The ion plating of carbon

Thin Solid Films, 45 (1977)553-561

ElsevierSequoia S,A., Lausanne--Printed in the Netherlands

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T H E ION P L A T I N G O F CARBON* D. G. TEERAND M. SALAMA Department of Aeronautical and Mechanical Engineering, University of Salford, Salford M5 4 W T ( Gt. Britain)

(ReceivedMarch 31, 1977;acceptedApril 9, 1977)

Ion-plated coatings of carbon have been deposited on several metal substrates. The coatings are very adherent and in contrast with vacuum-evaporated films have a highly graphitic crystal structure. They are wear resistant and have a low friction coefficient. The method o f deposition and the crystallographic and tribological studies will be described.

1. INTRODUCTION Ion-plated metal films generally have excellent adhesion, even when the film and substrate material do not exhibit solid solubility. This adhesion is a great advantage for low friction films on bearing surfaces where high friction coefficients result once the film has been removed. Spalvins et aL 1 found that ion-plated gold films on nickel and tool steel substrates gave lower friction than vacuum-evaporated films did when tested in a vacuum environment and that the low friction continued for almost twice the rubbing distance. Wisander 2 compared the friction and wear characteristics o f ion-plated and electroplated films of lead, indium and tin on steel, run in a liquid hydrogen environment; he found that the ion-plated films were superior in every case. Sherbiney 3 compared indium, lead and silver films deposited by vacuum evaporation and ion plating in a pin-on-disc machine in normal atmospheres. In all cases, the ion-plated films gave slightly lower friction and what is more important they retained the low friction for much longer rubbing distances. The low friction in these experiments was due to the use o f soft films on hard substrates rather than to any intrinsic property of the film material. Materials with hexagonal layer structures such as molybdenum disulphide and graphite have low frictions when used as solids or as surface coatings. Low friction coatings o f MoS 2 can be produced by sputtering 4, but it is doubtful if MoS2 can be deposited using evaporative ion-plating techniques. Carbon coatings have been produced by vacuum evaporation and by high temperature decomposition of carbon-containing gases such as methane. Also carbon coatings have been deposited using ion beams 5, 6 and diamond-like structures have been obtained. However, no ion-plated *Paper presentedat the International Conferenceon MetallurgicalCoatings, San Francisco,California, U.S.A., March 28-April 1, 1977.

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carbon coatings have been reported, and it was therefore decided to study the properties of ion-plated carbon coatings with tribological applications in mind. 2.

THE ION-PLATING METHOD

Two methods were used to produce the carbon films. The first was suitable only for relatively thin films ( < 1 ~tm) and consisted of a conventional ion-plating method but with contacting pointed carbon rods (through which a high current was passed) as the vapour source. This is a standard evaporation source for producing carbon films for electron microscope replicas. In order to increase the maximum thickness produced by this method, four similar sources could be operated sequentially. In the second method a differentially pumped 15 kW electron beam g ~ w a s used to evaporate the carbon. Much thicker films were possible using this method. The substrates were of copper and mild steel. The carbon rods used in the first method were spectrographically pure highly graphitic carbon. The carbons used in the electron beam gun material were all of high purity, but the graphiticity Varied from very low to high. The ion-plating method used was standard. Various bias voltages from - 500 V to - 5 kV were studied, as was a range of argon gas pressures. The substrates were cleaned by ion bombardment for 30 min prior to deposition. Deposition was carried out under the same argon gas pressure and bias voltage conditions that were used for the prior cleaning. The substrates were mounted on a water-cooled electrode, and the bulk temperature of the substrates was measured using a thermocouple. The bulk temperature did not rise above 400 °C in any of the ion-plating conditions used. 3. INITIALOBSERVATIONSON THE COATINGS The ion-plated carbon coatings were matt dark grey and showed no sign of detachment from the substrate. In contrast, vacuum-deposited carbon films were black and shiny and had bubbled away from the substrate under their internal stresses. A typical example of a vacuum-deposited carbon film is shown in Fig. 1.

Fig. 1. A typicalvacuum-depositedcarbon film.

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Sections through the ion-plated films were prepared either by electroplating a retaining layer of copper onto the film and sectioning and polishing down to alumina powder, as shown in Fig. 2, or by fracturing the samples after immersion in liquid nitrogen, as shown in Fig. 3. The granular nature of the ion-plated coatings is apparent in both types of section. The surface structure of the coatings can be seen in the micrographs of the scratch and wear tests. One rather curious result was obtained when carbon was deposited onto a copper substrate under a glow discharge of - 3 kV bias and 10 ~tm argon gas pressure but without any prior glow discharge cleaning. The substrate appeared to be covered in a black cotton wool. Scanning electron microscopy (SEM) revealed that the carbon had grown as a tangle of whiskers in the form of flat ribbons. These are shown in Fig. 4. The reasons for this type of growth have not been investigated further. 4. ADHESION TESTS The adhesion of the ion-plated carbon coatings was tested initially by means of the tape test which produced no adhesive failure. Further tests were carried out by diamond scratching under controlled loading conditions on a Leitz microhardness tester and by examining the scratches by SEM. In all cases there was little or no detachment of the coating. Figure 5 is o f a scratch under 500 g loading on a copper substrate. The diamond has penetrated into the copper and has ploughed a deep groove. Figure 6 is of a scratch under 200 g loading on a steel substrate. Here the steel has been plastically deformed but the coating has deformed with the steel and has remained attached to the steel. 5. CRYSTAL STRUCTURE

The structures of the coatings were studied by means of Debye-Scherrer powder patterns obtained from material scraped from the surface, by X-ray diffractometry and by reflection electron diffraction (RED). Both the vacuum-deposited films and the "whisker" growths gave patterns of broad diffuse bands that are typical of a carbon o f low graphiticity. The ion-plated films all gave Debye-Scherrer patterns of sharp lines that are typical o f a highly graphitic carbon. This was true when the source material was of high or low graphiticity. Figure 7 is a part of such a pattern from a carbon film ion plated onto copper. The lattice parameters calculated for such patterns were a = 2.465 A, c = 6.705 A--almost identical with those given for graphite 7 (a = 2.463/~, c = 6.714 A). The patterns obtained from the ion-plated films on steel were identical but also included lines due to Fe3C. R E D patterns obtained from the ion-plated coatings were again typical o f a highly graphitic carbon, as shown in Fig. 8. In this, and all other R E D patterns obtained, the basal plane (002) ring is much weaker than would be expected from a randomly oriented graphite coating, indicating that there is a deficiency o f crystals with their basal planes parallel to the substrate surface. This is a surprising result as the usual preferred orientation for deposited graphite is one with basal planes parallel to the surface.

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Fig. 5. A scratch under 500 g loading on a copper substrate. Fig. 6. A scratch under 200 g loading on a steel substrate.

Fig. 7. Part o f a Debye-Scherrer pattern from a carbon film ion plated onto copper.

Fig. 8. The R E D pattern o f an ion-plated coating.

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6. FRICTIONTESTS

The coefficient of friction of the coatings was measured on a modified Leitz microhardness tester. The pyramidal diamond indenter was replaced by a hemispherically ended steel pin of radius 1.08 mm and of Vickers hardness 850. The flat ion-plated samples were mounted on a specially constructed stage supported by four cantilever springs. This stage was itself mounted on the normal specimen stage o f the microhardness tester. The steel pin was loaded against an ion-plated sample, and the specimen stage was driven by rotating the standard micrometer stage controls by means o f a motor drive. The ion-plated sample was thus moved under the loaded pin, and the frictional force between the pin and sample caused a deflection of the leaf springs that was proportional to the frictio~a. The deflection was measured by a transducer whose signal was fed to a recorder. The arrangement is shown in Fig. 9. After one frictional pass, the pin was raised, the stage was returned to its original starting position, the pin was again loaded against the sample and a further frictional pass was made. This could be repeated for many passes, and the wear track could be examined microscopically at all stages. ill,--..

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The variation of friction with number of passes is shown in Fig. 10 for an ionplated carbon film on copper for two different loads. In each case the friction dropped during the initial "running-in" period to a steady low friction value. The higher load produced a greater plastic deformation in the copper substrate, giving a higher coefficient of friction. Longer runs were carried out on a standard pin-in-disc machine. Again the steel pin was hemispherically ended, of radius 3.97 mm and of Vickers hardness 850. The speed of rotation was 100 rev m i n - 1. Figure 11 shows the coefficient of friction plotted against the number of revolutions for loads of 0.5 kg and 2 kg. The friction trace has no sudden increase to failure but shows a gradual increase to high values that is typical of ion-plated metal films1. SEM of the rubbed ion-plated carbon coatings revealed that the films were worn away gradually, and no detachment of the film was detected. Figures 12(a) and 12(b) are typical of the wear tracks. Figure 12(b) may show evidence of the "roof tile" orientation found on rubbed transferred graphite films and on l'ubbed bulk carbons a, 9.

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7. DISCUSSION

The vacuum-deposited carbon films were non-adherent, had high internal stresses and were of low graphiticity. The ion-plated films were adherent, had low internal stresses and were of high graphiticity. The differences in adhesion and internal stress were detected on numerous occasions for vacuum-deposited and ionplated metal coatings. However, it is believed that a difference in crystal structure between vacuum-deposited and ion-plated coatings has not been reported previously.

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Pyrolytic carbon is deposited from a carbon-bearing gas such as methane onto a heated substrate. Substrate temperatures from 1000 to 2200 °C have been used I°, but in order to produce highly graphitic carbons a further heat treatment at 3000 °C of material produced at more than 2000 °C was necessary 11. The bulk temperature of the substrates in the ion-plating deposition was always less than 400 °C, but the temperature at the surface must have been considerably higher, owing to the high kinetic energy of the depositing carbon atoms. The high surface temperature appears to have been the decisive factor in causing the growth of highly graphitic carbon. The R E D patterns indicate that the ion-plated carbon films have a preferred orientation with the basal planes tending to be normal to the surface, whereas pyrolytic carbon films tend to deposit with the basal planes parallel to the substrate. An orientation with basal planes normal to the surface is consistent with a true outward growth mechanism, as is the formation of whiskers on the copper substrate which was not cleaned by ion b o m b a r d m e n t prior to deposition. Studies of.the nucleation and early stages of growth of ion-plated metal films 12 indicate that surface diffusion and the sideways growth of nuclei typical of vacuum evaporation occurs to a much lesser extent in ion plating. The orientation of ion-plated carbon films as revealed by R E D may give some confirmation to these views. However, much more detailed studies of the orientation of the ion-plated carbon films are required. ACKNOWLEDGMENTS The authors would like to thank Dr. R. D. Arnell for m a n y useful discussions, Mr. A. J. K i r k h a m for his work on the ion-plating apparatus and Professor J. H. Hailing for his enthusiastic support and encouragement. One of us (M.S.) is indebted to K. S. Paul Ltd. for financial support. REFERENCES

1 2 3 4

T. Spalvins,J. S. Przybyswewskiand D. H. Buckley,NASA Tech. Note TN-D-3707, 1966. D.W. Wipander, NASA Tech. Note TN-D-6455, 1971. M.G. Sherbiney, Ph.D. Thesis, Universityof Salford, 1975. D.H. Buckleyand T. Spalvins, NASA Spec. Publ. SP-5111, 1972.

5 S. AisenbergandR. Chabot, J. Appl. Phys.,42(1971)2953.

6 E.G. Spencer, P. H. Schmidt, D. C. Joy and F..I. Sansalone, Appl. Phys. Lett., 29 (1976) I 18. 7 A S T M Powder Diffraction File, Card No. 23-65. 8 P.V.K. Porgess and H. Wilman, Proc. Phys. Soc., London, 76 (1960) 513. 9 J.W. Midgleyand D. G. Teer, Nature (London), 189 (1961) 735. 10 J.C. Boknos, Chem. Phys. Carbon, 5 (1969) 1-118. 11 A.W. Moore, Chem. Phys. Carbon, 11 (1973) 69-187. 12 D.G. Teer, Int. Conf. on Metallurgical Coatings, San Francisco, California, March 28-April 1,1977.