Abradable coatings are designed to preferentially
abrade when contact is made with a mating part. Thermal spray abradable
coatings have low structural integrity so they are readily abraded when
they come into contact with a moving surface with higher structural
integrity. The coatings are designed not to damage the mating surface.
There are a number of different abradable coating
types, to cater for different operating temperatures, corrosive
environments and mating materials. The coating structure can be either:
Very porous with many unreacted or unmelted particles (which
would by normal standards be considered as a very poor coating)
produced by very careful selection of spraying parameters to achieve
the right degree of abradability. These coatings can be difficult to
reproduce accurately and need strict monitoring.
Dense and uniform structured coatings with additives such as
polymers, graphite, bentonite and boron nitride. These additives weaken
the matrix material of the coating and can provide some degree of dry
lubrication. Abradability is controlled mainly by the concentration of
additives and not by changes to the spraying parameters, which aids in
providing more consistent coatings
The main application of thermal spray abradable coatings is for gas
path clearance control in gas turbine engines. In an ideal situation,
gas turbine engines would be manufactured with minimal clearance in the
gas seal locations, but in the real world these clearances are very
much larger to permit operation without catastrophic interaction
between rotor and the casing. The clearance at the gas seal locations
has a significant effect on the gas turbine engine efficiency which
directly translates to fuel consumption. Thermal spray abradable
coatings successfully reduce these clearances to a minimum, without the
risk of catastrophic failure.
Schematic of a Typical Gas Turbine Engine
These coatings are used in the original manufacture to provide the
best possible seals in the first instance. They are also used during
overhaul to repair any excessive clearance produced during service.
Substantial improvement in engine performance and efficiency can be
realised by operating with clearances smaller than those achieved in
the original manufacture where abradable coatings were not used.
This technology is now spreading to other applications, such as land
based turbines and rotor seals in turbochargers for internal combustion
engines.
Thermal spray abradable coatings commonly used in order of
increasing temperature capability:
