5.5 Surface characterization of laser-treated samples
A close inspection of the grainy surface after the laser treatment and
washing verifies the existence of bound ceramic particles. No pure metal
surface can develop such a modification under laser irradiation (compare
Fig. 1). Individual particles side by side are visible with the optical
microscope and the SEM. Figure 5 illustrates the possibility to use the
fixation method for covering large surface areas by moving the sample
through the laser beam. Here parallel lines of fixed ceramic particles
were produced. The left side of the image shows the GB14 surface after
the laser treatment with a fluence slightly above 0.4
J/cm2. Along the centre of the laser beam path the
maximum intensity causes some ablation in the ceramic powder, visible as
dark lines in this microscopic image. The sample under the microscope
was illuminated from the left side, emphasizing the partial roughness of
the original surface. Nevertheless, the right side of Fig. 5 shows the
fixed ceramic powder after the excess GB14 was washed away. Fixed grains
are bright in front of the darker metal. The bound particles seem to be
fixed with a very constant density along the lines. The regular density
variation perpendicular to the lines is due to an incomplete overlap of
the parallel lines of the laser irradiation. A reduction of this
distance results in an even higher homogeneity.
Another example clearly shows how simple the fixation method works.
Figure 6 depicts a sample which was only covered in the upper part with
dip-coated Ca10. The lower part is pure metal. All images show the
sample after it was moved through the laser beam (vertical movement) and
after washing. From the left to the right only the magnification of the
images is enhanced. The fluence near the ablation threshold for the
ceramic first caused a strong ablation in the Ti6Al4V (Fig. 6a,b,d,e)
which has a lower ablation threshold. Where the laser beam touched the
edge of the ceramic layer, the metal ablation was locally enhanced (Fig.
6b,c) serving as a mark here. Further following the trace of the laser
beam upwards, a streak of fixed Ca10 grains is visible.
The fixation works best at the laser fluence, as close as possible, but
slightly below the ablation threshold of the metal. This is strong
evidence for the assumption that a local melting of the metal binds the
particles.
Figure 7 contrasts the Ti6Al4V without and with fixed Ca10. It is the
same sample as in Fig. 6. The two pictures from the optical microscope
(Fig. 7a,b) and the two SEM images (Fig. 7c,d) have the same scale,
respectively. The remaining roughness of the grinded Ti6Al4V plates
(Fig. 7a,c) does not disturb the Ca10 fixation. The grains with a
diameter in the range of 1 - 2 µm are big enough to homogeneously cover
the small irregularities of the metal (Fig. 7b,d). The lower part of the
SEM image (Fig. 7d) with the bound ceramic includes the previous coating
edge, proving the direct and very abrupt transition between metal
ablation and ceramic fixation.
The four pictures with different magnification in Fig. 8 present another
sample of fixed Ca10, partly with a higher resolution compared to Fig.
7. It is clearly visible that the Ca10 particles are sharp-edged to make
clear that their shape and structure was not changed by the laser
treatment. This becomes even more plausible when taking into account
that during the irradiation these fixed grains were still shielded by
other particles above. The top layer of up to 40 µm thickness is not
ablated, not melted, and possibly remains unaltered. Thus the bottom
layer is most likely unharmed, too. The radiation reaching the grains at
the metal surface travelled through a layer as thick as at least 20
times the diameter of the average particle. A slightly different
perspective of the same sample as in Fig. 8 is shown in Fig. 9, only
tilted by 40 degrees from the normal. The perspective view underlines
the remaining roughness of the surface, a property regarded as
supportive for cell growth as desired for bone implants.
The necessary proof that the chemical composition of the fixed particles
is unaltered can be seen in the measurements of energy dispersive X-ray
spectroscopy (Symietz, 2011). Figure 10 is an example of an energy
dispersive X-ray spectroscopy (EDX) measurement, integrated over the
complete GB14 surface shown in the picture inset. Signals from the metal
are also detected and are due to the existence of only a “monolayer”
of ceramic particles bound to the Ti6Al4V. But the ratio between the
counts from the ceramic elements supports qualitatively an unchanged
GB14 composition.
So far one can assume that the proper material is fixed unharmed.
Another most important question needs to be answered concerning the
unknown bonding strength. Ultrasonic cleaning shows that the particles
are fixed quite well. An additional test is the scratching of the fixed
grain layer. A diamond tip scratching under defined forces over the
surface of a sample of Ca10 fixed to Ti6Al4V was performed. Figure 11
presents pictures from the optical microscope of three traces of the
diamond tip. The mechanical forces were one, five and ten Newton. One
can clearly recognize the fixed grains as little dark spots. They do not
disappear inside the visible trench the diamond tip has cut at the two
lower forces. At F = 10 N though, there seems to be a very shiny and
smooth track dug deep into the metal. One is led to believe from this
picture that the Ca10 grains are removed from the metal. EDX
measurements prove that this is wrong. There is a clear EDX signal of
all the elements Ca10 consists of, not just next to the scratch but
everywhere inside the seemingly polished trace. In the optical
microscope just the topography is seen, the chemistry is invisible to
the eye. The fact that Ca10 still exists in all the scratch lines shows
that the particles are overrun by the diamond tip and pressed inside the
metal. The ceramic grains must remain intact, because if they had been
destroyed and spread along the direction the tip is pulled, a linearly
smeared out and stretched signal from the debris would appear. Instead
the EDX count distribution is the same inside and outside the scratch
lines. If and how much the particles are overrun and partly covered by
the metal cannot be seen directly. A thin covering of metal over the
ceramic would go undetected by EDX because of the depth sensitivity. The
most important message from this test is that such a strong mechanical
friction does not remove the fixed particles from the metal.
Another experiment led to more proof of the high binding strength. It is
intended to look at the contact line between the fixed grains and the
metal more closely. The substances were not suitable for a polishing
procedure. Too much material was smeared over the surface, disturbing
the delicate transition zone. Instead, the freeze fracturing of a sample
resulted in interesting SEM images. Figure 12 presents a sample,
actually the very same one as depicted in Figs. 8 and 9, but here it was
additionally covered electrolytically, first with a very thin gold
layer, followed by a thick layer of nickel. This protective cover layer
was meant to stabilize the thin Ca10 layer for breaking the sample
plate. The sample was made as thin as possible by producing a saw cut
from the side of the titanium alloy. Immediately after cooling in liquid
nitrogen, the sample was broken at the predetermined breaking point. The
Ti6Al4V looked distorted at the edges, caused by the mechanical force
during the break. This made a detailed study of the surface impossible.
Instead the Ca10 grains are still bound to the Ti6Al4V under some big
portions of the nickel cover that had come off and remained high above
the ceramic particles. The striking similarity to Fig. 9 shows that the
direct cover of gold and nickel, grown in contact to the Ca10 particles
and unintentionally being bent upwards, did not pull the Ca10 away from
the titanium alloy. The fixation to Ti6Al4V was stronger than the
binding to the new nickel cover.
All these results show that the calcium alkali phosphate powder can be
fixed to Ti6Al4V, and that the fixation is very strong. It remains to be
proven that the titanium alloy itself does not lose its mechanical
stability. This will be documented below in section 5.6.