Mo 2 BC coating synthesis, properties and interfacial interactions

In this Ph. D. thesis, low temperature phase formation and application potential of Mo₂BC protective coatings for Al forming are studied. Additionally, structural and mechanical properties of isostructural X₂BC (X= Mo, Ti, V, Zr, Nb, Hf, Ta and W) phases as well as their phase stability are investigated. Furthermore, the effect of Si additions on the thermal stability of sputtered amorphous Al₂O₃ thin films is studied. Emmerlich et al. [1] reported the formation of Mo₂BC coatings at a substrate temperature of 900°C by combinatorial magnetron sputtering. This synthesis temperature limits the choice of substrate materials severely. Here, utilizing high power pulsed magnetron sputtering (HPPMS), the synthesis temperature was reduced to 380°C. Since the crystallization of amorphous Mo₂BC powder was observed at 820°C, the plasma was analyzed to identify the cause of the significantly reduced synthesis temperature. The flux and energy characteristics of ionized plasma species and their effect on growth of Mo₂BC thin films were studied for direct current (DCMS), pulsed DC and HPPMS with nominal target peak power densities ranging from 6.1 up to 1082.5 W/cm². It is shown that the population of energetic ionized species increases and energy distribution functions extend to larger energies in HPPMS regime compared to DCMS. Additionally, two orders of magnitude larger instantaneous ion fluxes in HPMMS regime were measured compared to DCMS. Thus, low temperature synthesis of Mo₂BC is surface diffusion controlled. The synthesis strategy reported here greatly expands the range of technologically interesting substrate materials for application. To investigate the application potential of Mo₂BC protective coatings for Al forming, interfacial interactions between Mo₂BC and Al are theoretically and experimentally investigated. Based on ab initio simulations and calculations, chemical bonds formed across the pristine as well as the oxidized Mo₂BC(040) and Al interfaces are stronger than the Al s - Al s intra-cluster bonds. The formation of interfacial bonds is shown to be the onset of Al adhesion. This is validated experimentally by correlative compression-torsion-tribometer-experiments and subsequent composition analysis based on energy dispersive x-ray spectroscopy elemental maps. A systematic ab initio study was carried out on Mo₂BC-type phases wherein Mo is replaced by other transition metals, namely Ti, V, Zr, Nb, Hf, Ta and W. Structural and mechanical properties of all here mentioned ternary phases were calculated. Finally, based on correlative calorimetry and diffraction experiments a significant Si induced increase in thermal stability for sputtered amorphous alumina thin films was identified. Density functional theory molecular dynamics simulations and synchrotron X-ray diffraction experiments for amorphous Al₂O₃ with and without Si incorporation suggests that the experimentally identified enhanced thermal stability of amorphous alumina with addition of Si is due to the formation of shorter and stronger Si – O bonds as compared to Al – O bonds.