Mineralogy and reactions of aircraft engine deposits and relationship to geographical regions of operation

Modern aircraft engines might experience increased degradation when operating in regions with high contents of atmospheric mineral dust. Whilst not safety critical, this accelerated degradation might result in an increased maintenance burden. There are several distinct mechanisms of degradation caused by ingested dust in different engine parts. Particular focus has been on the damage caused by deposits on high pressure turbine (HPT) blades. Here deposits melt, infiltrate and chemically interact with porous thermal barrier coatings. This study examines the sensitivity of this kind of degradation to ingested mineral dust composition, which varies according to the geographical region of operation.

On exit from the compressor, minerals ingested into the engine can follow two different air flow paths. One is with air separated from the main flow, before entry into the combustor, which is used to cool the HPT blades. Minerals within this flow may be deposited in the HPT shank cavity and experience temperatures of ~800^°C. The second pathway is with the main air flow through the combustor. Minerals following this route experience higher temperatures (>1200^°C) and may be deposited on the HPT blade surface. A comparison of shank cavity with surface deposits isolates the effect that passage through the combustor and residency on the HPT blades has on the chemistry of the deposits. Here, we make this comparison for engines that have operated in different geographical regions.

We have analysed 8 shank cavity and HPT blade surface deposits from aircraft engines using XRD and SEM-EDS to obtain mineralogical and chemical compositions. Shank cavity deposits from a further 56 engines have also been analysed to obtain a sense of compositional variability across different operational regions. The mineral phases present in the shank cavity deposits are similar across all engines analysed and include several minerals, e.g., anhydrite and melilite, that formed in the engine from breakdown reactions of ingested dust. However, the relative abundance of these minerals varies, reflecting the likely composition of atmospheric dust in the regions of operation. The blade deposits are dominated by minerals formed by reactions between ingested minerals and thermal barrier coatings on the HPT blades, including garnets, spinels, and melilite. However, the relative abundance of these minerals also varies across regions.

Our ongoing work compares the chemistry of shank cavity deposits with HPT blade deposits using the minerals and textures to help constrain the processes causing the chemical changes. Concurrently, we seek to compare the chemistry of the shank cavity deposits with ingested dust composition. We aim to establish the extent to which the composition of the HPT deposits, and hence degradation, may be predicted from what is ingested at the front of the engine. With this knowledge, degradation mitigation strategies can then be tailored to operational region.