The elevated temperature mechanical properties of silicon nitride/boron nitride fibrous monoliths.

A unique, all-ceramic material capable of non-brittle fracture via crack deflection has been characterized from 25$\sp\circ$C through 1400$\sp\circ$C. This material, called fibrous monoliths (FMs), was comprised of unidirectionally aligned 250 $\mu$m diameter cells of silicon nitride surrounded by 10 $\mu$m thick cell boundaries of boron nitride. Six weight percent yttria and two weight percent alumina were added to the silicon nitride to aid in densification. TEM experiments revealed that the sintering aids used to densify the silicon nitride cells were migrating into the boron nitride cell boundary during hot-pressing and that a fine network of micro-cracks existed between basal planes of boron nitride. Elevated temperature four point bending tests were performed on fibrous monolith ceramics from room temperature through 1400$\sp\circ$C. Peak strengths of FMs averaged 510 MPa for specimens tested at room temperature through 176 MPa at 1400$\sp\circ$C. Work of fractures ranged from 7300 J/m$\sp2$ to 3200 J/m$\sp2$ under the same temperature conditions. The interfacial fracture energy of boron nitride, $\Gamma\sb{\rm BN}$, as a function of temperature has been determined using the Charalambides method. The fracture energy of boron nitride is approximately 40 J/m$\sp2$ and remained constant from 25$\sp\circ$C through 950$\sp\circ$C. A sharp increase in $\Gamma\sb{\rm BN}$, to about 60 J/m$\sp2$, was observed at 1000$\sp\circ$C-1050$\sp\circ$C. This increase in $\Gamma\sb{\rm BN}$ was attributed to interactions of the crack tip with the cell boundary glassy phase. Subsequent measurements at 1075$\sp\circ$C indicated a marked decrease in $\Gamma\sb{\rm BN}$ to near 40 J/m$\sp2$ before plateauing at 17-20 J/m$\sp2$ in the 1200$\sp\circ$C-1300$\sp\circ$C regime. The Mode I fracture toughness of silicon nitride was also determined using the single edge precracked beam method as a function of temperature. The He and Hutchinson model relating crack deflection at an interface to the Dundurs' parameter was applied to the current data set using the temperature dependent fracture energies of the boron nitride and the silicon nitride. A more refractory fibrous monolith was fabricated in an effort to extend the high temperature properties of SN/BN fibrous monoliths. Only 4 wt.% yttria was added to the silicon nitride to aid in densification. The presence of residual carbon following binder burnout was proposed to be responsible for the formation of melilite, a phase known to undergo severe oxidation between 900$\sp\circ$C-1100$\sp\circ$C. When residual carbon was removed prior to hot-pressing with a post-binder burnout heat treatment at 400$\sp\circ$C in air this phase was not present. A room temperature strength of 553 MPa and a work of fracture of 6700 J/m$\sp2$ was observed. A strength of 293 MPa was measured at 1400$\sp\circ$C.PhDApplied SciencesMaterials scienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131110/2/9825358.pd