Deposition of DLC film with adhesive W-DLC layer on stainless steel and its tribological properties

Tribological properties of a diamond-like carbon (DLC) coating with an adhesive tungsten-containing DLC (W-DLC) layer were investigated. The coatings were deposited onto AISI316L steel substrates and Si wafers using plasma enhanced chemical vapor deposition and tungsten co-sputtering of the metal target. Methane and argon gases were used as the precursor of the coatings. In this study, three types of coatings were evaluated: DLC/W-DLC on AISI316L (DLC-1), DLC/W-DLC on Si wafer (DLC-2), and DLC on Si wafer (DLC-3). The structural characterizations were performed by transmission electron microscopy and tapping mode atomic force microscopy. At the boundary between the W-DLC layer and the AISI316L substrate, microscopic decohesion or delamination was not observed. The surface roughness of the DLC-1 coating was greater than that of the DLC-2 coating. This feature was derived from the surface roughness of the initial surface of the AISI316L substrate. Friction tests were performed using a rotation-type ball-on-flat configuration tribometer. The observed friction of the DLC-1 coating was unstable compared with the DLC-2 or DLC-3 coatings. This was due to wear debris which had risen to the friction surface resulting in unstable friction on the DLC-1 coating. During the friction studies, the top DLC layer was removed from the adhesive W-DLC layer because the adhesive strength at this part was not enough. In order to achieve the low and stable friction of the DLC coating with the W-DLC layer on AISI316L, it is necessary to improve the smoothness of the surface and the adhesion between the DLC coating and the W-DLC layer.     

Microstructural design for fabrication of strain sensor utilizing tungsten-doped amorphous carbon coatings

Tungsten-containing diamond-like carbon (W-DLC) coatings were fabricated with an aim of producing new strain sensor applications. W-DLC coatings with various metal concentrations were prepared by chemical vapor deposition and DC magnetron co-sputtering of a tungsten metal target. The electrical resistance was shown to be proportional to the value of compression and tensile strain. The expression ΔR/R = Kε, was applied to the experimental results in order to determine the gauge factor of each coating, and this gauge factor varied according to the metal concentration. Transmission electron microscope was employed for structural analyses and it was shown that the structure of the W-DLC coatings consisted of nanometer size grains dispersed into an amorphous carbon host matrix. The electrical properties of the W-DLC coatings were simulated using a model based on a composite insulator-metal cluster structure. These results were in agreement with the experimental ones, and a high gauge factor can be obtained with low metal concentration or increase in size of metal cluster.     

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.     

Me-DLC films as material for highly sensitive temperature compensated strain gauges

In technical applications strain gauges are widely used. Apart from conventional polymer foil based strain gauges that are glued to the work piece surface, sputtered strain gauges are already commercially used in special applications. Those sputter strain gauges are typically made of NiCr alloy and the sensor layer is as sensitive to strain as the ones used in the glued strain gauges with a gauge factor of 2, but neglecting problems of creeping and swelling of the involved polymer materials. Diamond-like carbon (DLC) films offer significantly higher strain sensitivity, but usually they are also very sensitive to temperature effects. Using metal doped diamond-like carbon (Me-DLC), higher strain sensitivity than conventional metal based systems, in combination with thermal compensation, is possible. The influence of different process parameters on the gauge factor and temperature coefficient of resistance (TCR) of DLC and Me-DLC films produced in industrial sputtering systems was investigated. Gauge factors up to 13 in combination with a high negative TCR in the range of a few thousand ppm/K were reached with sputtered DLC films. The substrate bias voltage in particular showed a strong influence on the resulting gauge factor of the films. For Me-DLC films different deposition methods (dc and rf sputtering) and various doping metals (Ag, Ni, Ti, and W) were investigated. Using dc sputtering of the Me-DLC films only Ni-DLC showed gauge factors slightly higher than 2. Furthermore, only for Ni-DLC zero crossing of the TCR was observed by variation of the metal content. Using rf excitation especially Ni-DLC films showed gauge factors exceeding values of 15 in combination with a TCR close to zero.     

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.     

Friction-induced physical and chemical interactions among diamond-like carbon film,steel ball and water and/or oxygen molecules

Friction and wear behaviors of diamond-like carbon (DLC) film sliding against steel ball were investigated on a ball-on-disk test rig at different relative humidity (RH) in a nitrogen environment. The worn surface morphology of the steel ball was observed on a scanning electron microscope (SEM), while the chemical states of some typical elements on the worn surface of DLC film were investigated by means of X-ray photoelectron spectroscopy (XPS). The result showed that the DLC film recorded continuously increased friction coefficient and wear rate with increasing relative humidity from 5% to 100%. In dry nitrogen (RH < 5%), thick and compact transferred carbon-rich layer was observed covering on the worn surface of steel ball, while the chemical states of the original and worn film surface showed no obvious change. In humid nitrogen, distinct changes of the chemical states on the worn surface of DLC film took place, indicating that tribochemical reactions such as the oxidation of DLC film and the interactions between DLC film and steel ball were involved in the friction process. Therefore, it was proposed that the friction and wear behaviors of DLC film were dependent on the friction-induced physical and chemical interactions among DLC film, steel ball and water and/or oxygen molecules. The roles of environment in the friction and wear behaviors of DLC film were discussed in terms of the friction-induced physical and chemical interactions.     

Ultrasensitive strain sensors made from metal-coated carbon nanofiller/epoxy composites

A set of resistance-type strain sensors has been fabricated from metal-coated carbon nanofiller (CNF)/epoxy composites. Two nanofillers, i.e., multi-walled carbon nanotubes and vapor growth carbon fibers (VGCFs) with nickel, copper and silver coatings were used. The ultrahigh strain sensitivity was observed in these novel sensors as compared to the sensors made from the CNFs without metal-coating, and conventional strain gauges. In terms of gauge factor, the sensor made of VGCFs with silver coating is estimated to be 155, which is around 80 times higher than that in a metal-foil strain gauge. The possible mechanism responsible for the high sensitivity and its dependence with the networks of the CNFs with and without metal-coating and the geometries of the CNFs were thoroughly investigated.     

A 400 μm thickness diamond-like carbon preparation

A 400 μm thick diamond-like carbon (DLC) film was prepared on an aluminum alloy (A5052) substrate by a hybrid process of plasma-based ion implantation and deposition using toluene as a precursor gas. The plasma-based ion implantation during deposition relaxed the residual stress in DLC film to almost 0, indicating the production of stress-free DLC. The carbon ion implantation from the methane and acetylene plasmas to the substrate surface, prior to deposition, resulted in an interface graded in carbon composition as well as the formation of amorphous-like structure at the carbon ion-implanted layer that should work as a buffer for stress-relaxation. As a result, a supra-thick DLC film more than 400 μm in thickness was prepared on the substrate.     

Synthesis of amorphous carbon films by plasma-based ion implantation with simultaneous application of DC and pulse bias

Carbon films were synthesized on a Si wafer by simultaneous application of pulse bias and DC bias by a plasma-based ion implantation system using an electron cyclotron resonance (ECR) plasma source with a mirror field. The relationship between the pulse biasing voltage and the properties of carbon films was investigated. The hardness and tribological properties of the carbon film improved as the pulse bias voltage was decreased from −10 kV to −2 kV. Diamond-like carbon (DLC) films with a low friction coefficient were formed by simultaneous application of a low pulse bias voltage, such as −2 kV, and a DC bias. During the friction test of the DLC film, excellent tribological properties were observed under a high conducted load, such as 20 N, which shows that not only the friction coefficient but also the durability during the friction test was improved. The improvement of the tribological property was attributed to the formation of a mixed layer at the interface between the DLC film and the Si substrate.     

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.     

Temperature Dependence of Diamondlike Carbon Film Tribological Characteristics

The tribological characteristics of a diamondlike carbon (DLC) film deposited on high-speed steel were investigated systematically by using a ball-on-flat reciprocating tribometer over a range of temperatures (from −40° to 20°C). The results indicated that the temperature dependence of the DLC film's tribological behavior was associated with the counterpart material. DLC presented favorable tribological behavior while sliding on itself. However, when a steel ball slides against the DLC film, there is evidence that the heat generated has a significant impact on friction and wear. Microanalysis of wear tracks on the films showed that multiple wear mechanisms took place during testing. At higher temperatures, material transfer dominated the wear behavior, while fatigue-induced microcracking was the predominant wear mechanism at low temperatures. Raman analysis indicated that the DLC film was mechanically worn rather than removed by tribochemical interactions between the friction pairs.     

X-ray reflectivity analysis on initial stage of diamond-like carbon film deposition on Si substrate by RF plasma CVD and on removal of the sub-surface layer by oxygen plasma etching

The multi-layered structure of thin diamond-like carbon (DLC) films was investigated by X-ray reflectivity (XRR) analysis. Thin DLC films were deposited on Si substrate by RF plasma chemical vapor deposition (CVD) from acetylene source gas with short duration of plasma operation from 0.08 to 4.99 s. It was confirmed from XRR analysis that the thin DLC film on Si substrate had 3 layers consisting of a subsurface layer on the grown surface, a mixing layer at the interface to Si substrate, and a bulk-DLC layer sandwiched between the 2 layers. The 3 layers had been formed in 0.08 s at beginning of deposition with distinctive bulk-DLC layer of 1.7 nm thick already appeared due to extremely higher deposition rate only at the initial stage of CVD. The thickness of bulk-DLC layer increased with increasing CVD duration while both the mixing layer of higher density and the sub-surface layer of extremely low density continuously existed. By oxygen plasma etching, it was confirmed by XRR analysis that the sub-surface layer was clearly removed and another layer of lower density than the bulk DLC appeared.     

Structure of the silver containing diamond like carbon films: Study by multiwavelength Raman spectroscopy and XRD

In the present study structure of silver containing diamond like carbon (DLC:Ag) films deposited by reactive magnetron sputtering was investigated by X-ray diffractometry (XRD) and multiwavelength Raman spectroscopy. In the case of the DLC:Ag films containing low amount of silver, crystalline silver oxide prevails over silver. While at higher Ag atomic concentrations formation of the silver crystallites of the different orientations was observed. Surface enhanced Raman scattering (SERS) effect was detected for high Ag content in the films. For UV excited Raman spectra sp³ bonded carbon related Raman scattering T peak at ~ 1060 cm^− 1 was detected only for the films with the highest amount of silver (34.3 at.%). The dependence of the Raman scattering spectra parameters such as position of the G peak, G peak full width at half maximum (FWHM(G)), D/G peak area ratio on Ag atomic concentration in DLC:Ag film as well as Raman scattering spectra excitation wavelength were studied. The dependence on Ag amount in film was more pronounced in the case of the Raman scattering spectra excited by higher wavelength laser beam, while in the case of the spectra excited by 325 nm and 442 nm laser beams only weak dependence (or no dependence) was observed. Overall tendency of the decrease of the dispersion of the G peak with the increase of Ag atomic concentration was found. Thus sp³/sp² bond ratio in DLC:Ag film decreased with the increase of Ag atomic concentration in the films.     

A diamond-like carbon film as a self-alignment layer for LCDs

Diamond-like carbon (DLC) films without H deposited with a DC magnetron in-line sputtering system have shown sufficient self-alignment properties towards liquid crystals (LC). The DLC film was successfully used as an alignment layer for LC without any alignment processes such as rubbing or atomic beam bombardment or UV irradiation. From the observations of the test cells, the LC director was aligning parallel to the substrate movement direction of the in-line sputtering system. The alignment property of the DLC films has been demonstrated by a contrast ratio value of close to 200. It appears that DLC film may have anisotropic structure that is interacting with LC to align.     

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.     

Spinose carbon nanotubes grown on graphitized DLC film by low frequency r.f. plasma-enhanced chemical vapor deposition

Spinose carbon nanotubes (SCNTs) are grown on silicon substrates covered with diamond-like carbon film and iron catalyst film (Fe/DLC/Si structure) by low frequency r.f. plasma-enhanced chemical vapor deposition (LFRF-PECVD). During the pre-treatment of the Fe/DLC/Si substrate, there are three processes happened, namely, iron film spalled to small islands, the DLC film graphitized, and the iron island reacted partially with the graphitized DLC (GDLC), which can be deduced from the Raman spectroscopy and SEM pictures. SCNTs film grew from C₂H₂---H₂ mixture under low plasma density. The good contact of carbon nanotube with GDLC film was acquired by the accumulation of the graphite sheets and the reaction between the iron particles and GDLC film. The homogeneous spines with the length of approximately 15 nm and the thickness of <5 nm burgeoned from the defects at the wall of carbon nanotube and distributed uniformly, which were in fact thin bent or rolled-up graphite sheets.     

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.     

The effect of annealing on mechanical and tribological properties of diamond-like carbon multilayer films

The diamond-like carbon (DLC) multilayer films have been deposited by plasma CVD deposition onSi wafer substrate. The deposited films have then been post-annealed in vacuum at 250 °C for 2 h. Changes in internal stress, hardness, critical load, friction coefficient and wear have been investigated toassess the influence of annealing on mechanical and tribological properties of DLC multilayer films. At the same time, DLC single layerfilms are also deposited and annealed in the same method for a comparison.The results show that there is 28–33% decrease in internal stress and 10–13% decrease in hardness of theDLC single layer films after the anneal treatment. However, for the DLC multilayer films, there is 41–43% decreasein internal stress and less than 2% decrease in hardness. In addition, the annealed DLC multilayer filmhas the same friction and wear properties as that un-annealed film. This result indicates that the anneal treatment isan effective method for the DLC multilayer films to reduce the internal stress and to increase the critical load.The by-effect of the annealing, decrease of hardness and wear resistance of the multilayer film, can be restrictedby the multilayer structure.     

Femtosecond-laser-induced nanostructure formation and surface modification of diamond-like carbon film

This paper reports the pump and probe experiment for in situ reflectivity measurements in the femtosecond laser ablation that brings about nanoscale modification of diamond-like carbon (DLC) film. The characteristic reflectivity changes observed demonstrate that the formation of periodic nanostructure is preceded by a change in bonding structure of DLC in the ablation at low fluences. We have observed a coherent nonlinear wave-mixing signal that can resolve the ultrafast interaction processes for the nanoscale modification on the film surface. Based on the results obtained, a model of the interaction process is proposed.     

Interfacial electrical properties of diamond-like carbon/gallium arsenide heterostructures

Deposition onto GaAs of dielectric film with good interfacial properties is difficult owing to the high surface state densities at the GaAs surface. Study of diamond-like carbon (DLC)/GaAs heterostructures is worthwhile because of the advantageous properties of insulating DLC film and the low temperatures involved in its preparation. In this paper, we compare the electrical interfacial properties of DLC/GaAs with those of DLC/Si and DLC/Ge structures. The DLC films were prepared by r.f. plasma-assisted chemical vapour deposition using a mixture of propane and n-butane. Capacitance measurements were taken in the frequency range 400 Hz to 1 MHz. The interface trap density distribution N_ss(E) was estimated for the DLC---Si interface using Terman's method. Compared with the DLC---Si and DLC---Ge interfaces, the DLC---GaAs interface shows qualitatively different behaviour. There is an indication that the interfacial layer at the DLC---GaAs interface is more conducting than the DLC film, giving rise to Maxwell-Wagner type relaxation. As a result, the effective permittivity of the DLC film is one order of magnitude higher. The formation of this interfacial layer could be related to pre-etching of the GaAs substrate using argon plasma.