uniformly distributed throughout the composite matrix, and formed a bridge-like structure between the splats of chromium oxide, which improved the properties of composite coating.

Preparation and Properties of WC-Based Alloy Coatings with Controlled Co and Cr via High Velocity Oxy-Fuel Spray Technique.

WC based alloy coatings included different mass percent of Co and Cr have been synthesized on high carbon steel by using a facile high velocity oxy-fuel spray method. The mechanical nature of the coating films has been investigated by micro vickers hardness and fracture toughness. X-ray diffraction (XRD) and EDX analyses indicate that the three different samples (WC-10Co-4Cr, WC-17Co, and WC-12Co) consist of pure WC, W, Cr, and Co constituents as well as W2C and Co6W6C phases. The SEM and image analysis results show that WC-10Co-4Cr condition has higher porosity than those of WC-17Co, and WC-12Co coatings. WC-17Co coating showed the highest value in the hardness and fracture toughness test among three different samples. The obtained results revealed that the mechanical properties of WC based alloy coatings synthesized by a facile high velocity oxy-fuel spray method is very sensitive to Co content.

Thermal-Sprayed Coatings on Bushing and Sleeve-Pipe Surfaces in Continuous Galvanizing Sinking Roller Production Line Applications

This paper describes thermal spray techniques for making hard coatings on bushing and sleeve component surfaces. Specifically, plasma-arc welding was used to produce 5-mm thick Co-Cr alloy welding overlays on the bushing, while a high-velocity oxy-fuel spraying technique and laser re-melting technique were used to produce thinner coatings of Co-Cr-Ni+WC of about 1mm thickness on the sleeve-pipe counterparts. The surface-treated components were then submerged in liquid zinc to study the corrosive behaviour of the surface coating and substrate. Both the scanning electron microscope and energy dispersive spectrometer analyses were used to study the microstructure and phase composition of both coatings and substrates prior to and after corrosion experiments. The results show that the microstructure of the bushing consists of γ-cobalt solid solution as well as the eutectic structure of γ-cobalt and carbides, which have good corrosive resistance against molten zinc. Meanwhile, the microstructure of the sleeve pipe consists of a Co-Cr solid solution with various forms of carbides, which displays the combined properties of toughness with good corrosive resistance to molten zinc.

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.

Heat treatment effect on wear behaviour of HVOF-sprayed near-nanostructured coatings

This study investigates the effect of heat treatment on changes in microstructure and wear behaviour of WC-NiCr coatings. Two feedstock powders with a similar chemical composition and different particle sizes (near nano-structured WC-17NiCr and microstructured WC-15NiCr) were used. High-velocity oxyfuel spraying technique was used to deposit coatings on to a mild steel substrate using identical spraying parameters. Coated samples were then heat treated in a nitrogen atmosphere at 500 and 700°C. The effect of heat treatment on changes in hardness and wear performance of the coatings was studied using microstructural analysis, micro-hardness indentation and abrasive wear tests. The results showed that the heat treatment increased the hardness of both coatings and a corresponding increase in wear resistance was recorded. The formation of a brittle CrWO4 phase in the microstructured coating resulted in brittle fracture of the coating and this gave lower wear resistance compared to the nanostructured coatings.

High Temperature Behavior of Cr3C2-NiCr Coatings in the Actual Coal-Fired Boiler Environment

Erosion-corrosion is a serious problem observed in steam-powered electricity generation plants, and industrial waste incinerators. In the present study, four compositions of Cr3C2-(Ni-20Cr) alloy coating powder were deposited by high-velocity oxy-fuel spray technique on T-91 boiler tube steel. The cyclic studies were performed in a coal-fired boiler at 1123 K ± 10 K (850 °C ± 10 °C). X-ray diffraction, scanning electron microscopy/energy dispersive X-ray analysis and elemental mapping analysis techniques were used to analyze the corrosion products. All the coatings deposited on T-91 boiler tube steel imparted hot corrosion resistance. The 65 pctCr3C2 -35 pct (Ni-20Cr)-coated T-91 steel sample performed better than all other coated samples in the given environment.

Protection of SAE 213 T-22 Boiler Steel Tube Material with Various Power Coatings using HVOF Spray Technique

Corrosion is an unintentional gradual degradation of metal that occurs because of chemical or electrochemical attach. The corrosive nature of the gaseous environment at high temperature may cause rapid material degradation and result in premature failure of components. Boiler tubes in power plant are subjected to a wide variety of failure due to high temperature corrosion fatigue. For the reduction of corrosion either a material of required mechanical properties is used, which is impossible for a single material to have all these properties or provide the coating to existing material with surface coating methods, which improves the existing properties in economical way and reduce the cost of replacement. In this paper an attempt has been made by conducting the experimental work on SAE213 T-22 boiler steel tubes with different coating powders having composition of WC-10% CO-4% Cr and WC-12% Cr by adopting the High Velocity Oxy Fuel (HVOF) thermal spray technique to protect the boiler tubes from hot corrosion in gaseous environment and do the various calculations to made the comparison between uncoated and coated tube materials, by creating the artificial environment in the lab.

Fabrication and Characterization of Aluminium Matrix Composites by High Velocity Oxy-Fuel Thermal Spraying

The High Velocity Oxy-fuel (HVOF) spray technique has been used by many researchers to deposit composite coatings, but there is less available literature discussing the fabrication of bulk metal matrix composites by this technique. In the current study, aluminum matrix composites with dispersions of alumina (Al2O3) and silicon carbide (SiC) particulates are fabricated by HVOF spray technique. The study comprises the steps of providing the substrate, preparing a mixed powder comprising a first metal powder as a matrix and a second metal powder comprising a ceramic and selected as reinforcement. The detailed procedures to prepare the bulk by using the same are reported. An in-depth characterization of the composite formed has been performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Hardness of the composite formed is measured on Vickers microhardness tester. This study showed the possibility of fabricating bulk metal matrix composites of larger size by HVOF spray technique.

Microstructural Characterization and Properties Evaluation of Ni-Based Hardfaced Coating on AISI 304 Stainless Steel by High Velocity Oxyfuel Coating Technique

The present study concerns a detailed investigation of microstructural evolution of nickel based hardfaced coating on AISI 304 stainless steel by high velocity oxy-fuel (HVOF) deposition technique. The work has also been extended to study the effect of coating on microhardness, wear resistance and corrosion resistance of the surface. Deposition has been conducted on sand blasted AISI 304 stainless steel by HVOF spraying technique using nickel (Ni)-based alloy [Ni: 68.4 wt pct, chromium (Cr): 17 wt pct, boron (B): 3.9 wt pct, silicon (Si): 4.9 wt pct and iron (Fe): 5.8 wt pct] of particle size 45 to 60 μm as precursor powder. Under the optimum process parameters, deposition leads to development of nano-borides (of chromium, Cr2B and nickel, Ni3B) dispersion in metastable and partly amorphous gamma nickel (γ-Ni) matrix. The microhardness of the coating was significantly enhanced to 935 VHN as compared to 215 VHN of as-received substrate due to dispersion of nano-borides in grain refined and partly amorphous nickel matrix. Wear resistance property under fretting wear condition against WC indenter was improved in as-deposited layer (wear rate of 4.65 × 10−7 mm3/mm) as compared to as-received substrate (wear rate of 20.81 × 10−7 mm3/mm). The corrosion resistance property in a 3.56 wt pct NaCl solution was also improved.

High temperature oxidation studies on HVOF sprayed NiCrAl coatings on superalloys

High velocity oxyfuel (HVOF) spray coating technology is widely used for depositing wide range of powders on the surface of the materials which require protection in the high temperature environment, without significantly heating the substrate materials. HVOF spray technique has been used to deposit NiCrAl coating on the Ni and Fe based superalloys in the present work. The coating is expected to form protective oxidation resistant scales on the superalloys when it is exposed to corrosive environment as manifested in aero and land based turbine engines. Oxidation kinetics was established for both bare and NiCrAl coated superalloys at 900°C for 100 cycles by thermogravimetric analysis to assess the protective nature of the latter. The scales formed on the coated and bare superalloys, upon subjected to air oxidation at high temperature, were analysed using X-ray diffraction (XRD) and field emission scanning electron microscope attached with energy dispersive spectrometer (FESEM/EDS) to elucidate the oxidation mechanisms. The coating was found to be effective in enhancing the oxidation resistance of all the superalloys at 900°C under cyclic conditions, due to the formation of protective oxide scales, as observed in the present work. The EDS analysis of the oxide scales revealed the presence of NiO, Cr2O3 and Al2O3.

The Corrosion and Wear Performance of Microcrystalline WC-10Co-4Cr and Near-Nanocrystalline WC-17Co High Velocity Oxy-Fuel Sprayed Coatings on Steel Substrate

The study of near-nanocrystalline cermet composite coating was performed by depositing near-nanocrystalline WC-17Co powder using the high velocity oxy-fuel spraying technique. The WC-17Co powder consists of a core with an engineered near-nano-scale WC dispersion with a mean grain size 427 nm. The powder particle contains 6 wt pct of the ductile phase Co matrix mixed into the core to ensure that the reinforcing ceramic phase WC material is discontinuous to limit debridement during wear, while the remainder of the binding phase (11 wt pct) is applied as a coating on the powder particle to improve the ductility. The tribological properties of the coating, in terms of corrosion resistance, microhardness, and sliding abrasive wear, were studied and compared with those of an industrially standard microcrystalline WC-10Co-4Cr coating with a WC mean grain size 3 μm. Results indicated that the WC-17Co coating had superior wear and corrosion resistance compared to the WC-10Co-4Cr coating. The engineered WC-17Co powder with a duplex Co layer had prevented significant decarburization of the WC dispersion in the coating, thereby reducing the intersplat microporosity necessary for initiating microgalvanic cells. The improved wear resistance was attributed to the higher hardness value of the near-nanocrystalline WC-17Co coating.

Application of EDS microanalysis in the identification of inhomogeneities in surface protective layers on ductile cast iron parts

In this study, the results of the application of both scanning electron microscopy and EDS microanalysis in investigations of the process of the protective layer formation on the surface of ductile cast iron (the substrate material) turbine blades are presented. The turbine blades, designated for operation in an aggressive environment, were coated with protective chromium carbide layers using plasma spraying methods. The first turbine blade was coated using a standard plasma spraying technique, followed by superficial remelting treatment, while the second one was coated using a modified plasma technology, i.e., HVOF (High Velocity Oxy Fuel) spraying technique.

Abrasive wear behavior of thermally sprayed diamond reinforced composite coating deposited with both oxy-acetylene and HVOF techniques

Diamond and diamond-based coatings have long been studied for their exceptional properties. Although a great deal of research has been carried out in this field, little is known about their tribological wear behavior. In the present work, diamond reinforced composite (DRC) coatings of varying diamond content was deposited on mild steel substrates using both oxy-acetylene (OA) and high velocity oxy fuel (HVOF) thermal spraying techniques. The high stress abrasive wear behavior of these coatings is studied by performing two body abrasion tests for varying experimental parameters. It is observed that the HVOF-sprayed coatings suffered abrasion at a relatively low wear rate. The reasons for variations observed in the wear rate as a function of displacement during abrasion and grit size could be attributed to the deterioration of abrasive particles and the particle size effect respectively. While the disparity in the wear rates with respect to composition of the coatings was primarily controlled by the diamond content in the coating. The abrasive wear mechanism was found to be the same in both the coatings except that the coating deposited by HVOF spray technique, offered better abrasion resistance and therefore abraded at a slower rate. This is possibly due to lower porosity in the coating and higher bond strength between reinforced diamond particulates and the bronze matrix in HVOF-sprayed specimens.

Preparation of pore gradient silicon nitride ceramics by a high-velocity oxy-fuel spraying technique

The preparation of pore gradient silicon nitride (Si3N4) ceramics was investigated by thermally spraying dense Si3N4 coatings onto the porous Si3N4 ceramic substrates via a high-velocity oxyfuel (HVOF) spraying method. Spray powders with excellent processability were developed and produced by spray drying and sintering in a nitrogen atmosphere. After sieving, the powders with average particle size of 45-75 μm were selected. Optimization of spray parameters such as spray distance and hydrogen flow rate was carried out in order to find the most decisive factor necessary for the production of dense and well-adhering coatings and pore gradient stuctures. Zirconium phosphate bonded Si3N4 porous ceramics were used as the substrate and coatings were obtained only when spray distance was 200 mm. A pore gradient structure was obtained at lower H2 flow rate, while poor-adhering coatings were obtained at higher H2 flow rate due to the etching. The hardness is significantly improved after Si3N4 coatings were successfully applied.

Sliding wear performance of high-velocity oxy-fuel spray Al2O3/TiO2 and Cr2O3 coatings

High-velocity oxy-fuel (HVOF) spray ceramic oxide coatings have immense potential in industrial applications. However, they are not widely used yet due to the lack of an adequate scientific database created after testing these coatings for specific industrial applications. Two such ceramic coating powders, Al2O3+(40%)TiO2 and Cr2O3, were deposited on AISI 309 SS stainless steel by the HVOF spray technique, in order to enhance its wear resistance. This stainless steel is used in many components of thermal power plants in India, where it suffers one or more types of wear. The as-sprayed coatings were characterized by XRD and SEM analyses. Subsequently, the sliding wear behaviours of the uncoated, HVOF spray Al2O3+(40%)TiO2 and Cr2O3 coated AISI 309 SS were investigated according to ASTM standard G99-03 on a pin-on-disc wear test rig. Cumulative wear rate and coefficient of friction (μ) were calculated for the coated as well as the uncoated specimens for 30, 50, and 70 N normal loads at a constant sliding velocity of 1 m/s. Some of the worn-out surfaces were characterized by SEM analysis. Both the as-sprayed coatings exhibited typical splat morphology of a thermal spray process. The XRD analysis indicated the formation of Al2O3 and TiO2 phases for the Al2O3+(40%)TiO2 coating, and Cr2O3 phase for the Cr2O3 coating. It has been concluded that HVOF spray Al2O3+(40%)TiO2 and Cr2O3 coatings can be useful in minimizing the wear problem of AISI 309 SS. These coatings were found to be successful in retaining their surface contact with the substrate after the wear tests. The HVOF spray Cr2O3 coating can be recommended as a slightly better choice to reduce the wear of AISI 309 SS in comparison with the Al2O3+(40%)TiO2 coating.

A study of high-speed slurry erosion of NiCrBSi thermal-sprayed coating

The test specimens of NiCrBSi coating were prepared using the high velocity oxy-fuel spraying technique with a post-thermal treatment. Their high-speed slurry erosion characteristics have been systematically studied. SUS304 stainless steel was selected as the comparison material. Experimental results show that the NiCrBSi sprayed coating exhibits a much better slurry erosion resistance than the SUS304 stainless steel. The erosion rate for the NiCrBSi sprayed coating slightly increases with the impinged angle. However, a maximum erosion rate appears at an impinged angle of around 30° for SUS304 stainless steel. Both NiCrBSi sprayed coating and SUS304 stainless steel exhibit impinged surfaces with lots of furrows at an impinged angle of 30°. At a high impinged angle of 90°, the SUS304 stainless steel exhibits an impinged surface with lots of overlapping and irregular concavities, while, these features are less obvious for NiCrBSi sprayed coating. The hardness of SUS304 stainless steel increases significantly with increasing impinged angle during the high-speed slurry erosion. But, there is no obvious work hardening for the NiCrBSi sprayed coating due to its extra-high hardness and less plastic deformation, even at a high impinged angle of 90°.

Influence of Cr alloying on the microstructure of thermally sprayed quasicrystalline Al–Cu–Fe coatings

The present work reports the structural development of Al–Cu–Fe and Al–Cu–Fe–Cr coatings deposited by the high velocity oxy-fuel thermal spraying process and the influence of Cr alloying on the phase selection of Al–Cu–Fe coatings at various deposition temperatures. The porosity levels of the Al–Cu–Fe and Al–Cu–Fe–Cr coatings of the study are demonstrated to be lower than those reported for corresponding plasma-sprayed coatings. The results show that high velocity oxy-fuel spraying technique produces Al–Cu–Fe coatings that are phase structurally similar to plasma-sprayed Al–Cu–Fe coatings reported in literature. Al–Cu–Fe coatings are composed of a crystalline β-AlFe phase and a quasicrystalline i-Al 65Cu 20Fe 15 phase as well as an oxidised form of either or both of these phases. Addition of Cr to Al–Cu–Fe alloys introduces coatings that are made up of the crystalline θ-Al 2Cu phase and two quasicrystalline phases, the i 1-Al 80Cr 13.5Fe 6.5 and i 2-Al 13Cr 3Cu 4 phases. The formation of these icosahedral phases in Al–Cu–Fe–Cr alloys has not been reported before, although the occurrence of quasicrystal approximants with compositions close to those of the i 1-Al 80Cr 13.5Fe 6.5 and i 2-Al 13Cr 3Cu 4 phases has been demonstrated. On the basis of our results we propose that the icosahedral phase structure is greatly stabilised by the Cr addition to Al–Cu–Fe alloys.