XAFS Analysis of the Structural Change in Si-Zr-C-O Fiber Accompanied by Thermal Decomposition

100 %

1 / 1

100 %

1 / 1

As the operating gas temperature of gas turbine e n g i n e s i n c r e a s e s y e a r b y y e a r i n r e s p o n s e t o issues related to energy and the environment, the m a t e r i a l s t e m p e r a t u r e o f s u c h h i g h - t e m p e r a t u r e parts as the turbine blades, stator, and combustor l i n e r a r e n e a r i n g t h e l i m i t f o r a d i a b a t i c m e t a l l i c m a t e r i a l s e v e n w h e n c o o l i n g i s a p p l i e d . F o r t h i s r e a s o n , t h e r e i s m u c h i n t e r e s t i n s t u d i e s o n t h e a p p l i c a t i o n o f c e r a m i c m a t e r i a l s i n g a s t u r b i n e e n g i n e s , a n d c o n t i n u o u s f i b e r - r e i n f o r c e d c e r a m i c m a t r i x c o m p o s i t e ( C F C M C ) m a t e r i a l s , w h i c h a r e e x p e c t e d t o e x h i b i t h i g h d a m a g e t o l e r a n c e , a r e h i g h l y p r o m i s i n g ( F i g . 1 ) [ 1 ] . A n e x p e r i m e n t h a s been carried out at beamline BL01B1 [2]. XAFS Analysis of the Structural Change in Si-Zr-C-O Fiber Accompanied by Thermal Decomposition Figure 2(a) is a schematic illustration o f t h e C F C M C s t r u c t u r e . W e a r e n o w w o r k i n g o n p r o c e s s d e v e l o p m e n t , p a r t s fa br ic at io n te ch no lo gy de ve lo pm en t, an d m a t e r i a l s e v a l u a t i o n t e c h n o l o g y development centered around SiC-based co mp os it e ma te ri al s ( Si -Z r- C- O / Si C ) th at u s e S i - Z r - C - O a m o r p h o u s f i b e r a s t h e reinforcing fiber [3-7]. T h e d e c l i n e i n s t r e n g t h o f S i - Z r - C - O / S i C m a t e r i a l s i n a h i g h - t e m p e r a t u r e envi ronm ent, as is show n in Fig. 2(b) , is a t t r i b u t e d m a i n l y t o t h e d e c l i n e i n f i b e r strength due to the thermal decomposition and cry sta lli zat ion of the Si- Zr- C-O fib er, w h i c h i s m a i n l y r e s p o n s i b l e f o r t h e stre ngt h of the Si- Zr-C -O / SiC com pos ite . Also, in ambient air, embrittlement caused b y f i b e r / m a t r i x b o u n d a r y o x i d a t i o n m a y have an effect [4]. F i g u r e 3 i s a n o u t l i n e o f t h e S i - Z r - C - O / S i C fabrication process developed by Kawasaki Heavy Industries, Ltd. The Si-Zr-C-O fiber is prepared as a woven pre-form. A thin gradient layer of C/ SiC is d e p o s i t e d o n t h e s u r f a c e o f e a c h f i l a m e n t i n t h e pre-form by the CVD method. After the pre-form is impregnated with polycarbosilane xylene solution, it is pyrolized to form the SiC matrix [3,4]. As shown in Fig. 3 , the Si-Zr -C-O fiber is expos ed to a high temperature under reduced pressure and inert gas pressure in the fabrication process. Moreover, the o p e r a t i n g e n v i r o n m e n t o f g a s t u r b i n e s i s h i g h - p r e s s u r e a m b i e n t a i r . F o r t h e s e r e a s o n s , u n d e r s t a n d i n g t h e s t r u c t u r a l c h a n g e s i n t h e S i - Z r - C - O f i b e r i n t h e s e d i f f e r i n g e n v i r o n m e n t s s h o u l d p r o v i d e a g u i d e l i n e f o r o p t i m i z i n g t h e p r o c e s s c o n d i t i o n s s o a s t o c o n t r o l t h e d e g r a d a t i o n o f ma te ri al s in th e fa br ic at io n st ag e or fo r im pr ov in g t h e m a t e r i a l s c o n f i g u r a t i o n s o a s t o i n c r e a s e durability. Fig. 1. Development status of heat resistant materials [1]. Ceramic Matrix Composite Temperature ( ° C) Carbon/Carbon Composite 1950 1960 1970 1980 1990 2000 2010 2020 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 TBC Superalloys Year Intermetallic Compound Single Crystal Superalloys Directionally Solidified Superalloys Conventional Superalloys Fiber Reinforced Superalloys or Intermetallic Compound Oxide Dispersed Superalloys 83 Fig. 3. Schematic illustrations of the fabricating procedure for our Si-Zr-C-O / SiC composite. Fig. 2. Schematic illustrations of our Si-Zr-C-O / SiC composite and its deterioration mechanism in ambient atmosphere. Figure 4 shows the XANES spectra at Zr K -edge of the Si-Zr-C-O fiber after heat treatment at 1673 K , a n d t h e z i r c o n i u m f o i l . I n t h e S i - Z r - C - O f i b e r heat-treated in ambient air and argon, the spectra w e r e n o t m u c h d i f f e r e n t f r o m t h a t o f t h e a s - f a b r i c a t e d f i b e r . H o w e v e r , i n t h e S i - Z r - C - O f i b e r heat-treated in a vacuum, the spectra differed from that of the as-fabricated fiber after just four hours, and underwent an even more pronounced change w h e n t r e a t e d f o r 1 6 h o u r s . T h i s i n d i c a t e s t h a t some kind of the change in the electronic structure of Zr occurred for the fiber heat-treated in vacuum Fiber Coating by Chemical Vapor Deposition (CVD) Ar or N 2 N 2 P Impregnation (Heating & Pressurizing) Dry Pyrolysis Xylene Pore the Slurry Polycarbosilane (PCS) SiC powder SiC H R (PCS) Pyrolysis Slurry Making Polymer Impregnation and Pyrolysis(PIP) method Repeat to densify (6 times) SiC Fiber SiCl 4 (gas) + CH 4 (gas) SiC + 4HCl 3D-Woven Preform R H R | | | – Si – C – Si – | | | R H R < 760 Torr, 1173 K - 1573 K 5 atm, 1173 K - 1573 K R H R | | | – Si – C – Si – | | | R H R O 2 Decomposition in Si-Zr-C-O fiber Si - Zr 0.005 - C 1.44 - O 0.24 → SiC + 0.005 ZrC + 0.195 C+ 0.24 CO(g) Oxidation on Fiber Surface Si - Zr 0.005 - C 1.44 - O 0.24 + 1.725 O 2 → SiO 2 + 0.005 ZrO 2 + 1.44 CO(g) SiO 2 Si-Zr-C-O Fiber CO Stress Concentration due to Oxide Forming Fiber Deterioration due to Decomposition CO PIP-SiC CVD-C CVD-SiC CVD-SiC Si-Zr-C-O Fiber SiO 2 ZrC 50 nm Si-Zr-C-O Fiber PIP-SiC Matrix CVD C/SiC Interface Crack or Pore Crack (b) Deterioration mechanism of our Si-Zr-C-O/SiC composite. (a) Schematic illustration of our Si-Zr-C-O/SiC composite. TEM image in the vicinity of fiber/matrix interface in the Si-Zr-C-O/SiC composite after oxidation at 1473 K for 500 hours. 84 Xylene Normalized Absorption ( arb. unit) 18120 18140 18160 18180 18200 Energy ( keV ) Zr K -edge Zr Foil Si–Zr–C–O fiber as–fab. 1 6 7 3 K 1 6 h r i n v a c . 1 6 7 3 K 4 h r i n v a c . 1 6 7 3 K 1 6 h r i n a i r 1 6 7 3 K 4 h r i n a i r 1 6 7 3 K 1 6 h r i n A r 1 6 7 3 K 4 h r i n A r F i g . 4 . X A N E S s p e c t r a o f S i - Z r - C - O f i b e r s a f t e r h e a t t r e a t m e n t , c o m p a r e d with as-fabricated fiber and Zr foil. in a short time. Fi gu re 5 sh ow s th e Fo ur ie r tr an sf or ma ti on of XAFS spectra at Zr K -edge. A significant difference exists between the spectrum of the Si-Zr-C-O fiber heat-treated in ambient air and argon and that heat- treated in a vacuum. The peak near 3 Å , which is characteristic of the fiber after vacuum treatment, is b e l i e v e d t o b e z i r c o n i u m b e c a u s e i t c a n a l s o b e seen in metallic zirconium foil. The peak near 4.2 Å in the sample after vacuum treatment is fairly strong despite the great distance, a n d i t i s b e l i e v e d t o b e z i r c o n i u m w i t h a h i g h e r a t o m i c w e i g h t . T h e s e r e s u l t s r e v e a l t h a t t h e b e h a v i o r o f z i r c o n i u m i n t h e c r y s t a l l i z a t i o n s t a g e varies according to the heat-treatment atmosphere. B e c a u s e t h e t h e r m a l d e c o m p o s i t i o n o f S i - Z r - C - O f i b e r i s a c c o m p a n i e d b y t h e f o r m a t i o n o f c a r b o n m o n o x i d e g a s a s s h o w n b y t h e e q u a t i o n i n F i g . 2 ( b ) , t h e r e a c t i o n c a n b e e x p e c t e d t o p r o g r e s s fa st er in a va cu um th an in an ar go n at mo sp he re . Com par ing the low er thr ee spe ctr um in det ail , the s p e c t r u m o f t h e S i - Z r - C - O f i b e r h e a t - t r e a t e d i n ambient air and that of as-fabricated fiber are quite s i m i l a r , b u t t h a t h e a t - t r e a t e d i n a r g o n i s s l i g h t l y different from the former two. It has been reported that when Si-Ti-C-O fiber is heat-treated in ambient air, an oxide film formed on t h e f i b e r s u r f a c e i n h i b i t s C O g a s e m i s s i o n accompanied with the thermal decomposition of Si- T i - C - O f i b e r , a n d c o n s e q u e n t l y t h e t h e r m a l d e c o m p o s i t i o n i s c o n t r o l l e d [ 8 ] . T h e s a m e o x i d e film up to 0.5 μ m thick on its surface was found on S i - Z r - C - O f i b e r h e a t - t r e a t e d i n a m b i e n t a i r . T h e r e f o r e i t c a n b e s u p p o s e d t h a t t h e d i f f e r e n c e be tw ee n th e sp ec tr um of th e Si -Z r- C- O fi be r he at - treated in ambient air and that of argon is related to t h e r a t e o f t h e t h e r m a l d e c o m p o s i t i o n , a l t h o u g h further study will be required to conclude this. F i g u r e 6 s h o w s a c o m p a r i s o n o f t h e F o u r i e r transformation of XAFS spectra at the Zr K -edge of S i - Z r - C - O / S i C c o m p o s i t e s , c o n t a i n i n g S i - Z r - C - O f i b e r a t a b o u t 4 0 v o l . % , b e f o r e a n d a f t e r h i g h - t e m p e r a t u r e e x p o s u r e u n d e r v a r i o u s c o n d i t i o n s . T h e s p e c t r a f o r t h o s e c o m p o s i t e s , e v e n w h e n exposed to such a high temperature as 1773 K for a s l o n g a s 4 0 0 h o u r s , e x h i b i t e d n o m a j o r differences, and were close to that of the Si-Zr-C-O fiber heat-treated in an argon atmosphere in Fig. 5 r a t h e r t h a n t h a t i n a m b i e n t a i r . T h e s e r e s u l t s suggest that the CVD process, which is conducted i n a r e d u c e d p r e s s u r e a t m o s p h e r e , o r p y r o l y s i s step in inert gas, are effective for certain structural changes of the Si-Zr-C-O fiber. 85 Zr K -edge Zr Foil as–fab. 1673 K 4 hr in vac. 1673 K 4 hr in air 1673 K 4 hr in Ar Si–Zr–C–O fiber R (Å) 0 1 2 3 4 5 FT k 3 χ (k) ( arb. unit) Si–Zr–C–O / SiC composite Zr K -edge before exposure 1773 K 4 00 hr in air 1573 K 50 hr in air 1573 K 50 hr in Ar R (Å) 0 1 2 3 4 5 FT k 3 χ (k) ( arb. unit) K e n i c h i r o h I g a s h i r a a , K o z i N i s h i o b a n d Etsuya Yanase b (a) Kawasaki Heavy Industries, Ltd. (b) The New Industry Research Organization E-mail: igashira @ ati.khi.co.jp Fig. 6. Fourier transformation of XAFS s p e c t r a f o r S i - Z r - C - O / S i C c o m p o s i t e s before and after heat treatment. Fi g. 5. Fo ur ie r tr an sf or ma ti on of XAFS spectra for Si-Zr-C-O fibers a f t e r h e a t t r e a t m e n t , c o m p a r e d with as-fabricated fiber and Zr foil. References [1 ] D. W. Pe tr as ek et al . , Me ta l Pr og re ss 2, 13 0 (1986) 27. [ 2 ] K . I g a s h i r a , K . N i s h i o a n d E . Y a n a s e , t o b e submitted in J. of the Ceramic Soc. of Japan. [3] K. Igashira et al. , J. Jpn. Inst. Metals, No. 8, 62 (1998) 766. [4] K. Igashira et al. , J. Jpn. Inst. Metals, No. 12, 60 (1996) 1229. [5] K. Igas hira et al. , ECCM -8, ed. By V. Cri vel li, Woodhead Publishing 4 (1998) 39. [6] K. Nishio et al. , J. Gas Turbine Soc. of Japan, No. 98, 25 (1997) 94. [7] K. Nishio et al. , Transactions of the ASME 121 (1999) 12. [8] K. Kakimoto et al. , J. Jpn. Inst. Metals, No. 8, 57 (1993) 957. 86