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Abradable-coatings, applied generally by reliable Thermal Spray techniques, are special seal materials.
They are designed for gas path clearance control of gas turbine engines.
They are intended to provide smooth running in and improved performance.
The clearance considered is that between the tips of the blades mounted on rotating disks, and static shrouds, assembled on the engine casing.
Abradable materials are easily worn in to their final shape and dimension, by rubbing, in operation, against the moving parts, with minimal wear of the blade tips.
Gas turbine engines may include several stages in the compressor section, and then on the same shaft some more stages in the turbine section. Each stage operates at a different temperature.
Abradable-coatings are designed to preferentially abrade and dissolve when running in or when occasional contact is made with a mating part.
For best engine efficiency the clearance between the rotating and the static parts should be kept to a minimum because the fluid passing through that space is wasted, as it does not perform any useful work.
Thermal spray Abradable-coatings are designed to have low structural integrity so that they are readily abraded when they come into contact with a high speed moving surface made of stronger materials. The coatings should not damage the mating surfaces.
The debris must be tiny and soft, should find their way out of the engine with the air and gas flow, without eroding other parts of the engine.
It is therefore not a simple task that of developing suitable Abradable-coatings materials and finding proper processes for their application in reliable and proven ways.
In operation, heat expansion influences all dimensions and moreover rotating parts are subjected to centrifugal forces.
Also several temporary or permanent factors may affect concentricity, thus permitting, thanks to the Abradable-coatings, occasional local contact to be endured without major damages.
Abradable-coatings in fact allow for closer clearances, but also self adjust in place, if needed, as a consequence of unexpected disturbances occurring during normal operation.
The final clearance cannot be inspected directly in a running engine, by simple techniques.
It can be estimated by measurements taken when the engine is still and cold.
However the real performance must be deduced from calculations from engine efficiency data.
Abradable-coatings are used also in most rotating machinery like stationary gas turbines, turbo compressors, turbo chargers, and centrifugal pumps.
The goal of developing suitable materials and application techniques is to produce seals that do not cause blade wear, that maintain as smooth as possible shroud surfaces and that remain serviceable for thousands of operating hours.
Two of the most important properties of good Abradable-coating materials are hardness and lubricity.
These materials should be not so hard to cause wear of the mating part by which they are being rubbed, but also not so soft to be easily eroded by whatever particles may be contained in the engine gas stream.
Lubricity allows the rubbing material to smoothly cut into Abradable-coatings, rather than tear away large pieces.
The coating must also have satisfactory high temperature oxidation and corrosion resistance to protect the base material onto which it is applied.
The matrix materials used for embedding the abradable particles range from
- Aluminum-Silicon for low temperature applications,
- MCrAlY (where M = Co, Ni or Co/Ni, Chromium Aluminum Yttrium) for mid range temperatures prevailing in the compressor stages,
- to Yttrium-stabilised Zirconia (a high temperature ceramic material) for high temperature turbine section stages.
Low temperature Abradable-coatings include polymer materials to generate porosity when burnt-out in a subsequent heat treatment.
They may include also solid lubricants and release agents to ease the separation of abraded particles.
The exact composition of the materials is obviously proprietary of the developers. The users perform long practical tests in simulated or actual operation to qualify products and procedures to their specific requirements.
For operating temperatures above 900°C, only ceramic-based abradables, that wear in a brittle manner, are used.
The blade tips must be possibly reinforced by a strongly embedded abrasive grit to cut into the ceramic based abradable.
Very good results are obtained by laser remelting of blade tips with simultaneous injection of suitable hard particles.
Abradable-coatings are applied by high velocity oxy fuel (HVOF) thermal spray and by atmospheric plasma spraying (APS), generally preferred because their controllable parameters assure reproducible quality.
These coatings are sprayed relatively dense and, therefore, require a sacrificial phase that can be burnt out to generate porosity.
Thermally sprayed coatings offer advantages over other materials, as they can be directly applied, easily removed and repaired, with a large range of coating materials to choose from, and with good performance proven by long term experience.
For additional information on these application techniques see our page on Thermal-spray.
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Coating System for Aircraft Industry
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