Multipass Scratch Behavior of Borided and Nitrided H13 Steel

AISI H13 steel was subjected to boriding and nitriding; its performance was evaluated by the progressive load scratch test, and then by the multipass scratch test (MPST), to evaluate the accumulated damage. Boriding and nitriding were carried out at a temperature of 800 and 580 °C, respectively, both treatments for 1 and 5 h of exposure time. In scratch test, a load range of 3-90 N was used for borides; a higher range (3-180 N) was chosen for nitrides. Critical loads (L_c) were estimated based on optical microscope observations of the scratch tracks. Failure mechanisms and residual depths were examined by scanning electronic microscope and optical profilometry. Finally, using fractions of L_c, MPST was applied on uncoated and coated samples, for 25, 50, 75 and 100 unidirectional scratch cycles. The coefficient of friction (COF) evolution was recorded and analyzed. A decrease in COF was observed as cycles go through; it was initially higher in uncoated sample; moreover, it was observed that normal load was determinant in its behavior, regardless of the treatment time. Nitrided samples exhibited less catastrophic failures over borided ones, also showed a better volume loss/load ratio performance, from 6 to 8 times better compared to borided samples.