Ceramic-reinforced HEA matrix composites exhibiting an excellent combination of mechanical properties.

CoCrFeNi is a well-studied face centered cubic (fcc) high entropy alloy (HEA) that exhibits excellent ductility but only limited strength. The present study focusses on improving the strength-ductility balance of this HEA by addition of varying amounts of SiC using an arc melting route. Chromium present in the base HEA is found to result in decomposition of SiC during melting. Consequently, interaction of free carbon with chromium results in the in-situ formation of chromium carbide, while free silicon remains in solution in the base HEA and/or interacts with the constituent elements of the base HEA to form silicides. The changes in microstructural phases with increasing amount of SiC are found to follow the sequence: fcc → fcc + eutectic → fcc + chromium carbide platelets → fcc + chromium carbide platelets + silicides → fcc + chromium carbide platelets + silicides + graphite globules/flakes. In comparison to both conventional and high entropy alloys, the resulting composites were found to exhibit a very wide range of mechanical properties (yield strength from 277 MPa with more than 60% elongation to 2522 MPa with 6% elongation). Some of the developed high entropy composites showed an outstanding combination of mechanical properties (yield strength 1200 MPa with 37% elongation) and occupied previously unattainable regions in a yield strength versus elongation map. In addition to their significant elongation, the hardness and yield strength of the HEA composites are found to lie in the same range as those of bulk metallic glasses. It is therefore believed that development of high entropy composites can help in obtaining outstanding combinations of mechanical properties for advanced structural applications.

Effect of Annealing Heat Treatment on the Composition, Morphology, Structure and Mechanical Properties of the W-S-N Coatings.

Alloyed-transition metal dichalcogenide (TMD) coatings have been under investigation as multi-environment lubricants for the past few decades. These coatings display very low coefficient of friction properties at elevated temperatures. Studies on the annealing of these low-friction coatings are missing in the literature. For the first time, in this study, the annealing of the W-S-N dry lubricant coatings was carried out to study its effects on the composition, morphology, crystal structure and hardness of the coatings. The W-S-N coatings were deposited by direct current (DC) reactive magnetron sputtering. The analysis was carried out for as-deposited, 200 °C and 400 °C annealed coatings. The as-deposited coatings have N content in the range of 0-25.5 at. %. The coatings are compact and the densification increased with the increase in N-alloying. All the coatings are crystalline except the highest N-alloyed coating which is X-ray amorphous. A maximum hardness of 8.0 GPa was measured for the coating alloyed with 23 at. % N. Annealing did not affect the composition and morphology of the coatings, while some variations were observed in their crystal structure and hardness. The maximum hardness increased from 8 GPa to 9.2 GPa after 400 °C annealing of the 23 at. % N-alloyed coating.

On the Microstructural, Mechanical and Tribological Properties of Mo-Se-C Coatings and Their Potential for Friction Reduction against Rubber.

Friction and wear contribute to high energetic losses that reduce the efficiency of mechanical systems. However, carbon alloyed transition metal dichalcogenide (TMD-C) coatings possess low friction coefficients in diverse environments and can self-adapt to various sliding conditions. Hence, in this investigation, a semi-industrial magnetron sputtering device, operated in direct current mode (DC), is utilized to deposit several molybdenum-selenium-carbon (Mo-Se-C) coatings with a carbon content up to 60 atomic % (at. %). Then, the carbon content influence on the final properties of the films is analysed using several structural, mechanical and tribological characterization techniques. With an increasing carbon content in the Mo-Se-C films, lower Se/Mo ratio, porosity and roughness appeared, while the hardness and compactness increased. Pin-on-disk (POD) experiments performed in humid air disclosed that the Mo-Se-C vs. nitrile butadiene rubber (NBR) friction is higher than Mo-Se-C vs. steel friction, and the coefficient of friction (CoF) is higher at 25 °C than at 200 °C, for both steel and NBR countersurfaces. In terms of wear, the Mo-Se-C coatings with 51 at. % C showed the lowest specific wear rates of all carbon content films when sliding against steel. The study shows the potential of TMD-based coatings for friction and wear reduction sliding against rubber.

Mechanical Properties and Vacuum Tribological Performance of Mo-S-N Sputtered Coatings.

MoS2 is the most widely used dry lubricant for low friction applications in vacuum environments. However, due to its lamellar nature it exfoliates during sliding, leading to high wear, high coefficient of friction (COF), and low stability. Here, we report the mechanical properties and the vacuum (10-4 Pa) tribological performance of nitrogen-alloyed transition-metal-dichalcogenide (TMD-N) coatings. The coatings were deposited using a hybrid deposition method, that is, reactive direct current (DC) sputtering of MoS2 target assisted by an additional plasma source. The tribological tests were performed at relatively low contact stresses to replicate real industrial needs. The interaction between different mating surfaces (coating versus steel, coating versus coating) has been reported. Additionally, the effects of loads on the sliding properties were also studied for coating versus coating interactions. A maximum hardness of 8.9 GPa was measured for the 37 atom % N-alloyed coating. In all mating conditions, the pure MoS2 coating had COF in the range of 0.1-0.25 and the least specific wear rates were found to be 3.0 × 10-6 mm3/N·m for flat and 2.5 × 10-6 mm3/N·m for cylinder. As compared to MoS2 coating, the COF and specific wear rates decreased with N additions. The COF was in the range of 0.05-0.1 for Mo-S-N coatings, while coating versus coating displayed the lowest specific wear rates (8.6 × 10-8 mm3/N·m for flat and 4.4 × 10-8 mm3/N·m for cylinder). Finally, the increase in load resulted in a decrease of COF, but an increase in the wear rate was observed. The detailed mechanism behind the behavior of the COF for the different mating conditions was presented and discussed. This work brings some important issues when testing transition metal dichalcogenide-based coatings under low contact stress conditions more appropriate for simulating real service applications.