High Velocity Oxy-fuel Spraying Research Articles

Comparison of microstructure and adhesion strength of plasma, flame and high velocity oxy-fuel sprayed coatings from an iron aluminide powder

In this study an iron aluminide powder with the feedstock powder composition of Fe-28Al-5Cr and a particle size of 45μm to 75μm was thermally sprayed onto AlSi10Mg and AlSi12CuNiMg substrates by flame spray (FS), atmospheric plasma spray (APS) and high velocity oxy-fuel spray (HVOF) processes. The combination of the utilized materials is due to lightweight design and is, therefore, different from most of the previous studies, which dealt with the application of iron aluminide coatings onto steels. Coatings were analyzed in terms of microstructural investigations using SEM coupled with EDX measurements in the as sprayed condition and after a heat treatment of 100h at 500°C in argon atmosphere. Phase analysis was performed by XRD measurements in the as sprayed and heat treated condition. The FS and APS coatings contained different amounts of a bcc solid solution α(Fe, Al, Cr), FeO, Fe3O4, FeAl2O4 and γ-Al2O3. It was found that the FeO rearranged to bcc Fe and Fe3O4 during this heat treatment. The HVOF coating retained 90% of feedstock powder material and a low fraction of oxide and Al-depleted phases. The microhardness was determined to be 277HV0.4 (FS), 394HV0.4 (APS) and 479HV0.4 (HVOF) which was associated to the different constituting phases. Adhesion strength was measured using the tensile adhesion test (TAT) and the achieved adhesion was 13.8MPa (APS), 30.3MPa (FS) and exceeded 58MPa (HVOF).

Effect of Feedstock Size on the Properties of WC-17Co Coatings

Abstract High velocity oxy-fuel (HVOF) spray technique has been employed to fabricate WC-17Co coatings. To achieve a high performance of mechanical properties and abrasive wear resistance, four types of WC-17Co powders with different feedstock particle sizes were investigated. The results show that the feedstock powder with finer particle size has higher velocity and temperature during the spraying process, resulting in a denser coating and a stronger splat-splat bonding. The micro-hardness of WC-17Co coating increases with the decreasing of feedstock size. The coating deposited by finer feedstock powder has superior abrasive wear resistance, due to the smaller porosity and boundary flaws on coating surface. However, coating fracture toughness could be affected by feedstock size if it is too small. Among all those investigated coatings, the one with medium particle size and narrow size distribution exhibits both excellent mechanical properties and wear resistance.

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.

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.

Effect of amorphicity of HVOF sprayed Fe-based coatings on their corrosion performances and contacting osteoblast behavior

Fe53Cr19Zr7Mo2C18Si coatings with amorphous/nanocrystalline hybrid structures were fabricated by high velocity oxy-fuel spray. Enhanced amorphicity of the coatings, 55.85%, has been achieved by the spray quenching of the starting feedstock with an amorphicity of 36.82%. Further examination of the coating after post-spray crystallization annealing at 750°C was also conducted. Microstructure, microhardness, wear/corrosion resistance and biocompatibility of the coatings were systematically studied. Results show that the nanocrystallization treatment brought about increased microhardness and enhanced anti-wear performances. However, electrochemical and in vitro cell culture testing suggest that the full crystalline structure does not favor the anti-corrosion and biological performances while presence of the amorphous structure enhances corrosion resistance of the coatings and promotes attachment and proliferation of osteoblast cells on their surfaces. The results shed some light on developing amorphous metallic coatings for potential biomedical applications.

Microstructure and Sliding Wear Performance of Cr7C3-(Ni,Cr)3(Al,Cr) Coating Deposited from Cr7C3 In Situ Formed Atomized Powder

This work is aimed at developing a new type of Cr7C3-(Ni,Cr)3(Al,Cr) coating for parts used in heavy-duty diesel engines. The feedstock, in which the stripe-shaped Cr7C3 was in situ formed, was firstly prepared by vacuum melting and gas atomization and then subjected by high-velocity oxy-fuel spraying to form the coatings. The carbon content, microstructure and phase constitution of the powders, as well as the sprayed coatings, were analyzed by chemical analysis, SEM and XRD. The hardness and sliding wear performance of the sprayed coatings were also tested and compared to a commercial Cr3C2-NiCr coating used on piston rings. The results showed that the content of carbon in feedstock was almost the same as designed, and that the volume content of in situ formed Cr7C3 was increased with carbon and chromium added. The major phases of the powders and sprayed coatings are Cr7C3 and Cr-alloyed Ni3Al. Only a small amount of carbon lost during the spraying process. As Cr7C3 content increased in the coatings, the microhardness at room temperature was firstly increased to about 1000Hv0.3. The microhardness of the coatings stayed almost constant, while the testing temperature was raised up to 700 °C for 0.5 h, which illustrates the potential application of the investigated coatings under high temperature conditions. The coatings containing 70 and 77 vol.% Cr7C3 showed the most promising wear resistance, lower friction coefficient and better tribological compatibility to gray cast iron counterpart than other tested Cr7C3-(Ni,Cr)3(Al,Cr) coatings and the reference Cr3C2-NiCr coating.

A study of processing and slurry erosion behaviour of multi-walled carbon nanotubes modified HVOF sprayed nano-WC-10Co-4Cr coating

Abstract In the present research work, multi-walled carbon nanotubes (MWCNTs) were mixed with WC-10Co-4Cr nanostructured feedstock powder in order to study its effect on the slurry erosion behaviour of as-sprayed coating. The deposition of coatings was performed by using a high velocity oxy-fuel (HVOF) spray gun (Praxair TAFA JP-5000). Jet-type slurry erosion testing was performed to study the erosion performance and behaviour of as-sprayed and CNTs modified nano-WC-CoCr coatings. Important coating properties such as porosity, micro-hardness, surface roughness and indentation fracture toughness have been measured. The surface morphology of thermal spray powders, deposited coatings and worn-out surfaces has been studied by using scanning electron microscopy (SEM) images. The worn-out surfaces revealed that the material was removed by micro-cutting and ploughing of CoCr matrix, leading to the loosening and pull-out of WC grains in case of 30° impingement angle erosion testing. The 90° impingement angle erosion testing resulted in fracturing of WC grains followed by eruption of binder matrix by interlinking of cracks originated from fractured WC grains. The introduction of MWCNTs improved the fracture toughness of nano-WC-CoCr coating by ‘splat-bridging’ feature. The introduction of MWCNTs favoured the plastic deformation around the bridging by suppressing the brittle fracture of coating material. The increase in fracture toughness resulted in enhancement of erosion resistance of MWCNTs modified nano-WC-CoCr coating.

Suspension High Velocity Oxy-Fuel (SHVOF)-Sprayed Alumina Coatings: Microstructure, Nanoindentation and Wear

Suspension high velocity oxy-fuel spraying can be used to produce thermally sprayed coatings from powdered feedstocks too small to be processed by mechanical feeders, allowing formation of nanostructured coatings with improved density and mechanical properties. Here, alumina coatings were produced from submicron-sized feedstock in aqueous suspension, using two flame combustion parameters yielding contrasting microstructures. Both coatings were tested in dry sliding wear conditions with an alumina counterbody. The coating processed with high combustion power of 101 kW contained 74 wt.% amorphous phase and 26 wt.% crystalline phase (95 wt.% gamma and 3 wt.% alpha alumina), while the 72-kW coating contained lower 58 wt.% amorphous phase and 42 wt.% crystalline phases (73 wt.% was alpha and 26 wt.% gamma). The 101-kW coating had a dry sliding specific wear rate between 4 and 4.5 × 10−5 mm3/Nm, 2 orders of magnitude higher than the 72-kW coating wear rate of 2-4.2 × 10−7 mm3/Nm. A severe wear regime dominated by brittle fracture and grain pullout of the coating was responsible for the wear of the 101-kW coating, explained by mean fracture toughness three times lower than the 72-kW coating, owing to the almost complete absence of alpha alumina.

Hot corrosion behavior of NiCoCrAlYTa coating deposited on Inconel alloy substrate by high velocity oxy-fuel spraying upon exposure to molten V2O5-containing salts

The corrosion behavior of the HVOF-sprayed NiCoCrAlYTa coating upon exposure to molten V2O5 and molten mixture of 50wt.%V2O5+50wt.% Na2SO4 at 800–1000°C was discussed. The hardness, elastic modulus and contact stiffness of the coating were determined by nano-indentation test. Results indicate that the coating exhibits good corrosion resistance, which is ascribed to the formation of Al and Cr oxide protective film. The generation of hard phases Al2O3 and Cr2O3 with loose structure and Laves phase Cr2Ta lead to the increase of the hardness and elastic modulus of the corroded coatings and the decrease of contact stiffness.

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.

Combined slurry and cavitation erosion resistance of HVOF spray coated SS 410 steel

The hydro turbine materials surface is degraded due to the slurry erosion and cavitation. The solid particles carried by water impacting the material results in slurry erosion. The damage occurred due to slurry erosion is the concern, when considered individually. The erosion damage is observed to be severe when slurry erosion and cavitation are combined. The hydro turbine material, martensitic stainless (SS 410) is surface modified with 80Ni-Cr by High Velocity Oxy Fuel spray process. The coated material subjected to post thermal treatment at a temperature of 950 ° C, soaked at 1 h, 2 h and 3 h are subjected to combined slurry and cavitation erosion test. The cavitation is created by using Cavitation Inducers. The tests are conducted by using silica sand as the erodent with three different sizes of 150, 200 and 300 μm. The results are compared with the as-received specimen. The results confirmed the effect of heat treatment on the end results, as the coated thermal treated specimens showed better erosion resistance against the as-received specimen. The eroded specimens are characterized by Scanning Electron Microscope. The thermal treated HVOF coated specimens shown the better erosion resistance.

Sliding Wear Behavior of High Velocity Oxy-Fuel Sprayed WC-CO Coatings

The Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface and the feedstock is heated by electrical or chemical means. High velocity oxy-fuel (HVOF) spraying is seen as the pre-eminent process for deposition of such coatings. The coating that is formed is not homogenous and typically contains a certain degree of porosity, and, in the case of sprayed metals, the coating will contain oxides of the metal. This paper examines the effect of WC–Co coatings on the wear behaviour, WC were thermally sprayed by a high-velocity oxy-fuel (HVOF) system from WC–12% Co powder. Sliding wear resistance of this coating was calculated using pin-on-disk tribometer (ASTM G99-90). The morphologies of WC-Co coating were observed by X-ray diffraction (XRD), Energy dispersive analysis and Scanning electron microscope (SEM). Micro hardness is calculated by using Vickers Hardness test. And it was found that WC-Co coatings show better wear resistant properties.

Suspension high velocity oxy-fuel spraying of TiO2: A quantitative approach to phase composition

A range of coatings from a water based suspension of anatase has been prepared by suspension high velocity oxy-fuel spraying with the aim to study effects of heat power of the flame on phase composition, microstructure and surface topography. Three most commonly used approaches of quantitative phase analysis have been scrutinized with respect to their applicability and as some of the coatings showed presence of preferred orientation and it was argued that quantitative Rietveld refinement is the most accurate method for phase composition determination. Coatings had a layered duplex anatase/rutile microstructure with fraction of rutile increasing exponentially with heat power. Spraying at the lower heat power led to a lower surface roughness and higher power resulted in surfaces with pronounced humps, which were distributed homogeneously on the surface. The emergence of humps is related to an increase in macroscopic surface area of up to 30% with respect to the flat coating.

Impact of processing conditions and feedstock characteristics on thermally sprayed MCrAlY bondcoat properties

One of the options to manufacture MCrAlY bondcoats (M=Co, Ni) for thermal barrier coating systems is High Velocity Oxy-Fuel spraying (HVOF). In this work, particle diagnostics were applied to investigate the impact of processing conditions and feedstock characteristics on the relevant bondcoat properties. The results showed that compromises must be made on the oxygen/fuel ratio, spray distance, and particle size distribution to strike a balance between low oxidation and dense microstructures.These limitations initiated the development of the High Velocity Atmospheric Plasma Spray process (HV-APS) as a further alternative process. In this work, HV-APS process parameters were developed for a three cathode torch in combination with a 5mm diameter high speed nozzle. A one-dimensional calculation of the expansion through this nozzle to atmospheric pressure yielded supersonic conditions with a Mach number of 1.84. The calculated plasma temperatures at the nozzle exit and in the expanded jet are 8400K and slightly above 5200K, respectively, which is low compared to conventional APS processes.A very fine powder with a median particle size of 18μm was identified to be most suitable. Although the spray conditions were relatively cold, reasonable deposition efficiencies up to 61% and rather dense coatings were achieved using this feedstock. The as-sprayed porosity was ≈2% which was reduced by the subsequent vacuum heat treatment to <1%. The oxygen content determined by chemical analysis for a sample sprayed at a spray distance of 100mm was 0.41±0.04wt%.Moreover, reference samples were manufactured by Low Pressure Plasma Spraying (LPPS). The oxidation behavior was compared in isothermal and cyclic oxidation tests. The oxidation rates of the HV-APS coatings were found to be significantly lower than those of LPPS coatings. The thermally grown oxide scale showed less yttrium incorporation and better adherence in case of HV-APS. The latter is suggested to be related to a unique new distribution of Y-rich nano-sized oxide precipitates. The cyclic oxidation test confirmed the better oxidation resistance of the HV-APS coatings.

Identifying Indicators of Progress in Thermal Spray Research Using Bibliometrics Analysis

We investigated the research publications on thermal spray in the period of 1985-2015 using the data from Web of Science, Scopus and SciVal®. Bibliometrics analysis was employed to elucidate the country and institution distribution in various thermal spray research areas and to characterize the trends of topic change and technology progress. Results show that China, USA, Japan, Germany, India and France were the top countries in thermal spray research, and Xi’an Jiaotong University, Universite de Technologie Belfort–Montbeliard, Shanghai Institute of Ceramics, ETH Zurich, National Research Council of Canada, University of Limoges were among the top institutions that had high scholarly research output during 2005-2015. The terms of the titles, keywords and abstracts of the publications were analyzed by the Latent Dirichlet Allocation model and visually mapped using the VOSviewer software to reveal the progress of thermal spray technology. It is found that thermal barrier coating was consistently the main research area in thermal spray, and high-velocity oxy-fuel spray and cold spray developed rapidly in the last 10 years.

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 ℃.

Microstructure Evolution and Its Effect on the Wear Performance of HVOF-Sprayed Conventional WC-Co Coating

In this work, a conventional tungsten carbide 12% cobalt (WC-12Co) coating was deposited by using a liquid fuel JP-8000 high velocity oxyfuel spray system. The properties of the coating namely phase content, microstructure, hardness, porosity, and fracture toughness were examined. The microstructure evolution and its influence on the abrasive wear behavior of the coatings were evaluated in detail by in-situ scanning electron microscopy and a comprehensive model for decarburization of WC has been established using x-ray diffraction and transmission electron microscopy analyses.

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.

Fabrication and properties of NiCr/CNTs nanocomposite coatings prepared by High Velocity Oxy-Fuel Spraying

Abstract Nickel-Chromium (NiCr)/CNTs composite coatings were deposited onto mild steel substrates by High Velocity Oxy–Fuel Spraying (HVOF). The composite powder feedstock was prepared by: (1) mixing by ball milling NiCr powder with 0, 0.5, 1 wt% CNTs for 48 h: (2) growing carbon nanotubes on NiCr powder using chemical vapor deposition (CVD) technique at 650 °C for 60 min under ethanol atmosphere. The microstructure of NiCr-CNTs powder was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Chemical compositions of the different powders were studied using X-ray Diffractometry (XRD) and Energy Dispersive X-ray spectroscopy (EDX) techniques. Porosity and Vickers hardness of the coatings were also measured. Sliding wear tests were carried out using a pin-on-disk apparatus. Dense coatings which contained mainly Nickel phase could be achieved. Thickness of the coatings was about 300–350 μm with a similar % porosity of 0.49–1.01 vol%. Vickers hardness tests show that the hardness of Nickel-Chromium/CVDCNTs was 20% higher than that of a pure Nickel-Chromium coating leading to a better wear resistance. Polarization test showed that carbon nanotubes could improve the corrosion resistance of the coating. The main objectives of this study are to prepare the NiCr/CNTs composite feedstock powders and also to evaluate the properties of HVOF coatings from those powders.