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Thermal spray, being a cost- and time-efficient process, is used extensively in industrial and engineering sections for mass production of desired coating structures, allowing to deposit a wide range of materials on various substrates. Conventionally, powder feedstocks are used in plasma and high-velocity oxy-fuel (HVOF) thermal spray that has limitations such as limited feedstock particle size (10-100 µm), clogging and limited options for coating materials. Liquid feedstocks, in the form of suspensions or precursor solutions could potentially resolve these issues by allowing nano- and submicron particles to be deposited, where unlike dry feedstock, the liquid medium helps in reducing the friction and avoiding the clogging. Also, liquid feedstocks, especially precursor solutions, provide the opportunity to deposit a wide range of coating materials with better control over coating microstructure, material composition and stoichiometry by varying the properties of the feedstock. Despite benefits, liquid feedstock has its own complexities, such as complex feedstock preparation, thermo-physical reactions during interaction with the energy source and gases. Therefore, it becomes essential to understand how different suspension and solution precursor feedstock properties affect the coating microstructures and properties. This review paper covers a detailed discussion on the role of different process parameters such as feedstock properties, injection methods, different torches and surface properties, affecting the coating quality and performance and related recent developments and challenges are discussed. This would be beneficial in optimizing the spray parameters to obtain coatings with desired microstructures. The later part of the review focuses on the economic aspect of the suspension/solution precursor-based plasma and HVOF spray methods and their various applications.

There are many sets of information in the literature (e.g., papers, books and websites) about the great achievements that are expected for aerospace gas turbine engines by the employment of ceramic matrix composites (CMCs) and thermally sprayed environmental barrier coatings (EBCs) in their hot zones (e.g., combustion chambers, vanes, shrouds, blades and afterburners). Among these achievements, it is typically highlighted (i) turbine weight reduction, (ii) reduced fuel consumption, (iii) higher operation temperatures, (iv) superior thrust-to-weight ratio and (v) lower emission of toxic gases to the atmosphere. Although these achievements are true, they are generally not well-explained to the reader on how together they come to be. In addition, according to “conventional wisdom”, some of these engineering feats are in fact opposing each other (e.g., higher operation temperatures versus lower emissions). The objective of this tutorial paper is to present the reader how these feats are achieved by the concomitant combination of imaginative engineering. It will explain the non-stop driving force for increasing combustion temperatures; show the basic concepts of CMCs, the paramount need of EBCs, and the complexity of creating EBC architectures via air plasma spray (APS). Finally, highlights on how EBCs/CMCs are tested at high temperature will be provided. The content of this paper shall be understood by anyone with basic knowledge in materials processing and surface engineering.

Thermal barrier coating is critical for thermal insulation technology, making the underlying base metal capable of operating at a melting temperature of 1150 °C. By increasing the temperature of incoming gases, engineers can improve the thermal and mechanical performance of gas turbine blades and the piston cylinder arrangement. Recent developments in the field of thermal barrier coatings (TBCs) have made this material suitable for use in a variety of fields, including the aerospace and diesel engine industries. Changes in the turbine blade microstructure brought on by its operating environment determine how long and reliable it will be. In addition, the effectiveness of multi-layer, composite and functionally graded coatings depends heavily on the deposition procedures used to create them. This research aims to clarify the connection between workplace conditions, coating morphology and application methods. This article presents a high-level overview of the many coating processes and design procedures employed for TBCs to enhance the coating’s surface quality. To that end, this review is primarily concerned with the cultivation, processing and characteristics of engineered TBCs that have aided in the creation of specialized coatings for use in industrial settings.

Glassy deposits mainly comprising of calcium–magnesium–alumina–silicate (CMAS) accelerate the spallation of thermal barrier coatings (TBCs). In this work, an Al2O3 layer was produced on yttria-stabilized zirconia (YSZ) coating by plasma spray–physical vapor deposition (PS–PVD). The effects of processing parameters during PS–PVD process on the microstructures of the deposited Al2O3 coatings were investigated. A homogeneous Al2O3 coating with porosity less than 1% was deposited at the spray distance of 1400 mm, which is much denser than the coating produced by atmospheric plasma spray (APS) and other PS–PVD coatings sprayed at 1000 and 1900 mm. The CMAS corrosion test at 1250 °C showed that the APS Al2O3 layer was dissolved in CMAS after 24 h thermal exposure, whereas a dense reaction layer mainly composed of anorthite formed on the top region of the PS–PVD Al2O3 layer effectively protected the underlying Al2O3 layer from CMAS infiltration. Thermal cycling behavior of the PS–PVD Al2O3/YSZ TBC and the APS Al2O3/YSZ TBC at 1050 °C was compared. Bulk spallation of the APS TBC occurred within 300 h thermal cycling, while little spallation was observed on the PS–PVD TBC even after 500 h thermal cycling, indicating superior durability of the PS–PVD coating.

Yb2Si2O7/Si environmental barrier coatings (EBCs) were produced by air plasma spray (APS) and low-pressure plasma spray (LPPS) processes. The phase composition, microstructure, and bonding strength of APS and LPPS EBCs were investigated. Thermal cycling tests were performed in air and in steam atmosphere respectively at 1316 °C for both APS and LPPS EBCs. There is no coating failure in air atmosphere for both APS and LPPS EBCs after 900 cycles. In contrast, APS EBCs have an average life of 576 cycles in a steam cycling test in 90% H2O + 10% air at 1316 °C while LPPS EBCs survived 1000 cycles without failure. The superior durability of the LPPS EBCs compared to APS EBCs in the same steam cycling environment is attributed to the significantly reduced thermally grown oxide (TGO) growth rate because of the denser and crack-free microstructure, higher bonding strength, and reduced coefficient of thermal expansion (CTE) mismatch (less Yb2SiO5 phase) in the LPPS Yb2Si2O7/Si EBCs.

The plasma spray-physical vapor deposition (PS-PVD) process is a promising method to manufacture thermal barrier coatings (TBCs). It fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). The durability of PS-PVD manufactured columnar TBCs is strongly influenced by the compatibility of the metallic bondcoat (BC) and the ceramic TBC. Earlier investigations have shown that a smooth BC surface is beneficial for the durability during thermal cycling. Further improvements of the bonding between BC and TBC could be achieved by optimizing the formation of the thermally grown oxide (TGO) layer. In the present study, the parameters of pre-heating and deposition of the first coating layer were investigated in order to adjust the growth of the TGO. Finally, the durability of the PS-PVD coatings was improved while the main advantage of PS-PVD, i.e., much higher deposition rate in comparison to EB-PVD, could be maintained. For such coatings, improved thermal cycling lifetimes more than two times higher than conventionally sprayed TBCs, were measured in burner rigs at ~1250 °C/1050 °C surface/substrate exposure temperatures.

The processing conditions, microstructural and tribological characterizations of plasma-sprayed CoNiCrAlY-BN high temperature abradable coatings are reported in this manuscript. Plasma spray torch parameters were varied to produce a set of abradable coatings exhibiting a broad range of porosity levels (34-62%) and superficial Rockwell hardness values (0-78 HR15Y). Abradability tests have been performed using an abradable-seal test rig, capable of simulating operational wear at different rotor speeds and seal incursion rates (SIRs). These tests allowed determining the rubbing forces and quantifying the blade and seal wear characteristics for slow and fast SIRs. Erosion wear performance and ASTM C633 coating adhesion strength test results are also reported. For optimal abradability performance, it is shown that coating hardness needs to be lower than 70 and 50 HR15Y for slow and fast blade incursion rate conditions, respectively. It is shown that the erosion wear performance, as well as, the coating cohesive strength is a function of the coating hardness. The current results allow defining the coating specifications in terms of hardness and porosity for targeted applications.

The aim of this paper is to define and set up an experimental procedure, based on active thermography, for the characterization of coatings for industrial applications. This procedure is intended to be a fast and reliable method, alternative to the consolidated one described in International Standards. In more detail, a classical active thermographic set up, and not a dedicated apparatus, was used for that aim, and data processing techniques referred to the analytical approach described in Standards. The active thermography procedure provided the measurement of the surface temperature of specimens undergoing a thermal excitation, applied by means of a laser pulse (Pulsed Technique). Temperature data processing, according to and adapting the Standard procedures, allowed to obtain thermal conductivity and diffusivity information. In particular, two coating processes (Atmospheric and Suspension Plasma Spray) applied to the same base material, Inconel 601, and the same coating material were investigated. These results were compared in terms of thermal properties variation with respect to base and coated materials, and in terms of different coating processes (APS and SPS). Obtained results were also compared to those available in literature.

Since the COVID-19 pandemic, efforts in the field of surface decontamination have been redoubled. Finding innovative self-cleaning devices has become a challenge, and several solutions have been proposed in the market in recent years. In this work, an optimized powder/suspension plasma spray process at atmospheric pressure, using a Triplex Pro 210TM torch, is implemented to produce Cu-TiO2 surface coatings on stainless steel. The purpose is to investigate the potential improvement of antibacterial efficacy by the reactive surface species generated from TiO2 photoactivity under irradiation. A water-based suspension, prepared with AnalaR NORMAPURTM TiO2, is used as a precursor to incorporate the photocatalyst into an antibacterial copper matrix. Surface antibacterial tests according to ASTM 2180 standards were performed, and experiments were performed in treated contaminated water. Sub-stoichiometric blue TiO2 coatings showed complete bacterial elimination after 90 min of visible light irradiation, and Cu-TiO2 surface coatings were even able to disinfect the surfaces under white light, making the application interesting for bacterial destruction under natural illumination. These materials are also intended for application in water treatment, including both pathogens and chemical micropollutants, which is a pressing issue facing many countries today.

In this work, optical emission spectroscopy is used to study plasma–liquid precursor interactions in a plasma spray process. A mapping of the plasma jet is performed with a bundle of seven optical fibers while injecting various liquid precursors. The decomposition of two suspensions containing a titania (TiO2) powder in different solvents and that of one solution containing titanium butoxide is analyzed inside a radio frequency thermal plasma. For each precursor, the evolution of both temperature and titanium density along the plasma jet is observed. Two different plasma compositions were used to study their effects on the precursor decomposition. For each experiment, x-ray diffraction was performed on the collected powder to correlate OES observations with the structure and composition of the powder. Comparing these results brings a new understanding of the precursor decomposition inside the plasma, while the noted contrasts between water and ethanol as solvent, and between the use of a powder and that of an alkoxide as a source of titanium, help to assess the effect of these parameters on the plasma spray process.