ICMCTF2014 Session B3: Deposition Technologies for Diamond Like Coatings

Research and development of nanostructured materials with improved, tailor-designed properties is a fundamental need for the growth and advance of automotive, aerospace, chemical, biomedical and electronic industries among others. Diamond-like carbon (DLC) films have attracted significant attention recently due to their low friction, high hardness, high elastic modulus, chemical inertness, biocompatibility, and high wear resistance. These films are mostly obtained by plasma decomposition of a hydrocarbon-rich atmosphere. The major disadvantage of hard DLC coatings deposition and, therefore, their technical applications is that there is often a relatively low adhesion of these films on metallic substrates caused by very high total compressive stress on these coatings. In order to overcome the high residual stress and low adherence of DLC films on steel substrates, a thin amorphous silicon interlayer was deposited as an interface.

Amorphous silicon interlayer and DLC films were grown by employing an asymmetrical bipolar pulsed-DC PECVD system, using silane and acetylene atmospheres, respectively. DLC films were analyzed according to their microstructure, mechanical, and tribological properties as a function of self-bias voltage. The chemical composition and hydrogen content of the films were probed by means of Raman scattering spectroscopy. The total stress was evaluated through the measurement of the substrate curvature, using a profilometer, while nanoindentation experiments helped determine the films' hardness. The friction coefficient and critical load were determinated by using a tribometer. The corrosion resistance was evaluated by electrochemical potentiodynamic polarization techniques.

The use of an amorphous silicon interlayer improved the DLC films deposition onto steel substrates, producing good adhesion, low compressive stress, and a high hardness. The composition, microstructure, mechanical and tribological properties of the DLC films were strongly dependent on the self-bias voltages. All tests confirmed the importance of the intensity of ion bombardment during film growth on the mechanical and tribological properties of the films. Experimental results suggested that the surface roughness and hardness of the films are often closely related to the friction of the surface and to the wear resistance of the coatings. Also, they demonstrated that DLC coatings improve steel electrochemical corrosion resistances. The deposition rates combined with mechanical, tribological, and corrosion resistance properties of these films make them suitable candidates for specific industrial applications.

Among other coatings and surface treatments DLC (diamond-like carbon) coatings play an important role due to a unique combination of excellent properties like high hardness, high wear resistance and low friction coefficients.

Diamond-like carbon (DLC) coatings, especially amorphous hydrogenated a-C:H and metal containing a-C:H:Me coatings are well established in the automotive industry as a solution to reduce or even eliminate wear problems of highly loaded components. With the actual strong focus on reducing fuel consumption and minimizing the CO2 emission the reduction of friction losses becomes a major intent. DLC coatings have a high potential for significant friction reduction both under dry and lubricated operation.

The term DLC describes a coating material class covering a broad range of physical and chemical properties. Well defined combinations of coating properties can be adjusted using different deposition processes and parameters. A brief overview about the variety of DLC coatings, the corresponding deposition techniques and the coating properties and application examples will be presented.

Some objectives of further development are adhesion improvement, increase of hardness and high coating quality on complex geometries. A promising technique to fulfill these requirements is the deposition of a-C:H coatings by reactive d.c. magnetron sputter deposition from a graphite target with acetylene as reactive gas. The a-C:H coating deposition was carried out working with varying parameters of substrate bias (d.c. and pulsed d.c.), acetylene flows and ion current densities. The hardness clearly depended on the hydrogen contents and the hardest coatings (up to 50 GPa) could be prepared at hydrogen contents of about 10 atom %.

In addition further results of recent developments of effective DLC-based coating solutions will be presented.

The depositions using this new technology were demonstrated at an industrial scale unbalanced magnetron sputtering equipment with 4 sputtering cathodes, 6-axes planetary rotary substrate table and with effective loading space of 450mm in diameter and 400mm in height. After etching and interlayer formation, depositions of 1-3micron thick DLC coatings were carried out under 1-3 Pa of C2H2 and by applying MF-AC power of a few kW range. The deposition rate of 1-5microns/Hr was demonstrated on substrates on 3-fold rotation fixtures under full load conditions. The hardness of DLC coating was measured as 20 to 28GPa and the tribological properties comparable to DLC coatings from the other deposition technologies were obtained.