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Corrosion of ceramic matrix compositesT. Scanu, Ph. Colomban

To cite this version:T. Scanu, Ph. Colomban. Corrosion of ceramic matrix composites. Journal de Physique IV Colloque,1993, 03 (C7), pp.C7-1927-C7-1930. �10.1051/jp4:19937308�. �jpa-00251952�

JOURNAL DE PHYSIQUE IV Colloque C7, supplBment au Journal de Physique 111, Volume 3, novembre 1993

Corrosion of ceramic matrix composites

T. SCANU and Ph. COLOMBAN

ONERA-OM, BR 72, 92322 Chdtillon, France and LASIR, CNRS, 2 rue Henn Dunant, 94320 Thiais, France

Air s table ceramic matr ix composites a r e promising f o r thermostructura l applications such a s a i r c ra f t engine pa r t s . Turbine pa r t s a r e subject t o both sulphuric acid and sodium molten sa l t s corrosion due t o sulphate t r aces in engine fuel and t o t h e NaCl a i r content. The chemical stabil i ty is a very important cr i ter ion but t h i s point has not received much attention t o date. We repor t here a study of acidic and sodium corrosion of various aluminosillcate matrices : LAS matrices (LizOAlzO32-6SiOz,nPz05) in the amorphous, /3 eucryptite and 6 spodumene forms, BAS matr ix (BaOA1~032SiOz) in the form of monoclinic and hexagonal celsian, NASICON matr ix (Na3ZrzSizPOiz) and mullite matr ix . Microstructure damages and ion exchange have been analysed by X-ray diffraction, IR absorption, scanning electron microscopy and Raman microprobe. Drastic corrosion is observed f o r 13 spodumene containing composites with the formation of s t rong hydrogen bond o r with t h e cell expansion due t o ~ i / ~ a + exchange. Medium acidic a t tack occurs f o r glassy LAS, f3 eucryptite, BAS and NASICON matr ix composites. On the other hand, 6 eucryptite, NASICON and monoclinic celsian res is t t o alkaline melts. Mullite matr ix composites a r e never corroded.

INTRODUCTION

The ma t r ix of ceramic matr ix composites is usually selected in reference with t h e melting point and by t h e following c r i t e r i a : high Young's modul us, low thermal expansion. An important cri terion t o obtain high mechanical properties is tha t t h e thermal dilatation of the ma t r ix i s lower o r a t least equal t o tha t of the f iber , in o rde r t o avoid the crack format ion during the elaboration and the thermal cycling of t h e composite. Matrix compositions a r e thus chosen among loose-packed frameworks. Loose-packed f ramework promotes also ionic conductivity and ion exchange. Ion exchange is relatively r a r e in dense solids but i t i s common in loose-packed s t ructures and this phenomenon is intimately re la ted t o the (superlionic conductivity (1,2). Loose-packed f r an~eworks a r e a lso needed f o r low thermal expansion : t he ro ta t ion of molecula~- bricks (e .g . Si04, A104, PO4, AlO6, . . . I counter- balances the bond length increase with temperature , t he resulting unit-cell parameters and the unit-cell volume being thus s table o r even decreased, anisotropically, over a large temperature range. On the other hand, dense s t ruc tu res exhibit always high thermal expansion coefficients. In s t ruc tu res where the host framework is sufficiently loosely packed alkali o r earth-alkali ions diffuse, easily (1,3). Glasses a r e usually poorer ion exchangers and poorer ionic conductors than crystall ine phases of similar composition and s t ructure . Ion exchange induces change of the unit-cell parameters , directly f r o m the exchange o r indirectly with the phase t rans i t ions resulting f rom the back t o t h e s t ab le phases. A dilatation is generally associated t o the formation of cracks and then t o a d ras t i c lowering of the mechanical properties, whereas the contraction may preserve the mechanical properties (4,s). Furthermore, t he ion exchanged material can be metastable and, f o r example, hydrogen substi tuted s t ruc tu res have a lower thermal stabil i ty (2) . We repor t here a s tudy of the sulphuric acid and sodium n i t r a t e corrosion of various matrices. The ion exchange and the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19937308

JOURNAL DE PHYSIQUE IV

Fig. 1 : Photomicrographs of various matrices of ceramic matr ix composites before (a', a") and a f t e r acidic (a, b', b", c") o r alkaline (b, c ' ) corrosion : a , b, 6 spodumene (keati te) LAS mat r ix ; a ' , b', c ' amorphous P containing LAS ma t r ix ; a", c", BAS ma t r ix ; c" NASICON mat r ix (backscattered electron mode).

corresponding micros t ructure changes have been analysed by X-ray di f f rac t ion, scanning electron microscopy, chemical analysis and vibrational spect ra .

EXPERIMENTAL

The fabr ica t ion of 2D S i c Nicalon NLM202 woven fabr ics reinforced composites w a s previously reported : t w o routes have been used, a conventional melting route f o r "crystalline" LAS mat r ix (6) and a new sol-gel route f o r amorphous LAS, monoclinic (celsian) BAS, NASICON and mullite matr ices (7). Alkaline and acid corrosion were performed by immersion of polished samples in molten n i t r a t e o r in boiling concentrated sul fur ic acid, f o r t imes comprised between 5 min and 10 hours. Crushed samples (-0.5mm) a r e used f o r Li loss measurements.

RESULTS AND DISCUSSION

LAS Matr ices . The nominal compositions of our (3 eucryptite and (3 spodumene ("keatite") matr ices a r e close t o LiAlSizO6 and LiAlSis08, respectively. After acidic a t tack, w e observe a d ras t i c contraction (>8%) of (3 spodumene c parameter whereas the changes f o r (3 eucryptite phase a r e moderate, according t o the l i tera ture (8 ,9) . Although immersion of (3 eucryptite in molten NaN03 induces no change, a crushing of the (3 spodumene ma t r ix i s observed, according t o previous works (10,111 : t h e c parameter increases f rom 9.04 t o 9.14A. Fig. 1 shows typical photomicrographs of (3 spodumene matrix-Sic f ibe r composites. Af ter contact with boiling acid, polished samples exhibit parallel cracks which give evidence of the large s ize of (3 spodumene crystals. The cracks resul t f rom the contraction due t o t h e formation of s t rong hydrogen bonds between Si(A1104 t e t r ahedra (12). The cracks reveal t h e dendrit ic crystall ization around SIC f ibers . With the ~ i ' / ~ a + exchange, t h e large expansion of c parameter (-10%) crushed the matrix. Figs. la ' ,b ' ,c ' show changes observed f o r glassy, P containing LAS matr ix . After acid o r alkaline a t t acks we observe a network of i r regular cracks. Some cracks a r e roughly parallel t o the S i c woven f ab r i c sheets which reveals the s t r e s s due t o the f iber /matr ix dilatation mismatch. This indicates a contraction of the amorphous s t ruc tu re a f t e r both H+ or ~ a + exchange. As shown on Fig. 2, t he L~+/H+ exchange is slower in amorphous LAS (here P f r e e ) than in (3 eucryptite o r (3 spodumene phases. The unexchangeable Li+ ions of our (3 eucryptite sample a r e due to the presence of an amorphous second phase.

Fig. 2 : a ) Evolution of the Li content (%, in weight) versus leaching t ime in boiling pure H2S04 f o r amorphous LAS (am), (3 eucryptite LAS (Euc) and (3 spodumene LAS (spo). Comparison of Raman spec t r a of 13 spodumene LAS/SiC composite recorded before (up) o r a f t e r (down) acidic corrosion is given in b.

1930 JOURNAL DE PHYSIQUE IV

As shown on Fig. 2b, the L~' /H+ exchange can be controlled by Raman microspectroscopy using the frequency sh i f t of the main band, e.g. f rom 493cm-' in Li (3 spodumene t o 471cm-I in H-form (13). BAS m a t r i x . Whereas no a t tack is visible f o r monoclinic celsian/SiC composites immersed in molten NaN03 f o r a f e w hours, a par t ia l dissolution is observed a f t e r contact with boiling H2S04 (Fig. lb") f o r both hexagonal and monoclinic celsians. NASICON m a t r i x . This matr ix i s s table in molten NaN03. On the other hand, sulphuric acid induces t h e cracking of the matr ix according to a possible ion exchange (14) (Fig. lc"). Mulli te m a t r i x . No change has been observed.

CONCLUSION

The following chemical stabil i ty scales can be proposed . in acidic atmosphere : mullite>>>BAS, amorphous LAS>NASICON, (3 eucryptite>@ spodumene . in presence of molten alkali s a l t s : mullite, 6 eucryptite, NASICON>BAS>>>(3 spodumene. The 6 spodumene phase usually selected according t o mechanical c r i t e r i a i s t hus the phase t o be prohibed f rom t h e corrosion point of view. The use of ref ractory , chemically s table matrices (e.g. mullite) reinforced by nano or rnicroprecipitates appears t o be a be t t e r solution whatever a higher dilatation coefficient (15).

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