K. Vercammen

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).

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.

Abrasive wear by TiO2 particles on hard and on low friction coatings

Abstract The ball cratering test was used to study the abrasive wear resistance of a wide variety of coatings against TiO2 particles. The rutile TiO2, used as white pigment, causes mild abrasive wear in mixers, dyes, injection moulds and other components in the paint and plastic processing. The tests yield a clear ranking of the coatings. The abrasive wear due to the rutile particles is also compared to the wear induced by SiC particles commonly used for ball cratering tests. Differences in the wear mechanism using the latter are demonstrated. Furthermore, the calculation methods to derive the coating and substrate wear coefficients from the wear volume measurements have been critically reviewed. It can be concluded that the methods are especially useful for determining the wear coefficient in the coating. However, the substrate wear coefficient cannot be accurately determined when the wear resistance of the substrate is much lower than that of the coating. It is shown that the ball cratering test is not restricted to polished samples, but remains useful for moderate roughness as applicable to the desired components.

Friction and Wear of Diamond-Like Carbon Coatings Lubricated With Biodegradable Oils

The tribological behaviour of coated machine components lubricated with a biodegradable saturated ester and unsaturated ester oil has been studied. Different diamond-like carbon-based coatings (DLC) were selected as low friction coatings: pure DLC, Si-doped, Ti-doped and W-doped DLC. The performance of the studied ester oils has been compared with sunflower oil and mineral oil lubrication as reference. The oils were all additivated in the same way with conventional anti-wear (AW) and extreme pressure (EP) additives. Different tests have been performed: disc on disc, ball on disc and block on ring tests give an overview of the influence of different sliding modes. The effect of the lubricant on a coated system compared to the use of the coated system without lubrication is clear for the wear, but less obvious for the friction. It was found that DLC/DLC contacts lubricated with biodegradable lubricants show similar friction as steel/steel contacts. The effect of the different types of base oil is independent of the coating type. Additive activity which was clear in steel/steel contacts was not obvious in DLC/DLC contacts.Copyright