Understanding the Mechanisms of Adhesion Formation with AlSi based Abradables used in the Compressors of Gas Turbines

AlSi abradables are commonly used in the compressors of jet engines to minimise tip clearances and improve efficiency. Results from abradable testing rigs have shown the abradability of different coatings vary with changing hardness and incursion rates, with Metco 601 (AlSi-Polyester) bonding to the blade (adhesions) at high incursions rates (0.2 – 2µm/pass) but leading to blade wear at low incursion rates (0.02µm/pass). The wear response at high incursion rates is well documented, but the low incursion rate response is not well understood.
In this thesis in-situ tools have been developed for the low-speed spindle test rig at the University of Sheffield to monitor blade profile, temperature and abradable temperature simultaneously. These tools have been applied to rub tests with M601 to show that there is a relationship between abradable temperature and adhesion formation. These tools were applied further in a study targeting rubs at low incursion rates. It was found that the change in behaviour at low incursion rates was a consequence of the ability of the abradable to dissipate thermal energy, with the behaviour changing with changes in the coatings thermal properties.
These findings were applied further through the development of a sample heater block, allowing the bulk temperature of the abradable to be controlled during a test. Analysis of the in-situ data collected at different temperatures allowed the key criteria which influence adhesion formation with M601 to be determined.
To validate if these criteria could be applied to the wider range of AlSi abradables, elevated temperature testing was also completed on M320 (AlSi-hBN) and XPT268 (AlSi-Polyester-hBN). The wear of these materials was also found to follow the criteria proposed, but it was also identified that the addition of hBN had a further impact on the properties of the adhesions formed beyond what could be explained by this work.