Evaluation of cutting performance on Pramet carbide inserts with several coatings, made by Platit AG

Szabolcs BIRÓ ¹ – Tibor DR. CSELLE ² –

Václav DR. HÁJEK ³ – Sándor dr. SIPOS ⁴

1  Institute Engineer, University Óbuda, Bánki Donát Fac. of Mech. Eng.

2 – PhD, CEO, Platit AG (Switzerland)

3 – PhD, Project Manager, Pivot a. s. (Czeh Republic)

4  Associate Professor, University Óbuda, Bánki Donát Fac. of Mech. Eng.

Introduction

One of the most prosperous fields of the tool innovation – beside the development of tool construction and the expansion of the circle of the usable tool materials – is the field of evolution of materials and methods of the coatings where an explosion-like development can be noticed. The development of various capacity of coating units makes it possible for us to choose from coating types, we never had before. Beside the universal applicable coatings types there are special coating systems (PM HSS hob, drilling of Ni-base alloys etc.) as well, developed for a given task, increasing the cutting performance of the tools significantly.

In co-operation of the research teams’ of Budapest Tech Bánki Donát Faculty of Mech. Eng. and Platit AG (Switzerland) and its subsidiary company Pivot a.s., we have managed to explore the influence of PVD coatings, deposited on carbide turning inserts, made on the cutting performance. Our present evaluation contains the results of investigations, carried out on the coatings, made by the subsidiary in Sumperk on the hard coating unit, coded π300. In order to compare the cutting performance of the coatings better, different types of coating layers have been deposited on the same type (so called basic) insert, made by Pramet a.s.. In this present work we are going to report about the test results of various nanocomposites: for example multilayer coating nACo-MT and nACo³ coating. Our examinations have been extended to the nACoX4 coating system, developed firstly in the world in May 2009, having nanocomposite structure with oxynitride covering layer, as well [1].

1. Main aims and experimental procedures

A great number of investigations has been recently performed by us to evaluate the cutting performance, including the systematical analysis of the active areas and contact zone of the coated insert under different conditions.

1.1. Main goals for testing procedure

The cutting performance has been studied with below listed PVD coatings of 3… 13 μm thickness, layered on identical carbide inserts, made by PRAMET a.s. (Czech Republic). Pramet inserts were basic carbide material, coded H10F, consisting of extra-fine (grain size of 0,8 μm) 90% tungsten carbide (WC) and 10% cobalt.

Three deposition layers with different thin film characteristics have been tested by us to study the cutting performance of carbide inserts in turning process. The physical characteristics of coatings, deposited by π300 machine and latest developed coating systems, are listed in Table 1.

Table 1.

The main goal is to clear up the occurrence of chip formulation and chip breakage, long-lasting behaviour of inserts, mainly their surface finish producing capability by a detailed study. The most important questions are as follows:

• Which coating type can give longer tool life of diamond shaped (CNMG) turning inserts?

• How does the layer constitution and hardness influence the cutting performance of tested inserts?

• Which is the suitable coating material for different purposes?

1.2. Testing methods

The investigations were based on international directives and Hungarian standards, as well as our experiences, gained during our previous experimental series [1].

Table 2.

Although the measuring system allows to measure three components (tangential or cutting force, radial or passive force and longitudinal or feed force), we have measured only two main components on a KISTLER 5015 dynamometer and Dynoware software. The cutting force (F_c) is exerted in a vertical plane tangent to the workpiece. Since the passive force component (F_p) acts perpendicularly on the workpiece, it can deform the machine tool - workpiece - tool (MT-W-T) system.

We have wished to create connection between the observed tool wear and surface roughness parameters. On the observation of flank wear, carried out parallel by stereomicroscope with 45x magnification, we were able to create connection between them.

The cutting conditions have been determined on the basis of our previous examinations (see in Table 2.). Since the cutting performance of any cutting tools depends mainly on wear, this was carefully tested by the above mentioned stereomicroscope. During the whole testing process flank wear land width criteria was equal to 0,2 mm.

2. Testing results

The term “cutting performance” has a complex meaning, which can be featured by the following parameters:

• main parameters, like wear, wear rate and tool life,

• additional parameters like cutting force components, cutting power, cutting temperature, material removal rate, surface quality (including numerous parameters of machined surface roughness and waviness), chip form and sizes etc.

By analysing the gained results, generated under different conditions, we have striven to draw the most important conclusions, suitable for workshop practice. Although these results could be interesting for every user, in this report we have not enough space to show them in details.

2.1. Wear process of coated inserts

As already mentioned, we have measured the cutting performance at several cutting speeds. Since the substrates, insert geometries and edge preparations were identical on the tested inserts, we can conclude that the perceptible variance has been caused by the different coating layers.

The main character of the deterioration of cutting tool is the wear land, observed on sevaral parts of cutting inserts. We have observed two wear types, namely flank wear, caused by the well known abrasion mechanism of hard particles of the workpiece material, as well as the crater wear, resulting from the adhesive and diffusive action between the chip and tool material. We have focused our investigations on measuring of the flank wear and also examined the propagation of the crater wear.

Three types of the coating layers, mentioned above, will be presented in the following paragraphs. It is worth to analyse the wear process and the development of the wear intensity in details [2].

nACo-MT type coating layers

The standard nACo (TiAlN / SiN_x) nanocomposite coating has appeared on the product line of Platit AG in 2003. Its multilayered version has been tested by us. On the microscopic photos, placed at our disposal, the multilayer structure of the deposited coating is well visible in Figure 1. This type of coating layer has been tested at four different cutting speed values, applying a large range of speed values (v_c = 200 – 315 m/min). The wear curves, belonging to the different cutting speed values, can be seen in Figure 1.

The main character of the deterioration of cutting tools is the flank land wear, observed on the main cutting edge. All of the estimated wear - time curves have typical development, containing moderate growth and rapid wear escalation. This cutting condition has caused a slight cutting action with a very low tool wear rate. After rapid wear during the first few (approximately 2) minutes the wear settles down to a steady-state rate (see Figure 1.). The wear escalation has spreaded over not only the flank land, but an intensive crater formulated on the rake face near to the chip breaking grooves. Thus the real wear criteria of the tested inserts were broken up of main cutting edge. At low cutting speed this breaking has occured when flank wear width was equal to cca. VB ≈ 0,2 mm , but at high speed (v_c ≥ 300 m/min) this symptom has settled down at VB ≤ 0,15 mm wear width. This coating type has produced wear values, practically the same, as forecasted by our wear process predicting software [3], already used in our earlier publications, therefore the tool monitoring can be applied well to the nACo-MT layers.

nACo³ type coating layers

The triplet structure nanocomposite coating nACo³ has appeared in 2007. It contains three layers: an approx. ~ 200 nm adhesion layer on the substrate, in the middle there is a conventional, ~ 1,4 mm thick, AlTiN layer with very high Al-content, and an approx. ~ 0,5 mm thick nACo-layer is on the top. According to the Table 1., this coating type has the smallest thickness, but the greatest microhardness. This type of coating layer has been tested at four different cutting speed values, applying an increased range of speed values (v_c = 300 – 350 m/min). The wear curves, belonging to the different cutting speed values, can be seen in Figure 2.

All of the estimated wear - time curves have typical development, containing moderate growth and rapid wear escalation. After rapid wear during the first few (approximately 1 ... 3) minutes the wear settles down to a steady-state rate (see Figure 2.). For this reason after a 6...17 minutes of cutting time, the wear land width was less than VB_c = 0,08 ... 0,16 mm. From this time on, the wear process changes radically, the crater wear becomes dominant and the wear process of the insert escalates to the end. This coating type has produced a wear process, unpredictable by us, so the tool monitoring could not be carried out in the way, as described in the literature [3].

nACoX4 type coating layers

This coating combination has 4 layers, the order (starting from the internal layer to the external one) is: TiN, nACo, AlCrN and one AlCrON-layer (Figure 3.). This coating structure has been tested at three different cutting speed values, applying an appropriate cutting speed range (v_c = 315 – 350 m/min). The wear curves, belonging to different cutting speed values, can be also seen in the Figure 3.

All of the estimated wear - time curves have typical development: after rapid wear during the first few (approximately 1 … 2) minutes the flank land wear has achieved the value of 0,1 … 0,13 mm, what means 50-60% of the wear criteria! After that the wear settles down to a steady-state rate (see Figure 3.). These heavy cutting conditions have caused a moderate tool wear rate. The wear escalation process has started on the flank land at the applied cutting speed, especially in form of notch wear and it will accompany the whole wear process. This will be completed by the crater wear, spreading towards the edges gradually. The wear process of this coating type can be described by a wear-time function, so the tool monitoring can be made as mentioned earlier [3].

2.2. Tool lives of nanocomposite layered inserts

The investigations, carried out by us, show the combination of the wear forms (flank wear, crater wear). According to the cutting theory it is the source of non-Taylorian curves and it is its explanation as well [4]. The cutting times in accordance with the criteria of tool life VB=0,2 mm have diffused in a really wide range, between 6 ... 44 minutes, decisively due to chipping of edges.

Table 3.

Tool lives of the coating types, tested by us, belonging to the different cutting speed values, can be seen in Figure 4. The wear proces and durability of insert, occurred at a constant cutting speed value of (v_c=315 m/min) can be seen here as well.

The main results of our all examinations are collected in Table 3. In this, the most important data and characteristics of the cutting performance have been summarised by us, based on the literature [5].

3. Summary, conclusions

Based on the measurements, carried out by devices, the experiences, gained by us, and based on Figure 4. and Table 3. we were able to draw the following conclusions:

• tool life versus cutting speed showed a non-taylorian form in the tested velocity range. The equation, determined for each coating type, is given in Table 3.

• by applying the highest speed, the cutting performance of the tested coated inserts changed dramatically;

• good performance can be obtained by using nACo-MT coating layers. It can be connected with the biggest thickness of the coating layer: it can provide a really effective protection, especially at lower cutting speed values. The widespread use of this coating type in the industrial practice can be recommended, even though it has the shortest tool life. The wear process of the coating can be modelled well with the software, used by us, therefore the process security is guaranteed. It had the shortest tool life therefore it forms the basis of our investigation, it means, it is the etalon quality (during any comparison will be 100%);

• better results can be reached by using nACo³ coating layer, due to the largest microhardness and the smoothest surface roughness (i.e. topographical condition) of deposited layer. By a constant tool life it can achieve (by approx. 5%) higher cutting speed, but at a given cutting speed value (of 315 m/min) the tool life increases by approx. 38%. The widespread use of this coating type is hindered by the fact that the coating can easily suffer - due to its rigidity – an unpredictable, catastrophic wear.

• the best results of the coating structures, containing oxinitride top layers, can be obtained by using nACoX4. The best coating has achieved at least double tool life, compared to the coating with code nACo-MT. It can be connected with the really appropriate composed layer structure and not with the bit bigger layer thickness or microhardness of the deposited layer. In our opinion the TiN adhesion layer can provide a better adherence to the layer(s), to be deposited later. By a constant tool life it can achieve a higher speed (by approx. 10%), even more, in case of a given cutting speed it has higher tool life (by 80%). It means that the costs of the tool (price, costs of exchange, etc.) reduce, the operational costs decrease. Therefore is the widespread use of oxynitride coatings in the industrial practice recommended by us. There is one more reason for it: the wear process of this coating can be modelled well with the software, developed by us, therefore the process security can be guaranteed.

The cutting performance degradation of turning inserts with different coatings can be generally characterised by flank wear width. We have noticed two wear types, namely flank wear, caused by the well known abrasion mechanism of hard particles of the workpiece material, as well as the crater wear, resulted from the adhesive and diffusive action between the chip and tool material. Analysing the results, it can be concluded that

• by increasing the flank wear land width of tested inserts both cutting force and the passive one have increased slightly. This fact can be reasoned by the crater formulation, which helps to keep the cutting edge sharp;

• the propagation of the crater wear is also connected with the adhesive - fatigue wear machanism, concentrated on rake face. Since these small particles can adhere easily with chip material, they can cause the thermal-adhesive fatigue of tool material. The wear process after 7  10 minutes of cutting time becomes stronger, which can be explained by the above mentioned wear mechanisms;

• the deterioration process of cutting edge acts inconsistently on the surface roughness parameters (R_a , R_m , R_z), produced by different coating sytems, and has caused unacceptable poor surface finish when wear process has reached a critical state;

• different coating layers, deposited on Pramet carbide could only have delayed the crater formulation progress, acting on rake face of insert. This can be explained mainly by the composition of base carbide material. The cutting performance index definitely refers to the degree of protection, which can be caused by nanocomposite structure coating and oxynitride top-layered coating system.

Three types of coating layers have been reported in the previous paragraphs. Considering the fact that the external layer of the coating, having the best performance is made of oxynitride, it is worth to analyse the deterioration process, accompanying the wear of the inserts, separately. It can be topic for a next lecture in the future.

References

[1]Investigation on cutting performance of carbide inserts with several types of coatings made by PLATIT AG. (Special Report), Budapest Tech, 2006.

[2]T. Cselle and others: Oxinitride coatings for test on TH Budapest

36th International Conference On Metallurgical Coatings And Thin Films (ICMCTF 2009 ) April 27-May 1, 2009 Town and Country Hotel San Diego, California, USA

[3]Sz. Biró – Zs. Kisa – P. Uracs – S. dr. Sipos: New results of investigations carried out on tools made by the IMC Group XXIII. microCAD International Scientific Conference,

19-20 March 2009., Miskolc, Section M., p. 27 – 33.

[4]http://forgacsolaskutatas.hu/elmelet/nem_taylori.xml

[5]http://forgacsolaskutatas.hu/elmelet/forgacsolokepesseg.xml