## 1 Introduction
Indicative systems with reinforcing phases, such as Al\({}_{2}\)O\({}_{3}\)[19; 20; 21; 22; 23], SiO\({}_{2}\)[24], Cr\({}_{2}\)O\({}_{3}\)[25], ZnO [26], SiC [27; 28; 29], B\({}_{4}\)C [30], WC [31], CNTs [32; 33], Graphite [34], BN [35], Diamond [21; 33], Si3N4 [27; 36], CeO\({}_{2}\)[37], MoS\({}_{2}\)[38; 39], PTFE [40; 41], and combinations of them [42; 43] are reported. The hardness of coatings is of crucial importance and its optimization and improvement is strongly associated with other important properties, such as friction behavior and wear resistance. Various research efforts that are reported in the literature show that the increase of the embedded particle concentration leads to an increase of the overall coating hardness in the case that hard reinforcing particles are co-deposited (carbides, oxides, nitrides) [23; 24; 25; 26; 27; 37; 38] and they may decrease the overall hardness, if soft particles are co-deposited (PTFE for example) [40; 41]. However, the main hardness increase is achieved with the post heat treatment. The nanoindentation technique can also been applied for the determination of the mechanical properties of nanocomposites, and specifically of surface layers and coatings. This technique can provide valuable data regarding the mechanical behavior of materials. Several papers have compared the nanoindentation results with the results that were acquired from classical tensile tests and especially for the elastic modulus calculation [44; 45; 46]. The nanoindentation tests have been used in the past for the investigation of coatings materials. More specifically, [PERSON] et al. [47] used nanoindentation tests for the assessment of the mechanical property response of the Ni-P coatings and composites and for the mechanical response of Ni-P-TiO2 coatings. They observed that density and elastic modulus were both improved by the insertion of TiO\({}_{2}\) nanoparticles, with this improvement, nevertheless, fading by exceeding and optimum concentration above which particle agglomeration phenomena lead to a gradual decrease of such properties. [PERSON] et al. [17] also studied the mechanical propertiesof the Ni-P electroplated coatings that were reinforced with SiC particles. This showed that the particle presence does not significantly alter the modulus of elasticity of the plain coatings and that the hardness was slightly improved. They associated behavior with the effectivity the particles have in their strengthening effect, which is related with their size and/or their agglomeration behavior. The same authors [48] also studied, with the use of nanoindentation testing, the effects that talk phyllosilicate particles have on the mechanical properties of Ni-P coatings. They observed that, for the as plated condition, the presence of these particles decrease the hardness due to their intrinsic soft nature. Improvement was observed after heat treatment, which was rather an effect of matrix crystallisation and precipitation phenomena than being due to the presence of particles. [PERSON] et al. [49] studied the effect of Multi Wall Carbon Nanotubes (MWCNTs) at different annealing conditions within the same frame of improving the mechanical properties of Ni-P coatings. Their nanoindentation results showed that the presence of MWCNTs improves the hardness up to certain annealing temperature range above which grain growth and coarsening phenomena of the precipitated Ni3P phases lead to a gradual reduction. [PERSON] et al. [50] examined the effect of TiO\({}_{2}\) nanoparticles on the mechanical properties of Ni electrodeposited coatings. They observed that, the smaller the particle size, the more effective the strengthening effect of particles. However, this enhancement, as the authors showed, is not independent of other crucial processing parameters that can alter other microstructural features, such as grain size and boundaries, which also may play a vital role in the overall mechanical response. [PERSON] et al.