B Sarhan et al. Composites: Part A 32(2001)1095-1103 with relatively high porosity and can be regarded as a from the matrix to the fibre and increases the strength of the porous-matrix composite. The thickness of the carbon(fugi composites significantly (for instance in carbon/zro tive)-layer applied in our study is relatively large(200 nm), composites 230 MPa after hot-pressing and in carbon/ omparing with that of Keller et al. and therefore a lower Al O3-composites 180 MPa compared to 135 MPa of the fibre/matrix-interaction is to be expected(see Fig. 6(a)). ref Despite the presence of a large fugitive layer, the continuous Sintering between fibre and matrix in the reference heat-treatment at 1300.C leads to an almost total closure of sample decreases the damage tolerance as the strength the gap between the fibre and matrix increases in expense of damage tolerance. In the case of In contrast, our carbon(fugitive)ZrOz-interphase com- double layers, the behaviour of the oxide layer and its inter posite yields strength degradation after removal of the carbon ction with the matrix and fugitive layer is decisive for the as well as after heat-treatment at 1300C. This result is characteristics of the interphase. In those composites where comparable with that of the dense matrix composites of Keller a reasonable fugitive layer was maintained, crack deflection et al. where chemical interactions at the fibre/matrix-interface and fibre sliding at the fibre/matrix- interface were probable were held responsible for the strength reduction. We believe The efficiency of the fugitive/oxide-double layer system however, in the case of fugitive/LrO2-interphase composite, depends on the materials and the thermal loading condition the ZrO2-layer sinters and contracts in length heavily, clos-(e. g. cyclic or continuous thermal exposure). This may be ing the gap and causing notch-like defects on the fibre which due to the occurrence of different interactive mechanisms as a result reduce the damage tolerance as well as the within the matrix and at the interface. Under cyclic strength. This behaviour of monoclinic ZrO, was attributed heat-treatment conditions, the composites are found to be to the high thermal expansion coefficient(13 x 10 K ), more stable despite high temperature and relatively long compared to that of mullite(4.5×10-°K-) Cyclic heat-treatment in turn seems to break the sintering-related contact points within the matrix and at the interface during successive cooling down periods, yield References ing better maintenance of the gap and the damage tolerance In the case of ZrOr-interphase, repeated volume changes by reversible monoclinic-tetragonal phase-transformation [1] Chawla KK, Xu ZR, Ha JS, Schmuicker M, Schneider H Appl Compos Mater1997;4:263. under cyclic conditions affected the interfacial relations [2] Hay RS, Petry MD, Keller KA, Cinibulk MK, Welch JR Mater Re resulting in better sliding ability of the fibre Dc Symp Proc1995:365:377-82. [3] Milz C. PhD-study, Characterisation e silicate fibre to be used at high-temperature in fibre- opposites. Shaker 5. Conclusions Verlag, 2000 [4] Keller KA, Mah T, Parthasarathy TA, Cooke CM. Fugitive interfacial Presence of a dense oxide layer behind a carbon (i.e carbon coatings for oxide/oxide composites. J Am Ceram Soc fugitive)-layer plays a supporting role in the load transfer 2000;83(2):329-36, &                   -     1 &2       &   1')) 2    ,   !       ,  3        1 J (122 0        &        8+))
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