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High current Ni- and Fe-ion implantation into an Al surface to modify the mechanical properties
MATERIALS CHEM;S-T$W&lD Materials Chemistry and Physics 5-l ( 1998 1 309-3 12
High current Ni- and Fe-ion implantation into an Al surface to modify the mechanical properties B.X. Liu *, KY. Gao
Abstract The surface modification of Al wasstudiedby highcurrentNi- andFe-ionimplantationusinga metalvaporvacuumarc ion source.The strengthening intermetallic aluminides were formed after Ni- and Fe-ion implantation to doses in the order of 10’7-10’8 ions cm-‘. The depth profiles of the hardening phases were found to depend on the ion current density as well as the dose. In the case of Ni-implantation, when implantingwith a currentdensityof 89 p.A cm-’ to a dose of 6 X lOI ions cm-‘, the Al,nTi phasein the nearsurfacewasenrichedup to
85 vol.% and extendedinto a depthof about3000A. In the caseof Fe-ionimplantation.a dual implantationresultedin an almostunique AI,,Fe, compound standing in the near surface layer of 1000 .k in thickness and spreading to 4500 A in depth. Accordingly, the microhardness of the implanted Al films was measured and found to be considerably increased. The mechanisms responsible for the formation of intermetallic
aluminides andsurfacehardeningarealsodiscussed. Keywords:
0 199sElsevierScienceS.A. All rightsreserved.
Surface hardening: Intermrtallic compounds; Aluminides: Ion implantation
1. Introduction Aluminum and its alloys are widely usedas construction materialsowing to their excellent propertiesin many aspects and would be usedin more arcasif their surfacehardnessand wear resistancewere improved considerably. Ductile materials, like aluminum alloys, covered with a hard surfacelayer can achieve a better performance in wear and/or corrosion resistances[ I]. Generally, intermetallic compoundsare of relatively hard materialsand Ni- andFe-ion implantation into Al can form somealuminidesasa hard coating in the surface layer, asthere are many Al-N1 and AI-Fe intermetallic compounds that exist in their respective phasediagrams.Using metal vapor vacuum arc (MEVVA) ion source,it ispossible to implant both Ni andFe ions with a very high current density of up to about 120 JLAcm-” [ 2,3 ] and to a dosein the order of 10’7-‘8ions cm-’ within a feasibly short time. In addition, implantation with a high current density can trigger thermal annealingsimultaneously, which is of benefit for the formation of some expected Al-based intermetallic compounds [ 41.The objective of this study was to investigate the possibility of improving the surface propertiesof Al thin films by high current Ni- and Fe-ion implantation with an intention to * Corresponding author. Tel.: + 86-010-62785768: Fax: i 86-010.62771160; E-mail:[email protected] 0?53-0584/98/$19.@l 0 1998 Elsevier Science S.A. All rights reserved. PffS0?51-0584t98)00070-~
form a hard aluminide( s) surfacelayer extending deeperthan in the caseof conventional implantation techniques. 2. Experimental The samplesin this study were pure Al films with a total thicknessof 4000 or 7000 A depositedon thermally grown SiO, on Si wafers in an electron-gun evaporation with a vacuum level of lo-” Pa. The sampleswere then loaded on to a steel-madesample-holderin the target chamber of the MEVVA implanter with a vacuum level of 3 X IO-” Pa. As the implanter hasno analyzing magnet, the extracted Ni ions were analyzed to consist of 30% Ni+, 64% Ni2+, and 6% Ni” I, respectively, and the Fe ions were analyzed to consist of 25% Fe+? 68% Fe’*, and 7% Fe3+, respectively. During implantation, no deliberatecooling wastakenforthe samples. The sampleswith 4000 A Al films were implanted with Ni ions, at an extraction voltage of 4.5kV with the current densities varying from 2.5 to 89 FA cm-’ to dosesof (OS6) X IO” ions cm-‘. Some of the sampleswith 7000 A Al films were implanted by Fe ions at 30 kV with the current densitiesvarying from 2.5to 101 p,A cn-’ to dosesranging from 5 X lOI to 1 X 10” ions cm-‘, Some other samples with 7000 A Al films were doubly implanted with Fe ions, i.e. first with a current density of 76 andthen 25 p,A cm ~’ to a doserangeof ( l-3) X 1017ions cm-‘.
310
B.X. Liu, K. Y. Gno /Moieriols
Table I Formation
of Al,Ni
and A1,3Fe4 phases with good crystallinity
Current density (&A cme2) Ion dose ( X 10” cm-‘) Ni- or Fe-ion implanting time (min) Aluminide and its crystallinity (by XRD) Note: fine means almost all the diffraction
89 3 22 Al,Ni
Chrmis?p
as confirmed 89 6 38 AI,Ni
fine
fine
3. Results and discussion 3. I. Formation of Ni- rind Fe-alrminides In the samples implanted by Ni and Fe ions with a current density from 25 to 101 FA cmw2 to an ion dose from 3 X lOI to 1 X 10” ions cme2, Ni- and Fe-aluminides were formed immediately after implantation.Table 1summarizesthe XRD identification results that confirmed the formation of Ni-AI and Fe-Al phasesin the Al samples.Fig. 1 showsa typical XRD pattern of a sample implanted by Fe ions with 101 PAcm V-2at a doseof 1X lOI ions cm-‘. One seesthat almostall the strong diffraction linesof the AI,,Fe, phaseare shown, indicating a good crystallinity. While for Ni-ion implantation with a current density of 89 PA crnm2 to a dose of (3-6) X lOI ions cm-*, almost all the diffraction lines of the A13Niphasewereobservedin the XRD pattern,indicating a fine crystallinity of the formed compound. The RBS spectraof the Al films alsoconfirm the formation of aluminides. Almost all RBS patterns show clearly the plateausat the frontier of Al lines, suggestingthe formation
20
I
I I
I
1 25
t 35
* 30 Two
Fig. 1. XRDspectrumofthe atadoseoflX10’8ionscm-2.
Theta
sampleimplanted
54 (1998) 309-312
by XRD analysis 101 3 5 AlL3Fe4 good
101 6 25 Al,,Fe,
101 10 30 Alj3Fe4 fine
good
16125 313 20150 Al13Fe4 very good
lines were detected, good means all the strong lines were shown, and very good means the situation
X-ray diffraction (XRD) was conducted with a step-scan measurement using a D/max-RE? diffractometer with Cu radiation at 40 kV and 100 mA. Rutherford backscattering spectrometry (RBS) was performed with 2.023 MeV He ions. The HXD-IOOOA digital microhardness scale was employed to measure the Vickers/Knoops hardness of the samples.
21
and Phpks
I 40
I 45
I 50
(degree) by Fe-ionswith
101 PA cm-’
in between.
of an aluminide. In a later section, one of the RBS spectra will be shown. Under above experimentalconditions, the Ni and Fe concentration in the implanted region can reach about 25 at.‘%, which is aroundthe value to fulfill the chemicalstoichiometry requirement for fomring the corresponding 1:3 Ni- and Fealuminides. The temperature rise of the Al films during implantation can be estimatedby the method suggestedby Zhu and Liu [ 41. Accordingly, when implanting with acurrent density of25 ~.L*A cm-‘, the temperatureswere estimated to rise to around250°C (45 kV, Ni ) , and 200°C ( 30 kV, Fe), which were high enough to enable the A13Ni and A1,3Fe4 phaseto be formed as shown by our experimental results earlier. When implanting with aneven higher current density, e.g. 101 p,A crnm2(30 kV, Fe), the temperaturerise could go up to ashigh as55O”C,which wasbelieved to be of benefit for improving the crystallinity of the forming aluminides.All thesewere shown by their correspondingXRD resultslisted in Table 1. 3.2. Depth profiles of the implantedNi- and Fe-ions 3.2.1. High doseirnplnntation to extend modijed lqe~ thickness Fig. 2 shows the concentration profiles of Ni in the Al matrix deduced from the RBS spectra. At a fixed dose of 3 X lOI ions cm-*, curve (a) corresponding to a current
’ 0 : QO@, 1 Oo cl O” 0
1000
bb l
c
Od 0
0
2000 3000 Depth (A) Fig. 2. The concentration vs. depth profiles of Ni in Ni-ion films with various current densities and ion doses: (a) 3X10”cm-2: (b) 5lkAcm-‘, 3XlO”cm-I; (c) 3 X 10” cm-‘; (d) 89 PA cm-‘, 6~ 10” cm-‘.
4000 implanted Al 25 &A cm-‘, 89kAcm-‘.
Channel
Number
Fig. 3. RES of the sample implanted 3 X 10” cm-’ and then 25 p.A cm-‘/3
by Fe-ions with firat 76 PA cm-‘/ X 10” cm-*.
density of 25 FA cm -’ is an expanding Gaussian distribution. Curves (b) and (c) correspond to the current densities of 5 1 and 89 FA cmv2, respectively, in which a nearly flat concentration profile extending to 3000-4000 A implies that significant ion mixing and thermal diffusion have taken place, and that the higher the current density, the greater the amount of diffusion. Curve (a) also shows that about 78 ~01% of the A13Ni phase is located in a surface layer 1000 A thick. While for curve (c), a uniform A13Ni phase was spread over a considerably broad region of 4000 A in thickness and its volume percentage versus the pure Al was about 25 vol.%. At a fixed current density of 89 PA cm-‘, though the depth profiles of Al,Ni shown by curves (c) and (d), corresponding to doses of 3 X lOI and 6 X 10” cmT2, respectively, are comparable, the higher dose implantation resulted in an increase of volume percent of Al,Ni in a 1000 A-thick surface layer up to a value of 57-85 vol.%, which is much higher than that of the 3 X lOI cm-’ implanted one. In other words, implanting with high current density to a very high dose can lead to the formation of Al,Ni in the near surface, as well as to a depth extending to around 4000 A. This was also true for the cases of Fe-ion implantation. For instance, the concentration profiles of the samples implanted with Fe ions with current densities of 25 and 101 FA cm-’ at ion doses of 3 X lo”, 6 X lo”, 1 X lOi* ions cmF2 are similar to the case of Ni-implantation, discussed earlier. 3.2.2. DunI implnrztation to extend modified layer thickness
According to the above results, by implanting with a low current density, the higher the ion dose, the deeper the depth of the modified layer, because of the longer time of implantation required to enhance the diffusion of the implanted Fe Table 2 The microhardness
of the Al films implanted
Current density (FA cm-‘) Ion dose ( X IO” ctnS2) Ni-implanted. load 2 g Fe-implanted, load 2 g
by Ni- and Fe-ions
0 0 114 38.4
Vickers and Knoops hardnesses were used for Ni-ion a The current density is 89 PA cm-‘.
25 3 118 48.2 and Fe-ion
or Ni ions into the Al matrix. In fact, the depth was several times larger than that deduced by TRIM code calculation. While implanting with a high current density, a medium dose required a shorter time of implantation than that in the previous case and a relatively thinner modified layer was obtained, which was approximately the value calculated by TRIM code. On the contrary, in this case, the concentration of implanted ions as well as the so-formed hardening aluminides can be very high. Considering the respective features involved in the above two different conditions, a dual implantation of Fe-ions into Al was designed to attain a relatively thick modified layer together with a high concentration of the hardening Al,,Fe, compound standing in the near surface of the Al matrix. In the dual implantation process, the samples were first implanted by Fe ions with a high current density of 76 p.A cme2 to a dose of 3 X lOI ions cm-‘, and followed by an implantation with a low current density of 25 IJ-A cmw2 to a dose of 3 X 10” ions cm-‘. Fig. 3 is an RBS spectrum for a sample treated by this dual implantation. One seesfrom the figure that an extended modified layer of about 4500 A together with an almost unique Al ,,Fej compound in the near surface of 1000 A in thickness are obtained, which is believed to improve the surface troboligical property of Al. 3.3. Microhnrdness of the modifiedAl films
Either Vickers or Knoops microhardness wasmeasured for the Al samples after ion implantation with various ion beam parameters and the results are summarized in Table 2. It can be seen that the microhardness of the implanted films was considerably increased. Generally, the Knoops microhardness of the Fe-implanted Al films could increase up to 70% after a dual implantation. While the Vickers microhardness of the Ni-implanted Al films could increase by about 40% after high dose implantation. It is worthwhile noting the case for Fe ion implantation, that when the modified layer was deeper, the increase of the microhardness was considerably higher than for those with thinner modified layers, and that the concentration of the hardening Al, ,Fe, compound in the surface layer helped in increasing the microhardness. In addition, another round microhardness measurement was also performed by using a load of 5 g and the results showed a same trend of microhardness increment. As the modified layers were relatively thin, which was achieved by ion implantation, the microhardness measured by using high loading might be influenced by the substrate underneath, e.g. SiO, substrate in our case.
under various conditions 25 6 52.1 implanted
51 3 131 50.7
51 6 59.2
samples, respectively
101 3 145 B 50.6 (in a unit of kg mm-*)
101 6 163 ’ 54.9
101 10 61.3
76125 313 67.8
312
B.X. Liu, K. Y. Gao / Matrrinls
Chemistt~
4. Conclusions The major hardening phases formed by high current Niand Fe-ion implantation into Al were of Al,Ni and AI,,Fe, compounds, respectively. During implantation. the temperature rise caused by ion beam heating resulted in a simultaneous thermal annealing, which was responsible for the formation of the aluminide compounds with a fine crystallinity. The Al,Ni and Al ,,Fe, phases so formed in turn improved the surface tribological property of the Al films, as evidenced by a considerable increase of the microhardness. A process of dual implantation by varying ion beam parameters provided an opportunity to extend the modified layer thickness as well as to raise the surface concentration of the hardening aluminide compound standing in the near surface of the Al films.
and Physics 54 (I 998) 309-312
Acknowledgements This study was partially supported by the National Natural Science Foundation of China. The authors would like to express their thanks to the researchers at &heInstitute of Low Energy Nuclear Physics of Beijing Normal University, and at Laboratory of Heavy Ion Physics of Peking University as well as the researchers at the Institute of Materials Research of Tsinghua University for their kind help. References [ 11 C.W. Draper, J.M. Poare. Int. Mater. Res. 30 ( 1985) 85. [ 21 LG. Brown, J.E. Gavin, R.A. MacGill, Appl. Phys. Lett. 62 (1985) 358. [3] X.J. Zhang, F.S. Zhou, H.X. Zhang, S.J. Zhang, Q. Li, Z.E. Han, R:y. Sci. Instrum. 64 (1992) 2431. [4] D.H. Zhu, B.X. Liu, J. Appl. Phys. 77 (12) ( 1995) 6257.
High current Ni- and Fe-ion implantation into an Al surface to modify the mechanical properties B.X. Liu *, KY. Gao
Abstract The surface modification of Al wasstudiedby highcurrentNi- andFe-ionimplantationusinga metalvaporvacuumarc ion source.The strengthening intermetallic aluminides were formed after Ni- and Fe-ion implantation to doses in the order of 10’7-10’8 ions cm-‘. The depth profiles of the hardening phases were found to depend on the ion current density as well as the dose. In the case of Ni-implantation, when implantingwith a currentdensityof 89 p.A cm-’ to a dose of 6 X lOI ions cm-‘, the Al,nTi phasein the nearsurfacewasenrichedup to
85 vol.% and extendedinto a depthof about3000A. In the caseof Fe-ionimplantation.a dual implantationresultedin an almostunique AI,,Fe, compound standing in the near surface layer of 1000 .k in thickness and spreading to 4500 A in depth. Accordingly, the microhardness of the implanted Al films was measured and found to be considerably increased. The mechanisms responsible for the formation of intermetallic
aluminides andsurfacehardeningarealsodiscussed. Keywords:
0 199sElsevierScienceS.A. All rightsreserved.
Surface hardening: Intermrtallic compounds; Aluminides: Ion implantation
1. Introduction Aluminum and its alloys are widely usedas construction materialsowing to their excellent propertiesin many aspects and would be usedin more arcasif their surfacehardnessand wear resistancewere improved considerably. Ductile materials, like aluminum alloys, covered with a hard surfacelayer can achieve a better performance in wear and/or corrosion resistances[ I]. Generally, intermetallic compoundsare of relatively hard materialsand Ni- andFe-ion implantation into Al can form somealuminidesasa hard coating in the surface layer, asthere are many Al-N1 and AI-Fe intermetallic compounds that exist in their respective phasediagrams.Using metal vapor vacuum arc (MEVVA) ion source,it ispossible to implant both Ni andFe ions with a very high current density of up to about 120 JLAcm-” [ 2,3 ] and to a dosein the order of 10’7-‘8ions cm-’ within a feasibly short time. In addition, implantation with a high current density can trigger thermal annealingsimultaneously, which is of benefit for the formation of some expected Al-based intermetallic compounds [ 41.The objective of this study was to investigate the possibility of improving the surface propertiesof Al thin films by high current Ni- and Fe-ion implantation with an intention to * Corresponding author. Tel.: + 86-010-62785768: Fax: i 86-010.62771160; E-mail:[email protected] 0?53-0584/98/$19.@l 0 1998 Elsevier Science S.A. All rights reserved. PffS0?51-0584t98)00070-~
form a hard aluminide( s) surfacelayer extending deeperthan in the caseof conventional implantation techniques. 2. Experimental The samplesin this study were pure Al films with a total thicknessof 4000 or 7000 A depositedon thermally grown SiO, on Si wafers in an electron-gun evaporation with a vacuum level of lo-” Pa. The sampleswere then loaded on to a steel-madesample-holderin the target chamber of the MEVVA implanter with a vacuum level of 3 X IO-” Pa. As the implanter hasno analyzing magnet, the extracted Ni ions were analyzed to consist of 30% Ni+, 64% Ni2+, and 6% Ni” I, respectively, and the Fe ions were analyzed to consist of 25% Fe+? 68% Fe’*, and 7% Fe3+, respectively. During implantation, no deliberatecooling wastakenforthe samples. The sampleswith 4000 A Al films were implanted with Ni ions, at an extraction voltage of 4.5kV with the current densities varying from 2.5 to 89 FA cm-’ to dosesof (OS6) X IO” ions cm-‘. Some of the sampleswith 7000 A Al films were implanted by Fe ions at 30 kV with the current densitiesvarying from 2.5to 101 p,A cn-’ to dosesranging from 5 X lOI to 1 X 10” ions cm-‘, Some other samples with 7000 A Al films were doubly implanted with Fe ions, i.e. first with a current density of 76 andthen 25 p,A cm ~’ to a doserangeof ( l-3) X 1017ions cm-‘.
310
B.X. Liu, K. Y. Gno /Moieriols
Table I Formation
of Al,Ni
and A1,3Fe4 phases with good crystallinity
Current density (&A cme2) Ion dose ( X 10” cm-‘) Ni- or Fe-ion implanting time (min) Aluminide and its crystallinity (by XRD) Note: fine means almost all the diffraction
89 3 22 Al,Ni
Chrmis?p
as confirmed 89 6 38 AI,Ni
fine
fine
3. Results and discussion 3. I. Formation of Ni- rind Fe-alrminides In the samples implanted by Ni and Fe ions with a current density from 25 to 101 FA cmw2 to an ion dose from 3 X lOI to 1 X 10” ions cme2, Ni- and Fe-aluminides were formed immediately after implantation.Table 1summarizesthe XRD identification results that confirmed the formation of Ni-AI and Fe-Al phasesin the Al samples.Fig. 1 showsa typical XRD pattern of a sample implanted by Fe ions with 101 PAcm V-2at a doseof 1X lOI ions cm-‘. One seesthat almostall the strong diffraction linesof the AI,,Fe, phaseare shown, indicating a good crystallinity. While for Ni-ion implantation with a current density of 89 PA crnm2 to a dose of (3-6) X lOI ions cm-*, almost all the diffraction lines of the A13Niphasewereobservedin the XRD pattern,indicating a fine crystallinity of the formed compound. The RBS spectraof the Al films alsoconfirm the formation of aluminides. Almost all RBS patterns show clearly the plateausat the frontier of Al lines, suggestingthe formation
20
I
I I
I
1 25
t 35
* 30 Two
Fig. 1. XRDspectrumofthe atadoseoflX10’8ionscm-2.
Theta
sampleimplanted
54 (1998) 309-312
by XRD analysis 101 3 5 AlL3Fe4 good
101 6 25 Al,,Fe,
101 10 30 Alj3Fe4 fine
good
16125 313 20150 Al13Fe4 very good
lines were detected, good means all the strong lines were shown, and very good means the situation
X-ray diffraction (XRD) was conducted with a step-scan measurement using a D/max-RE? diffractometer with Cu radiation at 40 kV and 100 mA. Rutherford backscattering spectrometry (RBS) was performed with 2.023 MeV He ions. The HXD-IOOOA digital microhardness scale was employed to measure the Vickers/Knoops hardness of the samples.
21
and Phpks
I 40
I 45
I 50
(degree) by Fe-ionswith
101 PA cm-’
in between.
of an aluminide. In a later section, one of the RBS spectra will be shown. Under above experimentalconditions, the Ni and Fe concentration in the implanted region can reach about 25 at.‘%, which is aroundthe value to fulfill the chemicalstoichiometry requirement for fomring the corresponding 1:3 Ni- and Fealuminides. The temperature rise of the Al films during implantation can be estimatedby the method suggestedby Zhu and Liu [ 41. Accordingly, when implanting with acurrent density of25 ~.L*A cm-‘, the temperatureswere estimated to rise to around250°C (45 kV, Ni ) , and 200°C ( 30 kV, Fe), which were high enough to enable the A13Ni and A1,3Fe4 phaseto be formed as shown by our experimental results earlier. When implanting with aneven higher current density, e.g. 101 p,A crnm2(30 kV, Fe), the temperaturerise could go up to ashigh as55O”C,which wasbelieved to be of benefit for improving the crystallinity of the forming aluminides.All thesewere shown by their correspondingXRD resultslisted in Table 1. 3.2. Depth profiles of the implantedNi- and Fe-ions 3.2.1. High doseirnplnntation to extend modijed lqe~ thickness Fig. 2 shows the concentration profiles of Ni in the Al matrix deduced from the RBS spectra. At a fixed dose of 3 X lOI ions cm-*, curve (a) corresponding to a current
’ 0 : QO@, 1 Oo cl O” 0
1000
bb l
c
Od 0
0
2000 3000 Depth (A) Fig. 2. The concentration vs. depth profiles of Ni in Ni-ion films with various current densities and ion doses: (a) 3X10”cm-2: (b) 5lkAcm-‘, 3XlO”cm-I; (c) 3 X 10” cm-‘; (d) 89 PA cm-‘, 6~ 10” cm-‘.
4000 implanted Al 25 &A cm-‘, 89kAcm-‘.
Channel
Number
Fig. 3. RES of the sample implanted 3 X 10” cm-’ and then 25 p.A cm-‘/3
by Fe-ions with firat 76 PA cm-‘/ X 10” cm-*.
density of 25 FA cm -’ is an expanding Gaussian distribution. Curves (b) and (c) correspond to the current densities of 5 1 and 89 FA cmv2, respectively, in which a nearly flat concentration profile extending to 3000-4000 A implies that significant ion mixing and thermal diffusion have taken place, and that the higher the current density, the greater the amount of diffusion. Curve (a) also shows that about 78 ~01% of the A13Ni phase is located in a surface layer 1000 A thick. While for curve (c), a uniform A13Ni phase was spread over a considerably broad region of 4000 A in thickness and its volume percentage versus the pure Al was about 25 vol.%. At a fixed current density of 89 PA cm-‘, though the depth profiles of Al,Ni shown by curves (c) and (d), corresponding to doses of 3 X lOI and 6 X 10” cmT2, respectively, are comparable, the higher dose implantation resulted in an increase of volume percent of Al,Ni in a 1000 A-thick surface layer up to a value of 57-85 vol.%, which is much higher than that of the 3 X lOI cm-’ implanted one. In other words, implanting with high current density to a very high dose can lead to the formation of Al,Ni in the near surface, as well as to a depth extending to around 4000 A. This was also true for the cases of Fe-ion implantation. For instance, the concentration profiles of the samples implanted with Fe ions with current densities of 25 and 101 FA cm-’ at ion doses of 3 X lo”, 6 X lo”, 1 X lOi* ions cmF2 are similar to the case of Ni-implantation, discussed earlier. 3.2.2. DunI implnrztation to extend modified layer thickness
According to the above results, by implanting with a low current density, the higher the ion dose, the deeper the depth of the modified layer, because of the longer time of implantation required to enhance the diffusion of the implanted Fe Table 2 The microhardness
of the Al films implanted
Current density (FA cm-‘) Ion dose ( X IO” ctnS2) Ni-implanted. load 2 g Fe-implanted, load 2 g
by Ni- and Fe-ions
0 0 114 38.4
Vickers and Knoops hardnesses were used for Ni-ion a The current density is 89 PA cm-‘.
25 3 118 48.2 and Fe-ion
or Ni ions into the Al matrix. In fact, the depth was several times larger than that deduced by TRIM code calculation. While implanting with a high current density, a medium dose required a shorter time of implantation than that in the previous case and a relatively thinner modified layer was obtained, which was approximately the value calculated by TRIM code. On the contrary, in this case, the concentration of implanted ions as well as the so-formed hardening aluminides can be very high. Considering the respective features involved in the above two different conditions, a dual implantation of Fe-ions into Al was designed to attain a relatively thick modified layer together with a high concentration of the hardening Al,,Fe, compound standing in the near surface of the Al matrix. In the dual implantation process, the samples were first implanted by Fe ions with a high current density of 76 p.A cme2 to a dose of 3 X lOI ions cm-‘, and followed by an implantation with a low current density of 25 IJ-A cmw2 to a dose of 3 X 10” ions cm-‘. Fig. 3 is an RBS spectrum for a sample treated by this dual implantation. One seesfrom the figure that an extended modified layer of about 4500 A together with an almost unique Al ,,Fej compound in the near surface of 1000 A in thickness are obtained, which is believed to improve the surface troboligical property of Al. 3.3. Microhnrdness of the modifiedAl films
Either Vickers or Knoops microhardness wasmeasured for the Al samples after ion implantation with various ion beam parameters and the results are summarized in Table 2. It can be seen that the microhardness of the implanted films was considerably increased. Generally, the Knoops microhardness of the Fe-implanted Al films could increase up to 70% after a dual implantation. While the Vickers microhardness of the Ni-implanted Al films could increase by about 40% after high dose implantation. It is worthwhile noting the case for Fe ion implantation, that when the modified layer was deeper, the increase of the microhardness was considerably higher than for those with thinner modified layers, and that the concentration of the hardening Al, ,Fe, compound in the surface layer helped in increasing the microhardness. In addition, another round microhardness measurement was also performed by using a load of 5 g and the results showed a same trend of microhardness increment. As the modified layers were relatively thin, which was achieved by ion implantation, the microhardness measured by using high loading might be influenced by the substrate underneath, e.g. SiO, substrate in our case.
under various conditions 25 6 52.1 implanted
51 3 131 50.7
51 6 59.2
samples, respectively
101 3 145 B 50.6 (in a unit of kg mm-*)
101 6 163 ’ 54.9
101 10 61.3
76125 313 67.8
312
B.X. Liu, K. Y. Gao / Matrrinls
Chemistt~
4. Conclusions The major hardening phases formed by high current Niand Fe-ion implantation into Al were of Al,Ni and AI,,Fe, compounds, respectively. During implantation. the temperature rise caused by ion beam heating resulted in a simultaneous thermal annealing, which was responsible for the formation of the aluminide compounds with a fine crystallinity. The Al,Ni and Al ,,Fe, phases so formed in turn improved the surface tribological property of the Al films, as evidenced by a considerable increase of the microhardness. A process of dual implantation by varying ion beam parameters provided an opportunity to extend the modified layer thickness as well as to raise the surface concentration of the hardening aluminide compound standing in the near surface of the Al films.
and Physics 54 (I 998) 309-312
Acknowledgements This study was partially supported by the National Natural Science Foundation of China. The authors would like to express their thanks to the researchers at &heInstitute of Low Energy Nuclear Physics of Beijing Normal University, and at Laboratory of Heavy Ion Physics of Peking University as well as the researchers at the Institute of Materials Research of Tsinghua University for their kind help. References [ 11 C.W. Draper, J.M. Poare. Int. Mater. Res. 30 ( 1985) 85. [ 21 LG. Brown, J.E. Gavin, R.A. MacGill, Appl. Phys. Lett. 62 (1985) 358. [3] X.J. Zhang, F.S. Zhou, H.X. Zhang, S.J. Zhang, Q. Li, Z.E. Han, R:y. Sci. Instrum. 64 (1992) 2431. [4] D.H. Zhu, B.X. Liu, J. Appl. Phys. 77 (12) ( 1995) 6257.