Fig. 4a shows the hardness behavior of the 3 μm coating system over nitrided and non nitrided substrates. Examining this plot one can identify the second and the third stage of the hardness behavior across the coating thickness. In the third stage, it is clear that the hardness across the coating thickness is higher when the coating system is over nitrided substrates. Furthermore, the hardness across the 3 μm coating thickness over nitrided case A and B shows practically
the same behavior, meaning that neither case surface hardness nor case depth influences the hardness distribution throughout
this coating thickness. On the other hand, the hardness behavior of the 8.3 μm coating thickness over nitrided and non nitrided substrates shows a small difference among the three substrate conditions as shown in Fig. 4b. Comparing the two coating thickness