The effect of contaminants from processing equipment on the heat treatment of Mg-PSZ

During bulk milling and processing of magnesia partially stabilized zirconia (Mg-PSZ), iron, chromium, and molybdenum for stainless steel fixtures and piping as well as tungsten from the spray drier nozzle and valves may contaminate a ceramic slurry. It is known that these elements cause degradation of the ceramic components during heat treatment but the mechanism is uncertain. This study used a systematic comparison to help identify possible mechanisms. Selected metal oxides (1 mol%) were added to high purity zirconia and magnesia powder and processed. Isothermal heat treatments at 1400 and 1320 deg. C. for 0.1, 1, 2, 5, and 10 h were conducted in order to produce a temperature-time-transformation (TTT) diagram for comparison with undoped Mg-PSZ. There are two possible mechanisms for degradation. The first is that all the metal oxide dopants quickened the precipitate growth rate, thereby shifting the nose of the TTT diagram to shorter times. The chromium, molybdenum and tungsten oxides enhanced grain boundary monoclinic growth with increasing time at temperature. 13 refs., 1 tab., 7 figs