Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
resistance to thermal shock and has application in load-bearing components
such as high-temperature bearings and engine parts. The diffusivity of silicon
nitride is very low and the chief difficulty is in fabricating fully dense material.
Reaction bonded silicon nitride
Reaction bonded silicon nitride (RBSN) is fabricated from silicon powder.
The silicon powder is processed to the desired shape by pressing, slip casting
or other suitable process and then placed in a furnace under a nitrogen
atmosphere and heated to approximately 1400°C. The reaction
3 Si + 2 N 2 → 6 Si 3 N 4
takes place. Approximately 60% weight gain occurs during nitriding, but
less than 0.1% dimensional change, so that excellent dimensional control is
possible upon the finished product. Because of the need to allow access of
nitrogen during processing, the structure of the product will be that of an
interconnecting network of voids, and RBSN is typically only 70–80% of
full density. Because no grain boundary glassy phases are present, RBSN
does not suffer rapid degradation of strength at elevated temperature and it
is useful as a static component in high-temperature applications.
4.3.5 Sialons
Silicon nitride can be ‘alloyed’ with aluminium oxide because the AlO 4
tetrahedron is similar in size to the SiN 4 tetrahedra in silicon nitride. It is not
surprising, therefore, that up to two-thirds of the silicon can be replaced by
aluminium as long as sufficient nitrogen is replaced by oxygen to preserve
charge neutrality.
Sialon is much more resistant to oxidation than silicon nitride because a
surface protective film of ‘mullite’ (3Al 2 O 3 .2SiO 2 ) is developed rather than
the weaker coat of silica formed upon RBSN.
A high density is achieved by the use of yttria or yttria plus alumina as
densifying agents in sintering, by the formation of an yttria–sialon glass in
the grain boundaries. This glass may subsequently be crystallized in the
grain boundaries of the sialon, so that these materials then have very good
creep properties.
4.3.6 Carbide ceramics
Silicon carbide
Reaction-sintered silicon carbide (RSSC) is prepared from a mixture of graphite
and silicon carbide powders, which is infiltrated with molten silicon at