| Abrasives can be broadly defined as hard materials that
can be used to shape and/or finish other (relatively soft)
materials by grinding or abrading action. According to
archaeological evidences, usage of abrasives by man dates
back to nearly 1500 BC. Also, there are clear proofs that
Egyptians were carrying out grinding operations as early a~
1500 BC.
Initially, natural minerals such as flint, emery, corundum,
garnet and diamond were used as abrasives.
However, since the 1890s synthetic abrasives like silicon
carbide and fused aluminas have almost entirely replaced
the above natural materials.
Amongst the synthetic abrasives, alumina based or aluminous
materials, both fused and sintered, are the most
popular and commonly used. These abrasives are derived
from bauxite and Bayer alumina. The properties of the arc
fus~d aluminas can he varied by controlling the chemical
composition (particularly the amount and type of impurities),
microstructure, grain size and shape and chemicaV
thermal treatment of the grain. In fact, the properties can
be almost customised for a given set of grinding/abrading
applications.
Arc fusion, however, is a difficult and complicated process
to achieve well controlled chemistries, and microhomogeneities
of the abrasive grains. Therefore, the techniques
of ceramic processing and powder metallurgy have
been innovatively adapted to produce sintered abrasives.
Sintered alumina abrasives are materials derived from
bauxite and/or Bayer alumina. The properties can be tailor-
made for a given application by precise control over the
chemical composition and the sintering conditions.
In recent years (1990s), the principles of colloid chemistry
and sintering have been applied to manufacture the solgel
or seeded gel alumina abrasives. In this case, the ultrafine
crystallite size of the grain (making it very tough) and
the high purity of alumina (giving it high hardness) have
resulted in the production of alumina abrasives which exhibit
grinding performances far superior (say 3 to 10
times) to their conventional, fused alumina counterparts.
The salient aspects of the developments in alumina
abrasives are presented in this paper. |
