Aims: The main objective of the research was to investigate alternative processes, respect to hydroxylapatite plasma spray coating, in order to obtain metallic bio-materials presenting good osteointegration ability. An innovative process consisting of mechanical and thermochemical treatments was tested and a surface and mechanical characterization performed on treated samples.
Methods: The material investigated was the Ti-6Al-7Nb alloy. The surface modification process consists of grit blasting, passivation, alkali etching and thermal treatment performed in air or in vacuum. Crystallographic structure was investigated by XRD and TEM. Surface morphology and composition were assessed by SEM, EDS and AES analysis. Bioactivity was tested by soaking in standard SBF solution. Metal ion release measurements were performed by GFAA-ICP technique on withdrawn solution after soaking samples in SBF. Scratch and fatigue tests were performed as mechanical characterization of the material.
Results: The alkali etching strongly modifies the surface morphology of titanium and its alloys producing a microporous layer and a drastic increment in surface wettability. The use of previous passivation treatment modifies the surface crystallographic structure, forms a graded interface between the surface and the substrate, enhances the surface layer adhesion and scratch resistance, increases the corrosion resistance of the material and causes a low metal ion release. The use of a vacuum atmosphere during heat treatment inhibits rutile formation and scratch tests evidenced low damage on it. During soaking in SBF the formation of a reaction layer and of precipitated crystals containing Ca and P was detected on the treated samples. The precipitate morphology resembles that of apatite. The fatigue strength was 260 MPa for the treated series, while it was 460 MPa in the case of the grit blasted series without any additional treatment and therefore significantly higher.
Conclusions: It can be concluded that the surface of treated samples shows chemical, structural and morphological modifications. The passivation pre-treatment causes the formation of different crystallographic phases and of a smoother interface with the substrate. The treated samples evidenced a quite low metal ion release and interacted with SBF solution, showing a moderate bioactivity. The disadvantage of this process is the decrease in fatigue strength. This aspect suggests that when surface etching and modifications are performed with the aim of enhancing metal osteointegration ability, a careful investigation of their influence on the fatigue resistance must be performed.
