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Characterization of the mechanical properties of carbon metal multilayered films
RiAMOND RELATED MATERIALS ELSEVIER
Diamond and Related Materials 4 (1995) 843-847
Characterization J. Koskinen
of the mechanical properties multilayered films
of carbon metal
a, H. Ronkainen a, J.-P. Hirvonen a, R. Lappalainen b, K.A. Pischow a VTT, Manufacturing Technology, FIN-02044, Helsinki, Finland
’
b University of Helsinki, Department of Physics, FIN-00014 Helsinki, Finland ’ Helsinki University of Technology, Laboratory of Processing and Heat Treatment of Materials, FIN-02150 Espoo, Finland
Abstract Hard amorphous carbon films exhibit excellent wear resistance and low friction. The hard carbon films are brittle and have a high internal stress. Also the abrupt change of elastic modulus at the interface of the film and metallic substrate results in low adhesion and reduces load carrying capacity. Composite film structures with alternating layers of hard carbon and metallic films has been shown to possess unique mechanical properties such as enhanced fracture toughness and high hardness. Elastic mismatch may also be avoided by careful control of composition. The stress relaxation, when possible, enables the growth of thicker films. Multilayer films with alternating layers of hard carbon and TIN, have been deposited by using an arc discharge deposition apparatus. The carbon plasma is generated with a pulsed plasma source. The titanium and TIN films are deposited by using a d.c. arc source equipped with a particle filtering. Films with a thickness of about 0.5 pm were deposited with 10 layers of carbon and TIN,. The substrate materials were AISI440B stainless steel and (100) silicon. The film composition was determined by nuclear resonance analysis, Rutherford backscattering, scanning electron microscopy and scanning force microscopy. The wear resistance and load carrying capacity was tested by using a pin-on-disc test. The friction coefficient of the multilayer films was observed to be lower than for the pure diamond-like carbon films, while the wear rate of the multilayer film slightly increased and the wear rate of the counter surface was increased about ten fold. Keywords: Interfacial
layers; Tribology;
Diamond-like
carbon
1. Introduction Physical vapour deposited (PVD) diamond-like carbon (DLC) films have been reported to exhibit excellent tribological properties such as extremely low wear rate and low coefficient of friction. The diamond-like features of the DLC films are shown to correlate with an intrinsic compressive stress in the film which restricts the applicability of the DLC films [l]. On the other hand, the abrupt changes of the elastic constant at the interface of the DLC film and the substrate (e.g. metal) result into delamination of the film while loading the film-substrate system. By the introduction of metal doped and multilayer films thicker films with lower internal stresses have been deposited [2,3]. In this paper multilayer films with a structure of alternating layers of DLC and TIN, have been deposited by using the vacuum arc deposition method. DLC films free of hydrogen impurities are clearly harder that the amorphous hydrogenated carbon (a-C:H) films and have 092%9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(94)05262-X
a microstructure sometimes characterized as amorphous diamond [4]. Such hydrogen free films also posses very high compressive stress, up to tens of GPa [ 11. Thus it is of large interest to enhance the applicability of such hard DLC films by applying multilayer structures. The multilayer films were characterised by using scanning electron microscopy (SEM), ion beams and scanning force microscopy (SFM). The tribological properties were tested by using a pin-on-disc apparatus.
2. Experimental Substrates
Coatings were deposited on stainless steel substrates AISI440B (X 90 CrMoV 18) and silicon (100) wafers. The hardness of the stainless steel was 620 Hv. The steel samples were mechanically polished to a surface roughness (R,) 0.03 pm. Prior to deposition they were
degreased ultrasonic
in TEC, agitation.
acetone
and
isopropanol
using
Film deposition The deposition of the DLCTiN, multilayers was carried out by using vacuum arc deposition. The stainless steel vacuum chamber is evacuated by a diffusion oil pump to a base pressure of about 3 x 10m4 Pa. An argon ion beam from a broad beam ion source is used to sputter clean the substrates prior to deposition. The metal plasma arc source is a continuous current arc (SO-ZOO A) device with a titanium cathode with a diameter of 50 mm. A curved magnetic field of about 10 mT is used to deflect the plasma 90“ in order to reduce the number of metal droplets hitting the substrate. Prior to deposition the substrates were slightly etched (about lo-20 nm) with the argon ion source. Then the samples were coated with alternating layers of TIN, and DLC by sequential runs of the metal arc and the carbon arc. The metal arc was run in a nitrogen atmosphere (1 x lo-’ Pa during arc). The deposition rates were for TIN, film and 12 nm min - ’ about 100nm mini’ (0.1 nm pulse- ‘) for DLC respectively. For this study three different coatings were prepared with the layer thicknesses listed in Table 1.
Trihological
tests
The friction and wear performance of the coatings was assessed by pin-on-disc tests. The tests were carried out using polished balls made of AISI 52100 steel (100 Cr 6). The surface roughness (R,) of the balls was 0.016 urn. The tests were performed unlubricated under atmospheric conditions. The relative humidity during testing was controlled at 50 f 5% and the temperature at 21 k 2 ‘C. The normal force was 5 N, the sliding velocity 1.5 m s-i and the sliding distance 2000 m. The load created a nominal hertzian pressure of 0.78 GPa in the contact. The pin wear volumes were calculated from the ball wear scar diameters and the disc wear as an average value from four profilograms taken across the disc wear track. The tests were carried out two to three times and the friction and wear values reported are given as mean values of the results. The load carrying capacity of the coatings was evaluated by carrying out tests with increasing load. The load was increased from 5 N first to 10 N, then to 22 N and finally to 40 N after 500 m of sliding at each step. The total sliding distance also in this case was 2000 m. These tests were carried out only once.
3. Results Film characterization The film composition was determined by Rutherford backscattering (RBS) with 2.0 MeV He+ and H+ beams. The cross sections of the He and H scattering were corrected for the light elements [S]. Nitrogen profiling was performed by using the nuclear reaction ‘“N(~,wT)‘~C at the proton energy of E,=429 keV 161. The films and the wear surfaces were also characterized with SEM and energy dispersive spectroscopy (EDX). SFM analyses were performed using a DME Rasterscope 3000 equipped with both STM and SFM facilities. In the SFM work a micro-fabricated cantilever with a spring constant of 0.02 N m-l was used and the applied force was from 0.52.0 nN. The hardness of the films was measured with a conventional microhardness tester. The loads used were 10 and 20 g and both Vickers and Knoop indenters were used.
The RBS measurements indicate a regular multilayer structure as can be seen in Fig. 1. The nitrogen content in the TIN, layers was about 20 + 2 at.%. Stoichiometric TiN could be deposited by using a higher nitrogen gas pressure (4 x 1O-2 Pa) during the deposition. The hardness values and internal stress values measured for the samples are listed in Table 2. Although the total thicknesses of the films vary a clear tendency of softening of the film with increasing amount of TIN,
Table I Film compositions Coating No.
TIN, thickness
DLC thickness
(nm)
(nm)
33 5
380 26 70
1 2 3
Number layers
of
Total thickness (pm)
1 IO 10
0.38 0.60 0.75
Channel Fig. 1. RBS spectrum
number
of coating
No. 2 on silicon.
J. Koskinen et al. JDiamond and Related Materials 4 ( 1995) 843-847 Table 2 Hardness Sample No.
1 2 3
and internal
845
stress results of the samples Vickers hardness on AISI440B
Knoop hardness on Si substrate
(20 g)
(20 g)
Internal compressive stress (GPa)
1300 i_ 140 890 + 100 1200 * 170
5000 * 400 (0.5 pm) 1500 + 20 _
3.6 i 0.5 2.1 +_0.2 (delaminated
content in the film. Also the internal stress decreases respectively. The coating No. 3 on silicon delaminated during a late stage of the deposition leaving fragments of the film on the substrate. It is noteworthy that during delamination large amounts of silicon substrate material was locally delaminated along with the multilayer film. Up to 5 pm deep cavities were found at the delaminated surface by using surface profilometer and SFM. SFM spectra taken from the silicon samples indicate the presence of rounded shape particles with a diameter of 2-3 pm, presumably titanium droplets from the arc cathode (Fig. 2). These droplets must have drifted as electrically charged massive particles with the curved magnetic field and/or bounced from the walls of the deposition chamber in order to reach the sample surface. Signs of film debris can be observed in contact with the droplet showing that the droplet has arrived to the
sample surface at an earlier stage of the film growth and film has delaminated from the droplet during the deposition. The results from the tribological tests show that the friction coefficient values in the end of the tests are lower for the multilayer coatings compared to single layer DLC coating as can be seen in Fig. 3. The friction behaviour of the single layer and multilayer coatings differ from each other. The single layer coating reached a stable value rather rapidly, whereas for the multilayer coatings the friction coefficient fluctuated rather much before stabilising (Fig. 4). On the other hand, both the wear of the coated discs and particularly the wear of the pins were increased for the multilayer coatings (Fig. 3). The wear of the discs was difficult to measure, because the wear was mostly polishing type of wear causing only removal of the outermost surface roughness of the surface. The difficul-
5000.0 nm Fig. 2. SFM micrograph
of a droplet
on Si)
on coating
No. 1 on silicon.
J. Koskinen et al.lDimlond and R&ted
846 2 _
0.2
Y-
Mutmicrls 4 al
i 1995) 843-847
I
-E 0.15
:$ J= ;
0.1
0.5
6 is .Lt
0.05
0 Sliding distance [m]
2000
Sliding distance (ml
2000
0.5
Fig. 3. The coefficient of friction the discs in pin-on-disc tests.
and the wear rates of the pins and
ties in wear determination also effected the scatter of the wear results of the coated discs. The scatter in the wear volumes typically varied from 15-25%, but for the single layer coating the scatter of the disc wear was as high as 120% and for the coating with thick a-C layers (No. 3) about 80%. Owing to difficulties in wear volume determination, the surface roughness profile was also determined from the surface profiles. This was accomplished as follows: The profile y(x) was smoothened by using a fast fourier transformation (FFT) filtering routine to define a baseline for the surface. This curve had a maximum curvature corresponding to a minimum radius of curvature of 2 m. Then this baseline was subtracted from the original profile and converted to the corresponding absolute value ly(x)l. This curve was finally smoothened with the above mentioned FFT filtering routine. Thus obtained curve is similar to a continuous R, value of the surface. By this kind of analysis it is possible to observe that the wear track was polished to a smoothness which is about 0.020 pm compared to the value of about 0.050 pm at the original coated surface. The smoothness of the wear tracks corresponded inversely to the wear rates of the coatings. The smoothness values from steady load (5 N) tests and from the load carrying capacity tests are presented in Fig. 5. The wear volumes in the load carrying capacity tests were generally higher than in the tests carried out with 5 N load. Only the pin sliding against the single layer coating (coating No. 1) showed less wear in the load
I
_..._.__~
~~
~__
Sliding distance [ml
2000
Fig. 4. The typical friction graphs of (a) the single layer coating No. 1. (b) the multilayer coating No. 2 and (c) the multilayer coating No. 3.
Coating 1
Fig. 5. The smoothness values of the coatings in the tests with steady 5 N load and in the tests with increasing 5-40 N load.
J. Koskinen et al./Diamond and Related Materials 4 (3995) 843-847
carrying capacity test. The friction coefficient typically dropped immediately after the load was increased, but showed an increasing trend when the test was carried out further with higher loads. The wear surfaces were examined by optical microscopy and by SEM. The pins sliding against the single layer coating showed some layer formation on the pin wear surface, whereas the pins sliding against the multilayer coatings showed merely metallic appearance. Also in front of the contact area of the pin there was only a slight sliding deposit, when pin was slid against the multilayer coatings. EDX analyses showed that some titanium was transferred to the pin wear surface from the multilayer coatings. In the wear tracks of the single layer coating (No. 1) traces of wear debris of the same type as found in the sliding deposit on the pin, was discovered. On the wear tracks of the multilayer coatings, some removal of coating was detected.
4. Discussion
847
contact surfaces [9]. A metal oxide transfer layer formed on the pin wear surface has low adhesion to the carbon film. The increase of the titanium at the contact layer may enhance the inertness of the transfer layer accounting the lower coefficient of friction of the TIN,-DLC multilayers. The very mild wear of hard DLC films is difficult to quantify, and could therefore be characterized as smoothing. In many tribological applications such low wear rates are desirable or necessary and thus a quantitative smoothing analysis can be informative in analyzing the wear data. In conclusion, it has been demonstrated that multilayer structure DLC films have excellent wear resistant and low friction properties. Since the hydrogen free DLC films are harder and more wear resistant compared to the hydrogenated films there seems to be more potential to modify the properties of these films yet possessing diamond-like properties comparable to those of hydrogenated films after the modifications.
and conclusion
The wear rates of the pin was increased to a great extent when TiN,-layers are included in the coating structure. Earlier we have reported the wear behaviour of steel pins sliding against TIN coating, which showed two orders of magnitude higher wear rates for the pins sliding against TIN coating, compared to pins sliding against a-C coatings [ 71. In this case the increased wear of the pins probably originate from the action of TiNlayers in the coatings. The wear resistance of the coating itself also seemed to be reduced with multilayer structure. The coefficient of friction was decreased with TiN,-layered structures. For the amorphous hydrogenated carbon coatings it has been discovered that Ti-containing coatings have lower friction coefficient [S]. For the multilayered coatings the free Ti included in the structure can also have a similar effect in these tests. Possibly the incorporation of Ti enhances the growth of a transfer layer on the pin resulting in lowered coefficient of friction. The coating surface structure is dominated by the droplet formations. These droplets can be carbon or titanium in the case of multilayer coatings. These droplet formations obviously have an effect on the coating behaviour, particularly on the friction behaviour. The Ti-droplets in the multilayer coatings can for example have a friction reducing effect, when combined to hard carbon coating, as described above. It has earlier been presented that the low coefficient of friction may be a result of the transfer layers at the
Acknowledgements The SEM and EDX analysis of Tom Gustafsson and the indentation tests made by Seija Kiema are gratefully acknowledged.
References G.A.J. Amaratunga, WI. Milne, P. Hewitt, Cl1 VS. Veerasamy, P.J. Fallon, D.R.McKenzie and C.A. Davis, Diamond Relat. Mater., 2 (1992) 782-787. [21 A. Matthews and S.S. Eskildsen, Diamond Relat. Mater., 3 ( 1994) 902-911. K. Bewilogua and H. Dimigen, Mater. Manuf. c31 M. Grischke, Proc., 8 (4/5) (1993) 407-417. c41 D.C. Green, D.R. McKenzie and P.B. Lukins, in J.J. Pouch and A. Alterovitz (eds.), Materials Science Forum, Tram Tech, Zurich, Vols. 52 and 53, 1990, pp. 1033124. and E. Rauhala, Nucl. Instrum. Methods B, 64 c51 J. Saarilahti (1992) 734-738. A. Anttila and E. Sirvio, Proc. C61 J.P. Hirvonen, R. Lappalainen, 1st Int. Conf. on Plasma Surface Engineering, Sept. 1988, Garmisch-Partenkirchen, DGM Informations Gesellschaft, Oberursel, 1989, pp. 721-728. c71 H. Ronkainen, S. Varjus and J. Koskinen, Tribological properties of diamond-like carbon films, Proc. 5th Nordic Symp. on Trihology, 8-11 June, 2992, Helsinki, Tribologia, II (4) (1992) 133-141. et al., unpublished. cg1 H. Ronkainen J. Koskinen, J. Likonen, S. Varjus and c91 H. Ronkainen, J. Vihersalo, Characterization of wear surfaces in dry sliding of steel and alumina on hydrogenated and hydrogen-free carbon films, Diamond Relat. Mater., 3 (1994) 1329-1336.
Diamond and Related Materials 4 (1995) 843-847
Characterization J. Koskinen
of the mechanical properties multilayered films
of carbon metal
a, H. Ronkainen a, J.-P. Hirvonen a, R. Lappalainen b, K.A. Pischow a VTT, Manufacturing Technology, FIN-02044, Helsinki, Finland
’
b University of Helsinki, Department of Physics, FIN-00014 Helsinki, Finland ’ Helsinki University of Technology, Laboratory of Processing and Heat Treatment of Materials, FIN-02150 Espoo, Finland
Abstract Hard amorphous carbon films exhibit excellent wear resistance and low friction. The hard carbon films are brittle and have a high internal stress. Also the abrupt change of elastic modulus at the interface of the film and metallic substrate results in low adhesion and reduces load carrying capacity. Composite film structures with alternating layers of hard carbon and metallic films has been shown to possess unique mechanical properties such as enhanced fracture toughness and high hardness. Elastic mismatch may also be avoided by careful control of composition. The stress relaxation, when possible, enables the growth of thicker films. Multilayer films with alternating layers of hard carbon and TIN, have been deposited by using an arc discharge deposition apparatus. The carbon plasma is generated with a pulsed plasma source. The titanium and TIN films are deposited by using a d.c. arc source equipped with a particle filtering. Films with a thickness of about 0.5 pm were deposited with 10 layers of carbon and TIN,. The substrate materials were AISI440B stainless steel and (100) silicon. The film composition was determined by nuclear resonance analysis, Rutherford backscattering, scanning electron microscopy and scanning force microscopy. The wear resistance and load carrying capacity was tested by using a pin-on-disc test. The friction coefficient of the multilayer films was observed to be lower than for the pure diamond-like carbon films, while the wear rate of the multilayer film slightly increased and the wear rate of the counter surface was increased about ten fold. Keywords: Interfacial
layers; Tribology;
Diamond-like
carbon
1. Introduction Physical vapour deposited (PVD) diamond-like carbon (DLC) films have been reported to exhibit excellent tribological properties such as extremely low wear rate and low coefficient of friction. The diamond-like features of the DLC films are shown to correlate with an intrinsic compressive stress in the film which restricts the applicability of the DLC films [l]. On the other hand, the abrupt changes of the elastic constant at the interface of the DLC film and the substrate (e.g. metal) result into delamination of the film while loading the film-substrate system. By the introduction of metal doped and multilayer films thicker films with lower internal stresses have been deposited [2,3]. In this paper multilayer films with a structure of alternating layers of DLC and TIN, have been deposited by using the vacuum arc deposition method. DLC films free of hydrogen impurities are clearly harder that the amorphous hydrogenated carbon (a-C:H) films and have 092%9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(94)05262-X
a microstructure sometimes characterized as amorphous diamond [4]. Such hydrogen free films also posses very high compressive stress, up to tens of GPa [ 11. Thus it is of large interest to enhance the applicability of such hard DLC films by applying multilayer structures. The multilayer films were characterised by using scanning electron microscopy (SEM), ion beams and scanning force microscopy (SFM). The tribological properties were tested by using a pin-on-disc apparatus.
2. Experimental Substrates
Coatings were deposited on stainless steel substrates AISI440B (X 90 CrMoV 18) and silicon (100) wafers. The hardness of the stainless steel was 620 Hv. The steel samples were mechanically polished to a surface roughness (R,) 0.03 pm. Prior to deposition they were
degreased ultrasonic
in TEC, agitation.
acetone
and
isopropanol
using
Film deposition The deposition of the DLCTiN, multilayers was carried out by using vacuum arc deposition. The stainless steel vacuum chamber is evacuated by a diffusion oil pump to a base pressure of about 3 x 10m4 Pa. An argon ion beam from a broad beam ion source is used to sputter clean the substrates prior to deposition. The metal plasma arc source is a continuous current arc (SO-ZOO A) device with a titanium cathode with a diameter of 50 mm. A curved magnetic field of about 10 mT is used to deflect the plasma 90“ in order to reduce the number of metal droplets hitting the substrate. Prior to deposition the substrates were slightly etched (about lo-20 nm) with the argon ion source. Then the samples were coated with alternating layers of TIN, and DLC by sequential runs of the metal arc and the carbon arc. The metal arc was run in a nitrogen atmosphere (1 x lo-’ Pa during arc). The deposition rates were for TIN, film and 12 nm min - ’ about 100nm mini’ (0.1 nm pulse- ‘) for DLC respectively. For this study three different coatings were prepared with the layer thicknesses listed in Table 1.
Trihological
tests
The friction and wear performance of the coatings was assessed by pin-on-disc tests. The tests were carried out using polished balls made of AISI 52100 steel (100 Cr 6). The surface roughness (R,) of the balls was 0.016 urn. The tests were performed unlubricated under atmospheric conditions. The relative humidity during testing was controlled at 50 f 5% and the temperature at 21 k 2 ‘C. The normal force was 5 N, the sliding velocity 1.5 m s-i and the sliding distance 2000 m. The load created a nominal hertzian pressure of 0.78 GPa in the contact. The pin wear volumes were calculated from the ball wear scar diameters and the disc wear as an average value from four profilograms taken across the disc wear track. The tests were carried out two to three times and the friction and wear values reported are given as mean values of the results. The load carrying capacity of the coatings was evaluated by carrying out tests with increasing load. The load was increased from 5 N first to 10 N, then to 22 N and finally to 40 N after 500 m of sliding at each step. The total sliding distance also in this case was 2000 m. These tests were carried out only once.
3. Results Film characterization The film composition was determined by Rutherford backscattering (RBS) with 2.0 MeV He+ and H+ beams. The cross sections of the He and H scattering were corrected for the light elements [S]. Nitrogen profiling was performed by using the nuclear reaction ‘“N(~,wT)‘~C at the proton energy of E,=429 keV 161. The films and the wear surfaces were also characterized with SEM and energy dispersive spectroscopy (EDX). SFM analyses were performed using a DME Rasterscope 3000 equipped with both STM and SFM facilities. In the SFM work a micro-fabricated cantilever with a spring constant of 0.02 N m-l was used and the applied force was from 0.52.0 nN. The hardness of the films was measured with a conventional microhardness tester. The loads used were 10 and 20 g and both Vickers and Knoop indenters were used.
The RBS measurements indicate a regular multilayer structure as can be seen in Fig. 1. The nitrogen content in the TIN, layers was about 20 + 2 at.%. Stoichiometric TiN could be deposited by using a higher nitrogen gas pressure (4 x 1O-2 Pa) during the deposition. The hardness values and internal stress values measured for the samples are listed in Table 2. Although the total thicknesses of the films vary a clear tendency of softening of the film with increasing amount of TIN,
Table I Film compositions Coating No.
TIN, thickness
DLC thickness
(nm)
(nm)
33 5
380 26 70
1 2 3
Number layers
of
Total thickness (pm)
1 IO 10
0.38 0.60 0.75
Channel Fig. 1. RBS spectrum
number
of coating
No. 2 on silicon.
J. Koskinen et al. JDiamond and Related Materials 4 ( 1995) 843-847 Table 2 Hardness Sample No.
1 2 3
and internal
845
stress results of the samples Vickers hardness on AISI440B
Knoop hardness on Si substrate
(20 g)
(20 g)
Internal compressive stress (GPa)
1300 i_ 140 890 + 100 1200 * 170
5000 * 400 (0.5 pm) 1500 + 20 _
3.6 i 0.5 2.1 +_0.2 (delaminated
content in the film. Also the internal stress decreases respectively. The coating No. 3 on silicon delaminated during a late stage of the deposition leaving fragments of the film on the substrate. It is noteworthy that during delamination large amounts of silicon substrate material was locally delaminated along with the multilayer film. Up to 5 pm deep cavities were found at the delaminated surface by using surface profilometer and SFM. SFM spectra taken from the silicon samples indicate the presence of rounded shape particles with a diameter of 2-3 pm, presumably titanium droplets from the arc cathode (Fig. 2). These droplets must have drifted as electrically charged massive particles with the curved magnetic field and/or bounced from the walls of the deposition chamber in order to reach the sample surface. Signs of film debris can be observed in contact with the droplet showing that the droplet has arrived to the
sample surface at an earlier stage of the film growth and film has delaminated from the droplet during the deposition. The results from the tribological tests show that the friction coefficient values in the end of the tests are lower for the multilayer coatings compared to single layer DLC coating as can be seen in Fig. 3. The friction behaviour of the single layer and multilayer coatings differ from each other. The single layer coating reached a stable value rather rapidly, whereas for the multilayer coatings the friction coefficient fluctuated rather much before stabilising (Fig. 4). On the other hand, both the wear of the coated discs and particularly the wear of the pins were increased for the multilayer coatings (Fig. 3). The wear of the discs was difficult to measure, because the wear was mostly polishing type of wear causing only removal of the outermost surface roughness of the surface. The difficul-
5000.0 nm Fig. 2. SFM micrograph
of a droplet
on Si)
on coating
No. 1 on silicon.
J. Koskinen et al.lDimlond and R&ted
846 2 _
0.2
Y-
Mutmicrls 4 al
i 1995) 843-847
I
-E 0.15
:$ J= ;
0.1
0.5
6 is .Lt
0.05
0 Sliding distance [m]
2000
Sliding distance (ml
2000
0.5
Fig. 3. The coefficient of friction the discs in pin-on-disc tests.
and the wear rates of the pins and
ties in wear determination also effected the scatter of the wear results of the coated discs. The scatter in the wear volumes typically varied from 15-25%, but for the single layer coating the scatter of the disc wear was as high as 120% and for the coating with thick a-C layers (No. 3) about 80%. Owing to difficulties in wear volume determination, the surface roughness profile was also determined from the surface profiles. This was accomplished as follows: The profile y(x) was smoothened by using a fast fourier transformation (FFT) filtering routine to define a baseline for the surface. This curve had a maximum curvature corresponding to a minimum radius of curvature of 2 m. Then this baseline was subtracted from the original profile and converted to the corresponding absolute value ly(x)l. This curve was finally smoothened with the above mentioned FFT filtering routine. Thus obtained curve is similar to a continuous R, value of the surface. By this kind of analysis it is possible to observe that the wear track was polished to a smoothness which is about 0.020 pm compared to the value of about 0.050 pm at the original coated surface. The smoothness of the wear tracks corresponded inversely to the wear rates of the coatings. The smoothness values from steady load (5 N) tests and from the load carrying capacity tests are presented in Fig. 5. The wear volumes in the load carrying capacity tests were generally higher than in the tests carried out with 5 N load. Only the pin sliding against the single layer coating (coating No. 1) showed less wear in the load
I
_..._.__~
~~
~__
Sliding distance [ml
2000
Fig. 4. The typical friction graphs of (a) the single layer coating No. 1. (b) the multilayer coating No. 2 and (c) the multilayer coating No. 3.
Coating 1
Fig. 5. The smoothness values of the coatings in the tests with steady 5 N load and in the tests with increasing 5-40 N load.
J. Koskinen et al./Diamond and Related Materials 4 (3995) 843-847
carrying capacity test. The friction coefficient typically dropped immediately after the load was increased, but showed an increasing trend when the test was carried out further with higher loads. The wear surfaces were examined by optical microscopy and by SEM. The pins sliding against the single layer coating showed some layer formation on the pin wear surface, whereas the pins sliding against the multilayer coatings showed merely metallic appearance. Also in front of the contact area of the pin there was only a slight sliding deposit, when pin was slid against the multilayer coatings. EDX analyses showed that some titanium was transferred to the pin wear surface from the multilayer coatings. In the wear tracks of the single layer coating (No. 1) traces of wear debris of the same type as found in the sliding deposit on the pin, was discovered. On the wear tracks of the multilayer coatings, some removal of coating was detected.
4. Discussion
847
contact surfaces [9]. A metal oxide transfer layer formed on the pin wear surface has low adhesion to the carbon film. The increase of the titanium at the contact layer may enhance the inertness of the transfer layer accounting the lower coefficient of friction of the TIN,-DLC multilayers. The very mild wear of hard DLC films is difficult to quantify, and could therefore be characterized as smoothing. In many tribological applications such low wear rates are desirable or necessary and thus a quantitative smoothing analysis can be informative in analyzing the wear data. In conclusion, it has been demonstrated that multilayer structure DLC films have excellent wear resistant and low friction properties. Since the hydrogen free DLC films are harder and more wear resistant compared to the hydrogenated films there seems to be more potential to modify the properties of these films yet possessing diamond-like properties comparable to those of hydrogenated films after the modifications.
and conclusion
The wear rates of the pin was increased to a great extent when TiN,-layers are included in the coating structure. Earlier we have reported the wear behaviour of steel pins sliding against TIN coating, which showed two orders of magnitude higher wear rates for the pins sliding against TIN coating, compared to pins sliding against a-C coatings [ 71. In this case the increased wear of the pins probably originate from the action of TiNlayers in the coatings. The wear resistance of the coating itself also seemed to be reduced with multilayer structure. The coefficient of friction was decreased with TiN,-layered structures. For the amorphous hydrogenated carbon coatings it has been discovered that Ti-containing coatings have lower friction coefficient [S]. For the multilayered coatings the free Ti included in the structure can also have a similar effect in these tests. Possibly the incorporation of Ti enhances the growth of a transfer layer on the pin resulting in lowered coefficient of friction. The coating surface structure is dominated by the droplet formations. These droplets can be carbon or titanium in the case of multilayer coatings. These droplet formations obviously have an effect on the coating behaviour, particularly on the friction behaviour. The Ti-droplets in the multilayer coatings can for example have a friction reducing effect, when combined to hard carbon coating, as described above. It has earlier been presented that the low coefficient of friction may be a result of the transfer layers at the
Acknowledgements The SEM and EDX analysis of Tom Gustafsson and the indentation tests made by Seija Kiema are gratefully acknowledged.
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