J. P. Viricelle et al. Composites Science and Technology 61(2001)607-614 C/SiC(1)/B4C(1) SiC(3) 1020304050607080 SiC (5) Fig. 4. Scanning electron mi h in the bulk of the composite 70 elsewhere [15]. The atomic composition of matrix I is 一C very homogeneous throughout the layers SiBC(1) and SiBC(2)(Figs. 5 and 6). X-ray diffraction patterns(Cu, Ka) performed on particular samples where only matrix I has been deposited shows a poor crystallisation state Fig. 7)and a structure based on silicon carbide. The pyrolysis of these samples at 1500C in helium leads to the segregation of the constituents and crystallisation of licon carbide boron carbide and carbon (Fig. 8) More detailed investigations on the SiBC structure have been performed by high-resolution electron microscopy [14] and reveal that it is composed of Sic crystallites surrounded by an amorphous phase. Boron may be Point (au) present either in Sic lattice or as an amorphous boron Fig. 6. Composition profile of SiBC(2) layer. carbide phase. No trace of crystallised boron carbide has been observed. it is also known that the maximum solid solubility of boron in silicon carbide is about 0.2 wt%[16], and that boron can be incorporated either in Matrix 1 silicon or carbon sites [17]. However, by considering the boron content of matrix 1, the presence of an amor- -"--"Carbon phous boron carbide phase seems to be the more prob able hypothesis. From the mean chemical composition it is possible to calculate a theoretical molar composi tion by taking into account the main phases: SiC, B4C, C. The resulting molar composition shows an excess of carbon 102030405060708090 4. Results and discussion Prior to this study, the oxidation behaviour of each Fig. 7. X-ray diffraction pattern of matrix I SiBC constituent of the composite(fiber, matrices I and 2) had been investigated in similar conditions [18-20]. The occurs through the lateral faces( 8 mm x 4 mm). As a results obtained are necessary to understand the whole at, oxidation of our specimens is greatly enhanced composite behaviour. As indicated previously, the sam- compared to the final materials used in applications ples used for the composite oxidation study are coupons However, the interest is an elucidation of the phenom- (8 mm x8 mm x 4 mm) cut from larger plates. Thus, ena which facilitates their identification and under the access of oxygen in the bulk of the material mainl /  ?(.@ -     (    0     0     (  ' J0 .  + G 9    ! &         /    (      /     J0 7             -        (.,,
          0 0           !       J0 C =     0             0      ?(B@                                            A              E   6/                  ,' /1 ?(+@!                ?(7@ %/  !   0       (!                         J      !                60       3 ! B!  -   0   /               !            5 !   (  '    0     ?(C4',@ -               /    :     !                C  C  B     0    - !     0    6         0      C  B  :    !         0        5       %/  !              /      5    0 J0 B 0   0   6     J0 .  5  (   J0 +  5  '   J0 7 G 9     (  , -%   . /
%%  %'  0""1 "!23 +,)