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The enhanced adhesion properties of metallic thin films on metal substrates by ion implantation
Vacuum/volume 39/numbers Printed in Great Britain
2-4jpages
405 to 407/l
0042-207X/89$3.00+.00 Pergamon Press plc
989
The enhanced adhesion properties of metallic films on metal substrates by ion implantation Zuwu
Lai”, Renhe Liu, Jiming
Chengdu,
Wang
and Baifu Xie, Southwest
institute
ofApplied
Electronics,
thin PO Box 527,
PRC
A new method for preparing well bonded metallic thin coating films on metal substrates by low energy ion implantation is described. The implantation N’ energy is 100 keV, and the thickness of gold film is 300 A. The adhesion is significantly enhanced when the implantation dose reaches 5x 1015 ions cmm2. The adhesion is determined by wearing and peeling tests. The performance of corrosion-resistance will be improved also. For further improvement, the multilayers may be used.
1. Introduction In order to improve the surface performances of some metal materials, such as corrosion-resistance, wear-resistance and electric breakdown, etc., thin metallic coating films are widely used in practice. But ordinarily these thin films are not firm enough, it is easy to fray them by friction. Therefore the adhesion of metallic thin films to substrates is an important concern in application. The use of ion beam implantation as an effective method for surface modification has increasing interest’. But, in general, rather high implantation energy will be required. Besides this, a uniform, pinhole-less and wear-resistant surface is especially important for corrosion-resistant application. The enhanced adhesion phenomena of thin films on substrates by means of ion implantation have been reported in recent years’. In this article, we will describe a new method for preparing firm metallic thin coating films on metal substrates. We selected steel as the substrate material and gold as the coating thin film material here, because the adhesion of gold film to steel is not very good and the gold itself is non-corrosive. Thus it is rather easy to compare the performances of adhesion and corrosion for ion implanted coating layers. For further improvement bf the surface performance, a multilayer will be used. This method opens up broad prospects for preparing high quality coating layers in corrosionresistant applications. 2. Experimental method 2.1. General description. In our experiment, the most common ion implantation facility was used. The ion source was nitrogen gas, the specimens were perpendicular to the N+ ion beam with energy 100 keV, and beam current 5G200 PA. The scanning area of the ion beam was 80 x 80 mm. There were two sorts of steel specimens in our experiments, a circular one with diameter 18 mm, and a rectangular one with dimensions 5 x 18 mm. The thickness of all the specimens was 1 mm. These specimens were put in the vacuum chamber so as to evaporate a thin gold film with thickness 300 8, onto them. After * Present address : Graduate School, Physics, PO Box 2101, Beijing, PRC.
China Academy
of Engineering
implanting the N’ ions to a given dose, the surface performances of these specimens were tested. 2.2. Implantation energy and film thickness. A reasonable implantation energy depends on the film thickness. Typical experimental results are shown in Table 1. It is shown that when the film thickness is 500 A, the enhanced adhesion effect has not been observed even at the implanted dose up to 1 x 10” ions cm-*, under 80-100 keV. But the adhesion of the thin film with thickness 300 A will be significantly enhanced when the implanted dose reaches 4 x 10” ions cm-*. So in our experiment, the implanted energy and film thickness were fixed at 100 keV and 300 A, respectively. 2.3. Wearing test. The wearing test apparatus is shown in Figure I. The specimen was fixed on the turning holder which was driven by a small motor with a speed of 30 turns mini. A teflon bar coated with wiping paper was put on the specimen and pressed down with a load. Then the wear-resistant performance may be presented by the turning number which will cause the gold film to be frayed. 2.4. Peeling test. The peeling test machine was used for determining the critical value of the adhesive strength of the thin film. A steel bar with diameter 8 mm and length 60 mm was glued onto the surface of the specimen by means of high quality glue. The bar was positioned in the center of the surface of the specimen
Table 1. Implanted energy (kev)
Implanted dose (ions cm-‘)
Unimplanted 80 80 80 100 80 100
1x 3x 5x 8x 1x 4x
10’6 lOI lOI 1016 10” 10’5
Film thickness
Wearing
test
(A)
Turns
(9)
500 500 500 500 500 500 300
-1 -5lOO
60 60 60 60 100 100 60
Pressure
405
Zuwu Lai et al: Adhesion properties of metallic thin films
Implanted
Figure 1. The schematic holder, (3) specimen, (7) load
diagram of wearing test. (1) Motor, (2) turning (4) wiping paper, (5) Teflon bar, (6) press holder,
Figure 3. The wearing resistance
200 z s r E
150
dose
(tons
cmm2)
vs the implanted
ion dose
c 10'5 Imptanted
Figure 2. The schematic diagram of peeling test. (1) Steel specimen, gold layer by ion implantation, (3) glue, (4) steel bar, (5) holder.
(2)
and perpendicular to it, as shown in Figure 2. When the down pressure reached a certain critical value, the gold layer was pulled apart. The critical pressure means the adhesive strength of the gold layer. 2.5. Corrosion test. In order to test the corrosion-resistant performance of the specimen, we used HNO, of concentrations 30, 60 and 65%, respectively, and examined them continuously in a certain time interval. As the steel may be corroded by nitric acid, while gold not, the corrosion of the specimen was done through the pinholes on the gold layer. The corrosion-resistant performances were determined in comparison with the specimen surfaces of ion implanted and unimplanted gold layers with various concentrations of HNO,.
Figure 4. The curve of the adhesion
dose (ions
strength
cm’)
vs the implanted
ion dose.
tation dose was up to 6 x 10” ions cm-‘. These specimens were immersed in 30% HNO, for 1 h. For further improvement of corrosion-resistance, a multilayer may be used. The second gold film will be evaporated on the previous implanted gold film under a dose of 1 x 10 ions cm ‘. Then the corrosion-resistant performance will be improved significantly. The multilayer is very compact and the thickness is increased. The above results indicate that the wear-resistance, adhesion strength and corrosion-resistance all increase rapidly when the implanted dose is up to 4-6 x 10” ions cm 2.
4. Analysis and discussion The boundary of the gold layer with steel substrate has been analyzed by SIMS method after implantation. The hybrid of gold
3. Results Figure 3 shows the wearing-resistance (in wearing turns) vs the implanted ion dose. From Figure 3, one can clearly see that the wear-resistance increases rapidly as the implanted dose reaches 4-6 x 10” ions cm-‘. The gold layer will not be worn out at higher dosage. Figure 4 shows the adhesion strength vs the implanted ion dose by the peeling test. Figure 5 shows the corrosion boundary of the implanted and unimplanted parts of the specimen, which was immersed in 50% HNO, for 1 min. For corrosion treatments mentioned above, it may be seen that the surfaces will be corroded very seriously at unimplanted part. There are etching pits in the damaged region. But there was no corrosion observed when the implan406
Figure 5. The corrosion implanted
part.
of the boundary.
(A) Implanted
part;
(B) un-
Zuwu
Lai et al: Adhesion
properties
of metallic thin films
5. Conclusions
\
I
IO 0
I
I
I
2
3
Min
Figure 6. The profiling
of the interface.
(1) Unimplanted
; (2) implanted.
layer with steel substrate at the surface can be observed. The profile of their distribution is given in Figure 6. From this figure one can see that the gold penetrates into the steel substrate, and the steel is scattered back into gold layer. Thus the gold and steel will be very well mixed at the interface, such as when welded. Therefore the gold layer is very well bonded to the substrate. The mixing process can be explained by a cascade of collisions of the gold and steel atoms3. In our case, there is low energy implantation and the implanting ions N+ carry the energy and interact with the gold atoms through screened Coulomb collisions, and transfer the energy to the gold atoms. If the energy is high enough, a cascade of collisions will occur. So, if the implanting energy is too low or the gold layer is too thick, the energy of the gold atoms is not enough to form the cascade process and to penetrate into the substrate to form the mixing and the enhanced adhesion effect will not be observed. When the thickness of the gold layer is rather too thick, the implantation energy must be raised correspondingly. In the meantime, the implanted dose must be high enough, but if dose is too high, such as up to 1 x 10” ions cm-*, the corrosion-resistance will be worse. So we must carefully select reasonable values of the implanted energy, layer thickness and implanted dose.
(1) The new method is very effective and simple, especially for corrosion-resistance. Usually, the ion implantation method means that the ions of certain elements are injected into the surface of some metal materials directly. The implanted energy must be higher and preparing of some metallic ion sources, such as platinum, rhodium, molybdenum and tantalum are more difficult. Besides this, for a big workpiece the uniformity of the implanted layer is not perfect and will not be very thick. So the enhanced adhesion method of preparing high quality coating layers is more advantageous than the usual ion implantation method for corrosion-resistance. (2) Under correct choice of implanted energy and thickness of the coating layer, the wearing-resistance, adhesion strength and corrosion-resistance increase significantly when the implanted dose reaches up to a certain critical value. But too high an implanted dose is not necessary and harmful to corrosion-resistance. There exists an optimum condition. In our case (100 keV, 300 A), the choice of implanted dose is l-5 x lOI ions cm-*. (3) The enhanced adhesion effect is due to the mixing process, and the nitrogen ions act only as the energy transporter. So it is not the only requisite element used. Other ions, such as B+, Ar+, etc., can all be used for implanting also. (4) This method can be applied to any thin layer and substrate materials, so it opens up broad prospects, especially for corrosion-resistant applications. (5) In order to further improve the corrosion-resistant performance, multilayers may be used. The multilayers are made by means of evaporating or electroplating a new coating layer on the previously implanted layer. In general, the outside coating layer is well enough bonded to the previous, but, if necessary, it can be hardened also by ion implantation again. References ‘N E Harltey, W E Swindleharst, G Deamaley and J F Turner, J Mater Sci, 8,900 (1973). ‘J E Griffith, Y Qiu and T A Tombrello, Nucl Instrum Meth, 198, 607 (1982). ‘S Jacobson, B Johnsson and B Sundgvist, Thin Solid Films, 107, 89 (1983).
407
2-4jpages
405 to 407/l
0042-207X/89$3.00+.00 Pergamon Press plc
989
The enhanced adhesion properties of metallic films on metal substrates by ion implantation Zuwu
Lai”, Renhe Liu, Jiming
Chengdu,
Wang
and Baifu Xie, Southwest
institute
ofApplied
Electronics,
thin PO Box 527,
PRC
A new method for preparing well bonded metallic thin coating films on metal substrates by low energy ion implantation is described. The implantation N’ energy is 100 keV, and the thickness of gold film is 300 A. The adhesion is significantly enhanced when the implantation dose reaches 5x 1015 ions cmm2. The adhesion is determined by wearing and peeling tests. The performance of corrosion-resistance will be improved also. For further improvement, the multilayers may be used.
1. Introduction In order to improve the surface performances of some metal materials, such as corrosion-resistance, wear-resistance and electric breakdown, etc., thin metallic coating films are widely used in practice. But ordinarily these thin films are not firm enough, it is easy to fray them by friction. Therefore the adhesion of metallic thin films to substrates is an important concern in application. The use of ion beam implantation as an effective method for surface modification has increasing interest’. But, in general, rather high implantation energy will be required. Besides this, a uniform, pinhole-less and wear-resistant surface is especially important for corrosion-resistant application. The enhanced adhesion phenomena of thin films on substrates by means of ion implantation have been reported in recent years’. In this article, we will describe a new method for preparing firm metallic thin coating films on metal substrates. We selected steel as the substrate material and gold as the coating thin film material here, because the adhesion of gold film to steel is not very good and the gold itself is non-corrosive. Thus it is rather easy to compare the performances of adhesion and corrosion for ion implanted coating layers. For further improvement bf the surface performance, a multilayer will be used. This method opens up broad prospects for preparing high quality coating layers in corrosionresistant applications. 2. Experimental method 2.1. General description. In our experiment, the most common ion implantation facility was used. The ion source was nitrogen gas, the specimens were perpendicular to the N+ ion beam with energy 100 keV, and beam current 5G200 PA. The scanning area of the ion beam was 80 x 80 mm. There were two sorts of steel specimens in our experiments, a circular one with diameter 18 mm, and a rectangular one with dimensions 5 x 18 mm. The thickness of all the specimens was 1 mm. These specimens were put in the vacuum chamber so as to evaporate a thin gold film with thickness 300 8, onto them. After * Present address : Graduate School, Physics, PO Box 2101, Beijing, PRC.
China Academy
of Engineering
implanting the N’ ions to a given dose, the surface performances of these specimens were tested. 2.2. Implantation energy and film thickness. A reasonable implantation energy depends on the film thickness. Typical experimental results are shown in Table 1. It is shown that when the film thickness is 500 A, the enhanced adhesion effect has not been observed even at the implanted dose up to 1 x 10” ions cm-*, under 80-100 keV. But the adhesion of the thin film with thickness 300 A will be significantly enhanced when the implanted dose reaches 4 x 10” ions cm-*. So in our experiment, the implanted energy and film thickness were fixed at 100 keV and 300 A, respectively. 2.3. Wearing test. The wearing test apparatus is shown in Figure I. The specimen was fixed on the turning holder which was driven by a small motor with a speed of 30 turns mini. A teflon bar coated with wiping paper was put on the specimen and pressed down with a load. Then the wear-resistant performance may be presented by the turning number which will cause the gold film to be frayed. 2.4. Peeling test. The peeling test machine was used for determining the critical value of the adhesive strength of the thin film. A steel bar with diameter 8 mm and length 60 mm was glued onto the surface of the specimen by means of high quality glue. The bar was positioned in the center of the surface of the specimen
Table 1. Implanted energy (kev)
Implanted dose (ions cm-‘)
Unimplanted 80 80 80 100 80 100
1x 3x 5x 8x 1x 4x
10’6 lOI lOI 1016 10” 10’5
Film thickness
Wearing
test
(A)
Turns
(9)
500 500 500 500 500 500 300
-1 -5
60 60 60 60 100 100 60
Pressure
405
Zuwu Lai et al: Adhesion properties of metallic thin films
Implanted
Figure 1. The schematic holder, (3) specimen, (7) load
diagram of wearing test. (1) Motor, (2) turning (4) wiping paper, (5) Teflon bar, (6) press holder,
Figure 3. The wearing resistance
200 z s r E
150
dose
(tons
cmm2)
vs the implanted
ion dose
c 10'5 Imptanted
Figure 2. The schematic diagram of peeling test. (1) Steel specimen, gold layer by ion implantation, (3) glue, (4) steel bar, (5) holder.
(2)
and perpendicular to it, as shown in Figure 2. When the down pressure reached a certain critical value, the gold layer was pulled apart. The critical pressure means the adhesive strength of the gold layer. 2.5. Corrosion test. In order to test the corrosion-resistant performance of the specimen, we used HNO, of concentrations 30, 60 and 65%, respectively, and examined them continuously in a certain time interval. As the steel may be corroded by nitric acid, while gold not, the corrosion of the specimen was done through the pinholes on the gold layer. The corrosion-resistant performances were determined in comparison with the specimen surfaces of ion implanted and unimplanted gold layers with various concentrations of HNO,.
Figure 4. The curve of the adhesion
dose (ions
strength
cm’)
vs the implanted
ion dose.
tation dose was up to 6 x 10” ions cm-‘. These specimens were immersed in 30% HNO, for 1 h. For further improvement of corrosion-resistance, a multilayer may be used. The second gold film will be evaporated on the previous implanted gold film under a dose of 1 x 10 ions cm ‘. Then the corrosion-resistant performance will be improved significantly. The multilayer is very compact and the thickness is increased. The above results indicate that the wear-resistance, adhesion strength and corrosion-resistance all increase rapidly when the implanted dose is up to 4-6 x 10” ions cm 2.
4. Analysis and discussion The boundary of the gold layer with steel substrate has been analyzed by SIMS method after implantation. The hybrid of gold
3. Results Figure 3 shows the wearing-resistance (in wearing turns) vs the implanted ion dose. From Figure 3, one can clearly see that the wear-resistance increases rapidly as the implanted dose reaches 4-6 x 10” ions cm-‘. The gold layer will not be worn out at higher dosage. Figure 4 shows the adhesion strength vs the implanted ion dose by the peeling test. Figure 5 shows the corrosion boundary of the implanted and unimplanted parts of the specimen, which was immersed in 50% HNO, for 1 min. For corrosion treatments mentioned above, it may be seen that the surfaces will be corroded very seriously at unimplanted part. There are etching pits in the damaged region. But there was no corrosion observed when the implan406
Figure 5. The corrosion implanted
part.
of the boundary.
(A) Implanted
part;
(B) un-
Zuwu
Lai et al: Adhesion
properties
of metallic thin films
5. Conclusions
\
I
IO 0
I
I
I
2
3
Min
Figure 6. The profiling
of the interface.
(1) Unimplanted
; (2) implanted.
layer with steel substrate at the surface can be observed. The profile of their distribution is given in Figure 6. From this figure one can see that the gold penetrates into the steel substrate, and the steel is scattered back into gold layer. Thus the gold and steel will be very well mixed at the interface, such as when welded. Therefore the gold layer is very well bonded to the substrate. The mixing process can be explained by a cascade of collisions of the gold and steel atoms3. In our case, there is low energy implantation and the implanting ions N+ carry the energy and interact with the gold atoms through screened Coulomb collisions, and transfer the energy to the gold atoms. If the energy is high enough, a cascade of collisions will occur. So, if the implanting energy is too low or the gold layer is too thick, the energy of the gold atoms is not enough to form the cascade process and to penetrate into the substrate to form the mixing and the enhanced adhesion effect will not be observed. When the thickness of the gold layer is rather too thick, the implantation energy must be raised correspondingly. In the meantime, the implanted dose must be high enough, but if dose is too high, such as up to 1 x 10” ions cm-*, the corrosion-resistance will be worse. So we must carefully select reasonable values of the implanted energy, layer thickness and implanted dose.
(1) The new method is very effective and simple, especially for corrosion-resistance. Usually, the ion implantation method means that the ions of certain elements are injected into the surface of some metal materials directly. The implanted energy must be higher and preparing of some metallic ion sources, such as platinum, rhodium, molybdenum and tantalum are more difficult. Besides this, for a big workpiece the uniformity of the implanted layer is not perfect and will not be very thick. So the enhanced adhesion method of preparing high quality coating layers is more advantageous than the usual ion implantation method for corrosion-resistance. (2) Under correct choice of implanted energy and thickness of the coating layer, the wearing-resistance, adhesion strength and corrosion-resistance increase significantly when the implanted dose reaches up to a certain critical value. But too high an implanted dose is not necessary and harmful to corrosion-resistance. There exists an optimum condition. In our case (100 keV, 300 A), the choice of implanted dose is l-5 x lOI ions cm-*. (3) The enhanced adhesion effect is due to the mixing process, and the nitrogen ions act only as the energy transporter. So it is not the only requisite element used. Other ions, such as B+, Ar+, etc., can all be used for implanting also. (4) This method can be applied to any thin layer and substrate materials, so it opens up broad prospects, especially for corrosion-resistant applications. (5) In order to further improve the corrosion-resistant performance, multilayers may be used. The multilayers are made by means of evaporating or electroplating a new coating layer on the previously implanted layer. In general, the outside coating layer is well enough bonded to the previous, but, if necessary, it can be hardened also by ion implantation again. References ‘N E Harltey, W E Swindleharst, G Deamaley and J F Turner, J Mater Sci, 8,900 (1973). ‘J E Griffith, Y Qiu and T A Tombrello, Nucl Instrum Meth, 198, 607 (1982). ‘S Jacobson, B Johnsson and B Sundgvist, Thin Solid Films, 107, 89 (1983).
407