Wear, 53 (1979) 371- 375 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
Short Communication Effect of a glow discharge on the wear of metals in sliding contact
M. 0. KESTNER* Borg- Warner Corporation, (U.S.A.)
Roy C. Ingersoll Research Center, Des Plaines, Ill. 60018
(Received April 11,1978)
A glow discharge has been used to prepare metallic surfaces substantially free of adsorbed films derived from the atmosphere to which a lubricant may be applied more uniformly and adherently. As a result of an enhanced surface affinity for lubricating oils, wear was significantly reduced in the cases studied. 1. Introduction
The effectiveness of a liquid lubricant is determined principally by the magnitude of the interaction between lubricant molecules and the substrate metal. Consequently, the metallic surface should possess a high surface energy in order to ensure that the adsorption of lubricant is promoted. Although microscopically clean and outgassed metal surfaces have high surface energies (200 - 2000 erg cm-‘), metal surfaces encountered in industrial environments are of much lower energy owing to the formation of oxide and contaminating films which result from prolonged atmospheric exposure [ 11. These contaminant films, composed of water vapor, hydrocarbons and other miscellaneous species, decrease surface wettability and hinder the application of an adherent lubricant film. Conventional solvent cleaning techniques are of little avail in the removal of adsorbed films and result simply in the substitution of one contaminant for another. In this report we describe a method for the removal of adsorbed surface films (not oxides) which results in a surface of enhanced affinity for a lubricant and a corresponding decrease in wear. 2. Experimental The glow discharge apparatus is shown diagramatically in Fig. 1 and consists of a vacuum chamber connected to a 1500 V 150 mA d.c. power supply. Argon gas was used to provide an inert atmosphere. Typically, test specimens were treated at an argon pressure of 50 - 100 Torr at a constant *Present address: Apollo Chemical Corp., 35 South Jefferson Road, Whippany, N.J. 07981, U.S.A.
Fig. 1. The glow discharge apparatus.
current of 150 mA for a period of 5 min and lubricated in situ with Texaco TL2311 oil. The wear test specimens comprised three hemispherical brass pins running against a steel flat. The pins were fabricated from 6741 Mueller brass and the flats from 4027 carbonitrided steel. The steel surfaces were lapped with 6 pm diamond paste and all specimens were rinsed ult~on~c~y in isopropyl alcohol and dried in air prior to use. Specimens were wear tested on a drill press modified for this purpose. Testing was conducted in a constant volume of lubricant at a sliding speed of 50 ft min-l and an applied lever arm (1O:l) load of 10 lbf. The volume V of material removed from the pin specimens was calculated from measurements of the change h in pin height and the diameter c of the wear area using the equation nh
V= 24 (4P + 3c2) Adv~c~g and receding aqueous contact angles were measured in triplicate using a contact angle goniometer equipped with a protractor eyepiece. All measurements were made using triply distilled water in air at 25 “C. Contact angles on lubricated surfaces were performed upon dry lubricant films prepared by blotting the surfaces free of excess oil.
373 TABLE 1 Contact angle measurements upon lubricated and glow discharge treated surfaces*
None Glow discharge
%ontact angles were not measured on brass pins owing to their unsuitable geometries. bValues of f?A and OR after five successive 3 min ultrasonic rinsings in MEK.
3. Results and discussion
Measurements of aqueous advancing (0 A) and receding (0 a ) contact angles provide comparisons of surface wettability and uniformity [ 21. Table 1 lists contact angles determined upon lubricated controls and compares these values with those obtained on specimens treated with a glow discharge and subsequently lubricated. Also listed are values of 0 i and 8 k which represent the contact angles obtained upon the same surfaces after five successive rinsings in methyl ethyl ketone (MEK) to remove the lubricant film. Glow discharge treated and lubricated specimens were observed to possess higher values of 8 A and less hysteresis (0 A - 0 R) than lubricated controls demonstrating that more lubricant is adsorbed more uniformly on the former surfaces. Additionally, successive rinsings in MEK altered the value of 8 A on glow discharge treated surfaces by only 3.0% while the same angle on lubricated controls fell 24.0% from its initial value. Similar behavior was observed for 0 a. The fact that the lubricant film is more difficult to remove from treated surfaces can only be attributed to its increased adherence. The data of Table 2 and graph of Fig. 2 offer compelling evidence of the effect of a glow discharge upon surface wettability. Prior to treatment the steel surfaces possess an average eA of 77.5” which is characteristic of a relatively non-wetting contaminated surface. Immediately after treatment, Ba falls to approximately 36% of its initial value owing to the removal of adsorbed films during the discharge process. That these films are atmospheric in origin is shown by the relaxation of the activated surface in Fig. 2 where e A is seen to increase as atmospheric contaminants replenish the film. Both electron microscope and X-ray diffraction techniques failed to produce any evidence suggesting that the alteration of surface wettability was due to a change in composition or morphology of the substrate metal. As a result of the enhanced properties of lubricant films on glow discharge treated surfaces, wear was significantly reduced. The average wear volume of 27 glow discharge treated brass pins was V = 0.256 mma (u = 0.120) compared with an average wear volume of V = 0.650 mm’ (u = 0.195) on 21 untreated pins used as controls. This difference represents an
Effect of glow discharge treatment wettability of 4027 steel Sample
1 2 3 Average
72.6 33.3 76.6 77.5
37.3 24.3 22.3 27.9
aMeasured in air within 2 min of plasma treatment; actual values may be slightly lower.
210 >” a
Fig. 2. The relaxation
of a 4027 steel surface
by glow discharge.
improvement of 60.6% in wear performance. No measurable wear was observed on the carbonitrided steel flats. Similar results have been obtained in our laboratories for various cast irons and steels in sliding contact in other lubricants, suggesting that the glow discharge treatment may be generally applied. The improvements in wear performance are undoubtedly due to faster and more complete adsorption of lubricants onto glow discharge cleaned surfaces. The post-treatment decrease in 0 A is indicative of an increase in the surface free energy of the metal and the exposure of fresh adsorption sites leading to a more uniform and adherent lubricant film. The results of O’Kane and Mittal  support the effectiveness of a glow discharge in removing organic contaminants from metal surfaces. Wettability measurements made on plasma treated surfaces demonstrated that the activated surfaces possessed a high rate of adsorption of atmospheric contaminants and improved water wettability. Tamai et al.  have reported
that the adhesion of benzene, n-hexane, and cyclohexane on copper and steel surfaces was greatly improved when treated with an electric discharge from a Tesla coil in air. Additionally, superior lubricity was observed for these treated surfaces. It may be concluded that glow discharges, and plasmas in general, offer a superior method for the removal of adsorbed films from metal surfaces by means of electronic and ionic bombardments. Surfaces treated in this manner possess excellent wettabilities and high rates of adsorption leading to improved lubricity. As a result, wear is significantly reduced with no compositional or morphological changes in the substrate metals.
References 1 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965: 2 R. Johnson and R. Dettre, Wettability and contact angles. In E. Matijevic (ed.), Surface and Colloid Science, Wiley-Interscience, New York, 1969. 3 D. F. O’Kane and K. L. Mittal, Plasma cleaning of metal surfaces, J. Vat. Sci. Technol., 11(3) (1974) 567 - 569. 4 Y. Tamai, M. Suzuki and Y. Momose,The exe-electron effect on adsorption and lubricity of liquid hydrocarbons on metals, Tohoku Daigaku Hisuiyochi Kagaku Kenkyusho Hokoku, 18 (2) (1968) 143 - 147.