Biomedical Engineering Reference
In-Depth Information
and to describe the effect of ultrasonic vibration on the movement of
particles.
.
Other casting defects such as porosity and clusters caused by
nanoparticle agglomeration, can be significant. For example, gas
trapped between nanoparticles will be released in ultrasonic vibration.
Without an effective de-gassing process, this would be deleterious to the
mechanical properties of bulk material (Cao et al., 2008b).
Solid-phase MMNC ex-situ fabrication processes
In solid-phase MMNC ex-situ fabrication, the nanoreinforcement and metal
matrix are mixed in a solid state. The basic process is to mix ceramic and
metal particles to fabricate nanocomposite particles and then the particles
are consolidated by different methods such as hot pressing, hot extrusion,
cold pressing and sintering. Solid-phase fabrication processes always involve
a number of steps prior to final consolidation. Goussous et al. (2009) also
reported the use of back pressure equal-channel angular pressing to
consolidate milled Al and carbon black powders.
Mechanical milling is widely used to produce well-mixed ceramic and
metal powders. The process means that powders are pre-treated to their
proper size. Treated powders are mixed in the appropriate proportion and
loaded into a mill along with the grinding medium (generally steel balls).
The mix is milled until a steady state is reached. In the mechanical fraction
process, the ceramic particles are continually fractured until the size of the
milled particles reaches a critical value (Zhang, 2004). Mechanical milling
can produce a uniform dispersion of reinforcement particles in the matrix
through a repeated process of cold welding, fracturing, and rewelding, until
the reinforcement particles are well embedded into the matrix particles. The
technology is also widely used in the reinforcement of oxide dispersion
strengthened alloys (ODS) through uniform dispersion of very fine oxide
particles in the metal matrix. The advantages of milling are simplicity,
versatility, and economic viability (Suryanarayana et al., 2001). Besides, in
most milling processes, not only are the nanoparticles normally distributed
but also the grain size is usually at nanoscale due to severe deformation.
Results have shown that even after hot isostatic pressing, the grain size can
still be kept between 100 and 400 nm (Ferkel and Mordike, 2001). Besides
mechanical milling, Nouri et al. (2012) used a combined mechanical milling
and solution method to produce well-mixed MWNTs and Al powders,
which were consolidated to be MMNCs. The friction stir process is another
reported successful method to produce Mg-based nanocomposites (Lee
et al., 2006) in which the nanoparticles were uniformly dispersed.
Hot press, hot extrusion, cold press, and sintering methods are widely
used to consolidate mixed nanocomposite powders. Some modified
