- Email: [email protected]
Development of new technologies and practical applications of plasma immersion ion deposition (PIID)
Surface & Coatings Technology 204 (2010) 2869–2874
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Development of new technologies and practical applications of plasma immersion ion deposition (PIID) Ronghua Wei Southwest Research Institute®, San Antonio, Texas, USA
a r t i c l e
i n f o
Available online 10 February 2010 Keywords: DLC Mesh Pulsed glow discharge Tube ID coatings Erosion Wear Corrosion
a b s t r a c t This paper reviews the latest research and development at Southwest Research Institute (SwRI®) in plasma immersion ion deposition. SwRI has developed a few new technologies including high rate deposition of diamond-like carbon (DLC) coatings, thick DLC for improved erosion and corrosion resistance and the deposition of the inner surface of long pipes. These technologies are based on hollow cathode discharge (HCD) plasma process rather than the conventionally used glow discharge process. In the HCD process, electrons generated on the part experience multiple collisions before they can escape to the anode; therefore, an intensive plasma is generated, and hence a high current can be obtained. As a result a much higher deposition rate and a much thicker coating can be achieved than those obtained from conventional PIID processes. In this paper, we will discuss the principle of the HCD process, the experimental results of the DLC coatings in comparison with the DLC coating produced using conventional PIID and some practical applications of these technologies. In addition to the meshed PIID, SwRI has also been working on conventional, but large scaled PIID. A few issues and applications will be presented at the end of this paper. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Plasma source ion implantation (PSII) raised significant interest since the technology first appeared because of its promise of treating large 3-D components without the need of sample manipulation, thereby potentially reducing the processing cost [1,2]. Later the process has been commonly referred as plasma immersion ion implantation (PIII) [3–5]. After over two decades of effort, even though the research is still active [6–10], PIII has not been widely accepted by industry for its intended tribological applications due to the inherent shortcoming of ion implantation, i.e. the implanted species can only penetrate to a depth no more than 200 nm. This treated layer is deemed too shallow for most mechanical components. However, along with the research in PIII came another process from the late 1990s — plasma immersion ion deposition (PIID) [11–14]. Using this process, 3-D components can be deposited with a much thicker layer of hard coatings. In particular, diamond-like carbon (DLC) coating obtained using PIID has become the focal point by the research community because it can be readily deposited on a variety of surfaces. Due to its high hardness (10– 25 PGa) and sufficient thickness (0.5–2 µm), DLC has found practical applications particularly for automotive and biomedical industries [15–21]. It should be pointed out, although many techniques can be used to prepare DLC coatings including direct
E-mail address: [email protected]. 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.01.046
ion beam deposition, magnetron sputtering deposition, cathodic arc deposition, and RF plasma deposition [22–24], the pulsed glow discharge (PGD)-based PIID offers a much simpler approach and hence a potential for lower cost [11,12,25–28]. In addition, the deposition system hardware is also quite simple. SwRI has been developing the PIII and PIID technologies since 1990. The focus is on practical applications. In this paper, we will present the latest development related to novel technology and practical application examples at SwRI. 2. DLC deposition using a mesh method 2.1. Principle of meshed PIID In general, the DLC deposition rate using the PGD-based PIID process is low, much less than 1 µm/h, depending on the deposition parameters and system setup. To increase the deposition rate, SwRI has developed a hollow cathode discharge (HCD)-based mesh method [29]. Shown in Fig. 1A is a schematic of the meshed PIID process. When the voltage pulses are applied to the cage, plasma is generated inside the cage. The pulsed negative voltage also draws ions from all directions to the surfaces of the parts. As in conventional PIID, if argon gas is used, ion sputter cleaning can be accomplished. If a carbonaceous gas such as methane or acetylene is used, a DLC coating can be deposited on the parts. Shown in Fig. 1B is a photograph of the meshed PIID process. It is noted that the plasma luminescence is much stronger inside the cage than the outside. In fact, this process is similar to the HCD process in tubes [30,31]. The
2870
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
gas and hence generate more plasma. When these electrons finally lose their energy, they will escape from the mesh and go to the wall to complete the circuit loop. The plasma density generated by the mesh method is much higher than that generated by conventional glow discharge method, because in the latter case, the secondary electrons will go to the chamber wall directly with few collisions with the neutrals. As a result of the high plasma density in the meshed PIID processing, a high deposition rate can be reached. This process also increase the coating uniformity compared to conventional PIID. This can be understood as follows. When a part is placed in plasma, a plasma sheath is formed. The plasma sheath thickness can be written as:d = CV 3 = 4 = j1 = 2 where V is the applied voltage, j the current density, and C a constant proportional to the atomic mass unit of the ions. In the conventional PIID case, when the voltage is applied to the parts, it takes a few milliseconds to about a half of a microsecond for the voltage to reach the typical peak value of a few kV. The sheath, which starts close to the parts, moves out quickly. As a result, the coating conformity deteriorates. In the case of the meshed PIID process, the plasma is inside the cage and the potential is only a few eV above the parts throughout the application of the voltage. Hence the plasma sheath is very small (conformal). Moreover, in the meshed PIIP process, the plasma density is much higher than that produced by the conventional PIID process. Therefore, the current density (j) to the parts is much higher, leading to a smaller sheath dimension or more conformal deposition. Shown in Fig. 1C are the voltage and current waveforms for the meshed PIID process. The increase of the current with time is a characteristic of hollow cathode discharge. It is also noted that the peak current is much higher than that in conventional PIID.
2.2. Coating uniformity, thickness and deposition rate As shown in Fig. 1B a number of the parts are being coated with DLC, including a tube of 58 mm in diameter by 110 mm long representing 3-D components. By sectioning the tube longitudinally and using scanning electron microscopy, the coating thickness distribution along the inner diameter (ID) surface and the outer diameter (OD) surface is shown in Fig. 1D. As can be seen, the coating thickness varies from 2 to 9 µm. The coating on the inner surface is thicker than the outer surface due to the hollow cathode effect even if the tube was in the meshed cage. The coating does not seem to be uniform. However, considering both surfaces can be coated simultaneously with a coating that is sufficiently thick for most applications, it may still be acceptable. In contrast, if the conventional PIID process is used to deposit the coating on both the ID and OD simultaneously, the coating coverage will be completely unacceptable. It should also be pointed out that using the meshed PIID method insulating components including ceramics and polymeric materials can be coated easily.
Fig. 1. (A) Schematic of meshed PIID process, (B) a photograph of the process in progress, (C) voltage (top) and current (bottom) waveforms of the meshed PIID process (the scaling factors for the voltage and current waveforms are 2 kV/V and 10A/V, respectively), and (D) coating thickness distribution on both sides of the tube.
secondary electrons generated at the component surfaces by the impact from the incoming high energy ions are trapped inside the mesh. These electrons, in turn, will have collisions with the neutral
Fig. 2. Raman spectra for conventional DLC and meshed DLC coatings.
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
2871
Depending on the deposition parameter selection, DLC coatings including Si-containing DLC and Si–O-containing DLC up to 50 µm of total thickness have been obtained with the deposition rate of up to 6.5 µm/h on 304 stainless steel and 1018 carbon steel.
2.3. Structure characterization For comparison purposes, conventional PIID DLC was prepared on Si and steel samples using our “standard” deposition conditions (C2H2 gas at 15 mtorr, pulsed voltage of 4 kV, frequency of 2 kH, and pulse width of 20 µs). The Si samples were used for microstructural analyses, while the steel samples were used for tribological and corrosion testing. Raman spectroscopy was used to compare the DLC coatings prepared using the conventional PGD PIID process and the meshed PIID under similar conditions. Shown in Fig. 2 are the spectra. Qualitatively, the DLC films produced by both methods are similar. Quantitative analysis showed that the Id/Ig is about 1.2 and 0.8 for the conventional PIID and meshed PIID, respectively, indicating an increase in the sp3/sp2 ratio for the DLC prepared using the meshed
Fig. 4. (A) small ID, curve tube and (B) large ID, short tube PIID DLC coatings.
PIID [32–34]. Certainly, the structural properties of the meshed DLC films depend on the processing parameters.
2.4. Tribological characterizations Two tribological tests were conducted to evaluate the DLC films prepared using meshed PIID. The first was a wear test and the second was an erosion test. In the wear test, 1018 carbon steel was deposited with DLC using the mesh method and a ball-on-disc wear test was conducted in ambient environment, dry sliding with 1 N load against a ceramic ball. Shown in Fig. 3A is the coefficient of friction (COF) of the DLC film prepared using the meshed PIID method. As a comparison, the COFs for conventional DLC is also shown. Both DLC films show much lower COF than the uncoated 1018 steel (not shown, µ = 0.6–0.7), while the DLC prepared using the meshed PIID shows
Fig. 3. (A) Coefficient of friction, (B) erosion resistances and (C) corrosion resistance (in ohm) of 1018 steel, standard DLC and meshed PIID DLC coatings.
Fig. 5. Deposition of DLC on ID of long pipe using HCD PIID.
2872
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
Fig. 6. SEM images of DLC deposited long pipe ID. (A) low magnification and (B) high magnification cross-sectional views near center; (C) low magnification and (D) high magnification cross-sectional views near one end; and (E) and (F) morphological views of the corresponding locations at the center and the end, respectively.
slightly higher COF (µ = 0.12) than the conventional DLC, which may be the result of higher sp3 content. DLC films may be used for the protection of components under two-phase low situation such as fluid and sand. The erosion resistance of the DLC films was determined using a micro sand blaster with alumina powder of average grain 50 µm at the 90° incident angle at a back pressure of 5 psi. Shown in Fig. 3B is the mass loss for selected DLC films prepared using the meshed PIID method after a total of 2 min of testing. The data for the conventional DLC, as well as the uncoated substrate, are also shown. The DLC coating prepared using the meshed PIID exhibits lower mass loss than that for the conventional DLC. Certainly the erosion resistance depends on the coating thickness and the coating quality. 2.5. Corrosion resistance The corrosion resistance of the deposited samples was evaluated using electrochemical impedance spectroscopy. Equivalent circuit models were used to calculate the polarization and pore resistance values. The results are shown in Fig. 3C. Higher polarization resistance implies better corrosion resistance. As can be seen, the DLC deposited
using the meshed PIID method has much higher corrosion resistance than the standard DLC. One of the reasons may be that the thickness of the meshed PIID DLC is much larger. The detailed study on the meth PIID may be referred to Ref. [29]. 3. DLC deposition on the inner diameter of tubes and pipes 3.1. Deposition of short tubes As is known, PIID process is a variation of plasma enhanced chemical vapor deposition (PECVD). Besides the 3-D processing capability, another advantage of the PIID technology over physical vapor deposition (PVD) processes lies in that the ID of tubular structures can be deposited fairly easily. For typical size of tubes (20–100 mm in diameter by 0.1–3 m long), the ID can be deposited with DLC using various methods including hollow cathode discharge [35–37]. If the ID is too small and the material is paramagnetic, magnetic field may be used to generate plasma inside the tube [31]. Because DLC coatings are hard and chemically inert, they may be deposited on the ID of tubes for transporting crude oil or natural gas to reduce wear and corrosion from the medium with small amounts of sand and water.
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
2873
unlike the voltage waveform shown in Fig. 1C. It will decrease as the current rises, a characteristic of hollow cathode discharge. On the other hand, if the tube diameter is large, for instance, greater than 75 mm, low pressure glow discharge may be obtained. At higher pressures, the discharge will switch to the hollow cathode mode. 3.2. Deposition of long pipes Long pipe may also be deposited with DLC films. Shown in Fig. 5 is a large diameter (150 mm), long curved carbon steel pipe (2 m) being deposited with DLC inside the SwRI large PIID chamber (1.2 m in diameter by 2.5 m long). To further study the DLC coating adhesion and uniformity in even longer pipes with a smaller diameter, a carbon steel pipe of ∼ 85 mm in diameter by 3.5 m long was used. Because the pipe was rough, a light polishing was performed with sand paper, but the polishing was very limited due to the length and the roughness. After the polishing, it was first cleaned with acetone and alcohol; then it was pumped down from both sides using two vacuum pumps. It was sputter cleaned with Ar for 4 h, until the arcing diminished, to ensure the oxide was removed. Following that, a Si-DLC was deposited first and finally a DLC coating was deposited. To study the DLC coating adhesion to a rough surface, the large tube was sectioned and samples were taken for the SEM study. Shown in Fig. 6A and D are the low and high magnification cross-sectional SEM images taken from one end and the center, while Fig. 6E and F are the morphological images of the pipe at the corresponding locations. As can be seen, the steel substrate is very rough, but that the conformity of the coating, consisting of a bond layer of Si and a layer of DLC, is excellent. The coating even filled in the defects of the steel substrate. The coating adhesion is also excellent. No delamination of the coating occurred from the steel substrate due to metallographic sectioning. From the morphological views the surface is indeed very rough, but the coverage of the DLC film seems to be very good. No broken surface or uncoated areas can be observed. The morphology of the pipe near the end of the pipe is not the same as in the center of the pipe. This may be the result of mechanical polishing, which might have be done easily near the end. In addition, it is known the ion bombardment near the end is much stronger, which may have resulted in the morphology. 4. DLC deposition on large components
Fig. 7. DLC deposition on (A) large cutting blade and (B) on embossing tools, and (C) large number of parts.
In an early publication, the PIID process was demonstrated for depositing DLC on a large number of pistons [37,38]. At SwRI, the deposition of large components or a large number of small components is pursued. In the following, a few examples will be given. 4.1. Deposition of long cutting blade
They may also be used for tubes in steam turbines in which the twophase flow (water droplets and steam) causes serious erosion and corrosion. Shown in Fig. 4 are two photographs of short tubes that are being deposited with DLC coatings. One is a smaller diameter (25 mm) curved tube while the other a larger diameter (100 mm). In general the smaller the diameter, the higher the pressure is needed to generate plasma inside the tube. For a tube of 25 mm in diameter by 300–400 mm long, the breakdown pressure is about 50 mtorr either for Ar or acetylene. For small diameter tubes, it is nearly impossible to generate low pressure glow discharge inside the tube characterized by the low discharge current. When the pressure is high and once breakdown occurs, a high current is obtained. The current increases nearly monotonically with the pulse, until it is switched off, similar to the current waveform shown in Fig. 1C. If the discharge is very intense, after breakdown the pulse voltage cannot remain constant,
In the food industry, frozen meat needs cutting and the cutting blade experiences wear and corrosion problems. DLC coating was attempted to protect the cutting blade. The blade was approximately 2 m long, made of unspecified hard steel. The intended DLC was about 2 µm thick with a Si bond layer to increase the adhesion. Shown in Fig. 7A is a photograph of the blade during the DLC deposition process. 4.2. Deposition of large number of tools Embossing tools are used to transfer designed patterns to other metals commonly in the form of a thin foil. Embossing tools are generally expensive due to the patterning process. They experience wear. In some cases, the adhesion between the embossing tool and the foil may be very strong as that the foil is torn during the embossing process. To reduce wear and adhesion, DLC film is used. Typically about 100 nm of DLC is sufficient. Shown in Fig. 7B is a
2874
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
photograph of the PIID process in which about 40 pieces of embossing plates are being deposited with DLC. These plates are hung on both sides of the large worktable vertically mounted inside the deposition chamber to minimize the dust accumulation. SwRI has deposited about 2000 pieces. 4.3. Deposition of large number of small fuel injectors Fuel injectors are often coated with DLC or Me-DLC (Me = W or Cr) using magnetron sputtering process, in which a triple rotation worktable is commonly used for coating uniformity. The inherent advantage of the PIID process is its conformal coating capability. For large scaled production, it is also important to understand the packing density, or the minimum spacing beyond which the DLC can be deposited uniformly. Shown in Fig. 7C is a photograph of fuel injectors that are deposited with DLC using the PIID process. As can be seen, the injectors are approximately 3 mm in diameter by 30 mm long. They are divided into three groups of injectors and mounted on the table. In the first group, the distance between two injectors (center to center) is 12.5 mm, and in the second group, the distance is 25 mm, while in the last group the distance is 37.5 mm. After the deposition, the injectors were removed and the quality of the coating was examined. It was observed that the coating on the injectors mounted in the middle of the first group (12.5 mm spacing) was unacceptable. Because of the lack of the ion bombardment, the DLC coating was sooty and could be wiped off. In contrast, the coating in the other two groups exhibited excellent quality — shiny, hard and uniform. 5. Conclusion In this paper, we presented an overview on recent development of diamond-like carbon coatings at SwRI. A hollow cathode discharge based mesh PIID was presented. The coating wear resistance, coefficient of friction, erosion resistance and corrosion resistance were studied and found to be comparable to those from conventional PIID. The major advantages of this method over the conventional PIID method include the high deposition rate (up to 6.5 times), capability for thick coating (up to 50 µm) and much conformal coating coverage. In addition, this paper also discussed the deposition of tubes and pipes various shape and size. The PIID process may be the easiest way to the coating of ID of tubes and pipes. Up to 3.5 m long carbon steel pipes have been deposited with a fairly uniform coating. Finally, the coating of large components or a large number of components was also discussed with some actual application examples. Because of the maturity and its simplicity, it is believed the PIID process will find more applications in industry. References [1] J.R. Conrad, T. Castagna, Bull. Am. Phys. Soc. 31 (1986) 1479. [2] J.R. Conrad, R.A. Dodd, F.J. Worzala, X. Qiu, Plasma source ion implantation: a new, cost-effective, non-line-of-sight technique for ion implantation of materials, Surface and Coatings Technology 36 (3–4) (December 15 1988) 927. [3] R. Hutchings, G.A. Collins, J. Tendys, Surf. Coat. Technol. 51 (20) (1992) 489; G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Heat Treatment Metals 4 (1995). [4] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, C.H. Van Der Valk, Development of a plasma immersion ion implanter for the surface treatment of metal components, Surface and Coatings Technology 84 (1–3) (October 1996) 537. [5] J.R. Conrad, Introduction, in Handbook of Plasma Ion Implantation and Deposition, in: A. Anders (Ed.), ISBN 0-471-24698-0, 2000, p. 1, John Wiley & Sons, Inc. [6] Yong Luo, Shirong Ge, Fretting wear behavior of nitrogen ion implanted titanium alloys in bovine serum lubrication, Tribology International 42 (9) (September 2009) 1373. [7] Kuan-Wei Chen, Jen-Fin Lin, Wen-Fa Tsai, Chi-Fong Ai, “Plasma immersion ion implantation induced improvements of mechanical properties, wear resistance,
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15] [16] [17] [18] [19]
[20]
[21]
[22] [23] [24] [25] [26] [27]
[28]
[29]
[30] [31]
[32] [33] [34] [35] [36] [37]
[38]
and adhesion of diamond-like carbon films deposited on tool steel,” Surface and Coatings Technology, In Press, Corrected Proof. H.B. He, H.Y. Li, S.K. Lyu, “Tribological behavior of a 20CrMo alloy implanted with nitrogen ions by plasma source ion implantation,” Wear, In Press, Accepted Manuscript. H.B. He, H.Y. Li, S.K. Lyu, “Tribological behavior of a 20CrMo alloy implanted with nitrogen ions by plasma source ion implantation,” Wear, In Press, Accepted Manuscript. L.D. Yu, S. Aukkaravittayapun, D. Boonyawan, S. Sarapirom, T. Vilaithong, Recent development in surface modification of industrial materials by ion beams and plasma at Chiang Mai University, Surface and Coatings Technology 203 (17–18) (June 15 2009) 2771. J. Chen, J. Blanchard, J.R. Conrad, R.A. Dodd, Structure and wear properties of carbon implanted 304 stainless steel using plasma source ion implantation, Surface and Coatings Technology 53 (3) (November 10 1992) 267. K.C. Walter, M. Nastasi, C. Munson, Surface and Coatings Technology 93 (1997) 287. X.M. He, K.C. Walter, M. Nastasi, S.-T. Lee, X.S. Sun, Optical and tribological properties of diamond-like carbon films synthesized by plasma immersion ion processing, Thin Solid Films 355–356 (November 1 1999) 167. K. Sridharan, S. Anders, M. Nastasi, K.C. Walter, A. Anders, O.R. Monteriro, W. Ensinger, Nonsemiconductor applications of PII&D, in: A. Anders (Ed.), Handbook of Plasma Ion Implantation and Deposition, John Wiley & Sons, Inc, ISBN: 0-47124698-0, 2000, p. 553. A.H. Lettington, in: R.E. Clausing, et al., (Eds.), Diamond and Diamond-like Films and Coatings, New York, Plenum Press, 1991, p. 491. A. Matthews, S.S. Eskildsen, Diamond and Related Materials 3 (1994) 902. C. Donnet, Surf. Coat. Technol. 80 (1996) 151. A. Grill, Diamond and Related Materials 8 (1999) 428. P.K. Chu, J.Y. Chen, L.P. Wang, N. Huang, Plasma-surface modification of biomaterials, Materials Science and Engineering: R: Reports 36 (5–6) (March 29 2002) 143. P. Yang, N. Huang, Y.X. Leng, Z.Q. Yao, H.F. Zhou, M. Maitz, Y. Leng, P.K. Chu, Wettability and biocompatibility of nitrogen-doped hydrogenated amorphous carbon films: effect of nitrogen, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 242 (1–2) (January 2006) 22. E. Salgueiredo, M. Vila, M.A. Silva, M.A. Lopes, J.D. Santos, F.M. Costa, R.F. Silva, P.S. Gomes, M.H. Fernandes, Biocompatibility evaluation of DLC-coated Si3N4 substrates for biomedical applications, Diamond and Related Materials 17 (4–5) (April–May 2008) 878. Y. Catherine, in: R.E. Clausing, et al., (Eds.), Diamond and Diamond-like Films and Coatings, New York, Plenum Press, 1991, p. 193. N.A. Sanchez, C. Rincon, G. Zambrano, H. Galindo, P. Prieto, Thin Solid Films 373 (2000) 247. G. Fedosenko, J. Engemann, D. Korzec, Surf. Coat. Technol. 133–134 (2000) 535. K.C. Walter, H. Kung, T. Levine, J.R. Tesmer, P. Kodali, B.P. Wood, D.J. Rej, M. Nastasi, J. Koskinen, J.-P. Hirvonen, Mater. Res. Soc. Symp. Proc. 354 (1995) 87. J.V. Nantese, I.G. Brown, N.W. Cheung, G.A. Collins, MRS Bull. 21 (1996) 52. André Anders, From plasma immersion ion implantation to deposition: a historical perspective on principles and trends, Surface and Coatings Technology 156 (1–3) (July 1 2002) 3. Wei, R., Arps, J. and Lanning, B., Plasma and Ion Beam Surface Engineering for Practical Applications, presented at the 46th Annual SVC Technical Conference, San Francisco, California, May 3–8, 2003, to be published by Society of Vacuum Coaters. Ronghua Wei, Marta Jakab, Vasiliki Poenitzsch and Kent Coulter, Erosion, Corrosion and Wear Resistance and Microstructure of Diamond-like Carbon (DLC) Coatings Prepared using a Mesh Method, Presented at the 52nd Annual Society of Vacuum Coaters Technical Conference, April 28 – May 3, 2009, Santa Clara, USA, to be published by SVC in 2009. S.M. Malik, R.P. Fetherston, J.R. Conrad, J. Vac. Sci. Technol. A15 (1997) 2875. Ronghua Wei, Christopher Rincon, Thomas L. Booker, James H. Arps, Magnetic field enhanced plasma (MFEP) deposition of inner surfaces of tubes, Surf. Coat. Technol. 188–189 (2004) 691. J.W. Ager III, IEEE Transactions on Magnetics 29 (1) (1993) 259. J.K. Lee, M. Smallen, J. Enguero, H.J. Lee, A. Chao, IEEE Transactions on Magnetics 29 (1) (1993) 276. B. Marchon, P.N. Vo, IEEE Transactions on Magnetics 27 (6) (1991) 5160. K. Baba, R. Hatada, Nucl. Instrum. Methods Phys. Res., B, Beam Interact. Mater. Atoms 148 (1999) 69. K. Baba, R. Hatada, Nucl. Instrum. Methods Phys. Res., B, Beam Interact. Mater. Atoms 206 (2003) 704. Carter P. Munson, R.J. Faehl, I. Henins, M. Nastasi, W.A. Reass, D.J. Rej, J.T. Scheuer, K.C. Walter, B.P. Wood, Recent advances in plasma source ion implantation at Los Alamos National Laboratory, Surface and Coatings Technology 84 (1–3) (October 1996) 528. J.V. Mantese, I.G. Brown, N.W. Cheung, G.A. Collins, Plasma immersion ion implantation, MRS Bulletin (August, 1996) 52.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Development of new technologies and practical applications of plasma immersion ion deposition (PIID) Ronghua Wei Southwest Research Institute®, San Antonio, Texas, USA
a r t i c l e
i n f o
Available online 10 February 2010 Keywords: DLC Mesh Pulsed glow discharge Tube ID coatings Erosion Wear Corrosion
a b s t r a c t This paper reviews the latest research and development at Southwest Research Institute (SwRI®) in plasma immersion ion deposition. SwRI has developed a few new technologies including high rate deposition of diamond-like carbon (DLC) coatings, thick DLC for improved erosion and corrosion resistance and the deposition of the inner surface of long pipes. These technologies are based on hollow cathode discharge (HCD) plasma process rather than the conventionally used glow discharge process. In the HCD process, electrons generated on the part experience multiple collisions before they can escape to the anode; therefore, an intensive plasma is generated, and hence a high current can be obtained. As a result a much higher deposition rate and a much thicker coating can be achieved than those obtained from conventional PIID processes. In this paper, we will discuss the principle of the HCD process, the experimental results of the DLC coatings in comparison with the DLC coating produced using conventional PIID and some practical applications of these technologies. In addition to the meshed PIID, SwRI has also been working on conventional, but large scaled PIID. A few issues and applications will be presented at the end of this paper. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Plasma source ion implantation (PSII) raised significant interest since the technology first appeared because of its promise of treating large 3-D components without the need of sample manipulation, thereby potentially reducing the processing cost [1,2]. Later the process has been commonly referred as plasma immersion ion implantation (PIII) [3–5]. After over two decades of effort, even though the research is still active [6–10], PIII has not been widely accepted by industry for its intended tribological applications due to the inherent shortcoming of ion implantation, i.e. the implanted species can only penetrate to a depth no more than 200 nm. This treated layer is deemed too shallow for most mechanical components. However, along with the research in PIII came another process from the late 1990s — plasma immersion ion deposition (PIID) [11–14]. Using this process, 3-D components can be deposited with a much thicker layer of hard coatings. In particular, diamond-like carbon (DLC) coating obtained using PIID has become the focal point by the research community because it can be readily deposited on a variety of surfaces. Due to its high hardness (10– 25 PGa) and sufficient thickness (0.5–2 µm), DLC has found practical applications particularly for automotive and biomedical industries [15–21]. It should be pointed out, although many techniques can be used to prepare DLC coatings including direct
E-mail address: [email protected]. 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.01.046
ion beam deposition, magnetron sputtering deposition, cathodic arc deposition, and RF plasma deposition [22–24], the pulsed glow discharge (PGD)-based PIID offers a much simpler approach and hence a potential for lower cost [11,12,25–28]. In addition, the deposition system hardware is also quite simple. SwRI has been developing the PIII and PIID technologies since 1990. The focus is on practical applications. In this paper, we will present the latest development related to novel technology and practical application examples at SwRI. 2. DLC deposition using a mesh method 2.1. Principle of meshed PIID In general, the DLC deposition rate using the PGD-based PIID process is low, much less than 1 µm/h, depending on the deposition parameters and system setup. To increase the deposition rate, SwRI has developed a hollow cathode discharge (HCD)-based mesh method [29]. Shown in Fig. 1A is a schematic of the meshed PIID process. When the voltage pulses are applied to the cage, plasma is generated inside the cage. The pulsed negative voltage also draws ions from all directions to the surfaces of the parts. As in conventional PIID, if argon gas is used, ion sputter cleaning can be accomplished. If a carbonaceous gas such as methane or acetylene is used, a DLC coating can be deposited on the parts. Shown in Fig. 1B is a photograph of the meshed PIID process. It is noted that the plasma luminescence is much stronger inside the cage than the outside. In fact, this process is similar to the HCD process in tubes [30,31]. The
2870
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
gas and hence generate more plasma. When these electrons finally lose their energy, they will escape from the mesh and go to the wall to complete the circuit loop. The plasma density generated by the mesh method is much higher than that generated by conventional glow discharge method, because in the latter case, the secondary electrons will go to the chamber wall directly with few collisions with the neutrals. As a result of the high plasma density in the meshed PIID processing, a high deposition rate can be reached. This process also increase the coating uniformity compared to conventional PIID. This can be understood as follows. When a part is placed in plasma, a plasma sheath is formed. The plasma sheath thickness can be written as:d = CV 3 = 4 = j1 = 2 where V is the applied voltage, j the current density, and C a constant proportional to the atomic mass unit of the ions. In the conventional PIID case, when the voltage is applied to the parts, it takes a few milliseconds to about a half of a microsecond for the voltage to reach the typical peak value of a few kV. The sheath, which starts close to the parts, moves out quickly. As a result, the coating conformity deteriorates. In the case of the meshed PIID process, the plasma is inside the cage and the potential is only a few eV above the parts throughout the application of the voltage. Hence the plasma sheath is very small (conformal). Moreover, in the meshed PIIP process, the plasma density is much higher than that produced by the conventional PIID process. Therefore, the current density (j) to the parts is much higher, leading to a smaller sheath dimension or more conformal deposition. Shown in Fig. 1C are the voltage and current waveforms for the meshed PIID process. The increase of the current with time is a characteristic of hollow cathode discharge. It is also noted that the peak current is much higher than that in conventional PIID.
2.2. Coating uniformity, thickness and deposition rate As shown in Fig. 1B a number of the parts are being coated with DLC, including a tube of 58 mm in diameter by 110 mm long representing 3-D components. By sectioning the tube longitudinally and using scanning electron microscopy, the coating thickness distribution along the inner diameter (ID) surface and the outer diameter (OD) surface is shown in Fig. 1D. As can be seen, the coating thickness varies from 2 to 9 µm. The coating on the inner surface is thicker than the outer surface due to the hollow cathode effect even if the tube was in the meshed cage. The coating does not seem to be uniform. However, considering both surfaces can be coated simultaneously with a coating that is sufficiently thick for most applications, it may still be acceptable. In contrast, if the conventional PIID process is used to deposit the coating on both the ID and OD simultaneously, the coating coverage will be completely unacceptable. It should also be pointed out that using the meshed PIID method insulating components including ceramics and polymeric materials can be coated easily.
Fig. 1. (A) Schematic of meshed PIID process, (B) a photograph of the process in progress, (C) voltage (top) and current (bottom) waveforms of the meshed PIID process (the scaling factors for the voltage and current waveforms are 2 kV/V and 10A/V, respectively), and (D) coating thickness distribution on both sides of the tube.
secondary electrons generated at the component surfaces by the impact from the incoming high energy ions are trapped inside the mesh. These electrons, in turn, will have collisions with the neutral
Fig. 2. Raman spectra for conventional DLC and meshed DLC coatings.
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
2871
Depending on the deposition parameter selection, DLC coatings including Si-containing DLC and Si–O-containing DLC up to 50 µm of total thickness have been obtained with the deposition rate of up to 6.5 µm/h on 304 stainless steel and 1018 carbon steel.
2.3. Structure characterization For comparison purposes, conventional PIID DLC was prepared on Si and steel samples using our “standard” deposition conditions (C2H2 gas at 15 mtorr, pulsed voltage of 4 kV, frequency of 2 kH, and pulse width of 20 µs). The Si samples were used for microstructural analyses, while the steel samples were used for tribological and corrosion testing. Raman spectroscopy was used to compare the DLC coatings prepared using the conventional PGD PIID process and the meshed PIID under similar conditions. Shown in Fig. 2 are the spectra. Qualitatively, the DLC films produced by both methods are similar. Quantitative analysis showed that the Id/Ig is about 1.2 and 0.8 for the conventional PIID and meshed PIID, respectively, indicating an increase in the sp3/sp2 ratio for the DLC prepared using the meshed
Fig. 4. (A) small ID, curve tube and (B) large ID, short tube PIID DLC coatings.
PIID [32–34]. Certainly, the structural properties of the meshed DLC films depend on the processing parameters.
2.4. Tribological characterizations Two tribological tests were conducted to evaluate the DLC films prepared using meshed PIID. The first was a wear test and the second was an erosion test. In the wear test, 1018 carbon steel was deposited with DLC using the mesh method and a ball-on-disc wear test was conducted in ambient environment, dry sliding with 1 N load against a ceramic ball. Shown in Fig. 3A is the coefficient of friction (COF) of the DLC film prepared using the meshed PIID method. As a comparison, the COFs for conventional DLC is also shown. Both DLC films show much lower COF than the uncoated 1018 steel (not shown, µ = 0.6–0.7), while the DLC prepared using the meshed PIID shows
Fig. 3. (A) Coefficient of friction, (B) erosion resistances and (C) corrosion resistance (in ohm) of 1018 steel, standard DLC and meshed PIID DLC coatings.
Fig. 5. Deposition of DLC on ID of long pipe using HCD PIID.
2872
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
Fig. 6. SEM images of DLC deposited long pipe ID. (A) low magnification and (B) high magnification cross-sectional views near center; (C) low magnification and (D) high magnification cross-sectional views near one end; and (E) and (F) morphological views of the corresponding locations at the center and the end, respectively.
slightly higher COF (µ = 0.12) than the conventional DLC, which may be the result of higher sp3 content. DLC films may be used for the protection of components under two-phase low situation such as fluid and sand. The erosion resistance of the DLC films was determined using a micro sand blaster with alumina powder of average grain 50 µm at the 90° incident angle at a back pressure of 5 psi. Shown in Fig. 3B is the mass loss for selected DLC films prepared using the meshed PIID method after a total of 2 min of testing. The data for the conventional DLC, as well as the uncoated substrate, are also shown. The DLC coating prepared using the meshed PIID exhibits lower mass loss than that for the conventional DLC. Certainly the erosion resistance depends on the coating thickness and the coating quality. 2.5. Corrosion resistance The corrosion resistance of the deposited samples was evaluated using electrochemical impedance spectroscopy. Equivalent circuit models were used to calculate the polarization and pore resistance values. The results are shown in Fig. 3C. Higher polarization resistance implies better corrosion resistance. As can be seen, the DLC deposited
using the meshed PIID method has much higher corrosion resistance than the standard DLC. One of the reasons may be that the thickness of the meshed PIID DLC is much larger. The detailed study on the meth PIID may be referred to Ref. [29]. 3. DLC deposition on the inner diameter of tubes and pipes 3.1. Deposition of short tubes As is known, PIID process is a variation of plasma enhanced chemical vapor deposition (PECVD). Besides the 3-D processing capability, another advantage of the PIID technology over physical vapor deposition (PVD) processes lies in that the ID of tubular structures can be deposited fairly easily. For typical size of tubes (20–100 mm in diameter by 0.1–3 m long), the ID can be deposited with DLC using various methods including hollow cathode discharge [35–37]. If the ID is too small and the material is paramagnetic, magnetic field may be used to generate plasma inside the tube [31]. Because DLC coatings are hard and chemically inert, they may be deposited on the ID of tubes for transporting crude oil or natural gas to reduce wear and corrosion from the medium with small amounts of sand and water.
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
2873
unlike the voltage waveform shown in Fig. 1C. It will decrease as the current rises, a characteristic of hollow cathode discharge. On the other hand, if the tube diameter is large, for instance, greater than 75 mm, low pressure glow discharge may be obtained. At higher pressures, the discharge will switch to the hollow cathode mode. 3.2. Deposition of long pipes Long pipe may also be deposited with DLC films. Shown in Fig. 5 is a large diameter (150 mm), long curved carbon steel pipe (2 m) being deposited with DLC inside the SwRI large PIID chamber (1.2 m in diameter by 2.5 m long). To further study the DLC coating adhesion and uniformity in even longer pipes with a smaller diameter, a carbon steel pipe of ∼ 85 mm in diameter by 3.5 m long was used. Because the pipe was rough, a light polishing was performed with sand paper, but the polishing was very limited due to the length and the roughness. After the polishing, it was first cleaned with acetone and alcohol; then it was pumped down from both sides using two vacuum pumps. It was sputter cleaned with Ar for 4 h, until the arcing diminished, to ensure the oxide was removed. Following that, a Si-DLC was deposited first and finally a DLC coating was deposited. To study the DLC coating adhesion to a rough surface, the large tube was sectioned and samples were taken for the SEM study. Shown in Fig. 6A and D are the low and high magnification cross-sectional SEM images taken from one end and the center, while Fig. 6E and F are the morphological images of the pipe at the corresponding locations. As can be seen, the steel substrate is very rough, but that the conformity of the coating, consisting of a bond layer of Si and a layer of DLC, is excellent. The coating even filled in the defects of the steel substrate. The coating adhesion is also excellent. No delamination of the coating occurred from the steel substrate due to metallographic sectioning. From the morphological views the surface is indeed very rough, but the coverage of the DLC film seems to be very good. No broken surface or uncoated areas can be observed. The morphology of the pipe near the end of the pipe is not the same as in the center of the pipe. This may be the result of mechanical polishing, which might have be done easily near the end. In addition, it is known the ion bombardment near the end is much stronger, which may have resulted in the morphology. 4. DLC deposition on large components
Fig. 7. DLC deposition on (A) large cutting blade and (B) on embossing tools, and (C) large number of parts.
In an early publication, the PIID process was demonstrated for depositing DLC on a large number of pistons [37,38]. At SwRI, the deposition of large components or a large number of small components is pursued. In the following, a few examples will be given. 4.1. Deposition of long cutting blade
They may also be used for tubes in steam turbines in which the twophase flow (water droplets and steam) causes serious erosion and corrosion. Shown in Fig. 4 are two photographs of short tubes that are being deposited with DLC coatings. One is a smaller diameter (25 mm) curved tube while the other a larger diameter (100 mm). In general the smaller the diameter, the higher the pressure is needed to generate plasma inside the tube. For a tube of 25 mm in diameter by 300–400 mm long, the breakdown pressure is about 50 mtorr either for Ar or acetylene. For small diameter tubes, it is nearly impossible to generate low pressure glow discharge inside the tube characterized by the low discharge current. When the pressure is high and once breakdown occurs, a high current is obtained. The current increases nearly monotonically with the pulse, until it is switched off, similar to the current waveform shown in Fig. 1C. If the discharge is very intense, after breakdown the pulse voltage cannot remain constant,
In the food industry, frozen meat needs cutting and the cutting blade experiences wear and corrosion problems. DLC coating was attempted to protect the cutting blade. The blade was approximately 2 m long, made of unspecified hard steel. The intended DLC was about 2 µm thick with a Si bond layer to increase the adhesion. Shown in Fig. 7A is a photograph of the blade during the DLC deposition process. 4.2. Deposition of large number of tools Embossing tools are used to transfer designed patterns to other metals commonly in the form of a thin foil. Embossing tools are generally expensive due to the patterning process. They experience wear. In some cases, the adhesion between the embossing tool and the foil may be very strong as that the foil is torn during the embossing process. To reduce wear and adhesion, DLC film is used. Typically about 100 nm of DLC is sufficient. Shown in Fig. 7B is a
2874
R. Wei / Surface & Coatings Technology 204 (2010) 2869–2874
photograph of the PIID process in which about 40 pieces of embossing plates are being deposited with DLC. These plates are hung on both sides of the large worktable vertically mounted inside the deposition chamber to minimize the dust accumulation. SwRI has deposited about 2000 pieces. 4.3. Deposition of large number of small fuel injectors Fuel injectors are often coated with DLC or Me-DLC (Me = W or Cr) using magnetron sputtering process, in which a triple rotation worktable is commonly used for coating uniformity. The inherent advantage of the PIID process is its conformal coating capability. For large scaled production, it is also important to understand the packing density, or the minimum spacing beyond which the DLC can be deposited uniformly. Shown in Fig. 7C is a photograph of fuel injectors that are deposited with DLC using the PIID process. As can be seen, the injectors are approximately 3 mm in diameter by 30 mm long. They are divided into three groups of injectors and mounted on the table. In the first group, the distance between two injectors (center to center) is 12.5 mm, and in the second group, the distance is 25 mm, while in the last group the distance is 37.5 mm. After the deposition, the injectors were removed and the quality of the coating was examined. It was observed that the coating on the injectors mounted in the middle of the first group (12.5 mm spacing) was unacceptable. Because of the lack of the ion bombardment, the DLC coating was sooty and could be wiped off. In contrast, the coating in the other two groups exhibited excellent quality — shiny, hard and uniform. 5. Conclusion In this paper, we presented an overview on recent development of diamond-like carbon coatings at SwRI. A hollow cathode discharge based mesh PIID was presented. The coating wear resistance, coefficient of friction, erosion resistance and corrosion resistance were studied and found to be comparable to those from conventional PIID. The major advantages of this method over the conventional PIID method include the high deposition rate (up to 6.5 times), capability for thick coating (up to 50 µm) and much conformal coating coverage. In addition, this paper also discussed the deposition of tubes and pipes various shape and size. The PIID process may be the easiest way to the coating of ID of tubes and pipes. Up to 3.5 m long carbon steel pipes have been deposited with a fairly uniform coating. Finally, the coating of large components or a large number of components was also discussed with some actual application examples. Because of the maturity and its simplicity, it is believed the PIID process will find more applications in industry. References [1] J.R. Conrad, T. Castagna, Bull. Am. Phys. Soc. 31 (1986) 1479. [2] J.R. Conrad, R.A. Dodd, F.J. Worzala, X. Qiu, Plasma source ion implantation: a new, cost-effective, non-line-of-sight technique for ion implantation of materials, Surface and Coatings Technology 36 (3–4) (December 15 1988) 927. [3] R. Hutchings, G.A. Collins, J. Tendys, Surf. Coat. Technol. 51 (20) (1992) 489; G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Heat Treatment Metals 4 (1995). [4] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, C.H. Van Der Valk, Development of a plasma immersion ion implanter for the surface treatment of metal components, Surface and Coatings Technology 84 (1–3) (October 1996) 537. [5] J.R. Conrad, Introduction, in Handbook of Plasma Ion Implantation and Deposition, in: A. Anders (Ed.), ISBN 0-471-24698-0, 2000, p. 1, John Wiley & Sons, Inc. [6] Yong Luo, Shirong Ge, Fretting wear behavior of nitrogen ion implanted titanium alloys in bovine serum lubrication, Tribology International 42 (9) (September 2009) 1373. [7] Kuan-Wei Chen, Jen-Fin Lin, Wen-Fa Tsai, Chi-Fong Ai, “Plasma immersion ion implantation induced improvements of mechanical properties, wear resistance,
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15] [16] [17] [18] [19]
[20]
[21]
[22] [23] [24] [25] [26] [27]
[28]
[29]
[30] [31]
[32] [33] [34] [35] [36] [37]
[38]
and adhesion of diamond-like carbon films deposited on tool steel,” Surface and Coatings Technology, In Press, Corrected Proof. H.B. He, H.Y. Li, S.K. Lyu, “Tribological behavior of a 20CrMo alloy implanted with nitrogen ions by plasma source ion implantation,” Wear, In Press, Accepted Manuscript. H.B. He, H.Y. Li, S.K. Lyu, “Tribological behavior of a 20CrMo alloy implanted with nitrogen ions by plasma source ion implantation,” Wear, In Press, Accepted Manuscript. L.D. Yu, S. Aukkaravittayapun, D. Boonyawan, S. Sarapirom, T. Vilaithong, Recent development in surface modification of industrial materials by ion beams and plasma at Chiang Mai University, Surface and Coatings Technology 203 (17–18) (June 15 2009) 2771. J. Chen, J. Blanchard, J.R. Conrad, R.A. Dodd, Structure and wear properties of carbon implanted 304 stainless steel using plasma source ion implantation, Surface and Coatings Technology 53 (3) (November 10 1992) 267. K.C. Walter, M. Nastasi, C. Munson, Surface and Coatings Technology 93 (1997) 287. X.M. He, K.C. Walter, M. Nastasi, S.-T. Lee, X.S. Sun, Optical and tribological properties of diamond-like carbon films synthesized by plasma immersion ion processing, Thin Solid Films 355–356 (November 1 1999) 167. K. Sridharan, S. Anders, M. Nastasi, K.C. Walter, A. Anders, O.R. Monteriro, W. Ensinger, Nonsemiconductor applications of PII&D, in: A. Anders (Ed.), Handbook of Plasma Ion Implantation and Deposition, John Wiley & Sons, Inc, ISBN: 0-47124698-0, 2000, p. 553. A.H. Lettington, in: R.E. Clausing, et al., (Eds.), Diamond and Diamond-like Films and Coatings, New York, Plenum Press, 1991, p. 491. A. Matthews, S.S. Eskildsen, Diamond and Related Materials 3 (1994) 902. C. Donnet, Surf. Coat. Technol. 80 (1996) 151. A. Grill, Diamond and Related Materials 8 (1999) 428. P.K. Chu, J.Y. Chen, L.P. Wang, N. Huang, Plasma-surface modification of biomaterials, Materials Science and Engineering: R: Reports 36 (5–6) (March 29 2002) 143. P. Yang, N. Huang, Y.X. Leng, Z.Q. Yao, H.F. Zhou, M. Maitz, Y. Leng, P.K. Chu, Wettability and biocompatibility of nitrogen-doped hydrogenated amorphous carbon films: effect of nitrogen, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 242 (1–2) (January 2006) 22. E. Salgueiredo, M. Vila, M.A. Silva, M.A. Lopes, J.D. Santos, F.M. Costa, R.F. Silva, P.S. Gomes, M.H. Fernandes, Biocompatibility evaluation of DLC-coated Si3N4 substrates for biomedical applications, Diamond and Related Materials 17 (4–5) (April–May 2008) 878. Y. Catherine, in: R.E. Clausing, et al., (Eds.), Diamond and Diamond-like Films and Coatings, New York, Plenum Press, 1991, p. 193. N.A. Sanchez, C. Rincon, G. Zambrano, H. Galindo, P. Prieto, Thin Solid Films 373 (2000) 247. G. Fedosenko, J. Engemann, D. Korzec, Surf. Coat. Technol. 133–134 (2000) 535. K.C. Walter, H. Kung, T. Levine, J.R. Tesmer, P. Kodali, B.P. Wood, D.J. Rej, M. Nastasi, J. Koskinen, J.-P. Hirvonen, Mater. Res. Soc. Symp. Proc. 354 (1995) 87. J.V. Nantese, I.G. Brown, N.W. Cheung, G.A. Collins, MRS Bull. 21 (1996) 52. André Anders, From plasma immersion ion implantation to deposition: a historical perspective on principles and trends, Surface and Coatings Technology 156 (1–3) (July 1 2002) 3. Wei, R., Arps, J. and Lanning, B., Plasma and Ion Beam Surface Engineering for Practical Applications, presented at the 46th Annual SVC Technical Conference, San Francisco, California, May 3–8, 2003, to be published by Society of Vacuum Coaters. Ronghua Wei, Marta Jakab, Vasiliki Poenitzsch and Kent Coulter, Erosion, Corrosion and Wear Resistance and Microstructure of Diamond-like Carbon (DLC) Coatings Prepared using a Mesh Method, Presented at the 52nd Annual Society of Vacuum Coaters Technical Conference, April 28 – May 3, 2009, Santa Clara, USA, to be published by SVC in 2009. S.M. Malik, R.P. Fetherston, J.R. Conrad, J. Vac. Sci. Technol. A15 (1997) 2875. Ronghua Wei, Christopher Rincon, Thomas L. Booker, James H. Arps, Magnetic field enhanced plasma (MFEP) deposition of inner surfaces of tubes, Surf. Coat. Technol. 188–189 (2004) 691. J.W. Ager III, IEEE Transactions on Magnetics 29 (1) (1993) 259. J.K. Lee, M. Smallen, J. Enguero, H.J. Lee, A. Chao, IEEE Transactions on Magnetics 29 (1) (1993) 276. B. Marchon, P.N. Vo, IEEE Transactions on Magnetics 27 (6) (1991) 5160. K. Baba, R. Hatada, Nucl. Instrum. Methods Phys. Res., B, Beam Interact. Mater. Atoms 148 (1999) 69. K. Baba, R. Hatada, Nucl. Instrum. Methods Phys. Res., B, Beam Interact. Mater. Atoms 206 (2003) 704. Carter P. Munson, R.J. Faehl, I. Henins, M. Nastasi, W.A. Reass, D.J. Rej, J.T. Scheuer, K.C. Walter, B.P. Wood, Recent advances in plasma source ion implantation at Los Alamos National Laboratory, Surface and Coatings Technology 84 (1–3) (October 1996) 528. J.V. Mantese, I.G. Brown, N.W. Cheung, G.A. Collins, Plasma immersion ion implantation, MRS Bulletin (August, 1996) 52.