The H/E values of the plasma nitrided substrate and of the TiN (A2) coating, deposited in the hybrid process, are very similar ∼ 0.05, leading to a gentle transition of mechanical properties. It is worth noticing that the thickness of the TiN
coating obtained in the hybrid treatment is also 2.5 μm. According to Mathews et al. H3/E2 is an indicator of the coating resistance to plastic deformation. The H3/E2 values for the TiN (A2) film (H = 20 GPa and E = 340 GPa), for the plasma nitrided H13 steel (H = 12 GPa and E = 210 GPa) and for the non-nitrided H13 steel (H = 6 GPa and E = 210 GPa) are 0.04, 0.07 and 0.005 respectively. The H3/E2 values for the TiN (A2) coating and for the plasma nitrided steel are quite similar, in contrast with the value for the non-nitrided steel which is one order of magnitude lower. This great difference accounts for the poor adhesion properties of the TiN coating to the non-nitrided quenched and tempered H13 tool steel.