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       ,     &  , 8))  '))  -      ,      @))  1J 812  122 -    ,    =          , 1  . # 2         ,            8+))
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