P. Bohac

Hardness, intrinsic stress, and structure of the a-C and a-C:H films prepared by magnetron sputtering

Abstract The 1.5-μm thick carbon films prepared by magnetron sputtering of a carbon target in Ar, Ar +CH 4 and Ar+O 2 gas mixtures show that the higher their intrinsic stress, the higher their microhardness and the lower their resistivity. The a-C films obtained without ion bombardment (unbiased; the mean free path of sputtered C atoms larger or equal to the cathode–anode distance) show microhardness up to 25 GPa. The biased a-C films (i.e. with ion bombardment) achieve the highest microhardness (up to 50 GPa) and the lowest resistivity (0.01 Ω cm). An increase in Ar pressure, or the optional addition of O 2 , results in a decrease in the microhardness and intrinsic stress and an increase in the film resistivity. In comparison to a-C films, by adding CH 4 to Ar up to certain limit, the microhardness and intrinsic stress of these a-C:H films increase and subsequently decrease steeply. It was specified by analysis of the electron diffraction patterns of thin films (30–60 nm) deposited under the same conditions that the radius of the first co-ordination sphere of C atoms for all the films is in a good agreement with the value for graphite. The prime interplanar distance for biased a-C films is considerably lower than that for unbiased ones and for graphite. Our data indicate the sp 2 -bonded carbon structure of the deposited hard carbon films, in which the prime interplanar distance is reduced due to intrinsic stress. Thus, it is more suitable to explain the hardness origin as a consequence of the film nanostructure rather than the presence of sp 3 bonds .

Thermal stability of microhardness and internal stress of hard a-C films with predominantly sp2 bonds

Abstract Annealing in vacuum at temperatures up to 820 °C was used to study the thermal stability of mechanical properties of magnetron-sputtered thick (approx. 1.5 μm) a-C films. A predominance of sp2 bonds was characteristic for all these films. The microhardness, internal stress, electron diffraction, Raman and optical spectra of three sets of films with different initial microhardness (H≈50, 20 and 10 GPa, respectively) were compared. Annealing of the hardest film up to 500 °C led to an increase in microhardness accompanied by a decrease in internal stress. Internal stress did not relax completely for hard films, even after annealing up to 820 °C, and the microhardness remained rather high (∼40 GPa). Both the high internal stress and the specific film nanostructure are responsible for the high microhardness of these sp2-bonded films.

Study of the structure of hard graphite-like amorphous carbon films by electron diffraction

AbstractThe structure of thin hard carbon films was investigated by transmission electron diffraction using a filter of inelasticallyscattered electrons. It was shown for the first time that in the amorphous graphite-like carbon films under considerable compressivestress the interplane distance d 002 could be shortened down to approximately 0.300 nm. Also, substantiation was given for asimple procedure for the estimation of the first coordination sphere radius in amorphous carbon films. This radius was determinedfrom the maximum of the corresponding diffraction halo. It was shown that this procedure gave the radius of the first coordinationsphere very close to its real value for graphite-like structure but did not work so well for a diamond-like structure. 2002Elsevier Science B.V. All rights reserved. Keywords: Graphite-like carbon; Amorphous film; Structure; Electron diffraction 1. IntroductionHigh hardness of hydrogen-free carbon films is usu-ally linked with the presence of sp -like bonds and these

Interaction of oxygen with a-C:H, a-C films and other carbon materials

Magnetron sputtered films of a-C:H, a-C, as well as of a-COx, and glassy carbon and graphite were annealed in air and vacuum at different temperatures (up to 770 °C). The a-COx films (new type of amorphous graphite-like films) contain at most 18–22% of atomic oxygen depending on the way of their preparation. Annealing in air results in the removal of H from a-C:H films and in the incorporation of O in both a-C:H and a-C films as evidenced by IR spectra. Raman spectra show the up-shift of the G band (to approx. 1605 cm−1) in a-C and a-C:H films. Annealing of a-COx films at 770 °C in vacuum shifts the G band down to ∼1595 cm−1 and decreases oxygen content sharply down to 5 at.%. Annealing of graphite and glassy carbon in air at 600 °C does not change their Raman spectra. The electron diffraction patterns for the a-COx films are almost the same as for the a-C films, implying that oxygen occupies mainly the positions along the edges of graphite layers in every cluster and acts as an etchant. Its action decreases the intrinsic stress and leads to short-range order increase.

Oxidation of graphite-like carbon films with different microhardness and density

Abstract Evolution of microhardness, internal stress and thickness of three types of carbon (a-C) films (#1–3) with predominantly sp 2 bonds with different microhardness ( H =60, 24 and 20 GPa) and density after annealing in air for 1 h at temperatures of 100, 200, 300, 350 and 400 °C was investigated. The stable behavior of these quantities up to 400 °C demonstrated only superhard films from set #1. The films from set #2 ( H =24 GPa) showed the noticeable decrease in these values after annealing at 350 °C. The largest decrease of all the measured values after annealing at 350 °C showed the films from set #3. The films from series #2 and 3 fully vaporized after annealing at 400 °C. A considerable decrease has been observed not only in film thickness, but also in the density of films from set #3 after annealing at temperatures more than 300 °C, that indicates a modification of the film structure. Significant penetration of oxygen into film body without a loss of film thickness was observed in the range of 250–300 °C only for film #3. It was registered by the increase of resistivity, D band intensity in Raman spectra and sharp increase of the IR transmittance. Absorption bands at approximately 1750 and 1610 cm −1 which correspond to the vibrations of CO and CC appear in these IR spectra and simultaneously the G band in Raman spectra shifted to 1605 cm −1 . The film obtained has a graphite-like structure and its composition corresponds approximately to formula CO 0.15 CO 0.18 . This structure is very unstable and very quickly vaporized starting from 350 °C.

Degradation and stress evolution in a-C, a-C:H and Ti-C:H films

Abstract Presented in this contribution are some observations of the stress evolution and degradations of diamond-like a-C, a-C:H and Ti-C:H (10–20 at.% of Ti) films deposited by magnetron sputtering of the C or Ti target in the Ar or Ar+CH 4 gas mixture. The compressive stresses increased during several days for all the Ti-C:H films regardless of their structure (amorphous or a-C:H with TiC nanocrystalline inclusions) and composition (10–20 at.% of Ti). This phenomenon was not observed for a-C and was slightly displayed for a-C:H films having high hydrogen content. The growth of stress was accompanied by a decrease of optical constants, n and k . Simultaneously, Raman spectra and composition of Ti-C:H films remained unchanged. The degradation in ambient air during several hours or days takes place mostly for the films prepared by reactive sputtering in the Ar+CH 4 gas mixture, while a-C films, sputtered from the C target in argon, demonstrated stable behavior despite their high compressive stress level.

Electron Spectroscopy of COx Magnetron Sputtered Films

COx films were deposited by magnetron sputtering of a graphite target within two different routes: (i) by sputtering in an Ar+O2 gas mixture or (ii) by sputtering in pure Ar followed by annealing at 300 °C in air. Surface composition and chemical bonding was studied by X-ray induced photoelectron and Auger electron spectroscopy (XPS/XAES), and electron energy loss spectroscopy (EELS) in a low energy loss region. Oxygen content reached ~20 at. % in the COx layers, independent on the route used. The C 1s spectral line shapes indicate C–O and C=O bonding states, regardless dominating C–C and CHx bonds. Substantial differences were found in the XAES and reflection electron energy loss spectra (REELS) recorded from surface regions of the samples. The both spectra indicate dominating sp2 bonding of carbon atoms in an analyzed volume of COx films sputtered in Ar and oxidized in air at elevated temperature whereas for those deposited in an Ar+O2 mixture the sp3 bonding prevails over sp2.

Utilization of Acoustic Emission in Scratch Test Evaluation

The scratch test is a well-established instrumental method for assessment of the cohesive-adhesive parameters of thin films and coatings. Its evaluation is classically performed using the microscopic analysis of residual scratch and the indenter depth-change record. However, these analysis methods can be insufficient for detection of the very first film-to-substrate adhesion failures. To overcome this difficulty, an independent method of detection of acoustic emission signals can be employed. The detection system of acoustic emission, developed in our laboratory, utilizes a special holder and continuous recording during the whole scratch test. The piezoelectric sensor with 2 MHz sampling rate and sophisticated software allow a thorough post-process analysis of recorded acoustic emission signal. Failure events can be observed on microsecond scale and their frequency spectra can be evaluated.The demonstration of the acoustic emission probe detection capability is performed on the model layers. Comparison of the acoustic emission record to residual scratch image and indenter depth-change record shows a detection sensitivity of the method. Analysis of failure mode dynamics at the appropriate time scale is outlined.

Effect of Au drops on the phase velocity dispersion of Rayleigh waves in Si crystals

Effect of Au drops on the phase velocity dispersion of Rayleigh waves in Si crystals We have measured the phase velocity dispersion of the Rayleigh waves on a (001)Si surface covered by a thin island film of Au. The effect of the film on the propagation of the acoustic surface waves is explained using a model of discontinuous film having zero elastic moduli. The experimental data are in good coincidence with the proposed model.