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Machinability and scratch wear resistance of carbon-coated WC inserts
Materials Science and Engineering B 193 (2015) 146–152
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Machinability and scratch wear resistance of carbon-coated WC inserts B. Pazhanivel ∗ , T. Prem Kumar, G. Sozhan Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 4 July 2014 Received in revised form 28 November 2014 Accepted 1 December 2014 Available online 11 December 2014 Keywords: Cutting inserts Surface modification Machinability Chemical vapor deposition Nanocarbons
a b s t r a c t In this work, cemented tungsten carbide (WC) inserts were coated with nanocarbons/carbides by chemical vapor deposition (CVD) and their machinability and scratch wear resistance were investigated. The hardness and surface conditions of the WC substrate were studied before and after coating. The CVDgenerated nanocarbons on the insert surfaces were examined by SEM, FE-SEM and TEM. The electron microscopic images revealed that the carbons generated were multi-walled carbon nanotubes (MWCNTs) or carbides depending on the experimental conditions. In both the cases, the cutting edges of the inserts had dense deposits. Scratch wear test with the coated inserts showed that the co-efficient of friction was 0.1 as against 0.2 for the uncoated inserts under a ramp load of 1–13 N. The machinability characteristics of commercially available TiCN-coated inserts and the carbon-coated WC inserts were compared by using a CNC machine and a Rapid I vision inspection system. It was found that the carbidecoated inserts exhibited machinability with better surface finish comparable to that of the TiCN-coated inserts while the MWCNT-coated inserts showed inferior adhesion properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Machining tools such as cutting tool inserts, end mills and drills come with hard surface coatings for enhanced tool life, machining speed, cutting performance and surface finish. Surface coating also ensures that the cutting tool withstands high working temperatures and high hardness of the work piece [1]. Cemented carbide (WC) tools enriched with cobalt as a binder are among the most commonly used tool materials for machining applications. Cobalt in the insert provides high fracture strength [2]. Among the most popular hard coatings are those of aluminum oxide (Al2 O3 ) [3], titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN) and chromium nitride (CrN2 ) [4,5]. Even nanostructured multilayer coatings have been studied. For example, Moreno et al. [6] coated WC inserts 300 layers of TiN/TiAlN and demonstrated high tool life. Carbon-based coatings have also been employed, which have been demonstrated to withstand oxidation temperature of 500–700 ◦ C and working temperatures of 1050 ◦ C and more [7]. Moreover, a deposit of elemental carbon, due to its lubricity, could help improve surface finish during machining. An additional advantage with carbon is that as
a coating material it is rather inexpensive. Diamond coatings are especially attractive for cutting tool application due to their high hardness and wear resistance, low frication, good thermal conductivity and low coefficient of thermal expansion [8]. In terms of strength, CNTs, graphene, polycrystalline diamond and monocrystalline diamond compare well with naturally occurring diamonds. Carbon nanostructures have particularly attractive mechanical and thermal properties that can be made use of for such coatings. For example, carbon nanotubes have good chemical stability, and possess high tensile strength (∼50 GPa) and shear strength (∼500 MPa). They also have high thermal conductivity (2000–4000 W/m/K), which should ensure fast heat dissipation during machining. Moreover, tungsten carbide has the ability to support the growth of carbon nanostructures under conditions of chemical vapor deposition (CVD) [9]. In this study, WC inserts coated with carbon by a CVD method were examined for their machinability. It must be noted that WC inserts have hardness values close to those of naturally occurring diamonds.
2. Experimental 2.1. WC inserts
∗ Corresponding author. Tel.: +91 4565 241473; fax: +91 4565 227999. E-mail address: [email protected] (B. Pazhanivel). http://dx.doi.org/10.1016/j.mseb.2014.12.006 0921-5107/© 2014 Elsevier B.V. All rights reserved.
The micro-hardness of the WC inserts as tested with Ever One SVD M3 hardness tester was 2400 kg/mm2 . The chemical
B. Pazhanivel et al. / Materials Science and Engineering B 193 (2015) 146–152
147
Fig. 1. EDAX patterns showing the elemental composition (except carbon) of WC insert.
composition of the insert (presented in Fig. 1 and Table 1) is as follows: W (82.84%); Co (12.72%); and O (4.44%). This is the composition of the insert as determined by EDAX, which cannot detect carbon. Addition of cobalt as a binder gives the insert a compressive strength of 7.5 kN/mm2 . With a grain size of less than 2.5 , it has a fracture strength of 11–13 MPa [10].
2.2. Deposition of carbon nanostructures by CVD method Deposition of carbon nanostructures was carried out in a low-pressure chemical vapor deposition equipment. A schematic diagram of the CVD unit is shown in Fig. 2. The inserts were thoroughly cleaned with acetone to remove soil that might have been introduced by handling. They were then introduced into an 80-cm long, 4-cm diameter quartz tube fixed-bed flow reactor. After a 5min flushing of the reaction chamber with 99.997% argon, a stream of hydrogen (99.999%) was passed through the tube at 400 ◦ C at a flow rate of 200 sccm at a tube pressure of 100 Torr for 15 min. Any oxide impurity on the insert sample was removed by a 15min 900 ◦ C heat treatment in a reducing atmosphere (hydrogen: 400 sccm; argon: 100 sccm; under 600 Torr tube pressure). Such a surface pre-treatment is an important step considering that it improves adhesion properties [11]. MWCNTs were deposited on the reduced, oxide-free sample as follows. A mixture of methane (99.99%, 50 sccm), hydrogen (100 sccm) and argon (100 sccm) was passed over the inserts at 900 ◦ C for 60 min at a tube pressure of 100 Torr. Subsequently, the sample was cooled under flowing argon. The deposition of the carbide was carried out under the following conditions: methane (40 sccm), hydrogen (100 sccm) and
Table 1 Weight and atom percentages of elements constituting WC inserts as determined by EDAX. Element O Co W Total
Net counts 1041 10518 24,249
Weight %
Atom %
4.44 12.72 82.84
29.41 22.87 47.72
100.00
100.00
argon (100 sccm) under a tube pressure of 100 Torr by a120-min deposition at 900 ◦ C. 2.3. Characterization of the deposits Microscopic analyses were performed by using a Hitachi S3000H scanning electron microscope, a Carl Zeiss field-emission scanning electron microscope (Supra 55 VP) and an FEI transmission electron microscope (Tecnai 20 G2 ). X-ray diffraction patterns were recorded on a Bruker Advance 8 X-ray diffractometer fitted with a Cu-K␣ radiation source. AFM images were recorded on an Agilent Technologies 5500 model atomic force microscope under contact mode in open air. 2.4. Hardness and adhesion strength Scratch tests were conducted with a 0.2 mm diamond stylus point on Ducom scratch wear testing machine. 6-mm stroke lengths were made on the inserts with the indenter such that the traction force was 0.2 mm/s and the ramp load started at 1 N and ended at 13 N. The adhesion formula of dynamically loaded scratch tester for thin-film adhesion measurements [12] was used to calculate the adhesion of the film with the substrate. A =
1 1/2 R
(Wc H)1/2
or A =
2Wc Rb
A – adhesion strength (N/mm2 ); R – radius of stylus point (mm): 0.2 mm; Wc – detected critical load (N): 3 N; b – width of the track caused by indenter (mm): 0.05 mm; H – hardness of the substrate (N/mm2 ): 2300 HV kg/mm2 . A = 234.325 N/mm2 An adhesion value of 234.33 N/mm2 was obtained, which is 3/4th of the value of the shear strength of SS316 (320 N/mm2 ).
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Fig. 2. A schematic of the Atomate CVD unit.
Table 2 Machining parameters. S. no.
Feed (mm/rev)
Speed (m/s)
Depth of cut (mm)
Ra (m)
1 2 3
0.1 0.2 0.3
90 100 110
0.2 0.3 0.5
0.76 1.86 3.26
image of the scratch mark (Fig. 6). The figure shows that the diamond stylus with a ramp load of 1–13 N scratches out the film layer adjacent to the stylus. However, a closer examination reveals that the stylus could not penetrate the carbide layer even under a load
2.5. Machining studies The machining parameters employed in the measurements are presented in Table 2. The machining studies were performed in order to determine a finish cutting parameter using commercially available TiCN coated inserts. The cutting parameters were set on a computer numerical controlled machine (CNC Lathe TNS25 600ABC, PMT Machines). The machining material was SS 316 austenitic steel, and the machining parameters were: spindle speed: 90, 100 and 110 m/min; depth of cut: 0.2, 0.3 and 0.5 mm; and feed rate: 0.1, 0.2 and 0.3 mm. The best surface finish obtained at a spindle speed 90, depth of cut of 0.2 mm and a feed rate of 0.1 mm/s was had an Ra value of 0.76 m. 3. Results and discussion Fig. 3 compares the surface morphologies of the pristine WC insert and the MWCNT-deposited insert. Although the pristine sample shows no definite morphology, the MWCNT-deposited sample shows an abundance of MWCNTs. The CNTs have an average diameter of 40–50 nm and an average length of 6 m. The aspect ratio (length/diameter) is a critical parameter that can determine the adherence of the coating for mechanically demanding applications. With the ends of the CNTs pulling away from the substrate during machining, it is only to be assumed that the larger the aspect ratio the poorer will be the bonding. Fig. 4 is a TEM image of the carbon deposit showing predominantly of MWCNTs with wall thickness of about 6 nm. The poor adhesion of CNT deposits has already been reported [13]. Therefore, further tests were conducted with the inserts with carbide deposit. The scratch results obtained with the carbide-coated inserts are presented in Fig. 5. The red line is the normal load plotted at each point whereas the black line indicates the co-efficient of friction. The blue and green lines represent traction force and acoustic emission, respectively. The average co-efficient of friction of the coated surface is 0.1 m. The friction co-efficient of WC reported in the literature is 0.1 m [14]. Deviations from the average noted in the curve for the co-efficient of friction suggest that the indent needle tended to penetrate into the substrate material through the coating. The adhesion of the carbide under different load conditions with brittle fracture due to traction force can be seen from the SEM
Fig. 3. SEM images of pristine WC inset (top) and MWCNT-coated WC inset (middle and bottom).
B. Pazhanivel et al. / Materials Science and Engineering B 193 (2015) 146–152
Fig. 4. A typical TEM image of the carbon deposit on WC inset.
of 13 N. An FE-SEM image of the carbide-deposited insert is given in Fig. 7a. Fig. 7b is a typical AFM image of the sample. Fig. 8 compares the XRD patterns of the bare WC insert and the carbide-coated WC insert. The different reflections obtained with the WC insert match with those for tungsten carbide (JCPDS file
149
No. 0050728). The bare insert also shows diffraction peaks due to cobalt (2 = 44.37◦ and 75.37◦ , JCPDS file No. 0011259). While the peaks due to cobalt disappear upon carbon coating, new peaks corresponding to Co3 C and Co2 W4 C appear. Thus, during the coating process, the surface also gets covered with a layer of the carbides (10 m, as measured by DeFelsco Positractor 6000 film thickness measurement instrument). The coated sample also gave diffraction peaks corresponding to carbon (JCPDS file No. 0010640), confirming the presence of a carbon deposit. The machining study under the optimized finish parameters (spindle speed: 90 m/min; depth of cutting: 0.2 mm; feed rate: 0.1 mm/s) was tested on a stainless steel SS316 rod (diameter: 20 mm; length: 40 mm) and with the carbide-coated insert as well as with a commercially available insert (TiCN-coated). A better surface finish (the arithmetic mean roughness, Ra = 0.64–0.84 m) was achieved with the carbide-coated insert. The Ra value obtained with the commercial sample was only 1.04–1.42 m. This marks a significant difference of nearly 0.8 m in the value of Ra from that of surface machined by the commercially available insert. It may be noted that the value of Ra obtained with the carbidecoated insert is much lower than commonly reported values. For example, Shunmugesh et al. [15] reported Ra values of 1.8–3.2 m with Stallion 200 CNC machine for mild steel turning. The surface finish and roughness parameters of the machined job promise a potential application for the carbide-coated material, especially for
Fig. 5. (a) Scratch test, (film) = 0.1, and (substrate) = 0.2. Start load: 1 N; finish load: 13 N, Critical load: 3 N. Stroke length: 6 mm. Scratch velocity: 0.1 mm/s. (b) An enlarged image showing details of scratch test results. (For interpretation of the references to color in text near the reference citation, the reader is referred to the web version of this article.)
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Fig. 7. FE-SEM (a) and AFM (b) images of carbide-coated inserts.
must be noted that the wear behavior of coatings is due to built-up edges at the rake surfaces of tools (Fig. 10(c)). The crater wear, flank wear and chipping of edge are due to direct frictional contact with the metal surface. The minimum lifetime of the cutting edge was 20 min of machining. This can be achieved by hard surface coatings that prevent breakage of adjacent layers at the cutting edge and renders them intact all through the machining operation.
Fig. 6. SEM images of scratches made with 0.05 mm diamond stylus. MWCNTcoated (left) and flaky carbon-coated (right) inserts.
low-carbon (0.02–0.05%) austenitic steels. The improved performance of the carbide-coated insert may be attributed to the better lubricity and nanoelasticity of the carbide deposit. Thus such coatings help realize good surface finish, free from nano-chatter and vibrations. The coatings facilitate improved surface finish at a lower machining cutting parameter. The surface finish is also reflected in the maximum peak value Ry of 3.18 m. The plots for the surface roughness tests are presented in Fig. 9. 3.1. TiN-coated and carbide-coated WC inserts Fig. 10(a) shows images of the multi-layer-coated (Al2 O3 –TiCN–CrN–TiN) and carbide-coated WC inserts obtained with Image 67X. After machining, both the inserts show visible geometrical wear at the edges, called crater wear [16]. The carbidecoated inserts performed machining similar to commercial inserts that have different types of surface coatings. We observe that the delamination at the edges and its immediate neighborhood surface is due to insufficient adhesion of the coating (Fig. 10 (b)). It
Fig. 8. XRD patterns of the WC insert before and after flaky carbon coating.
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Fig. 9. Surface roughness of workpiece machined using (a) carbide-coated WC inserts, Ra = 0.64 m Ry = 3.18 m for 4 mm length; and (b) TiCN-coated WC inserts, Ra = 1.42 m Ry = 7.07 m for 4 mm length.
Fig. 10. Vision inspections (Image 67X) of (a) TiN-coated inserts before machining; (b) carbide-coated inserts before machining; (c) TiN-coated after machining; (d) carbidecoated inserts after machining; and (e) carbide-coated WC inserts after machining, showing damages to the built-up edges.
4. Conclusions In this study, we investigated the machining ability and wear resistance of WC inserts coated with carbon obtained by chemical
vapor deposition under two sets of experimental conditions. The surface morphologies of the coated inserts were examined by SEM, TEM and FE-SEM. Scratch wear tests were conducted with a diamond stylus to evaluate the adhesion strengths of the coatings to
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the inserts. Machining studies with the inserts were conducted on a CNC machine and the results were compared with those of a commercially available TiCN-coated insert. The coatings were distinct: one, of multi-walled carbon nanotubes and the other, of carbides. The surface coating helps in increasing tool life and surface finish while its adherence protects the insert from crater wear and flank wear and break away of edge. The surface finish at a spindle speed of 90 m/min, a depth of cut of 0.2 mm and a feed rate of 0.1 mm was found to be excellent, with an arithmetic mean roughness of Ra = 0.64 m and a maximum peak Ry of 3.18 m. The Ra value is lower by 0.8 m as compared with the commercial insert. Acknowledgments BP acknowledges with gratitude the help rendered by Messrs A. Rathish Kumar, R. Ravishankar and J. Kennedy in recording electron microscopic images. References [1] M. Yasuoka, P. Wang, R. Murakami, Surf. Coat. Technol. 206 (2012) 2168. [2] G. Vega, H. Arturo, Graduate Thesis. http://scholarcommons.usf.edu/etd/3121
[3] K. Jiang, K. Sarakinos, S. Konstantinidis, J.M. Schneider, J. Phys. D 43 (2010) 325202. [4] G. Strnad, J. Buhagiar, Sci. Bull. Univ. Targu Mures 7 (2010) 1841. [5] L.N.L. de Lacalle, A. Lamikiz, J.F. de Larrinoa, I. Azkona, in: J.P. Davim (Ed.), Machining of Hard Materials, Springer-Verlag, London, 2011, p. 33. [6] L.H. Moreno, J.C. Ciacedo, F. Martinez, G. Bejarano, T.S. Battaille, P. Prieto, J. Braz. Soc. Mech. Sci. Eng. 32 (2010) 114. [7] V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis, C. Galiotis, Carbon 46 (2008) 833. [8] Inspektor, Int. J. Refract. Metal. Hard Mater. 15 (1997) 49. [9] A. Hirata, N. Yoshioka, Tribol. Int. 37 (2004) 893. [10] A. Jawaid, S. Koksal, S. Sharif, Tribol. Trans. 43 (2000) 325. [11] F.H. Sun, Z.M. Zhang, M. Chen, H.S. Shen, Diam. Relat. Mater. 12 (2003) 711. [12] H. Kim, LLE Rev. 22 (1985) 85. [13] A. Mangione, G. Lanzara, L. Torrisid, F. Caridi, Radiat. Effects Defects Solids 165 (2010) 746. [14] J. Umeda, T. Mimota, K. Kondoh, B. Fugetsu, Trans. Join. Weld. Res. Inst. 41 (2012) 49. [15] K. Shunmugesh, K. Panneerselvam, M. Pramod, A. George, Res. J. Eng. Sci. 3 (2014) 1. [16] C.Y.M. Lim, S.C. Lim, K.S. Lee, 13th Int. Conf. Surface Modification Technologies, Singapore, 1999.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Machinability and scratch wear resistance of carbon-coated WC inserts B. Pazhanivel ∗ , T. Prem Kumar, G. Sozhan Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 4 July 2014 Received in revised form 28 November 2014 Accepted 1 December 2014 Available online 11 December 2014 Keywords: Cutting inserts Surface modification Machinability Chemical vapor deposition Nanocarbons
a b s t r a c t In this work, cemented tungsten carbide (WC) inserts were coated with nanocarbons/carbides by chemical vapor deposition (CVD) and their machinability and scratch wear resistance were investigated. The hardness and surface conditions of the WC substrate were studied before and after coating. The CVDgenerated nanocarbons on the insert surfaces were examined by SEM, FE-SEM and TEM. The electron microscopic images revealed that the carbons generated were multi-walled carbon nanotubes (MWCNTs) or carbides depending on the experimental conditions. In both the cases, the cutting edges of the inserts had dense deposits. Scratch wear test with the coated inserts showed that the co-efficient of friction was 0.1 as against 0.2 for the uncoated inserts under a ramp load of 1–13 N. The machinability characteristics of commercially available TiCN-coated inserts and the carbon-coated WC inserts were compared by using a CNC machine and a Rapid I vision inspection system. It was found that the carbidecoated inserts exhibited machinability with better surface finish comparable to that of the TiCN-coated inserts while the MWCNT-coated inserts showed inferior adhesion properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Machining tools such as cutting tool inserts, end mills and drills come with hard surface coatings for enhanced tool life, machining speed, cutting performance and surface finish. Surface coating also ensures that the cutting tool withstands high working temperatures and high hardness of the work piece [1]. Cemented carbide (WC) tools enriched with cobalt as a binder are among the most commonly used tool materials for machining applications. Cobalt in the insert provides high fracture strength [2]. Among the most popular hard coatings are those of aluminum oxide (Al2 O3 ) [3], titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN) and chromium nitride (CrN2 ) [4,5]. Even nanostructured multilayer coatings have been studied. For example, Moreno et al. [6] coated WC inserts 300 layers of TiN/TiAlN and demonstrated high tool life. Carbon-based coatings have also been employed, which have been demonstrated to withstand oxidation temperature of 500–700 ◦ C and working temperatures of 1050 ◦ C and more [7]. Moreover, a deposit of elemental carbon, due to its lubricity, could help improve surface finish during machining. An additional advantage with carbon is that as
a coating material it is rather inexpensive. Diamond coatings are especially attractive for cutting tool application due to their high hardness and wear resistance, low frication, good thermal conductivity and low coefficient of thermal expansion [8]. In terms of strength, CNTs, graphene, polycrystalline diamond and monocrystalline diamond compare well with naturally occurring diamonds. Carbon nanostructures have particularly attractive mechanical and thermal properties that can be made use of for such coatings. For example, carbon nanotubes have good chemical stability, and possess high tensile strength (∼50 GPa) and shear strength (∼500 MPa). They also have high thermal conductivity (2000–4000 W/m/K), which should ensure fast heat dissipation during machining. Moreover, tungsten carbide has the ability to support the growth of carbon nanostructures under conditions of chemical vapor deposition (CVD) [9]. In this study, WC inserts coated with carbon by a CVD method were examined for their machinability. It must be noted that WC inserts have hardness values close to those of naturally occurring diamonds.
2. Experimental 2.1. WC inserts
∗ Corresponding author. Tel.: +91 4565 241473; fax: +91 4565 227999. E-mail address: [email protected] (B. Pazhanivel). http://dx.doi.org/10.1016/j.mseb.2014.12.006 0921-5107/© 2014 Elsevier B.V. All rights reserved.
The micro-hardness of the WC inserts as tested with Ever One SVD M3 hardness tester was 2400 kg/mm2 . The chemical
B. Pazhanivel et al. / Materials Science and Engineering B 193 (2015) 146–152
147
Fig. 1. EDAX patterns showing the elemental composition (except carbon) of WC insert.
composition of the insert (presented in Fig. 1 and Table 1) is as follows: W (82.84%); Co (12.72%); and O (4.44%). This is the composition of the insert as determined by EDAX, which cannot detect carbon. Addition of cobalt as a binder gives the insert a compressive strength of 7.5 kN/mm2 . With a grain size of less than 2.5 , it has a fracture strength of 11–13 MPa [10].
2.2. Deposition of carbon nanostructures by CVD method Deposition of carbon nanostructures was carried out in a low-pressure chemical vapor deposition equipment. A schematic diagram of the CVD unit is shown in Fig. 2. The inserts were thoroughly cleaned with acetone to remove soil that might have been introduced by handling. They were then introduced into an 80-cm long, 4-cm diameter quartz tube fixed-bed flow reactor. After a 5min flushing of the reaction chamber with 99.997% argon, a stream of hydrogen (99.999%) was passed through the tube at 400 ◦ C at a flow rate of 200 sccm at a tube pressure of 100 Torr for 15 min. Any oxide impurity on the insert sample was removed by a 15min 900 ◦ C heat treatment in a reducing atmosphere (hydrogen: 400 sccm; argon: 100 sccm; under 600 Torr tube pressure). Such a surface pre-treatment is an important step considering that it improves adhesion properties [11]. MWCNTs were deposited on the reduced, oxide-free sample as follows. A mixture of methane (99.99%, 50 sccm), hydrogen (100 sccm) and argon (100 sccm) was passed over the inserts at 900 ◦ C for 60 min at a tube pressure of 100 Torr. Subsequently, the sample was cooled under flowing argon. The deposition of the carbide was carried out under the following conditions: methane (40 sccm), hydrogen (100 sccm) and
Table 1 Weight and atom percentages of elements constituting WC inserts as determined by EDAX. Element O Co W Total
Net counts 1041 10518 24,249
Weight %
Atom %
4.44 12.72 82.84
29.41 22.87 47.72
100.00
100.00
argon (100 sccm) under a tube pressure of 100 Torr by a120-min deposition at 900 ◦ C. 2.3. Characterization of the deposits Microscopic analyses were performed by using a Hitachi S3000H scanning electron microscope, a Carl Zeiss field-emission scanning electron microscope (Supra 55 VP) and an FEI transmission electron microscope (Tecnai 20 G2 ). X-ray diffraction patterns were recorded on a Bruker Advance 8 X-ray diffractometer fitted with a Cu-K␣ radiation source. AFM images were recorded on an Agilent Technologies 5500 model atomic force microscope under contact mode in open air. 2.4. Hardness and adhesion strength Scratch tests were conducted with a 0.2 mm diamond stylus point on Ducom scratch wear testing machine. 6-mm stroke lengths were made on the inserts with the indenter such that the traction force was 0.2 mm/s and the ramp load started at 1 N and ended at 13 N. The adhesion formula of dynamically loaded scratch tester for thin-film adhesion measurements [12] was used to calculate the adhesion of the film with the substrate. A =
1 1/2 R
(Wc H)1/2
or A =
2Wc Rb
A – adhesion strength (N/mm2 ); R – radius of stylus point (mm): 0.2 mm; Wc – detected critical load (N): 3 N; b – width of the track caused by indenter (mm): 0.05 mm; H – hardness of the substrate (N/mm2 ): 2300 HV kg/mm2 . A = 234.325 N/mm2 An adhesion value of 234.33 N/mm2 was obtained, which is 3/4th of the value of the shear strength of SS316 (320 N/mm2 ).
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Fig. 2. A schematic of the Atomate CVD unit.
Table 2 Machining parameters. S. no.
Feed (mm/rev)
Speed (m/s)
Depth of cut (mm)
Ra (m)
1 2 3
0.1 0.2 0.3
90 100 110
0.2 0.3 0.5
0.76 1.86 3.26
image of the scratch mark (Fig. 6). The figure shows that the diamond stylus with a ramp load of 1–13 N scratches out the film layer adjacent to the stylus. However, a closer examination reveals that the stylus could not penetrate the carbide layer even under a load
2.5. Machining studies The machining parameters employed in the measurements are presented in Table 2. The machining studies were performed in order to determine a finish cutting parameter using commercially available TiCN coated inserts. The cutting parameters were set on a computer numerical controlled machine (CNC Lathe TNS25 600ABC, PMT Machines). The machining material was SS 316 austenitic steel, and the machining parameters were: spindle speed: 90, 100 and 110 m/min; depth of cut: 0.2, 0.3 and 0.5 mm; and feed rate: 0.1, 0.2 and 0.3 mm. The best surface finish obtained at a spindle speed 90, depth of cut of 0.2 mm and a feed rate of 0.1 mm/s was had an Ra value of 0.76 m. 3. Results and discussion Fig. 3 compares the surface morphologies of the pristine WC insert and the MWCNT-deposited insert. Although the pristine sample shows no definite morphology, the MWCNT-deposited sample shows an abundance of MWCNTs. The CNTs have an average diameter of 40–50 nm and an average length of 6 m. The aspect ratio (length/diameter) is a critical parameter that can determine the adherence of the coating for mechanically demanding applications. With the ends of the CNTs pulling away from the substrate during machining, it is only to be assumed that the larger the aspect ratio the poorer will be the bonding. Fig. 4 is a TEM image of the carbon deposit showing predominantly of MWCNTs with wall thickness of about 6 nm. The poor adhesion of CNT deposits has already been reported [13]. Therefore, further tests were conducted with the inserts with carbide deposit. The scratch results obtained with the carbide-coated inserts are presented in Fig. 5. The red line is the normal load plotted at each point whereas the black line indicates the co-efficient of friction. The blue and green lines represent traction force and acoustic emission, respectively. The average co-efficient of friction of the coated surface is 0.1 m. The friction co-efficient of WC reported in the literature is 0.1 m [14]. Deviations from the average noted in the curve for the co-efficient of friction suggest that the indent needle tended to penetrate into the substrate material through the coating. The adhesion of the carbide under different load conditions with brittle fracture due to traction force can be seen from the SEM
Fig. 3. SEM images of pristine WC inset (top) and MWCNT-coated WC inset (middle and bottom).
B. Pazhanivel et al. / Materials Science and Engineering B 193 (2015) 146–152
Fig. 4. A typical TEM image of the carbon deposit on WC inset.
of 13 N. An FE-SEM image of the carbide-deposited insert is given in Fig. 7a. Fig. 7b is a typical AFM image of the sample. Fig. 8 compares the XRD patterns of the bare WC insert and the carbide-coated WC insert. The different reflections obtained with the WC insert match with those for tungsten carbide (JCPDS file
149
No. 0050728). The bare insert also shows diffraction peaks due to cobalt (2 = 44.37◦ and 75.37◦ , JCPDS file No. 0011259). While the peaks due to cobalt disappear upon carbon coating, new peaks corresponding to Co3 C and Co2 W4 C appear. Thus, during the coating process, the surface also gets covered with a layer of the carbides (10 m, as measured by DeFelsco Positractor 6000 film thickness measurement instrument). The coated sample also gave diffraction peaks corresponding to carbon (JCPDS file No. 0010640), confirming the presence of a carbon deposit. The machining study under the optimized finish parameters (spindle speed: 90 m/min; depth of cutting: 0.2 mm; feed rate: 0.1 mm/s) was tested on a stainless steel SS316 rod (diameter: 20 mm; length: 40 mm) and with the carbide-coated insert as well as with a commercially available insert (TiCN-coated). A better surface finish (the arithmetic mean roughness, Ra = 0.64–0.84 m) was achieved with the carbide-coated insert. The Ra value obtained with the commercial sample was only 1.04–1.42 m. This marks a significant difference of nearly 0.8 m in the value of Ra from that of surface machined by the commercially available insert. It may be noted that the value of Ra obtained with the carbidecoated insert is much lower than commonly reported values. For example, Shunmugesh et al. [15] reported Ra values of 1.8–3.2 m with Stallion 200 CNC machine for mild steel turning. The surface finish and roughness parameters of the machined job promise a potential application for the carbide-coated material, especially for
Fig. 5. (a) Scratch test, (film) = 0.1, and (substrate) = 0.2. Start load: 1 N; finish load: 13 N, Critical load: 3 N. Stroke length: 6 mm. Scratch velocity: 0.1 mm/s. (b) An enlarged image showing details of scratch test results. (For interpretation of the references to color in text near the reference citation, the reader is referred to the web version of this article.)
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Fig. 7. FE-SEM (a) and AFM (b) images of carbide-coated inserts.
must be noted that the wear behavior of coatings is due to built-up edges at the rake surfaces of tools (Fig. 10(c)). The crater wear, flank wear and chipping of edge are due to direct frictional contact with the metal surface. The minimum lifetime of the cutting edge was 20 min of machining. This can be achieved by hard surface coatings that prevent breakage of adjacent layers at the cutting edge and renders them intact all through the machining operation.
Fig. 6. SEM images of scratches made with 0.05 mm diamond stylus. MWCNTcoated (left) and flaky carbon-coated (right) inserts.
low-carbon (0.02–0.05%) austenitic steels. The improved performance of the carbide-coated insert may be attributed to the better lubricity and nanoelasticity of the carbide deposit. Thus such coatings help realize good surface finish, free from nano-chatter and vibrations. The coatings facilitate improved surface finish at a lower machining cutting parameter. The surface finish is also reflected in the maximum peak value Ry of 3.18 m. The plots for the surface roughness tests are presented in Fig. 9. 3.1. TiN-coated and carbide-coated WC inserts Fig. 10(a) shows images of the multi-layer-coated (Al2 O3 –TiCN–CrN–TiN) and carbide-coated WC inserts obtained with Image 67X. After machining, both the inserts show visible geometrical wear at the edges, called crater wear [16]. The carbidecoated inserts performed machining similar to commercial inserts that have different types of surface coatings. We observe that the delamination at the edges and its immediate neighborhood surface is due to insufficient adhesion of the coating (Fig. 10 (b)). It
Fig. 8. XRD patterns of the WC insert before and after flaky carbon coating.
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Fig. 9. Surface roughness of workpiece machined using (a) carbide-coated WC inserts, Ra = 0.64 m Ry = 3.18 m for 4 mm length; and (b) TiCN-coated WC inserts, Ra = 1.42 m Ry = 7.07 m for 4 mm length.
Fig. 10. Vision inspections (Image 67X) of (a) TiN-coated inserts before machining; (b) carbide-coated inserts before machining; (c) TiN-coated after machining; (d) carbidecoated inserts after machining; and (e) carbide-coated WC inserts after machining, showing damages to the built-up edges.
4. Conclusions In this study, we investigated the machining ability and wear resistance of WC inserts coated with carbon obtained by chemical
vapor deposition under two sets of experimental conditions. The surface morphologies of the coated inserts were examined by SEM, TEM and FE-SEM. Scratch wear tests were conducted with a diamond stylus to evaluate the adhesion strengths of the coatings to
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the inserts. Machining studies with the inserts were conducted on a CNC machine and the results were compared with those of a commercially available TiCN-coated insert. The coatings were distinct: one, of multi-walled carbon nanotubes and the other, of carbides. The surface coating helps in increasing tool life and surface finish while its adherence protects the insert from crater wear and flank wear and break away of edge. The surface finish at a spindle speed of 90 m/min, a depth of cut of 0.2 mm and a feed rate of 0.1 mm was found to be excellent, with an arithmetic mean roughness of Ra = 0.64 m and a maximum peak Ry of 3.18 m. The Ra value is lower by 0.8 m as compared with the commercial insert. Acknowledgments BP acknowledges with gratitude the help rendered by Messrs A. Rathish Kumar, R. Ravishankar and J. Kennedy in recording electron microscopic images. References [1] M. Yasuoka, P. Wang, R. Murakami, Surf. Coat. Technol. 206 (2012) 2168. [2] G. Vega, H. Arturo, Graduate Thesis. http://scholarcommons.usf.edu/etd/3121
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