Mechanism for the κ-Al2O3 to the α-Al2O3 transition and the stability of κ-Al2O3 under volume expansion

The kappa-Al2O3 metastable phase is an important material for producing cutting tools. However, at temperatures above 1000 K the kappa phase transforms into the stable modification alpha-Al2O3. We have investigated mechanisms for this transformation by means of molecular dynamic simulations using pair potentials. We have found that for the temperature range above 1000 K the mean square displacement of the atoms at the free surface changes its behavior drastically. Since, as was calculated, all other possible driving mechanisms of the phase transition such as pressure and/or temperature without a free surface are not sufficient to cause the transition, the free surface is the major factor initiating the unwanted transition. To hinder the transition one has to slow down the diffusion at the free surface. As an alternative to chemical vapor deposition of thin films of kappa-Al2O3 phase at surfaces of cutting tools, it is found that it is thermodynamically possible to obtain kappa-Al2O3 in a stable phase at a volume expansion of the alpha-Al2O3 phase at a negative pressure of about - 40 kbar.