Magnetron sputtering of binary, ternary and multicomponent thin film borides and carbides

Design of new thin film materials with improved properties and functionality is an important research field in materials science. In this thesis, I have used magnetron sputtering to deposit new thin film materials, which should combine high hardness and high ductility with other properties such as low friction or a good corrosion resistance. The films have been characterized with a range of techniques to determine the correlation between deposition parameters, microstructure and properties. The first part of the thesis is focussed on binary and ternary molybdenum-containing boride thin films with potential low-friction properties. It was found that the binary Mo-B films exhibited a nanocrystalline structure of ~ 16 nm large MoB2-x grains surrounded by an amorphous tissue phase. The special microstructure resulted in a very high hardness (29 ± 2 GPa), but limited ductility. An attempt to improve the ductility by formation of crystalline layered ternary borides such as Mo2BC and Mo2FeB2 failed due to difficulties to reach high deposition temperatures. However, the addition of carbon to the binary Mo-B films resulted initially in a reduced grain size of the MoB2-x grains. For carbon concentrations >23 at.%, a completely amorphous film was formed. Transmission electron microscopy (TEM) studies revealed that these films contained Mo-rich regions surrounded by a Mo-poor amorphous BCx phase and therefore best can be described an amorphous nanocomposite. The mechanical properties of the Mo-B-C films were strongly correlated to the amount of the softer amorphous BCx phase. Also the Mo-Fe-B films were amorphous with a hardness ranging from 19 to 26 GPa and a limited ductility. Tribological studies of the Mo-B, Mo-B-C and Mo-Fe-B films showed very high coefficients of friction (>0.8). Molybdenum and boron oxides were identified on the surface by Raman spectroscopy, but no indication of lubricating effects from these oxides could be observed. The results suggest that Mo-based borides have limited applications in future low-friction applications. In the second part of the thesis, a high entropy alloy (HEA) of TiCrNbTaW was studied with and without the addition of carbon. The results showed that a HEA phase with bcc structure could be deposited in a wide composition and temperature range. Small amounts of carbon were dissolved into the bcc phase, while higher carbon contents resulted in a multicomponent carbide phase. The correlation between structure and properties were studied in detail. It was found that some of the films exhibited very high hardness combined with high ductility suggesting potential application of these materials as wear-resistant coatings. Finally, the corrosion resistance of these complex alloys were investigated in a very acidic environment. They were found to exhibit excellent corrosion resistance superior to hyper-duplex stainless steels