Adhesion and wear studies of magnetron sputtered NbN films Kulwant Singh a,n , N. Krishnamurthy a , A.K. Suri b a FRMS, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b Materials Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India article info Article history: Received 2 May 2011 Received in revised form 12 December 2011 Accepted 29 December 2011 Available online 10 January 2012 Keywords: Coating Adhesion Friction Wear abstract NbN films were deposited on MS, SS and HSS substrates by reactive DC magnetron sputtering. Effects of N 2 flow and substrate biasing were studied on the deposition rate, crystal structure, surface hardness, adhesion and tribology. Scratch test was carried out for adhesion evaluation. Online recording of friction, depth of indentation and acoustic emission was carried out. Critical loads for cohesive and adhesive failures were observed. Tribological evaluation was performed on wear and friction machine with reciprocating ball-on-plate configuration at 3and 6 N loads and at 5 and 15 Hz frequencies against hard chrome steel balls of 12.7 mm diameter. Coatings deposited at N 2 /Ar flow ratio of 0.20–0.30 and at substrate bias voltage of 50 to 75 V showed higher hardness, better adhesion and low coefficient of friction. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Magnetron Sputtering, a physical vapor deposition technique, is widely used for deposition of compound coatings [1, 2]. Niobium nitride (NbN) films in the initial years were investigated more because of their superconducting properties rather than their mechanical properties. Early research towards synthesis of NbN films was directed to increase their superconducting transi- tion temperatures [3, 4]. However, NbN films are attractive in wear applications too because of their good thermal expansion match with widely used tool steels. Good mechanical properties coupled with chemical inertness, wear resistance, high melting point, temperature stability and high electrical conductivity make NbN films a suitable material for protective coating [5], field emission cathode [6] and diffusion barrier in microelectronic devices [7]. NbN thin films have been prepared by various deposition techniques including reactive magnetron sputtering [8–10], ion beam assisted deposition [11], pulsed laser deposition [12] and cathodic arc deposition [13, 14]. After the introduction of superlattice coatings, NbN was used as one layer component. Superlattice coatings such as TiN/NbN [15], TaN/NbN [16] and CrN/NbN [17] have been investigated for use as hard, wear resistant and corrosion protective coatings. All of these super- lattice films possess super-hardness effects, which exhibit an anomalous increase in their hardness and wear resistance. However, there is, only limited information on the tribological behavior of single-layer NbN coatings [18–20]. Singer et al. [18] reported data on hardness, wear resistance, crystal structure etc. for sputtered NbN and Nb 2 N thin films. Zhitomirsky et al. [21] prepared NbN coatings using vacuum arc deposition and reported that the chemical composition and mechanical properties were affected by the nitrogen pressure and NbN coatings exhibit high scratch load and high microhardness. NbN films can be used as wear resistant coatings due to its high hardness and good wear resistance. NbN is considered to be suitable for applications in microelectronics and sensors, and in micromechanics and superconducting electronics. Due to their high critical current density, good mechanical properties and transition temperature between 16 and 17 K, NbN films could be successfully used in several superconducting microelectronics applications. In the present study, NbN coatings have been studied on various steel substrates. NbN films were deposited on mild steel (MS), stainless steel (SS) and high speed steel (HSS) substrates by reactive DC magnetron sputtering at various nitrogen (N 2 ) flow rates and substrate biasing. Effect of N 2 flow and substrate biasing was studied on deposition rate, crystal structure, surface hard- ness, scratch adhesion and tribological aspects. 2. Experimental procedure NbN films were deposited using a reactive DC magnetron sputtering on MS, SS and HSS substrates. A niobium (99.9% min purity) metallic target 160 mm diameter and 4 mm thick was mechanically clamped to a planar sputter source mounted hor- izontally on the base of the chamber. The chamber was evacuated to a base pressure of 2 10 6 mbar. The distance between the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International 0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.12.023 n Corresponding author. Tel.: þ91 22 25595378; fax : þ91 22 25505151 E-mail address: [email protected] (K. Singh). Tribology International 50 (2012) 16–25
Transcript
Tribology International 50 (2012) 16–25
Contents lists available at SciVerse ScienceDirect
NbN films were deposited on MS, SS and HSS substrates by reactive DC magnetron sputtering. Effects of
N2 flow and substrate biasing were studied on the deposition rate, crystal structure, surface hardness,
adhesion and tribology. Scratch test was carried out for adhesion evaluation. Online recording of
friction, depth of indentation and acoustic emission was carried out. Critical loads for cohesive and
adhesive failures were observed. Tribological evaluation was performed on wear and friction machine
with reciprocating ball-on-plate configuration at 3and 6 N loads and at 5 and 15 Hz frequencies against
hard chrome steel balls of 12.7 mm diameter. Coatings deposited at N2/Ar flow ratio of 0.20–0.30 and at
substrate bias voltage of �50 to �75 V showed higher hardness, better adhesion and low coefficient of
friction.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Magnetron Sputtering, a physical vapor deposition technique,is widely used for deposition of compound coatings [1, 2].Niobium nitride (NbN) films in the initial years were investigatedmore because of their superconducting properties rather thantheir mechanical properties. Early research towards synthesis ofNbN films was directed to increase their superconducting transi-tion temperatures [3, 4]. However, NbN films are attractive inwear applications too because of their good thermal expansionmatch with widely used tool steels. Good mechanical propertiescoupled with chemical inertness, wear resistance, high meltingpoint, temperature stability and high electrical conductivity makeNbN films a suitable material for protective coating [5], fieldemission cathode [6] and diffusion barrier in microelectronicdevices [7]. NbN thin films have been prepared by variousdeposition techniques including reactive magnetron sputtering[8–10], ion beam assisted deposition [11], pulsed laser deposition[12] and cathodic arc deposition [13, 14]. After the introduction ofsuperlattice coatings, NbN was used as one layer component.Superlattice coatings such as TiN/NbN [15], TaN/NbN [16] andCrN/NbN [17] have been investigated for use as hard, wearresistant and corrosion protective coatings. All of these super-lattice films possess super-hardness effects, which exhibit ananomalous increase in their hardness and wear resistance.However, there is, only limited information on the tribologicalbehavior of single-layer NbN coatings [18–20]. Singer et al. [18]
All rights reserved.
x : þ91 22 25505151
reported data on hardness, wear resistance, crystal structure etc.for sputtered NbN and Nb2N thin films. Zhitomirsky et al. [21]prepared NbN coatings using vacuum arc deposition and reportedthat the chemical composition and mechanical properties wereaffected by the nitrogen pressure and NbN coatings exhibit highscratch load and high microhardness.
NbN films can be used as wear resistant coatings due to itshigh hardness and good wear resistance. NbN is considered to besuitable for applications in microelectronics and sensors, and inmicromechanics and superconducting electronics. Due to theirhigh critical current density, good mechanical properties andtransition temperature between 16 and 17 K, NbN films couldbe successfully used in several superconducting microelectronicsapplications.
In the present study, NbN coatings have been studied onvarious steel substrates. NbN films were deposited on mild steel(MS), stainless steel (SS) and high speed steel (HSS) substrates byreactive DC magnetron sputtering at various nitrogen (N2) flowrates and substrate biasing. Effect of N2 flow and substrate biasingwas studied on deposition rate, crystal structure, surface hard-ness, scratch adhesion and tribological aspects.
2. Experimental procedure
NbN films were deposited using a reactive DC magnetronsputtering on MS, SS and HSS substrates. A niobium (99.9% minpurity) metallic target 160 mm diameter and 4 mm thick wasmechanically clamped to a planar sputter source mounted hor-izontally on the base of the chamber. The chamber was evacuatedto a base pressure of 2�10�6 mbar. The distance between the
Fig. 1. Variation in deposition rate of Nb-N coating with (a) N2/Ar flow ratio (Ar
flow was fixed at 20 sccm) and (b) substrate biasing (N2/Ar¼0.20).
K. Singh et al. / Tribology International 50 (2012) 16–25 17
target and the substrate was fixed at 60 mm. The sputteringpressure was kept at 5�10�3 mbar by admitting a stream ofmixed gas of argon (Ar) and N2 into the chamber. Flow of Ar gaswas fixed at 20 sccm while N2 flow was varied from 0–14 sccm.Substrate biasing was kept constant at �50 V for coatingsdeposited at different nitrogen flow ratios. The power to thetarget was supplied through a stabilized d. c. power supply of0–1000 V (maximum current value: 6 A). Substrate biasing wasvaried from 0 to �150 V in a step of 25 V, keeping the N2/Ar flowratio constant at 0.20. Biasing was applied by means of astabilized d. c. power supply of variable voltage (0–300 V) andcurrent (0–500 mA). The rectangular samples of size 40�25 mmand 3 mm in thickness were polished, cleaned thoroughly anddegreased in alkaline solution ultrasonically prior to thedeposition.
Variation in nitrogen flow and substrate bias voltage wasselected on the basis of abundant literature available for thedeposition of hard nitride coatings. Most of the useful variation inthe properties of various nitride coatings has been found to liewithin a range of deposition parameters. To keep the thickness ofthe coatings constant (for comparative evaluation of the coat-ings), deposition time was varied in such a way that the coatingthickness remains within 710%.
After deposition of coatings on samples, mass modificationswere evaluated. Thickness of the coatings was calculated by massmodification, using the bulk density value. Actual coating thick-nesses were found out by micro-abrasion (Ball-cratering techni-que) using a CSM Calotest instrument by rotating AISI 52100 hardchrome steel ball against coated specimen using suspension ofdiamond particles. The crystal structure of the films was investi-gated by X-ray diffraction (XRD) using CuKa radiation. Surfacehardness was measured by a microhardness tester (Future TechFM-7 model) using Knoop indenter at a load of 25 gf. Fivereadings were performed for each of the sample and the averagevalues have been reported. Adhesion was evaluated by a scratchtester (CSEM, Revetest). Critical loads for cohesive and adhesivefailure for coatings on different substrates were observed. Theeffect of N2 flow and substrate biasing on critical loads wasinvestigated. Loading rates of 10 N/min, 30 N/min 50 N/min and80 N/min were employed on some of the samples to study theeffect of loading rate on critical loads. Loading rate did not givemuch impact on the variation in critical loads. To study the effectsof deposition parameters on Nb-N films during scratch tests, aconstant loading rate of 10 N/min and a constant scratching speedof 1.03 mm/min was employed for all the tests. The scratch lengthwas also kept constant at 4 mm. The scratch indenter used was a200 mm tip radius Rockwell type diamond indenter. Friction force,depth of indentation and acoustic emission signals for all thescratched samples were recorded online. The scratch tracks werevisualized in the optical microscopy immediately after the tests tovisualize the scratch patterns and pictures were taken at differentloads. The starting load in each test was 1 N while maximumload was varied from 10 N to 60 N. Tests were performed in alinearly progressive mode from 1 N start load to a predefinedmaximum load.
Tribological evaluation was performed in a Plint make (TE-70)wear and friction machine with reciprocating ball-on-plate con-figuration. Coated samples were tested at 3 N and 6 N loads and at5 and 15 Hz frequencies against hard chrome steel balls (AISI52100) of 12.7 mm diameter at room temperature. 5 and 15 Hzfrequencies correspond to the sliding speeds of 10 and 30 mm/s,respectively. Two different durations of 10 and 30 minutes wereselected for the wear tests. Wear test parameters are listed inTable 1. Coefficient of friction and wear rate were studied withrespect to load and sliding speed for coatings deposited at variousN2/Ar flow ratios.
3. Results and discussion
3.1. Thickness
It was intended to get the same coating thickness but, usingdifferent bias voltage values or different nitrogen flow values tosee changes in the coating properties. Therefore, duration ofcoating deposition was varied to get approximately same coatingthickness. In the deposition using biasing there was a continuousion bombardment at the substrate, causing the reduced effective
0.01200
1400
1600
1800
2000
Substrate biasing -50VSur
face
har
dnes
s (H
K25
)
N2 / Ar
1800
2000
2200
2400
e ha
rdne
ss (H
K25
)
N2 /Ar = 0.20
0.80.70.60.50.40.30.20.1
K. Singh et al. / Tribology International 50 (2012) 16–2518
deposition rate, which in turn reduced the coating thickness andtherefore more time was required at higher bias voltages to getthe same coating thickness. Coating thickness of about 1.8 mm710% was obtained for coatings deposited at various parameters.A variation of 5–15% in the coating thickness was found in thecalculated and actual values due to the density of the coatingsbeing lower than the bulk value.
3.2. Deposition rate
3.2.1. Effect of nitrogen flow
Deposition rates of Nb-N coatings have been plotted in Fig. 1aas a function of N2/Ar flow ratio. Coatings were deposited withoutand with biasing. For biasing, a constant substrate bias voltage of�50 V was applied. Deposition rate of Nb-N films decreased withthe increase in N2/Ar flow ratio. Maximum deposition rate wasobserved when there was no nitrogen introduced in to thechamber (N2¼0). Without substrate biasing, deposition rate was20 nm/min, which decreased to 8.4 nm/min when N2/Ar flowratio was increased in steps from zero to 0.70. Similarly deposi-tion rate for NbN coatings, deposited with substrate biasing at -50V, was 16 nm/min; which decreased successively to 6.7 nm/min with the increase in N2/Ar flow ratio from zero to 0.70.Deposition rate reduced almost linearly in both the cases withevery increase in N2 flow. This was due to the well known effectof nitride formation at the target called target poisoning, observedin many other studies [22–24]. Further, it was seen that thereduction in deposition rate followed three different linear pathsin both the cases—with biasing or without biasing. The threelinear decrease in deposition rates corresponded to the transitionof phases from Nb to hcp b-Nb2N; hcp b-Nb2N to cubic d-NbN andcubic d-NbN to hcp d0-NbN as was revealed by XRD, discussed inthe Section 3.3.
3.2.2. Effect of substrate biasing
Biasing the substrate causes continuous ion bombardment atthe substrate. This imparts energy and improves not only adhe-sion but also coating density [25]. However, deposition rate getsreduced. Fig. 1b shows deposition rate plotted against the sub-strate bias voltage keeping the flow ratio of nitrogen constant(N2/Ar¼0.20). The deposition rate decreased from 15.9 nm/min to6.0 nm/min when the bias voltage was increased from 0 to�150 V. Decrease in deposition rate with the increase in sub-strate biasing was due to the ion-bombardment effect at thesubstrate. With every increase in substrate biasing increased
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.05N2/Ar
Fig. 2. X-ray diffraction patterns of Nb-N coatings at various N2/Ar flow ratios
(substrate biasing was fixed at �50 V).
ion-bombardment at the substrate with higher energy ions takesplace; detaching the entrapped gas atoms and loosely bondedparticles from the substrate. In most physical vapor depositioncases increased substrate biasing results in the reduction ofdeposition rate due to the ion-bombardment and densificationeffects [26, 27].
3.3. X-ray diffraction
X-ray diffraction patterns of Nb-N films deposited at variousN2/Ar flow ratios are shown in Fig. 2. Coating deposited at N2/Arflow ratio of 0.05 shows hexagonal b-Nb2N as the major phasewith (101) preferred orientation. With increase in N2/Ar flow ratio
01200
1400
1600
Sur
fac
Substrate Biasing (-V)150125100755025
Fig. 3. Surface hardness of Nb-N as a function of (a) N2/Ar flow ratio (Ar flow
¼20 sccm) and (b) substrate biasing (N2/Ar¼0.20).
Table 2Surface hardness of NbN coating on MS, SS and HSS
substrates at a load of 25gf.
Substrate HK25
MS 1084
SS 2040
HSS 2620
K. Singh et al. / Tribology International 50 (2012) 16–25 19
to 0.10 the major phase becomes cubic d-NbN with preferredorientation of (111). At N2/Ar flow ratio of 0.30 hexagonal d0-NbNphase appears, though the major phase is still cubic d-NbN butnow with preferred orientation of (200). With further increase inN2/Ar flow ratio the hexagonal d0-NbN phase increases andbecomes major phase at N2/Ar flow ratio of 0.70. Similar phasechanges with increase in N2 flow were observed by Zenghu et al.[28] and Wen et al. [29]. In all the coatings, substrate peaks wereidentified as the major peaks.
3.4. Hardness
3.4.1. Effect of nitrogen flow
Knoop microhardness values for Nb-N coatings on SS, taken ata load of 25gf, have been plotted as a function of increasing N2/Arflow ratio in Fig. 3a (substrate biasing was kept constant at�50 V). Surface hardness was found to increase rapidly withthe increase in N2 flow. This was due to the compound formation.Surface hardness reached a maximum value of 2040 HK at a N2/Arflow ratio of 0.20 and then decreased with the further increase inN2/Ar flow. The variation in the hardness of the coating wasaccompanied with the observed changes in the crystalline struc-ture of the coating. From the XRD spectra it can be seen that themaximum hardness obtained, for coatings deposited at N2/Ar flowratio of 0.2, corresponds to FCC structure. At lower N2 flow, thelow hardness was due to the presence of b-Nb2N phase [18].When N2/Ar ratio was higher than 0.20, the hardness decreased
20µm
Fig. 4. Scratch patterns for NbN coating on MS (N2/Ar ¼0.20, Vb¼�50 V) at various l
and the major phase present was hcp d0-NbN phase. Similarchanges in the hardness with the variation in N2 have beenreported by Zenghu et al. [28]. Grain size also influences thehardness of the materials. Hardness changes as per the Hall-Petchrelationship. Grain size has been found to increase with theincrease in the partial pressure of nitrogen [30], [31, 32]. Thedecrease in hardness at higher partial pressures of nitrogen couldbe attributed due to the increase in grain sizes.
Table 2 shows the hardness of NbN coating deposited at N2/Arratio of 0.20 and �50 V substrate biasing on MS, SS and HSSsubstrates. HSS substrate showed the maximum hardness of2630HK25.
3.4.2. Effect of substrate biasing
Fig. 3b shows the surface hardness of NbN coatings on SSsubstrate deposited at various substrate bias voltages, keepingN2/Ar flow ratio constant at 0.20. Hardness increased continu-ously with the increase in substrate bias voltage to �150 V. Theincrease in hardness was due to the increased ion bombardmenton the substrate with every increase in substrate biasing that ledto the increased coating density. Hardness of the coatingincreased consistently from 1692HK25, for coating deposited atzero biasing, to 2346HK25 for coating deposited at substratebiasing of �150 V. Work by Kim et al. [9] showed the similarresults, where hardness increased consistently with the increasein substrate bias potential up to �200 V; this was attributed tothe development and refinement of dome structure with the
K. Singh et al. / Tribology International 50 (2012) 16–2520
increase in bias potential up to �200 V. An increased substratebias voltage raises the kinetic energy of the Arþ ions and niobiumparticles. A bombardment of the growing film with highlyenergized niobium particles and Arþ ions caused a dense struc-ture. As a result, the hardness of the film increased.
3.5. Scratch adhesion test
In order to have a real observation during scratch tests, thehorizontal displacement rate i.e scratch speed (1.03 mm/min) andthe load rate (10 N/min) were kept same for all the tests. Severaltypes of observations were revealed as the scratch progressedfrom 1N start load to predefined maximum load in a linearlyincreasing mode. These were—top layer removal, pile-up on thesides, visibility of small cracks to long wide cracks, pores, chip-ping, delamination of the coatings etc. Fig. 4 shows the scratchpatterns for NbN coating on MS sample taken at the loads of 2, 7,12, 16.5, 20 and 30 N. Fig. 5 shows the scratch patterns for NbNcoating on SS taken at the loads of 1, 12, 22, 28, 32 and 38 N. Fig. 6shows the scratch patterns for NbN coating on HSS taken at theloads of 1, 27, 36, 45, 53 and 60 N. The scratch photographs weretaken at many similar loads; however pictures with similarobservations (at different loads) have only been depicted here.These figures show the top layer removal, cracks, pile-up on the
Fig. 5. Scratch patterns for NbN coating on SS (N2/Ar ¼0.20, Vb¼�50 V) at various loa
sides, pores, chipping, partial and complete delamination of thecoating as the load increased. Prominent difference was visiblewith respect to the coating on different substrates. MS samplesshowed the poor adhesion as cracks, chipping, delamination etc.took place at much lower loads, while HSS sample showed thebest adhesion among the three substrates—showing these phe-nomena occurring at much higher loads. Fig. 7 shows the typicalonline recorded graphs for loading, friction, depth of indentationand acoustic emission during scratch test for NbN coating on MS,SS and HSS. Online recording of all the coated and scratchedsamples was performed. However, only typical spectrum fromone of the tests on each substrate has been reproduced here. Fromthese spectra, the depth of indentation and coefficient of frictiondata was evaluated. Acoustic emission signals corroborated thecritical loads for various types of observations as revealed in themicroscope (Figs. 4–6).
3.5.1. Critical loads
Two critical loads Lc1 and Lc2 have been defined for the failureof the coatings. Lc1 the first critical load corresponds to the initialcohesive failure of the coating such as appearance of first cracksor pores within the coating. And Lc2, the second critical loadcorresponds to adhesive failure of the coating i.e. first observationof adhesive failure such as chipping, partial delamination, pores
K. Singh et al. / Tribology International 50 (2012) 16–25 21
or some such phenomena, where substrate beneath coating getsexposed. Lc1 and Lc2 for NbN coatings on various substrates arelisted in Table 3. Lc1 and Lc2 for coatings on MS samples wereobserved to be between 6–8 N and 9–12 N loads, respectively.HSS coated substrates showed the maximum critical loads - Lc1
ranged from 14 to18 N and Lc2 varied from 24 to 36 N.
3.5.2. Coefficient of friction
Coefficient of friction (m), as observed in the scratch adhesiontests, increased with the increasing load. Table 4 lists the m valueat different loads for the three types of substrates. The variation inm was more pronounced for NbN coatings on MS and SSsubstrates, while for HSS coated samples, the variation in m wasleast. m varied from 0.23 to 0.45 for MS samples, 0.25–0.40 for SSsamples and 0.12 to 0.22 for HSS samples with the increase inload from 20 to 60N. This was expected since HSS is harder thanMS or SS. At any particular load, m was found to vary within a verylimited range for coatings deposited at various N2/Ar flow ratios.For example, for NbN coatings on SS sample, m varied from 0.22 to0.25 at 30 N load for coatings deposited at different N2/Ar flowratios.
3.5.3. Effect of loading rate
Effect of loading rate on Lc1 and Lc2 was found to have littleimpact. Lc1 was found to shift from 7 N at 10 N/min to 7.5 N at30 N/min and further to 8.2 N at 50 N/min. Similarly Lc2 shifted
from 8.2 N to 9 N and further to 9.5 N with the similar increase inapplied loading rates. For SS sample, Lc1 changed from 15 to 17 Nand Lc2 from 22 N to 24 N, when the applied load rate wasincreased successively from 10 N/min to 80 N/min. For HSScoated sample, the Lc1 shifted from 15 N to 15.5, 16 and 17 Nwhen the load rate was increased from 10 to 30, 50 and 80 N/min,respectively. Lc2 for coating on HSS sample was found to increasefrom 34 N to 34.5, 35 and 36 N, respectively, for the similarincrease in applied loading rates.
3.5.4. Depth of penetration
Depth of penetration has not much relevance here. Depth ofpenetration depends on how hard is the base substrate. Depth ofpenetration includes elastic as well as plastic deformation duringloading. After unloading most of it (elastic portion) is recovered.The combined value of elastic and plastic suppression at 30Napplied load showed that HSS samples had the least value whileMS samples showed the maximum value. At 30N load, on anaverage, MS samples had 20–30 mm depth of penetration, SSsamples had 12–25 mm depth of penetration and HSS sampleshad 6.5–10 mm depth of penetration. However, the length tra-velled by the indenter before the Lc2 value reaches (i.e. adhesivefailure of the coating) could be of more relevance. The maximumscratch lengths sustained by the MS, SS and HSS substrates wereobserved to be 0.8 mm, 1.5 mm and 3.6 mm before the Lc2 valuereached.
Fig. 7. Online recorded graphs for loading, coefficient of friction, depth of
indentation and acoustic emission during scratch test for NbN coatings (N2/Ar
¼0.20, Vb¼�50 V) on (a) MS, (b) SS and (c) HSS.
Table 3Critical loads for NbN coating on various substrates.
Critical Load MS SS HSS
Lc1 (N) 6–8 8–15 14–18
Lc2 (N) 9–12 12–25 24–36
Table 4m at different loads for NbN coating on MS, SS and HSS during scratch test.
Load(N) MS SS HSS
20 0.23 0.20 0.12
30 0.28 0.25 0.15
40 0.35 0.30 0.18
60 0.45 0.40 0.22
Table 5Effect of biasing on critical loads during scratch
test for Nb-N coatings on SS.
Biasing (�V) Lc1 Lc2
0 6.5 10.5
25 10.5 20
50 12.0 24
75 15.6 26
100 7.0 11
Table 6Effect of N2 flow on critical loads during scratch
test for Nb-N coatings on SS.
N2/Ar flow ratio Lc1 Lc2
0.10 11 18
0.20 12 24
0.30 11 25
0.40 10 20
0.50 8 20
0.60 8 18
0.70 7 14
K. Singh et al. / Tribology International 50 (2012) 16–2522
3.5.5. Effect of biasing
Increase in biasing voltage up to �75 V (in a step of 25 V),keeping other factors constant during deposition of Nb-N coating
on SS substrate, led to the consistent increase in Lc1 and Lc2.Further increases in the bias voltage decreased adhesion. The Lc1
and Lc2 values were found to be 14.6 and 26 N, respectively forcoatings deposited at �75 V biasing. However, at �100 V, Lc1 andLc2 both dropped drastically to 7 and 11 N, as shown inTable 5.The reason for this behavior could be cited to thegeneration of excess compressive stresses due to the bombard-ment by high energy ions, and the consequent detrimental of thecoating adhesion.
3.5.6. Effect of N2 flow
Nb-N coatings deposited on SS at N2/Ar flow ratios of 0.10 to0.70 (keeping the biasing voltage constant at �50 V) wereevaluated for scratch adhesion. Results with respect to criticalloads are shown in Table 6. Coatings deposited at N2/Ar flowratios of 0.20 and 0.30 showed better adhesion with highercritical loads—Lc1 was 11–12 N, and Lc2 was 24–25 N. Thesecoatings had also shown the higher hardness. At higher flow rates,critical loads decreased. Zhitomirsky et al. [21] had shown similar
K. Singh et al. / Tribology International 50 (2012) 16–25 23
results, where critical load as well as hardness initially increasedwith the increase in the nitrogen pressure and then decreasedgradually with the further increase in nitrogen.
3.6. Wear tests
3.6.1. Coefficient of friction
Typical graphs of coefficient of friction (m) with respect towear time for NbN coatings deposited on SS at 0.20, 0.30 and 0.40of N2/Ar flow and tested at 3 N load and 15 Hz frequency areshown in Fig. 8. During the initial ramping, friction fluctuates.This was due to the interaction of coating and the ball material. Inthe initial periods, m increased to a peak value and then reducedgradually to a lower stable value. High initial m was due to theadhesive wear taking place between the ball and the coated platethat led to the transfer of material from ball to the coated surface.The friction reduced as the material transfer from the ballstabilized after a large area of the coated sample was coveredwith the transferred material from the ball. After certain duration,this transfer covered the test area and then abrasive wear takesplace. The stable value of friction increased after some time as thetest progressed. During the steady state, the transferred material
Fig. 8. Coefficient of friction (m) Vs sliding time for NbN coatings deposited at
N2/Ar flow ratio of (a) 0.20, (b) 0.30 and (c) 0.40; wear tested @ 3N and 15Hz for 600 s.
worn out slowly by abrasive removal as the wear test progressed.m varied from 0.45 (for coatings deposited at N2/Ar flow ratio of0.20) to 0.52 (for coatings deposited at N2/Ar flow ratio of 0.30and 0.40) and further to 0.55 (for coatings deposited at N2/Ar flowratio of 0.60). Fig. 9 shows the variation in m with sliding time at5 and 15 Hz frequencies (keeping other factors constant) for Nb-Ncoatings deposited at N2/Ar flow ratio of 0.50. It can be seen from thegraph that with the increase in frequency from 5 to 15 Hz, coefficientof friction increased from 0.5 to 0.55 m for coatings deposited atdifferent N2/Ar flow ratios are plotted against load (at sliding speedsof 10 and 30 mm/s) and against sliding speed (at 3 and 6 N loads) inFigs. 10 and 11, respectively. Coefficient of friction decreased with theincrease in load and increased with the increase in sliding speed.Havey et al. [19] had shown the steady value of m as 0.6 for NbNcoatings against Si3N4 ball and 0.7 against SS ball in a ball-on-flattribometer. Fontalvo et al. [20] observed the mean value of m as0.8 for alumina ball tested against NbN coating tested at roomtemperature in a ball-on-disk tribometer. They cited the reason forthe observed higher m value that the coating might had worn outduring the tests and consequently concomitant higher amount ofproduced wear debris interacted with the surfaces in contact.
In the present study, coatings deposited at N2/Ar flow ratios of0.20 and 0.30 showed the lowest values of m ranging between0.38 to 0.41 tested at 6 N load and 5 Hz frequency.
3.6.2. Wear rate
Due to the transferred material from ball to the coated surface,it was difficult to measure the wear of the coated plate. Therefore,wear of the counter body (ball) was measured. Wear volume ofthe ball was calculated by measuring the wear scar diameter andusing formula–
V ¼ pd4=64r
Fig. 9. Coefficient of friction Vs sliding time for NbN coatings (N2/Ar ¼0.50) wear
tested at (a) 5 Hz and (b) 15 Hz; (Load¼ 6 N).
0.2
0.3
0.4
0.5
0.6
0.7
5Sliding Speed in mm/sec
3 N
0.10
0.20
0.40
0.50
0.60
0.2
0.3
0.4
0.5
0.6
0.7
Sliding Speed in mm/sec
6 N
0.10
0.20
0.40
0.50
0.60
352515
5 352515
µµ
N2/Ar
N2/Ar
Fig. 11. Coefficient of friction (m) Vs sliding speed at (a) 3 N and (b) 6 N load for
coatings deposited at various N2/Ar flow ratios.
2
3
4
5
6
7
8
Wea
r Rat
e x1
0-4 m
m3 /
N.M
Sliding Speed in mm/Sec
3 N
0.10
0.20
0.30
0.40
0.50
0.60
2
3
4
5
6
7
8
5
Wea
r Rat
e x1
0-4 m
m3 /
N.M
Sliding Speed in mm/Sec
6 N
0.10
0.20
0.30
0.40
0.50
0.60
7
N2/Ar
N2/Ar
10 15 20 25 30 35
5 10 15 20 25 30 35
Fig. 12. Ball wear rate Vs sliding speed at (a) 3 N and (b) 6 N load for coatings
deposited at various N2/Ar flow ratios.
2
3
4
5
6
7
8
Wea
r Rat
e x1
0-4 m
m3 /
N.M
Load (N)
Sld. Speed 10mm/Sec
0.10
0.20
0.30
0.40
0.50
0.60
2
3
4
5
6
7
8
2.5
Wea
r Rat
e x1
0-4 m
m3 /
N.M
Load (N)
Sld. Speed 30mm/Sec
0.10
0.20
0.30
0.40
0.50
0.60
3.5 4.5 5.5 6.5
2.5 3.5 4.5 5.5 6.5
N2/Ar
N2/Ar
Fig. 13. Ball wear rate Vs load at sliding speed of (a) 10 and (b) 30 mm/s for
coatings deposited at various N2/Ar flow ratios.
0.2
0.3
0.4
0.5
0.6
0.7
Load in N
Sld. Speed 10 mm/s
0.10
0.20
0.30
0.40
0.50
0.60
0.2
0.3
0.4
0.5
0.6
0.7
2.5Load in N
Sld. Speed 30 mm/s
0.10
0.20
0.40
0.50
0.60
3.5 4.5 5.5 6.5
2.5 3.5 4.5 5.5 6.5
µµ
N2/Ar
N2/Ar
Fig. 10. Coefficient of friction (m) Vs load at (a) 10 and (b) 30 mm/s sliding speed
for coatings deposited at various N2/Ar flow ratios.
K. Singh et al. / Tribology International 50 (2012) 16–2524
Where, V¼wear volume (mm3), d¼wear scar diameter (mm) andr¼ball radius (mm). Wear rate was calculated by the formula –
Wear rate¼Wear volume=ðload� sliding distanceÞ
Sliding distance was measured in mm.
Fig. 12 shows the variation in wear rate of the ball with respectto sliding speed at 3 and 6 N loads, while Fig. 13 shows the wearrate with respect to load at 10 and 30 mm/s of sliding speeds.Coatings deposited at N2/Ar flow ratio of r0.10 yielded thelowest ball wear rate; this was due to the metallic nature of thecoating and presence of Nb2N phase. Nb2N phase has been found
K. Singh et al. / Tribology International 50 (2012) 16–25 25
to show higher abrasion resistant by Singer et al. [18]. Besides, thelower adhesion tendency of the ball to the hexagonal Nb2N phasemight play a role as well. Coatings deposited at N2/Ar flow ratiosof 0.20–0.60 showed higher wear of the counter body (ball). It isevident from the figures that wear rate of the ball increased withthe increase in sliding speed; and wear rate decreased with theincrease in load. The increase in wear rate with the increase insliding speed is due to the longer sliding distance in case of higherspeed samples because duration of sliding is same for both slidingspeeds [19]. Coatings deposited at N2/Ar flow ratios of 0.20–0.60showed higher wear of the counter body.
4. Conclusions
NbN coatings were deposited on MS, SS and HSS substrates byreactive DC magnetron sputtering. N2/Ar flow ratio was varied from0–0.70 and substrate bias voltage varied from zero to �150 V insteps. Coatings were studied for their thickness, hardness, phases,adhesion and wear. The deposition rate decreases with increasingN2/Ar flow ratio and substrate bias, due to target poisoning and ion-bombardment of the coating, respectively. The N2/Ar flow ratio alsodetermined the crystal structure of the coating, which changed fromb-Nb2N to d-NbN to d0-NbN with increasing N2/Ar flow ratio ratio.The microstructure and phases determine both hardness and wear ofthe coating, while substrate bias also plays a role in adhesion andwear response of the coating. Hardness as well as adhesion washigher for coatings deposited at N2/Ar flow ratio of 0.20–0.30.Decrease in hardness at higher partial pressures of nitrogen couldbe attributed to the increase in grain size. Biasing causes ionbombardment on the growing film, which leads to reduction in grainsize and improvement in adhesion and coating density, which in turninfluences wear. Over a critical substrate bias of �75 V, bothadhesion and wear of the coating worsens. This is due to the changesin the microstructure and generation of excess compressive stresses.Coatings deposited at N2/Ar flow ratio of 0.20–0.30 also showed thelowest values of coefficient of friction during wear tests. Wear rate ofthe ball increased with the increase in sliding speed; and wear ratedecreased with the increase in load. The increase in wear rate withthe increase in sliding speed is due to the longer sliding distance.
NbN films having Nb2N and d-NbN as preferred orientationoffer suitable options as wear resistant coatings. Further investi-gation for NbN films with respect to wear resistance properties isrequired to be carried out at higher loads and for longer durations.
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