- Email: [email protected]
Chemical properties of ion beam sputter coated metal tubes
Nuclear Instruments
and Methods in Physics Research
B 127/128
(1997) 775-778
RIUMI B
Beam Interactions with Materials 8 Atoms
EISEVIER
Chemical properties of ion beam sputter coated metal tubes W. Ensinger
*
University of Augsburg, Institute of Physics. 86135 Augsburg. Germuny
Abstract The inner walls of iron and aluminum tubes were coated with chromium and tantalum by means of conical sputter targets which were moved through the tube while they were bombarded with an argon ion beam. In electrochemical tests in corrosive aqueous solutions of buffered acetic acid and salt brine the coatings appeared well adhering and able to considerably reduce corrosion of the tubes.
1. Introduction Tubes of various materials are in widespread industrial use. Often corrosion or wear performance is not sufficient, beacause the tube base material does not fulfil the requirements of a particular application. In analogy to flat workpieces, tubes can be protected from chemical and mechanical degradation when they are coated. For this purpose, conventional methods such as galvanic deposition and chemical vapour deposition can be used. The material to be deposited is transported by a medium, either liquid or gaseous, into the tube. In principle, it is also possible to use physical vapour deposition techniques. However, the line-of-sight character of a number of PVD techniques renders the deposition difficult because the material to be deposited has to enter the tube under very flat angles to the wall normal. The problem of coating inner tube walls by PVD techniques can be solved by introducing the source of the material to be deposited into the tube. A sputter target which is hit by an ion beam entering the tube parallel to its axis is moved through the tube [ 11. Thus, material from the target is deposited onto the tube walls and forms a coating. This paper deals with protection of metal tubes from aqueous corrosion which may occur when corrosive solutions are transported through the tube such as in chemical industry. 2. Experimental
The device for sputter coating tubes consists of an ion implanter which provides energetic ions for sputtering and of a target chamber equipped with a holder for the tubes and a conical sputter target which is moved through the
* Tel: + 49-821-598-3445. Fax: + 49-821-598-3425. E-mail: [email protected]
0168-583X/97/$17.00
tubes by means of a worm-drive. Figs. 1 and 2 show schematic presentations of the set-up and the processes which take place inside the tube. A beam of ions from the ion source is collimated by means of two screens with circular apertures. The ion beam is directed into the tube along the axis of the tube, and impinges onto the conical sputter target. Atoms from the target am deposited onto the inner walls of the tube. The sputter target is moved through the tube to provide a complete inner coating of the tube. The facility has been described in detail elsewhere
1121. Tubes of iron (purity 99.6%) (inner diameter 10 mm, length 100 mm) and pure aluminum (99.5%) (inner diameter 8 mm, length 30 mm> were prepared by drilling holes in rods. The holes were polished by SIC coated rods and rotating cotton buds with diamond paste. Prior to coating, they were degreased in acetone and isopropanole in an ultrasonic bath. Conical sputter targets of chrome and tantalum with diameters from 6 to 8 mm and 4.5 to 60” cone angle were moved through the tubes with speeds from 0.005 to 0.4 mm/s. A beam of argon ions which was collimated by two screens with chrome and tantalum inserts with apertures of 6 and 8 mm was directed into the tubes onto the sputter targets. The ion currents which were determined by charge collection at the sputter targets ranged from 10 to 60 p.A/cm2. For evaluating the corrosion behaviour of the samples, electrochemical methods were used. For preparing appropriate specimens, the tubes were cut in half or in four segments. The samples were connected to a wire with conductive glue. Contact area, sample backside and edges were covered with stop-off resin. In a three-electrode set-up with Standard Calomel reference Electrode (SCE) and platinum counter electrode, the samples were potentiodynamically polarized, with a potential scan rate of 36 V/h in case of iron and 0.2 V/h in case of aluminum.
0 1997 Elsevier Science B.V. All rights reserved
PN SOISS-583X(97)00005-0
IVb. SYNTHESlS
OF METALS
776
W. Ensinger/Nucl.
Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 775-778 flan e with fe ecf& rough
beam collimator
conical sputter target
4
voltage
Fig. 1. Schematicpresentationof set-up for sputter coating tube inner walls.
The current response was recorded logarithmically. The corrosive solutions were aerated buffered acetic acid of pH 5.6 and aqueous neutral 2 wt% NaCl solution at a temperature of 300 K. The composition of the films was determined by Auger electron spectroscopy combined with 3 keV argon ion etching. The sensitivity factors were gained from measurements of the pure metals. For evaluating the adhesion of the films to the substrates, pin pull measurements on identically prepared flat samples were carried out.
materials as the respective sputter targets to avoid contaminations by forward sputtering. The argon ion beam itself was cleaned from impurities from the ion source by mass separation in a magnet. As a consequence, Auger electron analysis did not show any relevant amounts of foreign metal. Fig. 3 shows an Auger concentration profile of an approximately 0.14 p,rn thick chromium film on aluminum. The film is mainly contaminated with oxygen and carbon. The incorporation of these arise from leakage of the chamber, desorption of mainly water from the chamber walls and hydrocarbons from the pumping system. In the 100
3. Results and dicussion
Al
r
For this study, pure metals were used to avoid the influence of alloying elements. The beam collimating screens in front of the tubes were made of the same
/
60
60
40
sputtering and deporition film
20
0 ion
reflection + ion beam mixing, ion implantation beam collimator
Fig. 2. Schematic inside the tube.
presentation
of processes
which
rake
place
10
20
30
sputter etch time [mid Fig. 3. Auger electron spectroscopic chromium film on aluminum.
concentration
profile
of a
W. Ensinger/
Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 775-778
interface between film and substrate, an increased amount of oxygen can be found which is assigned to the native oxide at the surface of aluminum. The profiles of tantalum look similar, likewise those of the iron substrates. In the latter, the amount of oxygen in the interface is slightly smaller, in the tantalum films the contaminations were slightly more than in the chrome films. Additionally a small amount of nitrogen was found. With a base pressure in the range of lo-’ Pa, the target chamber exhibits the usual gas pressure of an ion implanter for metal modification. The contaminations from gaseous species are in an expected level. An important feature of thin films is their adherence to the substrate. For an estimation, pin-pull tests with 2 mm diameter pins were carried out. To avoid the influence of the curvature of the tube surface, flat samples were mounted in the chamber and sputter coated from the sputter targets under identical process conditions. In all cases, the limit of the test with values around 80 h4Pa was reached. This is the value of the adhesive/cohesive forces of the epoxy glue, i.e. when the glue itself fails. This value is certainly not very high for metal/metal adhesion, but it demonstrates a certain adherence which is strong enough for a good corrosion performance. For comparison, a chromium film deposited by electron beam evaporation on the same sample gave a value of only 8 MPa. In this case, the interface between film and substrate failed rather than the glue. As mentioned above, the corrosion was tested by electrochemical polarization measurements. In case of iron, a slightly acidic medium was used, dilute acetic acid. In order to avoid changes in the pH-value which would influence the corrosion it was buffered with sodium acetate to a pH value of 5.6. In this environment, iron suffers from active corrosion with high rates [3]. This is expressed in the polarization curves as high anodic dissolution currents.
-1 f
c
-71 3 t I ’ -0.8 -0.6
a
0 + ’ t ’ 8 ’ v ( 1 ’ -0.4 -0.2 0.0
potential
IV]
Fig. 4. Polarization curve of iron and chromium in acetate buffer pH 5.6, (a) uncoated iron, (b) iron coated with chromium, (c) chromium.
-0.76
-0.72
717
-0.68
potential
-0.64
-0.60
IV1
Fig. 5. Polarization curve of aluminum in NaCl solution, uncoated, (b) coated with tantalum.
The coating materials, both chromium and tantalum, are protected by a highly stable and dense oxide film. They corrode passively with dissolution rates orders of magnitude lower in comparison to iron. The anodic dissolution currents in the polarization curves are therefore essentially currents from iron ions of the substrate through micropores. Fig. 4 shows in comparison a polarization curve of uncoated iron, iron coated with chromium and pure chromium. The dissolution currents of the coated sample are reduced by one to two orders of magnitude indicating a reduced corrosion. The behaviour is in between iron and pure chromium which shows considerably lower currents. A tantalum coating leads to similar effects with the currents being smaller by another factor of three to five in comparison to chromium. In case of aluminum as tube material, there is no coating necessary for the dilute only slightly acidic acetate buffer. Aluminum itself is passivated by an oxide film. Only when aggressive anions such as sulfide or chloride are present, corrosion can take place. In this case, the native oxide film is locally dissolved. In these places, aluminum starts to corrode rapidly forming deep pits. Pitting corrosion is therefore a major problem with aluminum. Both chromium and tantalum are not susceptible to pitting corrosion. Hence, they can serve as a protective coating. Fig. 5 shows in comparison the anodic currents of uncoated aluminum and a sample coated with tantalum. Owing to the large amount of chloride ions, the aluminum sample suffers immediately from rapid dissolution when it is anodically polarized. No passivation is oberved. The tantalum film is able to reduce the dissolution currents, although a passivation with considerably lower currents is also not oberved. It appears that both in the case of iron and aluminum the microporosity of the films is of major importance. IVb. SYNTHESISOF METALS
778
W. Ensinger/Nucl.
Instr. and Meth. in Phys. Res. B 127/
128 (1997) 775-778
Table 1 Combinations of ion beam sputter coatings and tube materials for corrosion protection tube material
coating
effect on corrosion behaviour
pure iron
Cr. Ta Cr. Ta Pt Tii(0) Ta FeAI, NiTi B
reduced reduced reduced reduced reduced reduced reduced
pure aluminum tantalum ball bearing steel titanium lowalloy steel stainless steel
corrosion in dilute acetic acid pitting corrosion in salt brine hydrogen embrittlement in sulfuric acid corrosion in dilute acetic acid corrosion in hot nitric acid corrosion in dilute acetic acid corrosion in sulfuric acid
With film thicknesses typically below 0.5 pm, as in the present case, the degree of complete surface coverage and flaws in the film, particularly at rough surfaces, play a major role. Adhesion is apparently no problem, because no delamination of the films was observed under the microscope. The reason for a good adhesion may be found in the energetic and ballistic conditions of the process [4]. First, the sputtered particles are hyperthermal with energies of several eV. The majority of sputtered atoms has energies of 2 to 3 eV, but the range of energies may extend up to several tens of eV. Second, reflected highly energetic argon ions impinge onto the growing film and penetrate, depending on their energy and angle of incidence, through the first monolayers into the interface between film and substrate. There, they deposit energy and momentum and may lead to a strengthening of the bonding forces [5]. The same mechanisms, namely ion assist during deposition, usually lead to film growth with a low void density and low intrinsic microporosity [6]. This is also important and beneficial for a good corrosion performance. Further combinations of tube materials and coatings are listed in Table 1, including examples of alloy coatings.
4. Summary Sputtering of corrosion resistant materials by an ion beam onto the inner walls of metal tubes leads to well adherend coatings which are, to a certain degree, able to protect the tubes from aqueous corrosion. Bombardment of
reference this work this work
Dl [41 [41 171 171
the growing film by reflected ions is presumably the reason for the observed good adhesion of the films. The main problem so far is seen in macropores when the films are formed on rough surface without in-vacua cleaning. An increase in film thickness will certainly improve the situation. During the process, the tubes remained at temperatures below 400 K. This renders this process suitable for temperature-sensitive material such as heat-treated steels and aluminum alloys. When, on the other hand, an elevated temperature can be tolerated, it can be expected that the performance of the films with respect to adhesion, porosity, and other features such as hardness and purity can further be improved. With large-area ion sources and arrays of tubes, an industrial application of this technique on small sized tubes seems possible.
References [I] W. Ensinger, Rev. Sci. Instr. 67 (1996) 318. [2] W. Ensinger, Surf. Coat. Technol. 84 (19%) 434. [3] L.L. Shreir (ed.), Corrosion (Newnes-Butterworth, London, 1976). [4] W. Ensinger, Surf. Coat. Technol. 86-87 (1996) 438. [5] J.E.E. Baglin, in: P. Mazzoldi and G.W. Arnold (eds.), Ion Beam Modification of Insulators. Elsevier, Amsterdam, 1986, p. 585. [6] F.A. Smidt, Int. Mater. Rev. 35 (1990) 61. [7] W. Ensinger, Thin Solid Films, submitted.
and Methods in Physics Research
B 127/128
(1997) 775-778
RIUMI B
Beam Interactions with Materials 8 Atoms
EISEVIER
Chemical properties of ion beam sputter coated metal tubes W. Ensinger
*
University of Augsburg, Institute of Physics. 86135 Augsburg. Germuny
Abstract The inner walls of iron and aluminum tubes were coated with chromium and tantalum by means of conical sputter targets which were moved through the tube while they were bombarded with an argon ion beam. In electrochemical tests in corrosive aqueous solutions of buffered acetic acid and salt brine the coatings appeared well adhering and able to considerably reduce corrosion of the tubes.
1. Introduction Tubes of various materials are in widespread industrial use. Often corrosion or wear performance is not sufficient, beacause the tube base material does not fulfil the requirements of a particular application. In analogy to flat workpieces, tubes can be protected from chemical and mechanical degradation when they are coated. For this purpose, conventional methods such as galvanic deposition and chemical vapour deposition can be used. The material to be deposited is transported by a medium, either liquid or gaseous, into the tube. In principle, it is also possible to use physical vapour deposition techniques. However, the line-of-sight character of a number of PVD techniques renders the deposition difficult because the material to be deposited has to enter the tube under very flat angles to the wall normal. The problem of coating inner tube walls by PVD techniques can be solved by introducing the source of the material to be deposited into the tube. A sputter target which is hit by an ion beam entering the tube parallel to its axis is moved through the tube [ 11. Thus, material from the target is deposited onto the tube walls and forms a coating. This paper deals with protection of metal tubes from aqueous corrosion which may occur when corrosive solutions are transported through the tube such as in chemical industry. 2. Experimental
The device for sputter coating tubes consists of an ion implanter which provides energetic ions for sputtering and of a target chamber equipped with a holder for the tubes and a conical sputter target which is moved through the
* Tel: + 49-821-598-3445. Fax: + 49-821-598-3425. E-mail: [email protected]
0168-583X/97/$17.00
tubes by means of a worm-drive. Figs. 1 and 2 show schematic presentations of the set-up and the processes which take place inside the tube. A beam of ions from the ion source is collimated by means of two screens with circular apertures. The ion beam is directed into the tube along the axis of the tube, and impinges onto the conical sputter target. Atoms from the target am deposited onto the inner walls of the tube. The sputter target is moved through the tube to provide a complete inner coating of the tube. The facility has been described in detail elsewhere
1121. Tubes of iron (purity 99.6%) (inner diameter 10 mm, length 100 mm) and pure aluminum (99.5%) (inner diameter 8 mm, length 30 mm> were prepared by drilling holes in rods. The holes were polished by SIC coated rods and rotating cotton buds with diamond paste. Prior to coating, they were degreased in acetone and isopropanole in an ultrasonic bath. Conical sputter targets of chrome and tantalum with diameters from 6 to 8 mm and 4.5 to 60” cone angle were moved through the tubes with speeds from 0.005 to 0.4 mm/s. A beam of argon ions which was collimated by two screens with chrome and tantalum inserts with apertures of 6 and 8 mm was directed into the tubes onto the sputter targets. The ion currents which were determined by charge collection at the sputter targets ranged from 10 to 60 p.A/cm2. For evaluating the corrosion behaviour of the samples, electrochemical methods were used. For preparing appropriate specimens, the tubes were cut in half or in four segments. The samples were connected to a wire with conductive glue. Contact area, sample backside and edges were covered with stop-off resin. In a three-electrode set-up with Standard Calomel reference Electrode (SCE) and platinum counter electrode, the samples were potentiodynamically polarized, with a potential scan rate of 36 V/h in case of iron and 0.2 V/h in case of aluminum.
0 1997 Elsevier Science B.V. All rights reserved
PN SOISS-583X(97)00005-0
IVb. SYNTHESlS
OF METALS
776
W. Ensinger/Nucl.
Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 775-778 flan e with fe ecf& rough
beam collimator
conical sputter target
4
voltage
Fig. 1. Schematicpresentationof set-up for sputter coating tube inner walls.
The current response was recorded logarithmically. The corrosive solutions were aerated buffered acetic acid of pH 5.6 and aqueous neutral 2 wt% NaCl solution at a temperature of 300 K. The composition of the films was determined by Auger electron spectroscopy combined with 3 keV argon ion etching. The sensitivity factors were gained from measurements of the pure metals. For evaluating the adhesion of the films to the substrates, pin pull measurements on identically prepared flat samples were carried out.
materials as the respective sputter targets to avoid contaminations by forward sputtering. The argon ion beam itself was cleaned from impurities from the ion source by mass separation in a magnet. As a consequence, Auger electron analysis did not show any relevant amounts of foreign metal. Fig. 3 shows an Auger concentration profile of an approximately 0.14 p,rn thick chromium film on aluminum. The film is mainly contaminated with oxygen and carbon. The incorporation of these arise from leakage of the chamber, desorption of mainly water from the chamber walls and hydrocarbons from the pumping system. In the 100
3. Results and dicussion
Al
r
For this study, pure metals were used to avoid the influence of alloying elements. The beam collimating screens in front of the tubes were made of the same
/
60
60
40
sputtering and deporition film
20
0 ion
reflection + ion beam mixing, ion implantation beam collimator
Fig. 2. Schematic inside the tube.
presentation
of processes
which
rake
place
10
20
30
sputter etch time [mid Fig. 3. Auger electron spectroscopic chromium film on aluminum.
concentration
profile
of a
W. Ensinger/
Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 775-778
interface between film and substrate, an increased amount of oxygen can be found which is assigned to the native oxide at the surface of aluminum. The profiles of tantalum look similar, likewise those of the iron substrates. In the latter, the amount of oxygen in the interface is slightly smaller, in the tantalum films the contaminations were slightly more than in the chrome films. Additionally a small amount of nitrogen was found. With a base pressure in the range of lo-’ Pa, the target chamber exhibits the usual gas pressure of an ion implanter for metal modification. The contaminations from gaseous species are in an expected level. An important feature of thin films is their adherence to the substrate. For an estimation, pin-pull tests with 2 mm diameter pins were carried out. To avoid the influence of the curvature of the tube surface, flat samples were mounted in the chamber and sputter coated from the sputter targets under identical process conditions. In all cases, the limit of the test with values around 80 h4Pa was reached. This is the value of the adhesive/cohesive forces of the epoxy glue, i.e. when the glue itself fails. This value is certainly not very high for metal/metal adhesion, but it demonstrates a certain adherence which is strong enough for a good corrosion performance. For comparison, a chromium film deposited by electron beam evaporation on the same sample gave a value of only 8 MPa. In this case, the interface between film and substrate failed rather than the glue. As mentioned above, the corrosion was tested by electrochemical polarization measurements. In case of iron, a slightly acidic medium was used, dilute acetic acid. In order to avoid changes in the pH-value which would influence the corrosion it was buffered with sodium acetate to a pH value of 5.6. In this environment, iron suffers from active corrosion with high rates [3]. This is expressed in the polarization curves as high anodic dissolution currents.
-1 f
c
-71 3 t I ’ -0.8 -0.6
a
0 + ’ t ’ 8 ’ v ( 1 ’ -0.4 -0.2 0.0
potential
IV]
Fig. 4. Polarization curve of iron and chromium in acetate buffer pH 5.6, (a) uncoated iron, (b) iron coated with chromium, (c) chromium.
-0.76
-0.72
717
-0.68
potential
-0.64
-0.60
IV1
Fig. 5. Polarization curve of aluminum in NaCl solution, uncoated, (b) coated with tantalum.
The coating materials, both chromium and tantalum, are protected by a highly stable and dense oxide film. They corrode passively with dissolution rates orders of magnitude lower in comparison to iron. The anodic dissolution currents in the polarization curves are therefore essentially currents from iron ions of the substrate through micropores. Fig. 4 shows in comparison a polarization curve of uncoated iron, iron coated with chromium and pure chromium. The dissolution currents of the coated sample are reduced by one to two orders of magnitude indicating a reduced corrosion. The behaviour is in between iron and pure chromium which shows considerably lower currents. A tantalum coating leads to similar effects with the currents being smaller by another factor of three to five in comparison to chromium. In case of aluminum as tube material, there is no coating necessary for the dilute only slightly acidic acetate buffer. Aluminum itself is passivated by an oxide film. Only when aggressive anions such as sulfide or chloride are present, corrosion can take place. In this case, the native oxide film is locally dissolved. In these places, aluminum starts to corrode rapidly forming deep pits. Pitting corrosion is therefore a major problem with aluminum. Both chromium and tantalum are not susceptible to pitting corrosion. Hence, they can serve as a protective coating. Fig. 5 shows in comparison the anodic currents of uncoated aluminum and a sample coated with tantalum. Owing to the large amount of chloride ions, the aluminum sample suffers immediately from rapid dissolution when it is anodically polarized. No passivation is oberved. The tantalum film is able to reduce the dissolution currents, although a passivation with considerably lower currents is also not oberved. It appears that both in the case of iron and aluminum the microporosity of the films is of major importance. IVb. SYNTHESISOF METALS
778
W. Ensinger/Nucl.
Instr. and Meth. in Phys. Res. B 127/
128 (1997) 775-778
Table 1 Combinations of ion beam sputter coatings and tube materials for corrosion protection tube material
coating
effect on corrosion behaviour
pure iron
Cr. Ta Cr. Ta Pt Tii(0) Ta FeAI, NiTi B
reduced reduced reduced reduced reduced reduced reduced
pure aluminum tantalum ball bearing steel titanium lowalloy steel stainless steel
corrosion in dilute acetic acid pitting corrosion in salt brine hydrogen embrittlement in sulfuric acid corrosion in dilute acetic acid corrosion in hot nitric acid corrosion in dilute acetic acid corrosion in sulfuric acid
With film thicknesses typically below 0.5 pm, as in the present case, the degree of complete surface coverage and flaws in the film, particularly at rough surfaces, play a major role. Adhesion is apparently no problem, because no delamination of the films was observed under the microscope. The reason for a good adhesion may be found in the energetic and ballistic conditions of the process [4]. First, the sputtered particles are hyperthermal with energies of several eV. The majority of sputtered atoms has energies of 2 to 3 eV, but the range of energies may extend up to several tens of eV. Second, reflected highly energetic argon ions impinge onto the growing film and penetrate, depending on their energy and angle of incidence, through the first monolayers into the interface between film and substrate. There, they deposit energy and momentum and may lead to a strengthening of the bonding forces [5]. The same mechanisms, namely ion assist during deposition, usually lead to film growth with a low void density and low intrinsic microporosity [6]. This is also important and beneficial for a good corrosion performance. Further combinations of tube materials and coatings are listed in Table 1, including examples of alloy coatings.
4. Summary Sputtering of corrosion resistant materials by an ion beam onto the inner walls of metal tubes leads to well adherend coatings which are, to a certain degree, able to protect the tubes from aqueous corrosion. Bombardment of
reference this work this work
Dl [41 [41 171 171
the growing film by reflected ions is presumably the reason for the observed good adhesion of the films. The main problem so far is seen in macropores when the films are formed on rough surface without in-vacua cleaning. An increase in film thickness will certainly improve the situation. During the process, the tubes remained at temperatures below 400 K. This renders this process suitable for temperature-sensitive material such as heat-treated steels and aluminum alloys. When, on the other hand, an elevated temperature can be tolerated, it can be expected that the performance of the films with respect to adhesion, porosity, and other features such as hardness and purity can further be improved. With large-area ion sources and arrays of tubes, an industrial application of this technique on small sized tubes seems possible.
References [I] W. Ensinger, Rev. Sci. Instr. 67 (1996) 318. [2] W. Ensinger, Surf. Coat. Technol. 84 (19%) 434. [3] L.L. Shreir (ed.), Corrosion (Newnes-Butterworth, London, 1976). [4] W. Ensinger, Surf. Coat. Technol. 86-87 (1996) 438. [5] J.E.E. Baglin, in: P. Mazzoldi and G.W. Arnold (eds.), Ion Beam Modification of Insulators. Elsevier, Amsterdam, 1986, p. 585. [6] F.A. Smidt, Int. Mater. Rev. 35 (1990) 61. [7] W. Ensinger, Thin Solid Films, submitted.