Coating new cilox

7/29/2019 Coating new cilox

1/4

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 cBNcoating 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 m at temperatures far below 1000 C, indexable inserts were coated with a B 4C 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 significantlyharder 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 themachining 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.

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

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), pooradhesion, 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 > 1m 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 m), 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 cuttingtools. 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 m

on silicon substrates (fig. 1) and nearly 1 m on cemented carbide

cutting inserts has been successful.

Si

B4C

B-C-N gradient layer

cBN layer

1 m

Fig. 1: SEM cross section image of a cBN layer system with a cBNlayer thickness of approximately 1.5 m

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 alayer 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.

* The development and analysis of cBN coatings is a joint research projectbetween the Institute for Machine Tools and Factory Management (IWF),Berlin (a) and the Fraunhofer Institute for Surface Engineering and ThinFilms (IST), Braunschweig (b).


2/4

roughness

iR m

1.8

0.6

0PVD cBN-K10

RmaxR

R

z

a

PVD TiAlN-K10 CVD TiCN-K10

(a) PVD cBN-K10 (b) PVD TiAlN-K10 (c) CVD TiCN-K10

6 m6 m 6 m

Fig. 2: Roughness and surface structure of the rake face of a cBN toolcoating in comparison with a PVD TiAlN and a CVD TiCNcoating

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].

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.

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].

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 completelayer 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 TiAlN-

coating, 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 (formerlyGGG 50), and hardened steel X 155 CrVMo 121 (D2). The

experiments were carried out under dry cutting conditions. Tab. 2

summarizes the experimental conditions.

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 m. 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.

cBN on silicon cBN on tools TiN

Hardness [GPa] 55 - 65 55 - 60 20 25

Vickers-hardness by

Fischerscope / HV0.06

5800 5100 2600

Elastic-Modulus [GPa] 500 - 550 500 - 550 250 - 260

Abrasive wear

[m3m-1N-110-15]

~ 0.4 ~ 0.6 5 - 7

Friction coefficient against

steel [-]

0.4 0.4 0.7

Critical load in scratch test

[N]

20 25 > 50

maximum cBN thickness

reached up to now withthis technique [m]

2.5 0.8 -

Tab. 1: Mechanical properties of various tool coatings

Machining Process External cylindrical turning

Machine Tool VDF 180-C VDF 180-C TNS 30Workpiece material 34 CrNiMo 6 GJS-500-7 X 155 CrVMo 121

Material number 1.6582 0.7050 1.2379

Hardness 38 HRC 200 HB 30 62 HRC

Workpiece shape Shafts

100 x 300 mm

Hollow shafts

150 x 65 x 195 mm

Shafts

30 x 200 mm

Cutting tools CNMA 120408

Cutting material cBN-K10 TiAlN-K10 TiCN-K10 CC PCBN

Coating type PVD PVD CVD - -

Cutting edge sharp rn = 50 m rn = 50 m 0.21 x 17 0.12 x 25

Workpiece

materials

1.6582

0.7050

1.2379

1.2379 1.2379 0.7050 0.7050

Cutting

velocities

[m/min]

20, 50, 75,

100, 125,

180, 1000

20 20 1000 1000

Feeds

[mm]

0.1, 0.3, 0.8,

1.1

0.1 0.1 0.1 0.1

Depths of cut

[mm]

0.5, 2.0 0.5 0.5 0.5 0.5

Tab. 2: 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 tocatastrophic 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


3/4

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.

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 aresult 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 andf= 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

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.

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.

feed f

cuttingvelocitycv

200

m/min

100

50

00 1.20.3 0.6 mm

process: cylindrical turning workp iece: 34 CrNiMo 6depth of cut:

p= 2.0 mm cutting mat.: cBN-K10

ISO code: CNMA 120408 lubricant: drya

crater wearlimit

plastic deformationlimit

fracturelimit

built-upedge limit

safezone

6 -6 -6 95 80 0.8 mm

rrs00 r

Fig. 3: The failure mode diagram for cBN-coated cutting tools inmachining of 34 CrNiMo 6 maps the forms of wear as functionsof cutting parameters

(a) crater wear (b) plastic deformation (c) fracture

(d) built-up edge (e) uniform wear

600 m 600 m 600 m

600 m600 m

Fig. 4: Different forms of tool wear on cBN-coated cutting tools aftermachining 34 CrNiMo 6 for 30 seconds: (a) crater; (b) plasticdeformation; (c) tool fracture; (d) build-up edge; (e) uniformand continuously growing flank wear

process: cylindrical turning workpiece: GJS-500-7cutting velo.: c = 1000 m/min feed: f = 0.1 mmdepth of cut: p= 0.5 mm lubricant: dryISO code: CNMA 120408 hardness: 300 HB 30

v

a

6 -6 -6 95 80 0.8 mm

rrs00 r

cutting time ct

widthofflank

wearlandVB

m

450

150

00 30075 150 s

CC PCBNcBN-K10

Fig. 5: Width of flank wear land in dependence on cutting time forvarious cutting materials in machining spheroidal graphite castiron


4/4

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 cBN-

coated 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.

Conclusion and Outlook

The results of some of the first cutting experiments with cBN-

coated cemented carbide inserts were presented in this paper.

After an explanation of the approach used here to deposit adherentcBN-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 andcontinuously 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.

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.

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 TiAlN-

coatings.

roughness

iR

process: cylindrical turning workp iece: 1.2379

cutting velo.: c = 20 m/min feed: f = 0.1 mmdepth of cut: p= 0.5 mm lubricant: dryISO code: CNMA 120408 hardness: 62 HRC

va

6 -6 -6 95 80 0.8 mm

rrs00 r

cuttingforces

iF

m

12

4

0PVD cBN-K10

N

600

200

0PVD cBN-K10 PVD TiAlN-K10 CVD TiCN-K10

PVD TiAlN-K10 CVD TiCN-K10

FcF

F

f

p

RmaxR

R

z

a

Fig. 6: Cutting forces and workpiece roughness in dependence oncutting materials in machining hardened steel

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

1 Wentorf Jr., R. H.: Chem. Phys., 26 (1957) 956

2 Kester, D. J.; Messier, R.; Vac, J.: Sci. Technol. A 12 (6) (1994) 3074

3 Tanabe, N. et al.: Diamond Relat. Mater. 1 (1992) 151

4 Schtze, A.; Bewilogua, K.; Lthje, H.; Kouptsidis, S.; Jger, S.: Surf.Coat. Technol. 74-75 (1995) 717

5 Ikeda, T.; Kawate, Y.; Hirai, Y.; Vac., J.: Sci. Technol. A8(4) (1990)3168

6 Murakawa, M.; Watanabe, S.; Miyake, S.: Diamond Films Technol. 1(1991) 55

7 Mirkarimi, P. B.; McCarty, K. F.; Medlin, D. L.: Materials Scienceand Engineering, R21 (1997) 47

8 Weber, A.; Bringmann, U.; Nikulski, R.; Klages, C.-P.: DiamondRelat. Mat. 2 (1993) 201

9 Litvinov, D.; Clarke, R.: Appl. Phys. Lett. 74 (1999) 955

10 Matsumoto, S.; Zhang, W.: Jpn. J. Appl. Phys. Vol. 39 (2000) 442

11 Donner, W.; Dosch, H.; Ullrich, S.; Erhardt, H.; Abernathy, D.: Appl.Phys. Lett. 73 (1998) 777

12 Boyen, H.-G.; Widmayer, P.; Schwertberger, D.; Deyneka, N.;Ziemann, P.: Appl. Phys. Lett. 76 (2000) 709

13 Yamamoto, K.; Keunecke, M., Bewilogua, K.: New Diamond andfrontier carbon technology, Vol. 10, No. 4, 2000

14 Yamamoto, K.; Keunecke, M., Bewilogua, K.: Thin Solid Films, 398-399 (2001) 142-149

15 Yamamoto, K.; Keunecke, M., Bewilogua, K.; Czigany, Z.; Hultmann,

L.: Surface and Coating Technology, 142-144 (2001) 881- 888

Documents

Coating new cilox