Proceedings of the 7th International Conference Coatings in Manufacturing Engineering, 1-3 October 2008, Chalkidiki, GreeceEdited by: K.-D. Bouzakis, Fr.-W. Bach, B. Denkena, M. Geiger,

Published by: Laboratory for Machine Tools and Manufacturing Engineering (ΕΕΔΜ),

Aristoteles University of Thessaloniki and of the Fraunhofer Project Center Coatings in Manufacturing (PCCM),

a joint initiative by Fraunhofer-Gesellschaft and Centre for Research and Technology Hellas

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EFFECT OF DRY MICRO BLASTING CONDITIONS OF PVD FILMS’ PROPERTIES

AND ON THE CUTTING PERFORMANCE OF COATED TOOLS

K.-D. Bouzakis1,3, S. Gerardis1,3, G. Skordaris1,3, G. Katirtzoglou1,3,

S. Makrimallakis1,3, M. Pappa1,3, N. Michailidis1,3, F. Klocke2,3, E. Bouzakis2

1. Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering Department, Aristoteles University of Thessaloniki, 54124, Hellas

2. Laboratory of Machine Tools and Production Engineering, Technical University of Aachen, Steinbachstr.53, D-52056 Aachen, Germany

3. Fraunhofer, Project Center Coatings in Manufacturing, A joint initiative by the Fraunhofer and the Centre for Research and Technology Hellas (CERTH)

ABSTRACT The conduct of micro-blasting on PVD films has been documented as an efficient method to improve the cutting performance of coated tools. The increase of tool life is correlated to the applied conditions during micro-blasting. In this way the determi-nation of the optimum condition during micro-blasting is of great importance. In the frame of the present paper, the effect of various blasting conditions such as of pres-sure and duration on the film properties and cutting performance is examined. Through a novel method, combining a FEM-based simulation’s algorithm of the penetration of a blasting grain into the coating material and the related XRD meas-urements at certain depths from the film surfaces, the film strength properties strati-fication versus the coating thickness after micro-blasting were determined. Ball cra-tering tests were carried out at the flank and rake near the cutting edge to investi-gate the alteration of coating thickness due to micro-blasting. Moreover the influence of dry micro-blasting conditions on the cutting edge radius and topomorphy is exam-ined by means of confocal and EDX-measurements respectively. The pre-described investigations enable an accurate estimation of the effect of dry micro-blasting con-ditions on the tool life. Milling experiments were conducted to investigate the cutting performance of coated tools, subjected to films’ micro-blasting. KEYWORDS: Micro-blasting, Wear, Cutting performance

1. INTRODUCTION Nowadays the coating manufactures tend to conduct micro-blasting on PVD films surface in order to enlarge the tool life /1,2,3/. The key-issue in such procedure is the application of appro-priate blasting condition. Variation of micro-blasting pressure and durtime is expected to affect significantly the wear resistance of the coated tools. The main goal of this paper is not only to determine optimum micro-blasting conditions for a specific coating, but also to interpret the oc-curring wear behaviour thoroughly.

2. DETERMINATION OF THE PVD FILMS’ SUPERFICIAL HARDNESS DUE

TO MICRO-BLASTING AT VARIOUS PRESSURES AND DURATIONS Nanohardness measurements were conducted in all examined micro-blasted PVD films at a maximum indentation load of 15mN, keeping all conditions constant during PVD films’ micro-blasting apart from the applied pressure, as illustrated in Figure 1a. As it can be observed in

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Figure 1: a) Nanohardness measurements and b) maximum indentation depth versus the mi-cro-blasting pressure.

Figure 1b, the micro-blasting pressure decreases the indentation depth, thus improving the su-perficial film hardness. Similar investigations were conducted in the case of micro-blasted PVD films with variable exercised micro-blasting duration (see Figure 2a). According to the related results (see Figure 2b), as the duration gets higher, the film superficial hardness is increased.

Through a method, based on residual stress measurements by X-ray diffraction (XRD) tech-niques and by a developed FEM-based algorithm, describing the continuous penetration of indi-vidual blasting grains into the coating material /4/, the coating strength properties distributions such as of the yield strength can be determined. The distribution of the yield strengths at vari-ous coating depths from the film surface of the examined coating, subjected to micro-blasting at

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Figure 2: a:) Nanohardness measurements and b:) maximum indentation depth versus the micro-blasting duration.

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Figure 3: Yield strength versus the coating depth at various micro-blasting pressures.

various pressures is shown in Figure 3. Depending upon the applied micro-blasting pressure, the width of the overstressed region within the coating due to micro-blasting is modified. Thus, although the increase of the yield strength remains invariable from micro-blasting pressure su-perficially, the increased micro-blasting pressure leads to higher plastic deformations and con-sequently films’ yield strengths at a distance from film surface over ca. 0.5 µm. 3. EFFECT OF THE MICRO-BLASTING PRESSURE AND DURATION ON COATING

THICKNESS, CUTTING EDGE RADIUS AND TOPOMORTHY According to recent publication /5/, the film thickness distribution on the rake and flank influence significantly the wear propagation in cutting processes. Thicker film on the tool rake in compari-son to the one on the flank, and moreover a thick and uniformly deposited film in the cutting wedge region enhances significantly the cutting performance in milling. The conduct of micro-blasting on PVD films might influence the distribution of the coating thickness near the cutting edge. In order to examine this effect, appropriate ball cratering tests were carried out. A charac-teristic ball-cratering imprint is shown in the upper part of Figure 4. Those imprints were evalu-ated with the aid of confocal extracted diagrams, describing the alterations of the crater depth versus the crater width (see bottom part of figure 4). According to the related results, shown in Figure 5, the micro-blasting procedure leads to a slightly decrease of the coating thickness on the rake, whereas the coating thickness on the flank remains almost invariable. More specifi-cally, at higher exercised pressures and durations during micro-blasting, this effect is more in-tense.

One more crucial tool parameter, affecting its cutting performance is the constructive cutting edge radius. As it has been already reported /6/, larger cutting edge radii reduce the tool me-chanical stresses, contributing to a significant increasing of the coated tool lifetime. The effect of micro-blasting conditions on the magnitude of the cutting edge radius was studied thoroughly by means of the conduction of confocal measurements along the cutting edge, as exhibited in Fig-ure 6. In this way, the fluctuations of the cutting edge radius versus the cutting edge position after PVD films’ micro-blasting at various pressures were registered. Thus, the mean values of

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Figure 4: Ball cratering test on flank and rake surfaces.

Figure 5: Coatings’ thickness on flank and rake surfaces.

the cutting edge radius were captured for all investigated cases. According to the extracted re-sults, shown in Figure 7, micro-blasting procedure on PVD films leads to cutting edge radius increase and this effect is more intense, as the pressure and duration get higher.

An overview concerning the effects of PVD films’ micro-blasting on the coating thickness of the cutting edge as well as on the cutting edge radius is shown in Figure 8. On the one hand, in-creased micro-blasting pressure and duration enlarge the cutting edge radius and due to this fact a cutting performance enhancement is expected. On the other hand the coating thickness on the cutting wedge is decreased drastically at higher pressures and durations, deteriorating

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Figure 6: Confocal measurements along the cutting edge.

Figure 7: Average cutting edge radius versus the micro-blasting pressure and duration.

the tool life. There is an optimum combination of micro-blasting pressure and duration, which should be applied.

Further investigations were focused on the influence of PVD films’ micro-blasting on the cutting edge topomorphy. It has to be pointed out that the cutting edge is undamaged after Physical Vapour Deposition, as shown in Figure 9a. The conduct of micro-blasting at pressures over ca. 0.4 MPa and duration over 6 sec can cause local coating material removals on the cutting edge, which are monitored through SEM investigations and can be observed in Figure 9b. Related EDX-measurements verify this fact (see Figure 9c), as the chemical elements W and Co start appearing at these measurements.

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Figure 8: Characteristics parameters of the cutting edge radius. 4. WEAR BEHAVIOR OF COATED TOOLS IN MILLING WITH MICRO-BLASTED FILMS AT

VARIOUS PRESSURES AND DURATIONS In order to investigate the effect of PVD films’ micro-blasting conditions on the cutting perform-ance, milling tests were carried out. The experiments were performed using a three-axis nu-merically controlled milling centre. The applied tool-workpiece system in the milling experiments is illustrated in the upper part of the Figure 10. A prescribed of successive cuts was set before every inspection of cutting insert status. The occurring chip characteristics are shown in the same figure. The tool wear condition was examined by means of Scanning Electron Microscopy (SEM) and energy dispersive X-ray microanalyses. Figure 11 demonstrates the flank wear ver-sus the accumulated number of cuts of the examined coatings, subjected to micro-blasting at various pressures and a constant duration of 4 sec. The coated insert subjected to micro-blasting at 0.2 MPa exhibited the best cutting performance, reaching a tool life of 170x103 cuts up to a flank wear width of approximately 0.2 mm. On the other hand, there is a slightly shorter tool life of a coated insert after micro-blasting at 0.4 MPa of almost 165 x103 cuts up to the same flank wear width of 0.2mm. The worst cutting performance, appeared at the coated insert subjected to micro-blasting at 0.6MPa. Characteristic SEM micrographs of the worn cutting edge after 70x103 cuts in all examined cases of the applied pressures are shown in the bottom figure part.

Corresponding milling experiments were carried out in order to investigate the effect of PVD films’ micro-blasting duration at a constant pressure of 0.4 MPa on the wear behavior. Accord-ing to the results, exhibited in Figure 12, the best cutting performance appeared at the coated inserts subjected to a micro-blasting duration of 4 sec, while the achieved lifetime of the coated tools after micro-blasting at 2 and 6 sec are almost equal to the as deposited case.

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Figure 9: SEM micrographs and EDX analyses at the cutting edge of all examined cases.

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Figure 10: Milling kinematics and undeformed chip geometry.

Figure 11: Flank wear development versus the number of cuts for all examined cases micro-blasted at various pressures.

An overview of the achieved number of cuts up to a flank wear width of 0.2mm, depending upon the applied micro-blasting pressure and duration is presented in Figure 13. There is a significant increase of cutting tool performance after films’ micro-blasting at 0.4 MPa and 4 sec. In all other cases, the increase of the tool life is shorter compared to the previous one, while a worsening of the wear behaviour compared to the as deposited films is appeared at a micro-blasting pressure of 0.6 MPa. The wear behaviour alterations can be attributed to the modification of the coating thickness on the cutting edge, as well as to the alteration of the cutting edge radius and topo-morphy after PVD-films’ micro-blasting at various conditions.

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Figure 12: Flank wear development versus the number of cuts for all examined cases micro-blasted at 0.4MPa pressure for various durations.

Figure 13: Overview of the achieved number of cuts up to VB=0.2 mm versus the micro-blasting pressure and duration.

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5. CONCLUSIONS In the present paper the effect of the applied conditions such as of pressure and duration during PVD films’ micro-blasting on the cutting performance is investigated thoroughly. According to the obtained results, micro-blasting on PVD film leads to a modification of the cutting edge radius and topomorphy as well as of the coating thickness on the cutting edge. Milling ex-periments indicate that there is an optimum combination of the applied micro-blasting pressure and duration, maximizing the coated tool lifetime. 6. REFERENCES

1. E. Bouzakis, Steigerung der Leistungsfähigkeit PVD-beschichteter Hartmetallwerkzeuge durch Strahlbehandlung. Dissertation, RWTH Aachen University, 2008.

2. F. Klocke, T. Schroeder, E. Bouzakis, A. Klein, Manipulation of coating and subsurface properties in reconditioning of WC–Co carbide cutting tools, Surface and Coatings Technol-ogy, 202 (2003) 1194-1198.

3. Z. Mohammadi, A.A. Ziaei-Moayyed, A. Sheikh-Mehdi Mesgar, Grit blasting of Ti–6Al–4V alloy: Optimization and its effect on adhesion strength of plasma-sprayed hydroxyapatite coatings, Journal of Materials Processing Technology, 194 (2007) 15-23.

4. Paper- A FEM based analytical-experimental method to determine strength properties gra-dations in coatings after micro-blasting procedures.

5. K.-D. Bouzakis, S. Hadjiyiannis, G. Skordaris, I. Mirisidis, N. Michailidis G. Erkens, Wear development on cemented carbides inserts, coated with variable film thickness in the cutting wedge region, Surface and Coatings Technology, 188-189 (2004) 636-643.

6. K.-D. Bouzakis, N. Michailidis, G. Skordaris, S. Kombogiannis, S. Hadjiyiannis, K. Ef-stathiou, E. Pavlidou, G. Erkens, S. Rambadt, I. Wirth, Optimization of the cutting edge roundness and its manufacturing procedures of cemented carbide inserts, to improve their milling performance after a PVD coating deposition, Surface and Coatings Technology, 163-164 (2003) 625-630.

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