Flame-sprayed Coatings Research Articles

Effects of Laser Remelting on Frictional Properties of Supersonic Flame-Sprayed Coatings

In this study, Cr3C2-Al2O3-NiCr coatings were prepared on INCONEL 600 alloy surfaces using the supersonic flame spraying technique, followed by a laser remelting treatment. In this way, this study further explored what impacts laser remelting has on coating performance. To this end, optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to carry out microstructural characterization. Energy-dispersive X-ray spectroscopy (EDS) was applied to conduct an analysis of the coatings’ elemental distribution while X-ray diffraction (XRD) was used to determine the coating phases. To measure the microhardness of the coatings, a microhardness tester was applied. In addition, the study investigated the samples’ electrochemical corrosion resistance and friction-wear performance under different surface conditions. According to the results, laser remelting enhanced the coating density, improved metallurgical bonding with the substrate, and optimized the carbide distribution, thereby enhancing corrosion and wear resistance in both air and corrosive media. However, excessive laser power hinders Cr3C2 nucleation, leading to diminished coating hardness and wear resistance in Cr7C3 formation.

Robot-aided tailoring of flame spray coatings for high-temperature barrier application

The flame spraying of refractory ceramics such as alumina is a promising technique for the modification of manifold substrate materials for high-temperature application. Its high potential for automation and feedstock design allows for swift adaption to variable demands. Consequentially, a tailoring of flame-sprayed barrier coatings can be realised in terms of the phase composition and the mechanical performance. In order to isolate the effects of the addition of zirconia and titania, as well as the coating thickness, the present study focused the biaxial flexure of self-supporting flame-sprayed alumina bodies, produced by a fully automated and robot-aided flame spray station (on the lab-scale).

Enhanced wear resistance of AlCoCrFeMo high entropy coatings (HECs) through various thermal spray techniques

The remarkable mechanical and wear properties of high entropy alloys (HEAs) have opened up exciting possibilities for their use in aerospace applications. This study explores the potential of AlCoCrFeMo high entropy coatings (HECs) fabricated using three thermal spray techniques, namely low-pressure cold spray (LPCS), flame spray (FS), and high-velocity oxy fuel spray (HVOF) as wear resistant surfaces. The coatings were investigated in terms of their microstructures, phases, mechanical properties, and sliding wear characteristics from room temperature up to 350 °C. Ex-situ characterization was performed using XRD for phase analysis and SEM-EDS for cross-sectional microscopy and phase compositions of the coatings. All three coatings exhibited a typical lamellar structure with the formation of BCC solid solution phase and variations in porosity and oxide content. The HVOF sprayed coatings showed higher hardness values compared to the FS and LPCS coatings, which can be attributed to their fine splats microstructure, lower porosity, and oxide inclusions compared to the other two coatings. In terms of tribological performance, the LPCS coatings exhibited overall lower frictional coefficient than the FS and HVOF coatings at both tested temperatures. However, the HVOF coatings exhibited relatively lower wear rates, correlating well with observations from ex-situ analysis, highlighting lower material transfer and decreased formation of debris particles compared to LPCS and FS coatings. These findings suggest that the HEA materials have great potential as next-generation tribological interfaces under high-temperature wear conditions, emphasizing the importance of designing materials with improved microstructural features.

Formation, Transformation, and Electrical Performance of Magnéli Phases Obtained by Flame Spraying from TiO2 Particles

In this paper, the formation and transformation of Magnéli phases (TinO2n-1) during manufacturing of flame-sprayed coatings from two powders consisting of anatase nanoparticles and submicrometric particles with Magnéli phases is analyzed, respectively. These powders were thermally sprayed on a ceramic substrate through oxidizing and neutral flame. Crystalline phases in powders and coatings were identified by x-ray diffraction and quantified by the Rietveld method, verifying the presence of identified crystalline phases in coatings by x-ray photoelectron spectroscopy. Finally, the electrical conductance of coatings was measured by the four-point probe method. Results indicate that Magnéli phases are produced from anatase nanoparticles sprayed using both the oxidizing and neutral flames, and when the neutral flame and finest particles are used, a high quantity of Ti4O7 and Ti5O9, recognized thanks to their high electrical conductivity, is obtained. In contrast, when the coatings were fabricated from the powder containing Magnéli phases, they were oxidized as much in oxidizing as in neutral flame, producing a partial transformation from Ti4O7 and Ti5O9 to both the rutile phase and TiO in low quantity. In spite of this partial transformation, the coatings maintain high electrical conductivity thanks to Ti4O7 and Ti5O9 Magnéli phases remaining and the TiO produced.

Effect of Feedstock Powder Intrinsic Characteristics on the Tribological Behavior of Inductively Remelted NiCrBSi Flame-Sprayed Coatings

Ni-based alloys are among the materials of choice in developing high-quality coatings for ambient and high-temperature applications that require protection against intense wear and corrosion. The current study aims to develop and characterize NiCrBSi coatings with high wear resistance and improved adhesion to the substrate. Starting with nickel-based feedstock powders, thermally sprayed coatings were initially fabricated. Prior to deposition, the powders were characterized in terms of microstructure, particle size, chemical composition, flowability, and density. For comparison, three types of powders with different chemical compositions and characteristics were deposited onto a 1.7227 tempered steel substrate using oxyacetylene flame spraying, and subsequently, the coatings were inductively remelted. Ball-on-disc sliding wear testing was chosen to investigate the tribological properties of both the as-sprayed and induction-remelted coatings. The results reveal that, in the case of as-sprayed coatings, the main wear mechanisms were abrasive, independent of powder chemical composition, and correlated with intense wear losses due to the poor intersplat cohesion typical of flame-sprayed coatings. The remelting treatment improved the performance of the coatings in terms of wear compared to that of the as-sprayed ones, and the density and lower porosity achieved during the induction post-treatment had a significant positive role in this behavior.

Considerations on the Wear Behavior of Vacuum-Remelted ZrO2-Reinforced Self-Fluxing Ni-Based Thermally Sprayed Alloys.

Without proper post-processing (often using flame, furnace, laser remelting, and induction) or reinforcements' addition, Ni-based flame-sprayed coatings generally manifest moderate adhesion to the substrate, high porosity, unmelted particles, undesirable oxides, or weak wear resistance and mechanical properties. The current research aimed to investigate the addition of ZrO2 as reinforcement to the self-fluxing alloy coatings. Mechanically mixed NiCrBSi-ZrO2 powders were thermally sprayed onto an industrially relevant high-grade steel. After thermal spraying, the samples were differently post-processed with a flame gun and with a vacuum furnace, respectively. Scanning electron microscopy showed a porosity reduction for the vacuum-heat-treated samples compared to that of the flame-post-processed ones. X-ray diffraction measurements showed differences in the main peaks of the patterns for the thermal processed samples compared to the as-sprayed ones, these having a direct influence on the mechanical behavior of the coatings. Although a slight microhardness decrease was observed in the case of vacuum-remelted samples, the overall low porosity and the phase differences helped the coating to perform better during wear-resistance testing, realized using a ball-on-disk arrangement, compared to the as-sprayed reference samples.

Sliding wear behavior of high entropy alloy coatings deposited through cold spraying and flame spraying: A comparative assessment

High entropy alloys (HEAs) are a promising class of advanced materials composed of multi-component elements that possess outstanding mechanical and corrosion properties, which enable their use in many industrial applications. However, limited studies have focused on the tribological behavior of HEAs fabricated by means of thermal spray technologies. In this study, AlCoCrFeMo equimolar HEAs were fabricated successfully using flame spraying and cold spraying techniques to determine the phase compositions, microstructures, properties, and sliding wear resistance. The results show the existence of oxides in the flame-sprayed HEA coatings due to in-flight oxidation and body-centered cubic (BCC) phases. Oxide evolutions improve the hardness of flame-sprayed HEA coatings compared to that of cold-sprayed coatings. The sliding performance was investigated at room temperature by varying the normal loads (i.e., 5 N, 10 N) and with constant sliding speed (i.e., 3.1 cm/s) using Al2O3 counterface. The cold-sprayed coatings showed overall lower coefficient of friction (i.e., ∼0.5) with numerous fluctuations compared to the flame-sprayed coatings. However, the flame-sprayed coatings showed improved wear resistance when compared to HEA deposited by means of cold spray. This correlated well with the observations of ex situ analysis, which showed greater material transfer and increased formation of debris on the cold-sprayed coatings compared to the flame-sprayed coatings. The results obtained in this study suggest that flame-sprayed HEA materials are promising candidates for next-generation tribological interfaces under specific conditions, pointing to new materials designed with improved microstructural features and enhanced mechanical properties.

Hybridized-flow flame spray (HFFS) process for the development of superior ceramic coatings

The present study deals with a novel, effective and single step thermal spray coating process development. The process is named as ‘Hybridized-Flow Flame Spray (HFFS) process’. The process is very much suitable for the development of ceramic coatings with superior properties when compared to conventional Powder Flame Spray (PFS) coatings. The HFFS process is capable in addressing the shortcomings (porosity, density, hardness, surface quality) of PFS coatings. The physical, mechanical and metallurgical analysis of PFS and HFFS coatings are done using density measurement kit, 3-D optical profilometer, Vickers and scratch hardness, and scanning electron microscope (SEM). The results show significant improvement in the properties. The increment in density (≈24%), hardness (Vickers ≈ 37% and scratch ≈ 126%); and decrement in coating thickness (≈35%) and average surface roughness (≈47%) of HFFS sprayed coatings are observed. The results are benchmarked with those of PFS coatings. The improved responses are attributed to the densification of coating structure and improved microstructural changes due to the presence of hybridized flow in the HFFS.

Evaluation of metallic structures and material properties of thermal sprayed coatings using stainless-steel wires

ABSTRACT To evaluate the fundamental differences between the standard wire flame-sprayed coatings of stainless-steel wires, the differences in their characteristics are discussed from a metallurgical point of view. Austenitic (SUS304 and SUS316) and martensitic (SUS420J2) stainless steels were used in this study. The SUS420J2 and SUS304 sprayed coatings were composed of austenite and martensite, whereas the SUS316 sprayed coating was composed of only austenite. The dense thermally sprayed coating of SUS316 was due to oxygen distribution, which corresponds to marble-pattern oxidised structures. Additionally, an increase in the carbon content was observed after thermal spraying. However it is difficult to determine the distribution of carbon; therefore, it is considered that carbon distribution is supersaturated at a certain amount and carbon exists as a solid solute in the austenite phase. Finally, the increase in the carbon content is related to the progress of corrosion.

Flame spraying of Al/Fe3Al-Fe3AlCx composites powders obtained by vertical ball milling

Aluminum composite coatings deposited by flame spray with nanometric materials as reinforcement have been used to produce high-performance coatings. The addition of carbon to Fe3Al, forming carbides Fe3AlCx (phase κ), enhances the properties of Fe3Al and contributes toward the increase in the microhardness of the coating, thus justifying the use of intermetallic as reinforcement to the Al coating. This study aims to obtain mechanically cladded Al (micro)/Fe3Al-k (nano) powder by a vertical ball mill (VBM) with two different compositions of reinforcement (20% and 40%) for producing powder flame spray coatings. The effect of the VBM powder processing route on phase formation, powder morphology, and coating microhardness was evaluated. The characteristics of the powders were investigated through their being subjected to milling times of 5, 10, 15, 20, and 30 min in VBM. The processing time of 15 min was the most suitable for obtaining Al composites with 20% and 40% Fe3Al-κ with no significant changes in particle size, no solid-state reactions, and Al particles cladded with Fe3Al–κ. Deposits of Al/Fe3Al-κ composite by flame spray were reached without separating the Fe3Al-κ nanoparticle from the Al microparticle during flight while forming composite coatings with aluminides particles dispersed throughout the coating. The coating formed presented up to 600 μm of thickness and microhardness of 197 ± 47Hv for 20% and 158 ± 49Hv for 40%.

Use of Different Process Gases for Manufacturing Isolating Alumina Coatings by Flame Spraying with Cords

Besides conventional industrial demands, thermally sprayed coatings are increasingly used for innovative products. Such an application is the additive manufacturing of electrical components in automotive engineering. In particular, heating units are currently manufactured by a combination of various spray technologies. At present, simpler spraying processes like flame spraying are investigated with regard to their suitability as a future cost-effective alternative for fabricating isolating alumina coatings. In the present study, alumina cords were flame-sprayed using compressed air and argon as atomizing gases. The results demonstrate finely dispersed microstructures and a more regular and partially even higher surface and volume resistivity compared to past investigations in the literature as well as conventionally plasma-sprayed coatings despite a significantly reduced coating thickness. The content of alpha phase is clearly higher than for plasma-sprayed coatings, regardless of the atomizing gas used. Moreover, flame-sprayed coatings using argon reveal a higher resistivity in comparison to coatings sprayed with air. While the atomizing gas is found to mainly influence the ideal stand-off distance, the phase composition is not changed severely. In addition to the phase composition and kinematics, it can finally be concluded that humidity plays a major role in the coating properties.

Statistical Analysis of the Flexural Strength of Free-Standing Flame-Sprayed Alumina Coatings Prior and After Thermal Shock

The impact of thermal shock upon the scatter of flexural strength data of free-standing thin flame-sprayed Al2O3 coatings was examined. As published in the literature, thermally sprayed ceramic systems exhibit superior thermal shock resistance and against this background, the statistical evaluation of strength data prior and after the thermal shock was intensified. Thus, the Weibull parameters m and σ0 were compared before and after thermal shock from 1000 °C to room temperature. The flexural strength was determined utilising the ball-on-three-balls method. Although the general scale of the Weibull modulus m was rather low (< 6), considerable differences were shown after thermal shocking as the Weibull modulus decreased by a factor of about 3.

Investigation of low-pressure cold-gas dynamic spraying of polyamide-12 (PA12) on steel surfaces

Due to their good tribological properties and corrosive resistance, thermoplastic coatings are becoming increasingly important in industrial applications. Thermal spray technologies in general and flame spraying in particular belong to the most common application methods. However, the high thermal energy inevitably leads to the risk of thermal decomposition, especially in the case of materials such as polyamide-12, which have a small discrepancy between the melting and degradation temperatures. Furthermore, flame-sprayed coatings often are highly porous, reducing its effectiveness as a corrosive barrier. As an alternative approach, polyamide-12 coatings were applied to steel substrates using a conventional low-pressure cold-gas dynamic spray system. Various mixtures including ceramic powders (Al2O3) as well as different injection concepts were investigated and evaluated. The focus was put on the resulting coating morphology. Furthermore, the cold-gas spraying system was modified with a continuous temperature control for the gas temperature to enable a more precise adjustment based on the melting temperature of the material in order to reduce the risk of thermal degradation.

Microstructure and Phase Composition of NiCrBSi-TiB2 Vacuum Furnace Fused Flame-Sprayed Coatings

Several studies have revealed that NiCrBSi reinforced with different intermetallic compounds exhibits characteristics that allow the material to be successfully used as a thermally sprayed coating for applications that require wear and corrosion protection. One of such materials is TiB2, as it has an elevated melting point, high elasticity modulus, increased hardness and good thermal stability. NiCrBSi was mechanically mixed with 5%, 10%, 15% and 20% vol. TiB2, flame sprayed and subsequently fused in a vacuum furnace. A remelting process under controlled atmosphere offer important advantages like good gas extraction and precise adjustment of temperature. The best performing batch of samples were selected for further analyse of microhardness and porosity measurements. The experimental results show that the thermal cycle created with the aid of the data offered by differential thermal analysis lead to a reduced porosity without affecting the hardness. X-ray diffractometry was used to characterize the phase composition while scanning electron microscopy was employed to investigate the microstructure and phase distribution. Further theoretical aspects regarding the influence of the phase composition on the coatings properties, like hardness or sliding wear conclude the work.

Lastnosti plamensko nanešenih premazov Al2O3/TiO2 in ZrO2/CaO

Lastnosti plamensko nanešenih premazov Al2O3/TiO2 in ZrO2/CaO

Microstructure and Mechanical Properties of Microwave Post-processed Ni Coating

Flame-sprayed coatings are widely used in the industries attributed to their low cost and simple processing. However, the presence of porosity and poor adhesion with the substrate requires suitable post-processing of the as-sprayed deposits. In the present work, post-processing of the flame-sprayed Ni-based coating has been successfully attempted using microwave hybrid heating. Microwave post-processing of the flame-sprayed coatings was carried out at 2.45 GHz in a 1 kW multimode industrial microwave applicator. The microwave-processed and as-sprayed deposits were characterized for their microstructure, porosity, fracture toughness and surface roughness. The properties of the coatings were correlated with their abrasive wear behavior using a sliding abrasion test on a pin-on-disk tribometer. Microwave post-processing led to healed micropores and microcracks, thus causing homogenization of the microstructure in the coating layer. Therefore, microwave post-processed coating layer exhibits improved mechanical and tribological properties compared to the as-sprayed coating layer.

Effect of Preparation on the Correct Interpretation of Thermal Spray Coatings on the Basis of Selected Atmospheric Plasma and Flame Spray Coatings

Abstract For quality assurance, thermal spray coatings undergo, metallographic examination, among other things, Here, for example, unmelted particles, pores, oxides, phase fractions, homogeneity, separations, cracks and impurities are estimated. The correct evaluation of thermal spray samples is heavily influenced by embedding and preparation methods.

Microporosity and delamination mechanisms in thermally sprayed borosilicate glass coatings

Conventional enamelling requires both the feedstock powder and the substrate component to be heated one or more times in a furnace at 800–900 °C. The process can degrade the substrate and limit the size of the component to the furnace dimensions, which are serious restrictions on the technology. This study concerns the use of combustion- flame spraying as an alternative technique for enamelling. In this process, the heat source (the flame) is separated from the substrate, which enables much lower substrate temperatures and avoids thermal damage. It also removes the need for furnace treatment and opens up the possibility of on-site enamelling and repair. However, experimental trials showed that thick flame-spray coatings delaminated during cooling and had high microporosities due to quenching stresses at the glass-steel interface and inadequate splat flow of small feedstock particles. The research shows that these adverse mechanisms could be overcome by pre-heating the substrate surface to the dilatometric softening temperature and removing fines from the feedstock powder. The control of these two parameters was found to double the adhesion strength, provide coatings of very similar hardness and fracture toughness to conventional enamel aswell as deposit coatings of over 1mmin thickness for heavy-duty corrosion protection. Thermal spraying is well established for ceramic and metal coatings but the fundamentally different structure of glasses requires a different approach. An advantage of combustion flame spraying shown up by this research is that the high energy of the flame accelerates the particles to a high velocity and the resulting impact forces promote the flow of the glass. As a result, lower substrate temperatures may be used with reduced risk of degradation or higher viscosity glasses may be deposited with enhanced properties. The influence of the type of thermal-spray technique on coating quality is also discussed

Flame-sprayed coatings as de-icing elements for fiber-reinforced polymer composite structures: Modeling and experimentation

The development of embedded de-icing elements for polymer-based composite materials, coupled with mathematical models that describe their performance, is of interest to the aerospace, communications, and energy industries. Nickel–chromium–aluminum–yttrium (NiCrAlY) coatings were deposited on to fiber-reinforced polymer composite (FRPC) plates by using a flame spraying process. Electric current was supplied to the metal alloy coatings to generate energy by way of Joule heating (or resistive heating) and to enable the coatings to act as heating elements for the FRPC structures. De-icing tests were performed at ambient temperatures of −5°C, −15°C, and −25°C, after liquid water was sprayed on the samples. Heat transfer models were developed to predict the heating and melting times of the ice during the de-icing process with the flame-sprayed coatings. The models were based on the separation of variables method for a finite length-scale melting problem and Stefan’s problem applied to a semi-infinite medium. It was found that a coating that was on the order of 100μm thick was effective for melting accumulated ice on polymer composite structures that were exposed to cold environments. The results of the finite length-scale model and its agreement with experimental data suggest that a heat conduction model based on the separation of variables method may be applied to free boundary problems to predict phase change phenomena induced by thermal-sprayed coatings.

A Study on Wear Behaviour of NiCrBSi Flame Sprayed Coatings

TThe present study is an attempt to observe and evaluate the dry rolling wear behavior [1] of three types of Ni-base coatings on an Amsler type wear machine, against low-alloyed steel discs. The tested coatings were obtained by flame spraying (on the same low-alloyed steel substrate as the discs) using commercial Ni-base self-fluxing powders: 1060-00 and 1355-20 type (manufactured by Hoganas) and JK 586 type (manufactured by Deloro Stelite Co). In order to increase the strength of the layers, the deposition was followed by fusing [2]. The tests were conducted on pairs of coated – uncoated discs at the same load and the same speed in all three cases. In order to evaluate the wear behavior, there were observed and measured the surface roughness and the rolling path profiles. A severe wear regime with adhesion as the main phenomenon was observed in case of the low-alloyed steel discs.