LASER PROCESSING OF ORTHOPEDIC BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 312

1. As deposited LENS ™ TNZT + 2B has borides precipitates that contain
compositionally segregated regions of higher Ti(Zr) or Ta(Nb) weight
percent,
2. Using Si 3 N 4 balls, there is TiB precipitate pullout resulting in accelerated
third body abrasive wear with higher friction coeffi cients.
3. Using 440C stainless steel balls drastically improved the friction coeffi -
cients of as deposited TNZT + 2B, eliminating the effect of “three body
abrasive wear” and resulting in superior wear resistance when compared
to Ti - 6Al - 4V ELI.
4. Oxide fi lm improves the wear resistance of both TNZT + 2B and Ti-6Al-
4V ELI.

9.4 CONCLUSIONS

The present article demonstrates the feasibility and advantages of using laser
engineered net shape processing (LENS™) for orthopedic biomaterials. This
relatively new manufacturing technology allows for the deposition of metallic
biomaterials with refi ned microstructures and in some cases preferential crystal-
lographic texture, which in turn lead to enhanced mechanical properties. In this
article, the specifi c focus has been on the laser deposition of newer generation
beta titanium alloys, such as those based on the Ti-Nb-Zr-Ta system. These newer
generation alloys not only exhibit a much lower elastic modulus ( ∼ 50 - 60 GPa) as

compared to conventionally used Ti - 6Al - 4V ( 110 GPa), but also consist of com-

pletely biocompatible alloying additions with minimal toxicity. Along the growth
direction during deposition, the LENS ™ deposited Ti - 35Nb - 7Zr - 5Ta alloys
exhibit a higher strength as compared with alloys of similar composition pro-
cessed via more conventional techniques, while still maintaining a low modulus.
The corrosion resistance of these laser-deposited alloys is also comparable to, if
not better than, conventionally processed Ti-6Al-4V ELI and commercially pure
Ti (Grade 2). Preliminary in vitro biocompatibility studies also indicate that these
laser-deposited alloys are comparable to conventionally used alloys and might
actually lead to better cell differentiation. Furthermore, by introducing a homo-
geneous distribution of hard ceramic precipitates, such as borides, into the same
metallic Ti - 35Nb - 7Zr - 5Ta base matrix to form metal - matrix composites, the wear
resistance of these alloys can be substantially enhanced. A higher wear resistance
would allow the use of these materials in a number of applications including the
femoral head part of the hip implant, or in other words, the ball part of the ball-
socket joint. This in turn leads to the possibility of manufacturing a unitized, com-
positionally and functionally-graded, femoral (hip) implant with site specifi c
properties. In such an implant the femoral stem would consist of a hollow Ti-
35Nb-7Zr-5Ta based structure with a low elastic modulus, while the femoral head
would be graded from a core consisting of the softer and tough base Ti-35Nb-7Zr-
5Ta alloy while the outer surface of the head consisted of wear-resistant boride-

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