Preparation and Characterization of Glow Disharge Modifies Titanium Surfaces
The main theme of the present thesis is controlled preparation and characterization of oxidized titanium surfaces. Such surfaces are of general technological interest. Overall goals of this work are to understand the role of the surface properties of titanium when it is used as a biomaterial, and to develop methods to prepare surfaces with carefully controlled properties for applications of titanium in the biomaterial area.
The major part of this thesis work has been concerned with glow discharge plasma preparation of titanium surfaces and associated analyses of these surfaces by surface spectroscopies and microscopy. Preparations were also made by wet-chemical methods (electrochemical polishing and anodic oxidation).
Both the electrochemically prepared and plasma oxidized titanium surfaces have been used in biological evaluation experiments in vitro and in vivo.
A process for plasma preparation has been developed that allows uniform cleaning and subsequent oxidation of cylindrical or screw-shaped metallic samples. The scheme was evaluated by applying Ar plasma cleaning to various types of titanium samples and then reoxidizing them either in an oxygen plasma or by thermal oxidation in O2. The results were monitored by Auger electron spectroscopy and x-ray photoelectron spectroscopy ex situ. By proper control of the process, all traces of previous preparation steps can be removed. Subsequently a new, clean oxide overlayer of stoichiometric TiO2 can be deposited. These oxides have uniform thickness in the range 0.5-150 nm and a low tendency to become contaminated by hydrocarbons in air. Similarly, TiN-films are formed on freshly cleaned titanium in a nitrogen plasma. When exposed to air, these nitrides are covered by an extremely thin oxide.
The above work was extended by a more detailed and sophisticated control of the plasma treatment. A new plasma cell, attached to an x-ray photoelectron spectrometer, allows in situ control of each step in multistep preparations (e.g., cleaning followed by oxidation or nitriding). The system was evaluated by investigating the effect of the plasma process parameters (discharge voltage, gas pressure and gas composition, sample temperature, and treatment time) on the properties of the prepared surfaces.
In this system it was confirmed by XPS measurements, that Ar plasmas give clean surfaces, which can be reoxidized to clean and stoichiometric TiO2. The reduction of such oxides in a mild Ar plasma, and the reoxidation in an O2 plasma or thermally, in O2 gas, were studied by following the core level and valence band spectra by XPS.
In conclusion, glow discharge preparation of titanium implants (or any metallic implant) offers superior flexibility, cleanliness, and control over the end result when compared to thermal, electrochemical or wet-chemical treatments. However, careful process control is required to prevent unexpected and undesired surfaces.