Fiber Push-Out Nanoindentation Study of BN Interface in SIC/SIC Composites Exposed to High Temperatures

Abstract

Interfacial properties of boron nitride fiber coatings are studied in melt infiltrated Hi-Nicalon/BN(CVI)/SiC composites subjected to room temperature cyclic fatigue loading and long term annealing to 1000-1200°C in air. Interfacial shear strength was determined from single fiber push-out tests. Hi-Nicalon fibers with diameters of 13-14.5 μm were pushed out from specimens, with thicknesses ranging from 110-280 μm using a spherical tip with a 1 μm radius and 90° conical shape. Interfacial shear strength was calculated from sliding load, fiber diameter and specimen thickness. Due to significant scattering, 30 individual push tests in every sample were used to obtain the average interfacial shear strength. The virgin specimen has interfacial shear strength of 20 MPa which is higher than tensile tested specimens (12 MPa). It was shown that strength of room temperature tensile and fatigue tested samples correlates with their interfacial shear strength obtained from fiber push-out study. Annealing of a virgin specimen for 100 hours at 1000°C slightly increase shear strength up to 21.5 MPa while annealing at 1100°C and 1200°C led to significant increase of shear strength up to 29 and 39 MPa correspondingly. This effect is associated with BN degradation at temperatures >1000°C.