An abradable coating is a coating made of an abradable material – meaning if it rubs against a more abrasive material in motion, the former will be worn whereas the latter will face no wear.
Abradable coatings are used in aircraft jet engines in the compressor and turbine sections where a minimal clearance is needed between the blade tips and the casing.
In typical turbo machinery, the clearance between blade tips and the casing must account for thermal expansion as well as changes in concentricity due to shock loading events. To prevent catastrophic tip to casing contact, conservatively large clearances must be employed.
The role of abradable coatings is not only to allow for closer clearances, but to automatically adjust clearances, in-situ, to accept physical events and/or thermal scenarios that may be found in a devices operational history.