EFFECT OF AGING TREATMENTS AT HIGH-TEMPERATURES ON THE MICROSTRUCTUREAND MECHANICAL-BEHAVIOR OF 2D NICALON C/SIC COMPOSITES .1. AGING UNDER VACUUM OR ARGON/

Two-dimensional Nicalon/C/SiC composites processed by chemical vapour infiltration have been aged at 1100-1500 degrees C under vacuum or arg on. The composites experience a weight loss Delta m/m(0) related to th e decomposition of the SiO2xC1-x phase of the fibres and to side-react ions between the fibre decomposition gaseous products (particularly Si O) and carbon (from the fibre and the interphase), as assessed by elec tron probe microanalysis, and Auger electron spectroscopy, analyses. T his decomposition/reaction process results in an evolution of the mech anical behaviour (characterized through tensile/push-out tests) correl ated with the weight loss. Mild ageing conditions (Delta m/m(0) < 1 . 3%) result in a weakening of the fibre-matrix bonding due to some inte rfacial decohesion with a decrease in both the proportional limit and failure strength, as well as an increase in the failure strain. More s evere ageing conditions (Delta m/m(0) approximate to 2%) yield a parti al consumption of the interphase with the formation of large SiC-cryst als on the fibre surface weakening the fibres, the composite still exh ibiting a non-linear mechanical behaviour. Finally, for extremely seve re ageing conditions (3 < Delta m/m(0) < 7%), the interphase is totall y consumed and the fibres undergo a pronounced decomposition/crystalli zation process, the composite becoming brittle with a very low load-be aring capability. (C) 1997 Elsevier Science Limited. All rights reserv ed.