The formation of Nickel coatings on stainless steel substrates and YSZ (Yttria-Stabilized Zirconia) on NiCrAlY in the Atmospheric Plasma Spray (APS) process is investigated. Coating formation over a substrate with an arbitrary shape (an inclined step in this paper) is considered. The topography of the coatings, as well as their microstructure, e.g., porosity, average thickness, and average roughness, are evaluated. An algorithm, which is based on the Monte-Carlo stochastic model, is employed. The significant difference between the coating temperature and that of the substrate leads to the formation of residual thermal stresses. These stresses are analyzed using Object Oriented Finite-element software (OOF) developed by the National Institute of Standards and Technology (NIST). An image of the cross-section of the coating is imported into the code, which utilizes an adaptive meshing technique and Finite- Element Method to calculate residual thermal stresses. The maximum stress in the coatings occurs at the interface between the coating and the substrate. The coatings' topography and microstructure are compared with those of the experiments.

Thermal barrier coatings have provided a revolution in the industry as they allow a higher operating temperature of equipment, improving the efficiency of gas turbines. However, one of the biggest challenges in terms of increasing the lifespan of TBC systems is the attack of fused silicates or simply CMAS (Calcium-Magnesium-Alumina-Silicate). CMAS are particles from the environment that can penetrate the TBC structure and cause delamination of the coating when exposed to high temperatures during thermal cycling. In this study, a plasma sprayed YSZ coating in the as coated and surface treated condition were given CMAS depositions from various preparation methods, and then subjected to thermal cycles at different evaluation temperatures and exposure times. The permeability of the ceramic layer and the penetration path of CMAS at different temperature levels were evaluated, as well as the penetration characteristics in relation to the microstructure of the ceramic layer. X-Ray diffraction and Scanning Electron Microscopy were used to characterize the applied CMAS and the penetration kinetics and conditions. Samples with longer exposure time had a considerable volume increase. The conditions to guarantee the formation of the silicate and its consequent wettability are also discussed.

In the study, Axial Suspension Plasma Spray (SPS) was used to produce a range of columnar microstructures from Yttria Stabilized Zirconia (YSZ) suspension after an extensive experimental design. The optimized microstructure was applied to a multi-layer GZ/YSZ system, in which both layers were sprayed with SPS. In addition to SPS, a new GZ coating using Axial Solution Precursor Plasma Spray (SPPS) was developed and deposited on top of the SPS GZ coating. The durability in the furnace cycling test (FCT), as well as the consequences of CMAS infiltration into the columnar coatings was extensively studied on different microstructures. Preliminary CMAS test on the SPS coatings infiltrated them completely, leading to delamination. To minimize the detrimental effect of CMAS on the underlying SPS, the dense solution precursor GZ layer was aimed to act as a sealant to protect the underlying columnar SPS-GZ layer from molten CMAS infiltration.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution suspension plasma sprayed YSZ TBC coating was applied on the spike contour of an additively manufactured aerospike engine demonstrator. The engine was designed for a hydrogen peroxide / kerosene 6 kN thrust at 2.0 MPa chamber pressure and was manufactured from nickel-based superalloy Inconel 718 powder using the laser powder bed fusion process (LPBF). Due to the novelty of the application of suspension sprayed YSZ thermal protection coatings on additively manufactured Inconel 718 components, extensive tests were necessary to characterize the interaction between the coating and the component.

In this work, the possibility of controlling the thermally sprayed TBC microstructure is in order to improve the overall TBC system performance. The control is possible primarily by metallic bond coat surface microtexturization prior to ceramic top coat spraying. Such pretreated bond coat was modeled to investigate the influence of the substrate topography on the plasma stream behavior as well as the feedstock particle thermophysical properties and trajectories in the substrate closest proximity. The microscale computational domain was considered here. It was extracted from entire spraying domain and located in the microtextured substrate boundary layer. Then, advanced flow models were introduced to the governing equations to define heat flux to the substrate, turbulent flow, and plasma jet / feedstock droplets interaction. Feedstock discrete phase was defined by the means of Discrete Phase Model (DPM) including particle drag laws and DPM source modelling. The motivation for this study was to model and investigate the influence of the bond coat microtexturization on the behavior of the feedstock particles in the substrate boundary layer. This opens the possibility of better understanding the TBC build-up mechanism and strictly controlling the microstructure of such TBCs.

Previous work conducted on atmospheric plasma spraying has shown the importance of including the measured gun voltage in the modeling procedure to improve the outputs prediction quality. Given a set of controllable input parameters, the produced coating specifications are influenced by the gun voltage measured during the spraying process. As the gun voltage can only be measured once the coating process has started, making predictions about the expected voltage is necessary to better select the process inputs that produce a coating with desired specifications. We suggest that the gun voltage is related to the status of the manufacturing equipment. Exploiting voltage information, we propose a modeling and configuration procedure that uses Gaussian process regression and Kalman filtering to reduce the impact of session-to-session equipment changes as well as in-session equipment wearing. We then demonstrate this approach in simulation and experiments, using an industrial atmospheric plasma spraying set-up to produce YSZ coatings.

Hot section components of stationary gas turbines such as turbine blades are coated with thermal barrier coatings (TBCs) to increase the high thermal strain tolerance thereby the improvement of the performance for the gas turbines. TBCs represent high-performance ceramics and are mostly composed of yttria-stabilized zirconia (YSZ) in order to fulfil the function of thermal insulation. The microstructure of conventional TBCs should be porous to decrease heat conduction. Besides porous TBCs, the recently developed vertically segmented thermal barrier coatings (s-TBCs) feature outstanding thermal durability. In this work, process parameter development for atmospheric plasma spraying (APS) of s-TBCs is presented. Within the experiments, relevant process parameters such as powder feed rate, surface speed and pathway strategy have been optimized. The aim of this work is to achieve a combination of low internal residual stress and high adhesive tensile strength for s-TBCs. For the formation of vertical cracks, the heat input into the powder feedstock material and the substrate must be controlled precisely.

During the thermal cycling process in MCrAlY-YSZ thermal barrier coating system, stresses are produced at bondcoat (BC)-topcoat (TC) interface due to the mismatch of the thermal expansion coefficients of the two coating layers. The stresses at the interface are not a single value and can be affected by the coatings’ microstructure. In this paper, finite element (FE) modeling method was used to study the behavior of the stress distribution at the coatings’ interface. The influence of the pore structure in the ceramic TC and the micro bulge structure at the metal BC surface was investigated. The results showed that both structures can change the stress distribution. The pores played a “stone-in-river” role, which trapped higher stress around them and simultaneously reduced the size of the macro stress zones in TC. The micro bulges at the TC/BC interface also trapped high stresses which could cause more interaction between TC cracks and BC roughness.

Thermal spraying is a complex physical process consisting of three main sub-systems: flame/plume generation, powder/flame/plume interaction and coating build-up. While mathematical and CFD models provide valuable insight about the individual modules of a thermal spray process, it is very difficult to gain overall insight of the whole process and dependencies between different inputs and outputs using mathematical and CFD analysis, due to very complex and interconnected nature of the thermal spray process. In this work, a sophisticated experiment has been conducted to collect enough data for the sake of developing data-driven model of a plasma spray process. Metco 204 powder feedstock material and F4 gun have been used. An optimized number of data samples has been chosen by applying common industrial input parameters in the experiment. The developed neural network model is able to predict the coating quality parameters with acceptable average accuracy of above 90% on test data by considering all relevant measurement error deviations of the process analysis methods. A sophisticated user-interface has been developed to enable the use of the model for coating parameter development as well as the designing recipe for target coating characteristics. The developed model can be used for different purposes: parameter development, off-line coating quality control, and eventually adaptive coating control.

Repair methods are of great interest to the aeronautic industry, especially for turbines. Deposition techniques that can quickly and easily repair small localised areas of damage in Thermal Barrier Coatings (TBCs) on combustion chambers could be financially worthwhile. In a first approach, a Low-Power Plasma Reactor (LPPR) operating at low pressure (< 1000 Pa, 240 W) was tested to locally deposit effective Yttria partially Stabilised Zirconia (YSZ) as TBC; however, a vacuum chamber would be more difficult to implement on an industrial scale. For this reason, a new LPPR (< 1 kW) operating at atmospheric pressure with solution precursors was investigated. The precursors were injected in the plasma afterglow to be sprayed and deposited onto parts of combustion chambers. As the afterglow temperature was cooler than for most thermal spray processes, spray distance was less than 10 mm. As such, YSZ deposition could be performed locally in hard-to-reach areas. YSZ coating characteristics were studied by FTIR and SEM analyses. For example, YSZ coatings exhibited the expected stoichiometry, a precursor conversion of 98 mol%, good adherence, and a porosity evaluated at approximately 30 vol%. In addition, YSZ coating thickness could be greater than 200 μm.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution, the influence of heat treatment and surface conditioning of the additively manufactured Inconel 718 substrates on the thermocycling performance of suspension sprayed YSZ coatings was investigated. The different surface conditions included as-built, sandblasted and milled substrate surfaces with and without heat treatment. YSZ coatings were applied using suspension plasma spraying (SPS) with commercial available suspensions. Thermal cycling tests (FCT) at 1100°C, 1300 °C, and 1500 °C were applied to coating systems until failure occurred. The microstructures of the samples were characterized before and after thermal cycling. The performance of the coatings was mainly influenced by the coating morphology and FCT test conditions and less by the state of the AM substrates. Columnar-like YSZ SPS sprayed coatings on AM Inconel 718 substrates seemed to be a promising candidate for rocket engine applications.

Most of ductile metals can be deposited by cold spray (CS). For brittle ceramic, such solid-state deposition process is still questionable, but some recent work on Ti0 2 or hydroxyapatite powders have shown that micrometric ceramic powder could be deposited by CS. In this work, it is claimed that the nature and the porous architecture of a ceramic powder with agglomerated ultra-fine grains play an important role on the impact behaviour. The aim of this work is to investigate the deformation behaviour of ceramic agglomerated powders under high velocity impact. Two different powders, respectively 3YSZ and Y 2 O 3 , were selected in order to study their architectures (particle size, porosity, density, crystallite size, etc.). Cold spray “splats” experiments, with various spraying distances to vary the particles velocities upon impact, were carried out to observe the deformation and fragmentation. In case of Y 2 O 3 , cold spray with dynamic vacuum surrounding atmosphere up to 3kPa were also prepared to evaluate the role of the atmosphere on the resulting impact. In parallel, in situ SEM micro-compression tests at 10 −2 s −1 on cross-sectioned 3YSZ particles involving flat-punch nano-indentation and micropillar compression were performed. By modelling the compression tests, the aim is to identify a Drücker-Prager behaviour law suitable for an agglomerated ceramic powder under quasi-static compression. Such deformation behaviour could help to better understand the compaction behaviour of agglomerated powders.

Deposition of hybrid plasma-sprayed coatings employing both dry powder and liquid feedstocks enables preparation of innovative coating architectures. Using this technique, miniature domains of additional (secondary) material may be introduced via the liquid feedstock route into the more conventional powder-deposited coating, providing potential benefits for the coating functionality. In this contribution, we have explored the tribological properties of hybrid coatings sprayed from alumina powder with additions of chromia (Cr 2 O 3 ), zirconia (ZrO 2 ), yttria-stabilized zirconia (YSZ), and titania (TiO 2 ) delivered from liquid feedstocks. The coatings were subjected to dry sliding wear testing and a subsequent analysis of the wear tracks to determine their wear resistance and coefficient of friction, as well as a qualitative assessment of the wear mechanisms. The hybrid coating doped with the chromia addition matched the remarkable wear resistance of highly-dense suspension-sprayed coatings. This is a significant result, especially when considering the order of magnitude better production efficiency of the hybrid coatings.

In-flight particle state has been shown to play a crucial role in determining the properties of thermally sprayed coatings. Therefore, process control strategies have been suggested to keep the in-flight particle state constant in order to decrease the variability of coating quality. Such strategies are already used to some extent in industrial applications, where the particle state is measured before starting the coating of a part to make a go/no-go decision based on whether the values are inside a predefined process window. This paper shows the importance of the evaluation procedure of the in-flight particle state, with focus on process control applications. The paper demonstrates this on the example of atmospheric plasma spraying (APS) of yttria-stabilized zirconia (YSZ), using a commercially available sensor system Accuraspray for measuring the ensemble particle temperatures and velocities as descriptors of the in-flight particle state. It is concluded that the stabilization time of the particle jet might be different from what is practically considered. Moreover, the paper investigates an approach of monitoring the in-flight particle state during a coating run without having to install the sensor system on the robot arm.

Recent development of plasma spraying with liquid feedstocks enabled deposition of novel thermal barrier coatings (TBCs) with top-coats incorporating various desirable features, such as columnar microstructure or ultrafine porosity. Moreover, different materials may be relatively easily combined, e.g., by alternating feedstocks in the feed line or using feedstock mixtures. Coatings with gradient change of chemistry/microstructure towards the free-surface can be also prepared by gradual change of the feedstock composition, which may be potentially beneficial for example to mitigate stresses at the macroscopic interfaces between TBC layers (typically bond-coat/top-coat or within layered top-coat). In this study, three experimental TBCs with gradient top-coats were successfully deposited by hybrid water/argon-stabilized WSP-H plasma torch, i.e., one version of coating with 8 wt.% yttria stabilized zirconia transitioning into gadolinium zirconate Gd 2 Zr 2 O 7 (YSZ→GZO) and two versions of Al 2 O 3 transitioning into YSZ (Al 2 O 3 →YSZ). Thermal cycling fatigue (TCF) test with peak temperature of 1100 °C showed outstanding thermal shock resistance of the YSZ→GZO coating and mediocre to poor resistance of both Al 2 O 3 →YSZ coatings.

In suspension plasma spraying (SPS); the use of water based suspensions provides a cheaper; safer and more environmentally friendly alternative to organic liquids. However; due to the physical properties of water; producing a water based SPS coating with desirable microstructure has so far been elusive. In this study; the effects of pH and dispersant on the rheology and stability of YSZ water based suspensions were investigated. PEI; PBTCA and α-Terpineol were used as dispersant polymers. The stabilized suspensions were deposited by Axial III plasma spray system and the relationship between suspension parameters and the atomized droplet size and the final coating microstructure was studied. The results showed that a combination of Terpineol dispersant with pH adjustment to 2.5; could lead to a SPS coating with columnar microstructure having 17.4 vol.% porosity.

Intensive R&D work of more than one decade has demonstrated many unique coating properties, particularly for oxide ceramic coatings fabricated by suspension thermal spraying technology. Suspension spraying allows producing yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC) with columnar microstructure, similar to those produced by electron-beam physical vapor deposition (EB-PVD), and vertically cracked morphologies, with a generally low thermal conductivity. Therefore, suspension sprayed YSZ TBCs are seen as an alternative to EB-PVD coatings and those produced by conventional air plasma spray (APS) processes. Nonetheless, the microstructure of the YSZ topcoat is strongly influenced by the properties of the metallic bondcoat. In this work, direct laser interference patterning (DLIP) was applied to texture the surface topography of Ni-alloy based plasma sprayed bondcoat. Suspension plasma spraying (SPS) was applied to produce YSZ coatings on top of as-sprayed and laser-patterned bondcoat. The samples were characterized in terms of microstructure, phase composition and thermal cycling performance. The influence of the bondcoat topography on the properties of suspension sprayed YSZ coatings is presented and discussed.

In this study, a novel self-healing concept is considered in order to increase the lifetime of thermal barrier coatings (TBCs) in modern gas turbines. For that purpose, SiC healing particles were introduced to conventional 8YSZ topcoats by using various plasma spray concepts, i.e., composite or multilayered coatings. All topcoats were sprayed by SG-100 plasma torch on previously deposited NiCrAlY bondcoats produced by conventional atmospheric plasma spraying. Coatings were subjected to thermal conductivity measurements by laser flash method up to 1000°C, isothermal oxidation testing up to 200h at 1100°C and finally thermal cyclic fatigue (TCF) lifetime testing at 1100°C. Microstructural coating evaluation was performed by scanning electronic microscope (SEM), in the as-produced and post high-temperature tested states. This was done to analyze the self-healing phenomena and its influence on the high-temperature performance of the newly developed TBCs.

The porous architecture of coatings has a significant influence on the coating performances and thus should be properly designed for the intended applications. For simulating the coating properties, it is necessary to determine the numerical representation of the coating microstructure. In this study, YSZ coatings were manufactured by suspension plasma spray (SPS). Afterwards, the porous architecture of as-prepared coatings was investigated by the combination of three techniques, imaging analysis, Ultra Small Angle X-ray Scattering (USAXS), and X-ray transmission. A microstructural model for reconstructing the porous architecture of the SPS coating was subsequently computed according to the collected experimental results. Finally, the coating thermal properties were simulated based on the model and were compared with the experimental results.

This study presents the results of thermal cycling experiments on thermal barrier coatings deposited using hybrid water/argon-stabilized plasma (WSP-H) torches. Topcoats produced from YSZ suspensions and powders were successfully prepared and evaluated by thermal fatigue testing. Quad-layer coatings with topcoats consisting of yttria stabilized zirconia, gadolinium zirconate, and yttrium aluminum garnet were also prepared and tested at high temperatures and thermal gradients. The results obtained show the potential of WSP-H technology for applications where protection of large components or deposition of thick coatings are required.