Chromalloy Keeps Aircraft Engines Aloft

Advanced thermal barrier coatings ensure high-revving gas turbines ‘stay cool’ under pressure.

Traditional hard chrome electro-plating and Type II anodizing

technologies have long beenemployed in the surface preparationor final coating of various aircraftcomponents—landing gears andother functional parts chief amongthem. But for other key sections ofthe aircraft—namely the criticalblades comprising the workhousegas turbine engines—repair stationsand OEMs typically opt for the high-er performance thermal barrier coat-ings. Generally applied via electronbeam physical vapor deposition, orEBPVD, these thermal barrier coat-ings—proponents say—are betterequipped to handle the extremelyhigh temperatures of today’sadvanced gas turbine engines, whichcan exceed 2400°F.

One highly regarded specialist inthis area is Chromalloy, a leading

independent supplier of technolog-ically advanced repairs, coatings,and FAA-approved re-engineeredparts for turbine airfoils and criti-cal engine components for com-mercial airlines, military craft,industrial turbine engines, andaero-derivative applications. A pio-neer in the development of com-mercially viable aluminide coatings,Chromalloy has introduced a seriesof innovative and proprietaryprocesses that allow engines to per-form at improved efficiency levels,at higher operating temperaturesand under severe environmentalconditions.

Dr. Ravi Shankar, director of coat-ing and process technologies atChromalloy, offers his perspective onwhy these thermal barrier “ceramic”coatings are so effective for thesedemanding applications (See Figure

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1). “Ceramic coatings are inherentlybrittle. The EBPVD process creates amicrostructure consisting of ceramiccolumns with gaps or porosity inbetween the columns, and thisimparts on the coatings a thermalcycling resistance to the rapid cyclesthat occur in gas turbine engines.Other types of processes (i.e., powdercoating, liquid paint, etc.) will pro-duce coatings that have porosity, butthey are only suitable for lower tem-peratures or limited temperatureranges. They simply would not sur-vive the very rapid temperature expo-sures that are so prevalent in a gasturbine engine.”

In that sense, one might considerceramic finishes to be “smart” coat-ings, so to speak. “Thermal barriercoatings are used to essentially allowthe engine temperature to run a littleharder—the technology essentially‘fools’ the blade into thinking thatit’s running colder,” Dr. Shankarexplained. “Use of thermal barriercoatings has allowed the operatingtemperatures of the high pressureturbine vanes and blades to increasesignificantly, thereby minimizing thedeleterious effects on the part basemetal.”

Shankar points to other benefits.Not only has the overall efficiency ofthe gas turbine improved due tothese thermal barrier coatings, but

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Chromalloy’s electronicbeam physical vapor depo-sition center inOrangeburg, N.Y.

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there have also been significantincreases in the time intervalsbetween required overhaul andmaintenance, he explained. Thisresults in significant cost savings tothe gas turbine operator or repairstation.

While Chromalloy says it is respon-sible for ushering in more recentinnovations in this particular field, itwon’t take responsibility for the ini-tial development of thermal barriercoatings (TBCs). That honor, accord-ing to Shankar, goes to the NASA,the National Aeronautics and SpaceAdministration, which developedTBCs back in the 1970s. Back thenTBCs were originally applied viaplasma spraying—a more “conven-tional” application method. Whileapplication of ceramic coatings viaplasma spray is still very much in usetoday, industry observers say opera-tors tend to relegate that process forturbines used in the power genera-tion industry, as the blades/vanes inthose applications usually rotate atmuch slower speeds compared to air-craft turbine engines—and slowerspeeds translate into less centrifugalforce.

By comparison, the EBPVD

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process employed in the coating ofthe more rapidly moving engine tur-bines grew in popularity as operatorsrealized the benefits of the applica-tion process. As Shankar describes it:the electron beam melts a pool ofceramic ingots, which physicallyevaporate and condense on the part.As the part rotates over the moltenpool, it generates a microstructure(similar to a toothbrush) with ceram-ic columns. To that end, as the basemetal expands and cools in a gas tur-bine engine, these ceramic columnsexpand and close without cracking.“Because the coatings are applied at avery high temperature, the bondingis chemical, so it’s stronger than theplasma spray coating,” Shankar said.“And because each ceramic column isso dense, it provides good erosionresistance that can withstand thehigh-speed rotation of the blade.”

The plasma spray method differsmarkedly from EBPVD, according toShankar. Under the conventionalplasma spraying procedure, he saysoperators essentially melt ceramicpowders (using the plasma gun),which are then sprayed onto the part.Much like an organic paintingprocess, layers of porosity are built in

between as operators apply themonto metallic bond coatings thatprovide mechanical bonding.Because of this, Shankar argues, theyhave slightly lower erosion and ther-mal cycling resistance—which, atleast in the aerospace industry, haslimited their use to stationary partssuch as nozzle guide vanes.

NEXT GENERATION INNOVATIONSHaving virtually mastered therequirements of today’s demandingcommercial and military aerospaceapplications, Chromalloy has itssights set on the next big hurdle—theeven higher-burning gas engines oftomorrow. Manufacturers havemade no secret of their goal toincrease the gas temperature to levelsgreater than what was previouslyachievable, so the engines can pro-duce more power and thrust.

To that end, Chromalloy is puttinga lot of stock in one of its newestinnovations, a patented coatingcalled Low K RT-35™, a lower ther-mal conductivity coating that allowsfor the same coating thickness foroptimal insulation, but enables theturbine blades to run cooler while

Partners in Research and Development Chromalloy makes it a point to scout out opportunities for joint ventures andstrategic partnerships both within and outside the traditional supply chain. Onesuch opportunity surfaced in 2011, when it signed on as an “organizing industrymember” with the Commonwealth Center for Advanced Manufacturing(CCAM), a public/private collaboration of member companies and Virginia’s pre-mier research universities: the University of Virginia, Virginia Tech and VirginiaState University. CCAM is primarily tasked with development of new manufacturing technologiesand transference of processes from the research lab into the production envi-ronment. The center’s organizing industry members, led by titans such as Rolls-Royce—the venerable turbine engine manufacturer—comprise diverse indus-tries. Other prominent partners include Canon Virginia, Northrop GrummanShipbuilding, Siemens and Sandvik. In 2012 CCAM unveiled its new facility on 20 acres adjacent to the Rolls-Roycejet engine manufacturing facilities in Virginia. Rolls-Royce donated land for the60,000-square-foot CCAM facility, which houses computational and large-scaleproduction labs as well as open production space for heavy equipment and sur-face coating processes. Dr. Ravi Shankar, director of coating and process tech-nologies for Chromalloy, recently told Metal Finishing magazine that his compa-ny is conducting both proprietary research as well as some generic researchwith the partners of CCAM. While the advancement of surface engineering isthe focal point of this research and development, innovation in precision partsmanufacturing is also high on the list. For more information on the program, please visit www.ccam-va.com.

Figure 1. Aerospace turbine engine blade withthermal barrier coating. Ceramic coatings,which are electrically non-conductive, cannotbe reproduced by electroplating or anodizing.

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ing, applied via EBPVD, of course,was successfully flight tested by aprominent Asian airline. The coatingwas demonstrated to enhance ther-mal insulation and provide greaterprotection for erosion and thermalcycling initially on coupons andpins, according to Shankar.(Additional planes incorporating thespecially coated blades are continu-ing to fly, with no negative feedback,he reports.)

Chromalloy’s Low K RT-35™ coat-ing was ultimately certified by theFAA in 2010 for use on the PW4000second-stage high-pressure turbineblade. Certification followed a seriesof tests confirming the low thermalconductivity (50% lower, to be moreprecise), high thermal cycle durabili-ty, high sintering resistance, highthermal–chemical stability, and goodphase stability of the coating. What’smore, Low K RT-35™ increases oxi-dation and corrosion resistance ofthe underlying bond coating, therebyextending the life of the engine com-ponents.

The next challenge for Chromalloycenters on the potential for the devel-opment of coatings that entail moreusage of composites—materials thatare being considered for commercialaircraft design. According toShankar, some OEMs have tried tomake turbine/compressor compo-nents with composites for enginetests. In those cases, Chromalloy hasprovided coatings for experimentalpurposes. But Shankar admits they

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the engine temperatures heat up (SeeFigure 2).

According to Chromalloy, the newLow K RT-35™ coating was the resultof a multi-year process that culmi-nated with certification for commer-cial aircraft engines. The coating wasengineered to address a very specificchallenge: When operating tempera-tures escalate in advanced gas tur-bine engines—especially tempera-tures exceeding 2400°F—the conven-tional “7YSZ” thermal barrier coatedparts showed rapid deterioration dueto insufficient thermal protection. Inaddition, researchers found its ownsintering reduced the thermal barriercoating’s compliance, causing addi-tional stresses resulting from volumechanges due to phase transforma-tion at the higher temperatures. Toaddress this issue, Chromalloy andother developers produced new ther-mal barrier coatings to provide lowerthermal conductivity to more effec-tively insulate the thermal transfer tothe components.

Chromalloy’s Low K RT-35™ coat-

haven’t really gone to the stage wherethey have been commercialized andmass produced. “I think it will be afew years before composites get intoroutine production,” he surmised.

So, for the time being, Chromalloyis sticking to its recipe for success bycontinuing to work with the OEMsand customers in commercial avia-tion and other market segments toaddress both present and emergingmanufacturer and operator needs.

“As turbine manufacturers increasethe internal operating temperaturesof engines to provide more powerand improvements to the engineoperation,” Shankar notes, “the needfor new coatings will continue.”

ABOUT CHROMALLOYChromalloy employs more than 4,000people and maintains sales and produc-tion operations in 17 countries. The com-pany supplies components, coatings, andadvanced manufacturing services to origi-nal equipment manufacturers, and it alsooffers extensive engineering and compo-nent repair capabilities for aviation,marine, and land-based aero-derivativeand heavy industrial turbine engines.

Now in its 61st year of operation,Chromalloy has built up a repertoire ofcoatings for aero engines and energy gen-eration alike. Chromalloy is also focusingon the manufacture of replacement parts,offering a full array of castings, coatingsand repairs, developed both independentlyand in conjunction with OEMs.

For more information, please visitwww.chromalloy.com.

Figure 2. An aerospace turbine engine bladecoated with Chromalloy’s Low K RT-35 coating.

Figure 3. Air plasma spray robot applies thermal barrier coating.

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