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.

The efficiency of internal combustion engines is gaining increased interest due to the impact of fuel consumption on greenhouse gas emissions and the goals of countries to minimize emissions. Thermal barrier coatings (TBCs) have shown great potential in improving the efficiency of internal combustion engines. The TBCs, applied on the surface of the piston, apart from thermal isolation, should also follow the surface temperature variations in the combustion chamber, reducing the energy loss and not affecting volumetric efficiency, and thus accomplish a raise in fuel efficiency. This characteristic of the TBC can be associated with the thermal properties, but the best performance test for TBCs is the single cylinder engine test. The single cylinder engine test is an expensive and time demanding procedure, making it not easily accessible. The purpose of this work was to develop a thermal swing test method to evaluate the applicability of TBCs in the combustion chamber of an internal combustion engine. This was carried out by measuring the temperature variation on the surface of the coating (thermal swing response) exposed to heat pulses from a high velocity air fuel (HVAF) spray torch. The TBCs were tested as sprayed (AS) and after grinding them to reduce roughness (RR) in order to ensure similar thickness and roughness along the different TBCs. Characterization of the coating microstructure was carried by scanning electron microscopy (SEM) together with image analysis techniques, and the thermal properties were measured by laser flash analysis (LFA). By correlating the thermal swing response with the microstructure and thermal properties of the coatings, it was determined that the coatings with large open pores exhibited the highest thermal swing response, which was as high as 200 degrees C.

Axial plasma spray is one of the thermal spray techniques to deposit multifunctional advanced coatings. The present work explores the use of this process to deposit thin, continuous, and adherent Ca5 (PO4)3OH (hydroxyapatite, HAp) coatings and characterize its microstructure, phases, hardness and adhesion strength. Three different suspension-deposited HAp coatings were investigated and compared with powder-deposited HAp coating on a Ti6Al4V substrate. The effect of mean solute particle size and solid-loading in the suspension has been explored on the evolution of microstructure, phase content and mechanical properties of axial suspension plasma sprayed (ASPS) coatings. Phase-characterization has shown retention of hydroxyapatite phase and coating crystallinity in the deposited coatings, whereas the adhesion strength of the HAp coating decreased from -40 MPa to -13 MPa when bioglass was added to the feedstock material. The lower solid load content and lower mean solute particle size in the suspension were found to be beneficial in achieving porous, rougher, and welladhering coatings. This work concludes that ASPS can potentially deposit thin HAp coatings (< 50 ?m) with high adhesion strength.

Gadolinium zirconate (GZ)-based TBCs comprising GZ as the top layer and yttria stabilized zirconia (YSZ) as the base layer, are attractive double-layered thermal barrier coatings (TBCs) for high temperature gas turbine engine application. This work attempts to understand the influence of individual layer microstructure on the durability of GZ/YSZ double-layered TBCs processed by suspension plasma spray (SPS). Two different spray parameters were chosen to obtain a combination of three microstructurally distinct GZ/YSZ double-layered TBCs i.e. GZ porous (P)/YSZ porous (P), GZ dense (D)/YSZ porous (P) and GZ dense (D)/YSZ dense (D). Thermal diffusivity of the as-deposited coatings was measured using Laser Flash Analysis (LFA) technique and the thermal conductivity of the TBCs was calculated. The GZ/YSZ double-layered TBC specimens were subjected to two different durability tests, i.e. thermal cyclic fatigue (TCF) and burner rig test (BRT). Sintering behavior of the individual layer TBC microstructures was evaluated by comparing the porosity evolution in as-deposited and TCF tested TBCs. Fracture toughness measurements performed on each layer of the double-layered TBCs were correlated with the durability results. Thermal cycling results amply demonstrate that the individual layer microstructure of GZ/YSZ double-layered TBC influenced its durability. Detailed failure analysis of the TCF and BRT failed specimens revealed similar failure modes for GZ (P)/YSZ (P), GZ (D)/YSZ (P) and GZ (D/YSZ (D) TBCs under identical thermal cyclic test conditions. However, failure modes differed when subjected to different thermal cyclic test conditions (TCF and BRT) and the probable causes are discussed. Findings from this work provide key insights on designing durable GZ/YSZ double-layered TBCs.