Silica, boron nitride, aluminum nitride, alumina composite, article and method of making same

Multiphase ceramic composites containing 47% to 52% by weight aluminum nitride, 23% to 27% by weight silica, 3% to 7% alumina and the balance boron nitride wherein the boron nitride is one phase in the composition and the reaction products of aluminum nitride, silica and alumina are the other phase in the composite. The boron nitride phase is uniformly distributed in the form of discrete particles throughout the reaction products of aluminum nitride, silica and alumina. A window transparent to electromagnetic radiation in a predetermined frequency spectrum is made by forming a homogeneous, finely-divided mixture of the foregoing composite and compacting the homogeneous, finely-divided mixture at an elevated temperature and pressure for a sufficient time to form reaction products of the aluminum nitride, silica and alumina. In preferred embodiments, the homogeneous, finely-divided mixture of aluminum nitride, silica, boron nitride and alumina is densified at a temperature of 1760 DEG C. and at a pressure of about 3775 p.s.i. for about 1 hour.