Undoped Diamond-like Carbon Films Research Articles

Low friction and wear of Nitrogen-doped diamond-like carbon films deposited on NBR with different flows of N2 by DC-MS

Utilizing a diamond-like carbon (DLC) coating on rubber surfaces is an innovative approach to improve the wear resistance and tribological characteristics of rubber. In this paper, a series of N-doped diamond-like thin films (N-DLC) were deposited on the surface of nitrile butadiene rubber (NBR) substrate by varying the flow rate of nitrogen using direct current magnetron sputtering (DC-MS) technology, and the effects of nitrogen flow rate on the surface morphology, structure, surface energy and tribological properties of N-DLC films were systematically investigated. The results show that the friction coefficients of the N-DLC films are all smaller than those of the undoped DLC films, and most importantly, the samples maintain a stable, ultra-low coefficient of friction of about 0.057 and a low wear rate of the order of 10−9 in the tribological tests at a nitrogen flow rate of 3 sccm, exhibiting excellent tribological performance. This is attributed to several key factors: higher hardness, better film-base bonding, lower surface roughness, and lower friction interface adhesion. This recognition provides insight into the preparation of N-DLC films on rubber to achieve low frictional wear.

Dielectric response and current-voltage characteristic of Mo-doped diamond-like carbon thin films at room temperature

Herein we have reported the effect of molybdenum (Mo) doping and acetylene (C2H2) gas flow rate, during deposition, on the electrical properties of diamond like carbon (DLC) films. The frequency dependent dielectric response of the undoped and doped DLC films has been discussed in details. Mo doping also increases the dielectric constant value owing to interfacial polarization and the corresponding change with C2H2 gas flow rate has been attributed to the formation of sp2 bonding over sp3 bonding, favoring the emergence of a graphite-like phase. The ac conductivity of DLC films has been observed to increase with frequency following a power law behavior. However, with 29 % Mo doping we have noticed two plateau regions having different slopes which has been explained on the basis of the jump relaxation model (JRM). The theoretical fitting of the experimental data indicates occurrence of ionic conduction along with polaron hoping. The dc contribution part (σdc) of the measured ac conductivity is found to vary with the acetylene (C2H2) gas flow dependent sp2 bonding formation. The σdc is extracted to be 2351 S/m for the 29 % Mo doped DLC films. From the Nyquist plot in impedance spectroscopy single relaxation phenomena have been observed for the no or low Mo doping but with 29 % Mo doping both the grain and grain boundary effect are prominent. This also suggests the presence of sp2 hybridization and formation of dangling bonds between the metal carbide and DLC film. From the current voltage characteristics, the development of high sp2 bonding and high electron injection into the DLC matrix with subsequent change in nature of the DLC from semiconducting to metallic with high Mo doping has been reported. From the capacitance voltage characteristics study it has been found that either increase in acetylene gas flow during deposition or Mo doping increases the space charge and trap carrier densities.

Comparison of mechanical and tribological properties of diamond-like carbon coatings doped with Europium and Gadolinium produced by HiPIMS

Hydrogen-free diamond-like carbon (DLC) thin films can be employed for numerous applications, including engine components, once they exhibit good properties for that. They have been used as coatings due to their attractive properties, including high hardness, high wear resistance, and low CoF even under high load/pressure. One of the major limitations of hard hydrogen-free ta-C coatings applications is related to the low reactivity with oil additives used nowadays. The Ionic liquids (ILs) emerged as a novel class of lubricants that can be used in future lubricated systems due to possessing unique physical properties, like high thermal stability and conductivity. To improve the lubrication performance of DLC with ILs, the films were doped with rare earth metals such as Gd and Eu. These non-carbide-forming elements can be introduced in the DLC matrix, incorporated as single atoms, and improve the surface adsorption and reactivity of phosphorus-based IL, and consequently, the lubricating properties of DLC/ILs sliding contacts. However, many demanding applications require good performance across the different regimes (from boundary to hydrodynamic regime). Across all regimes, the boundary is the most challenging, where metal-to-metal contact occurs. Therefore, it is essential to study the influence of the doping elements on the doped-DLCs performance under the boundary regime. For comparison, a pure-DLC was used. In this study, Eu and Gd-DLC tribological systems are characterized by pin-on-disk test in dry conditions and by scratch test. Also, mechanical properties are studied through nano-indentation. Results show that samples doped with a low atomic concentration of Eu or Gd (1%–3%), despite having a CoF higher than un-doped DLC films, show typical values for pure-DLC coatings, like low specific wear rate (<10−16 m3/Nm) and high hardness (23 GPa), permitting, in the future, the combination of novel nanostructured alloyed-DLCs and ILs needed to achieve the optimal lubrication performance.

Phosphorus doping of diamond-like carbon films by radio frequency CVD-cum-evaporation technique

Diamond-like carbon (DLC) films have been deposited by cracking methane gas in radio frequency plasma. Phosphorus was incorporated in DLC films by simultaneously evaporating phosphorus during the synthesis of DLC with methane gas by using capacitatively coupled plasma chemical vapour deposition (CVD) technique. The films were characterized by FESEM, XRD, XPS and Raman studies. The band gap of the pristine DLC films was found to be ~2.61eV and it decreased from 2.61eV to 2.31eV with increased incorporation of P in the DLC films. Raman studies indicated the presence of two broad peaks located at ~1360–1396cm−1 (D-line) and ~1542–1548cm−1 (G-line). The peak positions of the D-lines shifted in towards lower wave number with increased incorporation of phosphorus in them. This indicated that the DLC films became sp2 rich with increased incorporation of phosphorus. Electrical resistivity measurements have been carried out. The films were n-type as indicated by hot-probe technique. The values of electrical resistivity of the undoped and doped DLC films were 2.0×102Ω-cm and 0.7×102Ω-cm, respectively. Value of carrier concentrations (Nd) obtained were 2.5×1015cm−3 and 2.2×1016cm−3 for undoped and phosphorus doped DLC film with 3.4 at.% P in DLC, respectively.

Oxygen doping effect on the wettability of diamond-like carbon thin films

Diamond like carbon (DLC) thin films as a great coating to protect biomedical tools and optical devices are being developed. Modifying the surface properties, such as wettability, is necessary in some application. In this research, oxygen doping effect on the DLC films wettability was investigated. Oxygen doped diamond-like carbon (DLC) thin films were deposited by the RF-PECVD method with different oxygen concentrations in feeding gas (0.0, 1.9, 3.8, 5.6, 7.4 and 9.1vol%). The film's structure was studied by Raman spectroscopy (RS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The chemical composition was determined by a combination of elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS). Surface roughness and morphology of the films were analyzed by an atomic force microscopy (AFM). Wettability of the films was characterized by measuring the contact angles of the films with water. The results showed that by increasing the incorporation of oxygen into the films' structure, the contact angles of water with films were reduced. The increase in the oxygen content of the films, from zero to 11at%, caused to reduce water contact angles from 78.4° to 66.0°. By adding the oxygen into the film's structure, CO and CC bonds content increased in the films. Due to the more polarizability of CO and CC bonds in comparing with CC and CH bonds, a polar component of films' surface energy was increased by adding the oxygen to the films' structure. The surface roughness of the un-doped and doped DLC films were so low (less than 1nm) that they had no roles in the films' surface energy. Doping the DLC films with oxygen caused to increase wettability, without no significant change in the morphology and surface roughness.

The role of substrate bias and nitrogen doping on the structural evolution and local elastic modulus of diamond-like carbon films

Diamond-like carbon (DLC) films are synthesized on Si using plasma enhanced chemical vapor deposition. The role of substrate bias and nitrogen doping on the structural evolution and local elastic modulus of DLC films are systematically investigated. Raman spectroscopic studies reveal that the amount of graphitic C = C sp2 bonding increases with substrate bias and nitrogen doping. The density and hydrogen concentration in the films are found to vary from 0.7 to 2.2 g cm−3 and 16 to 38 atomic %, respectively, depending upon the substrate bias and nitrogen concentration in the DLC films. Atomic force acoustic microscopy (AFAM) analysis shows a direct correlation between the local elastic modulus and the structural properties estimated by Raman spectroscopy, Rutherford back scattering and elastic recoil detection analysis. AFAM analysis further confirms the evolution of soft second phases at high substrate biases (⩾ −150 V) in undoped DLC films. Further, N doping leads to the formation of such soft second phases in DLC films even at a lower substrate bias of −100 V. The AFAM studies provide direct microscopic evidence for the ‘sub-implantation growth model’, which predicts the formation of graphitic second phases in the DLC matrix at high substrate biases.

Deposition of boron doped DLC films on TiNb and characterization of their mechanical properties and blood compatibility

Diamond-like carbon (DLC) material is used in blood contacting devices as the surface coating material because of the antithrombogenicity behavior which helps to inhibit platelet adhesion and activation. In this study, DLC films were doped with boron during pulsed plasma chemical vapor deposition (CVD) to improve the blood compatibility. The ratio of boron to carbon (B/C) was varied from 0 to 0.4 in the film by adjusting the flow rate of trimethylboron and acetylene. Tribological tests indicated that boron doping with a low B/C ratio of 0.03 is beneficial for reducing friction (μ = 0.1), lowering hardness and slightly increasing wear rate compared to undoped DLC films. The B/C ratio in the film of 0.03 and 0.4 exhibited highly hydrophilic surface owing to their high wettability and high surface energy. An in vitro platelet adhesion experiment was conducted to compare the blood compatibility of TiNb substrates before and after coating with undoped and boron doped DLC. Films with highly hydrophilic surface enhanced the blood compatibility of TiNb, and the best results were obtained for DLC with the B/C ratio of 0.03. Boron doped DLC films are promising surface coatings for blood contacting devices.

Interpretation of friction and wear in DLC film: role of surface chemistry and test environment

In spite of the large amount of tribological work carried out to explain the friction and wear mechanism in diamond-like carbon (DLC) films, some of the core issues relating to the evolution of reactive species across sliding interfaces and their role on the friction and wear mechanism remain unclear. The phase composition, film density and hydrogen content present in a DLC film can be tailored by substrate biasing during film deposition to achieve a nearly vanishing friction coefficient. Furthermore, nitrogen doping in DLC films significantly improves wear resistance, and sliding occurs in a nearly wearless regime. Undoped and nitrogen-doped DLC films exhibit a nearly frictionless value with ultra-low wear behavior when tests are performed in argon, nitrogen and methane atmospheres. The antifriction and antiwear properties of the DLC films were improved with the reduction of adsorbed oxygen impurities on the film surface. This behavior was understood by correlating the oxygen impurities present at the surface/subsurface region of the DLC film while using x-ray photoelectron spectroscopy and depth-resolved Auger electron spectroscopy.

Characteristic of silicon doped diamond like carbon thin films on surface properties and human serum albumin adsorption

Diamond-like carbon (DLC) coatings are useful for creating biocompatible surfaces for medical implants. DLC and silicon doped DLC have been synthesised using plasma enhanced chemical vapour deposition (PECVD). The effects of surface morphology on the interaction of human serum albumin (HSA) with doped and undoped DLC films have been investigated using a range of surface analysis techniques using Raman spectroscopy and atomic force microscopy (AFM). Raman spectra of doped DLC show that silicon doped DLC reduces the growth range of the ID/IG ratio, with a significant red-shift of the G peak position. Following exposure to protein, for undoped DLC the peaks at 1664cm−1 and around 1241cm−1 can be attributed to amide I and III, respectively, with an increase in the surface morphology of the surfaces giving some indication of the protein structure on the surfaces. Results indicate that HSA exhibit the majority of β-sheet during the adsorption on the surfaces. The results showed that the silicon incorporation DLC tends to increase of surface roughness and the adsorbed level of HSA is higher with higher levels of silicon doping of the DLC. Therefore, doping DLC may provide a method of controlling the adsorption of protein.

Kinetics and thermodynamics of human serum albumin adsorption on silicon doped diamond like carbon

To gain a better understanding of protein adsorption onto biomaterial surfaces is required for the control of biocompatibility and bioactivity. Various samples of diamond like carbon (DLC) and silicon-doped DLC were synthesised using plasma enhanced chemical vapour deposition (PECVD). The effects of surface morphology on the interaction of human serum albumin (HSA) with doped and undoped DLC films was investigated using spectroscopic ellipsometry (SE) and other surface analysis techniques. The results highlighted an increase in both contact angle and hydrophobicity with increasing silicon dopant levels. A reduction on the contact angle values.After adsorption of HSA, the films showed a reduction in the contact angle with a significant change in the cosΔ and this gap increased with increasing surface coverage of HSA. The adsorption kinetics of HSA were also investigated and revealed that the maximum adsorption occurred at pH 5.0 and the process involved chemisorption. The experimental isotherm data were analysed using the Langmuir and Freundlich‎ models. The amount of HSA adsorbed increased with contact time and reached saturation ‎after 30 min. The adsorption ‎process was found to be pseudo first order with respect to the bulk concentration and was dependent on both the concentration of protein and surface characteristics of the samples. The amount of adsorbed HSA was higher with higher levels of silicon doping of the DLC. Therefore, doping DLC may provide an approach to controlling the protein adsorption.

Influence of transition metal doping on the tribological properties of pulsed laser deposited DLC films

Doped or alloyed diamond-like carbon (DLC) films exhibit superior properties compared to undoped DLC films. But the choice of dopant plays a vital role in tailoring specific properties of the DLC film desired for specific application. In the present work, a comparative study has been carried out in order to bring out the effect of transition metal (TM) doped DLC film on the tribological properties. Nanocomposite DLC/TM (TM=Ag, Ti and Ni) films were deposited by nanosecond pulsed laser deposition (PLD) technique on to AISISS304 substrates. Films microstructure and chemical behavior was studied by glancing incidence X-ray diffraction (GI-XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). The sp2 and sp3 fraction in the films are well described by electron energy loss spectroscopy (EELS) analysis. The Raman spectra of DLC/TM films showed peak shift towards lower wavenumber indicating the reduction of internal compressive stress. Formation of TM nanoclusters and TiC phases were confirmed in DLC/TM films by GI-XRD, XPS and HRTEM. DLC film doped with nanocrystalline Ag showed low friction behavior due to the formation of large amount of sp2 lubricant phase. However, high friction coefficient measured in DLC film doped with Ni and Ti. Such a high friction is explained by the presence of hard TiC and sp3 phase in the DLC/Ti and DLC/Ni films respectively.

Comparison between FTIR and XPS characterization of amino acid glycine adsorption onto diamond-like carbon (DLC) and silicon doped DLC

Diamond-like carbon (DLC) coatings are extremely useful for creating biocompatible surfaces on medical implants. DLC and silicon doped DLC synthesized on silicon wafer substrate by using plasma enhanced chemical vapour deposition (PECVD). The adsorption of glycine onto prepared samples has been investigated with a range of surface analysis techniques. The effects of surface morphology on the interaction of glycine with doped and undoped DLC films have been investigated. The chemical composition of the surface before and after adsorption was analyzed using X-ray photoelectron spectroscopy (XPS). For undoped DLC the spectra show peaks at 285eV (C 1s), 532eV (O 1s) and in the case of Si-doped DLC films a band at ∼100eV (Si 2p) is observed. Following exposure to solutions containing (0.001M) glycine, for undoped DLC the peaks at ∼285.0, ∼399 and ∼532eV reduced in intensity and for Si-DLC samples, the peak at 100eV was reduced. This gives an indication of the quantitative change in the amounts of C, N and O on the surfaces. From Fourier transform infrared (FTIR) spectrum, the peaks occur the following functional groups were assigned as COOR, COŌ, NH3+, NH2, CH and CCN. Both XPS and FTIR spectroscopy confirm that glycine was bound onto the surfaces of the DLC and Si-DLC films via interaction of ionized carboxyl groups and the amino group did not play a significant role in the adsorption of glycine. These results from SE show that an adsorbed layer of glycine is higher at low silicon doping whilst increased doping levels led to a reduced adsorption compared to undoped DLC. Therefore doping of DLC may provide an approach to controlling the protein adsorption.

Study of Human Serum Albumin Adsorption and Conformational Change on DLC and Silicon Doped DLC Using XPS and FTIR Spectroscopy

Diamond-like carbon (DLC) coatings are extremely useful for creating biocompatible surfaces on medical implants. DLC and silicon doped DLC synthesised on silicon wafer substrate by using plasma enhanced chemical vapour deposition (PECVD). The effects of surface morphology on the interaction of HSA with doped and undoped DLC films have been investigated. The chemical composition of the surface before and after adsorption was analysed using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). Results showed that silicon incorporation DLC tends to increase of sp3/sp2 hybridization ratio by decreasing sp2 hybridized carbon bonding configurations. Following exposure to solutions containing (0.250 μg/ml) HSA, the results indicated that significant changes in the C, N and O levels on the surfaces with reducing of the Si2p band at 100 eV. From FTIR spectrum, the peaks occur the following functional groups were assigned as amide I and II groups at 1650 cm-1 and 1580 cm-1. Both XPS and FTIR spectroscopy confirm that HSA was bound onto the surfaces of the DLC and Si-DLC films via interaction of ionized carboxyl groups and the amino group did not play a significant role in the adsorption of protein. These results from peak intensity show that an adsorbed layer of HSA is higher at high level (19%) silicon doping. Therefore doping of DLC may provide an approach to controlling the protein adsorption.

Characterization of boron-doped diamond-like carbon prepared by radio frequency sputtering

Instead of the sophisticated deposition processes and boron sources reported in literature, the study used the radio frequency magnetron sputtering method to prepare boron-doped diamond-like carbon (DLC) films with p-type conduction. The adopted sputtering targets were composed of boron pellets buried in a graphite disc. The undoped DLC films prepared exhibited n-type conduction, based on the Hall-effect measurement. For boron content ≥ 2.51 at.%, the films showed semiconductor behavior converted from n-type to p-type conduction after annealing at 450 °C. B-DLC films with a boron content of 5.91 at.% showed a maximum carrier concentration of 1.2 × 10 19 cm −3, a mobility of 0.4 cm 2/V s, and an electrical resistivity of 1.8 Ω cm. The results of XPS and Raman spectra indicated that the motion of boron atoms was thermally activated during post-annealing, helping promote the formation of C–B bonds in the films. Moreover, the doping of boron in DLC films decreased sp 3 bonding and facilitated carbon atoms to form sp 2 bonding and graphitization.

Laser Raman and X-ray photoelectron spectroscopy of phosphorus containing diamond-like carbon films grown by pulsed laser ablation methods

Mechanically hard, electrically conducting diamond-like carbon (DLC) films containing 0 to ∼26 at.% phosphorus have been deposited on both Si and quartz substrates by pulsed laser ablation of graphite/phosphorus targets (containing varying percentages of phosphorus), at a range of substrate temperatures ( T sub=25–400 °C), in vacuum. Laser Raman spectra (514.5 nm excitation wavelength) of the as-deposited films yield the band maxima and relative intensities of the D and G peaks ( I(D) and I(G), respectively), as a function of T sub, P:C ratio and incident laser fluence, F. Nanocrystalline graphitisation was deduced to occur at higher substrate temperatures, consistent with previous results for undoped DLC and CN x films. P containing DLC films deposited at lower T sub values exhibit higher I(D)/ I(G) ratios than the corresponding undoped DLC films. Measured trends in G-peak positions and I(D)/ I(G) ratios are consistent with values for the C-sp 3 content obtained by deconvoluting the C 1s peak in the corresponding X-ray photoelectron spectra. XPS also reveals a decrease in P content in films grown at higher T sub and at higher F, suggesting that P atoms are more mobile than C atoms, and thus less strongly accommodated, during the film growth process.

Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique

We have fabricated silicon field-emitter arrays coated with nitrogen-doped diamond-like carbon (DLC) films which were deposited by a filtered cathodic vacuum arc technique. The DLC coating on silicon emitters improved the emission characteristics including turn-on electric field and emission current. The turn-on electric fields of DLC-coated silicon emitters are 6.8, 4.0, 1.9, and 0.8 V/μm when the nitrogen flow rates are at 0, 5, 7, and 10 sccm, respectively. The turn-on electric field of the non-coated silicon emitter is 16.0 V/μm. The work functions of DLC-coated silicon emitters have been estimated from Fowler–Nordheim plots and were reduced from 1.86 eV in undoped DLC films to 0.47 eV in 10 sccm of nitrogen doped DLC-coated emitter. As the nitrogen doping concentration of the DLC-coated silicon emitters is increased, the field emission properties improved.

Silicon doped diamond-like carbon films as a coating for improvement of electron field emission

To improve the electron field emission parameters of silicon tips diamond-like carbon (DLC) films doped with silicon have been used as a coating. The idea was to enhance the field emission efficiency using DLC film and increase the DLC film conductivity to make easier electron transport from the silicon tip through the DLC film. The emission properties of in situ silicon doped DLC films have been investigated. Undoped and silicon doped DLC films were grown by plasma enhanced chemical vapor deposition from CH4:H2 and CH4:H2:SiH4 mixtures, respectively. In situ gas-phase doping allowed us to deposit DLC films with different concentrations of silicon in them. The concentration of SiH4 in the gas mixture was varied from 0% to 50%. To characterize the electron conductivity of undoped and silicon doped DLC films metal–insulator–semiconductor (Al–DLC–Si) capacitors were prepared on flat silicon wafers. The strong influence of silicon doping on DLC film properties and electron field emission was revealed. The conductivity of the DLC films increased and the electron field emission changed after doping of the DLC films with silicon. It is shown that silicon doped DLC films are a prospective coating for improvement of electron field emission from silicon tip arrays.

Properties of plasma enhanced chemical vapor deposition diamond-like carbon films as field electron emitters prepared in different regimes

The influence of the diamond-like carbon (DLC) films deposited in different conditions on electron field emission of silicon tips has been investigated. Arrays of square based pyramidal cathodes were fabricated in n-type Si(100) using wet etching in an acid mixture. Undoped and nitrogen doped DLC films were grown by plasma enhanced chemical vapor deposition method from CH4:H2 and CH4:H2:N2 gas mixtures, correspondingly. During nitrogen doped DLC films deposition the nitrogen content in gas mixture was varied from 0% to 45%. The DLC films thickness was changed in range from 10 to 100 nm. Refractive indexes of DLC films were lower for thinner layers. The effect of preparation conditions of the DLC films on their electron emission properties (work function, threshold voltage, etc.) has been studied. For comparison of the emission properties of the structures with DLC films deposited in different conditions the threshold (onset) voltages, field enhancement factors, effective emission areas, calculated from Fowler–Nordheim plots, have been used. It was confirmed that for DLC coatings the electron field emission was substantially increased in comparison with the emission from silicon tips without the coatings, although the nonmonotonous dependence of effective work function on the refractive index of DLC films has been observed. In this context, optimization experiments with respect to the minimal value of the effective work function, which in some cases was less than 1 eV, have been performed. It was found that for the structures with nitrogen doped DLC films the emission appears at lower voltages and the calculated effective work function is 0.1 eV less than for case of the best undoped DLC films. The obtained results can be used for the development of the high efficient field emission cathodes. The model for explanation of the experimental results is presented and the role of carbon–carbon, carbon–hydrogen, and carbon–nitrogen bonds is discussed.

Electron Field Emission Characteristics of Planar Field Emission Array with Diamondlike Carbon Electron Emitters

The electron emission characteristics of planar field emission arrays (FEAs), containing undoped and boron-doped diamondlike carbon (DLC) films as emitters, were investigated. The planar DLC FEAs require only 13.3 V/µm to turn on the electron field emission, whereas the boron-doped planar DLC FEAs requires an even lower electric field (9.8 V/µm) to trigger the electron emission. The boron-doped DLC films also possess an electron emission property highly superior to that of the undoped DLC films and exhibit a stable electron emission current of 938 µA under a 20 V/µm bias voltage, which corresponds to a high emission current density of (Je)B-DLC=128 mA/cm2. These superior properties suggest that the boron-doped DLC FEAs are potentially useful as electron emitters in flat panel displays.