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Al2O3 nanolaminated films: correlation to microstructure
Surface and Coatings Technology 125 (2000) 196–200 www.elsevier.nl/locate/surfcoat
Mechanical properties of Al/Al O nanolaminated films: 2 3 correlation to microstructure M. Ben Daia a, *, P. Aubert a, S. Labdi a, C. Le Paven-Thivet a, P. Houdy a, J.L. Bozet b a Laboratoire Multicouches Nanome´triques (LMN), Universite´ d’Evry, Bd F. Mitterand, 91023 Evry cedex, France b Universite´ de Lie`ge, Service d’Elements de Machine et de Tribologie, Rue E. Solvay 21, 4000 Lie`ge, Belgium
Abstract Wear resistance and the hardness of Al/Al O nanolaminated films were investigated in this study. Monolithic films and 2 3 multilayers were deposited on a silicon substrate with two different substrate temperatures: T =25°C and T =−90°C. The period s s thickness of multilayers was lowering from 40 to 2 nm. From nanoindentation measurements, it appears that the hardness of multilayers has an intermediate value between those of metal (Al ) and ceramic (Al O ). The tribological test was conducted by 2 3 the pin-on-disc method. The T =25°C deposited multilayers, as well as single films, demonstrated poor wear resistance. The best s wear resistance was obtained for multilayers deposited at the lowest substrate temperature (T =−90°C ). The results are in good s agreement with structural characterization. X-ray reflectometry demonstrated that the multilayer character of Al/Al O is more 2 3 pronounced for T =−90°C. © 2000 Elsevier Science S.A. All rights reserved. s Keywords: Al/Al O multilayers; Nanoindentation; Tribological properties 2 3
1. Introduction
2. Experimental details
In the field of research of new coatings for hard protective and wear resistance, multilayer films exhibit a large enhancement of mechanical properties compared to single layers. Wear resistance and hardness of multilayers are reported in many studies [1–3]. All these show a significant dependence of mechanical properties on the microstructure of multilayers. According to Koehler [4], the strength of multilayers depends, in addition to layer thickness, on the difference in the shear moduli of the components. The metal– ceramic multilayers, in particular, are expected to demonstrate a high strength and hardness. Only a few papers have been published on the investigation of the mechanical properties of Al/Al O multi2 3 layers [5]. Our paper focuses on the mechanical behavior of multilayers of alternating amorphous alumina with polycrystalline aluminum, with period thickness l less than 40 nm, correlated to morphological and structural properties.
The multilayers and monolithic films were produced by R.F. sputtering onto (100) silicon wafers. Two substrate temperatures were used, T =25°C and s T =−90°C. For the two sets of multilayers, the period s thickness ranged from l=40 nm to l=2 nm. Details of condition deposition can be found elsewhere [6 ]. Table 1 summarizes the physical characteristics of the samples. The friction and test wear was carried out using a pin-on-disc device at room temperature and humidity controlled at 25%. The normal load, kept constant at 1 N, was applied through an alumina ball of 6 mm diameter. The specimen was mounted on a rotating specimen holder. The nominal diameter of wear track and rotation speed were chosen to give a sliding speed of 0.005 m/s. The experiment was stopped when the ball reached the silicon substrate. This is accompanied by a change in friction coefficient m (0.6–0.7) and a specific silicon cracking sound. The wear track was checked by optical microscopy. The volume of material lost during wear was determined in order to compute the wear rate. Investigation of the mechanical properties was also performed using an indentation load–depth sensing apparatus. The experimental procedure and analysis
* Corresponding author. Tel.: +33-1-69-763016. E-mail address: [email protected] (M. Ben Daia)
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 54 5 - 9
197
M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200 Table 1 Physical characteristics of samples Film type
Period (l)
Individual film thickness ratio (nm)a
Total film thickness (nm) b
25
Monolithic Al Monolithic Al O 2 3 Al/Al O multilayers 2 3
−90
Monolithic Al Monolithic Al O 2 3 Al/Al O multilayers 2 3
– – 40 20 10 5 2 – – 40 20 5 2
– – 20/20 10/10 5/5 2.5/2.5 1/1 – – 20/20 10/10 2.5/2.5 1/1
540 75 215 225 195 240 280 540 73 140 175 130 70
Substrate temperature (°C )
a Individual thicknesses were estimated using deposition rate. b Thickness of films was measured using cross-section high resolution scanning electron microscopy.
used are described elsewhere [7]. Measurements of hardness were obtained with a Berkovich diamond indenter and the hardness was calculated using the true calibrated shape of the indenter. 10 indents were made on each sample and averaged. A simple cycle of loading and unloading was used in all the experiments, and the loading rate was equal to 2×P /min where P was max max the maximum load (the time for one cycle was 1 min).
3. Results 3.1. Structural characterization Alumina films are amorphous at any substrate temperature. The films are very smooth, which is suitable for multilayers film engineering. Aluminium films are polycrystalline, with a weak (111) texture. As shown by high resolution scanning electronic microscope (SEM ) surface observation, Al films are rough and granular, Fig. 1. We have studied the influence of substrate temperature on the morphology of aluminum films from low temperature (T =−90°C ) to high temperature (T =600°C ) and s s found that the grain size increases with temperature [6 ]. Nevertheless, no significant difference was seen between films deposited at low temperature T =−90°C and at s T =25°C. Equivalent diameters we are of the same s order: we=350 nm for thick films (200 nm thick), Fig. 1(a); and we=30 nm for thin films (10 nm thick), Fig. 1(b). The latter value is expected to arise in aluminium layers in Al/Al O multilayers. Al/Al O multilayers 2 3 2 3 also exhibit a granular surface, in agreement with the Al one. Cross-section high resolution SEM has not shown any laminated structure [6 ]. In fact, big grains, characterics of columnar growth, have been observed. Since aluminum layers are very rough, no perfect
interfaces are expected in Al/Al O multilayers. 2 3 Nevertheless, the multilayering character of Al/Al O 2 3 structures occurs in some multilayers, as shown by X-ray reflectometry ( XRR) experiments on 20 nm-periodic samples. In XRR, the appearance of Bragg peaks is the signature of a periodic structure. For T =−90°C, four s Bragg peaks are observed, Fig. 2(a), whereas at T =25°C two peaks are seen Fig. 2(b). The existence s of the multilayer structure in Al/Al O has also been 2 3 demonstrated by secondary ionic mass spectrometry (SIMS ) [8]. It is clear that the multilayering character of Al/Al O composites increases when the substrate 2 3 temperature decreases. This is well correlated to the fact that aluminum grain size decreases with substrate temperature, leading to more and more abrupt interfaces. It can also be expected that diffusion of aluminum and oxygen atoms is reduced at low temperatures. 3.2. Tribological tests The tribological tests were performed for all the samples. The wear rate was calculated as a function of period thickness for the two sets of multilayers and monolithic films. The results are summarized in Table 2. For monolithic films, a weak resistance to wear is seen for the two substrate temperatures. The multilayers deposited at T =25°C demonstrate similar behavior. s When comparing the two sets of multilayers, Fig. 3, it appears, on one hand, that there is no significant difference for multilayers with l≥10 nm. On the other hand, the samples at T =−90°C l=5 nm and l=2 nm have s a low wear rate and it seems that the wear resistance is greatly enhanced in these samples. Fig. 4 (a) and (b) shows the evolution of friction coefficient m with number of cycles during sliding for multilayers with l=5 nm and l=2 nm. In the case of T =25°C, the friction coefficient, initially started at 0.2– s
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M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200
(a)
(a)
(b)
(b) Fig. 1. Surface high resolution SEM observation of: (a) 200 nm thick Al films; (b) 10 nm thin Al film. The two thin films were deposited at T =−90°C. s
0.4, increased dramatically to achieve a value of m=1. This was accompanied with severe mode of wear. For T =−90°C, there were two stages of wear. First s the friction coefficient increased rapidly to achieve a
Fig. 2. X-ray reflectometry spectra of: (a) T =−90°C deposited s Al/Al O multilayers with d=20 nm; (b) T =25°C deposited 2 3 s Al/Al O multilayers with c=20 nm. 2 3
maximum value of 0.7 for the l=5 nm sample and 0.55 for the l=2 nm sample. This initial stage was followed by a steady state. The steady friction coefficient was equal to 0.6 for the l=5 nm sample and 0.54 for the l=2 nm sample, and this slight difference can explain the difference in wear rate observed for these samples.
Table 2 Wear rate and hardness measurements T =25°C s
Wear rate 10−2 (mm3 mm−1 N−1)
Hardness (GPa)
T =−90°C s
Wear rate 10−2 (mm3 mm−1 N−1)
Hardness (GPa)
Al Al O 2 3 l=40 nm l=20 nm l=10 nm l=5 nm l=2 nm
800 63 555 1012 1140 863 588
2.88±0.64 15.88±1.4 6.08±0.51 7.36±0.82 – 7.60±0.42 7.98±1.02
Al Al O 2 3 l=40 nm l=20 nm l=5 nm l=2 nm
900 73 392 443 1 0.4
2.07±0.45 15.57±1.32 7.87±0.32 9.35±0.87 10.08±0.91 9.23±1.11
M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200
199
Fig. 3. Wear rate vs. period thickness plotted for Al/Al O multilayers. 2 3 Fig. 5. Hardness of monolithic and multilayer Al/Al O films. 2 3
is no large difference in hardness of T =−90°C multis layers as function of period l. The hardness of all multilayers has to be compared with the rule of mixture value of 8.82 GPa calculated with single layers hardness for T =−90°C. s 4. Discussion (a)
(b) Fig. 4. Friction coefficient vs. number of cycles for multilayers deposited at: (a) T =25°C; (b) T =−90°C. s s
3.3. Hardness measurements Fig. 5 illustrates the hardness of multilayers compared to the monolith films hardness. The alumina films exhibited the highest value of hardness while the value of hardness of multilayers was between the aluminum hardness value and the alumina value. When comparing the T =25°C deposited multilayers with the s T =−90°C ones, it appears that the hardness of multis layers with l≥5 nm is more important for the second group. In the case of l=2 nm, no conclusion is given due to experimental error. It should be noted that there
The results of T =25°C deposited multilayers is cons sistent with the reduced multilayering character of these samples. The hardness of Al/Al O has been reported 2 3 by Ding et al. [5], but the range of period thickness is very different from the range investigated in this study. The maximum hardness value from Ding’s work (4.83±0.08) was obtained for 70 nm (Al )/10 nm (Al O ). This value is less than those obtained in this 2 3 paper for multilayers with l≤40 nm. The lack of hardness when decreasing the period of layering in this system of multilayers is in apparent contradiction with behavior reported for many multilayers with a well defined interface. If the multilayering structure is observed for T =−90°C for l=10 nm, the interface s roughness is not yet known for these samples. Further work will be necessary to estimate the composition mixing in these samples. The wear rate of T =−90°C deposited multilayers s with l≤5 nm was found to be the lowest. In particular, the wear resistance of T =−90°C deposited Al/Al O s 2 3 with l=2 nm is 182% higher than the monolithic Al O films ones. On the other hand, the hardness is 2 3 #50% lower. This in negative correlation with the fact that in ceramic films, the wear resistance depends on its hardness. However, the metal phase in multilayers allows inhibition of microcracking in these materials during sliding compared with hard Al O films. This can par2 3 tially explain the apparent contradiction in correlation of hardness and wear resistance behavior of Al/Al O 2 3 multilayers.
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M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200
5. Conclusion
Acknowledgements
Multilayers alternating polycrystalline aluminium and amorphous alumina were fabricated using RF-reactive sputtering. The temperature of substrate T during deposition greatly influenced the morphology s and structure of the films. XRR experiments showed an improvement of the multilayering character with decreasing T , despite the granular surface of Al layers. s For T =25°C deposited Al/Al O multilayers, no s 2 3 signicative difference in wear resistance was seen when comparing to Al O monolithic films. 2 3 A large enhancement of wear resistance was obtained for T =−90°C deposited multilayers with decreasing s period thickness. A comparison of the structure of the two sets of multilayers (T =25°C and T =−90°C ) suggests that s s tribological properties are greatly enhanced for films with good quality of multilayering which is Al/Al O 2 3 deposited at T =−90°C. s
We gratefully acknowledge M. Jeandin for useful discussion, M. Ne´lis for help on hardness measurements, P.A. Albouy for XRR experiments, and G. Renou for sample preparation.
References [1] W.D. Sproul, J. Vac. Sci. Technol. A 12 (1994) 4. [2] H. Kung, T.R. Jervis, J.-P. Hirvonen, J.D. Embry, T.E. Mitclell, M. Nastasi, Philos. Mag. A 71 (1995) 4. [3] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996). [4] J.S Koehler, Phys. Rev. B 2 (1970) 547. [5] Y. Ding, Z. Farhat, D.O. Northwood, A.T. Alpas, Surf. Coat. Technol. 68 (1994) 459. [6 ] C. Le Paven-Thivet, C. Malibert, Ph. Houdy, P.A. Albouy, Thin Solid Films 336 (1998) 373. [7] N.X. Randall, C. Julia-Schmutz, J.M. Soro, J. von Stebut, G. Zacharie, Thin solid Films 308/309 (1997) 297–303. [8] C. Le Paven-Thivet et al., unpublished data.
Mechanical properties of Al/Al O nanolaminated films: 2 3 correlation to microstructure M. Ben Daia a, *, P. Aubert a, S. Labdi a, C. Le Paven-Thivet a, P. Houdy a, J.L. Bozet b a Laboratoire Multicouches Nanome´triques (LMN), Universite´ d’Evry, Bd F. Mitterand, 91023 Evry cedex, France b Universite´ de Lie`ge, Service d’Elements de Machine et de Tribologie, Rue E. Solvay 21, 4000 Lie`ge, Belgium
Abstract Wear resistance and the hardness of Al/Al O nanolaminated films were investigated in this study. Monolithic films and 2 3 multilayers were deposited on a silicon substrate with two different substrate temperatures: T =25°C and T =−90°C. The period s s thickness of multilayers was lowering from 40 to 2 nm. From nanoindentation measurements, it appears that the hardness of multilayers has an intermediate value between those of metal (Al ) and ceramic (Al O ). The tribological test was conducted by 2 3 the pin-on-disc method. The T =25°C deposited multilayers, as well as single films, demonstrated poor wear resistance. The best s wear resistance was obtained for multilayers deposited at the lowest substrate temperature (T =−90°C ). The results are in good s agreement with structural characterization. X-ray reflectometry demonstrated that the multilayer character of Al/Al O is more 2 3 pronounced for T =−90°C. © 2000 Elsevier Science S.A. All rights reserved. s Keywords: Al/Al O multilayers; Nanoindentation; Tribological properties 2 3
1. Introduction
2. Experimental details
In the field of research of new coatings for hard protective and wear resistance, multilayer films exhibit a large enhancement of mechanical properties compared to single layers. Wear resistance and hardness of multilayers are reported in many studies [1–3]. All these show a significant dependence of mechanical properties on the microstructure of multilayers. According to Koehler [4], the strength of multilayers depends, in addition to layer thickness, on the difference in the shear moduli of the components. The metal– ceramic multilayers, in particular, are expected to demonstrate a high strength and hardness. Only a few papers have been published on the investigation of the mechanical properties of Al/Al O multi2 3 layers [5]. Our paper focuses on the mechanical behavior of multilayers of alternating amorphous alumina with polycrystalline aluminum, with period thickness l less than 40 nm, correlated to morphological and structural properties.
The multilayers and monolithic films were produced by R.F. sputtering onto (100) silicon wafers. Two substrate temperatures were used, T =25°C and s T =−90°C. For the two sets of multilayers, the period s thickness ranged from l=40 nm to l=2 nm. Details of condition deposition can be found elsewhere [6 ]. Table 1 summarizes the physical characteristics of the samples. The friction and test wear was carried out using a pin-on-disc device at room temperature and humidity controlled at 25%. The normal load, kept constant at 1 N, was applied through an alumina ball of 6 mm diameter. The specimen was mounted on a rotating specimen holder. The nominal diameter of wear track and rotation speed were chosen to give a sliding speed of 0.005 m/s. The experiment was stopped when the ball reached the silicon substrate. This is accompanied by a change in friction coefficient m (0.6–0.7) and a specific silicon cracking sound. The wear track was checked by optical microscopy. The volume of material lost during wear was determined in order to compute the wear rate. Investigation of the mechanical properties was also performed using an indentation load–depth sensing apparatus. The experimental procedure and analysis
* Corresponding author. Tel.: +33-1-69-763016. E-mail address: [email protected] (M. Ben Daia)
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 54 5 - 9
197
M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200 Table 1 Physical characteristics of samples Film type
Period (l)
Individual film thickness ratio (nm)a
Total film thickness (nm) b
25
Monolithic Al Monolithic Al O 2 3 Al/Al O multilayers 2 3
−90
Monolithic Al Monolithic Al O 2 3 Al/Al O multilayers 2 3
– – 40 20 10 5 2 – – 40 20 5 2
– – 20/20 10/10 5/5 2.5/2.5 1/1 – – 20/20 10/10 2.5/2.5 1/1
540 75 215 225 195 240 280 540 73 140 175 130 70
Substrate temperature (°C )
a Individual thicknesses were estimated using deposition rate. b Thickness of films was measured using cross-section high resolution scanning electron microscopy.
used are described elsewhere [7]. Measurements of hardness were obtained with a Berkovich diamond indenter and the hardness was calculated using the true calibrated shape of the indenter. 10 indents were made on each sample and averaged. A simple cycle of loading and unloading was used in all the experiments, and the loading rate was equal to 2×P /min where P was max max the maximum load (the time for one cycle was 1 min).
3. Results 3.1. Structural characterization Alumina films are amorphous at any substrate temperature. The films are very smooth, which is suitable for multilayers film engineering. Aluminium films are polycrystalline, with a weak (111) texture. As shown by high resolution scanning electronic microscope (SEM ) surface observation, Al films are rough and granular, Fig. 1. We have studied the influence of substrate temperature on the morphology of aluminum films from low temperature (T =−90°C ) to high temperature (T =600°C ) and s s found that the grain size increases with temperature [6 ]. Nevertheless, no significant difference was seen between films deposited at low temperature T =−90°C and at s T =25°C. Equivalent diameters we are of the same s order: we=350 nm for thick films (200 nm thick), Fig. 1(a); and we=30 nm for thin films (10 nm thick), Fig. 1(b). The latter value is expected to arise in aluminium layers in Al/Al O multilayers. Al/Al O multilayers 2 3 2 3 also exhibit a granular surface, in agreement with the Al one. Cross-section high resolution SEM has not shown any laminated structure [6 ]. In fact, big grains, characterics of columnar growth, have been observed. Since aluminum layers are very rough, no perfect
interfaces are expected in Al/Al O multilayers. 2 3 Nevertheless, the multilayering character of Al/Al O 2 3 structures occurs in some multilayers, as shown by X-ray reflectometry ( XRR) experiments on 20 nm-periodic samples. In XRR, the appearance of Bragg peaks is the signature of a periodic structure. For T =−90°C, four s Bragg peaks are observed, Fig. 2(a), whereas at T =25°C two peaks are seen Fig. 2(b). The existence s of the multilayer structure in Al/Al O has also been 2 3 demonstrated by secondary ionic mass spectrometry (SIMS ) [8]. It is clear that the multilayering character of Al/Al O composites increases when the substrate 2 3 temperature decreases. This is well correlated to the fact that aluminum grain size decreases with substrate temperature, leading to more and more abrupt interfaces. It can also be expected that diffusion of aluminum and oxygen atoms is reduced at low temperatures. 3.2. Tribological tests The tribological tests were performed for all the samples. The wear rate was calculated as a function of period thickness for the two sets of multilayers and monolithic films. The results are summarized in Table 2. For monolithic films, a weak resistance to wear is seen for the two substrate temperatures. The multilayers deposited at T =25°C demonstrate similar behavior. s When comparing the two sets of multilayers, Fig. 3, it appears, on one hand, that there is no significant difference for multilayers with l≥10 nm. On the other hand, the samples at T =−90°C l=5 nm and l=2 nm have s a low wear rate and it seems that the wear resistance is greatly enhanced in these samples. Fig. 4 (a) and (b) shows the evolution of friction coefficient m with number of cycles during sliding for multilayers with l=5 nm and l=2 nm. In the case of T =25°C, the friction coefficient, initially started at 0.2– s
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M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200
(a)
(a)
(b)
(b) Fig. 1. Surface high resolution SEM observation of: (a) 200 nm thick Al films; (b) 10 nm thin Al film. The two thin films were deposited at T =−90°C. s
0.4, increased dramatically to achieve a value of m=1. This was accompanied with severe mode of wear. For T =−90°C, there were two stages of wear. First s the friction coefficient increased rapidly to achieve a
Fig. 2. X-ray reflectometry spectra of: (a) T =−90°C deposited s Al/Al O multilayers with d=20 nm; (b) T =25°C deposited 2 3 s Al/Al O multilayers with c=20 nm. 2 3
maximum value of 0.7 for the l=5 nm sample and 0.55 for the l=2 nm sample. This initial stage was followed by a steady state. The steady friction coefficient was equal to 0.6 for the l=5 nm sample and 0.54 for the l=2 nm sample, and this slight difference can explain the difference in wear rate observed for these samples.
Table 2 Wear rate and hardness measurements T =25°C s
Wear rate 10−2 (mm3 mm−1 N−1)
Hardness (GPa)
T =−90°C s
Wear rate 10−2 (mm3 mm−1 N−1)
Hardness (GPa)
Al Al O 2 3 l=40 nm l=20 nm l=10 nm l=5 nm l=2 nm
800 63 555 1012 1140 863 588
2.88±0.64 15.88±1.4 6.08±0.51 7.36±0.82 – 7.60±0.42 7.98±1.02
Al Al O 2 3 l=40 nm l=20 nm l=5 nm l=2 nm
900 73 392 443 1 0.4
2.07±0.45 15.57±1.32 7.87±0.32 9.35±0.87 10.08±0.91 9.23±1.11
M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200
199
Fig. 3. Wear rate vs. period thickness plotted for Al/Al O multilayers. 2 3 Fig. 5. Hardness of monolithic and multilayer Al/Al O films. 2 3
is no large difference in hardness of T =−90°C multis layers as function of period l. The hardness of all multilayers has to be compared with the rule of mixture value of 8.82 GPa calculated with single layers hardness for T =−90°C. s 4. Discussion (a)
(b) Fig. 4. Friction coefficient vs. number of cycles for multilayers deposited at: (a) T =25°C; (b) T =−90°C. s s
3.3. Hardness measurements Fig. 5 illustrates the hardness of multilayers compared to the monolith films hardness. The alumina films exhibited the highest value of hardness while the value of hardness of multilayers was between the aluminum hardness value and the alumina value. When comparing the T =25°C deposited multilayers with the s T =−90°C ones, it appears that the hardness of multis layers with l≥5 nm is more important for the second group. In the case of l=2 nm, no conclusion is given due to experimental error. It should be noted that there
The results of T =25°C deposited multilayers is cons sistent with the reduced multilayering character of these samples. The hardness of Al/Al O has been reported 2 3 by Ding et al. [5], but the range of period thickness is very different from the range investigated in this study. The maximum hardness value from Ding’s work (4.83±0.08) was obtained for 70 nm (Al )/10 nm (Al O ). This value is less than those obtained in this 2 3 paper for multilayers with l≤40 nm. The lack of hardness when decreasing the period of layering in this system of multilayers is in apparent contradiction with behavior reported for many multilayers with a well defined interface. If the multilayering structure is observed for T =−90°C for l=10 nm, the interface s roughness is not yet known for these samples. Further work will be necessary to estimate the composition mixing in these samples. The wear rate of T =−90°C deposited multilayers s with l≤5 nm was found to be the lowest. In particular, the wear resistance of T =−90°C deposited Al/Al O s 2 3 with l=2 nm is 182% higher than the monolithic Al O films ones. On the other hand, the hardness is 2 3 #50% lower. This in negative correlation with the fact that in ceramic films, the wear resistance depends on its hardness. However, the metal phase in multilayers allows inhibition of microcracking in these materials during sliding compared with hard Al O films. This can par2 3 tially explain the apparent contradiction in correlation of hardness and wear resistance behavior of Al/Al O 2 3 multilayers.
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M. Ben Daia et al. / Surface and Coatings Technology 125 (2000) 196–200
5. Conclusion
Acknowledgements
Multilayers alternating polycrystalline aluminium and amorphous alumina were fabricated using RF-reactive sputtering. The temperature of substrate T during deposition greatly influenced the morphology s and structure of the films. XRR experiments showed an improvement of the multilayering character with decreasing T , despite the granular surface of Al layers. s For T =25°C deposited Al/Al O multilayers, no s 2 3 signicative difference in wear resistance was seen when comparing to Al O monolithic films. 2 3 A large enhancement of wear resistance was obtained for T =−90°C deposited multilayers with decreasing s period thickness. A comparison of the structure of the two sets of multilayers (T =25°C and T =−90°C ) suggests that s s tribological properties are greatly enhanced for films with good quality of multilayering which is Al/Al O 2 3 deposited at T =−90°C. s
We gratefully acknowledge M. Jeandin for useful discussion, M. Ne´lis for help on hardness measurements, P.A. Albouy for XRR experiments, and G. Renou for sample preparation.
References [1] W.D. Sproul, J. Vac. Sci. Technol. A 12 (1994) 4. [2] H. Kung, T.R. Jervis, J.-P. Hirvonen, J.D. Embry, T.E. Mitclell, M. Nastasi, Philos. Mag. A 71 (1995) 4. [3] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996). [4] J.S Koehler, Phys. Rev. B 2 (1970) 547. [5] Y. Ding, Z. Farhat, D.O. Northwood, A.T. Alpas, Surf. Coat. Technol. 68 (1994) 459. [6 ] C. Le Paven-Thivet, C. Malibert, Ph. Houdy, P.A. Albouy, Thin Solid Films 336 (1998) 373. [7] N.X. Randall, C. Julia-Schmutz, J.M. Soro, J. von Stebut, G. Zacharie, Thin solid Films 308/309 (1997) 297–303. [8] C. Le Paven-Thivet et al., unpublished data.