Biomedical Engineering Reference
In-Depth Information
substrate and friction stir processing to consolidate the coating. The
drawbacks for thermal spraying are that it produces a relative low density, a
softened heat-affected zone, and is a difficult process for bulk material.
6.4.2
In-situ
MMNC fabrication
In-situ techniques involve a chemical reaction resulting in the formation of a
very fine and thermodynamically stable reinforcing ceramic phase within a
metal matrix (Hsu et al., 2006). To produce reinforcement in the in-situ
fabrication process, either elevated temperature or mechanical energy is
needed to accelerate the reaction rate. Most of the technologies can be
categorized as mechanochemical, i.e. combined process of chemical reaction
and mechanical compaction.
Friction stir processing (FSP) can be an in-situ production process. For
the production of intermetallic reinforced in-situ composites, FSP can
provide: severe deformation to promote mixing and refining of the
constituent phases in the material; elevated temperature to facilitate the
formation of intermetallic phase in-situ; hot consolidation to form a fully
dense solid (Hsu et al., 2006).
High-energy ball milling and mechanical stirring processes are also used
to contribute energy for the in-situ chemical reaction process. Due to the
mechanical energy released in the process, chemical reactions and phase
transformations take place (Ying and Zhang, 2000). Ying and Zhang
successfully used this technology to fabricate Al 2 O 3 /Cu metal matrix
nanocomposite. In Ying and Zhang's experiment, mechanical alloying was
used to produce Cu(Al) solid solution powder. The powder was then milled
with CuO powder, with a reaction between Cu(Al) and CuO occurring.
Al 2 O 3 nanoparticles were observed. Yamasaki et al. produced TiN/Fe by
milling Fe-Ti in a nitrogen gas atmosphere (Yamasaki et al., 2003).
Besides mechanical alloying, molecular level mixing, electroless deposi-
tion, and other thermal-chemical methods have been reported as in-situ
nanocomposite powder routes. Daoush et al. (2009) reported on CNT/Cu
nanocomposite powders fabricated by electroless deposition copper on CNT
and the powders were then sintered to make MMNCs. Shehata et al. (2009)
used a reaction process with a mechanical milling process and successfully
distributed Al 2 O 3 particles uniformly in a Cu matrix. The abrasive wear
resistance of the Cu matrix was improved by the addition of Al 2 O 3
nanoparticles. In a liquid reaction process, Tu et al. (2002) managed to
fabricate TiB 2 in a melting Cu matrix by the reaction [Ti]+B 2 O 3 +3C
￿ ￿ ￿ ￿ ￿ ￿
TiB 2 +3CO. After the reaction process, the molten metal was rapidly
solidified and the particulates uniformly distributed.
Advantages of the in-situ process are that the reinforced surfaces
generated tend to remain free of contamination. The particles are usually
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