Stability of Ultrafine Tetragonal ZrO ₂ Coprecipitated with Al ₂ O ₃ by the Spray‐ICP Technique

The transformation of ultrafine powders (particle size, 0.01 to 0.04 μm) of the system ZrO 2– Al 2 O 3 , prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultrahigh temperature, was investigated. The powders were composed of metastable tetragonal ZrO 2 ( mt‐ ZrO 2 ) and γ‐Al 2 O 3 . On heating, the mt‐ ZrO 2 (or tetragonal ZrO 2 , t ‐ZrO 2 ) was retained up to 1200°C. At 1380°C the transformation to monoclinic ZrO 2 ( m ‐ZrO 2 ) occurred and the amount of the m ‐ZrO 2 decreased with the increase in Al 2 O 3 content, thus indicating the stabilization of the t ‐ZrO 2 by the Al 2 O 3 , which seems to be explained in terms of the retardation of grain growth.