COOLING RATE EFFECTS ON THE TETRAGONAL TO MONOCLINIC PHASE-TRANSFORMATION IN AGED PLASMA-SPRAYED YTTRIA PARTIALLY-STABILIZED ZIRCONIA

This study undertook to define the progression of the phase separation during aging of plasma-sprayed 8 wt.% Y2O3 partially stabilized ZrO2 using a matrix of aging times and temperatures (1288-degrees-C (2350-degrees-F), 1371-degrees-C (2500-degrees-F) and 1482-degrees-C (2700-degrees-F) for 100 h and 1288-degrees-C for 500 h and cooling rates. Both bulk coatings and crushed specimens were evaluated. Residual stress was found to be responsible for the partial suppression of the tetragonal to monoclinic (t --> m) transformation during air quenching. This study illustrated the importance of cooling rate on the t --> m transformation, the thermal expansion behavior and the flexure strength of the material. Thus, successful coating design for high temperature and/or long duration thermal exposure requires consideration of the cooling cycle and its effect on material behavior.