Abstract
Ceramic matrix composites (CMCs) have been prepared and optimized as already described in part I of this paper. The fibrous preform made of Hi-Nicalon S fibers was densified by a matrix composed of Si2N2O prepared inside the CMC by reacting a mixture of Si and SiO2 under high nitrogen pressure. This part describes the oxidation resistance and mechanical properties of the optimized CMC. The CMC submitted to oxidation in wet oxygen at 1400 °C for 170 h exhibited an oxidation gradient from the surface to almost the center of the sample. In the outer part of the sample, Si2N2O, Si3N4 and SiC were oxidized into silica in the cristobalite-crystallized form. The matrix microstructure looks similar to the original one at the center of the sample, while at the surface large pores are observed and the fiber/matrix interphase is consumed by oxidation. The elastic modulus and the hardness measured at room temperature by nano-indentation are, respectively, 100 and 8 GPa. The elastic modulus measured at room temperature by tensile tests ranges from 150 to 160 GPa and the ultimate yield strength from 320 to 390 MPa, which corresponds to a yield strain of about 0.6%. The yield strength identified by acoustic emission is about 40 MPa.
Highlights
We looked at variation in the elastic modulus as a function of the maximum load applied (Figure 12) and as a consequence of the damage undergone by the Ceramic matrix composites (CMCs)
Figure imposed to to the the test test piece piece during duringthis thiscycle. We have checked both the oxidation resistance at high temperature and tensile resistance at room temperature of CMCs densified by Si2 N2 O
The matrix microstructure looks similar to the original one at the center of the sample, while at the surface large pores are observed and the fiber/matrix interphase is consumed by oxidation
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. SiC is an oxidation-resistant material at high temperatures due to its ability to form a continuous protective oxide scale (SiO2 ) under highly oxidative environments (passive oxidation). If the time-dependent law is parabolic at 1300 ◦ C, this is no longer the case above 1350 ◦ C These latter authors have determined that above 1350 ◦ C, the oxidation law can be derived from a diffusion law of O2 through the amorphous SiO2 film in between the α-cristobalite crystals, the size of the latter increasing with temperature. Assuming that water vapor has no specific oxidation influence on Si2 N2 O, it should mainly affect the formed silica through enhanced SiO2 vaporization by formation of gaseous hydroxides, as previously mentioned in the case of SiC oxidation
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