Grain size effect of pre- and post-coating treated cemented carbides on PVD films' adhesion and mechanical properties

697Mat.-wiss. u. Werkstofftech. 2013, 44, No. 8 DOI 10.1002/mawe.201300178

Grain size effect of pre- and post-coating treated cementedcarbides on PVD films’ adhesion and mechanical properties

Einfluss der Karbidkorngr�ße vor- und nachbehandelterHartmetallsubstrate auf die Schichthaftung und die mechanischenEigenschaften der PVD-Beschichtung

K.-D. Bouzakis1, 2, N. Michailidis2, 3, G. Skordaris1, 2, A. Tsouknidas1, 2, S. Makrimallakis1, 2, E. Bouzakis1,2

The grain size of coated cemented carbide tools may significantly affect the tool life by influenc-ing either the coating adhesion to the substrate or the coating growth during deposition. In thepresent study three variously grained cemented carbides were coated with two different PVDfilms. Various pre- and post-treatments were applied, while their effect on the films’ mechanicalproperties and adhesion was assessed by nanoindentations and inclined impact tests. The coat-ings deposited on ultra-fine-grained substrates exhibited the highest mechanical propertiesoverall. The research revealed elevated adhesive properties for the fine-grained substrate com-pared to the normal and ultra-fine-grained one. The film adhesion of normal and fine-grainedsubstrate can be further enhanced through micro-blasting. Post-treatments of the coatingswere beneficial in all substrate cases, as brushing of the film surface removed droplets while atthe same time inducing surface residual stresses at the coating resulting in a hardness increase.

Keywords: PVD coatings / cemented carbides / grain size /

Die Karbidkorngr�ße von beschichteten Hartmetallwerkzeugen kann die Schichthaftung und dasSchichtwachstum maßgeblich beeinflussen und dadurch die Werkzeugstandzeit bestimmen. Inder vorliegenden Studie wurden drei Hartmetallsubstrate mit verschiedenen Korngr�ßen, die mitzwei PVD-Schichten beschichtet wurden, untersucht. Der Einfluss unterschiedlicher Vor- undNachbehandlungen auf die mechanischen Eigenschaften der Beschichtung sowie auf die Schicht-haftung wurde mit Hilfe von Nanoindenter-Untersuchungen und Impact-Tests beurteilt. DieHartstoffschichten die auf Ultrafeinkorn-Sorten (UF) abgeschieden wurden, wiesen insgesamtdie h�chsten mechanischen Eigenschaften auf. Dar�ber hinaus zeigten die Versuchsergebnisseeine verbesserte Schichthaftung bei der Feinstkorn-Sorte (F) gegen�ber den Feinkorn- (N) und Ul-trafeinkorn-Sorten. Die Schichthaftung k�nnte bei fein- und feinstk�rnigen Hartmetallsubstra-ten durch eine Mikrostrahlbehandlung mit Festk�rpern angepasst werden. Nachbehandlungender beschichteten Substrate mittels B�rsten waren vorteilhaft f�r alle unterschiedlichen Substra-te. Dies ist sowohl auf die Entfernung von Droplets aus der Schichtoberfl�che als auch auf die In-duzierung von Druckeigenspannungen zur�ckzuf�hren.

Schl�sselw�rter: PVD-Schichten / Hartmetall / Karbid-Korngr�ße /

1 Introduction

Both substrate and coating materials exert a significant effect onthe tool wear during machining. The use of cemented carbidetools with different grain size may influence not only the tool per-formance, but also the film growth during the vapor depositionprocesses and hence, the mechanical properties of the deposited

film [1–3]. In this context, the selection of the appropriate sub-strate pre- and post-treatment is pivotal for achieving the opti-mum tool life. In the present paper, two different PVD films, anAlTiN and a multi-layer AlTiN/TiSiN, were deposited on threecemented carbide substrates with average grain sizes and compo-sitions as follows:

Normal (N) grain size (~1.2 lm) containing 87% WC, 12% Coand 1% various carbides.

Fine (F) grain size (~0.8 lm) containing 89% WC, 10% Co and1% various carbides.

1 Laboratory for Machine Tools and Manufacturing Engineering,Mechanical Engineering Department, Aristoteles University of The-ssaloniki, Thessaloniki, Greece

2 Fraunhofer Project Center for Coatings in Manufacturing, in Centrefor Research and Technology Hellas in Thessaloniki and in FraunhoferInstitute for Production Technology in Aachen, Germany

3 Physical Metallurgy Laboratory, Mechanical Engineering Department,Aristotle University of Thessaloniki, Thessaloniki, Greece

i 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.wiley-vch.de/home/muw

Corresponding author: K.-D. Bouzakis, Laboratory for Machine Toolsand Manufacturing Engineering, Mechanical Engineering Department,Aristoteles University of Thessaloniki, 54124 Thessaloniki, GreeceE-mail: [email protected]

K.-D. Bouzakis et al. Mat.-wiss. u. Werkstofftech. 2013, 44, No. 8

Ultra-fine (UF) grain size (~0.5 lm) containing 86.5% WC,12% Co and 1.5% various carbides.

Al60Ti40N was deposited by a cathodic vacuum arc process attemperatures in the range of 500 8C in a nitrogen atmosphere,having a columnar structure [4]. The multi-layer AlTiN/TiSiNcoating was produced at similar AlTiN compositions and processconditions, by alternating AlTiN and TiSiN layers [5]. Both coat-ings had an overall thickness of 2 lm.

Micro-blasting of the substrates was conducted, prior to thecoating deposition, to improve the coating adhesion [6–8], at apressure of 3 bar for 5 s vertical to the substrates’ surface using~10 lm large Al2O3 particles. Brushing of the deposited coatingled to a further improvement of the coating performance. Thescope of the paper was to investigate the effect of the substrategrain size of well-adherent coatings (on pre-treated substrates),since the effect of the pre-treatment on the coating adhesion hasbeen already investigated [8]. Additionally, appropriate coatingpost-treatments may improve the films performance [7].

Nanoindentations were employed to determine the mechani-cal properties of the substrates and coatings for all examinedcombinations. Inclined impact tests and Rockwell indentationsrevealed the effect of the substrate grain size on the coating adhe-sion strength.

2 Film and substrates’ mechanical properties

In order to evaluate the effect of the cemented carbide grain sizeon the mechanical properties of the applied substrates, macro-and nanoindentations were conducted. Starting from the ultra-fine grained to the normal-grained substrate, the Rockwell Chardness is decreased, Figure 1a. The respective load-displace-ment nanoindentation diagrams confirm the same tendency,Figure 1b. The three curves represent the average of 320 nanoin-dentation measurements taken per grain size.

The cemented carbides’ grain size affects significantly thePVD AlTiN coating structure, Figure 2. The metastable solid sol-ution phase of (Al,Ti)N is characterized by its cubic NaCl (B1)structure. Cemented carbides (WC) possess a similar cubic crys-tal system. In this way, it can be assumed, that during the film

deposition on ultra-fine cemented carbides, the nucleation rateof potentially formed transient junctions such as AlTiN on theWC-carbide surfaces is increased compared to the correspondingone of normal grain size substrate. Due to this fact, in the lattercase, the coating exhibits a coarser structure and its mechanicalproperties are expected to be poorer.

Arc PVD coatings are usually characterized by increasedroughness due to droplets formed during deposition. Post-treat-

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Figure 1. Rockwell C hardness and nanoindentation results conducted on the cemented carbides with different grain size.

Bild 1. Rockwell C H�rtemessungen und Nanoindenter-Untersuchungen auf Hartmetall-Substraten mit verschiedenen Karbid-Korngr�ßen.

Figure 2. Structure of the coatings deposited on the cemented car-bide and interaction with different WC grain size.

Bild 2. Struktur der abgeschiedenen Beschichtungen auf Hartmetall-substrate mit unterschiedlichen Karbid-Korngr�ßen.

Mat.-wiss. u. Werkstofftech. 2013, 44, No. 8 Grain size effect of pre- and post-coating treated cemented carbides

ments like brushing are supposed to have beneficial effects onsurface integrity due to droplets’ removal. In addition, residualcompressive stresses are induced into the film, thus leading tocoating hardness and strength properties improvement. The pos-itive effect of brushing on the film surface topography is revealedwhen comparing the film surface prior and after brushing, Fig-ure 3. To capture the film mechanical properties, due to thedifferent substrate grain size, nanoindentations were performedon the AlTiN and AlTiN/TiSiN films. As brushing of the coatingleads to droplets’ removal, the developed scatter in the nanoin-dentation load-displacement the maximum indentation depthafter brushing is registered compared to the corresponding oneof the unbrushed coating. This can be attributed to the coatingsurface deformation during brushing, resulting in compressiveresidual stresses and consequently to hardness increase.

An overview of the maximum indentation depth, measured ata load of 15 mN on both coatings, is summarized in Figure 4 forall substrate cases. A strong relationship of the maximum inden-tation depth attained on the coatings with the substrate's grainsize can be observed, as the films grown on the normal-grained

substrates seem to be softer than the ones on the fine-grained,while coatings deposited on the ultra-fine-grained cemented car-bides withstand better the penetration of the Berkovich indenter.

3 Film adhesion assessment by Rockwellindentations

To evaluate the effect of the different cemented carbide grainsizes after micro-blasting on the film adhesion, Rockwell Cindentations were conducted on the coated specimens. TheRockwell imprints were scanned by white light confocal micro-scopy for both the AlTiN and AlTiN/TiSiN coatings, Figure 5. Inthe case of the AlTiN film deposited on an ultra-fine grained sub-strate, large radial cracks were observed in the imprint vicinity.Moreover, the substrates with fine and normal grain sizes exhibitpractically no cracks or detachments close to the imprint’s vicin-ity. According to these results, in all the examined cases, theadhesion may be characterized Class 1 [9]. Respective investiga-tions were conducted for AlTiN/TiSiN coating deposited on

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Figure 3. Micrographs of surfaces of an AlTiN coated UF substrate prior and after brushing and corresponding nanoindentation measurements.

Bild 3. Mikroskopische Aufnahmen von AlTiN-beschichteten Ultrafeinkorn-Substraten vor und nach dem B�rsten sowie entsprechende Nanoh�r-temessungen.


cemented carbides with different grain sizes and again all theimprints could be categorized as Class 1.

4 Film adhesion evaluation by inclined impacttests

For assessing quantitatively the film adhesion in the differentsubstrate grain sizes, inclined impact tests were conducted. Theinclined impact test is a convenient tool for predicting the coat-

ings’ adhesive properties. In this experimental method, therepetitive impacts are applied on an oblique coated surface,which is loaded vertically and tangentially simultaneously, Fig-ure 6. Due to the perpendicular loading direction during theRockwell indentation, the developing shear loads which are crit-ical for revealing film adhesion problems are relatively small.However, the inclined impact test renders possible the exerciseof shear loads on the film surfaces, leading to a more clear andrealistic determination of the film adhesion. The developedimprints can be evaluated with the aid of micrographs based on

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Figure 4. Overview of obtained nanoindentation depths on AlTiN and AlTiN/TiSiN coated specimens of various substrates.

Bild 4. �bersicht der erreichten Eindringtiefen bei AlTiN- und AlTiN/TiSiN-beschichteten Hartmetallproben aus verschiedenen Substraten.

Figure 5. Rockwell C imprints on all examined specimens coated with AlTiN and AlTiN/TiSiN films.

Bild 5. Rockwell C Eindr�cke auf den mit AlTiN- und AlTiN/TiSiN-beschichteten Proben.


confocal white light measurements, as it is displayed at the rightfigure part.

The remaining imprint depths, captured by such measure-ments, versus the number of impacts on AlTiN coated substratesare exhibited in the upper part of Figure 7. According to theattained results, AlTiN coatings deposited on cemented carbidesubstrates with fine grain size, withstand more effectively therepetitive oblique impact loads compared to those deposited onultra-fine or normal grain size substrates. Hereupon, theremoval of films from normal grain size substrates is compara-ble to that of fine-grained substrates. In contrast, the failure ofthe AlTiN film deposited on ultra-fine substrates leads to themost accelerated film removal, due to the comparably lowestadhesion. The latter can be explained taking into account thedeteriorated Co-free carbide surfaces of ultra-fine cemented car-bides after micro-blasting. Related tests were conducted in thecase of AlTiN/TiSiN films deposited on cemented carbide sub-strates with various grain sizes, Figure 7. According to theattained results, in all grain size cases, larger remaining imprintdepths are registered, even at the lower impact force of 10 daN,when AlTiN/TiSiN films are employed, compared to correspond-ing ones in the case of AlTiN films. Moreover, it is obvious thatthe ultra-fine cemented carbide substrates are associated with apoor film adhesion, which leads to a comparably more intensefilm removal during inclined impact test. For the tested loadsduring the inclined impact test, no plastic deformation of the

substrate occurs. However, when poor adhesion exists, the devel-oping stresses in the film are increasing, leading to a film prema-ture failure versus the number of impacts.

A comparison of the required number of impacts for achievinga remaining imprint depth equal to 0.5 lm, for AlTiN andAlTiN/TiSiN films for all the spectrum of investigated substrategrain sizes are shown in Figure 8. It is obvious that AlTiN coat-ings deposited on ultra-fine substrates exhibit the worst perform-ance due to their deteriorated adhesion. Moreover, a larger num-ber of impacts is required, when fine grained substrates areapplied compared to normal ones, due to their improvedmechanical properties and adhesion. The same figure shows theobserved tendencies in the case of AlTiN/TiSiN films. It has to bepointed out that AlTiN/TiSiN coatings are characterized by theirincreased hardness but also brittleness compared to AlTiN films.Thus, the same remaining imprint depth is achieved at compara-bly lower number of impacts in all investigated cases, since thecrack propagation in the film structure is comparably moreintense.

5 Film adhesion on substrates of various grainsizes

The coating adhesion and subsequently the cutting performanceof coated cemented carbide tools depend significantly on the

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Figure 6. Inclined impact test on coated specimens and determination of the remaining imprint depth.

Bild 6. Geneigter Impact-Test an beschichteten Proben und Ermittlung der verbleibenden Abdrucktiefe.


applied mechanical pre-treatments [10]. Micro-blasting ofground substrates is an efficient method for improving filmadhesion [6–8, 11]. This can be explained considering the effect

of micro-blasting on the substrate surfaces with regard to experi-ment results [6]. The individual carbides are revealed throughthe Co-binder removal from the cemented carbide surfaces. The

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Figure 7. Remaining imprint depth versus the number of impacts in inclined impact test for AlTiN and AlTiN/TiSiN coating on HM specimens.

Bild 7. Verbleibende Abdrucktiefen in Abh�ngigkeit von der Lastspielzahl beim geneigten Impact-Test an AlTiN- und AlTiN/TiSiN-beschichtetenHartmetallproben.

Figure 8. Comparison of the attained inclined impact test results for AlTiN and AlTiN/TiSiN coated specimens.

Bild 8. Vergleich der erzielten Ergebnisse beim Impact-Test an AlTiN- sowie an AlTiN/TiSiN-beschichteten Proben.


positive micro-blasting effects cannot be obtained in the case ofultra-fine cemented carbide, when micro-blasting grains arecomparably larger than the average WC carbide grain size, Fig-ure 9. Due to the dense WC grains distribution, Co-binderremoval is not possible. To overcome this problem a reduction ofthe grain size used in micro-blasting is necessary. After micro-blasting in the fine-grained substrate, a larger amount of WC-car-bides is revealed compared to the normal ones, also due to itssmaller Co content. Since the film is favorably built on the WC-carbides, the adhesion of the ultra-fine-grained substrates,micro-blasted by the indicated micro-blasting materials’ grainsize is comparably deteriorated.

6 Conclusions

In the present paper, normal, fine-grained and ultra-fine-grainedcemented carbide substrates were coated with two different PVDcoatings, an AlTiN and an AlTiN/TiSiN one. Nanoindentationand inclined impact test results revealed that the coating depos-ited on the ultra-fine-grained substrate possesses increasedmechanical strength compared to the film deposited on the nor-mal and fine-grained substrates. Rockwell indentations validatedthe substrates’ hardness and yielded to the conclusion of a goodadhesion of the deposited coatings on all the examined substrategrain sizes. These results exhibit the potentials to furtherimprove the cutting performance of coated tools by an appropri-ate combination of substrate grain size and proper pre- and post-treatment.

Acknowledgements

Part of the activities presented in this paper was funded by theSeventh Framework Programme (FP7) in the frame of the Quick-Pro project (262272).

7 References

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[8] K.-D. Bouzakis, G. Skordaris, E. Bouzakis, A. Tsouknidas,S. Makrimallakis, S. Gerardis, G. Katirtzoglou, CIRP Ann.-Manuf. Techn. 2011, 60, 587.

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Received in final form: June 17th 2013 T 178

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Figure 9. Micro-blasting effects on cemented carbide surface structure with different WC grain size.

Bild 9. Einfluss der Mikrostrahlbehandlung mit Festk�rpern auf die Oberfl�chenstruktur von Hartmetallsubstraten mit verschiedener Karbid-Korngr�ße.

Grain size effect of pre- and post-coating treated cemented carbides on PVD films' adhesion and mechanical properties

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