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In this study, newly developed NiCrB and conventional NiCrTi coatings were produced by arc spraying using compressed air and nitrogen as atomization gases. In this way, four coatings with different oxide contents were produced. The coatings were investigated in terms of their phase compositions, oxygen contents, and microstructures. The results showed that the oxygen contents in coatings were pronouncedly reduced by nitrogen-atomized compared to air-atomized. The oxygen contents in the nitrogen-atomized coatings were about one-fourth of those in the corresponding air-atomized coatings, respectively. No significant oxide phases were observed in the XRD patterns of the nitrogen-atomized coatings. Moreover, the corrosion behavior of the coatings was studied using a hot corrosion test in molten Na2SO4-10 wt.% NaCl salt at T = 800 °C. All coatings were significantly corroded under this test condition. However, NiCrB coatings exhibited higher corrosion resistance than NiCrTi coatings. The NiCrB and NiCrTi coatings prepared by nitrogen atomization corroded more severely in the initial stage than the corresponding coatings prepared by air atomization. The NiCrB coating with air-atomized showed the best corrosion resistance due to the low chromium content and the addition of boron. [ABSTRACT FROM AUTHOR]

In this study, a laser remelting process was applied to the FeCrCoNiTiAl0.6 high entropy alloy coating in order to improve the density and the surface quality of the coating. The coating was fabricated by high-velocity-oxygen-fuel (HVOF) technology. The microstructure and phase composition of the coating were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), X-ray diffractometer (XRD) and confocal scanning laser microscope (CSLM). Moreover, the wear behavior of the coating was evaluated by use of a ball-on-disc test. The coating was denser after laser remelting treatment by eliminating the previous lamellar structure. The microstructure of the laser-remelted coating exhibits two body-centered cubic (BCC) phases, which is different from the HVOF coating. In addition, aluminum oxide formed during laser remelting. Different from the wear mechanism of the HVOF coating, which comprised abrasion and fatigue, the major wear of the laser remelted coating was abrasion. [ABSTRACT FROM AUTHOR]

A series of NiCrB coating was deposited using the twin wire arc spraying technique with a newly designed NiCrB (B: 0–4 wt%) cored wire, aiming to protect the boiler tubes used in the waste-to-energy plant. The hot corrosion behavior of the coatings was investigated with the molten salt (Na 2 SO 4 + 10% NaCl, wt%) at 800 °C for 200 h using the thermogravimetric technique. The SEM/EDS and XRD results showed that the addition of element B alleviated the oxidation tendency in the as-deposited coatings. Sufficient free stage Cr atoms diffused to coating surface timely to form protective scale, which inhibited further rapid corrosion, owing to low oxides in the coating. In addition, the corrosion resistance was influenced by the B content of the coating. In the present study, the coating with 1–2% wt% B had the best hot corrosion performance. • A novel material system of NiCrB coating was deposited using the twin wire arc spraying with cored wire for hot corrosion. • The boron element has a positive effect on decreasing the oxidation during the arc spraying. • The B promoted the formation of successive oxide scale and decreased the scale thickness. [ABSTRACT FROM AUTHOR]

Hydroxyapatite (HA) is widely used as surface coatings on biomedical metal implants because of its excellent biocompatibility and osteoconductivity. In this study, HA coatings were deposited onto Ti-6Al-4V substrates by micro-plasma spraying and then heat treated at 650 °C in a muffle furnace for 3 h and 6 h. XRD results showed that, the HA peaks were sharper than those of corresponding as-sprayed coatings and the amorphous diffuse background was reduced after heat treatment. The transformation from impurity phases such as tetra-calcium phosphate (TTCP) and tri-calcium phosphate (TCP) and amorphous calcium phosphate (ACP) to crystalline HA occurred during the heat treatment. Moreover, the texture intensity was not increased during the re-crystallization of ACP and the transformation of impurity phases into HA. The SEM results showed that nanograins were formed on the coating surface. In addition, with the increase of the holding time, the nanograins become coarse. • Three kinds of HA coatings with different phase structure were fabricated by micro-plasma spraying. • The effect of heat treatment on the crystallinity, microstructure and surface morphology of HA coatings was investigated. • After heat treatment, nanograins of HA were formed on the coating surface and the nanograins became coarse with the increase of the holding time. [ABSTRACT FROM AUTHOR]

Si-rich Mo-Si-B powders (Mo-62Si-5B at%) were deposited on Nb-Si based alloy by laser cladding assisted with ultrasonic vibration. The obtained coatings mainly consist of MoSi 2 and (Mo, X) 5 Si 3 (X = Nb and Ti). With the introduction of ultrasonic vibration during laser cladding, the lath-shaped MoSi 2 phase transforms into cellular gradually. Meanwhile, the discrepancy on the microstructure of the top region and the bottom region is eliminated gradually, indicating that the cavitation and acoustic streaming of ultrasonic vibration not only refines the microstructure, but also homogenizes the element and phase distribution in the coatings. High-temperature oxidation experiment at 1250 ℃ were conducted to assess the oxidation resistance of the coatings. Compared with Nb-Si alloy, the coated specimens have much lower oxidation weight increase. Furthermore, with the assistance of ultrasonic vibration, the oxidation resistance of the coating is enhanced further. The oxide scales on the coatings consist of two layers: an upper amorphous aluminoborosilica with TiO 2 particles dispersed and a lower crystalline SiO 2 layer. With the introduction of ultrasonic vibration, the microstructure of the coatings is refined, resulting in more phase and grain boundaries in the coatings. Therefore, the diffusion of the elements to the surface is accelerated and the protective oxide scale can be formed quickly. • Si-rich Mo-Si-B coatings were prepared on Nb-Si based alloy by laser cladding assisted with ultrasonic vibration. • The ultrasonic vibration refines the microstructure and homogenizes the phase distribution in the coatings. • The oxide scale on the coatings consists of an upper amorphous aluminoborosilica and a lower crystalline SiO 2 layer. • The ultrasonic vibration improves the oxidation resistance of the coatings because of more phase and grain boundaries. [ABSTRACT FROM AUTHOR]

Silicide coatings and B-modified silicide coatings were prepared on Nb-Si based alloys using halide activated pack cementation. The hot corrosion behavior of the coatings exposed to Na 2 SO 4 and Na 2 SO 4 + NaCl mixture salts at 900 °C was investigated. The results show that B-modified silicide coatings possess better hot corrosion resistance than the silicide coatings. The hot corrosion products of silicide coatings show a regional microstructure, including uplift regions of AlNaSiO 4 imbedded with Al 2 O 3 phases, and depressed silica regions with NaNbO 3 and TiO 2 particles. The addition of B to the silicide coatings promotes the formation of the uniform and dense boroaluminosilicate scale with better fluidity during hot corrosion, which impedes the self-sustaining cyclic chlorination/sulfidation reactions, and thus, improves the hot corrosion resistance of the silicide coatings. • The hot corrosive scale of silicide coatings possesses high viscosity, resulting in a regional fluctuating microstructure. • The addition of B improves the hot corrosion resistance of the silicide coatings. • B-modified silicide coatings have a flatter hot corrosive scale than silicide coatings due to the better fluidity. • The boroaluminosilicate scale on B-modified silicide coating suppresses the chlorination/sulfidation reactions. [ABSTRACT FROM AUTHOR]

Abstract: A series of Fe–xCr cored wires (x =15, 20, 25, 30, 35 and 40% in weight) were used to produce protective coatings for components used in high-temperature environments by wire-arc spraying. The microstructure, morphology and high-temperature oxidation behavior of the Fe–xCr coatings were investigated. The Fe–xCr coatings presented a homogeneous structure with thin oxide inclusions and low porosity. Thermogravimetric analysis was used to evaluate the high-temperature oxidation behavior of the coatings at a temperature of 650°C under cyclic oxidation conditions. The results showed that all the Fe–xCr coatings exhibited significantly lower weight gain than the uncoated substrate. As Cr content of the coatings increased, the thickness of oxide scales decreased and oxidation resistance improved. [Copyright &y& Elsevier]

Thermal sprayed high entropy alloy coatings have garnered substantial attention for their remarkable mechanical properties and wear resistance. In this study, we explored the wear behavior and mechanisms of Al 0.6 TiCrFeCoNi coatings produced through atmospheric plasma spray (APS) at both room temperature and 500 °C. The microstructural and mechnical characterization of the APS-sprayed coating was performed using XRD, OM, SEM, CSLM and hardness testing techniques. The dry sliding wear test of the coating against Al 2 O 3 ball were conducted on a pin-on-disc tribometer at room temperature and 500 °C. The coating exhibited a single body-centered cubic (BCC) phase, with a typical lamellar structure marked by oxides and pores. Wear tests showed that the APS-sprayed coating exhibits superior wear resistance at elevated temperatures compared to room temperature due to the formation of compact oxide layer on the wear track. Detailed investigations on wear track reveal that abrasive wear, oxidation wear and fatigue wear are the prominent wear mechanisms on both conditions, while tribo-oxidation is dominated at 500 °C. Furthermore, We demonstrated that the looser, more porous microstructure of the APS-sprayed coating contributes to its enhanced wear resistance, preventing crack propagation and increasing strain tolerance. This provides a promising strategy for outstanding wear resistant coatings by designing looser lamellar coating structure. [Display omitted] • APS-sprayed Al 0.6 TiCrFeCoNi HEA coating exhibited superior wear resistance at elevated temperatures. • The superior wear resistance is attributed to the formation of a compact oxide layer. • At elevated temperature, in-situ oxidation and debris gathering and compaction devoted to the formation of oxide layer. • Loose lamellar structure in thermal sprayed coating reduces fatigue effect by inhibiting crack propagation. [ABSTRACT FROM AUTHOR]

Abstract For the aim of thermal barrier application in vehicle engines, a specific FeCrNbBSiC cored wire of consideration was produced to fabricate amorphous coating by wire-arc spraying. The microstructure and thermal conductivity of the as-deposited coating were characterized in detail. The results show that the FeCrNbBSiC coating (FB coating) primarily consists of amorphous phase owing to the combination of proper composition design and rapid quenching of wire-arc spraying. However, a fraction of crystalline phases coexists with the amorphous matrix, due to the both effects of partially incomplete alloying and continuous annealing. Depending on the expected amorphous structure, thermal conductivity of the developed FB coating is as low as 2.15 W/mK at room temperature, almost half of the referenced 316L stainless steel coating (SS coating, 4.20 W/mK), and displays relatively slow and limited increase with elevated temperature up to 500 °C. By means of temperature distribution analysis, an obviously higher temperature is found on the top surface of FB coating covered piston, which implies a benefit of enhancing heat energy conversion. Simultaneously, the temperature of aluminium alloy substrate could also be reduced by covered coating as the role of thermal insulation. Therefore, the wire-arc sprayed FeCrNbBSiC amorphous coating developed here with excellent thermal barrier performance and economic advantage could be a promising innovation and candidate for metal-based thermal barrier coatings applied in vehicle engines. Highlights • Innovative scheme of metal-based thermal barrier coatings was designed. • Wire-arc sprayed FeCrNbBSiC amorphous coating with high T x was obtained. • Incomplete alloying and continuous annealing result in crystalline phases formed. • Amorphization endows excellent thermal barrier ability for this coating (κ = 2.15 W/mK). • Application of this coating improves power efficiency and protects substrate effectively. [ABSTRACT FROM AUTHOR]

Abstract In this study, an Al 0.6 TiCrFeCoNi high entropy alloy coating was produced by high-velocity-oxygen-fuel spraying. The microstructure and phase composition of the coating were investigated, moreover the hardness and the fracture toughness of the coating were determined. The wear behavior of the coating at different temperatures was evaluated using a pin-on-disc test. The results show that the coating was very dense and consisted of lamellae. Two BCC phases with similar lattice parameters were detected in the as-sprayed coating. The wear of the coating against an Al 2 O 3 counter body was mainly caused by abrasion at all temperatures. The role of fatigue wear increased with increasing test temperature. In addition, tribo-reaction played an important role at the test temperature of T = 500 °C, which led to the lowest friction coefficient of the coating under the given test conditions. Highlights • The wear behavior of Al 0.6 TiCrFeCoNi HVOF coating was investigated at room temperature, T=300°C and T=500°C in detail. • The wear behavior of the coating changed significantly with the increasing test temperatures. • The wear behavior of the coating changed significantly with the increasing test temperatures. • The wear mechanism changed with temperatures, tribo-reaction played an important role at test temperature of T=500°C. [ABSTRACT FROM AUTHOR]

• Long-term heat exposure above critical temperature can induce continuous degeneration of thermal insulation property for Fe-based amorphous coating, revealing an importance of kinetic effect. • The coupling effect of microstructure transformation and structural sintering is disclosed, leading to a particular insight into the degeneration mechanism. • The low atomic diffusivity of amorphous phase retards the progress including crystallization transition, grain growth and pore healing, resulting in a sluggish degeneration of thermal insulation property. Fe-based amorphous coatings have been proposed to perform a promising thermal barrier application, however, the uncertain degeneration of properties induced by thermal stimulation is significantly concerned owing to their metastable nature. Herein, Fe 57 Cr 15 Nb 4 B 20 Si 4 amorphous coating with a low thermal conductivity was employed to investigate the degeneration of thermal insulation property and related mechanisms during long-term heat exposure. Excellent stability can be defined for the amorphous coating when annealed below glass transition temperature, despite a slight increase of thermal conductivity induced by structural relaxation. Extraordinary increase of thermal conductivity is found when prolonged annealing time at a critical temperature of 600 °C. The sluggish structural sintering dominates thermal conductivity to increase, whereas the effect of precipitated ultrafine nanocrystals is little. The cooperation of grain coarsening and structural sintering leads to a dramatical increase of thermal conductivity at the initial stage of 850 °C annealing, while the relatively low increase of thermal conductivity with prolonged duration is ascribed to the further grain growth. The obtained results demonstrate a comprehensive understanding on the thermal evolution of Fe-based amorphous coatings and form a basis for future works aiming to shed further light on the degeneration of related metallic coatings at high temperatures. [ABSTRACT FROM AUTHOR]

The correlation between the thermal transport properties and microstructure features of amorphous coatings is crucial for reliable thermal insulation applications. In this work, an Fe 48 Cr 15 Mo 14 C 15 B 6 Y 2 amorphous coating prepared by high-velocity oxygen-fuel spraying (HVOF) was employed to reveal this relationship under long-term heating. From the combined effects of the amorphous phase and unique coating structure, the proposed coating exhibits an extremely low thermal conductivity of 1.92 W/(m∙K), granting a significant potential for being as metal-based thermal barrier coatings. Isothermal annealing experiments below the glass transition temperature demonstrate that the thermal insulation property can maintain stability with prolonged annealing time, despite a slight degradation relative to the as-sprayed coating which is ascribed to structural relaxation. Although nearly full crystallization can be slowly induced by annealing at the critical temperature of 600 °C, the thermal conductivity displays a distinct two-stage variation with annealing time, which is primarily attributed to the sluggish sintering of the coating structure. Moreover, grain growth and rapid structural sintering result in a drastic degradation of thermal insulation in the early stage of annealing at 850 °C, whereas grain growth upon prolonged annealing governs further variation of thermal conductivity. The results of this study not only provide important insights into the degeneration mechanism of thermal insulation of Fe-based amorphous coating, but also present an inspiration for exploring the degeneration mechanism of the most metallic coatings serving at high temperatures. • The Fe-based amorphous coating exhibits an extremely low thermal conductivity of κ = 1.92 W/(m∙K). • The degeneration mechanism of thermal insulation property of Fe-based amorphous coating is disclosed. • The low atomic diffusivity retards the bridge-connection of interlamellar pores, resulting in a sluggish sintering kinetics. [ABSTRACT FROM AUTHOR]