Abstract:
The advantages of plasma sprayed hydroxyapatite coating of medical
implants are rapid adhesion of the implants onto the bone with long term
biocompatibility. In addition, plasma spraying techniques are the most convenient
methods to easily and rapidly produce such coatings. However, there are some
inherent problems such as weak bond strength between coatings and metal substrates.
Moreover, the relationship between spray parameters and the resulting structures and
properties are not well understood at present. The aim of this research is to study the
effect of spray parameters on the structures and mechanical properties of the
coatings. The parameters of the coatings studied include the effect of hydrogen flow
rate at 2, 3 and 4 fe/hr and the powder feed rate at 25-30, 40-45 and 75-80 g/min.
The influences of surface preparation techniques as well as surface roughness in the
range 1.5-2.0, 4.5-5.0 and 10.0-12.0 um were also studied together with the
influences ofpassivation treatment and preheating of substrates.
The results showed that hydroxyapatite powder melt more easily when
hydrogen flow rate was increased. This is evident from higher average hardness and
3 lower porosities III the coatings. At hydrogen flow rate of 3 ft3/hr, maxImum
thickness was achieved in the range between 300 to 600 um. X-ray diffraction
analysis showed that the phase of hydroxyapatite remained unchanged under our
experimental conditions, only slight line-broadening was found. The results also
showed that increasing surface roughness yielded higher bond strength. Surface
preparation using alumina grit showed higher bond strength than those using steel
grits at the same roughness. In this experiment, it was found that the highest bond
strength of 31.4 MPa 'was achieved when the stainless steel surface was grit blasted
using alumina to 4.5-5.0 um. Passivation of stainless steel substrate resulted in
icrease in bond strength significantly from about 14.3 MPa for the non-passivated
samples to about 31.4 MPa. Preheating of substrate led to lower bond strength.