Vishnubhai Vitthalbhai Patel

Picosecond optical studies of amorphous diamond and diamondlike carbon: Thermal conductivity and longitudinal sound velocity

A picosecond pump‐probe technique is used to measure the room‐temperature thermal conductivity κ and longitudinal sound velocity cl of amorphous diamond (a‐D) and diamondlike carbon (DLC) thin films. Both κ and cl were found to decrease with film hydrogen content. Depending on the film deposition technique, κ is in the range 5–10×10−2 W cm−1 K−1 for a‐D, and 3–10×10−3 W cm−1 K−1 for DLC. Values of cl were found to be in the range 14–18×105 cm s−1 for a‐D, and 6–9×105 cm s−1 for DLC.

Low dielectric constant films prepared by plasma-enhanced chemical vapor deposition from tetramethylsilane

Dielectric films have been prepared by radio-frequency plasma-enhanced chemical vapor deposition from mixtures of tetramethylsilane with oxygen. The films have been characterized as-deposited and after annealing at 400 °C to determine the thermal stability of their properties. Rutherford backscattering and forward recoil elastic scattering have been used for determination of the composition of the films. Optical properties were characterized by Fourier transform infrared spectroscopy and measurements of the index of refraction and optical gap. The electrical properties were measured in a Si/insulator/metal configuration. It has been found that the index of refraction decreases and the optical gap and dielectric constant increase with increasing oxygen concentration in the gas feed. While the materials did not show a mass or composition loss after annealing, the annealing resulted in a reduction of the dielectric constant of the films. Dielectric constants as low as 3.1 have been obtained after annealing t...

Tribochemistry of diamond-like carbon coatings in various environments

Abstract Diamond-like carbon films deposited on silicon wafers by r.f.-plasma-assisted chemical vapour deposition were friction tested in controlled atmospheres in a reciprocating pin-on-plate configuration using a steel sphere. Friction experiments were carried out in a vacuum range from 10 -7 to 50 Pa, in dry nitrogen and in ambient air. Analytical investigations of the wear process were peformed using transmission electron microscopy-electron energy loss spectroscopy and secondary ion mass spectroscopy. In all cases a transfer film was observed to form on the steel pin during the first 100 cycles, associated with relatively high values of the friction coefficient (0.2–0.3) at this stage. Beyond N =100 cycles the friction coefficient decreased to 0.006–0.008 in a vacuum below 10 -1 Pa and to 0.01–0.07 in a vacuum of 10–50 Pa and in dry nitrogen. The shearing ability of the interfacial film depends strongly on the nature of the atmosphere during friction, which affects the surface composition of the sliding counterfaces. A high vacuum is associated with ultralow friction and low wear. A poor vacuum and an inert atmosphere are associated with low friction and moderate wear. Ambient air is associated with relatively high friction and severe wear, coupled with the formation of roll-shaped debris of amorphous carbon containing iron oxide precipitates.

Optical and tribological properties of heat-treated diamond-like carbon

Diamond-like carbon, or DLC, films, prepared by the rf plasma decomposition of acetylene, have been deposited at substrate temperatures of 100 to 250 °C, with the substrate at a negative bias of 80 V dc. The DLC films have been annealed in vaccum at temperatures up to 600 °C for 3–4 h. The optical properties of the as-deposited and annealed films have been characterized by ellipsometry and Fourier transform infrared spectroscopy. The wear resistance of the thin DLC films and their friction coefficients have been characterized by a specially designed tribotester. The stresses in the films have also been determined. No significant differences were found between the index of refraction and IR absorption spectra of the as-deposited films, or films annealed at up to 390 °C. The DLC films begin losing hydrogen after annealing above 390 °C and only sp 2 bond carbon is observed after annealing at 590 °C. The wear behavior of the as-deposited films was identical for all deposition temperatures. DLC films deposited at 250 °C were more stable and could withstand higher annealing temperature than films deposited at lower temperatures, and remained wear resistant after anneling at 390 °C.

Inhomogeneous carbon bonding in hydrogenated amorphous carbon films

Hard‐carbon films prepared by the rf‐plasma decomposition of acetylene have been investigated by high‐resolution 13C nuclear magnetic resonance spectroscopy, x‐ray photoelectron spectroscopy (XPS), and the H(15Nα,γ)C nuclear resonant reaction. It was found that the ratio of sp2:sp3 bound carbon was 1.6, and that virtually all sp3 carbon atoms are, in fact, bound to one or more hydrogen atoms. Bulk layers contain about 40% hydrogen; however, results of the measurements of the hydrogen concentration, as well as those of XPS, confirm that the composition and properties of these carbon films are a strong function of their distance from the initial growth interface, and are spatially varying over the first 40 nm.

The respective role of oxygen and water vapor on the tribology of hydrogenated diamond-like carbon coatings

The tribological behavior of diamond-like carbon coatings (DLC) strongly depends on the chemical nature of the test environment. The present study proposes to explore the influence of water vapor and oxygen on the friction behavior of a hydrogenated DLC coating exhibiting ultralow friction in ultrahigh vacuum (friction coefficient below 0.01). Using a UHV tribometer, reciprocating pin-on-flat friction tests were performed in progressively increasing or decreasing partial pressures of pure oxygen and pure water vapor. The maximum gaseous pressures of oxygen and water vapor were 60 hPa and 25 hPa (1 hPa = 100 Pa), respectively, the second value corresponding to a relative humidity (RH) of 100% at room temperature. It was found that, for the pressure range explored, oxygen does not change the ultralow friction behavior of DLC observed in UHV. Conversely, water vapor drastically changes the friction coefficient at pressures above 0.5 hPa (RH = 2%), from about 0.01 to more than 0.1. Electron energy loss spectroscopy and in situ Auger electron spectroscopy have been performed to elucidate the friction mechanisms responsible for the tribological behaviors observed with the two different gaseous environments. In all cases no significant oxidation has been observed either inside the wear scars or in the wear debris particles. Ultralow friction is systematically associated with a homogeneous carbon-based transfer film. The higher friction observed at partial pressure of water vapor higher than 0.5 hPa, is associated with a thinner transfer film. Consequently friction seems to be controlled by the transfer film whose kinetics of formation strongly depends on the partial pressure of water vapor.

Stresses in diamond-like carbon films

Abstract 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.

Porosity in plasma enhanced chemical vapor deposited SiCOH dielectrics: A comparative study

The low dielectric constant (k) of plasma enhanced chemical vapor deposited SiCOH films has been attributed to porosity in the films. We have shown previously that the dielectric constant of such materials can be extended from the typical k values of 2.7–2.9 to ultralow-k values of k=2.0. The reduction in the dielectric constants has been achieved by enhancing the porosity in the films through the addition of an organic material to the SiCOH precursor and annealing the films to remove the thermally less-stable organic fractions. In order to confirm the relation between dielectric constant and film porosity the latter has been evaluated for SiCOH films with k values from 2.8 to 2.05 using positron annihilation spectroscopy, positron annihilation lifetime spectroscopy, small angle x-ray scattering, specular x-ray reflectivity, and ellipsometric porosimetry measurements. It has been found that the SiCOH films with k=2.8 had no detectable porosity, however the porosity increased with decreasing dielectric con...

Wear-resistant fluorinated diamondlike carbon films

Fluorinated diamondlike carbon (FDLC) films have been deposited on Si wafers by rf plasma-assisted chemical vapor deposition, under a variety of conditions. The films have been characterized by FTIR and index of refraction measurements, RBS and FRES analysis for determination of film composition, and stress measurements from the bending of the wafers by the deposited films. Friction and wear measurements have been performed using pin-on-flat and pin-on-disk testers in ambient air, at maximum Hertzian contact pressures ranging from 320 to 1100 MPa. By adjusting the deposition parameters, the properties of the FDLC films could be changed from soft films, with no significant wear resistance, to films containing more than 20% F and having wear resistance comparable to unfluorinated DLC. The tribological properties of the FDLC films are discussed in relation to their physical properties, as determined by the deposition conditions.

Tribological properties of diamond-like carbon and related materials

Abstract Amorphous hard carbon films are often called diamond-like carbon (DLC) owing to their hardness, extreme wear resistance, and generally low friction coefficients. The tribological properties of DLC span a wide range of values depending on the deposition condition but they have several common features: friction coefficients which are extremely low in ultrahigh vacuum or dry nitrogen but high in dry oxygen, and very high wear resistance in vacuum and in ambient atmosphere, depending however on the deposition conditions. Loss of hydrogen from hydrogenated DLC results in loss of wear resistance and an increase in the friction coefficient in vacuum. The paper will present the tribological properties of pure DLC films and related materials, in different evaluation environments. The properties will be related to the deposition method and conditions and the mechanisms proposed for explaining the very low friction coefficients of DLC will be discussed. The effects of thermal annealing on the tribological properties of DLC will also be addressed.