Dr. William Brindley Surface Engineering for Gas Turbine Engines
Dr. William Brindley - Manager Repair Center for Excellence
Rolls-Royce Corp.


Coatings have become more and more important for Gas Turbine components. Each component of a Turbine must meet a lifetime. Traditionally the first priority for designing such components was to develop materials to meet the mechanical and physical requirements, and the thought of utilizing coatings to achieve these goals was a "last step" in the design process. Times have changed however, and now coatings are no longer considered last, and many times are the first consideration in designing a component. It has become widely accepted that coatings are a necessary requirement for modern gas turbine components. Now people ask: What coating to use?, what the base metal-coating compatibility is?, and will it work?.

The most frequent consideration in engineering a coating for an application is cost. Other important considerations are its effect on base metal fatigue, base metal compatibility, and environmental issues (including negative effects on rearward components). While TBC (Thermal Barrier Coatings) get most of the press, coatings are very important for all stages of the gas turbine. We will discuss some of them from the front of the engine to the back.

In the compressor special abradable coatings are used to create air seals. It must be kept in mind that abraded particles travel through the rest of the turbine, so careful engineering is required to not let these become a problem. At the engine inlet erosion control coatings are used, and for turbine vanes there is a need for Thermal Barrier Coatings. Fretting is another problem that is addressed with coatings. Titanium to titanium wear surfaces would fret badly. A plasma sprayed Cu-Ni-In coating is found to greatly reduce this problem for todays engines, but better solution will be needed as requirements increase.

Compressors are frequently housed in Custom 450 which corrodes badly by salt spray (particularly of there are small amounts of SO2 in the environment). Coatings can reduce this problem. Another problem (particularly for the T-56 compressor in desert warfare) is erosion from sand ingestion. Hard coatings (such as TiN) are effective to fight such erosion. In some cases use of this coating has reduced inspections from a 300 hr to 2000 hr frequency.

Another problem in the compressor is the use of Al-graphite as a chipbreaker in abradable seal coatings. Unfortunately Al carried downstream can degrade high temperature capabilities in the combustor section. A program is currently underway to eliminate Al in the compressor. Another approach to reduce dust erosion is to put abrasive on the blade tips and allow use of harder (less erodible abradable coatings) in the seals.

In combustors the trend has been to use plasma sprayed MCrAlY coatings and with the use of less cooling air some plasma sprayed zirconia as the thermal barrier coatings. As operating temperatures get higher and higher the trend is to seek coatings with less thermal conductivity, and manage the heat transfer mechanisms so the onset of sintering does not occur. Today turbine operating temperatures routinely exceed the melting temperature of the component metals. Thermal barrier coatings must protect the component metals from seeing these temperatures. Important properties of such coatings include, creep, oxidation resistance, TMF, fatigue, fretting and erosion. Theoretically some TBC coatings can effectively block a 300 F disparity between operating temp and metal melting point. However, these coating can fall off and since they cannot regrow, serious consequences occur. For this reason excellent bond coats are needed. Currently there are no practical test beds to evaluate such coating properties. NASA has a laser rig for high thermal gradient measurement that give limited useful information. In current bond coatings such a BNi-50Al 15Pt, it has been found that the addition of hafnium can cause the Aluminum to diffuse into (rather than out of) the coating. Thus "gamma and gamma prime" coatings with platinum and hafnium are of special interest. "Gamma and gamma prime" coatings match the structure of the superalloys they are applied to.

Finally there is a great need for an good NDE (non-destructive evaluation) method for estimating remaining life in coatings. Much expensive processing (and many environmental issues) can be greatly reduced if coating could be used to their full useful life. One promising possibility is a "photo luminescence piezo spectroscopy" technique current being developed.

In summary, designers are now understanding the importance of coatings and the various aspects of engineering proper coatings for each application.