Suspension Plasma Spraying Thermal Barrier Coatings Research Articles

In Situ Monitoring of Cracking Mechanisms in Multi-Layered Suspension Plasma-Sprayed Thermal Barrier Coatings

In Situ Monitoring of Cracking Mechanisms in Multi-Layered Suspension Plasma-Sprayed Thermal Barrier Coatings

Lifetime Extension of Atmospheric and Suspension Plasma-Sprayed Thermal Barrier Coatings in Burner Rig Tests by Pre-Oxidizing the CoNiCrAlY Bond Coats

Lifetime Extension of Atmospheric and Suspension Plasma-Sprayed Thermal Barrier Coatings in Burner Rig Tests by Pre-Oxidizing the CoNiCrAlY Bond Coats

Understanding the effect of bondcoat surface treatment on enhanced lifetime of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are extensively used in gas turbine engines in power generation and aerospace applications. Improvements in TBC performance and lifetime are continuously pursued by the gas turbine industry. Suspension plasma spraying (SPS) has shown promising results in recent years as it could produce a porous columnar TBC microstructure exhibiting low thermal conductivity and high durability. Further improvements in lifetime and fundamental understanding of failure mechanisms would lead to their wider application.Lifetime of a TBC system is dependent not only on the topcoat microstructure and chemistry, but also on topcoat-bondcoat interface and bondcoat characteristics as they influence the growth rate of thermally grown oxide (TGO) layer as well as the mismatch stresses generated in the topcoat during operation. Moreover, in SPS TBCs, the column density and porosity in the topcoat are also affected by the bondcoat surface roughness which in turn affects TBC lifetime. In previous work, it was shown that bondcoat surface treatment by shot peening followed by light grit blasting can significantly enhance the cyclic lifetime of SPS TBCs.The objective of this work was to gain further insight into the lifetime improvements due to surface treatment of bondcoat. Commercial NiCoCrAlY powder was deposited by high velocity air fuel (HVAF) and high velocity oxy fuel (HVOF) spraying while SPS was used to deposit yttria stabilised zirconia topcoat. Before spraying the topcoat, the bondcoat samples were subjected to vacuum heat treatment, shot peening, and/or grit blasting. Residual stress measurements were performed by X-ray diffraction on samples with different variants of bondcoat treatments and compared with Almen strip tests. The lifetime was examined by thermal cyclic fatigue testing. The relationships between changes in bondcoat surface roughness, residual stress state and microstructure due to surface treatments, and resultant topcoat microstructure and lifetime were studied and discussed.

Effect of the microstructure of suspension plasma-sprayed thermal barrier coatings on their thermal cycling damage

The suspension plasma spray (SPS) technique has recently attracted attention due to the variety of microstructures achievable using submicron spray particles mixed with a solvent to form a suspension. Thermal barrier coatings (TBCs) with a columnar microstructure can be obtained using SPS. Because of its unique columnar microstructure achieving high strain tolerance, a top coat sprayed using this technique can efficiently suppress thermal cycling damage in SPS–TBCs. However, there are few reports on the effect of microstructure on the damage behavior of SPS–TBCs with a columnar structure. In this study, the effect of the top coat microstructure on the thermal cycling fatigue of SPS–TBCs was investigated. The size of the complex oxide formed on the top coat–bond coat interface and the length of the cracks at the interface (formed as a result of thermal cycling) increase with the column diameter in the top coat of SPS–TBCs. The thermal spray conditions affect the thermal cycling fatigue life of SPS–TBCs. The SPS–TBC with fine columnar structure exhibits superior thermal cycling fatigue life.

Understanding the influence of microstructure on hot corrosion and erosion behavior of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are bilayer systems comprising a 7–8 wt% yttria partially stabilized zirconia (YSZ) top coat deposited over a metallic bond coat. Suspension plasma spraying (SPS) is an advanced and attractive top coat processing technique due to its capability to yield a variety of microstructures, including the desired columnar microstructure for enhanced strain tolerance and durability. This work attempts to investigate the desirable microstructural features in an SPS processed TBCs to mitigate hot corrosion and minimize erosion related losses that are often responsible for coating degradation. SPS processed TBCs were deposited utilizing three different spray conditions to obtain distinct microstructural features (column density, interpass [IP] porosity bands, column width), porosity content, and mechanical properties. Apart from comprehensive characterization utilizing SEM, XRD and micro-indentation tests, the as-deposited TBCs were subjected to hot-corrosion tests in the presence of vanadium pentoxide and sodium sulfate as corrosive salts. Post-corrosion analysis revealed complete infiltration of the molten salts in all the investigated TBCs. However, the delamination cracks generated due to the infiltrated corrosive species were minimal in case of TBCs with higher fracture toughness. The differences in microstructure and mechanical properties also led to differences in erosion performance, with TBCs possessing minimal total porosity content and high fracture toughness best resisting erosion related damage. Post-erosion analysis revealed that the TBCs with higher fracture toughness and micro-hardness showed superior erosion resistance. Based on the erosion and corrosion results and subsequent post-mortem of failed specimens, plausible damage mechanisms are proposed. Findings from this work provide new insights on developing damage tolerant TBCs microstructures with enhanced durability when exposed to erosion and hot corrosion environments. • SPS TBCs with varying columnar microstructural features and mechanical properties were deposited. • Influence of parameters (suspension feed rate, spray distance, etc.) on microstructural features in SPS TBCs examined. • Desirable microstructural features were identified to minimize erosion and hot corrosion.

Thermal barrier coatings with novel architectures for diesel engine applications

The increased demands for higher efficiency and environmentally friendly diesel engines have led to a continuous search for new coating processing routes and new ceramic materials that can provide the required properties when applied on engine components such as pistons and exhaust manifolds. Although successful in gas turbine applications, thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) processes have not been employed so far in the automotive industry. This work aims to achieve a better understanding of the role of thermal conductivity and thermal effusivity on the durability of SPS TBCs applied to pistons of diesel engines. Three different coating architectures were considered for this study. The first architecture was yttria-stabilized zirconia (YSZ) lamellar top coat deposited by APS (Atmospheric Plasma Spray) and used as a reference sample in this study. The second architecture was a columnar SPS top coat of either YSZ or gadolinium zirconate (GZO) while the third architecture was an SPS columnar top coat, “sealed” with a dense sealing layer deposited on the top coat. Two types of sealing layers were used, a metallic (M) or a ceramic thermal spray layer (C). Laser Flash Analysis (LFA) was used to determine the thermal conductivity and thermal effusivity of the coatings. Two different thermal cyclic tests were used to test the TBCs behavior under cyclic thermal loads. Microstructure analysis before and after the thermal cyclic tests were performed using SEM in different microstructures and materials. The thermal cyclic test results were correlated with coatings microstructure and thermophysical properties. It was observed that the columnar coatings produced by SPS had an enhanced service life in the thermal cyclic tests as compared to the APS coatings.

Microstructure and failure analysis of suspension plasma sprayed thermal barrier coatings

Improvements in performance of thermal barrier coatings (TBCs) used in gas turbine engines are highly desired as they can result in higher engine efficiency leading to reduction of harmful emissions. Suspension plasma spraying (SPS) has been shown to produce high performance porous columnar TBCs that can provide low thermal conductivity and high durability. Apart from the topcoat microstructure and chemistry, the lifetime of TBCs is also dependent on bondcoat microstructure and chemistry, and topcoat-bondcoat interface roughness. In case of SPS TBCs, the interface roughness can significantly affect the columnar topcoat microstructure, thus making the bondcoat selection even more crucial.In this work, six different sets of samples were produced by fabricating bondcoats with conventional atmospheric plasma spraying (APS), high velocity air fuel (HVAF) spraying, or hybrid water/argon stabilised plasma (WSP-H) gun, and SPS topcoats using axial SPS (ASPS) or WSP-H spray guns. The objective of this study was to investigate the influence of varying the topcoat microstructure, bondcoat microstructure and topcoat-bondcoat interface roughness on oxide growth behaviour and thermal cyclic fatigue (TCF) lifetime of SPS TBCs. Samples after failure were investigated to understand the failure mechanism in each case.The results showed that changing the bondcoat spray process and spray gun resulted in significant variation in bondcoat surface roughness. A porous columnar structure was created by the ASPS process, while a feathery columnar structure was created by the WSP-H spray gun in this study. Samples with WSP-H bondcoat resulted in highest cyclic lifetime in this study, despite showing severe oxidation of the bondcoat as compared to APS and HVAF bondcoats. This result could be attributed to the very high bondcoat surface roughness in these samples that could have resulted in improved mechanical anchoring of the topcoat. The HVAF bondcoats showed the best oxidation resistance in this study.

Design of high lifetime suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) fabricated by suspension plasma spraying (SPS) have shown improved performance due to their low thermal conductivity and high durability along with relatively low production cost. Improvements in SPS TBCs that could further enhance their lifetime would lead to their widespread industrialisation. The objective of this study was to design a SPS TBC system with optimised topcoat microstructure and topcoat–bondcoat interface, combined with appropriate bondcoat microstructure and chemistry, which could exhibit high cyclic lifetime. Bondcoat deposition processes investigated in this study were high velocity air fuel (HVAF) spraying, high velocity oxy fuel spraying, vacuum plasma spraying, and diffusion process. Topcoat microstructure with high column density along with smooth topcoat–bondcoat interface and oxidation resistant bondcoat was shown as a favourable design for significant improvements in the lifetime of SPS TBCs. HVAF sprayed bondcoat treated by shot peening and grit blasting was shown to create this favourable design.

A study of suspension plasma-sprayed insulated pistons evaluated in a heavy-duty diesel engine

Thermal barrier coatings can be used to reduce the heat losses in heavy-duty diesel engines. A relatively new coating method for thermal barrier coatings is suspension plasma-spraying. Single-cylinder engine tests have been run to evaluate how heat losses to piston, cylinder head and exhausts as well as the specific fuel consumption are influenced by coating pistons with two different suspension plasma-sprayed thermal barrier coatings and one atmospheric plasma-sprayed thermal barrier coating, and comparing the results to those from an uncoated steel piston. The two suspension plasma-sprayed thermal barrier coatings showed reduced heat losses through the piston and less heat redirected to the cylinder head compared to conventional atmospheric plasma-sprayed thermal barrier coating, while one suspension plasma-sprayed coating with yttria-stabilized zirconia as top coat material showed increased exhaust temperature. However, the indicated specific fuel consumption was higher for all tested thermal barrier coatings than for an uncoated engine. The best performing thermal barrier coating with respect to indicated specific fuel consumption was a suspension plasma-sprayed coating with gadolinium zirconate as top coat material.

Influence of microstructure on the erosion behaviour of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are applied on the surface of hot parts of gas turbine engines to increase the turbine efficiency by providing thermal insulation and to protect the engine parts from the harsh environment. Typical degradation of TBCs can be attributed to bond coat oxidation, thermal stress etc. In addition to this, erosion can also lead to partial or complete removal of the TBCs especially when the engine operates under erosive environment such as flying over desert area, near active volcanic or offshore ocean environment. Suspension Plasma Spraying (SPS) is a promising technique for TBC applications by virtue of its ability to produce a strain-tolerant porous-columnar microstructure that combines the benefits of both electron beam physical vapor deposited (EB-PVD) as well as atmospheric plasma sprayed (APS) coatings. This work investigates the influence of various coating microstructures produced by SPS on their erosion behavior. Six different coatings with varied microstructures produced using different suspensions with distinct characteristics were studied and their erosion resistance was compared. Results showed significant influence of SPS TBCs microstructures on the erosion resistance. Furthermore, the erosion resistance of SPS TBCs showed a close correlation between fracture toughness and the erosion rate, higher fracture toughness favours superior erosion resistance.

Architecture designs for extending thermal cycling lifetime of suspension plasma sprayed thermal barrier coatings

Suspension plasma spraying (SPS) as a relatively new spraying technology has great potential on depositing high performance thermal barrier coatings (TBCs). In some cases, however, columnar SPS TBCs show premature failure in thermal cycling test. To explain the reasons of such failure, a failure mechanism for columnar SPS TBCs was proposed in this work. The premature failure of TBCs might be related to the radial stresses in the vicinity of top coat/bond coat interface. These radial stresses were introduced by the thermal misfit and the roughness of bond coat. According to this mechanism, two architecture designs of SPS TBCs were applied to improve the thermal cycling lifetime. One was a double layered top coat design with a lamellar atmospheric plasma sprayed (APS) sub-layer and a columnar SPS top-layer. The other one was a low roughness bond coat design with a columnar SPS top coat deposited on a low roughness bond coat which was grinded before the spraying. With both designs, lifetimes of SPS TBCs were significantly extended. Especially, a lifetime even better than conventional APS TBCs was achieved with the double layered design.

Development of bondcoats for high lifetime suspension plasma sprayed thermal barrier coatings

Fabrication of thermal barrier coatings (TBCs) by suspension plasma spraying (SPS) seems to be a promising alternative for the industry as SPS TBCs have the potential to provide lower thermal conductivity and longer lifetime than state-of-the-art allowing higher engine efficiency. Further improvements in lifetime of SPS TBCs and fundamental understanding of failure mechanisms in SPS TBCs are necessary for their widespread commercialisation. In this study, the influence of varying topcoat-bondcoat interface topography and bondcoat microstructure on lifetime was investigated. The objective of this work was to gain fundamental understanding of relationships between topcoat-bondcoat interface topography, bondcoat microstructure, and failure mechanisms in SPS TBCs.Seven sets of samples were produced in this study by keeping same bondcoat chemistry but varying feedstock particle size distributions and bondcoat spray processes. The topcoat chemistry and spray parameters were kept identical in all samples. Three-dimensional surface measurements along with scanning electron microscopy images were used to characterise bondcoat surface topography. The effect of varying interface topography and bondcoat microstructure on thermally grown oxide formation, stresses and lifetime was discussed.The results showed that varying bondcoat powder size distribution and spray process can have a significant effect on lifetime of SPS TBCs. Smoother bondcoats seemed to enhance the lifetime in case of SPS TBCs in case of same bondcoat chemistry and similar bondcoat microstructures. When considering the samples investigated in this study, samples with high velocity air-fuel (HVAF) bondcoats resulted in higher lifetime than other samples indicating that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.

Experimental visualization of microstructure evolution during suspension plasma spraying of thermal barrier coatings

This paper investigates the evolution of microstructure of thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) through a careful experimental study. Understanding the influence of different suspension characteristics such as type of solvent, solid load content and median particle size on the ensuing TBC microstructure, as well as visualizing the early stages of coating build-up leading to formation of a columnar microstructure or otherwise, was of specific interest. Several SPS TBCs with different suspensions were deposited under identical conditions (same substrate, bond coat and plasma spray parameters). The experimental study clearly revealed the important role of suspension characteristics, namely surface tension, density and viscosity, on the final microstructure, with study of its progressive evolution providing invaluable insights. Variations in suspension properties manifest in the form of differences in droplet momentum and trajectory, which are found to be key determinants governing the resulting microstructure (e.g., lamellar/vertically cracked or columnar).

Sintering resistance of advanced plasma-sprayed thermal barrier coatings with strain-tolerant microstructures

Sintering is one of the key issues in the high temperature service of thermal barrier coatings (TBCs), considering the continuously increasing operation temperature of gas-turbine for higher energy efficiency. Based on the conventional processing method of air plasma spraying (APS), suspension plasma spraying (SPS) technique has been developed recently, in order to improve the strain tolerance of TBCs. This strain tolerance of plasma-sprayed TBCs is largely effected by the sintering behavior, which is presently not fully understood. In this work, evolution of mechanical properties, in terms of Young’s modulus and viscosity, is systematically investigated by in-situ three-point bending test at 1200 °C on free-standing coatings, including micro-cracked APS, segmented APS, vertically cracked SPS and columnar structured SPS TBCs and correlated to the microstructural evolution. Based on experimental results, power law relations are proposed for the sintering induced mechanical evolution, which deepen the understanding of the sintering behavior of plasma-sprayed TBCs.

Failure Analysis of Multilayered Suspension Plasma-Sprayed Thermal Barrier Coatings for Gas Turbine Applications

Improvement in the performance of thermal barrier coatings (TBCs) is one of the key objectives for further development of gas turbine applications. The material most commonly used as TBC topcoat is yttria-stabilized zirconia (YSZ). However, the usage of YSZ is limited by the operating temperature range which in turn restricts the engine efficiency. Materials such as pyrochlores, perovskites, rare earth garnets are suitable candidates which could replace YSZ as they exhibit lower thermal conductivity and higher phase stability at elevated temperatures. The objective of this work was to investigate different multilayered TBCs consisting of advanced topcoat materials fabricated by suspension plasma spraying (SPS). The investigated topcoat materials were YSZ, dysprosia-stabilized zirconia, gadolinium zirconate, and ceria–yttria-stabilized zirconia. All topcoats were deposited by TriplexPro-210TM plasma spray gun and radial injection of suspension. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. Microstructure analysis of as-sprayed and failed specimens was performed with scanning electron microscope. The failure mechanisms in each case have been discussed in this article. The results show that SPS could be a promising route to produce multilayered TBCs for high-temperature applications.

Influence of Bondcoat Spray Process on Lifetime of Suspension Plasma-Sprayed Thermal Barrier Coatings

Development of thermal barrier coatings (TBCs) manufactured by suspension plasma spraying (SPS) is of high commercial interest as SPS has been shown capable of producing highly porous columnar microstructures similar to the conventionally used electron beam–physical vapor deposition. However, lifetime of SPS coatings needs to be improved further to be used in commercial applications. The bondcoat microstructure as well as topcoat–bondcoat interface topography affects the TBC lifetime significantly. The objective of this work was to investigate the influence of different bondcoat deposition processes for SPS topcoats. In this work, a NiCoCrAlY bondcoat deposited by high velocity air fuel (HVAF) was compared to commercial vacuum plasma-sprayed NiCoCrAlY and PtAl diffusion bondcoats. All bondcoat variations were prepared with and without grit blasting the bondcoat surface. SPS was used to deposit the topcoats on all samples using the same spray parameters. Lifetime of these samples was examined by thermal cyclic fatigue testing. Isothermal heat treatment was performed to study bondcoat oxidation over time. The effect of bondcoat deposition process and interface topography on lifetime in each case has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.

Bilayer Suspension Plasma-Sprayed Thermal Barrier Coatings with Enhanced Thermal Cyclic Lifetime: Experiments and Modeling

Suspension plasma spraying (SPS) has been shown as a promising process to produce porous columnar strain tolerant coatings for thermal barrier coatings (TBCs) in gas turbine engines. However, the highly porous structure is vulnerable to crack propagation, especially near the topcoat-bondcoat interface where high stresses are generated due to thermal cycling. A topcoat layer with high toughness near the topcoat-bondcoat interface could be beneficial to enhance thermal cyclic lifetime of SPS TBCs. In this work, a bilayer coating system consisting of first a dense layer near the topcoat-bondcoat interface followed by a porous columnar layer was fabricated by SPS using Yttria-stabilised zirconia suspension. The objective of this work was to investigate if the bilayer topcoat architecture could enhance the thermal cyclic lifetime of SPS TBCs through experiments and to understand the effect of the column gaps/vertical cracks and the dense layer on the generated stresses in the TBC during thermal cyclic loading through finite element modeling. The experimental results show that the bilayer TBC had significantly higher lifetime than the single-layer TBC. The modeling results show that the dense layer and vertical cracks are beneficial as they reduce the thermally induced stresses which thus increase the lifetime.

A comparative study on the performance of suspension plasma sprayed thermal barrier coatings with different bond coat systems

The performance of suspension plasma sprayed (SPS) yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal treatment at 1150°C was investigated. The NiCoCrAlY bond coats were applied by air plasma spray (APS) and high velocity oxygen fuel (HVOF) techniques. It was found that the microstructure of SPS TBCs depends on the surface morphology of the bond coat. The SPS TBCs with a rough APS bond coat exhibited a longer lifetime than those with a smooth HVOF bond coat. To understand this phenomenon, the evolution of the microstructure, mechanical properties and the residual stresses in the TBCs and TGO were systematically studied. Results showed that the surface roughness and oxidation behavior of the bond coat play dominant roles in the SPS TBC failure.

Thermal Conductivity Analysis and Lifetime Testing of Suspension Plasma-Sprayed Thermal Barrier Coatings

Suspension plasma spraying (SPS) has become an interesting method for the production of thermal barrier coatings for gas turbine components. The development of the SPS process has led to structures with segmented vertical cracks or column-like structures that can imitate strain-tolerant air plasma spraying (APS) or electron beam physical vapor deposition (EB-PVD) coatings. Additionally, SPS coatings can have lower thermal conductivity than EB-PVD coatings, while also being easier to produce. The combination of similar or improved properties with a potential for lower production costs makes SPS of great interest to the gas turbine industry. This study compares a number of SPS thermal barrier coatings (TBCs) with vertical cracks or column-like structures with the reference of segmented APS coatings. The primary focus has been on lifetime testing of these new coating systems. Samples were tested in thermo-cyclic fatigue at temperatures of 1100 °C for 1 h cycles. Additional testing was performed to assess thermal shock performance and erosion resistance. Thermal conductivity was also assessed for samples in their as-sprayed state, and the microstructures were investigated using SEM.