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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