Full Paper

Pulsed Nanocomposite TiAlN Coatings onComplex Shaped Tools for HighPerformance Cutting Operations

Kirsten Bobzin, Erich Lugscheider, Reimo Nickel, Philipp Immich,Stephan Bolz,* Fritz Klocke, Klaus Gerschwiler

The demand on high profitability in cutting operations has led to a variety of requirements forhigh performance tool coatings. Nanostructured coatings have shown most promising resultsin this connection. High oxidation resistance, hot hardness, and low friction are just a fewbenefits that these coatings offer. The deposition of nanostructured coatings is only possiblewithin a small deposition process window. Most cutting tool surfaces are complex shaped andinclude, for instance, small corner radii at the cutting edge or chip breakers. The local processwindow and the deposition parameters must be adapted to the actual shape of the cuttingtools in order to obtain a hard nanocomposite coating with adequate adhesion properties.Finally, the performance of these coatings has been studied in machining tests.

Introduction

Designing microstructure of hard coatings for cutting

application seems to be most convenient for enhancing

mechanical and tribological properties of a coating

material. Fast growing demands on the quality of thin

films results in new requirements due to high speed dry

cutting and hard machining. Established coating systems

like TiN possess low hot hardness as well as low oxidation

resistance. For severe cutting conditions they are replaced

by chemical resistant coatings such as (Ti1� xAlx)N which

is superior to conventional binary coatings due to its high

hot hardness up to 800 8C.[1,2] (Ti1� xAlx)N-coatings,

deposited by physical vapour deposition (PVD) exist as a

metastable mixed crystal system at low temperature. In

the transition zone they are present as a mixture of cubic

NaCl-structured TiN and hexagonal wurtzite structured

AlN.[3] The transition zone between cubic and hexagonal

K. Bobzin, E. Lugscheider, R. Nickel, P. Immich, S. BolzChair of Surface Engineering, RWTH Aachen University, Augus-tinerbach 4-22, D-52056 Aachen, GermanyFax: (þ49) 241 809 2264; E-mail: info1@iot.rwth-aachen.deF. Klocke, K. GerschwilerLaboratory for Machine Tools and Production Engineering, RWTHAachen University, Steinbachstraße 53B, D-52074 Aachen,Germany

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structure is of interest for cutting applications as the

formation of TiN and AlN domains results in an increase of

hardness by hindering the motion of dislocations.[4]

Though oxidation resistance of (TiAl)N-coatings rises with

increasing AlN-content, a limitation is set to DC sputtering

technique due to the formation of insulating films at the

targets. The pulsed MSIP technology offers the possibility

to support the glow discharge in front of the targets

independent from the AlN-content at the target surface.

Moreover, it is reported that nanocomposite TiAlN coat-

ings deposited by DC-MSIP may have grain sizes of about

30 nm.[5] The pulse technology permits the realisation of

dense plasmas with a high ionisation degree. High ion

energies result in the deposition of dense nanostructured

coatings with extremely small grain sizes and calculated

compressive stress. Smaller crystallite size as well as

compressive stress can increase the hardness of the

deposited films. Pulse technology additionally raises the

deposition rate.

Experimental Part

(Ti1�xAlx)N filmswere deposited at a total pressure of 500mPa by

reactive sputtering in a mixed atmosphere of argon and nitrogen

on WC-Co cutting inserts (CNMG 120408). Deposition was

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performed using four cathodes in a 2�2 dual cathode arrange-

ment on an industrial CemeCon CC800/9-Sinox coating unit. The

pulse sequence patterns were generated by melec pulse units

(SPIK 2000 A) using the standard melec bipolar pulse parameters.

The coatings were produced by cosputtering of two TiAl targets

and twoAl targets. The target size of all targetswas 88�500mm2.

The Al targets had a purity of 99.9%. For the Ti50Al50 targets the

purity was about 99.5% for the titanium and of 99.9% for the

cylindrical aluminium inserts within the sputter track. The coat-

ingswere deposited at a temperature of 500 8C. In order to increase

the AlN content of the coating, the power ratio at the Al targets

was varied while the TiAl targets were maintained at a constant

power density. A pulsed-bias voltage was applied at the substrate

holder with a frequency of 350 kHz and a pulse reverse time of

500 ns. During deposition the samples were moved in a planetary

motion to ensure a constant film thickness distribution at the

substrates. Before deposition the samples were ion etched in an

argon atmosphere. For this substrate cleaning process, an RF-

power source was used. The process parameters are listed in

Table 1. Coating composition was determined by energy dis-

persive spectrometry (EDS). Our TiAlN samples may contain three

possible phases TiN (cubic), (Ti1� xAlx)N (cubic) or AlN (wurzite,

cubic). The microstructure of the thin films was analysed by

grazing incidence X-ray diffraction (Seifert C3000 Diffraction

Table 1. Process parameters for the coating deposition.

Heating phase

Starting pressure 2 mPa

Heating power 16 kW

Heating time 4 500 s

Etching phase

Etching time 3 600 s

Bias Voltage �200 V

RF Power 1 500 W

RF 13.56 MHz

Argon flow 150 sccm

Coating phase

Coating time 3.6 h

Total pressure 500 mPa

Bias voltage �25 to �40 V

Frequency 350 kHz

Pulse reverse time 500 ns

Nitrogen flow 60 sccm

Argon flow Pressure controlled

Target power density 1.14–7.95 W � cm�2

(Al targets)

11.36 W � cm�2

(Ti50Al50 targets)

Cooling phase

Venting temperature 180 8C

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System) scanning the range of 20–1208 at an angle of incidence of

38. A CuKa radiation source was used (40 kV, 40 mA). For the

determination of the crystallite size, the Warren Averbach (WA)

method was used. Coatings deposited on WC-Co inserts were cut

out and thinned for TEM analysis to verify the determined

crystallite size. Nanoindentation (equipment: MTS Systems

Nanoindentation XP) was performed with a Berkovic indenter.

The indenter penetrated the coated surface perpendicular at a

constant load of 10 mN. Calculations of the Young’s modulus are

based on Oliver and Pharr’s equations and Poisson ratio was kept

constant at n¼ 0.25.[6,7] After elaborating the favoured AlN

content of the coating, the relationship between bias voltage,

hardness and grain size was analysed. Tests concerning the

thermal stability were carried out by annealing the coated sam-

ples in vacuum atmosphere from 600 to 1 000 8C. To test the cut-

ting performance a cutting test was accomplished by longitudinal

turning of Inconel 718. Synthetic ester without additiveswas used

as cooling lubricant. Concerning the cutting parameters a cutting

speed of 50 m �min�1, a feed rate of 0.15 mm and a cutting depth

of 0.3 mm were used.

Results and Discussion

(TiAl)N coatings with different Ti/Al ratios were deposited

by using different power ratios of the Al targets. With

regard to high oxidation resistance and high elevated-

temperature hardness a composition with a high AlN

content of 65 mol-% was generated for a target power

density of 7.95 W � cm�2 at the Al targets. To improve the

coating qualities, primarily the influence of the bias

voltage on coating properties such as grain size and

hardness had to be evaluated. The grain size was deter-

mined using the data fromanX-ray diffractogram. The line

shape of the Bragg peaks contains information about

the average grain size distribution. For the evaluation of

the crystallite size two well known methods, Debye–

Scherrer andWA can be used.[8,9] The Scherrermethod only

considers uniform crystallites. However, uniform crystal-

lites are rare and therefore, the WA method represents

an alternative. This method additionally considers both

crystallite size distribution and lattice microstrain. Based on

the WA method, a computerised program was developed

to determine the grain size of the coatings.[10] To confirm

the results of the calculated grain sizes by the program,

TEM investigations were carried out. To vary the grain size,

the bias voltage was altered. Figure 1 shows the effect of

the substrate bias voltage on the grain size and the

indentation hardness. In the case of the (Ti35Al65)N, coat-

ing, the grain size was about 10 nm for a pulsed substrate

bias of �25 V. It decreased very little for increasing bias

and reached the minimum value of approximately 6 nm

for the highest pulsed bias voltage of �40 V pulsed bias

applied in this test series. The indentation hardness

showed more change over the bias range than the grain

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Pulsed Nanocomposite TiAlN Coatings on Complex Shaped Tools . . .

Figure 1. Grain size and indentation hardness as a function of biasvoltage.

Figure 3. Grain size and indentation hardness as a function ofannealing temperature.

size did. With increasing bias the hardness increased and

reached 30 GPa for the highest bias voltage of �40 V. At

this voltage level, only a negligible bias effect on the sharp

cutting edges was observed.

It is known that, depending on the growth conditions,

the NaCl structured Ti1� xAlxN is metastable up to

approximately 60–70 mol-% of AlN.[11–13] A miscibility

gap between the cubic and the hexagonal phase in the

metastable TiN-AlN phase diagram expands with increas-

ing growth temperature.[14] For temperatures of about

500 8C used for the deposition process the material

conditions are far from equilibrium. Therefore, the super-

saturated phases of (Ti35Al65)N possess a high chemical

driving force for decomposition into their stable consti-

tuents.[15] This structural evolution was tested for the

coating deposited with a substrate bias voltage of �40 V

onWC/Co substrates by annealing in vacuum atmosphere

at different temperatures. Figure 2 presents the corre-

sponding XRD patterns for the coating as deposited as well

as for annealing temperatures in vacuum atmosphere up

Figure 2. XRD evolution of (Ti35Al65)N after annealing in vacuumatmosphere.

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to 1 000 8C. For annealing temperatures up to 800 8C the

system seems to be stable. Only cubic (fcc) peaks are

present. On increasing the annealing temperature (Ta)

above 800 8C peaks of the hexagonal close-packed struc-

ture like, e.g. (100) and (101) are found which are probably

formed by initiation of recrystallisation of the metastable

TiAlN. During annealing also indentation hardness and

grain size change with increasing annealing temperature

(Ta), as shown in Figure 3. The highest hardness value

corresponds to the smallest grain size for the as deposited

state. For increasing temperatures grain growth accom-

panied by a decrease in hardness is observed up to 800 8C.Regarding the coating which was annealed at 800 8C a

hardness of 23 GPa still corresponds to a rather small

crystallite size of 8 nm. For the decomposition of the

coating observed at temperatures higher than 800 8C,according to the XRD-patterns, a significant grain growth

of up to 25 nm occurs, resulting in a decrease of hardness

down to 18GPa. Results of the cutting test demonstrated in

Figure 4 show the cutting performance of the produced

Figure 4. Cutting test carried out in Inconel 718.

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nanocomposite coating compared to two commercial

finecrystalline TiAlN-coatings. The cutting test was

performed by longitudinal turning with coated WC/Co

inserts in Inconel 718. The cutting parameters are listed in

Figure 4. Regarding the wear on the flank side, the tool life

of the nanocomposite coating was approximately 33%

longer compared to the finecrystalline coatings. The better

cutting performancemay be a result of the nanocomposite

microstructure corresponding to good coating properties

like high hardness (30 GPa), high hot hardness and

resistance against recrystallisation (up to 800 8C), and

excellent adhesion which was determined in the scratch

test showing a critical load of 100 N.

Conclusion

The present paper reports about the properties of

nanocomposite TiAlN coatings dependent on their crystal-

lite sizes. It is proven that nanocomposite TiAlN coatings

deposited by MSIP pulsed technology exhibit extremely

small grain sizes down to 6 nm. The paper reveals a

correlation between grain size and coating hardness. For

decreasing crystallite size, hardness increases and for small

crystallites a high thermal stability is given. For tempera-

tures higher than 800 8C recrystallisation occurs resulting

in grain growth and decrease in hardness. The perfor-

mance of the nanocomposite coating has been proven in a

cutting test. The nanocomposite coating was found to be

more wear resistant than two commercial fine crystalline

coatings of the same system.

Plasma Process. Polym. 2007, 4, S673–S676

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Acknowledgements: The authors gratefully acknowledge thefinancial support of the German Research Foundation (DFG) withthe project number Lu 232 93-1.

Received: September 1, 2006; Revised: October 30, 2006;Accepted: November 10, 2006; DOI: 10.1002/ppap.200731703

Keywords: coatings; hardness; nanocomposites; pulsed dis-charges; sputtering

[1] S. Carvalho, E. Ribeiro, L. Rebouta, C. Tavares, J. P. Mendonca,A. Caetano Monteiro, N. J. M. Carvalho, J. Th. M. De Hosson,A. Cavaleiro, Surf. Coat. Technol. 2004, 177–178, 459.

[2] S. Veprek, S. Reiprich, Thin Solid Films 1995, 268, 64.[3] R. Cremer, M. Witthaut, D. Neuschutz,Miner. Met. Mater. Soc.

1998, 249.[4] P. Mayerhofer, C. Mitterer, H. Clemens, Adv. Eng. Mater. 2005,

12, 1071.[5] J. Musil, H. Hruby, Thin Solid Films 2000, 365, 104.[6] W. C. Oliver, G. M. Pharr, J. Mater. Res. 1992, 7, 1564.[7] G. M. Pharr, W.C. Oliver, MRS Bulletin 1992, 28.[8] P. Nesladek, Thesis, Technical University Munich, Munich

2000.[9] H. Natter.M. Schmelzer, M. Loffler, M.-S. Krill, C. E. Fitch,

A. Hempelmann, J. Phys. Chem. 2000, 104, 2467.[10] S. Bolz, Thin Solid Films 2006, in press.[11] S. Paldey, S. C. Deevi, Mater. Sci. Eng. A 2002, 342(1–2), 58.[12] F. Adibi, I. Petrov, L. Hultmann, U. Wahlstrom, T. Shimizu,

D. McIntyre, J. E. Greene, J.-E. Greene, J.-E. Sundgren, J. App.Phys. 1991, 69(9), 6437.

[13] A. Horling, L. Hultmann, M. Oden, J. Sjolen, L. Kerlsson, J. Vac.Sci. Technol. A 2002, 20(5), 1815.

[14] P. Spencer, Z. Metallkd. 2001, 92, 1145.[15] H. W. Hugosson, H. Hogberg, M. Algren, M. Rodma,

T. I. Selinder, J. App. Phys. 2003, 93, 4505.

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