substrate. Failure of the coating occurs with delamination due to
the growth of radial cracks and to pile-up formation in the
substrate. The A1 and A4 coatings show HF4 adhesion strength
quality and failure of the coating also occurs by delamination
due to the same mechanisms mentioned above, although in a
smaller extent. HF4 adhesion strength quality may be interpreted
as a transition between good adhesion and poor adhesion. None
of the specimens showed circular cracks in the neighborhood of
the indentation.
Plasma nitriding the H13 tool steel resulted in a 20 μm
diffusion layer without the formation of a white layer; the
adhesion strength quality of the coating deposited on the plasma
nitrided steel was HF1. Although radial cracks and substrate
pile-up could be seen near the indentation mark, they did not
lead to failure of the coating by delamination, indicating a very
good adhesion of the coating. The significant improvement of
the adhesion strength quality can be associated to the hardening
of the substrate due to plasma nitriding, increasing the load
bearing capacity of the substrate.
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.
4. Conclusions
Changing the amount of reactive gas in the rf magnetron
sputtering process using (Ti + N2) and (Ti +CH4) gases, allows