Stability of a Sapphire/Yttrium Aluminum Garnet Composite System

Sapphire filaments were vacuum hot-pressed at 1500°–1650°C in a yttrium aluminum garnet (YAG) powder matrix. The YAG powders were derived from alkoxide precursors. A fully dense composite with a YAG grain size of 1-5 pun was obtained. Some samples were heat-treated further for 75 h at 1400°C in air. No evidence of second-phases along interfaces, or of chemical reaction between the sapphire and YAG, was observed by optical microscopy, electron microprobe, or analytical TEM. However, triple junction equilibrium between YAG grain boundaries and YAG/sapphire interphase boundaries resulted in a cusped YAG/sapphire interface. Triple junction angles indicate that YAG grain boundaries and YAG/alumina interphase boundaries have nearly equal interface energies. Lack of a smooth YAG/sapphire interface is anticipated to cause poor pull-out properties.