Vacuum environments provide challenging conditions for tribological systems. MoS2 is one of the materials commonly known to provide low friction for both ambient and vacuum conditions. However, it also exhibits poor wear resistance and low ability to withstand higher contact pressures. In search of wear-resistant alternatives, superhard hydrogen-free tetrahedral amorphous carbon coatings (ta-C) are explored in this study. Although known to have excellent friction and wear properties in ambient atmospheres, their vacuum performance is limited when self-paired and with steel. In this study, the influence of the paired material on the friction behavior of ta-C is studied using counterbodies made from brass, bronze, copper, silicon carbide, and aluminum oxide, as well as from steel and ta-C coatings as reference materials. Brass was found to be the most promising counterbody material and was further tested in direct comparison to steel, as well as in long-term performance experiments. It was shown that the brass/ta-C friction pair exhibits low friction (µ < 0.1) and high wear in the short term, irrespective of ambient pressure, whereas in the long term, the friction coefficient increases due to a change in the wear mechanism. Al2O3 was identified as another promising sliding partner against ta-C, with a higher friction coefficient than that of brass (µ = 0.3), but considerably lower wear. All other pairings exhibited high friction, high wear, or both.

Thermally sprayed WC-based hardmetal coatings offer high hardness, good sliding wear and abrasion performance and find large applications in mechanical engineering, valve construction, or offshore applications. WC-Co coatings are mainly produced by high-velocity oxy-fuel spraying (HVOF) from conventional spray feedstock powders. In this work, suspension-HVOF spraying (S-HVOF) was used to produce dense-structured WC-12Co coatings and their microstructural, mechanical and tribological properties were investigated. Significant work was devoted to the development of appropriate aqueous suspensions starting from commercially available fine WC and Co raw powders feedstock. Suspension spraying was carried out using gas-fuelled HVOF TopGun system; for comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercially available spray powder. Microstructural characterization, x-ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered alumina and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of their microstructure, phase composition and coating-ball test couples.

It is widely known that tribological systems operating in vacuum environments face significant challenges. MoS2 is a well-known material for its low friction properties in both ambient and vacuum environments. However, it has a low wear resistance and a limited ability to withstand higher contact pressures. A possible alternative to MoS2 are super-hard hydrogen-free tetrahedral amorphous carbon coatings (ta-C). In normal atmospheric conditions, they have low friction and wear properties, but their performance in vacuum is limited. In this work, ta-C coatings are paired with brass in high vacuum conditions. Long-term tribological performance tests are conducted and analyzed using triboscopic imaging. It was found that the brass/ta-C friction pair exhibited an increase of the friction coefficient in the long term (up to 0.52) due to a change in wear mechanism. Different wear and healing behaviors could be found for both the brass counterbody and the ta-C discs using triboscopy.