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Intermediate‐Temperature Environmental Effects on Boron Nitride‐Coated Silicon Carbide‐Fiber‐Reinforced Glass‐Ceramic Composites

1997; Wiley; Volume: 80; Issue: 3 Linguagem: Inglês

10.1111/j.1151-2916.1997.tb02875.x

1551-2916

Ellen Y. Sun, Hua‐Tay Lin, John J. Brennan,

Concrete and Cement Materials Research

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.