Tool with wear-resistant cutting edge made of cubic boron nitride or polycrystalline cubic boron nitride, a method of manufacturing the tool and its use

For the improvement of the wear properties of tools with cutting edge of cubic boron nitride (CBN) or polycrystalline cubic boron nitride (PCBN) it is proposed to coat the CBN or PCBN body with a 0.5 to 6 .mu.m thick layer of one or more oxides of the metals zirconium and/or yttrium and/or magnesium and/or titanium and/or aluminum, preferably aluminum oxide. The wear-resistant coating is deposited from the gas phase at gas-phase temperatures up to 800.degree. C. Preferably for the coating the pulse-plasma CVD process is used. The so-coated tools are particularly suitable for the chip-forming precision machining of hard iron and hard steel materials.

CROSS REFERENCE TO RELATED APPLICATION 
This application is a national phase of PCT/DE92/00608 filed 23 Jul. 1992 
and based, in turn, upon German national application P41 26 851.2 of 14 
Aug. 1991 under the International Convention. 
FIELD OF THE INVENTION 
The present invention relates to a tool with a wear-resistant cutting edge 
made of cubic boron nitride or polycrystalline cubic boron nitride, 
whereby at least the surface of the tool is entirely or partially made of 
cubic boron nitride or polycrystalline cubic boron nitride. The invention 
also relates to a method of manufacture of the tool, as well as to its 
use. 
BACKGROUND OF THE INVENTION 
It has been long known to use tools of polycrystalline cubic boron nitride 
(PCBN) for the machining of particularly hard materials, for instance hard 
iron and steel materials, materials for cold or hot rolling, castings and 
the like. 
In addition to the widely used monoblocks of massive PCBN, which are 
available in various shapes and as a rule are clamped in the tool holder, 
there are also PCBN cutting inserts which can be soldered to the holder 
(inlays). These PCBN cutting inserts have a metallized face on one side, 
which makes it possible to solder them into a tool holder in the tool 
holders, mostly through brazing at 660.degree. C. to 840.degree. C. 
There have also been attempts to deposit cubic boron nitride (CBN) directly 
from the gas phase onto a substrate body through CVD (chemical vapor 
deposition), PVD (physical vapor deposition) or through a plasma-activated 
CVD process. 
So for instance in EP 0 209 137 it is proposed to deposit a layer of 
oriented-growth cubic boron nitride (CBN) from a gas phase onto a 
substrate which is provided with an intermediate layer of at least one 
nitride and/or oxynitride of the metals aluminum, gallium, indium and 
tellurium. 
The physical properties of CBN, particularly the high thermal stability, 
insures that materials which do not respond well to chip-forming machining 
can also be worked by turning, drilling and milling with high chip-removal 
volumes per time unit and considerable service life. 
However, according to K. Steinmetz (VDI-Z., Volume 192 (1987), No. 2, Pages 
64 to 69), it is a drawback of tools made of PCBN that considerable wear 
of the cutting edges occurs, especially during the finishing of workpieces 
made of hard iron materials, i.e. at smaller chipping cross sections, so 
that size, shape and surface quality requirements cannot be met. 
This effect is primarily due to the high heat conductivity of tools made of 
PCBN or CBN. 
Therefore Steinmetz suggested for such machining processes the use of a 
PCBN with a ceramic bonding phase. Due to the reduced heat conductivity as 
a result of this changed composition, a larger fraction of the machining 
heat is used for chip softening, so that better performance can be 
obtained during precision turning of hard steel parts. 
From JP 61-41768 A, in Patent Abstracts of Japan, C-359, Jul. 11, 1986, 
Vol. 10, No. 199, it is known to deposit, by avoiding a continuous layer, 
separate cubic BN crystals on a hard-metal alloy substrate, a cermet 
substrate, or a ceramic substrate, having a height of 0.1 to 10.mu.m and 
to fill the spaces free of boron nitride with carbides, nitrides, 
carbonitrides, borides, boron nitrides, oxides and oxycarbides of elements 
of the groups IVa to VIa, hexagonal BN, amorphous BN and Al.sub.2 O.sub.3. 
However a disadvantage resides in the fact that the singular boron nitride 
particles are not enough to insure a good cutting effect. Furthermore 
thermal stress can occur during cooling, as a result of the different 
heat-expansion coefficients of boron nitride and Al.sub.2 O.sub.3, which 
can lead to cracks. Finally the BN-particles tend to break away under 
load. 
It is known to deposit wear-resistant coatings directly onto the substrate 
according to the CVD process through gas phase reactions at high 
temperatures. In addition, plasma-activated CVD processes are known, which 
work at lower gas-phase temperatures. 
For instance DE 38 41 730 and the DE 38 41 731 describe a pulse-plasma CVD 
process for coating a metallic basic body with a nonconductive coating 
material, particularly Al.sub.2 O.sub.3 wherein to the basic body which is 
connected as cathode a pulsed direct voltage of 200 to 900 volt with a 
pulse duration of 50 .mu.s is established, whereby in the pulse pauses of 
80 .mu.s a residual voltage is maintained which is higher than the lowest 
ionization potential of the molecule participating in the CVD process, but 
not higher than 50% of the maximum voltage and wherein the coating is 
performed at gas-phase temperatures between 400.degree. C. and 800.degree. 
C. 
OBJECTS OF THE INVENTION 
It is now the object of the invention to provide a tool made of CBN or PCBN 
with a wear-resistant cutting edge, whose cutting-edge stability is 
considerably improved by avoiding the above-mentioned drawbacks. 
It is a further object of the present invention to provide an improved 
method of manufacturing the tool according to the invention. 
Finally it is the object of the present invention to provide an improved 
method of using the tool according to the invention. 
SUMMARY OF THE INVENTION 
The object of the invention is achieved by the tool according to the 
invention wherein a continuous CBN or PCBN layer extending over the entire 
tool or over a part thereof is coated with a thin layer consisting of one 
or more metal oxides. 
The thickness of the oxide layer amounts to 0.5 to 6 .mu.m, preferably 1 to 
3 .mu.m and/or of the CBN or PCBN layer can be up to 10 mm. 
Oxides of metals such as zirconium and/or yttrium and/or magnesium and/or 
titanium and/or aluminum are used for the oxide layer. 
Surprisingly it has been found that a tool of this design has not only 
exceptional wear-resistance properties, but that it is suitable for 
chip-forming precision machining of hard materials, particularly of hard 
iron and steel materials, due to its high cutting edge stability. 
The individual crystals of the PCBN are bonded under high pressure into a 
massive body by means of a metallic bonding agent (e.g. iron, nickel, 
cobalt) which can also contain hard-material components (e.g. TIC). Since 
PCBN is largely inert, during machining the atmospheric oxygen approaching 
the surface of the cutting body reacts preferably with the metallic 
components of the bonding agent and forms oxides, e.g. (iron oxide, nickel 
oxide, cobalt oxide), which can no longer perform the function of a 
bonding agent between the CBN-grains, due to insufficient mechanical 
properties (e.g. ductility, wetting ability). For this reason individual 
CBN grains can be detached under the stress during chip removal, which 
diminishes the stability of the cutting edge and as a result the 
mechanical cutting wear is considerably increased. 
The more the machining heat is diverted to the cutting body, the stronger 
this effect becomes. Since with the decrease of the cross section during 
machining the fraction of the heat which is used for chip softening 
diminishes, a stronger wear, respectively a lower stability of the cutting 
edge occurs, particularly in precision machining. This effect is 
additionally enhanced by the high temperatures occurring during chip 
removal, when machining hard iron or steel materials. 
If the PCBN is covered by a thin layer of oxide, the access of the 
atmospheric oxygen to the bonding agent is largely prevented, so that 
hardly any oxides form in the metals contained in the bonding agent. 
According to a particularly advantageous embodiment of the invention, the 
oxide layer of the invention is deposited on a massive body of PCBN 
(monoblock). 
Finally it is also possible to coat composite bodies, which are coated with 
CBN, whereby between the CBN layer and the substrate there can also be a 
CBN-free intermediate layer, with the oxide layer according to the 
invention. 
The intermediate layer consists of at least one nitride and/or oxynitride 
of the metals aluminum, gallium, indium or tellurium, preferably aluminum 
nitride is used. 
As the substrate body, hard metals or cutting ceramics or silicon nitride 
or silicon-aluminum-oxynitride or cermets or tool steels are used. 
The PCBN used in various embodiments of the invention contains bonding 
agent in a proportion of 0 to 50% by weight, preferably 10 to 25% by 
weight. Cobalt, nickel, tungsten and other metals, preferably cobalt, are 
used as bonding agents. 
According to a further, particularly advantageous development of the 
invention the oxide layer is not deposited on the entire surface covered 
by CBN or PCBN, but only there where the CBN or PCBN cutting edge is 
actually exposed to wear, so that the PCBN, respectively the CBN layer is 
only partially covered by the oxide layer. 
The object of the invention is further achieved through a process for the 
production of the tool with wear-resistant CBN or PCBN cutting edge, by 
applying the wear-reducing layer, consisting of one or more metal oxides 
onto the CBN or PCBN at gas-phase temperatures up to 800.degree. C. 
Preferably the gas phase temperature ranges from 400.degree. C. to 
600.degree. C. 
Furthermore with the process of the invention it is possible to produce 
oxide layers which bond very well with the substrate and are fully free of 
cracks. 
The particular advantage of the process of the invention consists in the 
fact that the tools with soldered or glued inlays can be coated as a 
complete tool, i.e. with already hardened PCBN, since in the process of 
the invention the gas-phase temperatures are selected so that they do not 
surpass the safe temperatures for the solder or adhesive. 
Due to this measure, damage to the applied oxide layer is avoided during 
the normally required subsequent insertion. 
Therefore according to the invention it is particularly advantageous to use 
the plasma-activated CVD process, particularly the pulse-plasma CVD 
process for the application of the oxide layer. 
Particularly good results have been obtained with the use of the 
pulse-plasma CVD process for the production of the oxide layer by setting 
the following operating parameters: 
______________________________________ 
Temperature of gas phase: 
400.degree. C. 
to 600.degree. C. 
Direct voltage: -300 V to -600 V 
Residual voltage: -20 V to -60 V 
Pulse duration: 30 .mu.s to 60 .mu.s 
Pulse pause: 40 .mu.s to 100 .mu.s 
______________________________________ 
The body to be coated was connected to be the cathode. 
Finally the object of the invention is achieved by the use of the tool 
according to the invention for the chip-forming machining of hard 
materials, particularly of hard iron and steel materials. 
According to the invention it is particularly advantageous to use the tool 
according to the invention as a cutting body for the precision machining 
of the mentioned materials.

SPECIFIC DESCRIPTION AND EXAMPLES 
The object of the invention is subsequently further explained with the aid 
of the embodiment examples. 
EXAMPLE 1 
In a depression of a hard-metal indexable insert, consisting of 94% by 
weight tungsten carbide and 6% by weight cobalt of the shape SNUN 120404 
(defined according to DIN 4987), a PCBN inlay of approximately 4.times.3 
mm is fastened with a solder melting at 800.degree. C. 
This indexable insert was coated with Al.sub.2 O.sub.3 by using the 
pulse-plasma CVD process according to DE 38 41 730 and 38 41 731 and with 
the following operating parameters: 
______________________________________ 
Gas phase temperature: 600.degree. 
C. 
Gas pressure: 250 Pa 
Direct voltage: -550 V 
Residual voltage: -40 V 
Pulse duration: 50 .mu.s 
Pulse pause: 80 .mu.s 
Coating time: 2 h 
______________________________________ 
Subsequent tests with the Al.sub.2 O.sub.3 -coated indexable insert have 
shown that the entire surface of the indexable insert, including the PCBN 
inlays, was coated with a 2 .mu.m thick, securely adhering layer of finely 
distributed, polycrystalline Al.sub.2 O.sub.3 of the alpha modification. 
After the coating process, the soldered-in inlay was strongly bonded with 
the hard-metal substrate, in unaltered condition. 
Turning tests have been carried out with the indexable insert of the 
invention with PCBN inlay on a steel 50CrMo4 with a Rockwell hardness of 
56 HRC, in continuous cutting compared with the same indexable inserts but 
which were not coated with Al.sub.2 O.sub.3 with PCBN inlay under the 
following test conditions: 
______________________________________ 
Cutting speed 130 m/min 
Cutting depth 0.5 mm 
Advance 0.08 mm/U. 
______________________________________ 
The turning tests were concluded when a wear mark width of 0.3 mm was 
reached. 
In the indexable insert with PCBN inlay not coated with Al.sub.2 O.sub.3 
this width of the wear mark was reached already after 19 min, while the 
indexable inserts with PCBN inlay and Al.sub.2 O.sub.3 -coating showed 
wear-mark widths of 0.3 mm only after 48 min. 
EXAMPLE 2 
An indexable insert of the shape SNUN 120408 (defined according to DIN 
4987), consisting of a hard metal of 94% by weight tungsten carbide and 6% 
by weight cobalt, was coated through a PVD process with a 8 .mu.m thick 
layer of CBN and subsequently coated with aluminum oxide under the test 
conditions described in Example 1 by the pulse-plasma CVD process 
according to DE 38 41 730 and DE 38 41 731. 
The subsequent testing of the indexable insert so coated showed that a 
layer of Al.sub.2 O.sub.3 with a thickness of 2.mu.m and with strong 
adherence to the CBN was deposited. Through an x-ray diffraction analysis 
it was established that the deposit is a very finely distributed, 
polycrystalline aluminum oxide of the alpha modification. 
The indexable insert of the invention was after that subjected to a 
chip-removal machining test according to Example 1 in comparison with an 
identical indexable insert, but which was not covered by Al.sub.2 O.sub.3. 
The wear-mark width of 0.3 mm appeared after 20 min in the indexable insert 
not coated with Al.sub.2 O.sub.3, but coated only with CBN, while in the 
indexable insert of the invention additionally coated with Al.sub.2 
O.sub.3 a corresponding wear mark appeared only after a use of 34 min.