Abstract
For the past decade, the aviation industry has been adopting sustainable aviation fuels (SAF) for use in aircraft to reduce the impact of aviation on climate change. Also, some nations look to SAF as an option for energy security for their military fleets. Understanding the critical impact of alternative fuel sources on hardware will provide the gas turbine industry with strategic options in sustainability and maintainability of the existing and new fleets. The alternative fuels with high hydrogen/carbon ratio (H/C) (such as synthetic paraffinic kerosenes (SPK)) could produce more water vapour content than the conventional jet fuels upon combustion, and this increased water vapour level could exert a significant impact over the long-term durability on hot section components such as the substrate blades, oxidation resistant coatings, thermal barrier coatings (TBCs), environmental barrier coatings (EBCs), resulting in an accelerated degradation of the turbine components. The possible detrimental effect of high-temperature water vapour on degradation and lifespan of hot section components was examined. Examples were specifically given on degradation and spallation of thermally grown oxides (TGO), formation of non-protective oxides and ceramics topcoats in TBCs. Results show that water vapour can lead to volatilization of TGO (Al2O3), and is responsible for the formation of non-protective oxides in both Pt-modified β-NiAl and MCrAlY coatings, leading to their early spallation. However, water vapour does not appear to directly affect the ceramic topcoat of the TBC. For EBCs coated on SiC-based substrates, the substrate recession via silica (TGO) volatilization was reviewed. These EBCs were observed undergoing degradation in highly hostile environments, e.g., constantly operating under high temperatures, pressures, and velocities condition in the presence of water vapour steam. The review intends to provide a perspective of high-temperature water vapour effect on the EBCs’ topcoat properties such as durability, degradation, crack nucleation and crack growth, and possible guidance for mitigating these degradation effects.
Highlights
The need to reduce the aviation industry’s impact on climate change and secure fuel supplies has driven investigation of sustainable aviation fuels (SAF, referred to as “alternative fuels”) for aircraft operations [1,2,3]
Alternative fuels with high hydrogen/carbon ratio (H/C), such as synthetic paraffinic kerosene (SPK) produced by the Fischer–Tropsch or alcohol-to-jet (ATJ) process could produce more water vapour content than the conventional jet fuels upon combustion, and this increased water vapour level could exert a significant impact over the long-term durability on hot section components such as the substrate blades, oxidation-resistant coatings, thermal barrier coatings and environmental barrier coatings, resulting in an accelerated degradation of the turbine components
The test results showed that oxidation is adversely affected by the increased water vapour levels, indicating that the growth of continuous and adherent alumina scale is considerably inhibited under water vapour environments, and more extensive transient oxidation occurs before the continuous α-Al2O3 formation takes place compared to the dry air, leading to more spinel phase formation
Summary
The need to reduce the aviation industry’s impact on climate change and secure fuel supplies has driven investigation of sustainable aviation fuels (SAF, referred to as “alternative fuels”) for aircraft operations [1,2,3]. The test results showed that oxidation is adversely affected by the increased water vapour levels, indicating that the growth of continuous and adherent alumina scale is considerably inhibited under water vapour environments, and more extensive transient oxidation occurs before the continuous α-Al2O3 formation takes place compared to the dry air, leading to more spinel phase formation They suggested that, initially, the hydrogen atoms from water vapour increase cation (Al3+) vacancy concentration, giving rise to a rapid growth of non-protective oxide NiO, and this results in more transient oxide formation and reduces the development of continuous α-Al2O3 scale. In the case where hydroxides or oxyhydroxides form, have higher volatility are normally observed than the corresponding oxides, leading to a loss of protection These results demonstrate that almost all aspects concerning oxide growth such as adsorption, dissociation and diffusion of reactants are affected in the presence of water vapour compared to the dry condition. It is known that upon the OH−1 ion concentration being increased, there is an associated increase in cation vacancies, and this, in turn, is responsible for the observed increase in oxidation rates
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