Microstructure and Mechanical Properties of Magnetron Sputtered Refractory Metal Thin Films

Sammanfattning: The design and development of new multifunctional materials that exhibit a combination of high hardness and ductility, as well as a high corrosion resistance and thermal stability, is one of the key challenges in the field of material science. The focus of this thesis is on the development of novel multifunctional magnetron sputtered CrNbTaTiW–C based thin films. Carbon was selected as an alloying element to investigate if it could modify the microstructure (via grain refinement) and improve the properties (e.g. the hardness and ductility).TaW-rich and near-equimolar high entropy alloys in the CrNbTaTiW system were selected as starting points for this study. The latter alloys were predicted, based on empirical design rules, to form a single-phase solid solution. In contrast, thermodynamic calculations showed that the films at equilibrium should be composed of a mixture of several phases at temperatures below 1100 °C.  Experimentally, however, a single-phase bcc structure was observed for the deposited films and it was concluded that the films were kinetically and not entropy stabilised. A hypothesis is that the kinetics during sputtering allow a ’direct’ phase selection by tuning the process parameters and evidence of this was found in the HfNbTiVZr alloy system.The CrNbTaTiW–C system is, however, complex and additional studies were carried out on the W–C and TaW–C systems. All metallic films crystallised in a bcc structure with a <110> texture and the column width of these films varied between 25 nm and 80 nm. The films were very hard (~ 13 GPa), which was explained by the small grain size. A single-phase bcc structure was also obtained upon the addition of 5-10 at.% carbon for all compositions except the near-equimolar CrNbTaTiW. X-ray diffraction indicated a unit cell expansion, which was attributed to the formation of a supersaturated solid solution. Additional atom probe tomography (APT) studies on selected samples confirmed the formation of such solid solutions. The supersaturated solid solution is not thermodynamically stable and an annealing study showed that heat treatment yielded segregation and clustering of carbon at the grain boundaries. The addition of carbon had a grain refining effect in the W–C system and the multicomponent CrNbTaTiW–C system. In general, the addition of carbon increased the hardness, which was mainly caused by a reduced grain size in line with the Hall-Petch relationship. Excellent mechanical properties of carbon supersaturated films were further confirmed in pillar tests on W–C films, which showed very high yield strength (~ 9 GPa) and no brittle fracture. The results show that carbon can be used as a chemical approach to control the grain size and properties of these films. Multicomponent carbides with a B1 structure were formed at high carbon concentrations (~ 40 at.%). The microstructure of these films depended strongly on the process parameters and a higher deposition temperature was found to increase the film density and hardness. The TaW-rich carbide exhibited a very high hardness of ~ 35 GPa and excellent corrosion resistance.