Phase and microstructural development in alumina sol-gel coatings on CoCr alloy

Phase transformation of gamma-Al2O3 to alpha-Al2O3 in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above similar to 1200 degreesC. Addition of alpha-Al2O3 seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the alpha-Al2O3 phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of alpha-Al2O3 grains at the expense of the precursor fine-grained gamma-Al2O3 matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was similar to 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol. (C) 2004 Kluwer Academic Publishers.