Antibacterial activity of DLC and Ag–DLC films produced by PECVD technique

Diamond-like carbon (DLC) films have been the focus of extensive research in recent years due to its potential application as surface coatings on biomedical devices. Doped carbon films are also useful as biomaterials. As silver (Ag) is known to be a potent antibacterial agent, Ag–DLC films have been suggested to be potentially useful in biomedical applications. In this paper, DLC films were growth on 316L stainless steel substrates by using Plasma Enhanced Chemical Vapour Deposition (PECVD) technique with a thin amorphous silicon interlayer. Silver colloidal solution was produced by eletrodeposition of silver electrodes in distilled water and during the deposition process it was sprayed among each 25 nm thickness layer DLC film. The antibacterial activity of DLC, Ag–DLC and silver colloidal solution were evaluated by bacterial eradication tests with Escherichia coli (E. coli) at different incubation times. With the increase of silver nanoparticle layers in Ag–DLC, the total compressive stress decreased significantly. Raman spectra showed the film structure did not suffer any substantial change due to the incorporation of silver. The only alteration suffered was a slightly reduction in hardness. DLC and Ag–DLC films demonstrated good results against E. coli, meaning that DLC and Ag–DLC can be useful to produce coatings with antibacterial properties for biomedical industry.     

Tribological behaviour of Ti containing nanocomposite DLC films under milli-Newton load range

In recent days nanocomposite films deposited by various CVD or PVD techniques are receiving intense attention because of their high hardness or high toughness. These films are considered to be potential layers for tribological applications. The present investigation is undertaken to evaluate the tribological behaviour of a nanocomposite Ti containing diamond like carbon film in the load range of milli-Newtons keeping in view the possible application in micro mechanical assemblies (MMA). Towards that purpose, films containing various Ti concentrations are deposited with the help of unbalanced magnetron sputter deposition technique. The tribological properties of these films are evaluated at 500 mN applied load. It is noted that the film containing 46 at % Ti has the maximum hardness in the order of 15.5 GPa and also very high plastic deformation. This film also shows a very low friction coefficient and wear rate.     

Investigation of substrate bias effects on the reactively sputtered ZrN diffusion barrier films

ZrN diffusion barrier films were prepared by DC reactive magnetron sputtering under different negative substrate bias. The composition, microstructure, resistivity and diffusion barrier properties of ZrN films, with respect to substrate bias, were studied by means of X-ray diffraction, electron probe microanalyzer, Auger electron spectroscopy, and four point probe method. Results showed that the deposition rate and impurity oxygen content of ZrN films were substantially influenced by the resputtering effects due to the ion bombardment on the film surface. The competition between surface energy and strain energy made the preferred orientation of ZrN films change from (1 1 1) to (2 0 0) and then back to highly (1 1 1) preferred orientation as a function of substrate bias. The application of negative substrate bias could effectively decrease the electrical resistivity due to the decrease of impurity oxygen content and the densification of films, resulting from the moderate-energy ion irradiation. The biased ZrN films could successfully be used as a diffusion barrier layer, between Cu and SiO₂, even up to the high temperature of 800 °C for 30 min.     

Characterization of magnetron sputtered carbon nitride films

A set of carbon nitride samples has been prepared by reactive magnetron sputtering. The only parameter varied was the nitrogen partial pressure p_N2. It turns out, however, that p_N2 has noticeable influence on the composition and the structure of the films only below 0.1 Pa. The composition of the bulk of the samples was investigated by elastic recoil detection (ERD), energy dispersive X-ray analysis (EDX), wavelength dispersive X-ray analysis (WDX) and Rutherford backscattering (RBS); although all of them give the same trend, some systematic differences were observed concerning the absolute values. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface is somewhat depleted in nitrogen. Sputter depth profiling could not be applied due to very strong preferential sputtering of nitrogen. X-ray diffraction (XRD) showed that the films are almost amorphous. Structural information was obtained from Fourier transform infrared spectroscopy (FTIR), EELS, and XPS; it turned out that the films are graphitic or paracyanogen-like with a density of approx. 2 g cm^-3. All of the characterization methods applied are discussed in view of the information they yield on carbon nitride films, and problems of their application to this special type of film.     

Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at various bias voltages V_b applied on Si substrate. We studied the optical properties of the films using in situ spectroscopic ellipsometry (SE) measurements in the energy region 1.5–5.5 eV. From the SE data analysis the dielectric function ε(ω) of the a-C films was obtained, providing information about the electronic structure and the bonding configuration of a-C films. Based on the SE data the films are classified in three categories. In Category I and II belong the films developed with V_b≥0 V (rich in sp² bonds) and −100≤V_b<0 V (rich in sp³ bonds), respectively. The dielectric function of the films belonging in these two categories can be described with two Lorentz oscillators located in the energy range 2.5–5 eV (π–π^*) and 9–12 eV (σ–σ^*). A correlation was found between the oscillator strength and the sp² and sp³ contents. The latter were calculated by analyzing the ε(ω) with the Bruggeman effective medium theory. In films deposited with V_b<−100 V (Category III), the formation of a new and dense carbon phase was detected which exhibits a semi-metallic optical behavior and the ε(ω) can be described with two oscillators located at ∼1.2 and ∼5.5 eV.     

Diamond-like carbon (DLC) films as electrochemical electrodes

Amorphous carbon can be any mixture of carbon bonds of sp³, sp², and even sp¹, with the possible presence of hydrogen. The group of mixture, of which there is a high fraction of diamond-like (sp³) bonds, is named diamond-like carbon (DLC). Unlike the crystalline carbon materials: diamond, graphite, carbon nanotube, fullerene and graphene, DLC can be deposited at room temperature without catalyst or surface pretreatment. Furthermore, its properties can be tuned by changing the sp³ content, the organization of sp² sites and hydrogen content, and also by doping. This paper firstly reviewed the electrochemical properties of DLC films and their applications.     

Nanotribological and nanomechanical properties of 5–80 nm tetrahedral amorphous carbon films on silicon

Nanoscratch testing has been used to investigate the tribological behaviour of 5, 20, 60 and 80 nm tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The nanoscratch behaviour of the films was found to depend on the film thickness, with 60 and 80 nm films undergoing border cracking and then at higher critical load a dramatic delamination event. 5 and 20 nm films have a lower critical load for onset of border cracks but do not undergo a clear dramatic failure, and instead are increasingly worn/ploughed through until film removal as confirmed by microscopic analysis. This is consistent with the thinner films having lower stress and reduced load-carrying ability. Nanoindentation confirms that the thicker films have enhanced load support and higher measured composite (film + substrate) hardness. The 80 nm film in particular can retain appreciable load support whilst deformed during indentation, as shown by its ability to alter the critical loads for contact-induced phase transformations in the Silicon substrate during unloading.     

Structural relaxation of sputtered amorphous carbon nitride films during thermal annealing

A study of the stress relaxation caused by post-deposition thermal annealing of carbon nitride thin films (CN_x) deposited onto Si substrates has been carried out. The intrinsic stress values were correlated with Fourier transform spectrometer (FTIR) and thermal desorption mass spectroscopy (TDMS) results. FTIR spectra showed the existence of N–Csp³, NCsp² and C≡N triple bonds in the deposited films and indicates the occurrence of their porous character. The analysis of the spectra versus annealing temperature (T_A) reveals two rearrangement mechanisms of the microstructure. Up to 200 °C, the reversion of NCsp² to N–Csp³ and CCsp² respectively, and then an increase of the connectivity of the C–C network for higher T_A. These dissociation/recombination mechanisms are used to describe the stress release occurring within the (CN_x) films upon heating.     

Biaxial elastic modulus of very thin diamond-like carbon (DLC) films

The biaxial elastic modulus of very thin diamond-like carbon (DLC) films was measured by the recently suggested free overhang method. The DLC films of thickness ranging from 33 to 1100 nm were deposited on Si wafers by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) or by the filtered vacuum arc (FVA) process. Because the substrate was partially removed to obtain sinusoidal free overhang of the DLC film, this method has an advantage over other methods in that the measured value is not affected by the mechanical properties of the substrate. This advantage is more significant for a very thin film deposited on a substrate with a large difference in mechanical properties. The measured biaxial elastic moduli were reasonable values as can be judged from the plane strain modulus of thick films measured by nanoindentation. The biaxial elastic modulus of the film deposited by r.f.-PACVD was 90±3 GPa and that of the film deposited by FVA process was 600±50 GPa. While the biaxial elastic modulus of the film deposited by FVA is independent of the film thickness, the film deposited by r.f.-PACVD exhibited decreased elastic modulus with decreasing film thickness when the film is thinner than 500 nm. Although the reason for the different behavior could not be clarified at the present state, differences in structural evolution during the initial stage of film growth seem to be the reason.     

Electrochemical corrosion and materials properties of reactively sputtered TiN/TiAlN multilayer coatings

TiN/TiAlN multilayers of 2 μm thickness were successfully prepared by reactive DC magnetron sputtering method. XRD pattern showed the (1 1 1) preferential orientation for both TiN and TiAlN layers. XPS characterization showed the presence of different phases like TiN, TiO₂, TiON, AlN and Al₂O₃. Cross sectional TEM indicated the columnar growth of the coatings. The average RMS roughness value of 4.8 nm was observed from AFM analysis. TiN/TiAlN coating showed lower friction coefficient and lower wear rate than single layer coatings. The results of electrochemical experiments indicated that a TiN/TiAlN multilayer coating has superior corrosion resistance in 3.5% NaCl solution.     

Effects of normal load on nanotribological properties of sputtered carbon nitride films

The nanotribological properties of amorphous carbon nitride (CN_x) films of ∼380 nm thickness were investigated, in the normal (contact) load range of 2–20 mN, using a Berkovich diamond indenter. The amorphous CN_x films tested in this work were grown on Si(100) substrates by reactive sputtering and energetic ion bombardment during deposition (IBD). The dependence of the friction behavior of the CN_x films on normal load (NL) was investigated in terms of nanomechanical properties, deformation mode and Atomic Force Microscopy (AFM) images of scratched surfaces, and the intensity of IBD. In films sputtered without IBD, the increase of the normal load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it remained constant. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and ploughing mechanisms contributed to the friction behavior in the intermediate and high-load ranges. Elastic and plastic deformation (PD) and delamination of the amorphous CN_x films occurred, depending on the normal-load ranges. On the other hand, films sputtered with high-energy IBD showed a load-dependent transition in both the scratch and the friction responses. Nanoscratching below 5 mN showed mainly elastic behavior of the film, while above 10 mN, a mixed elastic–plastic behavior was identified. Testing under a normal load of 20 mN resulted in local grooving at the film surface; however, in situ profiling of the scratch trace and AFM images showed no evidence of film failure. The increased load-carrying capacity, higher hardness and elastic response obtained with films grown with high-energy IBD, and the dominant friction mechanism at each load range illustrate the normal load dependence of the nanotribological properties of the sputtered CN_x films.     

Instability of diamond-like carbon (DLC) films during sliding in aqueous environment

The instability of diamond-like carbon (DLC) film deposited on Ti-6Al-4V alloy substrate using the r.f.-PACVD method was investigated under sliding conditions in an aqueous environment. Significant adhesive wear was observed when tested in this environment, while normal abrasive wear occurred in an ambient air of relative humidity about 25%. A critical test was performed to elucidate the reason for the instability which limits the biomedical applications of the DLC coating. By employing a multi-step coating process, it was shown that the instability is closely related to the penetration of water molecules to the interface via through-film defects or pinholes. These results suggest that the stability of DLC film in aqueous environment can be improved by controlling the through-film defects in the DLC coating layer.     

Impedance spectroscopy study of electrochromism in sputtered iridium oxide films

The a.c. impedance response of sputtered iridium oxide films (SIROFs) was studied at room temperature in 1M H₂SO₄ between 1mHz and 50mHz. The spectra were recorded as a function of applied potential in the range of electrochromic properties from 0.0 to 1.0V vs SCE and before and after an electrochemical treatment consisting of alternatively colouring and bleaching the electrode. The spectra were analysed with help of an equivalent circuit. Between 0.4 and 1.0V, the spectra can be interpreted as due to electrochemical proton insertion in a single phased compound. From the data, hydrogen chemical diffusion coefficients with values ranging from 2 × 10^–8 to 1.1 × 10^–7cm²s^–1 are found. It is shown that this parameter increases fourfold after the cycling treatment and significantly decreases with the amount of inserted hydrogen. Below 0.4V spectrum changes are observed over the intermediate frequency range studied, indicating some changes of the interfacial reactivity which remain to be clarified.     

Biological activity of silver-doped DLC films

The technique of combined DC metal arc and carbon pulsed arc was used to deposit thin solid films containing up to 6.5 at.% of silver. The microstructure and antibacterial properties of silver-doped diamond-like carbon (DLC) films have been investigated. Silver nanoparticle of flat disk shape located inside of an amorphous carbon matrix revealed excellent antibacterial properties concerning Staphylococcus аureus bacteria. Titanium substrates with DLC films doped with silver have an inhibiting effect on growth of some tumors, in particular, on rat neoplastic С6 glioma. This result is new and opens further possibility for application of DLC:Ag composite in medicine.     

Photocurrent spectroscopy study of passive films on hafnium and hafnium-tungsten sputtered alloys

Anodic and air-formed films on sputtered Hf and W-Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH)₂. This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials (V_f>10 V). In the case of W-Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising in different electrolytes. A PCS analysis of films grown on sputtered alloys is performed on the basis of previously proposed correlation between the band gap of anodic films and difference of electronegativity of their constituents.     

Preparation and properties of sputtered “glassy” carbon films

Some remarkable “glassy” carbon films with densities from 0.5 to 1.79 have been prepared by sputtering from carbon targets. Conditions for the preparation of various carbon films by rf sputtering and data on the density, mechanical properties, microhardness, microstructure and crystal structure of the films formed are presented. Optimum sputtering conditions for obtaining mechanically stable films up to 40μ thick are described. It is shown that density and microhardness of the carbon can be controlled within wide limits, and both increase with decreasing deposition rate. The higher density carbon films adhere poorly to glass, while the low density carbon films on glass are stable. All sputtered carbon films are non-crystalline and have a grainy microstructure. Potential applications exclude corrosion protection but include novel catalyst supports.     

Transfer-free growth of graphene on SiO2 insulator substrate from sputtered carbon and nickel films

Here we demonstrate the growth of transfer-free graphene on SiO₂ insulator substrates from sputtered carbon and metal layers with rapid thermal processing in the same evacuation. It was found that graphene always grows atop the stack and in close contact with the Ni. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions with monolayer surface coverage of up to 40% and overall graphene surface coverage of over 90%. Transfer-free graphene is produced on SiO₂ substrates with the removal of Ni in acid when Ni thickness is below 100 nm, which effectively eliminates the need to transfer graphene from metal to insulator substrates and paves the way to mass production of graphene directly on insulator substrates. The characteristics of Raman spectrum depend on the size of Ni grains, which in turn depend on the thickness of Ni, layer deposition sequence of the stack and RTP temperature. The mechanism of the transfer-free growth process was studied by AFM in combination with Raman. A model is proposed to depict the graphene growth process. Results also suggest a monolayer self-limiting growth for graphene on individual Ni grains.     

Properties of CNx films prepared by PECVD

For this project, CN_x films were prepared by the plasma-enhanced chemical vapour deposition (PECVD) method. Two kinds of substrate were chosen for film deposition: Cr–Ni–Mo steel (0.095 C, 17.25 Cr, 12.04 Ni, 2.49 Mo) and silicon. The substrates were prepared in the shape and size needed for the planned measurements. The surfaces for film deposition were polished and the substrates cleaned in organic solvents by means of ultrasound before treatment. The following CN_x film properties were investigated: thickness, internal stress, dependence of the friction coefficient on the load, refraction index and contamination. For the investigation of these properties the following methods or instruments were chosen: Talystep profilometer, the optical method of stress measurement based on macroscopic sample bending, tribometer HEF, ellipsometry, Rutherford backscattering spectroscopy (RBS) and electron probe microanalysis (EPMA). The thickness values fell in the range of hundredths of nanometres. The compressive stress of the films was below 0.8 GPa. The friction coefficient decreased when the load value increased. There was remarkable oxygen contamination in the prepared films.     

Mechanical properties of PECVD thin ceramic films

Silicon dioxide (thickness 350 nm and 969 nm) and silicon nitride (thickness 218 nm) films deposited on silicon substrate using plasma enhanced chemical vapor deposition process were investigated using a Berkovich nanoindenter. The load-depth measurements revealed that the oxide films have lower modulus and hardness compared to the silicon substrate, where as the nitride film has a higher hardness and slightly lower modulus than the substrate. To delineate the substrate effect, a phenomenological model, that captures most of the ‘continuous stiffness measurement’ data, was proposed and then extended on both sides to determine the film and substrate properties. The modulus and hardness of the oxide film were around 53 GPa and 4–8 GPa where as those of the nitride film were around 150 GPa and 19 GPa, respectively. These values compare well with the measurements reported elsewhere in the literature.     

Piezoresistive properties of diamond like carbon films containing copper

In the present study diamond like carbon films containing copper (DLC:Cu) were deposited by reactive magnetron sputtering. Direct current (DC) sputtering and high power pulsed magnetron sputtering (HIPIMS) were used. The influence of the composition and structure on piezoresistive properties of DLC:Cu films was investigated. Structure of DLC:Cu films was investigated by Raman scattering spectroscopy and transmission electron microscopy (TEM). Chemical composition of the films was studied by using energy-dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Particularly analysis of XPS O1s spectra revealed oxidation of Cu nanoparticles. Piezoresistive gauge factor of DLC:Cu films was in 3–6 range and decreased with the increase of copper atomic concentration. Tendency of the decrease of the gauge factor of DLC:Cu films with the increased D/G peak area ratio (decreased sp³/sp² carbon bond ratio) was observed. It was found that resistance (R) of DLC:Cu films decreased with the increase of Cu atomic concentration by logarithmic law. It is shown that a quasilinear increase of piezoresistive gauge factor with log(R) is in good accordance with percolation theory. Temperature coefficient of resistance (TCR) of DLC:Cu films was negative and decreased with copper amount in Cu atomic concentrations ranging up to ~ 40%. Very low TCR values (zero TCR) were observed only for DLC:Cu films with low gauge factor that was close to the gauge factor of the metallic strain gauges. Role of some possible mechanisms: copper amount as well as Cu cluster size on the value of gauge factor is discussed.