Climate change mitigation requires advances in energy efficiency. Thermal barrier coatings (TBCs) are widely employed to improve thermal efficiency in gas turbines and internal combustion engines, and engineering coatings’ porosity is an important route to control the heat transfer to metallic components. In this study, porous gadolinium zirconate (Gd2Zr2O7, GZO) coatings were produced using air plasma spraying (APS) with polyester as a pore-forming agent. A systematic investigation was carried out to evaluate the influence of stand-off distance, polyester particle size, and polyester content on the microstructure, porosity, thermal conductivity, and erosion resistance of the composite coatings. Results demonstrated that porosity increased with spraying distance and higher pore former content, reaching up to 50% porosity at 20 wt.% polyester addition. Enhanced porosity led to a significant reduction, up to 80%, in thermal conductivity compared to dense GZO coatings, with larger pores showing a more pronounced effect. However, erosion tests revealed that while moderate polyester additions improved erosion resistance due to crack-arresting effects, higher porosity levels reduced mechanical integrity and accelerated material loss, particularly after polyester burn-out post-treatment. These findings highlight the balance required between optimizing thermal insulation and maintaining erosion resistance, providing new insights into the design of high-performance porous TBCs.

Sediments often contain a complex mixture of organic and inorganic pollutants, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), aliphatic and aromatic hydrocarbons, phthalates, tributyltin (TBT), and metals. Since large quantities of contaminated sediments are regularly dredged, it is necessary to develop methods to simultaneously treat the different contaminants. These techniques should also handle complex sediment matrices where pollutants may be strongly bound, e.g., TBT in paint flakes and PAHs in tire particles.

This work has focused on the development of photoelectrocatalytic (PEC) processes to degrade organic pollutants (OPs) and simultaneously recover metals from marine, brackish and stormwater sediments. The remediation efficiencies were studied in real contaminated sediments and clear water solutions spiked with PAHs, PCBs, TBT, and metals. In spiked water, PEC yielded a > 99 % reduction of PAHs, PCBs, and TBT, and recovered 76 % of copper. The pollutant removal from the sediments by PEC was less efficient, especially for PAHs in stormwater sediment. However, the combination of PEC and H2O2 reduced other groups of OPs: TBT by 91 %, PCB by 82 %, and phthalates DINP and DEHP by 98 % and 85 %, respectively. The release of pollutants from sediments into the water phase is the key to successful PEC application.

Advanced thermal barrier coatings (TBCs) consisting of suspension plasma sprayed (SPS) topcoat and high-velocity oxy-fuel (HVOF) deposited bond coat were used in this study. Hastelloy-X substrates were coated with NiCoCrAlY bond coat using HVOF and yttrium stabilized zirconia (YSZ) with SPS techniques, respectively. Eight different topcoats were investigated with the standard HVOF bond coat for a lifetime and functional properties evaluation. Diverse microstructural features like interpass porosity, vertical and branching cracks in the coatings revealed a substantial influence on the lifetime and functional properties of the coatings. Thermal cyclic fatigue (TCF) and thermal shock tests were performed to evaluate the lifetime of the coatings and functional properties were assessed using laser flash analysis (LFA), air jet erosion tester, micro-indentation, etc. Specific emphasis on the sintering behavior of the microstructure and the mechanism behind the coating failure was detailed in this work. The findings demonstrate that HVOF-SPS systems offer a promising alternative to conventional coating technologies. 

This work represents a preliminary study of atmospheric plasma-sprayed (APS) YttriaStabilized Zirconia (YSZ)-based thermal barrier coatings (TBCs) deposited on forged and additivemanufactured (AM) HAYNES®282® (H282) superalloy substrates. The effect of different feedstockmorphologies and spray gun designs with radial and axial injection on APS-deposited YSZ layercharacteristics such as microstructure, porosity content, roughness, etc., has been investigated. Theperformance of TBCs in terms of thermal cycling fatigue (TCF) lifetime and erosion behaviour werealso comprehensively investigated. In view of the high surface roughness of as-built AM surfacescompared to forged substrates, two different types of NiCoCrAlY bond coats were examined: oneinvolved high-velocity air fuel (HVAF) spraying of a finer powder, and the other involved APSdeposition of a coarser feedstock. Despite the process and feedstock differences, the above two routesyielded comparable bond coat surface roughness on both types of substrates. Variation in porositylevel in the APS topcoat was observed when deposited using different YSZ feedstock powdersemploying axial or radial injection. However, the resultant TBCs on AM-derived substrates wereobserved to possess similar microstructures and functional properties as TBCs deposited on reference(forged) substrates for any given YSZ deposition process and feedstock. 

In future generation aviation, light weight, and thermally stable SiC/SiC ceramic matrix composites (CMCs) are considered the most promising structural materials to replace traditionally used Ni-based superalloys. However, in the presence of steam (a common combustion reaction product) and corrosive species (from ingestion of debris along with the intake air during take-off and landing), accelerated degradation of CMCs compromising its structural integrity is inevitable. Environmental Barrier Coatings (EBCs) are protective ceramic coatings consisting of rare earth (RE) silicates as a topcoat with silicon as a bond coat, and are widely used on CMCs, to impede their surface recession.

Thermal spray techniques are commonly employed to deposit EBCs, with highly crystalline, dense, and crack free coatings being desired for robust performance. In general, the high particle velocity and efficient energy transfer in axial feeding systems can result in coatings with higher density, reduced oxide content, and improved mechanical properties. In the present study, axial plasma sprayed ytterbium disilicate (YbDS) coatings deposited on silicon carbide (SiC) substrates using varying plasma spray parameters have been comprehensively characterized. Microstructure, porosity, and hardness have been studied for YbDS coatings deposited by varying nozzle diameter, carrier gas flow rate and stand of distance (SOD) during plasma spraying. Erosion and thermal cyclic fatigue performance of these coatings has also been investigated. 

The complex-shaped hot-section parts of new-generation turbine engines demand unique design solutions. Additive Manufacturing (AM) is an emergent production method that can produce metallic parts with complex geometries and minimal material wastage. In this work, the characteristics and performance behavior of Thermal Barrier Coatings (TBCs) deposited on forged and AM-built HAYNES®282® superalloy substrates were studied and compared. The bond coats were produced by High-Velocity Air-Fuel (HVAF) spraying using NiCoCrAlY powder feedstock and TBC top-coats by Suspension Plasma Spraying (SPS) using water- and ethanol-based suspensions of Yttria-Stabilized Zirconia (YSZ). The microstructural features, adhesion, Thermal Cycling Fatigue (TCF) lifetime, and thermal shock lifetimes of the TBCs were comprehensively investigated. The results showed that the deposition of bond coats reduced the roughness and asperities of the AM-built substrates. Depending on the type of suspension used and the spray parameters employed, the TBCs exhibited vertically cracked and columnar microstructures. However, no significant differences in TCF and thermal shock lifetimes of TBCs on AM and forged substrates were observed. It is demonstrated that TBC systems can be produced on AM-built metallic substrates, and the resulting TBCs can have similar microstructures and properties as TBCs deposited on conventional substrates.

Axial suspension plasma spraying (ASPS) is an alternative technique to atmospheric plasma spraying (APS), which uses a suspension of much finer powders (<5-micron particle size) as the feedstock. It can produce more refined microstructures than APS for biomedical implants. This paper highlights the influence of incorporated graphene nanoplatelets (GNPs) on the behavior of ASPS hydroxyapatite (HAp) coatings. The characterization of the ASPS coatings (HAp + varying GNP contents) was carried out using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), confocal Raman microscopy (CRM), white light interferometry (WLI), and contact angle measurements. The evaluation of the mechanical properties such as the hardness, roughness, adhesion strength, and porosity was carried out, along with a fretting wear performance. Additionally, the biocompatibility of the Hap + GNP coatings was evaluated using cytotoxicity testing which revealed a decrease in the cell viability from 92.7% to 85.4%, with an increase in the GNP wt.%. The visualization of the cell's components was carried out using SEM and Laser Scanning Microscopy. Furthermore, the changes in the genetic expression of the various cellular markers were assessed to analyze the epigenetic changes in human mesenchymal stem cells. The gene expression changes suggested that GNPs upregulated the proliferation marker and downregulated the pluripotent markers by a minimum of three folds.

Thermal barrier coatings (TBCs) are widely utilized in gas turbine engines for power generation. In recent years, the application of TBCs in automotive has been introduced to improve engine efficiency. Low thermal conductivity and high durability are desired coating properties for both gas turbine engines and automotive. Also, suspension plasma spraying (SPS) permits a columnar microstructure that combines both properties. However, it can be challenging to deposit a uniform columnar microstructure on a complex geometry, such as a gas turbine component or piston head, and achieve similar coating characteristics on all surfaces. This work’s objective was to investigate the influence of spray angle on the microstructure and lifetime of TBCs produced by SPS. For this purpose, SPS TBCs were deposited on specimens using different spray angles. The microstructures of the coatings were analyzed by image analysis for thickness, porosity, and column density. Thermal and optical properties were evaluated on each TBC. Lifetime tests, specifically designed for the two applications, were performed on all investigated TBCs. The lifetime results were analyzed with respect to the TBC microstructure and thermal and optical properties. This investigation showed that there is a limit to the spray angle that achieves the best compromise between TBC microstructure, thermal properties, optical properties, and lifetime. © 2022, The Author(s).

Internal combustion engines have a key role in the social and economic advancement of modern society but also a significant contribution to greenhouse gas emissions. For these engines, to preserve their role, a higher efficiency, that dramatically reduces the environmental impact, is necessary. To achieve increased engine efficiency, a technical solution is to lower the heat losses in the combustion chamber. Among them, the heat losses to the pistons are the preferential route, due to their extensive impact on fuel consumption. In this paper, porous thermal barrier coatings with large pores were applied to the pistons of diesel engines to improve engine efficiency. Atmospheric Plasma Spray (APS) process and porosity former TBC feedstock were employed to obtain high porosity coatings with large pores. Scanning Electron Microscopy (SEM) was utilized to investigate the microstructure of the coating in coupons and pistons. The optical properties of the coatings were explored with two methods: the spectral normal hemispherical reflectivity at room temperature (SNHRRT) and spectral normal emissivity at high temperature (SNEHT). The coatings’ behavior under thermal cyclic conditions was assessed by Flame Rig Test. Microstructure analysis was also performed before and after the test to identify the failure mechanisms. The engine efficiency was evaluated by measuring the Indicated Specific Fuel Consumption (ISFC) in a single-cylinder engine test. The results showed that porous coating with large pores combined with a higher emissivity can withstand the engine environment well and have the potential to provide enhancements in engine efficiency.