Synchronous plasma enhancement in RF-driven plasma source for ion implantation

We report an original method to increase periodically the plasma density in RF-driven plasma source for surface treatment of materials by ion implantation. The method consists of supplementary injection of ions, electrons and metastable atoms into the processing RF plasma using very short high voltage pulsed discharges applied on a separate electrode at the same repetition rate as the negative accelerating pulses applied on the target. Thus plasma density is periodically increased by an order of magnitude so that the synchronized negative pulses applied on the target for ion implantation find a background plasma about 10 times denser. The advantages of this new method were revealed by nitrogen implanted tests on copper and brass samples.     

Plasma distribution in the slender bore excited by coaxial rf electrode

A metal coaxial radio-frequency (rf) electrode has been utilized to produce plasma and deposit the coating onto the inner wall of the tubes. The inner electrode acts both as a plasma source and as a sputtering target to achieve plasma-based IBAD. The non-uniformity of thickness of deposited films and plasma density in the bore has been investigated for tubes with different lengths and inner diameters. The experimental results demonstrate that plasma density is higher at two ends of the tube and lower in the middle section of the tube. The uniformity of plasma density may increase with the inner diameter of tubes increasing, but will decrease with the length of tubes increasing. Copper films have been deposited using this technique and the variation of film thickness agrees with the distribution of plasma density along the bore axis. It is also observed that the uniformity of plasma density is higher than that of film thickness due to diffusion behavior of plasmas.     

Ion extraction from magnetically driven shunting arc and estimation of ion density at sheath boundary

Ion density of a magnetically driven shunting arc discharge is estimated from the target voltage characteristics of the pulse modulator circuit in plasma-based ion implantation (PBII). The voltage recovery time constant directly reflects the ion sheath characteristics, and the sheath resistance is related to the ion density inside the transient sheath. The measured characteristics are analyzed using an equivalent circuit of the pulse modulator in PBII. The estimated ion density decreases from 3 × 10⁸ to 4 × 10⁷ cm^− 3 with time after the arc ignition from 100 to 400 μs. This characteristic almost agrees with that of an ion current extracted from the arc plasma by applying a negative pulse voltage to the target.     

Secondary electron suppression in nitrogen plasma ion implantation using a low DC magnetic field

Experiments aiming at the reduction or even total suppression of secondary electrons during the plasma immersion ion implantation were carried out using a plasma device with low DC magnetic field. Comparison of ion implantations in B = 0 and another case with B = 43 G, indicated that the magnetic field was effective to suppress SE flow in the direction transversal to B but only partial suppression was attained in the longitudinal direction. However, these results are already significant since the efficiency of implantation was increased and the flow of SE to the walls became localized to the regions with B crossing the walls.     

Effect of ion implantation layer on adhesion of DLC film by plasma-based ion implantation and deposition

High adhesive diamond-like carbon (DLC) film on SUS304 was obtained using carbon ion implantation between DLC film and substrate material by plasma-based ion implantation and deposition (PBIID). Implantation of mixed silicon and carbon ions to the substrate resulted in much higher adhesion strength than that of the epoxy resin. Effect of ion implantation on adhesion of DLC film was studied by cross sectional STEM observation and EDS element analysis. Enhancement in adhesive strength by ion implantation of mixed carbon and silicon was ascribed to the formation of the multilayer interface consisting of mixed carbon and silicon ion implanted layer and the amorphous layer of carbon and silicon.     

Incident ion fluence gradients on the front and backside of flat samples

Plasma immersion ion implantation (PIII) is ideal for fast and efficient treatment into three-dimensional objects, as shown by experiments and simulations. In this presentation, a direct comparison of implantations into the front and backside of flat sample (disc, square and rectangle) at 5-15 kV pulse voltage with argon ions is performed with the spatial distribution of the incident ion fluence measured by spectroscopic ellipsometry on SiO₂/Si coupons. A strong influence of the supporting rod for the fluence distribution on the backside of the low symmetry samples, i.e. square and rectangle was observed, in contrast to no influence for the disc sample.     

Parametric study of nitrided AISI 304 austenite stainless steel prepared by plasma immersion ion implantation

This paper investigates the characteristics of plasma immersion nitrogen-ion implanted AISI 304 austenite stainless steel against such processing parameters as bias voltage (5-20 kV), substrate temperature (300-500 °C), and implantation fluence (1.4 × 10¹⁸-4.2 × 10¹⁸ cm^− 2). Characteristics of the as-implanted specimens under investigation included elemental depth profile, hardness depth profile, crystallographic structure, and corrosion behavior and were determined using glow discharge spectrometry (GDS), the Vickers hardness tester, X-ray diffractometry (XRD), and the potentiodynamic polarization test, respectively. The results show that nitrogen depth profiles strongly depend on these processing parameters and closely relate to the corresponding chromium depth profiles. The hardness depth profiles increase and widen as substrate temperature, bias voltage, and implantation fluence increase. In particular, an improvement in hardness is accompanied by a reduction in corrosion resistance when substrate temperature reaches 500 °C. The corrosion-resistance degrader, CrN, precipitates as substrate temperature exceeds 450 °C, a phenomenon which is clearly evident in the chromium depth profiles as well as the XRD results.     

Behavior of endothelial cells on micro-patterned titanium oxide fabricated by plasma immersion ion implantation and deposition and plasma etching

Titanium oxide films synthesized by plasma immersion ion implantation and deposition (PIII&D) were micro-patterned using argon plasma etching. Wells containing organized arrays of square holes of different depths separated by gaps of 25 μm were produced on the surface of the titanium oxide by etching for 30, 60, 90, and 120 min. The surface wettability and tension of the samples were evaluated. Human umbilical vein endothelial (HUVE) cells were cultured on both smooth and patterned surfaces. The effects of surface micro-patterning on the behavior of the HUVE cells were investigated. Our results show that the titanium oxide prepared by PIII&D has good cyto-compatibility and the micro-patterns influence both the surface energy and cell behavior. Our results demonstrate the feasibility of using micro-patterned surfaces to modulate HUVE cell behavior.     

Investigation of cytocompatibility of surface-treated cellulose nitrate films by using plasma immersion ion implantation

The alpha-particle sensitive colorless cellulose nitrate films (commercially available as LR 115 films from DOSIRAD, France) have been proposed as cell-culture substrates for alpha-particle radiobiological experiments. Cytocompatibility of the substrate is a key factor to the success of such experiments. The present work aims to investigate the cytocompatibility of surface-treated cellulose nitrate films by using plasma immersion ion implantation-deposition. The films were placed in a vacuum chamber, into which nitrogen gas was continuously bled and where the pressure was kept at 2 × 10^− 3 Torr. Implantation was carried out by igniting the nitrogen plasma at 100 W radio-frequency and applying high bias voltage in pulse with 20 μs pulse width and 50 Hz (with 20 kV or no voltage). HeLa cervix cancer cells were then cultured on both the plasma-treated and untreated cellulose nitrate films. Our tests showed that the plasma-treated films are in general more cytologically compatible.     

Influence of grain size on nitrogen diffusivity in austenitic stainless steel

Ion nitriding of austenitic stainless steel with the aim to improve the tribological properties while retaining the excellent corrosion resistance is a well-established method. At the same time, strongly varying microstructures can be obtained depending on the pretreatment. In this work, the influence of prior heat treatment in the temperature range between 950 and 1200 °C on the microstructure, especially the grain size, and the corresponding observed nitrogen diffusivity in austenitic stainless steel DIN 1.4301 (AISI 304, X6CrNi18.10) after nitrogen plasma immersion ion implantation (PIII) is studied. Cross-section and plan view samples were prepared and investigated. With increased annealing temperature, both larger grains and slower diffusion was observed, despite diffusion ranges much smaller than the average grain size. Another, still hidden effect of dislocation densities or other defects on both secondary parameters is suggested.     

Effects of pulse parameters on macro-particle production in pulsed cathodic vacuum arc deposition

The effects of the pulse parameters on the production of macro-particles in vacuum arc deposition are studied. A power supply that can provide either direct current or pulsed power is used and the influence of the current and duty cycle are independently investigated. Copper is used as the cathode and glass is used as the substrate. Optical microscopy and scanning electron microscopy with image processing software are used to analyze the macro-particles deposited on the substrate. Our results show the general trend that the density of macro-particles increases with the direct current but there is no obvious correlation with the duty cycles. The lowest degree of macro-particle contamination is observed at a duty cycle of about 40.5%.     

Diffusion, phase formation and segregation effects in Ti6Al4V after oxygen PIII

Plasma immersion ion implantation (PIII) of titanium leads to an excellent biocompati`bility of the surface due to the presence of a pure rutile layer. Ti6Al4V has improved mechanical properties due to the addition of vanadium and aluminium. These alloying elements alter the oxygen diffusion kinetics in the surface oxide, but with lesser affect for diffusion in the bulk material. Anatase is the dominant phase in the surface oxide layer. However, elevated temperatures lead to an additional Al segregation towards the surface.     

Effects of pulsing frequencies on macro-particle contamination during pulsed vacuum arc deposition

Offering high deposition rates and being suitable for thin film fabrication, vacuum arc deposition (VAD) has been investigated extensively. However, macro-particles (MPs) emitted from the vacuum arc degrade the properties of the deposited samples and various types of magnetic filters have been proposed to mitigate macro-particle contamination. Unfortunately, the resulting deposition efficiency is inevitably compromised. In this work, we used a direct current (DC) based pulsed vacuum arc to deposit copper on a glass substrate. DC ensures continuous arc burning to achieve better stability, and the pulsed power provides high instantaneous and discontinuous energy that affects the generation of MPs. Four sets of experiments were done, and the relationship between the pulsing frequencies and MPs deposited on the substrate was studied. Our results show that the MPs tend to be smaller when the pulsing frequency increases, and the MP ratios vary with the pulsing frequencies in a different way than duty cycles. Several factors related to this phenomenon are discussed.     

Properties of the TiN coatings on previously Ti ion-implanted magnesium alloy substrate

To enhance the mechanical properties of TiN coating on magnesium alloy, metal vapor vacuum arc (MEVVA) ion implantation was performed to modify magnesium alloy substrate before TiN film deposition. Implantation energy was fixed at 45 keV and dose was at 9 × 10¹⁷ cm^− 2. TiN coatings were deposited by magnetically filtered vacuum-arc plasma source on unimplanted and implanted substrate. The microstructure composition distribution and phase structure were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The chemical states of some typical elements of the TiN coating were analyzed by means of X-ray photoelectron spectroscopy (XPS). The properties of corrosion resistance of TiN coatings were studied by CS300P electrochemical-corrosion workstation, and the mechanism of the corrosion resistance was also discussed.     

Application of the induction plasma to the synthesis of two dimensional steam methane reforming Ni/Al2O3 catalyst

The purpose of the study is to develop a new protocol for preparing supported two dimensional catalysts with high external and low internal surface by combining thermal plasma spraying and thermal plasma chemical vapour deposition (TPCVD) processes. The method was tested for the production of an Al₂O₃ supported Ni catalysts used for methane steam reforming. The deposition of catalytic materials is made in 2 successive steps. The first step deals with alumina powder sprayed by induction plasma spraying (IPS) on a molybdenum substrate. The experimental conditions have been turned towards γ-Al₂O_3. The second stage deals with the deposition of nickel at the nanometric scale on the alumina layer using the thermal plasma chemical vapour deposition method. This last step was focused on the study of the influence of the nozzle type employed for the nickel solution spraying, the reactor internal pressure, the concentration and the flow rate of nickel salts solution, Ni(NO₃)₂·6H₂O, on the catalyst response. The formulations were tested as methane steam reforming catalysts.The results demonstrate that the catalyst morphology depends on plasma projection conditions and show the effectiveness of combining IPS and TPCVD processes for producing catalysts in methane steam reforming.     

Study of adhesion of TiN grown on a polymer substrate

TiN films were deposited on polycarbonate substrates by cathodic vacuum arc using the plasma immersion ion implantation and deposition (PIII&D) method. The biaxial intrinsic stress in the film deposited using PIII&D with 3 kV applied bias was 0.3 GPa — much lower than that found in films deposited without the application of high-voltage pulsed bias. It was found that the dominant mechanism for generating stress in the TiN film was thermal stress arising from the large difference between the thermal expansion coefficient of TiN and that of the polymer. Tensile testing was used to ascertain film adhesion and a model was used to estimate the adhesion between the film and the substrate. It was found that PIII&D strongly reduced the stress in the TiN film and increased the adhesion to the polycarbonate. The ultimate shear strength of adhesion is of the same order of magnitude as that of TiN on stainless steel.     

Austenite modification of AISI 316L SS by pulsed nitrogen ion beams generated in dense plasma focus discharges

We present the results of a surface modification of AISI 316L stainless steel by surface irradiation with high energy, pulsed nitrogen ion beams generated with 0.8 kJ dense plasma focus. The surface characterization was done using GAXRD, Auger electrons spectroscopy, TEM and optical microscopy. After the irradiation, we found a modification of a 1 μm thick surface layer, on which a gradual lattice expansion of the austenite with the number of irradiation pulses, i.e. with the total nitrogen ion fluence, was observed.In addition, ~ 40 nm close to the surface layer, a disordered lattice structure had been observed through TEM analysis. Those results can be explained in terms of the extreme thermal effect induced on the surface through the fast high energy release during the pulsed ion interaction with the steel surface, followed by an also rapid cooling down process which limits the nitrogen diffusion to the bulk.     

Hydrogen ion reduction in the process of iron rusting

Since the acceptance of the electrochemical rusting mechanism, oxygen reduction has been considered the main cathodic process, while H⁺ reduction has been overlooked for the past four decades because oxygen can be readily renewed due to the thin layer of solution film formed during atmospheric corrosion. This study shows that measurable hydrogen can be detected at the surface opposite to the corroding side of the specimen during wet-dry cycles, and a clear correlation exists between the quantities of hydrogen permeated through iron sheet and weight loss. Results suggest the intrinsic importance of H⁺ reduction that merits further investigation.     

Microstructure and apatite-forming ability of the MAO-treated porous titanium

Porous titanium was treated by micro-arc oxidation (MAO) in the aqueous electrolytes containing 0.1 and 0.2 M NaOH. The microstructure (including morphology, phase component, element composition and chemical species) and in vitro apatite-forming ability of the oxidized films formed on the inner-pore walls of porous titanium were investigated. It is found that continuous thin films with pore sizes of 20-60 nm are formed in both electrolytes. The films consist of an amorphous TiO₂ outmost layer, a coexisted intermediate layer of amorphous TiO₂ and rutile, and a Ti₂O₃ bottom layer, and tightly bond to the titanium substrate without any cracks. In vitro bioactivity assessment shows that both MAO films possess high apatite-forming abilities. It is also found that, compared with the film formed in the 0.1 M NaOH-containing electrolyte, the film formed in the 0.2 M NaOH-containing electrolyte has a higher roughness and more nanopores which help shorten apatite induction time. It is expected the MAO-formed bioactive porous titanium will not only be beneficial to bone ingrowth into the porous structure, but also be beneficial to achieve a tough chemical bonding at the bone/implant interface.