- Email: [email protected]
Dynamic evaluation of surface damage mode transitions in carbon nitride coatings due to sliding contact with a spherical diamond
Wear 225–229 Ž1999. 104–110
Dynamic evaluation of surface damage mode transitions in carbon nitride coatings due to sliding contact with a spherical diamond Dong F. Wang ) , Koji Kato Laboratory of Tribology, School of Mechanical Engineering, Tohoku UniÕersity, Sendai 980-8579, Japan
Abstract The influences of both the nitrogen incorporation parameters and the coating thickness on the surface damage mode transitions in carbon nitride coatings have been initially studied from the view points of critical loads and critical groove depth. An environmental scanning electron microscope, in which a traditional pin-on-disk tribotester was installed, has dynamically provided direct evidence that when and how the surface damage mode transitions do occur during the room-temperature sliding of ion beam assisted carbon nitride coatings deposited on single crystal silicon substrates against a spherical diamond. At a controlled relative humidity of 40%, the normal loads of all sliding tests were consecutively changed until 300 mN at a sliding speed of 10 mmrs. Based on a detailed study of seven combinations of nitrogen incorporation parameters and five kinds of thicknesses, i.e., 0, 10, 50, 100 and 200 nm, carbon nitride coatings showed that the sliding mainly consisted of ‘No grooving’, ‘Grooving without material removal’ and ‘Grooving with material removal’ three typical surface damage modes. The mode transitions were then experimentally studied in the light of critical loads and critical groove depth to the summarized surface damage modes and the influences of both the nitrogen incorporation parameters and the coating thickness were further discussed with a relation to nano-indentation hardness and internal stress. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Surface damage mode transitions; Nitrogen incorporation parameters; Coating thickness; Critical loads; Critical depth
1. Introduction Microelectromechanical systems, particularly surface micromachines, often include smooth and chemically active surfaces. Recent studies have revealed a profound influence of friction and surface damage on the whole performance including efficiency, power output and steady-state speed of silicon microdynamic devices w1–4x. Although carbon films show potential as a friction reducing coating for microdynamic devices w5–7x, nitrogen incorporation in carbon increases the fraction of sp 2 carbon bonds and may become good competitors for carbon films for a wide range of sliding elements, due to their low friction coefficients, better wear resistance, better durability, and reduced internal stresses but preserving hardness w8x. In fact, the friction properties of carbon nitride coatings have been extensively studied in an effort to achieve the
) Corresponding author. Fax: [email protected]
q 81-22-217-6955;
e-mail:
expected results w8–13x. But there have been no detailed reports published of surface damage of carbon nitride coatings, not to mention those studies based on in-situ observations. Hajek et al. w13x recently reported that, sliding against a rough, polycrystalline diamond-coated Si 3 N4 ball counterface eventually led to coating detachment of about 1 mm thick carbon nitride from the SiŽ100. substrate, on the basis of a macro-scale optical observation of the worn surface. As a matter of course, after interaction between two components ‘marks’, which can convey useful information about the contacting processes, are usually left on the sliding surfaces. It is then reasonably believed that only the in-situ observations of the contacting processes of these ‘marks’ can provide direct evidence to confirm rather than to infer the surface damage modes, and therefore, to identify the mode transitions. In this paper, however, with a pin-on-disk tribotester installed in the chamber of an environmental scanning electron microscope ŽE-SEM., the in-situ confirmation of the mode transitions in carbon nitride coatings has been accomplished, and the influences of both the nitrogen incorporation parameters and the coating thickness on the
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 5 2 - 4
D.F. Wang, K. Kator Wear 225–229 (1999) 104–110
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mode transitions have been initially studied from the view points of critical loads and critical groove depth.
2. Experimental 2.1. Coating synthesis An ion beam assisted deposition ŽIBAD. system as shown in Fig. 1, consisting of a cryogenically pumped chamber, a sputter deposition source, a low and high bucklet type ion source and a substrate holder, was used in this work. The substrate holder, with an ion beam irradiation diameter of 80 mm and an irradiation incident angle of 458, consists of a water-cooled copper plate and can be rotated at a speed of 4 rpm. The background and operating pressure in the vacuum chamber were better than 133.3 = 10y6 and 133.3 = 10y4 Pa, respectively. A high-purity carbon target Ž99.999%. was mounted on the cathode, and nitrogen with the addition of argon was used as the bombardment gas. Prior to actual deposition, SiŽ111. was cleaned in situ by bombardment-etching for 10 min with 1 keV and 100 mArcm2 nitrogen ions to remove any residual contaminants, and then held at ambient temperature during deposition. All carbon nitride coatings were grown to four different thicknesses of 10, 50, 100 and 200 nm. The coating thickness is firstly controlled by the deposition time and then measured by an atomic force microscope ŽAFM. for confirmation. Typical deposition parameters are: sputtering Ar ion energy 1 keV and Ar ion current 100 mA. The different nitrogen incorporation parameters are: assisted N ion energy 0.5–10 keV and N ion beam current density 10–40 mArcm2 , varying in deposition rates from 0.5 to 1.5 nmrmin. The amorphous structure with tiny crystals in the carbon nitride coatings has been observed and confirmed using a field emission transmission electron microscope ŽFE-TEM.. The X-ray photoelectron spectroscopy ŽXPS. and the second ion mass spectroscopy ŽSIMS. analyses have revealed that the level of nitrogen incorporation is
Fig. 2. Scheme of a pin-on-disk tribotester installed in an Environmental Scanning Electron Microscope ŽE-SEM..
about 10%, and the composition distribution of both carbon and nitrogen atoms normal to the coating surfaces are homogeneous. A detailed Raman spectroscopic analysis, however, has also further shown that higher nitrogen ion acceleration energy will incline to generating more sp 2 carbon bonds in carbon nitride coatings. 2.2. Nano-indentation and internal stress measurements The hardness values of carbon nitride coatings were investigated using a thin film hardness analyzer MHA-400 developed at NEC, Japan. The indentation hysteresis curves were interpreted by Doerner and Nix w14x approach, and no corrections for indenter geometry imperfections have been made. In order to evaluate the hardness values of coatings, the bare SiŽ111. was measured together with the coated substrates by increasing the applied load up to a pre-set maximum of 0.1 mN and fixing the penetration speed at 2.7 nmrs. Since the reference measurements reproducibly yielded values of about 6 GPa, the measured hardness values for carbon nitride coatings must therefor be treated as relative to the measured values for bare SiŽ111.. The internal stresses of carbon nitride coatings were calculated from the curvature resulting from the coating deposition with the following formula:
s s 4 Es ts2 dr3 Ž 1 y n . L2 t f
Ž 1.
where Es and n are elastic modulus and Poisson’s ratio of the substrate SiŽ111., ts and t f the thicknesses of the substrate and coating, respectively, d the substrate bending, and L the scanned length of the substrate. 2.3. Sliding tests, in-situ obserÕations and further examinations of surface damages
Fig. 1. Schematic illustration for an Ion Beam Assisted Deposition ŽIBAD. systems.
A pin-on-disk tribotester as shown in Fig. 2, installed in the chamber of an environmental scanning electron microscope ŽE-SEM., makes possible in-situ observations of contacting processes. The coating specimen was mounted on the disk. A spherical diamond pin, having a tip radius of 10 mm and an included angle of 608, was slid over the coating surface since a rigid and small wear Žor no wear.
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material is necessary to keep the contact condition steady. The normal load was consecutively varied up to 300 mN and a linear sliding speed of 10 mmrs was chosen. After sliding tests, the surface damage was further examined with a field emission scanning electron microscope ŽFE-SEM Model-4500. developed at Hitachi, Japan and a confocal scanning laser microscope ŽLasertec Model-1LM 21W., respectively. Particular attention was paid to the observations and measurements of grooves and to the element identification of carbon and nitrogen at the exact mode transition moment of surface damage for four typical regions of ‘no grooving’, ‘grooving before microcutting’, ‘grooving after microcutting’ and ‘wear particles by microcutting’.
3. Results and discussion 3.1. Three typical surface damage modes E-SEM images of various moments of a typical contacting process on carbon nitride coating with a thickness of 10 nm are shown in Fig. 3. On the basis of these continuous observations, the following three typical surface damage modes can then be confirmed: ‘No grooÕing’ mode Ž mode I .. No groove and wear particles can be clearly observed when normal load is less than certain value at this stage. This certain value is defined as the critical load P12 . ‘GrooÕing without material remoÕal’ mode Ž mode II .. When normal load increases, a groove is formed on the friction surface and microcracking also starts to occur bilaterally along the groove after the unloading of the diamond pin. Since the wear particles still cannot be observed, the groove is thus believed to be mainly resulted by a plastic mechanical process caused by the sliding of the diamond pin.
‘GrooÕing with material remoÕal’ mode Ž mode III .. After normal load exceeds a critical value, defined as P23 , the wear particles start to form by microcutting on loading ahead of the diamond pin or by microcracking induced delaminations after unloading. A further surface damage examination as shown in Fig. 4 for the left ‘mark’ clearly demonstrates a whole image of ‘grooving’ with and without material removal, as well as the very image of the mode transition as shown in Fig. 4C. 3.2. Influences of nitrogen incorporation parameters and coating thickness on critical loads of mode transitions Based on the in-situ observations of surface damage modes, the critical loads with different nitrogen incorporation conditions are compared in Fig. 5, which indicates that the relative ranking for both the critical loads, P12 and P23 , remains almost unchanged. It can be noted from Fig. 6 that the coating of relatively lower values of both nanoindentation hardness and compressive stress also shows relatively lower values of critical loads. The critical loads with different coating thicknesses are also compared in Fig. 7. It is noticeable that both the P12 and P23 are found to be increased with an increase in coating thickness. The variation of critical loads is consistent with that of measured nanoindentation hardness as shown in Fig. 8, but is found to be not obviously related with that of internal stress. Similar to the predicted schematic increasing behavior with substrate hardness w15x for the critical load corresponding to the failure between the coating and the substrate, both the critical loads of P12 and P23 in Figs. 5 and 7 are also found to be increased with the increased hardness values of carbon nitride coatings, in spite of different origins of hardness increment. In fact, the bombardment of the assisted ions with higher acceleration energy, i.e., 10, 3 even 1 keV, is believed to damage the deposition surface
Fig. 3. E-SEM images at various stages of a typical contacting process of a nitride coating with a thickness of 10 nm sliding against a spherical diamond pin, where image A showing ‘no grooving’, image B showing ‘grooving without material removal’, image C and D showing ‘grooving with material removal’, and the arrow showing the sliding direction.
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Fig. 4. A further surface damage examination after the in-situ observation of contacting process shown in Fig. 3, where image A and B showing the ‘Grooving without material removal’, image C showing the very point of mode transition, image D showing the ‘Grooving with material removal’, and E showing a whole image of the ‘Grooving’, and also indicating the corresponding positions observed in images B, C and D respectively.
and eventually leads to lower values of nanoindentation hardness and higher values of surface roughness w16x, while the variation of the nanoindentation hardness shown in Fig. 8 can be possibly considered as a result of an increase in coating thickness. It is also necessary, however, to pay more attention to the coating thickness effect on critical loads as shown in Fig. 7. Concerning the critical load P23 corresponding to the surface damage mode of ‘Grooving with material removal’ for instance, a 10 nm thick carbon nitride coating survived up to about 100 mN normal load, whereas, 200 nm thick coating had shown grooving with microcracking induced delaminations after applying a 115 mN normal load. In the light of this point and further taking account of the almost same friction coefficient of 0.10 for all test coatings w16x, it can be
anticipated that the ion beam assisted carbon nitride coatings of even smaller thicknesses can be applied in various tribological surfaces, without significantly affecting their tribological properties.
Fig. 5. The critical loads as a function of nitrogen incorporation parameters.
Fig. 6. Influence of nitrogen incorporation parameters on the measured values of nanoindentation hardness and internal stress.
3.3. Influences of nitrogen incorporation parameters and coating thickness on critical grooÕe depth of mode transitions As shown in Figs. 9 and 10, the critical groove depth of test coatings varies with nitrogen incorporation parameters
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Fig. 7. The critical loads as a function of coating thickness. Fig. 9. The critical depth of groove as a function of nitrogen incorporation parameters.
and coating thickness in a similar way of the critical loads as shown in Figs. 5 and 7. The variations of critical depth are also found to be related to those of nanoindentation hardness. Actually, both the deposition parameters and the thickness-dependent behaviors of the critical loads and critical depth, corresponding to the failure within the coating or between the coating and the substrate, have been extensively studied w15,17,18x that, i.e., the critical load increases or decreases with coating thickness. But they are all different from the case of the test carbon nitride coatings discussed here especially for the surface damage mode transition from ‘Grooving without material removal’ to ‘Grooving with material removal’ by microcutting, which is believed to be associated with a failure in the SiŽ111. substrate. For example, several 100 nm thick carbon nitride coatings by different nitrogen incorporation parameters as shown in Fig. 6 exhibit relatively higher internal stresses
Fig. 8. Influence of coating thickness on the measured values of nanoindentation hardness and internal stress.
of compressive nature measured as about 2 GPa, but no sign of the failure between the coating and the substrate, i.e., coating detachment from SiŽ111. was observed. This can be possibly explained by the following two empirically validated observations. The first is the critical groove depth is found to be always beyond the corresponding coating thickness as shown in Figs. 9 and 10. The second is that, based on the random results of an energy dispersive X-ray analysis as shown in Table 1 corresponding to the exact transition moment from mode II to mode III as summarized above in Section 3.1, both the carbon and nitrogen can be comparatively detected out for three typical regions of ‘no grooving’, ‘grooving before microcutting’ and ‘wear particles by microcutting’, whereas, no nitrogen can be identified for the region of ‘grooving after microcutting’. As for the theoretical possibilities, some interesting points can also be discussed as follows. Based on a local
Fig. 10. The critical depth of groove as a function of coating thickness.
D.F. Wang, K. Kator Wear 225–229 (1999) 104–110
109
Table 1 An energy dispersive X-ray analysis for the exact transition moment from surface damage mode II to mode III as summarized in Section 3.1
yield map, showing the positions of yield initiation for hard coating under sliding contact and theoretically proposed by Diao et al. w19x as a function of the ratio of coating yield strength to the substrate yield strength and the ratio of coating thickness to half-contact width at m s 0.25 and EfrEs s 2.0, only can ‘yield in the substrate’ occur for all tested carbon nitride coatings in particular with the almost same friction coefficient of 0.10 w16x. On the other hand, since the ion beam assisted deposition ŽIBAD. is a process in which coatings are bombarded concurrently or sequentially with deposition by a beam of energetic ions from a separate ion source, both the cleaning of the substrate sputtering prior to deposition and the formation of the mixed interlayer, made of substrate atoms, deposited atoms and assisted ions during deposition, can enhance the coating adhesion performance w20x. So it is also possible to believe that the observed microcutting induced wear particles can be formed initially by the microcracking in the substrate and finally by the propagation of the microcracking into the coating rather than just along the interface with a better adhesion. A recently published paper w21x, concerning an investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings on SiŽ100. substrates, revealed that cracking initiation was influenced by the
thickness and hardness of the coating, whereas, cracking propagation was influenced by the compressive stress in the film. According to our present observations, however, it is still unclear to understand the real function of the internal stress in surface damage mode transitions, especially when trying to explain the surface damage mode transition from ‘Grooving without material removal’ to
Fig. 11. The critical load of ‘Grooving with material removal’ for different nitrogen incorporation parameters and different coating thickness as a function of the corresponding critical depth of groove.
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‘Grooving with material removal’ by microcutting, occurred on loading ahead of the diamond pin as has been shown in Figs. 3 and 4 for the 10 nm thick carbon nitride coating with a tensive internal stress of 0.6 GPa. In the case of 200 nm thick carbon nitride coating with quite lower compressive internal stress, the mechanism of the mode transition for ‘Grooving with material removal’ from microcutting on loading to microcracking induced delaminations after unloading also needs further studied. But the observed linear relation of the critical depth to the critical load as shown in Fig. 11 probably implied a coating hardness dependent microcracking initiation behavior.
4. Conclusions Corresponding to the transitions among the three surface damage modes, mainly summarized as ‘No grooving’, ‘Grooving without material removal’ and ‘Grooving with material removal’, both the critical loads and the critical groove depth of carbon nitride coatings due to sliding against a spherical diamond were found to be influenced by the nitrogen incorporation parameters and the coating thickness with an apparent correlation with the measured values of nanoindentation hardness. Based on the in-situ observations of contacting processes, further examinations of surface damages and a detailed composition identification, the surface damage mode transition from ‘Grooving without material removal’ to ‘Grooving with material removal’ by microcutting was believed to be probably associated with a failure in the single crystal silicon substrate.
Acknowledgements One of the authors ŽDong F. Wang. would like to thank Dr. Noritsugu Umehara and Dr. Koshi Adachi for their helpful discussions, and Mr. Kanemi Fukurai, Dr. Mikiko Nakajima, Dr. Junguo Xu, Mr. Kazuhiro Nakamura and Dr. Sinan Wang for their experimental assistance. Sincere thanks are also given to Zhejiang University, Chinese Education Committee and Japanese Ministry of Education, Science and Culture for the award of a scholarship as well as granting him to study and undertake this research at Tohoku University. Special thanks are also given to Prof. Zhi.Y. Mao at Zhejiang University for his continuous encouragement. Finally, authors also want to express their sincere thanks to Prof. Yuji Enomoto of Department of Advanced Machinery at Mechanical Engineering Laboratory and Dr. Serge J. Fayeulle of Advanced Mechanical Integration Group at Seagate Storage Products for lots of valuable comments in the preparation of the final revised paper.
References w1x R.S. Muller, Microdynamics, Sensors and Actuators A 21–23 Ž1990. 1–8. w2x K.J. Gabriel, F. Behi, R. Mahadevan, M. Mehregany, In-situ friction and wear measurements in integrated polysilicon mechanisms, Sensors and Actuators A 21–23 Ž1990. 184–188. w3x K. Deng, W.H. Ko, G.M. Michal, A preliminary study on friction measurements in MEMS, Proc. 6th Int. Conf. Solid State Sensors and Actuators, Transducers ’91, San Fransisco, CA, 24–27 June 1991, pp. 213–216. w4x K. Komvopoulos, Surface engineering and microtribology for microelectromechanical systems, Wear 200 Ž1996. 305–327. w5x S. Suzuki, T. Matsuura, M. Uchizawa, S. Yura, H. Shibata, Friction and wear studies on lubricants and materials applicable to MEMS, Proc. IEEE Micro Electro Mechanical Systems, Nara, Japan, 30 January–2 February 1991, pp. 143–147. w6x K. Den, W.H. Ko, Application of diamond-like carbon film for microdynamic devices, Proc. IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, 22–25 June 1992, pp. 98–101. w7x M.R. Houston, R.T. Howe, K. Komvopoulos, R. Maboudian, Diamond-like carbon films for silicon passivation in microelectromechanical devices, Mater. Res. Soc. Symp. Proc. 383 Ž1995. 391–402. w8x A. Grill, Tribology of diamond-like carbon and related materials: an updated review, unpublished research report. w9x R. Prioli, S.I. Zanette, A.O. Caride, D.F. Franceschini, F.L. Freire Jr., Atomic force microscopy of amorphous hydrogenated carbon– nitrogen film deposited by radio-frequency-plasma decomposition of methane–ammonia gas mixtures, J. Vac. Sci. Technol. A 14 Ž4. Ž1996. 2351–2355. w10x J. Koskinen, J.P. Hirvonen, J. Levoska, P. Torri, Tribological characterization of carbon–nitrogen coatings deposited by using vacuum arc discharge, Diamond and Related Materials 5 Ž1996. 669–673. w11x E.C. Cutiongco, D. Li, Y.W. Chung, C.S. Bhatia, Tribological behavior of amorphous carbon nitride overcoats for magnetic thinfilm regid disks, J. Tribol. 118 Ž1996. 543–548. w12x A. Khurshudov, K. Kato, D. Sawada, Microtribological characterization of carbon nitride coatings, Tribol. Lett. 2 Ž1996. 13–21. w13x V. Hajek, K. Rusnak, J. Vlcek, L. Martinu, H.M. Hawthorne, Tribological study of CN x films prepared by reactive DC magnetron sputtering, Wear 213 Ž1997. 80–89. w14x M.F. Doerner, W.D. Nix, A method for interpreting the data from depth-sensing indentation instruments, Journal of Materials Research 1 Ž4. Ž1986. 601–609. w15x K. Holmberg, A. Matthews, Coatings tribology, in: D. Dowson ŽEd.., Tribology Series, Vol. 28, Elsevier, 1994. w16x D.F. Wang, K. Kato, N. Umehara, Mechanical characterization of ion beam assisted carbon nitride coatings, to be presented at the 26th Int. Conf. on Metallurgical Coatings and Thin Films ŽICMCTF’99., San Diego, CA, USA, April 12–16, 1999. w17x S.J. Bull, Failure mode maps in the thin film scratch adhesion test, Tribol. Inter. 30 Ž7. Ž1997. 491–498. w18x B.K. Gupta, B. Bhushan, Micromechanical properties of amorphous carbon coatings deposited by different deposition techniques, Thin Solid Films 270 Ž1995. 391–398. w19x D.F. Diao, K. Kato, K. Hayashi, The local yield map of hard coating under sliding contact, in: D. Dowson et al. ŽEds.., Thin Films in Tribology, pp. 419–427, 1993. w20x D.S. Rickerby, A. Matthews ŽEds.., Advanced Surface Coatings, A Handbook of Surface Engineering, Blackie and Son, NY, 1991. w21x P. Kodali, K.C. Walter, M. Nastasi, Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings, Tribol. Inter. 30 Ž8. Ž1997. 591–598.
Dynamic evaluation of surface damage mode transitions in carbon nitride coatings due to sliding contact with a spherical diamond Dong F. Wang ) , Koji Kato Laboratory of Tribology, School of Mechanical Engineering, Tohoku UniÕersity, Sendai 980-8579, Japan
Abstract The influences of both the nitrogen incorporation parameters and the coating thickness on the surface damage mode transitions in carbon nitride coatings have been initially studied from the view points of critical loads and critical groove depth. An environmental scanning electron microscope, in which a traditional pin-on-disk tribotester was installed, has dynamically provided direct evidence that when and how the surface damage mode transitions do occur during the room-temperature sliding of ion beam assisted carbon nitride coatings deposited on single crystal silicon substrates against a spherical diamond. At a controlled relative humidity of 40%, the normal loads of all sliding tests were consecutively changed until 300 mN at a sliding speed of 10 mmrs. Based on a detailed study of seven combinations of nitrogen incorporation parameters and five kinds of thicknesses, i.e., 0, 10, 50, 100 and 200 nm, carbon nitride coatings showed that the sliding mainly consisted of ‘No grooving’, ‘Grooving without material removal’ and ‘Grooving with material removal’ three typical surface damage modes. The mode transitions were then experimentally studied in the light of critical loads and critical groove depth to the summarized surface damage modes and the influences of both the nitrogen incorporation parameters and the coating thickness were further discussed with a relation to nano-indentation hardness and internal stress. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Surface damage mode transitions; Nitrogen incorporation parameters; Coating thickness; Critical loads; Critical depth
1. Introduction Microelectromechanical systems, particularly surface micromachines, often include smooth and chemically active surfaces. Recent studies have revealed a profound influence of friction and surface damage on the whole performance including efficiency, power output and steady-state speed of silicon microdynamic devices w1–4x. Although carbon films show potential as a friction reducing coating for microdynamic devices w5–7x, nitrogen incorporation in carbon increases the fraction of sp 2 carbon bonds and may become good competitors for carbon films for a wide range of sliding elements, due to their low friction coefficients, better wear resistance, better durability, and reduced internal stresses but preserving hardness w8x. In fact, the friction properties of carbon nitride coatings have been extensively studied in an effort to achieve the
) Corresponding author. Fax: [email protected]
q 81-22-217-6955;
e-mail:
expected results w8–13x. But there have been no detailed reports published of surface damage of carbon nitride coatings, not to mention those studies based on in-situ observations. Hajek et al. w13x recently reported that, sliding against a rough, polycrystalline diamond-coated Si 3 N4 ball counterface eventually led to coating detachment of about 1 mm thick carbon nitride from the SiŽ100. substrate, on the basis of a macro-scale optical observation of the worn surface. As a matter of course, after interaction between two components ‘marks’, which can convey useful information about the contacting processes, are usually left on the sliding surfaces. It is then reasonably believed that only the in-situ observations of the contacting processes of these ‘marks’ can provide direct evidence to confirm rather than to infer the surface damage modes, and therefore, to identify the mode transitions. In this paper, however, with a pin-on-disk tribotester installed in the chamber of an environmental scanning electron microscope ŽE-SEM., the in-situ confirmation of the mode transitions in carbon nitride coatings has been accomplished, and the influences of both the nitrogen incorporation parameters and the coating thickness on the
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 5 2 - 4
D.F. Wang, K. Kator Wear 225–229 (1999) 104–110
105
mode transitions have been initially studied from the view points of critical loads and critical groove depth.
2. Experimental 2.1. Coating synthesis An ion beam assisted deposition ŽIBAD. system as shown in Fig. 1, consisting of a cryogenically pumped chamber, a sputter deposition source, a low and high bucklet type ion source and a substrate holder, was used in this work. The substrate holder, with an ion beam irradiation diameter of 80 mm and an irradiation incident angle of 458, consists of a water-cooled copper plate and can be rotated at a speed of 4 rpm. The background and operating pressure in the vacuum chamber were better than 133.3 = 10y6 and 133.3 = 10y4 Pa, respectively. A high-purity carbon target Ž99.999%. was mounted on the cathode, and nitrogen with the addition of argon was used as the bombardment gas. Prior to actual deposition, SiŽ111. was cleaned in situ by bombardment-etching for 10 min with 1 keV and 100 mArcm2 nitrogen ions to remove any residual contaminants, and then held at ambient temperature during deposition. All carbon nitride coatings were grown to four different thicknesses of 10, 50, 100 and 200 nm. The coating thickness is firstly controlled by the deposition time and then measured by an atomic force microscope ŽAFM. for confirmation. Typical deposition parameters are: sputtering Ar ion energy 1 keV and Ar ion current 100 mA. The different nitrogen incorporation parameters are: assisted N ion energy 0.5–10 keV and N ion beam current density 10–40 mArcm2 , varying in deposition rates from 0.5 to 1.5 nmrmin. The amorphous structure with tiny crystals in the carbon nitride coatings has been observed and confirmed using a field emission transmission electron microscope ŽFE-TEM.. The X-ray photoelectron spectroscopy ŽXPS. and the second ion mass spectroscopy ŽSIMS. analyses have revealed that the level of nitrogen incorporation is
Fig. 2. Scheme of a pin-on-disk tribotester installed in an Environmental Scanning Electron Microscope ŽE-SEM..
about 10%, and the composition distribution of both carbon and nitrogen atoms normal to the coating surfaces are homogeneous. A detailed Raman spectroscopic analysis, however, has also further shown that higher nitrogen ion acceleration energy will incline to generating more sp 2 carbon bonds in carbon nitride coatings. 2.2. Nano-indentation and internal stress measurements The hardness values of carbon nitride coatings were investigated using a thin film hardness analyzer MHA-400 developed at NEC, Japan. The indentation hysteresis curves were interpreted by Doerner and Nix w14x approach, and no corrections for indenter geometry imperfections have been made. In order to evaluate the hardness values of coatings, the bare SiŽ111. was measured together with the coated substrates by increasing the applied load up to a pre-set maximum of 0.1 mN and fixing the penetration speed at 2.7 nmrs. Since the reference measurements reproducibly yielded values of about 6 GPa, the measured hardness values for carbon nitride coatings must therefor be treated as relative to the measured values for bare SiŽ111.. The internal stresses of carbon nitride coatings were calculated from the curvature resulting from the coating deposition with the following formula:
s s 4 Es ts2 dr3 Ž 1 y n . L2 t f
Ž 1.
where Es and n are elastic modulus and Poisson’s ratio of the substrate SiŽ111., ts and t f the thicknesses of the substrate and coating, respectively, d the substrate bending, and L the scanned length of the substrate. 2.3. Sliding tests, in-situ obserÕations and further examinations of surface damages
Fig. 1. Schematic illustration for an Ion Beam Assisted Deposition ŽIBAD. systems.
A pin-on-disk tribotester as shown in Fig. 2, installed in the chamber of an environmental scanning electron microscope ŽE-SEM., makes possible in-situ observations of contacting processes. The coating specimen was mounted on the disk. A spherical diamond pin, having a tip radius of 10 mm and an included angle of 608, was slid over the coating surface since a rigid and small wear Žor no wear.
106
D.F. Wang, K. Kator Wear 225–229 (1999) 104–110
material is necessary to keep the contact condition steady. The normal load was consecutively varied up to 300 mN and a linear sliding speed of 10 mmrs was chosen. After sliding tests, the surface damage was further examined with a field emission scanning electron microscope ŽFE-SEM Model-4500. developed at Hitachi, Japan and a confocal scanning laser microscope ŽLasertec Model-1LM 21W., respectively. Particular attention was paid to the observations and measurements of grooves and to the element identification of carbon and nitrogen at the exact mode transition moment of surface damage for four typical regions of ‘no grooving’, ‘grooving before microcutting’, ‘grooving after microcutting’ and ‘wear particles by microcutting’.
3. Results and discussion 3.1. Three typical surface damage modes E-SEM images of various moments of a typical contacting process on carbon nitride coating with a thickness of 10 nm are shown in Fig. 3. On the basis of these continuous observations, the following three typical surface damage modes can then be confirmed: ‘No grooÕing’ mode Ž mode I .. No groove and wear particles can be clearly observed when normal load is less than certain value at this stage. This certain value is defined as the critical load P12 . ‘GrooÕing without material remoÕal’ mode Ž mode II .. When normal load increases, a groove is formed on the friction surface and microcracking also starts to occur bilaterally along the groove after the unloading of the diamond pin. Since the wear particles still cannot be observed, the groove is thus believed to be mainly resulted by a plastic mechanical process caused by the sliding of the diamond pin.
‘GrooÕing with material remoÕal’ mode Ž mode III .. After normal load exceeds a critical value, defined as P23 , the wear particles start to form by microcutting on loading ahead of the diamond pin or by microcracking induced delaminations after unloading. A further surface damage examination as shown in Fig. 4 for the left ‘mark’ clearly demonstrates a whole image of ‘grooving’ with and without material removal, as well as the very image of the mode transition as shown in Fig. 4C. 3.2. Influences of nitrogen incorporation parameters and coating thickness on critical loads of mode transitions Based on the in-situ observations of surface damage modes, the critical loads with different nitrogen incorporation conditions are compared in Fig. 5, which indicates that the relative ranking for both the critical loads, P12 and P23 , remains almost unchanged. It can be noted from Fig. 6 that the coating of relatively lower values of both nanoindentation hardness and compressive stress also shows relatively lower values of critical loads. The critical loads with different coating thicknesses are also compared in Fig. 7. It is noticeable that both the P12 and P23 are found to be increased with an increase in coating thickness. The variation of critical loads is consistent with that of measured nanoindentation hardness as shown in Fig. 8, but is found to be not obviously related with that of internal stress. Similar to the predicted schematic increasing behavior with substrate hardness w15x for the critical load corresponding to the failure between the coating and the substrate, both the critical loads of P12 and P23 in Figs. 5 and 7 are also found to be increased with the increased hardness values of carbon nitride coatings, in spite of different origins of hardness increment. In fact, the bombardment of the assisted ions with higher acceleration energy, i.e., 10, 3 even 1 keV, is believed to damage the deposition surface
Fig. 3. E-SEM images at various stages of a typical contacting process of a nitride coating with a thickness of 10 nm sliding against a spherical diamond pin, where image A showing ‘no grooving’, image B showing ‘grooving without material removal’, image C and D showing ‘grooving with material removal’, and the arrow showing the sliding direction.
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Fig. 4. A further surface damage examination after the in-situ observation of contacting process shown in Fig. 3, where image A and B showing the ‘Grooving without material removal’, image C showing the very point of mode transition, image D showing the ‘Grooving with material removal’, and E showing a whole image of the ‘Grooving’, and also indicating the corresponding positions observed in images B, C and D respectively.
and eventually leads to lower values of nanoindentation hardness and higher values of surface roughness w16x, while the variation of the nanoindentation hardness shown in Fig. 8 can be possibly considered as a result of an increase in coating thickness. It is also necessary, however, to pay more attention to the coating thickness effect on critical loads as shown in Fig. 7. Concerning the critical load P23 corresponding to the surface damage mode of ‘Grooving with material removal’ for instance, a 10 nm thick carbon nitride coating survived up to about 100 mN normal load, whereas, 200 nm thick coating had shown grooving with microcracking induced delaminations after applying a 115 mN normal load. In the light of this point and further taking account of the almost same friction coefficient of 0.10 for all test coatings w16x, it can be
anticipated that the ion beam assisted carbon nitride coatings of even smaller thicknesses can be applied in various tribological surfaces, without significantly affecting their tribological properties.
Fig. 5. The critical loads as a function of nitrogen incorporation parameters.
Fig. 6. Influence of nitrogen incorporation parameters on the measured values of nanoindentation hardness and internal stress.
3.3. Influences of nitrogen incorporation parameters and coating thickness on critical grooÕe depth of mode transitions As shown in Figs. 9 and 10, the critical groove depth of test coatings varies with nitrogen incorporation parameters
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Fig. 7. The critical loads as a function of coating thickness. Fig. 9. The critical depth of groove as a function of nitrogen incorporation parameters.
and coating thickness in a similar way of the critical loads as shown in Figs. 5 and 7. The variations of critical depth are also found to be related to those of nanoindentation hardness. Actually, both the deposition parameters and the thickness-dependent behaviors of the critical loads and critical depth, corresponding to the failure within the coating or between the coating and the substrate, have been extensively studied w15,17,18x that, i.e., the critical load increases or decreases with coating thickness. But they are all different from the case of the test carbon nitride coatings discussed here especially for the surface damage mode transition from ‘Grooving without material removal’ to ‘Grooving with material removal’ by microcutting, which is believed to be associated with a failure in the SiŽ111. substrate. For example, several 100 nm thick carbon nitride coatings by different nitrogen incorporation parameters as shown in Fig. 6 exhibit relatively higher internal stresses
Fig. 8. Influence of coating thickness on the measured values of nanoindentation hardness and internal stress.
of compressive nature measured as about 2 GPa, but no sign of the failure between the coating and the substrate, i.e., coating detachment from SiŽ111. was observed. This can be possibly explained by the following two empirically validated observations. The first is the critical groove depth is found to be always beyond the corresponding coating thickness as shown in Figs. 9 and 10. The second is that, based on the random results of an energy dispersive X-ray analysis as shown in Table 1 corresponding to the exact transition moment from mode II to mode III as summarized above in Section 3.1, both the carbon and nitrogen can be comparatively detected out for three typical regions of ‘no grooving’, ‘grooving before microcutting’ and ‘wear particles by microcutting’, whereas, no nitrogen can be identified for the region of ‘grooving after microcutting’. As for the theoretical possibilities, some interesting points can also be discussed as follows. Based on a local
Fig. 10. The critical depth of groove as a function of coating thickness.
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Table 1 An energy dispersive X-ray analysis for the exact transition moment from surface damage mode II to mode III as summarized in Section 3.1
yield map, showing the positions of yield initiation for hard coating under sliding contact and theoretically proposed by Diao et al. w19x as a function of the ratio of coating yield strength to the substrate yield strength and the ratio of coating thickness to half-contact width at m s 0.25 and EfrEs s 2.0, only can ‘yield in the substrate’ occur for all tested carbon nitride coatings in particular with the almost same friction coefficient of 0.10 w16x. On the other hand, since the ion beam assisted deposition ŽIBAD. is a process in which coatings are bombarded concurrently or sequentially with deposition by a beam of energetic ions from a separate ion source, both the cleaning of the substrate sputtering prior to deposition and the formation of the mixed interlayer, made of substrate atoms, deposited atoms and assisted ions during deposition, can enhance the coating adhesion performance w20x. So it is also possible to believe that the observed microcutting induced wear particles can be formed initially by the microcracking in the substrate and finally by the propagation of the microcracking into the coating rather than just along the interface with a better adhesion. A recently published paper w21x, concerning an investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings on SiŽ100. substrates, revealed that cracking initiation was influenced by the
thickness and hardness of the coating, whereas, cracking propagation was influenced by the compressive stress in the film. According to our present observations, however, it is still unclear to understand the real function of the internal stress in surface damage mode transitions, especially when trying to explain the surface damage mode transition from ‘Grooving without material removal’ to
Fig. 11. The critical load of ‘Grooving with material removal’ for different nitrogen incorporation parameters and different coating thickness as a function of the corresponding critical depth of groove.
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‘Grooving with material removal’ by microcutting, occurred on loading ahead of the diamond pin as has been shown in Figs. 3 and 4 for the 10 nm thick carbon nitride coating with a tensive internal stress of 0.6 GPa. In the case of 200 nm thick carbon nitride coating with quite lower compressive internal stress, the mechanism of the mode transition for ‘Grooving with material removal’ from microcutting on loading to microcracking induced delaminations after unloading also needs further studied. But the observed linear relation of the critical depth to the critical load as shown in Fig. 11 probably implied a coating hardness dependent microcracking initiation behavior.
4. Conclusions Corresponding to the transitions among the three surface damage modes, mainly summarized as ‘No grooving’, ‘Grooving without material removal’ and ‘Grooving with material removal’, both the critical loads and the critical groove depth of carbon nitride coatings due to sliding against a spherical diamond were found to be influenced by the nitrogen incorporation parameters and the coating thickness with an apparent correlation with the measured values of nanoindentation hardness. Based on the in-situ observations of contacting processes, further examinations of surface damages and a detailed composition identification, the surface damage mode transition from ‘Grooving without material removal’ to ‘Grooving with material removal’ by microcutting was believed to be probably associated with a failure in the single crystal silicon substrate.
Acknowledgements One of the authors ŽDong F. Wang. would like to thank Dr. Noritsugu Umehara and Dr. Koshi Adachi for their helpful discussions, and Mr. Kanemi Fukurai, Dr. Mikiko Nakajima, Dr. Junguo Xu, Mr. Kazuhiro Nakamura and Dr. Sinan Wang for their experimental assistance. Sincere thanks are also given to Zhejiang University, Chinese Education Committee and Japanese Ministry of Education, Science and Culture for the award of a scholarship as well as granting him to study and undertake this research at Tohoku University. Special thanks are also given to Prof. Zhi.Y. Mao at Zhejiang University for his continuous encouragement. Finally, authors also want to express their sincere thanks to Prof. Yuji Enomoto of Department of Advanced Machinery at Mechanical Engineering Laboratory and Dr. Serge J. Fayeulle of Advanced Mechanical Integration Group at Seagate Storage Products for lots of valuable comments in the preparation of the final revised paper.
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