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Intermediate‐Temperature Environmental Effects on Boron Nitride‐Coated Silicon Carbide‐Fiber‐Reinforced Glass‐Ceramic Composites
Author(s) -
Sun Ellen Y.,
Lin HuaTay,
Brennan John J.
Publication year - 1997
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1997.tb02875.x
Subject(s) - materials science , composite material , flexural strength , borosilicate glass , silicon carbide , ultimate tensile strength , ceramic , fracture toughness , boron carbide , fiber , microstructure , silicon nitride , layer (electronics)
The environmental effects on the mechanical properties of fiber‐reinforced composites at intermediate temperatures were investigated by conducting flexural static‐fatigue experiments in air at 600° and 950°C. The material that was studied was a silicon carbide/boron nitride (SiC/BN) dual‐coated Nicalon‐fiber‐reinforced barium magnesium aluminosilicate glass‐ceramic. Comparable time‐dependent failure responses were found at 600° and 950°C when the maximum tensile stress applied in the bend bar was 60% of the room‐temperature ultimate flexural strength of as‐received materials. At both temperatures, the materials survived 500 h fatigue tests at lower stress levels. Among the samples that survived the 500 h fatigue tests, a 20% degradation in the room‐temperature flexural strength was measured in samples tested at 600°C, whereas no degradation was observed for the samples tested at 950°C. Microstructure and chemistry studies revealed interfacial oxidation in the samples that were fatigued at 600°C. The growth rate of the Si‐C‐O fiber oxidation product at 600°C was not sufficient to seal the stress‐induced cracks, so that the interior of the material was oxidized and resulted in a strength degradation and less fibrous fracture. In contrast, the interior of the material remained intact at 950°C because of crack sealing by rapid silicate formation, and strength/toughness of the composite was maintained. Also, at 600°C, BN oxidized via volatilization, because no borosilicate was formed.