Accession Number:

ADA291272

Title:

Effects of Thermal Cycling on Thermal Expansion and Mechanical Properties of SiC Fiber-Reinforced Reaction-Bonded Si3N4 Composites

Descriptive Note:

Technical memo.

Corporate Author:

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CENTER

Personal Author(s):

Report Date:

1994-10-01

Pagination or Media Count:

21.0

Abstract:

Thermal expansion curves for SiC fiber-reinforced reaction-bonded Si3N4 matrix composites SiCRBSN and unreinforced RBSN were measured from 25 to 1400 deg C in nitrogen and in oxygen. The effects of fibermatrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fiber direction were between the measured curves for the strongly and weakly bonded composites, but those normal to the fiber direction for both bonding cases were similar to that of the unreinforced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature, and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly bonded SiCRBSN composites the most, resulting in loss of straln capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fiber and RBSN matrix due to oxidation of the carbon-rich fiber surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the maln reasons for the poor mechanical performance of these composites.

Subject Categories:

  • Ceramics, Refractories and Glass
  • Inorganic Chemistry
  • Mechanics
  • Thermodynamics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE