Effects of implantation temperature and volume flow rate ratio of nitrogen and hydrogen on nitrogen concentration distribution, mechanical properties, fatigue life, fracture toughness, and tribological behavior of plasma-nitrided P20, 718 and 420 steels

In the present study, three steel materials, AISI P20 DEM NO.3, ASSAB 718 and AISI 420, were selected as the mold substrate materials in plastic injection forming. The plasma immersion ion implantation (PIII) system with an electron cyclotron resonance microwave source was applied to prepare the specimens by varying the implantation temperature (400 °C, 460 °C and 520 °C) and the volume flow rate ratio (4:1, 1:1 and 1:3) of nitrogen to hydrogen in the gas mixture (N2:H2). For the specimens with the same substrate material, the mean hardness was either invariant to or lowered by increase in the total penetration depth of nitrogen. The hardness was lowered by increasing the distance of the peak position of nitrogen concentration from the implantation surface. Both the fracture toughness and the fatigue life of a specimen at the nitrided layer were elevated by increasing the implantation temperature. The most significant increase in each of these two mechanical properties due to the temperature increase gives rise in the P20 specimens. Varying the N2:H2 ratio shows a fatigue life sequence of (FL)4:1 > (FL)1:3 ≥ (FL)1:1 for all three substrate materials. X-ray diffraction (XRD) was applied to determine the phase structures at the nitrided layer which was formed by changing either the implantation temperature or the N2:H2 ratio. The (α-Fe + CrN) phase in the nitrided later can elevate the fatigue life and fracture toughness of the specimen. However, the increase in the hardness is due to the combined effect of all main phases formed in the nitrided later. The scratch wear resistance of a specimen with ion implantations is significantly enhanced compared with that exhibited in the “pure” specimen (without nitrided layer). However, most of the specimens with ion implantations showed severer adhesive wear than corresponding “pure” specimens.