5.4 The principle of laser-induced fixation
Fixing ceramic particles to a metal plate was found possible for the
substances used to improve bone implants. The fine powder of the two
calcium alkali phosphates, GB14 and Ca10, forms a dip coated layer on
the titanium alloy Ti6Al4V. The femtosecond laser pulses do not harm the
surface of the 20 – 40 µm thick ceramic layers if the laser fluence is
below the ablation threshold of Fth = 0.4
J/cm2. The laser radiation is partly absorbed in the
ceramic layer, but mostly scattered on the particles of the layer. The
sufficiently thin layer permits enough radiation to reach the metal
surface, causing a local melting of the Ti6Al4V. This effect was first
found around craters generated by much higher radiation levels. Figure 3
shows an example of such a spot after a strong laser irradiation. Both
pictures have the same scale. These images were taken after the sample
was washed. A simple wash is already possible just by holding the sample
into water. The loosely bound ceramic powder immediately separates from
the metal. In order to ensure a complete removal of the unbound ceramic,
an ultrasonic treatment in water is necessary. After this thorough
“cleaning” and subsequent drying, the ablation of the Ti6Al4V is
visible as a distinct hole, appearing as a brightly reflecting area in
the image of the optical microscope (Fig. 3, left). Around the crater
there is an area that looks different than the rest of the metal
surface. There is a zone with a grainy appearance showing bright spots
on a darker background. More information can be obtained from the
scanning electron microscope (SEM) image (Fig. 3, right). Around the
deep and structured crater there is a large area with a diameter of more
than 100 µm, consisting of coarse grains. The concentration of these
grains decreases radially from the ablation centre. The spatial Gaussian
profile of the laser beam corresponds to the distribution of the
different modifications. In the area of the Ti6Al4V crater the fluence
was high enough to ablate both ceramic and metal. Around the hole the
ceramic grains were not removed. Instead, the ceramic grains in direct
contact with the Ti6Al4V were bound tightly to the metal surface. Only
the unbound ceramic particles were washed off by the ultrasonic
treatment.
Before giving a closer description of the fixed grain layer below in
section 5.5, this newly found method of fixing loose ceramic grains on a
metal surface without damaging or melting the top particle layer itself
is described. Figure 4 illustrates the process in three steps. First a
preliminary fixation by dip coating the metal plate into a water based
slurry is made. The dried powder layer can easily be scratched off the
surface. There is just enough binding strength between the individual
particles, as well as between the particles and the metal, to guarantee
a safe handling for the following step. It is also very important that
the ceramic layer is of uniform thickness, a requirement which is hard
to fulfil with loose and dry powder. Furthermore, the slurry composition
and the drying process need to result in a layer without drying cracks
or surface peeling. A homogeneous layer of powder particles has to
remain in contact with the metal. The second step is the laser
irradiation normal to the surface. The fluence must be adjusted to a
value which does not ablate the ceramic powder, but which is high enough
to melt an underlying metal layer (after passing through the complete
powder layer). This surface melting binds the particles in touch with
the metal. The third and final step is a wash in an ultrasonic water
bath. This is already a good test of the bonding strength between the
remaining ceramic grains and the metal. Due to this fixation by surface
melting, only a thin layer can be bound, having an average thickness of
the order of the particle size.