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.