Comprehensive wear model for tools with nanocomposite PVD coatings

12^th International Conference on Tools

University of Miskolc, Hungary

September 6 – 8. , 2007

ICT2007

dr. Sándor SIPOS^, Dr. Tibor CSELLE (PhD) ^, Sándor CSUKA^

Abstract

The present paper summarises the methods of physical vapour depositions as well as importance and possible performance of nanocomposite coatings. After a detailed summary of testing conditions this article briefly introduces the gained results and analyses the wear mechanisms of end mills which are deposited with various coating layers. This paper contains a comprehensive wear model, which could describe several micromechanisms of high temperature tribological system. With clearing up the intensity of degradation (used up) process worn tools can be modelled under several machining conditions. Some connections have been worked out for describing several symptoms (e.g. increasing feed forces during machining) in conjunction with tool wear. Lastly the paper presents the most important functions for tool monitoring of end milling cutters.

Introduction

Nowadays the most important field of the world wide competition is the tooling of manufacturing. The number of companies is increasing which aspire not only research getting to know and adopting new technologies, but after a successfully finished product development phase they begin to develop these further. The main fields of tool innovation are advanced design, up to date tool materials and new coating layers deposited with various system methods. In the latter case the trend can be noticed that the role of universal coatings is decreasing, but the usage-oriented vapour deposition systems (so called tailored coatings) are suddenly growing up.

In co-operation the research team's of Banki Polytechnic and Platit AG (Switzerland) as its subsidiary companies, have managed to explore the influence of PVD coatings deposited to carbide turning inserts as well as to cobalt alloyed (HSS-Co8) and powder metallurgical sintered (HSS-PM) high speed steel end mills on the cutting performance.

1. Previously gained results in various coatings

The conventional (Ti, Al)-based PVD coatings (e.g. TiAlCN, AlTiN, etc.) have conquered the market of high performance milling tools in the last years. These can be featured by the following characteristics: high hardness and hot hardness, high oxidation resistance and low heat conductivity. The superhard nanocomposite coatings are based on Ti, Al and Si elements, and two different phases are emerged in the plasma: the nanocrystalline AlTiN grains (cca. 3 nm) will be embedded into the amorphous Si₃N₄-matrix. These forms of coating can be characterised by the following: the structure enables extremely high hardness (40-50 GPa), which can be maintained at very high temperatures (up to ~1100°C). The enormous warm hardness makes for the coating possible machining without coolant.

Five deposition layers with different thin film characteristics have been used to study the wear behaviour PVD coatings of various thickness layered (2-4 μm) on identical Co alloyed high speed steel substrates. Two conventional PVD-coatings are TiAlCN respectively AlTiN layered, and superhard multilayered nanocomposites are as follows: nACRo-2 μm, nACRo-3 μm and nACRo-2,4 μm. All coatings are deposited on ∅10 diameter end mills (made by Fraisa AG., Switzerland).

Some results of our tests, carried out to study the wear mechanisms by using above mentioned coatings can be summarised as follows (Figure 1.):

• Although conventional TiAlCN coating’s hardness and the abrasive wear resistance increases, but with limited thermal stability can result only low protective effects on the tools. Due to the further increase of the maximal application temperature and higher oxidation resistance advanced AlTiN coating protects much better. The good performance of AlTiN has been particularly presented at non commonly used cutting speeds.

• • Nanocomposite coated end mills surpass at the tool/workpiece interface with enhancement of frictional characteristics. The tool life volumes of tested layers have achieved an increase of 20-50% at a 60 m/min cutting speed, but at higher speeds even higher improvement reached.

• The tool life was extremely high at cutting speed 80 m/min with an nc-AlCrN+a-Si₃N₄ nanocomposite coating, due to the extremely high stability up to high thermal loads.

2. Main aims and experimental procedures

The friction and wear of milling cutters limit often detrimentally the performance of machining process, thus process improvement may be achieved by understanding and modelling these mechanisms. A great number of investigations have been recently performed to evaluate milling process, including the systematically analysis the active areas and contact zone of the tools under different conditions.

Main aims are listed as follows:

• Which substrate material can give longer tool life of nanocomposite coated end mills?

• How does the layer thickness influence cutting performance of milling cutters?

• Which model can describe behaviour of milling tools?

Conditions of carried out test are shown in Figure 2.

The experiments have been carried out as conventional shoulder milling. The coating layer wear mechanisms was measured periodically during interruption of milling process, to measure flank wear of the main (peripheral) cutting edge and its corner wear. The components of cutting force (feed and perpendicular of feed direction, so called normal force), acting on the plane of workpiece, have been measured immediately before of wear investigation. Commercial end mills diameter 10 mm with four flutes were used, wear limits wereVB=0,6 mm at the corner of peripheral edge.

3. Results of investigations

After systematically carried out examinations the main aims have been reached, so we can present the most important results of milling tests.

3.1. Tool life of different nanocomposite coatings

The most important results of the tool life investigations were collected in Figure 3. After calculating milling length (L_T = 10^–3 *T *v_f [m]), we are able to draw the following conclusions:

1.) substrates, based on HSS-Co material could not perform at higher cutting speed; on the other side, powder metallurgical high speed steel (HSS-PM) shown an uniform performance,

2.) end mills, made from HSS-PM substrate with a nACo nanocomposite layer afford remarkable performance,

3.) at higher cutting speed composites, containing AlCrN nanocrystalline have reached longer milling path.

3.2. Wear mechanisms of nanocomposite layers deposited by PVD method

Based on observation of wear of the cutting tools with stereomicroscope (and CCD camera), it is clear that different wear mechanisms took place on coated end mills.  A worn edge of end mills is presented in Figure 4.

Hard particles in forms of inclusions, oxides and carbides in workpiece material and carbide particles of the tool material are supposed to be the source of abrasive wear mechanism, as it is shown evident as in Figure 4. If the substrate material has lower hardness or the peripheral edges have incorrect geometry, after a few minutes milling could be appeared an extreme wear intensity rate, and high cutting forces cause breakage of the corner and/or cutting edge (mechanical failure).

Sticking up of the workpiece material (so called build up) can not be clearly observed on relief flank. This is consistent with the dependence of wear mechanisms on cutting speed. Higher cutting speeds are favourable for abrasive wear and the existence of the coating at the wear land boundaries prevents it from adhesion.

Thermo-chemical wear mechanism was occurred in the extremely complex environment during milling. The cutting temperatures, estimated as ≥650°C, might develop these in the vicinity of the cutting edge, causing the failure of the cutting tools. The starting point of the controlled temperature failure was observed either at the corner or at a place on the main edge where the tool is in contacts with the surface of workpiece.

An extreme high hardness, low adhesion and moderate friction of nanocomposite coatings as well as chemical inertness of the deposited layer with workpiece material could contribute to a reduction of wear intensity rates and an increased tool life.  According to literature [4] we could describe flank wear intensity rate (I_VB, μm/min) of tested end mills by the following formula:

IVB(tc)=C1⋅e(A+B⋅tc+C⋅tc2) μm/min,(1)

and from the calculated intensity rates flank wear propagation as follows:

VB(tc)=C2⋅tc⋅IVB(tc) mm,(2)

were

t_c – elapsed cutting time, min,

I_VB(t_c) – calculated temporary wear intensity rate, μm/min,

A, B, C – constants for wear intensity rate model,

C₁ , C₂ – constant of measurements.

In the most cases the observed (measured) flank wear and presented wear model are adequate for describing the complex micro mechanisms of nanocomposite layered end mills, except those extreme wear intensity rates, which could be reach as much as 250 μm/min. We could prove the elaborated wear model, plotted in Figure 5. The above-mentioned model was validated by a high number of experiments, which in the overwhelming majority of cases shown good agreement with the predictions.

3.3. Tool monitoring by force measurements

In the phenomenon of cutting performance degradation are included all of those undesirable processes and effects, which are connected with the tool wear propagation or indirectly could play role in them. Based on measurements of feed and normal force components and on the parallel observation of flank wear of tested end mills by stereomicroscope, we were able to bring into connection both of them. After defining the so called corrected flank wear width (1+VB), we could describe the most important formula of tool monitoring as follows:

Fˉf(VB)=CFf⋅(1+VB)xFf [N],

where:

Fˉf – average feed force component of applied end mills, N

C_Ff – average feed force of sharp milling cutter (constant),

x_Ff – estimated exponent of corrected wear width, which is connected to an increase of feed force component, having an effect on degradation process.

The cutting performance degradation model was validated by a high number of experiments, which shown good agreement with the predictions and can be successfully applied in both cases of the cutting experiments, carrying out end milling and the tool monitoring manufacturing processes with simple supervision condition.

4. Summary

We carried out test series in order to determinate the cutting performance of different coatings. Analysing the results, gained by us, we can conclude that

• hard coating layer changes frictional contact conditions between workpiece and tool materials, resulting in application of much higher cutting conditions,

• the coating reduces considerably the cutting forces at enormous cut work material volume and abrasive wear even when the coating was worn out at the vicinity of the cutting edge,

• on the one hand our investigations confirmed that the powder metallurgical substrates excel in their performances, on the other hand several nanocomposite coatings, tested by us, could be used in accordance with applied milling condition (especially cutting speed),

• in the most cases the presented wear model is adequate for describing the complex micromechanisms of nanocomposite layered end mills,

• the cutting performance degradation model, based on measuring of feed force component, was validated by a high number of experiments, which shown good agreement with the predictions.

Other wear mechanisms as mentioned in the introduction part could contribute to wear of the milling tools.  However, with more accurate investigations the complex micromechanisms of wear should be discovered by applying scanning electron microscope.

5. References

[1] Reports on cutting performance of several conventional and nanocomposite layers deposited PVD method at PLATIT AG., Budapest, 2004-2007.

[2] R&D activities of Budapest Polytechnic in the fields of tool qualifying

Poster program: “Innovativeness in higher education” (edited by Scientific Society of Mechanical Engineering)

International Fair MACH-TECH, Budapest, April 19-22. 2005.

[3] Sándor Csuka: Investigations of wear characteristics of nanocomposite coated tools

Paper for State Scientific Students' Association, Győr, 2007. pp. 48.

[4] Sándor Sipos: Investigation of cutting performance of coated high speed steels made in Hungary (Doctoral Dissertation), NME, Miskolc, 1986. pp. 99 (appendix: pp. 97)

*- dr. Sándor, SIPOS Associate Professor,

Banki Donat Mechanical Engineering Faculty, Budapest Tech,

H-1081. Budapest, Népszínház u. 8. E-mail: sipos.sandor@bgk.bmf.hu

** - Tibor, Dr. CSELLE (PhD.), CEO

Platit AG., Moosstrasse 68. CH-2540, Grenchen, Switzerland

*** - Sándor, CSUKA III. graduate student

Bánki Donát Mechanical Engineering Faculty, Budapest Tech,

H-1081 Budapest, Népszínház u. 8.