Plasma immersion ion implantation (PI3) and rf plasma nitriding of a microalloyed steel

Plasma immersion ion implantation (PI3) is a hybrid surface treatment combining high-energy nitrogen implantation with plasma nitriding. More specifically, in PI3 the sample, whilst immersed in a radiofrequency (r.f.) plasma, is negatively biased to the implantation voltage, typically about 45 kV pulses with a duration of 100 mu s. The high voltage is responsible for nitrogen implantation. However, it is believed that additional nitrogen would be thermochemically absorbed on the surface during the high-voltage off-period owing to the presence of a d.c. self-bias developed on the sample still immersed in the r.f. plasma. The exact contribution of this self-bias to the total nitrogen mass transfer is not known at this time, but it is suspected to be significant, judging by the fact that the total nitrogen content of steels after PI3 treatment is always many times greater than the nominated implantation dose. In an effort to gain an idea about the extent of thermochemically driven nitrogen absorption, the following investigation was undertaken. Samples of a commercially available microalloyed steel were treated by PI3 and r.f. plasma nitriding under identical conditions. Based on extensive microstructural characterization of nitrided samples, it has been found that the PI3 process produced surface layers with higher hardness and thicker cases than r.f. plasma nitriding.