Measuring the thickness of ultra-thin diamond-like carbon films

This paper examines the challenge posed by the measurement of thickness of sub-50 nm diamond-like carbon (DLC) films deposited onto silicon substrates. We compared contact profilometry (CP), optical profilometry (OP), contact atomic force microscopy (CAFM), tapping atomic force microscopy (TAFM) and X-ray reflectometry (XRR). Generally, CP, CAFM, TAFM and XRR give similar thickness values except for the case of the more compliant samples measured by CP and CAFM. Moreover, the theoretically precise XRR technique gives significant standard deviation due to the layering of the DLC film. For those transparent samples, OP always gives an erroneous measurement. These metrological artefacts are compared to calculations of mechanical deformation (CP and CAFM), energy dissipation (TAFM) and thin film interferences (OP). The OP artefact is used to extract the film’s refractive index, in good agreement with literature values. Finally, the comparative data obtained in this study also shows that the density and refractive index of the 10 nm thick films are constituently lower than those of the 50 nm thick films. This scaling effect, which is consistent with known growth mechanisms for DLC, further complicates the measurement of thickness by optical techniques.     

Tribological behavior of nano-undulated surface of diamond-like carbon films

Tribological behavior of nano-undulated diamond-like carbon (DLC) films of the surface roughness ranging from 0.6 to 13.7 nm was investigated in an ambient air of 50% relative humidity. The nano-undulated DLC films were prepared by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) using nanosized Ni dots on a Si (100) substrate. The friction coefficient between the DLC film and the steel ball was characterized by a ball-on-disk type wear rig. Auger and Raman spectroscopy analysis of the debris revealed that the tribochemical reaction with environment was significantly suppressed as the surface roughness increased. Even if the rough surface increased the wear rate of the steel ball and thus the concentration of Fe in the debris, neither the oxidation of Fe nor the graphitization of the carbon in the debris occurred on the rough surface. However, the frictional behavior was affected by several factors: the composition and the size of debris, plowing effect of the rough surface, and the presence of the transfer layer on the wear scar surface.     

Tribological properties of graded diamond-like carbon films on Ti ion-implanted aluminum substrate

Titanium ions are implanted into the aluminum substrate prior to deposition of the Ti interlayer and DLC graded film produced by magnetron sputtering in order to enhance the structural continuity between the soft substrate and graded DLC film. The Ti-implanted substrate not only strengthens loading support but also improves adhesion with the overlying film. The total thickness of the functionally graded film on the Al substrate reaches over 10 μm and it has a hardness value of ~ 11 GPa. The graphitic network in DLC film helps to reduce the friction coefficient while the graded design disperses the stress during wear and loading.     

硬质合金刀具类金刚石涂层的摩擦磨损性能

以等离子体化学气相沉积技术在硬质合金刀具表面制备了类金刚石(DLC)涂层.研究了DLC涂层刀具和无涂层刀具的硬度,不同载荷、不同转速下两种刀具的摩擦磨损性能,以及在水润滑和油润滑条件下DLC涂层刀具的滑动摩擦行为.结果表明,DLC涂层刀具的平均硬度为2 099.9 HV,比无涂层刀具提高了48.3%;DLC涂层刀具的摩擦因数明显低于无涂层刀具,其磨损率随着载荷的增加而增大,随转速的增大而减小;油润滑比水润滑能更有效减缓摩擦作用.     

Stresses in diamond-like carbon films

Internal stresses have been measured in diamond-like carbon (DLC) films deposited by d.c. plasma assisted chemical vapor deposition from methane, acetylene, or cyclohexane, and in nitrogen containing DLC films deposited from acetylene, or cyclohexane and nitrogen. The total hydrogen content in the films and the fraction of bound hydrogen have been analyzed by forward recoil elastic scattering and Fourier transform infrared spectroscopy respectively. It was found that in pure DLC films the stresses increase with increasing fraction of unbound hydrogen. The highest compressive stresses were obtained in the films deposited from methane and the lowest stresses in films deposited from cyclohexane. In the nitrogen containing DLC films the stresses decrease with increasing nitrogen content in the films. Stresses as low as 0.22 GPa were obtained in the films deposited from cyclohexane and nitrogen at a ratio of 1/15 in the plasma.     

Bacterial adhesion on silicon-doped diamond-like carbon films

In this paper the surface properties of silicon-doped diamond-like carbon films with various Si contents on 316 stainless steel substrate by a magnetron sputtering technique were investigated. X-ray photoelectron spectroscopy was applied to determine the surface chemical composition of the films. Atomic force microscopy was used for the determination of surface roughness and topography. The sp² contents in the films were determined with Auger electron spectroscopy, which were 67.1%, 34.2% and 25.0% for silicon contents 1%, 2% and 3.8%. The sp³/sp² ratio increases with increasing the silicon contents in the films. Contact angles of three test liquids on the films were obtained with a Dataphysics OCA-20 contact angle analyzer. Surface free energies of the films and their dispersive and polar components were calculated using van Oss acid–base approach. Staphylococcus aureus was used for bacterial adhesion test. The experimental results showed that bacterial adhesion decreased with increasing the silicon content or with increasing sp³/sp² ratio in the films.     

Fracture behavior of diamond-like carbon films on stainless steel under a micro-tensile test condition

We investigated the stability of the DLC film coated on 304 stainless steel substrate by r.f. PACVD method. Fracture and spallation behaviors of the coating were observed during micro-tensile test of the film/substrate composite. As the tensile deformation proceeded, the cracks of the film appeared in the perpendicular direction to the tensile axis. Further deformation resulting in the local necking with shear band of 55° inclined to the tensile axis, induced the spallation of the film, which was initiated at the cracks of the film, and was aligned along the slip directions. We found that both the cracking and the spallation behaviors are strongly dependent on the pretreatment condition, such as Ar plasma pretreatment or Si buffer layer deposition. The spallation of the film was significantly suppressed in an optimized condition of the substrate cleaning by Ar glow discharge. These results show that the spallation behavior during the tensile test can be used to estimate the interfacial strength of the coating with relatively poor adhesion.     

Iodine doping of amorphous diamond-like carbon films

Amorphous diamond-like carbon (a:DLC) films have been doped by incorporation of iodine during the films deposition. XPS and AES analysis shows the existence of iodine atoms with constant concentration of 0.9% along the iodine doped DLC film (a:I-DLC). The optical and electronic properties of the doped films were studied. Optical measurements in the visible light show that iodine affects the interband absorption of the a:DLC films. Iodine causes decreasing of the optical energy gap, from 1.07 to 0.78 eV and affects the density of states at the conducting band. Like the optical measurements, electrical measurements show that iodine also decreases the activation energy of the films from 0.34 to 0.22 eV. This shows that although both gaps decrease, the optical energy gap remains different from that of electrical gap, also after doping.     

Fast deposition of diamond-like hydrogenated carbon films

Diamond-like hydrogenated carbon films have been formed at low temperatures using methane and acetylene as precursor gases. The source used was of a cascaded arc type employing Ar and Ar/H₂ as carrier gases. Energies of ion species and ion densities in the plasma were measured with a mass energy probe and a Langmuir probe.The films produced were characterized in terms of sp³ content, refractive index, relative hydrogen content, hardness and adhesion. The variation of these parameters is presented as functions of precursor gas flow, process pressure, and surface temperature.Deposition rates up to 30 nm/s have been achieved using acetylene as precursor gas at substrate temperatures below 100 °C. Experiments with acetylene showed deposition rates seven times greater than with methane. The typical sp³ content of 55–78% in the films was determined by X-ray-Excited Auger Electron Spectroscopy (XAES) technique. The hardness and reduced modulus were determined by nanoindentation. Preliminary Atomic Force Microscopy (AFM) studies of the films showed a roughness below 3 nm (R_a).     

Heat resistance of fluorinated diamond-like carbon films

The heat resistance of fluorinated diamond-like carbon (F-DLC) films produced by Plasma Immersion Ion Processing (PIIP) technique was investigated by annealing F-DLC coatings in a vacuum furnace. The growth rate for the F-DLC films was approximately 0.6 μm/h. In order to see the possible change in the composition and properties of the F-DLC films, Rutherford Backscattering Spectrometry (RBS), nanoindentation and contact angle measurements were performed before and after the heat treatments. The results show that the composition and properties of the F-DLC films were unchanged up to heat treatment at 300°C for up to 30 min. Blistering and film delamination occurred for samples treated at 400°C.     

Thermal gas effusion from diamond-like carbon films

Good-quality diamond-like carbon films (6 at.% H₂, 2400 kgf/mm² microhardness, 2.7 eV bandgap, higly insulating) have been obtained by the DC glow discharge decomposition of acetylene. Mass spectroscopic thermal effusion measurements were carried out on the films deposited under different deposition conditions. Analyses of hydrogen in conjunction with hydrocarbon effusing species yield information on the microstructure and nature of C---H bonding configurations. It is shown to be a useful analytical tool to study hydrogenated amorphous carbon films of different microstructures varying from polymer-like to diamond-like.     

Characterization of crystalline diamond incorporated diamond-like carbon films

The purpose of this paper is to show the production and characterization of diamond-like carbon (DLC) films with incorporated crystalline diamond (CD), produced by plasma enhanced chemical vapor deposition. CD-DLC films were characterized by scanning electron microscopy, X-ray diffraction, atomic force microscopy and Raman scattering spectroscopy. Wetting contact angle, stress and friction coefficient were also evaluated. Our results demonstrated CD-DLC films are more hydrogenated and hydrophobic, with higher fiction coefficient. The stress values kept almost constantly.     

Electrically conductive properties of tungsten-containing diamond-like carbon films

Tungsten-containing diamond-like carbon films with different metal concentrations were investigated. The films of several hundred nanometers in thickness were deposited on the silicon wafer using RF-PECVD (radio frequency plasma enhanced chemical vapor deposition) method. During deposition, metal component was co-sputtered using DC magnetron of tungsten target. The six samples with the concentration of 3.8, 6.1, 8.0, 16.3, 24.3 and 41.4 at.% of tungsten were made. The structural analyses were performed by TEM (transmission electron microscope) and Raman spectroscopy. These results indicated that tungsten clusters were well dispersed in amorphous carbon host matrix in the case of tungsten concentration from 3.8 to 24.2 at.%. However, no such a structure can be observed in the sample with 41.4 at.%. The AC electrical resistance was measured in the temperature range of 2–300 K using four-probe method in vacuum condition. The observed temperature dependence of electrical conductivity can be expressed by σ=σ₀exp−2(C₀/kT)^1/2 and tungsten concentration from 3.8 at.% to 24.2 at.%. In addition, the sample with 41.4 at.% showed the resistive superconducting transition at T_c of around  5.5 K.     

Humidity effect on friction behaviors of nano-undulated diamond-like carbon films

Humidity dependency of friction behavior of nano-undulated diamond-like carbon (DLC) films was investigated by a home-made ball-on-disk type tribometer under controlled relative humidity of 0, 50, and 90%. Nano-undulated DLC films with surface roughness ranging from 0.2 to 13.4 nm were prepared by deposition of DLC film on the Si substrate with Ni nanodots. Friction coefficient of the flat DLC surface increased with the relative humidity, while that of the nano-undulated surfaces revealed smaller dependence on the relative humidity. When the surface roughness increased to 13.4 nm, friction behavior was observed to be independent of the relative humidity. The analysis of chemical composition and atomic bond structure of the debris and the transfer layer revealed that the humidity dependence on the nano-undulated surface was minimized by suppressing the graphitization of the transfer layer even with high concentration of Fe in the debris.     

Application of aluminum nitride films for electronic devices

AlN films were grown by RF-magnetron reactive sputtering of Al-target in Ar+N₂ gaseous mixture. In the dependence on deposition conditions the films may be from nearly amorphous to fine crystalline structure. The <0001> texture for crystalline AlN is typical. The application of AlN films as a protective coating of thin-films thermoprinting devices; as a piezoelectric in devices based on surface acoustic waves and also as a material of cold cathodes is shown.     

Low surface energy coatings covalently bonded on diamond-like carbon films

In the present work, a chemical treatment with perfluorinated peroxides is proposed to obtain protective layers covalently linked to a diamond-like carbon (DLC) surface. The lubricant properties of perfluorinated compounds and the stability of the chemical modification of DLC surface simultaneously cooperate in this technical approach. Each fluorinated layer is deposed on an bare DLC surface by a dip coating application technique and the covalent linkage of the fluorinated layers is obtained by the thermal decomposition of the peroxidic moieties of the perfluorinated peroxides. Reactive perfluorinated radicals are generated close to the sp² sites of the DLC surface, allowing the formation of covalent bonds. The fluorinated peroxides used in this work belong to the class of the PFPE peroxides and to the class of the perfluorodiacyl (PFDA) peroxides. The effect of the fluorinated coatings on the DLC surface is studied using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), with contact angle (CA) measurements and, in particular, the friction forces are evaluated by means of lateral force microscopy (LFM).     

Study on the diamond-like carbon multilayer films for tribological application

In this paper, DLC multilayer films consisting of alternating layers of soft and hard carbon films were deposited on Si wafer by a plasma CVD deposition system. Different DLC multilayer films were prepared by varying the sub-layer thickness (from 1000 to 25 nm) and the ratio of hard to soft sub-layer (H/S) thickness (from 1:1 to 4:1). By using a ball-on-disk tribo-tester, the friction and wear properties of the DLC multilayer films were measured in vacuum, O₂ and dry-air environments respectively. By comparing with single-layer DLC film, the change of the multilayer structure has little influence on friction coefficient of the multilayer films. However, the wear rate of the DLC multilayer films is restricted effectively by constructed the multilayer structure in the film. The wear rate of the multilayer films is lower than that of the single film in reactive (O₂ and dry-air) environments. An DLC multilayer film with excellent wear resistance, approximately in the level of 10⁻⁸ mm³/Nm in different environments (dry-air, O₂ and vacuum), is obtained as the DLC multilayer film at a certain sub-layer thickness and ratio.     

Surface acoustic waves in diamond-like carbon films on LiNbO3

Diamond-like carbon (DLC) films have been reliably deposited on YZ LiNbO_3, and surface acoustic wave (SAW) velocity change has been observed. These relatively thin films increase the SAW velocity sufficiently, and they can be used for SAW devices. The cut-off behavior was observed at approximately 538 MHz in a 2-μm-thick DLC film on the LiNbO₃ substrate.     

Characterization of the mechanical properties of diamond-like carbon films

A recently suggested method to measure the elastic modulus of diamond-like carbon (DLC) films was reviewed. This method used a DLC bridge or free overhang which is free from the mechanical constraint of the substrate. Because of the high residual compressive stress of the DLC film, the bridge or the overhang exhibited a sinusoidal displacement on removing the mechanical constraint. Measuring the amplitude and wavelength of the sinusoidal displacement made it possible to measure the strain of the film which occurred by stress relaxation. Combined with independent stress measurement using the laser reflection method, this method allowed the calculation of the biaxial elastic modulus of the DLC film. This method was successfully applied to obtain the elastic properties of various DLC films from polymeric hydrogenated amorphous carbon (a-C:H) to hard tetrahedral amorphous carbon (ta-C) films. Since the substrate is completely removed from the measurement system, this method is insensitive to the mechanical properties of substrate. The mechanical properties of very thin DLC films could be thus measured and then can reveal the structural evolution of a-C:H films during the initial stages of deposition.     

Effect of oxygen plasma treatment on non-thrombogenicity of diamond-like carbon films

The non-thrombogenicity of oxygen-plasma-treated DLC films was investigated as surface coatings for medical devices. DLC films were deposited on polycarbonate substrates by a radio frequency plasma enhanced chemical vapor deposition method using acetylene gas. The deposited DLC films were then treated with plasma of oxygen gas at powers of 15 W, 50 W, and 200 W. Wettability was evaluated by water contact angle measurements and the changes in surface chemistry and roughness were examined by X-ray photoelectron spectroscopy and atomic force microscope analysis, respectively. Each oxygen-plasma-treated DLC film exhibited a hydrophilic nature with water contact angles of 11.1°, 17.7° and 36.8°. The non-thrombogenicity of the samples was evaluated through the incubation with platelet-rich plasma isolated from human whole blood. Non-thrombogenic properties dramatically improved for both 15 W- and 50 W-oxygen-plasma-treated DLC films. These results demonstrate that the oxygen plasma treatment at lower powers promotes the non-thrombogenicity of DLC films with highly hydrophilic surfaces.