Abstract
Evaluation of lifetime performance of Hi-Nicalon fiber-reinforced SiC ceramic composites was carried out in four-point bending over a temperature range of 700° to 1300°C in air. The effects of number of harness and type of coating on the lifetime response were also evaluated in the present study. The composites consisted of ∼40 vol. % Hi-Nicalon fiber (8 or 5 harness satin weave) with a 0.5 μm BN (or C) fiber coating and a melt-infiltration SiC ceramic matrix. Lifetime results indicated that the Hi-Nicalon/MI SiC composites exhibited a stress-dependent lifetime at stress levels above an apparent fatigue limit, similar to the trend observed in CG-Nicalon fiber-reinforced CVI SiC matrix composites. The observed stress-dependent lifetime behavior was independent upon the number of harness and type of coating employed. However, results indicated that the lifetime performance of Hi-Nicalon/MI SiC composites could be greatly enhanced via the use of a higher number of harness satin weave and/or BN fiber coating. For SiC composites with BN fiber coating at temperatures 1150°C the lifetime performance of SiC composites was limited by the onset of creep of Hi-Nicalon fibers. The lifetime governing processes in an oxidizing environment were generally attributed to the progressive oxidation of fiber coatings and subsequent formation of glassy phase(s), which formed a strong bond between fiber and matrix, resulting in embrittlement of the composite with time.
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