Corrosion protection of ion vapor deposition (IVD) Al-coated Al alloys by low-temperature plasma interface engineering Part II. DC cathodic polymerization under conditions of IVD (without using anode assembly)

DC cathodic polymerizations of trimethylsilane (TMS) were carried out in a bell-jar reactor without using an anode assembly, i.e., under the conditions similar to ion vapor deposition (IVD) operation. In order to initiate the DC glow discharge, a negative potential was applied to IVD Al-coated aluminum panels, the cathode, and grounded reactor wall, the anode. TMS plasma coatings obtained under such operation was studied in terms of refractive indices, linear polarization resistance, and adhesion performance to subsequent spray paint primers. Experimental results indicated that the TMS plasma coatings obtained without anode assembly have similar coating characteristics to those obtained by anode magnetron plasmas as used in Part I of this series, which showed excellent corrosion protection of IVD Al-coated aluminum alloys. As a result, the plasma interface engineered coating systems of IVD/plasma polymer/non-chromated primer obtained under such operation showed excellent corrosion protection of IVD Al-coated aluminum alloys, which outperformed the chromate conversion-coated IVD controls after 4 weeks of SO₂ and 12 weeks prohesion salt spray tests.     

Modification of wettability of a stainless-steel plate by cathodic plasma polymerization of trimethylsilane–oxygen mixtures

Wettability (by water) of a stainless-steel plate was modified by depositing an ultrathin layer of a plasma polymer of trimethylsilane (TMS) and an oxygen mixture. Plasma polymerization was carried out by cathodic glow discharge polymerization, in which a substrate stainless-steel plate was used as the cathode of the de glow discharge. The plasma polymer of TMS (oxygen mol fraction = 0) is very hydrophobic, but the addition of oxygen in the monomer mixture increased the hydrophilicity of the plasma polymer. The value of cos θ (θ: advancing contact angle of water) linearly increased with the mol fraction of oxygen in the monomer mixture. Oxygen in the monomer mixture acts as (1) an ablating agent of a carbon-based plasma polymer and (2) a comonomer for a silicon-based plasma polymer. The atomic composition of a plasma polymer, which can be expressed as SiC_xO_y, changes (x decreases and y increases) as the mol fraction of oxygen increases. © 1995 John Wiley & Sons, Inc.     

Direct current cathodic glow discharge polymerization of methane and butane

The deposition of a polymeric material on the surface of the cathode of a direct current (dc) glow discharge was investigated for methane and butane. The cathode region of a dc glow discharge is not a plasma in a strict sense. Consequently, the deposition of a polymeric material to the cathode surface differs significantly from so-called plasma polymerization of the same monomer (starting gas or vapor) that deposits on a substrate placed in a glow discharge plasma. Using methane and n-butane, the influence of the molecular weight of the monomer (M), volume flow rate, and discharge power on the deposition rate in a dc glow discharge were investigated and compared with those in an audio frequency and a radio frequency glow discharge. It was found that the deposition rate expressed in (thickness growth rate)/(M) is linearly proportional to the current density, which implies that cathodic polymerization is controlled by the cathode region parameter (not plasma parameters). The refractive indices (632.8 nm) for the cathodic polymers are in the range of 2.2–2.4 while those for plasma polymers are in the range of 1.5–1.7. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 237–245, 1998     

Corrosion protection of cold-rolled steel by low temperature plasma interface engineering: I. Enhancement of E-coat adhesion

An anode magnetron enhanced d.c. cathodic plasma treatment of a cold-rolled steel (CRS) plate and cathodic (plasma) polymerization were used to create interface engineered systems of cathodic E-coat/plasma polymer/plasma treated CRS. The adhesion of the E-coat and the corrosion protection characteristics of the systems were compared with the control, E-coat/Zn phosphate-chromate/alectro-galvanized steel (EGS). The adhesion of the E-coat to the plasma polymer coaled CRS was found to be excellent; the E-coat could not be removed by N-methyl pyrrolidone (60 °C) in 5 days, while the same E-coat applied on the control was removed in a few minutes. The plasma polymer of trimethylsilane (TMS) was found to yield excellent corrosion protection as used in the interface engineered systems, of which the corrosion (GM scab) test was found to be better than that for the control. The plasma pretreatment of the CRS surface prior to the cathodic polymerization of TMS was found to be a critically important factor. The corrosion-induced delamination of the first paint layer seems to play a key role in the corrosion of painted steel. It was demonstrated that good adhesion and durability of the entire interface system can produce a highly corrosion resistant painted CRS.     

Plasma interface engineered coating systems for magnesium alloys

In this study, a dc low-temperature plasma technique, including plasma treatment and plasma polymerization, was used to create interface engineered coating systems with a structure of Mg/plasma interlayer/cathodic electrocoating (E-coat) for machined AZ31B magnesium (Mg) alloy panels. The plasma interlayer deposited from trimethylsilane (TMS) precursor had a nano-scale thickness of ∼65 nm and well-controlled surface properties through subsequent plasma treatments in order to achieve different level of interfacial adhesion between the E-coat and the Mg substrates. The surface wettability of the plasma interlayer was monitored by water surface contact angle measurement. The interface adhesion of the coating system was evaluated using N-methylpyrrolidinone (NMP) paint removal test and ASTM tape test conducted under dry and wet conditions. Electrochemical impedance spectroscopy (EIS) was employed to investigate the effects of plasma interlayer properties including surface wettability and adhesion enhancement on corrosion protection properties of the coating systems. It was found that a more wettable interface enhanced the electrolyte penetration through the coating and thus reduced the corrosion resistance of the coating system. On the other hands, the improved interface adhesion had little effects on EIS results mainly due to the high chemical reactivity of the Mg alloy substrates.     

Plasma polymerization of tetrafluoroethylene. III. Capacitive audio frequency (10 kHz) and AC discharge

The plasma polymerization of tetrafluoroethylene (TFE) is studied in a capacitively coupled system with internal electrodes using a 10 kHz (af) and a 60 Hz (ac) source. The emphasis is on identifying conditions that are compatible with continuous coating of plasma polymer on a substrate moving through the center of the interelectrode gap. Operation at a pressure below 100 mTorr is most favorable for deposition of a substantial portion of the plasma polymer on this substrate. Plasma polymer deposited in this way is characterized by ESCA and by deposition rate data and compared to that deposited using rf power in both capacitively and inductively coupled systems. The polymers found in all systems are broadly similar and completely different from conventional poly(TFE). The distribution of power density in the various systems has been identified and compared. This is accomplished by using the known susceptibility of fluorine-containing polymers (including plasma polymer) to a high-power plasma as a probe of plasma power density within the interelectrode gap in the capacitively coupled system. The most active zone of the af or ac plasma is close to the electrode at a plasma pressure of approximately 40 mTorr. The use of a magnetic field leads to an intense localized glow such that etching by active fluorine atoms occurs at a specific locus on the electrode. By contrast, the low-pressure rf capacitively coupled glow discharge is the mildest of those investigated, and its most active zone is further from the electrode and much more diffusely localized by a magnetic field.     

Effect of Surface Energetics of Substrates on Adhesion Characteristics of Poly (p-xylylenes)

In investigating the effect of the surface energetics of substrate materials on the adhesion characteristics of poly(p-xylylene) and poly(chloro-p-xylylene) by the “Scotch Tape” method, it was found that if the substrates had not been preconditioned (treated with argon or a methane plasma), the adhesion was poor. The characteristics of water resistant adhesion that were observed when coated substrates were boiled in 0.9% sodium chloride solution were found to vary from excellent (when the polymer did not peel from the substrate after three cycles of 8 hours of boiling and 16 hours at room temperature) to poor (when the polymer peeled off almost immediately). It was noticed that water resistant adhesion depends on the hydrophobicity of the substrate material (the greater the hydrophobicity, the greater the adhesion) and is not related to the dry adhesive strength of poly(p-xylylene). The oxygen glow discharge treatment of the substrates decreased both the dry and wet adhesive strength of the polymer. The effect of the argon glow discharge treatment depended on the surface energetics of the substrate, and the methane glow discharge treatment increased both the dry and wet adhesive strength of the polymer. These preconditioning processes are discussed in terms of the sputtering of the material from the wall of the reactor in contact with the plasma and the deposition of the plasma polymer of the sputtered material on the substrate surface.     

Polymerization of styrene in an electrodeless glow discharge

The polymerization (polymer deposition) rate of styrene in an electrodeless glow discharge from styrene vapor and a mixture of styrene vapor and gas (H₂, He, A, and N₂) was investigated. The rate of polymerization, R, was found to be independent of the discharge power. The rate of polymerization of the pure monomer was found to be proportional to the square of monomer pressure p_M. The addition of gas increased the rate of polymerization depending upon the partial pressure of the gas, p_x, and R can be generally expressed by R = a[p_M]²{1 + b[p_x]}. The value of b is dependent of the type of gas and follows the order of N₂, > A > He > H₂. The distribution of polymer deposition was found to be nearly independent of the partial pressure of the gas and of the discharge power with N₂ and H₂ as plasma gas; however, with He and A, the distribution is highly dependent on the partial pressure of the gas and on the discharge power. The study strongly suggests that polymerization occurs in the vapor phase and that the growing polymer radicals deposit on the surface of the discharge vessel, yielding highly crosslinked polymer deposition.     

Preliminary experiment of surface hardening of polymers by glow discharge polymerization

Polymers including polyethylene, polycarbonate, and polytetrafluoroethylene were modified by glow discharge polymerization to enhance surface hardness. The surface hardness of the polymer substrates could be improved by glow discharge polymerizations of silicon-containing compounds. The mixture of tetramethylsilane (TMS) and oxygen was more effective than tetramethoxysilane to improve the surface hardness. The surface hardness improved by the glow discharge polymerization strongly depended on the nature of the polymer substrates to be modified. The adhesion between polymer films prepared from the TMS/O₂ mixture by glow discharge polymerization and the polymer substrates was good.     

A reactive-sputter-deposited TiSiN nanocomposite coating for the protection of metallic bipolar plates in proton exchange membrane fuel cells

To meet the needs of corrosion resistance and electrically conductivity for metallic bipolar plates that are employed in proton exchange membrane fuel cells (PEMFCs), a TiSiN nanocomposite coating was fabricated on to a Ti–6Al–4V substrate using reactive sputter-deposition through the double cathode glow discharge plasma technique. The microstructure of the TiSiN coating comprised nanocrystallite TiN grains embedded in an amorphous Si₃N₄ matrix. Electrochemical measurements were employed to investigate the corrosion behavior of the TiSiN coating in the simulated operating environments of a PEMFC, specifically 0.5 M H₂SO₄ solution containing different HF concentrations (namely 2, 4 and 6 ppm) at 70 °C pumped with H₂ at the anode and air at the cathode. With increasing HF concentration, a higher corrosion current density and lower corrosion potential were observed from both the coating and the uncoated substrate, indicating that the addition of HF accelerated their corrosion rates under these conditions. Compared to the uncoated substrate, the TiSiN coating showed a markedly higher corrosion resistance at all HF concentrations. The passive film that formed on the TiSiN coating, with a resistance of the order of magnitude of ~10⁷ Ω cm², displayed good electrochemical stability and was less affected by changes in HF concentration. For the TiSiN coating, the values of interfacial contact resistance (ICR) were 14.7 mΩ cm⁻² and 18.3 mΩ cm⁻², respectively, before and after 2.5 h potentiostatic polarization with 6 ppm HF under cathodic conditions under a compaction pressure of 140 N cm⁻². Both values are much lower than those for the bare substrate. Moreover, the TiSiN coating was shown to improve the hydrophobicity of Ti–6Al–4V that would help facilitate water management in the PEMFC operating environment. This coating, which exhibited excellent corrosion resistance, electro-conductivity and hydrophobicity, is therefore a promising material for protecting metallic bipolar plates from corrosive attack.     

Polymerization of hexamethyldisiloxane by plasma on activated charcoal: Investigation of parameters

The polymerization of hexamethyldisiloxane by plasma (glow discharge) on activated charcoal was investigated. The effects of discharge power, discharge duration, and monomer flow rate on polymer deposition were examined. Maximum deposition was observed when the applied power, monomer flow rate, and discharge duration were 20 W, 50 cm³/min, and 1 h, respectively. The deposited film was investigated by IR, SEM, and ESCA. The same tests were also carried out for plasma-induced polymer deposits.     

Effect of Surface Energetics of Substrates on Adhesion Characteristics of Poly (p-xylylenes)

In investigating the effect of the surface energetics of substrate materials on the adhesion characteristics of poly(p-xylylene) and poly(chloro-p-xylylene) by the “Scotch Tape” method, it was found that if the substrates had not been preconditioned (treated with argon or a methane plasma), the adhesion was poor. The characteristics of water resistant adhesion that were observed when coated substrates were boiled in 0.9% sodium chloride solution were found to vary from excellent (when the polymer did not peel from the substrate after three cycles of 8 hours of boiling and 16 hours at room temperature) to poor (when the polymer peeled off almost immediately). It was noticed that water resistant adhesion depends on the hydrophobicity of the substrate material (the greater the hydrophobicity, the greater the adhesion) and is not related to the dry adhesive strength of poly(p-xylylene). The oxygen glow discharge treatment of the substrates decreased both the dry and wet adhesive strength of the polymer. The effect of the argon glow discharge treatment depended on the surface energetics of the substrate, and the methane glow discharge treatment increased both the dry and wet adhesive strength of the polymer. These preconditioning processes are discussed in terms of the sputtering of the material from the wall of the reactor in contact with the plasma and the deposition of the plasma polymer of the sputtered material on the substrate surface.     

Evaluation of the corrosion protection performance of epoxy coatings containing Mg nanoparticle on carbon steel in 0.1 M NaCl solution by SECM and EIS techniques

The effect of corrosion protection performance of epoxy coatings containing magnesium (Mg) nanoparticles on carbon steel was analyzed using scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). Localized measurements such as oxygen consumption and iron dissolution were observed using SECM in 0.1 M NaCl in the epoxy-coated sample. Line profile and topographic image analysis were measured by applying ?0.70 and +0.60 V vs the Ag/AgCl/saturated KCl reference electrode as the tip potential for the cathodic and anodic reactions, respectively. The tip current at ?0.70 V for the epoxy-coated sample with Mg nanoparticles decreased rapidly, which is due to cathodic reduction in dissolved oxygen. The EIS measurements were conducted in 0.1 M NaCl after wet and dry cyclic corrosion test. The increase in the film resistance (R _f) and charge transfer resistance (R _ct) values was confirmed by the addition of Mg nanoparticles in the epoxy coating. Scanning electron microscope/energy-dispersive X-ray spectroscope analysis showed that Mg was enriched in corrosion products at a scratched area of the coated steel after corrosion testing. Focused ion beam–transmission electron microscope analysis confirmed the presence of the nanoscale oxide layer of Mg in the rust of the steel, which had a beneficial effect on the corrosion resistance of coated steel by forming protective corrosion products in the wet/dry cyclic test.     

Polymerization of organic compounds in an electrodeless glow discharge. VIII. Dependence of plasma polymerization of acrylonitrile on glow characteristic

The effects of experimental conditions (i.e., flow rate, pressure, discharge wattage, and glow characteristics) on the plasma polymerization of acrylonitrile were investigated. It was found that the glow characteristic is highly dependent on both flow rate and discharge wattage and that the plasma polymerization depends strongly on the glow characteristic. However, when experimental conditions are selected to maintain a fully developed glow in the tail flame portion of rf discharge, plasma polymerization is independent of discharge wattage and pressure. The polymer deposition rate is linearly proportional to the monomer flow rate. The deviations from this ideal situation are generally attributable to incomplete glow or partial glow under conditions which caused the deviation. The “character” of the glow largely determines the chemistry of the system. Consequently, the properties of polymers formed under different glow characteristics are also different.     

Plasma polymerization of some simple saturated hydrocarbons

Methane, ethane, propane, and n-butane were polymerized in a plasma created by a radio-frequency glow discharge. It was found that the kinetics of polymer deposition were similar for all monomers but methane. It is suggested that the deviation from “normal” behavior of methane may be due to the difference in mechanism of formation of reaction intermediates. The “characteristic map” for the plasma polymerization of ethane was determined. It was shown that, in addition to transparent films, powders may be formed at low pressures and low monomer flow rates and unstable discharges at high pressures and low flow rates. With increasing power input, the unstable regions are decreased while the powdery regions are increased. The rates of polymer deposition were found to depend on pressure, flow rate, and power. An empirical equation is proposed that relates the rates of deposition for ethane, propane, and n-butane to these parameters.     

Role of unsaturated groups in glow discharge polymerization

The role of unsaturated groups of the starting compounds in glow discharge polymerization was investigated by elemental analysis, infrared spectroscopy, and ESCA. Tetramethylsilane (TMS), trimethylvinylsilane (TMVS), and ethyltrimethylsilane (ETMS) were used as starting materials. Glow discharge conditions (W/FM value meaning the rf power input per mass of the monomers) as well as a degree of the unsaturation of the starting materials had strong influences upon the polymer deposition rate and the elemental composition of the formed polymers. The C_1s and the Si_2p spectra showed that the chemical composition of these polymers was distinguished by either presence or absence of the unsaturated groups in the starting compounds rather than kind of the unsaturated groups. The polymer formation, when the monomers containing double and triple bonds were served, is proceeded not only through these unsaturated bonds but also through the cleavage between Si and C atoms.     

Polymerization in an electrodeless glow discharge. III. Organic compounds without olefinic doublebond

The rates of polymer deposition from various organic compounds which do not contain an olefinic doublebond in an electrodeless glow discharge were studied. The polymerization rates of these unconventional monomers are by and large similar to those of olefinic monomers reported in the previous study (part II). The rate of polymer deposition R₀ from pure monomer flow can be characterized, according to the analysis used in part II, by R_o = apM² and R₀ = kF_w, where pM is the vapor pressure of the monomer, F_w is the weight basis flow rate of the monomer. Type A monomers which predominantly polymerize and type B monomers which decompose in a glow discharge were also found with these unconventional monomers. The effects of structural factors on the values of a amd k and on the classification of types A and B were examined. These structures and groups—aromatic, heteroaromatic, nitrogen-containing (e.g., >NH,? NH₂,? CN), Si-containing, and olefinic doublebond—favor the polymerization. These structures and groups—oxygen-containing chlorine, aliphatic hydrocarbon chains, and cyclic hydrocarbon chains—favor the decomposition of the monomer in a glow discharge. It is postulated that the polymerization of organic compounds proceed by the recombination of excited species (probably free radicals) created by glow discharge and reexcitation followed by further recombinations in the vapor phase and at the interface.     

Polymerization in an electrodeless glow discharge. II. Olefinic monomers

The rates of polymer deposition from various olefinic monomers in an electrodeless glow discharge were studied. The previously found empirical relationship (with styrene in part I) between the rate of polymer deposition R, the monomer pressure p_M, and gas pressure p_x in a steady-state flow system (i.e., R = a(p_M)² [1 + b(p_x)], R being nearly independent of the discharge power) was also found with all monomers investigated. (The effect of gas was examined with nitrogen in this study.) However, it was found that the polymer deposition is controlled by the monomer flow rate and R_o (in pure monomer flow) is proportional to the flow rate of monomer F_w (based on the weight); i.e., R_o = kF_w, where k is a characteristic rate constant of the polymerization. Olefinic monomers can be generally classified into two major groups, i.e., type A monomers which predominantly polymerize, and type B monomers which decompose in a glow discharge. Type B monomers have smaller values of a and k compared to type A monomers. The values of a and k for type A monomers both increase with increasing molecular weight of the monomer. The values of k for all monomers investigated are within roughly an order of magnitude, indicating that the reactivity levels of monomers are very similar in a glow discharge polymerization.     

Energetic deposition of carbon clusters with preferred orientation using a new mixed mode cathodic arc - Sputtering process

Carbon films were produced by triggering a cathodic arc from a magnetron glow discharge by the application of a high voltage pulse to the target. The arc is quenched rapidly to prevent the formation of large macroparticles normally produced by cathodic arcs. The films are smooth and, when grown with negative substrate bias, they contain graphitic nano-clusters which are preferentially oriented with their c-axis normal to the film surface. This orientation gives films with high in-plane conduction. We propose a mechanism for the formation of these oriented layers in which the clusters are charged, accelerated electrostatically and flattened on impact onto the growth surface.     

A novel hybrid composite coating of poly-3-amino-5-mercapto-1,2,4-triazole/TiO<Subscript>2</Subscript> on copper for corrosion protection

Poly-3-amino-5-mercapto-1,2,4-triazole/TiO₂ (p-AMTA/TiO₂) composite was effectively synthesized over the copper surface by cyclic voltammetric technique and used as a protective coating against corrosion. The resulting polymeric composite was characterized using Fourier transform infrared spectroscopy. The presence of TiO₂ particles in the polymer matrix was substantiated from X-ray diffraction pattern and energy-dispersive X-ray spectrum. The uniform dispersion of TiO₂ particles in the polymeric matrix was confirmed by the scanning electron microscope images. The protective effect of composite coating was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization methods in 3.5 % NaCl medium. Impedance measurements showed that charge transfer resistance (R _ct) values increased for polymeric composites which suggested the enhanced corrosion protection of copper. Further, the decrease in corrosion current density (i _corr) values and shifting of corrosion potential (E_corr) toward the cathodic direction confirmed the anticorrosive behavior of the polymeric composite. The reason for the higher protection of polymeric composite may be due to the well-dispersed TiO₂ particles in the polymer matrix exhibiting the enhanced barrier properties to protect copper surface from corrosion. The defects in the coatings can be reduced by embedded TiO₂ particles in the pores of the polymeric films to enhance the corrosion protection, consequently.