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SYNDRILL
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SYNDRILL
(right)
consists of a thin layer of polycrystalline diamond
integrally sintered at ultrahigh pressure and high
temperature to a cemented tungsten carbide backing. It is
available in various configurations, including discs up to
50.8mm in diameter and 8mm in thickness, all with a PCD
layer thickness of 0.8mm. Two types of SYNDRILL are
produced, both with a diamond grain size of 20-40µm:
SYNDRILL SRC is the standard
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SYNDRILL PCD products for rock
drilling
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material, while SYNDRILL SSP has
a slightly greater thermal stability. For further details of the
SYNDRILL product range, see the PCD and PCBN Product Catalogue,
2.1 and the SYNDRILL product information leaflet, 3.1.3
Fig. 1 (below)
is a scanning electron micrograph of a polished section through
the diamond layer of a SYNDRILL blank, showing its structure and
the extensive diamond-to-diamond bonding between the grains. Light
patches of cobalt are also visible between the grains. Cobalt
plays an important role as a solvent/catalyst in the synthesis of
this form of PCD; without it, no new intergrowth would occur
between the grains. Also visible in Fig. 1 is the interface
between the diamond layer and its tungsten carbide backing,
showing clearly the intimate bonding in this region.
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Wear (abrasion) resistance and
toughness are the two most important properties in assessing the
suitably of drilling materials. Abrasion resistance is primarily a
function of hardness, and, in this respect, diamond is superior to
all other known materials. because of the random orientation of the
diamond grains, PCD has a hardness equivalent to the anisotropic
single crystal's average (see
table below). The random
orientation of the staring material and the grain boundaries in PCD
are responsible for its high fracture toughness in comparison with
that of a diamond single crystal, which tends to cleave
preferentially along a crystal plane.
One different between PCD and conventional tool materials is that
the normal disparity between toughness and hardness does not apply
to the same extent with PCD. Despite PCD's marked superiority in
hardness, a comparison of toughness values places it between
carbides and ceramics (table
below).
PCD cutters provide a cutting
structure for those formations that respond well to failure by
shearing, such as rock types with unconfined crushing strengths of
less than 150MPa, eg mudstones, shales, sandstones, coal, limestones,
dolomites and marbles. Under steady state drilling conditions, PCD
wears by micro-chipping, so that the cutters continually provide
new, sharp cutting edges.
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