Erik Dekempeneer

Low friction and wear resistant a-C:H/a-Si1-xCx:H multilayer coatings

Abstract Amorphous hydrogenated carbon a-C:H and silicon alloyed a-C:H (a-Si 1− x C x :H) films were combined into a multilayer coating by alternatively adding silane to methane in a conventional capacitively coupled parallel plate r.f. plasma assisted chemical vapour deposition process (r.f. PACVD). The multilayer structures were revealed by Rutherford backscattering spectroscopy (RBS) and their mechanical properties were determined by depth sensing indentation. The tribological behaviour of various multilayer combinations was studied by ball-on-disk testing. The r.f. PACVD technique allowed to produce multilayer coatings with varying composition modulations and sharp interfaces. The mechanical properties of the a-C:H/a-Si 1− x C x :H multilayer coating were situated in between those of its constituents. By optimising the a-Si 1− x C x :H sublayer composition, sublayer thicknesses and thickness ratios, top layer composition and total thickness of the multilayer, a true low friction and wear resistant coating was obtained showing a friction coefficient below 0.1 under a relative humidity of 50% in air together with a wear resistance comparable with that of pure diamond-like carbon.

Study of RF PACVD diamond-like carbon coatings for space mechanism applications

Abstract Currently, sputtered molybdenum disulphide is an established coating in the field of space applications. However, when used in air, molybdenum disulphide loses much of its lubricating power, thus preventing in-air ground testing. In this work, the tribological properties in vacuum, dry N 2 and air of a-C:H films produced by radio frequency plasma assisted chemical vapour deposition (RF PACVD) were studied in order to assess their potential for applications in space. We demonstrated that diamond-like carbon (DLC) films deposited at low bias voltage show lubricating capacity under vacuum conditions. However, the shorter lifetime of these DLC films compared to MoS 2 under vacuum is considered to be an important limiting factor.

Thin tribological coatings: magic or design?

Surface engineering using applied coatings has become a well-established technology and is an extremely versatile means of improving component performance. Tribological coatings add physical properties, such as lubricity, hardness, or corrosion resistance, to lower-valued substrates that improve the overall quality of the component. In addition, the substrate can be designed for strength and toughness to avoid catastrophic failure of the component. This message is often misinterpreted by end-users of coated components, however, in the sense that it is believed that a thin surface coating can provide superior performance to a component made out of a cheap low quality bulk material. In addition, suppliers of tribological coatings often offer coatings on a trial and error basis without a systematic approach, resulting in few successes with many disappointments. In this paper some typical aspects of tribological coatings will be highlighted, emphasising that a coated component must be regarded as a composite structure. Both the coating and the substrate should be optimised taking into account mechanical, structural, chemical, electrical, thermal, and dimensional properties. Techniques are presented for the reliable testing of thin surface coatings, and a generic methodology is suggested using surface coating technology for systematic problem solving of friction and wear cases. Finally, an example of the potential of thin surface films for tribological applications is given.

Tribological and structural properties of amorphous BNC coatings

Abstract Amorphous BxNyCz:H films were prepared in a capacitively coupled r.f. PACVD reactor at deposition temperatures 0.5). As an exception, a striking similarity is found between a-B:H and a-C:H films with respect to their mechanical properties (hardness, Youngs modulus). Nevertheless, these a-B:H films also show a much lower wear resistance in a ball-on-disc test due to different friction mechanisms related to the formation of transfer layers. These results are correlated with the microstructural properties of the coatings as determined from infrared spectroscopy and Raman spectroscopy.

Scratch-resistant transparent boron nitride films

Abstract Transparent boron nitride (BN) coatings were deposited on glass and Si substrates in a conventional capacitively coupled RF PACVD system starting from diborane (diluted in helium) and nitrogen. By varying the plasma conditions (bias voltage, ion current density), coatings were prepared with hardness values ranging from 2 to 12 GPa (measured with a nano-indenter). Infrared absorption measurements indicated that the BN was of the hexagonal type. A combination of glancing-angle X-ray diffraction measurements and simulations shows that the coatings consist of hexagonal-type BN crystallites with different degrees of disorder (nanocrystalline or turbostratic material). High-resolution transmission electron microscopy analysis revealed the presence of an amorphous interface layer and on top of this interface layer a well-developed fringe pattern characteristic for the basal planes in h-BN. Depending on the plasma process conditions, these fringe patterns showed different degrees of disorder as well as different orientational relationships with respect to the substrate surface. These observations were correlated with the mechanical properties of the films.