Effect of a Boron Nitride Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites

Abstract

Typically, the debonding and sliding interface enabling fiber pullout for SiC‐fiber‐reinforced SiC‐matrix composites with BN‐based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress‐rupture times at higher stresses were observed in air at 815°3C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner‐rig exposure. Two primary factors were hypothesized for the occurrence of debonding and sliding between the BN interphase and the SiC matrix: a weaker interface at the BN/matrix interface than the fiber/BN interface and a residual tensile/shear stress‐state at the BN/matrix interface of melt‐infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration.

Single‐ and Multilayered Interphases in SiC/SiC Composites Exposed to Severe Environmental Conditions: An Overview

“…3)) and both interfacial shear stress and tensile stress were high. At lower fiber speed, crack deflection occurred at fiber surface (as a result of some surface crystallization) whereas for higher fiber speed it was observed at the SiC m /BN interface, these two scenarios corresponding to the “inside” and “outside” debonding reported by Morscher et al , 43 in related experiments. In the case of “outside” debonding, both the interfacial shear stress and tensile failure stress were lower but the lifetime in tensile static fatigue at 700°C in dry or wet air was dramatically improved (crack deflection occurring far from fiber surface) 42 …”

Borosilicate Glass‐Induced Fiber Degradation of SiC/BN/SiC Composites Exposed in Combustion Environments

“…The microscopy results shown here indicate that the iBN‐Sylramic fibers are more oxidation resistant than the Hi‐Nicalon Type S, and Sylramic fibers as long as the in situ BN layer remains in contact with the fiber. This greater oxidation resistance is manifest in the improved stress‐rupture properties observed for iBN‐Sylramic‐containing composites after testing at 1088 K in air . These stress‐rupture test conditions are less aggressive than the HPBR, however, and once the onset of BN oxidation occurs, borosilicate glass formation leads to rapid and significant degradation of the oxidation resistance.…”

Advances in SiC/SiC Composites for Aero‐Propulsion

“…To address this concern, NASA developed CMC thermal treatment processes that would allow the BN coating to shrink toward the fiber surface and debond from the matrix, thereby ensuring crack deflection to occur on the outside of the fiber-protective BN coating [29]. To address this concern, NASA developed CMC thermal treatment processes that would allow the BN coating to shrink toward the fiber surface and debond from the matrix, thereby ensuring crack deflection to occur on the outside of the fiber-protective BN coating [29].…”