Synthesis and characterisation of Mo-B-C thin films deposited by non-reactive DC magnetron sputtering

Thin films in the Mo-B-C system with varying carbon content (up to 37 at.%) were deposited using non-reactive DC magnetron sputtering. The phase composition and microstructure were determined and the potential use of the films in sliding electrical contact applications was evaluated. Films with lower than 23 at.% carbon content consisted of nanocrystalline MoB2 − x grains surrounded by an amorphous tissue phase (a-B for binary, and a-BCx for ternary films). With increasing carbon content grain sizes was found to decrease (from 16 to 5 nm), and above 23 at.% carbon the films deposited at room temperature were X-ray amorphous. Scanning transmission electron microscopy and energy dispersive X-ray spectroscopy reveal that these films contain Mo-rich and Mo-poor regions, and thus are two-phase amorphous nanocomposites. Low-carbon content samples exhibited a friction coefficient against the steel counter surface of 1.1; this was reduced to 0.8 for high carbon-content films. Analysis of the tribofilm revealed formation of molybdenum oxide and amorphous carbon, however without significant lubricating effect at room temperature. Hardness and elastic modulus decrease with carbon content from ~ 29 to ~ 22 GPa and ~ 526 to ~ 326 GPa. These values give an H/E ratio of 0.06 to 0.07, indicating brittle material. Resistivity was found to increase with carbon content from ~ 175 μΩ cm for binary Mo-B to ~ 395 μΩ cm for Mo-B-C thin film with 37 at.% of C. Therefore all the above results suggest that the Mo-B-C films are not suitable for sliding electrical contacts.