J. Meneve

Abrasion resistant low friction diamond-like multilayers

Over the past few years, we have investigated plasma deposited amorphous hydrogenated carbon (DLC) films modified with B, N and Si dopants. Whereas, all three elements have the advantage to induce stress relief in the films (combined with hardness reduction), Si appeared to be the most interesting alloying element for tribological applications (friction and wear). Following this work, multilayer coatings consisting of a stack of DLC layers alternated with Si-doped DLC films and more recently, diamond-like nanocomposite (Dylyn, a-C:H/a-Si:O) films were developed. A major advantage of the multilayer structure is the stress relief enabling the deposition of thick layers (10 μm) without any loss of good adhesion properties. These thick multilayers show improved abrasion resistance as well as extremely low friction properties (friction coefficient < 0.1 independent of the relative humidity). This paper discusses the mechanical and tribological properties of these multilayer coatings and presents a comparison to various other layer systems (TiN, CrN and hard Cr).

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.

Friction and wear behaviour of amorphous hydrogenated Si1−x Cx films

Amorphous hydrogenated Si1−x Cx films (a-Si1−x:H) (x=0.65-1) were deposited by r.f. plasma-assisted chemical vapour deposition. Their friction and wear properties were investigated with the help of a conventional ball-on-disk apparatus. These results are correlated with chemical (Si to C atomic ratio), structural (laser Raman Spectroscopy) and mechanical (internal stress, hardness and elastic modulus) properties. With increasing silicon content, the film material evolves from diamond-like carbon (DLC) to amorphous SiC-like material. Simultaneously, the values for hardness, elastic modulus and internal stress are reduced by 15%–30%. For a low normal load (1 N) in the ball-on-disk test, a-Si1−x:H films (0.7<x<0.9) show a very low wear rate of both the film and the counterbody, combined with a steady state low friction coefficient of 0.05 in a humid ambient atmosphere. For the higher loads (5 and 10 N), however, this low friction coefficient lasts for only a relatively short time. In this case, the harder DLC films perform tribologically better because of their higher wear resistance, low wear rate of the counterbody and generally low friction coefficient between 0.15 and 0.35 in a humid ambient atmosphere.

Structural, mechanical and tribological properties of plasma-assisted chemically vapour deposited hydrogenated CxN1−x:H films

Abstract Amorphous hydrogenated carbon nitride (a-C x N 1-x : H) films were prepared at low deposition temperatures (below 150°C) using a capacitively coupled r.f. plasma-assisted chemical vapour deposition process starting from CH 4 -N 2 gas mixtures. Films were deposited on glass, Si and steel substrates which were placed on the powered and grounded electrodes. The chemical composition of the films was analysed with electron probe microanalysis. By varying the CH 4 : N 2 partial pressure ratio, the N content of the films could be varied between 0 and 13 at.% at the powered electrode and between 0 and 35 at.% at the grounded electrode. Nanoindentation hardness measurements, film stress (bending beam test method) and wear measurements (against a steel ball in the ball-on-disc test) on films deposited at the powered electrode demonstrate that nitrogenation of amorphous carbon films leads to softer and less stressed materials. These results are explained in terms of the microstructural properties of our films which, according to our IR absorption spectroscopy and Raman spectroscopy measurements, can be interpreted on the basis of a familiar a-C: H cluster model.

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.

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.

Microstructure and mechanical properties of a-B1−xNx:H films prepared by r.f. PACVD

Abstract Amorphous B 1−x N x :H films were prepared in a capacitively coupled r.f. plasma-assisted chemical vapour deposition reactor at deposition temperatures below 200 °C starting from N 2 B 2 H 6 (10% in H 2 ) gas mixtures. Films were deposited on Si, steel and glass substrates which were placed on the grounded and powered electrodes. By varying the partial pressures of the gases, the composition parameter x was varied between 0 and 0.55. Optical emission spectroscopy was used to monitor H-, B- and N-related plasma species. The film microstructure was characterized by IR spectroscopy in the wavenumber range 400–4000 cm −1 . Compressive film stress (bending beam method) and hardness (nanoindentation method) were measured as a function of composition and bias voltage. Wear and friction behaviours were studied under humid and dry N 2 ambient conditions using a ball-on-disc apparatus. A comparison between a-B:H films and a-C:H (diamond-like carbon) films reveals similar mechanical properties but a very different wear behaviour. This result is explained in terms of different friction mechanisms related to the formation of transfer layers.