Hollow Cathode Arc Discharge Research Articles

Diamond-like films of tetrahedral amorphous carbon deposited by anodic arc evaporation of graphite

A physical vapor deposition process using anodic arc evaporation in combination with a hollow cathode arc discharge was applied to the evaporation of graphite for deposition of hydrogen-free carbon layers. The diamond-like carbon (DLC) films deposited on 100Cr6 steel substrates were investigated by nanoindentation, Raman spectrometry, FE-SEM, AFM and spectroscopic ellipsometry. The relationships between the process parameters and the coating properties are discussed. Coatings deposited without bias voltage at substrate temperatures <200 °C are very hard (61–75 GPa) with also very high Young's modulus (588–685 GPa). The evaluation of the Raman spectra indicated a high proportion of tetrahedral sp3 bonds in the range of 70–88 %. Obviously, the vapor particle energies are high enough to achieve such high hardness values even without application of a bias voltage. The coatings proved to be completely droplet-free and have a very low surface roughness as confirmed by FE-SEM and AFM. The deposition rates in the range of 4–18 nm/s are exceptionally high for such tetrahedral amorphous carbon (ta-C) coatings, which is a good prerequisite for industrial applications. The ta-C layers with very high hardness and smooth surface are well suited as wear resistant layers.

Plasma Enhanced High-Rate Deposition of Advanced Film Materials by Metal Reactive Evaporation in Organosilicon Vapors.

Dense homogeneous nanocomposite TiSiCN coatings with a thickness of up to 15 microns and a hardness of up to 42 GPa were obtained by the method of reactive titanium evaporation in a hollow cathode arc discharge in an Ar + C2H2 + N2-gas mixture with the addition of hexamethyldisilazane (HMDS). An analysis of the plasma composition showed that this method allowed for a wide range of changes in the activation degree of all components of the gas mixture, providing a high (up to 20 mA/cm2) ion current density. It is possible to widely change the chemical composition, microstructure, deposition rate, and properties of coatings obtained by this method, by changing the pressure, composition, and activation degree of the vapor-gas mixture. An increase in the fluxes of C2H2, N2, HMDS, and discharge current leads to an increase in the rate of coating formation. However, the optimal coatings from the point of view of microhardness were obtained at a low discharge current of 10 A and relatively low contents of C2H2 (1 sccm) and HMDS (0.3 g/h), exceeding which leads to a decrease in the hardness of the films and the deterioration of their quality, which can be explained by the excessive ionic exposure and the non-optimal chemical composition of the coatings.

Synthesis of Nanocomposite TiSiCN Coatings by Titanium Evaporation and Organosilicon Compound Activation in Hollow Cathode Arc Discharge.

TiSiCN coatings have been obtained by anode evaporation of titanium and the decomposition of hexamethyldisilazane in an arc discharge, using a self-heated hollow cathode, at the pressure rate of 1 mTorr of the Ar+N2 gas mixture. The proposed method makes it possible to independently and widely change the amount of metal and precursor vapor flows, the pressure and composition of the vapor-gas mixture and the degree of ionic interaction on the surface of the growing coating within a single discharge system. The paper presents the method and the results of the effect of a current discharge (10–50 A), and the flux of precursor vapours (0–1 g/h), on deposition rates, compositions, and properties of TiSiCN coatings deposited by an advanced combined PVD+PECVD method. Dense homogeneous TiSiCN coatings up to 6 µm thick and up to 27.5 GPa in hardness were obtained at 7.5 µm/h. The composition of the obtained coatings has been studied by X-ray diffraction and X-ray photoelectron spectroscopy, and it has been shown that the presented methods can form nanocomposite coatings with nanocrystallites TiC, TiN, and TiCxN1−x 3–10 nm in the amorphous matrix based on SiCN.

Structure and electrochemical behaviour of weldments of titanium Grade 1 in a bromine-containing solution

Purpose: The presented research aims to determine the microstructural changes in weldments of commercially pure titanium Grade 1 after welding by hollow cathode arc discharge in vacuum and related changes in the corrosion behaviour of the weldments. Design/methodology/approach: Macro and microstructure of weldments were studied using optical microscopy. Corrosion behaviour of untreated Grade 1 and heat-affected zone of weldments of Grade 1 was investigated using electrochemical testing, including open circuit potential measurements and potentiodynamic polarisation. As an aggressive environment, 1 M KBr water solution was used. Findings: Welding by hollow cathode arc discharge in vacuum leads to the formation of a coarse Widmanstätten structure in the heat-affected zone. This imperfect structure results in a passive layer with worsened protective properties, thus increasing the corrosion rate of weldments by up to two orders of magnitude compared to Grade 1 in as-received condition. The passive layer on the welded surfaces did not allow Grade 1 to acquire a stable corrosion potential during potenitodynamic polarization. Research limitations/implications: Titanium and its alloys are passivating metallic materials, and their corrosion resistance depends on the properties of a thin protective surface layer. Changes in the underlying metal microstructure can affect the passivation behaviour of titanium and the properties of this layer. Welding by hollow cathode arc discharge in vacuum alters the microstructure of heat-affected zone, thereby causing Widmanstätten microstructure to form. As the passive layer over that microstructure has worsened protective properties, we suggest additional heat treatment after welding to be applied. Future experimental research on this topic is needed. Originality/value: Welding by hollow cathode arc discharge in vacuum is a welding method allowing weldments to be done in a clean environment and even in space. In the specialised literature, information on the structure and corrosion resistance of weldments of commercially pure titanium Grade 1 welded by hollow cathode arc discharge in vacuum is missing. The present research fills in a tiny part of this gap in our knowledge.

Numerical simulation of the cathode erosion by ion sputtering in hollow cathode arc discharges

Cathode erosion by ion sputtering is one of the primary erosion mechanisms in hollow cathode discharges. In this paper, the moving mesh approach coupling with a two-dimensional plasma fluid model was employed to simulate the cathode erosion process for 100 h in a hollow cathode arc discharge. In the numerical model, plasma transport, arc flow and heating, and cathode thermal balance were considered. In addition, the electron emission, ion bombardment, and surface radiation were taken into account on the cathode wall. The normal erosion depth on the cathode surface was defined to characterize the degree of cathode erosion. The results showed that the ion sputtering erosion was sensitive to the ion flux toward the cathode wall and the distribution of the plasma potential. The external magnetic field could mitigate the erosion process. The changes of erosion depth and position under different discharge currents, gas flow rates, and intensities of the applied magnetic field were also investigated in this paper.

Development of a Mathematical Model of an Arc Plasma Discharge in Vacuum, Describing the Joint Solution of the Navier-Stokes Equations, Heat Transfer, with Equations of Plasma Theory

This article is devoted to the description of arc processes in a vacuum, relevant today for the melting of wire materials in the implementation of hybrid technologies of layer-by-layer synthesis. The directions of using an electric arc as a source of heat in creating arc additive technologies are indicated. One of the most promising methods for creating an arc discharge in vacuum using a non-consumable thermoelectronic hollow cathode is presented. A two-dimensional model has been developed for studying a hollow-cathode arc discharge in vacuum, where argon was used as the plasma-forming gas. All calculations were carried out in an axisymmetric formulation. To obtain a numerical solution, the COMSOL Multiphysics application software package was used. The obtained simulation results showed a satisfactory agreement with the literature and experimental data.

Numerical study of the power deposition in low-pressure hollow cathode arc discharges

Erosion is always the major factor for shortening the working lifetime of discharge devices with hollow cathode. In this paper, we employ a 2D self-consistent fluid model developed primarily in our previous work (Cong et al 2019 J. Phys. D: Appl. Phys. 52 045205) to carry out a systematic study of the properties of the low-pressure hollow cathode arc and its power depositions onto the cathode wall as well as into particles under different discharge conditions. The model considers the electron transport, electron energy transport, gas heating and flow, and cathode thermal balance. In the cathode thermal balance, apart from thermionic emission, ion bombardment, heat conduction, and surface radiation, the Coulomb collision between electrons and ions is additionally considered. The uniform axial applied magnetic field is also included in the present model. The results show that there exists an optimal discharge stage in the wide range of gas flow rate, in which the voltage and power depositions are almost constant. We also find that the contribution of power deposition from particles accounts nearly for half of the total power deposition onto the cathode. The discharge conditions such as gas flow rate, discharge current, and axial magnetic field can all change the pattern and area of the arc wetting the cathode wall, and therefore influence the properties or power depositions of the hollow cathode arc plasma.

Numerical model of a hollow cathode arc discharge formation in vacuum

Industrial application of hollow cathode arc (HCA) discharge necessitates understanding the processes of interaction between the plasma and the cathode and the anode, as well as related processes. The first step is to study the formation process of HCA discharge in vacuum with the micro-flow of plasma-forming gas through the hollow cathode. In this work presents a two-dimensional model describing the related processes of transfer of charged particles and the movement of plasma-forming gas flows. Electron density and mean electron energy are calculated by solving the drift-diffusion equations. The mass transfer equation for a multicomponent mixture is used to describe the mass transfer of heavy plasma particles. To calculate the electric field strength the Poisson equation is used. The emission of secondary electrons from the inner surface of the cathode is taken into account. The boundary conditions take into account the loss of charge as a result of chaotic motion and its occurrence due to thermal emission effects. The gas flow is determined by collisions and diffuse re-reflection from all surfaces assumed in accordance with Knudsen’s law. Calculations of plasma formation and the movement of plasma-forming gas flows were performed using the simulation package COMSOL Multiphysics.

A 2D model for low-pressure hollow cathode arc discharges in argon

A 2D fluid model is presented in this paper to study axisymmetric hollow cathode arc discharge, in which the equations of particle and particle energy transport, neutral gas flow and heating, and cathode thermal balance are included. This model can overcome the deficiency of other non-self-consistent models, where some boundary conditions must be predefined. In this work, the spatial variations of the electron density and temperature, gas temperature, and cathode wall temperature were investigated under various radii, thicknesses and surface emissivities of the hollow cathode. It is found that the cathode wall temperature was far below the measured wall temperature if the model neglected the contribution of the Joule heating of ions, while the calculated value was in good agreement with the measured one if the model took into account the contribution of the Joule heating of ions. This finding indicates that the Joule heating of ions played an important role and should not be neglected in the modeling of low-pressure hollow cathode discharges. The results also show that the domain and location of the plasma column inside the cathode tube were dependent on the pressure, pressure gradient, and gas temperature, and that the surface thermal radiation was a primary heat loss mechanism on the cathode. In addition, this paper also analyzed the formation of the cathode sheath and evaluated the sheath thickness.

Effect of humidity on the residual stress in silicon-containing plasma polymeric coatings

Residual stress measurements of thin films are common practice in device technology and are extremely important in particular for the characterization of thin film coatings. A largely ignored stress contribution is the difference in coefficient of hygroscopic expansion between the coating and substrate. This paper presents a rather novel approach to accurately evaluate the residual stress and coefficient of hygroscopic expansion of strongly curved specimens. Silicon-containing plasma polymer coatings with different carbon contents were deposited using hollow cathode arc discharge based PECVD. Samples of different layer composition were produced comprising silicon-containing plasma-polymer layers with a high carbon concentration and more “inorganic” SiO2 like layers with lower carbon concentration. All coatings show a compressive stress state. The highest stress was measured in the coating with the highest carbon content (239 ± 6 MPa) and decreases to 94 ± 31 MPa at lower carbon contents. Variation of the humidity showed that all coatings expand under influence of increasing relative humidity. The most inorganic coatings exhibits the highest expansion coefficient of 29.2 ± 2·10−6 (% r.h.)−1. The results obtained were compared with the results from contact angle measurements. An increase in the hygroscopic expansion corresponds with an increasing hydrophilicity of the coatings.

Influence of the applied power on the barrier performance of silicon‐containing plasma polymer coatings using a hollow cathode‐activated PECVD process

A hollow cathode arc discharge is used for the roll‐to‐roll deposition of silicon‐containing plasma polymer thin films on a polymer substrate. It is found that the fragmentation of the used monomer hexamethyldisiloxane (HMDSO) increases with increasing plasma power. The higher fragmentation was related to a reduced hydrogen content as a result of breaking CH bonds. This allowed for a higher degree of cross‐linking. The latter has a positive effect on the barrier performance of the coatings. A hollow cathode arc discharge with separate anode allowed the deposition of a plasma polymer with a water vapor transmission rate (WVTR) of 0.16 g m−2 day−1 (measured at 38 °C and 90% r.h.) on a PET substrate while maintaining a deposition rate of approximately 450 nm m min−1.

Spatially resolved Langmuir probe measurements of a magnetically enhanced hollow cathode arc plasma

Hollow cathode arc discharges are efficient plasma sources and are applied in substrate pretreatment or plasma-activated deposition processes. In order to generate large volume homogeneous plasmas to guarantee uniformity of plasma activation and coating properties, in the presented configuration a ring-shaped anode is positioned coaxially around the hollow cathode tube. A magnetic field is applied, which is axial within the cathode tube and spreads out in the deposition chamber. In order to characterize the hollow cathode plasma, spatially resolved Langmuir probe measurements have been carried out. The charge carrier density maximum on the cathode tube axis reaches values up to 1013cm−3. With increasing distance from the plasma source, the plasma density decreases and shows a smoother lateral profile. Maxwellian electron energy distribution functions are observed with spatially homogeneous electron temperatures in the range 1–4eV. Increasing the chamber pressure leads to higher plasma densities and lower electron temperatures. Reduction of the gas flow through the hollow cathode tube results in a strong rise of the plasma density over two orders of magnitude. The magnetic field supports the low gas flow mode and leads to higher plasma densities, too. The results of the Langmuir probe measurements are discussed by means of the active zone model and are further related to optical emission measurements performed in the vicinity of the hollow cathode orifice.

Hollow cathode arc discharge as an effective energy source for welding processes in vacuum

This paper presents the results of an investigation of thermal and physical properties of the hollow cathode arc discharge (HCAD) with respect to its application for welding processes in vacuum. The following main parameters of the arc discharge were studied: the external voltage–current (V–I) characteristics; plasma parameters inside the cathode cavity and in the arc external column and the radial heat flux density distribution into the anode. Langmuir electrical probes have been utilized to investigate plasma parameters. Electron energy distribution function was determined from the probe V–I characteristics by the computation of an inverse ill-posed problem. It was shown that, depending on welding parameters, HCAD can exist in two different forms: diffusive or constricted. At currents below 60 A, HCAD has the diffusive form, and with the increase in the current it changes to the constricted form. The discharge constriction phenomenon, we believe, could be explained by the appearance in the external plasma of high velocity electrons with energies from 12 to 22 eV. Parameters of the heat flux into the anode were investigated with spot and split-anode calorimeters. The heat flux density on the anode of the diffusive form of the discharge has a Gaussian distribution. The heat flux of the constricted form is significantly different from the diffusive one and can be approximated by the sum of two combined normal-circular heat sources with different power concentration coefficients. It was also found that the efficiency parameter of the discharge energy transfer to the anode can reach 0.7–0.86 of the discharge voltage, which confirmed that HCAD is a highly effective energy source for welding processes in vacuum. Examples of industrial applications of HCAD for welding, brazing and alloying in vacuum are presented.

Laser-triggered hollow-cathode plasma process for film growth

A method of generating a pulsed plasma plume of metallic species using a hollow-cathode arc discharge arrangement is presented. Electrical energy from a pulse-forming network (PFN) generates the transient plasma that evaporates material from the anode that is placed inside a hollow cathode. The discharge is triggered by thermionic electrons produced by a CO2 laser pulse that impinges on one of the electrodes. This plasma process has been used to deposit carbon films in a low-pressure argon or nitrogen ambient. Current pulses of 4–10ms in duration with peak currents of 350A have been produced by the PFN. Characteristics of the produced plasma have been studied by optical emission spectroscopy. The amount of energy imparted to the argon plasma is more than that for a nitrogen plasma. Comparison of on-axis intensity for the 426.9nm line of C+ for the two plasmas shows that the density of carbon ions generated in the nitrogen plasma is higher than that in the argon plasma. Films deposited by this method have fairly uniform thickness profiles that are of the form cos0.4θ for the argon plasma and cos2.2θ for the nitrogen plasma. This indicates that the nitrogen plasma is more forward directed than the argon plasma. Deposition rates of about 10–16Å∕pulse have been obtained for carbon films.

Structure and properties of Al 2O 3 layers deposited by plasma activated electron beam evaporation

The effects of process parameters such as substrate temperature, degree of plasma activation and pulsed bias voltage on the structure and properties of Al 2O 3 layers, deposited by electron beam (EB) evaporation at high deposition rates between 1.5 and 3 μm/min are presented. Layers deposited by reactive EB evaporation without plasma activation are amorphous to X-rays and have a porous columnar microstructure up to a substrate temperature of 700 °C. The hardness of these layers ranges between 2.5 and 8 GPa. A drastic improvement of the layer quality can be achieved by intense plasma activation during deposition process using a low voltage electron beam of a hollow cathode arc discharge (hollow cathode activated deposition; HAD process). Even at low substrate temperatures, dense amorphous layers with a hardness of 12 GPa could be deposited. At an enhanced substrate temperature of 700 °C the layers consist of the nanocrystalline γ phase with a grain size between 12 and 15 nm. The additional application of a pulsed bias voltage of 75 V results in the formation of a pronounced 〈110〉 fiber texture of columnar γ nanocrystallites with a lateral grain size between 30 and 40 nm. The development of the texture in the growth direction was investigated by cross-section TEM investigations. A further increase of bias voltage led to a line broadening of the X-ray diffraction reflexes, which can be interpreted by higher defect densities and lower grain size. The hardness of the dense, nanocrystalline γ Al 2O 3 layers ranges between 20 and 22 GPa. Thus, the application of the process for the deposition of wear resistant layers for cutting tools can be taken into account.

Hollow-cathode low-pressure arc discharges and their application in plasma generators and charged-particle sources

This paper presents the results of a study of hollow-cathode arc discharges which generate gas-discharge plasmas of densities 1010–1012 cm–3 in large volumes (∼1 m3) at low pressures (10–2–1 Pa) and at discharge currents of up to 200 A. Consideration is given to the design and peculiarities of hot-cathode and cold-cathode discharge systems. The parameters of plasma generators and charged-particle sources where use is made of arc discharges are cited and the problems of the most efficient application of such systems in technological processes of solid surface modification are discussed.

Plasma processes and film deposition using tetraethoxysilane

The fragmentation of tetraethoxysilane, Si(C 2H 5O) 4 (TEOS) has been investigated in a hollow cathode arc discharge plasma. Measurements of the molecular ions were performed with an energy dispersive mass spectrometer having an orifice in the plane of the substrate holder. During the measurements the plasma density and the electron temperature as well as the composition of the background plasma (Ar, Ar+He, Ar+N 2, Ar+C 2H 2, Ar+H 2, H 2+Ar) were systematically varied. The change of the molecular ion composition which mainly consisted of TEOS fragments was measured as a function of the parametrical changes. The results of these measurements were compared with the chemical composition, the deposition rate and the structure of films deposited under nearly identical conditions. A change in the magnetic field and thus a variation of the electron temperature caused a change of the film composition showing a significant increase in carbon content with an increase in electron temperature. It is assumed that this change in composition is caused by the increased production of lower molecular weight molecule fragments containing carbon with a high sticking coefficient.

Experiments and modelling of combined PVD and CVD processes using a hollow cathode arc discharge plasma

Abstract A plasma-based deposition method was developed which allows the formation of non-metallic coatings with a graded metal interlayer to improve the substrate adhesion of the non-metallic coating. For this a plasma-enhanced PVD process was combined with a CVD process using a hollow cathode arc discharge plasma produced from a LaB 6 cathode as thermal electron emitter. In the present experiments layers which consist of a combination of an Al interlayer with a graded transition to an a-C:H top coating were deposited. The metal atoms needed for the interlayer formation were introduced into the plasma by evaporation, ionised by the plasma and deposited on the substrate. Subsequently, by introducing hydrocarbon precursor gases into the same discharge, the a-C:H coating was deposited. During the deposition the energy of the ions can be controlled by applying a negative r.f. bias potential to the substrate. The a-C:H top coating had a thickness of up to 3 μm. The thickness of the Al and the mixed Al–C interface was 0.3–0.8 μm. Parallel to the experimental work a space-resolved numerical model was developed to analyze the plasma processes which lead to the deposition of the coating. The results of the calculations were density profiles of the relevant gas phase species and deposition rates which can be compared with experimental values.

Diamond growth in and above trenches in silicon

Abstract We have investigated the deposition of diamond films onto trench structures in silicon substrates with different aspect ratios (i.e. depth to width ratios). Deposition has been performed by microwave plasma-assisted CVD preceded by an in situ bias pretreatment to support nucleation. For comparison, bias pretreated samples have been coated ex situ by hot filament CVD and a hollow cathode arc discharge technique, respectively. The experimental observations suggest that diamond grows in the trenches with a nearly constant deposition rate until the moment when the diamond film above the trench is completely closed. Cross-sectional Raman spectroscopy has been applied to get a view inside trenches. These measurements have revealed differences in phase purity of diamond which has been deposited at distinct stages of growth. In general, very similar results have been achieved with the three different deposition techniques applied. Differences in film structure can be explained, assuming that diamond growth is controlled by the diffusion of hydrocarbons and atomic hydrogen from the plasma to the bottom side of the trench.

Plasma wall interaction for TiN x film deposition in a hollow cathode arc discharge

The present work discusses the relation between plasma parameters and the film microstructure. Both, plasma diagnostic and film characterization were carried out. The plasma in front of the substrate was analysed by Langmuir probe measurements and energy resolved mass spectrometry. Film properties were determined by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and X-ray reflectometry. TiN was used as film material. Thin TiN x films were deposited on Si (100) wafers using a hollow cathode arc evaporation device (HCAED). Films were grown at different nitrogen gas flows, substrate voltages and discharge powers. We observe a correlation between thin TiN x film properties and the characterics of the plasma. Increasing the N 2 gas flow as well as increasing negative substrate voltage at medium N 2 gas flow results in an increasing energy flux to the surface due to ion bombardment. An increase of the nitrogen content x can be observed with the increase of ion bombardment. Higher ion bombardment leads to a small broadening of the X-ray profile. The higher ion bombardment also induces a diminishing of film texture. With increasing nitrogen gas flow the strongly preferred (002) α-Ti and (111) δ-TiN x orientations change to polycrystalline TiN. Different discharge powers result in a nearly constant ion energy flux to the substrate and lead to the same film properties. These results demonstrate the dominant influence of ions on the properties of the deposited TiN x film.