Solid-particle erosion and strength degradation of Si 3N 4/BN fibrous monoliths

Abstract

Erosive damage was studied in Si 3N 4/BN fibrous monoliths (FMs) and the individual constituents of the cells and cell boundary, monolithic Si 3N 4 and BN. Unidirectional, 0/90°, and ±45° FMs were tested. Specimens were subjected to impact at 90° by angular SiC particles of average diameter 143 μm, traveling at 50–100 m/s. Steady-state erosion rates in the FMs were higher than predicted by a rule of mixtures based on erosion rates of the cell and cell-boundary phases. The relatively rapid FM erosion was attributed to chipping of the Si 3N 4 cells caused by radial cracks. Bending strengths were measured before and after erosion testing to steady state at 100 m/s. The strength of monolithic Si 3N 4 decreased 22%; the BN was not tested because insufficient material was available. Within experimental error, the strengths of the FMs were unaffected by erosion. Fracture data obtained approximately 1.5 years apart suggested that the FMs were susceptible to environmentally assisted slow crack growth.