Plasma films

The present paper is directed mainly at the question of water permeability through plasma films, the presence of water at a surface being considered essential for the Initiation and propagation of corrosion processes. 

As stated, the capability of plasma deposits to reduce the access of water to corrosion-sensitive surfaces may be an important motivation for their application in corrosion protection. In order to study this property, Kapton polyimide film was selected as the substrate because of its high inherent permeability to water and its ability to resist elevated temperatures. The response of Kapton film overcoated by PPHMDSO to the permeation of water vapor is shown in Fig. 1. Clearly, the presence of the organo-silicone plasma film greatly reduces water permeation. The magnitude of the effect is much enhanced when plasma polymers are produced at high T and p. 

The change in water vapor flux, AF, due to the presence of 0.5 pm plasma films may be expressed by... 

The permeation coefficients of the two thin film materials differ by an order of magnitude. To the extent then, that water initiates and propagates the corrosion of metal substrates, plasma films of... 

Plasma films are usually highly cross-linked, resistant to higher temperatures, resistant to abrasion and chemical attack, and are highly adherent to the surface. Adhesion to the surface is generally high both because the growing polymer complex can fit the surface contour and thus lock-itself in (physical adhesion), and because in many instances, the species are active enough to chemically react with the surface molecules to chemically bond to the surface. The surface can be prepared so that the chemical reaction is enhanced. 

Hydrophobicity - Before plasma treatment, silica powder is highly hydrophilic and immediately sinks in water. After plasma film deposition, the material floats on water for several hours. A significant reduction in polarity and in surface energy compared to untreated silica is found, down to the range of 28.4-47.7 mJ/m2. The water penetration into powder beds of untreated and plasma-treated silica is shown in Fig. 7. The untreated silica absorbs water very fast, whereas the plasma-treated silicas show a significantly decreased water penetration rate. The lowest rate is found for the polythiophene-coated silica (PTh-silica). 

In all the cases of poly acetylene, polythiophene, and polypyrrole coating, the amount of plasma-film deposition was different, caused by the difference in the structure of the three different monomers and their reactivity during the plasma process. PPy- and PTh-silica are more hydrophobic than PA-silica, probably due to the presence of different chemical moieties in the complex film structure deposited onto the silica surface. 

For the cyclic corrosion test, a layer of acrylosilane polymer coating (10-25 fim thick) was dip-coated onto the plasma-deposited substrates. The coated samples were then subjected to 25 scab cycles. The test results are plotted in Fig. 7. Corrosion performance (as described by the length of scribe creep) was correlated to the wattage used for plasma film deposition. As discussed in the previous section, the chemical structure and properties correlated with the deposition conditions, especially the power level applied. Therefore, atomic compositions for plasma polymers deposited at different power levels were also plotted in Fig. 7.A... 

Figure 9. SE 1 micrographs of surfaces of phosphated steel with and without deposition of plasma polymer of trimethylsilane. (a) No plasma film deposition (b) 2 min deposition (c) 8 min deposition.

The most extensive studies of plasma-polymerized membranes were performed in the 1970s and early 1980s by Yasuda, who tried to develop high-performance reverse osmosis membranes by depositing plasma films onto microporous poly-sulfone films [60,61]. More recently other workers have studied the gas permeability of plasma-polymerized films. For example, Stancell and Spencer [62] were able to obtain a gas separation plasma membrane with a hydrogen/methane selectivity of almost 300, and Kawakami et al. [63] have reported plasma membranes... 

The ability of the plasma BPSG film to passivate against sodium penetration was compared to the thermal film. Evidence was found of some sodium penetration in the plasma films, and none in the thermal ones. 

Figure 10.8 Scanned image of the surface of two alloy panels showing adhesion failure caused by the omission of O2 plasma treatment of the substrate prior to plasma film deposition and application of the primer (Deft 44-GN-72 MIL-P-85582 Type I Waterbased Chromated Control Primer), a) Panel after Skydrol LD4 fluid resistance test, which had the O2 plasma treatment prior to film deposition and primer application, b) Panel after scribed wet (24-h immersion in tap water) tape test, which had not been treated with the O2 plasma treatment prior to film deposition and primer application.

This reversal of the sequence from TMS/HFE to HFE/TMS is the most important issue. The second major issue is the effect of the O2 plasma treatment on the fluorine-containing contaminants. The XPS analysis of an initial sample revealed virtually no silicon on the alloy surface beneath the lifted primer but did indicate a rather substantial fluorine presence. The appearance of a strong silicon signal on the interface side of the removed primer indicated that the entire plasma film had likely delaminated at the interface with the alloy. Analysis of additional samples confirmed that the entire film and primer system had delaminated from the alloy panels. 

Figure 11.6c shows the monomer feed rate dependence of internal stress in VpMDSO plasma polymer films at different argon flow rates. The overall values of internal stress in plasma films obtained with argon flow rate at 1500 seem are much higher than those obtained at 750 seem. 

By the process described above, a plasma film could be obtained that had high enough electrical conductivity to allow direct electrodeposition of copper. The bulk resistivity of film measured by a four-point probe was 2.6 x 10 " ohm-cm for the copper-containing polymer film when deposition was stopped after 18 min at HOW. This value is critical if a uniform electrolytic deposit is to be obtained. For safety, deposition was carried out until a total film thickness of 150nm was obtained, giving a nearly pure metallic layer thick enough to allow subsequent electroplating. 

The final bulk resistivity was 7.8 x 10 " ohm-cm and 6.750 x 10 " ohm-cm for copper-containing and silver-containing films, respectively, taking the total thickness into account for calculation. The copper was then electroplated onto the composition-ungraded plasma film. 

The results of tensile lap-shear tests of coatings produced on FRP substrates for different process schemes with and without plasma pretreatment are shown in Table 21.1. Examination of failed surfaces showed that separation occurred at the interface between the substrate and the bottom of the plasma film. No failure occurred between the plasma film and electrocopper layer. 

Figure 31.5 Schematic drawing of plasma film on chemically cleaned alloy.

While a copper-enriched surface has the implication of always causing accelerated electrochemical corrosion, replacing the native, hydroxylated, mixed Al-Mg oxide layer with a thin stable oxide layer seems to allow the plasma films to tightly adhere to the alloy surface. This adhesion, coupled with the barrier properties of the films, appears to provide additional protection of the oxide layer from contact with corrosive agents. 

The chemical state of aluminum on the surface has a multitude of possible configuration designations. The state in the hydroxylated outer layer corresponds to various mineral phases such as AIO(OH) (boehmite), Al(OH)3, having a modified Auger parameter of 1460.6 on the acetone-cleaned surface and 1461.4 on both the (Aik) and (Dox) surfaces. When capped with a plasma polymer, depth profiles show that the state of the aluminum is seen to be consistent with the many oxides, as well as mixed states with plasma film components. 

Figures 31.23 and 31.24 show typical scanned images of SO2 and Prohesion salt spray-tested [7B] panels, respectively. Visual observation of these images reveals that the plasma-modified panels of [7B] have outperformed both control panels in the SO2 salt spray test. These plasma film combinations were prepared on deoxidized [7B] surfaces without any plasma cleaning pretreatment. Figure 31.23 also shows an image of a panel that had simply been deoxidized prior to the application of E-coat, which performed excellently in the SO2 salt spray test. Figure 31.25 compares the corrosion width obtained by the two methods. The comparisons shown in Figures 31.19, 31.22, and 31.25 indicates that the results obtained by the two methods do not match, partly due to the different duration of tests, and that samples which show good results in one test do not do as well in the other test.

Figure 32.18a and b summarizes the XPS results from three TMS plasma films produced in closed reactor system with and without addition of the second gas. Figure 32.19a and b summarizes the XPS results from three TMS plasma films produced in closed reactor system with and without addition of the second gas discharge treatment. These films were deposited on Alclad 7075-T6 panels that were... 

Figure 32.19 shows C/Si ratios formed from the XPS sputter depth profiles of the TMS plasma polymers with and without additional plasma treatment. As deposited, without a second plasma treatment, the closed system TMS plasma film has a surface that is carbon rich (with a C/Si ratio of about 4.7) and low oxygen content (with an O/Si ratio of about 0.7). From Figure 32.19a, it is observed that the as-deposited TMS plasma film shows a gradual structure change from the surface with more carbon (C/Si ratio of about 4.7) to lower carbon (C/Si ratio of about 1.7) in the bulk film. This also manifests itself as a higher C/Si ratio at the surface than the bulk value, which is unique to this film. 

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