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/JF Surface Coatings Tecnnology For Applications By Dorothy M. Comassar

Evolution in Design &Coatings The role of coatings In As operating temperatures in- 196O-J79 (Fighter Engine) creased in the 1970%more com- Simple aircraft appllca- ponent protection was required. tlons Is increasingly impw - Solid turbine blade era Additionally, corrosion-resistant - 1500 "F (+) (822 "C) turbine temp tant. Higher engine operat- coatings were essential on more Coatings: ing temperatures, new components, such as disks, Anodized al alloys and component life blades, and vanes. Plasma- Ceramiccoated stainless steel alloy requirements all drive the sprayed, wear-resistant coatings were also applied on compressor need for more durable Ceramic coated cases blade dovetails. AI-lacquer, heat-resistant systems. More and more, Throughout the 198Os, a typical compres sor case the engine component engine profile would include coat- designer 1s using new ings in every section of the engine. 1970479 (Fighter Engine) coatings systems to help The broad range of coating sys- . Expanded use of coatings tems addresses environmental Convection-cooled turbine provide the environmental conditions across the operating protection needed for these blade era spectrum of the engine. * 1800 "F (+) (990 "C) turbine temp increasingly stringent Coatings: demands.Thls article briefly Design Changes Corrosion-resistant coating for reviews the historical use Turbine airfoil design has evolved blades & vanes of coating systems in to allow increasing temperature Aluminum corrosion-resistant paint capability. (Seeexample in Fig. 1 .) aircraft turbine engine on casings The higher temperature environ- Co-dep on vanes, blades applications. Typical coat- ment has put increasing demands Corrosion-resistantcoatings on disks ing systems by engine on airfoil materials, as well as pro- Corrosion-resistant paint on shaft section (fan, compressor tective coating systems. Plasma-sprayed, wear-resistant and turbine) are described Improved cooling provided by coating (bearing load on shaft) the impingementifilm-cooled de- by material, coating pro- signs increase cooling capability; cess and environmental 1980-CF6 (Commercial Engine) all aspects, however-design, ma- - Coatings in every section of the challenges for each area. A terials, and manufacturing-are engine state-of-the-art process- being pushed to meet higher tem- All types of coatings I' Thermal Barrier Coating perature demands. .Impingementifilm cooled turbine (TBC)Aescribes coating As shown in Fig. 2, alloy devel- blade era opment has progressed signifi- process development - 2150°F(+)(1 186"C)turbinetemp cantly, providing additional tem- Coatings: challenges that typify future perature capability. Fan & compressor rub liners applications. Coating systems provide the Thermal barrier coatings additional protection required to Hot gas path seals satisfy total design life. Simple Dovetail & mid-span wear coatings aluminides satisfied early needs. Compressor erosion-resistant look at typical coating ap- Overlay coatings have been coatings plications in turbine en- added over the last 10 years for Oxidation & corrosion-resistant gines over the past few turther environmental protection. coatings EA decades shows increas- State-of-the-art systems, such as Labryinth seals ing use as well as complexity Thermal Barrier Coatings (TBC), In the 1960s, typical coatings were are being implemented today. simple anodized aluminum, aluminum Strategies for coating technolo- Historical perspective on design and coatings lacquer and ceramic coatings. gies are shown in Fig. 3. evolution over the past 30 )'ems.

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#)LY PRODUCTS CO p.0. Box 151, Atwood, California 92601 (714) 538-0701 FAX (714) 538-0691 FruO~allr:Clrcle114011 pwlpald readar rarvlcscard. . .. resistant coating, WC-Co, to prevent wearand resulting from thecon- tinuous loading and unloading during engine operation. = 3000 The shroud is an abradable material Turbine Temperature .-m structuredesigned to wear awayduring 5 - contact with the fan blade. It also may 2 be configured to act as a component of - 2000 convection cooled the noise attenuation system. kL d Compressor Section i Solld Blade Year The compressor section of the modern I- 1000 Ngas turbine is a labyrinth of structures that compresses the incoming air for combustion. The environmental tem- peratures range from about 300 "F to 7g. 1-Turbine technology trends. 1200 "F. To achieve the most efficient compressor, leakage intothesumpand Engine Section Profile principally the dovetail and mid-span around the blade tips must be mini- Of Coating Systems damper (Fig. 4). Fan rub liners also mized. The airfoils are designed with Coating systems are tailored to the provide a rub surface for the fan blade specific contours to maximize pumping environmental conditions to which they tips; therefore, they usually contain soft, capability with a minimum of drag in as are subjected. Engine operating tem- abradable coatings. The selection of few compression steps as possible. perature increases dramatically from lightweight, high-strength materials, Increasingly, the role of specialized the fan to the turbine sections. There- such as titanium-based alloys for the coatings is to maintain the designed fore, the profile of coatings axiallywithin fan blades and disks, necessitates ac- clearancesand contours overthe lifeof the engine accommodates the chang- commodation of titanium's poor-wear the engine. Ingested particulateserode ing environment. capabilities. The fan blade attachment the contours of the airfoils, causing a surfaces, therefore, are clad with mate- loss of efficiency by changing the airfoil Fan Section rials such as CuNiln to prevent galling contour, opening clearances at the In the fan section of the engine, fan and fretting. Vibration dampener struts blade tip, and reducing stall margins. blades are coated at high-wear areas, are coated with a strong, hard, wear- Particulates raise the risk of fatigue

Component Materials Trend 2000 Turbine Blade Materials

= 1800 .-m z c Solidification vE" 1600 "

2! Yea,* ca Improved Aluminides m . simple. IneXpBnElYe k .Thin (c 3 m119) E 1400 MCrAlY Coatings . Complex chamlrlrlss require c PVD or VPS . Cr and Y Improve oxIdalion reslsfance. scale adherence . Thlck (> 4 mlls) 1200 Tailored Coatings .MalCh PhySIcol and mechanical pmpedles 10 substrate , .Salld solullon or oxide dlspernlon 340 1960 1980 2000 Blrenglhenlng Year .MalCh lo SUbSlrale

July 1992 21 Typical Coating Scheme Typical Coating Scheme - Fan Airfoils and Liners Compressor Airfoils I - Honey Comb Fan Liner Coating at with Filler Tip and Around Leading wc Edge Impact Wear Resistance

CuNiln I I Wear Resistance

Fan Blade Comaressor Blade Front Stages - Titanium with WC Coating * Ooeratino Temoerature - 250°F I . Back Stages - In718 with CrC3 Coating 1 * Substrate Material - Titanium I Fig. 4-Typical coating scheme for the fan seclion. Fig. 5-Typical coaling scheme lor compressor section. failure by reducing the blade cross- damage by coating the pressure- walls vent wear of the rotor surface by the section and by defecting the highly near the tip with tungsten carbide for vanes. The blade mounts are coated stressed surfaces. lower temperature service, and with a with CuNiln and a solid film lubricant, The degree of severity of such par- carbide-based coating in the and mate against the same coating ticulate damage is a function of the higher temperature regions. combination of knife-edge surfaces engine location, the use of the aircraft, The seals for the blade tips are main- coated with AI,O, to wear into filled and and the locale where flown. As ex- tained with an abradable coating ap- unfilled stationary surfaces. pected, helicopter , which are plied in segments, or directly to the required to hover in their own dust casing, as dictated by the engine de- Turbine Section storms, are prime victims, and have sign. The materials are generally ther- Theturbinesection representsthe most special centrifugal separators incorpo- mal-sprayed graphite for lower challenging environment for the engine rated at the inlet to remove particu- temperature applications, and alumi- designer. Temperatures up to 2150 "F lates. Commercial engines ingest par- num bronze-nickel graphite or NiCrAl require both substrate materials and ticulates, especially when mounted in bentonitefor highertemperature needs. coating systems to be highly oxidation- low-hanging nacelles. Each material is designed to wear pref- and corrosion-resistant. To emphasize Typical coating schemes in the com- erentially with respect to the blade tips. again, new turbine airfoil alloys have pressor are illustrated in Fig. 5. The The rotors are coated with AI,O, be- grown in temperature capability, but airfoils are protected from particulate neath the case mounted vanes to pre- still requirethe added benefits provided by a coating system. Turbine airfoil coating systems include several pro- Turbine Airfoil Coating Systems cesses that provide corrosion and oxi- dation resistance (see table). As can CodeDoSition *Simple aluminide Pack be seen from Fig. 6, there is an in- Cementation creasing use of more advanced coat- External surfaces ing processes. There is a strong trend toward complexcoating systems (Fig. Chemical Vapor Deposition (CVD) *Simplealuminide positive flow gaseous 7), each of which may contain several system of the advanced processes. * Internal and external surfaces

Physical Vapor Deposition (PVD) * Electron beam PVD Current Coating Processes External surfaces The following is a brief description of some of the current coating processes __ Vacuum Plasma Spray (VPS) * Plasma spray inside an inert gas in use today. chamber under low-pressure environment Chemical Vapor Deposition (CVD) Classic chemical vapor deposition

Ceramic Plasma Spray -Air plasma deposition 01 ceramic ~ uses an external generator to supply a

Ceramic PVD -Complex ceramic gaseous system reactor with the reactive aluminum ha- ~ ~~ * Direct evaporation of ceramic lide. Internal and external surfaces are (reactive evaporation) under partial coated byCVDmethod.CVDallowsthe pressure uniform coating of complex geometric parts (not limited to line-of-sight).

22 Plating & Surface Finishing Physical Vapor Deposition (PVD) Heated raw material is evaporated from a source and condensed onto VPS the workpiece. The system features: PS ceramic 1. High deposition rate 2. Generally high workpiece tem- peratures during deposition E 3. Usually highvacuumenvironment PT AL 0m 4. Provides precision deposition and V molecular deposition. CVD AL Of interest is the characteristically CODEP different microstructure (see Fig. 8) L resultingfrom the two thermal barrier 119881 CODEP 8 others I/ coating processes. This variation in microstructure provides design flex- lncreasinq quantity ibility to accommodate changing en- vironmental demands. Operating experience indicates that not only the external surfaces of tur- Fig. 6-Use of advanced costing processes is increasing. bine airfoils require protection, but also the internal passages. The chemical Thermal Spray Processes depositedintheformofmoltendropletsonto vapor deposition process provides this These are bulk coating deposition tech- a relatively cool substrate. Thermal spray overall protection. The key advantage of nologies. A raw material is heated to its allows multi-component alloy deposition; the gaseous system is that it allows si- melting point, using a plasma. Material is involves both oxide and metallic Systems. multaneous deposition of both the inter- nal and external coating.

Coatings Enhance Engine Performance The continuing drive for improved engine performance in terms of specific fuel consumption (SFC), has driven the need for thermal barrier coatings (TBCs). These coatings exhibit superior tem- perature capabilityand allow the needed growth in engine perfor- mance. Characteristically, these coat- ings provide a thermal barrier to 1960s 1970s 1980s 1990s the substrate material, thereby en- Coating Complexity Increasing abling higher temperature capa- I bility with equivalent cooling air flow. The higher operating tem- Fig. 7-There is a strong trend toward the use of complex coating systems. perature results in improved SFC. I The importance of the TBC system becomes more evident I Columnar zirconia] ILamellar zirconia I as the material temperature limit I \ 1 I I 1 is approached (Fig. 9). TBCs will extend the usefulness of this last generation of superalloys, and bridge the gap between superal- loys and the next generation of materials. I- Thermal barrier coatings , present a number of challenges, '1, from a process technology stand- point. The TBC example is rep- - resentative of newer, more ad- --. vanced coating systems, in that it speaks to certain challenges ahead, and how they must be 'ig. 8-tIIUStration Of the different miCmStrUCtUre resulting from two thermal barrier costing r: approached from a process de- cesser, PVD (left) and plasma (right). velopment standpoint.

24 Plating & Surface Finishing ,. ,-

FAST, SIMPLE, SAFE AND ECONOMICAL WAY TO DESTROY CYANIDE (And a11 Oxidizable Organics)

FrasDUllr:Clrcla 115onpcctpaldrrd~rsrvlcec~rd What About Tomorrow? A look ahead to the engines of tomor- row shows that challenges abound, in- cluding: * The increased use of nonmetallic 4 materials I Hybrid components fabricated from I metals and ceramics Material Process technologies for construc- Thermal tion of these hybrid components that Capability require coated fibers, which are then Limit shaped and formed into components * In order to implement new coating processes, concurrent must be addressed, so that design, manufacturing and process develop- ment are all in step, providing a better Turbine Efficiency product in a shorter period of time. All this translates into a host of new ig. 9-Thermal barrier coatings oyslemz enhance lurbme ell!crency. challenges across a broad spectrum. The coating process technolo- gistwill befacedwithapproach- ing process development in :odep TEC 4 COatinQS for much the same way as with the YC Seals MMC's, CMC's - TBC example. The task, how- ever, will be much moredifficult as new materials-with which there is little or no experience- must be addressed. The inte- gration of product and process definition becom@simperative, because one is integral to the Coating / other in producing acceptably high-quality hardware (Fig. 10). life Extension 4 Performance 4 Coatings Integral with Careful attention must be paid Benefit the Design to our approach to process de- MMC - Metal Matrix Cornpasite CMC - velopment today, to help in po- Increased Reliability Needed sitioning for the challenges of w tomorrow. o

Current applications of thermal bar- expected results. Finally, quality assur- Dorothy Comassar riercoatings are primarilyceramic coat- ance tests, including nondestructive is manager, Manu- facturing Technology ing materials over a metal substrate. evaluation of coated hardware, must Laboratory, General Coating thickness may range from 0.050 be developed for specific TBC applica- Electric-Aircraft En- in. for turbine shrouds, to 0.005 in. for tions. Current NDE techniques include gines, OneNeumann turbine airfoils. eddy current for thickness measure- Way, Cincinnaii, OH 45215. She joined Applying the ceramic coating over ment and thermography for detection the metal-bond-coats and substrates cre- of dis-bond areas. Company as an en- ates significant technical challenges. The challenges of such a complex gineering assistant, Differences in coefficients of thermal system require an increasingly disci- having beena co-op student with GEs expansion require a strain-tolerant ce- plined approach to process develop- Nuclear Propulsion Depart- ramic to adhere during cyclic thermal ment, more so than in the past. These ment while aiiending the University of Cincin- shock in service. Sustained operation more-advanced processes are less pro- nati, from which she earneda BS in mechani- atelevatedtemperature requires a bond cess-tolerant, and the process operat- cal engineering. Ms. Comassar has had a coat with superior oxidation resistance. ing window must be carefully estab- long, distinguished career wiih GE-Aircraft Engines, wiih a wide range of responsibilities. In addition, stresses from the coating lished. Study of the fundamental rela- In 1983, she assumed her current position, process must be understood and con- tionships between process parameters, where she is responsible for manufacturing trolled to assure desired performance material properties and product perfor- process development acivities and iransition and durability in the engine. mance is imperative. The resultant pro- to AE Shops.

Processing materials, notablythe ce- cess understanding will allow the re- ~ ~______~ T~sisaneditedversionotapaperpresented atthe ramic powder size and morphology, quired high yields to be obtained from a AESF Aerospace Symposium, January 29, 1992. must be tightly controlled to achieve coating process that is in control. Orlando, FL.

26 Piafing & Surface Finishing