3.4.2 Hardness and elastic modulusI

3.4.2 Hardness and elastic modulusIn transition metal compounds, the chemical bond and cohesion energy are related to the mechanical properties, like Young's modulus and hardness. Fig. 5(a) displays nanoindentation measurements for the different samples as a function of the depth penetration. The indentation depth of the curve of the Mo-N depends of the nitrogen concentration in the film. The result is in good agreement with the variation of the compressive residual stress into the film and the information obtained from the microstructure images. The films at 20% and 50% nitrogen concentration in the plasma present high compressive stresses and dense microstructure areas, consequently the penetration of the indenter is lower than in the other films and inhibit crack propagation in the coating, while at 30% and 40% nitrogen concentration low compressive stresses and separate columnar areas would facilitate crack propagation and changes in the mechanical properties of the thin films. However, the indentation depth at 40% and 50% nitrogen concentration is greater of about 10% than for the other films and the substrate properties can affect the calculated hardness and elastic modulus of the films. The best way to estimate the hardness of a film is to deposit it on a substrate that has the same elastic modulus as that of the film, but unfortunately this occurs rarely. The measured hardness variation is explained by morphological changes. The extracted values of hardness H and elastic modulus E for each film are represented in Fig. 5(b) as a function of the N/Mo atomic ratio. A correlation between the hardness and elastic modulus with the penetration depth is observed. The values of H and E lay in the range between 13 and 18 GPa, and 198 to 240 GPa respectively, depending on the nitrogen partial pressure in the plasma and N/Mo ratio in the films.