Abrasive bodies

An abrasive body which comprises a cubic boron nitride compact bonded to a substrate. The compact has intergrowth between the cubic boron nitride particles which provides it with good thermal conductivity and has major surfaces on each of opposite sides, the one surface being bonded to the substrate and the other surface presenting a cutting edge. The substrate has a coefficient of thermal conductivity at least four times lower than that of the compact. The substrate is preferably made of aluminium oxide, Syalon or zirconia.

BACKGROUND OF THE INVENTION 
This invention relates to abrasive bodies. 
Cubic boron nitride abrasive compacts are well known in the art and are 
used in the cutting, grinding and otherwise abrading of various 
workpieces, particularly iron-containing workpieces. In use, they may be 
mounted directly on to a tool or bonded to a cemented carbide backing 
prior to mounting on to the tool. 
Cubic boron nitride compacts consist essentially of a mass of cubic boron 
nitride particles present in an amount of at least 70 percent, preferably 
80 to 90 percent, by volume of the compact bonded into a hard 
conglomerate. The compacts are polycrystalline masses and can replace 
single large crystals. 
Cubic boron nitride compacts invariably contain a second bonding phase 
which may contain a catalyst (also known as a solvent) for cubic boron 
nitride growth. Examples of suitable catalysts are aluminium or an alloy 
of aluminium with nickel, cobalt, iron, manganese or chromium. When the 
bonding matrix contains a catalyst, a certain amount of intergrowth 
between the cubic boron nitride particles occurs during compact 
manufacture. 
Cubic boron nitride compacts are made under conditions of temperature and 
pressure at which the cubic boron nitride particles are 
crystallographically stable. 
U.S. Pat. No. 3,982,911 describes another type of composite abrasive body 
comprising a layer of alloy-bonded cubic boron nitride crystals directly 
bonded to a metal substrate. The abrasive body is manufactured under 
relatively low pressure conditions which will not result in intergrowth 
occurring between the cubic boron nitride particles. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a cubic boron nitride 
compact having major surfaces on each of opposite sides thereof, the one 
surface being bonded to a substrate, and the other surface presenting a 
cutting edge, the compact comprising a first phase of a polycrystalline 
mass of intergrown cubic boron nitride particles and a second bonding 
phase and the substrate having a coefficient of thermal conductivity at 
least four times lower than that of the compact.

DETAILED DESCRIPTION OF THE INVENTION 
The abrasive body may have any suitable shape. It may have a disc shape or 
any other shape such as a segment of a disc, triangular, rectangular or 
square. A disc-shaped abrasive body may be fragmented into fragments of 
any suitable shape using known cutting techniques such as laser cutting or 
spark erosion. 
As mentioned above, one of the major surfaces of the compact will be bonded 
to a substrate, while the other major surface will present a cutting edge. 
The cutting edge may, in one embodiment, be a cutting point, for example 
the pointed end of the segment of a disc or a corner of a body of 
triangular, rectangular or square shape. 
In use, a cutting point or edge of the cubic boron nitride compact will 
perform the abrading operation. Heat will be generated at this point and 
will be conducted through the cubic boron nitride relatively rapidly to 
the substrate. The intergrowth between the cubic boron nitride particles 
ensures that the compact has a relatively high coefficient of thermal 
conductivity, i.e. of the order of 100 Wm.sup.-1 K.sup.-1. Because the 
substrate has a coefficient of thermal conductivity substantially less 
than that of the cubic boron nitride compact, the heat will be dissipated 
only slowly through the substrate and tend to concentrate in the thin 
compact. This, it has surprisingly been found, improves the abrading 
performance of the compact. 
In the prior art, cubic boron nitride compacts having intergrowth between 
the cubic boron nitride particles have been bonded either directly to a 
metal tool or to a cemented carbide substrate. Both such substrates have 
high coefficients of thermal conductivity and of the same order as that of 
the cubic boron nitride compact. The improvement in the abrading 
performance of the compact in the abrasive body of the invention over such 
prior art bodies is particularly surprising as it has always been believed 
that heat generated in the compact should be removed from the compact as 
quickly as possible to minimise any degradation of the cubic boron nitride 
particles occurring. 
The substrate will generally be larger in mass than the compact and should 
have good mechanical strength and a coefficient of thermal expansion close 
to that of the compact. Examples of suitable materials for the substrate 
are oxides, nitrides and Syalon (a commercially available 
silicon/aluminium/nitrogen/oxygen ceramic). Examples of suitable nitrides 
are boron nitride, aluminium nitride and silicon nitride; examples of 
suitable oxides are aluminium oxide and zirconia. Of these materials 
aluminium oxide, zirconia and Syalon are preferred. These materials will 
be in a sintered coherent form. 
It is important that the cubic boron nitride compact has intergrowth 
between the particles for it is this which contributes largely to the good 
thermal conductivity which such compacts possess. The second phase may be 
metallic in nature, and examples of such compacts are described in U.S. 
Pat. Nos. 3,743,489 and 3,767,371. The second phase may also be ceramic in 
nature and such compacts are preferred. Examples of such compacts are 
described in U.S. Pat. No. 3,944,398 where the second phase consists 
essentially of silicon nitride and a ceramic resulting from the 
interaction of aluminium with silicon nitride, or British Patent 
Publication No. 2,048,927 where the second phase consists essentially of 
aluminium nitride and/or aluminium diboride. 
Bonding of the compact to the substrate may be direct or through a metal or 
alloy bonding layer. Suitable metals and alloys for the bonding layer, 
when used, are well known in the art, as are the techniques and methods 
for bonding the compact to the substrate. The abrasive bodies of the 
invention have particular application as cutting tool inserts. 
An embodiment of the invention will now be described with reference to FIG. 
1 of the accompanying drawing. Referring to this drawing, there is shown a 
disc-shaped abrasive body 10 comprising a cubic boron nitride compact 12 
bonded to a substrate 14 which is larger in mass than the compact. The 
compact 12 has an upper major surface 16 and a lower major surface 18 
which is bonded to the substrate 14. As mentioned above, bonding between 
the surface 18 and the substrate 14 may be direct or through a metal or 
alloy bonding layer. In use, the circular edge 20 of surface 16 provides 
the cutting edge for the abrasive body. 
The abrasive body may be fragmented into fragments of any suitable shape 
using known cutting techniques such as spark erosion or laser cutting. For 
example, the abrasive body may be fragmented into a series of segments, 
one of which is illustrated by the dotted lines. In such a case, it is the 
point 22 of the segment which provides the cutting point. 
FIG. 2 illustrates an abrasive body similar to that of FIG. 1, save that it 
has a square shape. Like parts carry like numerals. The corners 24 provide 
cutting points in use. 
In an example of the invention, a cubic boron nitride compact of disc-shape 
(as illustrated by FIG. 1) was produced using the method described in 
British Patent Publication No. 2,048,927. The compact consisted of a 
polycrystalline mass of intergrown cubic boron nitride particles and a 
second bonding phase consisting essentially of aluminium nitride and/or 
diboride. The cubic boron nitride content of the compact was 85 percent by 
volume. The coefficient of thermal conductivity of the compact was 100 
Wm.sup.1 K.sup.-1. From this compact was cut a square-shaped compact using 
known laser cutting techniques. A substrate consisting of sintered 
coherent aluminium oxide and having a coefficient of thermal conductivity 
of 8.4 Wm.sup.-1 K.sup.-1 was produced. The substrate also had a square 
shape. 
A major surface of the compact was bonded to a major surface of the 
substrate to produce an abrasive body as illustrated by FIG. 2 of the 
accompanying drawing. Bonding between the compact and the substrate was 
achieved using an alloy bonding layer. The alloy was placed between the 
compact and substrate, a load applied to urge the compact and substrate 
together and the temperature raised to above the melting point of the 
alloy. Heating took place in a vacuum of 10.sup.-5 Torr to minimise 
degradation of the cubic boron nitride particles of the compact. The 
compact and substrate were firmly bonded together on returning to ambient 
temperature. The alloy of the bonding layer was a copper/manganese based 
layer. 
Using the method described above, a similar cubic boron nitride compact was 
bonded to a Syalon substrate and a zirconia substrate. Syalon has a 
coefficient of thermal conductivity of 23 Wm.sup.-1 K.sup.-1. Zirconia has 
a coefficient of thermal conductivity similar to that of aluminium oxide. 
The three abrasive bodies were compared with an unbacked cubic boron 
nitride compact of the type described in British Patent Publication No. 
2,048,927 in the machining of a workpiece made of a D3 tool steel. All 
three abrasive bodies out performed the unbacked compact.