Microwave plasma nitriding of a low-alloy steel

A low-alloy steel was nitrided in the downstream zone of an electron cyclot ron resonance (ECR) discharge at low pressure (0.15-0.25 Pa). Structure and properties of the nitrided surfaces were primarily controlled by the subst rate temperature T-s (300-550 degreesC), the nitrogen-hydrogen-argon gas mi xture composition, and the substrate bias voltage U-b (-1000 to +40 V) at a typical value of the incident microwave power P-i = 900 W, the distance of the substrate from output aperture of the ECR source was d = 250 mm, and t he nitriding time t = 2 h. Optical emission spectroscopy was used to study the behavior of significant atomic and molecular species, such as N-2, N-2( +), NH, H, Fe, Ar, and Art, in front of the nitrided sample. It has been fo und that due to a high plasma reactivity, nitriding is effective at substra te temperatures T(s)greater than or equal to 500 degreesC, when the surface hardness is about 1200 HV0.05 and a diffusion layer thickness up to 120 mu m has been achieved for t = 2 h. The presence of 10%-30% H-2 in a nitrogen- hydrogen gas mixture enhances the efficiency of nitriding in comparison wit h treatment in pure nitrogen under the same conditions. For T(s)greater tha n or equal to 500 degreesC, the process is effective even for substrates at positive potential (U-b = + 30 V) and for a lower amount of N-2 (10%-30%) in a nitrogen-hydrogen-argon gas mixture, for which no compound layer is fo rmed on the nitrided surface. However, the negative substrate bias voltage U-b enhances considerably the efficiency of nitriding only at lower substra te temperatures (Ts approximate to 400 degreesC), when the nitriding is rel atively weak. (C) 2000 American Vacuum Society. [S0734-2101(00)00906-4].