Modifications to blade tips in abradable contacts

In aero engines a thermally sprayed abradable material is applied to the inside of the casing in order to provide a seal around the blade tips of the compressor blades. During running and handling the blades cut a channel into the abradable material. This rub is poorly understood and often results in blade wear and poor sealing particularly in the later stages of the compressor. This thesis aims to investigate and characterise these rubs and provide recommendations for changes to the running and handling procedure, the abradable or the blade tip that will improve sealing and reduce blade wear.

The AlSi polyester vs Ti(6Al 4V) and NiCrAl bentonite vs Inconel 718 rubs have been investigated through high speed rub testing. The effect of incursion rate, blade speed and abradable hardness have been investigated. Wear mechanisms have been proposed and clarified by further high speed wear testing and microstructure modelling. In addition several possible tip modifications have been tested and a successful modification has been extensively characterised.

The wear mechanism was found to depend on the materials and the incursion rate while the abradable batch and, in the case of the NiCrAl bentonite abradable, the blade speed affected the severity of the mechanism. The AlSi based abradable cut at high incursion rates but wore by adhesion on to the blade an abrasion at low incursion rates. The NiCrAl bentonite abradable wore by local compaction and subsurface damage. At low incursion rate material was removed fast enough to accommodate the incursion but at higher incursion rates blade wear and compaction of the abradable were observed.