Although inner-diameter (ID) thermal barrier coating (TBC) processes are well established for large aero/land-based turbine liners, the relationships between the processes, microstructures, and properties of coatings applied to smaller, highly confined passages (ID < 200 mm), such as combustor liners, exhaust manifolds, and pipes that face comparable thermal loads, remain largely undocumented. This study examines 8 wt.

Abradable coating systems in aircraft gas turbines minimize the clearance between rotating blade tips and stationary liners, acting as sealants that improve engine efficiency. In this work, abradable liners made of a nickel-based superalloy substrate are coated with a CoNiCrAlY bondcoat, an intermediate Y2O3-stabilized ZrO2 (YSZ) layer, and a MgAl2O4 abradable top layer. The coated systems are evaluated using both thermal gradient cycling and incursion tests designed to replicate real operating conditions. To enhance bonding strength for thicker abradable coatings, laser ablation is applied to the bondcoat and YSZ layer to increase mechanical interlocking between layers. Furthermore, a combined dense and porous microstructure is introduced in the thick MgAl2O4 coating to lower the overall energy release rate while improving toughness near the interface. Three coating configurations are tested: (i) a standard high velocity oxygen fuel (HVOF) sprayed bondcoat with a standard atmospherically plasma sprayed (APS) YSZ/MgAl2O4 double-layer used as a reference, (ii) a laser-ablated HVOF bondcoat with a laser-ablated APS YSZ layer and a standard MgAl2O4 layer, and (iii) a laser-ablated HVOF bondcoat with a standard APS YSZ layer and a dense/porous MgAl2O4 layer. Both modified liners showed superior incursion performance and no failure after 1500 thermal cycles (125 h at 1390 degrees C).

This work employs burner rig testing to understand the modes of degradation of a CoNiCrAlY coated superalloy under thermo-cyclic service condition upon exposure to oxide, oxide-sulfate and sulfate deposits. The mixed oxide and oxide-sulfate deposits adhered to the Al2O3 TGO without reacting, likely due to the decomposition of sulfates and sequestration of reactive oxides into silicates. In contrast, sulfate-only deposits readily react with the TGO, forming less-protective calcium aluminates for all cycling conditions. While the TGO on the specimen heat treated isothermally remained protective, increased cycling frequency led to reaction product delamination depleting the Al-reservoir and oxidizing other coating elements.

Gadolinium zirconate (Gd2Zr2O7, GZO) has been investigated for two decades as a thermal barrier coating (TBC) material in gas turbines. The advantages of this material over conventional yttria-stabilized zirconia (YSZ) are its lower thermal conductivity, phase stability, and low sintering tendency at elevated temperatures above 1200 °C. However, a major disadvantage is the low fracture toughness of the material and the reaction with the thermally growing oxide layer (TGO). As a result, single-layer GZO coatings cannot be used as TBCs. To overcome these challenges, the present study focuses on the development of double-layer TBC systems, produced with a single gun to keep the manufacturing time low. The respective layers were optimized for specific requirements using a porous atmospheric plasma spray (APS)-YSZ interlayer covered by a suspension plasma sprayed (SPS)-GZO top layer. In furnace cycling tests at 1100 °C, the double-layer systems developed in this study were able to achieve a significant increase in thermal cycle life compared to single-layer APS-YSZ coatings. In addition, the failure mechanism after thermal cycling was investigated, and phase analysis was performed using XRD. The examination of Young’s modulus and the hardness of the layers revealed sintering in the GZO layer due to thermal cycling, while the APS layer showed only minor changes.

This study investigates failure mechanisms in a typical thermal barrier coating (TBC) system comprising an EB-PVD columnar top coat, an aluminide bond coat, and a Ni-based single crystal superalloy substrate, simulating gas turbine operating conditions using a burner rig. TBC degradation, initiated by interfacial defects from the LASAT method, was studied during thermal gradient cycling under fast and slow cooling. In-situ optical and infrared imaging, along with ex-situ SEM cross-sectional analysis, monitored failure mechanisms. The Laser Shock for Damage Monitoring (LASDAM) technique provided insights into gradient and cooling rate impacts on columnar TBC damage. Results showed significant effects of cooling rate on delamination and localized failure at blister sites, with LASDAM revealing significant overheating at damage sites. Analysis included full-field temperature and damage assessment, emphasizing blister-driven delamination under severe thermal gradients. Discussion focused on elastic stored energy effects, noting that fast cooling induced transient conditions where reversed temperature gradients increased damage, limiting TBC lifespan.

Environmental Barrier Coating systems (EBCs) were thermally cycled in a burner rig test facility. Yb2Si2O7 (YbDS) layer was deposited by air plasma spraying while two suspension plasma sprayed Yb2SiO5 (YbMS) microstructures were evaluated in duplex (YbDS/YbMS) systems: columnar and segmentation cracked. EBCs underwent 2000 cycles at a surface temperature of 1300 °C without signs of delamination failure. A porous YbMS layer formed at the base of intercolumnar gaps and segmentation cracks in the duplex systems, presumably due to reactions with entrapped water vapor. Furthermore, Yb2O3 depletion zones were evident at both the surface and YbDS/YbMS interface of the duplex EBCs.

Oxidation of the bond coat during turbine operation leads to additional stresses in the thermal barrier coating (TBC) system that promotes spalling of the thermal insulation. Therefore, the oxidation behavior of a TBC system plays an important role in the thermal cycling of a TBC system. To delay the loss of thermal insulation, research has typically focused for a long time on the composition and microstructure of the ceramic topcoats and metallic bond coats. More recently, heat treatment for the diffusion annealing of the bond coat has also become a focus of research. Several studies have shown that pre-oxidation of the bond coat prior to the application of the ceramic topcoat slows down the subsequent oxidation of the bond coat in service. The improved thermal cyclability has been demonstrated in studies for systems with atmospheric plasma-sprayed (APS), suspension plasma-sprayed (SPS) or electron beam physical vapor deposition (EB-PVD) top coatings. However, no study has directly compared the effects of pre-oxidation on different topcoats. Therefore, this study compared the effect of pre-oxidation on APS and SPS coatings with the same bond coat. For both topcoats, pre-oxidation slowed the subsequent TGO growth and thus increased the lifetime of the coatings. The improvement in lifetime was particularly pronounced for the systems with an SPS topcoat. Overall, the lifetime of the coatings with an APS topcoat was higher as the critical energy release rate within the coating was not exceeded in these coatings.

Improving the mechanical interlocking within the layers of an abradable and a thermal barrier coating (TBC) plays a key role in extending its lifetime. This can be achieved by increasing the surface profile of the bondcoat using laser ablation and flash coating. To this end, this work analyzes the impact of laser-ablated and atmo-spherically plasma-sprayed flash-coated CoNiCrAlY bondcoats on the furnace cycle lifetime of double-layered Y2O3-stabilized ZrO2/MgAl2O4 abradable coatings. The three different bondcoat modifications studied in this work are, namely, a laser-ablated vacuum plasma sprayed bondcoat with a square-shaped grid surface structure, a laser-ablated vacuum plasma sprayed bondcoat with an additional atmospheric plasma sprayed fine flash-coat for increased micro-roughness, and a standard vacuum plasma sprayed bondcoat with a coarse and a subsequent fine atmospherically plasma sprayed flash-coat. As a reference sample, a standard vacuum plasma sprayed bondcoat is used. A detailed analysis of the optimal laser parameters to manufacture homogenous laser -structured bondcoats is presented, while furnace cycle tests at 1100 degrees C are used to identify the performance of the different bondcoat/ceramic topcoat systems. The obtained results show that the cycling lifetime improved for all three modified bondcoats compared to the standard bondcoat. However, the two laser-structured bond -coats present the most significant increase by almost three times. X-ray diffraction and scanning electron mi-croscope results further reveal that the laser ablation process has no considerable effect on the material composition. Furthermore, roughness measurements highlight that the bondcoat's macro-roughness has a more pronounced effect on the cycling life than its micro-roughness. The furnace cycle tests, moreover, show that both laser-structured bondcoats have different failure modes, but similar cycling lifetime.

Dense environmental barrier coatings (EBCs) are an essential prerequisite to exploit the advantages offered by SiC-based fiber reinforced ceramic matrix composites (CMCs) to increase efficiency in gas turbines. Today's state-of-the art materials for application as EBCs are rare-earth (RE) silicates which, however, form amorphous phases upon rapid quenching from the melt. This makes their processing by thermal spray a challenge. Recently, high velocity oxygen fuel (HVOF) spraying was proposed as potential solution since the melting degree of the feedstock can be controlled effectively. This work studies the deposition of ytterbium disilicate (YbDS) at short stand-off distances and variant total feed rates and oxy-fuel ratios of the working gas. It was found that the overall degree of crystallinity could be kept at high level above 90%. The kinetic energy transferred by impinging particles was found to be an effective parameter to control the densification of the coatings. Porosities well below 10% were achieved while fully dense coatings were impeded due to the progressive accumulation of stresses in the coatings.

This study presents a new laser‐cladding‐based additive manufacturing technique named Clad2Z. Using a robot‐mounted confocal powder nozzle with axial infrared laser beam, ceramic columns with a diameter of 450 µm and an adjustable height are developed. Influence of laser parameters and robot movements on shape and microstructure is analyzed. As an example application, the common material yttria‐stabilized zirconia (YSZ) is used to deposit columnar‐structured thermal barrier coatings (TBCs). The excellent thermal cycling performance of the Clad2Z samples is demonstrated by burner rig tests and comparing lifetime and failure mechanism with conventional TBC systems.

Yttria-stabilized zirconia (YSZ) is the state-of-the-art top coat material for thermal barrier coatings (TBCs) applied on highly loaded gas turbine parts. During operation at high temperatures, stresses are induced by the thermal expansion coefficient mismatch between the ceramic TBC and the metallic substrate. As a consequence cracks can grow, propagate and finally lead to a spallation of the top coat. Using atmospheric plasma spraying (APS), so-called self-healing MoSi2 particles can be incorporated into the YSZ matrix to mitigate the propagation of cracks leading to a lifetime gain and possibly higher temperature capability of the TBC. In the present work, the healing process is realized by the oxidation of the self-healing particles, which introduces a volume expansion by a formation of reaction products, which can seal the cracks. The self-healing particles were introduced within the first 150 mu m of the YSZ coating matrix immediately on top of the bond coat. The degradation and lifetime of such systems were studied in furnace cycling and in burner rig tests, in which a temperature gradient through the sample was applied. The lifetime of the self-healing coatings was then compared to the lifetime of an YSZ coating without self-healing particles. In burner rig tests a clear lifetime extension of the self-healing TBCs was observed. The origin of this different behavior was investigated by microstructural analysis in scanning electron microscopy. A further insight into the failure mechanisms was gained by the analysis of a self-healing TBC cycled in a furnace cycling test only for about 55% of its expected lifetime.

Thermal barrier coatings (TBCs) are commonly used to protect gas turbine components from high temperatures and oxidation. Such coatings consist of ceramic top coats and metallic bond coats. The mismatch in thermal expansion of the top coat, the bond coat and the component material is one main factor leading to the failure of the coating system. Columnar-structured top coats offer an enhanced tolerance to the strain during thermal cycling. On a flat bond coated surface, these TBCs reach higher thermal cycling performance. However, on rough surfaces, as used for thermal spray coatings, the performance of these thermal barrier coatings seems to be restricted or even stays below the performance of atmospheric-plasma-sprayed (APS) thermal barrier coatings. This low performance is linked to out-of-plane stresses at the interface between the top coat and the bond coat. In this study, a thin additional oxide-dispersion-strengthened (ODS) bond coat with high alumina content provides a reduced mismatch of the coefficient of thermal expansion (CTE) between the top coat and the bond coat. Columnar suspension plasma sprayed (SPS), yttria-stabilized zirconia (YSZ) TBCs were combined with low-CTE ODS bond coats. The behavior of these TBCs was characterized with respect to thermal cycling performance and degradation in a burner-rig facility. The comparison showed an up-to-four-fold increase in the performance of the new system.

For more than 6 decades, thermal barrier coatings have been used to protect structural parts in both stationary and aviation gas turbines. These coatings allow the use of significant higher operation temperatures and hence increased efficiencies. In the 1970s, yttria-stabilized zirconia (YSZ) was identified as outstanding material for this application. As major deposition technologies both electron beam physical vapor deposition (EB-PVD) and atmospheric plasma spraying (APS) have been established. Although the topic is already rather old, there are still frequent activities ongoing to further improve the technology, both with respect to materials and microstructural issues also regarding the use of advanced coating technologies, especially in the field of thermal spray. The paper tries to summarize major developments in both fields, the materials and the processing focusing on thermal spray methods. The impact of both materials and processing are summarized by the results of burner rig tests for various systems. Furthermore, a short outlook on possible future directions of developments will be given.

Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200 degrees C. Above this temperature the deposited metastable tetragonal (t ') phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500 degrees C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates.

Suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) are the only thermal spray technologies shown to be capable of producing TBCs with columnar microstructures similar to the electron beam-physical vapor deposition (EB-PVD) process but at higher deposition rates and relatively lower costs. The objective of this study was to achieve fundamental understanding of the effect of different columnar microstructures produced by these two thermal spray processes on their insulation and lifetime performance and propose an optimized columnar microstructure. Characterization of TBCs in terms of microstructure, thermal conductivity, thermal cyclic fatigue lifetime and burner rig lifetime was performed. The results were compared with TBCs produced by the standard thermal spray technique, atmospheric plasma spraying (APS). Bondcoats deposited by the emerging high-velocity air fuel (HVAF) spraying were compared to the standard vacuum plasma-sprayed (VPS) bondcoats to investigate the influence of the bondcoat deposition process as well as topcoat–bondcoat interface topography. The results showed that the dense PS-PVD-processed TBC had the highest lifetime, although at an expense of the highest thermal conductivity. The reason for this behavior was attributed to the dense intracolumnar structure, wide intercolumnar gaps and high column density, thus improving the strain tolerance and fracture toughness.

In the high temperature combustion atmosphere inside of aircraft turbines, the currently used ceramic matrix composites require a protective environmental barrier coating (EBC) to mitigate corrosion of the turbine parts. Besides thermomechanical and thermochemical properties like matching thermal expansion coefficient (CTE) and a high resistance against corrosive media, mechanical properties like a high adhesion strength are also necessary for a long lifetime of the EBC. In the present work, the adhesion between an air plasma sprayed silicon bond coat and a vacuum plasma sprayed ytterbium disilicate topcoat was aimed to be enhanced by a laser surface structuring of the Si bond coat. An increase in interface toughness was assumed, since the introduction of structures would lead to an increased mechanical interlocking at the rougher bond coat interface. The interface toughness was measured by a new testing method, which allows the testing of specific interfaces. The results demonstrate a clear increase of the toughness from an original bond coat/topcoat interface (8.6 J/m2) compared to a laser structured interface (14.7 J/m2). Observations in the crack propagation indicates that the laser structuring may have led to a strengthening of the upper bond coat area by sintering. Furthermore, in addition to cohesive failure components, adhesive components can also be observed, which could have influenced the determined toughness.

Die Degradation von Warmedammschichten(thermal barrier coatings - TBCs) in Gasturbinen durch glasartige Calcium-Magnesium-Aluminiumsilikat (CMAS)Ablagerungen aus verschiedenen Quellen istseit vielen Jahren ein anhaltendes Problem. In dieser Studie wurde mit Hilfe derSynchrotron-Rontgen-Refraktions-Radiographie (SXRR), und vergleichend mit der Elektronenmikroskopie, das Eindringen von CMAS in die porose Struktur von atmospharisch plasmagespritzten (APS)TBCs sowie die Bildung und das Wachstum von Rissen unter thermisch zyklischer Belastung untersucht. Die Ergebnisse deuten darauf hin, dass die Infiltration sowie deren Kinetik im Brenner-Teststand wesentlich vom Benetzungsverhalten des CMAS beeinflusst werden. Trotz desoffensichtlichen Angriffs von CMAS auf die Korngrenzen hat die Wechselwirkung von Yttriumdioxid-stabilisiertem Zirkoniumdioxid (YSZ) mit intrudiertem CMAS keinen unmittelbaren Einfluss auf die Struktur und Dichte der innen Oberflachen(Risse, Poren). In einem spateren Stadium wird die Bildung von Rissen senkrecht zur auseren Oberflache in einer breiteren Zone der TBC-Schicht beobachtet.

Cr2AlC layers with thickness up to 100 mu m were deposited by high-velocity-atmospheric plasma spray (HV-APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr2AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria-stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400 degrees C and 1050 degrees C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Nevertheless, these results show a high potential of Cr2AlC and other Al-based MAX phases as bond coat material for high-temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed.

The chapter describes the different aspects of ceramic materials in gas turbines. The operation conditions such as high-pressure ratio and high temperatures result in improved efficiencies and make necessary the use of materials with high-temperature capability. In addition to the often used single-crystal alloys ceramic materials are discussed. Different bulk ceramics, for example, based on silicon nitride are described. A special focus is laid on ceramic matric composites, both oxide and nonoxide-based materials, which are of increasing interest for gas-turbine applications. In addition to the structural applications ceramics are also often used as coating material. Standard coating processes for protective coatings in gas turbines are described. Furthermore, thermal barrier coatings, a widely used coating system in gas turbines, and environmental barrier coatings as protective coatings for ceramic matrix composites are discussed in detail. Finally, also degradation and failure modes for the different high-temperature coating systems are the topics of this chapter.

Gadolinium zirconate (Gd2Zr2O7, GZO) as an advanced thermal barrier coating (TBC) material, has lower thermal conductivity, better phase stability, sintering resistance, and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (YSZ, 6-8 wt%) at temperatures above 1200 degrees C. However, the drawbacks of GZO, such as the low fracture toughness and the formation of deleterious interphases with thermally grown alumina have to be considered for the application as TBC. Using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS), double-layered YSZ/GZO TBCs, and triple-layered YSZ/GZO TBCs were manufactured. In thermal cycling tests, both multilayered TBCs showed a significant longer lifetime than conventional single-layered APS YSZ TBCs. The failure mechanism of TBCs in thermal cycling test was investigated. In addition, the CMAS attack resistance of both TBCs was also investigated in a modified burner rig facility. The triple-layered TBCs had an extremely long lifetime under CMAS attack. The failure mechanism of TBCs under CMAS attack and the CMAS infiltration mechanism were investigated and discussed.