Thermo-mechanical degradation and mitigation by molten volcanic ash wetting on thermal barrier coatings of jet engine turbine blades

Modern gas turbine engines employed in power and aerospace industries extensively utilize thermal barrier coatings (TBCs) to protect the structural integrity of engine components against any rapid degradation at extremely high temperatures (1300-1500 °C). TBCs typically consist of 7 wt% yttria-stabilized zirconia (YSZ/7YSZ) and are mainly produced by atmospheric plasma spray (APS) and electron beam-physical vapor deposition (EB-PVD). In the course of ever higher engine operation temperatures due to a continuous increase of aircraft engine efficiency, the coatings have become vulnerable to interactions with and degradation by atmospheric contaminants (environmental dust) such as volcanic ash and desert sand. In particular, damage to TBCs by molten volcanic ash pose a serious threat to the durability of YSZ TBCs. The deterioration of the TBCs, originate by surficial wetting of by molten volcanic ash, followed by infiltration into the TBCs and solidification of the melt, an increased thermo-mechanical stress within the coating occurs. This leads to crack formation due to the alternating thermal excursion during operation of the engine, which could potentially lead to the spallation of TBCs. In order to gain a deeper understanding of the initial cracking, the mechanical properties of solidified volcanic glass within the infiltrated TBCs were determined experimentally in this thesis. For the first time, thermal shock experiments (from 1300 °C to room temperature) were performed on silicate melt wetted TBCs, followed by a characterization of the resulting thermomechanical damage. To investigate the influence of different chemistries, 3 different volcanic ashes were used: Kilauea, Hawaii, USA (basaltic); Eyjafjallajökull, Iceland (trachy-andesitic); Cordon Caulle, Chile (rhyolitic). Mechanical properties of the volcanic ash glasses were determined and Eyjafjallajökull volcanic ash was chosen for this study, as it possessed a higher elastic modulus compared to Kilauea and Cordon Caulle volcanic ash. Low fracture toughness and a high elastic modulus within the infiltrated area of the TBCs during the thermal shock regime was attributed to the generation of microcracks. EB-PVD-TBCs are characterized by a columnar structure, whereas APS-TBCs have a lamellar layer structure. For jet engine turbine blades, EB-PVD coatings are preferred over APS-TBCs as they offer a higher in-plane strain tolerance. However, compared to the lamellar APS structure, they are more susceptible to infiltration of molten volcanic ash. In this thesis, the deposition of volcanic ash on EB-PVD TBCs was investigated experimentally. For this purpose, Eyjafjallajökull ash was deposited on EB-PVD TBCs by thermal spraying. The morphological development of the melting volcanic ash micro globules during infiltration into the columnar TBC structure was characterized by in-situ high-temperature dilatometry measurements. The results show the deposition dynamics of the volcanic ash particles on TBCs and thus allow conclusions on possible effects regarding interaction of volcanic ash with coated turbine blades. Finally, the mitigation potential of novel TBCs with hexagonal boron nitride (h-BN) additives against the deposition of molten volcanic ash was investigated in this dissertation. The experiments show that pure h-BN substrates have non-wetting properties under vacuum conditions at 1250 °C. The conclusions from these studies were useful for the subsequent comparison of conventional YSZ-TBCs with h-BN doped YSZ-TBCs under atmospheric conditions. It was shown that h-BN doped YSZ-TBCs are more resistant to molten volcanic ash than conventional YSZ-TBCs due to reduced infiltration and improved wetting resistance. This work serves as a proof of concept that more candidate materials exhibit resilience towards molten silicate attack.Heutige Gasturbinentriebwerke sind auf Wärmedämmschichten (thermal barrier coating - TBC) angewiesen um strukturelle Komponenten des Triebwerks vor extrem hohen Temperaturen (1300-1500 °C) zu schützen. TBCs bestehen typischerweise aus 7 Gew.-% Yttrium stabilisiertem Zirkonoxid (YSZ) und werden hauptsächlich durch atmosphärisches Plasmaspritzen (atmospheric plasma spray–APS) und Elektronenstrahlverdampfung (electron beam–physical vapor deposition – EB-PVD) hergestellt. Im Zuge der kontinuierlichen Effizienzsteigerung von Flugzeugtriebwerken durch immer höhere Betriebstemperaturen, wurden die Beschichtungen anfällig für Ablagerungen geschmolzener Partikel aus dem Luftstrom, wie Vulkanasche, Sand oder Staub. Insbesondere die Schädigung von TBCs durch geschmolzene Vulkanasche stellt eine ernsthafte Bedrohung für die Haltbarkeit von YSZ-TBCs dar. Aufgrund der oberflächlichen Benetzung mit einer silikatischen Schmelze und deren anschließenden Infiltration und Verfestigung kommt es zu einer erhöhten thermo-mechanischen Belastung innerhalb der Beschichtung. Dies hat auf Grund der thermischen Wechselbelastung während des Betriebes des Triebwerks eine Verfestigung der Vulkanasche zur Folge, die zum finalen Versagen der Beschichtung führen kann. Um ein tieferes Verständnis über die initiale Verfestigung der Vulkanasche zu erlangen, wurden im Rahmen dieser Dissertation die mechanischen Eigenschaften von verfestigtem Vulkanglas innerhalb der infiltrierten TBCs experimentell bestimmt. Dabei wurden erstmalig Wärmeschockexperimente (von 1300 °C bis auf Raumtemperatur) an Silikatschmelze benetzten TBCs durchgeführt, gefolgt von einer Charakterisierung der dadurch entstandenen thermomechanischen Schädigung. Um den Einfluß unterschiedlicher chemischer Zusammensetzungen zu untersuchen, kamen 3 verschiedene Vulkanaschen zum Einsatz: Kilauea, Hawaii, USA (basaltisch); Eyjafjallajökull, Island (trachy-andesitisch); Cordon Caulle, Chile (rhyolitisch). Es wurden die mechanischen Eigenschaften der Vulkanaschegläser und der Eyjafjallajökull-Vulkanasche bestimmt, da sie im Vergleich zu Kilauea- und Cordon Caulle-Vulkanasche ein höheres Elastizitätsmodul besaß. Als Ursache für die Ausbildung von Mikrorissen wurden eine niedrige Bruchfestigkeit sowie ein hohes Elastizitätsmodul innerhalb des infiltrierten Bereichs identifiziert. EB-PVD-TBCs sind durch eine säulenförmige Struktur gekennzeichnet, wohingegen APS-TBCs einen lamellaren Schichtaufbau besitzen. Bei Turbinenschaufeln von Strahltriebwerken werden EB-PVD-Beschichtungen bevorzugt gegenüber APS-TBCs verwendet, da sie eine höhere Flächendehnungstoleranz aufweisen. Allerdings sind sie im Vergleich zur lamellaren APS-Struktur anfälliger gegenüber einer Infiltrierung geschmolzener Vulkanasche. In dieser Arbeit wurde experimentell die Ablagerung von Vulkanasche auf EB-PVD TBCs untersucht. Zu diesem Zweck wurde Eyjafjallajökull-Asche mittels thermischen Sprühens auf EB-PVD-TBCs aufgebracht. Die morphologische Entwicklung der schmelzenden Vulkanasche-Mikrokügelchen während der Infiltration in die säulenförmige TBC-Struktur wurde mit Hilfe von in-situ-Hochtemperatur-Dilatometrie Messungen charakterisiert. Die Ergebnisse zeigen die Ablagerungsdynamik der Vulkanaschepartikel auf TBCs und erlauben somit Rückschlüsse auf mögliche Auswirkungen hinsichtlich einer Wechselwirkung von Vulkanasche mit beschichteten Turbinenschaufeln. Schließlich wurde im Rahmen dieser Dissertation das Minderungspotenzial neuartiger TBCs mit Zusätzen aus hexagonalem Bornitrid (h-BN) gegenüber Ablagerung von geschmolzener Vulkanasche untersucht. Die Experimente zeigen, dass reine h-BN-Substrate unter Vakuumbedingungen bei 1250 °C keine Benetzungseigenschaften aufweisen. Die Schlußfolgerungen aus diesen Studien waren nützlich für den anschließenden Vergleich von konventionellen YSZ-TBCs mit h-BN dotierten YSZ-TBC unter atmosphärischen Bedingungen. Dabei konnte gezeigt werden, dass h-BN dotierte YSZ-TBCs auf Grund einer verminderten Infiltration und einer verbesserten Benetzungsbeständigkeit widerstandsfähiger gegenüber geschmolzener Vulkanasche ist als konventionelle YSZ-TBCs. Diese Arbeiten zeigen, dass h-BN dotierte YSZ-TBCs und weitere noch zu untersuchende Materialkombinationen für eine Verbesserung der Widerstandsfähigkeit von TBCs gegenüber geschmolzener, atmosphärischer Partikel dienen können

Novel thermal barrier coatings resistant to molten volcanic ash wetting

Molten environmental deposits primarily emanating from volcanic ash pose a serious threat to aviation safety. When ingested into a jet engine, the volcanic ash melts and adheres to the surface of hot regions (i.e., combustion chamber, turbine blade, and nozzle guide vanes) of jet engines. Virtually, these hot zones in jet engines comprise a two-layer thermal barrier coating (TBCs). These ceramic TBCs provide thermal insulation to the underlying nickel-based super alloy substrate, but these coatings are more vulnerable to the damage caused by molten volcanic ash deposits. Particularly, in the pursuit of high output efficiency, turbine operating temperatures increasingly exceed 1250°C, leading to detrimental effects on the TBCs. Introducing rare-earth oxides (eg. Gadolinium oxide) into TBCs is regarded as one of the main migratory approach to prevent the damage by ash, because the infiltration silica-rich molten volcanic ash deposit is slowed down by crystallising the melt, preventing deeper infiltration into the coating. However, the initial phase of the damage progression of volcanic ash into the porous texture of TBC has become unavoidable. Here, we utilised thermal spray technology to produce a novel thermal barrier coating consisting of the mixture of the hexagonal boron nitride (h-BN, 30 vol.%) and yttria stabilized zirconia (YSZ, 70 vol. %) (BN-YSZ coating). Please click Additional Files below to see the full abstract

Gradient damage spreading of molten volcanic ash on thermal barrier coatings

Aviation safety and aero engine life are always threatened by dust or ash suspending in the air route which derive from inevitable natural phenomena (volcanic eruption and sand storm) and human productive activity (run way debris, industrial fumes, and coal ash emission). Those floating silicate ash with the low melt temperature (lower than 1100 ºC) will be easily ingested into jet engine and quickly melted due to the fact that the turbine inlet temperature of the current advanced jet engine at cruising altitude (1200-1450 ºC) far exceed the melting point of those silicate ash. Subsequently, these molten ash are deposited on the surface of thermal barrier coatings (TBCs). TBCs is a refractory ceramic layer deposited on the surface of super alloy and can protect these metal at the hot parts (such as combustion chamber, blade and nozzle) from high temperature. However, these silicate deposits will lead to serious spallation and even failure of TBCs. Once the TBCs exfoliate under stress or chemical corrosion because of ash deposition, the engine may stop running during the flight and cause air disaster. Therefore, silicate ash deposition undoubtedly pose a huge obstacle in the development of jet engine. Here, to comprehensively understand the effect of silicate deposits on TBCs, we investigated the subsurface-transverse spreading ring of re-melted volcanic ash (obtained from Tungurahua Volcano, Ecuador, 2014) with various droplet size on the APS TBCs and EB-PVD TBCs respectively at the temperature from 1200 ºC to 1600 ºC over a wide range of duration (Figs. 1a and b). Our results demonstrate that the gradient change of concentration of volcanic ash melt onto TBCs directly leads to the formation of spreading ring in the subsurface-transverse of molten volcanic ash located in the edge of main spreading area (Fig. 1c). These observations imply that the interaction process of molten silicate ash with TBCs is driven not only by vertical infiltration due to gravitation but also by horizontal spreading owing to capillary force. Notably, the infiltration depth of the ring area was deeper than that of the main liquid area, which closely resembles previously observed in ceramic plate (Figs. 1d and e). Overall, we summaries the influence of temperature, holding time and size of droplet on spreading radius and conclude the mechanism of vertical infiltration. Those work is the first step to improving the TBCs and serve as the basic of developing the new generation of aeroengines. Please click Additional Files below to see the full abstract

Thermo-mechanical degradation and mitigation by molten volcanic ash wetting on thermal barrier coatings of jet engine turbine blades

Modern gas turbine engines employed in power and aerospace industries extensively utilize thermal barrier coatings (TBCs) to protect the structural integrity of engine components against any rapid degradation at extremely high temperatures (1300-1500 °C). TBCs typically consist of 7 wt% yttria-stabilized zirconia (YSZ/7YSZ) and are mainly produced by atmospheric plasma spray (APS) and electron beam-physical vapor deposition (EB-PVD). In the course of ever higher engine operation temperatures due to a continuous increase of aircraft engine efficiency, the coatings have become vulnerable to interactions with and degradation by atmospheric contaminants (environmental dust) such as volcanic ash and desert sand. In particular, damage to TBCs by molten volcanic ash pose a serious threat to the durability of YSZ TBCs. The deterioration of the TBCs, originate by surficial wetting of by molten volcanic ash, followed by infiltration into the TBCs and solidification of the melt, an increased thermo-mechanical stress within the coating occurs. This leads to crack formation due to the alternating thermal excursion during operation of the engine, which could potentially lead to the spallation of TBCs. In order to gain a deeper understanding of the initial cracking, the mechanical properties of solidified volcanic glass within the infiltrated TBCs were determined experimentally in this thesis. For the first time, thermal shock experiments (from 1300 °C to room temperature) were performed on silicate melt wetted TBCs, followed by a characterization of the resulting thermomechanical damage. To investigate the influence of different chemistries, 3 different volcanic ashes were used: Kilauea, Hawaii, USA (basaltic); Eyjafjallajökull, Iceland (trachy-andesitic); Cordon Caulle, Chile (rhyolitic). Mechanical properties of the volcanic ash glasses were determined and Eyjafjallajökull volcanic ash was chosen for this study, as it possessed a higher elastic modulus compared to Kilauea and Cordon Caulle volcanic ash. Low fracture toughness and a high elastic modulus within the infiltrated area of the TBCs during the thermal shock regime was attributed to the generation of microcracks. EB-PVD-TBCs are characterized by a columnar structure, whereas APS-TBCs have a lamellar layer structure. For jet engine turbine blades, EB-PVD coatings are preferred over APS-TBCs as they offer a higher in-plane strain tolerance. However, compared to the lamellar APS structure, they are more susceptible to infiltration of molten volcanic ash. In this thesis, the deposition of volcanic ash on EB-PVD TBCs was investigated experimentally. For this purpose, Eyjafjallajökull ash was deposited on EB-PVD TBCs by thermal spraying. The morphological development of the melting volcanic ash micro globules during infiltration into the columnar TBC structure was characterized by in-situ high-temperature dilatometry measurements. The results show the deposition dynamics of the volcanic ash particles on TBCs and thus allow conclusions on possible effects regarding interaction of volcanic ash with coated turbine blades. Finally, the mitigation potential of novel TBCs with hexagonal boron nitride (h-BN) additives against the deposition of molten volcanic ash was investigated in this dissertation. The experiments show that pure h-BN substrates have non-wetting properties under vacuum conditions at 1250 °C. The conclusions from these studies were useful for the subsequent comparison of conventional YSZ-TBCs with h-BN doped YSZ-TBCs under atmospheric conditions. It was shown that h-BN doped YSZ-TBCs are more resistant to molten volcanic ash than conventional YSZ-TBCs due to reduced infiltration and improved wetting resistance. This work serves as a proof of concept that more candidate materials exhibit resilience towards molten silicate attack.Heutige Gasturbinentriebwerke sind auf Wärmedämmschichten (thermal barrier coating - TBC) angewiesen um strukturelle Komponenten des Triebwerks vor extrem hohen Temperaturen (1300-1500 °C) zu schützen. TBCs bestehen typischerweise aus 7 Gew.-% Yttrium stabilisiertem Zirkonoxid (YSZ) und werden hauptsächlich durch atmosphärisches Plasmaspritzen (atmospheric plasma spray–APS) und Elektronenstrahlverdampfung (electron beam–physical vapor deposition – EB-PVD) hergestellt. Im Zuge der kontinuierlichen Effizienzsteigerung von Flugzeugtriebwerken durch immer höhere Betriebstemperaturen, wurden die Beschichtungen anfällig für Ablagerungen geschmolzener Partikel aus dem Luftstrom, wie Vulkanasche, Sand oder Staub. Insbesondere die Schädigung von TBCs durch geschmolzene Vulkanasche stellt eine ernsthafte Bedrohung für die Haltbarkeit von YSZ-TBCs dar. Aufgrund der oberflächlichen Benetzung mit einer silikatischen Schmelze und deren anschließenden Infiltration und Verfestigung kommt es zu einer erhöhten thermo-mechanischen Belastung innerhalb der Beschichtung. Dies hat auf Grund der thermischen Wechselbelastung während des Betriebes des Triebwerks eine Verfestigung der Vulkanasche zur Folge, die zum finalen Versagen der Beschichtung führen kann. Um ein tieferes Verständnis über die initiale Verfestigung der Vulkanasche zu erlangen, wurden im Rahmen dieser Dissertation die mechanischen Eigenschaften von verfestigtem Vulkanglas innerhalb der infiltrierten TBCs experimentell bestimmt. Dabei wurden erstmalig Wärmeschockexperimente (von 1300 °C bis auf Raumtemperatur) an Silikatschmelze benetzten TBCs durchgeführt, gefolgt von einer Charakterisierung der dadurch entstandenen thermomechanischen Schädigung. Um den Einfluß unterschiedlicher chemischer Zusammensetzungen zu untersuchen, kamen 3 verschiedene Vulkanaschen zum Einsatz: Kilauea, Hawaii, USA (basaltisch); Eyjafjallajökull, Island (trachy-andesitisch); Cordon Caulle, Chile (rhyolitisch). Es wurden die mechanischen Eigenschaften der Vulkanaschegläser und der Eyjafjallajökull-Vulkanasche bestimmt, da sie im Vergleich zu Kilauea- und Cordon Caulle-Vulkanasche ein höheres Elastizitätsmodul besaß. Als Ursache für die Ausbildung von Mikrorissen wurden eine niedrige Bruchfestigkeit sowie ein hohes Elastizitätsmodul innerhalb des infiltrierten Bereichs identifiziert. EB-PVD-TBCs sind durch eine säulenförmige Struktur gekennzeichnet, wohingegen APS-TBCs einen lamellaren Schichtaufbau besitzen. Bei Turbinenschaufeln von Strahltriebwerken werden EB-PVD-Beschichtungen bevorzugt gegenüber APS-TBCs verwendet, da sie eine höhere Flächendehnungstoleranz aufweisen. Allerdings sind sie im Vergleich zur lamellaren APS-Struktur anfälliger gegenüber einer Infiltrierung geschmolzener Vulkanasche. In dieser Arbeit wurde experimentell die Ablagerung von Vulkanasche auf EB-PVD TBCs untersucht. Zu diesem Zweck wurde Eyjafjallajökull-Asche mittels thermischen Sprühens auf EB-PVD-TBCs aufgebracht. Die morphologische Entwicklung der schmelzenden Vulkanasche-Mikrokügelchen während der Infiltration in die säulenförmige TBC-Struktur wurde mit Hilfe von in-situ-Hochtemperatur-Dilatometrie Messungen charakterisiert. Die Ergebnisse zeigen die Ablagerungsdynamik der Vulkanaschepartikel auf TBCs und erlauben somit Rückschlüsse auf mögliche Auswirkungen hinsichtlich einer Wechselwirkung von Vulkanasche mit beschichteten Turbinenschaufeln. Schließlich wurde im Rahmen dieser Dissertation das Minderungspotenzial neuartiger TBCs mit Zusätzen aus hexagonalem Bornitrid (h-BN) gegenüber Ablagerung von geschmolzener Vulkanasche untersucht. Die Experimente zeigen, dass reine h-BN-Substrate unter Vakuumbedingungen bei 1250 °C keine Benetzungseigenschaften aufweisen. Die Schlußfolgerungen aus diesen Studien waren nützlich für den anschließenden Vergleich von konventionellen YSZ-TBCs mit h-BN dotierten YSZ-TBC unter atmosphärischen Bedingungen. Dabei konnte gezeigt werden, dass h-BN dotierte YSZ-TBCs auf Grund einer verminderten Infiltration und einer verbesserten Benetzungsbeständigkeit widerstandsfähiger gegenüber geschmolzener Vulkanasche ist als konventionelle YSZ-TBCs. Diese Arbeiten zeigen, dass h-BN dotierte YSZ-TBCs und weitere noch zu untersuchende Materialkombinationen für eine Verbesserung der Widerstandsfähigkeit von TBCs gegenüber geschmolzener, atmosphärischer Partikel dienen können

Estimation of CMAS Infiltration depth in EB-PVD TBCs: A new constraint model supported with experimental approach.

Two standard 7YSZ coatings were deposited by EB-PVD techniques and tested against CMAS infiltration at short time intervals (up to 8 min.) at 1250˚C in air. They exhibited different microstructures, i.e. porosities and microstructural features. Two species of CMAS with different compositions were used and their viscosities were determined using the concentric cylinder method and their contact angles were measured using high temperature heating microscopy. The theoretical viscosities, which were calculated using a statistical model based on the chemical composition of the melts, differed from the measured values of the viscosities by one order of magnitude. A large variation in the contact angles within a very short range of temperature (1243-1266°C) was observed as well. The porosity and surface area measurements were performed on both EB-PVD microstructures using the nitrogen physisorption method. Additionally, the produced coatings exhibited porosities of 14.5 and 29.5 percent and the infiltration experiments have shown that the more porous coating provides higher infiltration resistance. The effect of porosity on CMAS infiltration kinetics was investigated and the results elucidate that the porosity network plays a more preeminent role than the amount of porosity. The experimental infiltration results have been compared with calculated infiltration data using a novel mathematical approach proposed in previous studies in which the permeability of the coatings is assessed with two contrasting methods termed “concentric pipe” and “open pipe” models. The infiltration was calculated by incorporating the experimentally determined properties such as contact angle, viscosity and porosity. A fitting parameter has been derived from the equations for the geometry factor for both microstructures. The calculated and experimental results are in good agreement with the concentric pipe model supporting the validity of this CMAS infiltration model

Thermal spray coatings for molten salt facing structural parts and enabling opportunities for thermochemical cycle electrolysis.

Thermochemical water splitting stands out as the most efficient techniques to produce hydrogen through electrolysis at a high temperature, relying on a series of chemical reactions within a loop. However, achieving a durable thermochemical cycle system poses a significant challenge, particularly in manufacturing suitable coating materials for reaction vessels and pipes capable of enduring highly corrosive conditions created by high-temperature molten salts. The review summarises thermally sprayed coatings (deposited on structural materials) that can withstand thermochemical cycle corrosive environments, geared towards nuclear thermochemical copper-chlorine (Cu-Cl) cycles. An assessment was conducted to explore material composition and selection (structure-property relations), single and multi-layer coating manufacturing, as well as corrosion environment and testing methods. The aim was to identify the critical areas for research and development in utilising the feedstock materials and thermal spray coating techniques for applications in molten salt thermochemical applications, as well as use lessons learnt from other application areas (e.g., nuclear reaction vessels, boilers, waste incinerators, and aero engine gas-turbine) where other types of molten salt and temperature are expected. Assessment indicated that very limited sets of coating-substrate system with metallic interlayer is likely to survive high temperature corrosive environment for extended period of testing. However, within the known means and methods, as well as application of advanced thermal spray manufacturing processes could be a way forward to have sustainable coating-substrate assembly with extended lifetime. Spraying multi-layered coating (nano-structured or micro-structured powder materials) along with the application of modern suspension or solution based thermal spray techniques are considered to result in dense microstructures with improved resistance to high temperature thermochemical environment