Biomedical Engineering Reference
In-Depth Information
a)
Sintered Bead
Here gas-atomized spherical beads are sintered on to the metal part substrate.
Differing size beads produce differing pore sizes with a porosity of about 35%.
Commercially pure titanium beads are used for titanium alloy implants and Co-Cr
beads for Co-Cr implants. Such a coating is shown in Fig. 1.8.
Fig. 1.8
Sintered Bead Surface - 50X
These coatings have been in successful use for more than thirty years and are still
widely used today. Unfortunately, the high temperature sintering process adversely
affects the mechanical properties of the metal to which it is applied. Sintering
temperatures approach the melting point of the substrate. Local melting of the points
of contact between beads, however, occurs before the substrate material melts. This
high temperature substantially reduces (by about one third) the strength of the
substrate by increasing grain growth. Further, the addition of the beads creates local
stress magnification due to the stress concentration effect of the small radii at the
bead connections. Thus, the strength of the part is generally reduced to about only
one-third of its original strength by the addition of a sintered bead porous coating.
Further, sintered bead parts cannot be precipitation hardened.
Still by not coating areas of critical stress thus keeping stress below these lower
values of failure stress the loss in strength can be accommodated by proper design.
An example may be seen in Fig. 1. 9.
