THERMODYNAMICS OF TETRAGONAL ZIRCONIA FORMATION IN A NANOLAMINATE FILM

Zirconia-alumina transformation-toughening nanolaminates were fabricat ed by reactive sputter deposition. The average crystallite size and vo lume fraction of each zirconia polymorph were determined by x-ray diff raction. The volume fraction of tetragonal zirconia, the phase necessa ry for transformation toughening, was found to strongly depend upon th e zirconia layer thickness. An end-point thermodynamics model involvin g hemispherical cap zirconia crystallites was developed to explain thi s phenomenon. In excellent agreement with experimental results, the mo del predicts that unity volume fraction of tetragonal zirconia is prod uced in the nanolaminate when the zirconia layer thickness is less tha n the radius at which a growing zirconia crystallite spontaneously tra nsforms to the monoclinic phase. (C) 1996 American Institute of Physic s.