Anodization constitutes a surface modification route that allows the design of the structure, chemistry and electronic properties of the films formed by carefully selecting process variables. This versatility is used to modify the surface of permanent titanium implants to improve osseointegration rate. Post anodization heat treatments are often performed to improve the crystallinity of the surface films and this leads to increase of bioactivity. However, thermal treatments may alter not only the crystal structure of anodized oxide film, but also induce growth of thermal oxide or alter the structure of the anodic films.
Since anodic films on titanium may be inhomogeneous and porous, the effect of air heat treatments on such complex structures was evaluated in this work.
Flat specimens of commercially pure titanium Grade 2 were used. Anodising was performed in a two-electrode cell with Pt wire as counter electrode. Two acidic solutions were employed, i) TP: 1 mol L -1 H3PO4, ii) TPF: 1 mol.L -1 H3PO4 + 0.3 % HF; for potentiostatic anodization. Post anodization heat treatments at 553, 623, 673 and 723 K during 3 hours and cooling in furnace were conducted.
The surface morphology of the specimens was observed by scanning electron microcopy. The crystalline phases were determined by Raman spectroscopy and graze angle X-ray diffraction. Anodic polarization curves and electrochemical impedance spectroscopy (EIS) tests were performed in borax solution.
RESULTS AND DISCUSSION
DRX and Raman spectroscopy were complementary used to analyse the crystallographic phases detected and its spatial distribution. Not crystalline phases were detected on mechanical polished titanium (T), anatase was detected in most sites on TP and TPF presents a partial crystalline presence, with sites with anatase, rutile and amorphous film.
Changes in colour of heat treated samples are observed in all cases, indicating that a certain degree of change in the surface films has occurred. With Raman spectroscopy an increase in crystalline phase amount was detected in T, TP and TPF samples.
The oxide film formed in phosphoric (TP) behaves as a barrier, increasing the corrosion potential, diminishing the current density and increasing the total impedance. On the contrary, anodic films growth in phosphoric and fluoride solution, presents in anodic polarization experiments a similar response than non-anodized samples, but EIS evidences a complex structure of TPF anodic films characterized with two or even three time constants.
Thermal treatments in air have different effects on the modified surfaces. On polished titanium, the growth of a thermal oxide is evidenced both on anodic polarization curves and EIS. The thermal oxide layer behaves as a barrier to electronic transfer, acting in a similar way than anodic films. In TP films, the thermal treatments do not produce differences in current density during anodic polarization, while with EIS changes in the inner (compact) and outer (more porous) part of the film are evidenced. Finally, in TPF samples, thermal treatment results in deleterious effects on the barrier efficiency of the anodic layer. Porous TPF films behaves in similar way than polished titanium in anodic polarization tests, since the thermal treatment induce diminish in current densities. However, EIS results indicates a deleterious effect of the thermal treatment on the anodic layer, evidenced by a diminish in the total impedance in Bode plots, and the appearance of a capacitive film governing the EIS response. The electrochemical evidence suggests the rupture of the anodic film of TPF during thermal treatments, corroborating Raman and GDRX results and also SEM evidence.
It may be concluded that careful analysis of changes in structure both on the surface and in the substrate inner layer may be due in order to evaluate the influence of thermal treatments on the in vivo or in vitro response of surface modified titanium as a permanent implant, since is clear in this report that not only crystallization of amorphous portion of the surface oxide, but also growth, re-ordering and even destruction of anodic films may be occurring.