In the present thesis the electrodeposition of Ni-P coatings as well as the codeposition of SiC and WC particles (mean diameter of 1 μm and 200 nm, respectively) with Ni-P matrix from an additive-free modified Watts type bath using a rotating disc electrode and direct and pulse current regime, were studied. The aim of this work was the study of the effect of various electrolysis parameters on the structure, morphology and composition of the coatings, along with the study of the mechanical properties of the coatings focusing in their hardness and tribological performance. Furthermore, the structural properties were correlated with the examined mechanical properties. For the Ni-P coatings a dependence of phosphorous content of the alloy on the imposed conditions of the pulse current regime (i.e. the value of duty cycle and the frequency of the pulses) was revealed. The structure of the coating is characterized as amorphous, while under intense conditions of pulse current (low duty cycle values - high pulse frequencies) the production of the crystalline phase of Νi12P5 was achieved for the first time. Regarding the Ni-P/SiC and Ni-P/WC composite coatings the imposition of pulse current regime led to significant higher codeposition percentage of reinforcing particles, as well as to more homogeneous dispersion in the Ni-P matrix. It should be mentioned that the codeposition of Ni-P/WC coatings was achieved/recorded for the first time. Although SiC codeposition resulted in a decrease of phosphorous content in the matrix, this effect was observed in the case of WC particles. However, it was revealed that the Ni-P matrix of all composite coatings had also an amorphous structure. After thermal treatment at 400 oC the amorphous phase was crystallized in the steady phases of Ni, Ni3P and Ni2P. Moreover, during thermal treatment diffusion phenomena took place, as well as chemical reactions in the interfaces of substrate - coating and reinforcing particles - matrix. The hardness of the Ni-P coatings was in the range of 4,5-5,5 GPa, higher than that of pure nickel, due to the formation of the solid solution of nickel - phosphorous. After thermal treatment the hardness demonstrated a significant increase due to the formation of Ni crystals and the precipitation of Ni3P phase and reached the value of 12 GPa. The hardness values of the composite coating before thermal treatment varied between 5 and 7 GPa, while after thermal treatment were in the range of 12 and 18,5 GPa. The study of the tribological performance of the coatings revealed that the main wear mechanisms were those of abrasion and adhesion. In some cases of Ni-P coatings, fatigue was also detected. Regarding the wear volume loss, it was concluded that the imposition of pulse current, the codeposition of reinforcing particles and the thermal treatment have a beneficial effect on the wear resistance of the coatings. It should be mentioned that among all the coating that were studied in the present work the Ni-P/WC ones showed a superior wear resistance.