- Email: [email protected]
The modification of mechanical properties and adhesion of boron carbide sputtered films by ion implantation
2s
Nuclear Instruments
and Methods in Physics Research B 1 I7 (I 996) 408-414
__ k!Wl
__
B
Beam Interactions with Materials 6 Atoms
!!!I!!! EISEVIER
The modification of mechanical properties and adhesion of boron carbide sputtered films by ion implantation C.I. Chiang z’, 0. Meyer a’* , Rui M.C. da Silva b a Institut,fir
Nukleure
Frstkiirprrphy.tik.
b Depurtumento
Forschun~sxntrum
de Fisicu-ITN.
Kwlsruhe.
Estruda Nutimud
Postjbch
36 40. D-76021
Kdwuhe.
Germuny
IO. P-2685 Sucuuem, Portugul
Received 17 May 1996; revised form received 28 June I996 Abstract Mechanical and tribological properties of boron carbide films have been modified by nitrogen ion implantation in the near surface region. For nanocrystalline boron carbide films, hardness, friction coefficient and linear wear intensity were reduced from 3000 to 2800, from 0.18 to 0.09 and from 5.0 X lOma to 2.7 X IO-*, respectively. For amorphous boron carbide films the hardness and the linear wear intensity increased while the friction coefficient decreased. These modifications were attributed to the creation of defects and to the formation of boron nitride phases. The interface between boron carbide films and Si or WC substrates has also been modified by ion beam mixing and MeV-ion irradiation. Adhesion of B,C films to the substrate was assessed by measurements of critical loads in scratch tests. For films on Si-substrates the critical load increased from 10 N to I4 N after ion beam mixing and from IO N to 20 N after MeV-ion irradiation.
1. Introduction Boron carbide films are known to be superhard protective coatings owing to high hardness and wear resistance. However, low fracture and oxidation resistance are two main restrictions for the application of boron carbide films [1,2]. One of the possible ways for improving the fracture and oxidation resistance is to modify the film surface by ion beams. Ion implantation will on one hand create defects which change the stress state accordingly, and may hinder the crack propagation on the other hand new tough and oxidation resistant compounds may be formed, such as cubic boron nitride (c-BN). However as the synthesis of c-BN, both in bulk and thin film form, is found to be difficult in comparison with the synthesis of boron carbide, due to the competitive growth of hexagonal boron nitride (h-BN), an alternative method is the growth of c-BN on the surface of bulk boron carbide [3,4]. The same idea was applied to sputtered boron carbide films which were then implanted with nitrogen ions. A broad distribution of implanted ions was used by superposition of several combinations of ion energy and flux in order to get a more homogeneous profile. The modifications of mechanical and tribological properties will be reported here.
’ Corresponding author. Fax: + 49-7247-824624; email: [email protected]. ’ Permanent address: Materials R&D Center, CSIST, Taiwan, R.O.C. 0168-583X/96/$15.00 Copyright PI1 SO 168-583X(96)00364-3
Another major drawback of boron carbide films (and also of most of the ceramic hard coatings) is their low adhesion, especially on metallic or semimetallic substrates. Implantation techniques have been successfully applied for increasing the adhesion of metallic and polymer films on metallic substrates. In general, there are two methods based on different mechanisms for increasing adhesion, namely ion beam mixing [5] and MeV-ion irradiation [6]. In ion beam mixing energetic ions are implanted in the neighborhood of the interface between film and substrate, and dissipate their energy in the form of energy spikes, part of it is transferred to the neighboring film and substrate atoms. Both the binding strength and the number of bonds are then increased through ion induced diffusion (physically) and/or through the creation of new chemical bonds (chemically) in ion activated reactions. On the other hand MeV ion irradiation of the interface with light particles, such as electron and He+, with energies of several MeV, has commonly been called MeV-ion irradiation. For such light particles the corresponding projected ranges are in the order of several tens of pm which is thicker than most of the thin films. The energy loss in the interface region is mainly electronic. Hence the excitation of binding electrons is considered responsible for the enhancement of binding strength [6]. However, the exact mechanism is not yet fully understood. We applied both methods to study the influence of ion bombardment on adhesion of boron carbide films. Improvements of adhesion could be clearly observed.
0 1996 Elsevier Science B.V. All rights reserved
C.I. Chiang et al./Nucl.
Instr. and Meth. in Phys. Res. B I17
f 1996) 408-414
409
2. Experimental 2.1. Film preparation Boron carbide films were deposited by rf magnetron sputtering and analysed following the same procedure stated in our previous work [7]. A planar diode arrangement was used. The target consisted of hot pressed boron carbide disks. Both single crystal silicon of [OOI] orientation and polycrystalline WC-6%Co were used as substrates. The surfaces of the substrates were metallographitally polished and cleaned with alcohol immediately before deposition. Substrate temperature was controlled by a PID temperature controller by varying the current through the substrate holder made of tantalum. The base vacuum of the sputtering chamber was 5 X 10e6 mbar. Nanocrystalline boron carbide films were prepared using 200 W rf power at a substrate temperature of 1100°C and an argon pressure of 0.04 mbar. Amorphous boron carbide films were prepared at substrate temperatures of 500°C with the same values of rf power and argon pressure. 2.2. Ion beam modi$cation Our experiments of ion beam modification are classified as: (a) modification of mechanical and tribological properties of the film surface by ion implantation and (b) modification of adhesion of the film by ion beam mixing and MeV-ion irradiation. Implantations in the surface region at room temperature were conducted in ITN (Instituto Technologico Nuclear), Portugal. Implantation at higher temperatures together with the experiments for the modification of the interface were conducted in INFP-FZK (Institut fir Nukleare FestkGrperphysik, Forschungszentrum KdrIsruhe), Germany. 2.2.1. Ion implantation Both nanocrystalline and amorphous boron carbide films, 4 pm thick, were implanted with nitrogen ions N+ at room temperature and at 600°C. The implantation parameters are listed in Table 1. The distribution of implanted nitrogen can be calculated on the basis of a superposition principle [8]. The result is shown in Fig. 1. 2.2.2. Ion beam mixing In ion beam mixing experiments
N+ ions of 300 keV
0
200
400
Depth (nm)
Fig. I. Theoretical depth distribution of implanted nitrogen ions for boron carbide using the energy and flux combinations listed in Table 1.
energy were implanted to a total fluence of 2 X 10” ions/cm’. The maximum allowable energy of our equipment is 300 keV, which corresponds to a projected range of 0.4 pm. However films with a minimum thickness of 4 pm were usually required for applications such as wear resistance coating. In order to meet this requirement and in the energy limitation of the equipment, boron carbide films were first grown to a thickness of approximately 0.5 pm and then implanted at the film/substrate interface with 300 keV N+ ions, at room temperature. Following the implantation more boron carbide was deposited to a total thickness of 4 pm thick. 2.2.3. MeV-ion irradiation For MeV-ion irradiation samples were directly irradiated by 2 MeV He+ ions at room temperature to a total fluence of I x lOI ions/cm*. The beam diameter was 1 mm. The beam was scanned over an area of 10 X 10 mm* using a beam scanner, which was operated by varying the voltage of the deflection magnet in both x- and y-directions with fixed frequency. In order avoid the differences between batches of as-deposited films, only one half of the surface was implanted, the other half was covered by a shutter during ion bombardment. All the .‘ollowing analyses were conducted both on the implanted half and on the as-deposited half for comparison. 2.3. Composition.
thickness and surface roughness of the
jfilm Table 1 Implantation profile for 10% nitrogen in boron carbide Ion energy (IieV)
Ion flux (lOI ions/cm’)
50 100 180 300
7.01 10.6 13.6 16.0
Composition and thicknesses of the films, as well as the distribution of implanted species were determined by Rutherford Backscattering Spectrometry (RBS) using 2 MeV He+ ions. Glancing Angle Incidence X-ray Diffraction (GIXD) [9] at an angle of incidence of 3” was used to analyze the crystalline structure of the film. Phases were identified according to JCPDS (Joint Committee for Powder Diffraction Standard) 35-798. Binding states in the
C.I. Chiang et al./Nucl. Instr. and Meth. in Phys. Res. B 117 (1996) 408-414
410
Table 2 Core level bonds in eV for various phases
Substance
B Is
c Is
N Is
Refs.
B,C
186.6
281.8
_
h-BN c-8N B-B
190.2 190.7 187.9
_ _ _
398.2 398.7 _ _
[lOI [Ill 11 II
c-c N-N P-C,N,
_ _ _
284.4 _
404
286
400.1
mN and 6 rpm, respectively. By measuring the tangential force on the pin and the depth of the wear track after a certain number of revolutions, the friction coefficient f and the linear wear intensity I can be calculated according to:
[121 [I31 [I41 [I51
near surface region were obtained by X-ray Photoelectron Spectrometry (XPS) using Mg Km excitation. The core levels of the corresponding bonds of boron carbide, boron nitride, carbon nitride and elements B, C, N are listed in Table 2, [lo-IS]. The surface roughness was determined using an Atomic Force Microscope (AFM). The average area roughness for a test area of 5 X 5 mm2 was adopted following the definition listed in DIN 4760. 2.4. Hardness test Hardness of the films were measured by two processes: microvicker’s and nanoindentation process [16]. By microvicker’s test a Vicker’s pyramid indenter was used. The hardness value was calculated from the average value of the diagonals of the indentation. The hardness value will be expressed as HV 0.2 for a testing load of 0.2 kp according to DIN 50 133. If the film is harder than the substrate and the indentation depth is deeper than 1/IO of the film thickness, the measured hardness value is affected by the hardness of the substrate and will be called integrated hardness (according to IS0 45 16-1980). In order to measure the hardness in the implanted region, an indentation depth dependent measurement with indentation depth as small as 100 nm was used, called nanoindentation [ 161.In such nanoindentation experiments the dependence of indentation force (P) on indentation depth (h) for a complete loading and unloading cycle can be obtained. A Berkovich pyramid with a face angle of 65.3” was used as an indenter. The measured hardness value H in N/mm2, was calculated for each force step using the plastic indentation (penetration) depth (h,). The plastic indentation depth is equal to the total penetration minus the elastic penetration derived from the slope of the unload curve [ 161. 2.5. Friction and wear test The friction coefficient and linear wear intensity were determined by micropin-on-disk test. A rectangular pin was forced to contact a rotating disk under dry friction conditions. The normal force on the pin and rotational speed of the disk were kept constant with values of 200
where F,. F,, are the tangential and normal forces, respectively, t is the total depth of the wear track, and s is the total length traveled by the pin. The tangential and normal forces were measured by strain gauge with a resolution of 5 mN. A rectangular boron carbide bar of dimensions 1 X 1 X 15 mm3 and Vicker’s hardness (HV 0.2) of 2800 was used as pin. The contact surface of the pin was polished. The depth of the wear track after 2400 revolutions was measured by AFM. 2.4. Scratch test For the scratch test experiments a Rockwell diamond tip was forced into a specimen moving with a translational speed of 10 mm/min. The force at the point of contact was increased a rate of 150 N/min from zero until the detachment of the film initiated as detected by acoustic emission. This load is also known as the critical load (L,) and provides a measure of the adhesion. Only nanocrystalline films were tested. For each half of the specimen as deposited and implanted, three tests were conducted and the average value was adopted.
3. Results and discussion 3.1. Modification
in the surface region
Fig. 2 shows RBS spectra of a boron carbide film before and after implantation with nitrogen ions at room temperature. The B/C ratio as determined from the relative yield of He ions scattered by boron and carbon atoms is 3.8. A comparison of the measured projected range of the implanted nitrogen with the theoretically calculated projected range is listed in Table 3. There was no observable change of the RBS spectra for films implanted at 600°C. It can hence be. concluded that the measured projected range is very close to the theoretically predicted value up to a target temperature of 600°C. Since the energy resolution is about 20 keV for our RBS system, the depth resolution is 40 nm. We may assert that the thermally activated diffusion of implanted ions is less than 40 nm up to a target temperature of 600°C. This implies that the implanted nitrogen is either bound or trapped in the boron carbide lattice. The projected range reaches about 500 nm, but the depth distribution of the implanted species is not uniform. This could be improved however by using more
C.I. Chiang
et ul./Nucl.
41 I
Instr. and Meth. in Phys. Res. B 117 (1996) 408-414
2000 implantation
5 u E aJ jr:
1;
1000
2b
25
3b
3’5
Lb
2 Theta Fig. 3. GIXD pattern of an as-deposited nanocrysralline boron carbide film. The arrows indicate the positions of tbe diffraction peaks as listed in JCPDS 36-798. The subscript r stands for rhombohedral notation.
0
L
I 50
250
150 Channel
Fig. 2. RBS spectra of a nanocrystalline boron carbide film: (a) before (solid curve) and (b) after (dashed curve) implantation nitrogen ions at room temperature.
with
than four energy-flux combinations in order to achieve a more uniform profile. The as- deposited films are nearly stoichiometric. The compositional change, if any, is also less than the sensitivity limit for nitrogen in boron nitride. The GIXD pattern from an as deposited nanocrystalline boron carbide film is shown in Fig. 3. The two major diffraction peaks were identified as (11 I>, and (21 I), (r: rhombohedral) respectively. This shows that films were grown with a preferred (hll), orientation, for which the basal plane of the hexagonal lattice is nearly parallel to the substrate surface. The diffraction maxima all shifted to lower Bragg angles in relation to the JCPDS listed values, indicating the existance of compressive residual stresses in the basal plane. These stresses will be relaxed at higher target temperatures since no shifts were observed under these conditions. The exact reason for this kind of preferential growth is not clear. We suggest that this is a consequence of excess compressive stress parallel to the
Table 3 Projected range CR,) and standard deviation of the projected range (AR,) for nitrogen carbide films Ion energy (keV)
50
100 180 300
ions implanted Theoretical
into nanocrystalline
value
boron
Measured value
R, (nm)
AR, (nm)
R, (nm)
AR, (nm)
108 211 352 526
24 36 40 47
120 227 378 532
30 40 4.5 55
basal plane. The stress is relaxed at higher deposition temperatures. On the other hand, the results of XPS analysis show a very definite change of the binding state in the near surface region. Fig. 4 depicts the photoelectron spectrum obtained through the 1.25 keV photon irradiation. The binding energies of B Is, N is and C Is-line for B& h-BN and C,N, are identified with arrows in Fig. 4 according to Table 2. Owing to the presence of various bonds with closely neighbouring binding energies, only the superimposed curve can be seen. As shown by the solid curve, ihe surface for films before implantation is composed of free boron and carbon atoms together with boron carbide but no trace of boron nitride. After nitrogen implantation (dashed curve>, a peak appears at the expected position for the N 1s photopeak, i.e. new chemical bonds have been formed in the implanted region. The B Is photopeak shifts to higher binding energy, owing to the presence of these new bonds. Whether the new bond is boron nitride, carbon nitride, ternary C-B-N compounds or a mixture of these can not be determined because of the width of the photopeak. Measurements of Vicker’s hardness (HV 0.2) performed in the as- deposited nanocrystalline and amorphous boron carbide films on silicon substrates yielded the values of 2900 and 2400 respectively. Because the indentation depth is around 1.6 pm, which is larger than l/l0 of the film thickness, the influence of the substrate is inevitable. Thus, in order to overcome this influence, and to extract the effect of ion implantation, nanoindentation experiments were performed. The results are displayed in Fig. 5 where the dependence of hardness ( H) on the plastic penetration depth (h,) of an amorphous boron carbide film is shown before and after implantations. The initial stage of this curve (for h, < 200 nm) is sensitive to the roundness of the indenter tip and hence difficult to interpret. The hardness of the as-deposited films lies between 2400 and 2500 N/mm2 and is nearly independent of h, for h, larger than 400 nm while for the implanted film it is 3100 N/mm* for
C.I.
412
Chicmg
er d./Nucl.
Insrr.
and Meth.
in Phys. Rex. B I17
(1996)
408-414
0.4
..-
42
:, lmplonte~ i._,- ._.. ___~__..________._____ _______.___.._ 0 E 0
2000
1000 Time
(~1
Fig. 6. Friction coefficients (f‘) as a function of testing time of asdeposited (solid curve) and implanted (dashed curve) nanocrysWine boron carbide films.
413
399
E, kv) Fig. 4. (a) B Is, (b) N Is and (c) C Is photo electron energy spectrum from the 1.25 keV photon irradiation (MgK (1) of nanocrystalline boron carbide ftlm surfaces before (solid curve) and after (dashed curve) nitrogen implantation,
-E250C E z‘ ;
1500
400
800
1200
hp (nm)
Fig. 5. Hardness (H in N/mm’) of amorphous boron nitride films by nanoindentation: (a) before (solid curve) and (b) after (dashed curve) nitrogen ion implantation as a function of the plastic penetration depth (h, in nm).
h, = 400 nm and decreases with increasing h,. The hardness of implanted films is in general 15% - 20% higher than that of as-deposited films. In contrast the hardness of nanocrystalfine boron carbide films decreases 10% - 15% (from 3 100 to 2900 N/mm’) after implantation and the hardness of implanted films increases with indentation depth. According to the XPS results presented above, there is a large possibility of the formation of boron nitride or B-N-C compounds. It appears that this new phase, formed by nitrogen implantation, has an estimated hardness of 2800 N/mm’. It is harder than amorphous boron carbide (hardness 2400 N/mm21 and slightly softer than nanocrystalline boron carbide (hardness 2900 N/mm2). The friction coefficient of an amorphous boron carbide film is decreased from 0.11 to 0.09 after implantation. As shown in Fig. 6 for a nanocrystalline boron carbide film the friction coefficient also decreases from 0.18 to 0.09 after implantation. The typical roughness of as deposited amorphous films is only 0.2 nm, while for as-deposited nanocrystalline films it is 8 - 10 nm. After implantation the surface roughness of amorphous films does not change but the roughness for nanocrystalline films decreases to 0.1 - 0.5 nm which may be due to sputter etching or due to the formation of new amorphous phases. Since the friction coefficient decreases with increasing surface hardness but increases with increasing surface roughness of the film, other things being equal, and since the implanted surface becomes harder without drastically changing the roughness of amorphous films, the friction coefficient decreases accordingly. For nanocrystalline films, the hardness of the pin and film (disk) is comparable, the relative large ion induced “smoothing” effect results in the decrease of friction coefficient, as shown in Fig. 6. The linear wear intensity was determined from the depth of the wear track using AFM. It has an average value of 5.0 X IO-* for as deposited amorphous boron carbide films, and 3.7 X lo-* for as deposited nanocrystalline boron carbide films. After implantation it increases to 7.6 X lo-’ for amorphous films but decreases to 2.7 X IO-* for nanocrystalline films. The wear mode is mainly abrasive, with observable wear particles of submicrometer size. These wear particles are partly composed of boron
C.I. Chiang et al./Nucl. Instr. and Meth. in Phys. Res. B 117 (1996) 408-414
413
4. Conclusions
I
0
.
.
.
,
1
40
20 L (Nl
Fig. 7. Dependence of the acoustic emission signal intensity on the friction force L in scratch tests of nanocrystalline boron carbide films on G-substrates. Curve A: as-deposited; curve B: after ion beam mixing; curve C: after MeV-ion irradiation.
nitride or B-N-C-compounds and are relatively hard compared with the implanted half. The wear of implanted amorphous films increases due to the abrasive action of these hard particles. For hard nanocrystalline films the “smoothing” effect causes a decrease of the friction coefficient and thus results in a decrease of wear. 3.2. Modifications strate
The ability of ion beam modification to change the surface composition and mechanical properties, as well as to improve the adhesion of boron carbide films, gives a valuable solution to overcome the restrictions in various applications of boron carbide films. Specifically, the following conclusions can be made: (1) Boron nitride (or possible B-N-C compounds) can be formed in boron carbide by nitrogen ion implantation as determined by XPS analysis. (2) The surface hardness of the films changes accordingly. For an as-deposited amorphous film, the hardness is increased from 2400 to 2800 N/mm2 after iraplantation. Implantation also causes a decrease of the friction coefficient from 0.11 to 0.09 and an increase of the linear wear intensity from 3.7 X IO-’ to 7.6 X lo-‘. For nanocrystalline films, the hardness, friction coefficient and linear wear intensity all decrease after nitrogen ion implantation from 3000 to 2800 N/mm2, 0.18 to 0.09 and 5.0 X IO-* to 2.7 X 10m8, respectively. (3) The adhesion of nanocrystalline boron carbide films on Si-substrates increases from 10 to 14 N and from 10 to 20 N after ion beam mixing and MeV-ion irradiation, respectively.
at the interface between film and subAcknowledgements
Fig. 7 shows detachment results of scratch tests for nanocrystalline boron carbide films on silicon substrates before and after ion modification. The critical loads obtained in the scratch tests are summarized in Table 4. Planar detachment of the film was observed by optical microscopy in all cases. It can be seen that for films on silicon substrates the adhesion is clearly improved by both ion beam modification techniques, however the improvement through MeV-ion irradiation is more effective than through ion beam mixing. For films on WC-substrates the improvement is not conclusive for the conditions in use, because the difference in critical load lies in the range of statistical errors. For the ion beam mixing case the process of detachment is slow in comparison with the other cases.
The authors want to thank Mrs. D. Roller, E. Knaetsch and W. Seith for the operation of the Van de Graaff accelerator, Mrs. S. Massing and L.M. Redondo for conducting ion implantation, Dr. H. Klewe-Nebenius for the XPS experiments, Mr. E. Weppelmann for the nanoindentation and Dr. T. Bieger for the microtribological tests. This work has been supported by BMBF and by JNICT.
References
[II R. Telle, Chem. unser. Z. 22, Nr. 3 (1988) 93 (in German). El W. Stumpf, Gmelin Handbook, Boron suppl. 2 (1981) 117. 131N. Yu et al, Appl. Phys. Lett. 63 (1993) 1643. [41 R.R. Reeber, J.Q. Whitley, R.P. Kusy, R.J. Culbertson and Table 4 The critical load (L, carbide films
in N) as obtained
System as-deposited
B,C/Si B&/WC
10 4
by scratch tests on boron
Critical load (N) after modification ion beam mixing
MeV-ion irradiation
14 4
17 6
N. Yu, AIP Conf. Proc. 231 (Am. Inst. of Physics, New York, 1991) p. 647. [Sl S.J. Bull, Vacuum 43 (1992) 5 17. [d J.E.E. Baglin, Nucl. Instr. and Meth. B 65 (1992) 119. [71 C.I. Chiang, H. Holleck and 0. Meyer, Nucl. Instr. and Meth. B 91 (1994) 692. 181 G. Linker, KfK report 3 136 (1981). Forschungszentntm Karlsmhe, Postfach 3640, D-76021 Karlsruhe, Germany. [91 R.C. Buschert, P.N. Gibson, W. Gissler, J. Haupt and T.A. Crabb, Colloq. Phys. C7, suppl. no. 10 (1989) 169.
414
C.I. Chiang et ul./Nucl.
Instr. and Meth. in Phys. Res. B I17 (1996) 408-414
[IO] C. Vincent, H. Vincent, H. Mourichoux and J. Bouix. J. Mater. Sci. 27 (1992) 1892. [I I] H. Saitoh and W.A. Yarbrough, Diamond and Relat. Mater. I (1992) 137. [ 121 T. Goto and T. Hirai, J. Mater. Sci. Lett. 7 (1988) 548. [13] D. Briggs and M.P. Seah, Practical Surface Analysis, Vol. I (Wiley, New York, 1990).
[14] A. Nilsson et al., Surface Sci. 287/288 (1993) 758. [ISI 0. Burat, D. Bouchier, V. Stambouli and G. Gautherin, J. Appl. Phys. 68 (1990) 2780. [ 161 J. Pethica, R. Hutchings and W.C. Oliver, Philos. Mag. A 48 (1983) 593.
Nuclear Instruments
and Methods in Physics Research B 1 I7 (I 996) 408-414
__ k!Wl
__
B
Beam Interactions with Materials 6 Atoms
!!!I!!! EISEVIER
The modification of mechanical properties and adhesion of boron carbide sputtered films by ion implantation C.I. Chiang z’, 0. Meyer a’* , Rui M.C. da Silva b a Institut,fir
Nukleure
Frstkiirprrphy.tik.
b Depurtumento
Forschun~sxntrum
de Fisicu-ITN.
Kwlsruhe.
Estruda Nutimud
Postjbch
36 40. D-76021
Kdwuhe.
Germuny
IO. P-2685 Sucuuem, Portugul
Received 17 May 1996; revised form received 28 June I996 Abstract Mechanical and tribological properties of boron carbide films have been modified by nitrogen ion implantation in the near surface region. For nanocrystalline boron carbide films, hardness, friction coefficient and linear wear intensity were reduced from 3000 to 2800, from 0.18 to 0.09 and from 5.0 X lOma to 2.7 X IO-*, respectively. For amorphous boron carbide films the hardness and the linear wear intensity increased while the friction coefficient decreased. These modifications were attributed to the creation of defects and to the formation of boron nitride phases. The interface between boron carbide films and Si or WC substrates has also been modified by ion beam mixing and MeV-ion irradiation. Adhesion of B,C films to the substrate was assessed by measurements of critical loads in scratch tests. For films on Si-substrates the critical load increased from 10 N to I4 N after ion beam mixing and from IO N to 20 N after MeV-ion irradiation.
1. Introduction Boron carbide films are known to be superhard protective coatings owing to high hardness and wear resistance. However, low fracture and oxidation resistance are two main restrictions for the application of boron carbide films [1,2]. One of the possible ways for improving the fracture and oxidation resistance is to modify the film surface by ion beams. Ion implantation will on one hand create defects which change the stress state accordingly, and may hinder the crack propagation on the other hand new tough and oxidation resistant compounds may be formed, such as cubic boron nitride (c-BN). However as the synthesis of c-BN, both in bulk and thin film form, is found to be difficult in comparison with the synthesis of boron carbide, due to the competitive growth of hexagonal boron nitride (h-BN), an alternative method is the growth of c-BN on the surface of bulk boron carbide [3,4]. The same idea was applied to sputtered boron carbide films which were then implanted with nitrogen ions. A broad distribution of implanted ions was used by superposition of several combinations of ion energy and flux in order to get a more homogeneous profile. The modifications of mechanical and tribological properties will be reported here.
’ Corresponding author. Fax: + 49-7247-824624; email: [email protected]. ’ Permanent address: Materials R&D Center, CSIST, Taiwan, R.O.C. 0168-583X/96/$15.00 Copyright PI1 SO 168-583X(96)00364-3
Another major drawback of boron carbide films (and also of most of the ceramic hard coatings) is their low adhesion, especially on metallic or semimetallic substrates. Implantation techniques have been successfully applied for increasing the adhesion of metallic and polymer films on metallic substrates. In general, there are two methods based on different mechanisms for increasing adhesion, namely ion beam mixing [5] and MeV-ion irradiation [6]. In ion beam mixing energetic ions are implanted in the neighborhood of the interface between film and substrate, and dissipate their energy in the form of energy spikes, part of it is transferred to the neighboring film and substrate atoms. Both the binding strength and the number of bonds are then increased through ion induced diffusion (physically) and/or through the creation of new chemical bonds (chemically) in ion activated reactions. On the other hand MeV ion irradiation of the interface with light particles, such as electron and He+, with energies of several MeV, has commonly been called MeV-ion irradiation. For such light particles the corresponding projected ranges are in the order of several tens of pm which is thicker than most of the thin films. The energy loss in the interface region is mainly electronic. Hence the excitation of binding electrons is considered responsible for the enhancement of binding strength [6]. However, the exact mechanism is not yet fully understood. We applied both methods to study the influence of ion bombardment on adhesion of boron carbide films. Improvements of adhesion could be clearly observed.
0 1996 Elsevier Science B.V. All rights reserved
C.I. Chiang et al./Nucl.
Instr. and Meth. in Phys. Res. B I17
f 1996) 408-414
409
2. Experimental 2.1. Film preparation Boron carbide films were deposited by rf magnetron sputtering and analysed following the same procedure stated in our previous work [7]. A planar diode arrangement was used. The target consisted of hot pressed boron carbide disks. Both single crystal silicon of [OOI] orientation and polycrystalline WC-6%Co were used as substrates. The surfaces of the substrates were metallographitally polished and cleaned with alcohol immediately before deposition. Substrate temperature was controlled by a PID temperature controller by varying the current through the substrate holder made of tantalum. The base vacuum of the sputtering chamber was 5 X 10e6 mbar. Nanocrystalline boron carbide films were prepared using 200 W rf power at a substrate temperature of 1100°C and an argon pressure of 0.04 mbar. Amorphous boron carbide films were prepared at substrate temperatures of 500°C with the same values of rf power and argon pressure. 2.2. Ion beam modi$cation Our experiments of ion beam modification are classified as: (a) modification of mechanical and tribological properties of the film surface by ion implantation and (b) modification of adhesion of the film by ion beam mixing and MeV-ion irradiation. Implantations in the surface region at room temperature were conducted in ITN (Instituto Technologico Nuclear), Portugal. Implantation at higher temperatures together with the experiments for the modification of the interface were conducted in INFP-FZK (Institut fir Nukleare FestkGrperphysik, Forschungszentrum KdrIsruhe), Germany. 2.2.1. Ion implantation Both nanocrystalline and amorphous boron carbide films, 4 pm thick, were implanted with nitrogen ions N+ at room temperature and at 600°C. The implantation parameters are listed in Table 1. The distribution of implanted nitrogen can be calculated on the basis of a superposition principle [8]. The result is shown in Fig. 1. 2.2.2. Ion beam mixing In ion beam mixing experiments
N+ ions of 300 keV
0
200
400
Depth (nm)
Fig. I. Theoretical depth distribution of implanted nitrogen ions for boron carbide using the energy and flux combinations listed in Table 1.
energy were implanted to a total fluence of 2 X 10” ions/cm’. The maximum allowable energy of our equipment is 300 keV, which corresponds to a projected range of 0.4 pm. However films with a minimum thickness of 4 pm were usually required for applications such as wear resistance coating. In order to meet this requirement and in the energy limitation of the equipment, boron carbide films were first grown to a thickness of approximately 0.5 pm and then implanted at the film/substrate interface with 300 keV N+ ions, at room temperature. Following the implantation more boron carbide was deposited to a total thickness of 4 pm thick. 2.2.3. MeV-ion irradiation For MeV-ion irradiation samples were directly irradiated by 2 MeV He+ ions at room temperature to a total fluence of I x lOI ions/cm*. The beam diameter was 1 mm. The beam was scanned over an area of 10 X 10 mm* using a beam scanner, which was operated by varying the voltage of the deflection magnet in both x- and y-directions with fixed frequency. In order avoid the differences between batches of as-deposited films, only one half of the surface was implanted, the other half was covered by a shutter during ion bombardment. All the .‘ollowing analyses were conducted both on the implanted half and on the as-deposited half for comparison. 2.3. Composition.
thickness and surface roughness of the
jfilm Table 1 Implantation profile for 10% nitrogen in boron carbide Ion energy (IieV)
Ion flux (lOI ions/cm’)
50 100 180 300
7.01 10.6 13.6 16.0
Composition and thicknesses of the films, as well as the distribution of implanted species were determined by Rutherford Backscattering Spectrometry (RBS) using 2 MeV He+ ions. Glancing Angle Incidence X-ray Diffraction (GIXD) [9] at an angle of incidence of 3” was used to analyze the crystalline structure of the film. Phases were identified according to JCPDS (Joint Committee for Powder Diffraction Standard) 35-798. Binding states in the
C.I. Chiang et al./Nucl. Instr. and Meth. in Phys. Res. B 117 (1996) 408-414
410
Table 2 Core level bonds in eV for various phases
Substance
B Is
c Is
N Is
Refs.
B,C
186.6
281.8
_
h-BN c-8N B-B
190.2 190.7 187.9
_ _ _
398.2 398.7 _ _
[lOI [Ill 11 II
c-c N-N P-C,N,
_ _ _
284.4 _
404
286
400.1
mN and 6 rpm, respectively. By measuring the tangential force on the pin and the depth of the wear track after a certain number of revolutions, the friction coefficient f and the linear wear intensity I can be calculated according to:
[121 [I31 [I41 [I51
near surface region were obtained by X-ray Photoelectron Spectrometry (XPS) using Mg Km excitation. The core levels of the corresponding bonds of boron carbide, boron nitride, carbon nitride and elements B, C, N are listed in Table 2, [lo-IS]. The surface roughness was determined using an Atomic Force Microscope (AFM). The average area roughness for a test area of 5 X 5 mm2 was adopted following the definition listed in DIN 4760. 2.4. Hardness test Hardness of the films were measured by two processes: microvicker’s and nanoindentation process [16]. By microvicker’s test a Vicker’s pyramid indenter was used. The hardness value was calculated from the average value of the diagonals of the indentation. The hardness value will be expressed as HV 0.2 for a testing load of 0.2 kp according to DIN 50 133. If the film is harder than the substrate and the indentation depth is deeper than 1/IO of the film thickness, the measured hardness value is affected by the hardness of the substrate and will be called integrated hardness (according to IS0 45 16-1980). In order to measure the hardness in the implanted region, an indentation depth dependent measurement with indentation depth as small as 100 nm was used, called nanoindentation [ 161.In such nanoindentation experiments the dependence of indentation force (P) on indentation depth (h) for a complete loading and unloading cycle can be obtained. A Berkovich pyramid with a face angle of 65.3” was used as an indenter. The measured hardness value H in N/mm2, was calculated for each force step using the plastic indentation (penetration) depth (h,). The plastic indentation depth is equal to the total penetration minus the elastic penetration derived from the slope of the unload curve [ 161. 2.5. Friction and wear test The friction coefficient and linear wear intensity were determined by micropin-on-disk test. A rectangular pin was forced to contact a rotating disk under dry friction conditions. The normal force on the pin and rotational speed of the disk were kept constant with values of 200
where F,. F,, are the tangential and normal forces, respectively, t is the total depth of the wear track, and s is the total length traveled by the pin. The tangential and normal forces were measured by strain gauge with a resolution of 5 mN. A rectangular boron carbide bar of dimensions 1 X 1 X 15 mm3 and Vicker’s hardness (HV 0.2) of 2800 was used as pin. The contact surface of the pin was polished. The depth of the wear track after 2400 revolutions was measured by AFM. 2.4. Scratch test For the scratch test experiments a Rockwell diamond tip was forced into a specimen moving with a translational speed of 10 mm/min. The force at the point of contact was increased a rate of 150 N/min from zero until the detachment of the film initiated as detected by acoustic emission. This load is also known as the critical load (L,) and provides a measure of the adhesion. Only nanocrystalline films were tested. For each half of the specimen as deposited and implanted, three tests were conducted and the average value was adopted.
3. Results and discussion 3.1. Modification
in the surface region
Fig. 2 shows RBS spectra of a boron carbide film before and after implantation with nitrogen ions at room temperature. The B/C ratio as determined from the relative yield of He ions scattered by boron and carbon atoms is 3.8. A comparison of the measured projected range of the implanted nitrogen with the theoretically calculated projected range is listed in Table 3. There was no observable change of the RBS spectra for films implanted at 600°C. It can hence be. concluded that the measured projected range is very close to the theoretically predicted value up to a target temperature of 600°C. Since the energy resolution is about 20 keV for our RBS system, the depth resolution is 40 nm. We may assert that the thermally activated diffusion of implanted ions is less than 40 nm up to a target temperature of 600°C. This implies that the implanted nitrogen is either bound or trapped in the boron carbide lattice. The projected range reaches about 500 nm, but the depth distribution of the implanted species is not uniform. This could be improved however by using more
C.I. Chiang
et ul./Nucl.
41 I
Instr. and Meth. in Phys. Res. B 117 (1996) 408-414
2000 implantation
5 u E aJ jr:
1;
1000
2b
25
3b
3’5
Lb
2 Theta Fig. 3. GIXD pattern of an as-deposited nanocrysralline boron carbide film. The arrows indicate the positions of tbe diffraction peaks as listed in JCPDS 36-798. The subscript r stands for rhombohedral notation.
0
L
I 50
250
150 Channel
Fig. 2. RBS spectra of a nanocrystalline boron carbide film: (a) before (solid curve) and (b) after (dashed curve) implantation nitrogen ions at room temperature.
with
than four energy-flux combinations in order to achieve a more uniform profile. The as- deposited films are nearly stoichiometric. The compositional change, if any, is also less than the sensitivity limit for nitrogen in boron nitride. The GIXD pattern from an as deposited nanocrystalline boron carbide film is shown in Fig. 3. The two major diffraction peaks were identified as (11 I>, and (21 I), (r: rhombohedral) respectively. This shows that films were grown with a preferred (hll), orientation, for which the basal plane of the hexagonal lattice is nearly parallel to the substrate surface. The diffraction maxima all shifted to lower Bragg angles in relation to the JCPDS listed values, indicating the existance of compressive residual stresses in the basal plane. These stresses will be relaxed at higher target temperatures since no shifts were observed under these conditions. The exact reason for this kind of preferential growth is not clear. We suggest that this is a consequence of excess compressive stress parallel to the
Table 3 Projected range CR,) and standard deviation of the projected range (AR,) for nitrogen carbide films Ion energy (keV)
50
100 180 300
ions implanted Theoretical
into nanocrystalline
value
boron
Measured value
R, (nm)
AR, (nm)
R, (nm)
AR, (nm)
108 211 352 526
24 36 40 47
120 227 378 532
30 40 4.5 55
basal plane. The stress is relaxed at higher deposition temperatures. On the other hand, the results of XPS analysis show a very definite change of the binding state in the near surface region. Fig. 4 depicts the photoelectron spectrum obtained through the 1.25 keV photon irradiation. The binding energies of B Is, N is and C Is-line for B& h-BN and C,N, are identified with arrows in Fig. 4 according to Table 2. Owing to the presence of various bonds with closely neighbouring binding energies, only the superimposed curve can be seen. As shown by the solid curve, ihe surface for films before implantation is composed of free boron and carbon atoms together with boron carbide but no trace of boron nitride. After nitrogen implantation (dashed curve>, a peak appears at the expected position for the N 1s photopeak, i.e. new chemical bonds have been formed in the implanted region. The B Is photopeak shifts to higher binding energy, owing to the presence of these new bonds. Whether the new bond is boron nitride, carbon nitride, ternary C-B-N compounds or a mixture of these can not be determined because of the width of the photopeak. Measurements of Vicker’s hardness (HV 0.2) performed in the as- deposited nanocrystalline and amorphous boron carbide films on silicon substrates yielded the values of 2900 and 2400 respectively. Because the indentation depth is around 1.6 pm, which is larger than l/l0 of the film thickness, the influence of the substrate is inevitable. Thus, in order to overcome this influence, and to extract the effect of ion implantation, nanoindentation experiments were performed. The results are displayed in Fig. 5 where the dependence of hardness ( H) on the plastic penetration depth (h,) of an amorphous boron carbide film is shown before and after implantations. The initial stage of this curve (for h, < 200 nm) is sensitive to the roundness of the indenter tip and hence difficult to interpret. The hardness of the as-deposited films lies between 2400 and 2500 N/mm2 and is nearly independent of h, for h, larger than 400 nm while for the implanted film it is 3100 N/mm* for
C.I.
412
Chicmg
er d./Nucl.
Insrr.
and Meth.
in Phys. Rex. B I17
(1996)
408-414
0.4
..-
42
:, lmplonte~ i._,- ._.. ___~__..________._____ _______.___.._ 0 E 0
2000
1000 Time
(~1
Fig. 6. Friction coefficients (f‘) as a function of testing time of asdeposited (solid curve) and implanted (dashed curve) nanocrysWine boron carbide films.
413
399
E, kv) Fig. 4. (a) B Is, (b) N Is and (c) C Is photo electron energy spectrum from the 1.25 keV photon irradiation (MgK (1) of nanocrystalline boron carbide ftlm surfaces before (solid curve) and after (dashed curve) nitrogen implantation,
-E250C E z‘ ;
1500
400
800
1200
hp (nm)
Fig. 5. Hardness (H in N/mm’) of amorphous boron nitride films by nanoindentation: (a) before (solid curve) and (b) after (dashed curve) nitrogen ion implantation as a function of the plastic penetration depth (h, in nm).
h, = 400 nm and decreases with increasing h,. The hardness of implanted films is in general 15% - 20% higher than that of as-deposited films. In contrast the hardness of nanocrystalfine boron carbide films decreases 10% - 15% (from 3 100 to 2900 N/mm’) after implantation and the hardness of implanted films increases with indentation depth. According to the XPS results presented above, there is a large possibility of the formation of boron nitride or B-N-C compounds. It appears that this new phase, formed by nitrogen implantation, has an estimated hardness of 2800 N/mm’. It is harder than amorphous boron carbide (hardness 2400 N/mm21 and slightly softer than nanocrystalline boron carbide (hardness 2900 N/mm2). The friction coefficient of an amorphous boron carbide film is decreased from 0.11 to 0.09 after implantation. As shown in Fig. 6 for a nanocrystalline boron carbide film the friction coefficient also decreases from 0.18 to 0.09 after implantation. The typical roughness of as deposited amorphous films is only 0.2 nm, while for as-deposited nanocrystalline films it is 8 - 10 nm. After implantation the surface roughness of amorphous films does not change but the roughness for nanocrystalline films decreases to 0.1 - 0.5 nm which may be due to sputter etching or due to the formation of new amorphous phases. Since the friction coefficient decreases with increasing surface hardness but increases with increasing surface roughness of the film, other things being equal, and since the implanted surface becomes harder without drastically changing the roughness of amorphous films, the friction coefficient decreases accordingly. For nanocrystalline films, the hardness of the pin and film (disk) is comparable, the relative large ion induced “smoothing” effect results in the decrease of friction coefficient, as shown in Fig. 6. The linear wear intensity was determined from the depth of the wear track using AFM. It has an average value of 5.0 X IO-* for as deposited amorphous boron carbide films, and 3.7 X lo-* for as deposited nanocrystalline boron carbide films. After implantation it increases to 7.6 X lo-’ for amorphous films but decreases to 2.7 X IO-* for nanocrystalline films. The wear mode is mainly abrasive, with observable wear particles of submicrometer size. These wear particles are partly composed of boron
C.I. Chiang et al./Nucl. Instr. and Meth. in Phys. Res. B 117 (1996) 408-414
413
4. Conclusions
I
0
.
.
.
,
1
40
20 L (Nl
Fig. 7. Dependence of the acoustic emission signal intensity on the friction force L in scratch tests of nanocrystalline boron carbide films on G-substrates. Curve A: as-deposited; curve B: after ion beam mixing; curve C: after MeV-ion irradiation.
nitride or B-N-C-compounds and are relatively hard compared with the implanted half. The wear of implanted amorphous films increases due to the abrasive action of these hard particles. For hard nanocrystalline films the “smoothing” effect causes a decrease of the friction coefficient and thus results in a decrease of wear. 3.2. Modifications strate
The ability of ion beam modification to change the surface composition and mechanical properties, as well as to improve the adhesion of boron carbide films, gives a valuable solution to overcome the restrictions in various applications of boron carbide films. Specifically, the following conclusions can be made: (1) Boron nitride (or possible B-N-C compounds) can be formed in boron carbide by nitrogen ion implantation as determined by XPS analysis. (2) The surface hardness of the films changes accordingly. For an as-deposited amorphous film, the hardness is increased from 2400 to 2800 N/mm2 after iraplantation. Implantation also causes a decrease of the friction coefficient from 0.11 to 0.09 and an increase of the linear wear intensity from 3.7 X IO-’ to 7.6 X lo-‘. For nanocrystalline films, the hardness, friction coefficient and linear wear intensity all decrease after nitrogen ion implantation from 3000 to 2800 N/mm2, 0.18 to 0.09 and 5.0 X IO-* to 2.7 X 10m8, respectively. (3) The adhesion of nanocrystalline boron carbide films on Si-substrates increases from 10 to 14 N and from 10 to 20 N after ion beam mixing and MeV-ion irradiation, respectively.
at the interface between film and subAcknowledgements
Fig. 7 shows detachment results of scratch tests for nanocrystalline boron carbide films on silicon substrates before and after ion modification. The critical loads obtained in the scratch tests are summarized in Table 4. Planar detachment of the film was observed by optical microscopy in all cases. It can be seen that for films on silicon substrates the adhesion is clearly improved by both ion beam modification techniques, however the improvement through MeV-ion irradiation is more effective than through ion beam mixing. For films on WC-substrates the improvement is not conclusive for the conditions in use, because the difference in critical load lies in the range of statistical errors. For the ion beam mixing case the process of detachment is slow in comparison with the other cases.
The authors want to thank Mrs. D. Roller, E. Knaetsch and W. Seith for the operation of the Van de Graaff accelerator, Mrs. S. Massing and L.M. Redondo for conducting ion implantation, Dr. H. Klewe-Nebenius for the XPS experiments, Mr. E. Weppelmann for the nanoindentation and Dr. T. Bieger for the microtribological tests. This work has been supported by BMBF and by JNICT.
References
[II R. Telle, Chem. unser. Z. 22, Nr. 3 (1988) 93 (in German). El W. Stumpf, Gmelin Handbook, Boron suppl. 2 (1981) 117. 131N. Yu et al, Appl. Phys. Lett. 63 (1993) 1643. [41 R.R. Reeber, J.Q. Whitley, R.P. Kusy, R.J. Culbertson and Table 4 The critical load (L, carbide films
in N) as obtained
System as-deposited
B,C/Si B&/WC
10 4
by scratch tests on boron
Critical load (N) after modification ion beam mixing
MeV-ion irradiation
14 4
17 6
N. Yu, AIP Conf. Proc. 231 (Am. Inst. of Physics, New York, 1991) p. 647. [Sl S.J. Bull, Vacuum 43 (1992) 5 17. [d J.E.E. Baglin, Nucl. Instr. and Meth. B 65 (1992) 119. [71 C.I. Chiang, H. Holleck and 0. Meyer, Nucl. Instr. and Meth. B 91 (1994) 692. 181 G. Linker, KfK report 3 136 (1981). Forschungszentntm Karlsmhe, Postfach 3640, D-76021 Karlsruhe, Germany. [91 R.C. Buschert, P.N. Gibson, W. Gissler, J. Haupt and T.A. Crabb, Colloq. Phys. C7, suppl. no. 10 (1989) 169.
414
C.I. Chiang et ul./Nucl.
Instr. and Meth. in Phys. Res. B I17 (1996) 408-414
[IO] C. Vincent, H. Vincent, H. Mourichoux and J. Bouix. J. Mater. Sci. 27 (1992) 1892. [I I] H. Saitoh and W.A. Yarbrough, Diamond and Relat. Mater. I (1992) 137. [ 121 T. Goto and T. Hirai, J. Mater. Sci. Lett. 7 (1988) 548. [13] D. Briggs and M.P. Seah, Practical Surface Analysis, Vol. I (Wiley, New York, 1990).
[14] A. Nilsson et al., Surface Sci. 287/288 (1993) 758. [ISI 0. Burat, D. Bouchier, V. Stambouli and G. Gautherin, J. Appl. Phys. 68 (1990) 2780. [ 161 J. Pethica, R. Hutchings and W.C. Oliver, Philos. Mag. A 48 (1983) 593.