Pulsed DC unbalanced magnetron sputtering of hard solid lubricant coating for dry machining of aluminium alloy

1Gangopadhyay S., 2Acharya R., 3Chattopadhyay A. K., 3Paul S.

1Research Scholar, Department of Mechanical Engineering, IIT Kharagpur, PIN: 721 302 E-mail: soumya@mech.iitkgp.ernet.in

2M.Tech student, Department of Mechanical Engineering, IIT Kharagpur, PIN: 721 302 E-mail: acharya.ranadip@gmail.com

3Professor, Department of Mechanical Engineering, IIT Kharagpur, PIN: 721 302 E-mail: akc@mech.iitkgp.ernet.in

E-mail: spaul@mech.iitkgp.ernet.in

Abstract:- Low friction, hard and wear resistant TiN-MoSx composite coating was deposited by closed-field unbalanced magnetron sputtering (CFUBMS) with simultaneous activation of Ti and MoS2 targets in N2 gas environment. Ti-MoSx composite and pure MoSx coatings were also deposited for comparison of coating properties. The deposited films were characterized using Field Emission Scanning Electron Microscopy (FESEM), Grazing Incidence X-Ray Diffraction (GIXRD), scratch adhesion test, Vickers microhardness test and pin-on-disc test. The performance test of the coated tools was evaluated in dry turning of ISO: AlSiMg alloy. Analysis of the coated tools after machining by optical microscopy, FESEM and Energy Dispersive Spectroscopy (EDS) clearly showed reduced formation of built-up material over the rake surface of the coated insert compared to that of uncoated insert. This resulted in better surface finish of the machined workpiece. All the results clearly suggest the superior performance of TiN-MoSx composite coating over pure MoSx and Ti-MoSx composite coating. Keywords: TiN-MoSx composite coating, CFUBMS, aluminum alloy, surface finish.

1. INTRODUCTION

The costs of maintaining and eventually disposing of cutting fluids, combined with the health and safety concerns and stricter environmental regulation, have led to a heightened interest in either eliminating cutting fluid altogether, referred to as dry machining, or limiting the amount of cutting fluid applied in case of near dry machining or minimum quantity lubrication (MQL). Advances in the types of coatings applied to cutting tools have been the major factor in improving the feasibility of dry machining [1]. However, desirable performance can be expected if it is possible to develop a coating having excellent resistance to thermal and mechanical wear while generating less frictional heat during machining.

Solid lubrication has been known and used for

decades in specialized applications where liquid or grease lubrication is either undesirable or ineffective

[2]. The historical use of MoS2, the most commonly used solid lubricant material, in spacecraft can, therefore, be extended to industrial environments such as in dry machining or MQL. The superior anti-adhesive properties of MoS2 coating, in particular, has made it an attractive choice for machining sticky materials like aluminium and aluminium alloys under dry or near dry environment. However, it is a considerable challenge to sustain a soft coating like MoS2 with its low strength and poor oxidation resistance under atmospheric and humid condition along with lower coating adhesion. A great deal of research work has been carried out to enhance the tribological properties of MoS2 coating in humid environment by incorporation of different metal dopants like Au, Pb, Ti, W, Cr, Zr etc. [3,4]. Though some of these coatings have been found to be effective in drilling and milling operation [5,6], these could not be successfully used in continuous turning operation because of higher machining temperature

encountered [7]. Therefore, TiN being one of the versatile coating material for cutting tools owing to its high hardness and excellent wear resistance properties can be thought of a promising candidate for co-deposition with MoS2. Though, TiN-MoS2 composite coating with Ti-TiN graded interlayer has been reported to have good mechanical characteristics [8], its exploit as a coating material for cutting tools especially in dry machining application is yet to be studied.

Closed-field unbalanced magnetron sputtering

(CFUBMS) has become one of the widely used PVD technologies because of close compositional and microstructural control, good adherence and coating uniformity [4]. Recent advancement in pulsed dc magnetron sputtering has further widened the scope of improvement of properties of functional coatings mainly in terms of densification of coating and film-substrate adhesion due to effective arc suppression and higher substrate ion current density during deposition [9]. This deposition technology is particularly beneficial in reactive sputtering of oxides, nitrides (e.g. Al2O3, TiN etc.) [9,10] and non-reactive sputtering of dielectric materials like MoS2 [11].

In the present study, TiN-MoSx composite

coating was deposited by closed-field unbalanced magnetron sputtering (CFUBMS) using separate MoS2 and Ti target in N2 gas environment. Pulsed DC power supply was used for both the targets and substrate bias. Pure MoSx and Ti-MoSx composite coating have also been deposited for comparison of properties with TiN-MoSx composite coating. The coatings deposited on cemented carbide turning inserts were used in dry turning of ISO: AlSiMg aluminium alloy. The formation of built up layer on the tool surface and surface finish of machined workpiece have also been studied for both uncoated and coated tools.

2. EXPERIMENTAL DETAILS 2.1 Deposition of Coating

Deposition was carried out on cemented carbide inserts (ISO K10 grade, make: Widia, geometry: SPUN 12 03 08), C 40 steel discs for tribological studies and M2 grade HSS test coupons for other characterization tests. The steel substrates were polished to a roughness value of Ra = 0.05 µm and then thoroughly cleaned ultrasonically with trichloroethylene and isopropyl alcohol prior to deposition. Ti-MoSx and TiN-MoSx composite coatings were deposited by simultaneous activation

of vertically mounted Ti and MoS2 target with or without N2 gas environment. A hard TiN underlayer was also deposited prior to co-deposition to improve the load bearing capacity of the coating. Pure MoSx coating was also deposited for comparison of coating properties. However, in all cases a thin (~100 nm) Ti interlayer was deposited to promote improved film-substrate adhesion. All the coatings were deposited by a dual cathode closed-field unbalanced magnetron sputtering system (TOOL COATER, VTC-01A) manufactured by Milman Thin Films Pvt. Ltd., India. Both the targets as well as the substrate bias were powered with three pulsed DC power supplies, each of 10 kW with variable voltage and current controllers. The power supplies were operated with a pulse frequency (f) of 35 kHz and duty cycle of 90% both for the cathodes as well for the substrate bias throughout the entire deposition cycle and for all the coating architectures. Ar and N2 gas flow were individually controlled by mass flow controllers. All coating architectures were deposited at a working pressure of 0.3 Pa, substrate temperature of 2000C and pulsed substrate bias voltage of -50 V. 2.2 Physical Characterization

Surface morphology and structure of the coatings were studied by a high resolution Carl Zeiss, Supra 40 field emission scanning electron microscope (FESEM). The composition of the as-deposited films was determined by energy dispersive spectroscopy (EDS) coupled with FESEM. Grazing Incidence X-ray Diffraction (GIXRD) was used for the verification of the crystal phases of the coatings. Diffraction studies were carried out with a high resolution Philips, PANalytical PW 3050/60 X’Pert PRO instrument using Cu Kα radiation at an incident angle of 20. A 2θ scan range from 100 to 800 (for Ti-MoSx and TiN-MoSx composite coatings) and 100 to 600 (for pure MoSx coating) were selected. The voltage and current settings were 45 kV and 40 mA respectively. The samples were continuously scanned with a step size of 0.050 (2θ) and a count time of 2s per step. The data were later analysed with X’pert High score software and peaks were identified by comparing with standard JCPDS data files.

2.3 Scratch Adhesion Test

The film-substrate adhesion was studied by a TR-101M5, DUCOM scratch tester. The scratch tests were performed with a Rockwell C diamond stylus (0.2 mm radius) drawn across the surface of the coating at a constant linear speed of 12 mm/min. The normal load was varied linearly from 5 to 70 N.

Adhesion performance is usually quantified as the normal load required to delaminate the coating and is referred to as critical load Lc. This corresponds to the abrupt change in the friction or tangential force required to drive the diamond stylus across the coating. Hence, Lc was determined from the plot of friction force (or coefficient of friction) with respect to scratch length. However, further analysis by optical microscopy and EDS was also carried out to show the exact location of coating failure in order to confirm the critical load for each test.

2.4 Vickers Microhardness Test

Vickers microhardness tests were carried out in order to determine composite film hardness using a LM-700 Digital Indentation Tester, LECO with a Vickers Indenter, LECO (standardized in accordance with ASTM E92). At least five indentations were considered under 0.1 N load for each sample followed by observation under FESEM for better accuracy of measurement.

2.5 Pin-on-Disc Test

Pin-on-disc tests were carried out using a tribometer (TR-201-M3, DUCOM) to investigate friction and wear properties of various coating architectures studied. The material for the pin has been selected similar to that of workpiece selected for machining trial i.e. AlSiMg alloy. C 40 steel discs, each of 25 mm diameter, were deposited with different coating architectures studied. The tests were performed at 20 N loads at track diameter of 20 mm. The tests were continued for 15 min. with a linear speed of 30 m/min. (720 m of track length) or until coating failure. All the tests were carried out at room temperature (25-270C) and 50±5% relative humidity. The wear tracks were later studied with FESEM and EDS. 2.6 Performance Test in Dry Machining

The performance test of the coated cemented carbide inserts was evaluated in dry turning of AlSiMg alloy and then compared with the performance of an uncoated tool. The tests were carried out in a CNC turning centre (LMW, Pilatus 20T). The four different cutting speeds, V of 150, 200, 250, 300 m/min. were selected for the present study with constant feed, f of 0.1 mm/rev and depth of cut, d of 1 mm. The cutting tools after machining were analysed by optical microscopy and energy dispersive spectroscopy (EDS). The surface roughness of the machined workpiece was measured

by a surface profilometer (Make: Taylor Hobson, Model: Surtronic 3+). 3 RESULTS AND DISCUSSION 3.1 Physical Characterization

Figure 1 shows the FESEM fractograph and surface morphology of different coating architectures. It depicts the coating thickness in the range of 1- 1.5 µm and the formation of nanocrystalline grains during pulsed dc sputtering at the given conditions. Bulk EDS analysis on the top surface of the coating indicates the presence of around 25-30% of Ti (atomic %) in both TiN-MoSx and Ti-MoSx composite coating. However, other literatures available on TiN-MoSx composite coating reported much lower presence of MoSx phase (around 8%) [8, 12]. The GIXRD spectra for different coating architectures, as shown in Fig. 2, clearly reveals that application of pulsed power supply both at the targets as well as for the substrate bias during magnetron sputtering resulted in strong basal orientation of (002) plane even for pure MoSx coating with little trace of edge planes e.g. (102) and (103). However, co-deposition with Ti or TiN led to the suppression of reactive edge planes as evident from Fig. 2. Such Coating Architec

tures

Coating fractograph

Surface morphology

MoSx

TiN/ Ti-

MoSx

TiN/ TiN- MoSx

Fig. 1. Coating fractograph and surface morphology of different coating architectures

basal orientation of MoS2-based solid lubricant coatings is particularly more effective for tribological performance [11]. Figure 2 also indicates that the TiN-MoSx composite coating contains peaks for different TiN phases ((111), (200), (220)) as well as

one hexagonal MoSx (002) phase. Hence it can be concluded that MoSx and TiN phase tend to maintain separate entity in the proposed coating architecture and distinct presence of MoSx phase in the spectrum exhibits the existence of MoSx in compound form and not only as elemental Mo and S as suggested in other literatures [12]. However, in case of Ti-MoSx composite coating no separate peak for elemental Ti could be detected. This signifies that Ti could not be present in free form in co-deposited matrix [4].

0 10 20 30 40 50 60 70 80 9

0

1000

2000

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M oS 2-T iN codepositionM oS 2-T i codeposition

P ure M oS2

s fed

cb

Inte

nsity

(a.u

.)

2θ

aa: Mb: M

oS 2 (002)oS 2 (102)

c : M oS 2 (103)d : T iN (111)e : T iN (200)f: T iN (220)s : substrate

Fig. 2. GIXRD spectra of different coating architecture 3.2 Scratch Adhesion Test

Figure 3 shows average critical loads for different coating architectures. Each coating showed critical load in excess of 50 N. This improved film-substrate adhesion may be attributed to the enhanced ion

MoS2 Ti-MoS2 TiN-MoS20

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Coating Architecture

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6872

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Vic

kers

Mic

roha

rdne

ss (G

Pa)

Crit

ical

Loa

d (N

)

Critical Load Vickers Microhardness

Fig. 3. Plot for critical load (obtained in scratch test) and Vickers microhardness for

different coating architectures

current density at the substrate by the application of pulsed DC power for cathodes as well as for substrate bias. It is evident from Fig. 3 that adhesion of pure MoSx coating could be improved by composite

coating architectures. Figure 4 shows a representative optical micrograph showing the initiation of coating delamination in a scratch test for TiN-MoSx composite coating.

Fig. 4. Optical micrograph of scratch track showing

the initiation of coating delamination

3.3 Vickers Microhardness Test

The composite hardness of different coating architectures as obtained from Vickers microhardness test is shown in Fig. 3. It signifies that poor hardness of pure MoSx coating could be substantially augmented by Ti-MoSx or TiN-MoSx co-deposition. It is also evident that the microhardness obtained for the TiN-MoSx composite coating (>10 GPa) is higher than the microhardness obtained using DC magnetron sputtering (around 7.50 GPa), the latter containing even higher atomic percentage of TiN [8]. This improvement in hardness can be attributed to the denser coating structure resulted from higher energy ion flux associated with pulsed DC sputtering. A typical Vickers indentation on TiN-MoSx composite coating is shown in Fig. 5.

Fig.5. FESEM micrograph showing a Vickers indentation on TiN-MoSx composite

coating

3.4 Pin-on-Disc Test

Figure 6 shows the resulting representative coefficient of friction (μ) for different coating architectures against sliding distance under laboratory air condition while sliding against pin of AlSiMg alloy. It is shown that though pure MoSx coating

Coating delamination

Scratch direction

exhibited lowest coefficient of friction it failed after a sliding distance of around 200 m which was evident by the rise in friction curve. However, no such failure could be observed for composite coatings after a sliding distance of 720 m. A representative FESEM micrograph for TiN-MoSx composite coating is shown in Fig. 7. EDS analysis on wear track confirms the presence coating elements. Figure 6 also depicts that μ value of TiN-MoSx composite coating was less compared to that of Ti-MoSx coating. Superior tribological performance of composite coating could be attributed due to presence of self-lubricating MoSx phase through out the coating combined with excellent mechanical properties like adhesion and hardness because of the presence of hard and wear resistant TiN phase.

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 00 .0

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T i-M o S x

T iN -M o S x

M o S x

Coe

ffici

ent o

f fric

tion

(μ)

S l id in g D is ta n c e (m )

Fig. 6 Pin-on-disc test results showing the plot of coefficient of friction vs sliding

distance while sliding against a pin of AlSiMg alloy

Fig. 7 FESEM micrograph of a wear track for TiN- MoSx composite coating 3.5 Performance Test in Dry Machining

Figure 8 shows the condition of uncoated and coated cemented carbide inserts after machining of AlSiMg alloy with V of 300 m/min., f of 0.1 mm/rev and d of 1 mm. The optical micrographs clearly indicate that the maximum built-up of material occurred on uncoated tool and it was minimum for TiN-MoSx composite coating. Detailed EDS analysis

on the rake surface of the tools also confirms this observation. The excellent anti-sticking property of MoSx phase present through out the

(a) (b)

(c) (d)

Fig. 8. Optical micrographs of (a) uncoated and coated cemented carbide inserts with (b) pure MoSx, (c) Ti-MoSx, (d) TiN-MoSx

composite coating after dry turning of AlSiMg alloy

150 180 210 240 270 300

1.0

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R a(μm

)

Cutting Speed (m/min)

Uncoated Pure MoSx Ti-MoSx TiN-MoSx

Fig. 9 Plot of surface roughness vs. cutting speed in

dry turning of AlSiMg alloy composite coating combined with superior mechanical properties due to presence of TiN phase is primarily responsible for this. However, there is little tendency of aluminium sticking on the surface of pure MoSx coated tool. The comparatively poor adhesion and oxidation resistance of pure MoSx coating might lead to partial removal of coating at reasonably high cutting velocity. The reduced tendency of formation of built-up material on the coated tools was also reflected on the surface finish of the machined workpiece as indicated in Fig. 9. It can also be seen that increase in cutting speed from 150 to 200 m/min. for pure MoSx coated tool resulted

in some increase in surface roughness of machined surface. This is possibly due to partial removal of coating at higher cutting velocity. However, presence of a thin layer of solid lubricant coating, as detected by EDS analysis, even at higher cutting speed resulted in better surface finish compared to its uncoated counterpart. The best surface finish was all along obtained for TiN-MoSx composite coating because of minimum built-up material over the rake surface of the tool. 4. CONCLUSIONS

Remarkable improvement in mechanical and tribological properties could be achieved for TiN-MoSx composite coating deposited by pulsed dc magnetron sputtering thereby overcoming the limitation of pure MoSx coating like poor adhesion, low hardness and unacceptable tribological performance in humid atmosphere. The presence of TiN phase in the composite coating with superior mechanical properties like hardness, wear and oxidation resistance at elevated temperature makes it a better choice than Ti-MoSx composite coating. Encouraging result was also obtained in dry turning of aluminium alloy with TiN-MoSx composite coating where formation of built-up material could be successfully reduced finally resulting in much improved surface finish of the machined workpiece. 5. REFERENCES

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