Qingsong Yu

An Optical Emission Study on Expanding Low-Temperature Cascade Arc Plasmas

The optical emission spectra from expanding low-temperature cascade arc plasmas were studied. The objective of this study was to examine the distinctive features of low-temperature cascade arc plasmas in comparison with a radio frequency (RF) plasma source. The principal results obtained in this study were: (1) in an expanding cascade arc plasma jet, active heavy particles (mainly excited argon or helium neutral species under our operating conditions), rather than electrons, are responsible for the excitation of reactive species when a reactive gas is injected into the plasma jet, (2) the excitation of reactive species was found to be controlled by the electronic energy levels of these excited argon or helium neutrals, (3) changing the operating parameters affected only the emission intensities of excited species, and no effect on the emission nature of plasmas was observed.

Creation of polymerizable species in plasma polymerization

Plasma polymerization of trimethylsilane (TMS) was carried out and investigated in a direct current (dc) glow discharge. The formation of TMS plasma glow was carefully examined with optical photography as compared with an Ar dc glow discharge. It was found that there exists a significant difference in the nature of glow and how the glow is created in TMS glow discharge, which polymerizes or causes deposition, and that of monatomic gas such as Ar, which does not polymerize or deposit. In dc Ar discharge, the negative glow, which is the most luminous zone in the discharge, develops in a distinctive distance away from the cathode surface, and the cathode remains in the dark space. In a strong contrast to this situation, in TMS dc discharge, the primary glow that is termed as cathode-glow in this paper appears at cathode surface, while a much weaker negative glow as a secondary glow was observed at the similar location to where the Ar negative glow appears. The deposition results of plasma polymers and gas phase composition data of TMS in a closed reactor acquired by ellipsometry and residual gas analyzer (RGA) measurements clearly indicated that the cathode-glow in TMS glow discharge is mainly associated with chemically reactive species that would polymerize or form deposition, but the negative glow is related to species from simple gases that would not polymerize or deposit. Based on the glow location with respect to the cathode, it was deduced that the cathode-glow is due to photon emitting species created by molecular dissociation of the monomer that is caused by low energy electrons emanating from the cathode surface. The negative glow is due to the ionization and the formation of excited neutrals of fragmented atoms caused by high-energy electrons. Polymerizable species that would cause deposition of material (plasma polymers) are created mainly by the fragmentation of monomer molecules by low energy electrons, but not by electron-impact ionization of the monomer.

Engineering the surface and interface of Parylene C coatings by low-temperature plasmas

Abstract There is a need for the development of environmentally benign processes by which to protect aluminum alloys from corrosion. Vacuum-deposited Parylene C conformal coating is a very good candidate to provide such protection due to its excellent bulk properties: its moisture barrier, high mechanical strength and thermal stability. However, its poor adhesion to most smooth or non-porous substrates has restricted its application. In this study, low-temperature plasma deposition and treatment has proved to be a powerful approach to engineer the surface and interface of Parylene C coatings. After applying a special plasma polymer coating, which acts as an inter-layer to provide good adhesion to the substrate as well as to the subsequent primer, an excellent adhesion of Parylene C coating to a smooth 7075-T6 aluminum alloy has been achieved. After the surface has been functionalized by plasma treatment, the naturally hydrophobic Parylene polymer became paintable with both solventborne and waterborne spray primers.

Corrosion protection of ion vapor deposition (IVD) Al-coated Al alloys by low-temperature plasma interface engineering: Part I. DC cathodic polymerization with anode magnetron enhancement

Abstract Anode magnetron enhanced DC cathodic plasmas were used to treat ion vapor deposition (IVD) aluminum-coated 2024-T3 and 7075-T6 Al alloys for the creation of plasma interface-engineered systems of IVD/plasma polymer/primer. Cathodic electrocoat (E-coat) and three kinds of spray paints were employed as primers. Plasma treatment and polymerization on IVD Al-coated Al alloys provided an excellent adhesion base for succeeding primer coatings; extremely strong, water-insensitive adhesion was obtained between the plasma-treated IVD Al-coated Al alloys and primers. When evaluated by SO 2 and prohesion salt spray tests, these plasma interface-engineered IVD/plasma polymer/primer systems showed excellent corrosion protection characteristics. After 4 weeks of SO 2 salt spray testing, these plasma coating systems outperformed the two types of controls used in this study, i.e., chromate conversion coated and then cathodic electrocoated (BASF E-coat) or chromated primer (Deft 44-GN-36) coated IVD Al alloys. After 12 weeks of Prohesion salt spray testing, the plasma interface-engineered IVD/plasma polymer/primer systems showed corrosion test results comparable to the Deft primer-coated controls and outperformed the BASF cathodic electrocoated controls.

Corrosion protection of ion vapor deposition (IVD) Al-coated Al alloys by low-temperature plasma interface engineering: Part III—DC cathodic polymerization in a closed reactor system

Abstract DC cathodic polymerization of trimethylsilane (TMS) and its mixtures with argon was conducted in a closed reactor system. The TMS deposition behavior and plasma parameters were examined with discharge time during the deposition process. The chemical composition of TMS plasma polymers was investigated by X-ray photoelectron spectroscopy (XPS) analysis. It was found that the TMS plasma coatings obtained under such operations have a distinct chemical structure that gradually changes from silicon rich at the interface with the substrate to carbon rich at the top surface. The coating characteristics of TMS plasma polymers were evaluated in terms of refractive index, linear polarization resistance, and adhesion performance to subsequent spray paint primers. Experimental data indicated that DC cathodic polymerization of TMS in a closed reactor system produced plasma coatings with superior-coating properties, such as higher refractive index and stronger primer adhesion, to those obtained in a flow reactor system as employed in parts 1 and 2 of this series. As a result, the plasma interface engineered-coating systems of IVD/plasma polymer/non-chromated primer obtained under such an operation showed excellent corrosion protection of IVD Al-coated Al alloys which outperformed the chromate conversion-coated IVD controls after 4 weeks of SO 2 salt spray and 12 weeks of Prohesion salt spray tests.

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)

Abstract 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 SO 2 and 12 weeks prohesion salt spray tests.

XPS Analysis of the Aging of Thin, Adhesion Promoting, Fluorocarbon Treatments of DC Plasma Polymers

Plasma polymer treatment of aluminum alloys has recently been shown to improve adhesion of primer coatings, thereby reducing the corrosion of thusly protected panels to the levels afforded by conventional chromate conversion coating. One particular plasma polymer system, comprised of a ∼50 nm trimethylsilane DC plasma polymer capped by an ultrathin layer modified by DC hexafluoroethane plasma treatment, has shown tremendous adhesion increases to a wide variety of primers, yielding a coating that is virtually unremovable with conventional stripping applications. An application window was empirically deduced regarding this improved adhesion, indicating that the primer needed to be applied within 5 days of plasma treatment to display the tenacious adhesion to panels. In an effort to elucidate the differences between fresh and aged panels, an X-ray photoelectron spectroscopy (XPS) time study of this system was undertaken. Some direct correlation to this time frame was observed in the XPS data, indicating that a particular fluorocarbon structure in the films modified upon continued atmospheric exposure, rearranging the local bonding environment by introducing additional C—C bonding with an increase in oxygen incorporation.

Selective adsorption of fluorocarbons and its effects on the adhesion of plasma polymer protective coatings

Cathodic DC plasma deposited films have shown promise as intermediate adhesion and barrier layers for use in the interface engineering of corrosion protection systems on various materials. The surface treatment of plasma deposited trimethylsilane (TMS) films with various post-deposition plasma treatments can improve the adhesion of various paints to these films, which are usually strongly adhered to underlying substrates. Research into the application of these systems for corrosion protection of aluminum alloys included post-deposition treatments of the TMS films with hexafluoroethane (HFE) plasmas, which was seen to significantly improve the adhesion of primers. Oxygen plasma cleaning of the alloy surfaces, prior to deposition of the TMS film, is normally employed to remove organic contaminants. During testing of sample aluminum panels, one batch was processed without the oxygen plasma treatment and exhibited extensive adhesion failures. The investigation of these results shows that low levels of fluorocarbon contaminants readily react with the alloy surface and deposit a fluorine containing carbonaceous layer, which dramatically interferes with the adhesion of the plasma polymer to the alloys, but the adhesion with primer coatings remains tenacious. X-ray photoelectron spectroscopy (XPS) studies also show that the presence of even low levels of these contaminants in the chamber, during the oxygen cleaning process, is sufficient to induce the conversion of the surface from oxide to a mixture of oxide and fluoride. This conversion is considered detrimental to the corrosion resistance of these systems.

Durable bonding characteristics of thermoplastic olefins plasma-treated by low-temperature cascade arc torches

Low-temperature cascade arc torch (LTCAT) polymerization was applied to modify surface characteristics of thermoplastic olefins (TPOs). LTCAT has a unique feature that the activation of carrier gas (Ar) via creation of a plasma state and the deactivation of excited species that lead to surface modification or to the deposition of plasma polymer are spatially de-coupled. Consequently, the addition of a monomer or a reaction gas in the reaction chamber does not influence the first step of Ar plasma formation. The main active species in LTCAT are excited Ar neutrals, and no electric field exists in the reaction chamber. It was found that the beams of excited Ar neutrals are ideally suited for the surface modification of polymers because undesirable damage caused by the bombardment of energetic species can be avoided. The LTCAT of Ar was found to yield stable modification of TPO surfaces via post-plasma reaction of the treated surface with ambient oxygen. An excellent adhesion of a primer coating to surface of TPOs was obtained by treatment of the surface with Ar LTCAT, which passed the tape test after scribed samples were boiled in water for 8 h.

Electrochemical characterization of plasma polymer coatings in corrosion protection of aluminum alloys

Low-temperature plasma polymerization is a promising pretreatment technique to create environmentally friendly coating systems for corrosion protection of aluminum alloys. In this study, the pretreatment effects of plasma treatment and plasma polymerization on corrosion properties of alclad aluminum alloy 2024-T3 ([2A]) were investigated using electrochemical characterization techniques, including cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS). The [2A] panels were coated with an ultrathin layer (∼50nm) of plasma polymers in a direct current (dc) glow discharge of trimethylsilane or its mixtures with one of two diatomic gases (O2 and N2). The CP measurement results showed that the plasma polymer coated [2A] panels exhibited more negative corrosion potentials (Ecorr), smaller corrosion currents (Icorr), and no surface passivation when compared with uncoated [2A] control panels. The lower values of Icorr imply a higher corrosion resistance on the plasma polymer coated [2A]. When i...