Investigation of wear mechanisms in abradable-blade contacts

Abradable materials are thermally sprayed on the inside of casings in aero-engines to provide a seal between rotating blades and a casing. The use of such materials in aero engines have been an area of research interest over the past few decades as small reductions in clearances between stationary and rotating parts can lead to large increases in engine efficiency. The tight requirements for abradable coatings and increase in compressor temperature from stage to stage have led to the development of abradable materials specific for different compressor stages. In this thesis wear mechanisms observed in two types of contacts were investigated: between the Ti(6Al4V) blades and AlSi-polyester abradable used in lower temperature compressor stages and between the Inconel 718 blades and NiCrAl-bentonite abradable used in higher temperature compressor stages.

This was accomplished through commissioning a new high-speed testing rig with a capability to achieve speed up to 280m/s and instrumenting it with a front-on stroboscopic imaging system. The new imaging system allowed new insights into adhesion and abrasion mechanisms observed in tests with AlSi-polyester abradable samples. For adhesions, it was shown that the combination of bulk temperature and flash temperature concepts was able to explain most of the observations in this work. It was shown that the likelihood of adhesions formation increased with contact forces and decreased with blade tip thickness. For contacts with NiCrAl-bentonite abradable two distinct contact modes were identified. These contact modes were then explained through the balance between the incursion rate and the rate of abradable fracture. It was shown in this work, that blade length plays a significant role in the effectiveness of abradable surface fracture, with very long (low stiffness) blades leading to inefficient fracture.

The effect of angled blades was investigated with short (stiff) blades and compared their effectiveness when they were introduced to long blades in previous research. It was shown that with short blades, angles did not offer the same increase in abradable fracture effectiveness. It was then suggested that both an increase in stiffness and introduction of an angle to long (less stiff) blades improve abradable fracture in a similar manner, and with stiff blades angles do not offer further performance improvements.

The difference in blade wear performance between flat and angled blades was explained through the presence of low temperature abrasive wear and thermally driven abrasive wear. The low temperature abrasive wear rate was low and no wear was observed with flat blades. With angled blades, however, some wear occurred due to the very low thickness at the sharp blade tip. The thermally driven wear occurred for tests that transitioned to the high-force contact mode. This type of wear occurred earlier during a test and the wear rate was higher for flat blades than for angled blades. Finally, the effects of such simplifications such as using blades with a fixed rather than a variable chord thickness and performing tests at a single rather than a variable incursion rate were investigated. It was shown that both these simplifications can have an influence on test outcomes and that the effect of simplifications used on scaled test rigs needs to be accounted for when considering the applicability of results from such rigs to actual aero-engines.