Chapter 4
Tribological Evaluation and Analysis of Coating Materials Kazuhisa Miyoshi
Downloaded by CORNELL UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 23, 1992 | doi: 10.1021/bk-1992-0485.ch004
Lewis Research Center, National Aeronautics and Space Administration, Cleveland, OH 44135
A physical characterization of coating materials by analytical techniques such as x-ray photoelectron spectroscopy, Auger electron spectroscopy, ellipsometry, and nuclear reaction analysis can contribute to the understanding of adhesion and friction of the coatings and can partially predict the tribological properties of the coatings. This two-part paper describes the tribological properties and physical characteristics of (1) diamond like carbon (DLC) films and (2) silicon nitride (SiN ) films. Emphasis is to relate plasma deposition conditions to thefilmchemistry and composition and to the adhesion and friction of the films. With the DLC films, the higher the plasma deposition power, the less the hydrogen concentration and the greater the film density and the hardness. The friction behavior of DLCfilmsdeposited at higher deposition powers (200 to 300 W) is similar to that of bulk diamond. Even in a vacuum the DLC films effectively lubricate ceramic surfaces (Si N ) at temperatures to 500 °C. With SiN films, the silicon to nitrogen ratios and the amount of amorphous silicon depend on depositionfrequency.The presence of rich amorphous silicon in the high-frequency plasma-deposited SiN films increases their adhesion and friction above 500 °C in vacuum. x
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Amorphous hydrogenated carbon films (a-C:H), known as diamondlike carbon (DLC), and silicon nitride (SiN ) films are being created with the aid of recent developments in deposition processes and analytical methods (1-10). D L C and SiN films can be promising materials for hard, wear-resistant, solid lubricating films in tribological systems in both terrestrial and space environments. In the open literature, however, relatively little information is available on their potential as solid lubricating films and/or as wear-resistant films in tribological systems. To have effective lubricating films, however, we must first and foremost control the adhesion and friction properties of the films. x
This chapter not subject to U.S. copyright Published 1992 American Chemical Society In Surface Science Investigations in Tribology; Chung, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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Tribological Evaluation and Analysis of Coating Materials 59
4. MIYOSHI
The D L C and S i N films used in this investigation were grown by means of plasma chemical depositions (9-12). The properties of thin films are sensitive to the plasma deposition conditions (13-17). Films of the most diverse composition and structure may be formed. Changes in film compositions and film properties can be perceived with the aid of analytical techniques. The objective of this paper is to review the changes in chemistry, compositions, and properties of plasma-deposited D L C and plasma-deposited SiN films by the plasma deposition parameter settings and the subsequent effects on the adhesion and friction properties. Analytical methods such as Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), ellipsometry, and nuclear reaction analyses are used to identify the chemical and compositional characteristics of the thin films. Sliding friction experiments were conducted to examine the friction properties of the D L C and SiN films in contact with the hemispherical silicon nitride (Si N ) pins (1.6-mm radius) in dry nitrogen and/or in a 3xl0" Pa ultrahigh vacuum. x
Downloaded by CORNELL UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 23, 1992 | doi: 10.1021/bk-1992-0485.ch004
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ADHESION AND FRICTION OF DIAMONDLIKE C A R B O N FILMS D L C films (approximately 60 nm thick) were formed on hot-pressed, polycrystalline, magnesia-doped silicon nitride (Si N ) flat substrates by using the 30-kHz ac glow discharge from a planar plasma reactor at deposition powers of 50 to 300 W (0.8 to 5 kW/m (11-17). The gas source was methane (99.97 percent pure). Two sets of sliding friction experiments were conducted using hot-pressed, polycrystalline, magnesia-doped S i N pin specimens. In the first set, multipass sliding friction experiments were conducted with hemispherical S i N pins (1.6-mm radius) in a dry nitrogen environment with a load of 1 N (Hertzian contact pressure, 910 MPa) and at a sliding velocity of 8 mm/min at room temperature (18). The pin was made to traverse the surface of the D L C films. The motion was reciprocal. Reference experiments for friction were also conducted with a single-crystal (111) diamond flat and uncoated S i N flats in contact with hemispherical S i N pins in dry nitrogen. In the second set, single-pass sliding friction experiments were conducted with the as-coated D L C films deposited on S i N flats in contact with ion-sputter-cleaned hemispherical S i N pins (1.6-mm radius) in ultrahigh vacuum with loads up to 1.7 N (average Hertzian contact pressure, 1.5 GPa) and at a sliding velocity of 3 mm/min at temperatures to 700 °C (79). 3
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Dry Nitrogen Environment The coefficients of friction for a D L C film deposited at 50 W and for an uncoated S i N flat are plotted as a function of the number of repeated passes (Figure 1). The friction data presented in Figure 1 indicate that the coefficient of friction was considerably lower for the plasma-deposited D L C film than for the uncoated S i N flat. The presence of D L C films decreased friction. Figures 2(a) and (b) present typical plots of the coefficients of friction for plasmadeposited D L C films at low (50 W) and high (250 W) deposition power as a function of the number of repeated passes. The friction characteristics of the D L C films made by different deposition powers were of two types. With the D L C films deposited at 50 to 150 W, the first type of friction characteristics (Figure 2 (a)) was generally 3
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In Surface Science Investigations in Tribology; Chung, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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1 Downloaded by CORNELL UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 23, 1992 | doi: 10.1021/bk-1992-0485.ch004
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