Biomedical Engineering Reference
In-Depth Information
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resistance to oxidation.
Milling of soft metals/alloys and hard ceramic materials leads to the
formation of valuable composite powders with important new features.
Changing the ratio of the cermetal components produces final composite
products with widely different properties. These materials have much higher
mechanical, temperature and aggressive media resistance properties
compared to ceramics properties. The hardness of cermetals increases with
decreasing particle size (Chang et al., 1999; Zhang et al., 2000; Portnoy
et al., 2002).
12.2 Composite powder formation: bottom-up and top-
down techniques
Composite powders can be produced by two techniques: bottom-up and
top-down.
The first method is based on building nanostructures atom-by-atom,
layer-by-layer. These chemical processes could take place in liquid, solid as
well as in gas phases (Tjong and Chen, 2004). Examples of these are the
sol-gel method, melt spinning-melt quenching (MQ), chemical vapour
deposition (CVD), physical vapour deposition (PVD), plasma ablation,
laser pyrolysis, and so on. Unfortunately, there are often some disadvan-
tages, for example multi-stage processes, the high cost of the alkoxide pre-
cursors, solvent evaporation and necessity for thermal treatment at high
temperatures to provide coarse-grained products.
The top-down technique begins with macrostructured materials and uses
mechanical, chemical or other forms of energy to 'break' them into smaller
pieces. However, it is very important to develop methods which minimize
damage to the environment. One promising candidate is mechanochemistry,
often referred to as 'green' processes. Mechanical energy can be easily
explored for chemical syntheses of new functional materials like composite
powders. No doubt, this method is fast, economical and gives high-purity
products, which can often take nanostructure forms (Heegn et al., 2003). In
mechanochemical syntheses, the course of solid-state reactions can be
effectively controlled and regulated by choosing the precursors of syntheses
and/or milling variables.
12.2.1 Mechanical treatment
Mechanical treatment of solids uses mechanical energy through high-energy
ball milling, abrasion, fracture or welding to produce new materials or
generate products with desired features, for example powder mixtures
consisting of metal and ceramics (Courtney, 1994; Boldyrev et al., 1996).
