Plasma—polymer films

The presence of a third peak in the sulfur profile, near the film/substrate interface, indicates that some sulfur diffused through the plasma polymer films and reacted with iron, probably forming FeS at the interface. 

The following are some important characteristics of plasma polymer films 1 2 3 4... 

Another attempt by Tricas et al. to modify the surface of carbon black was by the plasma polymerization of acrylic acid [34]. Treatment with acrylic acid made carbon black hydrophilic. Plasma-coated carbon black was mixed with natural rubber and showed increased filler-filler interaction. The bound rubber content was reduced after the surface treatment of the filler. The authors also concluded that the surface of the carbon black was completely covered by the plasma polymer film, preventing the carbon black surface from playing any role in the polymer matrix. 

Characterization of this tumbler reactor was carried out via the deposition rate measurement of a plasma polymer film on silicon wafers under different conditions. In the longitudinal direction, the deposition rate decreases significantly when the plasma moves from the central plasma zone to the remote zone. With appropriate shielding, the decay in deposition rate in the longitudinal direction can be effectively reduced. By means of the stirring, a uniform distribution of the plasma deposition is achieved within the chamber. 

Quantitative characterization of plasma-polymer films, especially of ultrathin fluorinated carbon plasma polymer films, has been performed by ToF-SIMS to study changes in the surface composition and molecular distribution. CFX films on silicon and polyethylene terephthalate (PET) substrates were exposed to a pulsed Ar/CHF3 plasma by varying the deposition time from 10-90 s.111-113 The results show differences in film growth and CFX cross linking for the silicon and PET substrates.111... 

Friedrich JF, Retzko I, Kuhn G, Unger W, Lippitz A (2001) Metal doped plasma polymer films. In Mittal KL (ed) Metallized Plastics, vol 7. VSP, Utrecht p 117-142... 

Figure 4.10 C/Si ratios of plasma polymer films of TMS prepared in a flow system reactor (Tfs) and in a closed system reactor (Tcs), as generated by XPS depth profiling.

Postdeposition plasma modifications to the plasma polymer of TMS have been seen to greatly improve bonding to various primers and paints [18-20]. One particular system has been observed to have tremendous adhesion between plasma-coated A1 alloy panels and paint applied to them. This system involves cathodic DC plasma deposition of a roughly 50-nm primary plasma polymer film from TMS onto a properly pretreated alloy substrate, followed by the deposition of an extremely thin fluorocarbon film by DC cathodic deposition of hexafluoroethane (HFE). It was the superadhesion aspect of this particular system that triggered the series of ESR studies [3,21]. 

The internal stress in plasma polymer films is generally expansive, i.e., the force to expand the film is strained by external compressive stress. According to the concept presented by Yasuda et al. [1], the internal stress in a plasma polymer stems on the fundamental growth mechanisms of plasma polymer formation. A plasma polymer is formed by consecutive insertion of reactive species, which can be viewed as a wedging process. The internal stress is related to how frequently the insertion occurs as well as on the size of inserting species. The both factors are dependent on the operational factors of plasma polymerization. 

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. 

Figure 11.7b shows the internal stress in LPCAT films of cyclic siloxanes 1,3,5,7-tetramethylcyclotetrasiloxane (TMTSO) and 2,4,6,8-tetravinyl-2,4,6,8-tetra-methylcyclotetrasiloxane (TVTMTSO). The large siloxane ring structure in these two monomers did not provide any decrease of internal stress in resultant plasma polymer films, compared with simple siloxane monomers, i.e., TMTSO, HMDSO, and VpMDO. 

Figure 11.13 shows the dependence of refractive index in plasma polymer film on energy factor W FM)c FM)t. The refractive indices of all the plasma polymers showed an increasing dependence on the value of energy factor W FM)cl FM). ... 

If a comparison is made between Figures 11.11 and 11.13, a correlation of higher internal stress to larger refractive index in plasma polymer films was observed. In the plasma polymer films with larger refractive index, the internal stress that developed during the deposition process is more difficult to release afterward due to the tighter structure. 

Figure 11.14 Qualitative correlation of internal stress with refractive index of cascade arc torch plasma polymer films.

The results of tensile lap-shear and chemical composition tests for different process schemes are shown in Tables 21.2-21.4. Metal analyses of the initial plasma coating layer and those of the failed surfaces after tensile lap-shear tests confirm that the failure occurred at the interface between the substrate surface and the bottom of the plasma polymer film. These results show that the intermediate layer provided by grading of the metal content throughout the plasma polymer film can improve the strength between the polymer and metal films. The graded metal-containing plasma polymer film can join a polymer and a metal with strong adhesion, and also reduce... 

Surface conditioning by plasma pretreatment with oxygen and hydrogen is a crucial factor in obtaining very strong bonding at the interface between the substrate and composite plasma polymer film as a preplate for metallization. 

The shear strength of coatings metallized by employing composition-graded films is dependent on the extent and strength of chemical bonding between the substrate surface and the plasma polymer film. 

Deoxidized surfaces of [7B] with a plasma polymer coating ([7B] (Dox)/T) showed higher polarization resistance than the chemically deoxidized surfaces without a plasma polymer. This indicates that the added corrosion resistance offered by plasma polymer films is much higher than that of the barrier-type oxides, formed after chemical cleaning, alone. As compared to the chromate conversion-coated surfaces ([7B] CQ, the deoxidized and plasma polymer-coated ([7B] (Dox)/T) surfaces showed higher Rp values, suggesting that these surfaces have higher corrosion resistance. 

Figure 32.19 Cross-sectional depth profile of XPS measured C/Si ratios of TMS plasma polymer films prepared in a (a) closed reactor system and (b) flow reactor system with and without second surface treatment by O2 or Ar plasmas.

Figure 33.2 shows XPS spectra of the surfaces of the TMS plasma polymer film deposited on (Ar + H2) plasma-pretreated steel (a, b, c) and on O2 plasma-pretreated steel (d, e, f). As shown in the spectra, the surface of the plasma film is functional in nature with functional groups of C-OH, C=0, and Si-OH. Two films basically ended up with the same surface structure. This is also confirmed by XPS analysis of the film during the film aging in air after the film deposition, which indicated that the film surfaces were saturated with a fixed surface structure after a few hours of air exposure [4]. This is due to a well-known phenomenon that the residual free radicals of the plasma polymer surface reacted with oxygen after exposure to air [5]. Curve deconvolution of C Is peaks showed structures of C-Si, C-C, C-0, and C=0. The analysis clearly shows a silicon carbide type of structure, which is consistent with the IR results. The functional surfaces of TMS films provide bonding sites for the subsequent electrodeposition of primer (E-coat). 

Figure 33.2 XPS spectra of surfaces of TMS plasma polymer films deposited on (Ar + H2) plasma pretreated steel (a, b, c) and O2 plasma pretreated steel (d, e, f) adapted from reference [4].

It is clear from the study on pure iron that oxides participate in LCVD of TMS, and characteristics of plasma polymer films differ depending on the extent of oxides present on the surface when LCVD is applied. Oxides on the surface of pure iron are more stable than those on steel and hence more difficult to remove, but this can be effected by plasma pretreatment with (Ar + H2) mixture. SAIL by LCVD involving removal of oxides provides excellent corrosion protection of pure iron. The key factor of SAIL by LCVD for corrosion protection of metals in general is the handling of oxides, which depends on the characteristic nature of the metal oxide to be handled. Once strong chemical bonds were formed between nanofilm of plasma polymer, either through oxides or direct bonding to the substrate metal, the LCVD film acts as the barrier to corrosive species. 

---