Biomedical Engineering Reference
In-Depth Information
Chemical Vapor Deposition (CVD) is another very widely-used method for
titanium nitride coating. CVD relies on the reaction of metal halide (titanium
chloride) gas along with nitrogen. The addition of nitrogen lowers the free energy
of the reaction products so that titanium nitride is formed in preference to the
titanium metal.
21.6.1 Substrates
Titanium alloy (Ti6Al4V) (conforming to ISO 5832—Implants for surgery—
metallic materials: Part 3: Wrought titanium—6 aluminum—4 vanadium alloy)
was chosen as substrate for titanium nitride with artifi cial heart valve cages as one
of the prospective applications.
21.6.2 TiN Coating Processes
All samples went through a thorough cleaning process prior to coating. PVD with
vacuum evaporation was used for the coating of the samples in this study. Uniform
coating thickness in the range of one to two microns was achieved. In evaporation
process, the coating material is heated in a vacuum above its boiling point. The
molecules in the vapor phase condense on the substrate to form the coating.
21.7 COATING CHARACTERIZATION
Raman spectra of the DLC coatings were recorded by illuminating the samples
with a 40 mW power laser whose wavelength was 514.52 nm. The Raman spectra
of the DLC coatings indicated a broad maximum around 1525 cm
− 1
and a shoul-
der like feature at 1223 cm
− 1
but did not show any sharp features normally
observed in the case of pure diamond and graphite (Krishnan et al., 2002).
Energy dispersive X-ray microanalysis (EDX) was carried out on the TiN
surface (Figure 21.1) and the presence of titanium nitride was confi rmed. The
elemental composition of the surface was estimated from the EDX as titanium
(95.6%), aluminum (0.16%), vanadium (0.81%) and nitrogen (2.64%).
21.7.1 Surface Roughness Estimation
The major factors that affect the blood compatibility of any material are its
surface characteristics in terms of surface roughness, free charge, and impurity
levels. The surface roughness of the samples was studied using optical profi lo-
meter (Talysurf CLI 1000, Taylor Hobson, UK). The average surface roughness
was estimated from area profi les of six independent 0.2 mm
×
0.2 mm zones, each
acquired with 1.0
m spatial resolution. The waviness of the surfaces was cor-
rected based on the methods described in International standards (ISO 4287 and
ISO 4288). Seven equidistant profi les were extracted from the waviness-corrected
surface and the average and standard deviation of the Ra values estimated. A
μ
Search WWH ::
Custom Search