The phase relationships in the system ZrOz-MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high-temperature X-ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrOZ decreases the transition temperature for monoclinic $ tetragonal ZrOz, and 1 mol% of MgO is soluble in the monoclinic zirconia at -1070°C. The invariant eutectoid point is at 13.5 +-0.3 mol% MgO at 1406" rt 7"C, which is in fair agreement with previous results by Grain. The ordered phase MgzZr501z (6-phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZrsOI3 (y-phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.

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The need for structural materials that can withstand severe environments up to 4000°F has promulgated the investigation of sol-gel derived ceramic and composite coatings on carbon/carbon composite materials. Alumina and zirconia sols have been deposited via thermophoresis on carbon/carbon substrates.

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