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Ion Plating
Ion Plating D. G. TEER *
The technique of ion plating was first reported as such in 1963. Since that time the technique has been developed and is finding application in tribology, corrosion protection, electrical contacts etc. This paper discusses the technique and modifications, and a number of practical applications are given.
Surface coatings are required in a variety of applications. The purpose of the coatings might be to increase corrosion or erosion resistance, improve friction and wear or fatigue properties, to provide an electrically conducting surface, increase reflectivity, or might be simply to improve appearance. The various types of coating all have different requirements as to their properties but it is possible to list a number of general requirements for thin films. A successful surface coating is dense, pin-hole free, uniform in thickness and, probably most important, is adherent to the substrate. In 1938, Berghaus ~ described a vacuum coating technique in which vapour was deposited on a substrate in a glow discharge, the substrate being the cathode of the discharge. He claimed that the coatings had 'a perfect structure and adhering strength" even for thicker layers. In 1963, Mattox 2 reported a similar technique which he called 'ion plating'. He claimed that the technique produced films with excellent adhesion, even in those cases where the film and substrate material were mutually incompatible. Further, the fihns were uniform, with no build-up at sharp corners, and surfaces that were out of line of sight of the source were also coated, ie, the technique had good throwing power. These results of Mattox stimulated great interest and since 1963 many papers have been published confirming the h]gh adhesion and uniformity 3-6. Further work has indicated that the films have good corrosion resistance 7 and that the grain structure of the films obtaaned is superior to that obtained by alternative vapour deposition techniques 8 lo. Using ion plating, coatings of pure metals, or alloys 4 can be deposited on to metal or ceramic H substrates. Using a modification of the basic technique, known as reactive ion plating, hard ceramic coatings such as titanium nitride or tungsten carbide ]2 can be obtained. In the following account, the basic technique is described in some detail together with a number of modifications. The theory of ion plating is discussed, and a number of practical applications are given.
TECHNIQUE The essential features of an ion plating apparatus are shown in Fig 1. It comprises a vacuum chamber and pumping system, as in a conventional vacuum coater. In addition, there is an inert gas inlet into the chamber, the flow being regulated by a needle valve, and the specimen is mounted * Department of Mechanical Engineering, Umverslty of Salford, Salford, M5 4WT, UK.
Pwan~ gouge Cathode water
co~=ng system
High voltage dc supply ( 0 - 6 kV )
Substrate Metalhc shield Gloss jar Metalhc screen Rubber seol Baffle valve
Ionlzahon gouge
Diffusion pump
Two way valve Deslcant r ~ trap
tVapour ra~p ALr inlet valve
Rotory pump
Fig 1
Ion plating apparatus
on an insulated high tension electrode. The vapour source can be either a resistance heated filament or boat, or an electron beam gun. In a typical coating sequence, the specimen or substrate is attached to the HT electrode, the filament or boat is loaded with the coating material and the chamber is pumped down to a pressure of 10-5 torr or better. The chamber is then backfilled with argon to a pressure of 10 2 torr to 5 × 10 -2 torr and maintained at the selected pressure by control of the argon leak valve and by partial closure of the baffle valve. The bias voltage is connected to the HT electrode/specimen holder and maintained at 3 - 5 kV negative. This causes a glow discharge to be struck between the earthed parts of the apparatus and the specimen which is bombarded by the high energy argon ions produced in the discharge. The ton bombardment gradually removes the surface layers and thereby any surface contamination from the specimen, and is continued until the surface is clean. Typically, the sputter cleaning is complete in 30 mins. While maintaining the discharge, the vapour source is energised and the coating material is vapourised into the discharge and deposits on the specimen surface.
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This completes the deposition, the vapour source and the speclment bias voltage are switched off, and the vacuum chamber is opened to the atmosphere after closing the balL fie and needle valves. Perhaps the most useful development in ion plating has been the use of an electron beam gun to vapourise the coating material. By using an eb gun it is possible to provide coatings of high melting point metals such as molybdenum and tungsten. Also the rate of deposition can be increased considerably and it is much more convenient to produce thick coatings using an eb gun instead of a resistance heated source. Using a rod-fed eb gun it is possible to produce alloy coatings of closely controlled composition 4. The ion plating process as described above is suitable for deposition onto conducting substrates only. In order to coat insulators a simple modification is necessary. The insulating substrate is surrounded by a conducting mesh which is attached to the HT electrode. The discharge is struck between the mesh and the earthed parts of the apparatus but the energetic argon and depositing vapour atoms pass through the mesh to the substrate. A more sophisticated method for coating insulators is to use an RF bias and so to prevent the build up of positive charge on the specimen 13. In certain applications it is advantageous to maintain the specimen at a low temperature and water cooling of the HT electrode is straightforward and often sufficient. For lower deposition temperatures, liquid nitrogen cooling has been used 14. A high speciment temperature helps to promote rapid diffusion of the coating material in the substrate. Where this is desirable the necessary high temperature can often be achieved by increasing the argon gas pressure and consequently the ion current to the specimen ~s. If it is required to coat at low gas pressure but at high specimen temperature, then a specimen heater must be incorporated ~6. The exact design of the specimen mounting arrangement will always be important in any production facility and will depend on the size and shape of the specimen, area to be coated, uniformity desired, etc. There is little information m the current literature giving details of specimen mountings although a wide variety of specimens are being coated using the ion plating technique. For instance, White x2 reports coating rods 6 ft long with refractory compounds and also
05
04 E =L
_e 0 2 Front
/ 0 I ~-
0
Beck
I 5
I IO
I 15
I 20
I 25
I 30
Gos pressure, p. m Hg
Fig 2 Film thickness on front and back surfaces of a fiat plate specimen as a f u n c t i o n o f argon pressure
248
TRIBOLOGY international December 1975
refurbishing worn aircraft engine parts by replacing the worn material by an ion plated coating. Janninck, Heiden and Guttensohn 17 describe a holder which accommodated 1100 reed blade contacts for ion plating with gold, and Starkovlch ~8 gives details of a mass production method for coating femte cores by ion plating. Very hard ceramic films can be produced using mg technique by replacing the inert argon by a Films of titanium nitride, titanium carbide and bide have been produced by including nitrogen m the coating chamber 12'19'2°.
the ion platreactive gas. tungsten caror methane
The maximum size of the specimen which can be coated by ion plating is limited only by the physical size of the coatlng chamber and the availability of HT supphes capable of sufficient power. Ion platers are now available with 200 kW electron beam guns with bias supplies of over 40 kW capable of coating surfaces 20 in by 20 in 21. FILM CHARACTERISTICS
The two characteristics of Ion plating that have received most attention are the throwing power and the adhesion of the films. We will consider first the throwing power. Chambers and Carmichael 4 have published details of the relative thickness of films deposited onto the front and back surfaces of a flat plate as a function of gas pressure. These results have been confirmed by Teer and Sherbiney ~° whose results are shown in Fig 2. It can be seen that the ratio of film thickness on the front surface to that on the back surface, approaches unity at the higher gas pressures. What this means in practice is that back surfaces, internal surfaces of tubes, screw threads etc, can be coated without specimen rotation, as is necessary in vacuum deposition. It has been suggested that the good throwing power of the ion plating process is due to the ionised vapour atoms following field lines to the back and internal surfaces. However, Teer and Sherbiney ~° have now shown that the field has only a minor effect and that the throwing power is due to gas scattering effects. With regard to adhesion many investigators have reported excellent adhesion for ion plated films. In this laboratory we have used bend and tensile tests on coated specimens, and have been unable to cause failure of the adhesive bond. Scratch tests too, indicate an exceptionally strong adhesive bond. Fig 3 shows coated tensile specimens pulled to failure. The vacuum evaporated coating has become detached completely whereas the ion plated coating is still adherent up to the point of fracture. The result of a scratch test is shown in Fig 4. The ion plated coating can be seen along the side of the scratch and no adhesive failure is apparent. Scanning electron micrographs of tracks of a hemispherically ended indenter rubbed over coated surfaces are shown in Figs 5a and 5b. Again the difference in adhesion of the vacuum evaporated and the ion plated coatings is apparent. Mattox s has reviewed the requirements for good adhesion. These can be summarized by stating that the substrate surface must be cleaned of any contaminating films, and that adhesion is improved if the coating material can penetrate into the substrate material to form a graded interface. Ion plating satisfies both these requirements. The ion bombardment prior to and during the deposition, produces and maintains a clean surface. The coating atoms evaporated into the glow discharge, derive kinetic energy from the discharge and arrive at the substrate with a range of energies, the mean energy probably being about 300eV for typical ion
resistance. We will consider these properties in turn although in many cases the distinction becomes blurred. For instance, low friction is often accompanied by low wear, and for rubbing surfaces corrosion can be a major cause of wear. Low friction coatings
Soft metal films on hard substrates are used in many applications where it is impossible to use conventional lubricants, for instance, in vacuum or space environments z6. Spalvins e t al 3 found that ion plated gold films on nickel and tool steel substrates gave lower frictions than vacuum evaporated films when tested in a vacuum environment. Also the low friction continued for almost twice the rubbing distance as compared to the vacuum evaporated films. Wlsander 27 compared the friction and characteristics of ion plated and electroplated films of lead, indium and tin on steel, run in a liquid hydrogen environment. The ion plated films were superior m each case. Fig 3 SEver on steel tensile specimens, ion plated on the /eft, vacuum evaporated on the right
Jrfece of coating
Sherbiney 28 has compared indium, lead and silver films deposited by vapour deposition and ion plating, in a pro-on-disc machine in normal atmospheres. In all cases, the ion plated films gave slightly lower friction, but more importantly. they retained the low friction for much longer rubbing distances. A similar result was obtained by Arnell, Teer and Suliman z9 for lead films on steel substrates tested in ultra high vacuum. The reason for the lower friction coefficient
~ChOnthrough coating
ubstrate
Fig 4 Scanning electron micrograph of diamond scratch. Ion plated silver on copper substrate. Magnification 5000x
plating conditions 22. This high energy bombardment causes enhanced diffusion (and hence a graded interface) due to heating effects and also due to the production of a high density of vacancies m the sub-surface regions. The enhanced diffusion has been noted by Adler and Swaroop 23 for copper film on steel substrates, by Teer and Selim is for aluminlum on titanium and by Teer and Sherbiney ~° for indium on copper. These pairs of materials are all compatible, and so conventional dift'usion can take place. In the case of incompatible pairs the adhesion is also very good, but the depth of penetration is very much less 24. Ion implantation has been suggested as the major cause of the graded interface and the good adhesion. However, it should be reahsed that only a small percentage of the coating atoms are ionised 22 and also that under typical ion plating conditions few of these ions arrive at the substrate with the maximum possible energy 2s. Thus ion implantation is unlikely to cause penetration of the coating material beyond a depth of about 50A. APPLICATIONS
Most ion plated coatings have been applied to substrates in order to reduce friction and/or wear or to increase corrosion
Fig 5 Scanning electron micrographs of tracks caused by 0.015 in diameter, hemispherically ended pin. (a) Ion plated sEver on steel Magnification 700x. (b) Vacuum evaporated silver on steel Magnification 280x
TRIBOLOGY international December 1975
249
of the ion plated films is probably due to a hardenmg of the surface of the substrate caused by the penetration of the coating material. The greater endurance is assocmted with the improved adhesion of the ion plated films. A promising development In the field of lubrication by thin metal films is the use of two stage coatings, where a soft film such as lead is ion plated on to a fihn of a hexagonal close packed metal such as cobalt on a hard substrate 3°. The lead provides the low shear strength surface and the cobalt prevents any excessive wear of the substrate when the lead film is penetrated.
Wear resistant coatings There are few reports in the current literature of the wear resistance of ion plated hard metal flhn coatings. However, it IS now possible to ion plate with alloy steels, refractory metal alloys and the hexagonal closed packed metals and undoubtedly these will find increasing application as wear and erosion resistance flhns. The advantages offered by ion plating are the excellent adhesion and the uniformity of the coating, eliminating any post deposition inachming operations. A number of laboratories are lnvestigatmg ion plating ot refractory carbide and nitride fihns. White 12 describes the development of a nozzle used to direct a flow of abrasive shlrry onto steel, concrete and rock for cutting purposes. Although previous designs fabricated froln cemented carbides were unable to provide the necessary abrasion resistance, an ion plated coating, consisting of successive layers of refractory metal, carbide and nltride, was successful at less than half the anticipated thickness. White also reports a ten-fold increase in operating hfe of rotary engine parts, due to the use of ion plated coatings. Other applications under investigation are the coating of tool tips with refractory metal carbides and mtrides. Ion plating offers the possibility of deposmon at significantly lower substrate temperatures than are necessary in chemical vapour deposition. Titanium alloys are used extensively in the aircraft industry because of their high strength to weight ratio. However, titanium offers very poor wear resistance and ts also dip ficult to lubricate. Hard, wear resistant surfaces can be produced on titamuln by ion plating with alumlmum q. The enhanced diffusion caused by the ion bombardment produces an interface of titanlum-alumlnluln solid solutloll further hardened by TiAI3 precipitates. The solid solution has a hexagonal close packed structure with excellent wear resistance. The interface has been tested by rubbing with a hemispherical ended, hardened steel pin (VHN 800). The initial coefficient of friction is less than 0.1 and, after continued rubbing, the steel is worn rather than the titanium Is. Fig 6 shows a coated titanium disc which has been rubbed against a hardened steel ball, under a load of 3 lb at 30 rpm at a radius of 0.5 m for 8 h.
Fig 6 Scanning electron micrograph of wear track on titanium, ion plated with aluminium. Magnificat/nn 50>
13 ~ thick, and has operated in normal atlnosphere at 150°C without visible oxydatlon. Previously, similar fuel elements had been protected by electro-plated nickel but this tended to spall tinder thermal shock resulting in serious corrosion. Cadmium coatings have long been used to provide corrosion protection for steel and titanium fasteners. It is interesting to note that in a 1968 Ministry of Technology process specification 32 for the coating of carbon and low alloy steel fasteners with cadmium, the process described is an Ion plating process although the title 'ion plating' is not used. More recently It has been found that aluminium films provide better corrosion protection and Steube and McCrary sa describe a number of successful applications for ion plated alulnlnium coatings on aircraft and spacecraft parts, includnlg coatings for corrosion protection of titanium alloy fasteners. Corrosion protection for blades in the hot section of a gas turbine has been improved considerably in the past few years by using overlay coatings 34. These coatings of cobalt, chrommm, aluminiuln, yttrium alloys, have been deposited by vapour deposition 3s but require post deposition mechameal and heat treatlnent to Improve the colulnnar gram structure. Thts structure ls primarily due to the line of sight coating characteristics of vacuum deposition. Ion plating appears to be a more prolnising method of producing films of the required structure 36, and it is being developed for this application in a number of laboratories. CONCLUSIONS
Corrosion resistant fdms
Ion plating is capable of producing highly adhesive films of uniform thickness and with good grain structure. With the introduction of electron beam guns, it is possible to produce fihns of metals and alloys at high deposition rates Reactive ion plating offers the exciting possiblhties of very hard coatings of the refractory metal carbides. The technique is already finding application In trlbology, corrosion protection, electrical contacts, etc. It is probable that further important applications will be found in metal to metal, and metal to ceralnic joining, and in the production of "alloys" of normally incompatible materials.
The efficiency of ion plated fihns in provtdllrg corrosion resistance was first demonstrated by Mattox and Bland 7. A uranium fuel elelnent was ion plated with alulninlum film
Much development work IS still required into various aspects of ion plating but many proven applications exist awaiting commercial exploitation, lt ts one of the anns of thls paper
The ball has been worn to a depth of 0.007 in whereas the wear of the titanium surface is neghglble, the track being defined more by transferred steel than by titamum wear.
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TRIBOLOGY international December 1975
to stimulate both further development and commercM interest in the process.
REFERENCES 1
Berghaus, B., UK Patent Specification, 510, 993, 1938
2
Mattox, D.M., JApplPhys, 34, 1963, p2493
3
Spalvins, T, Przybyszewski, J. S. and Buckley, D. H., NASA TN D-3707, 1966
18
Starkovich, J., Sandla Corp Dev Report No SC-DR-68-188, 1968
19
Anderson, A., SA/c, Paper No 73052 presented at Automobde Engineering Meeting, Detroit, Michigan, May 1973
20
Stowell, W. R., Thin Sohd Films, 1974, pl 11
21
Maher and ltarker, Soc Vac Coater~ Tech l,'orum Proc, Chicago, 1973, pl
22
Teer, D. G., to bc pubhshed
23
Swaroop, B., and Adler, l . , J Vac Scz Technol, 10, 1973, p503
24
Ahmed, N. A. G., MSc Dissertation, University or" Salford, 1974
25
Davis, W. D. and Vanderslice, T. A., Phys Rev, 131, 1963, t)219
26
Bisson, E. E., Advanced Bearing Technology, NASA SP-38, 1964, p259
4
Chambers, D. L. and Carmichael, D. C., Research/DevelopmentMag, 22, 1971. p32
5
Mattox, D . M . , J V a c S c i T e c h n o l , 10,47 1973
6
NASA SP 5111 P, Proc Conf on 'Sputtering and Ion Plating', held at Lewis Res Center, March 1972
7
Mattox, D. M. and Bland, R. D., J Nucl Mats. 21, 1967, p349
27
Wisander, D. W., NASA TN D-6455, 1971
8
Bland, R. D,, Kominiak, J. and Mattox, D. M.. J l'ac Sci Technol, 11, 1974, p671
28
Sherbiney, M, G., PhD Thesis, Unwerslty of Salford, 1975
29
Arnell, R. D., Teer, D. G. and Suliman, F., to be pubhshed
30
Teer, D. G. and Arneil, R. D., UK Patent Apphcatlon 29701/74, 1974
9
Wan, C. T., Chambers, D. L. and Carmichael, D. C.
10
Teer, D. G., and Sherbiney, M. G., to bc pubhshcd
11
Mattox, D. M . , J A m Ceram Soc, 38, 1965, p385
31
Teer, D. G., UK Patent Apphcatlon 35448/74, 1974
12
White, G. W,, Research/Derelopment Mag, 24, 1973, p43
32
13
GordonDavy, J. andHanak, J . J . , J VacSct Technol, 11, 1974, p43
Ministry of Technology Process Specification DTD, 960, Nov 1968
33
14
Bland, R. D., Sandm Corp Report No, SC-TM-71-0526, 1971
Steube, K. E., and McCrary, L. E., J Vac Sci Technol, I 1, 1974, p362
15
Teer, D. G. and Selim, F., to be pubhqlcd
34
Goward, G.W.,JMetals. 22, 1970, p31
16
Bland, R. D.,J VacSciTechnol, 11, 1974, p906
35
17
Janninck, R, F., Heiden, C. R. and Guttensohn, A. E., J Vac Sci Technol, 11, 1974, p535
Boone, D. H. Strangman, T. E. and Wilson, L. W., J t'ac Sci Technol, 11, 1974, p641
36
Krutenat, R. C., US Patent 3,639,151, t:eb 1972
2nd InternationalConference and Exhibition on Computers in EngineeringandBuildingDesign Imperial College,London 23,/25March1976 Organized by the journal Computer Aided Design A forum for discussion of significant developments and applications in the use of the computer as a design tool. The conference will consist of three days of two parallel sessions. Invited review papers will introduce 48 papers in sessions in the following subject areas: • • •
Design linked to manufacture Building design Mechanical engineering
An exhibition is also bein{) held showing computer graphics equipment, and Other peripherals together with CAD software. Exhibitors participating include
Tektronix, Hewlett-Packa~d, Scicon, Repko (NV) and CAD Centre.
• •
Civil and structural engineering Graphics, drafting and displays
Conference Sponsors: I FIP WG 5.2, CAD Centre, Design Office Consortium, Construction Industry Research & Information Association, Institute of Civil Engineers, British Computer Society.
Further conference and exhibition details from: Mrs. M. Stacey, Conference Secretary, CAD 76, IPC Science & Technology Press Ltd, 32 High Street, Guildford, Surrey, England GU1 3EW. Tel: Guildford (0483) 71661. Telex: Scitechpress Gd. 859556.
TRIBOLOGY international December 1975
251
The technique of ion plating was first reported as such in 1963. Since that time the technique has been developed and is finding application in tribology, corrosion protection, electrical contacts etc. This paper discusses the technique and modifications, and a number of practical applications are given.
Surface coatings are required in a variety of applications. The purpose of the coatings might be to increase corrosion or erosion resistance, improve friction and wear or fatigue properties, to provide an electrically conducting surface, increase reflectivity, or might be simply to improve appearance. The various types of coating all have different requirements as to their properties but it is possible to list a number of general requirements for thin films. A successful surface coating is dense, pin-hole free, uniform in thickness and, probably most important, is adherent to the substrate. In 1938, Berghaus ~ described a vacuum coating technique in which vapour was deposited on a substrate in a glow discharge, the substrate being the cathode of the discharge. He claimed that the coatings had 'a perfect structure and adhering strength" even for thicker layers. In 1963, Mattox 2 reported a similar technique which he called 'ion plating'. He claimed that the technique produced films with excellent adhesion, even in those cases where the film and substrate material were mutually incompatible. Further, the fihns were uniform, with no build-up at sharp corners, and surfaces that were out of line of sight of the source were also coated, ie, the technique had good throwing power. These results of Mattox stimulated great interest and since 1963 many papers have been published confirming the h]gh adhesion and uniformity 3-6. Further work has indicated that the films have good corrosion resistance 7 and that the grain structure of the films obtaaned is superior to that obtained by alternative vapour deposition techniques 8 lo. Using ion plating, coatings of pure metals, or alloys 4 can be deposited on to metal or ceramic H substrates. Using a modification of the basic technique, known as reactive ion plating, hard ceramic coatings such as titanium nitride or tungsten carbide ]2 can be obtained. In the following account, the basic technique is described in some detail together with a number of modifications. The theory of ion plating is discussed, and a number of practical applications are given.
TECHNIQUE The essential features of an ion plating apparatus are shown in Fig 1. It comprises a vacuum chamber and pumping system, as in a conventional vacuum coater. In addition, there is an inert gas inlet into the chamber, the flow being regulated by a needle valve, and the specimen is mounted * Department of Mechanical Engineering, Umverslty of Salford, Salford, M5 4WT, UK.
Pwan~ gouge Cathode water
co~=ng system
High voltage dc supply ( 0 - 6 kV )
Substrate Metalhc shield Gloss jar Metalhc screen Rubber seol Baffle valve
Ionlzahon gouge
Diffusion pump
Two way valve Deslcant r ~ trap
tVapour ra~p ALr inlet valve
Rotory pump
Fig 1
Ion plating apparatus
on an insulated high tension electrode. The vapour source can be either a resistance heated filament or boat, or an electron beam gun. In a typical coating sequence, the specimen or substrate is attached to the HT electrode, the filament or boat is loaded with the coating material and the chamber is pumped down to a pressure of 10-5 torr or better. The chamber is then backfilled with argon to a pressure of 10 2 torr to 5 × 10 -2 torr and maintained at the selected pressure by control of the argon leak valve and by partial closure of the baffle valve. The bias voltage is connected to the HT electrode/specimen holder and maintained at 3 - 5 kV negative. This causes a glow discharge to be struck between the earthed parts of the apparatus and the specimen which is bombarded by the high energy argon ions produced in the discharge. The ton bombardment gradually removes the surface layers and thereby any surface contamination from the specimen, and is continued until the surface is clean. Typically, the sputter cleaning is complete in 30 mins. While maintaining the discharge, the vapour source is energised and the coating material is vapourised into the discharge and deposits on the specimen surface.
TRIBOLOGY international December 1975
247
This completes the deposition, the vapour source and the speclment bias voltage are switched off, and the vacuum chamber is opened to the atmosphere after closing the balL fie and needle valves. Perhaps the most useful development in ion plating has been the use of an electron beam gun to vapourise the coating material. By using an eb gun it is possible to provide coatings of high melting point metals such as molybdenum and tungsten. Also the rate of deposition can be increased considerably and it is much more convenient to produce thick coatings using an eb gun instead of a resistance heated source. Using a rod-fed eb gun it is possible to produce alloy coatings of closely controlled composition 4. The ion plating process as described above is suitable for deposition onto conducting substrates only. In order to coat insulators a simple modification is necessary. The insulating substrate is surrounded by a conducting mesh which is attached to the HT electrode. The discharge is struck between the mesh and the earthed parts of the apparatus but the energetic argon and depositing vapour atoms pass through the mesh to the substrate. A more sophisticated method for coating insulators is to use an RF bias and so to prevent the build up of positive charge on the specimen 13. In certain applications it is advantageous to maintain the specimen at a low temperature and water cooling of the HT electrode is straightforward and often sufficient. For lower deposition temperatures, liquid nitrogen cooling has been used 14. A high speciment temperature helps to promote rapid diffusion of the coating material in the substrate. Where this is desirable the necessary high temperature can often be achieved by increasing the argon gas pressure and consequently the ion current to the specimen ~s. If it is required to coat at low gas pressure but at high specimen temperature, then a specimen heater must be incorporated ~6. The exact design of the specimen mounting arrangement will always be important in any production facility and will depend on the size and shape of the specimen, area to be coated, uniformity desired, etc. There is little information m the current literature giving details of specimen mountings although a wide variety of specimens are being coated using the ion plating technique. For instance, White x2 reports coating rods 6 ft long with refractory compounds and also
05
04 E =L
_e 0 2 Front
/ 0 I ~-
0
Beck
I 5
I IO
I 15
I 20
I 25
I 30
Gos pressure, p. m Hg
Fig 2 Film thickness on front and back surfaces of a fiat plate specimen as a f u n c t i o n o f argon pressure
248
TRIBOLOGY international December 1975
refurbishing worn aircraft engine parts by replacing the worn material by an ion plated coating. Janninck, Heiden and Guttensohn 17 describe a holder which accommodated 1100 reed blade contacts for ion plating with gold, and Starkovlch ~8 gives details of a mass production method for coating femte cores by ion plating. Very hard ceramic films can be produced using mg technique by replacing the inert argon by a Films of titanium nitride, titanium carbide and bide have been produced by including nitrogen m the coating chamber 12'19'2°.
the ion platreactive gas. tungsten caror methane
The maximum size of the specimen which can be coated by ion plating is limited only by the physical size of the coatlng chamber and the availability of HT supphes capable of sufficient power. Ion platers are now available with 200 kW electron beam guns with bias supplies of over 40 kW capable of coating surfaces 20 in by 20 in 21. FILM CHARACTERISTICS
The two characteristics of Ion plating that have received most attention are the throwing power and the adhesion of the films. We will consider first the throwing power. Chambers and Carmichael 4 have published details of the relative thickness of films deposited onto the front and back surfaces of a flat plate as a function of gas pressure. These results have been confirmed by Teer and Sherbiney ~° whose results are shown in Fig 2. It can be seen that the ratio of film thickness on the front surface to that on the back surface, approaches unity at the higher gas pressures. What this means in practice is that back surfaces, internal surfaces of tubes, screw threads etc, can be coated without specimen rotation, as is necessary in vacuum deposition. It has been suggested that the good throwing power of the ion plating process is due to the ionised vapour atoms following field lines to the back and internal surfaces. However, Teer and Sherbiney ~° have now shown that the field has only a minor effect and that the throwing power is due to gas scattering effects. With regard to adhesion many investigators have reported excellent adhesion for ion plated films. In this laboratory we have used bend and tensile tests on coated specimens, and have been unable to cause failure of the adhesive bond. Scratch tests too, indicate an exceptionally strong adhesive bond. Fig 3 shows coated tensile specimens pulled to failure. The vacuum evaporated coating has become detached completely whereas the ion plated coating is still adherent up to the point of fracture. The result of a scratch test is shown in Fig 4. The ion plated coating can be seen along the side of the scratch and no adhesive failure is apparent. Scanning electron micrographs of tracks of a hemispherically ended indenter rubbed over coated surfaces are shown in Figs 5a and 5b. Again the difference in adhesion of the vacuum evaporated and the ion plated coatings is apparent. Mattox s has reviewed the requirements for good adhesion. These can be summarized by stating that the substrate surface must be cleaned of any contaminating films, and that adhesion is improved if the coating material can penetrate into the substrate material to form a graded interface. Ion plating satisfies both these requirements. The ion bombardment prior to and during the deposition, produces and maintains a clean surface. The coating atoms evaporated into the glow discharge, derive kinetic energy from the discharge and arrive at the substrate with a range of energies, the mean energy probably being about 300eV for typical ion
resistance. We will consider these properties in turn although in many cases the distinction becomes blurred. For instance, low friction is often accompanied by low wear, and for rubbing surfaces corrosion can be a major cause of wear. Low friction coatings
Soft metal films on hard substrates are used in many applications where it is impossible to use conventional lubricants, for instance, in vacuum or space environments z6. Spalvins e t al 3 found that ion plated gold films on nickel and tool steel substrates gave lower frictions than vacuum evaporated films when tested in a vacuum environment. Also the low friction continued for almost twice the rubbing distance as compared to the vacuum evaporated films. Wlsander 27 compared the friction and characteristics of ion plated and electroplated films of lead, indium and tin on steel, run in a liquid hydrogen environment. The ion plated films were superior m each case. Fig 3 SEver on steel tensile specimens, ion plated on the /eft, vacuum evaporated on the right
Jrfece of coating
Sherbiney 28 has compared indium, lead and silver films deposited by vapour deposition and ion plating, in a pro-on-disc machine in normal atmospheres. In all cases, the ion plated films gave slightly lower friction, but more importantly. they retained the low friction for much longer rubbing distances. A similar result was obtained by Arnell, Teer and Suliman z9 for lead films on steel substrates tested in ultra high vacuum. The reason for the lower friction coefficient
~ChOnthrough coating
ubstrate
Fig 4 Scanning electron micrograph of diamond scratch. Ion plated silver on copper substrate. Magnification 5000x
plating conditions 22. This high energy bombardment causes enhanced diffusion (and hence a graded interface) due to heating effects and also due to the production of a high density of vacancies m the sub-surface regions. The enhanced diffusion has been noted by Adler and Swaroop 23 for copper film on steel substrates, by Teer and Selim is for aluminlum on titanium and by Teer and Sherbiney ~° for indium on copper. These pairs of materials are all compatible, and so conventional dift'usion can take place. In the case of incompatible pairs the adhesion is also very good, but the depth of penetration is very much less 24. Ion implantation has been suggested as the major cause of the graded interface and the good adhesion. However, it should be reahsed that only a small percentage of the coating atoms are ionised 22 and also that under typical ion plating conditions few of these ions arrive at the substrate with the maximum possible energy 2s. Thus ion implantation is unlikely to cause penetration of the coating material beyond a depth of about 50A. APPLICATIONS
Most ion plated coatings have been applied to substrates in order to reduce friction and/or wear or to increase corrosion
Fig 5 Scanning electron micrographs of tracks caused by 0.015 in diameter, hemispherically ended pin. (a) Ion plated sEver on steel Magnification 700x. (b) Vacuum evaporated silver on steel Magnification 280x
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of the ion plated films is probably due to a hardenmg of the surface of the substrate caused by the penetration of the coating material. The greater endurance is assocmted with the improved adhesion of the ion plated films. A promising development In the field of lubrication by thin metal films is the use of two stage coatings, where a soft film such as lead is ion plated on to a fihn of a hexagonal close packed metal such as cobalt on a hard substrate 3°. The lead provides the low shear strength surface and the cobalt prevents any excessive wear of the substrate when the lead film is penetrated.
Wear resistant coatings There are few reports in the current literature of the wear resistance of ion plated hard metal flhn coatings. However, it IS now possible to ion plate with alloy steels, refractory metal alloys and the hexagonal closed packed metals and undoubtedly these will find increasing application as wear and erosion resistance flhns. The advantages offered by ion plating are the excellent adhesion and the uniformity of the coating, eliminating any post deposition inachming operations. A number of laboratories are lnvestigatmg ion plating ot refractory carbide and nitride fihns. White 12 describes the development of a nozzle used to direct a flow of abrasive shlrry onto steel, concrete and rock for cutting purposes. Although previous designs fabricated froln cemented carbides were unable to provide the necessary abrasion resistance, an ion plated coating, consisting of successive layers of refractory metal, carbide and nltride, was successful at less than half the anticipated thickness. White also reports a ten-fold increase in operating hfe of rotary engine parts, due to the use of ion plated coatings. Other applications under investigation are the coating of tool tips with refractory metal carbides and mtrides. Ion plating offers the possibility of deposmon at significantly lower substrate temperatures than are necessary in chemical vapour deposition. Titanium alloys are used extensively in the aircraft industry because of their high strength to weight ratio. However, titanium offers very poor wear resistance and ts also dip ficult to lubricate. Hard, wear resistant surfaces can be produced on titamuln by ion plating with alumlmum q. The enhanced diffusion caused by the ion bombardment produces an interface of titanlum-alumlnluln solid solutloll further hardened by TiAI3 precipitates. The solid solution has a hexagonal close packed structure with excellent wear resistance. The interface has been tested by rubbing with a hemispherical ended, hardened steel pin (VHN 800). The initial coefficient of friction is less than 0.1 and, after continued rubbing, the steel is worn rather than the titanium Is. Fig 6 shows a coated titanium disc which has been rubbed against a hardened steel ball, under a load of 3 lb at 30 rpm at a radius of 0.5 m for 8 h.
Fig 6 Scanning electron micrograph of wear track on titanium, ion plated with aluminium. Magnificat/nn 50>
13 ~ thick, and has operated in normal atlnosphere at 150°C without visible oxydatlon. Previously, similar fuel elements had been protected by electro-plated nickel but this tended to spall tinder thermal shock resulting in serious corrosion. Cadmium coatings have long been used to provide corrosion protection for steel and titanium fasteners. It is interesting to note that in a 1968 Ministry of Technology process specification 32 for the coating of carbon and low alloy steel fasteners with cadmium, the process described is an Ion plating process although the title 'ion plating' is not used. More recently It has been found that aluminium films provide better corrosion protection and Steube and McCrary sa describe a number of successful applications for ion plated alulnlnium coatings on aircraft and spacecraft parts, includnlg coatings for corrosion protection of titanium alloy fasteners. Corrosion protection for blades in the hot section of a gas turbine has been improved considerably in the past few years by using overlay coatings 34. These coatings of cobalt, chrommm, aluminiuln, yttrium alloys, have been deposited by vapour deposition 3s but require post deposition mechameal and heat treatlnent to Improve the colulnnar gram structure. Thts structure ls primarily due to the line of sight coating characteristics of vacuum deposition. Ion plating appears to be a more prolnising method of producing films of the required structure 36, and it is being developed for this application in a number of laboratories. CONCLUSIONS
Corrosion resistant fdms
Ion plating is capable of producing highly adhesive films of uniform thickness and with good grain structure. With the introduction of electron beam guns, it is possible to produce fihns of metals and alloys at high deposition rates Reactive ion plating offers the exciting possiblhties of very hard coatings of the refractory metal carbides. The technique is already finding application In trlbology, corrosion protection, electrical contacts, etc. It is probable that further important applications will be found in metal to metal, and metal to ceralnic joining, and in the production of "alloys" of normally incompatible materials.
The efficiency of ion plated fihns in provtdllrg corrosion resistance was first demonstrated by Mattox and Bland 7. A uranium fuel elelnent was ion plated with alulninlum film
Much development work IS still required into various aspects of ion plating but many proven applications exist awaiting commercial exploitation, lt ts one of the anns of thls paper
The ball has been worn to a depth of 0.007 in whereas the wear of the titanium surface is neghglble, the track being defined more by transferred steel than by titamum wear.
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to stimulate both further development and commercM interest in the process.
REFERENCES 1
Berghaus, B., UK Patent Specification, 510, 993, 1938
2
Mattox, D.M., JApplPhys, 34, 1963, p2493
3
Spalvins, T, Przybyszewski, J. S. and Buckley, D. H., NASA TN D-3707, 1966
18
Starkovich, J., Sandla Corp Dev Report No SC-DR-68-188, 1968
19
Anderson, A., SA/c, Paper No 73052 presented at Automobde Engineering Meeting, Detroit, Michigan, May 1973
20
Stowell, W. R., Thin Sohd Films, 1974, pl 11
21
Maher and ltarker, Soc Vac Coater~ Tech l,'orum Proc, Chicago, 1973, pl
22
Teer, D. G., to bc pubhshed
23
Swaroop, B., and Adler, l . , J Vac Scz Technol, 10, 1973, p503
24
Ahmed, N. A. G., MSc Dissertation, University or" Salford, 1974
25
Davis, W. D. and Vanderslice, T. A., Phys Rev, 131, 1963, t)219
26
Bisson, E. E., Advanced Bearing Technology, NASA SP-38, 1964, p259
4
Chambers, D. L. and Carmichael, D. C., Research/DevelopmentMag, 22, 1971. p32
5
Mattox, D . M . , J V a c S c i T e c h n o l , 10,47 1973
6
NASA SP 5111 P, Proc Conf on 'Sputtering and Ion Plating', held at Lewis Res Center, March 1972
7
Mattox, D. M. and Bland, R. D., J Nucl Mats. 21, 1967, p349
27
Wisander, D. W., NASA TN D-6455, 1971
8
Bland, R. D,, Kominiak, J. and Mattox, D. M.. J l'ac Sci Technol, 11, 1974, p671
28
Sherbiney, M, G., PhD Thesis, Unwerslty of Salford, 1975
29
Arnell, R. D., Teer, D. G. and Suliman, F., to be pubhshed
30
Teer, D. G. and Arneil, R. D., UK Patent Apphcatlon 29701/74, 1974
9
Wan, C. T., Chambers, D. L. and Carmichael, D. C.
10
Teer, D. G., and Sherbiney, M. G., to bc pubhshcd
11
Mattox, D. M . , J A m Ceram Soc, 38, 1965, p385
31
Teer, D. G., UK Patent Apphcatlon 35448/74, 1974
12
White, G. W,, Research/Derelopment Mag, 24, 1973, p43
32
13
GordonDavy, J. andHanak, J . J . , J VacSct Technol, 11, 1974, p43
Ministry of Technology Process Specification DTD, 960, Nov 1968
33
14
Bland, R. D., Sandm Corp Report No, SC-TM-71-0526, 1971
Steube, K. E., and McCrary, L. E., J Vac Sci Technol, I 1, 1974, p362
15
Teer, D. G. and Selim, F., to be pubhqlcd
34
Goward, G.W.,JMetals. 22, 1970, p31
16
Bland, R. D.,J VacSciTechnol, 11, 1974, p906
35
17
Janninck, R, F., Heiden, C. R. and Guttensohn, A. E., J Vac Sci Technol, 11, 1974, p535
Boone, D. H. Strangman, T. E. and Wilson, L. W., J t'ac Sci Technol, 11, 1974, p641
36
Krutenat, R. C., US Patent 3,639,151, t:eb 1972
2nd InternationalConference and Exhibition on Computers in EngineeringandBuildingDesign Imperial College,London 23,/25March1976 Organized by the journal Computer Aided Design A forum for discussion of significant developments and applications in the use of the computer as a design tool. The conference will consist of three days of two parallel sessions. Invited review papers will introduce 48 papers in sessions in the following subject areas: • • •
Design linked to manufacture Building design Mechanical engineering
An exhibition is also bein{) held showing computer graphics equipment, and Other peripherals together with CAD software. Exhibitors participating include
Tektronix, Hewlett-Packa~d, Scicon, Repko (NV) and CAD Centre.
• •
Civil and structural engineering Graphics, drafting and displays
Conference Sponsors: I FIP WG 5.2, CAD Centre, Design Office Consortium, Construction Industry Research & Information Association, Institute of Civil Engineers, British Computer Society.
Further conference and exhibition details from: Mrs. M. Stacey, Conference Secretary, CAD 76, IPC Science & Technology Press Ltd, 32 High Street, Guildford, Surrey, England GU1 3EW. Tel: Guildford (0483) 71661. Telex: Scitechpress Gd. 859556.
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