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Sputter deposited low-friction and tough Cr–Si3N4 nanocomposite coatings on plasma nitrided M2 steel
Vacuum 86 (2012) 1118e1125
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Sputter deposited low-friction and tough CreSi3N4 nanocomposite coatings on plasma nitrided M2 steel Harish C. Barshilia*, B. Deepthi, G. Srinivas, K.S. Rajam Surface Engineering Division, National Aerospace Laboratories (CSIR), Post Bag No. 1779, Bangalore 560 017, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 May 2011 Received in revised form 14 October 2011 Accepted 14 October 2011
Nanocomposite coatings of CreSi3N4 exhibiting low friction and high toughness were prepared on plasma nitrided AISI M2 steel substrates using an unbalanced magnetron sputtering system. The surface morphology and cross-sectional microstructure of the CreSi3N4 nanocomposite coatings were studied using field emission scanning electron microscopy (FESEM) techniques. CreSi3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited a dense microstructure with nanoindentation hardness and toughness values of 18 GPa and 2.0 MPam½, respectively. Nanoscratch measurements indicated that Cr eSi3N4 nanocomposite coatings exhibited good adhesion with a maximum critical load of 150 mN. Ballon-disc reciprocating tests at a load of 2 N showed that CreSi3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited an average friction coefficient of 0.30. FESEM studies of the wear tracks indicated that there was no significant wear loss and the CreSi3N4 nanocomposite coatings exhibited only mild wear due to oxidation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: M2 steel CreSi3N4 nanocomposite coating Plasma nitriding Magnetron sputtering Toughness Friction coefficient
1. Introduction Surface engineering techniques alter the near-surface composition and microstructure of materials and are used to develop a wide range of functional properties (e.g., mechanical, electrical, wear-resistant, corrosion-resistant, etc.) on various substrate surfaces such as steel, titanium and aluminum alloys. Plasma nitriding is a popularly used surface modification technique and involves plasma-activated thermo-chemical diffusion of nitrogen in materials like steel. The properties of the nitrided layer can be controlled by optimizing the process parameters such as substrate temperature, nitriding duration, gas mixture, applied voltage, etc. [1]. Plasma nitriding is known to improve the surface hardness of steels, depending on the composition of the steel and the nitriding process parameters [2e4]. However, the maximum hardness achieved as a result of plasma nitriding of engineering substrates (e.g., cutting tools) may not be sufficient for machining applications. Moreover, for these applications, the toughness as well as the friction coefficient also play an important role [5,6]. Therefore, it is necessary to enhance the surface mechanical properties of the plasma nitrided surfaces by other deposition techniques. Plasma nitriding is often combined with physical vapor deposition (PVD) and this is called duplex treatment [7,8]. It may be * Corresponding author. Tel.: þ91 80 25086494; fax: þ91 80 25210113. E-mail address: [email protected] (H.C. Barshilia). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.016
noted that the interface between the plasma nitrided substrates and PVD coating needs to be engineered judiciously for improving the load-bearing capacity of the coating-substrate system and to prevent premature failure of the PVD coating, thus leading to superior performance of the coating-substrate system. Hard coatings of TiN, CrN, TiAlN, etc. prepared on plasma nitrided steel and other substrates have been studied in the last decade [9e12]. In recent years, transition metal nitride based nanocomposite coatings have emerged as new generation hard coatings for various cutting tool applications [13]. Nanocomposite coatings exhibit higher toughness because of grain boundary sliding and migration [5,14]. In particular, metal-ceramic nanocomposites are expected to exhibit moderate hardness and high toughness [15]. The ceramic phase in the nanocomposite coating exhibits better mechanical properties such as higher hardness and the metal phase leads to ductility in the nanocomposite coating [15]. Studies on metalamorphous carbon and metal-amorphous hydrogenated carbon coatings can be found in the literature wherein improved toughness has been obtained due to addition of metallic phases [16,17]. Nanocomposite coatings prepared on plasma nitrided substrates are being explored of late since they exhibit a combination of properties resulting from plasma nitriding and PVD coating [18,19]. To the best of our knowledge, the mechanical and tribological properties of metal doped ceramic nanocomposite thin films on plasma nitrided steel substrates have not been studied. In the present study, CreSi3N4 nanocomposite coatings are prepared by
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magnetron sputtering on pulsed plasma nitrided AISI M2 steel substrates. The mechanical and tribological properties of CreSi3N4 coatings prepared at different Cr contents on plasma nitrided M2 steel substrates are also studied. 2. Experimental details Nanocomposite coatings of CreSi3N4 were prepared using magnetron sputtering on pulsed plasma nitrided high speed AISI M2 steel substrates (19 19 mm2). The chemical composition of the M2 steel substrate used in the present study was 82.7 at.% Fe, 5.2 at.% Cr, 4.5 at.% Mo, 3.5 at.% W, 3.1 at.% V and 1.0 at.% C. The metallographically polished substrates were cleaned in an ultrasonic agitator using isopropyl alcohol and acetone. The nitriding chamber was evacuated to a base vacuum of 5.0 101 Pa using a rotary pump. Pre-cleaning of the substrates was carried out at 380e400 V using a pulsed DC power supply (frequency ¼ 15.8 kHz with 75% on-time and 25% off-time) in a gas mixture of 20% N2 and 80% H2 and a pressure of 3.0 102 Pa. Subsequently, the substrates were subjected to sputter cleaning at a gas pressure of 4.0 102 Pa and a temperature of 250 C. After plasma cleaning, the substrates were heated to the desired temperature and plasma nitriding was carried out at a pressure of 8.0 102 Pa in a gas mixture of 20% N2 and 80% H2. The required temperature was maintained by adjusting the applied voltage. After plasma nitriding, the chamber was cooled down to about 80 C and the nitrided substrates were taken out. In order to optimize the plasma nitriding process, experiments were carried out at different temperatures (400e550 C) and durations (2e4 h). The plasma nitriding process parameters were optimized to eliminate the formation of white layer. Additionally, the plasma nitrided substrates were polished to remove few monolayers from the surface of the substrate, thus eliminating the presence of white layer before deposition of the sputtered coating. The plasma nitrided M2 steel substrates prepared at the optimized nitriding conditions (500 C, 2 h) were coated with CreSi3N4 nanocomposite coatings in an unbalanced magnetron sputtering system. Silicon substrates were also loaded along with the plasma nitrided substrates to prepare samples for XRD and cross-sectional studies. The nanocomposite coatings were deposited using high purity Cr (99.95%) and Si3N4 (99.9%) targets in Ar plasma. An asymmetric bipolar-pulsed DC power supply (frequency ¼ 100 kHz, pulse width ¼ 2976 ns, positive pulse bias ¼ þ37 V) was used to sputter the Cr target and Si3N4 target was sputtered using an RF power supply. Before deposition, the vacuum chamber was pumped down to a base pressure of 4.0 104 Pa using a turbomolecular pump backed by a rotary pump. The substrates were sputter cleaned at a substrate bias of 700 V and an Ar gas pressure of 2.0 Pa for 45 min. The targets were sputter cleaned for 5 min. A Cr interlayer was deposited on the substrates in order to improve the adhesion of the coatings. The CreSi3N4 nanocomposite coatings were deposited at a substrate temperature of 300 C, substrate bias of 50 V and an Ar gas pressure of 1.0 101 Pa. The Cr target power density was varied from 1.1 to 3.3 W/cm2 in order to study the tribological properties at different Cr contents while maintaining a constant power density of 7.9 W/cm2 for the Si3N4 target. X-ray diffraction (XRD) data of the plasma nitrided M2 steel substrates and CreSi3N4 nanocomposite coatings were recorded in qe2q geometry using PANalytical X’Pert Pro X-ray diffractometer with Cu Ka radiation source (l ¼ 0.15406 nm) operated at 45 kV and 40 mA. The surface morphology and cross-sectional microstructure of the plasma nitrided M2 steel substrates before and after deposition of CreSi3N4 coatings were studied using a Carl Zeiss Supra 40 VP field emission scanning electron microscope (FESEM) and elemental compositions of the coatings were determined using
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Oxford energy-dispersive X-ray spectrometer (EDS) attached to FESEM. Knoop micro-hardness measurements of the plasma nitrided substrates were carried out using a BUEHLER MICROMET-2103 micro-hardness tester under a load of 490 mN (50 gf). The mechanical properties of CreSi3N4 nanocomposite coatings were studied using nanoindentation hardness tester (CSM Instruments) at a load of 5 mN using a Berkovich diamond indenter. The details of the nanoindentation measurements are described elsewhere [20]. The toughness of CreSi3N4 nanocomposite coatings was evaluated from nanoindentation measurements carried out using a cube corner type diamond tip. The fracture threshold load was obtained from tests at different surface locations with peak loads ranging from 100 to 300 mN. The atomic force microscope (AFM) system attached to the nanohardness tester was used to image the surface in contact mode. The adhesive strengths and friction coefficients of CreSi3N4 nanocomposite coatings prepared by duplex treatment were evaluated using a CETR UMT nanoscratch tester. For the nanoscratch tests, a 2.5 mm diameter Berkovich indenter was used. The maximum applied load was 400 mN and scratch speed was 0.1 mm/s. For the wear tests, the nanoscratch tester was used in a ball-on-disc reciprocating configuration without the use of any lubricant. The wear tests were carried out using a 4 mm diameter WC ball (H ¼ 92 HRC) at a load of 2 N (Hertzian contact stress ¼ approximately 0.6 GPa), sliding speed of 6 mm/s, sliding amplitude of 3 mm, at room temperature (25 C) and 60% relative humidity. 3. Results and discussion 3.1. Chemical composition and structure The chemical composition obtained using EDS at the surface of plasma nitrided M2 steel substrate was 57% Fe, 3.1% Cr, 2.8% Mo, 2.7% W, 2.4% V, 21% N and 11% C. For CreSi3N4 nanocomposite coatings, the Cr content varied from 20 to 48 at.% when the Cr target power density was varied from 1.1 to 3.3 W/cm2, respectively. The XRD data of M2 steel substrate after plasma nitriding at 500 C for 2 h is shown in Fig. 1(a). The XRD pattern showed a high intensity reflection from the (110) plane and a relatively low intensity peak from the (200) plane of bct (body centered tetragonal) tempered martensite (a0 -Fe) phase at 2q ¼ 44.02 and 64.72 , respectively [21,22]. The XRD data of plasma nitrided substrates also showed diffraction peaks corresponding to iron nitrides, 0 3-Fe2e3N and g -Fe4N, indicating the formation of a dual phase compound layer [4,23]. The carbide phases observed in the XRD pattern originate due to the presence of carbide forming elements such as V, Cr, Mo and W in M2 steel [21,22]. These carbides include M6C and MC type complex carbide phases such as Fe4Mo2C, Fe3W3C, non-stoichiometric VC with dissolved Mo and W, etc. [24]. XRD patterns obtained for CreSi3N4 nanocomposite coatings prepared at 39 and 48 at.% Cr on silicon substrates are shown in Fig. 1(b). CreSi3N4 nanocomposite coatings exhibited a broad band centered at approximately 42.8 and no peaks corresponding to Cr or Si3N4 were observed in the XRD data. This indicates that the CreSi3N4 nanocomposite coatings were nearly amorphous or consisted of very small crystallites with crystallite size in the range of approximately 1e3 nm. The XRD data of Si3N4 coating prepared under similar deposition conditions did not show any characteristic peak in the 2q range of 10e70 indicating that the Si3N4 coating was amorphous (data not shown). 3.2. Surface morphology and cross-sectional studies Fig. 2 shows the FESEM micrographs of the surface and crosssection of M2 steel substrates plasma nitrided at 500 C for 2 h.
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The surface of the plasma nitrided substrate showed the presence of spheroidal precipitates of molybdenum, tungsten and vanadium carbides as shown in Fig. 2(a) [25]. The cross-sectional FESEM micrograph of plasma nitrided M2 steel substrate (Fig. 2(b)) showed the presence of a diffusion zone characterized by a dense matrix leading to the core. The depth of the diffusion zone was approximately 48 mm. Fig. 3 shows the FESEM micrographs of Si3N4 and CreSi3N4 nanocomposite coatings prepared at different Cr contents. Si3N4 coating exhibited a cauliflower-like morphology, as shown in Fig. 3(a). With addition of Cr in Si3N4, there was a significant change in the morphology of the coatings. CreSi3N4 nanocomposite coatings prepared at 20 at.% Cr displayed a dense featureless morphology, as evident from Fig. 3(b). There was no significant variation in the morphology of CreSi3N4 nanocomposite coatings with increase in the Cr content from 20 to 48 at.% (Fig. 3(c),(d)). A typical cross-sectional FESEM micrograph of CreSi3N4 nanocomposite coating (39 at.% Cr) is shown in Fig. 3(e) and it indicated that the coating exhibited a dense microstructure. The thin Cr interlayer is also visible in Fig. 3(e). From the cross-sectional FESEM micrograph, the thickness of the CreSi3N4 nanocomposite coating was found to be approximately 1.5 mm. 3.3. Mechanical properties
Fig. 1. X-ray diffraction data of (a) AISI M2 steel substrate plasma nitrided at 500 C for 2 h and (b) CreSi3N4 nanocomposite coating prepared on Si substrate at 39 and 48 at.% Cr.
The hardnesses of the non-nitrided and plasma nitrided M2 steel substrates (under optimized nitriding conditions: 500 C, 2 h) were approximately 9.6 and 14 GPa, respectively. For CreSi3N4 nanocomposite coatings, the hardness and elastic modulus decreased with increase in the Cr content as shown in Fig. 4. A maximum hardness of 24 GPa and elastic modulus of 246 GPa were obtained for CreSi3N4 nanocomposite coatings prepared at 20 at.% Cr. The decrease in mechanical properties of CreSi3N4 coating with increase in metal content is consistent with the results reported in the literature for metal-ceramic systems [26e28]. The hardness and elastic modulus values of Cr and Si3N4 coatings are also presented in Fig. 4 which correspond well with the values reported by other researchers [29,30]. Preliminary experiments on the determination of toughness of the nanocomposite coating were carried out by examining the surface radial cracks generated during nanoindentation. Toughness measurements were carried out on CreSi3N4 nanocomposite coatings using cube corner indentation method. Even though the indentation method suffers from limitations such as existence of
Fig. 2. FESEM micrographs of (a) surface morphology and (b) cross-section of AISI M2 steel substrate plasma nitrided at 500 C for 2 h.
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Fig. 3. FESEM micrographs of the surface morphology of (a) Si3N4 coating, and CreSi3N4 nanocomposite coating prepared at Cr contents of (b) 20 at.%, (c) 39 at.% and (d) 48 at.%. Fig. 3(e) shows the cross-sectional FESEM micrograph of CreSi3N4 nanocomposite coating prepared at a Cr content of 39 at.%.
cracking threshold, complex multiple crack configuration, residual stress, etc. this method is widely used for toughness evaluation of thin films due to its simplicity [15,31,32]. The use of cube corner indenter helps in reducing the threshold load to induce cracking [33]. The following equation was used to determine the toughness [34]:
1=2 E P Kc ¼ d ; H c3=2
(1)
where Kc is the fracture toughness, d is a constant related to indenter geometry (0.032 for cube corner diamond indenter), E is the elastic modulus of the coating, H is the hardness of the coating, P is the applied indentation load and c is the radial crack length
240
Hardness (GPa)
25
Cr 230
20
Si3N4 220
H 15
Elastic Modulus (GPa)
250
30
E Cr
Si3N4
210
10 10
20
30
40
50
Cr (at.%) Fig. 4. Variations of hardness and elastic modulus of CreSi3N4 nanocomposite coatings with Cr content. Also shown are the hardness and elastic modulus values of Cr and Si3N4 coatings.
(measured from the center of contact to the end of the corner radial crack). CreSi3N4 nanocomposite coatings prepared on plasma nitrided M2 steel substrates could not be used for the nanoindentation tests as the coatings exhibited significant roughness and therefore toughness measurements were done for coatings prepared on Si substrates. Fig. 5 shows the three-dimensional AFM images of the radial cracks observed at the perimeter of the indentation for Si3N4 and CreSi3N4 nanocomposite coatings prepared at different Cr contents on Si substrates. The cracking threshold load was 250 mN for the coating prepared at 20 at.% Cr. CreSi3N4 nanocomposite coatings prepared at 39 and 48 at.% Cr exhibited a higher threshold load of 300 mN for crack initiation. Si3N4 coating on the other hand exhibited the lowest cracking threshold load of 100 mN. For the toughness calculations, the crack length was measured as the projected distance between the center of the imprint and the fracture apex. The cracks propagating from the three corners of the indentation for Si3N4 and CreSi3N4 nanocomposite coating prepared at 20 at.% Cr are clearly visible in Fig. 5(a) and (b), respectively. In the case of CreSi3N4 nanocomposite coatings prepared at 39 and 48 at.% Cr, the cracks were very narrow and pile up of the material as a result of indentation was observed (Fig. 5(c) and (d)). It has to be mentioned that the nanoindentation measurements at a load of 300 mN may be influenced by the substrate. CreSi3N4 nanocomposite coating prepared at 48 at.% Cr exhibited the highest toughness value of 2.0 MPam½ as determined from Eq. (1). CreSi3N4 nanocomposite coatings prepared at 20 and 39 at.% Cr exhibited toughness values of 1.7 and 1.9 MPam½, respectively. It has to be noted that the increase in toughness of CreSi3N4 nanocomposite coating with increase in Cr content from 20 to 48 at.% is at the expense of hardness which decreased from 24 to 18 GPa, respectively. The toughness value obtained for Si3N4 coating was 1.3 MPam½. The improved toughness of CreSi3N4 nanocomposite coatings compared to Si3N4 coating may be attributed to the incorporation of metallic Cr in Si3N4 matrix. Higher toughness of metal-ceramic nanocomposite systems compared to conventional binary and ternary PVD coatings has been reported in the literature [15,35e37]. Addition of metallic phase introduces ductility in
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Fig. 5. Three-dimensional AFM images of radial fracture at the perimeter of the indentation imprint obtained on (a) Si3N4 coating, and CreSi3N4 nanocomposite coating prepared at Cr contents of (b) 20 at.%, (c) 39 at.% and (d) 48 at.%. The AFM images were recorded at the respective cracking threshold loads for (a), (b), (c) and (d) which were 100, 250, 300 and 300 mN, respectively.
brittle ceramic films. The ductile phase can lead to toughening of the composite in mainly two ways: (1) relieving the stress field around the crack tip by ductile phase deformation or blunting of the crack tip at a ductile particle and (2) bridging of the crack by the ductile phase behind the advancing crack tip [6,38]. However, it may be observed that the contribution of ductile phase to toughness enhancement has been very small. Similar results have been reported in the literature [26,38]. The main reason for this has been attributed to lack of plastic deformation of the ductile phase [38]. Several mechanisms such as, pull-out of the ductile phase, crack propagation in the ceramic matrix, failure of matrix adjacent to the ductile phase-matrix interface before failure of ductile phase, etc., which limit plastic deformation of the ductile phase during crack propagation [38].
track beyond Lc2, which confirms the scratch response of the coating observed from the nanoscratch profiles [39]. For CreSi3N4 nanocomposite coatings prepared at 20 at.% Cr (Fig. 6(a) and (d)), Lc1 and Lc2 values were 123 and 187 mN, respectively. At 39 at.% Cr, CreSi3N4 nanocomposite coatings exhibited improved adhesion with Lc1 and Lc2 values of 150 and 196 mN, respectively as shown in Fig. 6(b) and (e). However, at a Cr content of 48 at.%, the adhesive strength of CreSi3N4 nanocomposite coatings reduced (Lc1 ¼ 102 mN, Lc2 ¼ 177 mN) and almost circular type of spallation was observed on either sides of the scratch track (Fig. 6(f)), giving rise to periodic fluctuations in the friction force curve as shown in Fig. 6(c).
3.5. Tribological properties 3.4. Adhesive strength Fig. 6 presents the nanoscratch profiles and optical images obtained from the nanoscratch measurements carried out to study the adhesion strength of CreSi3N4 coatings prepared at different Cr contents on plasma nitrided M2 steel substrates. The total scratch length was 1 mm. The nanoscratch profiles show the variations of applied force (Fz) and friction force (Fx) with sliding time. In Fig. 5, the load at which initial damage of the coating occurs is denoted as Lc1 and the load at which complete failure of the coating takes place is denoted as Lc2. The critical loads for failure of the coatings were obtained by analyzing the nanoscratch profiles (Fig. 6(a)e(c)) and optical images (Fig. 6(d)e(f)) of the scratch traces. From Fig. 6(a)e(c), it can be observed that at Lc1, there was a slight deviation in the friction force curve and beyond Lc2, large variations were observed in the profile indicating complete failure of the coatings. The corresponding optical images in Fig. 6(d)e(f) showed compressive wedge-type spallation on either side of the scratch
The evolution of friction coefficients of Si3N4 and CreSi3N4 nanocomposite coatings prepared at different Cr contents on plasma nitrided M2 steel substrates are shown in Fig. 7. CreSi3N4 nanocomposite coatings exhibited relatively lower friction coefficients than Si3N4 coating. CreSi3N4 nanocomposite coating prepared at 48 at.% Cr exhibited an almost stable friction coefficient of 0.30 after 14,400 cycles. All the other coatings exhibited an increasing friction coefficient curve with increase in the number of sliding cycles. CreSi3N4 nanocomposite coatings prepared at 20 and 39 at.% Cr exhibited average friction coefficients of 0.37 and 0.39, respectively. Si3N4 coating prepared under identical conditions showed an average friction coefficient of 0.40. The friction coefficient of uncoated plasma nitrided M2 steel substrates was approximately 0.60 (data not shown). The above results suggest that the tribological properties of plasma nitrided M2 steel substrates could be significantly enhanced by the deposition of CreSi3N4 nanocomposite coatings.
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Fig. 6. Nanoscratch profiles obtained from nanoscratch measurements for CreSi3N4 nanocomposite coatings prepared at Cr contents of (a) 20 at.% (b) 39 at.% and (c) 48 at.%. The corresponding optical images are shown in (d), (e) and (f).
Fig. 8 shows the optical image and FESEM micrographs of a typical wear track on CreSi3N4 nanocomposite coating (48 at.% Cr) after 14,400 sliding cycles. The optical image of the whole wear track (length ¼ 3 mm, width ¼ 210 mm) is presented in Fig. 8(a). The wear track was smooth with no accumulated wear debris along the edges of the wear track except at the front and rear ends (regions I and III) of the track. Fig. 8(b)e(d) represent the FESEM micrographs of regions I, II and III, respectively. The two ends of the wear track
Fig. 7. Friction coefficient data for Si3N4, and CreSi3N4 nanocomposite coatings prepared at different Cr contents on plasma nitrided M2 steel substrates.
(regions I and III) showed dark gray patches suggesting accumulation of wear debris/formation of new phases. The worn area at the center of the wear track (region II) did not show any crack formation or delamination. The higher magnification FESEM micrographs of regions I, II and III are shown in Fig. 8(e)e(g). These micrographs reveal the presence of several cracked areas in regions I and III of the wear track. The cracking of the coating may be attributed to tensile stresses which arise during sliding of the sample against the WC ball and oxidation of the coating [40]. EDS data recorded in the dark areas of regions I and III indicated considerable decrease in the nitrogen content and an increase in the oxygen content confirming the formation of oxide phases. From the FESEM studies of the wear track on CreSi3N4 nanocomposite coating, it can be inferred that there was no considerable material loss due to wear and the coatings exhibited only mild wear by oxidation. The enhanced wear resistance of CreSi3N4 nanocomposite coatings prepared on plasma nitrided M2 steel substrates may be attributed to their good adhesion and moderate hardness as discussed in the previous sections. To summarize, in the present study we have developed a coating-substrate system with improved mechanical and tribological properties by duplex treatment. As mentioned before, the toughness of the CreSi3N4 nanocomposite coating was increased at the expense of coating hardness. However, in order to meet the complex requirements of engineering applications, coatings with moderate hardness, high toughness and low friction are essential.
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Fig. 8. (a) Optical image of the whole wear track on CreSi3N4 nanocomposite coating (48 at.% Cr), (b)e(d) FESEM micrographs of different regions of the wear track on CreSi3N4 nanocomposite coating, (e)e(g) High magnification FESEM micrographs of different regions of the wear track on CreSi3N4 nanocomposite coating.
4. Conclusions
References
Low-friction and tough CreSi3N4 nanocomposite coatings were deposited by magnetron sputtering on plasma nitrided AISI M2 steel substrates. The XRD data of CreSi3N4 nanocomposite coatings suggested that the coatings were nearly amorphous with very small crystallites. CreSi3N4 nanocomposite coating prepared at 48 at.% Cr exhibited moderate hardness (18 GPa) and a high toughness value 2.0 MPam½ compared to Si3N4 coating. Nanoscratch tests demonstrated the improved load-bearing capacity of CreSi3N4 nanocomposite coatings prepared on plasma nitrided substrates with a maximum Lc1 value of 150 mN for coatings prepared at 39 at.% Cr. Wear tests carried out in ball-on-disc reciprocating configuration at a load of 2 N showed that CreSi3N4 nanocomposite coatings prepared at 48 at.% Cr on M2 steel substrates exhibited an average friction coefficient of 0.30. Analysis of the wear tracks on CreSi3N4 nanocomposite coatings by FESEM studies revealed that the coatings exhibited only mild wear due to formation of oxides.
[1] Pye D. Practical nitriding and ferritic nitrocarburizing. Ohio: ASM International; 2003. [2] Ochoa EA, Figueroa CA. Influence of the microstructure on steel hardening in pulsed plasma nitriding. J Vac Sci Technol A 2008;26:328e32. [3] Wang J, Xiong J, Peng Q, Fan H, Wang Y, Li G, et al. Effects of DC plasma nitriding parameters on microstructure and properties of 304L stainless steel. Mater Character 2009;60:197e203. [4] Zagonel LF, Figueroa CA, Droppa Jr R, Alvarez F. Influence of the process temperature on the steel microstructure and hardening in pulsed plasma nitriding. Surf Coat Technol 2006;201:452e7. [5] Zhang S, Wang HL, Ong SE, Sun D, Bui XL. Hard yet tough nanocomposite coatings e present status and future trends. Plasma Process Polym 2007;4: 220e8. [6] Wang H, Zhang S. Toughness and toughening of hard nanocomposite coatings. In: Zhang S, editor. CRC handbook of nanostructured thin films and coatings. Boca Raton: CRC Press; 2011. p. 99e145. [7] Bell T, Dong H, Sun Y. Realising the potential of duplex surface engineering. Tribol Int 1998;31:127e37. [8] Rie KT, Broszeit E. Plasma diffusion treatment and duplex treatment-recent development and new applications. Surf Coat Technol 1995;76e77:425e36. [9] Podgornik B, Hogmark S, Sandberg O, Leskovsek V. Wear resistance and antisticking properties of duplex treated forming tool steel. Wear 2003;254: 1113e21. [10] Braga D, Dias JP, Cavaleiro A. Duplex treatment: W-Ti-N sputtered coatings on pre-nitrided low and high alloy steels. Surf Coat Technol 2006;200:4861e9. [11] Kamminga JD, Hoy R, Janssen GCAM, Lugscheider E, Maes M. First results on duplex coatings without intermediate mechanical treatment. Surf Coat Technol 2003;174e175:671e6. [12] Batista JCA, Godoy C, Buono VTL, Matthews A. Characterization of duplex and non-duplex (Ti, Al)N and Cr-N PVD coatings. Mater Sci Eng A 2002;336: 39e51. [13] Barshilia HC, Deepthi B, Rajam KS. Transition metal nitride based nanolayered multilayer coatings and nanocomposite coatings as novel superhard materials. In: Zhang S, editor. CRC handbook of nanostructured thin films and coatings. Boca Raton: CRC Press; 2011. p. 427e80. [14] Voevodin AA, Zabinski JS. Supertough wear-resistant coatings with ‘chameleon’ surface adaptation. Thin Solid Films 2000;370:223e31. [15] Zhang S, Sun D, Fu Y, Du H. Toughening of hard nanostructural thin films: a critical review. Surf Coat Technol 2005;198:2e8.
Acknowledgments The authors thank the Director, NAL for giving permission to publish these results. We thank Mr. Siju and Mr. Praveen Kumar for performing various measurements. Mr. V. Prakash is thanked for sample preparation and experimental work. Dr. Satyam Suwas from Indian Institute of Science, Bangalore is thanked for XRD measurements. This work was supported by the Council of Scientific and Industrial Research, New Delhi, India (Project Nos. FAC-00-01-11, SIP-SED-03 and NWP00-51-03).
H.C. Barshilia et al. / Vacuum 86 (2012) 1118e1125 [16] Dub S, Pauleau Y, Thiery F. Mechanical properties of nanostructured copperhydrogenated amorphous carbon composite films studied by nanoindentation. Surf Coat Technol 2004;180e181:551e5. [17] Pauleau Y, Thiery F. Design and characterization of nanostructured metal/ carbon composite films. Surf Coat Technol 2004;180e181:313e22. [18] Guruvenket S, Li D, Klemberg-Sapieha JE, Martinu L, Szpunar J. Mechanical and tribological properties of duplex treated TiN, nc-TiN/a-SiNx and nc-TiCN/ a-SiCN coatings deposited on 410 low alloy stainless steel. Surf Coat Technol 2009;203:2905e11. [19] Rahman M, Haider J, Dowling DP, Duggan P, Hashmi MSJ. Investigation of mechanical properties of TiN þ MoSx coating on plasma-nitrided substrate. Surf Coat Technol 2005;200:1451e7. [20] Barshilia HC, Rajam KS. Characterization of Cu/Ni multilayer coatings by nanoindentation and atomic force microscopy. Surf Coat Technol 2002;155: 195e202. [21] Nayal G, Lewis DB, Lembke M, Munz WD, Cockrem JE. Influence of sample geometry on the effect of pulse plasma nitriding of M2 steel. Surf Coat Technol 1999;111:148e57. [22] Akbari A, Mohammadzadeh R, Templier C, Riviere JP. Effect of the initial microstructure on the plasma nitriding behavior of AISI M2 high speed steel. Surf Coat Technol 2010;204:4114e20. [23] da Silva Rocha A, Strohaecker T, Tomala V, Hirsch T. Microstructure and residual stresses of a plasma nitrided M2 tool steel. Surf Coat Technol 1999; 115:24e31. [24] Serna MM, Rossi JL. MC complex carbide in AISI M2 high-speed steel. Mater Lett 2009;63:691e3. [25] Voort GFV. Tool steels. In: Metals handbook, vol. 9. Ohio: American Society for Metals; 1987. p. 256e72. [26] Voevodin AA, Hu JJ, Jones JG, Fitz TA, Zabinski JS. Growth and structural characterization of yttria-stabilized zirconia-gold nanocomposite films with improved toughness. Thin Solid Films 2001;401:187e95. [27] Soldan J, Musil J. Structure and mechanical properties of DC magnetron sputtered TiC/Cu films. Vacuum 2006;81:531e8.
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[28] Elangovan T, Kuppusami P, Thirumurugesan R, Sudha C, Mohandas E. Mangalaraj. A study on the influence of copper content in CrN/Cu nanocomposite thin films prepared by pulsed DC magnetron sputtering. J Nanoscience Nanotechnol 2009;9:5436e40. [29] Lu W, Komvopoulos K. Nanomechanical and nanotribological properties of carbon, chromium, and titanium carbide ultrathin films. J Tribol 2001;123: 717e24. [30] Vila M, Caceres D, Prieto C. Mechanical properties of sputtered silicon nitride thin films. J Appl Phys 2003;94:7868e73. [31] Kodali P, Walter KC, Nastasi M. Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings. Tribol Int 1997;8: 591e8. [32] Jedrzejowski P, Klemberg-Sapieha JE, Martinu L. Quaternary hard nanocomposite TiCxNy/SiCN coatings prepared by plasma enhanced chemical vapor deposition. Thin Solid Films 2004;466:189e96. [33] Fischer-Cripps AC. Nanoindentation. 2nd ed. New York: Springer-Verlag; 2004. [34] Lawn BR, Evans AG, Marshall DB. Elastic/plastic indentation damage in ceramics: the median/radial crack system. J Am Ceram Soc 1980;63: 574e81. [35] Neralla S, Kumar D, Yarmolenko S, Sankar J. Mechanical properties of nanocomposite metal-ceramic thin films. Compos Part B 2004;35:157e62. [36] Zhang S, Bui XL, Fu Y. Magnetron-sputtered nc-TiC/a-C(Al) tough nanocomposite coatings. Thin Solid Films 2004;467:261e6. [37] Tuan WH, Brook RJ. The toughening of alumina with nickel inclusions. J Eur Ceram Soc 1990;6:31e7. [38] Sun X, Yeomans JA. Ductile phase toughened brittle materials. J Mater Sci Technol 1996;12:124e34. [39] Bull SJ. Failure modes in scratch adhesion testing. Surf Coat Technol 1991;50: 25e32. [40] Zlatanovic M, Kakas D, Mazibrada L, Kunosic A, Munz WD. Influence of plasma nitriding on wear performance of TiN coating. Surf Coat Technol 1994;64: 173e81.
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Sputter deposited low-friction and tough CreSi3N4 nanocomposite coatings on plasma nitrided M2 steel Harish C. Barshilia*, B. Deepthi, G. Srinivas, K.S. Rajam Surface Engineering Division, National Aerospace Laboratories (CSIR), Post Bag No. 1779, Bangalore 560 017, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 May 2011 Received in revised form 14 October 2011 Accepted 14 October 2011
Nanocomposite coatings of CreSi3N4 exhibiting low friction and high toughness were prepared on plasma nitrided AISI M2 steel substrates using an unbalanced magnetron sputtering system. The surface morphology and cross-sectional microstructure of the CreSi3N4 nanocomposite coatings were studied using field emission scanning electron microscopy (FESEM) techniques. CreSi3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited a dense microstructure with nanoindentation hardness and toughness values of 18 GPa and 2.0 MPam½, respectively. Nanoscratch measurements indicated that Cr eSi3N4 nanocomposite coatings exhibited good adhesion with a maximum critical load of 150 mN. Ballon-disc reciprocating tests at a load of 2 N showed that CreSi3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited an average friction coefficient of 0.30. FESEM studies of the wear tracks indicated that there was no significant wear loss and the CreSi3N4 nanocomposite coatings exhibited only mild wear due to oxidation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: M2 steel CreSi3N4 nanocomposite coating Plasma nitriding Magnetron sputtering Toughness Friction coefficient
1. Introduction Surface engineering techniques alter the near-surface composition and microstructure of materials and are used to develop a wide range of functional properties (e.g., mechanical, electrical, wear-resistant, corrosion-resistant, etc.) on various substrate surfaces such as steel, titanium and aluminum alloys. Plasma nitriding is a popularly used surface modification technique and involves plasma-activated thermo-chemical diffusion of nitrogen in materials like steel. The properties of the nitrided layer can be controlled by optimizing the process parameters such as substrate temperature, nitriding duration, gas mixture, applied voltage, etc. [1]. Plasma nitriding is known to improve the surface hardness of steels, depending on the composition of the steel and the nitriding process parameters [2e4]. However, the maximum hardness achieved as a result of plasma nitriding of engineering substrates (e.g., cutting tools) may not be sufficient for machining applications. Moreover, for these applications, the toughness as well as the friction coefficient also play an important role [5,6]. Therefore, it is necessary to enhance the surface mechanical properties of the plasma nitrided surfaces by other deposition techniques. Plasma nitriding is often combined with physical vapor deposition (PVD) and this is called duplex treatment [7,8]. It may be * Corresponding author. Tel.: þ91 80 25086494; fax: þ91 80 25210113. E-mail address: [email protected] (H.C. Barshilia). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.016
noted that the interface between the plasma nitrided substrates and PVD coating needs to be engineered judiciously for improving the load-bearing capacity of the coating-substrate system and to prevent premature failure of the PVD coating, thus leading to superior performance of the coating-substrate system. Hard coatings of TiN, CrN, TiAlN, etc. prepared on plasma nitrided steel and other substrates have been studied in the last decade [9e12]. In recent years, transition metal nitride based nanocomposite coatings have emerged as new generation hard coatings for various cutting tool applications [13]. Nanocomposite coatings exhibit higher toughness because of grain boundary sliding and migration [5,14]. In particular, metal-ceramic nanocomposites are expected to exhibit moderate hardness and high toughness [15]. The ceramic phase in the nanocomposite coating exhibits better mechanical properties such as higher hardness and the metal phase leads to ductility in the nanocomposite coating [15]. Studies on metalamorphous carbon and metal-amorphous hydrogenated carbon coatings can be found in the literature wherein improved toughness has been obtained due to addition of metallic phases [16,17]. Nanocomposite coatings prepared on plasma nitrided substrates are being explored of late since they exhibit a combination of properties resulting from plasma nitriding and PVD coating [18,19]. To the best of our knowledge, the mechanical and tribological properties of metal doped ceramic nanocomposite thin films on plasma nitrided steel substrates have not been studied. In the present study, CreSi3N4 nanocomposite coatings are prepared by
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magnetron sputtering on pulsed plasma nitrided AISI M2 steel substrates. The mechanical and tribological properties of CreSi3N4 coatings prepared at different Cr contents on plasma nitrided M2 steel substrates are also studied. 2. Experimental details Nanocomposite coatings of CreSi3N4 were prepared using magnetron sputtering on pulsed plasma nitrided high speed AISI M2 steel substrates (19 19 mm2). The chemical composition of the M2 steel substrate used in the present study was 82.7 at.% Fe, 5.2 at.% Cr, 4.5 at.% Mo, 3.5 at.% W, 3.1 at.% V and 1.0 at.% C. The metallographically polished substrates were cleaned in an ultrasonic agitator using isopropyl alcohol and acetone. The nitriding chamber was evacuated to a base vacuum of 5.0 101 Pa using a rotary pump. Pre-cleaning of the substrates was carried out at 380e400 V using a pulsed DC power supply (frequency ¼ 15.8 kHz with 75% on-time and 25% off-time) in a gas mixture of 20% N2 and 80% H2 and a pressure of 3.0 102 Pa. Subsequently, the substrates were subjected to sputter cleaning at a gas pressure of 4.0 102 Pa and a temperature of 250 C. After plasma cleaning, the substrates were heated to the desired temperature and plasma nitriding was carried out at a pressure of 8.0 102 Pa in a gas mixture of 20% N2 and 80% H2. The required temperature was maintained by adjusting the applied voltage. After plasma nitriding, the chamber was cooled down to about 80 C and the nitrided substrates were taken out. In order to optimize the plasma nitriding process, experiments were carried out at different temperatures (400e550 C) and durations (2e4 h). The plasma nitriding process parameters were optimized to eliminate the formation of white layer. Additionally, the plasma nitrided substrates were polished to remove few monolayers from the surface of the substrate, thus eliminating the presence of white layer before deposition of the sputtered coating. The plasma nitrided M2 steel substrates prepared at the optimized nitriding conditions (500 C, 2 h) were coated with CreSi3N4 nanocomposite coatings in an unbalanced magnetron sputtering system. Silicon substrates were also loaded along with the plasma nitrided substrates to prepare samples for XRD and cross-sectional studies. The nanocomposite coatings were deposited using high purity Cr (99.95%) and Si3N4 (99.9%) targets in Ar plasma. An asymmetric bipolar-pulsed DC power supply (frequency ¼ 100 kHz, pulse width ¼ 2976 ns, positive pulse bias ¼ þ37 V) was used to sputter the Cr target and Si3N4 target was sputtered using an RF power supply. Before deposition, the vacuum chamber was pumped down to a base pressure of 4.0 104 Pa using a turbomolecular pump backed by a rotary pump. The substrates were sputter cleaned at a substrate bias of 700 V and an Ar gas pressure of 2.0 Pa for 45 min. The targets were sputter cleaned for 5 min. A Cr interlayer was deposited on the substrates in order to improve the adhesion of the coatings. The CreSi3N4 nanocomposite coatings were deposited at a substrate temperature of 300 C, substrate bias of 50 V and an Ar gas pressure of 1.0 101 Pa. The Cr target power density was varied from 1.1 to 3.3 W/cm2 in order to study the tribological properties at different Cr contents while maintaining a constant power density of 7.9 W/cm2 for the Si3N4 target. X-ray diffraction (XRD) data of the plasma nitrided M2 steel substrates and CreSi3N4 nanocomposite coatings were recorded in qe2q geometry using PANalytical X’Pert Pro X-ray diffractometer with Cu Ka radiation source (l ¼ 0.15406 nm) operated at 45 kV and 40 mA. The surface morphology and cross-sectional microstructure of the plasma nitrided M2 steel substrates before and after deposition of CreSi3N4 coatings were studied using a Carl Zeiss Supra 40 VP field emission scanning electron microscope (FESEM) and elemental compositions of the coatings were determined using
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Oxford energy-dispersive X-ray spectrometer (EDS) attached to FESEM. Knoop micro-hardness measurements of the plasma nitrided substrates were carried out using a BUEHLER MICROMET-2103 micro-hardness tester under a load of 490 mN (50 gf). The mechanical properties of CreSi3N4 nanocomposite coatings were studied using nanoindentation hardness tester (CSM Instruments) at a load of 5 mN using a Berkovich diamond indenter. The details of the nanoindentation measurements are described elsewhere [20]. The toughness of CreSi3N4 nanocomposite coatings was evaluated from nanoindentation measurements carried out using a cube corner type diamond tip. The fracture threshold load was obtained from tests at different surface locations with peak loads ranging from 100 to 300 mN. The atomic force microscope (AFM) system attached to the nanohardness tester was used to image the surface in contact mode. The adhesive strengths and friction coefficients of CreSi3N4 nanocomposite coatings prepared by duplex treatment were evaluated using a CETR UMT nanoscratch tester. For the nanoscratch tests, a 2.5 mm diameter Berkovich indenter was used. The maximum applied load was 400 mN and scratch speed was 0.1 mm/s. For the wear tests, the nanoscratch tester was used in a ball-on-disc reciprocating configuration without the use of any lubricant. The wear tests were carried out using a 4 mm diameter WC ball (H ¼ 92 HRC) at a load of 2 N (Hertzian contact stress ¼ approximately 0.6 GPa), sliding speed of 6 mm/s, sliding amplitude of 3 mm, at room temperature (25 C) and 60% relative humidity. 3. Results and discussion 3.1. Chemical composition and structure The chemical composition obtained using EDS at the surface of plasma nitrided M2 steel substrate was 57% Fe, 3.1% Cr, 2.8% Mo, 2.7% W, 2.4% V, 21% N and 11% C. For CreSi3N4 nanocomposite coatings, the Cr content varied from 20 to 48 at.% when the Cr target power density was varied from 1.1 to 3.3 W/cm2, respectively. The XRD data of M2 steel substrate after plasma nitriding at 500 C for 2 h is shown in Fig. 1(a). The XRD pattern showed a high intensity reflection from the (110) plane and a relatively low intensity peak from the (200) plane of bct (body centered tetragonal) tempered martensite (a0 -Fe) phase at 2q ¼ 44.02 and 64.72 , respectively [21,22]. The XRD data of plasma nitrided substrates also showed diffraction peaks corresponding to iron nitrides, 0 3-Fe2e3N and g -Fe4N, indicating the formation of a dual phase compound layer [4,23]. The carbide phases observed in the XRD pattern originate due to the presence of carbide forming elements such as V, Cr, Mo and W in M2 steel [21,22]. These carbides include M6C and MC type complex carbide phases such as Fe4Mo2C, Fe3W3C, non-stoichiometric VC with dissolved Mo and W, etc. [24]. XRD patterns obtained for CreSi3N4 nanocomposite coatings prepared at 39 and 48 at.% Cr on silicon substrates are shown in Fig. 1(b). CreSi3N4 nanocomposite coatings exhibited a broad band centered at approximately 42.8 and no peaks corresponding to Cr or Si3N4 were observed in the XRD data. This indicates that the CreSi3N4 nanocomposite coatings were nearly amorphous or consisted of very small crystallites with crystallite size in the range of approximately 1e3 nm. The XRD data of Si3N4 coating prepared under similar deposition conditions did not show any characteristic peak in the 2q range of 10e70 indicating that the Si3N4 coating was amorphous (data not shown). 3.2. Surface morphology and cross-sectional studies Fig. 2 shows the FESEM micrographs of the surface and crosssection of M2 steel substrates plasma nitrided at 500 C for 2 h.
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The surface of the plasma nitrided substrate showed the presence of spheroidal precipitates of molybdenum, tungsten and vanadium carbides as shown in Fig. 2(a) [25]. The cross-sectional FESEM micrograph of plasma nitrided M2 steel substrate (Fig. 2(b)) showed the presence of a diffusion zone characterized by a dense matrix leading to the core. The depth of the diffusion zone was approximately 48 mm. Fig. 3 shows the FESEM micrographs of Si3N4 and CreSi3N4 nanocomposite coatings prepared at different Cr contents. Si3N4 coating exhibited a cauliflower-like morphology, as shown in Fig. 3(a). With addition of Cr in Si3N4, there was a significant change in the morphology of the coatings. CreSi3N4 nanocomposite coatings prepared at 20 at.% Cr displayed a dense featureless morphology, as evident from Fig. 3(b). There was no significant variation in the morphology of CreSi3N4 nanocomposite coatings with increase in the Cr content from 20 to 48 at.% (Fig. 3(c),(d)). A typical cross-sectional FESEM micrograph of CreSi3N4 nanocomposite coating (39 at.% Cr) is shown in Fig. 3(e) and it indicated that the coating exhibited a dense microstructure. The thin Cr interlayer is also visible in Fig. 3(e). From the cross-sectional FESEM micrograph, the thickness of the CreSi3N4 nanocomposite coating was found to be approximately 1.5 mm. 3.3. Mechanical properties
Fig. 1. X-ray diffraction data of (a) AISI M2 steel substrate plasma nitrided at 500 C for 2 h and (b) CreSi3N4 nanocomposite coating prepared on Si substrate at 39 and 48 at.% Cr.
The hardnesses of the non-nitrided and plasma nitrided M2 steel substrates (under optimized nitriding conditions: 500 C, 2 h) were approximately 9.6 and 14 GPa, respectively. For CreSi3N4 nanocomposite coatings, the hardness and elastic modulus decreased with increase in the Cr content as shown in Fig. 4. A maximum hardness of 24 GPa and elastic modulus of 246 GPa were obtained for CreSi3N4 nanocomposite coatings prepared at 20 at.% Cr. The decrease in mechanical properties of CreSi3N4 coating with increase in metal content is consistent with the results reported in the literature for metal-ceramic systems [26e28]. The hardness and elastic modulus values of Cr and Si3N4 coatings are also presented in Fig. 4 which correspond well with the values reported by other researchers [29,30]. Preliminary experiments on the determination of toughness of the nanocomposite coating were carried out by examining the surface radial cracks generated during nanoindentation. Toughness measurements were carried out on CreSi3N4 nanocomposite coatings using cube corner indentation method. Even though the indentation method suffers from limitations such as existence of
Fig. 2. FESEM micrographs of (a) surface morphology and (b) cross-section of AISI M2 steel substrate plasma nitrided at 500 C for 2 h.
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Fig. 3. FESEM micrographs of the surface morphology of (a) Si3N4 coating, and CreSi3N4 nanocomposite coating prepared at Cr contents of (b) 20 at.%, (c) 39 at.% and (d) 48 at.%. Fig. 3(e) shows the cross-sectional FESEM micrograph of CreSi3N4 nanocomposite coating prepared at a Cr content of 39 at.%.
cracking threshold, complex multiple crack configuration, residual stress, etc. this method is widely used for toughness evaluation of thin films due to its simplicity [15,31,32]. The use of cube corner indenter helps in reducing the threshold load to induce cracking [33]. The following equation was used to determine the toughness [34]:
1=2 E P Kc ¼ d ; H c3=2
(1)
where Kc is the fracture toughness, d is a constant related to indenter geometry (0.032 for cube corner diamond indenter), E is the elastic modulus of the coating, H is the hardness of the coating, P is the applied indentation load and c is the radial crack length
240
Hardness (GPa)
25
Cr 230
20
Si3N4 220
H 15
Elastic Modulus (GPa)
250
30
E Cr
Si3N4
210
10 10
20
30
40
50
Cr (at.%) Fig. 4. Variations of hardness and elastic modulus of CreSi3N4 nanocomposite coatings with Cr content. Also shown are the hardness and elastic modulus values of Cr and Si3N4 coatings.
(measured from the center of contact to the end of the corner radial crack). CreSi3N4 nanocomposite coatings prepared on plasma nitrided M2 steel substrates could not be used for the nanoindentation tests as the coatings exhibited significant roughness and therefore toughness measurements were done for coatings prepared on Si substrates. Fig. 5 shows the three-dimensional AFM images of the radial cracks observed at the perimeter of the indentation for Si3N4 and CreSi3N4 nanocomposite coatings prepared at different Cr contents on Si substrates. The cracking threshold load was 250 mN for the coating prepared at 20 at.% Cr. CreSi3N4 nanocomposite coatings prepared at 39 and 48 at.% Cr exhibited a higher threshold load of 300 mN for crack initiation. Si3N4 coating on the other hand exhibited the lowest cracking threshold load of 100 mN. For the toughness calculations, the crack length was measured as the projected distance between the center of the imprint and the fracture apex. The cracks propagating from the three corners of the indentation for Si3N4 and CreSi3N4 nanocomposite coating prepared at 20 at.% Cr are clearly visible in Fig. 5(a) and (b), respectively. In the case of CreSi3N4 nanocomposite coatings prepared at 39 and 48 at.% Cr, the cracks were very narrow and pile up of the material as a result of indentation was observed (Fig. 5(c) and (d)). It has to be mentioned that the nanoindentation measurements at a load of 300 mN may be influenced by the substrate. CreSi3N4 nanocomposite coating prepared at 48 at.% Cr exhibited the highest toughness value of 2.0 MPam½ as determined from Eq. (1). CreSi3N4 nanocomposite coatings prepared at 20 and 39 at.% Cr exhibited toughness values of 1.7 and 1.9 MPam½, respectively. It has to be noted that the increase in toughness of CreSi3N4 nanocomposite coating with increase in Cr content from 20 to 48 at.% is at the expense of hardness which decreased from 24 to 18 GPa, respectively. The toughness value obtained for Si3N4 coating was 1.3 MPam½. The improved toughness of CreSi3N4 nanocomposite coatings compared to Si3N4 coating may be attributed to the incorporation of metallic Cr in Si3N4 matrix. Higher toughness of metal-ceramic nanocomposite systems compared to conventional binary and ternary PVD coatings has been reported in the literature [15,35e37]. Addition of metallic phase introduces ductility in
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Fig. 5. Three-dimensional AFM images of radial fracture at the perimeter of the indentation imprint obtained on (a) Si3N4 coating, and CreSi3N4 nanocomposite coating prepared at Cr contents of (b) 20 at.%, (c) 39 at.% and (d) 48 at.%. The AFM images were recorded at the respective cracking threshold loads for (a), (b), (c) and (d) which were 100, 250, 300 and 300 mN, respectively.
brittle ceramic films. The ductile phase can lead to toughening of the composite in mainly two ways: (1) relieving the stress field around the crack tip by ductile phase deformation or blunting of the crack tip at a ductile particle and (2) bridging of the crack by the ductile phase behind the advancing crack tip [6,38]. However, it may be observed that the contribution of ductile phase to toughness enhancement has been very small. Similar results have been reported in the literature [26,38]. The main reason for this has been attributed to lack of plastic deformation of the ductile phase [38]. Several mechanisms such as, pull-out of the ductile phase, crack propagation in the ceramic matrix, failure of matrix adjacent to the ductile phase-matrix interface before failure of ductile phase, etc., which limit plastic deformation of the ductile phase during crack propagation [38].
track beyond Lc2, which confirms the scratch response of the coating observed from the nanoscratch profiles [39]. For CreSi3N4 nanocomposite coatings prepared at 20 at.% Cr (Fig. 6(a) and (d)), Lc1 and Lc2 values were 123 and 187 mN, respectively. At 39 at.% Cr, CreSi3N4 nanocomposite coatings exhibited improved adhesion with Lc1 and Lc2 values of 150 and 196 mN, respectively as shown in Fig. 6(b) and (e). However, at a Cr content of 48 at.%, the adhesive strength of CreSi3N4 nanocomposite coatings reduced (Lc1 ¼ 102 mN, Lc2 ¼ 177 mN) and almost circular type of spallation was observed on either sides of the scratch track (Fig. 6(f)), giving rise to periodic fluctuations in the friction force curve as shown in Fig. 6(c).
3.5. Tribological properties 3.4. Adhesive strength Fig. 6 presents the nanoscratch profiles and optical images obtained from the nanoscratch measurements carried out to study the adhesion strength of CreSi3N4 coatings prepared at different Cr contents on plasma nitrided M2 steel substrates. The total scratch length was 1 mm. The nanoscratch profiles show the variations of applied force (Fz) and friction force (Fx) with sliding time. In Fig. 5, the load at which initial damage of the coating occurs is denoted as Lc1 and the load at which complete failure of the coating takes place is denoted as Lc2. The critical loads for failure of the coatings were obtained by analyzing the nanoscratch profiles (Fig. 6(a)e(c)) and optical images (Fig. 6(d)e(f)) of the scratch traces. From Fig. 6(a)e(c), it can be observed that at Lc1, there was a slight deviation in the friction force curve and beyond Lc2, large variations were observed in the profile indicating complete failure of the coatings. The corresponding optical images in Fig. 6(d)e(f) showed compressive wedge-type spallation on either side of the scratch
The evolution of friction coefficients of Si3N4 and CreSi3N4 nanocomposite coatings prepared at different Cr contents on plasma nitrided M2 steel substrates are shown in Fig. 7. CreSi3N4 nanocomposite coatings exhibited relatively lower friction coefficients than Si3N4 coating. CreSi3N4 nanocomposite coating prepared at 48 at.% Cr exhibited an almost stable friction coefficient of 0.30 after 14,400 cycles. All the other coatings exhibited an increasing friction coefficient curve with increase in the number of sliding cycles. CreSi3N4 nanocomposite coatings prepared at 20 and 39 at.% Cr exhibited average friction coefficients of 0.37 and 0.39, respectively. Si3N4 coating prepared under identical conditions showed an average friction coefficient of 0.40. The friction coefficient of uncoated plasma nitrided M2 steel substrates was approximately 0.60 (data not shown). The above results suggest that the tribological properties of plasma nitrided M2 steel substrates could be significantly enhanced by the deposition of CreSi3N4 nanocomposite coatings.
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Fig. 6. Nanoscratch profiles obtained from nanoscratch measurements for CreSi3N4 nanocomposite coatings prepared at Cr contents of (a) 20 at.% (b) 39 at.% and (c) 48 at.%. The corresponding optical images are shown in (d), (e) and (f).
Fig. 8 shows the optical image and FESEM micrographs of a typical wear track on CreSi3N4 nanocomposite coating (48 at.% Cr) after 14,400 sliding cycles. The optical image of the whole wear track (length ¼ 3 mm, width ¼ 210 mm) is presented in Fig. 8(a). The wear track was smooth with no accumulated wear debris along the edges of the wear track except at the front and rear ends (regions I and III) of the track. Fig. 8(b)e(d) represent the FESEM micrographs of regions I, II and III, respectively. The two ends of the wear track
Fig. 7. Friction coefficient data for Si3N4, and CreSi3N4 nanocomposite coatings prepared at different Cr contents on plasma nitrided M2 steel substrates.
(regions I and III) showed dark gray patches suggesting accumulation of wear debris/formation of new phases. The worn area at the center of the wear track (region II) did not show any crack formation or delamination. The higher magnification FESEM micrographs of regions I, II and III are shown in Fig. 8(e)e(g). These micrographs reveal the presence of several cracked areas in regions I and III of the wear track. The cracking of the coating may be attributed to tensile stresses which arise during sliding of the sample against the WC ball and oxidation of the coating [40]. EDS data recorded in the dark areas of regions I and III indicated considerable decrease in the nitrogen content and an increase in the oxygen content confirming the formation of oxide phases. From the FESEM studies of the wear track on CreSi3N4 nanocomposite coating, it can be inferred that there was no considerable material loss due to wear and the coatings exhibited only mild wear by oxidation. The enhanced wear resistance of CreSi3N4 nanocomposite coatings prepared on plasma nitrided M2 steel substrates may be attributed to their good adhesion and moderate hardness as discussed in the previous sections. To summarize, in the present study we have developed a coating-substrate system with improved mechanical and tribological properties by duplex treatment. As mentioned before, the toughness of the CreSi3N4 nanocomposite coating was increased at the expense of coating hardness. However, in order to meet the complex requirements of engineering applications, coatings with moderate hardness, high toughness and low friction are essential.
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Fig. 8. (a) Optical image of the whole wear track on CreSi3N4 nanocomposite coating (48 at.% Cr), (b)e(d) FESEM micrographs of different regions of the wear track on CreSi3N4 nanocomposite coating, (e)e(g) High magnification FESEM micrographs of different regions of the wear track on CreSi3N4 nanocomposite coating.
4. Conclusions
References
Low-friction and tough CreSi3N4 nanocomposite coatings were deposited by magnetron sputtering on plasma nitrided AISI M2 steel substrates. The XRD data of CreSi3N4 nanocomposite coatings suggested that the coatings were nearly amorphous with very small crystallites. CreSi3N4 nanocomposite coating prepared at 48 at.% Cr exhibited moderate hardness (18 GPa) and a high toughness value 2.0 MPam½ compared to Si3N4 coating. Nanoscratch tests demonstrated the improved load-bearing capacity of CreSi3N4 nanocomposite coatings prepared on plasma nitrided substrates with a maximum Lc1 value of 150 mN for coatings prepared at 39 at.% Cr. Wear tests carried out in ball-on-disc reciprocating configuration at a load of 2 N showed that CreSi3N4 nanocomposite coatings prepared at 48 at.% Cr on M2 steel substrates exhibited an average friction coefficient of 0.30. Analysis of the wear tracks on CreSi3N4 nanocomposite coatings by FESEM studies revealed that the coatings exhibited only mild wear due to formation of oxides.
[1] Pye D. Practical nitriding and ferritic nitrocarburizing. Ohio: ASM International; 2003. [2] Ochoa EA, Figueroa CA. Influence of the microstructure on steel hardening in pulsed plasma nitriding. J Vac Sci Technol A 2008;26:328e32. [3] Wang J, Xiong J, Peng Q, Fan H, Wang Y, Li G, et al. Effects of DC plasma nitriding parameters on microstructure and properties of 304L stainless steel. Mater Character 2009;60:197e203. [4] Zagonel LF, Figueroa CA, Droppa Jr R, Alvarez F. Influence of the process temperature on the steel microstructure and hardening in pulsed plasma nitriding. Surf Coat Technol 2006;201:452e7. [5] Zhang S, Wang HL, Ong SE, Sun D, Bui XL. Hard yet tough nanocomposite coatings e present status and future trends. Plasma Process Polym 2007;4: 220e8. [6] Wang H, Zhang S. Toughness and toughening of hard nanocomposite coatings. In: Zhang S, editor. CRC handbook of nanostructured thin films and coatings. Boca Raton: CRC Press; 2011. p. 99e145. [7] Bell T, Dong H, Sun Y. Realising the potential of duplex surface engineering. Tribol Int 1998;31:127e37. [8] Rie KT, Broszeit E. Plasma diffusion treatment and duplex treatment-recent development and new applications. Surf Coat Technol 1995;76e77:425e36. [9] Podgornik B, Hogmark S, Sandberg O, Leskovsek V. Wear resistance and antisticking properties of duplex treated forming tool steel. Wear 2003;254: 1113e21. [10] Braga D, Dias JP, Cavaleiro A. Duplex treatment: W-Ti-N sputtered coatings on pre-nitrided low and high alloy steels. Surf Coat Technol 2006;200:4861e9. [11] Kamminga JD, Hoy R, Janssen GCAM, Lugscheider E, Maes M. First results on duplex coatings without intermediate mechanical treatment. Surf Coat Technol 2003;174e175:671e6. [12] Batista JCA, Godoy C, Buono VTL, Matthews A. Characterization of duplex and non-duplex (Ti, Al)N and Cr-N PVD coatings. Mater Sci Eng A 2002;336: 39e51. [13] Barshilia HC, Deepthi B, Rajam KS. Transition metal nitride based nanolayered multilayer coatings and nanocomposite coatings as novel superhard materials. In: Zhang S, editor. CRC handbook of nanostructured thin films and coatings. Boca Raton: CRC Press; 2011. p. 427e80. [14] Voevodin AA, Zabinski JS. Supertough wear-resistant coatings with ‘chameleon’ surface adaptation. Thin Solid Films 2000;370:223e31. [15] Zhang S, Sun D, Fu Y, Du H. Toughening of hard nanostructural thin films: a critical review. Surf Coat Technol 2005;198:2e8.
Acknowledgments The authors thank the Director, NAL for giving permission to publish these results. We thank Mr. Siju and Mr. Praveen Kumar for performing various measurements. Mr. V. Prakash is thanked for sample preparation and experimental work. Dr. Satyam Suwas from Indian Institute of Science, Bangalore is thanked for XRD measurements. This work was supported by the Council of Scientific and Industrial Research, New Delhi, India (Project Nos. FAC-00-01-11, SIP-SED-03 and NWP00-51-03).
H.C. Barshilia et al. / Vacuum 86 (2012) 1118e1125 [16] Dub S, Pauleau Y, Thiery F. Mechanical properties of nanostructured copperhydrogenated amorphous carbon composite films studied by nanoindentation. Surf Coat Technol 2004;180e181:551e5. [17] Pauleau Y, Thiery F. Design and characterization of nanostructured metal/ carbon composite films. Surf Coat Technol 2004;180e181:313e22. [18] Guruvenket S, Li D, Klemberg-Sapieha JE, Martinu L, Szpunar J. Mechanical and tribological properties of duplex treated TiN, nc-TiN/a-SiNx and nc-TiCN/ a-SiCN coatings deposited on 410 low alloy stainless steel. Surf Coat Technol 2009;203:2905e11. [19] Rahman M, Haider J, Dowling DP, Duggan P, Hashmi MSJ. Investigation of mechanical properties of TiN þ MoSx coating on plasma-nitrided substrate. Surf Coat Technol 2005;200:1451e7. [20] Barshilia HC, Rajam KS. Characterization of Cu/Ni multilayer coatings by nanoindentation and atomic force microscopy. Surf Coat Technol 2002;155: 195e202. [21] Nayal G, Lewis DB, Lembke M, Munz WD, Cockrem JE. Influence of sample geometry on the effect of pulse plasma nitriding of M2 steel. Surf Coat Technol 1999;111:148e57. [22] Akbari A, Mohammadzadeh R, Templier C, Riviere JP. Effect of the initial microstructure on the plasma nitriding behavior of AISI M2 high speed steel. Surf Coat Technol 2010;204:4114e20. [23] da Silva Rocha A, Strohaecker T, Tomala V, Hirsch T. Microstructure and residual stresses of a plasma nitrided M2 tool steel. Surf Coat Technol 1999; 115:24e31. [24] Serna MM, Rossi JL. MC complex carbide in AISI M2 high-speed steel. Mater Lett 2009;63:691e3. [25] Voort GFV. Tool steels. In: Metals handbook, vol. 9. Ohio: American Society for Metals; 1987. p. 256e72. [26] Voevodin AA, Hu JJ, Jones JG, Fitz TA, Zabinski JS. Growth and structural characterization of yttria-stabilized zirconia-gold nanocomposite films with improved toughness. Thin Solid Films 2001;401:187e95. [27] Soldan J, Musil J. Structure and mechanical properties of DC magnetron sputtered TiC/Cu films. Vacuum 2006;81:531e8.
1125
[28] Elangovan T, Kuppusami P, Thirumurugesan R, Sudha C, Mohandas E. Mangalaraj. A study on the influence of copper content in CrN/Cu nanocomposite thin films prepared by pulsed DC magnetron sputtering. J Nanoscience Nanotechnol 2009;9:5436e40. [29] Lu W, Komvopoulos K. Nanomechanical and nanotribological properties of carbon, chromium, and titanium carbide ultrathin films. J Tribol 2001;123: 717e24. [30] Vila M, Caceres D, Prieto C. Mechanical properties of sputtered silicon nitride thin films. J Appl Phys 2003;94:7868e73. [31] Kodali P, Walter KC, Nastasi M. Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings. Tribol Int 1997;8: 591e8. [32] Jedrzejowski P, Klemberg-Sapieha JE, Martinu L. Quaternary hard nanocomposite TiCxNy/SiCN coatings prepared by plasma enhanced chemical vapor deposition. Thin Solid Films 2004;466:189e96. [33] Fischer-Cripps AC. Nanoindentation. 2nd ed. New York: Springer-Verlag; 2004. [34] Lawn BR, Evans AG, Marshall DB. Elastic/plastic indentation damage in ceramics: the median/radial crack system. J Am Ceram Soc 1980;63: 574e81. [35] Neralla S, Kumar D, Yarmolenko S, Sankar J. Mechanical properties of nanocomposite metal-ceramic thin films. Compos Part B 2004;35:157e62. [36] Zhang S, Bui XL, Fu Y. Magnetron-sputtered nc-TiC/a-C(Al) tough nanocomposite coatings. Thin Solid Films 2004;467:261e6. [37] Tuan WH, Brook RJ. The toughening of alumina with nickel inclusions. J Eur Ceram Soc 1990;6:31e7. [38] Sun X, Yeomans JA. Ductile phase toughened brittle materials. J Mater Sci Technol 1996;12:124e34. [39] Bull SJ. Failure modes in scratch adhesion testing. Surf Coat Technol 1991;50: 25e32. [40] Zlatanovic M, Kakas D, Mazibrada L, Kunosic A, Munz WD. Influence of plasma nitriding on wear performance of TiN coating. Surf Coat Technol 1994;64: 173e81.