Biomedical Engineering Reference
In-Depth Information
1.2
Design of thermal shock resistant and flame
retardant ceramic nanocomposites
Various design methodologies have been used by materials scientists for the
development of thermal shock resistant and flame retardant ceramic
nanocomposites. In such design it is necessary to consider the cause and
effect of various parameters such as processing, the functions of selective
ingredients, interface conditions, grain sizes, grain boundary sliding, creep
resistance, oxidation resistance, densification of ingredients, dispersions,
strengthening and toughening mechanisms, thermal expansion mismatch at
the grain boundaries, etc. Materials scientists are considering all such viable
parameters in the design of various types of ceramic nanocomposite
materials that will be particularly effective in thermal shock and flame
retardant applications.
1.2.1 Design of sintering techniques
Ceramic nanocomposites were first successfully designed by the chemical
vapour deposition (CVD) (Niihara and Hirai 1986). TiN particles or
whiskers of diameter approximately 5 nm were dispersed within Si
3
N
4
matrix grains. The CVD process is a very suitable method to disperse nano-
size second phases into matrix grains or at grain boundaries. However, this
process is not applicable for the fabrication of large and complex-shaped
components for mass production and it is also very expensive. There are
some low-cost but effective processes to obtain nano-sized ceramic powders
and nanocomposite powders, such as sol-gel, micro emulsion, autoignition,
co-deposition and high-energy ball milling (HEBM). One of the principal
problems is the ability to consolidate nanopowders to high relative density
without grain growth. To obtain dense bulk nanoceramics, it is essential to
decrease either the sintering temperature or retaining time at the highest
point, or to employ hot-pressing (HP), gas pressure sintering (GPS) or fast
consolidation techniques such as microwave sintering and spark plasma
sintering (SPS). Pressureless sintering can be employed to fabricate complex
shapes, and significant stabilization can be achieved by using a protective
powder bed while the products generally indicate low density and the
process requires large amounts of additives for densification (Munakata
et al. 1986). In hot-pressing, powder mixtures with additives are heated to
high temperatures under an applied uniaxial pressure. Traditional hot-
pressing uses 20-30MPa pressure, which enhances both the rearrangement
of particles and grain boundary diffusion. Hot-pressing offers the ability to
fabricate dense products, but also limits the products to simple shapes
(Hwang and Chen 1994). Other promising established techniques are
isostatic pressure and hot isostatic pressing (HIP). These techniques are very
