Hydrogenated Carbon Layers Research Articles

Thin amorphous hydrogenated carbon layers on polylactic acid and polyhydroxybutyrate biopolymers - influence of substrate chemistry on interlayer formation

Polymers are often modified by plasma treatment to adapt them to new applications. During the deposition of an amorphous hydrogenated (a-C:H) layer, a mixed phase (interlayer) of the polymer substrate and the deposited layer often forms. The interlayer is responsible among others for a good adhesion between the layer and the substrate. In this study, amorphous hydrogenated carbon films were deposited on the structurally similar biopolymers polylactic acid (PLA) and polyhydroxybutyrate (PHB) using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). In an industrially established process, layer thicknesses of up to 50 nm were produced in 10 nm steps. The surface morphology of each layer was investigated by ex situ atomic force microscopy. Chemical composition was explored first with surface sensitive synchrotron X-ray-based techniques such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure. Subsequently, IR measurements were performed for a more detailed analysis of the functional groups contained. Based on the data obtained (e.g. sp2/sp3 ratio, grain formation on the surface), it was possible to determine the transition of polymer to a-C:H layer and therefore the thickness of the interlayer dimension formed during coating. For PLA, the thickness is about 40 nm, for PHB 20 nm.

Controlling the sp2/sp3 ratio of amorphous hydrogenated carbon layers (a-C:H) on polylactide acid (PLA) at constant distance but angle-dependent deposition

The biopolymer polylactide acid (PLA) is suitable as an eco-friendly substitute for conventional polymers, but polymer surfaces often do not meet industrial requirements and thus severely limit applications. Surface adhesion, substrate related decomposition and barrier permeability can be modified with plasma-deposited thin amorphous hydrogenated carbon (a-C:H) coatings. Their properties depend significantly on the sp2/sp3 ratio, here to be controlled by changing the coating geometry between sample and plasma source. A 50 µm PLA film was a-C:H coated at different angle settings (30° intervals) for 2.5, 5.0, 7.5 and 10.0 min to analyze angle-dependent layer growth. Surface morphology was recorded by scanning electron and atomic force microscopy, and the surface free energy (SFE) and contact angle were determined. Modified barrier properties such as the water vapor transmission rate were investigated and showed a reduction in permeability of up to 47% for defined coating geometries and times. In addition, investigations of the top a-C:H layers by synchrotron assisted X-ray techniques revealed angle-dependent changes in the sp2/sp3 binding ratio. The angle-dependent coating not only changes the chemical composition of the layer, but also improves the barrier properties and extends the SFE of PLA by an increased polar fraction.

The Growth Behavior of Amorphous Hydrogenated Carbon a-C:H Layers on Industrial Polycarbonates—A Weak Interlayer and a Distinct Dehydrogenation Zone

Polycarbonate (PC) is a material that is used in many areas: automotive, aerospace engineering and data storage industries. Its hardness is of particular importance, but some applications are affected by its low wettability or scratch susceptibility. This can be changed either by blending with other polymers, or by surface modifications, such as the application of an amorphous hydrogenated carbon layer (a-C:H). In this study, individual a-C:H layers of different thicknesses (10–2000 nm) were deposited on PC by RF PECVD. Both the layer morphology with AFM and SEM and the bonding states of the carbon on the surface with synchrotron-assisted XPS and NEXAFS were studied. The aim was to investigate the coatability of PC and the stability of the a-C:H. Special attention was paid to the interlayer region from 0 to 100 nm, since this is responsible for the layer to base material bonding, and to the zone of dehydrogenation (from about 1000 nm), since this changes the surface composition considerably. For PC, the interlayer was relatively small with a thickness of only 20 nm. Additionally, a correlation was found between the evolving grain structure and the development of the C‒H peak according to NEXAFS C K-edge measurements.

Effect of Cellulose Nanocrystals on the Coating of Chitosan Nanocomposite Film Using Plasma-Mediated Deposition of Amorphous Hydrogenated Carbon (a–C:H) Layers

The substitution of petroleum-based polymers with naturally derived biopolymers may be a good alternative for the conservation of natural fossil resources and the alleviation of pollution and waste disposal problems. However, in order to be used in a wide range of applications, some biopolymers’ properties should be enhanced. In this study, biocompatible, non-toxic, and biodegradable chitosan (CS) film and CS reinforced with 10 wt% of cellulose nanocrystals (CN–CS) were coated with amorphous hydrogenated carbon layers (a–C:H) of different thickness. To investigate the effect of the nano-reinforcement on the a–C:H layer applied, mild radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) was used to coat the CS and its CN–CS bio-nanocomposite film. Both the surface characteristics and the chemical composition were analyzed. The surface morphology and wettability were examined by ex-situ atomic force microscopy (AFM) and contact angle measurements (CA), respectively. Hereby, the relationship between sp2/sp3 ratios on a macroscopic scale was also evaluated. For the investigation of the chemical composition, the surface sensitive synchrotron X-ray radiation techniques near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) as well as diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) were used.

Changing Contents of Carbon Hybridizations in Amorphous Hydrogenated Carbon Layers (a-C:H) on Sustainable Polyhydroxybutyrate (PHB) Exhibit a Significant Deterioration in Stability, Depending on Thickness

PHB is a biodegradable polymer based on renewable raw materials that could replace synthetic polymers in many applications. A big advantage is the resulting reduction of the waste problem, as well as the conservation of fossil resources. To arrange it for various applications, the surface is arranged by plasma-enhanced chemical vapor deposition (PECVD) with amorphous hydrogenated carbon layers (a-C:H). Here, on a 50 µm thick PHB-foil, a-C:H layers of different thicknesses (0–500 nm) were deposited in 50 nm steps. Surface topography was investigated by scanning electron microscopy (SEM), chemical composition by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and wettability checked by contact angle. In addition, layers were examined by synchrotron supported X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS), which revealed thickness dependent changes of the sp2/sp3 ratio. With increasing thickness, even the topography changes show internal, stress-induced phenomena. The results obtained provide a more detailed understanding of the predominantly inorganic a-C:H coatings on (bio)polymers via in situ growth.

Influence of plasma nitriding pretreatments on the tribo-mechanical properties of DLC coatings sputtered on AISI H11

The duplex treatment, consisting of plasma nitriding and the deposition of a DLC coating, was carried out on the hot-work tool steel AISI H11. The coating structure, composed of Cr-based interlayers and a hydrogenated carbon layer, was sputtered on non-nitrided, nitrided, as well as nitrided-repolished AISI H11 steel with an either annealed or quenched and tempered base condition to examine the influence of the pretreatment condition on the tribo-mechanical properties of the DLC coating.Besides the graded hardness profile, plasma nitriding leads to a roughness increase, which affects the microstructure as well as the mechanical properties of the DLC coating. The rougher surface favors a film growth of a carbon layer with larger cluster-like structures. As a result, these DLC coatings exhibit hardness values below 22 GPa, while the coating systems sputtered on substrates with smoother surfaces reach values of approximately 26 GPa and showed a good adherence. The heat treatment condition influences the load-bearing capacity of the nitrided substrate as the higher core hardness enhances the mechanical support of the coating and reaches the highest adhesion class HF1 in the Rockwell C tests. Due to the lower film adhesion and the low hardness of the DLC coatings sputtered on nitrided non-repolished AISI H11, high coefficients of frictions and wear coefficients of up to 0.59 and 3.19 ∗ 10−5 mm3/N∗m were determined in tribometer tests against WC/Co counterparts. In contrast, the nitrided repolished steel exhibits a low coefficient of friction of 0.12 as well as a low wear coefficient of 0.06 ∗ 10−5 mm3/N∗m. Therefore, a repolishing of the nitrided AISI H11 with quenched and tempered base condition ensures the highest load-bearing capability of the substrate as well as an improved friction and wear behavior of the DLC coating.

Correlation Between 3-D Surface Topography and Different Deposition Times of Engineered Ni@a-C:H Thin Films

This study is aimed at analyzing the surface micromorphology of amorphous hydrogenated carbon layers containing nickel nanoparticles (Ni-NPs@a-C:H) within their structure, which were sputtered by co-deposition of RF-Sputtering and RF-PECVD method from acetylene gas and nickel target on glass substrates. Prepared films were then used as research material following their deposition time at three different times of 3, 7 and 13 minutes. Series of height samples were taken with the help of atomic force microscopy (AFM) operating in a non-contact mode and examined in order to determine their surface engineering characteristics. The analysis of 3-D surface texture helps to understand their essential characteristics and their implications as well as graphicalmodels and their implementation in computer simulation.

Microstructure of nickel nanoparticles embedded in carbon films: case study on annealing effect by micromorphology analysis

The presented study is aimed at analyzing the surface texture of amorphous hydrogenated carbon layers containing nickel nanoparticles (Ni‐NPs@a‐C:H) within their structure, which were deposited by Radio Frequency (RF) sputtering and RF‐Plasma Enhanced Chemical Vapor Deposition (RF‐PECVD) methods on glass substrates. Prepared films were then used as research material following their annealing at two different temperatures of 250 °C and 350 °C in an inert argon atmosphere. Series of height samples were taken with the help of atomic force microscopy (AFM) operating in a non‐contact mode and examined in order to determine their fractal characteristics. Raw AFM data were first plane‐fitted to remove the surface bow exhibiting the so‐called residual surface, and then numerically processed to calculate the Areal Autocorrelation Function (AACF), which was later used to compute the Structure Function (SF). The log–log plots of the latter served for calculation of fractal properties of surfaces under investigation, including fractal dimension D, and pseudo‐topothesy K. The analysis of 3‐D surface texture helps to understand their essential characteristics and their implications as well as graphical models and their implementation in computer simulation. Copyright © 2016 John Wiley & Sons, Ltd.

Suppression of hydrogenated carbon film deposition and hydrogen isotope retention by nitrogen addition into cold remote H/D and CH4 mixture plasmas

Control of tritium retention and its removal from the first wall of future fusion devices are one of the most crucial issues for safety and effective use for fuel. Nitrogen addition into remote edge plasmas has been considered and tested as an effective method for suppression of carbon film deposition and reduction of hydrogen isotope absorption in the deposited films. In this paper we have investigated the scavenger effects of nitrogen injected into low temperature D2/CH4 plasmas on hydrogenated carbon film growth using a small helical device. The result of the deposition shows that the key reactive particles with CN and ND(H) bonds to suppression of hydrogenated carbon film growth and hydrogen isotope absorption are much slowly generated compared with hydrocarbon particles such as CD(H)x and C2D(H)x. This may be due to the slow atomic nitrogen diffusion into hydrogenated carbon layer and the chemical equilibrium between nitrogen absorption.

Surface effects on the diagnostic of carbon/nitrogen low-pressure plasmas studied by differentially pumped mass spectrometry.

In this work, the characterization of the species produced in reactive plasmas by differentially pumped mass spectrometry is addressed. A H2/CH4/N2 mixture (90 : 5 : 5) was fed into a direct current glow discharge and analysed by conventional and cryo-trap assisted mass spectrometry. The gaseous mixture was chosen because of its particular relevance in the inhibition of tritium-rich carbon film deposition in fusion plasmas (scavenger technique) and in the deposition of amorphous hydrogenated carbon films by plasma-assisted chemical vapour deposition. Important changes in the composition of the detected species upon surface modification of the reactor walls (stainless steel or covered by an amorphous hydrogenated carbon layer) or in the way they are sampled (length and spatial configuration of the stainless steel duct) were detected. They are analysed in terms of radical formation and recombination on the reactor walls or into the sampling duct, thus providing some insight into the underlying chemistry. In general, when the reactor walls are covered by an amorphous hydrogenated carbon layer, more hydrocarbons are produced, but the radical production is lower and seem to be less reactive than in stainless steel. Also, two sources of oxygen contamination in the plasma have been identified, from the native oxide layer in stainless steel and from unintended water contamination in the chamber, which modify considerably the detected species.

Development of a novel piezoresistive thin film sensor system based on hydrogenated carbon

Thin film sensor systems based on hydrogenated carbon have the advantage to combine two very important characteristics in order to be used in measurement engineering: Firstly, the sensory layer demonstrates piezoresistive behavior and secondly its good properties related to hardness and wear resistance lead in a tribologically stable system. Therefore, the thin film sensor systems can be applied into the main distribution of force within machine parts or used for universal interchangeable sensor systems, e.g. sensory washers. In this article the deposition of a self-contained thin film sensor system on a large technical component (spindle shaft) is shown. The spindle shaft with a length of 480 mm and an outer diameter of 90 mm is part of a belt driven machining spindle for planing machines in woodworking industries. In order to establish a measurement system, which allows monitoring the clamping force of the tool holder and the imbalance of the mounted tool, the thin film sensor system was directly applied to the front surface of the spindle shaft. For this application a novel self-contained thin film sensor system was developed, which consists of an alumina layer for electrical isolation, a chromium layer to establish internal sensor electrodes, a piezoresistive hydrogenated carbon layer (1 μm) and a second covering wear resistance and insulation layer (silicon and oxygen modified carbon layer). The piezoresistive sensor layer and the top layer are part of the diamond like carbon layer family (Robertson, Diam Relat Mater 12:79–84, 2003; Bewilogua et al. DLC based coatings for tribological applications, pp. 67–75, 2006; Biehl et al. Thin Solid Films 515(3):1171–1175, 2006, Novel measurement and monitoring system for forming processes based on piezoresistive thin film systems. Springer Verlag, pp. 879–883, 2010).

Novel measurement and monitoring system for forming processes based on piezoresistive thin film systems

The investigation of a novel sensor system, integrated in the main load region of forming machines, is the challenge. Therefore, it is important that the thin film system has an excellent tribological quality in combination with a piezoresistive behaviour. The layer system is deposited on the polished surface of a steel substrate. It has such geometries that it can be easily integrated in the drawing cushion of a deep drawing machine. The thin film sensor system exists out of a piezoresistive hydrogenated carbon layer. Onto this layer arrays of chromium structures are deposited. The structures are protected against wear by an insulating silicon doped hydrogenated carbon layer. The whole thin film system has a thickness of about 9 μm. During the forming process the steel plate is in direct touch with the sensor system and moves over it. The position of the steel is measured in dependence on the forming stadium. The sensor system works as a control system to ensure that the shape of the product is perfect and without any cracks or creases.

Comparison of diamond bias enhanced nucleation on Ir and 3C‐SiC: A high resolution electron energy loss spectroscopy study

AbstractBias enhanced nucleation (BEN) is the crucial process step to generate epitaxially oriented diamond nuclei on various substrates comprising elemental semiconductors, carbides and non‐carbide forming metals. Numerous studies on Si and SiC have shown that the diamond growth immediately starts during the biasing as soon as nuclei have been formed. In contrast, nucleation on Ir requires a stronger ion bombardment under which bulk diamond is etched and 3D crystal growth of the nuclei is suppressed. In the present work, we used high resolution electron energy loss spectroscopy (HR‐EELS) to study the vibrational features of the nucleation layers, which were formed by BEN on Ir(001) as compared to the carbon layers formed by BEN on 3C‐SiC. It was found that the HR‐EEL spectra taken for a BEN layer on iridium with successful diamond nucleation, as indicated by the observation of domains, significantly differs from a carbon nucleation layer without these patterns. The peaks at 300 and 450 meV due to overtones stemming from diamond's optical phonon represent a unique feature correlated with the presence of diamond nuclei. The appearance and strengthening of the corresponding loss lines after annealing is interpreted in terms of a weakly bonded amorphous or highly defective hydrogenated carbon layer that initially covers the nuclei. These findings confirm and complement models for the nucleation process, which had been developed on the base of former observations by various other techniques. The stronger signals from crystalline diamond in the spectra taken from BEN on 3C‐SiC confirm that the weaker ion bombardment leaves a more intact diamond crystal at the surface.

Stability of a-C:N:H Layers Deposited by RF Plasma Enhanced CVD

A series of amorphous hydrogenated carbon layers doped with nitrogen (a-C:N:H) was deposited on Si (001). The synthesis was performed from gaseous N2/CH4 mixture using PE CVD (RF CVD technique; 13,56 MHz). An influence of the processing conditions on layer-growth rate was analysed. Thickness of the layers deposited during 1 hour at various temperatures, pressures and RF powers were taken as a basis. It has been proved that the substrate temperature is a key parameter for the layer formation. Temperature rise results in the deposition rate decrease. This unfavourable effect may be reduced by application of increased gas pressure and/or higher plasma RF generator power. At optimal conditions (46 oC; 0,8 Tr; 60 W) the deposition rate reaches up to 600 nm/hour. FT-IR spectra of the layers were measured within 1250 - 4000 cm-1 and discussed with regard to the atomic structure. The intensities of the characteristic absorption bands were compared. The results show that the layers have various N/C ratios according to the applied processing conditions.

Plasma deposition of carbon layer: Correlations between plasma parameters, film structure and properties

The links between plasma parameters, discharge, film structure and properties are not really yet understood for amorphous hydrogenated carbon layers (a-C:H) plasma deposition. Here, a-C:H layers are deposited in a dual radio-frequency–multipolar microwave plasma excited by distributed electron cyclotron resonance reactor at low CH 4 pressure. This study deals with the plasma analysis, the film characterization and with plasma parameters effect on a-C:H films deposition, structure and properties. The discharge analysis shows that CH 4 is decomposed in CH y1 + H radicals and that C 2H 2 y2 are present in the discharge. As at low pressure, recombination can only take place on the surface, C 2 H 2 y2 desorbs from the surface. Moreover, C 2 species are observed attributed to C 2 H 2 y2 dissociation. The evolution of film composition with plasma power shows that the proportion of sp 2 CC decreases in contrast with those of CH bonds which increases. From these observations, a phenomenological model for a-C:H deposition can be proposed. Finally, properties are correlated with the film structure and the effect of MW plasma power can then be given.

Chemical sputtering of carbon by nitrogen ions

Chemical sputtering of amorphous hydrogenated carbon layers by nitrogen molecular ions was studied as a function of the ion energy in the range from 30to900eV. The sputtering yield shows only a very weak variation with energy in the range from 900 down to 50eV. For lower energies it decreases significantly. This behavior is interpreted as an indication of chemical sputtering.

Depth distribution of 0.4–1.6 keV deuterium ions implanted into polystyrene and hydrogenated carbon

At low energy, large-angle multiple scattering influences the depth distribution of ions implanted into materials. Therefore, Monte Carlo codes have been widely used to calculate the depth distribution of implanted ions. However, these codes give results which can sometimes be significantly different from experimental measurements. In addition, during the implantation of polymers and hydrogenated carbon layers, a given quantity of hydrogen atoms can be released, which changes the stopping power and influences the depth distribution. Finally, the local accumulation of electric charges also modify the ion distribution. In order to quantify the effect of each contribution, samples of hydrogenated carbon and polystyrene (PS) have been implanted with low energy (0.4–1.6 keV) deuterium ions. An experimental procedure is described which takes into account the effects mentioned above in order to measure the mean ranges (Rp) and standard deviations (σ) of the depth distributions by means of the ERD ExB technique. Results show that both the Rp and σ decrease with the quantity of hydrogen in the materials. The experimental data in PS films are not in good agreement with the computer simulations while those in the a-C:H layers are well reproduced using TRIM.SP with the krypton–carbon interatomic potential and the Oen–Robinson electronic energy loss model. Comparison of the experimental D depth profiles in PS with the D depth profiles and the damage profiles as simulated by TRIM.SP and with the H loss profile following the D implantation suggests that the implanted D could be trapped in the damage profile induced by the implantation.

Infrared analysis of thin films: amorphous, hydrogenated carbon on silicon

The infrared analysis of thin films on a thick substrate is discussed using the example of plasma-deposited, amorphous, hydrogenated carbon layers (a-C:H) on silicon substrates. The framework for the optical analysis of thin films is presented. The main characteristic of thin film optics is the occurrence of interference effects due to the coherent superposition of light multiply reflected at the various internal and external interfaces of the optical system. These interference effects lead to a sinusoidal variation of the transmitted and reflected intensity. As a consequence, the Lambert-Beer law is not applicable for the determination of the absorption coefficient of thin films. Furthermore, observable changes of the transmission and reflection spectra occur in the vicinity of strong absorption bands due to the Kramers-Kronig relation. For a sound data evaluation these effects have to be included in the analysis. To be able to extract the full information contained in a measured optical thin film spectrum, an experimentally measured spectrum has to be simulated using the full formalism including the Kramers-Kronig relation. Infrared absorption spectra and the resulting k spectra in the range of the CH vibrational bands around 3000 cm-1 are presented for a variety of a-C:H layers. The shape and the total intensity of the peak are quite sensitive to the film structure. Soft, polymerlike hydrocarbon layers are characterized by a well structured, intense IR absorption band, while hard, amorphous, hydrogenated carbon layers exhibit a structureless, broad IR absorption band with relative low intensity. The k spectra of the CH vibrational bands can be considered as fingerprint for the type of a-C:H film.

Deposition of amorphous hydrogenated carbon films due to hydrocarbon molecule ion-beam bombardment

Carbon deposition and hydrogen codeposition are investigated as functions of ion energy, fluence and target temperature at normal incidence on silicon and graphite by bombardment with mass selected CH 2 +, CH 3 +, CH 4 + and CD 4 + molecule beams. An amorphous hydrogenated carbon layer (a-C:H) is formed in a thickness range of 40 to 130 nm by CH 3 + and CH 2 + bombardment up to a fluence of 3 × 10 18/cm 2. The deposition process, the re-erosion phenomenon and the H C ratio of the a-C:H films are studied between 300 and 1100 K in the ion energy range from 0.15 to 3 keV by means of ion beam analysis and Auger Electron Spectroscopy (AES). The experimental results are compared with TRIDYN computer simulation and previous experimental results of carbon sputtering by atomic H + and C + beams in order to get a better understanding of the interaction between hydrocarbon ions and the carbon-based wall materials in fusion devices.

Oxidation and hydrogen isotope exchange in amorphous, deuterated carbon films

The hydrogen desorption and air oxidation of amorphous deuterated carbon (a-C:D) films deposited on Si areinvestigated. The results are compared with a hydrogenated carbon layer deposited on a silicon collector sample mounted on the wall of tokamak ASDEX Upgrade during 6 months' plasma operation. The a-C:D films, deposited on a silicon substrate by the RF glow-discharge method, are exposed to air at room temperature and between 500 and 800 K. The total amounts and depth profiles of both deuterium and carbon are measured by ERD (elastic recoil detection) and PES (proton enhanced scattering). It is found that the surface deuterium loss is accompanied by oxygen uptake and hydrogen isotope exchange in the film until deuterium is completely released. Further annealing leads to the removal of the whole carbon layer. In accordance with previous oxygen implantation studies the erosion is modeled to be due to the chemical reaction of carbon and deuterium with oxygen from air. The removal of the codeposited layer from the ASDEX-Upgrade sample proceeds much faster than that found for a-C:D films under identical treatment conditions. The removal rate strongly depends on film structure.