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

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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 0.05-0.2moll -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 Al 2 TiO 5 phase with heterogeneous precipitation of Al 2 TiO 5 ·TiO 2 and 3Al 2 TiO 5 ·Al 2 O 3 eutectics. Al- and Ti-enriched constituents of this structure show hardnesses of 1050-1480 and 300-845H K,0.02 , respectively. The layer growth rate increases with increasing electrolyte concentration, providing a maximum thickness of over 60μm and a surface roughness (R a ) of 3-4μm. 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.

Bibliographical metadata

Original languageEnglish
Pages (from-to)172-184
Number of pages13
JournalApplied Surface Science
Issue number1-4
Publication statusPublished - 15 Nov 2002