<p>The recently established theory has built clear connections between hardness and toughness and electron structure involving both valence electron concentration (VEC) and core electron count (CEC) in transition metal nitride (TMN) ceramics. However, the underlying deformation mechanisms remain unclear. Herein, we conduct in-depth analysis on microstructure evolution during deformation of the high VEC–CEC solution TiMoN coatings having desired combination of high hardness and toughness. The effects of solid solution, preferred orientation linked with symbiotic compressive stress, grain size and dislocations are systematically discussed. We discover that numerous dislocations have been implanted into the nanocrystals of the TiMoN coating during the high-ionization arc deposition. Using two-beam bright-field imaging, we count the dislocation density and confirm occurrence of dislocation multiplication to form effective plastic deformation, which contributes to significant strain hardening, comparable to solid solution hardening, fine-grain hardening and compressive stress hardening. The improved dislocation activities also play a crucial role in enhancing the toughness by providing extra energy dissipation paths. This work gains new insights into the origins of mechanical properties of ceramic coatings and possibility to tune them via defects.</p> Graphical abstract