Enhanced toughness in brittle ceramic materials, such as transition metal nitrides (TMN), is achieved by optimizing the occupancy of shear-sensitive metallic electronic-states. This is the major result of my theoretical research, aimed to solve an inherent long-standing problem for hard ceramic protective coatings: brittleness. High hardness, in combination with high toughness, is thus one of the most desired mechanical/physical properties in modern coatings. A significant part of this PhD Thesis is dedicated to the density functional theory (DFT) calculations carried out to understand the electronic origins of ductility, and to predict novel TMN alloys with optimal hardness/toughness ratios. Importantly, one of the TMN alloys identified in my theoretical work has subsequently been synthesized in the laboratory and exhibits the predicted properties.

The second part of this Thesis concerns molecular dynamics (MD) simulations of Ti, N, and TiNx adspecies diffusion on TiN surfaces, chosen as a model material, to provide unprecedented detail of critical atomic-scale transport processes, which dictate the growth modes of TMN thin films. Even the most advanced experimental techniques cannot provide sufficient information on the kinetics and dynamics of picosecond atomistic processes, which affect thin films nucleation and growth. Information on these phenomena would allow experimentalists to better understand the role of deposition conditions and fine tune thin films growth modes, to tailor coatings properties to the requirements of different applications. The MD simulations discussed in the second part of this PhD Thesis, predict that Ti adatoms and TiN2 admolecules are the most mobile species on TiN(001) terraces. Moreover, these adspecies are rapidly incorporated at island descending steps, and primarily contribute to layer-by-layer growth. In contrast, TiN3 tetramers are found to be essentially stationary on both TiN(001) terraces and islands, and thus constitute the critical nuclei for three-dimensional growth.

This thesis concerns computer simulations, using classical molecular dynamics, of transport processes related to TiN(001) growth. It is motivated from the challenge to understand transport processes at the atomic scale responsible for crystal and film growth and their different growth modes. Not even the most advanced experimental techniques are capable of resolving the sub ps time and sub-Ångström length-scales required. TiN belongs to an important class of transition metal nitrides, and is chosen here as a model system for such fundamental studies of surface transport. The simulations show that on terraces, Ti adatoms exhibit much higher migration rates than N adatoms. For TiN_x complexes, as x increases from 1 to 3, rotation becomes increasingly more prevalent than translation. This leads to surprisingly high mobilities of TiN₂ trimers, higher than that of N adatoms. On islands, Ti adatoms experience a significant funneling effect, resulting in short residence times. TiN dimers and TiN₂ trimers exhibit surprisingly high diffusivities and residence times even shorter than Ti adatoms. TiN₃ trimers, however, are essentially stationary on both terraces and islands and serve as nucleation clusters. Overall, Ti adatoms and TiN₂ trimers are the most efficient carriers of Ti and N atoms with and between TiN(001) surface layers. These results indicate that Ti/N flux ratios close to one promote layer-by-layer TiN(001) growth, whereas lower ratios result in surface roughening. Understanding of these phenomena enables experimentalists to tune  the growth processes to optimize material properties.

In this thesis I also carry out theoretical calculations to investigate the role of configurational order on the metallic sublattice in relation to toughness enhancement. My studies set out from the recent understanding that the toughness of transition metal nitrides can be enhanced by tuning the valence electron concentration. My results show that ordered alloys exhibit lower resistance to shear deformations than disordered alloys, and higher resistance to tensile deformation. The lower resistance to shear deformations is explained by the formation of fully bonding electronic states perpendicular to the applied stress. Using the Pugh-Pettifor criterion, it is shown that while configurational order has an effect on the ductility of the material, this is primarily governed by the valence electron concentration.

The atomic-scale dynamical processes at play during film growth cannot be resolved by even the most advanced experimental methods. As such, computational methods, and chiefly classical molecular dynamics, are the only available research tools to study these processes. The investigation of key dynamical processes during thin film growth yields a deeper understanding of the film growth evolution, ultimately allowing for the optimization of experimental parameters and tailoring of film properties. This thesis details the study of fundamental surface dynamics processes, and the role played by primary diffusing species, during TiN film growth, here employed as a model system for transition metal nitrides in general. It is found that Ti adatoms and TiN₂ admolecules are the fastest diffusing species, and the species which most rapidly descend from islands onto the growing film. Thus, they are the main contributors and players in driving the layer-by-layer growth mode. TiN₃ admolecules, in contrast, are essentially stationary and thereby promote multilayer growth. Large-scale growth simulations reveal that tailoring the incident N/Ti ratio and N kinetic energy significantly affects the growth mode and film microstructure.

The mechanical properties of ternary transition metal nitride and carbide alloys, investigated using density functional theory, are also discussed herein, in comparison to recent experimental results. By optimizing the valence electron concentration in these compounds, the occupation of shear-compliant d‑t_2g electronic states can be maximized. The investigation of M¹M²N alloys, where M¹ = Ti or V and M² = W or Mo, with different structures demonstrates that this optimization leads to enhanced ductility, and thereby toughness, in transition metal nitride alloys regardless of the degree of ordering on the metal sublattice. Estimations based on the calculation of the mechanical properties of the corresponding M¹M²C transition metal carbide alloys indicate that these materials remain brittle. However, charge density analysis and calculations of stress/strain curves reveal features commonly associated with ductile materials.