Oxidation Resistant Porous Material For Transpiration Cooled Vanes

2INTRODUCTIONT h i sr e s e a r c h and developmentworkwas c o n d u c t e d w i t h t h e u l t i m a t e g o a l o fp r o d u c i n gp o r o u sm e t a ls h e e tm a t e r i a lw i t hg o o dh i g ht e m p e r a t u r eo x i d a t i o nr e s i s t a n c ef o ra p p l i c a t i o ni nt h et r a n s p i r a t i o nc o o l i n go fgas t u r b i n ee n g i n e s .2.1BackgroundAs r e p o r t e di nr e f e r e n c e3 , t h et r e n di nb o t hm i l i t a r yand commercialgast u r b i n ee n g i n e si st o w a r d st u r b o f a ne n g i n e sh a v i n ga compact, h i g ht e m p e r a t u r egas g e n e r a t o r .To make suchenginescompact,lightweight,and w i t hs u p e r i o rs p e c i f i cf u e lc o n s u m p t i o n ,t h e r ei st h eneed t os i m u l t a n e o u s l yi n c r e a s eb o t hcompressorpressurer a t i o and t u r b i n ei n l e tt e m p e r a t u r e .T u r b i n ei n l e tt e m p e r ifa t u r e s may go t o t h o s e c o r r e s p o n d i n g t o s t o i c h i o m e t r i c f u e l - a i r m i x t u r em a t e r i a l s and c o o l i n gd e s i g n sc a nbe developedthatwill t o l e r a t e suchtemperatures re1 iably.Up t o t h e p r e s e n t t i m e , c o n v e c t i o n c o o l i n ghasbeent h ep r i m a r y means o fc o o l i n g gas t u r b i n ee n g i n e s ,w i t h some film c o o l i n g a u g m e n t a t i o n i n c r i t i c a lit i sr e g i o n s . A t t h es e v e r ec o o l i n gc o n d i t i o n se x p e c t e di nf u t u r ee n g i n e s ,will beinadequate,andmoreadvanced c o o l i n gl i k e l yt h a tc o n v e c t i o nc o o l i n gf i l m and t r a n s p i r a t i o n c o o l i n g will have t o be u t i l i z e d .schemes suchasT h e o r e t i c a l l yt r a n s p i r a t i o nc o o l i n gi s s u p e r i o r t o film c o o l i n g as showni n r e f e r e n c e 4, p r o v i d e dt h eo x i d a t i o np r o b l e m sd e s c r i b e db e l o wc a nb eo v e r come. A l s o good creep l i f e canbeaccomplishedw i t ht r a n s p i r a t i o nc o o l e db l a d e5.designs compared w i t h o t h e r c o o l i n g d e s i g n sas d e s c r i b e di nr e f e r e n c es,shroud1 inersand s i m i l a r componentshave been f a b r i c a t e df r o mf i n ew i r ea b o u t0.005 i n c hspace wound i n a g e o m e t r i cp a t t e r n and(0.13 mn) i nd i a m e t e r ,f l a t t e n e d ,f i n a l l y d i f f u s i o n bonded t o p r o v i d e a p o r o u ss t r u c t u r e - Poroloy" (See r e f e r ence 6 ) . The c h a r a c t e r i s t i c so fh i g hs u r f a c ea r e at ovolume r a t i o and smallp o r es i z e enhance c o o l i n ge f f i c i e n c yb u ta g g r a v a t eo x i d a t i o np r o b l e m s .O x i d a t i o nc o r r o s i o nw h i c hmay be c o n s i d e r e dm i n o rf o rs o l i do rs h e e tm e t a lcompon e n t sm i g h ts e r i o u s l ya f f e c tp o r o u sw a l lc o o l i n ga i r f l o w p e r m e a b i l i t y ands t r e n g t h .T h e r e f o r e ,o x i d a t i o nr e s i s t a n c ei sa p r i m a r yd e s i g nc r i t e r i o nf o rt r a n s p i r a t i o nc o o l i n gm a t e r i a ls p e c i f i c a t i o n .R e t e n t i o no fgood mechanicalp r o p e r t i e s ,e s p e c i a l l yd u c t i l i t y ,a f t e rc y c l i ch e a t i n gand a g i n g i s a l s oi m p o r t a n t as w e l l as t h eh i g ht e m p e r a t u r es t r e n g t ho ft h em a t e r i a l .A c c o r d i n g l y ,t h e s ep r o p e r t i e sa r ed e t e r m i n e di nt h i sw o r k .E a r l i e r workasdescribedi nr e f e r e n c e s 1 and 2 was d i r e c t e d t o w a r d1, s o l i df o r mI.rne f e r e n c eselectionofalloysfortestinginporousmetao x i d a t i o n r e s i s t a n c e ands h e e ts p e c i m e n sw e r ee v a l u a t e do nt h eb a s i so f-I." P o r o l o yi sa r e g i s t e r e dt r a d e name d e s c r i b i n g space-wound w i r ep o r o u sm e t a lon, F i l t e r D i v i s i o n .m a t e r i a l ,m a n u f a c t u r e db yTheBendixCorporat3

m e c h a n i c apl r o p e r t i e s .F o u rc a n d i d a t ea l l o y s(N 155, TD nickel-chromium, DH 242and H a s t e l l o y X ) w e r es e l e c t e do nt h eb a s i so ft h er e s u l t so ft h e s et e s t sforf u r t h e re v a l u a t i o n .As r e p o r t e di nr e f e r e n c e2 ,t h e s ea l l o y sw e r emade i n t o f i n emm) d i a m e t e r )a n dt e s t e df o ro x i d a t i o nr e s i s t a n c eandw i r e (0.005 inch(0.13m e c h a n i c a lp r o p e r t i e s .The t e s t i n go ft h ea l l o y si nw i r ef o r mwas a necessaryp r e l u d et of a b r i c a t i o no fp o r o u sm a t e r i a l sf r o mt h e s ea l l o y sb e c a u s et h eo x i d a t i o nb e h a v i o r a n dm e c h a n i c a lp r o p e r t i e sc o u l dd i f f e rs i g n i f i c a n t l ybetween ag i v e na l l o yi ns h e e tand w i r ef o r m .I na d d i t i o n ,t h ed i f f i c u l t yi nf a b r i c a t i o no f good q u a l i t y f i n e w i r e f r o m e a c h a l l o ywas assessed.I t was c o n c l u d e dt h a tTD nickel-chromium showed themostpromiseandshouldbeconsideredforevalua t i o n i n p o r o u sm e t a fl o r m .I na d d i t i o n ,i t was n o t e dt h a ta l l o yGE 1541 w h i c hhadbeent e s t e di ns h e e tb u tn o tw i r ef o r ms h o u l dbeconsideredbecauseof i t ss u p e r i o ro x i d a t i o np r o p e r t i e s .2.2ScopeBased on t h e e a r l i e r a l l o y s e l e c t i o nwork, t h i s i n v e s t i g a t i o n was i n i t i a t e dt oe v a l u a t eP o r o l o ymade from GE 1541, TD N i Cr and DH 242and t o s u p p l y NASA w i t hs h e e t so ft h etwobesto f t h e s em a t e r i a l s .D i f f i c u l t ywas e n c o u n t e r e di np r o c u r i n g good q u a l i t y TD N i Cr w i r e i n s u f f i c i e n t q u a n t i t y f o r f a b r i c a t i n gP o r o l o y and i n i t i a l o x i d a t i o n p e r m e a b i l i t y t e s tshowed l e s sp e r m e a b i l i t yr e t e n t i o nt h a nGE 1541. C o n s e q u e n t l yt h i sm a t e r i a lwas d r o p p e df r o mc o n s i d e r a t i o nDH 245 and H 875.B o t hw e r es e l e c t e df o ri nf a v o ro ft w oo t h e rc a n d i d a t ea l l o y s ,b e i n gr e a d i l ya v a i l a b l ei nf i n ew i r eformandf o rt h e i re x p e c t e dgood o x i d a t i o nr e s i s t a n c e ; DH 245, a n i c k e l - c h r o m i u mt y p ea l l o ys i m i l a rt oDH 242 e x c e p tf o rt h ea d d i t i o n o f aluminumandH 875, a n i r o n chromiumaluminum a l l o y w h i c h was showni n e a r l i e r w o r k( r e f e r e n c e1 ) t o havegoodo x i d a t i o nr e s i s t a n c e .P r e l i m i n a r ye v a l u a t i o no ft h ea l l o y si nw i r eand F'oroloy f o r ml e dt ot h ec o n c l u s i o nt h a to n l y t h e GE 1541 and H 875 a l l o y ss h o u l db ee v a l u a t e de x t e n s i v e l yi np o r o u smetalform.The reason f o rt h i sc o n c l u s i o nwas t h a tt h e s ea l l o y sw e r et h eo n l yones t h a t showed anypromise o f b e i n g a b l e t o o p e r a t e a t t e m p e r a t u r e s o f 1 8 0 0 " (982 C)andabove,w i t h o u to x i d a t i o ns u f f i c i e n tt ocause a s i g n i f i c a n t loss o fs h e e tp e r m e a b i l i t y .However, t h et e n a c i o u so x i d el a y e rf o r m e db yt h ea l l o y smade d i f f u s i o n b o n d i n g d i f f i c u l tand n e c e s s i t a t e d a processdevelopmentforf a b r i c a t i n g them i n t oP o r o l o y .I t was d i s c o v e r e dd u r i n gt h i sp r o c e s sd e v e l o p ment t h a tt h e GE 1541 a l l o ys u f f e r e da c c e l e r a t e do x i d a t i o na tt e m p e r a t u r el e v e l swhere o x i d a t i o ni sn o r m a l l ys l i g h ti nt h eb u l ka l l o y .T h i sp o o ro x i d a t i o nr e s i s t a n c ea p p e a r e dt ob ei n t r o d u c e dt ot h ea l l o yd u r i n gp r o c e s s i n gi n t ow i r ef o r m .An i n v e s t i g a t i o n was conducted t o d e t e r m i n e t h e c a u s e o f t h e l o s s o f o x i d a t i o nr e s i s t a n c e and s p e c i f i c a t i o n s w e r e a d o p t e d f o r t h e a l l o y i n w i r eand rodformtopreventsucha l o s sf r o mo c c u r r i n gi nt h ef u t u r e .E v a l u a t i o no ft h eG E 1541 and H 875 i n p o r o u sm e t a lf o r mc o n s i s t e do fc y c l i cf u r n a c eo x i d a t i o nt e s t sa t1800 and 2000 F(982and1093 C) f o re x p o s u r et i m e so f 4, 16, 64, 100, 200, 300, 400, 500 and 600 h o u r s .A f t e reachexposurec y c l e , specimenswereremoved f r o mt h ef u r n a c eand a i rc o o l e d .T e m p e r a t u r e sand t i m e sa sw e l la sg e n e r a lt e s tp r o c e d u r e sw e r et h esame as t h o s er e p o r t e di nreferences 1 and 2.The o x i d a t i o ne f f e c t s wereevaluatedbymeasurement o fw e i g h tg a i n ,p e r m e a b i l i t ychange, t h i c k n e s s change and room temperaturem e c h a n i c a lp r o p e r t i e sa f t e re x p o s u r e .E l e c t r o nbeam w e l dj o i n t sbetweenPoroloyand I N 100 s t r u tm a t e r i a lw e r ea l s oe v a l u a t e di nt h e s et e s t s .I na d d i t i o n ,4

t e n s i l e and s t r e s sr u p t u r ep r o p e r t i e sweredetermineda t 1800 F(982 C).Testsf o rp e r m e a b i l i t yf o l l o w i n go x i d a t i o na t1700and1800 F(927and982 C)ande l e c t r o n beam w e l de v a l u a t i o n sw e r emade f o r t h e TD N i C r and DH 242 a l l o y s .DH 242servedasreferenceo f c o m p a r i s o nf o rt h eThe r e s u l t s o b t a i n e d f o r t h eD H 242 i s t h e most commonly employed a l l o y f o r t r a n s p i r a t i o no t h e ra l l o y ss i n c e7).c o o l i n gt o d a y( r e f e r e n c e2.3PurposeThe purpose o f t h i s i n v e s t i g a t i o nwas t o e v a l u a t e t h e s e l e c t e d a l l o y s i nporousmetalformi n terms o f p o t e n t i a l a p p l i c a t i o n for t r a n s p i r a t i o nc o o l i n go f gas t u r b i n e vanes.T h i sp u r p o s e was t o beaccomplishedby(1)developingp r o c e d u r e sf o rp r o d u c i n go x i d a t i o nr e s i s t a n tp o r o u sm a t e r i a lf r o md i f f i c u l t - t o bond, o x i d a t i o nr e s i s t a n ta l l o y s ,( 2 )e v a l u a t i n ge l e c t r o nbeam w e l d j o i n t s o ft h e s em a t e r i a l st ot u r b i n eb l a d es t r u ta l l o y s ,a n d( 3 )e x p e r i m e n t a l l yd e t e r m i n i n gandmechanicalproperties.the effects of oxidationonpermeability5

3WIRE PROCUREMENT AND POROLOY PROCESS DEVELOPMENT3.1WireProcurementA l l w i r e f o r t h i s program wasmadet o a nominaldiameter o f 0.005 i n c h(0.13 mm) u n l e s so t h e r w i s en o t e d .The H 875, DH 242andDH 245 a l l o yw i r e sw e r epurchased t o c o m m e r c i a ls p e c i f i c a t i o n s from H o s k i n sM a n u f a c t u r i n g Company andD r i v e rH a r r i s Company, r e s p e c t i v e l y . A vacuum m e l t e dh e a to fGE 1541 wasp u r c h a s e df r o mt h eG e n e r a lE l e c t r i cCompany andprocessedintofinewirebyH o s k i n sM a n u f a c t u r i n g Company. The c h e m i c a la n a l y s i so fa l la l l o yw i r e sa r ep r e s e n t e di nT a b l e1 . The m e c h a n i c a lp r o p e r t i e so ft h ew i r e si nt h ea s - r e c e i v e d ,s i m u l a t e ds i n t e r e dc o n d i t i o nand a f t e r o x i d a t i o n a r e g i v e n i n T a b l e2.3.1.1A n a l y s i so fA c c e l e r a t e dO x i d a t i o n .- I t was d i s c o v e r e dd u r i n gt h edevelopment o fb o n d i n gp r o c e d u r e sf o rGE 1541 w i r e t h a t bondedsamplesunderwenta c c e l e r a t e do x i d a t i o na f t e rs h o r te x p o s u r e( 2 4h o u r s )i na i ra t1800 F (982 C).The f i r s t i m p r e s s i o n was t h a t some o ft h eb o n d i n gt e c h n i q u e sw e r ec o n t r i b u t i n gt ot h el o s so fo x i d a t i o nr e s i s t a n c ei nt h ea l l o ys i n c et h eb u l ka l l o yo x i d a t i o nr e s i s t a n c e was e x c e l l e n t .However, i t was q u i c k l yd i s c o v e r e d upon o x i d a t i o nt e s t i n go ft h eGE 1541 w i r e p r i o r t o b o n d i n gt h a tt h ep o o ro x i d a t i o nr e s i s t a n c ewas i n h e r e n ti nt h ew i r e .T h e r e f o r ean i n v e s t i g a t i o n was u n d e r t a k e nt od e t e r minethecauseo ft h ep r o b l e mand t o u s e t h i s i n f o r m a t i o n t o p r e p a r ea specific a t i o nf o rt h ew i r eand t h er o d( f r o mw h i c ht h ew i r ei s drawn)whichwouldp r e v e n td e g r a d a t i o no fo x i d a t i o nc h a r a c t e r i s t i c si nt h er e d u c t i o no ft h ea l l o yf r o mb u l kt ow i r ef o r m .The f i r s t s t e p i n t h e a n a l y s i swas m e t a l l o g r a p h i ce x a m i n a t i o n .P h o t o g r a p h so ft h ew i r es u r f a c er e v e a l e dt h ep r e s e n c eo ft r a n s v e r s ec r a c k sa l o n gt h ew i r ew h i c h had p o o r o x i d a t i o n r e s i s t a n c e w h i l e t h e w i r e w i t hgood o x i d a t i o n r e s i s t a n c e had none ( F i g u r e1 ) .E x a m i n a t i o no ft h e1/4 i n c h( 6 . 4mm) d i a m e t e r o df r o mw h i c ht h ew i r ewas drawn a l s o r e v e a l e d some s u r f a c e c r a c k s , b u t w i t hafrequency much l e s st h a no nt h ew i r e .W i r e sw i t hp o o ro x i d a t i o nr e s i s t a n c ew e r efound t o r e g a i n o x i d a t i o n r e s i s t a n c eif, priortooxidation,thewirewas e t c h e dt o remove a l lt r a c e so ft h es u r f a c ec r a c k s .T h i si n d i c a t e dt h a tt h ep o o ro x i d a t i o n r e s i s t a n c e was i n i t i a t e d by t h es u r f a c el a y e rc o n t a i n i n gt h ec r a c k s .Samples w i t h t r a n s v e r s e s u r f a c e c r a c k s w e r e f o u n d t o o x i d i z emore r a p i d l y whentheywereexposedt o h y d r o c h l o r i c a c i d and t h e n o x i d a t i o n - t e s t e d w i t h o u tc l e a n i n gt h er e s i d u e .T h e r e f o r e ,i m p r o p e rc l e a n i n gd u r i n gw i r ep r o c e s s i n gc o u l dh a v ec o n t r i b u t e dt oi t s failure.However, t h i sd i dn o te x p l a i nt h ep r e s e n c eo fI t was thoughtthatimproperdrawingprocedurescouldLea causet h ec r a c k s .( e . g . ,e x c e s s i v er e d u c t i o n s ,i m p r o p e rl u b r i c a t i o n ) .The e f f e c to fa n n e a l i n gf u r n a c ea t m o s p h e r ewas i n v e s t i g a t e d .A n n e a l i n gt h ew i r ei nh y d r o g e na t1900 F(1038 C)producedn e i t h e rc r a c k i n gn o rg r e a t l ya c c e l e r a t e do x i d a t i o n .However, t h ee f f e c to fn i t r o g e nwas pronounced. Asshown i nF i g u r e2 ,a n n e a l i n gi na n i t r o g e na t m o s p h e r ef o ro n l y1 / 2h o u ra t1900 F( 1 0 3 8 " )p r o d u c e dn i t r i d e st h r o u g h o u tt h ec r o s ss e c t i o no ft h ew i r e .Thesen i t r i d e se m b r i t t l e dt h ew i r e .C o n s e q u e n t l y ,e x p o s u r et on i t r o g e nwas mostp r o b a b l yt h ec a u s eo fs u r f a c ec r a c k i n gs i n c et h ew i r e sw e r ea n n e a l e di nc r a c k e dammonia d u r i n gp r o c e s s i n g .The e x p o s u r ed u r i n gp r o c e s s i n gwas o n l yt h e6IIllI II

e q u i v a l e n t o f a b o u to n em i n u t es t a t i ce x p o s u r eand t h e r e f o r e , n i t r i d e s w e r e n o tn o t e dt h r o u g h o u tt h ew i r e ' sc r o s ss e c t i o n .However, f o r m a t i o ni n a t h i ns u r f a c et o p r o d u c et h ec r a c k i n g .! J i r e sw h i c hw e r eh e a t e di nl a y e rw o u l db es u f f i c i e n tt h en i t r o g e na t m o s p h e r ei nt h el a b o r a t o r yw e r et h e ne x p o s e dt oa i ra t1 8 0 0 " (982 C) f o r 24 hoursandfoundt os u f f e ra c c e l e r a t e do x i d a t i o n( F i g u r e3).Therefore,exposureof thewireduringannealingtonitrogen-containingatmosp h e r e was a p p a r e n t l y t h e c a u s e o f t h e l o s s o f o x i d a t i o n r e s i s t a n c e s i n c e w i r ep r o c e s s i n gp e r f o r m e di na ni n e r t( n o n - n i t r o g e n )a t m o s p h e r ey i e l d e dgood q u a l i t yw i r e .E x a m i n a t i o no fw i r esamples d u r i n gv a r i o u ss t a g e so fw i r ed r a w i n gr e v e a l e d t h a t most o f t h e t r a n s v e r s e c r a c k s w h i c h o c c u r r e d i n p o o r q u a l i t y w i r ea p p e a r e da f t e rt h ef i n a ldraw i n d i c a t i n g t h i s t o bethemostcriticalstepintheprocess.The l o s s o f o x i d a t i o n r e s i s t a n c e a f t e r e x p o s u r et o n i t r o g e na p p e a r s t o bea r e s u l to fn i t r i d ef o r m a t i o nw i t ht h ea l l o y i n ge l e m e n t s .G e n e r a lE l e c t r i chasGE 1541 a l l o y l o s e s i t sgood o x i d a t i o nr e s i s t a n c e when t h ef o u n dt h a tt h ealuminumleveldropsbelow3.5 p e rc e n t .A p p a r e n t l yt h ef o r m a t i o no fn i t r i d e sd e p l e t e st h em a t r i xa l u m i n u mt oa c o n c e n t r a t i o nb e l o wt h i sv a l u e .3.1.2Rod and W i r eS p e c i f i c a t i o n . - Based o nt h er e s u l t so ft h i sa n a l y s i s ,s p e c i f i c a t i o n sw e r e drawnupf o r t h e GE 1541 w i r e and r o d( f r o mw h i c ht h ew i r ei s drawn) t op r e v e n tt h el o s so fo x i d a t i o nr e s i s t a n c ed u r i n gp r o c e s s i n g .Thes p e c i f i c a t i o n s were g i v e nB e n d i xR e s e a r c hL a b o r a t o r i e si d e n t i f i c a t i o nnumbersand a r ed e s i g n a t e da sPS-985 ( w i r e ) and PS-997 ( r o d ) . The essence of t h er o ds p e c i f i c a t i o ni st h a tt h ealuminumandy t t r i u mc o n t e n to ft h er o db es u f f i c i e n t l yp e rc e n t minimum y t t r i u m ) so t h a t i fh i g h ( 4 p e rc e n t minimumaluminumand0.5some d e p l e t i o no c c u r sd u r i n gw i r ed r a w i n g ,i t will n o td e g r a d et h eo x i d a t i o nc h a r a c t e r i s t i c so ft h eb u l ka l l o y .I na d d i t i o n ,t h er o dmustbethoroughlyi n s p e c t e d( v i s u a land d y ep e n e t r a n t )f o rs u r f a c ef l a w sand c r a c k s p r i o r t o w i r ed r a w i n gR. a d i o g r a p h i ce x a m i n a t i o nf o irn t e r n afll a w si sa l s or e q u i r e d .Them a j o rr e q u i r e m e n t s o f t h e w i r e s p e c i f i c a t i o n s a r e :1.no n i t r o g e n - c o n t a i n i n ga, n n e a l i n ga t m o s p h e r e sshouldbeemployedandreducingatmospheress h o u l d g e n e r a l l y beavoided;2.nophosphatew i r ed r a w i n gl u

schemes such as film and transpiration cooling will have to be utilized. Theoretically transpiration cooling is superior to film cooling as shown in reference 4, provided the oxidation problems described below can be over- come. Also good creep life can be accomplished with transpiration cooled blade