Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation |
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Authors: | A. L. Yerokhin A. Leyland A. Matthews |
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Affiliation: | Research Centre in Surface Engineering, University of Hull, Cottingham Road, Hull HU6 7RX, UK |
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Abstract: | 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 3Al2TiO5·Al2O3 eutectics. Al- and Ti-enriched constituents of this structure show hardnesses of 1050–1480 and 300–845 HK, 0.02, respectively. The layer growth rate increases with increasing electrolyte concentration, providing a maximum thickness of over 60 μm and a surface roughness (Ra) of 3–4 μm. 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. |
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Keywords: | Plasma electrolytic oxidation Aluminium titanate Kinetics Structure Titanium alloy |
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