Bonded films coating thickness

Hz for thick coatings such as reinforced coal tar enamel, being selected to minimise interference from commonly occurring frequencies while maximising the distance the signal will travel, some 5-10 km on a reasonably well-coated pipeline. For thin film, coatings, such as a fusion-bonded epoxy, a frequency of 200 Hz has been found more appropriate. 

When this resin was exposed as a thin film to the UV radiation of a medium pressure mercury lamp (80 W aiH), the crosslinking polymerization was found to develop extensively within a fraction of a second (18). The kinetics of this ultra-fast reaction can be followed quantitatively by monitoring the decrease of the IR absorption at 810 an-1 of the acrylic double bond (CHCH twisting). Figure 8 shows a typical kinetic curve obtained for a 20 pm thick film coated onto a NaCl disk and exposed in the presence of air to the UV radiation at a fluence rate of 1.5 x 10 6 einstein s-1 cm 2. 

The initial thickness of most molybdenum disulphide coatings has an important influence on the performance and life of the coating . The special case of sputtered films is considered in Chapter 10, and there is little information about thickness effects for in situ or transfer films. Many workers have investigated the effect of film thickness on bonded films, but, as was pointed out earlier, much of this work appears confusing, and sometimes contradictory, because of failure to understand and analyse the effects of running-in or burnishing on the consolidation and resulting structure of the films. 

If design considerations require it, a much thinner bonded film could be used, or a soft film could be burnished down to much reduced thickness. Under those circumstances it would be possible to use a smoother substrate, but coating performance deteriorates badly if the surface is too smooth, and a miminum acceptable surface roughness would probably be about 0.2 yt/m c.I.a., giving a combined roughness parameter of about 0.3 fjm. It seems probable that in such a case an initial, or a fully-burnished, coating thickness of 1 to 2/>m would give a useful life before any serious problem of asperity penetration arises. 

One form of film break-up has been mentioned previously, in which the surface of a relatively thick film becomes consolidated over a soft unconsolidated layer. This can lead to shear in the soft layer, especially with high non-conformal loading, and the consolidated layer will break away. Loss of adhesion and film break-up can also occur, especially with a bonded film, if the surface pre-treatment has been badly performed, so that the coating simply fails to adhere and breaks away. These cases may be considered as premature failures caused by poor film preparation. 

Bahun, C.J. and Jones, J.R., Influence of Load, Speed and Coating Thickness on the Wear Life of a Bonded Solid Lubricant, Lubric. Eng., 25, 351, (1969). Hopkins, V., Discussion, p. 6, on Rabinowicz, E., Variation of Friction and Wear of Solid Lubricant Films with Film Thickness, ASLE Trans., 10, 1, (1967). Whitehouse, G.D., Nandan, D. and Whitehurst, C.A., The Effect of Film Thickness on Friction Coefficients for Solid Lubricants CaF2, MoSj and Graphite, ASLE Trans., 13, 159, (1970). 

Thin films and coatings, irrespective of the deposition techniques used, always have internal stresses. The stresses are caused either by the thermal expansion mismatch between the coating and substrate or due to the non-equilibrium nature of the process which puts atoms out of position with respect to the minimums in the interatomic force fields. These two contributions are thermal and intrinsic stresses respectively. Small variations in the bond lengths and bond angles in covalently bonded ceramics lead to large internal stresses which have broad, structural and electronic consequences. Structurally, the stresses may lead to failure of adhesion or development of microcracks in the film. Since the intrinsic stresses increase with the coating thickness, failure of adhesion is observed frequently in the case of thick films. Electronically, the stresses affect the pizoelectric properties, optical absorption and optoelectronic properties and electronic dopability of the film or the substrate. 

Bonded Solid-Film Lubricants. Although a thin film of soHd lubricant that is burnished onto a wearing surface often is useful for break-in operations, over 95% are resin bonded for improved life and performance (62). Use of adhesive binders permits apphcations of coatings 5—20 p.m thick by spraying, dipping, or bmshing as dispersions in a volatile solvent. Some commonly used bonded lubricant films are Hsted in Table 12 (62) with a more extensive listing in Reference 61. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

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