Technical University Berlin, Institute for Machine Tools and Factory Management
CBN coatings on Cutting Tools *
Eckart Uhlmanna; Guenter Braeuerb; Eric Wiemanna; Martin Keuneckeb
Abstract
The machining of hard workpiece materials requires significantly harder cutting materials. Superhard cubic boron
nitride (cBN) is the hardest known material eligible for the machining of ferrous materials. The development of a cBN
coating for cutting tools, combining the advantages of coating and of cBN, is of great importance for many branches of
industry. Based on the first success of depositing adherent cBN films on cemented carbide substrates with a thickness of
up to 0.8 µm at temperatures far below 1000 °C, indexable inserts were coated with a B4C film as target and a cBN
facing. These coatings show excellent mechanical and physical properties. Cutting experiments with cBN coated
cemented carbide tools were carried out and the tool life, cutting forces and workpiece surface roughness were
measured. The results are presented for the machining of several workpiece materials.
Keywords: Production Process, CBN coatings, Wear
Introduction
The machining of hard workpiece materials requires significantly
harder cutting materials. In contrast to diamond, superhard cubic
boron nitride (cBN) is outstandingly eligible for the machining of
steels. In fact, it is the second hardest of all known materials.
Indeed, cBN is very expensive and only available in the form of
simply shaped inserts. However, the geometry of tools with
functional hard coatings can be specifically attuned to the
requirements of the machining task. Therefore, the development of
a cBN coating for cutting tools, combining the advantages of a
coating and of the cutting material cBN, is of great importance for
many branches of industry.
Since the percentage of hard and high-performance workpiece
materials increases, the problem of early tool wear during the
machining of these materials becomes more distinct. A solution
for the meeting of the increased requirements is the deposition of
special hard coatings. Particularly wear reduction coatings of
superhard materials such as diamond and cBN might increase tool
life substantially. However, steel materials cannot be machined
with diamond tools due to the reactivity of iron with carbon.
Today, the large segment of machining ferric materials is
dominated by cemented carbide tools with coatings of titanium
nitride (TiN), titanium carbonitride (TiCN) and titanium
aluminum nitride (TiAlN). But still, the performance of most
coatings is not sufficient.
deposited cBN films were limited to the thickness range of a few
hundred nanometers. This is mainly caused by an enormous
residual intrinsic compressive stress (up to 20 GPa), poor
adhesion, and a lack of long-term stability under ambient
conditions. Only few recent publications report on thicker cBN
films. However, films with a thickness of > 1µm could only be
synthesized only on silicon substrates at very high substrate
temperatures in the range of 1000 °C [9, 10], or film stress
reduction with expensive annealing [11], or intermitted deposition
in combination with high energy ion irradiation techniques [12].
Therefore, in spite of the mentioned successes in depositing thick
cubic boron nitride films (> 1 µm), the development of a cBN
coating on non-silicon substrate materials remains a scientific
challenge. Though, only the use of substrate materials other than
silicon leads to an application of cBN based coatings on cutting
tools. Our approach to obtain thicker cBN films leads to an
improvement of adhesion and a mechanical stabilization of the
cBN layer system without an essential reduction of stress [13].
The deposition of cBN films with thicknesses of more than 2 µm
on silicon substrates (fig. 1) and nearly 1 µm on cemented carbide
cutting inserts has been successful.
cBN layer
CBN coatings on silicon and cemeted carbide substrates
The cBN development started with the first synthesis of cBN bulk
material by Wentorf [1] in a high pressure and high temperature
process about 45 years ago. This technique is still in use in a more
elaborated status and provides efficient cutting inserts for the
machining of hardened steels, for example. This technique,
however, permits only cutting inserts with relatively simple
geometries. Therefore, research work on the synthesis of cBN
coatings has increased in the past few years.
Various PVD [2-6], CVD [7], and PECVD [8] processes have
succeeded in cBN film deposition. Unfortunately, nearly all
* The development and analysis of cBN coatings is a joint research project
between the Institute for Machine Tools and Factory Management (IWF),
Berlin (a) and the Fraunhofer Institute for Surface Engineering and Thin
Films (IST), Braunschweig (b).
1 µm
B-C-N gradient layer
B4C
Si
Fig. 1:
SEM cross section image of a cBN layer system with a cBN
layer thickness of approximately 1.5 µm
TEM investigations confirmed the assumption of nanocrystallinity
of the cBN layer and, additionally, the cubic phase of the layer
[14]. The progress is based on a modified sputter technique and a
layer system consisting of a boron carbide (B4C) target for
sputtering, a B-C-N gradient layer, and finally the cBN layer.
Among other properties, B4C is eligible for the use in a d.c.
sputtering process. The deposition of the coating system was
carried out in a radio frequency diode sputtering setup in
laboratory scale. Further description of the process and the
experimental setup is published elsewhere [13].
6 µm
The mechanical and tribological characterization of the coatings
were carried out by several different methods, like indentation
experiments for the determination of hardness and elastic
modulus, pin-on-disc tests for the friction coefficient, scratch tests
for the evaluation of the adhesion and abrasive wear experiments.
Some of these results are summarized in tab. 1. Further
information on structural and compositional features and on the
mechanical and tribological properties of thick cBN films has
already been published [14, 15].
Hardness [GPa]
Vickers-hardness by
Fischerscope® / HV0.06
Elastic-Modulus [GPa]
Abrasive wear
[m3m-1N-110-15]
Friction coefficient against
steel [-]
Critical load in scratch test
[N]
maximum cBN thickness
reached up to now with
this technique [µm]
Tab. 1:
cBN on silicon
55 - 65
5800
cBN on tools
55 - 60
5100
TiN
20 – 25
2600
500 - 550
~ 0.4
500 - 550
~ 0.6
250 - 260
5-7
0.4
0.4
0.7
20
25
> 50
2.5
0.8
-
Mechanical properties of various tool coatings
The values for a typical titanium nitride coating are added for a
comparison. The results confirm the outstanding properties of
cBN coatings. The hardness in combination with the very low
abrasive wear rates and the relatively low friction coefficient
emphasize the potential of cBN as a superhard and wear resistant
tool coating. In addition, fig. 2 shows the smooth surface of a cBN
caoting and compares it to a PVD TiAlN and a CVD TiCN
coating. Rmax of the cBN coating is only 1/10 and Ra even only
1/30 of the values of the other two coatings. However, the future
development leads to an increase of the cBN ratio in the complete
layer system and an enhancement of the adhesion of the films.
Experimental Setup
Turning tests were carried out on a VDF-180 C CNC inclined bed
turning lathe by Boehringer, Germany, and a TNS 30 by Traub,
Germany. The selected cutting materials were cemented carbides
of ISO specification K10 with PVD cBN coating, PVD TiAlNcoating, and CVD TiCN-Al2O3-TiN-coating, as well as Al2O3
oxide ceramics and polycrystalline cubic boron nitride. The
geometry of the indexable inserts was ISO code CNMA 120408.
Three different workpiece materials were machined: alloyed steel
34 CrNiMo 6, spheroidal graphite cast iron GJS-500-7 (formerly
GGG 50), and hardened steel X 155 CrVMo 121 (“D2”). The
experiments were carried out under dry cutting conditions. Tab. 2
summarizes the experimental conditions.
(a) PVD cBN-K10
6 µm
6 µm
(b) PVD TiAlN-K10
(c) CVD TiCN-K10
1.8
roughness Ri
The coating system was slightly changed for the deposition on
tool substrates, such as polished cutting inserts of cemented
carbides. In order to achieve a better adhesion of the complete
layer system to the substrate, an additional adhesive layer
consisting of titanium (Ti) was applied first to the substrate
followed by the above-presented layer system. The cutting
experiments and their characterization were carried out with this
changed layer sytem for tool substrates The results will be
presented in the following sections.
0.6
0
Fig. 2:
Rmax
Rz
Ra
µm
PVD cBN-K10
PVD TiAlN-K10
CVD TiCN-K10
Roughness and surface structure of the rake face of a cBN tool
coating in comparison with a PVD TiAlN and a CVD TiCN
coating
The criterion of tool wear assessment is the width of flank wear
land VB. The measurements were carried out on a centering
microscope MA 116 and an Intralux 20 HE double arm cold light
source, both made by Marcel Aubert, Switzerland. The
microscope has a 30x magnification. The measurements were
taken with a digital gauge made by Messwelk, Germany. The
dissolution is 10 µm. The different wear states of the tools were
documented by means of electron microscopic images taken with
the scanning electron microscope (SEM) Zeiss DSM 950 by Carl
Zeiss, Germany.
Machining Process
Machine Tool
Workpiece material
Material number
Hardness
Workpiece shape
External cylindrical turning
VDF 180-C
TNS 30
GJS-500-7
X 155 CrVMo 121
0.7050
1.2379
200 HB 30
62 HRC
Hollow shafts
Shafts
150 x 65 x 195 mm 30 x 200 mm
CNMA 120408
TiAlN-K10 TiCN-K10 CC
PCBN
PVD
CVD
rn = 50 µm rn = 50 µm 0.21 x 17° 0.12 x 25°
1.2379
1.2379
0.7050
0.7050
VDF 180-C
34 CrNiMo 6
1.6582
38 HRC
Shafts
100 x 300 mm
Cutting tools
Cutting material cBN-K10
Coating type
PVD
Cutting edge
sharp
Workpiece
1.6582
materials
0.7050
1.2379
Cutting
20, 50, 75, 20
velocities
100, 125,
[m/min]
180, 1000
Feeds
0.1, 0.3, 0.8, 0.1
[mm]
1.1
Depths of cut 0.5, 2.0
0.5
[mm]
Tab. 2:
20
1000
1000
0.1
0.1
0.1
0.5
0.5
0.5
Experimental conditions and parameters
Wear Behaviour
The wear on cBN-coated cutting tools is, like on other cutting
materials, dependent on machining and tool parameters: cutting
velocity, feed, depth of cut, sharpness of the cutting edge, tool
geometry (i.e. the presence of chip-breakers) and the type of
cutting stress (continuous or interrupted cutting, thermal shock).
One or a combination of several forms of wear, which, in an
advanced stage, may lead to overload or fatigue and finally to
catastrophic edge breakage, can initiate tool failure. In general, the
wear modes vary with tool composition and cutting edge geometry
in response to the cutting force and cutting temperatures under
given cutting conditions. However, a strong coating adhesion is
not always obtained with the development of new coatings.
The failure mode diagram in fig. 3 shows that all major forms of
wear can be generated with cBN coated tools in dependence on
cutting speed and feed, which serve the predictability, reliability,
and improvement of the wear behavior. On the basis of the
tool/workpiece interaction it can be seen that cutting velocity
depends on the thermo-chemical stability of the cutting material
and the feed depends on its mechanical strength or fracture
toughness. Increasing heat generates more heat dissipated to the
tool and high temperatures lead to oxidation or diffusion.
Increasing feed induces higher mechanical stresses on the tool.
The variables used in fig. 3 define the failure limits at continuous
cutting conditions, represented in the failure mode diagram, for
the machining of 34 CrNiMo 6 with cBN-coated tools.
applied to the substrate. The tool breakage in (c) is a result of
mechanical overload. It has to be mentioned that the necessary
feed of 1.1 mm is much more than it is usually used in cutting
processes because of the high workpiece roughness. First
indications of the built-up edge effect can be seen in (d), whereas
uniform and continuously growing flank wear dominate in (e). In
general, the cutting velocities used for generating these wear
forms is roughly half of TiAlN coatings while the feeds are
approximately the same.
600 µm
600 µm
(a) crater wear
(b) plastic deformation
600 µm
(c) fracture
200
crater wear
limit
cutting velocity vc
m/min
plastic deformation
limit
600 µm
(d) built-up edge
100
safe
zone
fracture
limit
Fig. 4:
50
0.3
0.6
1.2
mm
feed f
process:
cylindrical turning
depth of cut: ap = 2.0 mm
ISO code:
CNMA 120408
0
6°
Fig. 3:
0
-6°
workpiece: 34 CrNiMo 6
cutting mat.: cBN-K10
lubricant:
dry
s
r
r
rε
-6°
95°
80°
0.8 mm
The failure mode diagram for cBN-coated cutting tools in
machining of 34 CrNiMo 6 maps the forms of wear as functions
of cutting parameters
Tool breakage or fracture is the least desirable failure mode
because it is the most unpredictable and can be damaging to the
workpiece. With the present cBN-coated tools, fracture after a
cutting duration of 30 seconds was observed at a cutting velocity
of 75 m/min and a feed of 1.1 mm, whereas crater wear was a
result of the cutting parameters vc = 180 m/min and f = 0.3 mm.
Plastic deformation of the cutting edge could be found between
these two sets of parameters, more precisely at vc = 125 m/min and
f = 0.8 mm. The safe operating zone depicted in the failure mode
diagram is a region of gradual wear associated with reliable
performance. Mainly flank wear and some crater wear was
generated at vc = 100 m/min and f = 0.3 mm. A small built-up
edge was observed at vc = 50 m/min and f = 0.1 mm.
The SEM pictures in fig. 4 correspond to all five different forms
of tool wear. In (a), crater wear can be seen as a result of high
temperatures and abrasive wear on the rake face. Note that the
interface between the crater and the coated surface is very sharp.
A lack of coating adherence cannot be observed. In (b), the cutting
inserts shows signs of plastic deformation due to high forces at
elevated temperatures. Nonetheless, the coating is still perfectly
High-Speed Cutting
Although the highest cutting velocity used in machining alloyed
steel in fig. 3 and 4 is only 180 m/min, cBN-coated tools are also
eligible for the machining at high cutting speeds. Fig. 5 shows the
results for the machining of GJS-500-7 at a cutting velocity of
1000 m/min. It becomes clear that the tool life of the cBN-coated
cemented carbide insert is only slightly less than half the tool life
of expensive PCBN and oxide ceramic inserts. It has to be noted
that there is still the possibility to modify the geometry of the
coated insert and that the full potential of this technology is not
seized yet.
width of flank wear land VB
0
(e) uniform wear
Different forms of tool wear on cBN-coated cutting tools after
machining 34 CrNiMo 6 for 30 seconds: (a) crater; (b) plastic
deformation; (c) tool fracture; (d) build-up edge; (e) uniform
and continuously growing flank wear
built-up
edge limit
0
600 µm
450
cBN-K10
CC
PCBN
µm
150
0
0
75
150
300
s
cutting time tc
process:
cutting velo.:
depth of cut:
ISO code:
0
6°
Fig. 5:
cylindrical turning
vc = 1 000 m/min
ap = 0.5 mm
CNMA 120408
0
-6°
workpiece:
feed:
lubricant:
hardness:
s
r
r
-6°
95°
80°
GJS-500-7
f = 0.1 mm
dry
300 HB 30
rε
0.8 mm
Width of flank wear land in dependence on cutting time for
various cutting materials in machining spheroidal graphite cast
iron
Machining of Hardened Steels
Another important field of application of cBN tools is the turning
of hardened steel, which is a difficult machining task for new tool
coatings because of the high cutting forces. However, fig. 6 shows
that the cutting forces in turning D2 (hardness 62 HRC) with cBNcoated tools and PVD TiAlN-coated tools are almost the same and
that they lie significantly below those of CVD TiCN-coatings. In
addition, the surface roughness of the machined workpiece with
cBN-coated tools is much better than the roughness achieved with
other tool coatings.
cutting forces Fi
600
N
Fc
Ff
Fp
200
0
PVD cBN-K10
PVD TiAlN-K10
CVD TiCN-K10
roughness Ri
12
µm
Rmax
Rz
Ra
PVD cBN-K10
process:
cutting velo.:
depth of cut:
ISO code:
0
6°
Fig. 6:
Finally, the machining of hardened steel was analyzed and the
cutting forces and surface roughness were measured. In
comparison with other PVD and CVD tool coatings, the
workpiece roughness generated by cBN-coated tools is much
lower and the cutting forces, which are significantly lower than
those of CVD TiCN-coatings, are similar to those of PVD TiAlNcoatings.
Summing up, cBN tool coatings are eligible for the machining of
various workpiece materials even at high cutting velocities. The
machining of hardened steels is also possible. The achievable
workpiece roughness is better than that of other PVD and CVD
tool coatings. However, the applicable cutting velocities and
achievable tool lives of cBN-coatings are still lower than those of
some other tools. The next steps to counter these problems are the
increase of coating thickness and the implementation of a
rotational mechanism during coating.
References
4
0
Furthermore, the tool life of the machining of spheroid graphite
cast iron was presented in comparison with those of PCBN and
oxide ceramics. It could be proven that the tool life of cBN-coated
cutting tools is roughly 50 % of that of ceramic and 40 % of that
of expensive PCBN, whereby the tool geometry of the coated tool
may still be optimized.
PVD TiAlN-K10
cylindrical turning
vc = 20 m/min
ap = 0.5 mm
CNMA 120408
0
-6°
CVD TiCN-K10
workpiece:
feed:
lubricant:
hardness:
s
r
r
-6°
95°
80°
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Conclusion and Outlook
The results of some of the first cutting experiments with cBNcoated cemented carbide inserts were presented in this paper.
After an explanation of the approach used here to deposit adherent
cBN-coatings on cutting tool substrates at low temperatures, the
outstanding mechanical properties of cBN coatings were
summarized and compared to standard TiN-coatings. In particular,
the low surface roughness of the coating was compared with PVD
TiAlN-coatings and CVD TiCN coatings on the same cemented
carbide substrate.
Then, a failure mode diagram was introduced for the machining of
alloyed steel with cBN-coated inserts. It proved that cBN-coated
tools can generate all kinds of wear like common wear resistance
coatings: crater wear at high cutting velocities, plastic deformation
at high velocities and high feeds, tool fracture at high feeds, the
built-up edge effect at low cutting parameters, and uniform and
continuously growing flank wear in a so-called safe zone. This
result is important for the understanding of the wear behavior of
cBN coatings and for the enhancement of these tools.
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