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Properties and performances of innovative coated tools for turning inconel
ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 48 (2008) 815–823 www.elsevier.com/locate/ijmactool
Properties and performances of innovative coated tools for turning inconel Luca Settineria,, Maria Giulia Fagab, Beatriz Lergac a
Department of Production Systems and Business Economics, Politecnico di Torino, Turin, Italy b Institute for Science and Technology of Ceramics - CNR, Turin, Italy c Centro de Ingenierı´a Avanzada de Superficies AIN, Cordovilla-Pamplona, Spain
Received 24 July 2007; received in revised form 3 December 2007; accepted 11 December 2007 Available online 31 December 2007
Abstract Three innovative nanostructured coatings have been developed to be applied on cutting tools for continuous cutting of nickel-based super-alloys, in Minimum Quantity Lubrication (MQL) or dry conditions. The coatings, TiN+AlTiN, TiN+AlTiN+MoS2 and CrN+CrN:C+C, were applied by PVD techniques on WC-Co inserts, developing nanostructured layers, characterised by superior performances, as confirmed both by laboratory tests and machining experiments. Coatings surface qualification included SEM observations with EDS analysis, ball erosion test, nanoindentation and scratch tests, classic tribological evaluation by ball-on-disc set-up, surface texture analysis. Results were analysed in light of the outcome of machining experiments performed mainly in dry and MQL turning of Inconel 718. Ball-on-disc and scratch tests, as well as machining experiments, agreed in classifying the coatings in the following decreasing performance order: TiN+AlTiN+MoS2, followed by TiN+AlTiN, and by CrN+CrN:C+C. r 2007 Elsevier Ltd. All rights reserved. Keywords: Turning; Inconel 718; Coatings; Nanostructure; Dry cutting; MQL
1. Introduction Modern machine tools, with increased dynamic performances, stiffness and power, require cutting tools with highly improved properties, in particular on their surfaces. This is even truer in machining difficult-to-cut alloys, operation that requires the engineering of tools with elevated surface properties in terms of wear resistance, hardness, strength, toughness, and thermal stability at high temperature. Correlation between chemical, physical and mechanical characteristics of cutting tools’ surfaces and their performances in cutting operations is therefore a key issue for both tools manufacturers and users [1]. Nickel based alloys represent a class of increasing interest, since they are widely used by aerospace industry Corresponding author. Tel.: +39 011 5647230; fax: +39 011 5647299.
E-mail address: [email protected] (L. Settineri). 0890-6955/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2007.12.007
which has undergone an expansion in the last years. In particular, Inconel 718 has the suitable features for manufacturing jet engine and high-speed airframe parts such as wheels, buckets, spacers, bolts and fasteners. However, the highly abrasive carbide particles contained in the material cause abrasive wear, and the poor thermal conductivity leads to high cutting temperatures on the rake face. Furthermore, as most nickel-based superalloys, Inconel 718 has high chemical affinity with many materials used for cutting tools, and this causes diffusion wear. Other reasons of premature tool failure are welding and adhesion phenomena, generating damage on the tool rake face [2–4]. High cutting speeds, enhancing tool temperature up to plastic deformation threshold and promoting diffusion wear, lead to even worse problems. These effects are traditionally limited by the use of lubricant, but the current quest of dry or Minimum Quantity Lubrication (MQL) machining, aimed at control of pollution and part surface
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contamination, further compounds the severe burden of the tools [4,5]. In this work three coatings, TiN+AlTiN, TiN+AlTiN +MoS2 and CrN+CrN:C+C, deposited on WC-Co inserts have been fully characterised. The choice of AlTiN based coatings arises from their hardness, ranging between 15 and 30 GPa, depending on the deposition conditions, and from their high temperature oxidation resistance, depending on the Al–Ti ratio, while MoS2 was used as a further layer because of its lubricating properties. The CrN based coating has been chosen on account of its reduced tendency to weld to the workmaterial, improving resistance to adhesive wear [6]. Deposition techniques and parameters have been selected with the aim of obtaining nanostructured textures for the coatings, that allow for highly improved mechanical features. A full range of laboratory characterisation tests has been used to extract information over the coating chemical, physical and mechanical features. The wear mechanisms and tribological properties of the coatings have been studied and related to their machining behaviour, measured in continuous turning operations of Inconel 718. 2. Experimental set-up 2.1. Cutting tools, coatings and machining tests The hard metal composition of the inserts used as substrates in this study was an ISO K20 94 wt% WC6 wt% Co with an hardness HRA 91.970.7 (data declared by the supplier). Their geometry was ISO RCMN0800F. The coatings obtained by means of different PVD techniques were:
TiN+AlTiN (coating A in the following), deposited by Arc evaporation with a multilayer structure where TiN and AlTiN single layers have been deposited alternatively. The coating has 10 layers, 5 of TiN and 5 of AlTiN, the monolayer in contact with the substrate is TiN and the external monolayer is AlTiN. TiN+AlTiN+MoS2 (coating B in the following), deposited by a hybrid process, result of the combination of Arc evaporation, to deposit TiN and AlTiN, and Magnetron sputtering, to deposit MoS2. The coating has three layers, one of TiN, in touch with the substrate, one intermediate layer of AlTiN and an external low-friction layer of MoS2. CrN+CrN:C+C (coating C in the following), deposited by Magnetron sputtering, with a three-layer structure as shown by the formula.
For each coating, nine inserts have been produced for the machining tests and three samples for the characterisation tests. Such inserts were applied to Inconel 718 (HRCE50) in continuous turning under different cutting speed and lubrication conditions. Cutting tests have been performed
Table 1 Cutting conditions for the lathe-turning tests Lathe
CNC Graziano 101
Cutting speed, vc (m/min) Feed rate, f (mm/rev) Depth of cut ap (mm) Lubricant Work material
35, 70, 140 0.25 0.5 Wet, MQL, dry cutting Inconel 718, HRCffi50
on a CNC Graziano 101 instrumented lathe, with a peak power of 40 kW; cutting conditions are reported in Table 1. In particular, wet, dry and MQL conditions have been applied. In MQL conditions, the lubricant is sprayed on the cutter in form of an aerosol, with a consumption of several order of magnitude lower than in the traditional lubrication systems. A specially-designed system and lubricant must be used. In our case, an Accu-Lubes microlubrication system was used, with an Accu-Lubes LB 5000 lubricant. This is a safe lubricant, composed by vegetal oil without any toxic additive (such as chlorine sulphide, nitrites, phenols and so on). No lubricant residuals were produced and tools, machines, workpiece and chips remained clean, being immediately ready for the following operation. 2.2. Laboratory characterisation 2.2.1. Coating thickness Coating thickness was measured by a CSM CALOTESTs ball erosion test device, suitable for coatings with a thickness ranging between 0.1 and 50 mm. Ball erosion is a test method featuring coating thickness determination (for mono and multi-layer) with a broad range of measurement, and it is applicable to a wide variety of materials, and compatible with VDI 3198 standard. 2.2.2. Tribological tests Friction and wear tests were carried out on a CSEM High Temperature Tribometer. The chosen configuration was ball-on-disc, with a ball of Alumina, a trace diameter of 6 mm. The tests have been carried out at room temperature and at 800 1C. They were interrupted every 1000 revolutions to examine wear trace. The traces left after the wear tests were measured by means of the interferometric profiler described in the following. The complete test conditions are listed in Table 2. 2.2.3. Surface roughness In order to evaluate surface roughness and to measure the traces of the wear tests, an interferometric profilometer WYKO RST 500 was used. This equipment allows to scan three-dimensional images of the surfaces of the samples. The resolution reaches the order of the Angstrom. To measure the surface texture on a region large enough, a total magnification of 5.3 was chosen, which allows to
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was used for the observation of the coatings and of the wear traces obtained with the tribometer.
Table 2 Tribological test conditions Friction test
Wear test
1 12 Al2O3 5000 0.1 9 800
10 6 Al2O3 20000 0.1 9 800
3. Results Load (N) Ball diameter (mm) Ball material Cycles Linear speed (mm/s) Trace radius (mm) Test temperature (1C)
Table 3 Scratch test conditions Type
Linear, progressive load
Begin load (N) End load (N) Loading rate (N/min) Speed (mm/min)
1 100 50 5
investigate an area of 1.2 0.9 mm2. Five measurements in different points were made in each sample and the average value was computed.
3.1. Machining tests As an example, the evolution of the mean flank wear VBB versus cutting time at vc ¼ 35 m/min, in dry conditions and at vc ¼ 140 m/min in MQL conditions are respectively shown in Figs. 1 and 2. Other results have been omitted for lack of space. Experimental points in the figures represent the average values of three VBB measurements per insert type. Individual values of VBB represent the maximum width of flank wear, according to ISO 3685 standard, measured off-line, using a monitor connected to an optical fibre probe. Figs. 3–5 show the cutting lifecutting speed experimental results on the Taylor plane for the coated and uncoated tools. Maximum flank wear considered to declare the tool out of service was VB=0.20 mm. The results seem to follow closely the Taylor’s equation, although experimental data have not been considered sufficient to numerically estimate the parameters of the Taylor’s equation. A
Uncoated
B
C
Avg. flank wear VB (mm)
0.40
2.2.4. Nanoindentation Hardness was determined by means of a Nano Indenters XP, with a Berkovich indenter, which allows to perform indentations with loads increasing from 1 to 1000 mN. Final loads of 10 and 750 mN were applied. The former was applied in 20 steps and the latter in 100 steps, with unloading paths of 20 steps in both cases. The 10 mN test allows to appreciate the coating features, while the 750 mN test yields the properties of the substrate minimising the coating influence.
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.00
2.00
4.00
6.00
8.00
Cutting time (min )
2.2.6. Sem observations A Cambridge Instruments Stereoscan 360 Scanning Electron Microscope (SEM) with tungsten filament and thermoionic emission, 0.2–40 keV acceleration tension, 5 nm resolution, associated with a Link Analytical— Oxford Instruments INCA Energy 300 micro-analyser with a resolution of 133 eV for Energy Dispersion Analysis
Fig. 1. Wear curves. vc ¼ 35 m/min, dry conditions.
Uncoated
A
B
C
0.70 Avg. flank wear VB (mm)
2.2.5. Adhesion properties Adhesion properties were measured by a CSM Revetest scratch testing equipment. Major features are: diamondstylus scratch method, feedback-controlled force actuator, acoustic emission detection. The test conditions are showed in Table 3. Further adhesion properties have been assessed by Rockwell A indentation, used qualitatively by observing the cracks surrounding the indentation, to rank coatings on the basis of their adhesion to the substrate.
0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.00
2.00
4.00
6.00
Cutting time (min) Fig. 2. Wear curves. vc ¼ 140 m/min, MQL conditions.
8.00
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Uncoated
A
B
Table 4 Coatings’ structure and thicknesses
C
Cutting time (min)
10.00
1.00 10.00
100.00
1000.00
Coating
Identification
Thickness (mm)
Structure
TiN+AlTiN TiN+AlTiN+MoS2 CrN+CrN:C+C
A B C
3.870.1 3.970.1 3.870.1
10 layers 3 layers 3 layers
Table 5 Average friction coefficients vs. the Alumina ball of the Tribometer
Cutting speed (m/min)
Fig. 3. Cutting life vs. cutting speed. Dry conditions. Max flank wear VB ¼ 0.20 mm.
Coating
A
B
C
Uncoated
Friction coeff. at 20 1C Friction coeff. at 800 1C
0.49 0.61
0.12 0.48
0.65 0.79
0.69 0.78
Tests performed at 800 1C.
Uncoated
A
B
C
Cutting time (min)
10.00
1.00 10.00
100.00
1000.00
Cutting speed (m/min) Fig. 4. Cutting life vs. cutting speed. MQL conditions. Max flank wear VB ¼ 0.20 mm.
Uncoated
A
B
C
Cutting time (min)
10.00
1.00 10.00
100.00
1000.00
Cutting speed (m/min) Fig. 5. Cutting life vs. cutting speed. Wet conditions. Max flank wear VB ¼ 0.20 mm.
3.2. Surface properties of the coatings 3.2.1. Ball erosion test Coating average thicknesses measured by ball erosion test are shown in Table 4. The thicknesses values are similar, therefore the three coatings can be compared in their mechanical and tribological characteristics.
3.2.2. Ball-on-disc test Ball-on-disc test allows to investigate the friction coefficient and the tribological behaviour of a surface [6,7]. The values of the friction coefficients of the coatings against the Alumina ball, under the Coulomb’s hypothesis, are displayed in Table 5. The results are referred to tests carried out at room temperature and at 800 1C. Results are quite well scaled at room temperature, but as the temperature rises, they converge to a common higher value. In order to characterize the tribological behaviour of the samples, depth and width of the traces were measured with a profilometer, while the ball wear was measured with a high resolution digital camera interfaced with a PC. In the following, only high-temperature tests results are shown, since real cutting conditions generate high temperatures at the tool-workpiece interface and therefore on the tool-rake surface. In Fig. 6 the depth and width of the sample trace vs. the number of revolutions are shown. During the test the balls wear as well. The geometrical indicators used to quantify ball wear are: wear depth in the ball, that is the height reduction in the load application direction, and the area of the (ideally) plane surface created by the friction between the sample and the ball. Figs. 7 and 8 show respectively the wear depth and the wear area of the ball vs. the number of revolutions, as the tests were being carried on. However, the previous indicators cannot identify the tribological behaviour of the coatings, since in our case a typical third-body wear mechanism has been noticed, confirmed by the EDS analysis of the traces; an example is shown in Fig. 9. Therefore an indicator was needed that could take into account for both sample and ball wear increase during the test, and this was identified in the ratio between the ball wear area and sample wear depth vs. the number of revolutions, whose behaviour is shown in Fig. 10 [8,9].
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Depth of the sample wear
819
20
4
Ni Cr
3
15 Ni
cps
m
Fe
2 1
10
0 0
10
20
30
40
C
5
50
Fe
S Nb
O Cr
Number of laps (x103)
Ti
Cr
Ti
Al
S
Ni
Fe
Ti
0
Width of the sample wear
2
1500
4
6
8
Energy (keV)
Fig. 9. EDS spectrum of the wear track of coating C, after the tribological tests.
m
1000
500 B
A
C
900
0 0
10
20
30
40
50
Number of laps (x103) A
(mm)
C
600 B
300 Fig. 6. Sample trace depth (a) and width (b) vs. number of laps. Tests performed at 800 1C.
0
mm
B 0.12 0.10 0.08 0.06 0.04 0.02 0
A
20000
40000
Number of laps
C
Fig. 10. Ratio ball wear area/sample trace depth, vs. the number of revolutions. Tests performed at 800 1C.
0
10000
20000
30000
40000
50000
number of laps Fig. 7. Wear depth of the ball vs. the number of revolutions. Tests performed at 800 1C.
B
A
3.2.3. Nanoindentation tests The aim of the test is to evaluate the hardness of the coatings, by isolating the measurement from the influence of the substrate material. The nanoindentation usually shows a certain variability in the results, and this obliges to follow particular procedures to guarantee a proper repeatability. In any case, these test results have more a comparative than an absolute value. The results of some of the nanoindentation tests are reported in Table 6. 3.2.4. Adhesion tests Scratch test results are shown in Table 7 in terms of average critical load over ten repetitions, and minimum and maximum values. A clear prevalence of coating B can be observed. Fig. 11 shows the results of the Rockwell A indentation test. The results confirm stronger adhesion and toughness of coating B.
C
2 1.5 mm2
0
1 0.5 0 0
10000
20000
30000
40000
number of laps
Fig. 8. Wear area of the ball vs. the number of revolutions. Tests performed at 800 1C.
3.2.5. Roughness tests The outcome of the roughness tests are showed in Table 8, in terms of Ra, Ry, Rt, and Rz. Assessment length was 15 mm. As it was expected, the values are comparable
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Table 6 Results of the Nanoindentations test Samples
Loading conditions
Young’s modulus (GPa)
Hardness (GPa)
Displacement (nm)
Uncoated
Min load (10 mN) Max load (750 mN) Min load (10 mN) Max load (750 mN) Min load (10 mN) Max load (750 mN) Min load (10 mN) Max load (750 mN)
400.4 410.6 595.7 564.1 578.3 520.7 520.7 480.7
12.3 12.1 32 15.3 29 13.6 15.9 13.4
– 1008.7 – 919.0 – 708.4 – 963.2
A B C
Table 7 Results of the scratch tests in terms of critical load at delamination Coating
Identification
Avg. critical load at delamination (N)
Min/max critical load (N)
TiN+AlTiN TiN+AlTiN+MoS2 CrN+CrN:C+C
A B C
83 93 62
82/89 87/98 57/66
Results computed over ten repetitions.
for the three coatings, and similar to those of the uncoated sample, since the substrate preparation and finishing procedure was identical.
The wet conditions cutting tests, carried out using commercial lubrication fluids, showed a reduction of the difference between uncoated and coated inserts, as it can be seen from the comparison of Figs. 3–5. Further, tests highlighted that coating B showed better overall wear resistance compared to the other coatings. Analysis of wear mechanism of the coatings will be done in the following paragraph, after description of their surface properties. Lubrication condition can be ranked in decreasing performance order as: MQL, wet and dry. The phenomenon can be ascribed to the way the lubricant is spread on the tool using MQL; in fact, the small quantity vaporised on the cutting edge limits thermal shocks, increasing coating duration [8]. 4.2. Laboratory tests
4. Discussion 4.1. Turning tests Turning tests were aimed at evaluating coated tools’ performances under standard cutting speed conditions with respect to uncoated hard metal, commonly used with application to Inconel 718 [10]. The outcome of the cutting tests indicates that the adoption of coating A and most of all coating B have a very positive effect on cutting performance, especially as the cutting speed gets higher, as shown in Figs. 1 and 2. This is justified considering that as the cutting speed increases, the temperature also increases, causing volatile WOx formation, determining chemical alteration of the WC-Co material, which affects some properties like hardness. Since these degradation phenomena become important at temperatures of nearly 900 1C, it is expected that these effects arise when cutting speed is 60 m/min or more [11]. Wear morphologies of the tools at high cutting speed are shown in Fig. 12 for coatings C and A. It can be observed that, even if a limited wear is observed, coating delamination occurred, indicating some adhesion problems between coating and substrate. The phenomenon is even more evident in Fig. 13, in which the white areas indicate the uncoated substrate.
Laboratory characterisation tests have been run with the aim of assessing coating’s quality by means of wellestablished experimental procedures. Further, the comparison with the outcome of the cutting tests underlines the most important coating’s features for cutting and may help to establish an off-line experimental paradigm able to rank the coatings in the correct cutting performance order. Some important aspects of the laboratory characterisation results should be highlighted: 1) The comparison between the results of the tribological tests, reported in Figs. 6–8 and the results of the nanohardness tests, reported in Table 6, show that wear rates are not directly correlated to hardness, indicating some other phenomena, besides simple abrasion, to be important for the wear mechanisms. On the other hand, it is worthwhile to observe that wear rates at high temperature are quite in agreement with turning performances, as it can be observed by comparing Fig. 10 with Figs. 1–5. 2) Friction coefficients at high temperature cannot be used to differentiate the performances of the coatings, since they are quite similar, as shown in Table 5. 3) As shown in Fig. 6, coating C reached significantly higher values of wear if compared with the other two coatings.
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Fig. 11. Rockwell A indentation results: coating A (a); coating B (b); coating C (c).
Table 8 Results of roughness measurements Coating
Identification
Ra Avg./Rng
Ry Avg./Rng
Rt Avg./Rng
Rz Avg./Rng
TiN+AlTiN TiN+AlTiN+MoS2 CrN+CrN:C+C Wc-Co
A B C Uncoated
0.267/0.031 0.356/0.041 0.271/0.039 0.287/0.030
5.714/0.53 6.000/0.65 5.758/0.61 5.885/0.56
6.054/0.58 7.434/0.71 6.758/0.72 6.147/0.64
4.543/0.38 5.601/0.54 4.538/0.45 4.832/0.44
Average and range computed over 5 sampling intervals.
Fig. 12. Wear morphology after cutting at vc ¼ 140 m/min for coating C (a) and A (b).
Adhesion area Coating removal
Adhesion area Coating removal
Fig. 13. Wear morphology at the end of turning tests under vc ¼ 140 m/min for coating A. An important delamination phenomenon occurs.
4) EDS analysis pointed out that an important third-body wear phenomenon occurs, since traces of alumina powder coming from the ball could be found in the wear trace of the sample, as it is shown in Fig. 9, referred to coating C.
5) Fig. 10 shows that in the first part of the test wear proceeds faster on the ball than on the sample, while in the second part of the test sample wear starts to be significative and the ball-to-sample wear ratio decreases. This occurs when the alumina powder coming from the
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Thermal Crack
Fig. 14. Wear morphology for a sample of cutting tool coated with coating A. Cutting conditions: vc 140 m/min, dry, cutting time: 4.33 min VBB ¼ 0.24 mm.
ball starts to abrade the coatings, generating a thirdbody wear phenomenon. In the third part of the test, the ball-to-sample wear ratio becomes constant, indicating that the wear proceeds with the same rate in the two elements. Coating B achieves better performances in terms of ball-to-sample wear ratio. 6) Scratch test and Rockwell C indentation showed significantly higher adhesion and toughness values for the coating B, as the observation of Table 7 and Fig. 11 clearly points out. 4.3. Wear mechanisms Cutting performances of PVD coated tools are strongly affected by composition, microstructure, residual stresses and adhesion between coating and substrate, these features determining wear behaviour. For the studied coatings, wear analysis obtained via SEM–EDS indicated abrasion and welding as main wear mechanisms. A large amount of built up edge was found on the cutting edge of all the observed tools, leading in some cases to craterisation and delamination. Surprisingly, coating B showed better cutting performaces than coating A, although similar in nominal composition. Such behaviour is not entirely justified by the presence in coating B of the external layer of MoS2. EDS analysis, in fact, revealed a first important difference, since Al/Ti ratio was found more homogeneous along the coating B (60/40) than on coating A (ranging from 55/45 to 40/60). A higher percentage of aluminium determines a greater high temperature stability, because of the tendency to form a protective layer of Al2O3 during the machining operation, leading to higher abrasion resistance. In addition, in the case of coating A also some thermal fatigue cracks were found at high cutting speed (see Fig. 14). The low thermal conductivity of AlTiN limits heat dissipation and generates high thermal gradients that may lead to thermal cracks, especially if residual stresses are present in the coating. In the case of coating A, which is a multilayer coating with 10 alternating layers of different composition, residual stresses are likely to be present.
In the case of the CrN-based coating C, premature failure occurred, due to delamination, probably arising from poor adhesion to the substrate, as shown by adhesion test. When adhesive forces between chip and coating are higher than between coating and substrate, debonding rapidly occurs. 4.4. Laboratory characterisation-machining test correlation If we compare the outcome of the cutting tests with the results of the laboratory characterisation, it can be assessed that the tribological tests yield coherent results with the cutting tests, if the index shown in Fig. 10 is considered. In fact, the ball-to-sample wear ratio is able to clearly classify the coatings in the correct performance order. Good forecast properties are also shown by adhesion/toughness tests (see Table 7 and Fig. 11), that yielded the same classification order. The aforementioned tests can therefore be considered good candidates to establish a laboratory characterisation paradigm for cutting tool coatings to be used under severe cutting conditions. Nano-hardness test, on the other hand, did not give the same indications. It is certainly an important test to assess coatings’ quality, but it must not be forgotten that hardness is not always the most important feature for a cutting tool, especially under the cutting conditions here explored. Furthermore, the nanohardness tests referred to in this paper have been performed at room temperature, and we don’t have indications over the coatings’ hardness at the high temperatures reached during machining. 5. Conclusion This paper reports the results obtained in turning Inconel 718 using innovative coated inserts, produced via new PVD processes. All the tested tools performed better than the uncoated inserts, currently the state of the art for machining Inconel 718. Of the three experimental coatings, the AlTiN-based, produced via Arc evaporation, with a further layer of MoS2 deposited via Magnetron sputtering, was found
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more promising both in dry and in MQL condition. The reason can be ascribed to the optimal Al/Ti ratio, that gives the proper combination of abrasion resistance and adhesion to the substrate, and to the MoS2 layer, that limits the adhesion to the chip. The AlTiN-based coating with a multilayer structure performed second, due to the absence of the lubricating MoS2 layer, to the non-optimal Al/Ti ratio and to the occurrence of thermal fatigue cracks, probably caused by the multilayer structure. The CrN-based coating didn’t show a good behaviour, compared with the other coatings, mainly due to low adhesion and extended delamination. Some performance indexes obtained from the characterisation tests, such as the ball-to-sample wear ratio or the scratch test critical load, were able to clearly classify the coatings in the correct performance order. MQL was found to enhance tool performances, further limiting friction. As this lubrication system is inherently safe, the combination coating-MQL represents an interesting opportunity in machining difficult-to-cut materials. Acknowledgments The authors wish to thank Mr. J. Housden of Tecvac Ltd, Cambridge, England and Mr. V. Bellido-Gonzalez of Gencoa Ltd, Liverpool, England for their help in conducting the experimental work that lead to preparation of this paper.
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International Journal of Machine Tools & Manufacture 48 (2008) 815–823 www.elsevier.com/locate/ijmactool
Properties and performances of innovative coated tools for turning inconel Luca Settineria,, Maria Giulia Fagab, Beatriz Lergac a
Department of Production Systems and Business Economics, Politecnico di Torino, Turin, Italy b Institute for Science and Technology of Ceramics - CNR, Turin, Italy c Centro de Ingenierı´a Avanzada de Superficies AIN, Cordovilla-Pamplona, Spain
Received 24 July 2007; received in revised form 3 December 2007; accepted 11 December 2007 Available online 31 December 2007
Abstract Three innovative nanostructured coatings have been developed to be applied on cutting tools for continuous cutting of nickel-based super-alloys, in Minimum Quantity Lubrication (MQL) or dry conditions. The coatings, TiN+AlTiN, TiN+AlTiN+MoS2 and CrN+CrN:C+C, were applied by PVD techniques on WC-Co inserts, developing nanostructured layers, characterised by superior performances, as confirmed both by laboratory tests and machining experiments. Coatings surface qualification included SEM observations with EDS analysis, ball erosion test, nanoindentation and scratch tests, classic tribological evaluation by ball-on-disc set-up, surface texture analysis. Results were analysed in light of the outcome of machining experiments performed mainly in dry and MQL turning of Inconel 718. Ball-on-disc and scratch tests, as well as machining experiments, agreed in classifying the coatings in the following decreasing performance order: TiN+AlTiN+MoS2, followed by TiN+AlTiN, and by CrN+CrN:C+C. r 2007 Elsevier Ltd. All rights reserved. Keywords: Turning; Inconel 718; Coatings; Nanostructure; Dry cutting; MQL
1. Introduction Modern machine tools, with increased dynamic performances, stiffness and power, require cutting tools with highly improved properties, in particular on their surfaces. This is even truer in machining difficult-to-cut alloys, operation that requires the engineering of tools with elevated surface properties in terms of wear resistance, hardness, strength, toughness, and thermal stability at high temperature. Correlation between chemical, physical and mechanical characteristics of cutting tools’ surfaces and their performances in cutting operations is therefore a key issue for both tools manufacturers and users [1]. Nickel based alloys represent a class of increasing interest, since they are widely used by aerospace industry Corresponding author. Tel.: +39 011 5647230; fax: +39 011 5647299.
E-mail address: [email protected] (L. Settineri). 0890-6955/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2007.12.007
which has undergone an expansion in the last years. In particular, Inconel 718 has the suitable features for manufacturing jet engine and high-speed airframe parts such as wheels, buckets, spacers, bolts and fasteners. However, the highly abrasive carbide particles contained in the material cause abrasive wear, and the poor thermal conductivity leads to high cutting temperatures on the rake face. Furthermore, as most nickel-based superalloys, Inconel 718 has high chemical affinity with many materials used for cutting tools, and this causes diffusion wear. Other reasons of premature tool failure are welding and adhesion phenomena, generating damage on the tool rake face [2–4]. High cutting speeds, enhancing tool temperature up to plastic deformation threshold and promoting diffusion wear, lead to even worse problems. These effects are traditionally limited by the use of lubricant, but the current quest of dry or Minimum Quantity Lubrication (MQL) machining, aimed at control of pollution and part surface
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contamination, further compounds the severe burden of the tools [4,5]. In this work three coatings, TiN+AlTiN, TiN+AlTiN +MoS2 and CrN+CrN:C+C, deposited on WC-Co inserts have been fully characterised. The choice of AlTiN based coatings arises from their hardness, ranging between 15 and 30 GPa, depending on the deposition conditions, and from their high temperature oxidation resistance, depending on the Al–Ti ratio, while MoS2 was used as a further layer because of its lubricating properties. The CrN based coating has been chosen on account of its reduced tendency to weld to the workmaterial, improving resistance to adhesive wear [6]. Deposition techniques and parameters have been selected with the aim of obtaining nanostructured textures for the coatings, that allow for highly improved mechanical features. A full range of laboratory characterisation tests has been used to extract information over the coating chemical, physical and mechanical features. The wear mechanisms and tribological properties of the coatings have been studied and related to their machining behaviour, measured in continuous turning operations of Inconel 718. 2. Experimental set-up 2.1. Cutting tools, coatings and machining tests The hard metal composition of the inserts used as substrates in this study was an ISO K20 94 wt% WC6 wt% Co with an hardness HRA 91.970.7 (data declared by the supplier). Their geometry was ISO RCMN0800F. The coatings obtained by means of different PVD techniques were:
TiN+AlTiN (coating A in the following), deposited by Arc evaporation with a multilayer structure where TiN and AlTiN single layers have been deposited alternatively. The coating has 10 layers, 5 of TiN and 5 of AlTiN, the monolayer in contact with the substrate is TiN and the external monolayer is AlTiN. TiN+AlTiN+MoS2 (coating B in the following), deposited by a hybrid process, result of the combination of Arc evaporation, to deposit TiN and AlTiN, and Magnetron sputtering, to deposit MoS2. The coating has three layers, one of TiN, in touch with the substrate, one intermediate layer of AlTiN and an external low-friction layer of MoS2. CrN+CrN:C+C (coating C in the following), deposited by Magnetron sputtering, with a three-layer structure as shown by the formula.
For each coating, nine inserts have been produced for the machining tests and three samples for the characterisation tests. Such inserts were applied to Inconel 718 (HRCE50) in continuous turning under different cutting speed and lubrication conditions. Cutting tests have been performed
Table 1 Cutting conditions for the lathe-turning tests Lathe
CNC Graziano 101
Cutting speed, vc (m/min) Feed rate, f (mm/rev) Depth of cut ap (mm) Lubricant Work material
35, 70, 140 0.25 0.5 Wet, MQL, dry cutting Inconel 718, HRCffi50
on a CNC Graziano 101 instrumented lathe, with a peak power of 40 kW; cutting conditions are reported in Table 1. In particular, wet, dry and MQL conditions have been applied. In MQL conditions, the lubricant is sprayed on the cutter in form of an aerosol, with a consumption of several order of magnitude lower than in the traditional lubrication systems. A specially-designed system and lubricant must be used. In our case, an Accu-Lubes microlubrication system was used, with an Accu-Lubes LB 5000 lubricant. This is a safe lubricant, composed by vegetal oil without any toxic additive (such as chlorine sulphide, nitrites, phenols and so on). No lubricant residuals were produced and tools, machines, workpiece and chips remained clean, being immediately ready for the following operation. 2.2. Laboratory characterisation 2.2.1. Coating thickness Coating thickness was measured by a CSM CALOTESTs ball erosion test device, suitable for coatings with a thickness ranging between 0.1 and 50 mm. Ball erosion is a test method featuring coating thickness determination (for mono and multi-layer) with a broad range of measurement, and it is applicable to a wide variety of materials, and compatible with VDI 3198 standard. 2.2.2. Tribological tests Friction and wear tests were carried out on a CSEM High Temperature Tribometer. The chosen configuration was ball-on-disc, with a ball of Alumina, a trace diameter of 6 mm. The tests have been carried out at room temperature and at 800 1C. They were interrupted every 1000 revolutions to examine wear trace. The traces left after the wear tests were measured by means of the interferometric profiler described in the following. The complete test conditions are listed in Table 2. 2.2.3. Surface roughness In order to evaluate surface roughness and to measure the traces of the wear tests, an interferometric profilometer WYKO RST 500 was used. This equipment allows to scan three-dimensional images of the surfaces of the samples. The resolution reaches the order of the Angstrom. To measure the surface texture on a region large enough, a total magnification of 5.3 was chosen, which allows to
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was used for the observation of the coatings and of the wear traces obtained with the tribometer.
Table 2 Tribological test conditions Friction test
Wear test
1 12 Al2O3 5000 0.1 9 800
10 6 Al2O3 20000 0.1 9 800
3. Results Load (N) Ball diameter (mm) Ball material Cycles Linear speed (mm/s) Trace radius (mm) Test temperature (1C)
Table 3 Scratch test conditions Type
Linear, progressive load
Begin load (N) End load (N) Loading rate (N/min) Speed (mm/min)
1 100 50 5
investigate an area of 1.2 0.9 mm2. Five measurements in different points were made in each sample and the average value was computed.
3.1. Machining tests As an example, the evolution of the mean flank wear VBB versus cutting time at vc ¼ 35 m/min, in dry conditions and at vc ¼ 140 m/min in MQL conditions are respectively shown in Figs. 1 and 2. Other results have been omitted for lack of space. Experimental points in the figures represent the average values of three VBB measurements per insert type. Individual values of VBB represent the maximum width of flank wear, according to ISO 3685 standard, measured off-line, using a monitor connected to an optical fibre probe. Figs. 3–5 show the cutting lifecutting speed experimental results on the Taylor plane for the coated and uncoated tools. Maximum flank wear considered to declare the tool out of service was VB=0.20 mm. The results seem to follow closely the Taylor’s equation, although experimental data have not been considered sufficient to numerically estimate the parameters of the Taylor’s equation. A
Uncoated
B
C
Avg. flank wear VB (mm)
0.40
2.2.4. Nanoindentation Hardness was determined by means of a Nano Indenters XP, with a Berkovich indenter, which allows to perform indentations with loads increasing from 1 to 1000 mN. Final loads of 10 and 750 mN were applied. The former was applied in 20 steps and the latter in 100 steps, with unloading paths of 20 steps in both cases. The 10 mN test allows to appreciate the coating features, while the 750 mN test yields the properties of the substrate minimising the coating influence.
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.00
2.00
4.00
6.00
8.00
Cutting time (min )
2.2.6. Sem observations A Cambridge Instruments Stereoscan 360 Scanning Electron Microscope (SEM) with tungsten filament and thermoionic emission, 0.2–40 keV acceleration tension, 5 nm resolution, associated with a Link Analytical— Oxford Instruments INCA Energy 300 micro-analyser with a resolution of 133 eV for Energy Dispersion Analysis
Fig. 1. Wear curves. vc ¼ 35 m/min, dry conditions.
Uncoated
A
B
C
0.70 Avg. flank wear VB (mm)
2.2.5. Adhesion properties Adhesion properties were measured by a CSM Revetest scratch testing equipment. Major features are: diamondstylus scratch method, feedback-controlled force actuator, acoustic emission detection. The test conditions are showed in Table 3. Further adhesion properties have been assessed by Rockwell A indentation, used qualitatively by observing the cracks surrounding the indentation, to rank coatings on the basis of their adhesion to the substrate.
0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.00
2.00
4.00
6.00
Cutting time (min) Fig. 2. Wear curves. vc ¼ 140 m/min, MQL conditions.
8.00
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Uncoated
A
B
Table 4 Coatings’ structure and thicknesses
C
Cutting time (min)
10.00
1.00 10.00
100.00
1000.00
Coating
Identification
Thickness (mm)
Structure
TiN+AlTiN TiN+AlTiN+MoS2 CrN+CrN:C+C
A B C
3.870.1 3.970.1 3.870.1
10 layers 3 layers 3 layers
Table 5 Average friction coefficients vs. the Alumina ball of the Tribometer
Cutting speed (m/min)
Fig. 3. Cutting life vs. cutting speed. Dry conditions. Max flank wear VB ¼ 0.20 mm.
Coating
A
B
C
Uncoated
Friction coeff. at 20 1C Friction coeff. at 800 1C
0.49 0.61
0.12 0.48
0.65 0.79
0.69 0.78
Tests performed at 800 1C.
Uncoated
A
B
C
Cutting time (min)
10.00
1.00 10.00
100.00
1000.00
Cutting speed (m/min) Fig. 4. Cutting life vs. cutting speed. MQL conditions. Max flank wear VB ¼ 0.20 mm.
Uncoated
A
B
C
Cutting time (min)
10.00
1.00 10.00
100.00
1000.00
Cutting speed (m/min) Fig. 5. Cutting life vs. cutting speed. Wet conditions. Max flank wear VB ¼ 0.20 mm.
3.2. Surface properties of the coatings 3.2.1. Ball erosion test Coating average thicknesses measured by ball erosion test are shown in Table 4. The thicknesses values are similar, therefore the three coatings can be compared in their mechanical and tribological characteristics.
3.2.2. Ball-on-disc test Ball-on-disc test allows to investigate the friction coefficient and the tribological behaviour of a surface [6,7]. The values of the friction coefficients of the coatings against the Alumina ball, under the Coulomb’s hypothesis, are displayed in Table 5. The results are referred to tests carried out at room temperature and at 800 1C. Results are quite well scaled at room temperature, but as the temperature rises, they converge to a common higher value. In order to characterize the tribological behaviour of the samples, depth and width of the traces were measured with a profilometer, while the ball wear was measured with a high resolution digital camera interfaced with a PC. In the following, only high-temperature tests results are shown, since real cutting conditions generate high temperatures at the tool-workpiece interface and therefore on the tool-rake surface. In Fig. 6 the depth and width of the sample trace vs. the number of revolutions are shown. During the test the balls wear as well. The geometrical indicators used to quantify ball wear are: wear depth in the ball, that is the height reduction in the load application direction, and the area of the (ideally) plane surface created by the friction between the sample and the ball. Figs. 7 and 8 show respectively the wear depth and the wear area of the ball vs. the number of revolutions, as the tests were being carried on. However, the previous indicators cannot identify the tribological behaviour of the coatings, since in our case a typical third-body wear mechanism has been noticed, confirmed by the EDS analysis of the traces; an example is shown in Fig. 9. Therefore an indicator was needed that could take into account for both sample and ball wear increase during the test, and this was identified in the ratio between the ball wear area and sample wear depth vs. the number of revolutions, whose behaviour is shown in Fig. 10 [8,9].
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Depth of the sample wear
819
20
4
Ni Cr
3
15 Ni
cps
m
Fe
2 1
10
0 0
10
20
30
40
C
5
50
Fe
S Nb
O Cr
Number of laps (x103)
Ti
Cr
Ti
Al
S
Ni
Fe
Ti
0
Width of the sample wear
2
1500
4
6
8
Energy (keV)
Fig. 9. EDS spectrum of the wear track of coating C, after the tribological tests.
m
1000
500 B
A
C
900
0 0
10
20
30
40
50
Number of laps (x103) A
(mm)
C
600 B
300 Fig. 6. Sample trace depth (a) and width (b) vs. number of laps. Tests performed at 800 1C.
0
mm
B 0.12 0.10 0.08 0.06 0.04 0.02 0
A
20000
40000
Number of laps
C
Fig. 10. Ratio ball wear area/sample trace depth, vs. the number of revolutions. Tests performed at 800 1C.
0
10000
20000
30000
40000
50000
number of laps Fig. 7. Wear depth of the ball vs. the number of revolutions. Tests performed at 800 1C.
B
A
3.2.3. Nanoindentation tests The aim of the test is to evaluate the hardness of the coatings, by isolating the measurement from the influence of the substrate material. The nanoindentation usually shows a certain variability in the results, and this obliges to follow particular procedures to guarantee a proper repeatability. In any case, these test results have more a comparative than an absolute value. The results of some of the nanoindentation tests are reported in Table 6. 3.2.4. Adhesion tests Scratch test results are shown in Table 7 in terms of average critical load over ten repetitions, and minimum and maximum values. A clear prevalence of coating B can be observed. Fig. 11 shows the results of the Rockwell A indentation test. The results confirm stronger adhesion and toughness of coating B.
C
2 1.5 mm2
0
1 0.5 0 0
10000
20000
30000
40000
number of laps
Fig. 8. Wear area of the ball vs. the number of revolutions. Tests performed at 800 1C.
3.2.5. Roughness tests The outcome of the roughness tests are showed in Table 8, in terms of Ra, Ry, Rt, and Rz. Assessment length was 15 mm. As it was expected, the values are comparable
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Table 6 Results of the Nanoindentations test Samples
Loading conditions
Young’s modulus (GPa)
Hardness (GPa)
Displacement (nm)
Uncoated
Min load (10 mN) Max load (750 mN) Min load (10 mN) Max load (750 mN) Min load (10 mN) Max load (750 mN) Min load (10 mN) Max load (750 mN)
400.4 410.6 595.7 564.1 578.3 520.7 520.7 480.7
12.3 12.1 32 15.3 29 13.6 15.9 13.4
– 1008.7 – 919.0 – 708.4 – 963.2
A B C
Table 7 Results of the scratch tests in terms of critical load at delamination Coating
Identification
Avg. critical load at delamination (N)
Min/max critical load (N)
TiN+AlTiN TiN+AlTiN+MoS2 CrN+CrN:C+C
A B C
83 93 62
82/89 87/98 57/66
Results computed over ten repetitions.
for the three coatings, and similar to those of the uncoated sample, since the substrate preparation and finishing procedure was identical.
The wet conditions cutting tests, carried out using commercial lubrication fluids, showed a reduction of the difference between uncoated and coated inserts, as it can be seen from the comparison of Figs. 3–5. Further, tests highlighted that coating B showed better overall wear resistance compared to the other coatings. Analysis of wear mechanism of the coatings will be done in the following paragraph, after description of their surface properties. Lubrication condition can be ranked in decreasing performance order as: MQL, wet and dry. The phenomenon can be ascribed to the way the lubricant is spread on the tool using MQL; in fact, the small quantity vaporised on the cutting edge limits thermal shocks, increasing coating duration [8]. 4.2. Laboratory tests
4. Discussion 4.1. Turning tests Turning tests were aimed at evaluating coated tools’ performances under standard cutting speed conditions with respect to uncoated hard metal, commonly used with application to Inconel 718 [10]. The outcome of the cutting tests indicates that the adoption of coating A and most of all coating B have a very positive effect on cutting performance, especially as the cutting speed gets higher, as shown in Figs. 1 and 2. This is justified considering that as the cutting speed increases, the temperature also increases, causing volatile WOx formation, determining chemical alteration of the WC-Co material, which affects some properties like hardness. Since these degradation phenomena become important at temperatures of nearly 900 1C, it is expected that these effects arise when cutting speed is 60 m/min or more [11]. Wear morphologies of the tools at high cutting speed are shown in Fig. 12 for coatings C and A. It can be observed that, even if a limited wear is observed, coating delamination occurred, indicating some adhesion problems between coating and substrate. The phenomenon is even more evident in Fig. 13, in which the white areas indicate the uncoated substrate.
Laboratory characterisation tests have been run with the aim of assessing coating’s quality by means of wellestablished experimental procedures. Further, the comparison with the outcome of the cutting tests underlines the most important coating’s features for cutting and may help to establish an off-line experimental paradigm able to rank the coatings in the correct cutting performance order. Some important aspects of the laboratory characterisation results should be highlighted: 1) The comparison between the results of the tribological tests, reported in Figs. 6–8 and the results of the nanohardness tests, reported in Table 6, show that wear rates are not directly correlated to hardness, indicating some other phenomena, besides simple abrasion, to be important for the wear mechanisms. On the other hand, it is worthwhile to observe that wear rates at high temperature are quite in agreement with turning performances, as it can be observed by comparing Fig. 10 with Figs. 1–5. 2) Friction coefficients at high temperature cannot be used to differentiate the performances of the coatings, since they are quite similar, as shown in Table 5. 3) As shown in Fig. 6, coating C reached significantly higher values of wear if compared with the other two coatings.
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Fig. 11. Rockwell A indentation results: coating A (a); coating B (b); coating C (c).
Table 8 Results of roughness measurements Coating
Identification
Ra Avg./Rng
Ry Avg./Rng
Rt Avg./Rng
Rz Avg./Rng
TiN+AlTiN TiN+AlTiN+MoS2 CrN+CrN:C+C Wc-Co
A B C Uncoated
0.267/0.031 0.356/0.041 0.271/0.039 0.287/0.030
5.714/0.53 6.000/0.65 5.758/0.61 5.885/0.56
6.054/0.58 7.434/0.71 6.758/0.72 6.147/0.64
4.543/0.38 5.601/0.54 4.538/0.45 4.832/0.44
Average and range computed over 5 sampling intervals.
Fig. 12. Wear morphology after cutting at vc ¼ 140 m/min for coating C (a) and A (b).
Adhesion area Coating removal
Adhesion area Coating removal
Fig. 13. Wear morphology at the end of turning tests under vc ¼ 140 m/min for coating A. An important delamination phenomenon occurs.
4) EDS analysis pointed out that an important third-body wear phenomenon occurs, since traces of alumina powder coming from the ball could be found in the wear trace of the sample, as it is shown in Fig. 9, referred to coating C.
5) Fig. 10 shows that in the first part of the test wear proceeds faster on the ball than on the sample, while in the second part of the test sample wear starts to be significative and the ball-to-sample wear ratio decreases. This occurs when the alumina powder coming from the
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Thermal Crack
Fig. 14. Wear morphology for a sample of cutting tool coated with coating A. Cutting conditions: vc 140 m/min, dry, cutting time: 4.33 min VBB ¼ 0.24 mm.
ball starts to abrade the coatings, generating a thirdbody wear phenomenon. In the third part of the test, the ball-to-sample wear ratio becomes constant, indicating that the wear proceeds with the same rate in the two elements. Coating B achieves better performances in terms of ball-to-sample wear ratio. 6) Scratch test and Rockwell C indentation showed significantly higher adhesion and toughness values for the coating B, as the observation of Table 7 and Fig. 11 clearly points out. 4.3. Wear mechanisms Cutting performances of PVD coated tools are strongly affected by composition, microstructure, residual stresses and adhesion between coating and substrate, these features determining wear behaviour. For the studied coatings, wear analysis obtained via SEM–EDS indicated abrasion and welding as main wear mechanisms. A large amount of built up edge was found on the cutting edge of all the observed tools, leading in some cases to craterisation and delamination. Surprisingly, coating B showed better cutting performaces than coating A, although similar in nominal composition. Such behaviour is not entirely justified by the presence in coating B of the external layer of MoS2. EDS analysis, in fact, revealed a first important difference, since Al/Ti ratio was found more homogeneous along the coating B (60/40) than on coating A (ranging from 55/45 to 40/60). A higher percentage of aluminium determines a greater high temperature stability, because of the tendency to form a protective layer of Al2O3 during the machining operation, leading to higher abrasion resistance. In addition, in the case of coating A also some thermal fatigue cracks were found at high cutting speed (see Fig. 14). The low thermal conductivity of AlTiN limits heat dissipation and generates high thermal gradients that may lead to thermal cracks, especially if residual stresses are present in the coating. In the case of coating A, which is a multilayer coating with 10 alternating layers of different composition, residual stresses are likely to be present.
In the case of the CrN-based coating C, premature failure occurred, due to delamination, probably arising from poor adhesion to the substrate, as shown by adhesion test. When adhesive forces between chip and coating are higher than between coating and substrate, debonding rapidly occurs. 4.4. Laboratory characterisation-machining test correlation If we compare the outcome of the cutting tests with the results of the laboratory characterisation, it can be assessed that the tribological tests yield coherent results with the cutting tests, if the index shown in Fig. 10 is considered. In fact, the ball-to-sample wear ratio is able to clearly classify the coatings in the correct performance order. Good forecast properties are also shown by adhesion/toughness tests (see Table 7 and Fig. 11), that yielded the same classification order. The aforementioned tests can therefore be considered good candidates to establish a laboratory characterisation paradigm for cutting tool coatings to be used under severe cutting conditions. Nano-hardness test, on the other hand, did not give the same indications. It is certainly an important test to assess coatings’ quality, but it must not be forgotten that hardness is not always the most important feature for a cutting tool, especially under the cutting conditions here explored. Furthermore, the nanohardness tests referred to in this paper have been performed at room temperature, and we don’t have indications over the coatings’ hardness at the high temperatures reached during machining. 5. Conclusion This paper reports the results obtained in turning Inconel 718 using innovative coated inserts, produced via new PVD processes. All the tested tools performed better than the uncoated inserts, currently the state of the art for machining Inconel 718. Of the three experimental coatings, the AlTiN-based, produced via Arc evaporation, with a further layer of MoS2 deposited via Magnetron sputtering, was found
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more promising both in dry and in MQL condition. The reason can be ascribed to the optimal Al/Ti ratio, that gives the proper combination of abrasion resistance and adhesion to the substrate, and to the MoS2 layer, that limits the adhesion to the chip. The AlTiN-based coating with a multilayer structure performed second, due to the absence of the lubricating MoS2 layer, to the non-optimal Al/Ti ratio and to the occurrence of thermal fatigue cracks, probably caused by the multilayer structure. The CrN-based coating didn’t show a good behaviour, compared with the other coatings, mainly due to low adhesion and extended delamination. Some performance indexes obtained from the characterisation tests, such as the ball-to-sample wear ratio or the scratch test critical load, were able to clearly classify the coatings in the correct performance order. MQL was found to enhance tool performances, further limiting friction. As this lubrication system is inherently safe, the combination coating-MQL represents an interesting opportunity in machining difficult-to-cut materials. Acknowledgments The authors wish to thank Mr. J. Housden of Tecvac Ltd, Cambridge, England and Mr. V. Bellido-Gonzalez of Gencoa Ltd, Liverpool, England for their help in conducting the experimental work that lead to preparation of this paper.
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