Plasma spraying with liquid feedstock offers an exciting opportunity to obtain coatings with characteristics that are vastly different from those produced using conventional spray-grade powders. The two extensively investigated variants of this technique are suspension plasma spraying (SPS), which utilizes a suspension of fine powders in an appropriate medium, and solution precursor plasma spraying (SPPS), which involves use of a suitable solution precursor that can form the desired particles in situ. The advent of axial injection plasma spray systems in recent times has also eliminated concerns regarding low deposition rates/efficiencies associated with liquid feedstock. The 10–100 μm size particles that constitute conventional spray powders lead to individual splats that are more than an order of magnitude larger compared to those resulting from the fine (approximately 100 nm–2 μm in size) particles already present in suspensions in SPS or formed in situ in SPPS. The distinct characteristics of the resulting coatings are directly attributable to the above very dissimilar splats (“building blocks” for coatings) responsible for their formation. This chapter discusses the salient features associated with SPS and SPPS processing, highlights their versatility for depositing a vast range of ceramic coatings with diverse functional attributes, and discusses their utility, particularly for high-temperature applications through some illustrative examples. A further extension of liquid feedstock plasma processing to enable use of hybrid powder-liquid combinations for plasma spraying is also discussed. This presents a novel approach to explore new material combinations, create various function-dependent coating architectures with multi-scale features, and enable convenient realization of layered, composite, and graded coatings as demonstrated through specific examples.

Coatings are pivotal in combating problems of premature component degradation in aggressive industrial environments and constitute a strategic area for continued development. Thermal spray (TS) coatings offer distinct advantages by combining versatility, cost-effectiveness, and the ability to coat complex geometries without constraints of other in-chamber processes. Consequently, TS techniques like high-velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS) are industrially well-accepted. However, they have reached limits of their capabilities while expectations from coatings progressively increase in pursuit of enhanced efficiency and productivity. Two emerging TS variants, namely high-velocity air-fuel (HVAF) and liquid feedstock thermal spraying, offer attractive pathways to realize high-performance surfaces superior to those hitherto achievable. Supersonic HVAF spraying provides highly adherent coatings with negligible porosity and its low processing temperature also ensures insignificant thermal ‘damage’ (oxidation, decarburization, etc.) to the starting material. On the other hand, liquid feedstock derived TS coatings, deposited using suspensions of fine particles (100 nm–5 µm) or solution precursors, permits the production of coatings with novel microstructures and diverse application-specific architectures. The possibility of hybrid processing, combining liquid and powder feedstock, provides further opportunities to fine tune the properties of functional surfaces. These new approaches are discussed along with some illustrative examples.

In this work, three double layered thermal barrier coating (TBC) variations with different gadolinium zirconate (GZ) and YSZ thickness (400GZ/100YSZ, 250GZ/250YSZ and 100GZ/400YSZ respectively, where the prefixed numbers before GZ and YSZ represent the layer thickness in μm), were produced by suspension plasma spray (SPS) process. The objective was to investigate the influence of YSZ thickness on the thermal conductivity and thermal shock lifetime of the GZ/YSZ double layered TBCs. The as sprayed TBCs were characterized using SEM, XRD and porosity measurements. Thermal diffusivity measurements were made using laser flash analysis and the thermal conductivity of the TBCs was calculated. The double layered TBC with the lowest YSZ (400GZ/100YSZ) thickness showed lower thermal diffusivity and thermal conductivity. The double layered TBCs were subjected to thermal shock test at a TBC surface temperature of 1350 °C. Results indicate that the TBC with a higher YSZ thickness (100GZ/400YSZ) showed inferior thermal shock lifetime whereas the TBCs with low YSZ thickness showed comparatively higher thermal shock lifetimes. Failure of the TBCs after thermal shock test was analyzed using SEM and XRD to gain further insights.

The elastic modulus and fracture toughness of an air plasma sprayed thermal barrier coating (APS TBC) were measured using the micro-cantilever bending technique. The micro-cantilevers were machined by a focused ion beam with their central arms either parallel or normal to the bond coat/topcoat interface. Such orientations allowed direct measurements of both the in-plane and out-of-plane elastic moduli as well as mode I fracture toughness by bending. The calculated elastic modulus along the in-plane and out-of-plane direction is 144 GPa and 110 GPa, respectively, suggesting that the APS TBC is elastically anisotropic at microscale. The derived mode I fracture toughness along the plane parallel to the interface is 0.40 MPam. This relatively low toughness reflects the weak fracture resistance of the highly-flawed APS for short cracks at microscale. The measurements in this study can be incorporated into micromechanical life time prediction models of the APS TBCs. © 2019 Elsevier B.V.

A multi-component and multi-phase-field modelling approach, combined with transformation kinetics modelling, was used to model microstructure evolution during laser metal powder directed energy deposition of Alloy 718 and subsequent heat treatments. Experimental temperature measurements were utilised to predict microstructural evolution during successive addition of layers. Segregation of alloying elements as well as formation of Laves and δ phase was specifically modelled. The predicted elemental concentrations were then used in transformation kinetics to estimate changes in Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams for Alloy 718. Modelling results showed good agreement with experimentally observed phase evolution within the microstructure. The results indicate that the approach can be a valuable tool, both for improving process understanding and for process development including subsequent heat treatment.

Electron beam melting (EBM) is a powder bed additive manufacturing process where a powder material is melted selectively in a layer-by-layer approach using an electron beam. EBM has some unique features during the manufacture of components with high-performance superalloys that are commonly used in gas turbines such as Alloy 718. EBM has a high deposition rate due to its high beam energy and speed, comparatively low residual stresses, and limited problems with oxidation. However, due to the layer-by-layer melting approach and high powder bed temperature, the as-built EBM Alloy 718 exhibits a microstructural gradient starting from the top of the sample. In this study, we conducted modeling to obtain a deeper understanding of microstructural development during EBM and the homogenization that occurs during manufacturing with Alloy 718. A multicomponent phase-field modeling approach was combined with transformation kinetic modeling to predict the microstructural gradient and the results were compared with experimental observations. In particular, we investigated the segregation of elements during solidification and the subsequent "in situ" homogenization heat treatment at the elevated powder bed temperature. The predicted elemental composition was then used for thermodynamic modeling to predict the changes in the continuous cooling transformation and time-temperature transformation diagrams for Alloy 718, which helped to explain the observed phase evolution within the microstructure. The results indicate that the proposed approach can be employed as a valuable tool for understanding processes and for process development, including post-heat treatments. © 2019, The Author(s).

Superhydrophobic surfaces that are durable and can be easily manufactured are of high interest for many industrial applications. Measuring and understanding roughness in the context of superhydrophobicity is the first step in creation of a surface that does not require activation to be hydrophobic. In this study, the as sprayed surface of different cermet (WC-10Co4Cr and Cr3C2-25Ni20Cr) coatings produced by High Velocity Air Fuel (HVAF) spraying – have been investigated to assess their wetting ability. In order to address the challenges raised by the specific roughness profile of thermal spray surfaces, two routes have been adapted and used for surface characteristics analysis i.e. statistical and fractal. Results show that both methods have a strong correlation to wettability. Roughness parameters Sdq and Sdr show good correlation with advancing contact angle. Hausdorff Dimension of a sub-micrometer profile shows good relation with the contact angle and provides information for state of the droplet. To determine how to increase the contact angle of the coating surface, coating parameters such as CGS Density have been correlated with Hausdorff Dimension. Both methods provide good understanding in terms of wettability of rough cermet surfaces. © 2019 Elsevier B.V.

This work employed an axial suspension plasma spray (SPS) process to deposit two different gadolinium zirconate (GZ) based triple layered thermal barrier coatings (TBCs). The first was a 'layered' TBC (GZ dense/GZ/YSZ) where the base layer was YSZ, intermediate layer was a relatively porous GZ and the top layer was a relatively dense GZ. The second triple layered TBC was a 'composite' TBC (GZ dense/GZ + YSZ/YSZ) comprising of an YSZ base layer, a GZ + YSZ intermediate layer and a dense GZ top layer. The as sprayed TBCs (layered and composite) were characterized using SEM/EDS and XRD. Two different methods (water intrusion and image analysis) were used to measure the porosity content of the as sprayed TBCs. Fracture toughness measurements were made on the intermediate layers (GZ + YSZ layer of the composite TBC and porous GZ layer of the layered TBC respectively) using micro indentation tests. The GZ + YSZ layer in the composite TBC was shown to have a slightly higher fracture toughness than the relatively porous GZ layer in the layered TBC. Erosion performance of the as sprayed TBCs was evaluated at room temperature where the composite TBC showed higher erosion resistance than the layered TBC. However, in the burner rig test conducted at 1400 °C, the layered TBC showed higher thermal cyclic lifetime than the composite TBC. Failure analysis of the thermally cycled and eroded TBCs was performed using SEM and XRD. © 2018 Elsevier B.V.

In this work, three different TBC compositions comprising of yttria partially stabilized zirconia (8YSZ), yttria fully stabilized zirconia (48YSZ) and gadolinium zirconate (GZ) respectively, were processed by suspension plasma spray (SPS) to obtain columnar microstructured TBCs. Additionally, for comparison, lamellar microstructured, 7YSZ TBC was deposited by air plasma spray (APS) process. SEM analysis was carried out to investigate the microstructure and white light interferometry was used to evaluate the surface morphology of the as-sprayed TBCs. Porosity measurements were made using water intrusion and image analysis methods and it was observed that the SPS-YSZ and APS-YSZ TBCs showed higher porosity content than SPS-GZ and SPS-48YSZ. The as-sprayed TBC variations (APS-YSZ, SPS-YSZ, SPS-GZ, and SPS-48YSZ) were subjected to erosion test. Results indicate that the erosion resistance of APS-YSZ TBC was inferior to the SPS-YSZ, SPS-GZ and SPS-48YSZ TBCs respectively. Among the SPS processed TBCs, SPS-YSZ showed the highest erosion resistance whereas the SPS-48YSZ showed the lowest erosion resistance. SEM analysis of the eroded TBCs (cross section and surface morphology) was performed to gain further insights on their erosion behavior. Based on the erosion results and post erosion SEM analysis, erosion mechanisms for splat like microstructured APS TBC and columnar microstructured SPS TBCs were proposed. The findings from this work provide new insights on the erosion mechanisms of columnar microstructured TBCs and lamellar microstructured TBCs deposited by plasma spray. © 2019 Elsevier B.V.

Thermal barrier coatings (TBCs) play a vital role in allowing the gas turbine engines to operate at high temperatures. With higher operating temperatures (>1200°C), the standard TBC material, 7-8wt. % Yttria Stabilized Zirconia (YSZ), is susceptible to CMAS (Calcium Magnesium Alumino Silicates) degradation and undesirable phase transformation. New TBC materials such as gadolinium zirconate (GZ) have shown to be capable of overcoming the challenges faced by YSZ. However, GZ has inferior fracture toughness relative to YSZ. In this work, three double layered TBC variations with different GZ and YSZ thickness respectively (400GZ/100YSZ, 250GZ/250YSZ and 100GZ/400GZ respectively, where the prefix numbers represent thickness in urn) were produced by suspension plasma spray (SPS) process. In all the three double layered TBC variations, the overall TBC thickness with GZ as the top layer and YSZ as the base layer was kept the same (500 urn). The objective was to investigate the influence of YSZ thickness on the thermal cyclic fatigue performance of GZ/YSZ double layered TBC. The as sprayed TBCs were characterized by SEM, XRD and porosity measurements and later subjected to thermal cyclic fatigue test at 1100°C. It was observed that the GZ/YSZ double layered TBC with lowest YSZ thickness (400GZ/100YSZ) showed higher thermal cyclic lifetime whereas the TBC with thicker YSZ layer (100GZ/400YSZ) showed lowest thermal cyclic fatigue lifetime. The failure analysis of the thermally cycled TBCs revealed similar failure modes, i.e. spallation of the top coat due to horizontal crack propagation within the thermally grown oxide (TGO). Furthermore, the ceramic top coats in all the three TBC variations after failure showed the widening of column gaps. © 2018 ASM International® All rights reserved.