Degradation at 1200 degrees C of a SiC coated 2D-Nicalon/C/SiC composite processed by SICFILL (R) method

The thermal stability of a 2D-Nicalon/C/SiC composite nias studied through the variation of both mechanical properties and microstructure occurring du ring heat treating. The composite was processed by infiltration of SiC pref orms according to SIC-FILL(R) method. The material toughness was enhanced b y a carbon interphase put between the fibers and the matrix. In order to im prove the thermal stability a CVI layer was deposited on the carbon interph ase and the specimen surfaces were CVD covered by an external SiC seal coat ing about 165 mu m thick. The aging tests were carried out at 1200 degrees C in air or in non oxidizing environment (vacuum). Other specimens were the rmally cycled between 25 and 1150 degrees C. Three point bending tests and Charpy impact measurements were performed before and after these treatments . The composite microstructure was investigated by scanning electron micros cope (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), r eflectance infrared spectroscopy (FTIR) and surface area BET measurements. The as-processed material showed a modulus of rupture (MOR) of 483 MPa and appreciable toughness. These characteristics were retained after aging (200 h at 1200 degrees C) under vacuum. Air thermal treatments caused heavy los s of strength and increase of brittleness. Strong oxidation cccurred during these last treatments at both the carbon interlayer and the matrix, while the SiC external sample coating was not oxidized. The oxygen needed for com posite bulk oxidation flowed through the SiC coating due to the occasional presence of very few structural defects. (C) 2000 Elsevier Science Ltd. All rights reserved.