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Nanolayered gradient structures as an intermediate layer for diamond coatings
Dianlond and Related Materials, 3 ( 19941 1107 1111
1107
Nanolayered gradient structures as an intermediate layer for diamond coatings* J. Koskinen, J.-P. Hirvonen and S.-P. Hannula Technical Research Centre ~/'l,'inland, Metallurgy Laboratory, FIN-02151 Espoo (Finland)
K. Pischow Helsinki l_~niversity ~!['Technoh)gy, Laboratory (~]'Processing and Heat Treat ment ~!/ Materials, FI N-02150 Espoo (Finhmd)
H. Kattelus and I. Suni Technical Research Centre ( f Finland, Semiconductor Laboratory. FIN-02151 Espoo (Finland) ( Received September 24, 1993; accepted in final form November 23, 1993)
Abstract A molybdenum/titanium multilayered film was used as an intermediate film between a diamond-like carbon (DLC) coating and steel and silicon substrates. The elastic properties of the Mo/Ti intermediate layer were modified by using a gradient thickness of the titanium films. An attempt was thus made to obtain a smooth gradient of the elastic modulus from the substrate (steel, 210 GPa) to the DLC film (300 GPa). The Mo/Ti multilayer was deposited in a sputter deposition unit with two sputter targets and a rotating substrate holder. The hydrogen-free DLC coating was deposited with a pulsed arc discharge method. The Mo/Ti film was characterized by several sophisticated methods, such as cross-section scanning force microscopy, cross-section transmission electron microscopy, Rutherford backscattering spectroscopy and nano-indentation. The film has a layered structure with a columnar growth. Preliminary tests of the mechanical properties were performed by using a scratch test. On silicon the Mo/Ti film detached at a slightly higher critical load compared with the DLC film with no intermediate film. However, poor adhesion of the Mo/Ti film to the steel substrate prevented evaluation of the mechanical properties of the sample.
1. Introduction
Owing to the difference in elastic properties of diamond or diamond-like coatings and typical metallic substrates, stress concentrations are typically encountered at an interface. This results in problems in adhesion and endurance life of coatings subjected to mechanical loading, e.g. tribological or thermal stresses. Furthermore, other mechanical properties, such as fracture toughness, fracture elongation or stress relaxation, presumably play an important role at the interface. We can assume, a priori, that at the interface enhanced materials performance is required. Unexpected mechanical properties of materials having a nanometric microstructure have recently been reported [1]. Specifically, hardness and yield strength have been shown to obey a Hall-Petch type relationship down to a grain size of a few nanometres [2]. Furthermore, enhanced fracture toughness of typically brittle materials such as ceramics and even superplastic deformation at elevated temperatures has been reported [3]. Besides *Paper presented at Diamond Films '93, Albufeira, September 20 24, 1993.
0925 963594/$7.00 SSDI 0 9 2 5 - 9 6 3 5 ( 9 3 ) 0 0 1 6 8 - D
three-dimensional nanocrystalline materials with uniaxial grain structure there is a one-dimensional nanocrystalline structure which consists of a layered structure with the thickness of each individual layer on the nanometre scale. This kind of nanolayered structure has many properties in common with the uniaxial nanocrystalline materials. The tensile yield strength again obeys the Hall Petch type relation, i.e. the yield strength is proportional to the inverse of the square root of the grain size, in this case a wavelength of the layered structure [4]. For instance, an enhancement by almost a factor of seven of the yield strength compared with that calculated using a rule-of-mixture was observed in an AI-Cu system when the wavelength of the structure was decreased to 70 nm [5]. Typically, the adhesion of a coating on a base material is improved by a thin intermediate layer, such as titanium. A mismatch in the elastic properties can be accommodated by using a gradient structure with continuously varying concentrations of the constituents as an intermediate layer, provided that the elastic properties of the constituents have an appropriate difference. One method of producing an intermediate layer with both a gradient and nanometric characteristics is a
~ 1994
Elsevier Sequoia. All righ(s reserved
1108
J. Koskinen et al. / Nanolayered gradient structures for diamond coatings
layered structure with varying relative thicknesses of the sublayers of the constituents, and in which the sublayer thickness is on the nanometric scale. In this work we have applied this idea by producing a nanolayered gradient structure of molybdenum and titanium on a hardened steel substrate. These constituents were chosen because of an adequate difference in elastic constants. Molybdenum and titanium are also thermodynamically compatible, i.e. molybdenum and titanium do not form intermetallic compounds and this minimizes the possibility of a reaction between sublayers. In this paper we report a deposition and characterization of this kind of nanolayered gradient structure, including some results of preliminary adhesion and tribological tests.
2. Experimental details Coatings were deposited on hardened high-chromium tool steel, with a nominal composition of 1.55% C, 0.3% Si, 0.3% Mn, 12% Cr, 0.8% Mo and 0.8% V, and on silicon (100) wafers. The steel samples were mechanically polished and degreased in TEC, acetone and isopropanol using ultrasonic agitation. Deposits on the silicon substrates were used in Rutherford backscattering (RBS), nano-indentation and scratch tests. 2.1. Film deposition The Mo/Ti layers were deposited by using a dual target sputter unit. The deposition chamber was evacuated to a base pressure of 10 -4 Pa and backfilled with an argon flow of 10 sccm min -1 to a pressure of 1.33 Pa. Titanium was sputtered from a planar magnetron target with a diameter of 4 in at a d.c. power of 80 W at the beginning. Prior to the deposition of the nanolayered structure a 10 nm thick layer of titanium was deposited, because titanium is presumed to have good adhesion. Molybdenum was sputtered from a diode target with a diameter of 6 in at an r.f. power of 275 W. The composite Mo/Ti films were prepared by cyclically passing the samples beneath the two targets at such a rate and target power that the constituent Ti layers were 5 nm and the Mo layers 10 nm thick at the beginning of the deposition. This yields the local value of the Young's modulus of 210 GPa, which corresponds also to the Young's modulus of the steel substrate. During deposition the r.f. power was kept constant and the d.c. power reduced gradually to zero at the end of the deposition. This resulted in a gradual decrease of the thickness of the titanium sublayer. The value of the Young's modulus increases accordingly as a function of the increasing volume fraction of molybdenum reaching the value of pure molybdenum (330 GPa) at the surface. The total thickness of the nanolayered structure was
1.5 gm and the number of sublayers of each kind more than 100. Deposition of the DLC coatings was done in a separate coating unit and the samples were exposed to air before the DLC coating. The DLC films were deposited using a pulsed vacuum arc discharge method equipped with a curved magnetic field to deflect the plasma. The discharge arc is ignited by an ignition spark allowing a plasma discharge to take place between a graphite cathode and anode. A plasma plume is ejected through the orifice of the anode and is further deflected by a curvilinear magnetic field. The deflection of the plasma provides filtering of the particles originating from the cathode spots. The deposition rate was about 0.5 gm h -1. The ambient vacuum in the deposition chamber was about 3 x 10-6mbar. The substrate temperature during depositions was less than 60°C, as measured from the back of the substrate holder. The deposition method is described in more detail elsewhere [6]. Prior to deposition the samples were briefly sputteretched in an Ar ion beam (750 eV, 1 mA cm -2) for 1 rain by using a 3 cm Kaufman ion source. The thickness of the DLC film was 0.5 ~tm. DLC films on substrates without the Mo/Ti intermediate film were also deposited and used as reference. 2.2. Film characterization Several characterization methods were used to investigate both the intermediate Mo/Ti and the DLC films. RBS (4 MeV He +) was used to measure the composition in the films. The deposition rates in the sputtering system were calibrated by using the RBS results. 2.3. Sample preparation for X - S F M analysis The sample preparation is based on the hypothesis of producing first a completely smooth surface by polishing and secondly etching this to a topography reflecting the underlying bulk microstructure. The details of the sample preparation technique are published elsewhere [7,8]. The ion beam polishing was carried out by two TELETWIN ion guns with an Ar ÷ ion beam in two steps. First, two ion guns were applied at 10 keV ion energy and 3 mA ion current. The incident angle of the ion beams measured from the surface plane was 3° and the sample was rotated at 2rpm. The polishing time at this stage was about 60 minutes. Secondly, only one ion gun was used at 10 kV and 3 mA, and the sample was rocked _+45° measured from the normal to the sample surface. The aim of the second step is to push the smooth Si surface morphology over the Mo/Ti multilayer. Thus the ion bombardment occurred from the silicon wafer side. When an appropriately smooth surface was achieved on the area containing the Mo/Ti multilayer, the polishing was finished. A final polishing was applied by one ion gun working at 2.5 kV
J. Koskinen et al. / Nanolayered gradient structures for diamond coatings
and 1 mA for 10 min, while the sample was rocked the same way as before. The chamber pressure was at all times better than 2.5 x 10 -3 Pa. Ion beam etching was applied to develop a surface topography reflecting the bulk microstructure of the Mo/Ti multilayer. One ion gun was applied at 2 kV ion energy and 0.5 mA ion beam intensity. The incidence angle of the ion beam was 30 ° measured from the surface plane. The sample was rotated during the 10 rain etching time.
Energy 0.5
350
1109
(MeV) 1.0
I
1.5
i
I
300 250
,~ 2OO
\\
N 15o
E Z
100
50 C
2.4. Investigation methods SFM analyses were performed using a D M E 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-1 was used and the applied force was from 0.5 nN to 2.0 nN. X-TEM analysis of the samples was carried out by applying Phillips CM20 transmission electron microscopy. The mechanical properties of the films were evaluated by using scratch tests. The test apparatus has a spherical diamond tip (radius 200 p.m), which is drawn over the sample surface ( 1 4 m m rain -1) with continuously increasing normal force. The rate of loading was generally 40 N min -1 and 150 N min -~ for the harder substrates. The normal force tangential friction force and the acoustic emission from the diamond tip were constantly monitored. The condition of the diamond pin was inspected after each scratch using an optical microscope and the pin was cleaned if necessary. The normal load required to delaminate the DLC film (critical load) was measured by investigating the scratch with an optical microscope (200 x magnification). The wear scar was also inspected by using a surface profilometer (DEKTAK II). The hardness and the elastic modulus of the films were measured by using nano-indentation equipment. Elastic modulus values were obtained from the unloading slope.
3. Results The RBS measurements showed that the Mo/Ti film had a gradient composition of titanium with 0 at.% of titanium at the surface and 30-40 at.% of titanium at the substrate Mo/Ti film interface (Fig. 1). The layer structure of the Mo/Ti film could not be detected by the RBS spectrum. A cross-section scanning force microscope picture (Fig. 2) of the Mo/Ti multilayer shows a clear layered structure. Starting from the silicon substrate, on the right-hand side of the picture, a deeper etched (dark) layer having a thickness varying from 6 nm to 10 nm
0
~
Mo
v-
0
i
100
200
300
400
500
600
Channel Fig. 1. RBS m e a s u r e m e n t of the silicon s u b s t r a t e M o / T i DLC. The solid line is a s i m u l a t i o n with an a s s u m p t i o n of no Ti in the film. The d a s h e d line is a s i m u l a t i o n with a t i t a n i u m c o n c e n t r a t i o n which grows linearly from 0 to 40 a t . % from the surface t o w a r d s the substrate.
can be seen. This layer could well be the Ti layer deposited first on the silicon substrate, because the sputtering yield of titanium is slightly higher than that of silicon, the yields being 0.6 and 0.5, respectively. The following three layers are two fairly thick light layers separated by a very thin dark layer (lower) having a total thickness of about 40 nm. After that, the layers seem to be somewhat thinner, so that the thickness on a dark and white layer pair is about 15.2 nm on average, as can be seen in Fig. 2. This measurement is in good agreement with the nominal thickness of 15 nm consisting of 10 nm of Mo and 5 nm of Ti. However, the dark layer is very thin, and being deeper it has been sputtered faster than the light layer, which is higher. If the thicker layer is molybdenum it should be etched deeper owing to the higher sputtering yield of Mo compared with that of Ti, the yields being 0.9 and 0.6, respectively. Initially, the layers are very straight and parallel to the substrate surface, although after some 10-20 layers the appearance changes and the structure seems to be composed of vertical bunches having a slightly different layer orientation from each other. This appearance is typical of columnar growth. The lack of a sharp interface between the layers can be due to the sample preparation or due to a diffusion process during the deposition. The form and dimensions of the tip can also affect the results in the range where the measured features and dimensions of the tip are close to each other. A cross-sectional transmission electron micrograph of the Mo/Ti multilayer, having a thickness of 1620 nm, can be seen in Fig. 3, which shows also how a columnar structure starts to develop after about ten layers. The layer system starts with a 12 nm thick Ti layer and the first few layers are grown parallel to the substrate, with the thickness of the Mo layers being 12.5 nm (lighter lines) and the thickness of the Ti layer being 5.2 nm
J. Koskinen et al. / Nanolayered gradient structures for diamond coatings
1110 JPHBI
584.8
nm
Fig. 2. A 585 nm x 585 nm X-SFM scan of an Mo/Ti multilayer. The columnar growth can be seen on the left-hand side of the picture.
(darker lines). After a few layers the system breaks up to form a columnar structure. As the columnar structure develops, the layers become thinner, the Mo and Ti layers having thicknesses of about 10 nm and about 3 nm, respectively. The elastic moduli of the depositions were about 300 G P a for the DLC film and about 225 G P a on average for the Mo/Ti film. The corresponding hardness values were 30 G P a and 10 GPa, respectively. The indentation measurements did not indicate a gradient modulus for the Mo/Ti film. This could be due to the fact that the elastic and plastic deformations were probed at such a large depth that only an average value of the elastic modulus could be obtained. The scratch tests showed that the adhesion of the Mo/Ti film to the steel substrate was quite poor. The critical load L c was 2.5 N, whereas L c was 9 N for the reference sample without the Mo/Ti film. For deposition on silicon the critical loads were 14 N and 10.6 N for the sample with the Mo/Ti film and reference sample, respectively. On silicon a plastic deformation of the surface could
be observed at loads lower than the critical load for delamination.
4. Discussion and conclusion Poor adhesion of the Mo/Ti film to the steel was probably caused by insufficient precleaning of the steel substrate. This prevented adequate mechanical evaluation of the gradient interface structure. However, it was demonstrated that the hardness and elastic properties of the interface could be tailored using a multilayer film. The scratch tests on silicon samples indicated plastic behaviour of the Mo/Ti film. X-STM and X-TEM analyses clearly showed columnar growth in the Mo/Ti multilayer. After about 10 coating cycles the straight titanium lines become clearly curved. This columnar structure will evidently have an effect on those properties which should arise from the nanocrystalline structure of materials. It is of vital importance to find the deposition parameters leading to an undisturbed layered structure. By using a layered
d. Koskinen et al. / Nanolayered gradient structures jot diamond coatings
1111
structure, w i t h o n e a m o r p h o u s c o m p o n e n t , e.g. M o / S i C , it s h o u l d be p o s s i b l e to g r o w a n a n o c r y s t a l l i n e film,
Acknowledgments T h e T E M analysis was p e r f o r m e d by Dr. P.B. B a r n a at the R e s e a r c h I n s t i t u t e for T e c h n i c a l Physics R I T P , B u d a p e s t . N a n o - i n d e n t a t i o n m e a s u r e m e n t s w e r e perf o r m e d by Dr. K.J. M a c k f r o m N a n o I n s t r u m e n t s , Inc.
References
Fig. 3. X-TEM image of the Mo/Ti multilayer on silicon substrate.
1 F.H. Froes and C. Suryanarayana, J. Met., June { 1989) 12. 2 J.R. Weertman, M. Niedzielka and C. Youngdahl, in M. Nastasi, D.M. Parkin and H. Gleiter (eds.), Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures, Kluwer, Dordrecht, 1993, p. 241. 3 R. Lappalainen and R. Raj, in M. Nastasi, D.M. Parkin and H. Gleiter (eds.), Mechanical Properties and DeJormation Behavior o[ Materials Having Ultra-Fine Microstructures, Kluwer , Dordrecht, 1993, p. 401. 4 R.F. Bunshah, R. Nimmagadda, H.J. Doerr, B.A. Movchan, N.I. Grechanuk and E.V. Dabizha, Thin Solid Films, 72 (1980) 161. 5 S.L. Lehoczky, Phys. Rev. Lett., 41 {1978) 1814. 6 J. Koskinen, U. Ehrnsten, A. Mahiout, R. Lahtinen, J.-P. Hirvonen and S.-P. Hannula, Surf. Coat. Technol., 62 (1993) 356. 7 K.A. Pischow, E.O. Ristolainen and A.S. Korhonen, Proc. A M P T "93, Dublin, 1993, pp. 1887-1895. 8 K.A. Pischow, A.S. Korhonen, M. Adamik and P.B. Barna, presented at STM-93: J. Vac. Sei. B, in press.
1107
Nanolayered gradient structures as an intermediate layer for diamond coatings* J. Koskinen, J.-P. Hirvonen and S.-P. Hannula Technical Research Centre ~/'l,'inland, Metallurgy Laboratory, FIN-02151 Espoo (Finland)
K. Pischow Helsinki l_~niversity ~!['Technoh)gy, Laboratory (~]'Processing and Heat Treat ment ~!/ Materials, FI N-02150 Espoo (Finhmd)
H. Kattelus and I. Suni Technical Research Centre ( f Finland, Semiconductor Laboratory. FIN-02151 Espoo (Finland) ( Received September 24, 1993; accepted in final form November 23, 1993)
Abstract A molybdenum/titanium multilayered film was used as an intermediate film between a diamond-like carbon (DLC) coating and steel and silicon substrates. The elastic properties of the Mo/Ti intermediate layer were modified by using a gradient thickness of the titanium films. An attempt was thus made to obtain a smooth gradient of the elastic modulus from the substrate (steel, 210 GPa) to the DLC film (300 GPa). The Mo/Ti multilayer was deposited in a sputter deposition unit with two sputter targets and a rotating substrate holder. The hydrogen-free DLC coating was deposited with a pulsed arc discharge method. The Mo/Ti film was characterized by several sophisticated methods, such as cross-section scanning force microscopy, cross-section transmission electron microscopy, Rutherford backscattering spectroscopy and nano-indentation. The film has a layered structure with a columnar growth. Preliminary tests of the mechanical properties were performed by using a scratch test. On silicon the Mo/Ti film detached at a slightly higher critical load compared with the DLC film with no intermediate film. However, poor adhesion of the Mo/Ti film to the steel substrate prevented evaluation of the mechanical properties of the sample.
1. Introduction
Owing to the difference in elastic properties of diamond or diamond-like coatings and typical metallic substrates, stress concentrations are typically encountered at an interface. This results in problems in adhesion and endurance life of coatings subjected to mechanical loading, e.g. tribological or thermal stresses. Furthermore, other mechanical properties, such as fracture toughness, fracture elongation or stress relaxation, presumably play an important role at the interface. We can assume, a priori, that at the interface enhanced materials performance is required. Unexpected mechanical properties of materials having a nanometric microstructure have recently been reported [1]. Specifically, hardness and yield strength have been shown to obey a Hall-Petch type relationship down to a grain size of a few nanometres [2]. Furthermore, enhanced fracture toughness of typically brittle materials such as ceramics and even superplastic deformation at elevated temperatures has been reported [3]. Besides *Paper presented at Diamond Films '93, Albufeira, September 20 24, 1993.
0925 963594/$7.00 SSDI 0 9 2 5 - 9 6 3 5 ( 9 3 ) 0 0 1 6 8 - D
three-dimensional nanocrystalline materials with uniaxial grain structure there is a one-dimensional nanocrystalline structure which consists of a layered structure with the thickness of each individual layer on the nanometre scale. This kind of nanolayered structure has many properties in common with the uniaxial nanocrystalline materials. The tensile yield strength again obeys the Hall Petch type relation, i.e. the yield strength is proportional to the inverse of the square root of the grain size, in this case a wavelength of the layered structure [4]. For instance, an enhancement by almost a factor of seven of the yield strength compared with that calculated using a rule-of-mixture was observed in an AI-Cu system when the wavelength of the structure was decreased to 70 nm [5]. Typically, the adhesion of a coating on a base material is improved by a thin intermediate layer, such as titanium. A mismatch in the elastic properties can be accommodated by using a gradient structure with continuously varying concentrations of the constituents as an intermediate layer, provided that the elastic properties of the constituents have an appropriate difference. One method of producing an intermediate layer with both a gradient and nanometric characteristics is a
~ 1994
Elsevier Sequoia. All righ(s reserved
1108
J. Koskinen et al. / Nanolayered gradient structures for diamond coatings
layered structure with varying relative thicknesses of the sublayers of the constituents, and in which the sublayer thickness is on the nanometric scale. In this work we have applied this idea by producing a nanolayered gradient structure of molybdenum and titanium on a hardened steel substrate. These constituents were chosen because of an adequate difference in elastic constants. Molybdenum and titanium are also thermodynamically compatible, i.e. molybdenum and titanium do not form intermetallic compounds and this minimizes the possibility of a reaction between sublayers. In this paper we report a deposition and characterization of this kind of nanolayered gradient structure, including some results of preliminary adhesion and tribological tests.
2. Experimental details Coatings were deposited on hardened high-chromium tool steel, with a nominal composition of 1.55% C, 0.3% Si, 0.3% Mn, 12% Cr, 0.8% Mo and 0.8% V, and on silicon (100) wafers. The steel samples were mechanically polished and degreased in TEC, acetone and isopropanol using ultrasonic agitation. Deposits on the silicon substrates were used in Rutherford backscattering (RBS), nano-indentation and scratch tests. 2.1. Film deposition The Mo/Ti layers were deposited by using a dual target sputter unit. The deposition chamber was evacuated to a base pressure of 10 -4 Pa and backfilled with an argon flow of 10 sccm min -1 to a pressure of 1.33 Pa. Titanium was sputtered from a planar magnetron target with a diameter of 4 in at a d.c. power of 80 W at the beginning. Prior to the deposition of the nanolayered structure a 10 nm thick layer of titanium was deposited, because titanium is presumed to have good adhesion. Molybdenum was sputtered from a diode target with a diameter of 6 in at an r.f. power of 275 W. The composite Mo/Ti films were prepared by cyclically passing the samples beneath the two targets at such a rate and target power that the constituent Ti layers were 5 nm and the Mo layers 10 nm thick at the beginning of the deposition. This yields the local value of the Young's modulus of 210 GPa, which corresponds also to the Young's modulus of the steel substrate. During deposition the r.f. power was kept constant and the d.c. power reduced gradually to zero at the end of the deposition. This resulted in a gradual decrease of the thickness of the titanium sublayer. The value of the Young's modulus increases accordingly as a function of the increasing volume fraction of molybdenum reaching the value of pure molybdenum (330 GPa) at the surface. The total thickness of the nanolayered structure was
1.5 gm and the number of sublayers of each kind more than 100. Deposition of the DLC coatings was done in a separate coating unit and the samples were exposed to air before the DLC coating. The DLC films were deposited using a pulsed vacuum arc discharge method equipped with a curved magnetic field to deflect the plasma. The discharge arc is ignited by an ignition spark allowing a plasma discharge to take place between a graphite cathode and anode. A plasma plume is ejected through the orifice of the anode and is further deflected by a curvilinear magnetic field. The deflection of the plasma provides filtering of the particles originating from the cathode spots. The deposition rate was about 0.5 gm h -1. The ambient vacuum in the deposition chamber was about 3 x 10-6mbar. The substrate temperature during depositions was less than 60°C, as measured from the back of the substrate holder. The deposition method is described in more detail elsewhere [6]. Prior to deposition the samples were briefly sputteretched in an Ar ion beam (750 eV, 1 mA cm -2) for 1 rain by using a 3 cm Kaufman ion source. The thickness of the DLC film was 0.5 ~tm. DLC films on substrates without the Mo/Ti intermediate film were also deposited and used as reference. 2.2. Film characterization Several characterization methods were used to investigate both the intermediate Mo/Ti and the DLC films. RBS (4 MeV He +) was used to measure the composition in the films. The deposition rates in the sputtering system were calibrated by using the RBS results. 2.3. Sample preparation for X - S F M analysis The sample preparation is based on the hypothesis of producing first a completely smooth surface by polishing and secondly etching this to a topography reflecting the underlying bulk microstructure. The details of the sample preparation technique are published elsewhere [7,8]. The ion beam polishing was carried out by two TELETWIN ion guns with an Ar ÷ ion beam in two steps. First, two ion guns were applied at 10 keV ion energy and 3 mA ion current. The incident angle of the ion beams measured from the surface plane was 3° and the sample was rotated at 2rpm. The polishing time at this stage was about 60 minutes. Secondly, only one ion gun was used at 10 kV and 3 mA, and the sample was rocked _+45° measured from the normal to the sample surface. The aim of the second step is to push the smooth Si surface morphology over the Mo/Ti multilayer. Thus the ion bombardment occurred from the silicon wafer side. When an appropriately smooth surface was achieved on the area containing the Mo/Ti multilayer, the polishing was finished. A final polishing was applied by one ion gun working at 2.5 kV
J. Koskinen et al. / Nanolayered gradient structures for diamond coatings
and 1 mA for 10 min, while the sample was rocked the same way as before. The chamber pressure was at all times better than 2.5 x 10 -3 Pa. Ion beam etching was applied to develop a surface topography reflecting the bulk microstructure of the Mo/Ti multilayer. One ion gun was applied at 2 kV ion energy and 0.5 mA ion beam intensity. The incidence angle of the ion beam was 30 ° measured from the surface plane. The sample was rotated during the 10 rain etching time.
Energy 0.5
350
1109
(MeV) 1.0
I
1.5
i
I
300 250
,~ 2OO
\\
N 15o
E Z
100
50 C
2.4. Investigation methods SFM analyses were performed using a D M E 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-1 was used and the applied force was from 0.5 nN to 2.0 nN. X-TEM analysis of the samples was carried out by applying Phillips CM20 transmission electron microscopy. The mechanical properties of the films were evaluated by using scratch tests. The test apparatus has a spherical diamond tip (radius 200 p.m), which is drawn over the sample surface ( 1 4 m m rain -1) with continuously increasing normal force. The rate of loading was generally 40 N min -1 and 150 N min -~ for the harder substrates. The normal force tangential friction force and the acoustic emission from the diamond tip were constantly monitored. The condition of the diamond pin was inspected after each scratch using an optical microscope and the pin was cleaned if necessary. The normal load required to delaminate the DLC film (critical load) was measured by investigating the scratch with an optical microscope (200 x magnification). The wear scar was also inspected by using a surface profilometer (DEKTAK II). The hardness and the elastic modulus of the films were measured by using nano-indentation equipment. Elastic modulus values were obtained from the unloading slope.
3. Results The RBS measurements showed that the Mo/Ti film had a gradient composition of titanium with 0 at.% of titanium at the surface and 30-40 at.% of titanium at the substrate Mo/Ti film interface (Fig. 1). The layer structure of the Mo/Ti film could not be detected by the RBS spectrum. A cross-section scanning force microscope picture (Fig. 2) of the Mo/Ti multilayer shows a clear layered structure. Starting from the silicon substrate, on the right-hand side of the picture, a deeper etched (dark) layer having a thickness varying from 6 nm to 10 nm
0
~
Mo
v-
0
i
100
200
300
400
500
600
Channel Fig. 1. RBS m e a s u r e m e n t of the silicon s u b s t r a t e M o / T i DLC. The solid line is a s i m u l a t i o n with an a s s u m p t i o n of no Ti in the film. The d a s h e d line is a s i m u l a t i o n with a t i t a n i u m c o n c e n t r a t i o n which grows linearly from 0 to 40 a t . % from the surface t o w a r d s the substrate.
can be seen. This layer could well be the Ti layer deposited first on the silicon substrate, because the sputtering yield of titanium is slightly higher than that of silicon, the yields being 0.6 and 0.5, respectively. The following three layers are two fairly thick light layers separated by a very thin dark layer (lower) having a total thickness of about 40 nm. After that, the layers seem to be somewhat thinner, so that the thickness on a dark and white layer pair is about 15.2 nm on average, as can be seen in Fig. 2. This measurement is in good agreement with the nominal thickness of 15 nm consisting of 10 nm of Mo and 5 nm of Ti. However, the dark layer is very thin, and being deeper it has been sputtered faster than the light layer, which is higher. If the thicker layer is molybdenum it should be etched deeper owing to the higher sputtering yield of Mo compared with that of Ti, the yields being 0.9 and 0.6, respectively. Initially, the layers are very straight and parallel to the substrate surface, although after some 10-20 layers the appearance changes and the structure seems to be composed of vertical bunches having a slightly different layer orientation from each other. This appearance is typical of columnar growth. The lack of a sharp interface between the layers can be due to the sample preparation or due to a diffusion process during the deposition. The form and dimensions of the tip can also affect the results in the range where the measured features and dimensions of the tip are close to each other. A cross-sectional transmission electron micrograph of the Mo/Ti multilayer, having a thickness of 1620 nm, can be seen in Fig. 3, which shows also how a columnar structure starts to develop after about ten layers. The layer system starts with a 12 nm thick Ti layer and the first few layers are grown parallel to the substrate, with the thickness of the Mo layers being 12.5 nm (lighter lines) and the thickness of the Ti layer being 5.2 nm
J. Koskinen et al. / Nanolayered gradient structures for diamond coatings
1110 JPHBI
584.8
nm
Fig. 2. A 585 nm x 585 nm X-SFM scan of an Mo/Ti multilayer. The columnar growth can be seen on the left-hand side of the picture.
(darker lines). After a few layers the system breaks up to form a columnar structure. As the columnar structure develops, the layers become thinner, the Mo and Ti layers having thicknesses of about 10 nm and about 3 nm, respectively. The elastic moduli of the depositions were about 300 G P a for the DLC film and about 225 G P a on average for the Mo/Ti film. The corresponding hardness values were 30 G P a and 10 GPa, respectively. The indentation measurements did not indicate a gradient modulus for the Mo/Ti film. This could be due to the fact that the elastic and plastic deformations were probed at such a large depth that only an average value of the elastic modulus could be obtained. The scratch tests showed that the adhesion of the Mo/Ti film to the steel substrate was quite poor. The critical load L c was 2.5 N, whereas L c was 9 N for the reference sample without the Mo/Ti film. For deposition on silicon the critical loads were 14 N and 10.6 N for the sample with the Mo/Ti film and reference sample, respectively. On silicon a plastic deformation of the surface could
be observed at loads lower than the critical load for delamination.
4. Discussion and conclusion Poor adhesion of the Mo/Ti film to the steel was probably caused by insufficient precleaning of the steel substrate. This prevented adequate mechanical evaluation of the gradient interface structure. However, it was demonstrated that the hardness and elastic properties of the interface could be tailored using a multilayer film. The scratch tests on silicon samples indicated plastic behaviour of the Mo/Ti film. X-STM and X-TEM analyses clearly showed columnar growth in the Mo/Ti multilayer. After about 10 coating cycles the straight titanium lines become clearly curved. This columnar structure will evidently have an effect on those properties which should arise from the nanocrystalline structure of materials. It is of vital importance to find the deposition parameters leading to an undisturbed layered structure. By using a layered
d. Koskinen et al. / Nanolayered gradient structures jot diamond coatings
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structure, w i t h o n e a m o r p h o u s c o m p o n e n t , e.g. M o / S i C , it s h o u l d be p o s s i b l e to g r o w a n a n o c r y s t a l l i n e film,
Acknowledgments T h e T E M analysis was p e r f o r m e d by Dr. P.B. B a r n a at the R e s e a r c h I n s t i t u t e for T e c h n i c a l Physics R I T P , B u d a p e s t . N a n o - i n d e n t a t i o n m e a s u r e m e n t s w e r e perf o r m e d by Dr. K.J. M a c k f r o m N a n o I n s t r u m e n t s , Inc.
References
Fig. 3. X-TEM image of the Mo/Ti multilayer on silicon substrate.
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