Sputtering friction

Fig. 22—Friction coefficients between WC ball and TiN/Si3N4 nanocomposite coatings as function of the Si content. The coatings were deposited by reactive magnetron sputtering. The friction coefficients of the TiN/Si3N4 coatings were obtained under the load of 20 N. In the case of the TiN coating and the Si3N4 coating, the load is 5 N, because the two coatings will fail and peel off from the substrate under the load of 20 N.

The microscale friction and wear behavior of thin film of gold (Au) which was prepared in a vacuum sputtering apparatus was investigated. The substrate is Si(lOO) wafer. The film thickness is about 800 nm. For comparison, the microscale friction and wear of the substrate was also studied. 

First calculations of beryllium transport in PISCES have also been carried out. The axial profile of Bell emission can be well reproduced. The simulated amount of re-deposition of sputtered beryllium is relatively small (about 2%). The main reason for this is the radial transport (caused by diffusion, radial electrical field and friction) leading to a loss of beryllium, which cannot contribute to a re-deposition. However, additional calculations and a detailed comparison with experimental findings is necessary. The influence of the beryllium deposition at the graphite samples on the chemical erosion is one of the open questions still to be addressed and is a matter of particular interest for ITER. 

Except for the anomalous low friction reported for an ultra-pure sputtered film (see Section 10.5) and under radiation (see Section 7.4), typical minimum coefficients of friction in dry air or vacuum are 0.02 to 0.03. For fully disordered films in dry air the static coefficient of friction can be between 0.12 and 0.15, but because reorientation begins rapidly when movement starts any figures for the kinetic friction of disordered films must be suspect. 

The basic Brophy and Ingraham technique was studied by several other authors. Bayer and Trivedi " found that the effectiveness of the technique depended more on the nature of the coating than on its thickness, and that retained moisture in the electroplate was essential for effective conversion. They recommended a current density of 21.5 A/m for 5 minutes to produce a coating thickness of 1.25 to 2.5 tjm. Nishimura and co-workers found that the presence of air or water or both in the conversion gas improved the wear life. Table 9.5 compares the properties of the in situ films with those of burnished and sputtered films, and shows superior wear life for the in situ films. Their friction results were curious, in that they found that the initial films which were formed gave low friction in air or nitrogen but not in vacuum. Low friction in vacuum was obtained when the initial product was heated in vacuum to 400°C. 

Figure 10.3 Effect of Negative DC Bias on Coefficient of Sliding Friction for Sputtered Molybdenum Disulphide (Ref.274)...

The best coating performance is often obtained with hard substrates. This is consistent with the discussion of friction of thin films in Chapter 5, but with sputtered coatings there may be an additional factor involved. The endurance of sputtered films has been shown to depend critically on the strength of adhesion to the substrate, and if adhesion involves intermingling of sputtered atoms with those of the substrate, it seems likely that the strength of adhesion will be inherently greater with hard substrates. 

The oxidation problem restricts the performance of sputtered coatings even in dry nitrogen at elevated temperatures. Anderson and Roberts tested a sputtered molybdenum disulphide coating in nitrogen containing less than 15 ppm of oxygen, and found a marked deterioration in both friction and endurance at 400 C, which they ascribed to oxidation. 

There is very little information available about corrosion risks with the other lubricating dichalcogenides. Spalvins mentioned that the friction and endurance of a sputtered tungsten disulphide film deteriorated considerably when tested in the atmosphere instead of in vacuum, and the deterioration was associated with formation of sulphuric acid and corrosion of the substrate. These results are not very different from those obtained with molybdenum disulphide in the same period, and there seems to be no reason to expect the problem to be any greater or any less with the other dichalcogenides. 

Spalvins, T. and Przybyszewski, J.S., Deposition of Sputtered Molybdenum Disulfide Films and Friction Characteristics of Such Films in Vacuum,... 

Devine, M.J. and Sander, L.F., Interatomic Spacing Concept for Solid Lubricant - Metal Systems, Amer. Chem. Soc. Divn. of Pet. Chem., 13, (1968). Przybyszewski, J.S. and Spalvins, T., Friction and Contact Resistance During Sliding in Vacuum of Low Resistivity Metals Lubricated with Sputtered Molybdenum Disulfide Films, NASA TN D-5349, (July, 1969). 

Roberts, E.W., TheTribology of Sputtered Molybdenum Disulphide Films, Proc. I. Mech. E. Inti. Conf. Tribology - Friction, Lubrication and Wear Fifty Years On, London, (1-3 July, 1987), vol. 1, p. 503, Paper No. C172/87. 

Spalvins, T., Characteristic Morphological and Frictional Changes in Sputtered M0S2 Filnns, Proc. 3rd Inti. Conf. on Solid Lubrication, Denver, Colorado, (7-10 Aug. 1984), ASLE SP-14, p. 201. 

Bergmann, E., Malat, G., Muller, C. and Simon-Vermot, A., Friction Properties of Sputtered Dichaicogenide Layers, Tribology Inti., 14, 329, (1981). 

In frictional brass-coated steels quantification of continuously varying concentrations of Cu, Fe and Zn can be performed based on a linear combination of the RSFs and/or sputter rates for both Fe and brass [652], Technical layers in a Cr-Ni system with a thickness of 30 nm to 10 pm can be characterized with respect to composition and thickness with GD-MS using a penetration rate of up to 0.1 pm/s and a depth resolution of 10 nm [613]. 

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