c00.52es ELSEVIER Composites: Part A 32(2001)1095-1103 Effect of interphase characteristics on long-term durability of oxide-based fibre-reinforced composites B. Saruhana, *M. Schmucker M. Bartsch. Schneider K. Nubian.G. Wahl German Aerospace Centre, Institute for Materials Research, D-51147 Koln, Germany Technical University of Braunschweig, Institnte of Surface Technology, D-38108 Braunschweig, Germany Received 26 May 2000; revised 26 November 2000; accepted 30 November 2000 Abstract Mullite based fibre-reinforced composites having double layer fibre-coatings were produced and characterised. The multi-layer inter phases were produced by a CVD-process(carbon(fugitive)ZrO2 or Al2O3) on aluminosilicate Nextel 720 fibres. Composites were fabricated by infiltration of coated fibres with a pre-mullite slurry and hot-pressing in argon at 1300C. Short term heat-treatment of composites in air yielded a gap between the fibre and the oxide layer by oxidation of the carbon layer(so-called fugitive layer). The mposites were statically and cyclically heat-treated at 1300.C for 1000 h in order to identify the mechanical and microstructural changes Mechanical characterisation of the heat-treated composites was carried out by three-point bending. The effectiveness of the fugit determined by the oxide layer and its high-temperature stability in interaction with the matrix but it also depends on the loading Under cyclic heat-treatment conditions the composites are found to be more stable and damage tolerant than under constant high e. c 2001 Elsevier Keywords: B Interface/interphase: B. Mechanical properties; E. Chemical vapour deposition(CVD): B. Damage tolerance; Mullite 1. Introduction at very high temperatures, owing to the high diffusion rates of the oxides [21 Relying on the outstanding physical, chemical and ther In this study, two interphase combinations, based on the mal properties, mullite based fibre-reinforced composites multi-layer principle were used. The double layer interphase became favourite candidates for the high-temperature appli- systems, having carbon ZrO2- and carbon Al2O3- cation in the combustion chambers of gas turbine engines. double layers were produced by Chemical vapour deposi- These applications require damage tolerant, temperature- tion(CVD)-coating of Nextel 720 fibres successively and oxidation-resistant ceramic components. In order to The mechanical and microstructural effects of the double realise damage tolerance in ceramics, it is necessary to layer interphase systems were compared with that of a refer employ continuous fibre-reinforcement and possibly an ence composite which had only a carbon(fugitive)-layer at nert interface material (interphase). Thus, a suitable weak the fibre/matrix-interface bonding debonding and sliding at the fibre/matrix- interface can be achieved, which leads to fibre pull-out and conse- quently to the damage tolerance. These properties are to be 2. Materials and methods realised and maintained not only at room temperature, but also at elevated temperatures and over long terms. Non Coating of Nextel 720 fibre fabrics(8 harness Atlas) oxide interphases(e.g BN and C) are structurally suitable, were produced by a CVD-process, with special evaporation however,do not survive under long-term exposure at high and deposition equipment. The starting precursors were le to the lack of oxidation resistance [1]. zirconium and aluminium tetrametylheptandionate(tmhd)4 Among many suggested concepts, the fugitive coating, for oxide coating and propane for carbon coating.Carbon resulting in a gap at the fibre/matrix-interface delivered coating of the woven fibre mats was carried out with pure promising results. One drawback is the closure of the gap propane at 950C under 12.5 mbar pressure. Subsequently the ZrO2 and Al2O3-oxide coatings were vapour-deposited Corresponding author at about 510 and 610.C in a mixture of oxygen and argon E-mail address: bilge saruhan@dlr.de(B Saruhan). under 5 mbar, respectively. After completion of the fibre front matter o 2001 Elsevier Science Ltd. All rights reserved. PI:S1359-835X(01)00016-1
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1096 B Saruhan et al. /Composites: Part A 32(2001)1095-1103 Carbon Carbon Al2O3 51251E Fig. 1. Scanning electron micrographs of coated fibres:(a)carbon/ZrO -double coating; and( b) carbon/AlO] double coating. (a) (b Carbon Carbon Zro AlO Fig. 2. Scanning electron micrographs of the interphases after hot-pressing at 1300C for 15 min under 10 MPa pressure. (a) Reference sample b)carbon/ZrO,; and(c)carbon/Al_O3-
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B Sarhan ef al./ Composites: Part A 32(2001)1095-110 coating, the thickness of carbon coating was measured to microscope (Leitz LEO 982, Germany)and tested mechani- vary between 100 and 200 nm. The thickness of oxide coat- cally at room temperature by a three-point-bending test wit ings was in turn approximately 800 nm(Fig. 1(a)and (b)). a span of 20 mm, using a UTS-10 testing equipment with a The composites were prepared by aqueous slurry 200N load cell. The displacement in the middle of the infiltration of a submicrometer pre-mullite powder(Siral, 20 mm span was measured with one inductive strain Condea, Germany) into the unidirectional laid double gauge, neglecting the system compliance of the testing coated fibre tows and hot-pressing the composites in argon machine which was estimated to be very small compared at 1300C, for 15 min, under 10 MPa uniaxial pressure. The with that of the test samples. pyrolytic carbon layer was intact after hot-pressing. The omposites were heat-treated at 1200C for 2 h in air in order to obtain a gap between the fibre and the oxide layer 3. Results (so-called fugitive layer). Considering the application temperatures for the composites in combustion chambers, After hot-pressing in argon, the composites contained the composites were heat-treated at 1300C for 1000 h under intact carbon, carbon/monoclinic ZrO2 and carbon/AlO3- continuous-and cycling-heating conditions Thermal cycling interphases (Fig. 2(a)-(c)). The reference composite was carried out by heating up at a rate of 10 K/min to 1300C contained a 200 nm thick carbon interphase, after hot and holding at this temperature for I h before cooling down pressing in argon(Fig. 2a). The thickness of the double to room temperature. This cycle was repeated 1000 times. layer-coating varied after hot-pressing such that the carbon The mechanical and microstructural changes in the layer in carbon/monoclinic Zro2-composite was reduced to opposites were determined by microstructural investiga- 100 nm(initially 180 nm)and that of monoclinic ZrO2-layer tions and mechanical testing. The composites to 400 nm(initially 800 nm). The morphology of characterised microstructurally with a scanning elect clinic ZrO2-layer became somewhat porous(Fig 2b) Fig. 3. Scanning electron micrographs of the interphases after heat-treatment at 1200C for 2 h in air:(a) reference sample:(b)carbon/ZrO2; and (c) carbon/AlO3
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1098 B Saruhan et al. /Composites: Part A 32(2001)1095-1103 Table 1 Mechanical data of the composites and the thickness of the layers before and after heat-treatment Fiber coating As-hot-pressed Heat-treated at Continuous heat- Cyclic heat- 200°Cfor2 h treated at1300° treated at1300° for 1000 h for1000×1h Fracture strength 160 MPa 42.4 GPa 84.6 GPa Carbon(fugitive)ZrO2 Thickness of C-laye Thickness of oxide-layer 800 400nm 150nm Fracture strength 230 MPa 200 MPa 140 MPa 106 GPa 81 GPa Carbon(fugitive )AlO Thickness of C-layer 180 50 Thickness of oxide-lay 79 MPa 170 MPa 170 MPa 200 MPa Young's modulus 75 GPa 74 5 GPa 91 GPa 92 GPa The carbon layer in carbon/Al2O3-composites after no change compared with that of the as-coated one hot-pressing was about 150 nm, being only slightly thin- (Fig. 2(c)) ner than the thickness of the as-coated layer. The thick Heat-treatment of composites at 1200"C for 2 h in air ness of the AlO3-layer was reduced to 150 nm (initially yielded a gap between fiber and oxide layer by oxidation 800 nm). The morphology of the Al2Oj-layer showed of the carbon layer (fugitive layer)(Fig 3(a)-(c)). This 1300.1000K Fig. 4. Scanning electron micrographs of the interphases after continuous exposure at 1300.C in air for 1000 h: (a)reference sample; (b) fugitive/ZrO2: and (c)fugitive/Al_O3
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B Sarhan ef al./ Composites: Part A 32(2001)1095-110 process did not cause any warping or cracking of the fibre and causes formation of notch-like damage on fibres composites. The thickness of the fugitive layer in the refer-(Fig. 4(a)) ence composite was significantly smaller than the carbon The gap closure at the interface of the fugitive/ZrO2- yer after hot-pressing (about 80 nm double layer composites was very advanced already after 200 nm), immediately after the oxidation of carbon layer hot-pressing but led to a complete disappearance of the gap at 1200.C/2 h. The thickness of the fugitive and ZrO at 1300C, after 1000 h of continuous heating. The Zro- layers in the fugitive/ZrO2-composites were reduced to layer showed sintering-related morphological changes, result 0 nm and 200 nm, respectively. The thickness of the ing in a rough surface development and thickness reduction fugitive layer in the fugitive/Al2O3-double layer com-(Fig 4(b)). In the fugitive/Al2O3-double layer-composites posites showed no change. The thickness variations of also sintering necks formed after 1000 h continuous heating the interphases as well as numerical mechanical data to at 1300C. However, in this case, the interaction was not the composites before and after heat-treatment are listed only between the fibre and the Al2O3-layer, but also between in Table 1 the matrix and the Al2O3-layer. So that the thickness of the Generally the fugitive layer is thermally unstable and gap was heterogeneous; larger where sintering-related ses progressively as the heat-treatment temperature and contacts between the AlO3-layer and the matrix took duration increases. This is mainly because the matrix as well place, thinner or closed where the fibre and the Al2O as the oxide layer maintains a substantial sintering activity layer were sintered together(Fig 4(c)) after hot-pressing. At the interface of the reference compo- The interfacial relations after cyclic treatment at 1300.C site between the matrix and the fibre substantial sintering differ from those under continuous heat-treatment at the necks form at 1300C after 1000 h continuous exposure in same temperature and same exposure time,(Fig. 5(a)-(c)) air. This interaction between the fibre and the matrix The fugitive layer appeared to be maintained in all compo- educes the amount of alumina on the surface zone of the sites, although to different extents in each composite. The bR m Fig. 5. Scanning electron micrographs of the interphases after 1000 thermal cycles at 1300.C in air:(a) control sample:(b) fugitive/ZrO2; and(c)fugitive/ A12O3
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