Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation

The paper reports on a systematic investigation into the effects of process parameters on the growth kinetics and associated changes in the structure, phase composition and mechanical properties of surface layers formed on Ti-6Al-4V alloy by plasma electrolytic oxidation (PEO) treatment in 0.05-0.2 mol l(-1) solutions of sodium aluminate. Methods of gravimetric, SEM and XRD analysis, as well as microhardness and scratch testing, are employed to investigate mass transfer and phase-structure transformations in the surface layer. The probable mechanisms of layer formation are discussed, which comprise electrochemical oxidation of the Ti-electrode by OH- anions, complimented by chemical precipitation of Al(OH)(3) and plasma-induced transformations in the surface discharges. Running with a total yield efficiency of 20-30%, these processes lead to the formation of predominantly the Al2TiO5 phase with heterogeneous precipitation of Al2TiO5.TiO2 and 3Al(2)TiO(5).Al2O3 eutectics. Al- and Ti-enriched constituents of this structure show hardnesses of 1050-1480 and 300-845 H-K,H-0.02, respectively. The layer growth rate increases with increasing electrolyte concentration, providing a maximum thickness of over 60 mum and a surface roughness (R-a) of 3-4 mum. Increasing the electrolyte pH from 12.0 to 12.8 results in smoothing and thickening of the surface layer but a lower sample weight gain, associated with an enhancement of the Ti electro-oxidation process. Morphological changes during PEO formation of the surface layer include gradual transformation of the original fine grained but porous structure into a dense, fused morphology which is adversely affected by discharge-induced thermal stresses, causing a degradation of the layer adhesion strength. (C) 2002 Elsevier Science B.V. All rights reserved.