Chlorine-induced high-temperature corrosion and erosion-corrosion behavior of amorphous Fe-based coatings sprayed by high velocity air-fuel (HVAF) and high velocity oxy-fuel (HVOF) techniques were ...investigated. The coated specimens were first exposed to isothermal high-temperature corrosion at 600 °C in ambient air with and without KCl. The exposed specimens were then subjected to alumina erodent. The as-sprayed HVAF coating showed a more compact and uniform microstructure with a higher hardness leading to higher corrosion and erosion-corrosion resistance. After erosion, all the coatings similarly exhibited a combined brittle/ductile damage to surface oxide scale that previously formed in the corrosive environment. The corrosion and erosion-corrosion behavior of the coatings primarily relied on the uniformity of coatings' microstructure and distribution of alloying elements to form the protective oxide scale in the corrosive environment, which can resist against erodent in the erosive media.
•Deposition of an Fe-based amorphous powder by HVAF and HVOF processes•Higher hardness of the HVAF (956 ± 56 HV0.1) compared to HVOF coating (821 ± 72 HV0.1)•Higher corrosion and erosion resistance of the HVAF compared to HVOF coating•A combined brittle/ductile corrosion scale response found in both eroded-corroded coatings
Power generation from renewable resources has attracted increasing attention in recent years owing to the global implementation of clean energy policies. However, such power plants suffer from severe ...high-temperature corrosion of critical components such as water walls and superheater tubes. The corrosion is mainly triggered by aggressive gases like HCl, H
2
O, etc., often in combination with alkali and metal chlorides that are produced during fuel combustion. Employment of a dense defect-free adherent coating through thermal spray techniques is a promising approach to improving the performances of components as well as their lifetimes and, thus, significantly increasing the thermal/electrical efficiency of power plants. Notwithstanding the already widespread deployment of thermal spray coatings, a few intrinsic limitations, including the presence of pores and relatively weak intersplat bonding that lead to increased corrosion susceptibility, have restricted the benefits that can be derived from these coatings. Nonetheless, the field of thermal spraying has been continuously evolving, and concomitant advances have led to progressive improvements in coating quality; hence, a periodic critical assessment of our understanding of the efficacy of coatings in mitigating corrosion damage can be highly educative. The present paper seeks to comprehensively document the current state of the art, elaborating on the recent progress in thermal spray coatings for high-temperature corrosion applications, including the alloying effects, and the role of microstructural characteristics for understanding the behavior of corrosion-resistant coatings. In particular, this review comprises a substantive discussion on high-temperature corrosion mechanisms, novel coating compositions, and a succinct comparison of the corrosion-resistant coatings produced by diverse thermal spray techniques.
This paper investigates the thermal shock behavior of thermal barrier coatings (TBCs) produced by axial suspension plasma spraying (ASPS). TBCs with different columnar microstructures were subjected ...to cyclic thermal shock testing in a burner rig. Failure analysis of these TBCs revealed a clear relationship between lifetime and porosity. However, tailoring the microstructure of these TBCs for enhanced durability is challenging due to their inherently wide pore size distribution (ranging from few nanometers up to few tens of micrometers). This study reveals that pores with different length scales play varying roles in influencing TBC durability. Fracture toughness shows a strong correlation with the lifetime of various ASPS TBCs and is found to be the prominent life determining factor. Based on the results, an understanding-based design philosophy for tailoring of the columnar microstructure of ASPS TBCs for enhanced durability under cyclic thermal shock loading is proposed.
•A novel design strategy for tailoring ASPS columnar microstructure is proposed.•Porosity is classified in 4 classes: (>10μm), (10μm–1μm), (1μm–100nm) & (<100nm).•About 40%(1μm–100nm), 34%(10μm–1μm), 20%(>10μm) & 7%(<100nm) pores were present.•Fracture toughness was found to be the most prominent life-determining factor.•Pores (>10μm) had the most severe effect on lifetime than other porosity classes.
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This paper investigates the evolution of microstructure of thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) through a careful experimental study. Understanding the ...influence of different suspension characteristics such as type of solvent, solid load content and median particle size on the ensuing TBC microstructure, as well as visualizing the early stages of coating build-up leading to formation of a columnar microstructure or otherwise, was of specific interest. Several SPS TBCs with different suspensions were deposited under identical conditions (same substrate, bond coat and plasma spray parameters). The experimental study clearly revealed the important role of suspension characteristics, namely surface tension, density and viscosity, on the final microstructure, with study of its progressive evolution providing invaluable insights. Variations in suspension properties manifest in the form of differences in droplet momentum and trajectory, which are found to be key determinants governing the resulting microstructure (e.g., lamellar/vertically cracked or columnar).
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•GNP incorporated in alumina matrix to deposit composite coatings via SPS.•Raman analysis confirmed retention of GNP in as-processed composite coatings.•Improved mechanical and ...tribological properties achieved for composite coatings.•Wear mechanisms for composite and monolithic coating were revealed.
Graphene possesses high fracture toughness and excellent lubrication properties, which can be exploited to enhance tribological performance of coating systems utilized to combat wear. In this work, suspension plasma spray (SPS) process was employed to deposit a composite, graphene nano-platelets (GNP) incorporated alumina coating. For comparison, monolithic alumina was also deposited utilizing identical spray conditions. The as-deposited coatings were characterized in detail for their microstructure, porosity content, hardness, fracture toughness and phase composition. Raman analysis of the as-deposited composite coating confirmed retention of GNP. The composite coating also showed good microstructural integrity, comparable porosity, higher fracture toughness and similar alumina phase composition as the monolithic alumina coating. The as-deposited coatings were subjected to dry sliding wear tests. The GNP incorporated composite coating showed lower CoF and lower specific wear rate than the pure alumina coating. Additionally, the counter surface also showed a lower wear rate in case of the composite coating. Post-wear analysis performed by SEM/EDS showed differences in the coating wear track and in the ball wear track of monolithic and composite coatings. Furthermore, Raman analysis in the wear track of composite coating confirmed the presence of GNP. The micro-indentation and wear test results indicate that the presence of GNP in the composite coating aided in improving fracture toughness, lowering CoF and specific wear rate compared to the monolithic coating. Results from this work demonstrated retention of GNP in an SPS processed coating, which can be further exploited to design superior wear-resistant coatings.
High-temperature corrosion of critical components such as water walls and superheater tubes in biomass/waste-fired boilers is a major challenge. A dense and defect-free thermal spray coating has been ...shown to be promising to achieve a high electrical/thermal efficiency in power plants. The field of thermal spraying and quality of coatings have been progressively evolving; therefore, a critical assessment of our understanding of the efficacy of coatings in increasingly aggressive operating environments of the power plants can be highly educative. The effects of composition and microstructure on high-temperature corrosion behavior of the coatings were discussed in the first part of the review. The present paper that is the second part of the review covers the emerging research field of performance assessment of thermal spray coatings in harsh corrosion-prone environments and provides a comprehensive overview of the underlying high-temperature corrosion mechanisms that lead to the damage of exposed coatings. The application of contemporary analytical methods for better understanding of the behavior of corrosion-resistant coatings is also discussed. A discussion based on an exhaustive review of the literature provides an unbiased commentary on the advanced accomplishments and some outstanding issues in the field that warrant further research. An assessment of the current status of the field, the gaps in the scientific understanding, and the research needs for the expansion of thermal spray coatings for high-temperature corrosion applications is also provided.
•HVAF-sprayed coatings were exposed to O2+HCl atmosphere with and without KCl deposit•NiCrMo exposed without KCl showed the lowest kp value (2.5×10-7 mg2/cm4s)•With KCl, kp was much lower in NiCrMo ...(1.3×10-6 mg2/cm4s) than NiCrAlY (2.2×10-5 mg2/cm4s).•Formation of K2CrO4 followed by Cl- were the main reasons of failure at the early stages of corrosion exposure.•At the last stages, the protective chromia scale was interrupted due to the simultaneous presence of Cl- and Cl2.
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The oxidation performance of NiCrAlY and NiCrMo coatings thermally sprayed by high velocity air-fuel (HVAF) technique has been investigated in a chloridizing-oxidizing environment, with and without a KCl deposit, at 600 °C for up to 168 h. Both coatings protected the substrate in the absence of KCl due to formation of a dense Cr-rich oxide scale. In the presence of KCl, Cl−/Cl2 diffused through a non-protective and porous NiCr2O4 scale formed on NiCrAlY, leading to formation of volatile CrCl3. On the other hand, Mo in NiCrMo stimulated the formation of a more protective Cr-rich oxide scale which increased the corrosion resistance by reducing Cl−/Cl2 diffusion.
Suspension plasma spray (SPS) thermal barrier coatings are currently at an early stage of industrial adoption. There remain questions about the performance of SPS columnar coatings under different ...engine environmental conditions as it may influence which established engine coatings can be replaced by SPS coatings. One particular area of concern has been the erosion resistance of SPS coatings.
In this study a columnar SPS coating has been evaluated against three types of state of art air plasma spray coatings: conventional porous coating, high porosity coating and dense vertically cracked coating. Air-jet erosion testing was performed on coatings at a glancing angle of 30 degrees and with direct impact at 90 degrees. Coatings have been ranked according to their mass loss per unit erodent mass. Coatings were also evaluated for their microstructure, porosity content, hardness, and fracture toughness. The erosion damage created during testing has also been investigated using electron microscopy to observe the damage mechanism. The results of this study demonstrate that SPS coatings can outperform porous APS coatings in erosion resistance and could be considered a match for dense vertically cracked coatings. The SPS columnar coatings have shown a decreasing erosion rate with exposure time that suggest the influence of surface roughness on initial erosion behaviour.
•A SPS columnar TBC is benchmarked in erosion testing against porous and dense vertically cracked air plasma spray TBC's•SPS columnar coatings have superior erosion resistance to porous APS TBC's and comparable to dense vertically cracked TBC's•Measured erosion performance is dependent on surface roughness for the SPS coatings
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•Tailored microemulsion-gel for depot effect to treat psoriasis.•Target to improve permeability and cyclosporine retention using microemulsion.•Topical cyclosporine delivery will ...avoid systemic immunosuppression side effects.
Psoriasis is a widespread chronic disease affecting 1–3 % of total population. In major cases (>80 %), it is treated by topical application of corticosteroids. However, the topical route is very challenging due to physico-chemical nature of diseased stratum corneum and so no single treatment works for every patient. The oral route showed severe side effects due to systemic immunosuppression, which can be avoided by topical route. The aim of the research work was to investigate cyclosporine loaded microemulsion based gel for effective cyclosporine permeation and retention in the skin tissue for psoriasis treatment. The pseudo ternary phase diagram at three Smix ratios (2:1, 1:1, and 1:2; Tween 80: isopropyl alcohol) were constructed using isopropyl myristate as oil phase. The Smix at 2:1 ratio showed large microemulsion area. The transmission electron microscope images showed spherical non-aggregated oil globules with the size < 50 nm. The selected microemulsion (Cy-2-ME12O55SM) was incorporated in Carbopol 940 gel for topical application. The ex vivo diffusion study showed improved permeation (>24 h) with microemulsion-gel in comparison to cyclosporine suspension. The microemulsion-gel was non-irritating on the rabbit skin. In drug retention studies, microemulsion-gel showed high drug retention (trapping, 38.92 %) in the skin tissue, which was due to destabilization of microemulsion after penetration in the skin layer causing precipitation of cyclosporine. The depot effect due to cyclosporine precipitates could be helpful for sustained effect of cyclosporine for the effective treatment of psoriasis.
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