Biomedical Engineering Reference
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extensive plastic deformation accompanied with micro-plowing and micro-
cracking perpendicular to the sliding direction.
In addition, shear delamination and smearing with transfer of material from
the wear surface to the counterpart material was also observed. Surface topogra-
phy analysis showed that the roughness of the metastable Ti alloys tested were
higher than that for the Ti-6Al-4V alloy [39]. Also the TNTZ alloys were sub-
jected to different heat treatments in order to study the effect of heat treatment/
microstructure on wear resistance. The wear loss measurements indicate that
heat treatment is not a practical way to improve the wear resistance of the Tita-
nium alloys. The TNTZ and the Ti-6Al-4V alloys were then subjected to thermal
oxidation to study the effect of an oxide layer on wear resistance. Results indicate
that the presence of an oxide layer greatly improves the wear resistance of both
TNZT alloys as well as that of Ti-6Al-4V [40].
In addition to base titanium alloys, previous research has been devoted to
the study of hard boride reinforcements in the Ti matrix. Sliding wear properties
of TiB/Ti-6Al-4V metal matrix layer fabricated using laser cladding and laser
melt injection were evaluated in a previous study [41] and showed an improve-
ment in the tribological properties of this MMC layer. The results of the wear
studies reported highlighted the excellent wear resistance of the Laser engi-
neered TiB/Ti-6Al-4V MMC layers [41]. Hence these composites combine the
high strength and stiffness of the borides with the toughness and damage toler-
ance of the Ti-alloy matrix and offer attractive properties including increased
stiffness and substantially enhanced
wear resistance
. Using the laser-engineered
net shaping (LENS™) process, it has been successfully demonstrated that
in - situ
TiB reinforced Ti alloy composites can be fabricated in a single step
[42,43]. These laser-deposited composites exhibit a substantially refi ned distri-
bution of reinforcing TiB precipitates as compared to their conventionally pro-
cessed (e.g., ingot processing) counterparts. Furthermore, by employing novel
near-net shape processing technologies, such as LENS™, it becomes possible
to not only rapidly and effi ciently manufacture custom-designed implants, but
also to functionally-grade those to exhibit required site-specifi c properties.
Therefore, the current effort is directed towards the processing, characterization,
and, measurement of the tribological properties of laser-deposited, boride rein-
forced, TNZT composites based on the nominal matrix composition Ti-35Nb-
7Zr-5Ta (all in wt %). Sliding wear resistance and friction behavior were
examined under a wide range of post-processing and testing conditions such
as varying the concentrated interfacial load, the effect of ex-situ annealing on
the tribological properties, and the role of counterpart material. The tribologi-
cal properties have been correlated to the surface microstructure under these
conditions.
9.3.4.2 Titanium Boride Reinforced Ti-Nb-Zr-Ta Composites.
LENS ™
deposition of the metal-matrix composites were carried out from a powder feed-
stock consisting of a blend of elemental pure Ti, Nb, Zr and Ta powders with
Titanium Boride (TiB
2
) mixed in the ratio of 51 wt% Ti + 35 wt% Nb + 7 wt%
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