Metals - Orthopaedic Biomaterials in Research and Practice - page 191

Cast Sintered

Direct Preformed

FIGUre 7.10 surface structures.

Preformed structures . The last approach has been to form pads of
material, either in a random mesh or felt or in an orderly woven
structure, and then to bond the resulting “preform” to the surface
by diffusion. This approach has the advantage over a pure sintering
process in that the resulting coatings may be made much stronger,
and shorter periods at high temperature are required, thus better
preserving the properties of the base material. A design disadvan-
tage is the difficulty in coating complexly shaped surfaces.

These surface structural modification processes have profound con-
sequences on the behavior of the finished device for two reasons.

1. The interface between the surface structure and the base material
produces regions of stress concentration, greatly degrading fatigue
resistance. This effect may be reduced by careful design and con-
trol of processing but cannot be eliminated.

2. Thermal requirements of the processes for surface structure for-
mation may severely degrade the properties of the base material.
This is especially pronounced in the use of sintering for which tem-
peratures must be maintained at a high level to permit sufficient
diffusion between substrate and coating for adequate bonding.

It appears experimentally that all surface structural modification
reduces fatigue resistance. This reduction has been reported to be up to
75% in some materials systems. Significant design and process develop-
ment efforts are under way to reduce this effect to an absolute minimum,
but it will continue to be with us for the foreseeable future.

Comparison of orthopaedic alloys

It is clear that composition, properties, and processing of these alloys
vary widely. It is difficult to make generic comparisons because of the
previously stated close relationship between mechanical design and
selection of materials. However, as a general guide, Table 7.11 presents