Thermal Barrier Coatings for Jet Engines Improvement

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Thin Solid Films, 119 (1984) ,301-315

ME T AL L URGI CAL AND PROT E CT I VE COAT I NGS 301

THERMAL BARRIER COATINGS FOR JET ENGINE IMPROVEMENT*

G. JOHNER AND K. K. SCI- I1A/EITZER

MT U Motoren- und Turbinen-Union Miinchen G.m.b.H., Munich F.R .G. )

(Rec eived M arc h 30 , 1984; accep ted M ay 7 , 1984)

The aim of these investigations is to develop thermal barrier coatings (TBCs)with improved resistance against thermal cycling to prevent problems of over-

heating in the hot section of jet engines. Plasma-sprayed TBCs consisting of an

Ni-Cr-A1 bond coat and zirconia overlayers stabilized with different materials

(CaO, MgO or Y203) were tested in a burner rig under thermocyclic conditions, in a

hot gas centrifugal rig and in a modern jet engine. These tests demonstrate good

correlation between the failure mode of the coatings in the thermocyclic tests and in

engine tests. The best performance in all tests was shown by zirconia partially

stabilized with yttria according to microanalytical investigations of the unstressed

and stressed TBCs. Furthermore, heat treatments as well as ongoing optimization of

the spraying parameters of the TBCs demonstrate potential improvement.

1. INTRODUCTION

In the future there will be an ever-increasing demand for improved fuel

efficiency of aircraft gas turbine engines, both in the military and civil sectors. An

unavoidable concomitant of this demand will be higher gas temperatures. Direc-

tionally solidified eutectics, single-crystal materials and oxide-dispersion-strengthened superalloys are costly ways of achieving high gas temperature

resistance. A comparatively favourable method lies in the use of ceramic thermal

barrier coatings (TBCs) consisting, inter alia, of a metallic bond coat and a top coat

constituted mainly of ZrO2. Both coatings are preferably applied by plasma

spraying. The purpose of the present investigation is to obtain a TBC on internally

cooled turbine blades with best possible insulating properties and high reliability

against spalling during operation.

2. THEORETICALASPECTS

ZrOz is used as the base material of TBCs mainly because of its physical

* Paper presented at the InternationalConferenceon MetallurgicalCoatings,San Diego, CA, U.S.A.,April 9-13, 1984.

0040-6090/84/$3.00 © ElsevierSequoia/Printed n The Netherlands



302 G . J O H N E R , K . K . S C H W E I T Z E R

p r o p e r t i e s , s u c h a s it s t h e r m a l c o n d u c t i v i t y (2 ~ 1 .7 W m 1 K 1 f o r t h e b u l k

m a t e r i a l ) , t h e r m a l e x p a n s i o n c o e f f i c i e n t (ct ~ 9 × 1 0 - 6 m m 1 K 1) a n d m e l t i n g

p o i n t ( ~ = 2 7 1 0 ° C )1 .

T h e c r y s t a l s t r u c t u r e o f Z r O 2 a l t e r s w i t h t e m p e r a t u r e , c h a n g i n g f r o mm o n o c l i n i c to t e t r a g o n a l t o c u b i c a s t h e t e m p e r a t u r e i n c r e a se s , a n d v i c e v e r s a as t h e

t e m p e r a t u r e d e c r e a s e s a g a i n . T h e r a p i d d i f f u s i o n l e s s m a r t e n s i t i c c h a n g e f r o m

m o n o c l i n i c t o te t r a g o n a l s t r u c t u r e in t h e t e m p e r a t u r e r a n g e 9 5 0 - 1 4 0 0 ° C , a c c o m -

p an ie d by a v ol um e c o n tr a c t i o n of J/o-J,-/o~ /1-~o/2.3, n p ar t ic ula r , l ead s to m at er ia l

f a ti g u e a n d c o n s e q u e n t l y m e a n s t h a t Z r O 2 a l o n e is u n s u i t a b le a s a T B C . H o w e v e r ,

b y a ll o y i n g Z r O 2 w i t h o t h e r o x i d e s su c h a s Y 2 0 3 , M g O o r C a O , i t is p o s s ib l e to

o b t a i n m a t e r i a l s w i t h a p a r t l y o r e v e n e n t i r e ly s ta b l e c u b i c l a t ti c e s t r u c t u r e o f t h e

f l u o r i d e t y p e a l l t h e w a y f r o m r o o m t e m p e r a t u r e t o o v e r 2 0 0 0 c'C .

T h e s t a b i li z a t io n , c o m p l e t e o r p a r ti a l, i n w h i c h t h e t e t r a g o n a l a n d m o n o c l i n i c

p h a s e s a l so o c c u r i n a d d i t i o n t o t h e c u b ic p h a s e , d e p e n d s o n t h e a m o u n t o f a d d e d

o x i de . C o m p l e t e s t a bi li t y c e r t a i n ly s o lv e s t h e p r o b l e m o f m a r t e n s i t i c p h a s e

t r a n s f o r m a t i o n a n d v o l u m e c h a n g e , b u t t h is is o ff se t b y p o o r t h e r m a l s h o c k

b e h a v i o u r 4. T h e t h e r m a l s h o c k b e h a v i o u r , w h i c h is o n e o f t h e m o s t i m p o r t a n t

p r o p e r t ie s , c a n b e d e c i s iv e ly i m p r o v e d b y p a r t i a l s t a b i l iz a t i o n 5. T h i s p h e n o m e n o n

c a n b e e x p l a i n e d , f o r e x a m p l e , w i t h t h e a i d o f t h e m o d e l o f s t r e s s - in d u c e d p h a s e

t r a n s f o r m a t i o n , a l s o c a l le d p h a s e t r a n s f o r m a t i o n t o u g h e n i n g 2,6 s , a s f o ll o w s .

I f a c r a c k f r o n t m e e t s a t e t r a g o n a l p a r t i c le , t h e p a r t i c l e w ill u n d e r g o

t r a n s f o r m a t i o n i n t o t h e m o n o c l i n i c p h a s e. T h e r e s u l t in g i n c r e as e in v o l u m e o f t h e

p a r t i c l e s to p s c r a c k g r o w t h a s a r e s u lt o f c o m p r e s s i v e s t r es s es , o r l e a d s t o t h ef o r m a t i o n o f m i c r o c r a c k s ( le n g t h , 1 ~ m o r le ss) 9 i n t h e v i c i n i t y a c c o m p a n i e d b y

e n e r g y d i s s ip a t i o n o f th e p r i m a r y c r a c k . T h i s t r a n s f o r m a t i o n p r o c e s s c a n b e c a u s e d

j u s t a s e a s i ly b y t h e r m a l s t r e ss .

T h e i nf lu e n c e o f C a O , M g O a n d Y 2 0 3 a s st a b il iz e rs o n t h e t h e r m a l s h o c k

r e s i s ta n c e o f Z r O 2 t h e r m a l b a r r i e r c o a t i n g s c a n b e i l l u s t r a te d b y t h e e a r l i e r

e x p e r i m e n t s b y S t e c u r a 1° a n d S t e p k a e t a l . 11 , w h o o b t a i n e d t h e f o ll o w i n g re s u lt s

w i t h M A R - M - 2 0 0 a s s u b s tr a te a n d N i - 1 6 C r - 6 A I - 0 . 6 Y a s b o n d c o a t , w h e r e e a c h

h o t g a s c y c l e w a s f o r 1 h a t M a c h 0 .3 w i t h t h e t e m p e r a t u r e v a r i e d f r o m 2 8 0 t o 9 75 t o

2 8 0 ° C : Z r O 2 ' 5 . 4 C a O , s p a ll in g a f t e r 87 c y cl e s; Z r O z . 3 . 4 M g O , s p a ll in g a f t e r 4 5 0c y c l e s; Z r O 2 1 2 Y 2 0 3 , 6 5 0 c y c l es w i t h o u t p r o b l e m s . I n th e s e e x p e r i m e n t s , t h e Y 2 0 3 -

s t a b i l i z e d T B C p r o v e d t o h a v e t h e b e s t r e s i s t a n c e t o e r o s i o n a s w e l l . C o m p l e t e

s t a b i l i z a t i o n o f Z r O 2 i s a s s u m e d f r o m a b o u t 2 w t.~ o Y20312, 13

3. E X P E R I M E N T A L D E T A I L S

3 . 1 . T e s t m a t e r i a

F o u r d i ff e re n t t y p e s o f T B C w e r e a p p l i e d b y p l a s m a s p r a y i ng , n a m e l y

Z r O 2 5 C a O , Z r O z ' 2 4 M g O , Z r O 2 2 0 Y 2 0 3 a n d Z r O / ' 7 Y 2 0 3 . T h e b o n d c o a t w a s

s p r a y e d u s in g a n ( N i - 1 6 C r ~ - 6 A I c o m p o s i t e p o w d e r . T h e s u rf a c e o f t h e s u b s t r a t e w a s

b l a s te d w i t h a l u m i n i u m o x i d e p r i o r t o c o a t in g . T h e t h i ck n e s se s o f t h e b o n d a n d t o p

c o a t s w e r e 0 .1 m m a n d 0 .4 m m r e s p e c ti v e l y .

T h e s e c o a t i n g s w e r e s e l e c te d b e c a u s e f i rs t ly t h e r e f e re n c e s s h o w a l a c k o f

a g r e e m e n t a b o u t t h e i r p r o p e r t i e s a n d s e c o n d l y th e y o u g h t t o b e r e a d i l y av a i l ab l e

s in c e th e y a r e m a d e f r o m s t a n d a r d p o w d e r s .



T H E R M A L B A R R I E R C O A T I N G S F O R J E T E N G I N E I M P R O V E M E N T 303

3 . 2 . T e s t m e t h o d s

T o e n s u r e t h a t t e s t in g o f t h e T B C t o o k p l a c e u n d e r r e a li st ic e n g i n e c o n d i t i o n s

a s f a r a s p o s s ib l e, t h e l e a d i n g e d g e s o f t u r b i n e b l a d e s w e r e s i m u l a t e d b y u s i n g

h o l l o w e d - o u t r o u n d s p e c i m e n s o f I N 1 00 (F i gs . 1 a n d 2). T h e c o a t e d I N 10 0s p e c i m e n s w e r e s u b j e c t e d t o t h e r m o c y c l i c a n d t h e r m o m e c h a n i c a l s t r e s s e s i n a

s t r e a m o f h o t g a s u s i n g J P 4 f u el f o r c o m b u s t i o n ( F i gs . 1 a n d 3). T h e r m a l f a t i g u e

s t re s s e s we r e a p p l i e d b y t he c yc l i c i m p i n g e m e n t ( e ve r y 30 s) o f a s t r e a m o f h o t g a s

w i t h a v e l o c it y o f M a c h 1 a n d a t e m p e r a t u r e o f 1 6 70 K ( h e a t t r a n s f e r c oe f fi c ie n t a t

s t a g n a t i o n p o i n t % = 8 2 0 0 W m - 2 K - t), w h i l s t c o o l i n g a i r f l o w e d t h r o u g h t h e b o r e

o f t he s pe c i m e n . Co o l i n g ( e ve r y 30 s) wa s e f f e c t e d b y f r e e c o n ve c t i o n i n a t m o s p he r i c

s e c t i o n A A c o m l x t s f i o n[ c h o , , ~ I ~ ~ ~ I N ~ o

l

turn fi xtu re lOmm

F i g . 1 . S c h e m a t i c s e t -u p f o r t h e t h e r m a l f a t i g u e te s t .

F i g . 2. H o l l o w s p e c i m e n f o r t h e h o t s p i n t e s t.



3 0 4 G . JO H N E R , K . K . S C H W E I T Z E R

a i r ( ~o ~ 1 0 W m 2 K 1). T e s t i n g w a s c o n t i n u e d u n t i l t h e T B C s s t a r t e d t o s p a l l. T h e

t h e r m o m e c h a n i c a l s tr e ss e s w e r e a p p li e d b y s p i n n in g r o u n d s p e c im e n s m o u n t e d o n a

d i sc d r iv e n b y a c o m p r e s s e d a i r tu r b i n e a n d h o u s e d in a c a s i n g s u p p li e d w i t h h o t J P 4

g a s f r o m a c o m b u s t o r ( F ig . 3 ). T h e s p e c i m e n s , c o o l e d i n t e rn a l l y w i th c o m p r e s s e d a ir ,w e r e e a c h s p u n in h o t g a s a t a t e m p e r a t u r e o f 14 7 0 K f o r 5 a n d 3 0 m i n a t s p e e d s o f

2 2 5 0 0 a n d 3 0 0 0 0 r e v m i n 1. T h e s p e c i m e n s w e r e w e i g h e d b e f o r e a n d a f t e r t e s ti n g .

om

F i g. 3 . S p i n t es t d i sc o f M A R - M - 2 4 6 w i t h a t ta c h e d s p e c i m e n .

F o r t e s t in g t h e b e h a v i o u r o f th e T B C in a j e t e ng i ne , i n t e rn a l l y c o o l e d t u r b i n e

b l a d e s o f I N 1 00 w e r e c o a t e d in th e s a m e m a n n e r a s t h e r o u n d s p e c i m e n s .

T o v e r if y t h e t r a n s f e r a b il i ty o f t h e t e st r e su lt s, Z r O z . 5 C a O a n d Z r O 2 . 7 Y E O 3

T B C s w i t h w i d e ly d i ff e ri n g t h e r m a l f a t i g u e r e s is t a n c e w e r e t e s te d i n a c c o r d a n c e w i t h

t h e l i t e r a t u r e 1° '1 1 i n a m o d e r n m i l i t a r y a e r o e n g i n e .

3.3 . M e thods o f i nves t iga t ion

T h e r o u n d s p e c i m e n s a n d t u r b i n e b l a d e s w e r e s u b j e c t e d to m e t a l l o g r a p h i c

e x a m i n a t i o n t o d e t e r m i n e t h e e ff ec ts o f s t re s s o n t h e m o r p h o l o g y o f t h e T B C s . T h ee x a m i n a t i o n s w e r e c a r r i e d o u t w i t h th e a i d o f s c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M )

i n c o n j u n c t i o n w i t h e n e r g y - d i s p e r s iv e a n a l y s i s o f X - r a y s ( E D A X ) a n d a m i c r o p r o b e

a n a l y s e r. T h e p h a s e s t a b i l it y w a s d e t e r m i n e d w i t h o n e t y p e o f T B C , i n th e a s -

d e p o s i t e d c o n d i t i o n a n d a f t e r t h e t h e r m a l f a t i g ue t es t, b y m e a n s o f X - r a y fi ne

s t r u c t u r e a n a l y s i s ( u si n g a S i e m e n s K r i s t a ll o f l e x 4 i n s t r u m e n t ) . B e c a u s e , a s a r e s u l t o f

t h e r a p i d s o l id i fi c a ti o n , a s - d e p o s i t e d T B C s a r e c h a r a c t e r i z e d b y a m e t a s t a b l e p h a s e

s t r u c t u r e a n d u n e v e n d i s t r ib u t i o n o f t h e n a t u r a l s t r es s e s t h r o u g h o u t t h e c o a t i n g ,

h e a t t r e a t m e n t a n d m i c r o h a r d n e s s t e s ts w e r e c a r r i e d o u t. T h e p u r p o s e o f t h e h e a t

t r e a t m e n t w a s t o re l ie v e s t r e ss e s a n d t o o b t a i n u n i f o r m s t r e s se s i n t h e c o a t i n g .

F u r t h e r m o r e , h e a t in g a n d q u e n c h i n g w e r e u s ed t o i m p r o v e t h e s e g m e n t a t i o n o f t he

T B C . T h e s e i n v e s t i g a t io n s a r e c o n t i n u i n g in c l o se c o - o p e r a t i o n w i t h t h e T e c h n i c a l

U n i v e r s i t y o f M u n i c h 14. T e s t s a r e a l s o b e i n g c a r r i e d o u t i n to o b t a i n i n g u n i f o r m

p r o p e r t ie s t h r o u g h o u t t h e T B C b y a p p r o p r i a t e t e m p e r a t u r e c o n t r o l. F o r t h es e te sts ,

t he N i - C r - A 1 b o n d c o a t a n d th e Z r O / t o p c o a t h av e b ee n p l a s m a s p r a y e d w ith n o

i n t e r r u p t i o n i n t i m e o n t o s e v e r a l b l a d e s .



THERMAL BARRIER COATINGS FOR JET ENGINE IMPROVEMENT 305

4. RESULTS AND DISCUSSION

4 .1 . T e s t r e s u l t s

The the r ma l f a t ig u e a n d s p in t e s t s i n ho t JP 4 g a s de mo n s t r a t e t he s u pe r io rc h a r ac t er is ti c s o f th e Z r O 2 . 7 Y 2 0 3 T B C , a s s h o w n i n T a b l e I . T h e Z r O 2 . 5 C a O a n d

Zr O2 2 0Y 2 03 TBC s a r e c ha r a c t e r i ze d b y po o r r e s i s t a n c e to the r ma l f a t ig u e . I n

c o m p a r i s o n , t h e Z r O 2 . 2 4 M g O c o a t i n g is co n s i d e r a b ly m o r e r e s i st a n t t o f a ti g ue a n d

a l so to sp inn ing , bu t s ti ll does no t a t ta in the fa t igue res is tance o f the ZrO 2 7Y 203

coa t ing .

TABLE I

CHARACTERISTICS OF VARIOUS THERMAL BARRIER COATINGS

T BC Number o f cycles to s tar t Mater ia l removal a f t er spinn ing (g)of spa l ling

Zr O 2.5CAO ,~ 80 0.46-0.54, nearly completely spalled

Zr O 2.24MGO ~ 2000 0.14-0.19

ZrO2. 20Y20 3 ~ 60 Not established because of poor thermal

resistanceZrO2-7Y203 ,~ 4700 0.0-0.02

T h e s p in t e s t sp e c im e n s w i t h o u t a n y o u t w a r d s ig n s o f d a m a g e t o t h e c o a t i n g

(see Ta b le I ) we re spun aga in ( see Sec t ion 3), th is t ime w i th th e a dd i t ion o f 5 g o fI N 100 po w de r w i th a g r a in s ize o f le ss t ha n 45 ~ tm, to o b ta in in fo r ma t io n a b o u t t he

r e s is t a n ce o f Z r O 2 7Y 2 0 3 c o a t in g s to pa r t i c le e r o s io n . O f th r e e s pe c ime n s t e s t e d , a

ma te r i a l l o ss o f 0 .18 g w a s fo u n d o n o n e s pe c ime n o n l y . Th e f a c t t ha t tw o s pe c ime n s

e x h ib i t e d n o l o ss o f ma s s , de s p i t e s p in n in g p r io r t o t he e r o s io n t e st , i n d ic a t e s

c o n fo r m i ty w i th S te c u r a 1° a n d S te pk a e t a l . 1 ~ c o n c e r n in g the h ig h r e s i s t a n c e o f

Zr O 2 7Y 2 03 c o a t in g s to pa rt i c le e r o sio n .

Tu r b in e b l a de s w i th T BC s ( se e S e c t io n 3 .2 ) s ho w s ig n s o f da m a g e to the c o a t in g

a f t er o pe r a t io n in the e ng in e ( du r a t io n , 6 0 h ) s imi la r t o t ha t o f t he r o u n d s pe c ime n s

a f t er t he the r ma l f a tig u e te s t. Wh e r e a s o n l y o n e o f t e n b l a de s c o a te d w i thZr O 2 .7Y 2 03 e x h ib i t e d a pp r e c i a b l e l o s s o f ma te r i a l i n t he a r e a o f t he l e a d in g e dg e to

the c o n v e x s ide , a ll 11 b l a de s c o a te d w i th Z r O2 5C a O s u ff e re d e x te n sive da ma g e

(Fig. 4).

4 .2 . M i c r o s c o p e e x a m i n a t i o n

T h e d a m a g e t o t h e b l a d e c o a t e d w i th Z r O 2 . 5 C a O i n th e l e ad i n g e d g e a r e a is i n

the na tu re o f loca l ized spa l l ing (F ig. 5 ). Sep ara t io n has a pp aren t ly o ccur red w i th in

t h e T B C a n d n o t b e t w e e n t h e T B C a n d t h e b o n d c o a t. I n a d d i t io n , t h e r e is h a rd l y

a n y b l a c k e n in g o f t he s e a r e a s b y the r e s idu e s o f c o m b u s t io n , i n c o n t r a s t w i th the

a d j a c e n t c o a t in g s u r fa c e , i n d ic a tin g tha t t h is s pa l lin g p r e s u m a b l y o c c u r r e d to w a r ds

the e n d o f t he e n g in e r u n .

T h e Z r O 2 . 5 C a O c o a t i n g in t h e n e i g h b o u ri n g c o n v e x a r e a h a s b e e n g as e r o d e d

d o w n t o t h e b o n d c o a t , a n d t h is p h e n o m e n o n is b o r n e o u t b y t h e p o o r r e si st a nc e t o

e r o s io n o f t h is t ype o f TB C o b s e r ve d in the s p in te s t. I n c o n t r a s t , t he Z r O 2 .7Y 2 03

c o a t in g s ho w s o n l y d i s t in c t , pa r t i a l l y e x pa n de d , s e g me n ta t io n c r a c k in g in the




(a) (b)

c mJ

Fig. 4. Turbine blades coated with (a) ZrO2-7Y203 and (b) ZrO2-5CaO after 60 h in an engine.

Fig. 5. A ZrO 2-5CAO coating after a 60 h engine run; leading edge peeling has occurred.

Fig. 6. A ZrO 2.7Y203 coating after a 60 h engine run; the leading edge shows segmentation.

leading edge area (Fig. 6). Peeling or pitting of the coating was not found in the

convex area on n ine of the ten blades tested.

To clarify the re ason for the spalling of the Zr O2- 7Y2 0 3 coati ng of one blade,

microsections were prepared (Fig. 7). It was revealed that the poor quality of the

bo nd coat, nam ely insufficient thickness an d incomplet eness, in the leadi ng edge

area was to blame.




Fig. 7. A ZrO 2.7Y203 coating after a 60 h engine run; leading edge peeling has occurred. The bond coat

is too thin and is incomplete.

Fig. 8. ZrOz-5CaO coating after one thermal cycle. Cracking between the TBC and the bond coat has

occurred, and there is high porosity.

The l o w the r ma l f a t ig u e r e si s ta n c e o f t he Z r O 2 .5C a O c o a t in g c a n b e e x p la in e d

b y the f a c t t ha t c l ea r s e pa r a t io n o f t he TB C pa r a ll e l t o t he s u r f a ce n e a r t he b o n d c o a t

occu rs a f te r jus t one th e rm al cyc le ( see Sec t ion 3 .2 and Fig . 8 ). Fu r the rm ore ,

m ic r o f r a c to g r a phy ( Fig. 9 ) s ho w s the e x pe c te d s e pa r a t io n w i th in the TB C pa r a ll e l t o

the s u r f a c e o w in g to the s ma l l c o n ta c t s u r f a c e s b e tw e e n the in d iv idu a l c e r a mic

spla ts . B ecause o f excess ive s t ress ing a t po re edges and the n o tch e ffec t o f smal l

c racks , the rmal s t r es ses qu ick ly l ead to se r ious separa t ion pa ra l l e l to the su r face ,

w i th the r e s u l t t ha t c o m pl e t e a r e a s o f t he c o a t in g a r e r e m o ve d b y g a s fo r ce s .

The s e f in d ing s a r e s u p po r t e d b y o n g o in g in ve s t ig a t io n s in to e x t r e me l y de n s e

po r e l es s Z r O2 - 5C a O c o a t in g s w h ic h s ho w f ir st s ig n s o f s pa ll in g a f t e r mo r e tha n 2 000

the r ma l c yc le s fo r t he s a me de po s i t t h i c k n e ss a n d t e s t c o n d i t io n s .

TB C s o f Z r O 2 ' 2 4 M g O ha ve e s s e n ti a ll y b e t t e r r e s is t a n ce to the r ma l f a tig u e tha n

the Z r O 2 5C a O c o a t in g s . The u n s t r e ss e d c o a t in g is c ha r a c t e r ize d b y a c o m -

p a r a t i v e l y g o o d u n i o n b e t w e e n t h e b o n d c o a t a n d t h e t o p c o a t a n d b y m o d e r a t e

po r o s i ty (F ig . I 0 ). U n d e r g r e a t e r ma g n i f i ca t io n , t he mu l t ipha s e c ha r a c t e r o f t he TB C



308 G . JO H N E R , K . K . S C H W E I T Z E R

F i g . 9. A Z r O 2 .5 C A O c o a t i n g a f t e r 7 0 t h e r m a l c y c le s . T h e c o n t a c t s u r f a c e b e t w e e n t h e c e r a m i c c l u s te r s is

s m a l l , a n d t h e e x p e c t e d s e p a r a t i o n p a r a l le l t o t h e s u r f a c e h a s o c c u r r e d .

i s h ighl ighted by the v a r ious shades o f g rey (F ig . l 1). The da rk phase , ex tend ing in

the d i rec t ion o f s t r ia t ion and runn ing roug hly pa ra l l e l to the su r face , i s en r iched near

t h e b o n d c o a t. J u d g i n g b y t h e p h a se s y s te m Z r O z - M g O 15, t h is p h a s e is p r o b a b l y

f r e e Mg O. Th i s a s s u mpt io n i s c o n f i r me d b y the mic r o p r o b e e l e me n t ma ps in the

t r a n s it i o n b e tw e e n the TBC a n d the b o n d c o a t in t he imm e d ia t e v i c in i ty o f t he

s pa l le d s po t s ( Fig . 12 ) . The s t re s s e d Z r O z .2 4 M g O c o a t in g i s c ha r a c t e r i ze d b y

d i s t in c t c r a c k s in the v i c in i ty o f t he b o n d c o a t o r i n t he a r e a o f M g O e n r i c hme n t ( Fig.13), ind ica t ing tha t the re i s a r ecogn izab le cause . O pt im al the rm al fa t igue resis tance

o f Z r O z ' 2 4 M g O TB C s c a l ls fo r a s u n i fo rm a s po s s ib l e d i s t r ib u t io n o f t he fr e e M g O .

O f the T BC in ve s tig a te d , Z r O 2 - 7Y 2 0 3 s ho w s the h ig he s t t he r ma l f a tig u e

r e s i s t a n c e . The u n s t r e s s e d c o a t in g e x h ib i t s n u me r o u s s e g me n ta t io n c r a c k s a n d

c o m pa r a t iv e l y f e w po r e s ( F ig . 14 ) . The mu l t ipha s e s t r u c tu r e o f t he c o a t in g i s

F i g. 1 0. A Z r O 2 . 2 4 M g O c o a t i n t h e u n s t r e s s e d c o n d i t i o n ; t h e r e is g o o d u n i o n b e t w e e n t h e b o n d a n d t h e

t o p c o a t s .

F i g . 1 1 . A Z r O 2 24 M G O c o a t i n g i n t h e u n s t r e s s e d c o n d i t i o n , s h o w i n g d a r k s t r i a t i o n s w i t h e n r i c h m e n t

n e a r t h e b o n d c o a t .



T H E R M A L B A R R I E R C O A T I N G S F O R JE T E N G I N E I M P R O V E M E N T 3 0 9

(a) (b )

F ig . 12 . Z r O z . 2 4 M g O e l e m e n t m a p s f o r ( a ) m a g n e s i u m a n d ( b ) z ir c o n iu m ; s e e F ig . l l .

F i g . 1 3. A Z r O 2 " 2 4 M g O c o a t i n g a f t er 2 0 0 0 t h e r m a l c y cl es , s h o w i n g c le a r s ig n s o f d a m a g e t o t h e T B C i n

unp ee led areas .

F i g . 1 4. A Z r O 2 . 7 Y 2 0 3 c o a t i n g i n t h e u n st r es se d c o n d i t i o n , s h o w i n g n u m e r o u s s e g m e n t a t i o n c ra c k s.



3 1 0 G , J O H N E R , K . K . S C H W E I T Z E R

revealed under greater magnification (Fig. 15). Dark, and in some cases striated,

precipitations are shown to be rich in yttrium by the microprobe element maps.

Judging by the phase system ZrO2-YzO316, this is probably yttrium-rich

ZraY4012, present in addition to the monoclinic phase at room temperature.In addition to the above elements, scattered silicon enrichments, sometimes in

the immediate vicinity of the yttrium enrichments, were found. According to the

manufacturer of the spray powder, this is due to almost unavoidable impurities

(SiO2 content of the powder, 0.24 w t . ' ) .

After thermal fatigue stressing the coating is characterized by an increase in

segmentation, which is also visible macroscopically (Fig. 16). Horizontal separations

occur only near the surface and not between the bond coat and the TBC, with the

result that the coating appears still to be functionable. Numerous areas of the

coating indicate very compact good segmentation and an intact TBC (Fig. 17).

The noticeable resistance of this coating is confirmed by X-ray fine-structure

analysis, which has not yet been completed and which permits only qualitative

statements to be made at present ~4. X-ray diffraction analysis of the ZrOz-7Y203

spray powder reveals that, in addition to the predominant tetragonal phase, in this

( a ) ( b )

9?

I I. . . . . . . . . . . . 20 ~tm

Cc)

F i g . 1 5. ( a) M i c r o g r a p h a n d ( b) , (c ) e l e c t r o n s c a n n i n g m i c r o p r o b e a n a l y s e r p i c t u r e s o f Z r O 2 7 Y 2 0 3 f o r

( b) z i r c o n i u m a n d (c ) y t t r i u m .




f ~

Fig. 16. A ZrOz.7YzO3 coatingafter 4000 therm alcycles,showingonly slightdamage.

Fig. 17. A ZrO2.7Y20a coating after 4000 therm al cycles, showing almost complete freedom fromdefects.

c o n d i t i o n t h e m o n o c l i n i c p h a s e i s a l so p r e s e n t. I n t h e a s - d e p o s i t e d c o n d i t i o n o n f l a t

I N 7 18 s p e c i m e n s , o n l y t h e t e t r a g o n a l p h a s e i s p r e s e n t, w i t h o u t t h e o c c u r r e n c e o f t h e

X - r a y p e a k s o f t h e m o n o c l i n i c p h a s e . A f t e r 1 0 0 0 0 t h e r m a l f a t ig u e c y c le s ( se e S e c t i o n

3 .3 ) t h e m o n o c l i n i c p h a s e is a g a i n p r e s e n t i n a d d i t i o n t o t h e p r e d o m i n a n t t e t r a g o n a l

p h a s e . T h e s e f i n d i n g s a g r e e w e l l w i t h t h o s e o f B o c h a n d R o g e a u x ~7.T h e m a n u f a c t u r i n g p r o c e s s ( c r u s h i n g o f s o l id i fi e d m e l t a f t e r 7 0 0 h o f c o o l in g )

l e a d s t o t h e e x p e c t a t i o n t h a t t h e s p r a y p o w d e r i s i n p h a s e e q u i l i b r iu m a n d a c u b i c

p h a s e i s t h u s a l s o p r e se n t , b u t c a n n o t b e r e c o g n i z e d b e c a u s e o f l i n e a r o v e r l a p p i n g

w i t h t h e t e t r a g o n a l p h a s e .

T h e f a c t t h a t t h e m o n o c l i n i c p h a s e b e c o m e s r e c o g n i z a b l e i n t h is t y p e o f c o a t i n g

o n l y a f t e r a l a rg e n u m b e r o f t h e r m a l c y c le s i n d i c a te s t h a t t h e m e c h a n i s m o f




t r a n s f o r m a t i o n t o u g h e n i n g z ap p li es . T h e s u p e r i o r r e s i s ta n c e t o t h e r m a l f a t i gu e o f

t h e Z r O 2 . 7 Y 2 0 3 is t h u s u n d e r s t a n d a b l e , e s p e c ia l ly i n v ie w o f t h e a n o m a l o u s

b e h a v i o u r o f t h e Z r O z ' Y 2 0 3 a l lo y w i th r e g a r d t o s i n te r in g , as o b s e r v e d b y

A m m a n n is . W h i l s t t h e v o l u m e o f Y 2 0 3 - s t a b i li z e d Z r O 2 i n c re a s e s u n d e r t h e r m o -c y c l i c s tr e ss , o t h e r o x i d e s t a b il iz e r s c a u s e a r e d u c t i o n i n v o l u m e . C o n s e q u e n t l y , i t is

e a s il y i m a g i n a b l e t h a t s h r i n k a g e c r a c k s , c a u se d b y t h e m a n u f a c t u r i n g p r o c e s s , d o

n o t r e a c h t h e c r i ti c a l l e n g t h o w i n g t o t h e g e n e r a t i o n o f l ig h t c o m p r e s s i v e s t re s se s .

A n o t h e r a d v a n t a g e o u s c h a r a c t e r i s ti c o f t h e Z r O 2 - 7 Y 2 0 3 c o a t i n g i s it s v e r y

c o n s t a n t t h e r m a l c o n d u c t i v i t y (2 = 0 .9 5 W m 1 K - 1 i n t h e a s - d e p o s i te d c o n d i t i o n ;

2 = 1 .0 5 W m 1 K - ~ a f t e r 1 0 0 h o f a n n e a l i n g a t 1 27 3 K ) , w h e r e a s t h e o t h e r t w o

t y p e s o f T B C ( Z r O 2 . 5 C a O a n d Z r O 2 2 4 M g O ) a r e c h a r ac t e r i z e d b y a c le a r i nc r ea s e

i n t h i s i m p o r t a n t p r o p e r t y (e .g . f o r Z r O 2 . 5 C a O a s d e p o s i t ed 2 = 0 .6 W m - ~ K -

a n d a f t e r 1 0 0 h a n n e a l i n g a t 1 27 3 K 2 = 1 .6 W m ~ K - 1).

4 .3 . H e a t t r e a t m e n t

H e a t t r e a t m e n t i n c l u d i n g q u e n c h i n g t e s t s a r e c a r r i e d o u t o n T B C s w i t h t h e

o b j e c t i v e o f i n f l u e n c i n g t h e s e g m e n t a t i o n a n d n a t u r a l s t re s s c o n d i t i o n 14. I n i ti a l

e x p e r i m e n t s w i th Z r O / . 5 C a O c o a t i n g s o n fl a t I N 7 1 8 s p e c i m e n s e x h i b i t e d u ns a t is -

f a c t o r y b e h a v i o u r w i t h r e g a r d t o a r ti fi c ia l ly i n d u c e d s e g m e n t a t i o n c r a c k i n g b y

h e a t i n g b r i e fl y t o 9 7 0 ° C , f o l l o w e d b y c o o l i n g o f t h e c o a t i n g s u r f a c e in w a t e r ( F i g . 1 8 ).

T h e c r a c k f ir st r u n s p e r p e n d i c u l a r t o t h e s u rf a ce , t h e n a t a b o u t h a l f o f t h e c o a t i n g

t h i c k n e s s i t c h a n g e s d i r e c t i o n b y 4 5 ° t o c o n t i n u e p a r a l l e l t o t h e s u r f a c e n e a r t h e b o n d

c o a t . I t i s a s s u m e d t h a t d u r i n g p l a s m a s p r a y i n g t h e f i r s t l a y e r s a r e s u b j e c t e d t os h a r p e r c o o l i n g c o n d i t i o n s t h a n a r e t h e s u rf a c e la y e rs . C o n s e q u e n t l y , i t is i m p o s s i bl e

F i g . 1 8. A Z r O 2 - 5 C a O c o a t i n g w i th a s e g m e n t a t i o n c r a c k TM.



THERMAL BARRIER COATINGS FOR JET ENGINE IMPROVEMENT 3 1 3

f o r t h e n a t u r a l s tr e s se s to b e d i s t r ib u t e d e v e n l y th r o u g h o u t t h e c o a t i n g ; t h is i s b o r n e

o u t b y m i c r o h a r d n e s s m e a s u r e m e n t s ( V ic k e rs ' h a r d n e s s u n d e r a l o a d o f 0 .2 N ;

F i g. 1 9). T h e n o n - u n i f o r m r e s i d u a l s t re s s d i s t r i b u t io n m a y b e t h e c a u s e o f t h e c h a n g e

i n d i re c t i o n o f t h e c r a c k p r o p a g a t i o n . T h i s h y p o t h e s i s i s b o r n e o u t t o a c e r t a i n e x t e n tb y t h e r e s u l ts o f h e a t t r e a t m e n t a t 9 0 0 ° C f o r 2 0 m i n f o l l o w e d b y q u e n c h i n g i n w a t e r

( F i g . 2 0 ) , s i n c e t h e s e g m e n t a t i o n c r a c k s p r o p a g a t e p e r p e n d i c u l a r t o t h e s u r f a c e

a l m o s t w i t h o u t e x c e p ti o n a n d a r e d u c t i o n o f t h e m i c r o h a r d n e s s t h r o u g h o u t t h e

c o a t i n g o c c u r s ( F i g . 1 9 ) .

~ I I , o o o. 9 0 o I ~ 9oo~

2 8 0 o 1 I I I ~ _ 8 o o

~ ° ° l I L ~ F I ~oo

~ 600

~ o o T l ~oo1

E 3 1

°°1 o . : ]

H-+I Ii . ; / i

IiN, cr/A i [s ~ b ~ t~ 2 i~B C

a ) )

Fig. 19. Microhardness of a ZrOz.5CaO coating (a) before and (b) after heat trea tment (20 min at

900 °C; water quench).

i i i il ̧ ¸ i i l i i

~ i i i i ~ ~ ~ i i ~ . ~ i l l ~ ~ i l i i i

(:i,~:,T~~ ............ ....... ~ . . . . ~.......... ~

Fig. 20. A ZrO2.5CaO coating after heat trea tment (20 min at 900 °C; water quencM4).




5 . C O N C L U S I O N S

To improve the high temperature resistance of internally cooled 1N 100

substrates, four different types of TBC were applied by plasma spraying, namelyZrO2 5aO, ZrO2.24MgO, ZrO2'20Y203 and ZrOE.7Y203. In all cases the bond

coat was sprayed using an (Ni-16Cr)-6AI composite powder.

To ensure that testing of the TBC took place under realistic engine conditions

as far as possible, the leading edges of turbine blades were simulated by using

hollowed-out round specimens of IN 100. The coated IN 100 specimens were

subjected to thermocyclic and thermomechanical stresses in a stream of hot gas

using JP4 fuel for combustion (see Figs. 1-3). To verify the transferability of the test

results, ZrO2 5CaO and ZrO2.7Y203 TBCs with widely differing thermal fatigue

resistance were tested in a modern military jet engine. To determine the effects of

stress on the morphology of the TBCs the specimens and turbine blades were

subjected to metallographic examination. The phase stability was determined with

one type of TBC in the as-deposited condition and after the thermal fatigue test, by

means of X-ray diffraction analyses. Furthermore some heat treatment and

microhardness tests were carried out to reduce the residual stresses and to improve

the segmentation of the unstressed TBCs.

The main result was that the thermal fatigue and spin tests in hot JP4 gas

demonstrate the superior characteristics of ZrO 2 7Y203 TBC. The ZrO2' 5CaO and

ZrO2.20Y203 TBCs were characterized by poor resistance to thermal fatigue. In

comparison the ZrO2.24MgO coating was considerably more resistant to fatigueand also to spinning, but still did not attain the fatigue resistance of the ZrOz'7Y/O3

coating. Turbine blades with TBCs (see Section 3.2) show signs of damage to the

coating after 60 h of operation in the engine similar to that of the round specimens

after the thermal fatigue test. Whereas only one of ten blades coated with

ZrOz.7Y203 exhibited loss of material in the area of the leading edge, all 11 blades

coated with ZrO2.5CaO suffered extensive damage. The results of the microscopic

investigations show good conformance in the failure mode of the TBCs tested on the

specimens and the turbine blades. This means that the results obtained with the rig

tests show high transferability to engines.From the results of the microscopic investigations the widely differing

behaviour of the four different types of TBC in the rig tests becomes understandable.

Apart from the chemical composit ion of the TBC, these investigations show clearly

that only a dense sprayed coating with very few pores and many perpendicular

propagated segmentation cracks will have superior resistance against thermal

fatigue stress and particle erosion attack. Furthermore, the ongoing research in

post-treatment of the TBCs demonstrate how to achieve many well-propagated

segmentation cracks by annealing and quenching these coatings.

R E F E R E N C E S

1 W . D i e n s t , H o c h t e m p e r a t u r w e r k s t o f f k , W e r k s t o f f t e c h n i s h c h e V e r l a g s g e s e l l s c h a f t , K a r l s r u h e , 1 97 8,

p . 94 .

2 N . C l a u s s e n , U m w a n d l u n g s v e r s t ~ i r k t e k e r a m i s c h e W e r k s to f f e , Z . Werksto.JJ?ech., 13 (4) (1982)

138 147.



T HE RMAL BARRI ER C OAT I NGS F OR JE T E NGINE I MPROVE ME NT 315

3 J . F . L y n c h , C . G . R u d e r e r a n d H . W . D u c k w o r t h , E n g in eer in g P r o p er t i e s o f S e l ec t e d Cer a m i c

M a t e r i a l s , Am er ican Ce r am ic Soc ie ty , 1 9 66 .

4 P . G . V a l e n t i n e a n d R . D . M a i e r , M i c r o s t r u c tu r e a n d m e c h a n i c a l p r o p e r ti e s o f b u l k a n d p l a s m a -

s p r ay ed Y 20 3 p a r t i a l ly s t ab i l i zed z i r con ia , NA S A Co n t r a c t . R ep . 1 65 1 2 6, 1 9 8 0 ( N a t i o n al

A e r o n a u t i c s a n d S p a c e A d m i n i s tr a t i o n ).5 R . C. Ga r v ie and P . S . N ich o l s on , S t r uc tu r e and th e r m om ech a n ica l p r op e r t i e s o f p a r t i a l ly s t ab i li zed

z i r co n i a i n t h e C a O - Z r O 2 sy s te m , J . A m . Cer a m . S o c . , 5 5 (1972) 152-157.

6 G .K . Bans a l and A . H . Heue r , P r ec ip i t a t ion in p a r t i a l ly s t ab i li zed z i r con ia , J . A m . Cer a m . S o c . , 5 8

(1975) 235-238 .

7 R .F . Pabs t , I . Bogn a r and J . B . Z w is s le r , E r gebn i s s e zu r st a t i s ch en , d y nam is ch en und zy k li s ch en

E r m i id ung be i ke r amis ch en , W er ks to f fen , Ber . Dtsch . Keram. Ges . , 57 (1980) 13-16.

8 T . K . Gu p ta , F . F . L ange and J . H . Bech to ld , E f fect s o f s t r e s s- ind uced p h as e t r ans fo r ma t ions on th e

p r op e r t i e s o f p o ly c r y s t a l l i ne z i r con ia con ta in ing me ta s t ab le t e t r agona l p h as e , J . Mater . Sc i . , 13

(1978) 1464-1470 .

9 N . Claus s en , M ic r os t r uc tu r a l d e s igns o f Z r O z- tough ened ce r amics , Co n f . o n Z i r c o n i a , S t u t t g a r t ,

J u n e 2 1 - 2 3 , 1 9 8 3 , t o be p ub l i s h ed .1 0 S . S t ecu r a , E f fect s o f com p os i t i ona l ch anges o f t h e p e r fo r man ce o f a t h e r m a l ba r r i e r coa t ing s y s t em,

N A S A T e c h . M e m o . T M - 7 8 9 7 6 , 1 97 9 ( N a t i o n a l A e r o n a u t i c s a n d S p a c e A d m i n i s t ra t i o n ) .

11 F .S . S t ep ka , C . H . L iebe r t and S . S t ecu r a , Sum ma r y o f NA SA r e s ea r ch on th e r m a l ba r r i e r coa t ings ,

In t . A u tom ot ive Engineering Congr . , Detro i t , 1977 , in S A E P a p e r 77 0 3 43 , 1977.

12 R . E . B e n n e r a n d A . S . N a g e l b e rg , C h a r a c t e r i z a t i o n o f Z r O 2 - Y z O 3 t h e r m a l b a r r i e r c o a t i n g s b y

r aman s p ec t r o s cop y , Thin So l id F i lms , 84 (1981 ) 8 9-94.

1 3 R . J . B r a t to n , S. K . L au , C . A . And e r s s on and S . Y . L ee , Ce r am ic th e r m a l ba r r i e r coa t ings fo r ga s

tu r b ine eng ines, A S M E P a p er 8 2 -G T - 2 65 , 1980.

14 S. Ku nz, W~irmebehandlung yon Wi irmedi imm schich ten , Proje c t E4 .9 /1 ( Bund es min i s t e r ium f i i r

F o r s c h u n g u n d T e c h n o l o g i e g e f 6 rd e r te s P r o j e k t d e r D e c h e m a / G r u p p e 4 , T e c h n is c h e U n i v e r si t /i t

M i i n c h e n ) .1 5 D . V iech n ik i and V . S . S tub ican , M ech an i s m o f d ecom p os i t i on o f cub ic s o l id s o lu t ions in t h e s y s tem

Z r O 2 - M g O , J . A m . Cer a m . S o c . , 4 8 (1965) 292-297 .

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Co n f . o n Z i r c o n i a , S t u t t g a r t , J u n e 2 1 - 2 3 , 1 9 8 3 , t o be p ub l i s h ed .

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ma te r i a l s , P W A R ep ., 1 97 6 ( P r a t t and W h i tney Ai r c r a f t ) .

Thermal Barrier Coatings for Jet Engines Improvement

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