ICMCTF2010 Session D2-2: Diamond and Diamond-Like Carbon Materials

Diamond-like carbon ( DLC), also known as amorphous hydrogenated or non-hydrogenated carbon (a-C:H, a-C), is a class of coating materials with excellent mechanical, tribological and biological properties. Depending on the deposition conditions and by the addition of other elements into the DLC these properties can be varied within a certain range. In the present work DLC coatings were deposited by employing DC magnetron sputtering of graphite targets, rf magnetron sputtering of titanium and silicon targets, respectively, in an argon/acetylene atmosphere and by the use of a special ion gun (anode layer source (ALS)) working with acetylene as carbon precursor. The structure and the topography of the coatings deposited onto silicon, glass and PET substrates were investigated by Raman spectroscopy and atomic force microscopy. The surface energy and the contact angle of the different coatings were determined by the sessile drop technique. Protein adsorption was investigated by fluorescence spectroscopy, fluorescence microscopy and a quartz crystal micro-balance method (QCM) using a bovine serum albumin.

For a-C:H coatings deposited by the ALS the discharge voltage varied from 1 kV to 3 kV was found to be the critical parameter to control the bonding structure and sp³ content in the coatings. The metal content of the doped DLC coatings was adjusted by varying the sputtering power density and the C₂H₂/Ar ratio.

A newly developed filter module will be presented which was successfully developed from the laboratory state into an industrially usable LAM source. The principle of this filter for the separation of the charged plasma from macro- and micro-particles will be demonstrated. The advantages of this kind of filtering are the unlimited linear extension of the deposition area, a simple construction as well as the high efficiency. The transparency of the filter has been measured to be more than 60% in the case of super hard carbon (ta-C) film deposition. It is shown that the starting surface quality of components will be hardly changed by coating with ta-C up to a film thickness of more than 2 µm.

Structural properties of tetrahedral amorphous carbon (t-aC) deposited by the filtered cathodic vacuum arc (FCVA) technique is investigated. The films were prepared with 5 ms current pulses of 180 A and frequency of 3 Hz. The deposition substrate was polarized in a range of 0 to - 500 V bias voltage. Argon gas was incorporated in a series of films using an ion gun source to simultaneously bombard the films with a beam of argon ions with energy in the 0-180 eV range. Argon concentration in the films was determined by RBS measurements. A study of argon effusion, realized in the range from room temperature up to about 1000°C, shows that the structure of the films depends on the argon ion bombardment energy. The density of the films, also determined by RBS, was in the 2.5 to 3.0 g/cm³ range, depending on the substrate bias voltage. The films are extremely stressed (up to about 10 Gpa) as determined using the bending beam technique, which limited the deposition of films to about 50-70 nm. It was observed that the stress reduces significantly as a function of annealing temperature. Nanohardness measurements show that the hardness of the films prepared in the 100-200 V bias voltage is about 30 Gpa. This value is underestimated since the films are very thin. Raman measurements were used to investigate the structure of the films as a function of the bias voltage and annealing temperature.

Diamond-like amorphous carbon (DLC) surfaces have been patterned by a combination of colloidal lithography and pulsed-DC plasma-enhanced chemical vapour deposition. A self-assembled monolayer of silica sub-micron particles (~ 300 nm) was deposited on monocrystalline silicon (~5 cm²) by Langmuir-Blodgett, in order to act as sacrificial template for hole-mask lithography. A sub-micrometric pattern was generated on the substrate via ion beam etching (argon) of the colloid samples (550 eV), which was held at different incidence angles. As a result, in-plane anisotropy in the sub-micron range was generated. The fabrication was completed by plasma deposition of DLC thin film on the textured substrates. The samples were morphologically characterized by SEM and AFM. The surface properties evidenced the formed anisotropy, as shown by the directional dependences of friction coefficient (nanotribometer) and wettability (water contact angle). This fabrication technique finds applications in the industry of micromechanical devices, anisotropic tribological coatings, nanoimprint lithography, microfluidics, photonic crystals, and patterned surfaces for biomedicine.

Carbon based coatings, particularly diamond like carbon (DLC) surface films are known to mitigate aluminum adhesion and reduce friction at room temperature. This attractive tribological behaviour is useful for applications such as tool coatings for aluminum forming and machining. However, for those operations that are performed at elevated temperatures (e.g., hot forming) or generate frictional heat during the contact (e.g., dry machining) coatings are expected to maintain their tribological properties at high temperatures. The candidates for these demanding applications include boron carbide (B₄C) and DLC coatings reinforced with nanoparticles. An understanding of the micromechanisms of friction, wear and adhesion of carbon based coatings against aluminum alloys at high temperatures will help designing coatings with improved high temperature tribological properties. With this goal in mind, this study was focused on B₄C and DLC reinforced with WC nanoparticles coatings sliding against a 319 grade cast aluminum alloy by performing pin-on-disk experiments at temperatures up to 400°C. Experimental results have shown that the 319 Al/B₄C tribosystem generated coefficient of friction (COF) values ranging between 0.51 and 0.65, independent of the testing temperature However, increased amounts of aluminum adhesion was detected inside the B₄C coating’s wear tracks at elevated temperatures. Focussed ion beam (FIB) milled cross-sections of the wear tracks revealed that the coating failed due to shearing along the columnar grains. The 319 Al/WC-DLC tribosystem generated much lower COFs; in fact, the room temperature COF of 0.15 decreased to 0.06 at 200°C, in contrast WC-DLC wear increased slightly with the temperature. The wear and adhesion mechanisms of these coatings were further assessed using FIB, HRTEM and FTIR analyses and will be discussed in the meeting.

Surface roughness and dynamic growth behavior of TiC/a-C nanocomposite films, deposited by non-reactive pulsed-DC (p-DC) sputtering of graphite-targets, were studied using atomic force microscopy, cross-sectional scanning and transmission electron microscopy. Upon increasing the intensity of concurrent ion impingement by raising the frequency of p-DC sputtering, a transition from dynamic roughening to dynamic smoothing was revealed. Rough TiC/a-C films were intentionally grown on smooth surface at low pulse frequency (100 kHz) to simulate a rough finishing of industrial substrates and then rapid smoothening of such initial rough films at higher pulse frequency (350 kHz) was observed. From detailed analyses of surface morphology and growth conditions it was concluded that a transition in dominating growth mechanism from geometric shadowing at p-DC 100 kHz to surface diffusion driven by impact-induced atomistic downhill flow process due to enhanced impingement of Ar+ ions at 350 kHz occurs. It was shown that rapid smoothing of initially rough surfaces with RMS roughness ~ 6 nm to < 1 nm can be effectively achieved with p-DC sputtering at 350 kHz pulse frequency, leading to a transition from a strong columnar to a columnar-free microstructure. The observed dynamic smoothing phenomenon was applied to obtain ultra-smooth (RMS roughness ~ 0.19 nm) and ultra-low friction (µ~0.05) TiC/a-C:H nanocomposite films on rough steel substrates by p-DC sputtering of Ti-targets in an argon/acetylene atmosphere.