- Email: [email protected]
a-C:H coatings investigated by transmission electron microscopy technique
ACME-189; No. of Pages 7 archives of civil and mechanical engineering xxx (2014) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique L. Major * Institute of Metallurgy and Materials Sciences Polish Academy of Sciences, 25 Reymonta Street, PL-30-059 Krakow, Poland
article info
abstract
Article history:
Wear mechanisms of multilayer TiN/Ti/a-C:H coatings were investigated by transmission
Received 5 July 2013
electron microscopy technique. Mechanical properties of the TiN/Ti/a-C:H multilayer coat-
Accepted 18 January 2014
ing were tested by a ball-on-disk. Microstructure of the coatings was analyzed after the
Available online xxx
mechanical treatment. Two types of wear mechanisms were discovered:
Keywords:
- layer by layer remove and tribofilm formation.
- cracking Cracking was performed by the 'layers motion' mechanism. After a one layer step
Wear Micromechanics Electron microscopy Microanalysis
movement they were fixed again. Tribofilm was built of two types of fractions. One was homogenous, the second one had layered structure. The qualitative chemical EDS analysis showed, that titanium oxide dominated in the homogenous area, while in the layered structure partial graphitization process was found. The graphite formation during the wear process, from the carbon phase presented in the coating, is a beneficial phenomenon due to the fact that graphite is a very good lubricant. The presence of the graphite phase in the tribofilm has been confirmed by the qualitative EDS chemical analysis, electron diffraction pattern and the high resolution technique. The article presents new ideas and techniques in materials science, especially in surface engineering, as well as in mechanics and mechanisms of wear of multilayer coatings. # 2014 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Hard coatings produced by physical vapor deposition (PVD) are used to enhance the wear resistance of many tribological
materials, particularly those operating in severe environments [1–4]. Advanced coating concept like multilayer coatings become increasingly important for wear protection under extreme and complex loads. In multilayer PVD coatings, at one side a combination of single layer materials with different
* Tel.: +48 12 2952800; fax: +48 12 2952804. E-mail address: [email protected]. 1644-9665/$ – see front matter # 2014 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2014.01.007 Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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properties and functions, at the other side a specific interface to grain ratio can be realized. In conclusion, multilayer coatings with such alternating properties can combine high hardness with ability to deformation [5–14]. Described coatings would potentially find an application to protect medical tools. Phases selection has been done concerning this aspect. Layers of titanium nitride (TiN) were sequentially deposited together with layers of amorphous carbon phase (a-C:H) in the presented work. TiN is of special interest due to its corrosion resistance, high hardness and bio-compatibility [1]. a-C:H is characterized by a very low coefficient of friction and by biological inertness [15], which is crucial for the wear resistant coatings used in medicine. Detailed studies of deformation mechanisms of multilayer coatings for high-resolution cross sectional observations are still very rare owing to the complexity of necessary cross-section thin foil preparation. Reasonably, this work focuses on the micro- and nanoscale investigation of deformation in multilayer coatings on the TiN/Ti/a-C:H system.
2.
Experimental
The hybrid PLD (Pulsed Laser Deposition + magnetron sputtering) equipped with high purity titanium target (99.9 at.% Ti) and carbon target were used for multilayer coatings deposition. Multilayer TiN/Ti/a-C:H coatings were deposited using both sequential atmosphere change (nitrogen for TiN and argon for a-C:H) and sequential target change (Ti for TiN and C for a-C:H). Thin Ti layers were deposited in argon atmosphere from the same target as TiN. After some time of deposition the gas flow was gradually changed to nitrogen for TiN deposition. The target evaporation (ablation) and vapor activation take place by means of a high energetic focused pulsed laser beam. The high-energetic vapor (plasma) flux allows deposition of highly adhesive coatings even at room temperature. In addition the industrially scaled room-temperature PLD deposition process was linked with other vacuum coating technique-magnetron sputtering and ion-assisted deposition to combine the advantages of these techniques. Details of deposition process is described elsewhere [11]. Analyzed coatings were deformed by ball-on-disk wear test at 1 N load and 20,000 cycles. The load and cycles number were chosen basing on the preliminary tests as well as on the experience with similar coatings. On one side they may not lead to total coating removal, on the other side it should caused some measurable deformation. Before mechanical tests, behavior of coating under outside uploading has been modeled. The modeling was performed by finite elements method [16]. The microstructure of as deposited coatings as well as coatings after mechanical tests was characterized using TECNAI G2 F20 FEG (200 kV) transmission electron microscope (TEM). Phase analysis was performed by electron diffraction pattern and confirmed by identification of high resolution images (HRTEM). Energy Dispersive X-ray technique (EDS) was done for quantitative chemical analysis of the coatings. Thin foils for TEM analysis
have been prepared from a section of mechanically deformed place by the Focused Ion Beam (FIB) technique, using QUANTA 200 3D microscope.
3.
Results and discussion
Two ceramic phases formed a multilayer system in the presented work. To reduce of residual stress in TiN/a-C:H multilayer coatings, thin Ti-metallic interlayers (7 nm) were inserted between each TiN and a-C:H interface. Additionally metallic Ti buffer layer was deposited as a first layer from the substrate, to increase the adhesion properties of the coating (Fig. 1). The multilayer coating was subjected to tribology wear test. The test was based on ball-on-disk. Before it, the behavior of coating under outside uploading has been modeled. The modeling was performed by finite elements method. It was done for static tests, however it well corresponded also to dynamic once. Stress distribution in the coating in dependency on the distance from the penetrator has been presented. The closer to penetrator, the higher residual stress concentration at the coating/ substrate interface. The further the distance from the penetrator, residual stress were located at the top part of coating (Fig. 2). The calculation showed that the radial tensile stress occurred on the surface of the multilayer coating could be responsible for cracks formation. The coefficient of friction between the penetrator and a deformable surface of the multilayer coating would affect on the residual stress state in the first a-C:H layer. Microstructure analysis of coating after wear test indicated two, wear mechanisms, which occurred simultaneously. The first one was based on cracking the second on layer by layer remove and tribofilm formation. In cracking process, ceramic layers brittle cracked, while metallic Ti thin layers deformed plastically. The plastic deformation was realized in 458. It is a typical angle for plastic deformation of polycrystalline, metallic materials. Energy of brittle cracking in ceramics was reduced by the strain of Ti layers. The experimental results confirmed numerical modeling. The highest cracks concentration was located on the side of the wear track, and dependently on the distance form indenter, cracking was moved from the bottom to the top part of coating or in opposite direction (Fig. 3), which was well presented in numerical modeling. The stress concentration motion caused the 'layers motion'. There are many examples which confirm advantages of multilayer coatings in cracking resistance, however, in conventional multilayer coatings all layers need to be broken at the same time [17]. In the described case only one layer at a time was broken moved to the closest, neighboring layer and was fixed. The layers reunion was possible because of the presence of thin, plastically deformed metallic (titanium) layers at each TiN/a-C:H interface (Fig. 3). It was done because of localized high energy of cracking process.
Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 1 – TEM microstructure analysis of TiN/Ti/a-C:H multilayer coating on cross-section. (a) Bright field image of the total coating; (b) image in higher magnification; and (c) phase analysis done by diffraction patterns.
Fig. 2 – Finite elements modeling of residual stress distribution during mechanical test; (a) image of the complex model; (b) magnification of the fragment of the model where the highest stress was concentrated at the top part of the coating; and (c) magnification of the fragment of the model where the highest stress was concentrated at the bottom part of the coating. Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 3 – Bright field TEM microstructure image of TiN/Ti/a-C:H multilayer coating on cross-section, after mechanical ball-ondisk test, which revealed layers motion mechanism in the cracking process; (a) view of the total crack propagating through the coating; (b) fragment of the coating showing the layers reunion after cracking; and (c) fragment of the coating showing a role of plastically deformed Ti metallic layers.
The description of the second wear mechanism of the described coatings (layer-by-layer remove and tribofilm formation) (Fig. 4) showed that the wear of such type of multilayer coatings may be more predictable then single layered ones. They are removed layer by layer. On the presented image, part of the top layer was already removed, which was marked by arrows. During the wear process grains of coating become polished or crushed, producing a wear debris. This debris agglomerate, become pressed together and sinters to a tribofilm on the surfaces. The tribofilm fills up cavities, covers grains and smoothens out the topography. It may be a correlation between the build-up of the tribofilm and a decrease in friction [1,15,18]. The decrease of the coefficient of friction may
be connected with graphitization process [19]. The asdeposited DLC was found to consist of a dense, threedimensional network structure with a medium range order (3 nm) present. Diffraction pattern analysis showed that DLC was mainly amorphous. Two diffuse diffraction rings with d1 1 1 = 0.21 nm and d2 2 0 = 0.12 nm were observed, suggesting the presence of a short-range cubic diamond structure (sp3). Carbon layers during wear process are crashed, removed and mechanically deposit again at the top of the coating in the form of graphite. Morphologically, the wear debris was found to be a discontinuous segregation of nano-sized carbon particles. Diffraction pattern analysis showed that the debris consisted of graphite (sp2). A wear mechanism has been proposed based on the transformation of DLC to graphite.
Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
ACME-189; No. of Pages 7 archives of civil and mechanical engineering xxx (2014) xxx–xxx
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Fig. 4 – Bright field TEM microstructure image of TiN/Ti/a-C:H multilayer coating on cross-section, after mechanical ball-ondisk test, which revealed layer by layer remove + tribofilm formation.
The transformation is related to the frictional energy. Graphite is a very good lubricant. The partially graphitization process was observed in the case of described coating. The tribofilm contained two types of fractions. The TEM observations in the bright field revealed that one had homogenous character, the second one had layered structure (Fig. 5). The layered structure of the tribofilm which contains carbon may informed that it contained graphite. The phase analysis was performed by selected area electron diffraction pattern. It showed carbon phase content in the form of graphite (Fig. 5b). Qualitative chemical analysis of both types of tribofilm fractions was done by the Energy Dispersive X-ray Spectroscopy technique (EDS) (Fig. 6).
Results have been presented in the form of maps of selected elements (Fig. 6a). They informed that the highest oxygen concentration was in the area of the homogenous tribofilm, while the highest carbon concentration in the area of layered one. The results were confirmed by qualitative point analysis (Fig. 6b). Finally the partial graphitization was confirmed by high resolution transmission electron microscopy technique. The distance in between atomic columns well corresponded with theoretical values for graphite (Fig. 7). The graphite was found in between titanium oxide phase (TiO2-rutile) (Fig. 8). The titanium oxide in the form of rutile was formed in between graphite layers during the wear process.
Fig. 5 – Bright field TEM microstructure analysis of TiN/Ti/a-C:H multilayer coating on cross-section, after mechanical ball-ondisk test; graphite tribofilm formation; (a) TEM bright field image; and (b) phase analysis done by electron diffraction pattern. Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 6 – Qualitative EDS chemical analysis of tribofilm; (a) maps of selected elements; and (b) diagrams of point analysis.
Fig. 7 – HRTEM analysis of the graphite in the tribofilm. Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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references
Fig. 8 – HRTEM analysis of titanium oxide in the tribofilm (in between graphite layers).
4.
Conclusions
- Wear of multilayer TiN/Ti/a-C:H coatings was realized by two types of wear mechanisms which occurred simultaneously during the wear process: by cracking, by layer by layer remove and tribofilm formation. - Cracking was not performed suddenly. It was done by 'layers motion' mechanism. - During the wear process some amount of carbon phase which was removed from the coating was transformed into graphite – the naturally formed lubricant. - The partial graphitization process has been confirmed by bright field TEM technique, by qualitative chemical analysis done using EDS technique and finally by high resolution transmission electron microscopy analysis.
Acknowledgements - The research project was financed by the National Science Centre (Polish-Narodowe Centrum Nauki, NCN) No: 3066/B/ T02/2011/40. - Polish-Austrian exchange project (2012–2014) 023/2012/2013/ 2014; 8548/R 12/R 14. - Joanneum Research Forschungsges.m.b.H., Institute for Surface Technologies and Photonics, Functional Surfaces, Austria: PhD Eng. Juergen Lackner for coatings deposition. - AGH University of Science and Technology, Faculty of Mechanical Engineering and Robotics, Laboratory of Tribology and Surface Engineering, Krakow, Poland: PhD Eng. Marcin Kot for mechanical tests.
[1] R.F. Bunshah, Handbook of Hard Coatings. NP, William Andrew publishing, New Jersey, USA, 2001,ISBN 0-8155-1438-7. [2] D.S. Rickerby, A. Matthews, Advanced Surface Coatings. Handbook of Surface Engineering, Chapman and Hall, NY, USA, 1991. [3] J. Adamus, J.M. Lackner, L. Major, A study of the impact of antiadhesive coatings on the sheet-titanium forming processes, Archives of Civil and Mechanical Engineering 13 (2013) 64–71. [4] R. Major, F. Bruckert, J.M. Lackner, W. Waldhauser, M. Pietrzyk, B. Major, Kinetics of eukaryote cells adhesion under shear flow detachment on the PLD deposited surfaces, Bulletin of the Polish Academy of Sciences 56 (2008) 223–228. [5] J.M. Lackner, Ł. Major, M. Kot, Microscale interpretation of tribological phenomena in Ti/TiN soft-hard multilayer coatings on soft austenite steel substrates, Bulletin of the Polish Academy of Sciences 59 (3) (2011) 343–355. [6] L. Major, J.M. Lackner, J. Morgiel, TEM Investigations of Wear Mechanisms of Single and Multilayer Coatings, The Transmission Electron Microscope, 2012, pp. 179–1962 ISBN 979-953-307-311-7. [7] L. Major, W. Tirry, G. Van Tendeloo, Microstructure and defect characterization at interfacesin TiN/CrN multilayer coatings, Surface & Coatings Technology 202 (2008) 6075–6080. [8] L. Major, J. Morgiel, J.M. Lackner, M.J. Szczerba, M. Kot, B. Major, Microstructure design and tribological properties of Cr/CrN and TiN/CrN multilayer films, Advanced Engineering Materials 10 (7) (2008) 617–621. [9] E. Martinez, J. Romero, A. Lousa, J. Esteve, Nanoindentation stress–strain curves as a method for thin-film complete mechanical characterization: application to nanometric CrN/Cr multilayer coatings, Applied Physics A 77 (2003) 419–426. [10] L. Major, J. Morgiel, TEM analysis of wear of Ti/TiN multilayer coatings in ball-on-disc test, Key Engineering Materials 409 (2009) 123–127. [11] J.M. Lackner, Industrially-scaled large-area and high-rate tribological coating by pulsed laser deposition, Surface & Coatings Technology 200 (2005) 1439–1444. [12] M. Kot, Contact mechanics of coating-substrate systems: monolayer and multilayer coatings, Archives of Civil and Mechanical Engineering 12 (2012) 464–470. [13] M. Kot, P. Lacki, Contact mechanics of coating-substrate systems. I. Methods of analysis and FEM modeling of nanoindentation tests, Journal of the Balkan Tribological Association 18 (2012) 598–614. [14] P. Wiecinski, J. Smolik, H. Garbacz, K.J. Kurzydlowski, Microstructure and mechanical properties of nanostructure multilayer CrN/Cr coatings on titanium alloy, Thin Solid Films 519 (2011) 4069–4073. [15] V. Kumar, A.A. Bergman, A.A. Gorokhovsky, A.M. Zaitsev, Formation of carbon nanofilms on diamond for all-carbon based temperature and chemical sensor application, Carbon 49 (2011) 1385–1394. [16] P. Lacki, Friction conditions in the friction brake system of Mi-2 helicopter, Archives of Civil and Mechanical Engineering (2013) http://dx.doi.org/10.1016/j.acme.2013.04.005. [17] U. Wiklund, P. Hedenqvist, S. Hogmark, Multilayer cracking resistance in bending, Surface and Coatings Technology 97 (1997) 773–778. [18] J. Olofsson, S. Jacobson, The influence of grain size and surface treatment on the tribofilm formation on alumina components, Wear 292–293 (2012) 17–24. [19] Y. Liu, E.I. Meletis, Evidence of graphitization of diamond-like carbon films during sliding wear, Journal of Materials Science 32 (1997) 3491–3495.
Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique L. Major * Institute of Metallurgy and Materials Sciences Polish Academy of Sciences, 25 Reymonta Street, PL-30-059 Krakow, Poland
article info
abstract
Article history:
Wear mechanisms of multilayer TiN/Ti/a-C:H coatings were investigated by transmission
Received 5 July 2013
electron microscopy technique. Mechanical properties of the TiN/Ti/a-C:H multilayer coat-
Accepted 18 January 2014
ing were tested by a ball-on-disk. Microstructure of the coatings was analyzed after the
Available online xxx
mechanical treatment. Two types of wear mechanisms were discovered:
Keywords:
- layer by layer remove and tribofilm formation.
- cracking Cracking was performed by the 'layers motion' mechanism. After a one layer step
Wear Micromechanics Electron microscopy Microanalysis
movement they were fixed again. Tribofilm was built of two types of fractions. One was homogenous, the second one had layered structure. The qualitative chemical EDS analysis showed, that titanium oxide dominated in the homogenous area, while in the layered structure partial graphitization process was found. The graphite formation during the wear process, from the carbon phase presented in the coating, is a beneficial phenomenon due to the fact that graphite is a very good lubricant. The presence of the graphite phase in the tribofilm has been confirmed by the qualitative EDS chemical analysis, electron diffraction pattern and the high resolution technique. The article presents new ideas and techniques in materials science, especially in surface engineering, as well as in mechanics and mechanisms of wear of multilayer coatings. # 2014 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Hard coatings produced by physical vapor deposition (PVD) are used to enhance the wear resistance of many tribological
materials, particularly those operating in severe environments [1–4]. Advanced coating concept like multilayer coatings become increasingly important for wear protection under extreme and complex loads. In multilayer PVD coatings, at one side a combination of single layer materials with different
* Tel.: +48 12 2952800; fax: +48 12 2952804. E-mail address: [email protected]. 1644-9665/$ – see front matter # 2014 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2014.01.007 Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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properties and functions, at the other side a specific interface to grain ratio can be realized. In conclusion, multilayer coatings with such alternating properties can combine high hardness with ability to deformation [5–14]. Described coatings would potentially find an application to protect medical tools. Phases selection has been done concerning this aspect. Layers of titanium nitride (TiN) were sequentially deposited together with layers of amorphous carbon phase (a-C:H) in the presented work. TiN is of special interest due to its corrosion resistance, high hardness and bio-compatibility [1]. a-C:H is characterized by a very low coefficient of friction and by biological inertness [15], which is crucial for the wear resistant coatings used in medicine. Detailed studies of deformation mechanisms of multilayer coatings for high-resolution cross sectional observations are still very rare owing to the complexity of necessary cross-section thin foil preparation. Reasonably, this work focuses on the micro- and nanoscale investigation of deformation in multilayer coatings on the TiN/Ti/a-C:H system.
2.
Experimental
The hybrid PLD (Pulsed Laser Deposition + magnetron sputtering) equipped with high purity titanium target (99.9 at.% Ti) and carbon target were used for multilayer coatings deposition. Multilayer TiN/Ti/a-C:H coatings were deposited using both sequential atmosphere change (nitrogen for TiN and argon for a-C:H) and sequential target change (Ti for TiN and C for a-C:H). Thin Ti layers were deposited in argon atmosphere from the same target as TiN. After some time of deposition the gas flow was gradually changed to nitrogen for TiN deposition. The target evaporation (ablation) and vapor activation take place by means of a high energetic focused pulsed laser beam. The high-energetic vapor (plasma) flux allows deposition of highly adhesive coatings even at room temperature. In addition the industrially scaled room-temperature PLD deposition process was linked with other vacuum coating technique-magnetron sputtering and ion-assisted deposition to combine the advantages of these techniques. Details of deposition process is described elsewhere [11]. Analyzed coatings were deformed by ball-on-disk wear test at 1 N load and 20,000 cycles. The load and cycles number were chosen basing on the preliminary tests as well as on the experience with similar coatings. On one side they may not lead to total coating removal, on the other side it should caused some measurable deformation. Before mechanical tests, behavior of coating under outside uploading has been modeled. The modeling was performed by finite elements method [16]. The microstructure of as deposited coatings as well as coatings after mechanical tests was characterized using TECNAI G2 F20 FEG (200 kV) transmission electron microscope (TEM). Phase analysis was performed by electron diffraction pattern and confirmed by identification of high resolution images (HRTEM). Energy Dispersive X-ray technique (EDS) was done for quantitative chemical analysis of the coatings. Thin foils for TEM analysis
have been prepared from a section of mechanically deformed place by the Focused Ion Beam (FIB) technique, using QUANTA 200 3D microscope.
3.
Results and discussion
Two ceramic phases formed a multilayer system in the presented work. To reduce of residual stress in TiN/a-C:H multilayer coatings, thin Ti-metallic interlayers (7 nm) were inserted between each TiN and a-C:H interface. Additionally metallic Ti buffer layer was deposited as a first layer from the substrate, to increase the adhesion properties of the coating (Fig. 1). The multilayer coating was subjected to tribology wear test. The test was based on ball-on-disk. Before it, the behavior of coating under outside uploading has been modeled. The modeling was performed by finite elements method. It was done for static tests, however it well corresponded also to dynamic once. Stress distribution in the coating in dependency on the distance from the penetrator has been presented. The closer to penetrator, the higher residual stress concentration at the coating/ substrate interface. The further the distance from the penetrator, residual stress were located at the top part of coating (Fig. 2). The calculation showed that the radial tensile stress occurred on the surface of the multilayer coating could be responsible for cracks formation. The coefficient of friction between the penetrator and a deformable surface of the multilayer coating would affect on the residual stress state in the first a-C:H layer. Microstructure analysis of coating after wear test indicated two, wear mechanisms, which occurred simultaneously. The first one was based on cracking the second on layer by layer remove and tribofilm formation. In cracking process, ceramic layers brittle cracked, while metallic Ti thin layers deformed plastically. The plastic deformation was realized in 458. It is a typical angle for plastic deformation of polycrystalline, metallic materials. Energy of brittle cracking in ceramics was reduced by the strain of Ti layers. The experimental results confirmed numerical modeling. The highest cracks concentration was located on the side of the wear track, and dependently on the distance form indenter, cracking was moved from the bottom to the top part of coating or in opposite direction (Fig. 3), which was well presented in numerical modeling. The stress concentration motion caused the 'layers motion'. There are many examples which confirm advantages of multilayer coatings in cracking resistance, however, in conventional multilayer coatings all layers need to be broken at the same time [17]. In the described case only one layer at a time was broken moved to the closest, neighboring layer and was fixed. The layers reunion was possible because of the presence of thin, plastically deformed metallic (titanium) layers at each TiN/a-C:H interface (Fig. 3). It was done because of localized high energy of cracking process.
Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 1 – TEM microstructure analysis of TiN/Ti/a-C:H multilayer coating on cross-section. (a) Bright field image of the total coating; (b) image in higher magnification; and (c) phase analysis done by diffraction patterns.
Fig. 2 – Finite elements modeling of residual stress distribution during mechanical test; (a) image of the complex model; (b) magnification of the fragment of the model where the highest stress was concentrated at the top part of the coating; and (c) magnification of the fragment of the model where the highest stress was concentrated at the bottom part of the coating. Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 3 – Bright field TEM microstructure image of TiN/Ti/a-C:H multilayer coating on cross-section, after mechanical ball-ondisk test, which revealed layers motion mechanism in the cracking process; (a) view of the total crack propagating through the coating; (b) fragment of the coating showing the layers reunion after cracking; and (c) fragment of the coating showing a role of plastically deformed Ti metallic layers.
The description of the second wear mechanism of the described coatings (layer-by-layer remove and tribofilm formation) (Fig. 4) showed that the wear of such type of multilayer coatings may be more predictable then single layered ones. They are removed layer by layer. On the presented image, part of the top layer was already removed, which was marked by arrows. During the wear process grains of coating become polished or crushed, producing a wear debris. This debris agglomerate, become pressed together and sinters to a tribofilm on the surfaces. The tribofilm fills up cavities, covers grains and smoothens out the topography. It may be a correlation between the build-up of the tribofilm and a decrease in friction [1,15,18]. The decrease of the coefficient of friction may
be connected with graphitization process [19]. The asdeposited DLC was found to consist of a dense, threedimensional network structure with a medium range order (3 nm) present. Diffraction pattern analysis showed that DLC was mainly amorphous. Two diffuse diffraction rings with d1 1 1 = 0.21 nm and d2 2 0 = 0.12 nm were observed, suggesting the presence of a short-range cubic diamond structure (sp3). Carbon layers during wear process are crashed, removed and mechanically deposit again at the top of the coating in the form of graphite. Morphologically, the wear debris was found to be a discontinuous segregation of nano-sized carbon particles. Diffraction pattern analysis showed that the debris consisted of graphite (sp2). A wear mechanism has been proposed based on the transformation of DLC to graphite.
Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
ACME-189; No. of Pages 7 archives of civil and mechanical engineering xxx (2014) xxx–xxx
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Fig. 4 – Bright field TEM microstructure image of TiN/Ti/a-C:H multilayer coating on cross-section, after mechanical ball-ondisk test, which revealed layer by layer remove + tribofilm formation.
The transformation is related to the frictional energy. Graphite is a very good lubricant. The partially graphitization process was observed in the case of described coating. The tribofilm contained two types of fractions. The TEM observations in the bright field revealed that one had homogenous character, the second one had layered structure (Fig. 5). The layered structure of the tribofilm which contains carbon may informed that it contained graphite. The phase analysis was performed by selected area electron diffraction pattern. It showed carbon phase content in the form of graphite (Fig. 5b). Qualitative chemical analysis of both types of tribofilm fractions was done by the Energy Dispersive X-ray Spectroscopy technique (EDS) (Fig. 6).
Results have been presented in the form of maps of selected elements (Fig. 6a). They informed that the highest oxygen concentration was in the area of the homogenous tribofilm, while the highest carbon concentration in the area of layered one. The results were confirmed by qualitative point analysis (Fig. 6b). Finally the partial graphitization was confirmed by high resolution transmission electron microscopy technique. The distance in between atomic columns well corresponded with theoretical values for graphite (Fig. 7). The graphite was found in between titanium oxide phase (TiO2-rutile) (Fig. 8). The titanium oxide in the form of rutile was formed in between graphite layers during the wear process.
Fig. 5 – Bright field TEM microstructure analysis of TiN/Ti/a-C:H multilayer coating on cross-section, after mechanical ball-ondisk test; graphite tribofilm formation; (a) TEM bright field image; and (b) phase analysis done by electron diffraction pattern. Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 6 – Qualitative EDS chemical analysis of tribofilm; (a) maps of selected elements; and (b) diagrams of point analysis.
Fig. 7 – HRTEM analysis of the graphite in the tribofilm. Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007
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Fig. 8 – HRTEM analysis of titanium oxide in the tribofilm (in between graphite layers).
4.
Conclusions
- Wear of multilayer TiN/Ti/a-C:H coatings was realized by two types of wear mechanisms which occurred simultaneously during the wear process: by cracking, by layer by layer remove and tribofilm formation. - Cracking was not performed suddenly. It was done by 'layers motion' mechanism. - During the wear process some amount of carbon phase which was removed from the coating was transformed into graphite – the naturally formed lubricant. - The partial graphitization process has been confirmed by bright field TEM technique, by qualitative chemical analysis done using EDS technique and finally by high resolution transmission electron microscopy analysis.
Acknowledgements - The research project was financed by the National Science Centre (Polish-Narodowe Centrum Nauki, NCN) No: 3066/B/ T02/2011/40. - Polish-Austrian exchange project (2012–2014) 023/2012/2013/ 2014; 8548/R 12/R 14. - Joanneum Research Forschungsges.m.b.H., Institute for Surface Technologies and Photonics, Functional Surfaces, Austria: PhD Eng. Juergen Lackner for coatings deposition. - AGH University of Science and Technology, Faculty of Mechanical Engineering and Robotics, Laboratory of Tribology and Surface Engineering, Krakow, Poland: PhD Eng. Marcin Kot for mechanical tests.
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Please cite this article in press as: L. Major, Wear mechanisms of multilayer TiN/Ti/a-C:H coatings investigated by transmission electron microscopy technique, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2014.01.007