Clearance control coatings
Clearance control systems or gas path seal coatings are those used in selective areas of gas turbine engines to maintain tight tolerances between rotating and static parts. This is best accomplished when the rotating member cuts a path into the static component. Typically, the static member is coated with an abradable material while the rotating part is coated with a hard, abrasive material. The rotating member functions like a grinding wheel.

The abradable component will exhibit good adhesion and erosivity; be easily rubbed with the rub surface being smooth; and, lastly, debris should not be detrimental to the engine’s overall performance.

The deposition of abradable coatings is particularly suited to thermal spraying based upon current knowledge of parameter interactions. This awareness permits the deposition of coatings with predetermined density levels vital for them to be highly abradable without causing damage to the incurring member.

Coatings for engine cold sections (<1200°F [649°C]), low (fan) and high-pressure compressor (HPC), are generally applied over a nickel-aluminum bond coat. Abradable products include:

· Commercially pure aluminum
· Aluminum-silicon alloys
· Aluminum quasicrystal alloy
· Nickel graphite composites
· Nickel-aluminum alloys and composites
· Silicon-aluminum graphite composites
· Aluminum-bronze graphite composites
· Silicon-aluminum+polyester blends
· Silicon-aluminum+polyimide blends
· Aluminum-bronze+polyimide blends
· Nickel-chrome+polyester blends
· Nickel-chrome+polyurethane blends
· Nickel-chrome+bentonite blends
· Nickel-chrome-aluminum/bentonite blends
· Nickel-chrome+boron nitride blends
· Nickel-chrome+hollow spheres blends
· Nickel-chrome-iron+boron nitride blends

At the rear of the engine, in the high pressure and low-pressure turbine (HPT and LPT) sections, temperatures are very hot, much greater than 1200°F (649°C), necessitating the use of high materials which are easily rubbed. High temperature abradables include:

· MCrAlY type alloys
· Exothermic MCrAlY’s
· Nickel-chrome-aluminum/bentonite blends
· Yttria-zirconia + polyester blends

As with front-end abradables the abrasives deposited onto hot section components are generally applied over a nickel-aluminum composite bond coat. However, unlike the abradables the abrasives are dense. Abrasive materials include:

· Nickel clad alumina
· Nickel-chrome clad alumina
· Nickel-chrome+clad alumina blends
· Nickel-chrome-aluminum+alumina blends